Time course of sensitivity of golden hamsters to melatonin injections throughout the day.

Twenty-four groups of adult male golden hamsters, housed on 14 h of light, 14L:10D, received daily injections of melatonin, each at a different hour of the day. Injections (15 micrograms melatonin/0.1 ml ethanol:saline, 1:10, s.c.) persisted for 27 weeks. At the beginning of this investigation and every 2-4 weeks thereafter, laparotomies were performed on a few animals of each group and the testes measured to assess the efficacy of the melatonin injections in causing testicular regression. Two periods of melatonin sensitivity were identified. The first was brief, of 1-h duration, immediately prior to lights on (0500-0600 h). The second period extended for 6 h throughout late afternoon and early evening (1500-2100 h; lights out 2000 h). Regression was complete in all these groups except that injected at 1500 h in which the testes regressed to only 950 mg before recrudescence occurred. In all other affected groups, testes regressed to less than 400 mg. In those groups where melatonin did not cause full testicular regression, daily injections resulted in partial regression, usually to approximately 2000 mg prior to regrowth.