Spinal neurons involved in the generation of at-level pain following spinal injury in the rat

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

C-low threshold mechanoreceptor activation becomes sufficient to trigger affective pain in spinal cord-injured mice in association with increased respiratory rates

The mechanisms of neuropathic pain after spinal cord injury (SCI) are not fully understood. In addition to the plasticity that occurs within the injured spinal cord, peripheral processes, such as hyperactivity of primary nociceptors, are critical to the expression of pain after SCI. In adult rats, truncal stimulation within the tuning range of C-low threshold mechanoreceptors (C-LTMRs) contributes to pain hypersensitivity and elevates respiratory rates (RRs) after SCI. This suggests that C-LTMRs, which normally encode pleasant, affiliative touch, undergo plasticity to transmit pain sensation following injury. Because tyrosine hydroxylase (TH) expression is a specific marker of C-LTMRs, in the periphery, here we used TH-Cre adult mice to investigate more specifically the involvement of C-LTMRs in at-level pain after thoracic contusion SCI. Using a modified light-dark chamber conditioned place aversion (CPA) paradigm, we assessed chamber preferences and transitions between chambers at baseline, and in response to mechanical and optogenetic stimulation of C-LTMRs. In parallel, at baseline and select post-surgical timepoints, mice underwent non-contact RR recordings and von Frey assessment of mechanical hypersensitivity. The results showed that SCI mice avoided the chamber associated with C-LTMR stimulation, an effect that was more pronounced with optical stimulation. They also displayed elevated RRs at rest and during CPA training sessions. Importantly, these changes were restricted to chronic post-surgery timepoints, when hindpaw mechanical hypersensitivity was also evident. Together, these results suggest that C-LTMR afferent plasticity, coexisting with potentially facilitatory changes in breathing, drives at-level affective pain following SCI in adult mice.

Comparisons of motor and sensory abnormalities after lumbar and thoracic contusion spinal cord injury in male rats

Rodent models of contusion spinal cord injury (SCI) are widely studied for the mechanisms underlying functional deficits after SCI. Yet, how does lesion level affect SCI-induced motor and sensory dysfunctions remains unclear. Using a computer-controlled impactor (Impact One™, Leica) and the same parameters (diameter, 2.0 mm; Speed: 4.0 m/s; Depth: 1.5 mm; Dwell time: 0.1 s), we produced contusions at mid-thoracic (T10) and rostral-lumbar (L2) spinal cord in male rats, and compared locomotor and sensory dysfunctions within the same experimental setting. The time courses of locomotor deficit were comparable between thoracic (n = 8) and lumbar (n = 7) SCI rats, but the severity was greater after thoracic SCI especially during the first week post-injury, as indicated by the lower Basso, Beattle and Bresnahan open-field locomotion scores. Both groups showed similar heightened avoiding response (hyper-reactivity) to mechanical stimulation applied at the hindpaws from day 21-56 post-injury, as indicated by decreased paw withdrawal thresholds. Compared to lumbar SCI, thoracic SCI induced a greater decrease of paw withdrawal latency in hot-plate test from day 28-56 post-injury. In contrast, lumbar SCI rats showed a greater reduction of avoidance threshold to mechanical stimulation at the girdle region, and larger overgroomed area than thoracic SCI rats at day 14 post-injury. Thus, thoracic SCI may induce greater motor deficits and hindpaw heat hyper-reactivity than did lumbar SCI. In contrast, lumbar SCI may elicit greater at-level mechanical hyper-reactivity and overgrooming behavior than thoracic SCI. Future study needs to examine the specific pathological changes underlying different dysfunctions in two SCI models.

