Hyperexcitability Of Dorsal Horn Neurons Research Articles

Inflammatory-induced spinal dorsal horn neurons hyperexcitability is mediated by P2X4 receptors.

Introduction:Purinergic ionotropic P2X receptors (P2RX) are involved in normal and pathological pain transmission. Among them, P2X4 are expressed in dorsal root ganglion and in the spinal cord. Their activation during nerve injury or chronic peripheral inflammation modifies pain sensitivity that leads to the phenomenon of allodynia and hyperalgesia.Objectives:We study here, in vivo, the role of P2X4 on the excitability of dorsal horn neurons (DHNs) in naive or pathological context.Methods:We recorded DHNs in vivo in anesthetized wild-type or P2RX4−/− mice. We measured nociceptive integration and short-term sensitization by DHNs both in naive and inflamed mice.Results:Our results indicate that P2X4 alter neuronal excitability only in the pathological context of peripheral inflammation. Consequently, excitability of DHNs from inflamed P2RX4−/− mice remains similar to naive animals.Conclusion:These results confirm the prominent role of P2X4 in inflammatory pain context and demonstrate that P2X4 are also involved in the hyperexcitability of DHNs.

Short-term swimming exercise attenuates the sensitization of dorsal horn neurons in rats with NGF-induced low back pain.

Physical exercise has been shown to be an effective therapy for non-specific low back pain. The study investigated if swimming exercise is a means to reduce the spinal sensitization in an animal model of non-specific low back pain. In deeply anesthetized rats, dorsal horn neurons were recorded in spinal segment L2. To induce sensitization of dorsal horn neurons, two injections of nerve growth factor were made into the lumbar multifidus muscle at an interval of 5days. Swimming exercise for 30min was performed on the 5days between both NGF injections. A control group received the NGF injections without exercise treatment. Swimming exercise caused a significant decrease in the NGF-induced hyperexcitability of dorsal horn neurons. Compared to control, the proportion of neurons with input from deep somatic tissues and of convergent neurons with input from at least two types of different tissues decreased significantly (50% vs. 25% and 37% vs. 15%; both p<0.05). Swimming exercise also reduced the NGF-induced increase in neuronal resting activity. Both the proportion of active neurons and the mean discharge frequency of all neurons decreased significantly (60%, 76.3±23.1imp/min; vs. 25%, 51.7±35.1imp/min; both p<0.01). In our animal model of low back pain, short-term swimming exercise effectively reduced the latent sensitization of spinal dorsal horn neurons. Swimming exercise decreased the hyperexcitability of the neurons to low back input and lowered the resting activity of sensitized neurons. Physical exercise is a common treatment for low back pain. The possible mechanisms underlying the effects of exercise are probably multifold. This work shows that swimming exercise prevents sensitization of dorsal horn neurons, which may be one mechanism for the positive effects of exercise.

Recombinant interleukin-4 alleviates mechanical allodynia via injury-induced interleukin-4 receptor alpha in spinal microglia in a rat model of neuropathic pain.

Glial cells play important roles in the development and maintenance of neuropathic pain. In particular, activated microglia in the spinal cord facilitate the hyper-excitability of dorsal horn neurons after peripheral nerve injury via pro-inflammatory molecules. In this study, we investigated the possible involvement of the anti-inflammatory cytokine, interleukin-4 (IL-4), in neuropathic pain. We did not detect the expression of IL-4 mRNA in the rat dorsal root ganglion or spinal cord; however, peripheral nerve injury induced the expression of IL-4 receptor (IL-4R) alpha mRNA in the spinal cord. A histological analysis revealed that nerve injury induced IL-4R alpha mRNA in activated spinal microglia ipsilateral to the injury site. Additionally, the increases in IL-4R alpha were coincident with the increased expression of phosphorylated signal transducer and activator of transcription 6 (pSTAT6) in spinal microglia. Intrathecal administration of recombinant IL-4 suppressed mechanical hypersensitivity in neuropathic rats, and the analgesic effect of IL-4 was accompanied by further enhancement of pSTAT6 expression in spinal microglia. Taken together, these results suggest that the adaptive responses of microglia to nerve injury involve both inflammatory and anti-inflammatory signaling, including IL-4R alpha and pSTAT6. These findings support that utilizing the endogenous anti-nociceptive activity of IL-4R alpha may modify the cell lineage of pro-nociceptive microglia, thus providing a novel therapeutic strategy for neuropathic pain.

