3843905
Sex Differences in Context Fear Generalization and Recruitment of Hippocampus and Amygdala during Retrieval
Anxiety disorders are commonly associated with increased generalization of fear from a stress- or trauma-associated environment to a neutral context or environment. Differences in context-associated memory in males and females may contribute to increased susceptibility to anxiety disorders in women. Here we examined sex differences in context fear generalization and its neural correlates. We observed stronger context fear conditioning and more generalization of fear to a similar context in females than males. In addition, context preexposure increased fear conditioning in males and decreased generalization in females. Accordingly, males showed stronger cFos activity in dorsal hippocampus during memory retrieval and context generalization, whereas females showed preferential recruitment of basal amygdala. Together, these findings are consistent with previous research showing that hippocampal activity correlates with reduced context fear generalization. Differential competition between hippocampus and amygdala-dependent processes may thus contribute to sex differences in retrieval of context fear and greater generalization of fear-associated memory.
13781258
Twelve‐month and lifetime prevalence and lifetime morbid risk of anxiety and mood disorders in the United States
Estimates of 12‐month and lifetime prevalence and of lifetime morbid risk (LMR) of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM‐IV‐TR) anxiety and mood disorders are presented based on US epidemiological surveys among people aged 13+. The presentation is designed for use in the upcoming DSM‐5 manual to provide more coherent estimates than would otherwise be available. Prevalence estimates are presented for the age groups proposed by DSM‐5 workgroups as the most useful to consider for policy planning purposes. The LMR/12‐month prevalence estimates ranked by frequency are as follows: major depressive episode: 29.9%/8.6%; specific phobia: 18.4/12.1%; social phobia: 13.0/7.4%; post‐traumatic stress disorder: 10.1/3.7%; generalized anxiety disorder: 9.0/2.0%; separation anxiety disorder: 8.7/1.2%; panic disorder: 6.8%/2.4%; bipolar disorder: 4.1/1.8%; agoraphobia: 3.7/1.7%; obsessive‐compulsive disorder: 2.7/1.2. Four broad patterns of results are most noteworthy: first, that the most common (lifetime prevalence/morbid risk) lifetime anxiety‐mood disorders in the United States are major depression (16.6/29.9%), specific phobia (15.6/18.4%), and social phobia (10.7/13.0%) and the least common are agoraphobia (2.5/3.7%) and obsessive‐compulsive disorder (2.3/2.7%); second, that the anxiety‐mood disorders with the earlier median ages‐of‐onset are phobias and separation anxiety disorder (ages 15–17) and those with the latest are panic disorder, major depression, and generalized anxiety disorder (ages 23–30); third, that LMR is considerably higher than lifetime prevalence for most anxiety‐mood disorders, although the magnitude of this difference is much higher for disorders with later than earlier ages‐of‐onset; and fourth, that the ratio of 12‐month to lifetime prevalence, roughly characterizing persistence, varies meaningfully in ways consistent with independent evidence about differential persistence of these disorders. Copyright © 2012 John Wiley & Sons, Ltd.
25030446
A behavioural neuroscience perspective on the aetiology and treatment of anxiety disorders.
Over the past decades, behaviour and cognitive psychology have produced fruitful and mutually converging theories from which hypotheses could be derived on the nature and origin of fear and anxiety disorders. Notwithstanding the emergence of effective treatments, there are still many questions that remain to be answered. Here, I will argue that the burgeoning field of behavioural neuroscience may advance our understanding of fear, anxiety disorders and its treatments. Decades of fear-conditioning research across species have begun to elucidate the neurobiological mechanisms underlying associative fear learning and memory. The fear-conditioning paradigm provides a well-controlled and fine-grained research platform to examine these processes. Although the traditional fear conditioning paradigm was originally designed to unveil general principles of fear (un)learning, it is well-suited to understand the transition from normal fear to pathological fear and the mechanisms of change. This paper presents 1) a selection of fear conditioning studies on the generalization and persistence of associative fear memory as intermediate phenotypes of fear and anxiety disorders, and 2) insights from neuroscience on the malleability of fear memory with the potential to provide a long-term cure for anxiety and related disorders.
15432166
Overgeneralization of conditioned fear as a pathogenic marker of panic disorder.
OBJECTIVE
Classical conditioning features prominently in many etiological accounts of panic disorder. According to such accounts, neutral conditioned stimuli present during panic attacks acquire panicogenic properties. Conditioned stimuli triggering panic symptoms are not limited to the original conditioned stimuli but are thought to generalize to stimuli resembling those co-occurring with panic, resulting in the proliferation of panic cues. The authors conducted a laboratory-based assessment of this potential correlate of panic disorder by testing the degree to which panic patients and healthy subjects manifest generalization of conditioned fear.


METHOD
Nineteen patients with a DSM-IV-TR diagnosis of panic disorder and 19 healthy comparison subjects were recruited for the study. The fear-generalization paradigm consisted of 10 rings of graded size presented on a computer monitor; one extreme size was a conditioned danger cue, the other extreme a conditioned safety cue, and the eight rings of intermediary size created a continuum of similarity from one extreme to the other. Generalization was assessed by conditioned fear potentiating of the startle blink reflex as measured with electromyography (EMG).


RESULTS
Panic patients displayed stronger conditioned generalization than comparison subjects, as reflected by startle EMG. Conditioned fear in panic patients generalized to rings with up to three units of dissimilarity to the conditioned danger cue, whereas generalization in comparison subjects was restricted to rings with only one unit of dissimilarity.


CONCLUSIONS
The findings demonstrate a marked proclivity toward fear overgeneralization in panic disorder and provide a methodology for laboratory-based investigations of this central, yet understudied, conditioning correlate of panic. Given the putative molecular basis of fear conditioning, these results may have implications for novel treatments and prevention in panic disorder.
14533161
Generalized Anxiety Disorder Is Associated With Overgeneralization of Classically Conditioned Fear
BACKGROUND
Meta-analytic results of fear-conditioning studies in the anxiety disorders implicate generalization of conditioned fear to stimuli resembling the conditioned danger cue as one of the more robust conditioning markers of anxiety pathology. Due to the absence of conditioning studies assessing generalization in generalized anxiety disorder (GAD), results of this meta-analysis do not reveal whether such generalization abnormalities also apply to GAD. The current study fills this gap by behaviorally and psychophysiologically assessing levels of conditioned fear generalization across adults with and without GAD.


METHODS
Twenty-two patients with a DSM-IV-Text Revision diagnosis of GAD and 26 healthy comparison subjects were recruited and tested. The employed generalization paradigm consisted of quasi-randomly presented rings of gradually increasing size, with extreme sizes serving as conditioned danger cues (CS+) and conditioned safety cues. The rings of intermediary size served as generalization stimuli, creating a continuum of similarity between CS+ and conditioned safety cues across which to assess response slopes, referred to as generalization gradients. Primary outcome variables included slopes for fear-potentiated startle (electromyography) and self-reported risk ratings.


RESULTS
Behavioral and psychophysiological findings demonstrated overgeneralization of conditioned fear among patients with GAD. Specifically, generalization gradients were abnormally shallow among GAD patients, reflecting less degradation of the conditioned fear response as the presented stimulus differentiated from the CS+.


CONCLUSIONS
Overgeneralization of conditioned fear to safe encounters resembling feared situations may contribute importantly to the psychopathology of GAD by proliferating anxiety cues in the individual's environment that are then capable of evoking and maintaining anxiety and worry associated with GAD.
26209455
Neural circuits and mechanisms involved in fear generalization: Implications for the pathophysiology and treatment of posttraumatic stress disorder
Compelling evidence suggests that fear generalization (i.e. the transfer of fear from a particular stimulus to another one sharing similarities with the original stimulus) may contribute to the development of posttraumatic stress disorder (PTSD), for which current treatments are ineffective. Deficits in hippocampus-mediated pattern separation, the process by which memories are stored as unique representations that are resistant to confusion, have been solely proposed as a putative underlying marker of generalization. We delineate instead an enlarged scenario, wherein conditioned and generalized fear memories share a common neurocircuitry, with the hippocampus being the nub of contextual fear, and the prefrontal cortex of both cued and contextual fear. The potential contribution of the amygdala and insula will be highlighted as well. Finally, we will consider vulnerability factors that may contribute to the development of PTSD, and suggest avenues for novel therapeutics. A better understanding of the mechanisms behind fear generalization is fundamental to provide further insight into treatment of debilitating conditions such as PTSD.
38370436
Contextual fear conditioning, conjunctive representations, pattern completion, and the hippocampus.
Impaired contextual fear conditioning produced by damage to the hippocampus has been attributed to the loss of a conjunctive representation of the features of the context. There is, however, no direct evidence that conjunctive representations contribute to contextual fear conditioning. These experiments addressed this issue and found support for the conjunctive representation view. Two results made this point: (a) Preexposure to the conditioning context, but not to its separable features, facilitated contextual fear conditioning, and (b) generalization of fear conditioning to similar contexts was enhanced by preexposure to the context used to test for generalization. These results are interpreted as pattern completion to the preexposed context during the conditioning episode. They support the view that a conjunctive representation of context plays an important role in contextual fear conditioning and that the impairments produced by damage to the hippocampus result from the loss of this conjunctive contribution.
679003
Memory for context becomes less specific with time.
Context memories initially require the hippocampus, but over time become independent of this structure. This shift reflects a consolidation process whereby memories are gradually stored in distributed regions of the cortex. The function of this process is thought to be the extraction of statistical regularities and general knowledge from specific experiences. The current study examined this idea in mice by measuring the specificity of context memories during consolidation. In the first experiment, separate groups of animals were trained with a single shock and tested in the training context or a novel environment 1, 14, 28, or 36 d later. We found a systematic increase in generalization over this period. Initially, mice froze more in the training context, but fear of the novel environment grew over time until animals eventually froze an equivalent amount in both contexts. The second experiment demonstrated that the increase in generalization was due to a loss of detailed information about the context and not fear incubation. In this experiment, mice were exposed to the context and then trained with an immediate shock 1 or 36 d later. Animals trained 1 d after exposure acquired robust context fear that did not generalize across environments. In contrast, mice trained 36 d after exposure froze an equivalent amount in the training context and the novel environment. The same profile was observed in H-ras mutants that exhibit enhanced hippocampal plasticity and learning. These results suggest that context memories are specific early after training when they require the hippocampus, and become more general as they are permanently stored in the cortex.
