Multiple phases of relief from experimental mechanical allodynia by systemic lidocaine: responses to early and late infusions

Comparison of the effects of perioperative intravenous infusions of esmolol and lidocaine on the quality of postoperative recovery in patients undergoing functional endoscopic sinus surgery: a randomized, double-blind, noninferiority study

BACKGROUND

Perioperative intravenous lidocaine infusion can improve the quality of recovery (QoR) in patients undergoing functional endoscopic sinus surgery (FESS), but the effect of esmolol on recovery has been rarely studied. The aim of this study to compare the effects of esmolol and lidocaine on the QoR in patients with FESS.

METHODS

A total of 60 patients were randomly divided into Group E: intravenous esmolol (0.5 mg/kg for 1 min, followed by 3.0 mg/kg/h); Group L: intravenous lidocaine (2.0 mg/ kg for 10 min, followed by 2 mg/kg/h). The quality of recovery-15 (QoR-15) score was compared. Other parameters compared were the numeric rating pain scale (NRS), haemodynamic data, Surgical field conditions, intraoperative drug dosages, number of cases of remedial analgesia, time to awakening and incidence of postoperative sore throat (POST) as well as postoperative nausea and vomiting (PONV).

RESULTS

The mean difference in the QoR-15 score between Group E and Group L on postoperative day 1 (POD1) was - 1.37 (95% CI - 2.75 to 0.01; P < 0.001 for noninferiority), indicating the noninferiority of esmolol. Haemodynamic changes and intraoperative nitroglycerine dosages were significantly lower in Group E than in Group L (P < 0.05). The scores of surgical field quality (SSFQ) was higher in Group E than in Group L (P < 0.05).

CONCLUSION

Intravenous infusion of esmolol is not inferior to lidocaine in the quality of postoperative recovery in patients with FESS, and is more advantageous in terms of the quality of the surgical field, attenuation of intraoperative haemodynamic fluctuations, and postoperative awakening.

The Pharmacological Treatment of Neuropathic Pain in Children

The International Association for the Study of Pain (IASP) defines neuropathic pain as pain caused by a lesion or disease of the somatosensory nervous system. It is characterized as a clinical condition in which diagnostic studies reveal an underlying cause of an abnormality in the peripheral or central nervous system. Many common causes of neuropathic pain in adults are rare in children. The purpose of this focused narrative review is, to 1) provide an overview of neuropathic pain in children, 2) highlight unique considerations related to the diagnosis and mechanisms of neuropathic pain in children, and 3) perform a comprehensive analysis of the pharmacological treatments available. We emphasize that data for routine use of pharmacological agents in children with neuropathic pain are largely inferred from adult literature with little research performed on pediatric populations, yet have clear evidence of harms to pediatric patients. Based on these findings, we propose risk mitigation strategies such as utilizing topical treatments whenever possible, assessing pain phenotyping to guide drug class choice, and considering pharmaceuticals in the broader context of the multidisciplinary treatment of pediatric pain. Furthermore, we highlight important directions for future research on pediatric neuropathic pain treatment.

Exploring the Mystery of Osteoarthritis using Bioinformatics Analysis of Cartilage Tissue

BACKGROUND

Osteoarthritis (OA) is a kind of chronic disease relating to joints, which seriously affectsthe daily life activities of the elderly and can also lead to disability. However, the pathogenesis of OA is still unclear, which leads to limited treatment and the therapeutic effect far from people's expectations. This study aims to filter out key genes in the pathogenesis of OA and explore their potential role in the occurrence and development of OA.

METHODS

The dataset of GSE117999 was obtained and analyzed in order to identify the differentially expressed genes (DEGs), hub genes and key genes. We also identified potential miRNAs which may play a major role in the pathogenesis of OA, and verified their difference in OA by real-time quantitative PCR (RT-qPCR). DGldb was found to serve as an indicator to identify drugs with potential therapeutic effects on key genes and Receiver Operating Characteristic (ROC) analysis was used for identifying underlying biomarkers of OA.

RESULTS

We identified ten key genes, including MDM2, RB1, EGFR, ESR1, UBE2E3, WWP1, BCL2, OAS2, TYMS and MSH2. Then, we identified hsa-mir-3613-3p, hsa-mir-548e-5p and hsamir- 5692a to be potentially related to key genes. In addition, RT-qPCR confirmed the differential expression of identified genes in mouse cartilage with or without OA. We then identified Etoposide and Everolimus, which were potentially specific to the most key genes. Finally, we speculated that ESR1 might be a potential biomarker of OA.

CONCLUSION

In this study, potential key genes related to OA and their biological functions were identified, and their potential application value in the diagnosis and treatment of OA has been demonstrated, which will help us to improve the therapeutic effect of OA.

Lidocaine Infusions for Pain Management in Pediatrics

Lidocaine is an amino amide with a well-established role as a local anesthetic agent. Systemic intravenous administration expands its clinical use to include acute and chronic pain circumstances, such as postoperative pain, neuropathic pain, postherpetic neuralgia, hyperalgesia, visceral pain, and centrally mediated pain. For refractory pain that has not responded to conventional therapy or if further escalation of treatment is prevented by contraindications or side effects to standard therapies, a continuous infusion of lidocaine may be considered as a single intervention or as a sequence of infusions. Here, we review and evaluate published data reflecting the use of lidocaine continuous infusions for pain management in the pediatric population.

