CHARACTERIZATION OF THE FOOT WITHDRAWAL RESPONSE TO NOXIOUS RADIANT-HEAT IN THE RAT

The rat foot withdrawal response to noxious radiant heat has been used as a model of nociception that is particularly useful for measurement s of unilateral changes in nociceptive responses. The purpose of these studies was to characterize the foot withdrawal response to graded ra tes of noxious skin heating. Response latencies and both surface and s ubsurface temperatures produced by 6 different intensities of radiant heat were measured to determine whether response latency is an appropr iate measure of nociceptive threshold. With constant intensity heating , the temperature of the skin surface increased as logarithmic functio n of time, while subsurface temperature increased linearly with time. In contrast, a heating function that linearly increased the temperatur e at the skin surface increased the subsurface temperature as an expon ential function of time. These results and published reports of nocice ptive afferent recordings which used similar skin heating parameters, indicate that nociceptive foot withdrawal responses occur at about the same skin temperature as the activation of nociceptors. These results also indicate that since constant intensity heating produces linear i ncreases in the subsurface temperature, then response latency can be u sed as an accurate measure of changes in nociceptive threshold produce d by drug treatments. These observations lead to the conclusion that t he foot withdrawal response latency is a valid and useful measure of n ociceptive threshold in rodents.