These results suggest that expression of mTNF-alpha after injury is related to development of pain, and that reverse signaling through mTNF-alpha by sTNFR at that level reduces cellular markers of inflammatory response and pain-related behavior.

To dissect the molecular basis of the neuroimmune response associated with the genesis of inflammatory (nociceptive) pain, we constructed a herpes simplex virus-based gene transfer vector to express the antiinflammatory cytokine interleukin-10 (IL-10), and used it to examine the effect of IL-10 expression in activated microglial cells in vitro, and in inflammatory pain in vivo. IL-10 reduced the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and decreased the expression of full-length membrane spanning tumor necrosis factor-a (mTNFa) following lipopolysaccharide stimulation of microglia in vitro. IL-10 also reduced intracellular cleavage of mTNFa and release of the soluble cleavage product sTNFa. Similar effects on TNFa expression were observed when the cells were pretreated with a p38 MAPK inhibitor. In animals, injection of a dilute solution of formalin in the skin resulted in an increase in mTNFa in spinal dorsal horn, without detectable sTNFa. Local release of IL-10 achieved by gene transfer reduced the number of spontaneous flinches in the early and delayed phases of the formalin test of inflammatory pain. The effect of IL-10 on nocisponsive behavior correlated with a block in phosphorylation of p38 and reduced expression of 26 kDa mTNFa in spinal microglia. The results emphasize the key role played by membrane TNFa in the spinal neuroimmune response in pain caused by peripheral inflammation.