ATP receptors of microglia involved in pain

Microglia, activated when physiological homeostasis is threatened, play an important role as immune cells in the CNS. Activated microglia show a progressive series of changes in morphology, gene expression, function and number, and produce and release various chemical mediators, including proinflammatory cytokines that can produce immunological actions and modify neuronal function. Recently, accumulating evidence has indicated an important role for ATP receptors of activated microglia in neuropathic pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, cancer, diabetes or infection. The expression of the P2X₄ receptor, a subtype of ATP receptors, is enhanced in spinal microglia in a peripheral nerve injury model, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produces a reduction of the neuropathic pain. Several cytokines such as interleukin 6 (IL6) and tumour necrosis factor α (TNFα) in the dorsal horn are also increased after nerve lesion and have been implicated in contributing to nerve-injury pain. ATP can activate mitogen-activated protein kinase (MAPK) leading to the release of bioactive substances including cytokines from microglia. Thus, diffusible factors released from activated microglia by the stimulation of purinergic receptors may have an important role in the development of neuropathic pain.