The function of microglia through purinergic receptors: neuropathic pain and cytokine release

Microglia play an important role as immune cells in the central nervous system (CNS). Microglia are activated in threatened physiological homeostasis, including CNS trauma, apoptosis, ischemia, inflammation, and infection. Activated microglia show a stereotypic, 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 can also act on neurons to alter their function. Recently, a great deal of attention is focusing on the relation between activated microglia through adenosine 5'-triphosphate (ATP) receptors and neuropathic pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes, or infection. This type of pain can be so severe that even light touching can be intensely painful and it is generally resistant to currently available treatments. There is abundant evidence that extracellular ATP and microglia have an important role in neuropathic pain. The expression of P2X4 receptor, a subtype of ATP receptors, is enhanced in spinal microglia after peripheral nerve injury model, and blocking pharmacologically and suppressing molecularly P2X4 receptors produce a reduction of the neuropathic pain. Several cytokines such as interleukin-1beta (IL-1beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) in the dorsal horn are increased after nerve lesion and have been implicated in contributing to nerve-injury pain, presumably by altering synaptic transmission in the CNS, including the spinal cord. Nerve injury also leads to persistent activation of p38 mitogen-activated protein kinase (MAPK) in microglia. An inhibitor of this enzyme reverses mechanical allodynia following spinal nerve ligation (SNL). ATP is able to activate 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. Understanding the key roles of ATP receptors, including P2X4 receptors, in the microglia may lead to new strategies for the management of neuropathic pain.