Selectivity for opioid receptor subtypes of enkephalin analogues in isolated smooth muscle and in the analgesic effect in mice.

Selectivity for opioid receptor subtypes of enkephalin analogues (KK-1, -2, -3 and -4) of Tyr moiety on the N-terminal, and Phe-ol group on the C-terminal, connected with the methylene group (n=1-4) were examined in isolated smooth muscle preparations and in the analgesic effect in mice. In the longitudinal muscle preparations of guinea pig ileum (GPI), morphine, U-50488H and all the enkephalin analogues inhibited electrically evoked contractions, and the inhibitory effects of morphine, KK-1, KK-2 and KK-3 were antagonized by naloxone with relatively high pA2 values, while that of U-50488H and KK-3 were preferentially antagonized by norbinaltorphimine. In the rabbit vas deferens preparations (RVD), on the other hand, U-50488H, KK-3 and KK-4 showed weak inhibitory effects and the inhibition of U-50488H and KK-3 were antagonized by norbinaltorphimine. By intrace-rebroventricularly (i.c.v.) injection, enkephalin analogues produced analgesia in the acetic acid (AcOH) writhing test, and the effect of KK-1 and KK-2 as well as morphine was antagonized by 1 mg/kg naloxone, while those of U-50488H and KK-3 were sensitive to 1 mg/kg Mr2266. In conclusion, enkephalin analogues with a short methylene chain between the functional groups, KK-1 and KK-2, mainly exert their effect through opioid μ-receptors, while those of longer chain, KK-3 and KK-4, act through k-receptors preferentially, and KK-3 is situated in the alternating point of the selectivity for μ-and k-receptors