Steroidal and photoperiodic regulation of opiate binding in male golden hamsters.

This study was designed to investigate interactions between daylength and testosterone (T) in the regulation of 3H-naloxone binding which may contribute to seasonal changes in the negative feedback and behavioral effects of androgens in the golden hamster. Photoperiod influenced opiate binding in hamsters with intact gonads only in the medial amygdala. Castration elevated specific 3H-naloxone binding in the medial amygdala, medial preoptic nucleus, bed nucleus of the stria terminalis, and in the basolateral amygdaloid nucleus of hamsters exposed to either long (14L:10D) or short (5L:19D) days. Exposure to SD renders hamsters less sensitive to T maintenance in reversing these effects. Delay of T replacement until 5 weeks after castration eliminated the ability of this androgen to reverse the influence of castration upon opiate receptors in the medial amygdala. Pinealectomy markedly increased 3H-naloxone binding in short days in several brain areas. The data demonstrate that androgens and photoperiod interact to regulate 3H-naloxone binding, particularly in the medial amygdala. These effects may play a functionally relevant role in seasonal changes in the expression of sexual behavior and/or gonadotropin secretion.