The Known Biology of Neuropathic Pain and Its Relevance to Pain Management

ABSTRACT: Patients with neuropathic pain are heterogeneous in pathophysiology, etiology, and clinical presentation. Signs and symptoms are determined by the nature of the injury and factors such as genetics, sex, prior injury, age, culture, and environment. Basic science has provided general information about pain etiology by studying the consequences of peripheral injury in rodent models. This is associated with the release of inflammatory cytokines, chemokines, and growth factors that sensitize sensory nerve endings, alter gene expression, promote post-translational modification of proteins, and alter ion channel function. This leads to spontaneous activity in primary afferent neurons that is crucial for the onset and persistence of pain and the release of secondary mediators such as colony-stimulating factor 1 from primary afferent terminals. These promote the release of tertiary mediators such as brain-derived neurotrophic factor and interleukin-1β from microglia and astrocytes. Tertiary mediators facilitate the transmission of nociceptive information at the spinal, thalamic, and cortical levels. For the most part, these findings have failed to identify new therapeutic approaches. More recent basic science has better mirrored the clinical situation by addressing the pathophysiology associated with specific types of injury, refinement of methodology, and attention to various contributory factors such as sex. Improved quantification of sensory profiles in each patient and their distribution into defined clusters may improve translation between basic science and clinical practice. If such quantification can be traced back to cellular and molecular aspects of pathophysiology, this may lead to personalized medicine approaches that dictate a rational therapeutic approach for each individual.


Introduction
2][3][4][5] This heterogeneity of presentation also reflects the association of neuropathic pain with a diverse set of maladies.These include peripheral nerve trauma, brain or spinal cord injury, fibromyalgia, multiple sclerosis, spinal, cortical or brain stem cord stroke, post herpetic and trigeminal neuralgia, migraine, osteoarthritis, rheumatoid arthritis, autoimmune disease, complex regional pain syndromes I and II, viral infections such as HIV and COVID-19 and neuropathies associated with diabetes, chemotherapy, and cancer itself. 6igns and symptoms include hyperalgesia, mechanical, or coldinduced allodynia, bouts of spontaneous "electric shock like" pain and sometimes the persistent burning pain of causalgia 6 Some patients experience sensory disturbances.These may involve paresthesias, described as a crawling sensation, pricking or tingling 7 or anesthesia dolorosa where the area of injury is painful yet insensitive to touch. 8Neuropathic pain is frequently intractable, relatively insensitive to the action of opioids 9,10 and may present with comorbidities such as anxiety, irritability, sleep disorders, depression, and/or sensory abnormalities. 7,11Despite intensive efforts to find new drugs and targets over the past 30 years, the urgent need to find new treatments persists. 6,9,10,12Most of the current understanding is derived from peripheral nerve injury models in rodents.In most cases, the spared nerve injury (SNI) or chronic constriction injury (CCI) models are used. 13his review will overview the current understanding of pain induced in animal models by peripheral nerve injury.In view of the recognized knowledge gap between these basic science results and the various signs and symptoms and/or pain phenotypes seen in patients, 12 a brief outline of clinical and basic science strategies that seek to bridge this gap will be presented.

