Integration of chemosensory and hormonal cues is essential for sexual behaviour in the male Syrian hamster: role of the medial amygdaloid nucleus.

Mating behaviour in the male hamster requires chemosensory and hormonal cues, and copulation is abolished if either signal is interrupted. In addition, the integration of chemosensory stimuli with steroid signals is essential for mating. In castrated male hamsters, implantation ofa testosterone-filled cannula in the preoptic area stimulates mating behaviour. However, removal of the ipsilateral olfactory bulb prevents steroid facilitation of sexual activity. The present studies determined if the integration of chemosensory and hormonal cues necessary for mating behaviour is distributed within steroid-sensitive nuclei in the brain, or is restricted to the preoptic area. Specifically, the hypothesis was tested that the medial amygdala is capable of odour and hormone integration. Castrated male hamsters received an intracerebral implant of testosterone in the medial amygdala combined with removal of a single olfactory bulb, ipsilateral or contralateral to the implant. Mating behaviour did not increase after implant surgery and bulbectomy in either ipsilateral or contralateral bulbectomized males. In a second study, males were bulbectomized three weeks after implant surgery, to demonstrate the ability of testosterone in the medial amygdala to stimulate male sexual behaviour, and the loss of behaviour following bulbectomy. The results confirm that integration of odour and steroid cues is essential for mating in the male hamster. Moreover, the medial amygdaloid nucleus contributes to chemosensory and hormonal integration. However, compared with steroid stimulation in the preoptic area, the behavioural effects of testosterone in the medial amygdaloid nucleus are more sensitive to manipulations of the olfactory system, suggesting that the amygdala requires bilateral chemosensory input.