Assessment of the Effect of Perioperative Venous Lidocaine on the Intensity of Pain and IL-6 Concentration After Laparoscopic Gastroplasty

BACKGROUND AND OBJECTIVES

Opioids are associated with sedation and respiratory depression. The primary objective of this study was to assess pain intensity after gastric bypass with lidocaine. The secondary objective was to assess the IL-6 concentration, consumption of morphine, time to morphine request, time to extubation, and side effects.

METHODS

Sixty patients aged 18 to 60 years, with ASA (American Society of Anesthesiologists) scores of 2 or 3, who underwent bariatric surgery were allocated to two groups. Patients in group 1 were administered lidocaine (1.5 mg/kg) 5 min before the induction of anesthesia, and group 2 was administered 0.9% saline solution in an equal volume. Subsequently, lidocaine (2 mg/kg/h) or 0.9% saline was infused during the entire surgical procedure. Anesthesia was performed with fentanyl (5 μg/kg), propofol, rocuronium, and sevoflurane. Postoperative patient-controlled analgesia was provided with morphine. The following were evaluated: pain intensity, IL-6, 24-h consumption of morphine, time to the morphine request, time to extubation, and adverse effects.

RESULTS

The lidocaine group had a lower pain intensity than the saline group for up to 1 h, with no differences between groups in IL-6 and time to extubation. The lidocaine group consumed less morphine within 24 h, had a longer time until the first supplemental morphine request, and had a lower incidence of nausea.

CONCLUSIONS

Lidocaine reduced the intensity of early postoperative pain, incidence of nausea, and consumption of morphine within 24 h and increased time to the first morphine request, without reducing the plasma concentrations of IL-6.

Efficacy and Safety of Lidocaine Infusion Treatment for Neuropathic Pain: A Randomized, Double-Blind, and Placebo-Controlled Study

BACKGROUND AND OBJECTIVES

Lidocaine infusion therapy (LIT) is an effective treatment for relieving neuropathic pain (NeP). However, it remains unclear whether pain relief can be sustained through repeated lidocaine infusions. This study aimed to determine whether repeated intravenous administration of low-dose lidocaine could provide prolonged pain relief in patients with specific NeP conditions.

METHODS

This is a prospective, randomized, double-blind, placebo-controlled, parallel study. We compared the efficacy and safety of lidocaine infusions (3 mg/kg) in the LIT group and normal saline infusions in the control group once a week for 4 consecutive weeks in patients with postherpetic neuralgia or complex regional pain syndrome type II. The primary outcome was the difference in the percentage change in the 11-point numerical rating scale (NRS) pain score from baseline to after the final infusion. Secondary outcomes included pain scores during 4 weeks of follow-up and any complications.

RESULTS

Forty-two patients completed this study protocol. The percentage reduction in NRS pain scores after the final infusion was significantly greater in the LIT group compared with the control group (P = 0.011). However, this pain reduction was not detectable at the 4-week follow-up. The difference in the percentage change in NRS pain scores was especially prominent in the LIT group after the third and fourth infusions. None of the study participants experienced serious complications from the treatment.

CONCLUSIONS

Lidocaine infusion therapy (3 mg/kg of lidocaine administered over 1 hour) provided effective short-term pain relief, which was substantially prominent after repeated infusions were administered to patients with refractory NeP.

CLINICAL TRIAL REGISTRATION

This study was registered at ClinicalTrials.gov, identifier NCT02597257.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

The AMPK Activator A769662 Blocks Voltage-Gated Sodium Channels: Discovery of a Novel Pharmacophore with Potential Utility for Analgesic Development

Voltage-gated sodium channels (VGSC) regulate neuronal excitability by governing action potential (AP) generation and propagation. Recent studies have revealed that AMP-activated protein kinase (AMPK) activators decrease sensory neuron excitability, potentially by preventing sodium (Na⁺) channel phosphorylation by kinases such as ERK or via modulation of translation regulation pathways. The direct positive allosteric modulator A769662 displays substantially greater efficacy than other AMPK activators in decreasing sensory neuron excitability suggesting additional mechanisms of action. Here, we show that A769662 acutely inhibits AP firing stimulated by ramp current injection in rat trigeminal ganglion (TG) neurons. PT1, a structurally dissimilar AMPK activator that reduces nerve growth factor (NGF) -induced hyperexcitability, has no influence on AP firing in TG neurons upon acute application. In voltage-clamp recordings, application of A769662 reduces VGSC current amplitudes. These findings, based on acute A769662 application, suggest a direct channel blocking effect. Indeed, A769662 dose-dependently blocks VGSC in rat TG neurons and in Na_v1.7-transfected cells with an IC₅₀ of ~ 10 μM. A769662 neither displayed use-dependent inhibition nor interacted with the local anesthetic (LA) binding site. Popliteal fossa administration of A769662 decreased noxious thermal responses with a peak effect at 5 mins demonstrating an analgesic effect. These data indicate that in addition to AMPK activation, A769662 acts as a direct blocker/modulator of VGSCs, a potential mechanism enhancing the analgesic property of this compound.

