Role of central 5-HT3 receptors in the control of blood pressure in stressed and non-stressed rats.

The aim of the present study was to investigate the participation of central 5-HT(3) receptors in the control of blood pressure and heart rate (HR) of non-stressed and stressed rats. The pharmacological stimulation of brain 5-HT(3) receptors by third ventricle injections of the selective 5-HT(3) receptor agonist m-CPBG induced a significant decrease in blood pressure in non-stressed rats and impaired the hypertensive response induced by restraint stress. The blockade of brain 5-HT(3) receptors by the central administration of the selective 5-HT(3) antagonist ondansetron elicited a significant increase in blood pressure in non-stressed rats. Conversely, the hypertensive response induced by restraint stress was not affected by central administration of ondansetron. Additionally, baroreflex-mediated bradycardia during phenylephrine-induced hypertensive response was preserved in non-stressed animals receiving third ventricle injections of m-CPBG, while the baroreflex-mediated tachycardia that occurs during the hypotensive response induced by the administration of sodium nitroprusside was impaired. It is concluded that the serotoninergic component represented by the brain 5-HT(3) receptors exerts a tonic inhibitory influence on the central control of blood pressure in non-stressed rats, probably by a sympathoinhibitory-related mechanism. On the other hand, during stress, this central 5-HT(3)-dependent inhibitory drive is overwhelmed by the different neurochemical systems that harmonically trigger and sustain the hypertensive response.