TABLE 1.

Opioid antinociceptive tolerance using an 8-h model

Mice were either repeatedly administered vehicle over 8 h and then challenged with the opioid (vehicle + opioid) or repeatedly administered opioid over 8 h and then challenged with the opioid (opioid + opioid) as well as vehicle intracerebroventricularly. Meperidine (40 mg/kg s.c.), morphine (10 mg/kg s.c.), and fentanyl (0.2 mg/kg s.c.) were administered every 2 h for a total of four injections, with test doses of the opioid administered 2 h after the last injection. DAMGO (25.7 ng/kg i.c.v.) was administered every 1 h for a total of eight injections, with test doses of DAMGO administered 1 h after the last injection. All opioid + opioid groups received vehicle intracerebroventricular injections before the opioid test doses. Thirty minutes (20 min for DAMGO) after test doses were administered, tail-immersion latencies were determined for construction of dose-response curves as well as calculation of ED₅₀ values and potency ratios.