DIFFERENCES BETWEEN RECEPTORS FOR 5-HYDROXYTRYPTAMINE ON AUTONOMIC NEURONES REVEALED BY NOR-(–)-COCAINE

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Abstract
The interaction between nor-(.sbd.)-cocaine and 5-hydroxytryptamine (5-HT) at autonomic neuronal 5-hydroxytryptamine (5-HT) receptors was investigated on the rabbit heart and guinea-pig ileum. The chronotropic response to 5-HT mediated through activation of receptors on the terminal sympathetic fibers of the rabbit heart was antagonized selectively and surmountably by nor-(.sbd.)-cocaine. Schild analysis yielded a pA2 [competitive antagonistic activity] value for the interaction of 7.79 .+-. 0.11, n [no.] = 13. On the quinea-pig ileum induced to contract by activation of 5-HT receptors on the intramural cholinergic nerves, nor-(.sbd.)-cocaine was a selective and surmountable antagonist of 5-HT, although the concentrations needed were higher than those effective on the heart. The pA2 value for the interaction was 6.49 .+-. 0.07, n = 16. The differential blocking effect of nor-(.sbd.)-cocaine in the 2 tissues suggests that the receptors for 5-HT on the cholinergic nerves of the ileum and the sympathetic fibers of the heart are different. 5-Methoxytryptamine is apparently potent agonist on the ileum, yet inactive on the heart.