Antinociceptive effect of buprenorphine in μ₁-opioid receptor deficient CXBK mice

The antinociceptive effect of buprenorphine was examined in μ₁-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 β- funaltrexamine (β-FNA), a μ-opioid receptor antagonist, or naloxonazine (NXZ), a selective μ-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 β-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 δ- opioid receptor antagonist, norbinaltorphimine, a selective κ-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 μ₁- rather than μ₂-, δ- or κ-opioid receptors are involved in the antinociceptive effects of buprenorphine.