MELATONIN SIGNAL TRANSDUCTION AND MECHANISM OF ACTION IN THE CENTRAL-NERVOUS-SYSTEM - USING THE RABBIT CORTEX AS A MODEL

The cortex of the rabbit (Oryctolagus cuniculus) is rich in melatonin binding sites, and particularly abundant is the parietal cortex. Consequently, we characterized the putative melatonin receptor in the parietal cortex by a series of in vitro ligand-receptor binding experiments and biochemical and electrophysiological studies. The in vitro saturation and competition experiments demonstrated that the binding in the crude cortical membrane preparations was of high affinity and specificity. Guanine nucleotides (GDP), GTP, and GTP-gamma-S) inhibited the specific 2-[I-125]iodomelationin binding in a dose-dependent manner. Coincubation with a nonhydrolyzable GTP analog provoked a shift in the binding affinity; the numerical values of the K(d) increased from 20-30 to 200-600 pM. Melatonin, in nanomolar concentrations, was able to inhibit the forskolin-stimulated accumulation of cAMP in paretial cortex explants, and preincubation with pertussis toxin counteracted this effect of melatonin. Apparently, the melatonin binding site in the rabbit parietal cortex is linked to its second messenger via a pertussis toxin-sensitive G-protein, probably of the inhibitory G(i) class, similar to what has been described for different parts of the brain of other vertebrates. The experiments on the spontaneous firing activity of single neurons in the third to fourth layer of the parietal cortex in anesthetized animals showed that melatonin and its potent agonist 2-iodomelatonin exhibited gamma-aminobutyric acid (GABA)-like effects and were able alone, in nonomolar concentrations, to significantly slow the neuronal firing activity. Moreover, both melatonin and 2-iodomelationin potentiated the effect of GABA on the neuronal activity, leading to powerful inhibition of the tested neurons. Undoubtedly, the binding site in the rabbit parietal cortex possesses all of the characteristics of a functional receptor. We suggest that melatonin is involved in the control of fundamental cortical functions and that it acts in concert with GABA, one of the two major inhibitory neuro-transmitters in the central nervous system.