Clocinnamox dose-dependently antagonizes morphine-analgesia and [ [formula omitted]]DAMGO binding in rats

Clocinnamox is a long-lasting, nonequilibrium, μ-opioid receptor antagonist in mice and monkeys. The present studies examined the in vivo and ex vivo effects of clocinnamox in rats. Under control conditions, morphine dose-dependently increased tail-withdrawal latencies from 50°C water, with a mean ED 50 of 7.3±1.1 mg/kg. Clocinnamox antagonized the antinociceptive effects of morphine. 1.0 mg/kg clocinnamox displaced the morphine dose–response curve 4-fold to the right of the control curve and 10 mg/kg clocinnamox eliminated morphine's antinociceptive effects at doses up to 1000 mg/kg for at least seven days. There was a gradual recovery of the antinociceptive response to morphine; however, the morphine dose–response curve did not return to its original position by five weeks after 10 mg/kg clocinnamox. Whole brain membranes were prepared from separate groups of rats for determination of binding parameters of [ 3 H ][ d-Ala 2, N-Me-Phe 4,Gly 5-ol]-enkephalin (DAMGO). Clocinnamox dose-dependently decreased [ 3 H ]DAMGO binding ex vivo and the decreased binding was a result of changes in B max. The control B max for [ 3 H ]DAMGO was 234±8 fmol/mg protein; in membranes prepared from rats pretreated with 10 mg/kg clocinnamox, the B max value for [ 3 H ]DAMGO was 54±2 fmol/mg protein. The B max values for [ 3 H ]DAMGO binding after an injection of 10 mg/kg clocinnamox returned towards control values gradually, four weeks after clocinnamox the B max was 178±10 fmol/mg protein. These results suggest that clocinnamox is a long-lasting, nonequilibrium μ-opioid receptor antagonist in rats.