In 1994, the International Association for the Study of Pain defined neuropathic pain as “pain originating from lesions or dysfunction of the peripheral or central nervous system, or due to transient organ damage.” Removing the “transient organ damage” clause gives us the subtype of neuropathic pain. In 2001, neuropathic pain was further simplified to “pain caused by lesions or dysfunction of the peripheral or central nervous system.”
1. Causes of Neuropathic Pain
There are numerous causes of neuropathic pain, ranging from physical injuries to complex metabolic neuropathies. The relationship between neuropathic pain and clinical symptoms is complex; most patients with nerve damage do not experience pathological pain, while a small number may experience severe pain after central or peripheral nerve injuries, which can persist for a long time. Neuropathic pain can be triggered by innocuous or noxious stimuli to the nervous system and various diseases, including: ① Peripheral or central nervous system injuries, such as nerve compression, amputation, crush injuries, and spinal cord injuries; ② Neuropathic pain related to herpes zoster infection or HIV (human immunodeficiency virus); ③ Nerve compression, such as tumor compression or carpal tunnel syndrome; ④ Metabolic disorders, such as diabetic neuropathy or uremic neuropathy; ⑤ Ischemia, such as vascular infarction or stroke. Neuropathic pain is a mixed condition formed by a group of diseases with different causes and manifestations. Table 1-1 lists common causes of neuropathic pain; Table 1-2 provides some common conditions that lead to neuropathic pain. Currently, there is insufficient understanding of individual sensitivity to neuropathic pain following nerve injury, making it difficult to predict which patients with nerve damage will experience abnormal neuropathic pain. Therefore, it is also unclear why patients with similar clinical symptoms can have differing pain levels and qualities.
Table 1-1 Causes of Neuropathic Pain
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Cause |
Corresponding Neuropathic Pain |
|
Traumatic Mechanical Injury |
Compression-type neuropathy, nerve transection injury, burning pain, spinal cord pain |
|
Metabolic or Nutritional |
Injury, postoperative pain, phantom limb pain |
|
Viral |
Alcoholic neuropathy, beriberi, pellagra |
|
Neurotoxic |
Postherpetic neuralgia, HIV-related pain |
|
Non-viral Diseases |
Vincristine, cisplatin, thallium, arsenic, radiotherapy |
|
Ischemia |
Diabetes, malignant tumors, multiple sclerosis, trigeminal neuralgia |
|
Neurotransmitter Dysfunction |
Vasculitis, amyloidosis, congenital diseases, thalamic syndrome, post-stroke pain, complex regional pain syndrome |
Table 1-2 Common Conditions Associated with Neuropathic Pain
|
Peripheral |
Central |
|
Traumatic Mechanical Injury (including iatrogenic) Ischemic neuropathy Nerve root and nerve compression Polyneuropathy (hereditary, metabolic, toxic, inflammatory, infectious, tumor-like, nutritional, amyloidosis, and vasculitis) nerve plexus injury Phantom pain, phantom limb pain Postherpetic neuralgia Trigeminal and glossopharyngeal neuralgia Cancer-related neuropathic pain (tumor invasion of nerves, surgical nerve damage, radiotherapy damage to nerves) Scar pain |
Stroke (ischemic or hemorrhagic) Multiple sclerosis Spinal cord injury Spinal cord cavitation / medullary cavitation |
2. Classification of Neuropathic Pain
Regardless of the cause and the local anatomy of the lesions, many patients exhibit very similar clinical manifestations of neuropathic pain. The main characteristics include: ① Persistent spontaneous pain; ② Pain occurring in the area damaged by sensory nerve lesions; ③ Subthreshold (gentle) stimuli causing pain; ④ Hyperexcitability, with enhanced response to suprathreshold stimuli; ⑤ Possible referred pain and pain persisting after stimulus cessation; ⑥ Often includes sympathetic nervous activity. Neuropathic pain is often classified based on etiology (e.g., diabetic neuropathy, postherpetic neuralgia, post-traumatic neuralgia) or the anatomical location of the neuropathy (central pain or peripheral neuropathic pain).