Spinal Cord Stimulation for Pain Treatment After Spinal Cord Injury

In addition to restoration of bladder, bowel, and motor functions, alleviating the accompanying debilitating pain is equally important for improving the quality of life of patients with spinal cord injury (SCI). Currently, however, the treatment of chronic pain after SCI remains a largely unmet need. Electrical spinal cord stimulation (SCS) has been used to manage a variety of chronic pain conditions that are refractory to pharmacotherapy. Yet, its efficacy, benefit profiles, and mechanisms of action in SCI pain remain elusive, due to limited research, methodological weaknesses in previous clinical studies, and a lack of mechanistic exploration of SCS for SCI pain control. We aim to review recent studies and outline the therapeutic potential of different SCS paradigms for traumatic SCI pain. We begin with an overview of its manifestations, classification, potential underlying etiology, and current challenges for its treatment. The clinical evidence for using SCS in SCI pain is then reviewed. Finally, future perspectives of pre-clinical research and clinical study of SCS for SCI pain treatment are discussed.

Intra-spinal microstimulation may alleviate chronic pain after spinal cord injury

Chronic pain after spinal cord injury (SCI) is a form of central neuropathic pain that is debilitating and often refractory to current pharmacological treatments. Neurostimulation pain therapies, such as epidural spinal cord stimulation, have only moderate success in reducing SCI pain. The pathogenesis of SCI pain may involve a state of central neuronal hyperexcitability, especially in the spinal cord dorsal horn, that develops after injury. We hypothesize that the neuronal structures near the spinal cord injury site may be an important pain generator, and intraspinal microstimulation (ISMS) may normalize dorsal horn neuronal hyperexcitability and hence alleviate SCI pain. Specifically, ISMS may induce frequency-dependent conduction block on axons of afferent sensory neurons, in the spinothalamic tract and Lissauer's tract. ISMS may also facilitate primary afferent depolarization that elicits presynaptic inhibition of incoming afferent inputs. Together, these actions will reduce abnormal afferent inputs and ascending pain signals before they can reach the brain. Furthermore, ISMS may directly induce inhibitory postsynaptic potentials in dorsal horn neurons, and trigger the release of endogenous inhibitory neurotransmitters, opioids and serotonin to inhibit postsynaptic neurons and restore the compromised segmental pain inhibition after SCI. Finally, ISMS may alter the frequency and pattern of discharge such that the rostrally conducted impulses no longer code pain or activate brain areas concerned with pain signaling. Based on recent progress in understanding spinal learning and plasticity, we also postulate that repetitive or long-term ISMS may help the dorsal horn "reset" neuronal excitability and regain normal pain processing for a prolonged period. By finely tuning the stimulation parameters (e.g., intensity, pulse width, frequency), position, and geometry of ISMS electrode, multiple spinal structures (e.g., dorsal horn, dorsal column, spinothalamic tract) may be modulated to induce synergistic pain inhibition. Our hypothesis can be readily tested in preclinical models of SCI pain by using a combination of in vivo electrophysiological (neuronal activity) and animal behavioral (pain response) approaches. Since ISMS electrodes stimulate the spinal structures directly, we expect that the effective stimulus intensity and energy consumption can be lower than that for epidural spinal cord stimulation. The proposed hypothesis may provide insights and rationales for developing a novel neurostimulation pain therapy by directly inhibiting the pain generators in the spinal cord, and ISMS may be an alternative strategy to treat SCI pain.

Predifferentiated GABAergic neural precursor transplants for alleviation of dysesthetic central pain following excitotoxic spinal cord injury

