The putative melatonin receptor antagonist GR128107 is a partial agonist on Xenopus laevis melanophores

1 GR128107 (3-(1-acetyl-3-methyl-piperidine)-5-methoxyindole) has previousl y been reported to be a competitive melatonin receptor antagonist in blocki ng melatonin inhibition of [H-3]-dopamine release from rabbit retina, a res ponse mediated by the MT2 receptor subtype. 2 GR128107, like melatonin, induced a rapid (maximum response in 60-90 min) pigment aggregation in a clonal line of Xenopus laevis melanophores. GR128 107 behaved as a partial agonist (pEC(50) 8.58+/-0.03, n=3) with an E-max o f 0.83 (relative to melatonin, pEC(50) 10.09+/-0.03, n=3). 3 The concentration-response curve for pigment granule aggregation to both melatonin and GR128107 was displaced in a parallel, rightward manner by mel atonin receptor antagonists with very similar potencies; estimated pK(B) RJ 252 (against melatonin 4.60/against GR128107 4.54) < GR135533 (6.40/6.14) < Luzindole (6.45/6.49) < S20929 (6.58/6.65) < 4-P-PDOT (6.73/6.85). 4 Bath melatonin- and GR128107-induced pigment granule aggregation was prev ented by pretreatment of melanophores with pertussis toxin (10-1000 ng ml(- 1)). 5 Prolonged pre-treatment of melanophores with melatonin desensitized the p igment aggregation response to GR128107. In desensitized cells, the maximal aggregation produced by GR128107 was only 0.27+/-0.01 (n=4) and the pEC(50 ) was reduced (vehicle 8.57+/-0.12; melatonin pre-treated 7.84+/-0.09, n=4) . The maximal response to melatonin in desensitized melanophores was unchan ged but the pEC(50) was reduced (vehicle 10.49+/-0.03; melatonin pre-treate d 9.83+/-0.04, n=4). 6 These results demonstrate that GR128107 induces pigment granule aggregati on in Xenopus melanophores by activating a cell membrane melatonin receptor coupled via a pertussis toxin-sensitive G-protein. 7 The partial agonist activity of GR128107 in melanophores may be apparent because of the very high density of melatonin receptors in these cells (B-m ax 1223 fmol mg protein(-1)) compared to the low density of sites in rabbit retina (B-max 3.1 fmol mg protein(-1)). This suggestion is supported by th e finding that GR128107, like melatonin, acted as a full agonist and inhibi ted forskolin-stimulation of cyclic AMP accumulation in NIH-3T3 cells expre ssing a high density of human mt(1) or MT2 receptors.