Serotonin agonists and antagonists in experimental hypertension.

Serotonin (5-hydroxytryptamine; also called 5-HT) modifies cardiovascular activity by central as well as peripheral sites of action. When 5-HT is injected within the central nervous system, depending upon the dose and site of administration, either a pressor or a depressor effect is observed. Recent findings suggest that this depressor effect may be mediated by central "5-HT1-like" receptors, since certain compounds that exhibit a high affinity for the 5-HT1A binding site can reduce blood pressure by a central action in both hypertensive and normotensive animals. Peripherally, 5-HT elicits vasodilatation (both directly and indirectly via presynaptic sympathoinhibition and release of vasodilator substances from endothelium) or vasoconstriction (with associated amplification of noradrenaline response) of mainly "large" conductance arteries mediated by, respectively, "5-HT1-like" and 5-HT2 receptors. Of the various antagonists at 5-HT receptors, it is only ketanserin that effectively lowers arterial blood pressure. However, since it is unlikely that the very low concentrations of 5-HT in plasma exert a significant influence on the maintenance of peripheral vascular resistance, the blockade of 5-HT2 receptors by ketanserin does not seem to explain the reduction of blood pressure in hypertension. Indeed, apart from the undoubtedly potent 5-HT2 receptor blockade, ketanserin also has alpha 1-adrenoceptor antagonist, central vasomotor depressant, and "direct" vasodilator properties, which can explain its antihypertensive action.