Brain Research, 590 (1992) 29-38 © 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00 29 BRES 18016 Photoperiodic regulation of substance P immunoreactivity in the mating behavior pathway of the male golden hamster Jennifer Marie Swann and Nick Macchione Department of Biological Sciences, Rutgers State U~irersity, Newark, NJ 07102 (USA) (Accepted 31 March 1992) Key words: Medial nucleus of the amygdala; Bed nucleus of the stria terminalis; Medial preoptic area; Neuropeptide; Testosterone; Seasonal Mating behavior in the male golden hamster is regulated by both gonadal steroids and photoperiod. Gonadal steroids may regulate mating behavior by actions on the medial nucleus of the amygdala, bed nucleus of the stria terminalis, and medial preoptic area. Neurons in these areas actively accumulate gonadal steroids and lesions of these nuclei disrupt mating behavior in male hamsters. Photoperiodic regulation of mating behavior is regulated, at least in part, by decreased responsiveness to gonadal steroids. Therefore, we sought to determine if the changes induced by changes in gonadal steroids would mimic those induced by changes in photoperiod. The number of substance P-containing neurons in these areas decrease following castration and are restored with testosterone treatment suggesting that this peptide may mediate steroidal regulation of male mating behavior. To determine the effect of photoperiod on substance P, peptide containing neurons were counted in (1) enucleates (a -- 6), (2) ¢nudeated castrates treated with testosterone (n = 6), (3) castrates treated with testosterone (n -- 4), and (4) intact controls (n = 6). Bilateral enucleation caused a decrease in the number of substance P neurons in the medial nucleus, bed nucleus of the stria terminalis, and medial preoptic area (P < 0.05), Testosterone treatment prevented this decrease (P < 0.05). Thus, a decrease in daylength causes a decrease in substance P in the medial nucleus of the amygdala, the medial bed nucleus of the stria terminalis :rod the medial preoptic area that is mediated by changes in testosterone levels. INTRODUCTION In the male golden hamster, as in other mammals, copulatory behavior is regulated by both internal and external factors. The most significant internal factor is the level of circulating hormones, primarily androgens. Important external factors include those related to the proximity of the female and the time of year. Specifi- cally, reproductive behavior in the male hamster is stimulated by pheromones secreted by the female ~7 and testosterone 3 and inhibited by decreasing day- length t. Pheromones secreted by the female stimulate recep- tors in the vomeronasal organ and olfactory mucosa of the male 17'26 which project to the accessory and main olfactory bulbs, respectively 49. Neurons in both areas project directly and indirectly to the medial nucleus of the amygdala 43, which in turn projects to the bed nucleus of the stria terminalis, and the medial preoptic area 2°. This chemosensory system plays a crucial role in the regulation of copulatory behavior in the male hamster. Destruction of input from both vomeronasal organ and mucosa s') or ablation of the main and acces- sot), olfactory bulbs 2"s'at'eliminates male mating behav- ior. Similarly, bilateral lesions of the medial nucleus of the amygdala, the bed nucleus of the stria terminalis or the medial preoptic area abolish or severely disrupt normal copulation in the male hamster 2x38. The medial nucleus of the amygdala, the bed nu- cleus of the stria terminalis and the medial preoptic area may serve as sites for hormonal regulation of copulatory activity. Neurons in these areas concentrate gonadal steroids in a number of mammalian species 44 including the hamster '~'35. Androgen pellets implanted in these sites maintain or restore copulation in cas- trated hamsters 24. Similarly, a pellet of dihydrotestos- terone implanted in the medial nucleus of the amyg- dala of castrated, estrogen-primed male rats facilitates Correspondence: J.M. Swann, Department of Biological Sciences, 135 Smith Hall, Rutgers Univ°rsity, Newark, NJ 07102, USA. Fax (1) (2{)1) 648-1007.