The medial amygdaloid nucleus and medial preoptic area mediate steroidal control of sexual behavior in the male Syrian hamster.

An important goal of studies on steroid receptors in the brain is to understand the functions of specific populations of steroid receptor-containing neurons, particularly in the control of sexual behavior. The present study compared the ability of testosterone implants directed toward the medial amygdaloid nucleus (Me) or the bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA) to stimulate mating in castrated males. Twenty adult male hamsters were selected for vigorous sexual behavior, and baseline measures of copulatory behavior during the first 10 min of a 30-min mating behavior test were recorded on two occasions. Twelve weeks after castration, sexual behavior was recorded as before, and each male received a single intracranial implant constructed of 23-gauge tubing packed with crystalline testosterone placed stereotaxically into Me or BNST/MPOA (n = 10 each). Behavior was assessed on two occasions after surgery to determine if implants in Me or BNST/MPOA could stimulate sexual behavior above that observed after castration. In half of the males from each group, testosterone increased the males' interaction with the female, anogenital investigation, mounting, and reduced the latency to the first mount. After completion of behavioral testing, males were perfused and the brains processed for androgen receptor immunocytochemistry to determine the extent of brain regions influenced by the testosterone-filled cannula. In tissues stained rapidly in diaminobenzidine, androgen receptor-containing neurons were visible only in steroid responsive brain regions adjacent to the testosterone implant. This approach revealed that steroid receptors in both the posterior subdivision of Me and in the medial subregions of BNST/MPOA can mediate hormonal control of mating behavior in the male Syrian hamster. Implants placed outside these regions failed to stimulate mating. These results suggest that maintenance of copulatory behavior by gonadal steroids is not transduced by a single brain region, but that steroid effects can be elicited at multiple points along the mating behavior pathway.