Electroacupuncture potentiates the antinociceptive effect of intrathecal endomorphin-1 in the rat formalin test

Endomorphin-1 is a novel endogenous mu-opioid peptide. In this study, we ex amined the effects of 2 Hz electroacupuncture in the rat tail flick test an d the formalin test (a persistent noxious model). Moreover, we investigated if the electroacupuncture potentiated the effect of intrathecal endomorphi n-1. The results demonstrated that electroacupuncture alone produced a sign ificant antinociception in the tail flick test, but not in the formalin tes t, and that intrathecal endomorphin-1 dose-dependently suppressed the bipha sic nociceptive behavior in the formalin test. Electroacupuncture enhanced the antinociceptive effect of intrathecal endomorphin-1 in the formalin tes t, resulting in a significant leftward shift in the dose-response curves fo r intrathecal endomorphin-1 antinociception. The enhanced effect was antago nized by intraperitoneal naltrexone. The study suggests that electroacupunc ture may potentiate the intrathecal endomorphin-1 antinociception partially mediated by opioid receptors. (C) 2000 Elsevier Science Ireland Ltd. All r ights reserved.