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.

Early afferent activity from the facet joint after painful trauma to its capsule potentiates neuronal excitability and glutamate signaling in the spinal cord

Cervical facet joint injury induces persistent pain and central sensitization. Preventing the peripheral neuronal signals that initiate sensitization attenuates neuropathic pain. Yet, there is no clear relationship among facet joint afferent activity, development of central sensitization, and pain, which may be hindering effective treatments for this pain syndrome. This study investigates how afferent activity from the injured cervical facet joint affects induction of behavioral sensitivity and central sensitization. Intra-articular bupivacaine was administered to transiently suppress afferent activity immediately or 4days after facet injury. Mechanical hyperalgesia was monitored after injury, and spinal neuronal hyperexcitability and spinal expression of proteins that promote neuronal excitability were measured on day 7. Facet injury with saline vehicle treatment induced significant mechanical hyperalgesia (P<.027), dorsal horn neuronal hyperexcitability (P<.026), upregulation of pERK1/2, pNR1, mGluR5, GLAST, and GFAP, and downregulation of GLT1 (P<.032). However, intra-articular bupivacaine immediately after injury significantly attenuated hyperalgesia (P<.0001), neuronal hyperexcitability (P<.004), and dysregulation of excitatory signaling proteins (P<.049). In contrast, intra-articular bupivacaine at day 4 had no effect on these outcomes. Silencing afferent activity during the development of neuronal hyperexcitability (4hours, 8hours, 1day) attenuated hyperalgesia and neuronal hyperexcitability (P<.045) only for the treatment given 4hours after injury. This study suggests that early afferent activity from the injured facet induces development of spinal sensitization via spinal excitatory glutamatergic signaling. Peripheral intervention blocking afferent activity is effective only over a short period of time early after injury and before spinal modifications develop, and is independent of modulating spinal glial activation.

Early spread of hyperexcitability to caudal dorsal horn networks after a chemically-induced lesion of the rat spinal cord in vitro

Hyperexcitability of dorsal horn neurons has been shown to play a key role in neuropathic pain following chronic experimental spinal cord injury. With a neonatal in vitro spinal cord injury model, we show that a chemically-induced lesion leads to rapid gain-of-function of sublesional dorsal horn networks biased to hyperexcitation. The expression of the GABA synthetic enzyme GAD65 was significantly reduced at the same level of the spinal cord, suggesting a compromised inhibitory system. We propose that our model could be useful to test early approaches to contrast spinal cord injury-induced central sensitization of dorsal horn circuits.

P2X4R and P2X7R in neuropathic pain

AbstractNeuropathic pain is often a consequence of nerve injury or of diseases such as diabetes, infection, autoimmune disease, or cancer. Neuropathic pain can be agonizing, can persist over long periods, and, unfortunately, is often resistant to known painkillers. There is a rapidly growing body of evidence indicating that signaling by extracellular nucleotides through purinergic P2 receptors [ionotropic P2X receptors (P2XRs) and metabotropic P2Y receptors (P2YRs)] play crucial roles in neuropathic pain. Following peripheral nerve injury, the expression of purinergic receptors (e.g., P2X4R) is markedly upregulated specifically in microglia, a type of glial cells known as resident macrophages in the central nervous system. Activation of P2X4R and P2X7R in microglia causes release of diffusible factors (such as brain‐derived neurotrophic factor, interleukin‐1β, interleukin‐6, and tumor necrosis factor‐α) involved in nerve‐injury‐induced pain behaviors and hyperexcitability of dorsal horn neurons. Suppression of the function or expression of these purinergic receptors strongly suppresses neuropathic pain. Recent advances further our understanding of mechanisms by which microglial purinergic receptors contribute to the pathogenesis of neuropathic pain. WIREs Membr Transp Signal 2012, 1:513–521. doi: 10.1002/wmts.47For further resources related to this article, please visit the WIREs website.

