Melatonin, the Pineal Gland and Circadian Rhythms.

The rat pineal is a component of the circadian clock. Exogenous melatonin entrains the rat clock and does not require the presence of the pineal gland. The pineal gland is important for circadian rhythmicity. Pinealectomy exacerbates the disruptive effects of LL on rhythmicity. This may be due to a role of the pineal gland and melatonin in clock light sensitivity, since pinealectomized rats periods decelerate more rapidly in increasing light than do those of sham-operated rats. It may also be due to a role in coupling circadian components since multiple outputs of the clock are disrupted by pinealectomy equivalently. The rat circadian clock regulates a plethora of peripheral processes by modulation of sympathetic tone. This regulation is responsible for the circadian rhythm in heart rate. The avian visual system contains high affinity melatonin receptors. These receptors are responsible for a circadian modulation of metabolic and electrical activity in visual structures. Melatonin receptors are regulated by the circadian clock. Receptor binding is rhythmic, high during the day and low during the night, does not depend on the pineal gland or melatonin but is abolished by lesions of the suprachiasmatic nucleus. Finally, we have cloned two melatonin receptors from chick brain.