The role of the dorsal columns in neuropathic behavior: evidence for plasticity and non-specificity

Naked mole-rats lack cold sensitivity before and after nerve injury

Neuropathic pain is a chronic disease state resulting from injury to the nervous system. This type of pain often responds poorly to standard treatments and occasionally may get worse instead of better over time. Patients who experience neuropathic pain report sensitivity to cold and mechanical stimuli. Since the nociceptive system of African naked mole-rats contains unique adaptations that result in insensitivity to some pain types, we investigated whether naked mole-rats may be resilient to sensitivity following nerve injury. Using the spared nerve injury model of neuropathic pain, we showed that sensitivity to mechanical stimuli developed similarly in mice and naked mole-rats. However, naked mole-rats lacked sensitivity to mild cold stimulation after nerve injury, while mice developed robust cold sensitivity. We pursued this response deficit by testing behavior to activators of transient receptor potential (TRP) receptors involved in detecting cold in naïve animals. Following mustard oil, a TRPA1 activator, naked mole-rats responded similarly to mice. Conversely, icilin, a TRPM8 agonist, did not evoke pain behavior in naked mole-rats when compared with mice. Finally, we used RNAscope to probe for TRPA1 and TRPM8 messenger RNA expression in dorsal root ganglia of both species. We found increased TRPA1 messenger RNA, but decreased TRPM8 punctae in naked mole-rats when compared with mice. Our findings likely reflect species differences due to evolutionary environmental responses that are not easily explained by differences in receptor expression between the species.

Allodynia and hyperalgesia: review

The main purpose was to highlight the problem of hyperalgesia and allodynia. Main anatomic structures, which participate in nociception were mentioned in this article, with pathologic and pathophysiologic changes, that can be caused by hyperalgesia and allodynia. Main methods of diagnostics and assessment of mentioned symptoms were represented along with the modern approaches to treatment and prevention.

Characterization of Macrophage/Microglial Activation and Effect of Photobiomodulation in the Spared Nerve Injury Model of Neuropathic Pain

Objective

Neuropathic pain is common and debilitating with limited effective treatments. Macrophage/microglial activation along ascending somatosensory pathways following peripheral nerve injury facilitates neuropathic pain. However, polarization of macrophages/microglia in neuropathic pain is not well understood. Photobiomodulation treatment has been used to decrease neuropathic pain, has anti-inflammatory effects in spinal injury and wound healing models, and modulates microglial polarization in vitro. Our aim was to characterize macrophage/microglia response after peripheral nerve injury and modulate the response with photobiomodulation.

Methods

Adult male Sprague-Dawley rats were randomly assigned to sham (N = 13), spared nerve injury (N = 13), or injury + photobiomodulation treatment groups (N = 7). Mechanical hypersensitivity was assessed with electronic von Frey. Photobiomodulation (980 nm) was applied to affected hind paw (output power 1 W, 20 s, 41cm above skin, power density 43.25 mW/cm 2 , dose 20 J), dorsal root ganglia (output power 4.5W, 19s, in skin contact, power density 43.25 mW/cm 2 , dose 85.5 J), and spinal cord regions (output power 1.5 W, 19s, in skin contact, power density 43.25 mW/cm 2 , dose 28.5 J) every other day from day 7-30 post-operatively. Immunohistochemistry characterized macrophage/microglial activation.

Results

Injured groups demonstrated mechanical hypersensitivity 1-30 days post-operatively. Photobiomodulation-treated animals began to recover after two treatments; at day 26, mechanical sensitivity reached baseline. Peripheral nerve injury caused region-specific macrophages/microglia activation along spinothalamic and dorsal-column medial lemniscus pathways. A pro-inflammatory microglial marker was expressed in the spinal cord of injured rats compared to photobiomodulation-treated and sham group. Photobiomodulation-treated dorsal root ganglion macrophages expressed anti-inflammatory markers.

Conclusion

Photobiomodulation effectively reduced mechanical hypersensitivity, potentially through modulating macrophage/microglial activation to an anti-inflammatory phenotype.

Spatial and Cellular Expression Patterns of Erythropoietin-Receptor and Erythropoietin during a 42-Day Post-Lesional Time Course after Graded Thoracic Spinal Cord Impact Lesions in the Rat

