RESUMEN
Melatonin, known as the hallmark of darkness, is released during the dark by pineal glands and retina of adult animals. In neonates melatonin is secreted in a tonic manner by extra-pineal sources, including retina. Here we show that melatonin modulates the density and functional response of nicotinic acetylcholine receptors (nAChRs) in central and peripheral nervous system. Melatonin effect is restricted to α-bungarotoxin sensitive nAChRs in all models tested: rat sympathetic nerve terminals, cerebellum, and chick retina. In addition, it is shown that in adult animals, where the pineal gland releases melatonin in a rhythm manner, this hormone imposes a daily rhythm in the cholinergic function, whereas during the development of the retina, when melatonin is produced in a tonic manner, it is essential for the appearance of function nAChRs sensitive to α-bungarotoxin. This common pattern of action on different cell models that express α-bungarotoxin-sensitive receptors probably reflects a more general mechanism of regulation of these receptors.