A more detailed classification based on mechanisms is currently not feasible. For example, when peripheral nerve injury occurs, a large area of nerve innervation may exhibit mechanical allodynia, but the precise and comprehensive related mechanisms cannot yet be pinpointed. Currently, some animal models and human studies have proposed several potential pathophysiological mechanisms that may act alone or together to explain this phenomenon: ① Aδ/C fiber-related peripheral sensitization; ② Activation of certain silent nociceptors; ③ Synaptic transmission between peripheral mechanoreceptors and nociceptive afferents; ④ Loss of spinal dorsal root neuronal inhibition mediated by Aβ fibers; ⑤ Central sensitization; ⑥ Sprouting of dorsal root mechanoreceptive neurons; ⑦ Activation of descending facilitation systems from the brainstem.
3. Diagnosis
Neuropathic pain is part of a neurogenic disease caused by nervous system damage or dysfunction. The four diagnostic elements are as follows: ① Pathological evidence of known nerve injury; ② Nature of the pain, such as burning pain, radiating pain, stabbing pain, electric shock-like pain, etc., and may exhibit hyperalgesia or abnormal pain; ③ Functional deficits, sensory or motor deficits, or autonomic symptoms following nerve injury; ④ Response to conventional treatments, with only partial sensitivity to opioids or NSAIDs.
Diagnosing neuropathic pain first requires precise and detailed medical history collection, focusing on the onset of pain and potential associated diseases such as trauma or surgery. The medical history may provide information that leads to specific diagnoses such as trigeminal neuralgia or glossopharyngeal neuralgia. Many cases of neuropathic pain are not intermittent nor specifically symptomatic; they are usually persistent and may sometimes overlap with intermittent pain. All neuropathic pain occurs within the innervation area of damaged nerves or conduction pathways. When patients describe the pain area, if it matches the corresponding anatomical nerve distribution area, it can greatly aid in diagnosis. Phantom pain at the amputation site and ulnar nerve compression pain at the ulnar side of the hand are two typical examples of referred pain.
Patients with neuropathic pain often describe the pain as spontaneous or caused by abnormal stimuli, with such stimulus pain manifesting as abnormal pain perception (stimuli that are usually insufficient to produce pain can cause pain, often due to light mechanical or cold stimuli). Abnormal pain perception is not exclusive to neuropathic pain and can also occur in some non-neuropathic pain conditions such as sunburn, arthritis, and hysteria. Hyperalgesia refers to an increased perception intensity of normal pain stimuli, meaning mild stimuli cause intense pain. Neuropathic pain is often accompanied by sensory abnormalities and dysesthesias, which can be spontaneous or evoked.
A comprehensive examination of patients with neuropathic pain should include sensory, motor, and autonomic symptom signs, along with a detailed medical history to confirm or rule out the diagnosis. Because pain is a subjective sensation, the diagnosis of neuropathic pain largely relies on confirming sensory abnormalities in the areas innervated by the damaged nerve, nerve plexus, nerve root, or central pathways. A thorough bedside examination of autonomic sensory functions is extremely important. Patients’ sensory deviations include the range of hyperesthesia or hypoesthesia, as well as changes in the nature and spatiotemporal aspects of the symptoms, which are crucial for assessing the condition. Sensory examination is best conducted after all other information has been comprehensively collected, as the final step in a series of diagnostic tests, using various different styles of mapping to depict the areas of sensory disturbance and see if they correspond with the areas innervated by the damaged nerve. Detailed studies of patients with central pain from stroke or multiple sclerosis show that the pain condition seems unrelated to changes in somatic sensory pathways or motor systems, but the changes in sensory abnormalities related to central pain are correlated with the spinal-thalamic-cortical system, leading to changes in temperature and/or pain stimuli perception. When a series of centrally facilitated symptoms and signs such as pain and/or sensory disturbances appear, careful consideration, detailed examination, and differential diagnosis are necessary, taking into account possible non-neurogenic causes. Such situations occur only occasionally, often when symptoms and signs have developed over a period and are sometimes interpreted as variations in the distribution areas of nerves or nerve roots. Further examination of somatic sensory status, quantitative somatosensory testing (QST) techniques can be used to enhance standard clinical neurophysiological examinations, especially to identify small fiber system lesions and some positive signs, such as dynamic mechanical allodynia. QST techniques provide specific and graded assessments of changes in thresholds related to different somatic sensory pathways and defects or positive changes in suprathreshold stimuli responses. There are many methods for detecting such stimulus patterns, and the choice of testing methods and the number of tests to be used depend on time factors and considerations of the suitability of the patient being studied. When neuropathic pain is suspected, the most commonly used clinical methods are touch, vibration massage, and temperature testing. Quantitative methods for these tests include von Frey filaments, vibrametry, and methods based on Peltier element devices (assessing four different thermal stimuli: warmth, cold, heat pain, cold pain).
