Proinflammatory cytokines are produced predominantly by activated macrophages and are involved in the up-regulation of inflammatory reactions. IL-1β, IL-6, and TNF-α are the typical proinflammatory cytokines.
IL-1β is released primarily by monocytes and macrophages as well as by nonimmune cells, such as fibroblasts and endothelial cells, during cell injury, infection, invasion, and inflammation. Very recently, it was found that IL-1β is expressed in nociceptive DRG neurons. IL-1β expression is enhanced following crush injury to peripheral nerve and after trauma in microglia and astrocytes in the central nervous system (CNS). IL-1β can produce hyperalgesia following either intraperitoneal, intracerebroventricular or intraplantar injection. Moreover, IL-1β was found to increase the production of substance P and prostaglandin E2(PGE2) in a number of neuronal and glial cells. IL-1ra, a specific IL-1 receptor antagonist, competitively binds to the same receptor as IL-1β but does not transduce a cellular signal, thereby blocking IL-1β-mediated cellular changes. Administrations of IL-1ra and other anti-inflammatory cytokines have been demonstrated to prevent or attenuate cytokine-mediated inflammatory hyperalgesia and nerve-injury induced mechanical allodynia.
IL-6 has been shown to play a central role in the neuronal reaction to nerve injury. Suppression of IL-6R by in vivo application of anti-IL-6R antibodies led to reduced regenerative effects. IL-6 is also involved in microglial and astrocytic activation as well as in regulation of neuronal neuropeptides expression. There is evidence that IL-6 contributes to the development of neuropathic pain behavior following a peripheral nerve injury. For example, sciatic cryoneurolysis, a sympathetically-independent model of neuropathic pain involving repeatedly freezing and thawing a section of the sciatic nerve, results in increased IL-6 immunoreactivity in the spinal cord. In addition, intrathecal infusion of IL-6 induces tactile allodynia and thermal hyperalgesia in intact and nerve-injured rats, respectively.
TNF-α, also known as cachectin, is another inflammatory cytokine that plays a well-established, key role in some pain models. TNF acts on several different signaling pathways through two cell surface receptors, TNFR1 and TNFR2 to regulate apoptotic pathways, NF-kB activation of inflammation, and activate stress-activated protein kinases (SAPKs). TNF-α receptors are present in both neurons and glia. TNF-α has been shown to play important roles in both inflammatory and neuropathic hyperalgesia. Intraplantar injection of complete Freund's adjuvant in adult rats resulted in significant elevation in the levels of TNF-α, IL-1β, and nerve growth factor (NGF) in the inflamed paw. A single injection of anti-TNF-α antiserum before the CFA significantly delayed the onset of the resultant inflammatory hyperalgesia and reduced IL-1β but not NGF levels. Intraplantar injection of TNF-α also produces mechanical and thermal hyperalgesia. It has been found that TNF-α injected into nerves induces Wallerian degeneration and generates the transient display of behaviors and endoneurial pathologies found in experimentally painful nerve injury. TNF binding protein (TNF-BP), an inhibitor of TNF, is a soluble form of a transmembrane TNF-receptor. When TNF-BP is administered systemically, the hyperalgesia normally observed after lipopolysaccharide (LPS) administration is completely eliminated. Intrathecal administration of a combination of TNF-BP and IL-1 antagonist attenuated mechanical allodynia in rats with L5 spinal nerve transection.
A variety of cytokines are known to induce chemotaxis. One particular subgroup of structurally related cytokines is known as chemokines. The term chemotactic cytokines (CHEMOtactic CytoKINES) usually refers to this. These factors represent a family of low molecular weight secreted proteins that primarily function in the activation and migration of leukocytes although some of them also possess a variety of other functions. Chemokines have conserved cysteine residues that allow them to be assigned to four groups: C-C chemokines (RANTES, monocyte chemoattractant protein or MCP-1, monocyte inflammatory protein or MIP-1α, and MIP-1β), C-X-C chemokines (IL-8 also called growth related oncogene or GRO/KC), C chemokines (lymphotactin), and CXXXC chemokines (fractalkine).
Various chemokines including MIP-1α, MCP-1 and GRO/KC are up-regulated not only in models of neuroinflammatory and demylinating diseases, but also in various forms of CNS trauma and in injured peripheral nerve. Receptors for MCP-1, MIP-1α and GRO/KC are expressed on DRG neurons. Interestingly, mice lacking the CCR2 receptor completely fail to develop mechanical allodynia in the partial sciatic injury model although pain sensitivity in uninjured animals is normal. In the same study, normal mice showed a sustained upregulation of the receptors in both DRG and peripheral nerve after the injury. This suggests that the chemokines, including MCP-1 in particular, play very key roles in neuropathic pain as well as in neuroinflammatory conditions.