Effects of photoperiod, castration, and gonadotropin-releasing hormone (GnRH) on the number of GnRH receptors in male golden hamsters.

Maintenance of male golden hamsters on short photoperiod leads to testicular regression due to pineal gland-mediated inhibition of the hypothalamo-hypophyseal reproductive system. This study clarifies the dynamics of the action of GnRH at the pituitary level after short photoperiod-induced gonadal regression. In Exp 1, adult male golden hamsters were injected with BSA or varying doses of exogenous GnRH every 8 h for 3 days. There was no effect on the number of GnRH receptors, indicating that GnRH does not increase the number of its own receptors in hamsters as it does in rats. In Exp II, adult male golden hamsters were placed on a 14-h light, 10-h dark lighting schedule (LD 14:10) or LD 6:18. Nine weeks later, half of the animals on each photoperiod were castrated, and 10 weeks after initiation of the experiment, intact and castrated hamsters were decapitated. Intact hamsters on LD 6:18 had the expected reduction in serum LH levels and testicular weight compared to intact animals on LD 14:10. There was a postcastration rise in serum LH in both groups, but the increase was attenuated in the animals on short photoperiod. Castration of animals on LD 14:10 resulted in an increased number of GnRH receptors per pituitary, but this increase was at least partly due to an increase in pituitary weight. Intact animals on LD 6:18 had fewer pituitary GnRH receptors than intact hamsters on LD 14:10, but this decrease was not due solely to a decrease in pituitary weight. These results indicate that maintenance on short photoperiod results in a decreased number of pituitary GnRH receptors. In Exp III, intact animals on LD 6:18 responded to 1 microgram GnRH, sc, with an increment in LH secretion similar to that in intact animals on LD 14:10. Castration of animals on long photoperiod increased the LH response to GnRH, but castration of hamsters on short photoperiod did not. In conclusion, a reduced number of pituitary GnRH receptors may be involved in the testicular regression associated with short photoperiod in male golden hamsters.