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

The potency and affinity of two series of melatonin receptor ligands w ere examined using the pigment aggregation response in a clonal line o f Xenopus laevis melanophores and radioligand binding assays on native receptors in chicken brain, recombinant human mt, and MT2 and Xenopus laevis mel(lc) receptor subtypes. One series was based on melatonin a nd 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 v aried from one to five carbon atoms. All analogues in the melatonin se ries were full agonists in melanophores (pEC(50) 7.76-10.24), while al l compounds in the luzindole series were competitive melatonin antagon ists (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 affin ity 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 bye antagonists were MT2-sele ctive the most selective analogue (N-pentanoyl 2-benzyltryptamine, MT2 pK(i) 8.03) having 89- and 229-fold higher affinity than at mt(l) or mel(lc) receptor subtypes. The different structure-activity relationsh ips of these receptor agonists and antagonists is discussed with regar d to the possible binding sites of agonists and antagonists within the receptor protein.