Lack of antinociceptive activity of clonidine in two pain models: Comparison with other analgesics and CNS active drugs

The antinociceptive activity of clonidine was investigated in two models of pain using the modified Haffner tail clip test (MHA) in mice and the formalin test (FT) in rats. Morphine was used as a positive control for comparison. Clonidine at does from 0.5 mg/kg to 8 mg/kg SC was not effective in MHA while morphine administration resulted in a dose-dependent increase in the latency to bite the clip and an ED₅₀ of 1.2 mg/kg SC. No antinociceptive activity was observed for clonidine using the FT up to a dose of 4 mg/kg SC which causes severe behavioral depression. The ED₅₀ for morphine in FT was 1.7 mg/kg SC. From these studies we conclude that clonidine is not active in all tests used to predict analgesic efficacy in man. Additional analgesic and centrally acting drugs were also included for comparative purposes. Of the compounds tested only the opiates active in FT and MHA and cholinesterase inhibitors (active in MHA only) showed analgesic effects.