The effects of tracheal tube cuffs filled with air, saline or alkalinised lidocaine on haemodynamic changes and laryngotracheal morbidity in children: a randomised, controlled trial

We studied the effects of tracheal tube cuffs filled with air, saline or alkalinised lidocaine on haemodynamic changes during tracheal extubation and postoperative laryngotracheal morbidity in children. We randomly allocated 164 children aged 3-13 years undergoing general anaesthesia to one of four groups; tracheal tube cuffs filled with air (n = 41); saline (n = 41); alkalinised lidocaine 0.5% (n = 41); or alkalinised lidocaine 1% (n = 41). Intracuff pressure was monitored and maintained below 20 cmH₂ O. The mean (SD) increases in systolic blood pressure after tracheal extubation compared with before extubation were 10.9 (10.8) mmHg, 7.3 (17.7) mmHg, 4.1 (10.5) mmHg and 1.9 (9.5) mmHg in the air, saline, 0.5% and 1% alkalinised lidocaine groups, respectively (p = 0.021). The mean (SD) increases in diastolic blood pressure after tracheal extubation compared with before extubation were 3.9 (9.7) mmHg, 7.9 (14.6) mmHg, 0.7 (10.4) mmHg and 3.6 (6.9) mmHg in the air, saline, 0.5% and 1% alkalinised lidocaine groups, respectively (p = 0.019). The mean (SD) increases in heart rate after tracheal extubation compared with before extubation were 14.2 (7.6) beats.min^-1 , 15.5 (13.1) beats.min^-1 , 5.2 (9.6) beats.min^-1 and 4.1 (6.6) beats.min^-1 in the air, saline, 0.5% and 1% alkalinised lidocaine groups, respectively (p < 0.001). The incidence of sore throat 8 h after tracheal extubation was 22.0% in the air-filled group, 9.8% in the saline group, 4.9% in the 0.5% alkalinised lidocaine group and 2.4% in the 1% alkalinised lidocaine group, p = 0.015. We conclude that filling the tracheal tube cuff with alkalinised lidocaine-filled reduces the haemodynamic response to tracheal extubation and postoperative laryngotracheal morbidity in children.

Astrocyte sigma-1 receptors modulate connexin 43 expression leading to the induction of below-level mechanical allodynia in spinal cord injured mice

We have previously shown using a spinal cord injury (SCI) model that gap junctions contribute to the early spread of astrocyte activation in the lumbar spinal cord and that this astrocyte communication plays critical role in the induction of central neuropathic pain. Sigma-1 receptors (Sig-1Rs) have been implicated in spinal astrocyte activation and the development of peripheral neuropathic pain, yet their contribution to central neuropathic pain remains unknown. Thus, we investigated whether SCI upregulates spinal Sig-1Rs, which in turn increase the expression of the astrocytic gap junction protein, connexin 43 (Cx43) leading to the induction of central neuropathic pain. A thoracic spinal cord hemisection significantly increased both astrocyte activation and Cx43 expression in lumbar dorsal horn. Sig-1Rs were also increased in lumbar dorsal horn astrocytes, but not neurons or microglia. Intrathecal injection of an astrocyte metabolic inhibitor (fluorocitrate); a gap junction/hemichannel blocker (carbenoxolone); or a Cx43 mimetic peptide (43Gap26) significantly reduced SCI-induced bilateral below-level mechanical allodynia. Blockade of Sig-1Rs with BD1047 during the induction phase of pain significantly suppressed the SCI-induced development of mechanical allodynia, astrocyte activation, increased expression of Cx43 in both total and membrane levels, and increased association of Cx43 with Sig-1R. However, SCI did not change the expression of oligodendrocyte (Cx32) or neuronal (Cx36) gap junction proteins. These findings demonstrate that SCI activates astrocyte Sig-1Rs leading to increases in the expression of the gap junction protein, Cx43 and astrocyte activation in the lumbar dorsal horn, and ultimately contribute to the induction of bilateral below-level mechanical allodynia.

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 4 days 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 (4 hours, 8 hours, 1 day) attenuated hyperalgesia and neuronal hyperexcitability (P<.045) only for the treatment given 4 hours 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.

Neurophysiology of arthritis pain

Arthritis pain is a complex phenomenon involving intricate neurophysiological processing at all levels of the pain pathway. The treatment options available to alleviate joint pain are fairly limited and most arthritis patients report only modest pain relief with current treatments. A better understanding of the neural mechanisms responsible for musculoskeletal pain and the identification of new targets will help in the development of future pharmacological therapies. This article reviews some of the latest research into factors which contribute to joint pain and covers areas such as cannabinoids, proteinase activated receptors, sodium channels, cytokines and transient receptor potential channels. The emerging hypothesis that osteoarthritis may have a neuropathic component is also discussed.

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.

Axonal voltage-gated ion channels as pharmacological targets for pain

Upon peripheral nerve injury (caused by trauma or disease process) axons of the dorsal root ganglion (DRG) somatosensory neurons have the ability to sprout and regrow/remyelinate to reinnervate distant target tissue or form a tangled scar mass called a neuroma. This regenerative response can become maladaptive leading to a persistent and debilitating pain state referred to as chronic pain corresponding to the clinical description of neuropathic/chronic inflammatory pain. There is little agreement to what causes peripheral chronic pain other than hyperactivity of the nociceptive DRG neurons which ultimately depends on the function of voltage-gated ion channels. This review focuses on the pharmacological modulators of voltage-gated ion channels known to be present on axonal membrane which represents by far the largest surface of DRG neurons. Blockers of voltage-gated Na(+) channels, openers of voltage-gated K(+) channels and blockers of hyperpolarization-activated cyclic nucleotide-gated channels that were found to reduce neuronal activity were also found to be effective in neuropathic and inflammatory pain states. The isoforms of these channels present on nociceptive axons have limited specificity. The rationale for considering axonal voltage-gated ion channels as targets for pain treatment comes from the accumulating evidence that chronic pain states are associated with a dysregulation of these channels that could alter their specificity and make them more susceptible to pharmacological modulation. This drives the need for further development of subtype-specific voltage-gated ion channels modulators, as well as clinically available neurophysiological techniques for monitoring axonal ion channel function in peripheral nerves.

