ETHYLKETOCYCLAZOCINE DECREASES NORADRENALINE RELEASE AND BLOOD-PRESSURE IN THE RABBIT AT A PERIPHERAL OPIOID RECEPTOR

Rabbits were pithed and their sympathetic outflow was stimulated electrically via the pithing rod. Arterial blood pressure, heart rate, the endogenous plasma noradrenaline [norepinephrine] level, the plasma 3H-noradrenaline clearance and the noradrenaline release rate (the rate of entry of endogenous noradrenaline into the plasma) were determined. Ethylketocyclazocine 0.1 mg/kg + 0.02 mg/kg per h and 1 mg/kg + 0.2 mg/kg per h but not 0.01 mg/kg + 0.002 mg/kg per h decreased blood pressure, the endogenous plasma noradrenaline level and the noradrenaline release rate. The effects of ethylketocyclazocine 1 mg/kg + 0.2 mg/kg per h were antagonized by naloxone 1 mg/kg + 0.5 mg/kg per h. Given alone, naloxone caused no change. Ethylketocyclazocine evidently inhibits action potential-evoked release of noradrenaline from postganglionic sympathetic neurons, and can lower blood pressure by a peripheral effect, possibly mediated by opioid receptors at the terminal axons.