Structural Remodeling of Injured Peripheral Nerves
Following SNI of rodent peripheral nerves, degeneration of the axons of low threshold non-nociceptive afferents can lead to loss of sensation.Peripheral nociceptors then sprout into territories that were previously occupied by low threshold afferents.These nociceptors are transformed to exhibit a low activation threshold so that mild tactile stimulation now produces mechanical allodynia. 29 many cases, injury also provokes the sprouting of perivascular sympathetic fibers so that they interact and excite sensory nerve terminals and DRG cell bodies. 30,31These processes are especially relevant to the etiology of complex regional pain syndrome II. 32jury-Induced Peripheral Sensitization, the Importance of Spontaneous Activity, and the Generation of Secondary Mediators Immune cell-derived primary mediators sensitize peripheral nerve endings, axons, and cell bodies of primary afferents. 14Mediators also promote plasma extravasation and increase the permeability of the blood-brain barrier 33 and the blood-nerve barrier in the periphery. 34This and the chemoattractant profiles of various mediators facilitate the recruitment of immunocompetent leucocytes and lymphocytes to the site of injury. 15,20These myeloid and lymphoid cells themselves release a host of cytokines and chemokines thereby instigating a positive feedback process in the initiation and maintenance of neuroinflammation and pain.Neuroinflammation is defined as activation of the brain's innate immune system in response to an inflammatory challenge. 35,36atellite glial cells and resident macrophages in DRG 37-39 represent yet another source of inflammatory mediators.The actions of primary mediators such as IL-1β and TNF-α on DRG neurons culminate in marked changes in genes coding for neuropeptides, cytokines, chemokines, receptors, ion channels, signal transduction molecules, and synaptic vesicle proteins. 40,41Some of these gene products also function as secondary mediators that are released and effect the transfer of information between damaged peripheral nerves and various cell types in the spinal dorsal horn. 18rimary mediators also control the expression of long noncoding RNA's 42 and microRNA's in DRG.The latter are also upregulated by nerve injury 6 and post-transcriptionally regulate the protein expression of hundreds of genes in a sequence-specific manner. 43Transfer of microRNAs between cell types may be brought about by the release and uptake of exosomes. 44mportantly, altered function of ion channels as a result of the action of primary mediators leads to increased excitability of primary afferent neurons [45][46][47][48][49] and the generation of stimulus-independent spontaneous activity.1][52][53] This is illustrated by the effectiveness of topically applied lidocaine in the clinic. 54Altered ion channel function and peripheral hyperexcitability may even be involved in spinal cord injury 55 and central post-stroke pain. 56Although Na v 1.7, K v 7.2, Ca v 2.2, Ca v 3.2, and HCN2 channels have emerged as potential therapeutic targets for drug development, with the notable exception of gabapentinoid action on voltage-gated Ca 2þ channels, 9 pharmacological manipulation of these channels has failed to identify new therapeutic approaches. 57he observation that peripherally generated pain is often not suppressed by rhizotomy 58 seems at odds with the idea that stimulus-independent spontaneous activity is required for pain maintenance.It is possible, however, that pain seen after rhizotomy is related to deafferentation.This deafferentation pain may replace that which previously resulted from ectopic primary afferent activity. 58][62][63] In general however, attempts to block the action of inflammatory mediators to limit neuropathic pain in the clinic have met with limited success. 