Evidence for a presynaptic inhibitory action of 5-hydroxytryptamine on sympathetic neurotransmission to the myocardium.

A study was undertaken to determine the effect of 5-Hydroxytryptamine (5-HT) on sympathetic neurotransmission to the myocardium. Intravenous infusion of 5-HT (20 microgram/kg/min) produced a decrease in mean blood pressure and caused significantly impairment of cardioacceleration observed during the stimulation of right post-ganglionic cardiac sympathetic nerve. The pressor and tachycardic responses to norepinephrine as well as tyramine were potentiated during 5-HT infusion. The inhibitory effect of 5-HT on responses to cardiac nerve stimulation was not prevented by desipramine. Prior treatment with cyproheptadine prevented the depressor effect of 5-HT and significantly antagonized the inhibitory action of 5-HT on the positive chronotropic effect of cardiac nerve stimulation. Methysergide administration to a different group of dogs antagonized the effect of 5-HT on blood pressure, but did not alter the impairment of cardioacceleration caused by 5-ht during cardiac nerve stimulation. Neither cyproheptadine nor methysergide had any effect on the responses to cardiac nerve stimulation. These results suggest that 5-HT inhibits sympathetic neurotransmission of the myocardium via an action on 'trypaminergic' receptors which may be located on sympathetic nerve terminal and that these receptors can be selectively blocked by cyproheptadine but not by methysergide.