Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.58 no.5 Campinas Sept./Oct. 2008
Neuropathic pain - neurochemical aspects*
Dolor neuropático - aspectos neuroquímicos
Durval Campos Kraychete, TSAI; Judymara Lauzi Gozzani, TSAII; Angiolina Campos KraycheteIII
Adjunto de Anestesiologia - UFBA; Coordenador do Ambulatório de Dor -
IIEditor-Chefe da Revista Brasileira de Anestesiologia; Coordenador do Serviço de Dor da Faculdade de Ciências Médicas da Santa Casa de São Paulo, SP
IIIMédica Estagiária do Ambulatório de Dor - UFBA
OBJECTIVES: Neuropathic pain is caused by damage or inflammation of the
nervous system. It is a complex syndrome and its biological mechanisms, involving
inflammatory and immunologic theories, are not clear. The objective of this
review was to describe the main biologic factors associated with neuropathic
pain, making a logical association between hypotheses suggested in the literature.
CONTENTS: The main neuromediators, ion channels, and cells, including cells in the nervous system involved in neuronal excitation are described, and the possible activation sequence or interaction among those agents in the neoplastic change secondary to nerve damage are emphasized.
CONCLUSIONS: It was possible to conclude that the advances on the knowledge of the pathophysiology of neuropathic pain can determine new pharmacologic approaches for this syndrome.
Key Words: PAIN: neuropathic; PHYSIOLOGY: neurotransmitters.
Y OBJETIVOS: El dolor Neuropático lo causa la lesión o inflamación
del sistema nervioso. Es un síndrome complejo, con mecanismos biológicos
poco aclarados, que envuelve teorías inflamatorias e inmunes. El objetivo
de esta revisión fue describir los principales factores biológicos
relacionados con el dolor Neuropático, asociando de forma lógica
a las hipótesis sugeridas por la literatura.
CONTENIDO: Fueron descritos los principales neuromediadores, canales iónicos y células, incluyendo las del sistema inmune involucrados en la excitabilidad neuronal, como también la posible secuencia de activación o interacción de esos agentes en la alteración neuroplástica proveniente e la agresión al nervio.
CONCLUSIONES: De ese estudio, se pudo concluir que los avances en el conocimiento de la fisiopatología del dolor Neuropático, pueden determinar nuevos objetivos para el abordaje farmacológico de ese síndrome.
Neuropathic pain is defined as pain secondary to nerve damage or dysfunction and, more widely, as a consequence of damage or disease of the somesthetic system 1. It is a complex syndrome whose biological mechanisms are not clear, involving inflammatory and immunologic theories.
EXPERIMENTAL MODELS FOR THE STUDY OF NEUROPATHIC PAIN
Most experimental models described in the literature involve mice and were developed from traumatic, metabolic, or toxic peripheral lesions:
- Spinal nerve ligature (SNL) - one or more spinal nerves innervating the paw are ligated and cut.
- Partial sciatic ligature (PSL) - part of the sciatic nerve is ligated.
- Chronic Constrictive Injury (CCI) - includes the placement of four ligatures on the sciatic nerve tightened by a chrome suture.
- Spared nerve injury (SNI) - fibular and posterior tibial nerves are severed and the sural nerve is spared. This technique causes long-term behavioral changes.
Other methods include the intraperitoneal injection of streptozocin to mimic diabetic neuropathy, or paclitaxel and vincristine for chemotherapy induced neuropathy. Models of central pain use contusion (trauma using the force of impact of tissue dislocation), or ischemic lesions from slow compression by clamping or balloon insufflation. Cytotoxic methods use injections of glutamate analogues (cainate) or substances that cause lesion in specific areas of the gray matter. The techniques described are aimed at causing mechanical and thermal hyperalgesia 2,3.
