High affinity melatonin receptors in the vertebrate brain: implications for the control of the endogenous oscillatory systems.

Currently, the melatonin receptor is depicted as a membrane-associated protein, linked to a guanine nucleotide-binding protein (G-protein), and thus the melatonin receptor represents a member of a receptor superfamily, acting through G-proteins in the first step of their signal-transduction pathways. Although on a number of occasions specific binding of radioactive melatonin has been demonstrated in a wide variety of tissues and organs, to date, high affinity G-protein-regulated melatonin binding sites, suggestive for a functional melatonin receptor, have been convincingly confirmed in the brain only. There is a significant species variation in the distribution of the melatonin receptor in the vertebrate brain. The limited number of studies prevents any definitive conclusion in terms of phylogeny, though generally speaking, the lower vertebrates' brains tend to express melatonin receptors with wider distribution. Two sites have been consistently found to express high density of melatonin receptors: the pars tuberalis of the adenohypophysis and the hypothalamic suprachiasmatic nuclei (SCN). It must be pointed out, however, that there are some exceptions. Binding in the human pars tuberalis has not been reported, and apparently, the sheep and the mustelids' suprachiasmatic nuclei do not express detectable binding. The function of melatonin in pars tuberalis is unclear, and the control of the synthesis (and release) of paracrine factors that act at site(s) distant from the melatonin target cells, have been suggested.(ABSTRACT TRUNCATED AT 250 WORDS)