Gabapentinoids: Gabapentin and Pregabalin for Postoperative Pain Management

Editorial Postsurgical pain is normally perceived as nociceptive pain. Surgical trauma has been known to induce central and peripheral sensitization and hyperalgesia, which in untreated cases could lead to chronic postoperative pain after surgery. Indeed pain is one of the three most common medical causes of delayed discharge after ambulatory surgery, the other two being drowsiness and nausea/ vomiting. Antihyperalgesic drugs improve postoperative pain by preventing the development of central sensitization (1). The development of newer agents available for postoperative pain control create possibilities for better combinations in multimodal analgesia. The recent advances in postoperative pain management can be specifically grouped in the following areas: Finding exact molecular mechanisms, new pharmaceutical products and other routes and modes of analgesic delivery. For the years, opioids have been the mainstay of postoperative pain management but they have side effects. For this purpose the multimodal approach and non-opioid drugs have been suggested to improve postoperative analgesia and to reduce opioid related side effects (2). Gabapentinoids (gabapentin and pregabalin) were originally introduced as antiepileptics but have analgesic, anticonvulsant, and anxiolytic effects also. These easily tolerable drugs by patients have limited side-effects. Gabapentin an anti-epileptic drug binds to the alpha-2 delta subunit of the presynaptic voltage gated-calcium channels and inhibits calcium release so prevents the release of excitatory neurotransmitters involved in the pain pathways (2, 3). Gabapentin has demonstrated analgesic effect in diabetic neuropathy, post-herpetic neuralgia, and neuropathic pain. Several meta-analyses reveal that perioperative gabapentin helps to produce a significant opioid-sparing effect and probably decreses postoperative pain score relative to the control group (4, 5). Pregabalin is a structural analog of gamma-aminobutyric acid (GABA). It acts by presynaptic binding to the α -2-λ subunit of voltage-gated calcium channels that are widely distributed in the spinal cord and brain6. By this mechanism, pregabalin modulates the release of several excitatory neurotransmitters, such as glutamate, norepinephrine, substance P, and calcitonin gene-related peptide. It leads to inhibitory modulation of “overexcited” neurons and returning them to a “normal” state. Centrally, pregabalin could reduce the hyperexcitability of dorsal horn neurons that is induced by tissue damage (6). To sum up, pregabalin has a more appropriate pharmacokinetic profile than gabapentin, including dose-independent absorption and far more potent than gabapentin while producing fewer adverse effects (7-9). Pregabalin has efficacy of varying degree in neuropathic pain conditions such as postherpetic neuralgia, painful diabetic neuropathy, central neuropathic pain, and fibromyalgia. While some surveys do not demonstrate a significant analgesic effect in the acute, including postoperative pain control (9, 10), other studies suggest pregabalin to have effective sedative and opioid-sparing effects (11-13), and emphasize on its effectiveness in acute pain control. Since safe postoperative pain control is nessecary, established role of pregabalin as an analgesic adjuvant as a part of multimodal analgesia for acute pain control is in progress (7, 8, 14, 15). Its unique potency in reducing opioid requirements, prevention of opioid tolerance, enhancement the quality of opioid analgesia, decreased respiratory depression and anxiolysis, make it an attractive drug to consider for control of pain in the post-operative setting (15-17). Lots of meta-analyses and clinical trials show that perioperative pregabaline helps to produce a significant opioid-sparing effect and probably improves postoperative pain score relative to the control group (18-21). Having looked at these two drugs from different angles and aspects, one comes to this understanding that these multi-purpose drugs have found a strong and reliable place in acute pain service setting. So, Gabapentinoids are an effective tool in the treatment of postoperative pain.

Spatial and temporal activation of spinal glial cells: Role of gliopathy in central neuropathic pain following spinal cord injury in rats