18807513
The Hippocampus Plays a Selective Role in the Retrieval of Detailed Contextual Memories
BACKGROUND
It is widely believed that the hippocampus plays a temporary role in the retrieval of episodic and contextual memories. Initial research indicated that damage to this structure produced amnesia for newly acquired memories but did not affect those formed in the distant past. A number of recent studies, however, have found that the hippocampus is required for the retrieval of episodic and contextual memories regardless of their age. These findings are currently the subject of intense debate, and a satisfying resolution has yet to be identified.


RESULTS
The current experiments address this issue by demonstrating that detailed memories require the hippocampus, whereas memories that lose precision become independent of this structure. First, we show that the dorsal hippocampus is preferentially activated by the retrieval of detailed contextual fear memories. We then establish that the hippocampus is necessary for the retrieval of detailed memories by using a context-generalization procedure. Mice that exhibit high levels of generalization to a novel environment show no memory loss when the hippocampus is subsequently inactivated. In contrast, mice that discriminate between contexts are significantly impaired by hippocampus inactivation.


CONCLUSIONS
Our data suggest that detailed contextual memories require the hippocampus, whereas memories that lose precision can be retrieved without this structure. These findings can account for discrepancies in the literature-memories of our distant past can be either lost or retained after hippocampus damage depending on their quality-and provide a new framework for understanding memory consolidation.
17424188
Sex differences, context preexposure, and the immediate shock deficit in Pavlovian context conditioning with mice.
The acquisition of context fear in rats is affected by variables such as the sex of the animal, the placement to shock interval (PSI), and preexposure to the context. The current experiments assessed the effects of these variables on context conditioning in mice (C57BL/6). In Experiment 1, mice were placed in a chamber and received a single shock 5s, 20 s, 40s, 60s, 180s, or 720s later. Increasing the PSI produced corresponding increases in conditional freezing during the context test. In addition, male mice acquired more context conditioning than female mice did but only at intermediate PSIs. In Experiment 2, preexposure to the context before training alleviated the sex difference found with an intermediate PSI. The results are discussed in terms of configural learning theory and are argued to be contrary to the predictions of scalar expectancy theory.
18300519
Sex differences in the generalization of fear as a function of retention intervals.
In previous studies using male rodents, context change disrupted a fear response at a short, but not a long, retention interval. Here, we examined the effects of context changes on fear responses as a function of time in male and female rats. Males displayed context discrimination at all intervals, whereas females exhibited generalization by 5 d. Ovariectomized females with no hormone replacement displayed context discrimination at 5 d, whereas those receiving 17β-estradiol generalized their fear response to a neutral context. These results demonstrate that fear generalization for contextual cues occurs faster in female rats and is mediated, in part, by estrogens.
34680758
Sex and strategy use matters for pattern separation, adult neurogenesis, and immediate early gene expression in the hippocampus
Adult neurogenesis in the dentate gyrus (DG) plays a crucial role for pattern separation, and there are sex differences in the regulation of neurogenesis. Although sex differences, favoring males, in spatial navigation have been reported, it is not known whether there are sex differences in pattern separation. The current study was designed to determine whether there are sex differences in the ability for separating similar or distinct patterns, learning strategy choice, adult neurogenesis, and immediate early gene (IEG) expression in the DG in response to pattern separation training. Male and female Sprague‐Dawley rats received a single injection of the DNA synthesis marker, bromodeoxyuridine (BrdU), and were tested for the ability of separating spatial patterns in a spatial pattern separation version of delayed nonmatching to place task using the eight‐arm radial arm maze. Twenty‐seven days following BrdU injection, rats received a probe trial to determine whether they were idiothetic or spatial strategy users. We found that male spatial strategy users outperformed female spatial strategy users only when separating similar, but not distinct, patterns. Furthermore, male spatial strategy users had greater neurogenesis in response to pattern separation training than all other groups. Interestingly, neurogenesis was positively correlated with performance on similar pattern trials during pattern separation in female spatial strategy users but negatively correlated with performance in male idiothetic strategy users. These results suggest that the survival of new neurons may play an important positive role for pattern separation of similar patterns in females. Furthermore, we found sex and strategy differences in IEG expression in the CA1 and CA3 regions in response to pattern separation. These findings emphasize the importance of studying biological sex on hippocampal function and neural plasticity. © 2015 Wiley Periodicals, Inc.
11135003
Sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats: positive correlation between LTP and contextual learning
Three experiments investigated sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats. Experiment 1 revealed a robust sex difference in the magnitude of LTP induced at perforant path synapses in the dentate gyrus of pentobarbital-anesthetized rats. This sex difference in LTP was evident in rats of 35 and 60 days of age and was not the result of pre-LTP sex differences in perforant path synaptic transmission; 20-day-old rats did not show LTP. An analysis of field potentials evoked during LTP induction revealed a sex difference in the magnitude of N-methyl-D-aspartate (NMDA) receptor activation that was highly correlated with the magnitude of LTP. Experiment 2 showed that males condition more fear, measured as freezing, to the contextual conditional stimuli (CSs) of a conditioning chamber compared to their female counterparts. This sex difference in conditional freezing was apparent with both low and high unconditional stimulus (US, footshock) intensities. Experiment 3 revealed that the enhanced fear conditioning in males was specific to contextual CSs, and consisted of a more rapid rate of conditioning. Together, these experiments reveal a positive correlation between the magnitude of hippocampal LTP and a form of learning that depends on the hippocampus. Furthermore, they suggest a neural basis for sex differences in hippocampus-dependent learning tasks.
17093017
Long‐lasting regulation of hippocampal Bdnf gene transcription after contextual fear conditioning
Long‐term memory formation requires de novo protein synthesis and gene transcription. During contextual long‐term memory formation brain‐derived neurotrophic factor (BDNF) gene expression changes in conjunction with alterations of DNA methylation in the Bdnf gene. However, little is known about the molecular mechanisms underlying the maintenance and persistence of contextual long‐term memory. Here, we examined the transcription of specific Bdnf exons in the hippocampus for long periods after contextual fear conditioning. We found changes in transcription lasting for at least 24 h after contextual fear conditioning, with some sex‐specific effects. In addition, hypomethylation at a CpG site in CpG island 2 located at the end of Bdnf exon III sequence was detected at 0.5 h and maintained for up to 24 h after contextual fear conditioning. The identification of these long‐lasting changes in transcription and DNA methylation at the Bdnf gene suggests that BDNF might have a role for storage of contextual long‐term memory in the hippocampus.
16356967
Sex differences in contextual fear conditioning are associated with differential ventral hippocampal extracellular signal-regulated kinase activation
Although sex differences have been reported in hippocampal-dependent learning and memory, including contextual fear memories, the underlying molecular mechanisms contributing to such differences are not well understood. The present study examined the extent to which sex differences in contextual fear conditioning are related to differential activation of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK), a protein kinase critically involved in memory formation. We first show that male rats exhibit more long-term retention of contextual fear conditioning than female rats. During a tone test, females spent more time freezing than males, although both sexes exhibited robust retention of auditory fear learning. Using Western blot analysis, we then show that phosphorylated ERK levels in ventral, but not dorsal, hippocampus are higher in males than females, relative to same-sex controls, 60 minutes after fear conditioning. Post-conditioning increases in ERK activation were observed in the amygdala in both males and females, suggesting a selective effect of sex on hippocampal ERK activation. Together, these findings suggest that differential activation of the ERK signal transduction pathway in male and female rats, particularly in the ventral hippocampus, is associated with sex differences in contextual fear.
20822711
A selective increase in phosphorylation of cyclic AMP response element-binding protein in hippocampal CA1 region of male, but not female, rats following contextual fear and passive avoidance conditioning
Cyclic AMP response element-binding protein (CREB), a transcription factor on which multiple signal transduction pathways converge, has been implicated in long-term memory. We examined whether the sex difference in the performance of contextual fear or passive avoidance conditioning is associated with a change in the activation of CREB in the hippocampus, a neural structure important for long-term memory. The activation of CREB in different subregions within the hippocampus in male and female rats was determined immunohistochemically with an antibody that specifically recognizes the phosphorylated form of CREB (pCREB). With respect to the freezing time in contextual fear conditioning and the step-through latency in passive avoidance conditioning, male rats exhibited better performance than female rats. Phosphorylation of CREB (% pCREB) as revealed by the ratio of the pCREB-immunoreactive (pCREB-ir) cell number to the CREB-immunoreactive cell number was increased in the CA1 region, but not in CA3, CA4, or in the dentate gyrus following training for both types of conditioning in males. In females, such an increase in % pCREB was not found in any hippocampal subregion at any time after conditioning or by increasing the intensity of foot shock. Orchidectomy in males did not alter either the performance of contextual conditioning or conditioning-induced CREB phosphorylation in CA1. The close relationship between behavioral performance and CREB phosphorylation in the CA1 region suggests that hippocampal CREB is involved in the sex difference in some forms of learning and memory.
18974647
Context preexposure prevents forgetting of a contextual fear memory: implication for regional changes in brain activation patterns associated with recent and remote memory tests.
Contextual fear conditioning was maintained over a 15-day retention interval suggesting no forgetting of the conditioning experience. However, a more subtle generalization test revealed that, as the retention interval increased, rats showed enhanced generalized fear to an altered context. Preexposure to the training context prior to conditioning, however, prevented this enhanced generalized fear from developing. These results support the hypothesis that the memory representation of the context degrades as the memory ages and is responsible for enhanced generalization. The implications of these results for systems consolidation versus forgetting interpretations of regional changes in neural activation patterns that occur as memories age are discussed.
