Histamine and 5-hydroxytryptamine-inhibition of transmitter release mediated by H2- and 5-hydroxytryptamine receptors

The vasodilatation caused by histamine and 5-hydroxytryptamine may be due, at least in part, to their inhibitory action on adrenergic neurotransmission. The evidence for this is as follows: 1) contractions of isolated canine vascular strips caused by sympathetic nerve stimulation are depressed by these substances whereas contractions caused by norepinephrine are either unchanged or augmented; 2) histamine and 5-hydroxytryptamine inhibit the release of norepinephrine evoked by sympathetic nerve stimulation of isolated vascular strips previously incubated with the labeled transmitter. This inhibitory effect can be demonstrated using concentrations of the substinces less than those required to affect the smooth muscle cells directly. By contrast, neither histamine nor 5-hydroxytryptamine inhibits the displacement of neuronal norepinephrine by tyramine, suggesting that these substances interfere with the exocytotic process. Additional studies have identified the histamine-H2 receptor as the probable mediator of this prejunctional action of histamine, whereas the nature of the receptor for 5-hydroxytryptamine remains to be clarified.