Diseases of joints are among the most frequent causes of chronic pain. In the course of joint diseases the peripheral and the central nociceptive system develop persistent hyperexcitability (peripheral and central sensitization). This review addresses the mechanisms of spinal sensitization evoked by arthritis. Electrophysiological recordings in anaesthetized rats from spinal cord neurons with knee input in a model of acute arthritis showed that acute spinal sensitization is dependent on spinal glutamate receptors (AMPA, NMDA and metabotropic glutamate receptors) and supported by spinal actions of neuropeptides such as neurokinins and CGRP, by prostaglandins, and by proinflammatory cytokines. In several chronic arthritis models (including immune-mediated arthritis and osteoarthritis) spinal glia activation was observed to be coincident with behavioral mechanical hyperalgesia which was attenuated or prevented by intrathecal application of minocycline, fluorocitrate and pentoxyfylline. Some studies identified specific pathways of micro- and astroglia activation such as the purinoceptor- (P X -) cathepsin S/CX CR1 pathway, the mobility group box-1 protein (HMGB1) and toll-like receptor 4 (TLR4) activation, spinal NF魏B/p65 activation and others. The spinal cytokines TNF, interleukin-6, interleukin-1尾 and others form a functional spinal network characterized by an interaction between neurons and glia cells which is required for spinal sensitization. Neutralization of spinal cytokines by intrathecal interventions attenuates mechanical hyperalgesia. This effect may in part result from local suppression of spinal sensitization and in part from efferent effects which attenuate the inflammatory process in the joint. In summary, arthritis evokes significant spinal hyperexcitability which is likely to contribute to the phenotype of arthritis pain in patients.
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