SUMMARY / ABSTRACT We plan to investigate circuit defects underlying sensory hypersensitivity in Fragile X syndrome (FXS), the most common inherited form of intellectual impairment and the most common single gene cause of autism. In hyperarousal to sensory stimuli, affected individuals are deeply troubled by sounds, smells, sights, or touches that seem normal to others. This leads to maladaptive behaviors, including avoidance responses, such as tactile defensiveness. Virtually all individuals with FXS suffer from tactile defensiveness and Fmr1 knockout (Fmr1-/-) mice, an animal model of FXS, exhibit clear signs of sensory hyperarousal. Elucidating the types of circuit dysfunction that cause fragile X mice to interpret certain stimuli as aversive/threatening, and how this eventually leads to an avoidance response, represents a major knowledge gap in FXS research. To address this, we propose a novel symptom-to-circuit-to-neuron approach in the Fmr1-/- mouse model of FXS in order to investigate disruptions at the circuit and single neuron levels that result in altered sensory processing. In a recent study (He et al., J Neurosci, 2017), we demonstrated how, in response to repetitive tactile stimulation of whiskers, Fmr1-/- mice display a sensory avoidance behavior analogous to tactile defensiveness in humans. Using in vivo calcium imaging in somatosensory (S1) barrel cortex, we then showed that repetitive whisker stimulation results in a gradual reduction in neuronal firing in 2-week-old and in adult wild-type (WT) mice, but not in Fmr1-/- mice. Thus, one of the circuit defects that could explain tactile defensiveness in FXS is a loss of neuronal adaptation in cortical neurons (simply put, neurons in S1 cortex of fragile X mice are not be able to tune out persistent tactile stimuli). We now propose to test whether this loss of neuronal adaptation results from a dysfunction in parvalbumin (PV) or somatostatin (SST) GABAergic interneurons in S1 cortex, and then to delineate circuit alterations in brain regions that are both upstream or downstream from S1 cortex. These studies will allow us to generate a more detailed wiring diagram of sensory hyperarousal in FXS, by examining three stages of sensory processing: thalamus (input), cortex (integration), and amygdala (output). Throughout, we will investigate whether manipulating neuronal activity at each of these stages of sensory processing might ameliorate maladaptive behaviors associated with sensory hyperarousal in Fmr1-/- mice. Our experimental design employs cutting edge techniques, including in vivo two-photon calcium imaging, silicon microprobes, DREADDs, and Cre-Lox genetics, and seeks to address important knowledge gaps in FXS. Because many of the signaling pathways that are dysregulated in FXS are also implicated in other neurodevelopmental disorders, we believe that our unique symptomcircuit approach has a very high significance and is likely to be of broad importance to many types of autism and mental impairment.