2-Phenylmelatonin

2-Phenylmelatonin: a partial agonist at enteric melatonin receptors.[Pubmed:11097268]

Pharmacol Toxicol. 2000 Oct;87(4):156-60.

The effect of the melatonin receptor ligand, 2-Phenylmelatonin, has been assessed in isolated strips of the guinea-pig proximal colon. 2-Phenylmelatonin (0.01 nM-1 microM) caused a concentration-dependent contractile response. The potency value (-log EC50) was 9.3 +/- 1.0. The maximum effect was 25 +/- 4%, of that elicited by the maximally effective concentration (0.3 microM) of 5-HT and 43 +/- 3%, of that by the maximally effective concentration (10 microM) of melatonin. When used as an antagonist, 2-Phenylmelatonin (0.01 nM and 0.1 nM) concentration-dependently inhibited melatonin-induced contractions with depression of the maximum response by 25% and 54%, respectively. Higher (1 nM) 2-Phenylmelatonin concentrations failed to antagonize melatonin-induced response. Prazosin (0.3 microM), a selective antagonist of melatonin MT3 sites, antagonized melatonin-induced contractions to an extent similar to that induced by 0.01 nM 2-Phenylmelatonin (with 30% reduction of the maximum effect to melatonin). The combination of 0.3 microM prazosin and 0.01 nM 2-Phenylmelatonin caused antagonism similar in extent to that caused by each individual antagonist. 2-Phenylmelatonin at subnanomolar concentrations behaves as an antagonist of melatonin-induced contractile responses while at nanomolar/micromolar concentrations it behaves as a weak contractile agonist.

Ligand efficacy and potency at recombinant human MT2 melatonin receptors: evidence for agonist activity of some mt1-antagonists.[Pubmed:10455277]

Br J Pharmacol. 1999 Jul;127(5):1288-94.

NIH3T3 fibroblast cells transfected with the full-length coding region of the MT2 human melatonin receptor stably expressed the receptor that is coupled to a pertussis toxin-sensitive G protein and exhibits high affinity for melatonin (K(I) = 261 pM). The order of apparent affinity for selected compounds was: 4-phenyl-2-propionamidotetralin (4P-PDOT) > 2-Phenylmelatonin > 2-iodomelatonin > 2-bromomelatonin > 6-chloromelatonin > or = melatonin > luzindole > N-acetyl-tryptamine > or = N-[(2-phenyl-1H-indol-3-yl)ethyl]cyclobutanecarboxamide (compound 6) > N-acetylserotonin. 4P-PDOT exhibited a very high selectivity (approximately 22,000 times) for the MT2 receptor with respect to the mt1 receptor subtype, as tested in comparative experiments with membrane preparations from NIH3T3 cells stably transfected with the human mt1 receptor. MT2 melatonin receptors mediated incorporation of [35S]-GTPgammaS into isolated membranes via receptor catalyzed exchange of [35S]-GTPgammaS for GDP. The relative intrinsic activity and potency of the compounds were subsequently studied by using [35S]-GTPgammaS incorporation. The order of potency was equal to the order of apparent affinity. Melatonin and full agonists increased [35S]-GTPgammaS binding by 250% over basal (taken as 100%). Luzindole did not increase basal [35S]-GTPgammaS binding but competitively inhibited melatonin-stimulated [35S]-GTPgammaS binding, thus exhibiting antagonist action. The other two mt1 antagonists used here, 4P-PDOT and N-[(2-phenyl-1H-indol-3-yl)ethyl]cyclobutanecarboxamide, behaved as partial agonists at the MT2 subtype, with relative intrinsic activities of 0.37 and 0.39, respectively. These findings show, for the first time, important differences in the intrinsic activity of analogues between the human mt1 and MT2 melatonin receptor subtypes.

Pharmacological characterization of the human melatonin Mel1a receptor following stable transfection into NIH3T3 cells.[Pubmed:9647472]

Br J Pharmacol. 1998 Jun;124(3):485-92.

1. Mouse fibroblasts (NIH3T3) transfected with the full-length coding region of the Mel1a melatonin receptor stably expressed the receptor, coupled to a pertussis toxin-sensitive G-protein(s) and exhibiting high affinity and adequate pharmacological profile. 2. The receptor protein had the tendency of a strong coupling to the G-protein and therefore low-affinity state was induced by uncoupling the receptor from its G-protein in presence of high concentrations of NaCl (500-700 mM) and/or GTPgammaS (100 microM). Thereafter, the affinity of a series of melatonin analogues was determined to both, high- and low-affinity receptor states, thus providing a basis for the prediction of their efficacy, according to the ternary complex model. 3. The cells were subsequently used to study the agonist-induced G-protein activation, determined by calculating the rate of GDP-GTP exchange measured in presence of 35S-labelled GTPgammaS. The natural ligand melatonin induced a significant increase in the GDP-GTP exchange rate, the presence of GDP and NaCl being necessary to observe this effect. 4. The full agonists 2-Phenylmelatonin, 2-bromomelatonin and 6-chloromelatonin equally induced an increase of the GDP-GTP exchange. 5-Hydroxy-N-acetyltryptamine activated the GTP-GDP exchange to a much lesser extent (53%) than melatonin, thus behaving as a partial agonist. As predicted by the model, the melatonin antagonist (N-[(2-phenyl-1H-indol-3-yl)ethyl]cyclobutanecarboxamide) was without effect on basal G protein activation. Coincubation of this compound with melatonin induced a dose-dependent rightward shift in the melatonin concentration-effect curve, thus exhibiting the behaviour of a competitive and surmountable antagonist. 5. Using the equation proposed by Venter (1997) we were able to determine that there were no 'spare' receptors in the system. Therefore, the approach proposed in the present work can be successfully used for the determination of 'drug action' at the level of the human Mel1a melatonin receptor and evaluation of the efficacy of new selective melatonin analogues.

2-Substituted 5-methoxy-N-acyltryptamines: synthesis, binding affinity for the melatonin receptor, and evaluation of the biological activity.[Pubmed:8258829]

J Med Chem. 1993 Dec 10;36(25):4069-74.

A series of 2-substituted 5-methoxy-N-acyltryptamines was synthesized and their affinity for the melatonin receptor, isolated from whole quail brains, was tested in a succession of in vitro ligand-receptor binding experiments, using 2-[125I]iodomelatonin as a labeled ligand. Optimization of the C2 substituent and the N-acyl group resulted in compounds having picomolar affinity for the receptor (vs nanomolar affinity for melatonin). In two tests for evaluation of the biological activity (effects on the spontaneous firing activity of single neurons in the rabbit parietal cortex in situ, and the Syrian hamster gonadal regression model in vivo) most of the analogs behaved as agonists. Isopropyl substitution at C2 alone, or concomitantly with cyclopropyl substitution at the N-acyl position, resulted in much lower affinity and weaker biological effect, or lack of activity in the latter case. Of interest are the compounds 4d (R = phenyl, R1 = CH3) and 4g (R = phenyl, R1 = cyclopropyl), which expressed high affinity for the receptor and apparent antagonistic activity under the conditions of the experimental models employed, though the analog 4g (R = phenyl, (R1 = cyclopropyl) seemingly was a weak antagonist and in situ expressed mixed activity in the higher concentration range. Cyclopropyl substitution at the N-acyl position inevitably resulted in lower affinity for the receptor and weaker biological activity. These data demonstrate that the N-acetyl group is important for both affinity and agonist biological activity. The substituents at C2 are crucial for the affinity of the compound for the receptor and can be utilized to create putative high-affinity agonists or antagonists.