Contribution of Spinal Dopamine Receptors to the Hypotensive Action of Bromocriptine in Rats

Abstract

Summary

Bromocriptine, a dopamine (DA) receptor agonist, has been reported to have hypotensive effects in anesthetized and conscious normotensive rats but its mechanism of action is still not fully understood. Therefore, we studied the changes in mean arterial blood pressure (MAP) and heart rate (HR) elicited by an intravenous (i.v.) administration of bromocriptine (150μ.g/kg), in either pentobarbital-anesthetized or conscious normotensive rats, pretreated with either i.v. (0.3 mg/kg) or intrathecal (i.t.) (93 nmol) domperidone, a DA receptor antagonist that does not cross the blood-brain barrier. In these preparations, i.v. administration of bromocriptine elicited dose-dependent decreases in MAP and rises in HR. The hypotensive effect was antagonized partially by i.v. and fully by i.v. domperidone. However, the latter compound did not modify the tachycardia, which could be blocked by propranolol (0.5 mg/kg i.v.). In rats pretreated with the latter β-adrenoceptor antagonist, bromocriptine produced only a decrease in blood pressure that was inhibited by i.v. and i.t. domperidone. These results suggest that, in anesthetized and conscious normotensive rats, the hypotension induced by systemic administration of bromocriptine is fully mediated by DA2 dopamine receptors, which are located partly within the spinal cord and partly in the peripheral circulation.