A comparison of clonidine with morphine for antinociceptive and antiwithdrawal actions.

Clonidine was found to possess dose-dependent analgesic and antiwithdrawal activity. In mice, clonidine prolonged the tail flick latency and inhibited phenylquinone-induced writhing. In rats, it inhibited tail withdrawal from hot water and a pain response to pressure application on an inflamed paw. The effective doses of clonidine were different in the different tests employed, but they were always smaller than those of morphine. Naloxone failed to antagonize the analgesic actions of clonidine but effectively antagonized those of morphine. Phenoxybenzamine also did not alter the inhibition of tail flick-induced by clonidine. Clonidine suppressed morphine withdrawal body shakes in a dose-dependent manner as does morphine. This action of clonidine was not reversed by naloxone. In usual laboratory tests, clonidine appears to be an effective analgesic which antagonizes signs of morphine withdrawal.