Perioperative interventions to reduce chronic postsurgical pain

Approximately 10% of patients following a variety of surgeries develop chronic postsurgical pain. Reducing chronic postoperative pain is especially important to reconstructive surgeons because common operations such as breast and limb reconstruction have even higher risk for developing chronic postsurgical pain. Animal studies of posttraumatic nerve injury pain demonstrate that there is a critical time frame before and immediately after nerve injury in which specific interventions can reduce the incidence and intensity of chronic neuropathic pain behaviors-so called "preventative analgesia." In animal models, perineural local anesthetic, systemic intravenous local anesthetic, perineural clonidine, systemic gabapentin, systemic tricyclic antidepressants, and minocycline have each been shown to reduce pain behaviors days to weeks after treatment. The translation of this work to humans also suggests that brief perioperative interventions may protect patients from developing new chronic postsurgical pain. Recent clinical trial data show that there is an opportunity during the perioperative period to dramatically reduce the incidence and severity of chronic postsurgical pain. The surgeon, working with the anesthesiologist, has the ability to modify both early and chronic postoperative pain by implementing an evidence-based preventative analgesia plan.

Preventive analgesia by local anesthetics: the reduction of postoperative pain by peripheral nerve blocks and intravenous drugs

The use of local anesthetics to reduce acute postoperative pain has a long history, but recent reports have not been systematically reviewed. In addition, the need to include only those clinical studies that meet minimum standards for randomization and blinding must be adhered to. In this review, we have applied stringent clinical study design standards to identify publications on the use of perioperative local anesthetics. We first examined several types of peripheral nerve blocks, covering a variety of surgical procedures, and second, we examined the effects of intentionally administered IV local anesthetic (lidocaine) for suppression of postoperative pain. Thirdly, we have examined publications in which vascular concentrations of local anesthetics were measured at different times after peripheral nerve block procedures, noting the incidence when those levels reached ones achieved during intentional IV administration. Importantly, the very large number of studies using neuraxial blockade techniques (epidural, spinal) has not been included in this review but will be dealt with separately in a later review. The overall results showed a strongly positive effect of local anesthetics, by either route, for suppressing postoperative pain scores and analgesic (opiate) consumption. In only a few situations were the effects equivocal. Enhanced effectiveness with the addition of adjuvants was not uniformly apparent. The differential benefits between drug delivery before, during, or immediately after a surgical procedure are not obvious, and a general conclusion is that the significant antihyperalgesic effects occur when the local anesthetic is present during the acute postoperative period, and its presence during surgery is not essential for this action.

Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT(2A)R) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. We investigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) and NMDA, in 5-HT(2A)R-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT(2A)R agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented by mGluR1 antagonist AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] or NMDA antagonist d-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT(2A)R and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT(2A)R with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT(2A)R-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT(2A)R antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT(2A)R-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT(2A)R blockade. These results emphasize a role for 5-HT(2A)R in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT(2A)R-mediated facilitation.

Involvement of Nav 1.8 sodium ion channels in the transduction of mechanical pain in a rodent model of osteoarthritis

INTRODUCTION

A subgroup of voltage gated sodium channels including Nav1.8 are exclusively expressed on small diameter primary afferent neurons and are therefore believed to be integral to the neurotransmission of nociceptive pain. The present study examined whether local application of A-803467, a selective blocker of the Nav 1.8 sodium channel, can reduce nociceptive transmission from the joint in a rodent model of osteoarthritis (OA).

METHODS

OA-like changes were induced in male Wistar rats by an intra-articular injection of 3 mg sodium monoiodoacetate (MIA). Joint nociception was measured at day 14 by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious rotation of the joint both before and following close intra-arterial injection of A-803467. The effect of Nav1.8 blockade on joint pain perception and secondary allodynia were determined in MIA treated animals by hindlimb incapacitance and von Frey hair algesiometry respectively.

RESULTS

A-803467 significantly reduced the firing rate of joint afferents during noxious rotation of the joint but had no effect during non-noxious rotation. In the pain studies, peripheral injection of A-803467 into OA knees attenuated hindlimb incapacitance and secondary allodynia.

CONCLUSIONS

These studies show for the first time that the Nav1.8 sodium channel is part of the molecular machinery involved in mechanotransduction of joint pain. Targeting the Nav1.8 sodium channel on joint nociceptors could therefore be useful for the treatment of OA pain, avoiding the unwanted side effects of non-selective nerve blocks.

Inflammatory cytokine and chemokine expression is differentially modulated acutely in the dorsal root ganglion in response to different nerve root compressions

STUDY DESIGN

Inflammatory proteins were quantified in bilateral dorsal root ganglions (DRGs) at 1 hour and 1 day using a multiplexed assay after 2 different unilateral nerve root compression injuries.

OBJECTIVE

To quantify cytokines and a chemokine in the DRG after nerve root compression with and without a chemical injury to determine contributing inflammatory factors in the DRG that may mediate radicular nociception in clinically relevant nerve root pathologies.

SUMMARY OF BACKGROUND DATA

Inflammatory cytokines are known to relate to the behavioral hypersensitivity induced after injuries to the nerve root. However, the relative expression of these proteins in the DRG after cervical nerve root compression are not known.

METHODS

The right C7 nerve root underwent transient compression (10 gf) or transient compression with a chemical irritation (10 gf + chr). The chemical injury was also given alone (chr), and the nerve root was exposed (sham), providing 2 types of controls. Mechanical allodynia was measured to assess behavioral outcomes. Interleukin (IL)-1b, IL-6, tumor necrosis factor-a, and macrophage inflammatory protein 3 (MIP3) were quantified in bilateral DRGs at 1 hour and 1 day using a multiplexed assay.