SECONDARY HYPERALGesIA AND CENTRAL SENSITIZATION
Secondary hyperalgesia is due to damage of neural and non-neural tissues adjacent to the primary lesion, and it is associated with central sensitization. Thus, patients with neuropathic pain can feel mechanical allodynia on the skin related to nerve transmission through Aβ fibers. This occurs because when the noxious stimulus from Aδ fibers reaches the dorsal horn of the spinal cord (lamina I), it can activate wide dynamic range neurons (lamina V) and increase the synaptic efficacy of Aβ fibers. Therefore, the loss of tactile function in patients with neuropathic pain can lead to an end of the allodynia 4.
Role of Primary Afferents
Since receptors that are expressed predominantly in small-diameter nerve fibers, such as cannabinoid and neutrotrophic receptors reverse thermal and mechanical hyperalgesia after SNL when they are stimulated, this reinforces the hypothesis that neuropathic pain is related to trauma of the primary afferent 5,6.
Hypothesis of the Damaged Primary Afferent
In the lesion of the primary afferent, it has been widely documented that there is spontaneous and ectopic activity of the traumatic neuroma to thermal, chemical, and mechanical stimulus 7. Damage of fibers distal to the dorsal root ganglion causes local valerian degeneration associated with inflammatory phenomena and activation of macrophages, facilitating abnormal electric discharges from areas not affected by the lesion. An increase in the expression of TRVP1 receptors (transient receptor potential vanilloid-1), brain derived neurotrophic factors (BDNF), and excitatory neurotransmitters, such as the calcitonin gene-related peptide (CGRP) is seen. Since adjacent intact C fibers belong to the same damaged nerve or fasciculus, they develop an increased sensitivity to catecholamines and in the expression of Nav 1.8 sodium channel receptors. This could explain why L4 rhizotomy can alleviate the mechanical hyperalgesia secondary to the L5 lesion in mice 2,3. The injection of a local anesthetic directly in the L5 dorsal root ganglion in the model of SNL in mice reversed the changes secondary to neuropathic pain 8. This occurred despite the fact that the L5 lesion causes spontaneous neuronal activity preferentially in Aδ fibers, which suggests that the spontaneous activity of the Aδ fiber initiates the central sensitization and changes the phenotypic expression of the primary afferent (C fibers). A new expression of neuropeptides normally related with C fibers can occur, as well as an increase in the spontaneous activity of Aβ fibers 2,3.
Hypothesis of the Intact Primary Afferent
After damage of a peripheral nerve, spontaneous neural activity develops in primary afferents that share the same innervation with the severed nerves. The incidence of spontaneous neuronal activity is high, around 50%. However, the damage of a spinal nerve in rodents leads in intact nerves to an increased sensitivity to adrenaline and tumor necrosis factor (TNFα) 9. There is an increase in the response to heat and cold secondary to the increased expression of proteins for transitory receptor potential vanilloid (TRVP1) and cold-sensitive channels (TRPA1) in the dorsal horn of the spinal cord 10. There is also an increased expression of messenger ribonucleic acid (mRNA) for the calcitonin gene-related peptide (CGRP), brain-derived neurotrophic factor (BDNF), and purinergic receptors (P2X3) 2,3.
Pain Mediated by the Sympathetic Nervous System
The influence of the sympathetic nervous system on neuropathic pain is very relevant. Complex regional syndrome usually manifests itself with severe pain in one extremity. Patients present with edema, hyperalgesia, or changes in motor function, which might improve with a sympathetic block or with α-adrenergic receptors antagonists, indicating a sympathetic component. In animal models of neuropathic pain secondary to SNL at the level of L6, more than 60% of the intact nociceptors show spontaneous activity and more than 50% respond to α-adrenergic agonists 2,3.
