Comparative effects of intrahippocampal injection of dynorphin A(1-8), dynorphin A(1-13), dynorphin A(1-17), U-50,488H, and dynorphin B on blood pressure and heart rate in spontaneously hypertensive and normotensive Wistar-Kyoto rats.

We previously demonstrated centrally mediated hypotensive and bradycardic effects of dynorphin A(1-8) (DA1-8) on microinjection into various areas of the hippocampal formation (HF) of both anesthetized and conscious male normotensive and spontaneously hypertensive rats (SHR). The purpose of the present study was to determine whether other dynorphin fragments also had this activity. We microinjected DA1-8, dynorphin A(1-13), dynorphin A(1-17), dynorphin B (DB), and the nonpeptide kappa-opioid agonist U-50,488H into HF areas previously found to react to DA1-8, at doses ranging from 0.05 to 50 nmol. The subjects were male SHR and normotensive Wistar-Kyoto (WKY) rats in which arterial pressure and heart rate were monitored. Dose-related centrally mediated hypotension and bradycardia were found in both strains with all agents used, except for DB, which had no effects. Similarly injected drug vehicle was also without effect. In general, the responses were greater in SHR than in WKY rats. Preinjection of the active HF areas with 2 nmol of nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, which itself had no blood pressure or heart rate effects, abolished both the decrease in blood pressure and heart rate of all dynorphins and U-50,488H. The results demonstrated the equivalent abilities of all the dynorphin fragments studied, except DB, to cause HF-mediated hypotension and bradycardia. The results with U-50,488H and nor-BNI strongly implicate kappa-opiate receptor activation of the HF in these effects.