II - Prostaglandin hyperalgesia: The peripheral analgesic activity of morphine, enkephalins and opioid antagonists

Morphine, enkephalins, nalorphine, naloxone and pentazocine are shown to have a peripheral analgesic effect. In our modification of the Randall-Selitto test these substances were 50–100 times more potent than a standard local anaesthetic, lidocaine. At this peripheral site, naloxone did not antagonize the effect of morphine. Morphine had a marked analgesic effect on the hyperalgesia induced by PGE₂ and PGI₂, BaCl₂, Ca²⁺ ionophore A23187, isoprenaline but not on that induced by dibutyryl cyclic AMP. It was suggested that the peripheral analgesic effect of morphine is due to an inhibition of adenylate-cyclase activity.