Microglial/macrophage GRK2 determines duration of peripheral IL-1β-induced hyperalgesia: Contribution of spinal cord CX3CR1, p38 and IL-1 signaling

Chronic pain associated with inflammation is a major clinical problem, but the underlying mechanisms are incompletely understood. Recently, we reported that GRK2(+/-) mice with a similar to 50% reduction of GRK2 develop prolonged hyperalgesia following a single intraplantar injection of the pro-inflammatory cytokine interleukin-1 beta (IL-1 beta). Here we show that spinal microglia/macrophage GRK2 is reduced during chronic inflammation-induced hyperalgesia. Next, we applied CRE-Lox technology to create mice with low GRK2 in microglia/macrophages/granulocytes (LysM-GRK2(f/+)), or sensory neurons or astrocytes. Only mice deficient in microglial/macrophage/granulocyte GRK2 display prolonged IL-1 beta-induced hyperalgesia that lasts up to 8 days. Two days after intraplantar IL-1 beta, increased microglial/macrophage activity occurs in the lumbar but not thoracic spinal cord of GRK2-deficient mice. Intrathecal pre-treatment with minocycline, an inhibitor of microglia/macrophage activation, accelerates resolution of hyperalgesia independent of genotype and prevents transition to chronic hyperalgesia in GRK2(+/-) mice. Ongoing hyperalgesia in GRK2(+/-) mice is reversed by minocycline administration at days 1 and 2 after IL-1 beta injection. Similarly, IL-1 beta-induced hyperalgesia in LysM-GRK2(f/+) mice is attenuated by intrathecal administration of anti-CX3CR1 to abrogate fractalkine signaling, the p38 inhibitor SB239063 and the IL-1 antagonist IL-1ra. These data establish that chronic inflammatory hyperalgesia is associated with reduced GRK2 in microglia/macrophages and that low GRK2 in these cells is sufficient to markedly prolong hyperalgesia after a single intraplantar injection of IL-1 beta. Ongoing hyperalgesia is maintained by spinal microglial/macrophage activity, fractalkine signaling, p38 activation and IL-1 signaling. We propose that chronic inflammation decreases spinal microglial/macrophage GRK2, which prevents silencing of microglia/macrophage activity and thereby contributes to prolonged hyperalgesia. (C) 2010 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.