64directional Signalling between the Nervous and Immune Systems and "Neurogenic Neuroinflammation" The relationship between immune cells and neurons is bidirectional.][70][71][72] This "neurogenic neuroinflammation" 73 is brought about by the release of neuropeptides and glutamate from primary afferents and their interaction with their cognate receptors on immune cells, astrocytes, and microglia.72,74 Actions of Secondary Mediators and Transfer of Information from the Periphery to the Spinal Cord Most secondary mediators are released from primary afferent terminals.Substances such as colony-stimulating factor 1 (CSF-1), the chemokines CCL-21, CXCL-12, and Wnt3a and Wnt5a 6,18,28,75-78 activate their cognate receptors on spinal microglia and/or astrocytes and alter their properties. Activted glia thereby detects and mount an enduring response to peripheral nerve injury. Spnal microglia are affected in male rodents 77 whereas invading macrophages and adaptive immune cells such as T-lymphocytes are involved in females.[79][80][81] CCL-21 and CXCL-12 signal to activate astrocytes.78,82 The inflammatory mediator, IFN-γ is increased in spinal cord following peripheral nerve injury 83 and this may originate from invading T-lymphocytes.
Microglial-derived BDNF increases excitatory drive to excitatory dorsal horn neurons and inhibits that to inhibitory neurons by both presynaptic and postsynaptic mechanisms. 87,93,94This altered synaptic activity is capable of increasing spontaneous action potential discharge in excitatory neurons while reducing it in inhibitory neurons. 93DNF also enhances excitatory responses to N-methyl-d aspartate (NMDA) in rat spinal cord in vitro. 101This may involve potentiation of the function of presynaptic NMDA receptors on primary afferent terminals 102 with a resultant increase in excitatory glutamatergic transmission.This may contribute to the effectiveness of the NMDA blocker, ketamine in some patients. 54eripheral nerve injury reduces expression of the potassiumchloride exporter (KCC2) selectively in nociceptive dorsal horn neurons. 90,103The resulting accumulation of intracellular Cl − normally causes outward, inhibitory GABAergic synaptic currents mediated by Cl − influx to become inward excitatory currents mediated by Cl − efflux. 90In male rats, this downregulation of KCC2 is mediated by BDNF. 104Since the loss of GABAergic inhibition enables non-noxious Aβ fiber-mediated excitatory transmission to access the superficial spinal dorsal horn, this process contributes to the establishment of allodynia. 99ong-term potentiation (LTP) of synaptic transmission, sometimes known as "wind-up", contributes to central sensitization in the dorsal horn. 105,106LTP of C-fibre responses is augmented by BDNF 107 and LTP induced by nerve stimulation is occluded by BDNF pretreatment. 108The importance of these effects was recently underlined by the observation that spinal LTP as well as microglial activation and upregulation of BDNF are inhibited by antibodies to the secondary mediator CSF-1.This strongly implicates the CSF-1microglia-BDNF axis 18 in the generation of spinal LTP. 109s already mentioned, in females, changes in sensory processing in the dorsal horn involve the invasion of macrophages and Tlymphocytes. 80,81Yet as in males, this leads to attenuation of inhibition following the collapse of the Cl − gradient. 110In females, collapse of the Cl − gradient is also brought about by the neuropeptide, CGRP 111 which is released from primary afferent terminals. 112L-1β from microglia stimulates astrocytic production of both TNF-α and IL-1β itself 113 thereby amplifying the initial IL-1β signal.Spinal actions of IL-1β involve increases in excitatory synaptic transmission.65,66 This may involve a reduction in the ability of astrocytes to take up glutamate as a result of internalization of the astrocytic glutamate transporter (EAAT2). 114TNF-α also augments excitatory transmission in the dorsal horn 18,66 as well as LTP by an action on glial cells.115 Blockade of TNF-1 receptors attenuates neuropathic pain in male rodents but not in females.116 Although anti-TNF antibodies and anti-TNF drugs such as thalidomide are available, none seem particularly useful in pain management.117 IFN-γ from invading T-lymphocytes induces both tactile allodynia and altered microglia function. Genetc ablation of the interferon receptor (IFN-γR) impairs nerve injury-evoked activation of ipsilateral microglia and tactile allodynia.118 IFN-γ also increases dorsal horn excitability 119 and facilitates synaptic transmission between primary afferent C-fibres and Lamina 1 neurons via a microglial dependent mechanism.120