Erythropoietin (Epo) exhibits promising neuroregenerative potential for spinal cord injury (SCI), and might be involved in other long-term sequelae, such as neuropathic pain development. The current studies investigated the time courses and spatial and cellular patterns of Epo and erythropoietin receptor (EpoR) expression along the spinal axis after graded SCI. Male Long Evans rats received 100 kdyn, 150 kdyn, and 200 kdyn thoracic (T9) contusions from an Infinite Horizon impactor. Sham controls received laminectomies. Anatomical and quantitative immunohistochemical analyses of the EpoR/Epo expression along the whole spinal axis were performed 7, 15, and 42 postoperative days (DPO) after the lesioning. Cellular expression was investigated by double- and triple-labeling for EpoR/Epo with cellular markers and proliferating cells in subgroups of 5-bromo-2-deoxyuridine pre-treated animals. Prolonged EpoR/Epo-expression was confirmed by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Quantified EpoR/Epo immunoreactivities in pain-related spinal cord regions and ventrolateral white matter (VLWM) were correlated with the mechanical sensitivity thresholds and locomotor function of the respective animals. EpoR and Epo were constitutively expressed in the ventral horn neurons and vascular and glial cells in the dorsal columns (DC) and the VLWM. After SCI, in addition to expression in the lesion core, EpoR/Epo immunoreactivities exhibited significant time- and lesion grade-dependent induction in the DC and VLWM along the spinal axis. EpoR and Epo immunoreactive cells were co-stained with markers for astroglial, neural precursor cell and vascular markers. In the VLWM, EpoR- and Epo-positive proliferating cells were co-stained with glial fibrillary acidic protein (GFAP) and nestin. The DC EpoR/Epo immunoreactivities exhibited linear relationships with the behavioral correlates of post-lesional chronic pain development at DPO 42. SCI leads to long-lasting multicellular EpoR/Epo induction beyond the lesion core in the spinal cord regions that are involved in central pain development and regenerative processes. Our studies provide a time frame to investigate the effects of Epo application on motor function or pain development, especially in the later time course after lesioning.

Acute Pain Management

Acute pain management is improving steadily over the past few years, but training and professional education are still lacking in many professions. Untreated or undertreated acute pain could have detrimental effects on the patient in terms of comfort and recovery from trauma or surgery. Acute undertreated pain can decrease a patient's vascular perfusion, increase oxygen demand, suppress the immune system, and possibly risk increased incidence of venous thrombosis. Although acute postoperative pain needs to be managed aggressively, patients are most vulnerable during this period for developing adverse effects, and therefore, patient assessment and careful drug therapy evaluation are necessary processes in therapeutic planning. Acute pain management requires careful and thorough initial assessment and follow-up reassessment in addition to frequent dosage adjustments, and managing analgesic induced side effects. Analgesic selection and dosing must be based on the patient's past and recent analgesic exposure. There is no single acute pain management regimen that is suitable for all patients. Analgesics must be tailored to the individual patient.

Enhanced Expression of TREK-1 Is Related with Chronic Constriction Injury of Neuropathic Pain Mouse Model in Dorsal Root Ganglion

Neuropathic pain is a complex state showing increased pain response with dysfunctional inhibitory neurotransmission. The TREK family, one of the two pore domain K⁺ (K2P) channel subgroups were focused among various mechanisms of neuropathic pain. These channels influence neuronal excitability and are thought to be related in mechano/thermosensation. However, only a little is known about the expression and role of TREK-1 and TREK-2, in neuropathic pain. It is performed to know whether TREK-1 and/ or 2 are positively related in dorsal root ganglion (DRG) of a mouse neuropathic pain model, the chronic constriction injury (CCI) model. Following this purpose, Reverse Transcription Polymerase Chain Reaction (RT-PCR) and western blot analyses were performed using mouse DRG of CCI model and compared to the sham surgery group. Immunofluorescence staining of isolectin- B4 (IB4) and TREK were performed. Electrophysiological recordings of single channel currents were analyzed to obtain the information about the channel. Interactions with known TREK activators were tested to confirm the expression. While both TREK-1 and TREK-2 mRNA were significantly overexpressed in DRG of CCI mice, only TREK-1 showed significant increase (~9 fold) in western blot analysis. The TREK-1-like channel recorded in DRG neurons of the CCI mouse showed similar current-voltage relationship and conductance to TREK-1. It was easily activated by low pH solution (pH 6.3), negative pressure, and riluzole. Immunofluorescence images showed the expression of TREK-1 was stronger compared to TREK-2 on IB4 positive neurons. These results suggest that modulation of the TREK-1 channel may have beneficial analgesic effects in neuropathic pain patients.

Leukotriene enhances NMDA-induced inward currents in dorsal horn neurons of the rat spinal cord after peripheral nerve injury

BACKGROUND

LTB4 is classified as a leukotriene (LT), a group of lipid mediators that are derived from arachidonic acid. It is recognized that leukotrienes are involved in the pathogenesis of many diseases, including peripheral inflammatory pain. However, little is known about the effects of leukotrienes on the spinal dorsal horn during neuropathic pain. Previously, we reported that there was increased expression of 5-lipoxygenase (5-LO) at spinal microglia, and the leukotriene B4 receptor 1 (BLT1), a high affinity receptor of LTB4, in spinal neurons in spared nerve injury (SNI) model rats. In the present study, we examined the effects of LTB4 on spinal dorsal horn neurons in both naïve and SNI model rats using patch-clamp methods.