21997183
Neuronal Ensembles in Amygdala, Hippocampus, and Prefrontal Cortex Track Differential Components of Contextual Fear
Although the circuit mediating contextual fear conditioning has been extensively described, the precise contribution that specific anatomical nodes make to behavior has not been fully elucidated. To clarify the roles of the dorsal hippocampus (DH), basolateral amygdala (BLA), and medial prefrontal cortex (mPFC) in contextual fear conditioning, activity within these regions was mapped using cellular compartment analysis of temporal activity using fluorescence in situ hybridization (catFISH) for Arc mRNA. Rats were delay-fear conditioned or immediately shocked (controls) and thereafter reexposed to the shocked context to test for fear memory recall. Subsequent catFISH analyses revealed that in the DH, cells were preferentially reactivated during the context test, regardless of whether animals had been fear conditioned or immediately shocked, suggesting that the DH encodes spatial information specifically, rather then the emotional valence of an environment. In direct contrast, neuronal ensembles in the BLA were only reactivated at test if animals had been fear conditioned, suggesting that the amygdala specifically tracks the emotional properties of a context. Interestingly, Arc expression in the mPFC was consistent with both amygdala- and hippocampus-like patterns, supporting a role for the mPFC in both fear and contextual processing. Collectively, these data provide crucial insight into the region-specific behavior of neuronal ensembles during contextual fear conditioning and demonstrate a dissociable role for the hippocampus and amygdala in processing the contextual and emotional properties of a fear memory.
18957626
Memory for context is impaired by injecting anisomycin into dorsal hippocampus following context exploration
Pre-exposure to the context facilitates the small amount of contextual fear conditioning that is normally produced by immediate shock. This context pre-exposure facilitation effect provides a convenient way to study the rat's learning about context. We recently reported that anterograde damage to dorsal hippocampus prevents this facilitation. The present experiments strengthen this conclusion by showing that the protein synthesis inhibitor, anisomycin, injected bilaterally into the dorsal hippocampus following context pre-exposure also significantly reduces the facilitation effect. The same treatment given immediately after immediate shock, however, had no effect on facilitation. These results support theories that assume that, (a) contextual fear involves two processes, acquiring and storing a conjunctive representation of a context and associating that representation with fear; and (b) the hippocampus contributes to contextual fear by participating in the storage of the memory representation of the context.
659931
Gender specific requirement of GluR1 receptors in contextual conditioning but not spatial learning
The GluR1 subunit of the AMPA receptor is required for hippocampal-dependent memory formation, emotional learning and synaptic plasticity. Recent work has shown that GluR1-independent synaptic plasticity is mediated by nitric oxide. Nitric oxide activity is influenced by estrogen. It is unknown whether this gender-dependent effect conveys a gender dimorphic requirement of GluR1 for learning. This hypothesis was tested in two behavioral paradigms. In Experiment 1, the retention of contextual fear conditioning was impaired in male but not female GluR1 knockout mice. In Experiment 2, GluR1 knockout mice made significantly more arm entry errors during acquisition of a radial-arm watermaze task. This deficit was independent of gender. These results indicate that some forms of learning are gender dimorphic in GluR1 knockout mice. The results are discussed with reference to task and gender-specific interactions between GluR1 receptor intracellular signalling pathways.
21947789
Early life stress impairs fear conditioning in adult male and female rats
We demonstrated that neonatal isolation (1-h pup isolation; postnatal days 2-9) impairs context-induced fear conditioning in adult male rats and tends to enhance this effect and foot shock sensitivity in females. In this study, we examine the effects of brief (i.e., handling; 15 min) and prolonged (3 h) maternal separations (postnatal days 1-21) on fear conditioning and foot shock sensitivity in adult male and female rats. Identical training and test conditions from our prior study were employed so comparisons of the three early life stressors could be made. Context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz) were measured after 10 tone-shock training trials in Experiment 1. In Experiment 2, foot shock responses (flinch, jump, sonic vocalizations) to escalating shock levels were assessed. Brief maternal separation impaired context- and cue-conditioned fear in rats of both sexes as assessed by USVs. Prolonged maternal separation only impaired context fear in female rats. There were no effects on foot shock sensitivity. Results of this and other studies suggest that early life stress impairs fear conditioning in adult rats whereas stress experienced in adulthood has the opposite effect. These opposing effects may reflect developmental differences on stress-induced alterations on hippocampal regulation of the hypothalamic-pituitary-adrenal axis.
46462580
Effect of Sex on Fear Conditioning is Similar for Context and Discrete CS in Wistar, Lewis and Fischer Rat Strains
Enhanced fear in males relative to females, both innate and conditioned, is a well-described characteristic of behavior in the laboratory rat. In the case of aversive conditioning to foot shock in Long-Evans rats, it has been described that conditioning to general (nondiscrete) contextual cues is greater in male rats relative to female rats, whereas conditioning to a discrete, predictive stimulis (CS) is not. These findings have been combined with evidence for greater levels of hippocampal LTP in males in Sprague-Dawley rats to derive a model of hippocampal-LTP-mediated contextual and not CS, fear conditioning. The present study reports on an analysis of the effect of sex in contextual and discrete CS conditioning to foot shock, assessed via measurement of freezing behavior in a novel automated paradigm, in three rat strains: Wistar, Fischer, and Lewis. In Wistar rats, there was a consistent but nonsignificant tendency for males to demonstrate both more contextual and more CS conditioning than females; in Fischer rats, males demonstrated both more contextual and more CS conditioning than females; in Lewis rats, a markedly enhanced acquisition of freezing in males did not translate into a sex difference in either context or CS conditioning at expression. Therefore, within each strain the effect of sex was consistent between context and CS conditioning. These findings, taken together with the hippocampal LTP evidence, suggest that the latter mediates both contextual and discrete CS aversive conditioning, and contributes to sex differences in both these forms of conditioning, in those strains where these sex differences exist.
16416845
Sexual dimorphisms in the effect of low‐level p25 expression on synaptic plasticity and memory
p25, a degradation product of p35, has been reported to accumulate in the forebrain of patients with Alzheimer's disease. p25 as well as p35 are activators of cyclin‐dependent kinase 5 (Cdk5) although p25/Cdk5 and p35/Cdk5 complexes have distinct properties. Several mouse models with high levels of p25 expression exhibit signs of neurodegeneration. On the contrary, we have shown that low levels of p25 expression do not cause neurodegeneration and are even beneficial for particular types of learning and memory [Angelo et al., (2003) Eur J. Neurosci., 18, 423–431]. Here, we have studied the influence of low‐level p25 expression in hippocampal synaptic plasticity and in learning and memory for each sex separately in two different genetic backgrounds (129B6F1 and C57BL/6). Surprisingly, we found that low‐level p25 expression had different consequences in male and female mutants. In the two genetic backgrounds LTP induced by a strong stimulation of the Schaffer's collaterals (four trains, 1‐s duration, 5‐min interval) was severely impaired in male, but not in female, p25 mutants. Furthermore, in the two genetic backgrounds spatial learning in the Morris water maze was faster in female p25 mutants than in male transgenic mice. These results suggest that, in women, the production of p25 in Alzheimer's disease could be a compensation for some early learning and memory deficits.
9389494
Comparison of inbred mouse substrains reveals segregation of maladaptive fear phenotypes
Maladaptive fear, such as fear that is persistent or easily generalized to a nonthreatening stimuli, is associated with anxiety-related disorders in humans. In the laboratory, maladaptive fear can be modeled in rodents using Pavlovian fear conditioning. Recently, an inbred mouse strain known as 129S1/SvImJ, or 129S1 has been reported as exhibiting impairments in fear extinction and enhanced fear generalization. With a long-term goal of identifying segregating genetic markers of maladaptive fear, we used Pavlovian fear conditioning to characterize a closely related substrain designated as 129S6/SvEvTac, or 129S6. Here we report that, like 129S1 animals, 129S6 mice exhibit appropriate levels of fear upon conditioning, but are unable to extinguish fear memories once they are consolidated. Importantly, the maladaptive fear phenotype in this inbred stain can be segregated by sub-strain when probed using conditioning protocols designed to assess generalized fear. We find that unlike the 129S1 substrain, mice from the 129S6 sub-strain do not generalize conditioned fear to previously novel contexts and can learn to discriminate between two similar contexts when trained using a discrimination protocol. These results suggest that at least two forms of maladaptive fear (deficits in fear extinction and fear generalization) can be can be functionally segregated, further suggesting that the underlying neurobiology is heritable. Given the observation that two closely related sub-strains can exhibit different constellations of maladaptive fear suggests that these findings could be exploited to facilitate the identification of candidate genes for anxiety-related disorders.
7242970
Activation of ERβ modulates fear generalization through an effect on memory retrieval
Women are 60% more likely to suffer from an anxiety disorder than men. One hypothesis for this difference may be that females exhibit increased rates of fear generalization. Females generalize fear to a neutral context faster than males, a process driven, in part, by estrogens. In the current study, ovariectomized adult female Long-Evans rats were given acute injections of estradiol benzoate (15μg/0.1mL sesame oil) or sesame oil during a passive avoidance procedure to determine if estrogens increase fear generalization through an effect on fear memory acquisition/consolidation or through fear memory retrieval. Animals injected 1h prior to training generalized to the neutral context 24h later but not 7days after training. Generalization was also seen when injections occurred 24h before testing, but not when tested at immediate (1h) or intermediate (6h) time points. In Experiment 3, animals were injected with estrogen receptor (ER) agonists, PPT or DPN, to determine which ER subtype(s) increased fear generalization. Only the ERβ agonist, DPN, increased fear generalization when testing occurred 24h after injection. Our results indicate that estradiol increases fear generalization through an effect on fear memory retrieval mechanisms by activation of ERβ.