Failure to Resolve Chronic Neuroinflammation
All types of injury are capable of promoting inflammation and pain 121 and the interactions of inflammatory mediators with neurons, glia, immunocompetent leucocytes and lymphocytes, and macrophages 14 promote neuroinflammation.Since identified "off signals" actively suppress the classical signs of inflammation, 121,122 pain is usually short lasting or acute.The signals that actively resolve inflammation and pain include anti-inflammatory cytokines such as IL-10 and lipid-derived specialized pro-resolving mediators (SPMs). 123,124Despite this, the neuroinflammation associated with neuropathic pain may not resolve, thereby promoting the transition from acute pain to chronic pain. 12152][53]56 Excessive neuronal activity releases glutamate and neuropeptides which interact with glia and immune cells to provoke the generation of inflammatory mediators. 73It is possible that this incessant neurogenic neuroinflammation overcomes the resolution processes that normally terminate inflammation thereby contributing to the indefinite persistence of neuropathic pain.
In addition, the injury-induced structural changes in peripheral afferent 29 and sympathetic nerves 30,31 and in higher brain structures are almost certainly irreversible. 12These enduring changes also contribute to the chronic nature of neuropathic pain.

Changes in Central Sensory Pathways in Higher Brain Regions
6][127] Peripheral nerve injury promotes microglial activation in the contralateral thalamus, sensory cortex, and amygdala as would be expected from the anatomical projections of ascending sensory fibers.Brain regions not directly involved in either sensory or affective aspects of pain, such as the motor cortex, do not display microglial activation. 128Hyperactivity in parts of the anterior cingulate cortex and other limbic structures drives the anxiety and depression that represent a co-morbidity of chronic and neuropathic pain. 7,129lood-borne inflammatory mediators 130 from the site of peripheral injury increase the permeability of the blood-brain barrier. 33This allows CNS neurons to access blood cells and the cytokines and chemokines they produce. 131In addition, the selective activation of glia and immune cells in nociceptive pathways 125 likely reflects localized neurogenic neuroinflammation in response to enduring intense activity. 73

Alterations in Descending Control of Spinal Processing
Spinal nociceptive processing is subject to modulation by descending serotonergic and noradrenergic pathways. 6,132Descending inhibition is mediated via α 2 -adrenoceptors and 5HT 7 receptors whereas serotonergic activation of metabotropic 5HT 2 receptors and ionotropic 5HT 3 receptors facilitates transmission 7 .3][134] Actions on these descending controls are thus likely to underlie the efficacy of tricyclic antidepressants and serotonin-noradrenaline reuptake inhibitors in pain management. 7,10

Different Injuries and Different Etiologies
2][3][4] Thus while mechanical allodynia produced in animals by SNI 13 persists for many weeks, that produced by CCI is shortlived and recovery is seen in about 4 weeks. 13,37Similarly, changes in synaptic transmission in the superficial dorsal horn are more robust after sciatic CCI than after complete sciatic nerve section (axotomy). 92These findings are consistent with the observation that CCI promotes stronger and more long-lasting upregulation of TNF-α, IL-1β, and CCL-2 than axotomy by nerve crush. 135It has also been shown that the neuronal subtypes in the dorsal horn that are involved in generation of mechanical allodynia is defined by the nature of peripheral nerve injury. 136ore clinically relevant observations include reports that neuropathic pain associated with multiple sclerosis is characterized by loss of spinal neurons 137 but this effect is not seen with CCI. 138,139he above findings imply that different types of injury provoke the generation of different sets of mediators 18,140 and thus present different drug targets.

The Way Forward? Bridging the Gap between Basic Science and Clinical Practice
Given that patients with neuropathic pain are heterogeneous in pathophysiology, etiology, and clinical presentation 1,5 it is hardly surprising that injury-specific pathologies are found in animal models.41,142 Quantitative sensory testing (QST) may help to bridge the knowledge gap between clinical and laboratory findings.This involves formalization and quantification of a battery of neurological tests, such as response to von Frey filaments, vibration, heat, pressure, and cold as well as dynamic allodynia and wind-up ratio. 5indings are compared with datasets that represent normal responses to sensory tests.Neuropathic pain patients can then be grouped into clusters based on their sensory profiles and this may have a role in determining treatment. 143Technological improvements in microneurography have shown that the specific C-fibre subpopulation affected (mechanoinsensitive versus nonmechanoceptive) depends on the source of neuropathic pain and the type of neuropathy. 144,145Modern microneurography approaches will thus play a role in future refinement of QST.The validity of QST is supported by the observation that post hoc analysis of responders to treatments in clinical trials suggest that clinical effectiveness may cluster according to pain phenotype. 143Beyond this, it may also be possible to subcategorize patients according to their cytokine profile.It then may be possible to correlate precisely quantified signs and symptoms in each individual patient to pathophysiology at the cellular and molecular level.
Recent improvements in basic science approaches also seek to bridge the gap between the "bench and bedside".For example, improved methodologies are starting to differentiate probable pain in animal models from nociception or simple withdrawal reflexes. 57,146Also more attention is now paid to the genetics, environment, and sex of experimental animals 1,80 and improved methodologies are now available for bringing human tissue to the laboratory.These include the culture of human nociceptors either from surgical or post-mortem tissue or using human-induced pluripotent stem cell-derived nociceptors. 145,147aken together, these approaches will permit a rational and highly personalized medicine approach that will dictate the most appropriate therapeutic approach for each individual patient. 7,148,149nding.No financial support was provided for the writing of this review.
Disclosures.The author has no financial or other disclosures.
Statement of Authorship.PAS was responsible for conceiving, researching, and writing this article.