Pain is a protective evolutionary function that involves “unpleasant sensory and emotional experiences associated with, or resembling that associated with, actual or potential tissue damage” . Acute pain is an adaptive and essential survival behavior. Chronic pain is a pathological condition that poses a significant clinical, economic and social burden . Chronic pain is the most common clinical complaint in the United States affecting ∼10–20% of the U.S. population with an estimated annual cost of $600 billion, the most significant economic cost of any disease to-date . Neuropathic pain is defined as pain that is “initiated or caused by a primary lesion or dysfunction in the nervous system” . Neuropathic pain can be divided into either having peripheral origin or central origin and can be further divided into acute or chronic pain, the latter defined as pain lasting for longer than 3 months .Pain is a subjective sensory experience that cannot be directly measured nor quantified. Although pain is subjective and influenced by many physiological and psychological factors, measuring biomarkers of neuropathic pain provides an opportunity to identify objective markers of peripheral nerve damage and other pathology contributing to neuropathic pain. If used in combination, biomarkers related to pain mechanisms offer the possibility to develop objective painrelated indicators that may improve diagnosis, treatment, and understanding of pain pathophysiology . The pursuit of pain biomarkers has followed two largely separate general directions: physiological vs. brain neuroimaging. Physiological pain biomarkers research has followed multiple lines of investigation including genetic, vesicular micro-RNA, metabolic/molecular, and stress markers. Neuroimaging biomarker research in neuropathic pain research was initially motivated by research into brain areas activated by painful stimuli and that vary with pain severity . Brain activity that occurs in response to pain can also be observed in the absence of pain,ebb and flow tray which has led to conflicting evidence regarding brain activity related to pain.
Thus, some researchers are developing biomarkers based on the mechanisms underlying pain and pain perception and biomarkers that may predict response to medication and pain treatments allowing for prediction of personalized treatment responses . Toward the goal of identifying composite biomarkers for investigating neuropathic pain mechanisms and improving diagnosis and treatment response, we present a review of non-imaging and imaging pain biomarkers related to various neuropathic pain mechanisms, including opiate, inflammation, endocannabinoid mechanisms. In this review, we review mechanisms for neuropathic pain in general, but we focus on pain biomarkers for different types of peripheral neuropathies. Although various reviews of pain biomarkers exist, we focus on creating composite biomarkers through machine learning approaches that can most accurately identify people with neuropathic pain. Blocking opioidergic transmission reduces dopamine release in the nucleus accumbens that accompanies pain relief . The endogenous opioid system consists of four opioid peptide families: β-endorphin, enkephalins, dynorphins, Abbreviations: 2-AG, 2-arachidonoylglycerol; AEA, anandamide; CBR, G protein-coupled ECB receptor; CSF, cerebral spinal fluid; CRPS, complex regional pain syndrome; CRP, c reactive protein; DMN, default mode network; DHEA, dehydroepiandrosterone; ECB, endocannabinoid; FAAH, fatty acid amide hydrolase; MAGL, cytosolic monoacylglycerol lipase; MPF, medial prefrontal cortex; NA, nucleus accumbens; QST, quantitative sensory testing; SFN, small fiber neuropathy; sICAM-1, soluble intercellular adhesion molecule-1; TNF, tumor necrosis factor; WBC, white blood cell. and nociceptin/orphanin and 4 families of receptors: mu, delta, kappa, and nociceptin . Opioid receptors are expressed by central and peripheral neurons, by neuroendocrine , immune, and ectodermal cells .
