Injury and inflammation trigger the activation of proteases from the circulation, immune cells and epithelial tissues that regulate cells by cleaving protease-activated receptors (PARs), members of a family of four G protein coupled receptors (GPCRs).
These G protein-coupled receptors of nociceptive neurons can sensitize transient receptor potential (TRP) ion channels, which amplify neurogenic inflammation and pain. Protease-activated receptor 2 (PAR2), a receptor for inflammatory proteases, is a major mediator of neurogenic inflammation and pain.
PAR2 is co-expressed with substance P and calcitonin gene-related peptide by a subpopulation of primary spinal afferent neurons that control neurogenic inflammation and pain transmission. Activation of PAR2 on sensory nerve endings evokes the local release of these neuropeptides, which stimulate extravasation of plasma proteins, infiltration of neutrophils and vasodilation (neurogenic inflammation). PAR2 activation also promotes the central release of neuropeptides that activate second order spinal neurons that transmit pain. These mechanisms contribute to painful inflammation of the intestine, pancreas and joints. Therefore, it is of considerable interest to understand the mechanisms by which PARs regulate the activity of nociceptive neurons.
Members of the TRP family, including TRPV1, TRPV4 and TRPA1 mediate neurogenic inflammation and pain, and are major down-stream targets of PAR2 . Activation of these non-selective cation channels stimulates the influx of extracellular Ca2+ ions and the release of neuropeptides in peripheral tissues and the spinal cord, which induces neurogenic inflammation and pain. During injury and inflammation, several factors are generated that can directly activate these channels. Elevated temperatures, protons and lipid mediators activate TRPV1, mechanical shear stress, osmotic stimuli and lipid mediators activate TRPV4, and products of reactive oxygen species and reactive prostaglandin metabolites activate TRPA1. However, indirect mechanisms, particularly those triggered by GPCRs, play a prominent role in TRP channel activation. Many GPCRs that induce neurogenic inflammation and pain indirectly regulate TRP channels, which mediate their pro-inflammatory and pronociceptive actions.
Reference: Protease-activated Receptor-2 (PAR2) and Transient Receptor Potential Vanilloid 4 (TRPV4) Coupling is Required for Sustained Inflammatory Signaling
I think it is of particular interest, that TRP can be activated of Reactive Oxygen Species (ROS), because oxidative stress levels are raised in chronic fatigue syndrome and are associated with clinical symptoms
Some researchers believe that ME/CFS, fibromyalgia, irritable bowel syndrome and other pain syndromes share a common biochemistry via sensitization. This review article describes TRPA1 mediated neural cross-talk induced by oxidative stress as model for some pain syndromes:
TRPV1 and TRPA1 act as a nocisensor to mediate not only an afferent signal to the dorsal horn of the spinal cord, but also an efferent signal in the periphery through secretion of inflammatory agents, such as substance P and calcitonin gene-related peptide in nociceptive sensory neurons.
Peripheral inflammation produces multiple inflammatory mediators that act on their cognate receptors to activate intracellular signal transduction pathways and thereby modify the expression and function of TRPV1 and TRPA1 (peripheral sensitization). During tissue damage and inflammation, oxidative stress, such as reactive oxygen species or reactive carbonyl species is also generated endogenously.
The highly diffusible nature might account for the actions of free radical formation far from the site of injury, thereby producing systemic pain conditions without central sensitization through neural cross-talk.
Reference: Transient receptor potential A1 receptor-mediated neural cross-talk and afferent sensitization induced by oxidative stress: implication for the pathogenesis of interstitial cystitis/bladder pain syndrome
A thorough description of ROS and TRP is provided in this article:
Role of Reactive Oxygen Species and Redox in Regulating the Function of Transient Receptor Potential Channels
And further description of peripheral sensitization influenced by chemokines/TRP activation is found in this article: Chemokines as Pain Mediators and Modulators
This knowledge opens the possibility that TRPA1 antagonist can be used against inflammatory pain:
Patent application: NOVEL TRPA1 ANTAGONISTS