RESULTS

Bath application of LTB4 did not change AMPA receptor-mediated spontaneous excitatory postsynaptic currents (sEPSCs) or membrane potentials. However, we found that LTB4 enhanced the amplitude of NMDA receptor-mediated sEPSCs and significantly increased exogenous NMDA-induced inward currents in SNI model rats. This increase of inward currents could be inhibited by a selective LTB4 antagonist, U75302, as well as a GDP-β-S, a G-protein inhibitor. These results indicate that both increased LTB4 from spinal microglia or increased BLT1 in spinal neurons after peripheral nerve injury can enhance the activity of NMDA receptors through intracellular G-proteins in spinal dorsal horn neurons.

CONCLUSION

Our findings showed that LTB4, which may originate from microglia, can activate BLT1 receptors which are expressed on the membrane of spinal dorsal horn neurons during neuropathic pain. This glia-neuron interaction induces the enhancement of NMDA currents through intracellular G-proteins. The enhancement of NMDA receptor sensitivity of dorsal horn neurons may lead to central sensitization, leading to mechanical pain hypersensitivity.

The role of the dorsolateral funiculi in the pain relieving effect of spinal cord stimulation: a study in a rat model of neuropathic pain

Activation of the dorsal columns is relayed to supraspinal centers, involved in pain modulation, probably via the descending fibers in the dorsolateral funiculi (DLF). The present study examines the role of the DLF in the attenuation of pain-related signs by spinal cord stimulation (SCS). Several groups of rats were subjected to nerve injury and to chronic bilateral DLF lesions at C5-7 level. In each animal, two sets of miniature electrodes were implanted, a caudal system placed in the dorsal epidural space at low thoracic level and another implanted over the dorsal column nuclei, rostral to the lesions. Stimulation (50 Hz, 0.2 ms; 70 % of motor threshold) was applied for 5 min via either of the electrodes. Behavioral tests were used to assess the effects of SCS on the nerve injury-induced mechanical and cold hypersensitivity and heat hyperalgesia. Prior to application of SCS, antagonists to either of GABAA or B, 5-HT1 or 1-2 or α/β-adrenergic receptors were injected i.p. Both stimulations produced comparable decreases (80-90 % of the control) of neuropathic manifestations in rats with intact spinal cords. DLF lesions attenuated the effects of both types of stimulation by about 50 %. Pretreatment with receptor antagonists differentially counteracted the effects of rostral and caudal stimulation; the inhibition with rostral stimulation generally being more prominently influenced. These results provide further support to the notion of important involvement of brainstem pain modulating centers in the effects of SCS. A major component of the inhibitory spinal-supraspinal-spinal loop is mediated by fibers running in the DLF.

Comparison of intensity-dependent inhibition of spinal wide-dynamic range neurons by dorsal column and peripheral nerve stimulation in a rat model of neuropathic pain

BACKGROUND

Spinal cord stimulation (SCS) and peripheral nerve stimulation (PNS) are thought to reduce pain by activating a sufficient number of large myelinated (Aβ) fibres, which in turn initiate spinal segmental mechanisms of analgesia. However, the volume of neuronal activity and how this activity is associated with different treatment targets is unclear under neuropathic pain conditions.

METHODS

We sought to delineate the intensity-dependent mechanisms of SCS and PNS analgesia by in vivo extracellular recordings from spinal wide-dynamic range neurons in nerve-injured rats. To mimic therapeutic SCS and PNS, we used bipolar needle electrodes and platinum hook electrodes to stimulate the dorsal column and the tibial nerve, respectively. Compound action potentials were recorded to calibrate the amplitude of conditioning stimulation required to activate A-fibres and thus titrate the volume of activation.

RESULTS

Dorsal column stimulation (50 Hz, five intensities) inhibited the windup (a short form of neuronal sensitization) and the C-component response of wide-dynamic range neurons to graded intracutaneous electrical stimuli in an intensity-dependent manner. Tibial nerve stimulation (50 Hz, three intensities) also suppressed the windup in an intensity-dependent fashion but did not affect the acute C-component response.

CONCLUSIONS

SCS and PNS may offer similar inhibition of short-term neuronal sensitization. However, only SCS attenuates spinal transmission of acute noxious inputs under neuropathic pain conditions. Our findings begin to differentiate peripheral from spinal-targeted neuromodulation therapies and may help to select the best stimulation target and optimum therapeutic intensity for pain treatment.

Bortezomib-induced painful peripheral neuropathy: an electrophysiological, behavioral, morphological and mechanistic study in the mouse

Bortezomib is the first proteasome inhibitor with significant antineoplastic activity for the treatment of relapsed/refractory multiple myeloma as well as other hematological and solid neoplasms. Peripheral neurological complications manifesting with paresthesias, burning sensations, dysesthesias, numbness, sensory loss, reduced proprioception and vibratory sensitivity are among the major limiting side effects associated with bortezomib therapy. Although bortezomib-induced painful peripheral neuropathy is clinically easy to diagnose and reliable models are available, its pathophysiology remains partly unclear.

