Periodic Exposure to a Brief Light Signal Stimulates Neuroendocrine-Gonadal Activity in Golden Hamsters

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Abstract
The photoperiodic effects of a periodic light pulse on neuroendocrine-gonadal activity in the male golden hamster was examined using a night interruption paradigm. Adult male hamsters that had been housed 3 to 5 per cage on LD 14:10 were placed either in individual cages, each equipped with a running wheel, or maintained in communal housing conditions, without access to a running wheel. Animals were then transferred to either LD 6:18 or to a LD 6:18 light cycle with a periodic 10-second night interruption (8 hours after lights off) occurring once every two, four, or seven days. As expected, exposure to LD 6:18 for 11 weeks induced complete regression of the testes and seminal vesicles, accompanied by low serum levels of LH and FSH, in both individually and communally-housed animals. However, in individually-housed animals receiving a 10-second night interruption every other day on LD 6:18, paired testis and seminal vesicle weights, as well as serum gonadotropin levels, were maintained at values comparable to those normally observed in hamsters exposed to photostimulatory long days. Furthermore, the presentation of a periodic 10-second night interruption once every four or seven days to individually-housed animals with access to a running wheel was sufficient to partially or totally block the inhibitory effects of short days on neuroendocrine-gonadal activity. Communally-housed hamsters receiving a 10-second light pulse once every two, four, or seven days also exhibited paired testis and seminal vesicle weights, as well as serum gonadotropin levels, that were consistently higher than the values obtained for animals exposed only to LD 6:18. However, the efficacy of each night interruption paradigm in maintaining testicular and seminal vesicle weight in communally-housed animals was considerably less than that observed for animals with access to a running wheel. These results demonstrate that the periodic presentation of a brief photic stimulus once every two, four, or seven days during the dark phase of an inhibitory light cycle can influence the neuroendocrine events underlying the photoperiodic regulation of gonadal function. This experimental approach, whereby a discrete stimulus is periodically administered at a specific arcadian time to induce a photoperiodic response, should provide a valuable model for examining the physiologic mechanisms by which light regulates the neuroendocrine-gonadal axis in mammals.