Nav 1.3, 1.7, 1.8, and 1.9 sodium channels can be found in the dorsal root ganglion and are involved in the generation of action potentials and conduction of nociceptors. Nav 1.3 and 1.7 are sensitive to tetrodotoxin and the other two are not. There is an increase in the expression of Nav 1.3 in the dorsal root ganglion of damaged axons. This channel has kinetic properties that facilitate repetitive discharges. The β2 subunit regulates the opening of the channel and mice that do not express this subunit do not develop mechanical hyperalgesia after nerve damage. Therefore, the high density of voltage-dependent sodium channels in the damaged nerve can cause neuronal depolarization to last longer than the refractory period of adjacent sodium channels, allowing the antidromic propagation of the action potential (from proximal to distal) in the nerves. The frequency of fast pulses in large-caliber fibers can lead to central sensitization because they can stimulate adjacent nociceptive fibers. Mutations in Nav 1.7 sodium channels can reduce the excitability of the sympathetic nervous system and cause hyperexcitability in small-caliber fibers. This would explain the edema, erythema, and pain of erythromelalgia 11.
Role of Central Sensitization
Central sensitization involves homo- and heterosynaptic mechanisms. In homosynaptic sensitization, the test stimulus and conditioning are associated with the same afferent. This can be exemplified by the wind up phenomenon, when the continuous low-frequency stimulation of afferent C fibers causes an increase in the response of specific cells in the dorsal horn of the spinal cord. In heterosynaptic sensitization, the test stimulus and conditioning are related in a different way. In this case, nociceptive stimuli increase the synaptic efficacy of mechanoreceptors connected with Aβ fibers. Thus, neuropathic pain is accompanied by homo- and heterosynaptic sensitization, which was demonstrated in models of L5 lesion using the SNL technique. L5 myelinated fibers (Aβ and Aδ) develop spontaneous activity. The impulse from those fibers can lead to homosynaptic sensitization in the spinothalamic tract, explaining the persistent chronic pain. On the other hand, projection to the adjacent segment at the L4 level can lead to heterosynaptic sensitization. Therefore, when applying mechanical stimulus to Aβ fibers in the skin corresponding to the L4 path, it can lead to sensitization of afferent fibers in the cells related with L4 in the dorsal horn of the spinal cord. Thus intact L4 afferents show mechanical and thermal hyperalgesia 2,3.
Central sensitization occurs by an increase in synaptic efficacy or due to excessive release of excitatory neurotransmitters.
Release of glutamate is inhibited by the activation of GABAergic (type B), adenosine, and opioid (µ) 12 receptors. The reduction or failure of the function of those receptors can lead to neuronal hyperexcitability in the damaged nerve. Calcium channels a2d subunits in the dorsal root ganglion and in the spinal cord can also be increased 13, which cause the release of excitatory neurotransmitters. The phenotypic change of Ad fibers in neuropathic pain would similarly cause the pre-synaptic release of substance P facilitating sensitization of the dorsal horn of the spinal cord.
The release of substance P and CGRP, besides other excitatory neurotransmitters (aspartate and glutamate) in the synaptic cleft activates NMDA (N-methyl-D-aspartate) and AMPA (amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors leading to an increase of calcium influx into the cell, formation of the calcium-calmodulin complex, activation of the enzyme calcium-calmodulin kinase II, and the neuronal pathways of nitric oxide synthase and production of nitric oxide. This promotes a specific action of protein kinases (A and C), mitogen-activated protein kinases (ERK ½, JNK p38, MAPK), and the transcriptional factors CREB, ATF-2 (activating transcription factor-2) that when phosphorylated bind to c-fos and c-jun (immediate-early genes) promoters, resulting in the synthesis of nuclear gene products that undergo dimerization to form the AP-1 complex and several other genes that facilitate neuronal excitability and changes in the neuroplasticity of the dorsal horn 14. It is possible that the damaged nerve shows increased expression of mRNA for AMPA and NMDA receptors in the dorsal horn of the spinal cord. Gene transcription seems to be connected more to the activation of NMDA and neurokinin (NK1) receptors, besides the pre- and post-synaptic action of nitric oxide 14.
