New MT2 Melatonin Receptor-Selective Ligands: Agonists and Partial Agonists.

The search for melatonin receptor agonists and antagonists specific towards one of the receptor subtypes will extend our understanding of the role of this system in relaying circadian information to the body. A series of compounds derived from a hit compound discovered in a screening process led to powerful agonists specific for one of the isoform of the melatonin receptor namely, MT2. The compounds are based on a poorly explored skeleton in the molecular pharmacology of melatonin. By changing the steric hindrance of one substituent (i.e., from a hydrogen atom to a tributylstannyl group), we identified a possible partial agonist that could lead to antagonist analogues. The functionalities of these compounds were measured with a series of assays, including the binding of GTPγS, the inhibition of the cyclic AMP production, the ß-arrestin recruitment, and the cell shape changes as determined by cellular dielectric spectroscopy (CellKey®). The variations between the compounds are discussed.

Design, synthesis and pharmacological evaluation of novel naphthalenic derivatives as selective MT(1) melatoninergic ligands.

Novel heterodimer analogues of melatonin were synthesized, when agomelatine (1) and various aryl units are linked via a linear alkyl chain through the methoxy group. The compounds were tested for their actions at melatonin receptors. Several of these ligands are MT(1)-selective with nanomolar or subnanomolar affinity. In addition, while most of the derivatives behave as partial agonists on one or both receptor subtypes, N-[2-(7-{4-[6-(1-methoxycarbonylethyl)naphthalen-2-yloxy]butoxy}naphthalen-1-yl)ethyl]acetamide (36), a subnanomolar MT(1) ligand with an 11-fold preference over MT(2) receptors, is a full antagonist on both receptors. Our results also confirm that the selectivity seen for the MT(1) receptor arises predominantly from steric factors and is not a consequence of the bridging of melatonin receptor dimers.

Design and synthesis of 3-phenyltetrahydronaphthalenic derivatives as new selective MT2 melatoninergic ligands. Part II.

Following our studies of the melatoninergic receptors, we have developed new tetrahydronaphthalenic derivatives of melatonin that have been tested as selective melatonin receptors ligands. Regarding the role of the phenyl substituent to obtain selective ligands, modulation of selectivity and activity have been achieved by modifications of the acyl group and substitutions on the phenyl ring. Ten of the seventeen evaluated derivatives have MT(2) receptor affinity similar to that of melatonin. Moreover, we have achieved remarkable MT(2) selectivity over MT(1) (selectivity >100) and have been able to further extend the RSA of the tetrahydrophthalenic series. However, the compounds presented here display partial agonist or antagonist behavior instead of full agonist.

Evidence for superoxide anion generation in aortas of cholesterol-fed rabbits treated with L-arginine.

The inducible form of nitric oxide synthase (iNOS) is present in advanced atherosclerotic lesions. The aim of the present paper was to compare the functionality of iNOS in rabbits fed a 0.3% cholesterol-diet for 24 weeks (Baseline), and 36 weeks, with l-arginine (l-Arg) or vehicle supplementation (Saline) for the last 12 weeks. N-iminoethyl-l-lysine (l-NIL; 10 microM), a selective inhibitor of iNOS, potentiated the contractions to phenylephrine in aortas from Baseline, Saline and l-Arg rabbits confirming the presence of a functional iNOS. In l-Arg rabbits, the contractions induced by l-NIL were less pronounced than those noted in Baseline and Saline rabbits; superoxide dismutase (150 U/ml) significantly increased the phenylephrine-induced contractions only in the l-Arg rabbits. In the presence of NADPH, aortas from l-Arg rabbits produced more superoxide anions than aortas from saline rabbits as evidenced by the lucigenin-enhanced chemiluminescence technique. In conclusion, our results show functional and biochemical evidence for an increased superoxide anion production in atherosclerotic aortas from hypercholesterolemic rabbits treated with l-Arg for 12 weeks. These data may thus help to explain the lack of beneficial effects of l-Arg on atherosclerosis progression in long-term experimental hypercholesterolemia as well as in severely atherosclerotic humans.