Melatonin: role in development.

Melatonin is the mammalian fetus's window to periodicity of the outside world. Through melatonin, the fetus "knows" what time of year it is and, in all likelihood, also knows the time of day. The best known function of melatonin during development is to communicate information about photoperiod and thereby adaptively regulate reproductive development. A second likely function of melatonin during development, which may be related to but more widespread than the first, is to entrain the developing circadian pacemaker. Prenatal maternal entrainment occurs in all of the eutherian mammals in which it has been examined, and in Syrian hamsters exogenous melatonin during development causes entrainment. The broader distribution and greater abundance of melatonin receptors during development, relative to mature animals, suggests that developmental effects of melatonin are greater and more diverse. The human fetal suprachiasmatic nucleus expresses melatonin binding sites and is therefore likely to be affected by both endogenous and exogenous melatonin with consequences for the prenatal and postnatal expression and entrainment of circadian rhythms. Caution is warranted, not only concerning the use of exogenous melatonin during pregnancy and lactation but also concerning behavior that might disrupt the mother's endogenous melatonin rhythm.