37745521
Roles of testosterone and amygdaloid LTP induction in determining sex differences in fear memory magnitude
Women are thought to form fear memory more robust than men do and testosterone is suspected to play a role in determining such a sex difference. Mouse cued fear freezing was used to study the sex-related susceptibility and the role of testosterone in fear memory in humans. A 75-dB tone was found to provoke weak freezing, while 0.15-mA and 0.20-mA footshock caused strong freezing responses. No sex differences were noticed in the tone- or footshock-induced (naïve fear) freezing. Following the conditionings, female mice exhibited greater tone (cued fear)-induced freezing than did male mice. Nonetheless, female mice demonstrated indistinctive cued fear freezing across the estrous phases and ovariectomy did not affect such freezing in female mice. Orchidectomy enhanced the cued fear freezing in male mice. Systemic testosterone administrations and an intra-lateral nucleus of amygdala (LA) testosterone infusion diminished the cued fear freezing in orchidectomized male mice, while pretreatment with flutamide (Flu) eradicated these effects. Long-term potentiation (LTP) magnitude in LA has been known to correlate with the strength of the cued fear conditioning. We found that LA LTP magnitude was indeed greater in female than male mice. Orchidectomy enhanced LTP magnitude in males' LA, while ovariectomy decreased LTP magnitude in females' LA. Testosterone decreased LTP magnitude in orchidectomized males' LA and estradiol enhanced LTP magnitude in ovariectomized females' LA. Finally, male mice had lower LA GluR1 expression than female mice and orchidectomy enhanced the GluR1 expression in male mice. These findings, taken together, suggest that testosterone plays a critical role in rendering the sex differences in the cued fear freezing and LA LTP. Testosterone is negatively associated with LA LTP and the cued fear memory in male mice. However, ovarian hormones and LA LTP are loosely associated with the cued fear memory in female mice.
17064191
Hippocampal and extrahippocampal systems compete for control of contextual fear: role of ventral subiculum and amygdala.
Two neural systems, a hippocampal system and an extrahippocampal system compete for control over contextual fear, and the hippocampal system normally dominates. Our experiments reveal that output provided by the ventral subiculum is critical for the hippocampal system to win this competition. Bilateral electrolytic lesions of the ventral subiculum after conditioning, but not before conditioning, impaired contextual fear conditioning. Reversibly inactivating this region by bilateral injections of muscimol produced the same results-no impairment when the injection occurred prior to conditioning but a significant impairment when this region was inactivated after conditioning. Thus, the extrahippocampal system can support contextual fear conditioning if the ventral subiculum is disabled before conditioning but not if it is disabled after conditioning. Our experiments also reveal that the basolateral region of the amygdala (BLA) is where the two systems compete for associative control of the fear system. To test this hypothesis we reasoned that the extrahippocampal system would also acquire associative control over the fear system, even if the hippocampal system were functional, if the basal level of plasticity potential in the BLA could be increased. We did this by injecting the D1 dopamine agonist, SKF82958, into the BLA just prior to conditioning. This treatment resulted in a significant increase in freezing when the ventral subiculum was disabled prior to the test. These results are discussed in relationship to the idea that D1 agonists increase plasticity potential by increasing the pool of available extrasynaptic GluR1 receptors in the population of neurons supporting acquired fear.
7236821
The Fear Circuit Revisited: Contributions of the Basal Amygdala Nuclei to Conditioned Fear
The lateral nucleus (LA) is the input station of the amygdala for information about conditioned stimuli (CSs), whereas the medial sector of the central nucleus (CeM) is the output region that contributes most amygdala projections to brainstem fear effectors. However, there are no direct links between LA and CeM. As the main target of LA and with its strong projection to CeM, the basomedial amygdala (BM) constitutes a good candidate to bridge this gap. Consistent with this notion, it was reported that combined posttraining lesions of the basal nuclei [BM plus basolateral nucleus (BL)] abolish conditioned fear responses, whereas selective BL inactivation does not. Thus, we examined the relative contribution of BM and BL to conditioned fear using unit recordings and inactivation with muscimol microinfusions in rats. Approximately 30% of BM and BL neurons acquired robust responses to auditory CSs predicting footshocks. While most BL cells stopped firing at CS offset, BM responses typically outlasted the CS by ≥40 s, paralleling the persistence of conditioned fear after the CS. This observation suggests that BM neurons are not passive relays of rapidly adapting LA inputs about the CS. Surprisingly, independent inactivation of either BM or BL with muscimol did not cause a reduction of conditioned freezing even though an extinction recall deficit was seen the next day. In contrast, combined BL–BM inactivation did. Overall, there results support the notion that the basal nuclei are involved in conditioned fear expression and extinction but that there is functional redundancy between them.
227738
Fear renewal preferentially activates ventral hippocampal neurons projecting to both amygdala and prefrontal cortex in rats
Anxiety, trauma and stress-related disorders are often characterized by a loss of context-appropriate emotional responding. The contextual retrieval of emotional memory involves hippocampal projections to the medial prefrontal cortex and amygdala; however the relative contribution of these projections is unclear. To address this question, we characterized retrieval-induced Fos expression in ventral hippocampal (VH) neurons projecting to the prelimbic cortex (PL) and basal amygdala (BA) after the extinction of conditioned fear in rats. After extinction, freezing behavior (an index of learned fear) to the auditory conditioned stimulus was suppressed in the extinction context, but was “renewed” in another context. Hippocampal neurons projecting to either PL or BA exhibited similar degrees of context-dependent Fos expression; there were more Fos-positive neurons in each area after the renewal, as opposed, to suppression of fear. Importantly, however, VH neurons projecting to both PL and BA were more likely to express Fos during fear renewal than neurons projecting to either PL or BA alone. These data suggest that although projections from the hippocampus to PL and BA are similarly involved in the contextual retrieval of emotional memories, VH neurons with collaterals to both areas may be particularly important for synchronizing prefrontal-amygdala circuits during fear renewal.
45129788
The role of dorsal hippocampus and basolateral amygdala NMDA receptors in the acquisition and retrieval of context and contextual fear memories.
The authors used 3-phase context preexposure facilitation methodology to study the contribution of N-methyl-D-aspartate (NMDA) receptors in dorsal hippocampus (DH) and the basal lateral region of the amygdala (BLA) to (a) acquisition of the context memory, (b) retrieval of the context memory, (c) acquisition of context-shock association, and (d) retrieval of the context-shock association. The NMDA receptor antagonist D-2-amino-5 phosphonopentanoic acid (D-AP5) was injected into either the DH or BLA prior to (a) the context preexposure phase, (b) the immediate shock phase, or (c) the test for contextual fear. Antagonizing NMDA receptors in the DH impaired the acquisition of the context memory but did not affect its retrieval or retrieval of the fear memory. Antagonizing NMDA receptors with D-AP5 in the BLA impaired acquisition of the context-shock association but had no effect on the expression of fear. However, both DL-AP5 and L-AP5 reduced the expression of fear when they were injected into the amygdala prior to testing for contextual fear.
11646051
Male mice housed in groups engage in frequent fighting and show a lower response to additional bone loading than females or individually housed males that do not fight
Experiments to investigate bone's physiological adaptation to mechanical loading frequently employ models that apply dynamic loads to bones in vivo and assess the changes in mass and architecture that result. It is axiomatic that bones will only show an adaptive response if the applied artificial loading environment differs in a significant way from that to which the bones have been habituated by normal functional loading. It is generally assumed that this normal loading is similar between experimental groups. In the study reported here we found that this was not always the case. Male and female 17-week-old C57BL/6 mice were housed in groups of six, and a single episode (40 cycles) of non-invasive axial loading, engendering 2,200με on the medial surface of the proximal tibiae in sample mice, was applied to right tibiae on alternate days for two weeks. This engendered an adaptive increase in bone mass in females, but not males. Observation revealed the main difference in behaviour between males and females was that males were involved in fights 1.3 times per hour, whereas the females never fought. We therefore housed all mice individually. In females, there was a similar significant osteogenic response to loading in cortical and trabecular bone of both grouped and individual mice. In contrast, in males, adaptive increases in the loaded compared with non-loaded control bones was only apparent in animals housed individually. Our interpretation of these findings is that the frequent vigorous fighting that occurs between young adult males housed in groups could be sufficient to engender peak strains and strain rates that equal or exceed the stimulus derived from artificial loading. This indicates the importance of ensuring that physical activity is consistent between groups. Reducing the background level of the naturally engendered strain environment allows adaptive responses to artificial loading to be demonstrated at lower loads.
1323287
Relationship between Fos Production and Classical Fear Conditioning: Effects of Novelty, Latent Inhibition, and Unconditioned Stimulus Preexposure
The relationship between FOS production in the sensory cortex and limbic system and the ability of C57BL/6N mice to acquire context- and tone-dependent freezing were investigated after fear conditioning, which was achieved by exposure of mice to context only or context and tone (10 kHz, 75 dB) as conditioned stimuli (Cs) paired with an electric footshock (0.7 mA, constant) as unconditioned stimulus (Us). The effect of preexposure to Cs or Cs paired with Us on FOS production and learning was also tested. It was demonstrated that high simultaneous FOS production in the parietal cortex, hippocampus, and amygdala paralleled the ability of mice to acquire strong freezing responses to novel Cs. After contextual preexposure (latent inhibition), FOS production could be elicited in the central amygdala only by shock and in the basolateral amygdala only by tone. Under these conditions, the ability of mice to acquire contextual freezing was almost abolished, whereas tone-dependent freezing was reduced. Lacking FOS production in the central amygdala after preexposure to context followed by shock (Us preexposure effect) paralleled the inability of mice to acquire tone-dependent freezing, although the tone elicited FOS production in the basolateral amygdala. On the basis of these findings it was concluded that synchronous Cs- and Us-induced FOS production in several defined forebrain areas was accompanied with associative learning of novel stimuli, and that a subsequent low level of FOS production might have been responsible or indicative for delayed conditioning to those stimuli.
15149628
Cortical Representations Are Reinstated by the Hippocampus during Memory Retrieval
The hippocampus is assumed to retrieve memory by reinstating patterns of cortical activity that were observed during learning. To test this idea, we monitored the activity of individual cortical neurons while simultaneously inactivating the hippocampus. Neurons that were active during context fear conditioning were tagged with the long-lasting fluorescent protein H2B-GFP and the light-activated proton pump ArchT. These proteins allowed us to identify encoding neurons several days after learning and silence them with laser stimulation. When tagged CA1 cells were silenced, we found that memory retrieval was impaired and representations in the cortex (entorhinal, retrosplenial, perirhinal) and the amygdala could not be reactivated. Importantly, hippocampal inactivation did not alter the total amount of activity in most brain regions. Instead, it selectively prevented neurons that were active during learning from being reactivated during retrieval. These data provide functional evidence that the hippocampus reactivates specific memory representations during retrieval.
