COMPARATIVE ASPECTS OF THE PINEAL MELATONIN SYSTEM OF POIKILOTHERMIC VERTEBRATES/

The pineal gland of poikilothermic vertebrates originates as an evagin ation from the diencephalic roof between the habenular and the posteri or commissures, and associates with a parapineal organ to form the so- called pineal complex. The pinealocytes may be photosensitive, secreto ry or intermediate cells between both. Melatonin, the indoleamine secr eted by the pineal, exhibits a circadian secretory rhythm that conveys environmental information to the organism. The peak melatonin secreti on occurs during the night, although there are a few examples of an in crease in indoleamine secretion during the day, Melatonin is also synt hesized in other sites such as the retina, and it has been found in ma ny invertebrates and unicellular organisms. The rhythmic secretory pat tern of melatonin is responsible for many biological rhythms exhibited by lower vertebrates. These rhythms are abolished by pinealectomy in some species, but not in others, suggesting the existence of an extra- pineal pacemaker. The photoperiod and the temperature (especially in r eptiles) are the main environmental factors affecting the secretory rh ythm of melatonin. Poikilothermic vertebrates exhibit a circadian rhyt hmic color change, with nocturnal blanching, usually related to melato nin secretion. In amphibians, melatonin exhibits a potent skin lighten ing activity. However, in fishes and reptiles the melatonin effects va ry with the species, the developmental stage, and the pigment cell loc ation. Melatonin also exerts inhibitory or excitatory activity on the amphibian reproductive system, regulation of circadian locomotory acti vity in reptiles, and modulation of the amphibian metamorphosis. Melat onin has also a modulatory effect on the response of target cells to d ifferent hormones and high concentrations or prolonged exposure to the indoleamine may cause autodesensitization in various tissues. Binding sites of melatonin have been detected in the central nervous system a nd peripheral tissues of various vertebrates. The relative potencies o f melatonin analogues demonstrated two subtypes of melatonin receptors (ML-1 and ML-2). A transmembrane melatonin receptor has been cloned f rom Xenopus laevis melanophores; it belongs to the family of the G pro tein-coupled receptors and exhibits 85% homology with the mammalian ne rvous system receptor. Melatonin binding sites in the nucleus of many cell types and its potent intracellular anti-oxidant action suggest me chanisms of action other than through the G-protein coupled receptor.