Modification of clonidine-induced pressor responses in morphine-dependent and hypophysectomised rats

Pressor responses to successive doubling doses of clonidine were examined in anaesthetised normotensive (NT) and spontaneously hypertensive rats (SHR). The slope of the dose-response curves was similar in both strains of rat, but the average response to the same concentrations of clonidine was less in the SHR. Naloxone (2 mg/kg i.v.) did not alter the dose-response curve in either strain. However, in NT and SHR made morphine-dependent (3 X 75 mg morphine pellets s.c.) and in hypophysectomised rats the increase in blood pressure was significantly reduced compared to the untreated or sham operated control groups, with a decrease in both the slope of the dose-response curve and a decrease in the average response. These results suggest that opioid peptide systems can modify vascular reactivity although not via circulating peptides. The specificity of the effects remains to be established.