TNFα Mechanically Sensitizes Masseter Muscle Afferent Fibers of Male Rats

Hakim AW, Dong XD, Svensson P, Kumar U, Cairns BE. TNF alpha mechanically sensitizes masseter muscle afferent fibers of male rats. J Neurophysiol 102: 1551-1559, 2009. First published June 24, 2009; doi:10.1152/jn.00326.2009. Behavioral evidence in rats indicates that injection of tumor necrosis factor alpha (TNF alpha) into skeletal muscle results in a prolonged mechanical sensitization without gross inflammation. To investigate whether a peripheral mechanism could underlie this effect, in the present study, TNF alpha (1 or 0.1 mu g) was injected into the rat masseter muscle to assess its effect on the excitability and mechanical threshold (MT) of muscle nociceptors as well as on inflammation. Expression of TNFR1 (P55 receptors) and TNFR2 (P75 receptors) by the masseter muscle and trigeminal ganglion neurons that innervate that muscle was determined by Western blot and immunohistochemistry, respectively. The Evans blue dye technique was used at the end of the TNF alpha experiments to assess for plasma protein extravasation. In subsequent experiments to confirm the involvement of receptor activation in TNF alpha-induced effects, P55 or P75 receptor antibody was co-injected with TNF alpha. Intramuscular injection of 1 mu g TNF alpha did not excite nociceptors but did significantly decrease MT compared with vehicle control. There was no evidence of gross inflammation 3 h after injection of TNF alpha. Co-injection of TNF alpha with P55 or P75 receptor antibodies attenuated TNF alpha-induced mechanical sensitization. P55 and P75 receptors were expressed by 29 and 62% of masseter nociceptors, respectively. These findings indicate that TNF alpha induces mechanical sensitization of masseter nociceptors that is mediated through activation of peripheral P55 and P75 receptors. These results support the hypothesis that a peripheral receptor mechanism could contribute to TNF alpha-induced noninflammatory mechanical sensitization of skeletal muscle previously reported in behaving rats.