Serotonin-induced platelet aggregation predicts the antihypertensive response to serotonin receptor antagonists

Ligustrazine monomer against cerebral ischemia/reperfusion injury

Ligustrazine (2,3,5,6-tetramethylpyrazine) is a major active ingredient of the Szechwan lovage rhizome and is extensively used in treatment of ischemic cerebrovascular disease. The mechanism of action of ligustrazine use against ischemic cerebrovascular diseases remains unclear at present. This study summarizes its protective effect, the optimum time window of administration, and the most effective mode of administration for clinical treatment of cerebral ischemia/reperfusion injury. We examine the effects of ligustrazine on suppressing excitatory amino acid release, promoting migration, differentiation and proliferation of endogenous neural stem cells. We also looked at its effects on angiogenesis and how it inhibits thrombosis, the inflammatory response, and apoptosis after cerebral ischemia. We consider that ligustrazine gives noticeable protection from cerebral ischemia/reperfusion injury. The time window of ligustrazine administration is limited. The protective effect and time window of a series of derivative monomers of ligustrazine such as 2-[(1,1-dimethylethyl)oxidoimino]methyl]-3,5,6-trimethylpyrazine, CXC137 and CXC195 after cerebral ischemia were better than ligustrazine.

Serotonin and the heart

Serotonin is a naturally occurring vasoactive substance that has diverse cardiophysiological effects. These effects can be explained by the existence of serotonin receptor subtypes which mediate different biological actions. The vasoconstrictive actions of serotonin are mediated by 5-HT2 serotonergic receptors, and serotonin also amplifies the release and activities of other vasoconstrictors, such as angiotensin and norepinephrine. Abnormalities in the serotonergic system may play an important role in the pathophysiology of multiple cardiovascular disease states such as systemic hypertension, primary pulmonary hypertension and peripheral vascular disease. Selective 5-HT2 serotonergic receptor blockers have been developed which appear to be potent vasodilators with therapeutic potential in various cardiovascular disease states. The largest clinical experience has been collected with ketanserin, and other agents in this class are being investigated. Prolongation of the ECG QT interval with 5-HT2 serotonergic receptor blockers may pose a potential risk with these treatments in some patients.

Hypotensive and reflex bradycardic effects of ketanserin, but not of prazosin, enhanced selectively in aging conscious rats

To determine whether cardiovascular effects of ketanserin are altered differently with aging as compared with those of prazosin, we recorded blood pressure (BP) and heart rate (HR) changes produced by treatment with either drug in three age groups of conscious Sprague-Dawley rats. BP was decreased more by ketanserin in 24-month than in 4- or 14-month-old rats, but was decreased equally by prazosin in all age groups. Pressor responses to phenylephrine (PE) were consistently abolished by both drugs, indicating that the greater hypotensive effects of ketanserin in 24-month-old rats were not due simply to alpha 1-adrenergic blockade. By contrast, baroreflex sensitivity, determined from reflex HR responses to infused angiotensin or sodium nitroprusside (SNP), was altered differently in old rats by ketanserin but not by prazosin. Whereas enhancement of reflex bradycardia by prazosin occurred at all ages, it was demonstrable only with ketanserin in older rats. Moreover, reflex tachycardia was unaffected by prazosin but was reversed to bradycardia by ketanserin in older rats. Because these differences persisted even after the data had been normalized to compensate for differences in baseline pressures, effects on HR reflexes were considered age dependent for ketanserin but not for prazosin. Although the underlying mechanisms are not clear, the selective enhancement of reflex bradycardia and reversal of reflex tachycardia in old rats by ketanserin, but not by prazosin, could explain why hypotensive responses to ketanserin increase with age whereas those to prazosin do not.

Aging enhances serotonergic cardiovascular blockade by ketanserin in conscious rats

We recorded cardiovascular responses to serotonin (5-HT) and to two selective serotonergic agonists following ketanserin treatment in 3 groups of conscious rats aged 4, 14 or 24 months. The selective agonists were DOI (5-HT2 agonist), and phenylbiguanide (5-HT3 agonist). Before ketanserin treatment, pressor responses to 5-HT or DOI were larger while reflex bradycardic responses to 5-HT or phenylbiguanide were smaller in 14- and 24-month than in 4-month-old rats. Ketanserin treatment lowered blood pressure consistently, and the ensuing hypotension was more pronounced in 14- and 24-month than in 4-month-old rats. Pressor responses to DOI were attenuated similarly in all rats, but those to 5-HT were reversed to depressor responses whose magnitude was smaller in 14- and 24-month than in 4-month-old rats. On the other hand, bradycardic responses to 5-HT and phenylbiguanide were enhanced in 14- and 24- but not in 4-month-old rats. Our results indicate that even before ketanserin was given, old rats had enhanced pressor responses to 5-HT2 agonists together with weakened bradycardic responses to 5-HT3 agonists. Following ketanserin treatment, 5-HT2 pressor responses were blocked while 5-HT3 bradycardic responses were enhanced but only in old rats. These results are compatible with the interpretation that the more pronounced hypotension produced in old rats by ketanserin is due to two complementary effects on serotonergic receptors: blockade of 5-HT2 pressor responses coupled with selective enhancement of 5-HT3 bradycardic responses.