Methods for the Evaluation of Drug Action at the Human Melatonin Receptor Subtypes

NIH3T3 fibroblast cells transfected with the full-length coding regions of the mt1 and MT2 human melatonin receptors stably expressed the receptor, coupled to a pertussis-toxin-sensitive G protein and exhibiting high affinity for melatonin. Both mt1 and MT2 melatonin receptors mediated the incorporation of [35S]GTP (gamma) S into isolated membranes via receptor-catalyzed exchange of [35S]GTP (gamma) S for GDP. The relative intrinsic activity and potency of the compounds were subsequently studied by using [35S]GTP (gamma) S incorporation. The order of potency was equal to the order of apparent affinity. Melatonin and full agonists increased [35S]GTP (gamma) S binding. Luzindole did not increase basal [35S]GTP (gamma) S binding but competitively inhibited melatonin-stimulated [35S]GTP (gamma) S binding, thus exhibiting antagonist action. Two other mt1 antagonists, 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. For the first time, these findings show important differences in analogue intrinsic activity between the human mt1 and MT2 melatonin receptor subtypes.