THE PLANTAR TEST APPARATUS (UGO-BASILE BIOLOGICAL APPARATUS), A CONTROLLED INFRARED NOXIOUS RADIANT-HEAT STIMULUS FOR PRECISE WITHDRAWAL LATENCY MEASUREMENT IN THE RAT, AS A TOOL FOR HUMANS

In the present study, we precisely and automatically measured the with drawal latency to noxious radiant heat application in unrestrained mal e rats and in human subjects of both sexes, by means of the ''plantar test'' apparatus (Ugo Basile Biological Apparatus). The infrared light stimulus of this tool was applied underneath the hindpaws of rats and the middle fingers of human subjects. With one right and one left sti mulation every 10 min, we observed a decrease in latency over a 40-min testing period in rats; the latency reached a mean value of 5.08 +/- 0.25 sec after 40 min with a 36-W stimulus, which corresponded to 46.5 degrees C. In pilot experiments, also performed on rats, we showed th at the opiate morphine (10 mg/kg, i.p.) produced remarkable increases of the withdrawal latency only in ''naive'' animals (i.e., ones that h ad never experienced the plantar test stimulus) and not in animals ''h abituated'' to it. Among humans, we noted gender differences, such as less sensitivity to the infrared noxious radiant heat for women, parti cularly during the menstrual period. A difference from rats was that t here was no significant latency modification along the 40-min testing period for either women or men, with a mean latency of 5.61 +/- 0.18 s ec (47.5 degrees C) for the women and 4.39 +/- 0.10 sec (45.5 degrees C) for the men. These data confirm the reliability of the plantar test in rats, and demonstrate the possible use of an infrared source in hu man subjects as a noxious heat stimulus; the withdrawal reaction to th is stimulus is emphasized as a good index of nociception in humans.