During puberty and adolescence, the brain experiences significant structural changes that mediate the maturation of sex-specific sociosexual behaviors, such as sexual behavior and intermale aggression. These sex-specific sociosexual behaviors are regulated by sexually dimorphic brain regions such as the posterodorsal medial amygdala (MePD) and ventromedial hypothalamus (VMH). The MePD in laboratory rodents is larger in males than females and is responsible for processing chemosensory signals to initiate context-appropriate social behaviors. The VMH is also larger in males than females in rodent species, which is primarily driven by sex differences in the ventrolateral subdivision (VMHvl). The VMHvl is involved in gating male sexual behavior and intermale aggression. Many of these sociosexual behaviors do not emerge until the onset of puberty, suggesting that morphological changes in the MePD and VMHvl during puberty may facilitate their development. Recent studies have discovered that new cells are added to the hypothalamus and amygdala during puberty and a subset of pubertally born cells in the MePD is activated in response to a social encounter. The current dissertation sets out to expand on these findings and examine pubertal cytogenesis in the MePD and VMH using the mouse model. In Chapter 1A, a sex difference in the number of pubertally born cells was revealed in the adult MePD, with males having more pubertally born cells than females. This increase in pubertally born cells in males was dependent on the presence of functional androgen receptors (ARs) and pubertal and adult testosterone. In Chapter 1B, while I initially discovered a sex difference in pubertally born cells in the VMHvl, this finding was not replicated in subsequent studies. In addition, no sex differences were observed in the number of pubertally born cells in sexually monomorphic brain regions (i.e. dorsomedial and central subdivision of the VMH (VMHdm and VMHc, respectively) and dentate gyrus (DG)). These results suggest that the number of pubertally born cells in the adult VMH and DG may not be affected by gonadal hormones. In Chapter 2, I discovered that pubertal, but not adult, ARs promote the increase in pubertally born neurons, but not astrocytes, in the adult male MePD. However, pubertal and adult ARs did not influence the number of pubertally born neurons or astrocytes in the VMH or DG. In Chapter 3, I found that social interaction activated pubertally born cells in the adult male MePD, while social interaction with a male conspecific activated pubertally born cells in the adult male VMHvl and VMHdm. Social interaction did not activate pubertally born cells in the VMHc and DG. These studies demonstrate that gonadal hormones play an important role in regulating the proliferation and/or survival of pubertally born cells in the adult MePD, but not VMH or DG. Furthermore, I provide evidence that pubertally born cells in the MePD, VMHvl, and VMHdm are functionally incorporated into adult neural circuits. Thus, the addition and functional integration of pubertally born cells into neural circuits known to regulate social behavior may be a mechanism by which adult sociosexual behaviors emerge during puberty.
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