THE COMPLEX INTERACTION OF INFLAMMATORY MEDIATORS 

Interleukin-1 beta
Interleukin-6
Interleukin – 8
Interleukin –10
Prostaglandins
Tumor necrosis factor alpha
Nitric Oxide
Substance P (sP)
Gelatinase B or Matrix Metallo-Proteinase 9 (MMP-9)

We will now review the main inflammatory mediators and their complex interaction in induction, enhancement and propagation of persistent pain. We will also review some of the natural anti-inflammatory mediators produced by the body to cool down inflammation and the inflammatory response.

Interleukin-1 beta is a potent pain-generating mediator. Two pain producing pathways have been identified:  Inflammatory stimuli or injury to soft tissue induces the production of mediator Bradykinin, which stimulates the release of mediator Tumor necrosis factor alpha. The TNF-alpha induces production of (i) Interleukin -6 and Interleukin -1-Beta which stimulate the production of cyclooxygenase enzyme products, and (ii) Inflammatory mediator Interleukin -8, which stimulates production of sympathomimetics (sympathetic hyperalgesia) [11] [7].   Effects of Interleukin-1 beta include:

. Interleukin-1 beta stimulates inflammatory mediators prostaglandin E₂ (PGE₂), cyclooxygenase-2 (COX-2)  and matrix metalloproteases (MMPs) production [12] ^{, [13]} Interleukin-1 is a significant catalyst in cartilage damage. It induces the loss of proteoglycans, prevents the formation of the cartilage matrix [14] and prevents the proper maintenance of cartilage. Interleukin –1 is a significant catalyst in bone resorption^.  It stimulates osteoclasts cells involved in the resorption and removal of bone [15] [16] [17]

Interleukin-6

This is another potent pain-generating inflammatory mediator.  A significant amount of InterLeukin-6 is produced in the rat spinal cord following peripheral nerve injury that results in pain behaviors suggestive of neuropathic pain. These spinal IL-6 levels correlated directly with the mechanical allodynia intensity following nerve injury [18] . 

Interleukin – 8

This is a pain-generating inflammatory mediator.  In one study of patients with post herpetic neuralgia, the patients who received methylprednisolone, had interleukin-8 concentrations decrease by 50 percent, and this decrease correlated with the duration of neuralgia and with the extent of global pain relief [19] [8] (P<0.001 for both comparisons).

Interleukin –10

This is one of the natural anti-inflammatory cytokines, which also include Interleuken-1 receptor antagonist (IL-1ra), Interleukin –4, Interleukin –13 and transforming growth factor-beta1 (TGF-beta1). Interleukin-10 (IL-10) is made by immune cells called macrophages during the shut-off stage of the immune response. Interleukin-10 is a potent anti-inflammatory agent, which acts partly by decreasing the production of inflammatory cytokines interleukin-1 beta (Interleukin-1 beta), tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthetase (iNOS), by injured nerves and activated white blood cells, thus decreasing the amount of spinal cord and peripheral nerve damage [20] [21] . In rats with spinal cord injury (SCI), a single injection of IL-10 within half an hour resulted in 49% less spinal cord tissue loss than in untreated rats. The researchers observed nerve fibers traveling straight through the spared tissue regions, across the zone of injury. They also reported a decrease in the inflammatory mediator TNF-alpha, which rises significantly after SCI.

Prostaglandins

These are inflammatory mediators that are released during allergic and inflammatory processes. Phospholipase A2 enzyme, which is present in cell membranes, is stimulated or activated by tissue injury or microbial products. Activation of phospholipase A2 causes the release of arachidonic acid from the cell membrane phospholipid. From here there are two reaction pathways that are catalyzed by the enzymes cyclooxygenase (COX) and lipoxygenase (LOX). These two enzyme pathways compete with one another.  The cyclooxygenase enzyme pathway results in the formation of inflammatory mediator prostaglandins and thromboxane. The lipoxygenase enzyme pathway results in the formation of inflammatory mediator leukotriene. Because they are lipid soluble these mediators can easily pass out through cell membranes.

In the cyclooxygenase pathway, the prostaglandins D, E and F plus thromboxane and prostacyclin are made. Thromboxanes are made in platelets and cause constriction of vascular smooth muscle and platelet aggregation. Prostacyclins, produced by blood vessel walls, are antagonistic to thromboxanes as they inhibit platelet aggregation.

Prostaglandins have diverse actions dependent on cell type but are known to generally cause smooth muscle contraction. They are very potent but are inactivated rapidly in the systemic circulation. Leukotrienes are made in leukocytes and macrophages via the lipoxygenase pathway. They are potent constrictors of the bronchial airways. They are also important in inflammation and hypersensitivity reactions as they increase vascular permeability and attract leukocytes.

