DIFFERENTIAL REGULATION OF MELATONIN RECEPTORS IN SHEEP, CHICKEN AND LIZARD BRAINS BY CHOLERA AND PERTUSSIS TOXINS AND GUANINE-NUCLEOTIDES

G-proteins define both the pharmacological characteristics and the sig nalling pathways of G-protein-coupled receptors. Melatonin receptors h ave been shown to belong to this class of receptors through their sens itivity to modulators of G-protein function. This study reveals that 2 -I-125-iodomelatonin (I-125-MEL) binding to different target tissues i s differentially affected by agents which disrupt the G-protein cycle. GTP gamma S, pertussis (PTX) and cholera (CTX) toxins each reduce I-1 25-MEL binding to ovine pars tuberalis (oPT) and lizard brain membrane s, whereas chicken brain is affected only by GTP gamma S (guanosine 5' -O-(3-thiotriphosphate)) and CTX. In contrast, high affinity binding o f I-125-MEL in the ovine hippocampus was not affected by any of these agents. This finding, together with the fact that neural binding sites of the sheep brain were found to have markedly lower molecular mass t han those of the oPT on native gel electrophoresis (365 vs 525 kDa), s uggests that the neural I-125-MEL binding sites in sheep may not be G- protein coupled. Pharmacologically, however, the binding sites in the hippocampus and oPT could not be distinguished using 11 analogues of m elatonin. Therefore, these data support the notion not only of multipl e forms of melatonin receptor/G-protein complex, but of high affinity binding sites for I-125-MEL which do not display sensitivity to guanin e nucleotides.