Effects of epidural capsaicin with different contrention on pain threshold and nerve tissue structure in rats

Objective To evaluate the effects of epidural capsaicin on pain threshold and nerve tissue structure in adult rats. Methods Twenty-four adult male Wistar rats weighing 250-300 g were used in this study. A catheter was inserted into epidural space at L5,6 according to the method of Philippe. 0.4 ml of capsaicin 0.1% (group A), 0.25% (group B) or 0.5% (group C) or 10% Tween 80 (control group) was injected via the epidural catheter. Pain threshold was measured by thermal stimulation of the tail before (baseline) and on the 3rd, 7th and 14th day after epidural capsaicin. The animals were then killed. The lumbar segment (L4-6) of the spinal cord and spinal nerve roots were removed immediately for light and electron microscopic examination. Results Pain threshold was significantly higher in group A, B and C than in control group (P 0.05) . The degree of edema and damage to cell organelle were significantly greater in group B and C than in group A. Conclusion Epidural capsaicin can increase pain threshold but high doses of capsaicin are detrimental to spinal nerve.