When intact adult male golden hamsters are exposed to natural photoperiodic and temperature conditions during the winter months, their reproductive organs degenerate and plasma levels of LH decrease significantly. Pinealectomized hamsters kept under identical conditions do not experience the winter decline in reproductive activity. As spring approaches, the gonads of the non-pinealectomized hamsters regenerate to a functionally mature state. If hamsters with intact pineal glands are placed in near continuous darkness beginning in May (after having experienced reproductive atrophy in the previous winter), their reproductive systems do not regress. These results indicate that the neuroendocrine- gonadal axis may normally be refractory to pineal secretory products during the summer months or the pineal gland may be physiologically “exhausted” after a period of activity. The findings show that: (a) the photoperiod normally controls the seasonal reproductive rhythm in this species, and (b) the effects are mediated via the pineal gland.
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