In this study we used well-characterized immune-competent and immune-compromised mouse models of bortezomib-induced painful peripheral neuropathy. To characterize the drug-induced pathological changes in the peripheral nervous system, we examined the involvement of spinal cord neuronal function in the development of neuropathic pain and investigated the relevance of the immune response in painful peripheral neuropathy induced by bortezomib.

We found that bortezomib treatment induced morphological changes in the spinal cord, dorsal roots, dorsal root ganglia (DRG) and peripheral nerves. Neurophysiological abnormalities and specific functional alterations in Aδ and C fibers were also observed in peripheral nerve fibers. Mice developed mechanical allodynia and functional abnormalities of wide dynamic range neurons in the dorsal horn of spinal cord. Bortezomib induced increased expression of the neuronal stress marker activating transcription factor-3 in most DRG. Moreover, the immunodeficient animals treated with bortezomib developed a painful peripheral neuropathy with the same features observed in the immunocompetent mice.

In conclusion, this study extends the knowledge of the sites of damage induced in the nervous system by bortezomib administration. Moreover, a selective functional vulnerability of peripheral nerve fiber subpopulations was found as well as a change in the electrical activity of wide dynamic range neurons of dorsal horn of spinal cord. Finally, the immune response is not a key factor in the development of morphological and functional damage induced by bortezomib in the peripheral nervous system.

Peripheral neuropathy induces cutaneous hypersensitivity in chronically spinalized rats

BACKGROUND/OBJECTIVES

The present study was aimed at the issue of whether peripheral nerve injury-induced chronic pain is maintained by supraspinal structures governing descending facilitation to the spinal dorsal horn, or whether altered peripheral nociceptive mechanisms sustain central hyperexcitability and, in turn, neuropathic pain. We examined this question by determining the contribution of peripheral/spinal mechanisms, isolated from supraspinal influence(s), in cutaneous hypersensitivity in an animal model of peripheral neuropathy.

METHODS

Adult rats were spinalized at T8-T9; 8 days later, peripheral neuropathy was induced by implanting a 2-mm polyethylene cuff around the left sciatic nerve. Hind paw withdrawal responses to mechanical or thermal plantar stimulation were evaluated using von Frey filaments or a heat lamp, respectively.

RESULTS

Spinalized rats without cuff implantation exhibited a moderate decrease in mechanical withdrawal threshold on ~day 10 (P < 0.05) and in thermal withdrawal threshold on ~day 18 (P < 0.05). However, cuff-implanted spinalized rats developed a more rapid and significant decrease in mechanical (~day 4; P < 0.001) and thermal (~day 10; P < 0.05) withdrawal thresholds that remained significantly decreased through the duration of the study.

CONCLUSIONS

Our findings demonstrate an aberrant peripheral/spinal mechanism that induces and maintains thermal and to a greater degree tactile cutaneous hypersensitivity in the cuff model of neuropathic pain, and raise the prospect that altered peripheral/spinal nociceptive mechanisms in humans with peripheral neuropathy may have a pathologically relevant role in both inducing and sustaining neuropathic pain.

Spatiotemporal CCR1, CCL3(MIP-1α), CXCR4, CXCL12(SDF-1α) expression patterns in a rat spinal cord injury model of posttraumatic neuropathic pain

Object

Central neuropathic pain is a frequent challenging complication after spinal cord injury (SCI), and specific therapeutic approaches remain elusive. The purpose of the present investigations was to identify potential key mediators of these pain syndromes by analyzing detailed expression profiles of important chemokines in an experimental SCI paradigm of posttraumatic neuropathic pain in rats.

Methods

Expression of CCR1, CCL3(MIP-1α), CXCR4, and CXCL12(SDF-1α) was investigated in parallel with behavioral testing for mechanical and thermal nociceptive thresholds after standardized SCI; 100-kdyn (moderate injury) and 200-kdyn (severe injury) force-defined thoracic spinal cord contusion lesions were applied via an Infinite Horizon Impactor at the T-9 level. Sham controls received laminectomies. Hindlimb locomotor function as well as mechanical and thermal sensitivities were monitored weekly by standardized behavioral testing after SCI. Chemokine expression was analyzed by real-time reverse transcriptase polymerase chain reaction in the early (7 days postoperatively) and late (42 days postoperatively) time courses after SCI, and immunohistochemical analysis (anatomical and quantitative) was performed 2, 7, 14, and 42 days after lesioning. Double staining with cellular markers and pain-related peptides (substance P and CGRP) or receptors (TRPV-1, TRPV-2, VRL-1, and TLR-4) was performed. Based on data obtained from behavioral testing, quantified immunohistochemical chemokine expressions in individual animals were correlated with the respective mechanical and thermal sensitivity thresholds 6 weeks after SCI.

