Molecular and pharmacological evidence for MTI melatonin receptor subtype in the tail artery of juvenile Wistar rats

1 In this study reverse transcriptase-polymerase chain reaction (RT-PCR) has been used to identify mt(1) and MT2 receptor mRNA expression in the rat tail artery. The contributions of both receptors to the functional response to melatonin were examined with the putative selective MT2 receptor antagonists, 4-phenyl-2-propionamidotetraline (4-P-PDOT) and 2-benzyl-N-pentanoyltryptamine. In addition, the action of melatonin on the second messenger cyclic AMP was investigated. 2 Using RT-PCR, mt(1) receptor mRNA was detected in the tail artery from seven rats. In contrast MT2 receptor mRNA was not detected even after nested PCR. 3 At low concentrations of the MT2 selective ligands, neither 10 nM 4-P-PDOT (pEC(50) = 8.70 +/- 0.31 (control) vs 8.73 +/- 0.16, n = 6) nor 60 nM 2-benzyl-N-pentanoyltryptamine (pEC(50) = 8.53 +/- 0.20 (control) vs 8.83 +/- 0.38, n = 6) significantly altered the potency of melatonin in the rat tail artery. 4 At concentrations non-selective for mt(1) and MT2 receptors, 4-P-PDOT (3 mu M) and 2-benzyl-N-pentanoyltryptamine (5 mu M) caused a significant rightward displacement of the vasoconstrictor effect of melatonin. In the case of 4-P-PDOT, the estimated pK(B) (6.17 +/- 0.16, n = 8) is similar to the binding affinity for mt(1) receptor. 5 Pre-incubation with 1 mu M melatonin did not affect the conversion of [H-3]-adenine to [H-3]-cyclic AMP under basal condition (0.95 +/- 0.19% conversion (control) vs 0.92 +/- 0.19%, n = 4) or following exposure to 30 mu M forskolin (5.20 +/- 1.30% conversion (control) vs 5.35 +/- 0.90%, n = 4). 6 Based on the above findings, we conclude that melatonin receptor on the tail artery belongs to the MT1 receptor subtype, and that this receptor is probably independent of the adenylyl cyclase pathway.