We have previously shown that the antinociceptive effect of buprenorphine is enhanced in mice lacking the opioid receptor-like (ORL-1) receptor (Lutfy et al., 2003). In the present study, we tested the hypothesis that buprenorphine may cause the release of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL-1 receptor, which may attenuate the antinociceptive effect of buprenorphine, thereby inducing a sub-maximal antinociception in wild type mice. We first confirmed our previous finding by conducting a dose-response study for buprenorphine-induced antinociception in wild type and ORL-1 knockout mice. Mice were tested for baseline tail flick latency, injected with buprenorphine (0.3 mg/kg, s.c.) and tested for antinociception 15 min later. Immediately thereafter, mice were injected with the next dose of buprenorphine (1 mg/kg, s.c.) and tested 15 min later. Finally, mice received another injection of buprenorphine (3 mg/kg, s.c.) and tested 15 min later. Buprenorphine produced a sub-maximal antinociceptive effect in wild type mice. On the other hand, ORL-1 receptor knockout mice displayed maximal antinociception after the same buprenorphine treatment. We then tested whether the sub-maximal antinociceptive effect of buprenorphine in wild type mice was due to buprenorphine-induced release of OFQ/N. Mice lacking OFQ/N and their wild type littermates were tested for baseline tail flick latency and injected with buprenorphine (0.3, 1 and 3 mg/kg, s.c.) and tested, as described above. Our results showed that wild type and OFQ/N deficient mice displayed comparable buprenorphine-induced antinociception. Together, the present results suggest that the sub-maximal antinociceptive effect of buprenorphine in wild type mice is probably due to a direct interaction of buprenorphine with the ORL-1 receptor. (Supported in part by R01 DA016682 to KL)