Fig. 4. Effect of buprenorphine (A ), norbuprenorphine (B ), buprenorphine-3-glucuronide (C ), and norbuprenorphine-3-glucuronide (D ) on tidal volume. Unrestrained whole-body plethysmography was used to study the effect on tidal volume. Data were recorded for 30 min after subcutaneous injection of vehicle or buprenorphine (0.3 mg/kg; 0.6 μmol/kg), norbuprenorphine (1 mg/kg; 1.2 μmol/kg), buprenorphine-3-glucuronide (1 mg/kg; 0.6 μmol/kg), or norbuprenorphine-3-glucuronide (2.2 mg/kg, 3.8 μmol/kg). Norbuprenorphine-3-glucuronide caused a marked decrease in tidal volume 5 min after drug dose. Results are the mean ± SD (n = 4 per group). Asterisk , significantly different from control sample (P < 0.05) by two-way ANOVA.