Dual action mechanisms of KK-3, a newly synthesized leu-enkephalin derivative, in the production of spinal analgesic effects.

The action mechanism for the production of spinal analgesia of KK-3, tyrosyl-N-methyl-gamma-aminobutylyl-phenylalaninol, was examined by the tail pinch and tail flick methods. Intrathecal KK-3, 2.5, 5 and 10 nmol/mouse, dose-dependently produced an analgesic effect in both methods. In the tail pinch method, the analgesia was suppressed by 2 mg/kg but not by 1 mg/kg of naloxone; however, the analgesic effect was significantly antagonized by 1 and 2 mg/kg Mr2266, a kappa-antagonist. Meanwhile, both naloxone and Mr2266 failed to block the analgesic effect of KK-3 in the tail flick test. Intrathecal capsaicin, 0.3, 3 and 15 nmol/mouse, also produced a dose-dependent analgesic effect in the tail flick test, whereas no appreciable analgesia could be found in the tail pinch test. Neither naloxone nor Mr2266 blocked the analgesic effect of capsaicin. The results indicate that KK-3 may possess two separate pharmacological mechanisms for the production of analgesic effects on the spinal level: one is the depletion of substance P following its release from the spinal cord, and the other is the mediation through kappa-opioid receptors.