Integration of chemosensory and hormonal input in the male Syrian hamster brain.

Mating in the male Syrian hamster requires the interaction of chemosensory and hormonal stimuli. Chemosensory cues from the vomeronasal organ and olfactory mucosa are transmitted through limbic nuclei that contain receptors for gonadal steroid hormones, including the medial amygdaloid nucleus (Me) and medial preoptic area (MPOA). This pathway is essential for mating, as lesions that interrupt transmission of chemosensory cues to MPOA will abolish copulation. Likewise, gonadal steroids facilitate sexual behavior through Me and MPOA, as demonstrated using intracranial implants in the brains of castrate males. In addition, odor and hormonal signals must be integrated in the brain for copulation to occur. Mating is prevented when olfactory bulbectomy is performed ipsilateral to an intracranial testosterone implant, thereby preventing the interaction of odors and hormones. According to our current model, hormones may act as a gating signal to strengthen synaptic contacts along the chemosensory pathway, thereby permitting or enhancing transmission of chemosensory cues.