Abstract

This thesis investigated the role of gonadal steroids in the expression of appetitive and consummatory aspects of sexual behavior in the male rat, with particular attention to the involvement of the ventral tegmental area (VTA). Initial experiments established the time course of the loss and reinstatement of appetitive and consummatory behaviors following castration and subsequent testosterone (T) treatment, and further the excitatory role for sexual experience in "buffering" males against the inhibitory effects of castration. Additional experiments were conducted to examine the activation of different brain regions, including the VTA, following chemosensory, somatosensory, and hormonal stimulation in castrated, T-treated males, using Fos immunocytochemistry. Neural activation of the VTA was observed following multiple ejaculations, and subsequent studies revealed populations of dopamine and GABA neurons that had been activated differentially by copulatory stimulation. Lesions of the VTA disrupted both appetitive and consummatory sexual behaviors, but animals with more sexual experience prior to the lesions were less affected. Finally, intracranial implants of crystalline estradiol (E) into the VTA facilitated mounting in a majority of castrated males. T produced a smaller effect, whereas progesterone or cholesterol had no effect. These data indicate that the VTA plays a role in the hormonal stimulation of copulatory behavior. The effectiveness of E was particularly surprising, given the lack of neuronal aromatase and classical estrogen receptor in this region. However, the presence of glial aromatase and estrogen receptor Ý in this region could account for these effects, and suggest a novel mechanism by which dopamine and GABA neurons in this region might be regulated.