Results

After 200-kdyn lesions, the animals exhibited prolonged reduction in their nociceptive thresholds, while 100-kdyn groups showed pain-related behaviors only in the early time course after SCI. Investigated chemokines were widely induced after SCI, involving cervical, thoracic, and lumbar spinal cord levels far beyond the lesion core. CCR1 and CCL3 were induced significantly in the dorsal horns 2 days after lesioning and remained at high levels after SCI with significantly higher intensities after 200-kdyn than 100-kdyn contusions. CXCR4 and CXCL12 levels continuously increased from 2 to 42 days after moderate and severe lesions. Additionally, chemokines were induced significantly in dorsal columns, with highest density levels 42 days after 200-kdyn lesions. In dorsal horns, CCR1 was coexpressed with TRPV-1 while CXCR4 and CXCL12 were partially coexpressed with substance P and CGRP. In dorsal columns, CCL3/CCR1 colabeled with GFAP, TRPV-2, TRPV-1, TLR-4; CXCR4/CXCL12 coexpressed with GFAP, CD68/ED1, and TLR4. Chemokine immunoreactivity density levels, especially CCL3 and its receptor, correlated in part significantly with nociceptive thresholds.

Conclusions

The authors report lesion grade–dependent upregulation of different chemokines/chemokine receptors after spinal cord contusion lesions in pain-processing spinal cord regions in a clinically relevant model of traumatic SCI in rats. Prolonged chemokine induction further correlated with below-level pain development in the delayed time course after severe SCI and was coexpressed with pain-associated peptides and receptors, suggesting that chemokines play a crucial role in chronic central pain mechanisms after SCI.

Caudal granular insular cortex is sufficient and necessary for the long-term maintenance of allodynic behavior in the rat attributable to mononeuropathy

Mechanical allodynia, the perception of innocuous tactile stimulation as painful, is a severe symptom of chronic pain often produced by damage to peripheral nerves. Allodynia affects millions of people and remains highly resistant to classic analgesics and therapies. Neural mechanisms for the development and maintenance of allodynia have been investigated in the spinal cord, brainstem, thalamus, and forebrain, but manipulations of these regions rarely produce lasting effects. We found that long-term alleviation of allodynic manifestations is produced by discreetly lesioning a newly discovered somatosensory representation in caudal granular insular cortex (CGIC) in the rat, either before or after a chronic constriction injury of the sciatic nerve. However, CGIC lesions alone have no effect on normal mechanical stimulus thresholds. In addition, using electrophysiological techniques, we reveal a corticospinal loop that could be the anatomical source of the influence of CGIC on allodynia.

Chronic dizocilpine or apomorphine and development of neuropathy in two rat models I: behavioral effects and role of nucleus accumbens

Dopaminergic and glutamatergic inputs converge on nucleus accumbens (NAC) and affect the neuropathic pain. We tested the effects of daily systemic administration of dizocilpine (MK-801), a N-methyl-d-Aspartate (NMDA) noncompetitive receptor antagonist, or apomorphine (APO), a dopamine (DA) D1 and D2 receptor agonist, on neuropathic manifestations in the chronic constriction injury (CCI) and the spared nerve injury (SNI) models of mononeuropathy in rats. Six groups of rats were subjected to CCI or SNI neuropathy and 5-7 days later received daily intraperitoneal (ip) injections of saline, MK-801, or APO for two weeks. Tests for nociception and motor behaviors were performed at regular intervals. Tactile and cold allodynia were assessed using von Frey hairs or acetone drops, respectively. Heat hyperalgesia was assessed by the paw withdrawal test. Tests were performed before administering the daily injections. Another four groups of rats were subjected to SNI surgery, and then had their NAC (contralateral to the lesioned paw) perfused for two weeks with MK-801, saline, APO+ascorbic acid, or ascorbic acid alone using mini-osmotic pumps. Behavioral manifestations were assessed as above. Systemic daily injections of MK-801 and APO markedly attenuated the neuropathic manifestations in the CCI and SNI models with a minimal effect on cold allodynia. The same results were seen in the SNI model with chronic perfusion of NAC. Our results suggest that daily systemic administration of DA agonists and NMDA antagonists can attenuate neuropathic pain manifestations and that the NAC is involved in the modulation of neuropathic-like behaviors.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Ectopic discharge in Abeta afferents as a source of neuropathic pain

Ectopic discharge in axotomized dorsal root ganglion neurons is a key driver of neuropathic pain. However, the bulk of this activity is generated and carried centrally in large diameter myelinated Abeta afferents, a cell type that normally signals touch and vibration sense. Evidence is considered suggesting that following axotomy, Abeta afferents undergo a change in their electrical characteristics and also in the neurotransmitter complement that they express. This dual phenotypic switching renders them capable of (1) directly driving postsynaptic pain signaling pathways in the spinal cord, and (2) triggering and maintaining central sensitization.

