aNAcetyIpEndorphin(1_31) Disrupts the Diminishing Effect of Mastoparan on Opioid- and Clonidine-Evoked Supraspinal 1Antinociception in Mice

lntracerebroventricular (i.c.v.) administration to mice of the venom tetradecapeptide mastoparan (MP), which binds certain G transducer proteins, resulted in a long-lasting reduction of the supraspinal antinociception induced by opioids and clonidine, the alpha-2 adrenoceptor agonist. The “N-acetyl de-rivative of -endorphin-(1-31) (NAC-p-END; 1 pmol, I.C.V. per mouse) injected 1 hr before an equimolar dose of MP lessened the antagonist activity of the venom peptide on the antinoci-ception induced by D-pen25-enkephalin, [o-Ala]deltorphin II, o-A1a2-N-MePhe4-Gly-o15-enkephalin and clonidine when eval-uated 24 hr later with the tail-flick test. The impairment exerted by MP on these analgesic substances was not altered when NAC--END was administered after the venom peptide. There-fore, the antinociceptive effect depends on which peptide wa