Tumor necrosis factor alpha

This inflammatory mediator is released by macrophages as well as nerve cells. Very importantly, TNF-alpha is released from injured or herniated disks. During an inflammatory response, nerve cells communicate with each other by releasing neuro-transmitter glutamate. This process follows activation of a nerve cell receptor called CXCR4 by the inflammatory mediator stromal cell-derived factor 1 (SDF-1). An extraordinary feature of the nerve cell communication is the rapid release of inflammatory mediator tumor necrosis factor-alpha (TNF alpha). Subsequent to release of TNF-alpha, there is an increase in the formation of inflammatory mediator prostaglandin. Excessive prostaglandin release results in an increased production of neurotransmitter glutamate and an increase in nerve cell communication resulting in a vicious cycle of inflammation.  There is excitation of pain receptors and stimulation of the specialized nerves e.g. C fibers and A-delta fibers that carry pain impulses to the spinal cord and brain. 

Studies have established that herniated disk tissue (nucleus pulposus) produces a profound inflammatory reaction with release of inflammatory chemical mediators. Disk tissue applied to nerves may induce a characteristic nerve sheath injury [22] [9](24, 38, 41, 42), [23] [10] [24] [11] increased blood vessel permeability (9), and blood coagulation (24, 36). The primary inflammatory mediator implicated in this nerve injury is Tumor necrosis factor-alpha but other mediators including Interleukin 1-beta may also participate in the inflammatory reaction. Recent studies have also shown that that local application of nucleus pulposus may induce pain-related behavior in rats, particularly hypersensitivity to heat and other features of a neuropathic pain syndrome (23, 40).

Nitric Oxide

This inflammatory mediator is released by macrophages. Other mediators of inflammation such as reactive oxygen products and cytokines, considerably contribute to inflammation and inflammatory pain [19, 20] by causing an increased local production of Cyclooxygenase enzyme. The cyclooxygenase enzyme pathway results in the formation of inflammatory mediator prostaglandins and thromboxane. Concurrently to the increased production of the Cyclooxygenase–2 (COX-2) gene, there is increased production of the gene for the enzyme inducible nitric oxide synthetase (iNOS), leading to increased levels of nitric oxide (NO) in inflamed tissues [21]. In these tissues, NO has been shown to contribute to swelling, hyperalgesia (heightened reaction to pain) and pain [20, 22]. NO localized in high amounts in inflamed tissues has been shown to induce pain locally [59, 60] and enhances central as well as peripheral stimuli [61]. Inflammatory NO is thought to be synthesized by the inducible isoform of nitric oxide synthetase (iNOS).

Substance P (sP)

An important early event in the induction of neuropathic pain states is the release of Substance P from injured nerves which then increases local Tumor Necrosis Factor alpha (TNF-alpha) production. Substance P and TNF-alpha then attract and activate immune monocytes and macrophages, and can activate macrophages directly. Substance P effects are selective and Substance P does not stimulate production of Interleukin-1, Interleukin -3, or Interleukin -6.. Substance P and the associated increased production of TNF-alpha has been shown to be critically involved in the pathogenesis of neuropathic pain states. TNF-alpha protein and message are then further increased by activated immune macrophages recruited to the injury site several days after the primary injury. TNF-alpha can evoke spontaneous electrical activity in sensory C and A-delta nerve fibers that results in low-grade pain signal input contributing to central sensitization. Inhibition of macrophage recruitment to the nerve injury site, or pharmacologic interference with TNF-alpha production has been shown to reduce both the neuropathologic and behavioral manifestations of neuropathic pain states [25] [12].

Gelatinase B or Matrix Metallo-Proteinase 9 (MMP-9)

This enzyme is one of a group of metalloproteinases (which includes collagenase and stromelysin) that are involved in connective tissue breakdown. Normal cells produce MMP-9 in an inactive, or latent form. The enzyme is activated by inflammatory mediators such as TNF-alpha and interleukin-1 that are released by cells of the immune system (mainly neutrophils but also macrophages and lymphocytes) and transformed cells [26] [27] . MMP-9 helps these cells migrate through the blood vessels to inflammatory sites or to metastatic sites. Activated, MMP-9 can also degrade collagen in the extra cellular matrix of articular bone and cartilage and is associated with joint inflammation and bony erosions [28] . Consequently, MMP-9 plays a major role in acute and chronic inflammation, in cardiovascular and skin pathologies as well as in cancer metastasis. MMP-9 can also degrade a protein called beta amyloid. Normal cells produce MMP-9 in an inactive, or latent form, converting it to active enzyme when it is needed. But when normal brain cells producing MMP-9 fail to activate the enzyme, insoluble amyloid-b could accumulate in brain tissue. Previous research has shown that the undegraded form of amyloid-beta accumulates in the brain as senile "plaques" that signal the presence of Alzheimer's disease [29] .