Comparison of the structure-activity relationships of melatonin receptor agonists and antagonists: lengthening the N-acyl side-chain has differing effects on potency on Xenopus melanophores

Abstract

The potency and affinity of two series of melatonin receptor ligands were examined using the pigment aggregation response in a clonal line of Xenopus laevis melanophores and radioligand binding assays on native receptors in chicken brain, recombinant human mt₁ and MT₂ and Xenopus laevis mel_1c receptor subtypes. One series was based on melatonin and had a methoxy group at the 5-position of the indole ring, while the other was based on luzindole and lacked this substituent but did have a 2-benzyl moiety; the N-acyl group of each series of analogues was varied from one to five carbon atoms. All analogues in the melatonin series were full agonists in melanophores (pEC₅₀ 7.76–10.24), while all compounds in the luzindole series were competitive melatonin antagonists (pA ₂ 5.47–6.60). With the agonist series, increasing the N-acyl side-chain from one to three carbon atoms was well tolerated in both the functional and binding assays, but further lengthening of the side-chain progressively and dramatically reduced potency and affinity. In contrast, for the antagonist series neither potency nor binding affinity changed substantially with the length of the N-acyl chain, except at the recombinant MT₂ subtype where two of the analogues had a lower affinity. In binding assays, three of the five antagonists were MT₂-selective; the most selective analogue (N-pentanoyl 2-benzyltryptamine, MT₂ pK _i 8.03) having 89- and 229-fold higher affinity than at mt₁ or mel_1c receptor subtypes. The different structure-activity relationships of these receptor agonists and antagonists is discussed with regard to the possible binding sites of agonists and antagonists within the receptor protein.