Antinociceptive effect of buprenorphine in mu1-opioid receptor deficient CXBK mice

The antinociceptive effect of buprenorphine was examined in mu1-opioid receptor-deficient CXBK mice. I.p. administration of buprenorphine at a dose of 3 mg/kg produced marked antinociception in the tail-flick test in C57BL/6 mice, a progenitor strain of CXBK mice. The antinociceptive effect of buprenorphine in C57BL/6 mice was antagonized by pretreatment with either beta-funaltrexamine (beta-FNA), a mu-opioid receptor antagonist, or naloxonazine (NXZ), a selective mu1-opioid receptor antagonist. The antinociceptive effect of buprenorphine (3 mg/kg, i.p.) in CXBK mice was significantly less than that in C57BL/6 mice. Neither beta-FNA nor NXZ reduced the antinociceptive effect of buprenorphine in CXBK mice. There was no significant difference between the buprenorphine-induced antinociceptive effect in CXBK mice and NXZ-treated C57BL/6 mice. Furthermore, neither naltrindole, a selective delta-opioid receptor antagonist, nor norbinaltorphimine, a selective kappa-opioid receptor antagonist, had a significant effect on the antinociceptive effects of buprenorphine in both CXBK and C57BL/6 mice. These results support our previous hypothesis that mu1- rather than mu2-, delta- or kappa-opioid receptors are involved in the antinociceptive effects of buprenorphine.