Presynaptic inhibition by serotonin of cardiac sympathetic transmission in dogs.

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.