All opioid receptor types mediate analgesia but have differing side effects, mostly due to their variable regional expression and functional activity in different parts of central and peripheral organ systems. Endogenous opioids are particularly concentrated in circuits involved in pain modulation . Beta-endorphin levels in the CSF, blood and saliva have been investigated as possible pain biomarkers. Plasma Beta-endorphin has been used to investigate age responses to experimental pain . Patients with chronic neuropathic pain due to trauma or surgery have been shown to have lower levels of Beta endorphin in the CSF . Plasma and CSF Beta-endorphin have been investigated in patients with trigeminal neuralgia . Interestingly, Beta-endorphin in peripheral blood was related to levels in CSF; furthermore, the levels of Beta-endorphin were inversely correlated with the severity of pain symptoms . While chronic low back pain typically involves non-neuropathic pain mechanisms, it is interesting that plasma Beta-endorphin levels have been shown to be a promising biomarker for chronic back pain . In other non-neuropathic pain conditions, mu opioid receptors expressed on immune B cells was found to be a biomarker for chronic pain in fibromyalgia and osteoarthritis. In this study, the percentage of mu opioid receptors positive B cells was statistically lower in patients with moderate to severe pain than in pain-free subjects or mild pain subjects . In a heterogenous group of patients with pain, a composite biomarker was identified that uses emergent properties in genetics to separate patients with pain requiring extremely high opioid doses from controls . Negative studies for opiate mechanism pain biomarkers have shown that salivary Beta-endorphin is not a biomarker for neuropathic chronic pain propensity . Functional brain imaging performed on patients with nonneuropathic primary dysmenorrhea with mu-opioid receptor A118G polymorphism has been used to investigate pain sensitivity and opioid-analgesic treatment related to function in the descending pain modulatory system. Specifically, the functional connectivity of the descending pain modulatory system dependence upon mu-opioid receptor A118G polymorphisms was investigated. This study found that patient groups with different alleles for the A118G polymorphisms exhibited varying functional connectivity between the anterior cingulate cortex and periaqueductal gray .
Although magnetic resonance imaging provides information regarding structural and metabolic changes that provide insight into pain perception of the CNS, magnetic resonance imaging cannot image opioid function in cells in vivo at the molecular level. Such important opioid function information can be obtained through positron emission tomography and can be used to investigate pain opioid mechanisms . While the development of neuropathic pain has long been ascribed to the known contributors of central sensitization , the role of neuroinflammation regarding the initiation and maintenance of neuropathic pain has evolved tremendously over the last decade. Pro-inflammatory cytokines have been implicated in the generation of neuropathic pain states at both peripheral and central nervous system sites . Neuroinflammation of the peripheral nervous system is triggered by inciting damage to the peripheral nerves, either by trauma, metabolic disturbances , viral infection or surgical lesions leading to sprouting of new pain-sensitive fibers , excessive neuronal firing, and hypersensitization of primary afferent peripheral neurons. During a peripheral nerve injury, local cytokines recruits macrophages which secrete components of the complement cascade, coagulation factors, proteases, hydrolases, interferons, and other cytokines that ultimately facilitate degradation and phagocytosis of the pathogen and injured tissue. Peripheral neuroinflammatory mechanisms affect the damaged neuron and neighboring afferent neurons sharing the same innervation territory . Peripheral nerve injury causes neuroinflammation in the spinal cord . The neuroinflammation is triggered by hyperactivity of the injured primary afferent peripheral sensory neuron which increases neurotransmitters and neuromodulators, causing hyperactivity of postsynaptic nociceptive neuronal hyperactivity as well as the release of several inflammatory activators. A result of this lumbar spinal inflammation process is disruption of the blood-spinal cord barrier leading to increased permeability, which then leads to infiltration of immune cells such as T lymphocytes, macrophages, mast cells,4×8 flood tray and neutrophils from the periphery into the spinal cord and dorsal root ganglion . These mechanisms contribute to further release of inflammatory mediators which contribute to alterations in post-synaptic receptors. This neurotransmitter increase leads to hyperactivity of post-synaptic nociceptive neurons in the spinal cord and altered signaling up to the thalamus and cortex that may contribute to central sensitization and pain hypersensitivity . Nerve injury typically involves neuro-immune interaction involving glia . Glia are known to provide functional microenvironment modulating neuronal signal transduction, synaptic pruning, and neuroplasticity that contributes to central sensitization.
