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. Conse quently, we characterized the putative melatonin receptor in the parie tal cortex by a series of in vitro ligand-receptor binding experiments and biochemical and electrophysiological studies. The in vitro satura tion and competition experiments demonstrated that the binding in the crude cortical membrane preparations was of high affinity and specific ity. Guanine nucleotides (GDP), GTP, and GTP-gamma-S) inhibited the sp ecific 2-[I-125]iodomelationin binding in a dose-dependent manner. Coi ncubation with a nonhydrolyzable GTP analog provoked a shift in the bi nding affinity; the numerical values of the K(d) increased from 20-30 to 200-600 pM. Melatonin, in nanomolar concentrations, was able to inh ibit the forskolin-stimulated accumulation of cAMP in paretial cortex explants, and preincubation with pertussis toxin counteracted this eff ect of melatonin. Apparently, the melatonin binding site in the rabbit parietal cortex is linked to its second messenger via a pertussis tox in-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 sin gle neurons in the third to fourth layer of the parietal cortex in ane sthetized animals showed that melatonin and its potent agonist 2-iodom elatonin exhibited gamma-aminobutyric acid (GABA)-like effects and wer e able alone, in nonomolar concentrations, to significantly slow the n euronal firing activity. Moreover, both melatonin and 2-iodomelationin potentiated the effect of GABA on the neuronal activity, leading to p owerful inhibition of the tested neurons. Undoubtedly, the binding sit e in the rabbit parietal cortex possesses all of the characteristics o f 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 c entral nervous system.