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 previously been reported to be a competitive melatonin receptor antagonist in blocking melatonin inhibition of [3H]-dopamine release from rabbit retina, a response 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. GR128107 behaved as a partial agonist (pEC50 8.58+/-0.03, n=3) with an Emax of 0.83 (relative to melatonin, pEC50 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 melatonin receptor antagonists with very similar potencies; estimated pKB RJ252 (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. Both melatonin- and GR128107-induced pigment granule aggregation was prevented by pretreatment of melanophores with pertussis toxin (10-1000 ng ml(-1)). 5. Prolonged pre-treatment of melanophores with melatonin desensitized the pigment aggregation response to GR128107. In desensitized cells, the maximal aggregation produced by GR128107 was only 0.27+/-0.01 (n=4) and the pEC50 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 unchanged but the pEC50 was reduced (vehicle 10.49+/-0.03; melatonin pre-treated 9.83+/-0.04, n=4). 6. These results demonstrate that GR128107 induces pigment granule aggregation 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 (Bmax 1223 fmol mg protein(-1)) compared to the low density of sites in rabbit retina (Bmax 3.1 fmol mg protein(-1)). This suggestion is supported by the finding that GR128107, like melatonin, acted as a full agonist and inhibited forskolin-stimulation of cyclic AMP accumulation in NIH-3T3 cells expressing a high density of human mt1 or MT2 receptors.
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Biomedical Sciences Division, King's College London.
Sugden
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D000818:Animals; D001665:Binding Sites; D005576:Colforsin / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D007457:Iodine Radioisotopes; D008547:Melanophores / Q000378:metabolism*; D037342:Pertussis Toxin; D010880:Piperidines / Q000494:pharmacology*; D011956:Receptors, Cell Surface / Q000037:antagonists & inhibitors*; D018160:Receptors, Cytoplasmic and Nuclear / Q000037:antagonists & inhibitors*; D044094:Receptors, Melatonin; D013997:Time Factors; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D014982:Xenopus laevis
C119151:GR 128107; D007211:Indoles; D007457:Iodine Radioisotopes; D010880:Piperidines; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D044094:Receptors, Melatonin; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D000242:Cyclic AMP; D037342:Pertussis Toxin