Towards a theory of chronic pain

In this review, we integrate recent human and animal studies from the viewpoint of chronic pain. First, we briefly review the impact of chronic pain on society and address current pitfalls of its definition and clinical management. Second, we examine pain mechanisms via nociceptive information transmission cephalad and its impact and interaction with the cortex. Third, we present recent discoveries on the active role of the cortex in chronic pain, with findings indicating that the human cortex continuously reorganizes as it lives in chronic pain. We also introduce data emphasizing that distinct chronic pain conditions impact on the cortex in unique patterns. Fourth, animal studies regarding nociceptive transmission, recent evidence for supraspinal reorganization during pain, the necessity of descending modulation for maintenance of neuropathic behavior, and the impact of cortical manipulations on neuropathic pain is also reviewed. We further expound on the notion that chronic pain can be reformulated within the context of learning and memory, and demonstrate the relevance of the idea in the design of novel pharmacotherapies. Lastly, we integrate the human and animal data into a unified working model outlining the mechanism by which acute pain transitions into a chronic state. It incorporates knowledge of underlying brain structures and their reorganization, and also includes specific variations as a function of pain persistence and injury type, thereby providing mechanistic descriptions of several unique chronic pain conditions within a single model.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Neurochemical characterization of neuronal populations expressing protein kinase C gamma isoform in the spinal cord and gracile nucleus of the rat

Protein kinase C gamma (PKCgamma) is widely distributed throughout the CNS and is thought to play a role in long term hyper-excitability in nociceptive neurones. Here, we provide the first report of PKCgamma cells in the dorsal column nuclei of the adult rat. Retrograde labeling of PKCgamma cells from the thalamus with choleragenoid revealed that 25% of the PKCgamma positive gracile cells projected to the thalamus. Further, we have characterized the distribution of PKCgamma within gracile nucleus in terms of colocalization with various neurotransmitter receptors or enzymes and calcium binding proteins, and compared this with PKCgamma colocalization in cells of laminae I-III of the spinal cord. We show that approximately 90% of the PKCgamma cells in the gracile nucleus and 60% in the dorsal horn were immuno-positive for the AMPA receptor subunit glutamate 2/3 (GluR2/3). Little coexpression was seen with neurokinin 1 receptor, nitric oxide synthase (NOS) and the AMPA receptor subunit GluR1, markers of distinct neuronal subpopulations. In the spinal cord, a quarter of PKCgamma cells expressed calbindin, but very few cells did so in the gracile nucleus. Electrical stimulation at c-fiber strength of the normal or injured sciatic nerve was used to induce c-fos as a marker of postsynaptic activation in the spinal cord and gracile nucleus. Quantitative analysis of the number of PKCgamma positive gracile cells that expressed also c-fos increased from none to 24% after injury, indicating an alteration in the sensory activation pattern in these neurones after injury. C-fos was not induced in inner lamina II following c-fiber electrical stimulation of the intact or axotomized sciatic nerve, indicating no such plasticity at the spinal cord level. As dorsal column nuclei cells may contribute to allodynia after peripheral nerve injury, pharmacological modulation of PKCgamma activity may therefore be a possible way to ameliorate neuropathic pain after peripheral nerve injury.

Reversible attenuation of neuropathic-like manifestations in rats by lesions or local blocks of the intralaminar or the medial thalamic nuclei

BACKGROUND AND AIM

Thalamic somatosensory nuclei have been classified into medial and lateral systems based on their role in nociception. An imbalance between these two systems may result in abnormal somatic sensations and spontaneous pain. This study aims to investigate the effects of transient or permanent block of the medial and intralaminar nuclear groups on the neuropathic-like behavior in a rat model for mononeuropathy.

METHODS

Neuropathy was induced on one hind paw in different groups of rats following the spared nerve injury model. When the resulting hyperalgesia and allodynia (tactile and cold) reached a maximum plateau, the rats received either chemical or electrolytic lesion or lidocaine (2%) microperfusion, placed in the various thalamic nuclear groups.

RESULTS

All procedures produced transient but significant decrease of neuropathic manifestations. The magnitude and duration of decrease depended on the type and the site of the block. These effects can be ranked in increasing order as follows, electrolytic<chemical<lidocaine micro-perfusion according to the procedure, and as rostro-medial<ventro-median<parafascicular nuclei, according to the site of the block. Thermal hyperalgesia was the most affected while cold allodynia showed the least attenuation. Neuropathic manifestations returned to their pre-lesion levels after 2-3 weeks, along with frequently observed delayed hyper-responsiveness to the hotplate test.

CONCLUSION

The observed results demonstrate the involvement of the medial and intralaminar thalamic nuclei in the processing of neuropathic-like manifestations, and the reversibility of the effects suggests the flexibility of the neural network involved in supraspinal processing of nociceptive information.

