5-METHOXYTRYPTAMINE INHIBITS CYCLIC-AMP ACCUMULATION IN CULTURED RETINAL NEURONS THROUGH ACTIVATION OF A PERTUSSIS-TOXIN-SENSITIVE SITE DISTINCT FROM THE 2-[(125I)]IODOMELATONIN BINDING-SITE

Melatonin and 5-methoxytryptamine inhibited forskolin-stimulated cycli c AMP formation in cultured neural cells prepared from embryonic chick retina. Both methoxyindoles exhibited similar potency and efficacy, w ith EC(50) values of 0.8 nM for melatonin and 7.2 nM for 5-methoxytryp tamine. Inhibition of cyclic AMP formation by 5-methoxytryptamine or m elatonin was prevented by pretreatment with pertussis toxin. Pretreatm ent of cultures with 5-methoxytryptamine for 24 h reduced the subseque nt inhibitory cyclic AMP response to 5-methoxytryptamine but not that to 2-iodomelatonin. Putative melatonin receptors on cultured retinal c ells were labeled with 2-[I-125]iodometatonin. Melatonin displaced spe cific 2-[I-125] iodomelatonin with a K-i value (0.8 nM) similar to the EC(50) for inhibition of cyclic AMP formation. In contrast, 5-methoxy tryptamine only inhibited 2-[I-125] iodomelatonin binding at very hi h concentrations (K-i = 650 nM). Pretreating cultured cells for 24 h wi th 2-iodomelatonin or melatonin, but not with 5-methoxytryptamine, red uced subsequent 2-[I-125]iodomelatonin binding. Thus, 5-methoxytryptam ine appears to inhibit forskolin-stimulated cyclic AMP formation at a site distinct from the 2-iodomelatonin binding site.