The pineal complex of reptiles is a morphologically and functionally connected set of organs that originates as an evagination of the roof of the diencephalon. It is formed by two structures: the pineal organ and the parietal eye. The pineal gland is present in Chelonia, Squamata and Rhynchocephalia, but not in Crocodilia. The parietal eye is present in some species of lizards (Squamata) and in the tuatara (Rhynchocephalia). Both the pineal gland and the parietal eye are photosensitive. In particular, the parietal eye is an highly organized photoreceptive structure, with a well-defined lens, cornea and retina. The most important (and studied) secretory product of this complex is the hormone melatonin which is synthesized by both organs (pineal and parietal eye). The pineal organ is believed to be the neuroendocrine transducer of changes in photoperiod and environmental temperature and it has been demonstrated to have a functional role in many aspects of reptilian biology. Melatonin has an influence on the mechanisms controlling thermoregulation (behavioral and physiological), because its manipulation or removal may produce significant alterations of behavioral and physiological thermoregulatory parameters. The reptilian pineal complex may also possess self-sustained circadian oscillators which are involved in the circadian organization of these animals and in their reproduction. It is believed that many of the roles played by the pineal complex are mediated by the hormone melatonin, since exogenous administration of melatonin may affect the animal's physiology and/or behavior. The present paper will review the current knowledge about the neuroendocrinology and functional roles of the reptilian pineal complex.
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