Involvement of 5-hydroxytryptamine in the central control of respiration, blood pressure and heart rate in the anaesthetized rat.

5-Hydroxytryptamine (5-HT, 1 ng-25 μg) injected into the cerebral ventricles of urethananaesthetized rats produced a rise in blood pressure, a biphasic change in heart rate and a decrease in ventilation. Responses were largest after injections into the 3rd ventricle. Lateral ventricle injections produced smaller responses while 4th ventricle injections produced the least response. The cardiovascular responses were prevented or reduced by prior intraventricular injection of bromolysergic acid diethylamide. The pressor response was not blocked by hexamethonium (10 mg/kg i.v.) and was only partly reduced by pretreatment with either 6-hydroxydopamine (3 × 100 mg/kg i.v.) or atropine methonitrate (10 mg/kg i.v.). The pressor response was prevented by spinal transection at C1 or C3 but only slightly reduced by transection at C5 or C7. In animals curarized and artificially respired with air, the blood pressure response was reduced. It is conjectured that the rise in blood pressure may involve a direct effect of hypoxia or hypercapnia resulting from a decrease in respiratory activity which is in turn mediated by a direct action of 5-HT on structures lining the 3rd ventricles. There appears to be little or no involvement of the autonomic nervous system in the response. The fall in heart rate is mediated sympathetically.