In the spinal cord, neuron and glial cells actively interact and contribute to neurofunction. Surprisingly, both cell types have similar receptors, transporters and ion channels and also produce similar neurotransmitters and cytokines. The neuroanatomical and neurochemical similarities work synergistically to maintain physiological homeostasis in the normal spinal cord. However, in trauma or disease states, spinal glia become activated, dorsal horn neurons become hyperexcitable contributing to sensitized neuronal–glial circuits. The maladaptive spinal circuits directly affect synaptic excitability, including activation of intracellular downstream cascades that result in enhanced evoked and spontaneous activity in dorsal horn neurons with the result that abnormal pain syndromes develop.Recent literature reported that spinal cord injury produces glial activation in the dorsal horn; however, the majority of glial activation studies after SCI have focused on transient and/or acute time points, from a few hours to 1month, and peri-lesion sites, a few millimeters rostral and caudal to the lesion site. In addition, thoracic spinal cord injury produces activation of astrocytes and microglia that contributes to dorsal horn neuronal hyperexcitability and central neuropathic pain in above-level, at-level and below-level segments remote from the lesion in the spinal cord. The cellular and molecular events of glial activation are not simple events, rather they are the consequence of a combination of several neurochemical and neurophysiological changes following SCI. The ionic imbalances, neuroinflammation and alterations of cell cycle proteins after SCI are predominant components for neuroanatomical and neurochemical changes that result in glial activation. More importantly, SCI induced release of glutamate, proinflammatory cytokines, ATP, reactive oxygen species (ROS) and neurotrophic factors trigger activation of postsynaptic neuron and glial cells via their own receptors and channels that, in turn, contribute to neuronal–neuronal and neuronal–glial interaction as well as microglia–astrocytic interactions. However, a systematic review of temporal and spatial glial activation following SCI has not been done.In this review, we describe time and regional dependence of glial activation and describe activation mechanisms in various SCI models in rats. These data are placed in the broader context of glial activation mechanisms and chronic pain states. Our work in the context of work by others in SCI models demonstrates that dysfunctional glia, a condition called “gliopathy”, is a key contributor in the underlying cellular mechanisms contributing to neuropathic pain.

Perturbing PSD-95 Interactions With NR2B-subtype Receptors Attenuates Spinal Nociceptive Plasticity and Neuropathic Pain

Peripheral inflammation or nerve injury induces a primary afferent barrage into the spinal cord, which can cause N-methyl -aspartate (NMDA) receptor-dependent alterations in the responses of dorsal horn sensory neurons to subsequent afferent inputs. This plasticity, such as “wind-up” and central sensitization, contributes to the hyperexcitability of dorsal horn neurons and increased pain-related behavior in animal models, as well as clinical signs of chronic pain in humans, hyperalgesia and allodynia. Binding of NMDA receptor subunits by the scaffolding protein postsynaptic density protein-95 (PSD-95) can facilitate downstream intracellular signaling and modulate receptor stability, contributing to synaptic plasticity. Here, we show that spinal delivery of the mimetic peptide Tat-NR2B9c disrupts the interaction between PSD-95 and NR2B subunits in the dorsal horn and selectively reduces NMDA receptor-dependent events including wind-up of spinal sensory neurons, and both persistent formalin-induced neuronal activity and pain-related behaviors, attributed to central sensitization. Furthermore, a single intrathecal injection of Tat-NR2B9c in rats with established nerve injury-induced pain attenuates behavioral signs of mechanical and cold hypersensitivity, with no effect on locomotor performance. Thus, uncoupling of PSD-95 from spinal NR2B-containing NMDA receptors may prevent the neuronal plasticity involved in chronic pain and may be a successful analgesic therapy, reducing side effects associated with receptor blockade.

Systemic blockade of P2X3 and P2X2/3 receptors attenuates bone cancer pain behaviour in rats

Pain remains an area of considerable unmet clinical need, and this is particularly true of pain associated with bone metastases, in part because existing analgesic drugs show only limited efficacy in many patients and in part because of the adverse side effects associated with these agents. An important issue is that the nature and roles of the algogens produced in bone that drive pain-signalling systems remain unknown. Here, we tested the hypothesis that adenosine triphosphate is one such key mediator through actions on P2X3 and P2X2/3 receptors, which are expressed selectively on primary afferent nocioceptors, including those innervating the bone. Using a well-established rat model of bone cancer pain, AF-353, a recently described potent and selective P2X3 and P2X2/3 receptor antagonist, was administered orally to rats and found to produce highly significant prevention and reversal of bone cancer pain behaviour. This attenuation occurred without apparent modification of the disease, since bone destruction induced by rat MRMT-1 carcinoma cells was not significantly altered by AF-353. Using in vivo electrophysiology, evidence for a central site of action was provided by dose-dependent reductions in electrical, mechanical and thermal stimuli-evoked dorsal horn neuronal hyperexcitability following direct AF-353 administration onto the spinal cord of bone cancer animals. A peripheral site of action was also suggested by studies on the extracellular release of adenosine triphosphate from MRMT-1 carcinoma cells. Moreover, elevated phosphorylated-extracellular signal-regulated kinase expression in dorsal root ganglion neurons, induced by co-cultured MRMT-1 carcinoma cells, was significantly reduced in the presence of AF-353. These data suggest that blockade of P2X3 and P2X2/3 receptors on both the peripheral and central terminals of nocioceptors contributes to analgesic efficacy in a model of bone cancer pain. Thus, systemic P2X3 and P2X2/3 receptor antagonists with central nervous system penetration may offer a promising therapeutic tool in treating bone cancer pain.