44235451
Sex Differences in Animal Models: Focus on Addiction
The purpose of this review is to discuss ways to think about and study sex differences in preclinical animal models. We use the framework of addiction, in which animal models have excellent face and construct validity, to illustrate the importance of considering sex differences. There are four types of sex differences: qualitative, quantitative, population, and mechanistic. A better understanding of the ways males and females can differ will help scientists design experiments to characterize better the presence or absence of sex differences in new phenomena that they are investigating. We have outlined major quantitative, population, and mechanistic sex differences in the addiction domain using a heuristic framework of the three established stages of the addiction cycle: binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. Female rats, in general, acquire the self-administration of drugs and alcohol more rapidly, escalate their drug taking with extended access more rapidly, show more motivational withdrawal, and (where tested in animal models of “craving”) show greater reinstatement. The one exception is that female rats show less motivational withdrawal to alcohol. The bases for these quantitative sex differences appear to be both organizational, in that estradiol-treated neonatal animals show the male phenotype, and activational, in that the female phenotype depends on the effects of gonadal hormones. In animals, differences within the estrous cycle can be observed but are relatively minor. Such hormonal effects seem to be most prevalent during the acquisition of drug taking and less influential once compulsive drug taking is established and are linked largely to progesterone and estradiol. This review emphasizes not only significant differences in the phenotypes of females and males in the domain of addiction but emphasizes the paucity of data to date in our understanding of those differences.
8163498
Olfactory exposure to males, including men, causes stress and related analgesia in rodents
We found that exposure of mice and rats to male but not female experimenters produces pain inhibition. Male-related stimuli induced a robust physiological stress response that results in stress-induced analgesia. This effect could be replicated with T-shirts worn by men, bedding material from gonadally intact and unfamiliar male mammals, and presentation of compounds secreted from the human axilla. Experimenter sex can thus affect apparent baseline responses in behavioral testing.
13793525
Assessing Reproductive Status/Stages in Mice
The short reproductive cycle length observed in rodents, called the estrous cycle, makes them an ideal animal model for investigation of changes that occur during the reproductive cycle. Most of the data in the literature about the estrous cycle is obtained from rats because they are easily manipulated and they exhibit a clear and well‐defined estrous cycle. However, the increased number of experiments using knockout mice requires identification of their estrous cycle as well, since (in)fertility issues may arise. In mice, like rats, the identification of the stage of estrous cycle is based on the proportion of cell types observed in the vaginal secretion. The aim of this unit is to provide guidelines for quickly and accurately determining estrous cycle phases in mice.Curr. Protoc. Neurosci. 48:A.4I.1‐A.4I.8. © 2009 by John Wiley & Sons, Inc.
18777702
Conjunctive representations, the hippocampus, and contextual fear conditioning
The context in which events occur can be represented as both (1) a set of independent features, the feature representation view, and (2) a set of features bound into a unitary representation, the conjunction representation view. It is assumed that extrahippocampal (e.g., neocortical) areas provide a basis for feature representations, but the hippocampal formation makes an essential contribution to the automatic storage of conjunctive representations. We develop this dual-representation view and explore its implications for hippocampal contributions to contextual fear conditioning processes. To this end, we discuss how our framework can resolve some of the conflicts in the recent literature relating the hippocampus to contextual fear conditioning. We also present new data supporting the role of a key mechanism afforded by conjunctive representations—pattern completion (the ability of a subset of a memory pattern to activate the complete memory)—in contextual fear conditioning. As is implied by this mechanism, we report that fear can be conditioned to the memory representation of a context that is not actually present at the time of shock. Moreover, this result is predicted by our computational model of cortical and hippocampal function. We suggest that pattern completion demonstrated in animals and by our model provides a mechanistic bridge to human declarative memory.
2198786
Memory accuracy predicts hippocampal mTOR pathway activation following retrieval of contextual fear memory
Prior work suggests that hippocampus‐dependent memory undergoes a systems consolidation process such that recent memories are stored in the hippocampus, while older memories are independent of the hippocampus and instead dependent on cortical areas. One problem with interpreting these studies is that memory for the contextual stimuli weakens as time passes between the training event and testing and older memories are often less detailed, making it difficult to determine if memory storage in the hippocampus is related to the age or to the accuracy of the memory. Activity of the mammalian target of rapamycin (mTOR) signaling pathway is known to be important for controlling protein translation necessary for both memory consolidation after initial learning and for the reconsolidation of memory after retrieval. We tested whether p70s6 kinase (p70s6K), a key component of the mTOR signaling pathway, is activated following retrieval of context fear memory in the dorsal hippocampus (DH) and anterior cingulate cortex (ACC) at 1, 10, or 36 days after context fear conditioning. We also tested whether strengthening memory for the contextual stimuli changed p70s6K phosphorylation in these structures 36 days after training. We show that under standard training conditions retrieval of a recently formed memory is initially precise and involves the DH. Over time it loses detail, becomes independent of the DH and depends on the ACC. In a subsequent experiment, we preserved the accuracy of older memories through pre‐exposure to the training context. We show that remote memory still involved the DH in animals given pre‐exposure. These data support the notion that detailed memories depend on the DH regardless of their age. © 2013 Wiley Periodicals, Inc.
44963832
Calcium/calmodulin kinase kinase beta has a male-specific role in memory formation.
The calcium/calmodulin (CaM) kinase cascade regulates gene transcription, which is required for long-term memory formation. Previous studies with Camkk2 null mutant mice have shown that in males calcium/calmodulin kinase kinase beta (CaMKKbeta) is required for spatial memory formation and for activation of the transcription factor cyclic AMP-responsive element binding protein (CREB) in the hippocampus by spatial training. Here we show that CaMKKbeta is not required for spatial memory formation in female mice as female Camkk2 null mutants were not impaired in spatial memory formation and they had the same level of hippocampal CREB phosphorylation after spatial training as female wild-type mice. Furthermore, we show that male but not female Camkk2 null mutants were impaired in long-term potentiation (LTP) at hippocampal CA1 synapses. Finally, a transcriptional analysis of male Camkk2 null mutants led to the identification of a gene, glycosyl phosphatidyl-inositol anchor attachment protein 1 (GAA1), whose hippocampal mRNA expression was up-regulated by spatial and contextual training in male but not in female wild-type mice. Taken together, we conclude that CaMKKbeta has a male-specific function in hippocampal memory formation and we have identified male-restricted transcription occurring during hippocampal memory formation.
10942241
Perils and pitfalls of reporting sex differences
The idea of sex differences in the brain both fascinates and inflames the public. As a result, the communication and public discussion of new findings is particularly vulnerable to logical leaps and pseudoscience. A new US National Institutes of Health policy to consider both sexes in almost all preclinical research will increase the number of reported sex differences and thus the risk that research in this important area will be misinterpreted and misrepresented. In this article, I consider ways in which we might reduce that risk, for example, by (i) employing statistical tests that reveal the extent to which sex explains variation, rather than whether or not the sexes ‘differ’, (ii) properly characterizing the frequency distributions of scores or dependent measures, which nearly always overlap, and (iii) avoiding speculative functional or evolutionary explanations for sex-based variation, which usually invoke logical fallacies and perpetuate sex stereotypes. Ultimately, the factor of sex should be viewed as an imperfect, temporary proxy for yet-unknown factors, such as hormones or sex-linked genes, that explain variation better than sex. As scientists, we should be interested in discovering and understanding the true sources of variation, which will be more informative in the development of clinical treatments.
15144434
Minireview: Sex differences in adult and developing brains: compensation, compensation, compensation.
Despite decades of research, we do not know the functional significance of most sex differences in the brain. We are heavily invested in the idea that sex differences in brain structure cause sex differences in behavior. We rarely consider the possibility that sex differences in brain structure may also prevent sex differences in overt functions and behavior, by compensating for sex differences in physiological conditions, e.g. gonadal hormone levels that may generate undesirable sex differences if left unchecked. Such a dual function for sex differences is unlikely to be restricted to adult brains. This review will entertain the possibility that transient sex differences in gene expression in developing brains may cause permanent differences in brain structure but prevent them as well, by compensating for potentially differentiating effects of sex differences in gonadal hormone levels and sex chromosomal gene expression. Consistent application of this dual-function hypothesis will make the search for the functional significance of sex differences more productive.
144810913
Factors governing one-trial contextual conditioning
When a rat was placed in a chamber and shortly thereafter received a single footshock, it showed conditional freezing upon re-exposure to that chamber but not a different one (Experiment 1). Experiments 2–4 showed that the probability of this freezing decreased linearly with decreases in the delay between placement in the chamber and shock delivery. With very short delays (e.g., less than 27 sec), there was no freezing. Experiments 2 and 3 demonstrated that a 2-mm pre-exposure to the chamber, 24 h prior to shock delivery, reduced the minimum delay necessary to successfully condition freezing. Experiment 4 demonstrated that shorter delays were successful in conditioning freezing if a salient tone was a component of the contextual stimulus. The changes in freezing caused by delay interval and preexposure did not simply reflect the total time in the context, suggesting that there may be two requirements that place temporal restrictions on the conditioning of the freezing response. One is satisfied by sufficient exposure, whether or not that exposure is contiguous with shock. The second requirement is for a small amount of context exposure that is contiguous with shock.
25130503
False context fear memory in rats.
