Generation of melatonin rhythms.

In mammals, information about the environmental photoperiod is relayed from the retina to the suprachiasmatic nuclei (SCN) in the anterior hypothalamus and via the sympathetic nervous system to the pineal gland where it influences the secretion of melatonin. Light plays a dual role: to suppress the release of melatonin and to entrain the circadian rhythm generators in the SCN, which govern the endogenous melatonin rhythm. Under normal daily light-dark cycles melatonin secretion is confined to the dark period. In most photoperiodic species the daily pattern of secretion changes in response to changes in daylength, and this acts as a physiological time cue in the brain for the control of seasonal cycles in reproduction, moulting and other processes. To illustrate the underlying mechanisms that control the melatonin rhythm, results are presented from five experiments in which the blood plasma concentrations of melatonin were measured in Soay rams exposed to a variety of artificial changes in photoperiod including a switch from 16L:8D (16 h light:8 h dark) or 8L:16D to constant darkness, a switch from constant darkness to 1L:23D and a switch from 16L:8D to a 25 h or 23 h light-dark cycle. The results confirm that the melatonin rhythm is generated endogenously and will free-run under constant darkness with a period close to 24 h for at least 10 days. The rhythm can be entrained by exposure to IL:23D with the end of the light period acting as the 'melatonin-on' signal, and phase-shifts in the melatonin rhythm can be induced by phase-shifts in the light-dark cycle. The period for which melatonin concentrations are high each day (melatonin peak) also varies in duration under the different photoperiods, as a result of both the suppressive and the entraining effects of light. Two models explaining the control of melatonin peak duration are discussed.