Differential Effects of Melatonin on the Testes of Photoperiodic and Nonphotoperiodic Rodents1

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Abstract
Administration of melatonin via subcutaneous Silastic capsules caused a marked reduction in testicular weight, and suppressed spermatogenesis in two species of photoperiodic rodents: golden hamsters and grasshopper mice. In marked contrast, melatonin failed to exert any demonstrable effect on testis weight or spermatogenesis in two species of nonphotoperiodic rodents: laboratory rats and house mice. These findings suggest that melatonin, and by implication the pineal gland, may play a more importent role in regulating testicular function of rodents whose reproductive activity is markedly dependent upon seasonal changes in day length than of rodents whose reproductive performance is relatively insensitive to photoperiodic cues.