Four experiments used rats to study false context fear memories. In Experiment 1, rats were pre-exposed to a distinctive chamber (context A) or to a control environment (context C), shocked after a delay in a second chamber (context B) and tested either in B or A. Rats pre-exposed to A froze just as much as control rats in B but more than control rats in A. In Experiment 2, rats were pre-exposed to A or C, subjected to an immediate shock in B and tested in B or A. Rats pre-exposed to A froze when tested in A but did not freeze when tested in B and control rats did not freeze in either A or B. The false fear memory to the pre-exposed A was contingent on its similarity with the shocked B. In Experiment 3, rats pre-exposed to A and subjected to immediate shock in B froze when tested in A but did not freeze when tested in C and rats pre-exposed to C did not freeze when tested either in A or C. In Experiment 4, rats pre-exposed to A and subjected to immediate shock in B froze more when tested in A than rats whose pre-exposure to A began with an immediate shock. The results were discussed in terms of a dual systems explanation of context fear conditioning: a hippocampal-dependent process that forms a unitary representation of context and an amygdala-based process which associates this representation with shock.
30625564
Sexually divergent expression of active and passive conditioned fear responses in rats
Traditional rodent models of Pavlovian fear conditioning assess the strength of learning by quantifying freezing responses. However, sole reliance on this measure includes the de facto assumption that any locomotor activity reflects an absence of fear. Consequently, alternative expressions of associative learning are rarely considered. Here we identify a novel, active fear response (‘darting’) that occurs primarily in female rats. In females, darting exhibits the characteristics of a learned fear behavior, appearing during the CS period as conditioning proceeds and disappearing from the CS period during extinction. This finding motivates a reinterpretation of rodent fear conditioning studies, particularly in females, and it suggests that conditioned fear behavior is more diverse than previously appreciated. Moreover, rats that darted during initial fear conditioning exhibited lower freezing during the second day of extinction testing, suggesting that females employ distinct and adaptive fear response strategies that improve long-term outcomes. DOI: http://dx.doi.org/10.7554/eLife.11352.001
46044158
Molecular Specificity of Multiple Hippocampal Processes Governing Fear Extinction
Over many years, fear extinction has been conceptualized as one dominant process, new inhibitory learning, which serves to dampen previously acquired fear. Here we present an alternative view, that brain region-specific processing of representations, expectations and emotional attributes of the fear-provoking event, recruits unique mechanisms that interdependently contribute to the conditioning and extinction of fear. The co-occurrence of these mechanisms within the fear circuit can thus be tracked and differentiated at a molecular and cellular level. Among others, the transcriptional regulators cFos, cAMP-dependent response element binding protein (CREB), Zif268, and extracellular signal-regulated kinases (Erk) stand out as hippocampal nuclear markers signaling novelty, arousal, retrieval, and prediction error, respectively. Consistent with evidence from human studies, these findings indicate that, beyond inhibitory learning, fear extinction requires modification of the emotional attributes and expectations that define the threatening context. Given the likely dysregulation of one or more of these processes in anxiety disorders, a key research challenge for the future is the identification and enhancement of individual extinction mechanisms to target the specific components of fear. Environmental stimuli lacking affective properties (conditioned stimuli, CS) rapidly become threatening if presented with stressful events (unconditioned stimuli, US). Consequently, based on a CS-US association, the presentation of the CS triggers species-specific fear responses until the US consistently stops occurring. At that point, new learning takes place and the fear response declines, a phenomenon termed extinction. The view that extinction occurs because a new, inhibitory CS-noUS association gains control over behavior, has remained dominant in the field. The implications of impaired fear regulation in the development of anxiety disorders have stimulated intense research in this area. Rodent studies identified the circuits involved in the conditioning and extinction of fear of salient cues, generating data that were confirmed in humans with brain imaging approaches. Nevertheless, research with experimental animals has not fully taken advantage of human data in order to better interpret extinction mechanisms in the framework of learning, expectation and emotion governing fear-motivated behavior. The present article aims to summarize recent molecular evidence on fear extinction, focusing on hippocampal mechanisms and experimental models of contextual fear, and compare the results with other relevant fear paradigms and human imaging studies. Instead of conceptualizing extinction learning as one process, such as CS-noUS association or inhibitory learning, we propose that fear extinction reflects the behavioral output of several region-specific learning processes that modify different components of the conditioning memory. The significance of these findings is discussed in the framework of fear regulation and anxiety disorders.
35292886
Novelty, but not operant aversive learning, enhances Fos and Egr-1 expression in the medial prefrontal cortex and hippocampal areas of rats.
Immediate early genes (IEG) are presumed to be activated in response to stress, novelty, and learning. Evidence supports the involvement of prefrontal and hippocampal areas in stress and learning, but also in the detection of novel events. This study examined whether a previous experience with shocks changes the pattern of Fos and Egr-1 expression in the medial prefrontal cortex (mPFC), the hippocampal cornus ammonis 1 (CA1), and dentate gyrus (DG) of adult male Wistar rats that learned to escape in an operant aversive test. Subjects previously exposed to inescapable footshocks that learned to escape from shocks were assigned to the treated group (EXP). Subjects from Group Novelty (NOV) rested undisturbed during treatment and also learned to escape in the test. The nonshock group (NSH) rested undisturbed in both sessions. Standard immunohistochemistry procedures were used to detect the proteins in brain sections. The results show that a previous experience with shocks changed the pattern of IEG expression, then demonstrating c-fos and egr-1 induction as experience-dependent events. Compared with NSH and EXP an enhanced Fos expression was detected in the mPFC and CA1 subfield of Group NOV, which also exhibited increased Egr-1 expression in the mPFC and DG in comparison to NSH. No differences were found in the DG for Fos, or in the CA1 for Egr-1. Novelty, and not the operant aversive escape learning, seems to have generated IEG induction. The results suggest novel stimuli as a possible confounding factor in studies on Fos and/or Egr-1 expression in aversive conditions.
25492047
Sex-Related Difference in Amygdala Activity during Emotionally Influenced Memory Storage
We tested the possibility suggested by previous imaging studies that amygdala participation in the storage of emotionally influenced memory is differentially lateralized in men and women. Male and female subjects received two PET scans for regional cerebral glucose-one while viewing a series of emotionally provocative (negative) films, and a second while viewing a series of matched, but emotionally more neutral, films. Consistent with suggestions from several previously published studies, enhanced activity of the right, but not the left, amygdala in men was related to enhanced memory for the emotional films. Conversely, enhanced activity of the left, but not the right, amygdala in women was related to enhanced memory for the emotional films. These results demonstrate a clear gender-related lateralization of amygdala involvement in emotionally influenced memory, and indicate that theories of the neurobiology of emotionally influenced memory must begin to account for the influence of gender.
2078700
Effects of cue types on sex differences in human spatial memory
We examined the effects of cue types on human spatial memory in 3D virtual environments adapted from classical animal and human tasks. Two classes of cues of different functions were investigated: those that provide directional information, and those that provide positional information. Adding a directional cue (geographical slant) to the spatial delayed-match-to-sample task improved performance in males but not in females. When the slant directional cue was removed in a hidden-target location task, male performance was impaired but female performance was unaffected. The removal of positional cues, on the other hand, impaired female performance but not male performance. These results are consistent with results from laboratory rodents and thus support the hypothesis that sex differences in spatial memory arise from the dissociation between a preferential reliance on directional cues in males and on positional cues in females.
14655271
Sex Difference in Cue Strategy in a Modified Version of the Morris Water Task: Correlations between Brain and Behaviour
Background Sex differences in spatial memory function have been reported with mixed results in the literature, with some studies showing male advantages and others showing no differences. When considering estrus cycle in females, results are mixed at to whether high or low circulating estradiol results in an advantage in spatial navigation tasks. Research involving humans and rodents has demonstrated males preferentially employ Euclidean strategies and utilize geometric cues in order to spatially navigate, whereas females employ landmark strategies and cues in order to spatially navigate. Methodology/Principal Findings This study used the water-based snowcone maze in order to assess male and female preference for landmark or geometric cues, with specific emphasis placed on the effects of estrus cycle phase for female rat. Performance and preference for the geometric cue was examined in relation to total hippocampal and hippocampal subregions (CA1&2, CA3 and dentate gyrus) volumes and entorhinal cortex thickness in order to determine the relation between strategy and spatial performance and brain area size. The study revealed that males outperformed females overall during training trials, relied on the geometric cue when the platform was moved and showed significant correlations between entorhinal cortex thickness and spatial memory performance. No gross differences in behavioural performance was observed within females when accounting for cyclicity, and only total hippocampal volume was correlated with performance during the learning trials. Conclusions/Significance This study demonstrates the sex-specific use of cues and brain areas in a spatial learning task.
14355843
Sex differences in the strategies used by rats to solve a navigation task.
Rats were trained in a triangular-shaped pool to find a hidden platform, whose location was defined in terms of two sources of information, a landmark outside the pool and a particular corner of the pool. Subsequent test trials without the platform pitted these two sources of information against one another. This test revealed a clear sex difference. Females spent more time in an area of the pool that corresponded to the landmark, whereas males spent more time in the distinctive corner of the pool even though further tests revealed that both sexes had learned about the two sources of information by presenting cues individually. The results agree with the claim that males and females use different types of information in spatial navigation.
43689157
Information acquired by the hippocampus interferes with acquisition of the amygdala‐based conditioned‐cue preference in the rat
White and McDonald (1993, Behav Brain Res 55:269–281) previously reported that animals with amygdala lesions failed to acquire a conditioned‐cue preference (CCP) based on spatial cues, but that animals with fornix lesions exhibited larger CCPs of this type than normal animals. The present experments focused on the hippocampal interference with amygdala‐based CCP learning inferred from this finding. In experiment 1 we tested the hypothesis that this interference was due to the acquistion of information about the maze and its environment during a 10 min period of free exploration of the maze before the start of CCP training, hitherto given to all animals in these experiments. Normal animals that were not preexposed to the maze and animals that were preexposed to similar maze in a different room both exhibited larger CCPs than animals that were preexposed to the same maze in the same room as CCP training and testing. This suggests that normal animals acquire context‐specific information during the preexposure period and that this may be the cause of the hippocampus‐based interference with the amygdala‐mediated CCP. In experiment 2 we attempted to determine if the information thought to be acquired by the hippocampal memory system interferes with acquistion or expression of the CCP. As previously demonstrated, animals that received fornix lesions before preexposure (i.e., before the start of the experiment) exhibited larger than normal CCPs. Animals that received fornix lesions after preexposure but before CCP training and animals that received fornix lesions after CCP training but before testing both exhibited normal CCPs. These findings suggest that a hippocampally mediated process acts via a pathway that does not involve the fornix to inhibit either acquistion or expression, or both, of the CCP. These results imply the existence of some form of competition for behavioral control among neural systems that may be specialized for acquiring different types of information about the same situations. © 1995 Wiley‐Liss, Inc.
