EVIDENCE THAT THE ANTINOCICEPTIVE TAIL-FLICK RESPONSE IS PRODUCED INDEPENDENTLY FROM CHANGES IN EITHER TAIL-SKIN TEMPERATURE OR CORE TEMPERATURE

It has recently been hypothesized that tail-skin temperature may exert a profound influence on the latency of the tail-flick response to rad iant heat. Several recent reports in the literature urge investigators to assess tail temperatures concurrently when using the tail-flick te st and to adjust the tail-flick latency by a coefficient when a change in tail temperature is detected. Because much of the supporting evide nce of this hypothesis was strictly correlational, the purpose of the present study was to determine whether tail-skin temperature is an imp ortant factor contributing to the latency of the tail-flick response t o radiant heat. The effects of a series of pharmacological and non-pha rmacological manipulations on tail-skin temperature and response laten cies were assessed using either a low-intensity or high-intensity tail -flick stimulus. In addition, colonic temperature was evaluated. None of the drug treatments yielded a significant correlation between tail temperature and tail-flick latency. Of the seven drugs tested, only me camylamine produced a consistent change in tail-skin temperature. Alth ough mecamylamine significantly elevated tail temperature by more than 2-degrees-C, it failed to alter response latencies. Similarly tail su bmergence into 5-degrees-C water for 10 sec led to profound decreases in tail temperature ranging from -6.5 to -7.6-degrees-C while producin g only minimal increases in tail-flick latency. Conversely, submerging the tail in 38-degrees-C water or placing the animals over a heating pad maintained at 38-degrees-C increased tail temperatures at least 2- degrees-C without affecting response latencies. Inverse correlations w ere found between tail-flick latency and colonic temperature after mor phine, DELTA9-tetrahydrocannabinal (DELTA9-THC), and nicotine administ ration; however, these relationships do not appear to be causal. Sodiu m barbital produced far more hypothermia than any other agent, but did not produce any antinociception. Moreover, placing subjects in heated cages increased tail-skin temperature between 2 and 4-degrees-C and b locked the hypothermic effects of morphine and DELTA9-THC without redu cing the antinociceptive potencies of these agents. These findings ind icate that tail-skin and core temperatures have a negligible influence on the tail-flick response. We conclude that monitoring tail-skin or core temperatures when employing the tail-flick test is unnecessary an d altering tail-flick latencies to account for changes in tail tempera ture is unwarranted.