PRESYNAPTIC INHIBITION BY SEROTONIN OF CARDIAC SYMPATHETIC TRANSMISSION IN DOGS

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Abstract
The effect of serotonin on cardiac sympathetic transmission was investigated in vagotomized and cardiac decentralized dogs. Administration of serotonin in doses of 10-100 micrograms/kg i.v., during the resting unstimulated state caused tachycardia and pressor responses which were inhibited by cyproheptadine but not by guanethidine. The tachycardia was reduced by a beta-adrenoceptor antagonist, bufetolol. Serotonin in doses of 3-100 micrograms/kg depressed the elevated heart rate during maintained electrical stimulation of the cardiac sympathetic nerves. Cyproheptadine did not antagonize the serotonin-induced depression of the stimulation-elevated heart rate, while desipramine attenuated but did not abolish it. Serotonin did not have a significant effect on the heart rate elevated by maintained infusion of noradrenaline. The present results suggest that serotonin-induced depression of heart rate during sympathetic nerve stimulation is due to presynaptic inhibition by serotonin of cardiac sympathetic transmission which is not mediated via 'classic' tryptaminergic receptors.