13146231
Competition between Memory Systems: Acetylcholine Release in the Hippocampus Correlates Negatively with Good Performance on an Amygdala-Dependent Task
Lesions of the amygdala impair acquisition of a food conditioned place preference (CPP) task. In contrast, lesions of the fornix facilitate acquisition on this task, showing that an intact hippocampal system can interfere with learning an amygdala-dependent task. Our recent findings indicate that acetylcholine (ACh) release in the hippocampus increases while rats perform a hippocampus-dependent spontaneous alternation task. To the extent that ACh output in the hippocampus reflects activation of that brain area in learning and memory, the results obtained with fornix lesions suggest that ACh release in the hippocampus might be negatively correlated with learning on a CPP task. Using in vivo microdialysis, release of ACh was measured in the hippocampus while rats learned and were tested on an amygdala-dependent CPP task and a hippocampus-dependent spontaneous alternation task. Release of ACh in the hippocampus increased when rats were tested on either task. The magnitude of the increase in release of hippocampal ACh was negatively correlated with good performance on the amygdala-dependent CPP task. These findings suggest that ACh release may reflect activation and participation of the hippocampus in learning and memory, but in a manner that can be detrimental to performance on a task dependent on another brain area.
4321622
Optogenetic stimulation of a hippocampal engram activates fear memory recall
A specific memory is thought to be encoded by a sparse population of neurons. These neurons can be tagged during learning for subsequent identification and manipulation. Moreover, their ablation or inactivation results in reduced memory expression, suggesting their necessity in mnemonic processes. However, the question of sufficiency remains: it is unclear whether it is possible to elicit the behavioural output of a specific memory by directly activating a population of neurons that was active during learning. Here we show in mice that optogenetic reactivation of hippocampal neurons activated during fear conditioning is sufficient to induce freezing behaviour. We labelled a population of hippocampal dentate gyrus neurons activated during fear learning with channelrhodopsin-2 (ChR2) and later optically reactivated these neurons in a different context. The mice showed increased freezing only upon light stimulation, indicating light-induced fear memory recall. This freezing was not detected in non-fear-conditioned mice expressing ChR2 in a similar proportion of cells, nor in fear-conditioned mice with cells labelled by enhanced yellow fluorescent protein instead of ChR2. Finally, activation of cells labelled in a context not associated with fear did not evoke freezing in mice that were previously fear conditioned in a different context, suggesting that light-induced fear memory recall is context specific. Together, our findings indicate that activating a sparse but specific ensemble of hippocampal neurons that contribute to a memory engram is sufficient for the recall of that memory. Moreover, our experimental approach offers a general method of mapping cellular populations bearing memory engrams.
44969100
Engram cells retain memory under retrograde amnesia
Experimental recovery from retrograde amnesia When memory researchers induce amnesia, they normally assume that the manipulations prevent the memory engram from effective encoding at consolidation. In accordance with this, Ryan et al. found that after the injection of protein synthesis inhibitors, animals could not retrieve a memory. However, to their surprise, the memory could nevertheless be reactivated by light-induced activation of the neurons tagged during conditioning. Increased synaptic strength that is the result of cellular consolidation is thus not a critical requisite for storing a memory. Science, this issue p. 1007 Memory can be retrieved by optogenetic activation of the neurons involved in memory formation when memories would not normally have been stored. Memory consolidation is the process by which a newly formed and unstable memory transforms into a stable long-term memory. It is unknown whether the process of memory consolidation occurs exclusively through the stabilization of memory engrams. By using learning-dependent cell labeling, we identified an increase of synaptic strength and dendritic spine density specifically in consolidated memory engram cells. Although these properties are lacking in engram cells under protein synthesis inhibitor–induced amnesia, direct optogenetic activation of these cells results in memory retrieval, and this correlates with retained engram cell–specific connectivity. We propose that a specific pattern of connectivity of engram cells may be crucial for memory information storage and that strengthened synapses in these cells critically contribute to the memory retrieval process.
7394042
Direct Reactivation of a Coherent Neocortical Memory of Context
Declarative memories are thought to be stored within anatomically distributed neuronal networks requiring the hippocampus; however, it is unclear how neocortical areas participate in memory at the time of encoding. Here, we use a c-fos-based genetic tagging system to selectively express the channelrhodopsin variant, ChEF, and optogenetically reactivate a specific neural ensemble in retrosplenial cortex (RSC) engaged by context fear conditioning. Artificial stimulation of RSC was sufficient to produce both context-specific behavior and downstream cellular activity commensurate with natural experience. Moreover, optogenetically but not contextually elicited responses were insensitive to hippocampal inactivation, suggesting that although the hippocampus is needed to coordinate activation by sensory cues, a higher-order cortical framework can independently subserve learned behavior, even shortly after learning.
15893863
The Involvement of the Anterior Cingulate Cortex in Remote Contextual Fear Memory
Although the molecular, cellular, and systems mechanisms required for initial memory processing have been intensively investigated, those underlying permanent memory storage remain elusive. We present neuroanatomical, pharmacological, and genetic results demonstrating that the anterior cingulate cortex plays a critical role in remote memory for contextual fear conditioning. Imaging of activity-dependent genes shows that the anterior cingulate is activated by remote memory and that this activation is impaired by a null α-CaMKII mutation that blocks remote memory. Accordingly, reversible inactivation of this structure in normal mice disrupts remote memory without affecting recent memory.
41779721
Prefrontal microcircuit underlies contextual learning after hippocampal loss
Specific brain circuits have been classically linked to dedicated functions. However, compensation following brain damage suggests that these circuits are capable of dynamic adaptation. Such compensation is exemplified by Pavlovian fear conditioning following damage to the dorsal hippocampus (DH). Although the DH normally underlies contextual fear and fear renewal after extinction, both can be learned in the absence of the DH, although the mechanisms and nature of this compensation are currently unknown. Here, we report that recruitment of alternate structures, specifically the infralimbic and prelimbic prefrontal cortices, is required for compensation following damage to the hippocampus. Disconnection of these cortices in DH-compromised animals and immediate early gene induction profiles for amygdala-projecting prefrontal cells revealed that communication and dynamic rebalancing within this prefrontal microcircuit is critical. Additionally, the infralimbic cortex normally plays a role in limiting generalization of contextual fear. These discoveries reveal that plasticity through recruitment of alternate circuits allows the brain to compensate following damage, offering promise for targeted treatment of memory disorders.
14159880
Stimulation of Medial Prefrontal Cortex Decreases the Responsiveness of Central Amygdala Output Neurons
In extinction of auditory fear conditioning, rats learn that a tone no longer predicts the occurrence of a footshock. Recent lesion and unit recording studies suggest that the medial prefrontal cortex (mPFC) plays an essential role in the inhibition of conditioned fear following extinction. mPFC has robust projections to the amygdala, a structure that is known to mediate the acquisition and expression of conditioned fear. Fear conditioning potentiates the tone responses of neurons in the basolateral amygdala (BLA), which excite neurons in the central nucleus (Ce) of the amygdala. In turn, the Ce projects to the brainstem and hypothalamic areas that mediate fear responses. The present study was undertaken to test the hypothesis that the mPFC inhibits conditioned fear via feedforward inhibition of Ce output neurons. Recording extracellularly from physiologically identified brainstem-projecting Ce neurons, we tested the effect of mPFC prestimulation on Ce responsiveness to synaptic input. In support of our hypothesis, mPFC prestimulation dramatically reduced the responsiveness of Ce output neurons to inputs from the insular cortex and BLA. Thus, our findings support the idea that mPFC gates impulse transmission from the BLA to Ce, perhaps through GABAergic intercalated cells, thereby gating the expression of conditioned fear.
16295579
Gating of Fear in Prelimbic Cortex by Hippocampal and Amygdala Inputs
The prefrontal cortex (PFC) regulates emotional responses, but it is unclear how PFC integrates diverse inputs to select the appropriate response. We therefore evaluated the contribution of basolateral amygdala (BLA) and ventral hippocampus (vHPC) inputs to fear signaling in the prelimbic (PL) cortex, a PFC region critical for the expression of conditioned fear. In conditioned rats trained to press for food, BLA inactivation decreased the activity of projection cells in PL, and reduced PL conditioned tone responses. In contrast, vHPC inactivation decreased activity of interneurons in PL and increased PL conditioned tone responses. Consistent with hippocampal gating of fear after extinction, vHPC inactivation increased fear and PL pyramidal activity in extinguished, but not in conditioned, rats. These results suggest a prefrontal circuit whereby hippocampus gates amygdala-based fear. Thus, deficient hippocampal inhibition of PFC may underlie emotional disorders, especially in light of reduced hippocampal volume observed in depression and PTSD.
206986433
Anatomical substrates for direct interactions between hippocampus, medial prefrontal cortex, and the thalamic nucleus reuniens
The reuniens nucleus in the midline thalamus projects to the medial prefrontal cortex (mPFC) and the hippocampus, and has been suggested to modulate interactions between these regions, such as spindle–ripple correlations during sleep and theta band coherence during exploratory behavior. Feedback from the hippocampus to the nucleus reuniens has received less attention but has the potential to influence thalamocortical networks as a function of hippocampal activation. We used the retrograde tracer cholera toxin B conjugated to two fluorophores to study thalamic projections to the dorsal and ventral hippocampus and to the prelimbic and infralimbic subregions of mPFC. We also examined the feedback connections from the hippocampus to reuniens. The goal was to evaluate the anatomical basis for direct coordination between reuniens, mPFC, and hippocampus by looking for double-labeled cells in reuniens and hippocampus. In confirmation of previous reports, the nucleus reuniens was the origin of most thalamic afferents to the dorsal hippocampus, whereas both reuniens and the lateral dorsal nucleus projected to ventral hippocampus. Feedback from hippocampus to reuniens originated primarily in the dorsal and ventral subiculum. Thalamic cells with collaterals to mPFC and hippocampus were found in reuniens, across its anteroposterior axis, and represented, on average, about 8 % of the labeled cells in reuniens. Hippocampal cells with collaterals to mPFC and reuniens were less common (~1 % of the labeled subicular cells), and located in the molecular layer of the subiculum. The results indicate that a subset of reuniens cells can directly coordinate activity in mPFC and hippocampus. Cells with collaterals in the hippocampus–reuniens–mPFC network may be important for the systems consolidation of memory traces and for theta synchronization during exploratory behavior.
