Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats

Previous studies have demonstrated that injection of serotonin into the nucleus tractus solitarius (NTS) elicits hypotension and bradycardia in rats. The present study sought to further characterize this response and to examine the role of serotonergic mechanisms in the NTS in cardiovascular regulation in spontaneously hypertensive (SHR) rats. Injections of picomole amounts of serotonin into the NTS of chloralose-anesthetized normotensive Sprague-Dawley (S-D) or Wistar-Kyoto (WKY) rats produced hypotension and bradycardia that were eliminated by prior injection into the NTS of the selective 5HT(2) antagonist sarpogrelate. Bilateral injection of sarpogrelate did not alter blood pressure or reflex changes in heart rate in response to phenylephrine-induced increases in blood pressure or nitroprusside-induced decreases in blood pressure. In SHR rats, the depressor response produced by injection of serotonin into the NTS was markedly larger than in WKY rats, and was larger than depressor responses previously reported for other excitatory substances injected into the NTS. In SHR rats bilateral injection of sarpogrelate produced an increase in blood pressure, although it did not alter baroreceptor-evoked changes in heart rate. These results provide further support for the hypothesis that stimulation of 5HT(2) receptors in the NTS contributes to cardiovascular regulation independent of the baroreceptor reflex. Furthermore, this serotonergic system is altered in SHR rats, apparently acting tonically to reduce blood pressure.