Integration of chemosensory and hormonal cues is essential for mating in the male Syrian hamster.

Mating behavior in the male hamster is dependent upon both chemosensory and hormonal cues, and copulation is abolished if either signal is interrupted. Through reciprocal interactions of these signals, chemosensory stimuli increase circulating testosterone in the male, and the male's hormonal status influences his attraction to female pheromones. Furthermore, anatomical data suggest that these signals are transmitted through parallel pathways in separate subdivisions of the medial amygdaloid nucleus, bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). The aim of this study was to determine if the integration of chemosensory and hormonal cues is essential for mating. We combined an intracerebral implant of testosterone in BNST/MPOA with removal of a single olfactory bulb (UBx), ipsilateral or contralateral to the steroid implant. Previous studies have demonstrated that testosterone implants which stimulate androgen receptor-containing neurons in posteromedial BNST and MPOA can increase mounting in males castrated for 12 weeks. Moreover, unilateral bulbectomy alone does not prevent mating. In the present study, ipsilateral UBx prevents communication between hormonal and chemosensory circuits. Sexually experienced males were used. Twelve weeks after castration, a single olfactory bulb was removed, and each male received a testosterone-filled cannula (23 ga) directed at the MPOA. Two weeks later, sexual activity increased in six males with implants in BNST/MPOA and contralateral UBx, but copulation was not restored in eight males with ipsilateral UBx despite equivalent implant placement. This study demonstrates that communication between neurons receiving hormonal signals and chemosensory cues is required for mating behavior.