* The ML-1 binding site has high-affinity MEL receptors (K d<200 pM) with a consensus

rank order of drug potency in inhibiting [125I]MEL binding as follows: 2-iodomclatonin > 6-chloromelatonin ≥ MEL > 6-hydroxymelatonin > N-acetylserotonin >> 5-hydroxytryptamine. The ML-2 binding sites are characterized by a K d in the nanomolar range with a distinct pharmacological profile, notably a similar affinity for MEL and N-acetylserotonin.33,34 Due to difficulties in Inhibitors,research,lifescience,medical their characterization and to their low affinity, which does not seem to be compatible with the circulating MEL levels, less attention had been given to these ML-2 binding sites. The recent identification of MT, receptor reopens this question (see below). Molecular identification of MEL receptor subtypes The cloning of the first, high-affinity MEL receptor was a landmark in MEL receptor research history. This was achieved by using an expression cloning strategy to isolate the complementary Inhibitors,research,lifescience,medical DNA encoding for MEL receptor of Xenopus laevis dermal melanophores.35 This Xenoptis MEL receptor cDNA encoded a protein with seven putative transmembrane regions that led to its classification within the sellectchem superfamily of G-protein-coupled receptors.35 Identification of the Xenopus receptor sequence using homology-based screening methods led to the subsequent identification

of three types Inhibitors,research,lifescience,medical of vertebrate MEL Inhibitors,research,lifescience,medical receptors. Two receptor subtypes with highaffinity for MEL (initially termed Mel1a and Mel1b, but now called MT1 and MT2) have been cloned and characterized from sheep pars tuberalis (PT), human SCN and hamster and rat hypothalamus.36,37 In sheep PT, allelic isoforms of MT1 receptors (termed Mel1a(α) and Mel1a(β), respectively) have been identified.38 A third subtype of high-affinity

MEL receptor was cloned from a chicken brain library and termed the Mellc.39 The first Xenopus receptor to be isolated was a Inhibitors,research,lifescience,medical Mel1c receptor, which exists in two allelic isoforms, Mel1c(α) and Mel1c(β). So far, no mammalian homologue of the Mel1c receptor has been isolated, but cDNA fragments for a nonmammalian Mel1b have. All this molecular work has also demonstrated that, the characteristics of the high-affinity receptors are present in each of the three related receptors (MTl, MT2, and Mel1c). Very probably this is only the beginning GSK-3 of a long list. A receptor structurally related to the MEL receptors has already been isolated40,41 with a very interesting distribution of expression in neuroendocrine tissues.42 The natural ligand for this receptor has not. been identified (could it be a MEL metabolite?). More recently, a MEL receptor with a nanomolar affinity, called MT3, has also been isolated. The MT3 site is not a G-protein-coupled receptor, but corresponds to a binding on the enzyme quinone reductase.