CHRONIC PAIN AND IMMUNITY - MONONEUROPATHY ALTERS IMMUNE-RESPONSES INRATS

In order to investigate the possible relationship between chronic pain and the immune system, delayed-type hypersensitivity (DTH) and humora l immunity were assessed in Sprague-Dawley rats subjected to unilatera l peripheral mononeuropathy induced by sciatic ligation. Paw withdrawa l latency (PWL) time was measured twice during the experiment in anima ls subjected to sciatic nerve ligation or sham surgery. Sciatic nerve- ligated animals showed hyperalgesia in the leg subjected to neural lig ation when compared to the contralateral leg. No differences in PWL ti mes existed in sham-operated animals. In order to exclude possible alt erations in immune response due to the surgical procedure or to the hy peralgesia testing, a group of control animals, not subjected to surgi cal procedures or hyperalgesia testing, was also included in the exper iment. Three days post-sciatic ligation or sham surgery, both experime ntal and control animals were sensitized to keyhole limpet hemocyanin (KLH). A secondary sensitization followed 1 week after the initial imm unization. Fourteen days after the initial sensitization, KLH was inje cted into the hind foot pad and vehicle into the contralateral foot pa d in order to assess DTH. One group of rats subjected to sciatic nerve ligation was tested for DTH in the hind foot pad ipsilateral to the l igated nerve, while another group was tested in the contralateral foot pad. Twenty-four hours following foot pad injections, the thickness o f both paws was measured and animals were bled to test for anti-KLH im munoglobulins. Animals in which mononeuropathy was induced, but not sh am-operated or control animals, exhibited an enhanced DTH response to KLH. This enhanced DTH response occurred both ipsilateral and contrala teral to the ligated nerve. This increased response was blocked in bot h cases by the local anesthetic bupivicaine. Two sham-surgery groups a nd a normal control group were tested similarly. Gamma-immunoglobulin levels against KLH were significantly reduced in the hyperalgesic anim als when compared to control animals but were similar when compared to sham-operated animals. This study suggests that chronic nociception c auses significant alterations in immune function and strengthens the h ypothesis that chronic pain can influence the immune system.