It is important to note that sensory deviation signs in neuropathic pain are not entirely equivalent. Sensory changes were first described under the concept of neuropathic pain, but recent studies suggest that in some subgroups of patients with nociceptive pain, such as musculoskeletal pain patients, there are similar, transient, and various sensory disturbances, including loss of distribution boundaries in central pain areas and/or distal regions. Considering the results of somatic sensory examinations, the specific features suitable for true neuropathic pain (distinct boundaries of abnormal sensory areas) are reproducible during examinations. The physiological basis for sensory abnormalities in nociceptive pain remains unclear, but this phenomenon clearly indicates that the existence of sensory abnormalities under pain conditions is not a specific sign of neuropathic pain. Furthermore, pain with psychological origins, such as hysteria, is often reported to have sensory abnormalities, suggesting a prominent connection between the mental and physical realms. The importance of emphasizing bedside sensory examinations, carefully delineating areas of sensory abnormalities, and comparing them with the anatomical nerve distribution areas suspected from the collected medical history cannot be overstated.
Motor systems may exhibit functional impairments such as tremors and muscle weakness without damage, likely as a protective behavior of somatic motor reflexes or psychological restrictions. Spontaneous changes in signs are likely direct results of nerve injuries or harmful afferent spinal-cord reflexes. Physiological increases in sympathetic reflex activity are a result of pain, rather than the cause of pain as is commonly believed (sympathetically maintained pain).
Effects after intravenous administration should not be used as the sole diagnostic criterion, as responses to medications may overlap between neuropathic pain and other types of pain. However, efficacy testing can guide medication use and serve as a discussion and exploration of pathophysiological research.
In summary, the diagnosis of peripheral or central neuropathic pain should be based on medical history and symptom signs suggesting nerve lesion causes, as well as pain distribution areas consistent with neuroanatomy and sensory abnormalities in those areas (Table 1-3).
Table 1-3 Basis for Diagnosing Neuropathic Pain
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Detailed Medical History Collection Mapping of Pain Areas Careful and Logical Examination Detailed Examination Includes: Neurophysiological Examination (EMG, nerve conduction studies, sensory evoked potentials, F-responses) Quantitative Sensory Testing |
4. Acute Pain and Neuropathic Pain
Acute pain, also known as “nociceptive pain,” occurs when strong, harmful stimuli act on the skin or deep tissues. Harmful stimuli mainly refer to natural events: mechanical (pinching), thermal (heat or cold), chemical (acid or bee stings), or some man-made events (electric shock injuries). Acute pain leads to the activation of a specific type of primary sensory nerve fibers, nociceptors, triggering the firing of nerve impulses that are conducted along peripheral nerves, reaching the spinal cord or brainstem after passing through the dorsal root ganglion (DRG), where they activate secondary or tertiary neurons in the central nervous system, which are converted into the conscious perception of pain by the brain. Acute pain includes inflammatory pain and neuropathic pain, with chronic pain encompassing both types and mixed forms.
1. Inflammatory Pain is often defined alongside acute pain as “nociceptive pain.” Signs of inflammation, such as redness, swelling, and heat, are present in the skin or other tissues, accompanied by spontaneous pain and tactile sensitivity. Once harmful stimuli are removed, acute pain also disappears; however, if an inflammatory response has already occurred, pain and tactile sensitivity may persist for hours, days, months, or even years, such as in cases of scrapes, bruises, burns, mild infections, muscular joint pain, back pain, and tension headaches. Inflammatory pain typically shows good sensitivity to NSAIDs (such as aspirin) and opioid treatments (such as morphine).
2. Neuropathic Pain is pain caused by damage to the peripheral or central nervous system. Central pain includes post-stroke pain, post-paralysis pain, postherpetic neuralgia, diabetic peripheral neuropathy, phantom limb pain, and sciatica. Neuropathic pain is often characterized by burning sensations, sometimes sharp or electric shock-like pain in paroxysms. Treatment is more challenging; however, certain anticonvulsants, antidepressants, and antiarrhythmics can be effective, as well as local or systemic use of local anesthetics to block nerves. Opioids also have some effect on neuropathic pain, but the response is poorer compared to inflammatory pain.
