Immune Cells Trigger Pain

Bouts of neuropathic pain stemming from nerve damage can be provoked by normally innocuous sensations such as the touch of clothing on the skin. Japanese researchers now report that an immune system molecule contributes to the development of this type of sometimes debilitating pain, for which there is no current treatment (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0810420106). Kazuhide Inoue and colleagues at Kyushu University, in Fukuoka, knew from prior studies that the amount of the inflammatory cytokine IFN-γ in the spinal cord increases after a nerve injury. Their new findings show that IFN-γ binds to a receptor on spinal immune cells known as microglia, thereby activating the cells. The researchers believe IFN-γ prompts the cells to increase production of compounds including Lyn tyrosine kinase and the receptor P2X₄R, which increases the release of substances such as brain-derived neurotrophic factor, a protein that normally protects neurons. These changes trigger the hyperexcitability of spinal neurons that leads to neuropathic pain. Inoue and coworkers suggest that blocking IFN-γ signaling in spinal microglia could diminish neuropathic pain while leaving intact the normal pain-signaling pathways that alert a person to a harmful stimulus, such as a hot iron.