RESULTS

Ipsilateral allodynia at day 1 after 10 gf + chr was significantly increased over both 10 gf and chr (P < 0.049). Cytokines and MIP3 were not statistically increased over sham at 1 hour. By day 1 after 10 gf + chr, all proteins (IL-1β, IL-6, tumor necrosis factor-a, MIP3) were significantly increased over both normal and sham in the ipsilateral DRG (P < 0.036), and the cytokines were also significantly increased over chr (P < 0.029). Despite allodynia at day 1, cytokines at that time were not increased over normal or sham after either 10 gf or chr.

CONCLUSION

Nerve root compression alone may not be sufficient to induce early increases in proinflammatory cytokines in the DRG after radiculopathy and this early protein response may not be directly responsible for nociception in this type of injury.

Enduring prevention and transient reduction of postoperative pain by intrathecal resolvin D1

Postoperative pain slows surgical recovery, impacting the return of normal function for weeks, months, or longer. Here we report the antihyperalgesic actions of a new compound, resolvin D1 (RvD1), known to reduce inflammation and to suppress pain after peripheral nerve injury, on the acute pain occurring after paw incision and the prolonged pain after skin-muscle retraction. Injection of RvD1 (20-40ng) into the L5-L6 intrathecal space 30minutes before surgery reduces the postincisional primary mechanical hypersensitivity, lowering the peak change by approximately 70% (with 40ng) and reducing the area under the curve (AUC) for the entire 10-day postincisional course by approximately 60%. Intrathecal injection of RvD1 on postoperative day (POD) 1 reduces the hyperalgesia to the same level as that from preoperative injection within a few hours, an effect that persists for the remaining PODs. Tactile allodynia and hyperalgesia following the skin/muscle incision retraction procedure, measured at the maximum values 12 to 14days, is totally prevented by intrathecal RvD1 (40ng) given at POD 2. However, delaying the injection until POD 9 or POD 17 results in RvD1 causing only transient and incomplete reversal of hyperalgesia, lasting for <1day. These findings demonstrate the potent, effective reduction of postoperative pain by intrathecal RvD1 given before or shortly after surgery. The much more limited effect of this compound on retraction-induced pain, when given 1 to 2weeks later, suggests that the receptors or pathways for resolvins are more important in the early than the later stages of postoperative pain. Single intrathecal injections of resolvin D1 in rats before or 1 to 2days after surgery strongly reduce postoperative pain for several weeks.

Effect of systemic lidocaine on visceral and somatic nociception in conscious horses

REASONS FOR PERFORMING STUDY

Commonly used analgesics (nonsteroidal anti-inflammatory agents, opioids and alpha2-agonists) have unwanted side effects. An effective alternative with minimal adverse effects would benefit clinical equine pain management.

OBJECTIVES

To compare the effect of lidocaine or saline on duodenal and rectal distension threshold pressure and somatic thermal threshold in conscious mature horses.

HYPOTHESIS

Systemically administered lidocaine would increase somatic and visceral nociceptive thresholds.

METHODS

Lidocaine (2 mg/kg bwt bolus followed by 50 microg/kg bwt/min for 2 h) or saline was administered to 6 horses each carrying a permanently implanted gastric cannula, in a randomised, blinded cross-over design. Thermal threshold was measured using a probe containing a heater element placed over the withers which supplied heat until the horse responded. A barostatically controlled intraduodenal balloon was distended until a discomfort response was obtained. A rectal balloon was inflated until extruded or signs of discomfort noted.

RESULTS

Thermal threshold was increased significantly 30 and 90 mins after the start of lidocaine infusion. There was no change in duodenal distension pressure and a small but clinically insignificant change in colorectal distension pressure in the lidocaine group.

CONCLUSIONS

At the dose used, systemically administered lidocaine produced thermal antinociception but minimal changes in visceral nociception.

POTENTIAL RELEVANCE

At these doses, lidocaine may play a role in somatic analgesia in horses.

The effect of intravenous lidocaine on brain activation during non-noxious and acute noxious stimulation of the forepaw: a functional magnetic resonance imaging study in the rat

BACKGROUND

Lidocaine can alleviate acute as well as chronic neuropathic pain at very low plasma concentrations in humans and laboratory animals. The mechanism(s) underlying lidocaine's analgesic effect when administered systemically is poorly understood but clearly not related to interruption of peripheral nerve conduction. Other targets for lidocaine's analgesic action(s) have been suggested, including sodium channels and other receptor sites in the central rather than peripheral nervous system. To our knowledge, the effect of lidocaine on the brain's functional response to pain has never been investigated. Here, we therefore characterized the effect of systemic lidocaine on the brain's response to innocuous and acute noxious stimulation in the rat using functional magnetic resonance imaging (fMRI).

METHODS

Alpha-chloralose anesthetized rats underwent fMRI to quantify brain activation patterns in response to innocuous and noxious forepaw stimulation before and after IV administration of lidocaine.

RESULTS

Innocuous forepaw stimulation elicited brain activation only in the contralateral primary somatosensory (S1) cortex. Acute noxious forepaw stimulation induced activation in additional brain areas associated with pain perception, including the secondary somatosensory cortex (S2), thalamus, insula and limbic regions. Lidocaine administered at IV doses of either 1 mg/kg, 4 mg/kg or 10 mg/kg did not abolish or diminish brain activation in response to innocuous or noxious stimulation. In fact, IV doses of 4 mg/kg and 10 mg/kg lidocaine enhanced S1 and S2 responses to acute nociceptive stimulation, increasing the activated cortical volume by 50%-60%.

