Relevance of this research to public health. Exposure to stressful experiences, such as social isolation, early in development can contribute to vulnerability to stress-related psychiatric disorders during adulthood. One mechanism through which adverse early-life experiences may alter subsequent stress-related behavior is through long-term changes in the activity of the global monoaminergic systems, including serotonergic systems, that project to forebrain limbic circuits that regulate these behaviors. Consequently, understanding how adverse early-life experiences alter the activity of serotonergic systems projecting to forebrain circuits mediating stress- or anxiety-related behaviors is an important research objective. The proposed studies will investigate these relationships in a rat model of social isolation. Social isolation (SI) of rats has the most potent effects on subsequent behavior when it occurs during a critical phase from weaning to early adulthood. My previous studies have shown that isolation during early life alters corticotropin-releasing factor (CRF)-mediated serotonin (5-HT) release in the nucleus accumbens (NAc), a key limbic structure implicated as a substrate for many of the behavioral alterations observed with post-weaning SI, including increased anxiety. My additional studies suggest that isolation during the early part of development causes an up-regulation of CRF type 2 (CRF2) receptor levels in the dorsal raphe nucleus (dRN), an effect that may underlie the long-term alterations of CRF-induced serotonergic activity in the NAc observed in isolates. Furthermore, my findings suggest that the Si-induced alterations in fear, anxiety, and stress-related behavioral responses could be due to sensitization of stress-related, CRF- dependent activation of a dRN-NAc serotonergic circuit. The proposed studies will investigate three related objectives: 1) to determine the effects of SI and subsequent exposure to social defeat on behavior and c- Fos expression in 5-HT neurons in subdivisions of the dRN, 2) to investigate the effects of SI and subsequent exposure to social defeat on patterns of tryptophan hydroxylase 2 (tph2) gene expression in the dRN, and 3) to identify the effects of SI on in vitro tryptophan hydroxylase (TrpOH) activity-dependent responses in the dRN. These experiments will utilize the rat model of SI, and the techniques of behavioral testing and analysis, immunohistochemistry, in situ hybridization, neurochemistry, andelectrophysiology. These studies will increase our understanding of the mechanisms through which adverse early life experiences alter the effects of stress-related stimuli on serotonergic systems, which have been implicated in the pathophysiology of stress-related neuropsychiatric disorders.