5. Molecular Mechanisms of Nociception and Pain
(1) Chemical Mediators of Nociception
Using the genetic methods mentioned above, it is relatively easy to determine the action sites and characteristics of endogenous mediators that trigger pain perception. Damaged tissues release a series of molecules that initiate pain perception, with proteolytic cascade reactions acting on soluble precursor molecules to produce peptides related to changes in pain thresholds. Other mediators, such as lipids and nitric oxide (NO), transmit signals between and within cells, playing significant roles in inducing pain and altering pain thresholds.
1. Adenosine Triphosphate (ATP) exists in all cells at millimolar levels. Various harmful stimuli cause the release of intracellular ATP into the extracellular environment, activating G-protein coupled receptors (GPCRs) and ionotropic receptors on sensory neurons. The purinergic receptor P2X3 cation channel, activated by ATP, is expressed on nociceptive neurons and has been evaluated as an analgesic target in various studies, including antisense methods, ineffective mutant mice, and specific drug antagonists (North, 2003). Many studies have confirmed that this receptor plays an important role in inflammatory responses and neuropathic pain. Barclay et al. (2002) used intrathecal administration of antisense oligonucleotides to downregulate P2X3 receptor function, leading to a decrease in P2X3 protein levels in the primary afferent terminals of the spinal dorsal horn after seven days. This antisense treatment was also found to inhibit the development of hyperalgesia after partial sciatic nerve ligation and significantly reverse existing hypersensitivity within two days, consistent with the downregulation of P2X3 receptor protein and function. Despite these findings, there is currently no direct evidence of upregulation of P2X3 receptor expression in neuropathic pain. In fact, some studies have found downregulation of P2X3 after L5/L6 spinal nerve ligation in rats (Kage, 2002). A small number of small-diameter neurons show sensitive responses to α,β-methylene-ATP (a selective P2X3 agonist), while large-diameter neurons and some other small neurons exhibit delayed expression of functional P2X3 receptors. TNP-ATP is a potent P2X3 receptor antagonist, but its metabolic activity is unstable and it can also act on P2X1-4 subtypes. Nevertheless, studies have found that TNP-ATP can completely reverse pain hypersensitivity, even within a short duration of one hour (Tsuda, 2003). Recently, a highly potent stable P2X3 and P2X2/3 heteromer antagonist, compound A317491 (Jarvis, 2002), has been discovered to reverse mechanical allodynia and thermal hyperalgesia in rat models of neuropathic pain.
2. Kinins are released due to proteolytic cascade reactions, and the G-protein coupled bradykinin receptors B1 and B2 mediate the release of these blood-derived localized peptides, resulting in numerous effects. The kinin release enzyme-kinin system regulates the harmful responses of the circulatory system, contributing to inflammatory pain and wound healing in damaged tissues (Marceau and Regoli, 2004).
3. Prostaglandins are lipid mediators, especially prostaglandins, which have long been recognized for their important role in lowering pain thresholds. NSAIDs inhibit cyclooxygenase, blocking the metabolism of arachidonic acid (AA), thereby preventing the synthesis of prostaglandins. Many biological effects of prostaglandin-like substances are mediated by GPCRs and are the result of a series of activated protein kinases altering the characteristics of voltage-gated channels. Other shorter-acting lipids, such as hydroperoxyeicosatetraenoic acid (HPETE), are derivatives of AA that can directly act on ion channels like TRPV1 (capsaicin receptor), causing depolarization of sensory neurons (Hwang, 2000).
The expression of olfactory epithelium-associated GPCRs is believed to represent the way the entire GPCR family is related to sensory neurons. However, the ligands that activate these MAS-like receptors and their possible mechanisms in modulating the excitability of nociceptors remain unclear (Dong, 2001; Han, 2002).
Cannabinoids and opioids can inhibit pain pathways, and the CB1 receptors in sensory neurons and the central nervous system are currently considered effective targets for analgesic action in neuropathic pain (Fox, 2001). In some sciatic nerve ligation models, CB-selective agonists WIN55,212-2, CP-55,940, and HU-210 completely blocked the occurrence of mechanical allodynia within three hours of subcutaneous injection. Research by Zhang et al. (2003) indicated that chronic pain models induced by peripheral nerve damage, rather than peripheral inflammatory responses, led to high and specific expression of CB2 receptors in the lumbar spinal cord. Traditional opioid medications have clear and effective therapeutic effects on acute inflammatory pain and certain specific types of neuropathic pain, such as diabetic neuropathy (Rowbotham, 2003). The role of nociceptin/orphanin FQ systems in regulating neuropathic pain remains unclear and controversial. Earlier studies suggested that nociceptin has analgesic effects in models of neuropathic pain (Hao, 1998), while contrasting findings by Mabuchi (2001) using nociceptin/orphanin FQ antagonists JTC801 showed a reduction in thermal hyperalgesia associated with neuropathic pain.
