Effect of intrathecal and subepineural capsaicin on thermal sensitivity and autotomy in rats

In 79 Sprague-Dawley rats, we determined the effect of either intrathecal or subepineural capsaicin injection on: (1) latency of withdrawal of the hind foot to a nociceptive thermal stimulus (50 ± 1°C hot plate) and (2) the onset and severity of putative behavioral evidence of chronic pain in the rat (autotomy) which commonly appears following sciatic nerve section. Capsaicin (50 μg) was suspended in 5 μl of vehicle (10% Tween-80 in 0.9% saline) then injected either intrathecally at the level of the L4-5 vertebral interspace or subepineurally in the sciatic nerve at the level of the midfemur. Subepineural capsaicin consistently and efficiently produced thermal analgesia in the rat, while intrathecal capsaicin had no significant analgesic effect. In chronically denervated rats, both subepineural and intrathecal capsaicin decreased the latency to onset of first autotomy, and intrathecal capsaicin increased the severity of this behavior significantly. These data are consistent with the hypothesis that autotomy is the rat's response to abnormal sensations perceived in the denervated hind limb. Deafferentation of dorsal horn neurons appears to be of paramount importance in the production of autotomy while the relevance of peripherally originating spontaneous neuroma discharges to autotomy behavior is questioned.