Hypothalamic LHRH concentration decreases in intact but increases in castrated rats in constant light

In order to evaluate the effects of environmental lighting on the hypothalamic secretion of luteinizing hormone-releasing hormone (LHRH), intact and castrated male Wistar rats were kept either in periodic (LD) or in constant light (LL) for 1 week, and the hypothalamic LHRH concentration was measured by radioimmunoassay. In the intact rats the LHRH concentration was lower in constant than in periodic light (30 +/- 3 vs 61 +/- 3 pg mg-1, P less than 0.05), whereas in the castrated rats it was higher (16 +/- 2 vs 5 +/- 1 pg mg-1, P less than 0.05). The intact and castrated rats received melatonin (50 micrograms) or saline injections daily at either 09.00 h or 16.00 h in LD and LL for 1 week. In intact or castrated rats the melatonin injections were not able to prevent the change of hypothalamic LHRH in constant light. In castrated rats testosterone (125 or 250 micrograms) had no effect on the LHRH concentration under either lighting condition. The present results suggest that constant light reduces the synthesis of hypothalamic LHRH in intact rats but increases it in castrated rats. These effects of constant light do not appear to be related to melatonin.