The effect of the leucopyrokinin analogue: [2-8]-leucopyrokinin on central opioid receptors in rats

The antinociceptive effect of intracerebroventricular injections of 2–8]-leucopyrokinin (LPK), a truncated leucopyrokinin analogue, was determined in rats, by means of a tail immersion test. We found a significant antinociceptive effect of three i.c.v. doses of 2–8]-LPK: 1, 5 and 10 nmol. Pre-treating animals with naloxone hydrochloride (1 mg/kg i.p.) completely blocked the effect of two high doses of 2–8]-LPK. To determine the sub-types of opioid receptors involved in 2–8]-leucopyrokinin-induced analgesia we injected specific blockers of μ-, δ- and κ-receptors namely, β-funaltrexamine hydrochloride, naltrindole hydrochloride and nor-binaltorphimine dihydrochloride, respectively, prior to 2–8]-leucopyrokinin at equimolar doses. We conclude that the antinociceptive effect of 2–8]-leucopyrokinin is mediated mainly by central μ- and δ-opioid receptors.