Intraspinal quisqualic acid (QUIS) injury induce (i) mechanical and thermal hyperalgesia, (ii) progressive self-injurious overgrooming of the affected dermatome. The latter is thought to resemble painful dysesthesia observed in spinal cord injury (SCI) patients. We have reported previously loss of endogenous GABA immunoreactive (IR) cells in the superficial dorsal horn of QUIS rats 2 weeks post injury. Further histological evaluation showed that GABA-, glycine-, and synaptic vesicular transporter VIAAT-IR persisted but were substantially decreased in the injured spinal cord. In this study, partially differentiated GABA-IR embryonic neural precursor cells (NPCs) were transplanted into the spinal cord of QUIS rats to reverse overgrooming by replenishing lost inhibitory circuitry. Rat E14 NPCs were predifferentiated in 0.1 ng/ml FGF-2 for 4 h prior to transplantation. In vitro immunocytochemistry of transplant cohort showed large population of GABA-IR NPCs that double labeled with nestin but few colocalized with NeuN, indicating partial maturation. Two weeks following QUIS lesion at T12-L1, and following the onset of overgrooming, NPCs were transplanted into the QUIS lesion sites; bovine adrenal fibroblast cells were used as control. Overgrooming was reduced in >55.5% of NPC grafted animals, with inverse relationship between the number of surviving GABA-IR cells and the size of overgrooming. Fibroblast-control animals showed a progressive worsening of overgrooming. At 3 weeks post-transplantation, numerous GABA-, nestin-, and GFAP-IR cells were present in the lesion site. Surviving grafted GABA-IR NPCs were NeuN⁺ and GFAP⁻. These results indicate that partially differentiated NPCs survive and differentiate in vivo into neuronal cells following transplantation into an injured spinal cord. GABA-IR NPC transplants can restore lost dorsal horn inhibitory signaling and are useful in alleviating central pain following SCI.

Central Pain from Excitotoxic Spinal Cord Injury Induced by Intraspinal NMDA Injection: A Pilot Study

Background

The pathophysiological and neurochemical changes following spinal injury are not yet elucidated. This study was designed to evaluate the morphological changes of the dorsal horn of the spinal cord and profiles of pain behaviors following intraspinal injection of NMDA in rats.

Methods

Rats were randomized into three groups: a sham-operated control group and groups where the rats received 10 mM or 100 mM N-methyl-D-aspatate (NMDA) injected into their spinal dorsal horn. Following injection, hypersensitivity to cold and mechanical stimuli and excessive grooming behaviors were assessed serially for four weeks. Morphological changes of the spinal cord were evaluated four weeks after intraspinal injection.

Results

Few animals in the NMDA groups developed hypersensitivity to cold and mechanical stimuli. The number of groomers and the severity of excessive grooming were significantly higher in the 100 mM NMDA group than those values of the control and 10 mM NMDA groups. The size of the neck region (lamina III-IV) was significantly smaller in the 100 mM NMDA group than in the control and 10 mM NMDA groups.

Conclusions

In conclusion, intraspinal injection of NMDA in rats leads to the pathological sequela in the spinal cord and to excessive grooming behavior. These results support the use of NMDA and excessive grooming behavior after excitotoxic SCI as a model to study chronic pain after SCI.

Sensory stimulation prior to spinal cord injury induces post-injury dysesthesia in mice

Chronic pain and dysesthesias are debilitating conditions that can arise following spinal cord injury (SCI). Research studies frequently employ rodent models of SCI to better understand the underlying mechanisms and develop better treatments for these phenomena. While evoked withdrawal tests can assess hypersensitivity in these SCI models, there is little consensus over how to evaluate spontaneous sensory abnormalities that are seen in clinical SCI subjects. Overgrooming (OG) and biting after peripheral nerve injury or spinal cord excitotoxic lesions are thought to be one behavioral demonstration of spontaneous neuropathic pain or dysesthesia. However, reports of OG after contusion SCI are largely anecdotal and conditions causing this response are poorly understood. The present study investigated whether repeated application of sensory stimuli to the trunk prior to mid-thoracic contusion SCI would induce OG after SCI in mice. One week prior to SCI or laminectomy, mice were subjected either to nociceptive and mechanical stimulation, mechanical stimulation only, the testing situation without stimulation, or no treatment. They were then examined for 14 days after surgery and the sizes and locations of OG sites were recorded on anatomical maps. Mice subjected to either stimulus paradigm showed increased OG compared with unstimulated or uninjured mice. Histological analysis showed no difference in spinal cord lesion size due to sensory stimulation, or between mice that overgroomed or did not overgroom. The relationship between prior stimulation and contusion injury in mice that display OG indicates a critical interaction that may underlie one facet of spontaneous neuropathic symptoms after SCI.

