Biology of mammalian photoperiodism and the critical role of the pineal gland and melatonin

In mammals, photoperiodic information is transformed into a melatonin secre tory rhythm in the pineal gland (high levels at night, low levels during th e day). Melatonin exerts its effects in discrete hypothalamic areas, most l ikely through MT1 melatonin receptors. Whether melatonin is brought to the hypothalamus from the cerebrospinal fluid or the blood is still unclear. Th e final action of this indoleamine at the level of the central nervous syst em is a modulation of GnRH secretion but it does not act directly on GnRH n eurones; rather, its action involves a complex neural circuit of interneuro nes that includes at least dopaminergic, serotoninergic and aminoacidergic neurones. In addition, this network appears to undergo morphological change s between seasons.