Minocycline Attenuates Mechanical Allodynia and Central Sensitization Following Peripheral Second-Degree Burn Injury

Burn injury induces severe pain that can be refractory to existing pharmacotherapies. The underlying mechanism of burn pain remains unclear. We previously established an animal model and reported that unilateral burn injury induces chronic and bilateral mechanical allodynia, which is associated with central sensitization and microglial activation in the spinal cord dorsal horn. Modulation of the activity of microglia and p38 mitogen-activated protein kinase (MAPK) has been shown to ameliorate neuropathic pain in several nerve-injury pain models. In the present study, we show in this rat model that daily treatment with the microglial inhibitor minocycline (10 mg/kg), administered at the time of burn injury and for 7 days thereafter, significantly attenuates ipsilateral and contralateral allodynia as assessed up to 1 month following burn injury. These sensory changes are paralleled by significant suppression of evoked hyperexcitability of dorsal-horn neurons and of the expression of phosphorylated p38 (phospho-p38) in OX42+ microglial cells within the dorsal horn. Our results suggest that modulation of inflammation at early times after burn injury may have long-lasting effects, attenuating central neuropathic mechanisms which contribute to pain after burn injury. We demonstrate, in a rodent model of burn-associated pain, that the microglial inhibitor minocycline, delivered at the time of burn injury and for 1 week thereafter, has long-lasting effects, attenuating microglial activation and neuronal hyperresponsiveness in the dorsal horns, and ameliorating allodynia for at least 1 month.

Unilateral Focal Burn Injury Is Followed by Long-Lasting Bilateral Allodynia and Neuronal Hyperexcitability in Spinal Cord Dorsal Horn

Pain after burn injury can be intense and long lasting. Treatment is often ineffective, and there is a need for increased knowledge of the underlying pain mechanisms. In the present study, we established a unilateral partial-thickness burn injury model, which produces ipsilateral mechanical allodynia soon after injury, followed by contralateral allodynia. Chronic bilateral allodynia lasts up to 8 weeks postinjury in this model. In addition to the change in pain behavior, electrophysiological analyses showed that dorsal horn neurons become hyperexcitable and display significantly increased evoked activity with enlarged receptive fields, initially on the side ipsilateral to the injury, and subsequently on both sides of the spinal cord. It is known that, following nerve injury, activation of p38 mitogen-activated protein kinase (MAPK) pathways within spinal microglia contributes to the pathogenesis of pain. In our burn injury model, rapid and prolonged activation of phospho-p38-expressing microglia occurs bilaterally in the spinal cord dorsal horn. Taken together, these data demonstrate that a unilateral peripheral burn injury can produce long-lasting allodynia that can spread to the contralateral limb, together with dorsal horn neuronal hyperexcitability and microglial activation on both ipsilateral and contralateral sides of the spinal cord. Our results suggest that central neuropathic mechanisms can contribute to pain after burn injury. Mechanisms contributing to pain following burn injury are incompletely understood. In a novel animal model of burn injury, we have demonstrated hyperexcitability of second-order sensory neurons, activation of microglia, and chronic bilateral pain following the burn injury. This work identifies potential therapeutic targets to alleviate pain after burn injury.

Modality-specific hyperexcitability of dorsal horn neurons to mechanical stimuli in herpetic mice

Percutaneous inoculation of mice with herpes simplex virus type-1 produces marked dynamic allodynia in the zosteriform dermatome. In this study, we examined the electrophysiological excitability of the wide-dynamic range neuron in the spinal dorsal horn and the tibial nerve in response to mechanical (brush, punctum, and pinch) stimuli in mice with herpetic allodynia. The excitatory response of wide-dynamic range neurons to brush, but neither punctum nor pinch, stimulation of the zosteriform dermatome was increased in herpetic mice. The responses of the tibial nerve to all kinds of mechanical stimuli examined were decreased. These results suggest that dynamic allodynia in the affected dermatome is because of the increased excitability of wide-dynamic range neurons, but not primary afferents, to brush stimulation.

