Buprenorphine and gastrointestinal transit in rats: effect of naloxone on the biphasic dose-response curve.

1. Buprenorphine (0.01-10 mg/kg, subcutaneous [s.c.]) slowed the passage of a charcoal meal along the gastrointestinal tract in rats. The dose-response relationship was U-shaped. 2. When rats were pretreated with naloxone (0.30 mg/kg, s.c.), both the descending and ascending components of the buprenorphine dose-response curve were displaced to the right. 3. Buprenorphine-induced delay of transit was maximal at a dose of 0.10 mg/kg. In rats pretreated with naloxone, a 30-fold higher dose of buprenorphine was required for a comparable peak effect. 4. Moderate-high doses of buprenorphine may be acting on a functionally related binding site which non-competitively inhibits the usual buprenorphine-mu opioid receptor interaction.