Spinal pathways involved in supraspinal modulation of neuropathic manifestations in rats

Controversial results have been recently reported on the role of supraspinal centers in the modulation of nociceptive behavior in animal models of mononeuropathy. Our aim was to investigate the role of the various spinal pathways in the modulation of the neuropathic manifestations. Several groups of rats were subjected to selective spinal-tract lesions, either 2-3 weeks before or 2-3 weeks after the induction of mononeuropathy following the chronic constriction injury (CCI) or the spared nerve injury (SNI) models. Tactile and cold allodynias were assessed by Von Frey filaments and the acetone drops test, respectively. Thermal hyperalgesia was assessed by the paw withdrawal and the hot plate tests. The effects of unilateral and bilateral lesions of the dorso-lateral funiculus (DLF), the anterolateral column (ALC) or hemisection were tested over a period of 4-8 weeks. All spinal tract lesions produced reversible, but significant decrease of allodynia and hyperalgesia over a period of 1-3 weeks. The most pronounced effects were observed with bilateral lesions. The stronger attenuation was observed on thermal hyperalgesia, assessed by the paw withdrawal test, while cold allodynia was the least affected. Spinal lesions performed before the induction of neuropathy did not produce significant alterations in the temporal development of neuropathic manifestations. The present results allow the conclusion that all spinal tracts can be involved in the rostral transmission and the descending modulation of neuropathic manifestations. The recovery of symptoms following spinal lesions provides illustration on the plasticity of the neural network involved in the processing of the neuropathic syndromes.

Loss of spinal mu-opioid receptor is associated with mechanical allodynia in a rat model of peripheral neuropathy

The present study investigated whether the loss of spinal mu-opioid receptors following peripheral nerve injury is related to mechanical allodynia. We compared the quantity of spinal mu-opioid receptor and the effect of its antagonists, such as naloxone and CTOP, on pain behaviors in two groups of rats that showed extremely different severity of mechanical allodynia 2 weeks following partial injury of tail-innervating nerves. One group (allodynic group) exhibited robust signs of mechanical allodynia after the nerve injury, whereas the other group (non-allodynic group) showed little allodynia despite having suffered the same nerve injury. In addition, we investigated the quantity of spinal mu-opioid receptor and the effect of its antagonists on pain behaviors after the rats had recovered from mechanical allodynia 16 weeks following nerve injury. Immunohistochemical and Western blot analyses at 2 weeks after nerve injury indicated that spinal mu-opioid receptor content was more reduced in the allodynic group compared to the non-allodynic group. Intraperitoneal naloxone (2 mg/kg, i.p.) and intrathecal CTOP (10 microg/rat, i.t.) administration dramatically induced mechanical allodynia in the non-allodynic group. However, as in naïve animals, neither the loss of spinal mu-opioid receptors nor antagonist-induced mechanical allodynia was observed in the rats that had recovered from mechanical allodynia. These results suggest that the loss of spinal mu-opioid receptors following peripheral nerve injury is related to mechanical allodynia.

Transient attenuation of neuropathic manifestations in rats following lesion or reversible block of the lateral thalamic somatosensory nuclei

BACKGROUND AND AIMS

Nociceptive behavior in animal models for mononeuropathy has been shown to be altered by spinal tract lesions which suggest a possible supraspinal modulation. The thalamus constitutes a chief center for the processing of nociception. We have, therefore, investigated the effects of transient or permanent blocks of the lateral somatosensory thalamic nuclei (the ventrobasal complex) on the neuropathic manifestations in rats.

METHODS

Different groups of rats (n = 5-6) were subjected to mononeuropathy, following the spared nerve injury model, known to produce sustained heat hyperalgesia and tactile and cold allodynia which peaked about 2 weeks after nerve injury. This was followed by stereotaxic placement of either electrolytic or chemical lesions or implantation of mini osmotic pump for slow release of lidocaine in the ventrobasal complex.

RESULTS

Chronic electrolytic and chemical lesions or reversible block of the lateral somatosensory thalamus produced transient (1-2 weeks) attenuation of neuropathic manifestations along with a persistent decrease of the hot plate latency. The most pronounced effect was observed on heat hyperalgesia, and the least significant and short-lived effect was observed on cold allodynia.

CONCLUSION

We conclude that the lateral somatosensory thalamic complex is involved in the processing of neuropathic manifestations but cannot be considered as an obligatory or exclusive relay center for the neuropathic syndromes.

Laser evoked potentials and prepulse inhibition of the blink reflex in patients with Wallenberg's syndrome

Spinothalamic tract lesions in patients with Wallenberg's syndrome can be demonstrated by abnormalities in the laser evoked potentials (LEPs) to stimulation of the affected side. However, before reaching the structures generating LEPs, laser stimuli can induce effects at a subcortical level. We examined LEPs and laser-induced prepulse inhibition of the blink reflex in seven patients with Wallenberg's syndrome within a month after the infarct. All patients had abnormally elevated thresholds for temperature and pain sensation, and for pinprick pain induced by laser stimuli, in the affected vs the non-affected side. LEPs to stimulation of the affected side were abnormal because of absent, reduced or delayed responses. However, the same laser stimuli that were unable to induce LEPs generated normal inhibition of the blink reflex response when applied 250ms before a trigeminal nerve electrical stimulus. The percentage inhibition induced in the R2 response of the blink reflex by laser stimulation of the affected side was not different from that induced by stimulation of the non-affected side, or in control subjects. These results are compatible with either a different pathway for prepulse inhibition and evoked potentials or a reduced energy requirement of the sensory input generating prepulse inhibition in comparison to that generating evoked potentials.