Microglia and neuropathic pain

In contrast to physiological pain, pathological pain is not dependent on the presence of tissue-damaging stimuli. One type of pathological pain--neuropathic pain--is often a consequence of nerve injury or of diseases such as diabetes, AIDS, or cancer. Neuropathic pain can be agonizing, can persist over long periods, and, unfortunately, is often resistant to known painkillers. There is a rapidly growing body of evidence indicating that microglia, the CNS immune cells, have causal roles in the pathogenesis of pain hypersensitivity following nerve injury. We will review recent advances in our understanding of the mechanisms producing neuropathic pain, focusing on the roles of microglia-expressed molecules, including cell surface receptors, intracellular signaling molecules, and diffusible factors involved in nerve injury-induced pain behaviors and hyperexcitability of dorsal horn neurons. Elucidating how spinal microglia cause neuropathic pain may provide us with exciting insights into pain mechanisms and clues for developing new drugs for the treatment of neuropathic pain.

Role of the spinal cord NR2B-containing NMDA receptors in the development of neuropathic pain

Activation of N-methyl- d-aspartate (NMDA) receptors in the spinal dorsal horn has been shown to be essential for the initiation of central sensitization and the hyperexcitability of dorsal horn neurons in chronic pain. However, whether the spinal NR2B-containing NMDA (NMDA-2B) receptors are involved still remains largely unclear. Using behavioral test and in vivo extracellular electrophysiological recording in L5 spinal nerve-ligated (SNL) neuropathic rats, we investigate the roles of spinal cord NMDA-2B receptors in the development of neuropathic pain. Our study showed that intrathecal ( i.t.) injection of Ro 25-6981, a selective NMDA-2B receptor antagonist, had a dose-dependent anti-allodynic effect without causing motor dysfunction. Furthermore, i.t. application of another NMDA-2B receptor antagonist ifenprodil prior to SNL also significantly inhibited the mechanical allodynia but not the thermal hyperalgesia. These data suggest that NMDA-2B receptors at the spinal cord level play an important role in the development of neuropathic pain, especially at the early stage following nerve injury. In addition, spinal administration of Ro 25-6981 not only had a dose-dependent inhibitory effect on the C-fiber responses of dorsal horn wide dynamic range (WDR) neurons in both normal and SNL rats, but also significantly inhibited the long-term potentiation (LTP) in the C-fiber responses of WDR neurons induced by high-frequency stimulation (HFS) applied to the sciatic nerve. These results indicate that activation of the dorsal horn NMDA-2B receptors may be crucial for the spinal nociceptive synaptic transmission and for the development of long-lasting spinal hyperexcitability following nerve injury. In conclusion, the spinal cord NMDA-2B receptors play a role in the development of central sensitization and neuropathic pain via the induction of LTP in dorsal horn nociceptive synaptic transmission. Therefore, the spinal cord NMDA-2B receptor is likely to be a target for clinical pain therapy.

The role of spinal nuclear factor-kappa B in spinal hyperexcitability

In behavioral experiments, inhibition of nuclear factor-kappaB activation by systemic administration of the IkappaB kinase inhibitor S1627 has been shown to attenuate inflammatory and neuropathic pain. Here, we specifically investigated with electrophysiological recordings in anesthetized rats whether spinal application of S1627 influences hyperexcitability of dorsal horn neurons during an acute knee joint inflammation. Spinal application of S1627 before and early during development of inflammation totally prevented spinal hyperexcitability suggesting an important role of spinal nuclear factor-kappaB in this process. During established inflammation, however, S1627 did not reduce the responses of neurons to mechanical stimulation of the inflamed knee within 2.5 h after spinal administration, thus suggesting that spinal hyperexcitability is not maintained by continuous nuclear factor-kappaB activation.