On the other hand, a reduction in the expression of inhibitory receptors after nerve damage and neuronal sensitization would probably be secondary to facilitating mechanisms. Long-term depression is opposed to the long-term potentiation that occurs in GABA type inhibitor cells in the dorsal horn of the spinal cord and would lead to neuronal sensitization. The expression of potassium and chloride channels can be increased and if opened they could inhibit GABAergic neurons causing consequently nerve excitation. In experimental models of neuropathic pain GABAergic neurons can also undergo apoptosis or glycine-containing neurons can be lost resulting in facilitation of the nerve conduction 2,3.
CELLS AND NEUROPATHIC PAIN
Mast cells are present in peripheral nerves and can be activated after the increase of adenosine or bradykinin in the site of damage 15. This causes the release of granules containing histamine, proteases (tryptases and PAR-2 receptors), cytokines, and neurotrophic factors (NGF) capable of: direct excitation of nociceptors and dorsal root ganglion cells; facilitate the action of SP and CGRP; and provoke spontaneous burning pain 16. Synthesis, transcription, translation, and secretion of prostaglandins, cytokines, and chemokines 17 at the site of nerve damage besides recruitment of leukocytes including neutrophils and macrophages that affect directly the neurovegetative nervous system can also occur 18.
Neutrophils adhere to the vascular endothelium and migrate to the site of inflammation in the damaged nerve releasing lipoxigenases and cytokines that affect pain receptors directly. Thus, depletion of circulating neutrophils before experimental nerve lesion attenuates the resulting hyperalgesia 19. Neutrophils release chemokines and defensins that are chemotactic for macrophages and lymphocytes. On the other hand, macrophages that reside in the central and peripheral nervous systems phagocyte degenerated or dead neurons and Schwann cells 20. Macrophages release prostaglandins, cytokines (interleukin-6 IL-6, TNFα, and interleukin 1β IL-1β), and superoxide radicals -implicated on neuropathic pain, and the depletion of macrophages on mice with damaged nerves reduces hyperalgesia 21.
Lymphocytes responsible for cell-mediated immunity (T cells) can be found on the damaged site in models of neuropathic pain, expressing themselves in the periphery and in the central nervous system. Thus, mice that lack T cells develop less mechanical allodynia and thermal hyperalgesia when subjected to damage of the sciatic nerve 22.
T cells can be divided in CD4+ (helper) and CD8+ (cytotoxic), which provoke specific TH1 and TH2 reactions, respectively, according to the cytokines secreted. The TH1 response releases interferon-gamma and IL-2, and it is involved in cell-mediated inflammatory responses, while the TH2 response (IL-4, IL-5, IL-6, IL-9, and IL-10) is involved in the allergic response with the production of antibodies and inhibition of the synthesis of pro-inflammatory cytokines 23. Those responses could possibly have opposing effects in the evolution of neuropathic pain, since the transference of CD4+ lymphocytes increases the response to painful stimuli in mice, while the transference of CD8+ cells reduces hyperalgesia 22.
Glial and Schwann cells interact with neurons, promoting maintenance of the homeostasis, regulating the concentration of neurotransmitters and ions, and the extracellular pH. In neuropathic pain, microglial cells seem to play a fundamental role in the beginning of the lesion and astrocytes in its maintenance 24. Microglia are activated by several neuromediators, such as ATP, bradykinin, substance P, fractalkine, and Toll-like receptor 4 (TLR4) 25,26. ATP activates P2X4 receptors in the central nervous system; fractalkine is a chemokine expressed on the surface of spinal neurons that activates the microglial CX3CR1 receptor, and TLR4 receptors recognize molecules of different structures released during the nerve damage. However, it is not known how neuropathic pain develops after microglial activation. It is possible that glial cells release several excitatory neurotransmitters, such as prostaglandins, nitric oxide, cytokines, and chemokines, activating the sensitive afferent directly. Activation can also occur in the contralateral side of the body by propagation of calcium waves between neural junctions, facilitating the release of excitatory neuromediators 27.
