MELATONIN AS A CHRONOBIOTIC - PROS AND CONS

The pineal gland is a major component in the regulation of photoperiod ic 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 t he melatonin signal is that the duration of melatonin synthesis and re lease varies proportionally with the length of the night suggesting th at the duration of elevated melatonin is the most important signal con veying the photoperiodic message. In addition to the temporal coordina tion of saisonal phenomena, melatonin appears to be involved in the co ntrol of circadian rhythms in mammals. Activity-rest cycles of rats ca n be entrained by infusion or injection of melatonin at appropriate ti mes of the daily cycle. Rhythms of neuronal activity of the suprachias matic nuclei (SCN) of the hypothalamus can be inhibited and phase-shif ted by application of melatonin. The firing rate of SCN neurones of ra ts can be inhibited by iontophoretic application of melatonin. These d ata are in good agreement with the high density of melatonin receptors found in the SCN of most - but not all - mammalian species. On the ot her hand, activity-rest cycles appear to be perfectly normal in animal s which lack a daily melatonin rhythm. This, however, might be a labor atory artefact, and this system will probably fail when challenged in a complex situation.