Rat cold water tail-flick: a novel analgesic test that distinguishes opioid agonists from mixed agonist-antagonists.

The models currently used to assess antinociceptive efficacy in animals are far from ideal. Those procedures that detect both opioid agonists and mixed agonist-antagonists fail to differentiate between them unless the noxious stimulus is adjusted. Furthermore, changes in the sensitivity of the test often result in positive responses being elicited from agents that are either not analgesics or only weak ones, at best. The technique described in this report uses cold water as the noxious stimulus in rats. It is simple, requires no complicated instrumentation or training, correlates well with clinical efficacy in man, and allows separation of opioid agonists from mixed agonist-antagonists without detecting non-opioid agents.