CONCLUSION

The analgesic action of systemic lidocaine in acute pain is not reflected in a straightforward interruption of pain-induced fMRI brain activation as has been observed with opioids. The enhancement of cortical fMRI responses to acute pain by lidocaine observed here has also been reported for cocaine. We recently showed that both lidocaine and cocaine increased intracellular calcium concentrations in cortex, suggesting that this pharmacological effect could account for the enhanced sensitivity to somatosensory stimulation. As our model only measured physiological acute pain, it will be important to also test the response of these same pathways to lidocaine in a model of neuropathic pain to further investigate lidocaine's analgesic mechanism of action.

Injury discharges regulate calcium channel alpha-2-delta-1 subunit upregulation in the dorsal horn that contributes to initiation of neuropathic pain

Previous studies have shown that peripheral nerve injury in rats induces increased expression of the voltage gated calcium channel (VGCC) alpha-2-delta-1 subunit (Ca v alpha2 delta1) in spinal dorsal horn and sensory neurons in dorsal root ganglia (DRG) that correlates to established neuropathic pain states. To determine if injury discharges trigger Ca v alpha2 delta1 induction that contributes to neuropathic pain initiation, we examined allodynia onset and Ca v alpha2 delta1 levels in DRG and spinal dorsal horn of spinal nerve ligated rats after blocking injury induced neural activity with a local brief application of lidocaine on spinal nerves before the ligation. The lidocaine pretreatment blocked ligation-induced discharges in a dose-dependent manner. Similar pretreatment with the effective concentration of lidocaine diminished injury-induced increases of the Ca v alpha2 delta1 in DRG and abolished that in spinal dorsal horn specifically, and resulted in a delayed onset of tactile allodynia post-injury. Both dorsal horn Ca v alpha2 delta1 upregulation and tactile allodynia in the lidocaine pretreated rats returned to levels similar to that in saline pretreated controls 2 weeks post the ligation injury. In addition, preemptive intrathecal Ca v alpha2 delta1 antisense treatments blocked concurrently injury-induced allodynia onset and Ca v alpha2 delta1 upregulation in dorsal spinal cord. These findings indicate that injury induced discharges regulate Ca v alpha2 delta1 expression in the spinal dorsal horn that is critical for neuropathic allodynia initiation. Thus, preemptive blockade of injury-induced neural activity or Ca v alpha2 delta1 upregulation may be a beneficial option in neuropathic pain management.

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Spinal nerve ligation-induced activation of nuclear factor kappaB is facilitated by prostaglandins in the affected spinal cord and is a critical step in the development of mechanical allodynia

This study investigated the effect of 5th and 6th lumbar nerve (L5/L6) spinal nerve ligation (SNL) on activated nuclear factor kappaB (NFkBa) in nuclear extracts from the lumbar dorsal horn of the rat, and its relationship to prostaglandin (PG)-dependent spinal hyperexcitability and allodynia 3 days later. Male Sprague-Dawley rats, fitted with intrathecal (i.t.) catheters, underwent SNL- or sham-surgery. Paw withdrawal threshold (PWT), electromyographic analysis of the biceps femoris flexor reflex, and immunoblotting of the spinal cord were used. Both allodynia (PWT <or=4 g) and exaggerated A- and C-fiber-mediated reflex responses (AFRR and CFRR), featuring decreased activation thresholds and evoked hyperexcitability, were evident only in nerve-ligated animals. This was preceded by an increase in NFkBa in the ipsilateral lumbar dorsal horn at 12 h which was still present 3 days after SNL. The amount of NFkBa in the ventral horns was unchanged compared with sham-controls. Blocking the activation of spinal NFkappaB, either directly with ammonium pyrrolidedithiocarbamate (PDTC; 100 microg i.t.) or indirectly with S(+)-ibuprofen (100 microg i.t.) administered immediately after SNL, prevented the SNL-induced expression of spinal cyclooxygenase-2 and the development of spinal hyperexcitability and allodynia 3 days later. R(-)-Ibuprofen and vehicle had no effect. These results demonstrate that NFkappaB is not only activated by SNL, but that spinal PG generated in the affected spinal cord from the onset of nerve injury facilitates this process. NFkappaB is a critical antecedent in the development of spinal PG-dependent hyperexcitability and allodynia in the SNL model.

Tetrodotoxin for moderate to severe cancer pain: a randomized, double blind, parallel design multicenter study

Cancer pain is a serious public health issue and more effective treatments are needed. This study evaluates the analgesic activity of tetrodotoxin, a highly selective sodium channel blocker. This randomized, placebo-controlled, parallel design study of subcutaneous tetrodotoxin, in patients with moderate or severe unrelieved cancer pain persisting despite best available treatment, involved 22 centers across Canada. The design called for tetrodotoxin administered subcutaneously over Days 1-4 with a period of observation to Day 15 or longer. All patients could enroll into an open-label extension efficacy and safety trial. The primary endpoint was the proportion of analgesic responders in each treatment arm. Eighty-two patients were randomized, and results on 77 were available for analysis. There was a nonstatistically significant trend toward more responders in the active treatment arm based on the primary endpoint (pain intensity difference). However, analysis of secondary endpoints, and an exploratory post hoc analysis, suggested there may be a robust analgesic effect if a composite endpoint is used, including either fall in pain level, or fall in opioid dose, plus improvement in quality of life. Most patients described transient perioral tingling or other mild sensory phenomena within about an hour of each treatment. Nausea and other toxicities were generally mild, but one patient experienced a serious, adverse event, truncal and gait ataxia. This trial suggests tetrodotoxin may potentially relieve moderate to severe, treatment-resistant cancer pain in a large proportion of patients, and often for prolonged periods following treatment, but further study is warranted using a composite primary endpoint.