Continuous perfusion with morphine of the orbitofrontal cortex reduces allodynia and hyperalgesia in a rat model for mononeuropathy

Recent imaging reports demonstrate the activation of the orbitofrontal cortical (OFC) area during acute and chronic pain. The aim of this study was to compare the effects of chronic perfusion of this area with morphine on nociception in control rats and in rats subjected to mononeuropathy. Chronic perfusion of morphine, using miniosmotic pumps, produced significant and naloxone-reversible depression of tactile and cold allodynias and thermal hyperalgesia, observed in neuropathic rats, while it produced significant elevation and naloxone insensitive increase of acute nociceptive thresholds in control rats. The observed results support the idea that this area is a component of a flexible cerebral network involved in pain processing and perception.

Attenuation of neuropathic manifestations by local block of the activities of the ventrolateral orbito-frontal area in the rat

Clinical and recent imaging reports demonstrate the involvement of various cerebral prefrontal areas in the processing of pain. This has received further confirmation from animal experimentation showing an alteration of the threshold of acute nociceptive reflexes by various manipulations in the orbito-frontal cortical areas. The present study investigates the possible involvement of this area in the modulation of neuropathic manifestations in awake rats. Several groups of rats were subjected to mononeuropathy following the spared nerve injury model, known to produce evident tactile and cold allodynia and heat hyperalgesia. The activity of the ventrolateral orbital areas was selectively blocked by using either chronic or acute injection of lidocaine, electrolytic lesion, or chemical lesion with kainic acid or 6-hydroxydopamine (6-OHDA). The effects of these manipulations were compared with those following lesion of the somatic sensorimotor cortical areas. Local injection of lidocaine resulted in a reversible depression of all neuropathic manifestations while electrolytic or chemical lesions elicited transient attenuation affecting mainly the heat hyperalgesia and to a lesser extent the cold allodynia. The magnitude of the observed effects with the different procedures used can be ranked as follows: 6-OHDA<lesion<electrolytic lesion<kainic acid lesion<lidocaine injection. The observed effects were transient despite the permanence of the lesions while lesion of the somatosensorimotor cortices produced sustained reduction of the neuropathic manifestations. Our results correlate well with the established connections of the ventrolateral orbital area with the thalamic nucleus subnucleus involved in the procession of thermal nociception. The transient effects reported following permanent lesions in the orbital areas may reflect its flexible role in pain modulation. This observation provides further evidence on the plasticity of the neural networks involved in the regulation of nociceptive behavior.

Ascending pathways for mechanical allodynia in a rat model of neuropathic pain

To determine what the routes by which mechanical allodynia is transmitted following peripheral nerve injury, we assessed the effects of the dorsal column (DC) lesion performed before and 2 weeks after the partial injury of nerves innervating the tail on mechanical allodynia. Ipsilateral DC lesion 2 weeks after neuropathic surgery significantly, but not completely, attenuated mechanical allodynia. In addition, the DC lesion before peripheral nerve injury did not prevent the generation of mechanical allodynia, which was completely blocked by subsequent contralateral hemisection of the spinal cord. However, unlike mechanical allodynia, DC lesion did not change thermal allodynia. These results suggest that the signals for mechanical allodynia following peripheral nerve injury are transmitted via the ipsilateral DC and the contralateral pathway(s).

A single intravenous injection of KRN5500 (antibiotic spicamycin) produces long-term decreases in multiple sensory hypersensitivities in neuropathic pain

Neuropathic pain is a significant clinical problem. Currently, there are no drugs that produce complete amelioration of this type of pain. We have previously shown that KRN5500, a derivative of the antibiotic spicamycin, produces a prolonged (7-day), and significant reduction in neuropathic pain, but not nociceptive pain. Herein, we provide further evidence for the efficacy of this drug in inhibiting pain after IV injection in a spared nerve injury model of neuropathic pain. A single IV dose of the drug produces an increase in pain thresholds to punctuate mechanical stimuli and to cold stimuli over a period of 7 days, whereas IV injection of the vehicle is without any effect. No change in pain threshold was observed in the contralateral foot. In addition, a significant antiallodynic effect to mechanical stimuli was observed at 1, 2, 4, and 6 wk. The drug may be a potential candidate for cancer-related neuropathic pain as well as a marker for discovery of effective analgesics for neuropathic pain.

IMPLICATIONS

We examined the effect of a novel drug (KRN5500) on nerve damage pain. After the successful effects of this drug in a single human, we have shown that the drug infused as a single application at different doses in a rat model of nerve damage pain produces pain relief in this model for many weeks.