Opiate agonists inhibit neurogenic plasma extravasation in the rat

Neurogenic plasma extravasation (NPE) evoked by antidromic stimulation of the saphenous nerve was inhibited by more than 50% by i.p. administration of the synthetic enkephalin analogue (D-Met2, Pro5)-enkephalinamide, 0.3-10 mg/kg and, to a lesser extent, by morphine. The effect of enkephalinamide (10 mg/kg) was reversed by naloxone (1 mg/kg, s.c.). Evidence is presented that the effect of enkephalinamide was not secondary to haemodynamic changes or alteration of mast cell function. It is suggested that opiate agonists inhibit NPE through specific opiate receptors, located peripherally on sensory nerve endings, by inhibiting the release of mediator(s) responsible for NPE.