Transient cervical nerve root compression modulates pain: load thresholds for allodynia and sustained changes in spinal neuropeptide expression

Nerve root compression produces chronic pain and altered spinal neuropeptide expression. This study utilized controlled transient loading in a rat model of painful cervical nerve root compression to investigate the dependence of mechanical allodynia on load magnitude. Injury loads (0-110mN) were applied quasistatically using a customized loading device, and load thresholds to produce maintained mechanical allodynia were defined. Bilateral spinal expression of substance P (SP) and calcitonin gene-related peptide (CGRP) was assessed 7 days following compression using immunohistochemistry to determine relationships between these neuropeptides and compression load. A three-segment change point model was implemented to model allodynia responses and their relationship to load. Load thresholds were defined at which ipsilateral and contralateral allodynia were produced and sustained. The threshold for increased allodynia was lowest for acute (day 1) ipsilateral responses (26.29mN), while thresholds for allodynia on day 7 were similar for the ipsilateral (38.16mN) and contralateral forepaw (38.26mN). CGRP, but not SP, significantly decreased with load; the thresholds for ipsilateral and contralateral CGRP decreases corresponded to 19.52 and 24.03mN, respectively. These thresholds suggest bilateral allodynia may be mediated by spinal mechanisms, and that these mechanisms depend on the magnitude of load.

Chemical and mechanical nerve root insults induce differential behavioral sensitivity and glial activation that are enhanced in combination

Both chemical irritation and mechanical compression affect radicular pain from disc herniation. However, relative effects of these insults on pain symptoms are unclear. This study investigated chemical and mechanical contributions for painful cervical nerve root injury. Accordingly, the C7 nerve root separately underwent chromic gut exposure, 10gf compression, or their combination. Mechanical allodynia was assessed, and glial reactivity in the C7 spinal cord tissue was assayed at days 1 and 7 by immunohistochemistry using GFAP and OX-42 as markers of astrocytes and microglia, respectively. Both chromic gut irritation and 10gf compression produced ipsilateral increases in allodynia over sham (p<0.048); combining the two insults significantly (p<0.027) increased ipsilateral allodynia compared to either insult alone. Behavioral hypersensitivity was also produced in the contralateral forepaw for all injuries, but only the combined insult was significantly increased over sham (p<0.031). Astrocytic activation was significantly increased over normal (p<0.001) in the ipsilateral dorsal horn at 1 day after either compression or the combined injury. By day 7, GFAP-reactivity was further increased for the combined injury compared to day 1 (p<0.001). In contrast, spinal OX-42 staining was generally variable, with only mild activation at day 1. By day 7 after the combined injury, there were significant (p<0.003) bilateral increases in OX-42 staining over normal. Spinal astrocytic and microglial reactivity follow different patterns after chemical root irritation, compression, and a combined insult. The combination of transient compression and chemical irritation produces sustained bilateral hypersensitivity, sustained ipsilateral spinal astrocytic activation and late onset bilateral spinal microglial activation.

Intravenous Lidocaine: An Outdated or Underutilized Treatment for Pain?

BACKGROUND

Conventional analgesic treatment involves the use of oral and transdermal formulations of drugs that require repetitive administration for sustained pain relief to be achieved. Along with the potential of analgesia, the risk of ongoing side effects consequent on the use of these analgesics also exists and this may have a detrimental effect on the patient's quality of life. In contrast, an intriguing body of evidence suggests that short-term administration of intravenous lidocaine may produce pain relief that far exceeds both the duration of infusion and the half-life of the drug. When pain relief is produced, concomitant analgesic medication can be reduced, side effects from pain relieving medication minimized with a potential for very real improvement in the quality of life of the patient.

OBJECTIVE

To ascertain whether literature evidence supports the use of intravenous lidocaine in clinical practice.

DESIGN

A review of the currently available published evidence.

RESULTS

A reasonable body of evidence, along with extensive clinical experience, suggests that intravenous lidocaine can have a useful pain-relieving effect and is worth consideration in palliative care patients.

CONCLUSION

While this form of therapy is not commonplace in the terminally ill patient, it could be argued that its use has much merit in that field and should be considered.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Arthritis and pain. Future targets to control osteoarthritis pain

Clinical presentation of osteoarthritis (OA) is dominated by pain during joint use and at rest. OA pain is caused by aberrant functioning of a pathologically altered nervous system with key mechanistic drivers from peripheral nerves and central pain pathways. This review focuses on symptomatic pain therapy exemplified by molecular targets that alter sensitization and hyperexcitability of the nervous system, for example, opioids and cannabinoids. We highlight opportunities for targeting inflammatory mediators and their key receptors (for example, prostanoids, kinins, cytokines and chemokines), ion channels (for example, NaV1.8, NaV1.7 and CaV2.2) and neurotrophins (for example, nerve growth factor), noting evidence that relates to their participation in OA etiology and treatment. Future neurological treatments of pain appear optimistic but will require the systematic evaluation of emerging opportunities.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Multiple interacting sites of ectopic spike electrogenesis in primary sensory neurons

