Clocinnamox dose-dependently antagonizes morphine-analgesia and [³H]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 ED₅₀ 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 [³H][D-Ala²,N-Me-Phe⁴Gly⁵-ol]-enkephalin (DAMGO). Clocinnamox dose-dependently decreased [³H]DAMGO binding ex vivo and the decreased binding was a result of changes in B(max). The control B(max) for [³H]DAMGO was 234 ± 8 fmol/mg protein; in membranes prepared from rats pretreated with 10 mg/kg clocinnamox, the B(max) value for [³H]DAMGO was 54 ± 2 fmol/mg protein. The B(max) values for [³H]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.