Pineal Gland and Melatonin

This article succinctly summarizes what is known about the pineal gland and its secretory product melatonin. The pineal gland, a small dorsal outgrowth of the brain stem, is an end organ of the visual system; its function is determined by light and darkness as perceived by the retinas. The neural connections between the eyes and the pineal gland involve the peripheral sympathetic nervous system and an important synaptic relay in the suprachiasmatic nuclei, the biological clock. During darkness at night, the pineal gland, in response to norepinephrine released onto pineal cells by postganglionic sympathetic nerve fibers, produces melatonin and discharges it into the blood; as a consequence, blood levels of melatonin are elevated at night. The 24h cycle of blood melatonin functions in the regulation of circadian rhythms and in promoting nighttime sleep. In addition, melatonin inhibits growth of some tumors, has modulatory effects on the immune system, and functions as a direct free-radical scavenger and as an indirect antioxidant. Some of melatonin's actions are mediated via specific receptors while others are receptor independent.