[Thermal analgesia following intrathecal capsaicin administration in rats--detailed measurements of thermal analgesia over the lower body by a thermal probe].

This study was undertaken to examine the thermal pain thresholds over a wide area of the lower body surface following the intrathecal administration of capsaicin in rats. Thermal nociceptive thresholds measured under light halothane anesthesia were determined as skin twitch or escape response latencies to the heat stimulation (52.0 degrees C) by a thermal probe. Capsaicin (50 micrograms in 10 microliters) was injected through a chronically implanted catheter whose tip was near the lumbar enlargement of the spinal cord. The hot-plate test (52.0 degrees C) was also performed in all rats tested. Increase in thermal pain thresholds were consistently observed in the low back and abdominal region, while the hind paws did not always respond with prolonged skin twitch or escape latencies. Intensities of thermal analgesia at the sole of hind paws measured by hot-plate test correlated well with those by thermal probe test. In conclusion, intrathecal capsaicin definitely produced thermal analgesia, but its intensity was considerably variable in the hind paws. These results are in keeping with our previous finding that there was much variability in the effect of capsaicin assessed by the hot-plate test, indicating a possibility that capsaicin does not spread uniformly in the CSF because of its water insolubility or difficulty in penetrating to the large nerve roots innervating the hind paws.