On the other hand, Schwann cells interact with T cells expressing MHC class II molecules 28. Schwann cells secrete cytokines (IL-6, IL-1, TNFα), neurotrophic factors (NGF), prostaglandin E2, and ATP 29. They also express ion channels, and glutamate and cytokine receptors 30. Thus, it is possible that they contribute to the genesis of neuropathic pain.
Bradykinin and kalidine are formed in the blood and tissues. Bradykinin exerts its actions on B1 and B2 receptors, causes sensitization of peripheral nociceptors (desinhibiting vanilloid receptors - TRVP1), potentiates glutaminergic synaptic transmission in the spinal cord 31, stimulates the release of cytokines by macrophages, secretes neutrophil and monocyte chemotactic factors, and facilitates the release of histamine by mast cells. In mice, damage of the sciatic nerve increases the expression of B2 and B1 receptors in the dorsal root ganglion, with predominance of B1 receptors in the soma of myelinated axons. This suggests that B2 receptors are involved in central sensitization 31.
ATP and Adenosine
ATP is a classical neurotransmitter, but it is also released by damaged non-neuronal cells and tissues. It exerts its actions in purinergic receptors (P1 or P2). P2 receptors can be divided in P2X and P2Y, which are coupled to protein G and ion channels, respectively 32.
In experimental models of neuropathic pain P2X3 receptors are reduced (after axotomy or partial nerve ligature) or increased (chronic constrictive lesion); however, even in the face of reduction those receptors show increased sensitivity. Blockade of P2X3 receptors attenuates thermal and mechanical allodynia in mice 33. The expression of P2X4 receptors is also increased in microglia after nerve damage, and its pharmacological blockade reverses allodynia 34. P2X7 receptors can be found in T cells and macrophages. Mice that do not express this receptor do not develop neuropathic pain 35. On the other side, P2Y1 receptors show a 70% increase after damage of the sciatic nerve in mice 36.
Serotonin is a neurotransmitter synthesized and released by central nervous system (dorsal root ganglion, cytoplasm, and Schwann cells) neurons. In the periphery, serotonin is released by platelets and induces hyperalgesia by direct action in the primary afferent via the 5HT1A receptors 37. In partial nerve lesions in mice, serotonin contributes with mechanical hyperalgesia through 5HT2A and 5HT3 receptors 38.
Arachidonic acid metabolites include prostaglandins, thromboxanes, and leukotrienes. Prostaglandins PGE2 and PGI2 exert their action on protein G-coupled receptors (EP14 and IP, respectively) and induce hyperalgesia in peripheral 39 and central nervous system 40 nociceptors. The expression of cyclooxygenase (COX-2) in the damaged area, in the spinal cord (COX2 and COX1), and in the thalamus is increased in models of neuropathic pain, associated with the increase in the number of macrophages and in the production of PGE2 by mast cells 41,42. This phenomenon is related with the actions of IL1β, TNFα, nerve growth factor (NGF), MCP-1 (monocyte chemoattractant protein-1), and reactive oxygen species (ROS). Thus, PGE2 can depolarize directly cells with wide dynamic range in the dorsal horn of the spinal cord, activate tetrodotoxin-resistant sodium channels and voltage-dependent calcium channels, inhibit potassium channels, and increase the release of glutamate, substance P, or CGRP, or block glycine inhibition on specific nociceptive neurons. Treatment of cultures of dorsal horn neurons with PGE2 increases the expression of substance P NK1 receptors, and in models of nerve lesions it induces greater expression of basic fibroblast growth factor (bFGF), nerve growth factor, nitric oxide synthase (NOS), Nav 1.7 and 1.8 sodium channels, TRVP1 receptors (transitory receptor potential vanilloid-1), and metalloproteinases (MMPs) involved in axonal degeneration, loss of myelin sheath, recruitment of leukocytes and macrophages to the site of damage, and disruption of the hematoencephalic barrier. Those factors contribute for the maintenance of persistent chronic pain43. Despite the reversal of thermal and mechanical hyperalgesia after the subcutaneous injection of anti-inflammatories 38 in laboratory animals, in humans anti-inflammatories are ineffective in clinical practice for the treatment of neuropathic pain.