(2) Mechanical Sensing
1. Acid-sensing ion channels (ASICs) are a family of superfamily channels associated with mechanical sensing in mammals, with high expression in sensory neurons (Waldmann and Lazdunski, 1998). Currently, four different genes encoding ASIC subtypes (ASIC1-4) have been identified, along with two alternating variants (ASIC1 and ASIC2), totaling six known subtypes. Although protons are the only confirmed activators of ASICs, considering the homology between ASICs and MEC channels, along with their high expression in sensory neurons, researchers speculate that these channels may have a certain function in mechanical signal transduction (Lewin and Stucky, 2000). However, staining studies show that ASIC subtypes are distributed along nerve fibers rather than having a specific accumulation at nerve terminals; specific expression at sensory nerve endings is necessary for transmitting acid or mechanical stimuli. Nevertheless, immunological studies have found that many Aβ fiber nerve endings have immunoactive ASICs, which contradicts the long-known theory that low pH cannot lower the threshold of mechanical receptors (Lewin and Stucky, 2000). Therefore, Welsh et al. (2002) speculate that ASICs may belong to a multi-protein conduction complex like MEC-4 and MEC-10, thus masking the proton sensitivity of these channels.
Studies involving gene knockout mice do not support the claim that ASICs are mechanotransducers in mammals. Using sensory neuron cell body models, dorsal root ganglion neurons do exhibit current responses to mechanical stimuli (Drew, 2004). Comparing the responses of neurons categorized based on cell size, action potential duration, and isolectin B4 (IB4) binding ability between ASIC2 and ASIC3 knockout mice and wild-type mice shows distinctly different responses to mechanical stimuli, consistent with their phenotypes. Notably, there is a remarkable connection between action potential duration and mechanical sensitivity. It is speculated that low-threshold mechanoreceptors exhibit rapid adaptation to mechanical stimuli, while nociceptive receptors show smaller amplitude responses to slow or transient adaptation currents. There are no significant differences in amplitude and kinetics between neurons from ASIC2/3 knockout mice and wild-type mice. Ruthenium red (a capsaicin receptor blocker) can block mechanical current in a voltage-dependent manner, and this effect is comparable between the mutant and wild-type mice. Analysis of proton-gated currents shows that most low-threshold mechanoreceptors do not exhibit ASIC-like current characteristics but instead show continuous currents sensitive to low pH in both wild-type and ASIC2/3 double knockout mice. These findings further support the idea that another ion channel plays an important role in mechanical signal transduction in dorsal root ganglia. Lazdunski’s research group also studied the characteristics of ASIC2 knockout mice in auditory, cutaneous mechanical sensing, and visceral mechanical pain, with results failing to provide positive evidence for ASIC2’s role in mechanical sensing (Roza, 2004).
2. Transient receptor potential channels (TRP) have been confirmed to be related to mechanoreceptors in experiments with fruit flies (NOMPC or MAN) and Caenorhabditis elegans (OSM-9) mutants. So far, no closely homologous channels have been found in mammals, but TRPV4 shows moderate homology with OSM-9 (26% amino acids, Liedtke, 2003). TRPV4 is widely expressed in rodents, with the highest expression in the kidneys, and significant levels in the heart, liver, brain, and testes, although this channel is not expressed in sensory neurons themselves. Interestingly, it has been found to be expressed in places related to mechanical sensing, such as the cochlea, trigeminal ganglion, and Merkel cells. TRPV4 is heterogeneously expressed under hypotonic stimulation, lipids, and suitable temperatures. Studies have also found that the related TRPV1 channel can be opened by various stimuli, suggesting that this channel may serve as an integrator for multiple sensory stimuli. Behavioral studies measuring pain thresholds in response to compressive stimuli on mouse tails have shown that TRPV4 knockout mice have thresholds nearly double that of control groups, while von Frey filament testing shows similar thresholds between both groups. In summary, it remains unclear whether TRPV4 can be directly activated by mechanical stimuli or participates in the sensing of mechanical stimuli in situ; Suzuki (2003) reported surprising findings that contradict the sparse distribution of TRPV4 in DRG neurons.
