Epidural capsaicin produces prolonged segmental analgesia in the rat

The effect of epidural capsaicin injections on the thermal nociceptive threshold of unrestrained freely moving adult rats was examined. Capsaicin solution (1% 0.05 ml, 0.1 ml) was injected in a single dose or in two consecutive doses through an indwelling lumbar epidural catheter. Effects were compared with 1 ml 1% capsaicin injected intraperitoneally. Twelve rats served as sham-treated and vehicle controls. Nociceptive thermal stimuli were brief pulses of CO2 laser radiation directed at three body areas, hind limb, forelimb, and pinna. Capsaicin caused prolonged, segmental thermal analgesia. Maximal nociceptive threshold values in the hind limbs, attained within 24 h of epidural injection, were 2.5 (P less than or equal to 0.006) and 5.3 (P less than or equal to 0.0005) time control values for the 0.05-ml and 0.1-ml doses, respectively. Response thresholds in the forelimbs and pinna were unaffected. Two-stage epidural injection of capsaicin led to a roughly twofold elevation of threshold, as well as prolongation of the analgesia to about 14 days. Intraperitoneal injection of capsaicin resulted in elevation of nociceptive threshold which included all body areas tested. These results indicate that epidural application of capsaicin at the lumbar spinal level produced a profound and long-lasting segmental analgesia.