16718274
A Neural Circuit for Memory Specificity and Generalization
The Thalamus in Fear and Memory The medial prefrontal cortex (mPFC) mediates the cognitive control of many high-level brain functions. However, it is unclear which synaptic projections from the mPFC to subcortical regions are critical for maintaining the proper balance between retention and generalization of fear memory details. Using an array of behavioral, physiological, and anatomical techniques, Xu and Südhof (p. 1290) describe a neural circuit that controls memory generalization and specificity. This circuit involves the nucleus reuniens (NR), a thalamic nucleus of largely unknown function. Optogenetic activation of NR neurons in awake behaving mice revealed the role of the NR in fear memory generalization. Projections to and from the nucleus reunions in the thalamus regulate the specificity of contextual fear memory. Increased fear memory generalization is associated with posttraumatic stress disorder, but the circuit mechanisms that regulate memory specificity remain unclear. Here, we define a neural circuit—composed of the medial prefrontal cortex, the nucleus reuniens (NR), and the hippocampus—that controls fear memory generalization. Inactivation of prefrontal inputs into the NR or direct silencing of NR projections enhanced fear memory generalization, whereas constitutive activation of NR neurons decreased memory generalization. Direct optogenetic activation of phasic and tonic action-potential firing of NR neurons during memory acquisition enhanced or reduced memory generalization, respectively. We propose that the NR determines the specificity and generalization of memory attributes for a particular context by processing information from the medial prefrontal cortex en route to the hippocampus.
17440372
Sex-Specific Neuroanatomical Correlates of Fear Expression in Prefrontal-Amygdala Circuits
BACKGROUND
The neural projections from the infralimbic region of the prefrontal cortex to the amygdala are important for the maintenance of conditioned fear extinction. Neurons in this pathway exhibit a unique pattern of structural plasticity that is sex-dependent, but the relationship between the morphologic characteristics of these neurons and successful extinction in male and female subjects is unknown.


METHODS
Using classic cued fear conditioning and an extinction paradigm in large cohorts of male and female rats, we identified subpopulations of both sexes that exhibited high (HF) or low (LF) levels of freezing on an extinction retrieval test, representing failed or successful extinction maintenance, respectively. We combined retrograde tracing with fluorescent intracellular microinjections to perform three-dimensional reconstructions of infralimbic neurons that project to the basolateral amygdala in these groups.


RESULTS
The HF and LF male rats exhibited neuroanatomical distinctions that were not observed in HF or LF female rats. A retrospective analysis of behavior during fear conditioning and extinction revealed that despite no overall sex differences in freezing behavior, HF and LF phenotypes emerged in male rats during extinction and in female rats during fear conditioning, which does not involve infralimbic-basolateral amygdala neurons.


CONCLUSIONS
Our results suggest that the neural processes underlying successful or failed extinction maintenance may be sex-specific. These findings are relevant not only to future basic research on sex differences in fear conditioning and extinction but also to exposure-based clinical therapies, which are similar in premise to fear extinction and which are primarily used to treat disorders that are more common in women than in men.
23910096
Hippocampal cytosolic estrogen receptors regulate fear generalization in females
Generalization of fear responses is a symptom of many anxiety disorders and we have previously demonstrated that female rats generalize fear to a neutral context at a faster rate compared to males. This effect is due in part, to activation of ER and modulation of memory retrieval mechanisms resulting in fear generalization. Given that the effects of estradiol on fear generalization required approximately 24h, our data suggested possible genomic actions on fear generalization. To determine whether these actions were due to cytosolic versus membrane bound receptors, female rats were given infusions of ICI 182,780, a cytosolic estrogen receptor antagonist, into the lateral ventricle or dorsal hippocampus simultaneously with estradiol treatment or with an ER agonist (DPN). Infusions of ICI into the lateral ventricle or the dorsal hippocampus blocked fear generalization induced by peripheral or central treatment with estradiol or DPN, suggesting that estradiol acts through cytosolic ERβ receptors. In further support of these findings, intracerebroventricular or intra-hippocampal infusions of bovine serum conjugated estradiol (E2-BSA), activating membrane-bound estrogen receptors only, did not induce fear generalization. Moreover, rats receiving intra-hippocampal infusions of the ERK/MAPK inhibitor, U0126, continued to display estradiol-induced generalization, again suggesting that membrane-bound estrogen receptors do not contribute to fear generalization. Overall, these data suggest that estradiol-induced enhancements in fear generalization are mediated through activation of cytosolic/nuclear ER within the dorsal hippocampus. This region seems to be an important locus for the effects of estradiol on fear generalization although additional neuroanatomical regions have yet to be identified.
39309658
Acute stress impairs spatial memory in male but not female rats: influence of estrous cycle
We investigated how sex and estrous cycle influenced spatial recognition memory in the Y-maze after exposure to acute restraint stress. In Experiment 1, intact male and female rats were restrained for 1 h and then 2 h after the start of restraint, rats were trained on the Y-maze. After a 4 h delay, hippocampal-dependent spatial recognition memory was assessed. Acute stress produced opposite patterns between the sexes with spatial memory being impaired in males and facilitated in females. Serum corticosterone measures indicated that both sexes showed a robust corticosterone response after restraint and a moderate corticosterone response after Y-maze exposure. Serum corticosterone levels in response to restraint and Y-maze were not statistically different between the sexes. Experiment 2 examined the influence of the estrous cycle on spatial memory ability after acute stress. Acute stress facilitated spatial memory in females compared to controls, regardless of the estrous cycle phase (estrus and proestrus). Moreover, females in proestrus showed higher serum corticosterone levels during restraint compared to females in estrus. No differences in corticosterone levels were observed at baseline or following 2 h of recovery from restraint. These data show important differences in how sex and estrous cycle influence cognitive functions following acute stress.
23024484
Evidence for sex-specific shifting of neural processes underlying learning and memory following stress
Recent human research has been focused upon determining whether there is evidence that stress responses cause qualitative changes in neural activity such that people change their learning strategies from a spatial/contextual memory process through the hippocampus to a procedural stimulus-response process through the caudate nucleus. Moreover, interest has shifted to determining whether males and females exhibit the same type of stress-induced change in neural processing of associations. Presented is a select review of 2 different animal models that have examined how acute or chronic stressors change learning in a sex-specific manner. This is followed by a brief review of recent human studies documenting how learning and memory functions change following stressor exposure. In both cases, it is clear that ovarian hormones have a significant influence on how stress affects learning processes in females. We then examine the evidence for a role of acetylcholine, dopamine, norepinephrine, or serotonin in modulating this shifting of processing and how that may differ across sex. Conclusions drawn suggest that there may be evidence for sex-specific changes in amygdala and hippocampus neuromodulation; however, the behavioral data are still not conclusive as to whether this represents a common or sex-specific shift in how males and females process associations after stressor exposure.
3840236
Sex differences in the behavioural and hypothalamic–pituitary–adrenal response to contextual fear conditioning in rats
In recent years, special attention is being paid to sex differences in susceptibility to disease. In this regard, there is evidence that male rats present higher levels of both cued and contextual fear conditioning than females. However, little is known about the concomitant hypothalamic-pituitary-adrenal (HPA) axis response to those situations which are critical in emotional memories. Here, we studied the behavioural and HPA responses of male and female Wistar rats to context fear conditioning using electric footshock as the aversive stimulus. Fear-conditioned rats showed a much greater ACTH and corticosterone response than those merely exposed to the fear conditioning chamber without receiving shocks. Moreover, males presented higher levels of freezing whereas HPA axis response was greater in females. Accordingly, during the fear extinction tests, female rats consistently showed less freezing and higher extinction rate, but greater HPA activation than males. Exposure to an open-field resulted in lower activity/exploration in fear-conditioned males, but not in females, suggesting greater conditioned cognitive generalization in males than females. It can be concluded that important sex differences in fear conditioning are observed in both freezing and HPA activation, but the two sets of variables are affected in the opposite direction: enhanced behavioural impact in males, but enhanced HPA responsiveness in females. Thus, the role of sex differences on fear-related stimuli may depend on the variables chosen to evaluate it, the greater responsiveness of the HPA axis in females perhaps being an important factor to be further explored.
205511443
Finding the engram
Many attempts have been made to localize the physical trace of a memory, or engram, in the brain. However, until recently, engrams have remained largely elusive. In this Review, we develop four defining criteria that enable us to critically assess the recent progress that has been made towards finding the engram. Recent 'capture' studies use novel approaches to tag populations of neurons that are active during memory encoding, thereby allowing these engram-associated neurons to be manipulated at later times. We propose that findings from these capture studies represent considerable progress in allowing us to observe, erase and express the engram.
83876870
Molecular Mechanisms of Memory in Males and Females
Abstract In this chapter, we review differential molecular mechanisms in memory formation in female and male animals. Whereas delineating the receptors, intracellular signaling cascades, transcriptional mechanisms, and de novo protein synthesis required for memory formation has been a focus of research, the vast majority of this work has been solely in males. Evidence suggests, however, that females differ from males in strategies used to execute memory tasks, neural circuits engaged, and mechanisms required for memory formation. We discuss the influences of chromosomal complement and hormonal modulation in driving sex differences in intracellular signaling cascades and gene expression underlying memory processes. Finally, we explore how sex differences in molecular mechanisms of memory might be used as a comparative approach for understanding how different molecular network activation can give rise to the same functional outcome.