Photoperiodic Inhibition of the Reproductive System: A Prerequisite for the Induction of the Refractory Period in Hamsters

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Abstract
Maintenance of male hamsters on LD 6:18 results in an initial decline in testicular size which is followed by complete regrowth of the testes after about 20–25 weeks of short days. The animals are now considered to be in the refractory condition, since short days are no longer capable of inhibiting neuroendocrine-gonadal activity. The present investigation was designed to determine if an exposure to short days per se leads to the induction of the refractory condition or if changes in the reproductive system that occur during the initial exposure to short days lead to the onset of refractoriness. Melatonin-treated animals that did not show evidence of testicular atrophy during a 24 week exposure to a nonstimulatory LD 6:18 light cycle were not refractory to short days, while control animals which did undergo testicular regression and recrudescence in 24 weeks were refractory. These results indicate that an exposure to short days per se does not induce the refractory condition in hamsters and suggest that an inhibition of neuroendocrine-gonadal activity, which normally occurs during exposure to short days, is required for the initiation of the refractory state in hamsters.