Concentrations of CSF proinflammatory cytokines are increased in multiple neuropathic pain states . Most studies of pain syndromes have found elevations of proinflammatory and anti-inflammatory cytokines in painful conditions compared with healthy controls; furthermore, frequently higher levels of proinflammatory markers are associated with greater pain . Other markers such as soluble intercellular adhesion molecule- 1 , for example, have also been demonstrated to correlate with pain. sICAM-1 measured in serum correlates with patients’ self-reported pain levels in various pain conditions , distinguishing these patients from those with no or mild pain .Cytokines have also been demonstrated to be potent mediators of pain in peripheral neuropathy. In one peripheral neuropathy study, gene expression of pro- and anti-inflammatory cytokines was shown to be increased in patients compared to controls . Another study found neuropathic pain group was found to have higher serum levels of several markers including CReactive Protein and Tumor Necrosis Factor – α compared with two control groups. Furthermore, patients with painful neuropathy had higher sICAM-1 and CRP levels when compared to painless neuropathy . A meta-analysis comprehensively assessed the relationship between serum TNF- α levels and diabetic peripheral neuropathy in patients with type 2 diabetes, demonstrating increased serum TNF-α levels in patients with diabetic neuropathy compared to type 2 diabetic patients without neuropathy and compared with controls . Il- 17 is significantly upregulated in rat models of neuropathic pain, and mRNA expression levels of IL-1β and IL-6 are significantly enhanced in the spinal dorsal horn compared with controls . Moreover, functional recovery from neuropathic pain following a peripheral nerve injury relies on down regulation of IL-1 β and TNF- α responses .Another key pro-inflammatory neuropeptide, Substance P, is known to initiate biological inflammatory effects . In painful trigeminal neuralgia, levels of Substance P and other neuropeptides in the cerebrospinal fluid and blood of patients were found to have higher levels than that of controls; furthermore, blood levels of these markers correlated with those of the CSF . Another study investigating nonneuropathic experimental pain found altered substance P levels and dynamics when comparing older and younger adults . Compromised BBB can be identified with gadolinium-enhanced MRI as is seen in the setting of white matter lesions in multiple sclerosis. CNS-infiltration of circulating immune cells, such as monocyte infiltration into brain parenchyma, can be tracked with iron oxide nanoparticles and MRI. Pathological consequences of neuroinflammation such as apoptosis can be imaged with PET [99mTc] Annexin V or with iron accumulation with using MRI T2∗ relaxometry. These imaging techniques can be used to image human neuroinflammation which have potential to impact patient care in the foreseeable future . Integrated positron emission tomography-magnetic resonance imaging and the radioligand 11C-PBR28 for the translocator protein can be used to image regional brain volumes with glial activation. Given the putative role of activated glia in the establishment and or maintenance of persistent pain, pathophysiology, and management of a variety of persistent pain conditions the results from this technique are important to consider when considering imaging techniques for measuring CNS inflammatory effects of pain . There are three classifications of cannabinoids: phytocannabinoids , endocannabinoids , and synthetic cannabinoids. Similar to the opioid system, versions of the endocannabinoid system have been found in the vast majority of species with a nervous system . In particular, the ECB ligands 2-AG and AEA have been found throughout the animal kingdom . The ECB system regulates physiology across most organ systems and operates independently and interacts with the inflammatory system, the opiate system, the Vaniloid system, and with nuclear transcription factors . The ECB system works as a part of a negative feedback loop that regulates neurotransmitter and neuropeptide release in the nervous system. Endocannabinoid ligands are generated on-demand in response to high levels of activity and produce short-term inhibitory effects via their actions as retrograde transmitters at presynaptic inhibitory G protein-coupled receptors . The two most prevalent endocannabinoid ligands that bind endocannabinoid receptors are anandamide and 2- arachidonoylglycerol . The 2-AG basal level is ∼1,000 times greater than AEA in the brain . The enzyme acylphosphatidylethanolamine-phospholipase D is involved in the formation of AEA, and the enzyme diacylglycerol lipase is involved in 2-AG formation . Once synthesized and released, endocannabinoids are removed from the extracellular space through an endocannabinoid membrane transporter, subsequently AEA is hydrolyzed by the enzyme fatty acid amide hydrolase , and 2-AG is degraded by cytosolic monoacylglycerol lipase .