INCREASE OF NITRIC-OXIDE BY L-ARGININE POTENTIATES BETA-ENDORPHIN-OPIOID BUT NOT MU-OPIOID, DELTA-OPIOID OR KAPPA-OPIOID AGONIST-INDUCED ANTINOCICEPTION IN THE MOUSE

L-Arginine pretreated i.c.v. produced a time- and dose-dependent poten tiation of beta-endorphin-induced inhibition of the tail-flick respons e in ICR mice. However, the inhibition of the tail-flick response indu ced by morphine, DAMGO ([D-Ala2,NMePhe4,Gly5-ol]enkephalin), DPDPE ([D -Pen2,D-Pen5]enkephalin or U50,488H given i.c.v was not potentiated by i.c.v. pretreated L-arginine. The results indicate that L-arginine se lectively potentiates antinociception induced by epsilon-opioid recept or agonist, but not mu-, delta- or kappa-opioid receptor agonist. L-Ar ginine pretreated i.t. did not potentiate i.c.v. administered beta-end orphin-induced inhibition of the tail-flick response, indicating that the potentiating effect of L-arginine on beta-endorphin-induced antino ciception is located at the supraspinal sites but not at the spinal si tes.