Joint distraction magnitude is associated with different behavioral outcomes and substance P levels for cervical facet joint loading in the rat

The facet joint is a common source of pain in both the neck and low back, and can be injured by abnormal loading of the spinal joints. Whereas a host of nociceptive changes including neuronal activation, neuropeptide expression, and inflammatory mediator responses has been reported for rat models of joint pain, no such responses have been explicitly investigated or quantified for painful mechanical injury to the facet joint. Two magnitudes of joint loading were separately imposed in a rat model of cervical facet joint distraction: Painful and nonpainful distractions. Behavioral outcomes were defined by assessing mechanical hyperalgesia in the shoulders and forepaws. Substance P (SP) mRNA and protein levels were quantified in the dorsal root ganglion (DRG) and spinal cord at days 1 and 7 following distraction. Painful distraction produced mechanical hyperalgesia that was significantly greater (P < .010) than that for a nonpainful distraction. Painful distraction significantly increased spinal SP mRNA (P = .048) and SP protein expression in the DRG (P = .013) at day 7 compared to nonpainful distraction. However, spinal SP protein for painful distraction was significantly less (P = .024) than that for nonpainful distraction at day 1. Joint distractions producing different behavioral outcomes modulate SP mRNA and protein in the DRG and spinal cord, suggesting that SP responses may be involved with different temporal responses in painful joint loading.

PERSPECTIVE

SP mRNA and protein in the DRG and spinal cord are quantified at 2 time points after cervical facet joint distractions that separately do or do not produce mechanical hyperalgesia. Studies describe a role for SP to contribute to pain produced by mechanical joint loading.

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.

Dermatomal scratching after intramedullary quisqualate injection: correlation with cutaneous denervation

Central nervous system lesions cause peripheral dysfunctions currently attributed to central cell death that compromises function of intact peripheral nerves. Injecting quisqualate (QUIS) into the rat spinal cord models spinal cord injury (SCI) and causes at-level scratching and self-injury. Such overgrooming was interpreted to model pain until patients with self-injurious scratching after SCI reported itch motivated scratching that was painless because of sensory loss. Because self-injurious scratching is difficult to explain by central mechanisms alone, we hypothesized that QUIS injections damage peripheral axons of at-level afferents. QUIS was injected into thoracic spinal cords of 18 Long-Evans rats. Animals were killed 3 days after overgrooming began or 14 days after injection. Spinal cord lesions were localized and DRG-immunolabeled for ATF-3. At-level and control skin samples were PGP9.5-immunabeled to quantify axons. Eighty-four percent of QUIS rats overgroomed. Skin in these regions had lost two-thirds of epidermal innervation as compared with controls (P < .001). Rats that overgroomed had 47% less axon-length than nongrooming rats (P = .006). The presence of ATF-3 immunolabeled neurons within diagnosis-related groups of QUIS rats indicated death of afferent cell bodies. Overgrooming after QUIS injections may not be due entirely to central changes. As in humans, self-injurious neuropathic scratching appeared to require loss of protective pain sensations in addition to peripheral denervation.

PERSPECTIVE

This study suggests that intramedullary injection of quisqualic acid in rats causes death of at-level peripheral as well as central neurons. Self-injurious dermatomal scratching that develops in spinal-injured rats may reflect neuropathic itch and loss of protective pain sensations.

Locomotor dysfunction and pain: the scylla and charybdis of fiber sprouting after spinal cord injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.

Superficial NK1 expressing spinal dorsal horn neurones modulate inhibitory neurotransmission mediated by spinal GABA(A) receptors

