Chronic pain and immunity: mononeuropathy alters immune responses in rats

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Abstract
In order to investigate the possible relationship between chronic pain and the immune system, delayed-type hypersensitivity (DTH) and humoral immunity were assessed in Sprague-Dawley rats subjected to unilateral peripheral mononeuropathy induced by sciatic ligation. Paw withdrawal latency (PWL) time was measured twice during the experiment in animals subjected to sciatic nerve ligation or sham surgery. Sciatic nerve-ligated animals showed hyperalgesia in the leg subjected to neural ligation when compared to the contralateral leg. No differences in PWL times existed in sham-operated animals. In order to exclude possible alterations in immune response due to the surgical procedure or to the hyperalgesia testing, a group of control animals, not subjected to surgical procedures or hyperalgesia testing, was also included in the experiment. Three days post-sciatic ligation or sham surgery, both experimental and control animals were sensitized to keyhole limpet hemocyanin (KLH). A secondary sensitization followed 1 week after the initial immunization. Fourteen days after the initial sensitization, KLH was injected into the hind foot pad and vehicle into the contralateral foot pad 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 ligated nerve, while another group was tested in the contralateral foot pad. Twenty-four hours following foot pad injections, the thickness of both paws was measured and animals were bled to test for anti-KLH immunoglobulins. Animals in which mononeuropathy was induced, but not sham-operated or control animals, exhibited an enhanced DTH response to KLH.(ABSTRACT TRUNCATED AT 250 WORDS)