Melatonin and the seasonal control of reproduction

Many mammalian species from temperate latitudes exhibit seasonal variations in breeding activity which are controlled by the annual photoperiodic cycl e. Photoperiodic information is conveyed through several neural relays from the retina to the pineal gland where the light signal is translated into a daily cycle of melatonin secretion: high at night, low in the day. The len gth of the nocturnal secretion of melatonin reflects the duration of the ni ght and it regulates the pulsatile secretion of gonadotropin-releasing horm one (GnRH) from the hypothalamus. Changes in GnRH release induce correspond ing changes in luteinising hormone secretion which are responsible for the alternating presence or absence of ovulation in the female, and varying spe rm production in the male. It is not yet known where and how this pineal in doleamine acts to exert this effect. Although melatonin binding sites are p referentially localised in the pars tuberalis (PT) of the adenohypophysis, the hypothalamus contains the physiological target sites of melatonin for i ts action on reproduction. Melatonin does not seem to act directly on GnRH neurons; rather it appears to involve a complex neural circuit of interneur ons that includes at least dopaminergic, serotoninergic and excitatory amin oacidergic neurons. (C) Inra/Elsevier, Paris.