Lamina 1 projection neurones which express the NK1 receptor (NK1R+) drive a descending serotonergic pathway from the brainstem that enhances spinal dorsal horn neuronal activity via the facilitatory spinal 5-HT3 receptor. Selective destruction of these cells via lumbar injection of substance P-saporin (SP-SAP) attenuates pain behaviours, including mechanical and thermal hypersensitivity, which are mirrored by deficits in the evoked responses of lamina V-VI wide dynamic range (WDR) neurones to noxious stimuli. To assess whether removing the origin of this facilitatory spino-bulbo-spinal loop results in alterations in GABAergic spinal inhibitory systems, the effects of spinal bicuculline, a selective GABA(A) receptor antagonist, on the evoked neuronal responses to electrical (Abeta-, Adelta-, C-fibre, post-discharge and Input) and mechanical (brush, prod and von Frey (vF) 8 and 26 g) stimuli were measured in SAP and SP-SAP groups. In the SAP control group, bicuculline produced a significant dose related facilitation of the electrically evoked Adelta-, C-fibre, post-discharge and input neuronal responses. The evoked mechanical (prod, vF8 g and 26 g) responses were also significantly increased. Brush evoked neuronal responses in these animals were enhanced but did not reach significance. This facilitatory effect of bicuculline, however, was lost in the SP-SAP treated group. The generation of intrinsic GABAergic transmission in the spinal cord appears dependent on NK1 bearing neurons, yet despite the loss of GABAergic inhibitory controls after SP-SAP treatment, the net effect is a decrease in spinal cord excitability. Thus activation of these cells predominantly drives facilitation.

Spinal NK-1 receptor expressing neurons mediate opioid-induced hyperalgesia and antinociceptive tolerance via activation of descending pathways

Opioids can induce hyperalgesia in humans and in animals. Mechanisms of opiate-induced hyperalgesia and possibly of spinal antinociceptive tolerance may be linked to pronociceptive adaptations occurring at multiple levels of the nervous system including activation of descending facilitatory influences from the brainstem, spinal neuroplasticity, and changes in primary afferent fibers. Here, the role of NK-1 receptor expressing cells in the spinal dorsal horn in morphine-induced hyperalgesia and spinal antinociceptive tolerance was assessed by ablating these cells with intrathecal injection of SP-saporin (SP-SAP). Ablation of NK-1 receptor expressing cells prevented (a) morphine-induced thermal and mechanical hypersensitivity, (b) increased touch-evoked spinal FOS expression, (c) upregulation of spinal dynorphin content and (d) the rightward displacement of the spinal morphine antinociceptive dose-response curve (i.e., tolerance). Morphine-induced hyperalgesia and antinociceptive tolerance were also blocked by spinal administration of ondansetron, a serotonergic receptor antagonist. Thus, NK-1 receptor expressing neurons play a critical role in sustained morphine-induced neuroplastic changes which underlie spinal excitability reflected as thermal and tactile hypersensitivity to peripheral stimuli, and to reduced antinociceptive actions of spinal morphine (i.e., antinociceptive tolerance). Ablation of these cells likely eliminates the ascending limb of a spinal-bulbospinal loop that engages descending facilitation and elicits subsequent spinal neuroplasticity. The data may provide a basis for understanding mechanisms of prolonged pain which can occur in the absence of tissue injury.

Differential contribution of TRPV1 to thermal responses and tissue injury-induced sensitization of dorsal horn neurons in laminae I and V in the mouse

Our previous recordings from dorsal root ganglion and spinal lamina V neurons from TRPV1-mutant mice showed dramatic decreases in responses to temperatures near the activation threshold of this channel (43-49 degrees C). Somewhat unexpectedly, we only observed behavioral deficits in these mice at higher temperatures (50-58 degrees C). In the present study, we tested the hypothesis that the noxious heat-evoked pain behavior that persists in TRPV1-mutant mice reflects residual responsiveness of neurons in the superficial, but not deep, dorsal horn. To this end, we performed in vivo extracellular recordings of spinal nociresponsive neurons in laminae I and V in wild type (WT) and TRPV1 mutant mice. Neurons in WT and mutant mice from both laminae did not differ in their spontaneous activity or evoked responses to mechanical or cold stimuli. By contrast, most lamina I neurons from mutant mice responded to noxious heat with significantly higher thresholds than in WT mice. In contrast, lamina V neurons from mutant mice were virtually unresponsive to noxious heat before and after topical mustard oil-induced tissue injury. Interestingly, lamina I neurons in mutant mice displayed thermal sensitization following tissue injury, comparable in magnitude, but of shorter duration, than in WT mice. We conclude that TRPV1 is necessary for noxious heat-evoked responses of lamina V neurons, both before and after tissue injury. It is also an essential contributor to the normal activation threshold of lamina I neurons to noxious heat and for the full duration of thermal sensitization of lamina I neurons following injury. Finally, our results suggest that the processing of noxious thermal messages by neurons in lamina I involves convergent inputs from a heterogeneous population of primary afferent thermal nociceptors.

Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

Not all neuropathic pain patients gain relief from current therapies that include the anticonvulsant, gabapentin, thought to modulate calcium channel function. We report a neural circuit that is permissive for the effectiveness of gabapentin. Substance P-saporin (SP-SAP) was used to selectively ablate superficial dorsal horn neurons expressing the neurokinin-1 receptor for substance P. These neurons project to the brain as shown by retrograde labelling and engage descending brainstem serotonergic influences that enhance spinal excitability via a facilitatory action on 5HT(3) receptors. We show the integrity of this pathway following nerve injury contributes to the behavioural allodynia, neuronal plasticity of deep dorsal horn neurons and the injury-specific actions of gabapentin. Thus SP-SAP attenuated the tactile and cold hypersensitivity and abnormal neuronal coding (including spontaneous activity, expansion of receptive field size) seen after spinal nerve ligation. Furthermore the powerful actions of gabapentin after neuropathy were blocked by either ablation of NK-1 expressing neurones or 5HT(3) receptor antagonism using ondansetron. Remarkably, 5HT(3) receptor activation provided a state-dependency (independent of that produced by neuropathy) allowing GBP to powerfully inhibit in normal uninjured animals. This circuit is therefore a crucial determinant of the abnormal neuronal and behavioural manifestations of neuropathy and importantly, the efficacy of gabapentin. As this spino-bulbo-spinal circuit contacts areas of the brain implicated in the affective components of pain, this loop may represent a route by which emotions can influence the degree of pain in a patient, as well as the effectiveness of the drug treatment. These hypotheses are testable in patients.

Activation of the ERK1/2 signaling cascade by excitotoxic spinal cord injury

The role of the ERK1/2 signal transduction pathway and related transcription factors in the regulation of gene expression and pain behavior following excitotoxic spinal cord injury (SCI) was examined. Specifically, phosphorylation of ERK1/2, activation of transcription factors NF-kB, ELK-1, and CREB, and gene expression of the neurokinin-1 receptor and NMDA receptor subunits NR1 and NR-2A were investigated. Excitotoxic injury was produced by intraspinal injection of quisqualic acid (QUIS) in male Sprague-Dawley rats. Western blots were used to evaluate phosphorylation and activation of ERK1/2 and transcription factors using phospho-specific or total antibodies. Real-time PCR was used to evaluate gene expression of NK-1R, NR-1, and NR-2A. Assessment of excessive grooming behavior was used to evaluate the presence of spontaneous pain behavior. Excitotoxic spinal injury resulted in: (1) increased phosphorylation of ERK1/2; (2) increased activation of NF-kB and phosphorylation of ELK-1; and (3) increased gene expression for the NK-1 receptor and NR1 and NR-2A subunits of the NMDA receptor. Blockade of the ERK cascade with the MEK inhibitor PD98059 inhibited phosphorylation of ELK-1, activation of NF-kB and gene expression of NR1, NR-2A and NK-1R, and prevented the development of excessive grooming behavior. The results have shown that excitotoxic spinal injury leads to the injury-induced activation of the ERK-->ELK-1 and NF-kB signaling cascades and transcriptional regulation of receptors important in the development of chronic pain. Blockade of this intracellular kinase cascade prevented the onset of injury-induced pain behavior.

