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Thecytokine interleukin-1β(IL-1β) released by spinal microglia in enhanced response states contributes significantly to neuronalmechanisms of chronic pain. Here we examine the involvement of the purinergic P2X7 receptor in the release of IL-1β following activation of Toll-like receptor-4 (TLR4) in the dorsal horn, which is associated with nociceptive behavior and microglial activation.Weobserved that lipopolysaccharide (LPS)-induced release of IL-1β was prevented by pharmacological inhibition of the P2X7 receptor with A-438079, and was absent in spinal cord slices taken from P2X7 knock-out mice. Application of ATP did not evoke release of IL-1β from the dorsal horn unless preceded by an LPS priming stimulus,andthis releasewasdependentonP2X7receptor activation. Extensive phosphorylation ofp38MAPKin microglial cells in the dorsal horn was found to correlate with IL-1β secretion following both LPS and ATP. In behavioral studies, intrathecal injection of LPS in the lumbar spinal cord produced mechanical hyperalgesia in rat hindpaws, which was attenuated by concomitant injections of either a nonspecific (oxidized ATP) or a specific (A-438079) P2X7 antagonist. In addition, LPS-induced hypersensitivity was observed in wild-type but not P2X7 knock-out mice. These data suggest a critical role for the P2X7 receptor in the enhanced nociceptive transmission associated with microglial activation and secretion of IL-1β in the dorsal horn. We suggest that CNS-penetrant P2X7 receptor antagonists, by targeting microglia in pain-enhanced response states, may be beneficial for the treatment of persistent pain. [ABSTRACT FROM AUTHOR]

The development of new therapeutic approaches to the treatment of painful neuropathies requires a better understanding of the mechanisms that underlie chronic pain syndromes. There is increasing evidence that immune competent cells such as microglia contribute to the development of chronic pain states. Chemokines play a pivotal role in mediating neuronal-microglial communication which leads to increased nociception. Fractalkine (FKN) is structurally unique amongst the family of chemokines and their receptors and expressed both in the central nervous system and peripheral nerves, as well as in endothelial cells and lymphocytes. Signalling via the CX3CR1 receptor, FKN is able to mediate critical physiological functions necessary for immune regulation. In its soluble forms FKN mediates chemotaxis of immune cells whilst membrane bound FKN acts as an adhesion molecule mediating leukocyte capture and infiltration. As FKN/CX3CR1 is such a key signalling pair for homeostatic functions it is not surprising that it is implicated in a large number of diseases in which imbalance of the immune system is implied. Here we review the evidence that FKN/CX3CR1 mediates neuron-microglial communication in chronic pain states and is therefore key in the development of neuropathic pain. In addition, the contribution of FKN/CX3CR1 signalling to the pathogenesis and progression of two chronic inflammatory conditions, atherosclerosis and rheumatoid arthritis, are discussed.