The Role of Cytokines in the Initiation and Maintenance of Chronic Pain.

Recent advancements in the pain field have identified a central nervous system (CNS) neuroimmune response that may act as the driving force for neuronal hypersensitivity, the pathological correlate to chronic pain following peripheral nerve injury. Neuroimmune activation involves the activation of nonneuronal cells such as endothelial and glial cells, which when stimulated leads to enhanced production of a host of inflammatory mediators, such as cytokines. The central production of proinflammatory cytokines, such as interleukin-1beta (IL-1beta), IL-6 and tumor necrosis factor have been found to play a key role in the propagation of persistent pain states. In addition, chemotactic cytokines, chemokines, have also been recently identified in the CNS neuroimmune cascade that ensues after injury to a peripheral nerve. The extravasation of leukocytes from the blood to the site of perceived injury is defined as the neuroinflammatory aspect of this cascade. Chemokines directly control this leukocyte transmigration process. They are synthesized at the site of injury and establish a concentration gradient through which immune cells migrate. Recent studies have demonstrated leukocyte trafficking into the CNS following peripheral nerve or lumbar nerve root injury. With the use of selective cytokine inhibitors and neutralizing antibodies, tactile and thermal hypersensitivity is attenuated in animal models of neuropathy. A further understanding of the role of nonneuronal cells, the physiological mechanisms of CNS cytokines and chemokines, and their relevance to neuro- immune activation and neuroinflammatory processes may lead to the development of novel pharmacological agents for the treatment and prevention of chronic pain. (c) 2002 Prous Science. All rights reserved.