In addition to TRPV4, the role of TRPV1 in bladder mechanosensing and polycystin has been studied. Less related to the TRP family, TRPV1 may be involved in the mechanosensory function of ciliated renal epithelial cells (Nauli, 2004). Birder et al. (2002) confirmed that TRPV1 knockout mice, although morphologically normal in the bladder, had defects in spinal signaling for micturition reflex and bladder capacity perception. It is known that bladder distension can trigger ATP release, and the absence of TRPV1 reduces the amount of ATP released by distended bladder or low-tonic swollen urethral epithelial cells. Moreover, capsaicin-stimulated cultured urethral epithelial cells can trigger ATP release, suggesting that TRPV1 activation is a sufficient and necessary factor for ATP release. Currently, there are no reports from institutions indicating that TRPV1 can be opened by mechanical stimuli, and the absence of TRPV1 does not affect subcutaneous mechanical sensing (Caterina, 1997). Therefore, the role of TRPV1 in this pathway requires further research; perhaps mechanical stimuli open TRPV1 through chemical mediators (possibly lipids). Conducting electrophysiological analyses on mechanically stimulated urethral epithelial cells from this perspective may be a beneficial research direction.
Polycystin-1 (PC-1) regulates Ca2+ channels and K+ channels through G-protein signaling pathways (Delmas, 2004), while PC-2 is a Ca2+-permeable cation channel. Both have similar membrane topology structures to TRP channels, and mutations in either gene can lead to the occurrence of polycystic kidney disease. Nauli (2004) found that the normal function of these proteins is crucial for the mechanosensing of cilia in renal epithelial cells. In animals with dysfunctional PC-1, the normal intracellular Ca2+ increase induced by changes in fluid stress sensed by cilia is weakened or disappears; removing extracellular Ca2+ can inhibit this response in wild-type cells, and antibodies targeting the extracellular domain of PC-2 confirm that Ca2+ indeed enters cells through this channel. Researchers speculate that PC1 (with a larger extracellular domain) acts as a mechanoreceptor, subsequently activating the closely associated PC-2 channel. Finally, TRPA1 is believed to participate in primary mechanical sensing in the inner ear (Corey, 2004); this channel is also present in a small number of sensory neurons and may also be a site for mechanical sensing.
3. Chemically mediated mechanical sensing: When blood flow changes, endothelial cells release various factors such as NO, ATP, and substance P. Cockayne et al. (2002) found that bladder reflexes in mice lacking P2X3 receptors were significantly weakened, with reduced micturition frequency and increased bladder capacity; they also found that P2X3 receptors are normally distributed on sensory neurons innervating the bladder. Their subsequent studies showed that bladder distension triggers a cascade release of ATP, and P2X3 knockout animals exhibit poorer responses to sensory nerve fibers during bladder distension. Cook and McCleskey (2002) found that when keratinocytes and fibroblasts dissolve near sensory neurons, ATP acts on P2X receptors, causing depolarization of the neurons. This finding raises the possibility that some harmful mechanical stimuli may activate nociceptors by causing nearby cells to release ATP. Nakamura and Strittmatter (1996) previously proposed that the purinergic receptor P2Y1 might play a role in touch-induced impulses. They discovered P2Y1 during expression cloning screening of some DGR cRNAs in oocytes from clawed frogs, where external buffer solutions stimulated ATP release, activating this receptor.
(3) Temperature Receptors
Some TRP channels are also temperature-sensitive, providing the basis for nociceptive thermal perception in whole organisms or cultured sensory nerve cells. These channels exhibit different temperature activation thresholds (TRPV1 > 43℃, TRPV2 > 52℃, TRPV3 > 36℃, TRPV4 > 27℃-35℃, TRPM8 < 25℃-28℃, TRPA1 < 17℃), and are expressed in primary sensory neurons and some other tissues. Even more puzzling is that behavioral responses to nociceptive thermal stimuli are not mitigated in TRPV1 knockout mice; other TRP channels may more likely determine the perception of temperature by sensory neurons and skin in a synergistic manner (Peier, 2002; Woodbury, 2004).
