Article date: March 1999
By: Muy‐Teck Teh, David Sugden, in Volume 126, Issue 5, pages 1237-1245
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.
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).
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).
Both melatonin‐ and GR128107‐induced pigment granule aggregation was prevented by pre‐treatment of melanophores with pertussis toxin (10–1000 ng ml−1).
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).
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.
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.
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.
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).
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).
Both melatonin‐ and GR128107‐induced pigment granule aggregation was prevented by pre‐treatment of melanophores with pertussis toxin (10–1000 ng ml−1).
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).
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.
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.
British Journal of Pharmacology (1999) 126, 1237–1245; doi:10.1038/sj.bjp.0702404
DOI: 10.1038/sj.bjp.0702404
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