Characterization of novel groups of catecholaminergic cells in the male prairie vole extended amygdala

The neurobiology of copulatory and maternal behaviors has been well studied in mammals, but much less is known about gregarious and monogamous social behaviors. Prairie voles (Microtus ochrogaster) are one of few mammalian species that are highly gregarious, monogamous, and biparental, and have been a useful rodent model for studying the neural networks influencing these behaviors. Comparisons between the prairie vole brain and closely related nonmonogamous rodents have revealed species differences in neurotransmitter and receptor expression that contribute to differences in social behavior. Recently, large populations of cells expressing tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, were discovered in the prairie vole principal nucleus of the bed nucleus of the stria terminalis (pBST) and posterodorsal medial amygdala (MeApd). Large numbers of TH-immunoreactive (TH-ir) cells have never been described in these sites in other species. The pBST and MeApd are important for the expression of sociosexual behaviors, and these cells may affect the social behaviors typical of prairie voles. The experiments in this dissertation characterize the distribution, hormone regulation, immediate-early gene (IEG) responses, and neuroanatomical projections of these unique groups of cells in the male prairie vole brain.;The results of the first experiment (Chapter 2) indicated that several nonmonogamous species, laboratory rats, Syrian hamsters, and meadow voles, did not have these large populations of TH-ir cells, further supporting the idea that these cells may influence species-specific social behaviors. The results also indicated that male prairie voles had 3--5 times more TH-ir cells in the pBST and the MeApd than did females, and this sex difference was almost entirely due to differences in circulating gonadal hormones during adulthood. The experiments in Chapter 3 examined immediate-early gene expression in these TH-ir cells after males engaged in sociosexual behaviors, and these experiments revealed that these groups of TH-ir neurons likely influence gregariousness in male prairie voles, but may also be involved more specifically in mating and its facilitation of pair bonding. Finally, neuroanatomical tracing (Chapter 4) revealed that many TH-ir cells in the pBST and MeApd project to the medial preoptic area (MPO), an area important for the expression of many sociosexual behaviors.;These experiments demonstrate that TH expression in the pBST and MeApd remains plastic in adulthood and is sensitive to circulating gonadal hormones, much like other morphological and neurochemical features of the pBST and MeApd in rats, mice, and hamsters. Furthermore, these cell groups are unique in that they may promote the expression of all social behaviors, but also specific behaviors related to mating and pair bonding. They also provide a species-specific catecholaminergic input to the MPO, which has connections with other brain areas known to be important for pair bonding. These experiments have provided information necessary for many future experiments that can explore the precise function of these cells and their interactions with other neurochemicals. This catecholaminergic network is likely essential for the affiliative and monogamous behaviors typical of male prairie voles.