Nonopioid actions of U50,488 enantiomers contribute to their peripheral cutaneous antinociceptive effects

The ability of arylacetamide mu-opioid receptor agonists (kappa-ORAs) to block sodium channels by a nonopioid mechanism has been previously documented. The present experiments were undertaken to test whether two enantiomers of the arylacetamide kappa-ORA (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50,488), (+)-(1R,2R)U50,488 and (-)-(1S,2S)-U50,488, are antinociceptive in the formalin model by a peripheral, nonopioid receptor-mediated mechanism. Although both enantiomers have been previously shown to block sodium channels with comparable potencies, only (-)-(1S,2S)-U50,488 has activity at the mu-opioid receptor (KOR). In the formalin test, intrapaw administration of U50,488 enantiomers as well as lidocaine exhibited significant dose-related attenuation of formalin-induced flinching behavior. The rank order of potency of the drugs tested was (-)-(1S,2S)-U50,488 > (+)-(1R,2R)-U50,488 > lidocaine. The antinociception produced by lower doses of (-)-(1S,2S)- U50,488 was blocked by intrapaw nor-binaltorphimine as well as by antisense knockdown of the KOR. Such pretreatments, however, did not block the antinociception produced by (-)-(1R,2R) U50,488, lidocaine, or higher doses of (-)-(1S,2S)-U50,488. These data suggest that the sodium channel blocking effects of U50,488 and similar kappa-ORAs can contribute to their peripheral antinociceptive actions.