Central neuropathic itch from spinal-cord cavernous hemangioma: a human case, a possible animal model, and hypotheses about pathogenesis

Cavernous hemangiomas (cavernomas) of the spinal cord are rare congenital malformations that comprise less than 5% of all intramedullary lesions. Despite this rarity, we describe the third case of central neuropathic itch associated with intramedullary cavernoma. Since fewer than 10 cases of central spinal itch from all causes have been published, this concurrence suggests the possibility of a specific association. A middle-aged man developed chronic disabling neuropathic itch and pain affecting his left shoulder and arm after frank hemorrhage of a midcervical cavernoma. We hypothesize that the relatively rostro-dorsal location of his lesion increased its likelihood of causing itch as well as pain. The microscopic pathology of cavernomas, specifically their gliotic rim containing hemosiderin-laden phagocytes, fosters ectopic firing of nearby neurons and makes cranial cavernomas highly epileptogenic. We hypothesize that these pathological features predispose cavernomas to cause central itch if they are located near, but spare, the central itch projection neurons in lamina I of the dorsal horn. Quisqualate injections into the deeper layers (neck) of the dorsal horns of rats produce pathologically similar lesions. Such rats develop unilateral dermatomal hyperalgesia and self-injurious scratching and biting (autotomy). Although this pathological grooming is currently interpreted as a response to chronic pain, we propose that it more likely models scratching provoked by central neuropathic itch, as seen in our patient and others. Study of quisqualate-injected rats may provide leads towards new treatments for neuropathic itch.

Spinal Cord Injury: A Model of Central Neuropathic Pain

The condition of pain after spinal cord injury (SCI) affects the life quality of nearly 70% of individuals with SCI. Clinical studies over the past decade have provided important insights into the complexities of the clinical and psychosocial characteristics of this debilitating consequence of SCI. The use of experimental models developed to study at-level or below-level pain has provided an appreciation for the mechanism(s) responsible for the onset and progression of these conditions. Important to the studies related to SCI pain has been the focus on the molecular, biochemical, anatomical, and functional consequences of SCI that have identified potential therapeutic targets for the design of novel treatment strategies.

Bad news from the brain: descending 5-HT pathways that control spinal pain processing

The identification of opioid systems led to much of the early work on pain pharmacology being based on understanding inhibitory mechanisms of analgesia. However, hyperalgesia and allodynia are common clinical symptoms and therefore hyperexcitability must be a major component of pain. Thus, the emphasis of current research into pain has shifted to understanding excitatory pathways that underlie neuronal sensitization and potentiation. Although much evidence supports the presence of descending inhibitory mechanisms of pain, reports of facilitatory pathways from the brainstem have been scarce. In this article, we review evidence for facilitatory 5-HT pathways that link spinal cord and brainstem areas involved in mood and emotions. Because pain encompasses affective aspects, we suggest that these 5-HT pathways and other circuits are important in determining the levels of pain, the outcome of drug treatments and provide a mechanism whereby emotions can alter pain perception.

Spinal neurons that express NK-1 receptors modulate descending controls that project through the dorsolateral funiculus

Selective ablation of spinal neurons possessing substance P receptors (NK-1 receptors) using the selective cytotoxin conjugate substance P-saporin (SP-SAP) decreases hyperalgesia and central sensitization. The mechanisms by which NK-1 expressing neurons modulate the excitability of other dorsal horn neurons are unclear. Because the majority of NK-1 expressing spinal neurons project rostrally, it is possible that they are part of a spinal-supraspinal circuitry that contributes to descending modulation of excitability of spinal nociceptive neurons. We therefore determined whether ablation of spinal neurons that possess the NK-1 receptor altered descending systems that travel via the dorsolateral funiculus (DLF). Spontaneous activity and responses of dorsal horn neurons evoked by mechanical (von Frey monofilaments) and heat (35-51 degrees C) stimuli were determined before and after transection of the DLF and were compared in rats pretreated with intrathecal application of vehicle or SP-SAP. In vehicle-treated rats, transection of the DLF caused a 233% increase in mean spontaneous activity of neurons and enhanced their responses to mechanical and heat stimuli, whereas these increases in excitation were blocked in rats pretreated with SP-SAP. Importantly, SP-SAP alone had no effect on spontaneous or evoked activity in the absence of DLF transection. These results demonstrate that spinal neurons expressing the NK-1 receptor appear to play a pivotal role in regulating descending systems that modulate activity of nociceptive dorsal horn neurons.