Opioid peptides decrease noradrenaline release and blood pressure in the rabbit at peripheral receptors

Effects of dynorphin-(1-13), Leu5-enkephalin, D-Ala2,D-Leu5-enkephalin (DADLE), and for comparison bremazocine, on plasma noradrenaline concentration and mean arterial pressure (MAP) were studied in pithed rabbits. In the first series of experiments, the sympathetic outflow was stimulated electrically via the pithing rod at 2 Hz twice for 3 min each (S1, S2). Drugs were administered before S2. Bremazocine 10 micrograms/kg + 2 micrograms/kg/h and 100 micrograms/kg + 20 micrograms/kg/h, dynorphin 1 and 3 micrograms/kg/min, Leu5-enkephalin 100 micrograms/kg/min and DADLE 10 and 30 micrograms/kg/min all diminished the electrically-evoked increase in plasma noradrenaline and MAP. The effects were antagonized by naloxone. In the second series, an infusion of noradrenaline (2 micrograms/kg/min) was given twice for 3 min each (N1, N2). Drugs were administered before N2. Bremazocine 100 micrograms/kg + 20 micrograms/kg/h slightly enhanced the pressor effect of exogenous noradrenaline, whereas dynorphin 3 micrograms/kg/min, Leu5-enkephalin 100 micrograms/kg/min and DADLE 30 micrograms/kg/min caused no significant change. In the third series, the sympathetic outflow was stimulated continuously at 2 Hz, and the interaction of dynorphin and DADLE was studied. Dynorphin 1 microgram/kg/min and DADLE 10 micrograms/kg/min initially decreased MAP to a similar extent. The effect of DADLE faded with time. When, during continuous infusion of DADLE 10 micrograms/kg/min, and after return of MAP to the pre-DADLE level, dynorphin 1 microgram/kg/min or DADLE 10 micrograms/kg/min was infused additionally, the effect of dynorphin was unchanged, whereas that of DADLE was almost abolished. We conclude that the opioid peptides as well as bremazocine decrease action potential-evoked release of noradrenaline and, secondarily, blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)