Melatonin as the Hormone Which Mediates the Effects of the Pineal Gland on Neuroendocrine-Reproductive Axis of the Syrian Hamster

Abstract

Melatonin is the most widely investigated pineal compound. Since its discovery in 1958 by Lerner and colleagues, it has been the subject of numerous biochemical and physiological investigations. Its synthesis within the pineal gland has been extensively studied and a great deal is known of the factors which control its production (Smith, 1981). Although its effects on the endocrine system have been investigated in many different experimental paradigms (Cardinali, 1981), its sites of action and mechanisms remain obscure. Despite its prominence in the scientific literature, its specific role as a hormonal envoy of the pineal gland continues to be questioned (Brown et al., 1981a; Pevet et al., 1981a). If not melatonin, the hormonal product of the pineal could prove to be another indole (Reiter and Vaughan, 1977), a polypeptide (Benson, 1977; Benson et al., 1981), or another compound (Ebels, 1979). The following paragraphs summarize what is known of synthesis and physilogical actions of melatonin. The actions of the indole are extremely complex and, at times, appear to be contradictory. Many of these apparent contradictions can be explained by what has come to be known as the down-regulation hypothesis. Since many of the studies have dealt with the actions of melatonin on reproduction in the Syrian hamster, these are data that will be considered in greatest depth herein.