On the other side, leukotriene B4 (LTB4) produces hyperalgesia by releasing neuromediators from neutrophils 44. Besides, neurotrophic factors (NGF) also produce hyperalgesia by inducing the release of LTB4 from mast cells, and increase the recruitment of neutrophils 45.
Cytokines are small molecules that mediate the interactions among cells over distances. IL1β, IL-6, and TNFα are pro-inflammatory, induce the sequential production of each one, and are synergistic 46. The exogenous administration of those substances can also induce pain and hyperalgesia 47. Binding of IL1β to the IL1-RI receptor triggers a series of intracellular events activating transcriptional factors, inducing the expression of COX-2, nitric oxide synthase, IL1β, IL-6, and TNFα. Thus, IL1β affects nociceptors direct and indirectly 48, in nerve damage its production is increased in peripheral nerves 49, and in mice the administration of antibodies for the IL-1 receptor alleviates neuropathic pain.
Interleukin-6 is synthesized by mast cells, monocytes, lymphocytes, neurons, and glia. Lesions of the sciatic nerve promote the local increase in IL-6 expression, as well as in the dorsal horn of the spinal cord 50. In mice, the injection of IL-6 in the lateral ventricles produces thermal hyperalgesia 51. Despite controversial studies, in mice that do not produce IL-6 the development of mechanical allodynia after peripheral nerve lesion is delayed 52.
After acting on specific receptors, TNFα activates transcriptional factors (p36 MAPK and NFkβ) and releases COX-2-dependent factors. The intraplantar injection of TNFα in rodents induces mechanical hyperalgesia 53 and after topical and intraneural administration it induces thermal hyperalgesia and mechanical allodynia 54. Lesion of the primary afferent leads to an increase in TNFα in the dorsal horn of the spinal cord, locus ceruleus, and hippocampus 55.
Neurotrophins are dimeric proteins synthesized and released by several immune cells (mast cells and lymphocytes) and they are essential for the normal development of the nervous system in vertebrates 56. This family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neutrophin 3 and neutrophin 4/5. Glial cell-line derived neurotrophic factor (GDNF) is another protein with non-dimeric neurotrophic properties.
Neurotrophins act on specific receptors - tyrosine kinases (TrK) A, for NGF, B, for BDNF, and NT-4/5 and C, for NT-3, and their expression is greater in the embryo 57.
Nerve damage induces changes in the expression of growth factors. This change affects the tissue without innervation, Schwann cells, dorsal horn of the spinal cord, and dorsal root ganglion. However, after nerve damage the synthesis of those substances as well as of their receptors increases 58. In the experimental L5 SNL damage neurotrophic factor (GNF) is increased in the L4-innervated territory with the retrograde transport of neurotrophins to the dorsal root ganglion modifying the expression of brain derived neurotrophic factors (BDNF). Therefore, an influence of neurotrophic factors in damaged and intact neurons is seen. Neurotrophic factors (NGF) increase the expression of channels related with transduction of the mechanical stimulus (TRPV4). Thus, cold hyperalgesia can be secondary to the abnormal expression of TRPA1 and TRPM8 channels. Nerve growth factor besides causing direct sensitization of peripheral nociceptors affects immune and sympathetic cells 59, while BDNF facilitates the excitation of dorsal horn neurons 60. In experimental models, expression of those substances increases in the area related with the damaged nerve secondary to constriction as well as the corresponding dorsal root ganglion 61. The endoneural injection of neurotrpphic factor causes budding of synapses and signs of thermic hyperalgesia, while the perineural administration of this substance determines the forthcoming of mechanical hyperalgesia.