(4) Voltage-gated Channels
1. Sodium Channels: The sodium channel family in the mammalian nervous system consists of a group of ten structurally related genes. It has long been known that low concentrations of sodium channel blockers have strong analgesic effects (Strichartz, 2002). Studies on neuronal excitability, analyses of the expression of various channel subtypes in animal models, and gene knockout and antisense studies have confirmed the role of these channels in inflammatory responses and neuropathic pain. Sodium channels Na V1.8 and Na V1.9 are specifically expressed in the peripheral nervous system, predominantly distributed in nociceptive sensory neurons, and have received significant attention as targets for analgesic drug action. Na V1.7 is present in sympathetic and nociceptive sensory neurons, and studies using specific nociceptive receptor knockout mice confirm the critical role of this channel in inflammatory pain (Nassar, 2004). The embryonic sodium channel Na V1.3 and a β subunit (β3) are upregulated in DRG neurons during certain neuropathic pain conditions.
Na V1.3 is abundantly expressed in the adult central nervous system and has low expression in the normal adult peripheral nervous system. Axonal injury (axotomy) or other forms of nerve damage can induce re-expression of Na V1.3 and related β3 subunits in sensory neurons, but not in primary motor neurons (Waxman, 1994). This phenomenon can be reversed both in vitro and in vivo by high concentrations of glial-derived neurotrophic factor (GDNF), which is known to rapidly reactivate Na V1.3 from the non-activated state (Cummins, 2000). Axonal injury induces rapid activation of tetrodotoxin-sensitive (TTX-S) sodium channels in damaged neurons, which can also be inhibited by GDNF and nerve growth factor (NGF) (Cummins, 1997, 2000). GDNF inhibits Na V1.3 expression while alleviating the generation of ectopic action potentials, and behavioral changes related to thermal and mechanical hyperalgesia caused by CCI models are also relieved (Boucher, 2000). Moreover, experimental spinal cord injury shows upregulation of Na V1.3 in nociceptive sensory neurons with broad dynamic ranges, accompanied by hyperexcitability and pain. Antisense knockout of Na V1.3 in spinal cord injury animals reduces hyperexcitability and pain-related behaviors (Hains, 2003). Therefore, these studies suggest that Na V1.3 re-expression plays an important role in enhancing neuronal excitability and neuropathic pain following spinal or nerve injury.
Na V1.8 is primarily expressed in nociceptive sensory neurons (Djoihri, 2003), and the sodium ion flow that determines the depolarization phase of the action potential primarily occurs through this channel. The functional expression of this channel is regulated by inflammatory mediators, including NGF, and antisense studies and gene knockout studies support the role of this channel in inflammatory pain (Akopian, 1999). Antisense studies also suggest that this channel plays a role in the development of neuropathic pain (Lai, 2002), with Na V1.8 mutant mice exhibiting defects in the spread of ectopic action potential conduction (Roza, 2003); however, these mice show normal neuropathic pain behaviors at earlier time points.
Membrane-associated proteins (a class of proteins that can bind to membrane phospholipids after activation by calcium ions, involved in membrane transport and a series of other calcium-dependent activities) II/p11 bind to Na V1.3, promoting the insertion of functional channels into the cell membrane (Okuse, 2002), which may provide a target for regulating Na V1.8 expression and levels of Na V1.8 flow in sensory neurons. Na V1.9 is also expressed on sensory neurons (Dib-Hajj, 2002); it determines sustained sodium ion flow, with some overlap between activation and steady-state inactivation, suggesting that blocking Na V1.9 may be effective for pain treatment. Although normal levels of Na V1.9 expression depend on the supply of NGF or GDNF, due to the lack of Na V1.9 mutants and selective blockers, there is currently insufficient data to demonstrate the role of this channel in pain states (Cummins, 2000). Evidence shows that inflammatory mediators can upregulate the functional expression of Na V1.9 channels through G-protein coupled mechanisms. Existing studies have drawn significant attention to sodium channels as targets for analgesic drug action, but specific Na V1.3, Na V1.7, Na V1.8, and Na V1.9 antagonists have yet to undergo clinical testing.
