Parameters of the circadian rhythm of pineal melatonin secretion affecting reproductive responses in Siberian hamsters.

The major function of the mammalian pineal gland appears to be its central role in photoperiodism. The pineal hormone, melatonin, is synthesized and secreted primarily at night, under the control of a circadian oscillator that is entrained to the light-dark cycle. Both the circadian phase and the duration of the nocturnal peak of melatonin secretion are established primarily by interactions between the endogenous circadian oscillator and the daily photic cycle. The duration of the melatonin peak varies inversely with day length, and this relationship between day length and the duration of each circadian melatonin peak appears to be an integral part of the photoperiodic mechanism. When pinealectomized animals are given daily melatonin infusions of long duration, they exhibit physiologic responses that normally are observed during exposure to short day photoperiods; when administered short-duration melatonin infusions, the animals display long photoperiod-type responses. In addition to the importance of the duration of each melatonin peak, certain other parameters appear to be significant. If a long-duration infusion of melatonin is interrupted by a period of 2 hours or more without melatonin (i.e., to produce two short duration infusions), the responses are those typical for long day-exposed animals. Thus, to elicit short day-type responses, each long-duration melatonin peak must be relatively continuous; responses are not determined simply by the total time of exposure to melatonin in each circadian cycle. Also, long-duration melatonin peaks may not be effective to elicit photoperiod-type responses unless they are present at frequencies of nearly once every 24 hours or more.