Intramuscular injection of tumor necrosis factor-alpha induces muscle hyperalgesia in rats

The role of proinflammatory cytokines in neuropathic and inflammatory pain is well established. Recent studies Suggest that cytokines Such as turner necrosis factor-alpha (TNF) may also be involved in the development of muscle pain. To investigate the pathophysiology of intramuscular TNF, exogenous TNF (0.1 -10 mug), formalin (9%) or vehicle was injected into the gastrocnemius or biceps brachii muscles of rats. To quantify muscle hyperalgesia, changes in forelimb grip force or withdrawal thresholds to increasing pressure applied to the gastrocnemius muscle were measured. TNF evoked a time- and close-dependent Muscle hyperalgesia within several hours after injection that was totally reversed by systemic treatment with the non-opioid analgesic metamizol. Paw withdrawal thresholds or latencies to mechanical and thermal stimuli, respectively, were unchanged after intramuscular injection of TNF or formalin. In contrast to formalin, which induced significant Muscle tissue damage, macrophage infiltration, swelling and partial motor impairment demonstrated in rotarod tests, TNF induced neither histopathological tissue damage nor motor dysfunction. To investigate the effect of TNF and formalin on other potentially algesic mediators. muscles were analyzed for calcitonin-gene related peptide (CGRP). prostaglandin E-2 (PGE(2)) and nerve growth factor (NGF) 1 day after injection. TNF and formalin evoked intramuscular Upregulation of CGRP and NGF, whereas PGE(2) was increased exclusively after TNF injection. These findings allow us to speculate that endogenous TNF may play a role in the development Of muscle hyperalgesia. Targeting proinflammatory cytokines might be beneficial for the treatment of musculoskeletal pain syndromes. (C) 2003 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.