Analogues of diverse structure are unable to differentiate native melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores

Abstract

1 The pineal hormone melatonin exerts its biological effects through specific, high affinity G-protein coupled receptors. Recently, three melatonin receptor subtypes (Mella, Mellb and Mel,c) have been cloned. Neither the cloned subtypes, nor the native receptors have yet been compared in a detailed pharmacological analysis. 2 The present study examined the structure-activity relationships of a series of 21 melatonin analogues, by comparing their potency on the pigment aggregation response in Xenopus laevis melanophores with their affinity in radioligand binding competition studies in chicken retina and sheep pars tuberalis (PT), two tissues in which melatonin is known to mediate a biological response. 3 All but four of the analogues were full melatonin receptor agonists producing a concentration-related redistribution of pigment granules in cultured Xenopus melanophores. The remaining analogues produced little pigment aggregation at 10 gM. 4 Saturation studies with 2-[1251]-iodomelatonin identified a single binding site in the chicken retina and sheep PT membranes, with a KD of 36.6 + 2.8 and 37.3 + 4.3 pM, and a maximal number of binding sites (Bmax) of 16.6 + 0.5, and 40.1 + 1.7 fmol mg-' protein, respectively. 5 Comparison of the potency/affinity of the analogues for the binding sites gave a highly significant correlation in each case, retina/melanophore, r=0.97 (P<0.001, n= 17), PT/melanophore, r=0.97 (P<0.001, n=17) and PT/retina, r=0.98 (P<0.001, n=21). 6 Despite their large range in affinity and structural diversity these melatonin agonists were unable to distinguish between melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores.

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light‐mediated skin pigmentation

“…Of note, melatonin-induced melanosome aggregation of fish and amphibian melanophores is through binding with the melatonin receptor Mel1c (Andersson, Sköld, & Svensson, 2003;Aspengren et al, 2003;Mårtensson & Andersson, 1996;Pickering, Sword, Vonhoff, Jones, & Sugden, 1996;Svensson et al, 1993), which is present in fish, amphibians and birds, but disappeared during evolution in the lineages that gave rise to mammals (Ebisawa, Karne, Lerner, & Reppert, 1994).…”

An Updated Phylogenetic Analysis of Vertebrate Melatonin Receptor Sequences: Reflection on the Melatonin Receptor Nomenclature by the Nomenclature Subcommittee of the International Union of Pharmacology

“…Interestingly, our suggested classification-nomenclature system for vertebrate melatonin receptors can account for the failure in using different melatonin analogues to differentiate sheep pars tuberalis MEL 1A , chicken retinal Mel 1a and Mel 1c , as well as Xenopus melanophore Mel 1c receptors [7,17], since they are all of the same mt 1 receptor type. Moreover, it also offers an alternative explanation for hitherto unsuccessful attempts in cloning Mel 1c receptor from mammals, an unresolved tantalizing issue in melatonin receptor cloning.…”

Desensitization of pigment granule aggregation in Xenopus leavis melanophores: melatonin degradation rather than receptor down‐regulation is responsible

“…In any case, competition radioligand binding studies on each recombinant subtype and native receptors indicate about 70% of the melatonin receptors present would be occupied that at the concentration of melatonin used (1 n m ). Previous studies have shown that there is a very strong correlation between the binding affinity of melatonin agonists at recombinant Xenopus Mel 1c receptors and their potency in melanophores, However, as the agonists are ∼10‐fold more potent than expected from binding data, this suggests that only a proportion of the melatonin receptors present on melanophores need be activated to trigger a maximal pigment aggregation response (Pickering et al . 1997; Teh and Sugden 1999).…”