Upregulation of the phosphorylated form of CREB in spinothalamic tract cells following spinal cord injury: Relation to central neuropathic pain

Spinal cord injury (SCI) often leads to the generation of chronic intractable neuropathic pain. The mechanisms that lead to chronic central neuropathic pain (CNP) following SCI are not well understood, resulting in ineffective treatments for pain relief. Studies have demonstrated persistent hyperexcitability of dorsal horn neurons which may provide a substrate for CNP. We propose a number of similarities between CNP mechanisms and mechanisms that occur in long-term potentiation, in which hippocampal neurons are hyperexcitable. One biochemical similarity may be activation of the transcription factor, cyclic AMP response element-binding protein (CREB), via phosphorylation (pCREB). The current study was designed to examine whether tactile allodynia that develops in segments rostral to SCI (at-level pain) correlates with an increase in CREB phosphorylation in specific neurons known to be involved in allodynia, the spinothalamic tract (STT) cells. This study determined that, in animals experiencing at-level allodynia 35 days after SCI, pCREB was upregulated in the spinal cord segment rostral to the injury. In addition, pCREB was found to be upregulated specifically in STT cells in the rostral segment 35 days after SCI. These findings suggest one mechanism of maintained central neuropathic pain following SCI involves persistent upregulation of pCREB expression within STT cells.

Upregulation of Sodium Channel Nav1.3 and Functional Involvement in Neuronal Hyperexcitability Associated with Central Neuropathic Pain after Spinal Cord Injury

Spinal cord injury (SCI) can result in hyperexcitability of dorsal horn neurons and central neuropathic pain. We hypothesized that these phenomena are consequences, in part, of dysregulated expression of voltage-gated sodium channels. Because the rapidly repriming TTX-sensitive sodium channel Nav1.3 has been implicated in peripheral neuropathic pain, we investigated its role in central neuropathic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal contusion injury. Four weeks after injury when extracellular recordings demonstrated hyperexcitability of L3-L5 dorsal horn multireceptive nociceptive neurons, and when pain-related behaviors were evident, quantitative RT-PCR, in situ hybridization, and immunocytochemistry revealed an upregulation of Nav1.3 in dorsal horn nociceptive neurons. Intrathecal administration of antisense oligodeoxynucleotides (ODNs) targeting Nav1.3 resulted in decreased expression of Nav1.3 mRNA and protein, reduced hyperexcitability of multireceptive dorsal horn neurons, and attenuated mechanical allodynia and thermal hyperalgesia after SCI. Expression of Nav1.3 protein and hyperexcitability in dorsal horn neurons as well as pain-related behaviors returned after cessation of antisense delivery. Responses to normally noxious stimuli and motor function were unchanged in SCI animals administered Nav1.3 antisense, and administration of mismatch ODNs had no effect. These results demonstrate for the first time that Nav1.3 is upregulated in second-order dorsal horn sensory neurons after nervous system injury, showing that SCI can trigger changes in sodium channel expression, and suggest a functional link between Nav1.3 expression and neuronal hyperexcitability associated with central neuropathic pain.

Pain Sensitization in Male Chronic Pelvic Pain Syndrome: Why Are Symptoms so Difficult to Treat?

Male chronic pelvic pain syndrome (CPPS) is a chronic and debilitating symptom complex, and inconsistent outcomes of antibiotic and anti-inflammatory treatments have created an interest in investigating the neurological mechanisms of CPPS pain. Because chronic pelvic pain likely results in hyperexcitability of dorsal horn neurons, ie central sensitization, thermal algometry may be useful in the assessment of CPPS. We evaluated men with and without CPPS to determine if there were differences between the 2 groups in their responses to noxious heat stimuli. We recruited 66 healthy men without CPPS and 36 men with a history of CPPS for the study. Mean age of CPPS subjects was 43.1 years (range 18 to 62) and mean age of controls was 35.1 (range 21 to 61). All subjects with pain completed a National Institutes of Health-Chronic Prostatitis Symptom Index. We conducted thermal sensory tests with a small thermode programmed to deliver 2 series of 4 rapid bursts of noxious heat stimuli to the perineum and the anterior thigh. The subjects reported sensation on a computerized visual analog scale (COVAS) with a manual sliding lever. The average peak COVAS values and time to peak values from thigh and perineum of each series of thermal bursts were compared between CPPS and controls. The mean pain score on questionnaires was 9.7 (4 or greater indicating significant pain), mean urinary score was 4.7 (range 0 to 10) and mean quality of life impact score was 7.3 (range 3 to 13). Compared to controls men with CPPS reported higher mean peak COVAS values in the perineum. There was no difference between groups in the peak COVAS value on the thigh. Men with CPPS have altered heat sensation/pain sensitization in the perineum compared to controls. This finding is another that supports the resemblance of CPPS to other chronic pain syndromes and may help explain why CPPS is often refractory to treatment. Further investigations into male CPPS should focus on the mechanisms inducing and maintaining pain in an effort to treat this condition effectively.