Peripheral opioid receptors located on the rat saphenous nerve.

Morphine and ethylketocyclazocine inhibited neurogenic plasma extravasation (NPE) elicited by antidromic stimulation of the saphenous nerve of the rat. The inhibitory effects of the opiate agonists were antagonised by naloxone and N-methyl nalorphine and independent of any local anaesthetic actions since inhibition of NPE was observed on superfusion of the saphenous nerve with lignocaine, but not with morphine or ethylketocyclozocine. The opiate agonists also failed to inhibit extravasation induced by intradermal administration of substance P. The findings support the contention that specific opioid receptors are located on the peripheral part of primary afferent neurones and these receptors may contribute significantly to peripherally mediated antinociceptive effects of opioids.