Ethylketocyclazocine decreases noradrenaline release and blood pressure in the rabbit at a peripheral opioid receptor

Summary Rabbits were pithed and their sympathetic outflow was stimulated electrically via the pithing rod. Arterial blood pressure, heart rate, the endogenous plasma noradrenaline 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^–1+0.02 mg kg^–1 h^–1 and 1 mg kg^–1 + 0.2 mg kg^–1 h^–1 but not 0.01 mg kg^–1+ 0.002 mg kg^–1 h^–1 decreased blood pressure, the endogenous plasma noradrenaline level and the noradrenaline release rate. The effects of ethylketocyclazocine 1 mg kg^–1+ 0.2 mg kg^–1 h^–1 were antagonized by naloxone 1 mg kg^–1 + 0.5 mg kg^–1 h^–1. Given alone, naloxone caused no change. It is concluded that ethylketocyclazocine inhibits action potential-evoked release of noradrenaline from postganglionic sympathetic neurones, and hence can lower blood pressure, by a peripheral effect, possibly mediated by opioid receptors at the terminal axons.