Melatonin-Induced Testicular Growth in Golden Hamsters Maintained on Short Days

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Abstract
Melatonin, a putative pineal hormone, has been implicated in the control of neuroendocrine-gonadal activity in photoperiodic mammals. To study further the effect of melatonin on reproductive function, sexually mature adult male golden hamsters were implanted subcutaneously with either empty or 100 mm long melatonin-filled Silastic capsules before, or at various time intervals after, transfer to a nonstimulatory LD 6:18 light cycle. As expected, testicular regression occurred in control animals exposed to LD 6:18, but not in animals implanted with melatonin-filled capsules before they were placed on short days. If capsules were not implanted until after testicular atrophy had already occurred (i.e., either 6 or 9 weeks after transfer to LD 6:18), testicular growth was induced in melatonin-treated animals well in advance of spontaneous testicular recrudescence. In contrast to the stimulatory effect of melatonin on gonadal development in animals maintained on short days, melatonin inhibited testicular growth in animals that were moved from nonstimulatory short days to stimulatory (LD 14:10) long days. Gonadal growth eventually occurred in the melatonin-treated animals transferred to long days at a time corresponding to spontaneous testicular recrudescence in control animals maintained continuously on short days (i.e., between 16 and 23 weeks after initial transfer to LD 6:18). These results suggest that when the photoperiod stimulates neuroendocrine-gonadal activity, melatonin is inhibitory and when the photoperiod inhibits reproductive activity, melatonin is stimulatory.