Growth factors play a fundamental role in the development and maintenance of small neurons and the sympathetic nervous system. After nerve damage, α-adrenergic receptors are increased in the soma in intact axons, with the consequent sensitivity of those nerves to circulating adrenaline. In animal models, the blockade of damaged fibers by local anesthetics reduces significantly their spontaneous activity and nerve budding in the sympathetic nervous system up to five weeks after the lesion, with the blockade of NGF production by inhibiting tyrosine kinase.
Nitric Oxide and Superoxide Radicals
Superoxide radicals are released by mast cells and glia (astrocytes and microglia). On the other hand, nitric oxide has constitutive endothelial and neuronal forms, while the induced form is expressed in cells of the immunologic system. Nitric oxide causes hyperalgesia after injection in the skin and joints 162 and it is implicated in central sensitization 48, increasing the effects of PG2 in neuropathic pain models 63.
Lysophosphatidic acid (LPA) is derived from the lipid metabolism, it is released after tissue damage and might be involved in neuropathic pain. Platelets are the greatest producers of LPA and the plasma is its major source. Lysophosphatidic acid receptors (LPA1 to 4) are distributed in the central and peripheral nervous system. They are also coupled to protein Ga12/13 and when activated, they activate GTPase and RHoA and a series of second messengers, including Rho kinase or ROCK. The intrathecal injection of LPA in animals causes mechanical allodynia and thermal hyperalgesia. There is a drastic reduction of myelin-associated proteins (myelin basic and peripheral protein, MPB and PMP 22, respectively) and an increase in the expression of phosphokinase C and calcium channels (Cavα2δ1) with nerve degeneration, synaptic budding, increased neuronal excitability 64.
NEUROPATHIC PAIN AND NERVE DEGENERATION
Calcium influx is one of the first events after nerve damage. Calcium initiates tissue cicatrization grouping vesicles in the axolema around damaged distal and proximal nerve endings. The increase in intracellular calcium also activates calcium-dependent protein kinases and other proteases (UPS) that facilitate the release of cytokines and destruction of microtubules and microfilaments, which are important in the axonal transport of substances and in the mechanism of neuropathic pain. This leads to an accumulation of SP, CGRP, nitric oxide, neurotrophic factors, and sodium channels that, together, amplify neuronal excitability 65.
Although peripheral lesions of C fibers lead to a reduction of SP, neurotrophins (BDNF), receptors (VR1 and P2X3), and type N high-voltage calcium channels in the dorsal horn of the spinal cord, there is ascending regulation of type III TTX-sensitive (TTXs) channels and translocation of TTX-resistant (TTXr) sodium channels from the soma to the neuroma, increasing nerve excitability 66. The expression of Nav 1.3 TTXs channels, whose kinetic characteristic is to be easily activated and inactivated, can increase by two to 30-fold in models of dorsal root ganglion axotomy, ligature of the spinal nerve, chronic constrictive damage, diffuse nerve damage, diabetic neuropathy, and post-herpetic neuropathy 11.
When the nerve fiber is intact (inflammation), excitatory neuromediators increase in the dorsal horn of the spinal cord, as well as the expression of sodium channels (TTXr), which facilitates neuronal hyperexcitability and hinders the response to treatment with local anesthetics 66.
In both situations described, especially in lesions of C fibers, there can also be an increase in SP and BDNF in Aβ fibers (low threshold mechanoreceptors), as well as budding of those fibers at the site of afferent connections of C fibers (lamina II), widening the receptive field of the neuron and leading to the interpretation of innocuous peripheral mechanical stimuli as aggressive 67. This explains, for example, the mechanical allodynia that is seen in post-herpetic neuralgia 68. Sprouting of the sympathetic noradrenergic dorsal root ganglion axon around large-caliber neurons (Aδ fibers) would be another possibility, suggesting the hypothesis of activation of sensitive afferent fibers after sympathetic stimulation 69. Besides, some disproportion between excitatory and pain suppression pathways can be present, with reduction in the inhibitory activity of glycine, GABA, and opioids 66.
