Intracellular Mechanisms Of Melatonin Action

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3.1. G-Proteins

Melatonin receptors are coupled to the intracellular effectors via GTP-binding proteins. In the presence of GTP or non-hydrolysable GTP derivatives affinity of the melatonin receptors for 125I-melatonin is decreased (15). The effects of melatonin in the pituitary, including inhibition of LH release and the effects on 2nd messengers are abolished after preincubation with pertussis toxin (PTX; 27). This indicates that the receptors are coupled to G-protein(s) belonging to the Gi family. Also the effects of melatonin in SCN are mediated by Gi: preincubation with PTX blocks the effect of melatonin on spontaneous electric activity (8). Gi family consists of 5 subspecies: 3 Gi and 2 G0 proteins (2,17). It is not clear which of these G proteins is involved in transduction of the melatonin signal.

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