Specificity and nanomolar potency of melatonin on G-protein coupled melatonin MT1 and MT2 receptors expressed in HEK-293T human embryo kidney cells

This is a pre-registered study, i.e. a study whose hypotheses and experiments designed to address these hypotheses has been deposited in a database before starting the experiments. The study aims at assessing the Gs versus Gi coupling and the potency of melatonin in the human version of melatonin MT1 and MT2 G-protein-coupled receptors expressed in HEK-293T cells. The results show that these receptors are Gi but not Gs coupled. By using a standard procedure of modulation of 0.5 μM forskolin-induced cAMP levels, it was found that the potency of MT2 receptor-mediated actions is in the low nanomolar range, but the potency of MT1 is in the high nanomolar range. The potency of melatonin to stimulate the MT2 receptor is similar to that of a selective agonist, N-[2-(2-methoxy-6H-isoindolo[2,1-a]indol-11-yl)ethyl]butanamide (IIK7). Overall, the new data on the potency of melatonin on its receptors will provide a new look for melatonin research. It is important to consider this finding for appropriately addressing physiological or therapeutical effects based on melatonin potency. Thus, the low doses of melatonin used in the existing prolonged release preparations or in other supplements should be revisited