2. Potassium Channels: Potassium channels also play an important role in neuronal excitability, and in animal models of neuropathic pain, potassium channels are regulated to varying degrees at the transcriptional level. Ishikawa et al. (1999) used reverse transcription polymerase chain reaction (RT-PCR) technology to find that in the CCI pain model, the mRNA levels of Kv1.2, 1.4, 2.2, 4.2, and 4.3 in ipsilateral DRG decreased to 63%-73% of contralateral levels at three days post-surgery, and to 34%-63% at seven days; furthermore, Kv1.1 mRNA levels decreased to about 72% of contralateral levels at seven days, while Kv1.5, 1.6, 2.1, 3.1, 3.2, 3.5, and 4.1 mRNA levels did not show significant changes at any time. Interestingly, among these Kv channels on DRG, only Kv1.4 appears to be the only channel expressed in small-diameter sensory neurons, and its expression is significantly reduced in the Chung model of neuropathic pain (selective spinal nerve ligation) (Rasband, 2001). Passmore et al. (2003) demonstrated that KCNQ potassium ion currents (M currents) also play a role in determining pain thresholds. In rat models of neuropathic pain and inflammatory pain, retigabine (an anticonvulsant) enhanced M currents and reduced nociceptive sensory input to the spinal dorsal horn.
3. Calcium Channels and Neurotransmitter Release: A wealth of evidence demonstrates the important role of voltage-gated calcium channels in the pathogenesis of neuropathic pain, with various drugs acting on calcium channel subtypes to exert effective analgesic effects. Mice with mutations in N-type Cav2.2 channels show significant reductions in neuropathic pain behaviors after mechanical and thermal stimuli. Two effective analgesics for treating neuropathic pain selectively act on calcium channel subtypes: the snail toxin ω-conotoxin blocks Cav2.2-α subunits, while the widely used gabapentin has a high affinity for the α2δ calcium channel subunit (Gong, 2001). Voltage-gated calcium channels contain a single α subunit and share some structural homology with sodium channels, but other associated subunits are much more complex. The functional calcium channel complex primarily comprises the following five protein structures: α1 (170 kDa), α2 (150 kDa), β (52 kDa), δ (17-25 kDa), and γ (32 kDa).
Gabapentin has a high affinity for the α2δ1 site in brain tissue. Interestingly, the upregulation of α2δ1 during neuropathic pain shows a good correlation with gabapentin sensitivity (Luo, 2002), suggesting that the α2δ1 subunit is likely the target for gabapentin’s action. Not all neuropathic pain animal models with hyperalgesia show upregulation of α2δ1; Luo (2002) compared the expression levels of α2δ1 in rat DRG and spinal cord, and the sensitivity of gabapentin for treating hyperalgesia due to mechanical nerve injury (CCI, spinal nerve transection or ligation), metabolic diseases (diabetes), or chemical neuropathies (vincristine neurotoxicity). All types of nerve injuries exhibited hyperalgesia, but the upregulation of α2δ1 expression in DRG and/or spinal cord and sensitivity to gabapentin only occurred in mechanical injuries and diabetic neuropathies. These findings may partially explain why gabapentin is ineffective in some types of neuropathic pain patients.
Saegusa et al. (2002) further support the view of calcium channels as targets for analgesic drug action in studies on the characteristics of Cav2.2 gene knockout mice. Despite the widespread distribution of Cav2.2, transgenic mice in the Seltzer model have confirmed deficiencies in inflammatory responses and certain specific neuropathic pains, with these mutant mice showing abnormal stability in thermal and mechanical pain thresholds. The role of Cav2.2 in chronic pain aligns with the known effects of an N-type calcium channel blocker, ω-conotoxin, which has been found to have analgesic effects in animal models and humans. Intrathecal administration of ω-conotoxin can dose-dependently block existing thermal hyperalgesia and can reversibly block existing mechanical hyperalgesia. In this acute pain model, intrathecal administration of ω-conotoxin was more effective than intrathecal morphine, with a longer duration of action, but also increased side effects, making it unsuitable for clinical use.
(5) Sensory Neurons as Secondary Sensors of Tissue Injury
Increasing evidence shows that non-neuronal cells can release various mediators during tissue injury, affecting changes in pain thresholds and pain perception. Therefore, the expression of P2X receptors in macrophages and microglia plays a role in the development of neuropathic pain (Tsuda, 2003). Similarly, transgenic mice lacking temperature-sensitive TRP receptors in sensory neurons lose primary phenotypic effects, suggesting that keratinocytes may have some other heat-sensitive channels that transmit consistent signals with receptors on sensory neurons (Chung, 2003). As research on peripheral nociceptive mechanisms continues to deepen and new analgesics targeting peripheral sites are developed, a brighter future is anticipated.
