Effects of dopamine on blood pressure and heart rate after microinjection into the hypothalamus of spontaneously hypertensive and normotensive Wistar-Kyoto rats.

In anaesthetized spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats, 0.0125-0.4 mg/kg dopamine (DA) given intracerebroventricularly (icv) produced dose-related decreases in mean arterial blood pressure (MAP) and heart rate (HR), whereas intravenous DA produced dose-related increases in MAP and HR. SHR was significantly more responsive to icv DA compared with the normotensive controls. DA (0.02 mg/kg) significantly reduced MAP in a biphasic manner, when microinjected into the arcuate nucleus or third ventricle of SHR, with no significant changes in HR. No such effect occurred in WKY rats. Metoclopramide given concurrently with DA into the third ventricle attenuated the hypotensive response to DA. Saline injection had no significant effect on MAP and HR. The hypotensive responses to DA were not confounded by the spread of injected DA into the adjacent hypothalamic areas. These results support the hypothesis that there exists an abnormal sensitivity in SHR to centrally administered DA and that the arcuate appears to be the brain site involved in this abnormality.