ANALOGS OF DIVERSE STRUCTURE ARE UNABLE TO DIFFERENTIATE NATIVE MELATONIN RECEPTORS IN THE CHICKEN RETINA, SHEEP PARS TUBERALIS AND XENOPUSMELANOPHORES

1 The pineal hormone melatonin exerts its biological effects through s pecific, high affinity G-protein coupled receptors. Recently, three me latonin receptor subtypes (Mel(1a), Mel(1b) and Mel(1c)) have been clo ned. Neither the cloned subtypes, nor the native receptors have yet be en compared in a detailed pharmacological analysis. 2 The present stud y examined the structure-activity relationships of a series of 21 mela tonin analogues, by comparing their potency on the pigment aggregation response in Xenopus laevis melanophores with their affinity in radiol igand binding competition studies in chicken retina and sheep pars tub eralis (PT), two tissues in which melatonin is known to mediate a biol ogical response. 3 All but four of the analogues were full melatonin r eceptor agonists producing a concentration-related redistribution of p igment granules in cultured Xenopus melanophores. The remaining analog ues produced little pigment aggregation at 10 mu M. 4 Saturation studi es with 2-[I-125]-iodomelatonin identified a single binding site in th e chicken retina and sheep PT membranes, with a K-D of 36.6+/-2.8 and 37.3+/-4.3 pM, and a maximal number of binding sites (B-max) of 16.6+/ -0.5, and 40.1+/-1.7 fmol mg(-1) protein, respectively. 5 Comparison o f the potency/affinity of the analogues for the binding sites gave a h ighly 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 distingui sh between melatonin receptors in the chicken retina, sheep pars tuber alis and Xenopus melanophores.