Fig. (1).

Buprenorphine attenuates morphine-induced antinociception in wild type, but not ORL-1 receptor knockout, mice in the radiant heat tail flick assay. Mice were tested for baseline tail flick latency, injected with morphine (2.5 mg/kg, s.c.) and tested 30 min later. The same mice were injected with the next dose of morphine (5 mg/kg,) and tested 30 min later. After the tail flick latency was measured 30 min following the last dose of morphine (10 mg/kg, s.c.), mice were injected with buprenorphine (3 mg/kg, s.c.) and tested after a further 15-min delay. Data are means (± s.e.m.) of 5–6 mice/genotype.

*indicates a significant difference between wild type (WT) and knockout (KO) mice (t = 4.07; p<0.05).