Melatonin as a chronobiotic: PROS and CONS.
The pineal gland is a major component in the regulation of photoperiodic responses and hence, saisonality. All functions which appear to be controlled by the daylength can be influenced by modifying the output signal of the pineal gland, i.e. melatonin. An important property of the melatonin signal is that the duration of melatonin synthesis and release varies proportionally with the length of the night suggesting that the duration of elevated melatonin is the most important signal conveying the photoperiodic message. In addition to the temporal coordination of saisonal phenomena, melatonin appears to be involved in the control of circadian rhythms in mammals. Activity-rest cycles of rats can be entrained by infusion or injection of melatonin at appropriate times of the daily cycle. Rhythms of neuronal activity of the suprachiasmatic nuclei (SCN) of the hypothalamus can be inhibited and phase-shifted by application of melatonin. The firing rate of SCN neurones of rats can be inhibited by iontophoretic application of melatonin. These data are in good agreement with the high density of melatonin receptors found in the SCN of most--but not all-mammalian species. On the other hand, activity-rest cycles appear to be perfectly normal in animals which lack a daily melatonin rhythm. This, however, might be a laboratory artefact, and this system will probably fail when challenged in a complex situation.