In models of neuropathic pain in animals, the inhibitory effect of descending fibers originating in the periaqueductal gray matter and locus ceruleus can show a 50% reduction from the baseline. There is reduced efficacy of the opioid system in the spinal cord, with reduction of β-endorphins in the brain and spinal cord, with the consequent reduction of the analgesic effects of morphine, both by systemic and spinal administration. This suggests that opioid receptors or other factors that are necessary for the activation of the spinal opioid system are compromised after nerve damaged 2,3. On the other hand, programmed cell death can occur in dorsal horn neurons that express c-jun. This involves studies of the Bcl-2 and Bax gene families that inhibit and favor cell death, respectively. In mice with sectioned nerve and in which the expression of c-jun is increased, the Bcl-2/Bax relationship is reduced, indicating a tendency for apoptosis and deafferentation of post-synaptic spinal neurons 14.
The destruction of the posterior cord or the gracilis nucleus ipsilateral to the lesion can abolish mechanical allodynia in mice. On the other hand, allodynia can also be inhibited by the selective destruction of descending pathways of the brainstem and dorsolateral cord, indicating a role of ascending and descending pathways in changes secondary to neuropathic pain. Thermal allodynia can be abolished by the injection of lidocaine or cholecystokinin (CCK) receptor antagonists in the ventromedial area of the medulla (raphe magnus nucleus). Descending facilitation involving the median raphe nucleus in the medulla (NRM) and CCK can occur in nerve damage. Therefore, severing descendent fibers can reduce considerably mechanical or thermal allodynia in animals. Medulla-spinal impulses are also capable to activate post-synaptic sympathetic neurons that contribute for the maintenance of neuropathic pain 2,3,14.
As for the neurophysiological aspects of neuropathic pain in SNL, the spontaneous neuronal activity of somatosensorial regions of the thalamus (ventral posteromedial nucleus, VPM, and ventral posterior lateral nucleus, VPL) is increased and it can be inhibited by substances that block the expression of Nav 1.3 sodium channels. On the other hand, functional imaging of the thalamus contralateral to the neuropathic pain shows reduced metabolism. In the cerebral cortex, the behavior of 10% of the neurons is similar to the thalamic neurons, especially those located in the periphery of the deafferented zone of the primary somesthetic cortex. The electroencephalogram shows an increase in theta frequency (4 to 5 Hz) and the metabolism or the cortical output of the insular, posterior parietal, pre-frontal, and cingulate areas is elevated. The opposite is seen in the medial (Brodmann area 24), and anterior and perigenual (Brodmann areas 32 and 35) cingulate cortices. Some researchers demonstrated that the increased perigenual output after motor cortical stimulation of the Gasser ganglion reduces pain severity. This strengthens the hypothesis of the relationship between perigenual changes and failure of the descending inhibitory control. A reduction of opioid receptors in the periaqueductal gray matter, in medial thalamus, pre-frontal cortex, and ínsula due to internalization is also seen. This reduction is bilateral and symmetrical and preponderant in the ipsilateral side in the case of a central lesion 2.
Neuropathic pain is still a challenge for clinical and experimental researchers. Its mechanisms, which are complex and not completely understood result occasionally in dynamics with contradictory results. Understanding the neurobiology of neuropathic pain is a step towards the improvement in the treatment of this syndrome. This understanding may result in the development of drugs aimed at specific targets with effective responses.
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Correspondence to: Submitted em 16
de fevereiro de 2008 *
Received from Faculdade de Medicina da Universidade Federal da Bahia (UFBA),
Dr. Durval Campos Kraychete
Rua Rio de São Pedro, 327/401 Graça
40150-350 Salvador, BA
Accepted para publicação em 23 de junho de 2008
Submitted em 16
de fevereiro de 2008
* Received from Faculdade de Medicina da Universidade Federal da Bahia (UFBA), Salvador, BA