Ectopic discharge generated in injured afferent axons and cell somata in vivo contributes significantly to chronic neuropathic dysesthesia and pain after nerve trauma. Progress has been made toward understanding the processes responsible for this discharge using a preparation consisting of whole excised dorsal root ganglia (DRGs) with the cut nerve attached. In the in vitro preparation, however, spike activity originates in the DRG cell soma but rarely in the axon. We have now overcome this impediment to understanding the overall electrogenic processes in soma and axon, including the resulting discharge patterns, by modifying the bath medium in which recordings are made. At both sites, bursts can be triggered by subthreshold oscillations, a phasic stimulus, or spikes arising elsewhere in the neuron. In the soma, once triggered, bursts are maintained by depolarizing afterpotentials, whereas in the axon, an additional process also plays a role, delayed depolarizing potentials. This alternative process appears to be involved in "clock-like" bursting, a discharge pattern much more common in axons than somata. Ectopic spikes arise alternatively in the soma, the injured axon end (neuroma), and the region of the axonal T-junction. Discharge sequences, and even individual multiplet bursts, may be a mosaic of action potentials that originate at these alternative electrogenic sites within the neuron. Correspondingly, discharge generated at these alternative sites may interact, explaining the sometimes-complex firing patterns observed in vivo.

Dose-related antiallodynic effects of cyclic AMP response element-binding protein-antisense oligonucleotide in the spared nerve injury model of neuropathic pain

A transcription factor known as cyclic AMP response element-binding protein has been shown to be involved in the central sensitization in neuropathic pain and inflammation pain. The present study examined the roles of cyclic AMP response element-binding protein and of the phosphorylated cyclic AMP response element-binding protein in the maintenance of mechanical and cold allodynia induced by a neuropathic pain model, "spared nerve injury," in rats. First, the results of immunohistochemical study showed that phosphorylated cyclic AMP response element-binding protein, but not cyclic AMP response element-binding protein, increased bilaterally in the spinal dorsal horn 14 days following spared nerve injury, indicating a possible contribution of phosphorylated cyclic AMP response element-binding protein in spared nerve injury. Second, chronic intrathecal application of cyclic AMP response element-binding protein antisense oligodeoxynucleotide with three doses (10 microg/day, 20 microg/day and 40 microg/day) for 5 days demonstrated that the higher doses (20 and 40 microg) significantly attenuated both mechanical (bilaterally) and cold (ipsilaterally) allodynia, compared with sense oligodeoxynucleotide and the lower dose (10 microg). Western blot results showed that the alleviation in intensity of behavioral performance was accompanied by a significant reduction of total cyclic AMP response element-binding protein and phosphorylated cyclic AMP response element-binding protein in the spinal dorsal horn. Moreover, there were no differences in cyclic AMP response element-binding protein and phosphorylated cyclic AMP response element-binding protein between ipsilateral and contralateral dorsal horns. Our data demonstrate a close association between the expression of behavioral hypersensitivity and cyclic AMP response element-binding protein activation in the spinal dorsal horn following spared nerve injury, supporting the notion that phosphorylated cyclic AMP response element-binding protein may play an important role in the maintenance of chronic neuropathic pain.

Neuropathic pain: early spontaneous afferent activity is the trigger

Intractable neuropathic pain often results from nerve injury. One immediate event in damaged nerve is a sustained increase in spontaneous afferent activity, which has a well-established role in ongoing pain. Using two rat models of neuropathic pain, the CCI and SNI models, we show that local, temporary nerve blockade of this afferent activity permanently inhibits the subsequent development of both thermal hyperalgesia and mechanical allodynia. Timing is critical-the nerve blockade must last at least 3-5 days and is effective if started immediately after nerve injury, but not if started at 10 days after injury when neuropathic pain is already established. Effective nerve blockade also prevents subsequent development of spontaneous afferent activity measured electrophysiologically. Similar results were obtained in both pain models, and with two blockade methods (200mg of a depot form bupivacaine at the injury site, or perfusion of the injured nerve just proximal to the injury site with TTX). These results indicate that early spontaneous afferent fiber activity is the key trigger for the development of pain behaviors, and suggest that spontaneous activity may be required for many of the later changes in the sensory neurons, spinal cord, and brain observed in neuropathic pain models. Many pre-clinical and clinical studies of pre-emptive analgesia have used much shorter duration of blockade, or have not started immediately after the injury. Our results suggest that effective pre-emptive analgesia can be achieved only when nerve block is administered early after injury and lasts several days.

Transient cervical nerve root compression in the rat induces bilateral forepaw allodynia and spinal glial activation: mechanical factors in painful neck injuries

STUDY DESIGN

An in vivo rat model of transient cervical nerve root compression.

OBJECTIVES

To investigate the potential for cervical nerve root compression to produce behavioral hypersensitivity and examine its dependence on compression.

SUMMARY OF BACKGROUND DATA

Clinically, nerve root injury has been hypothesized as a potential source of neck pain, particularly because cervical nerve roots are at mechanical risk for injury during neck loading. Lumbar radiculopathy models of nerve root ligation show that mechanical allodynia and spinal glial changes depend on nerve root deformation magnitude. However, no investigation has been performed to examine cervical nerve root compression as a cause of pain.

METHODS

Two compressive forces (10 and 60 grams force [gf]) were transiently applied to the C7 nerve roots unilaterally using microvascular clips in separate groups (n = 12 each). Sham procedures were also performed in a separate group of rats (n = 12). Bilateral forepaw mechanical allodynia was monitored after surgery for 7 days. On day 7, spinal glial activation was assessed using immunohistochemistry to investigate its dependence on nerve root compressive force, in the context of behavioral hypersensitivity.

RESULTS

Bilateral allodynia was observed following injury, which was significantly (P < 0.042) increased over sham and baseline responses. No difference in allodynia was found between the 10 and 60 gf injuries. Astrocytic and microglial activation were observed in the ipsilateral dorsal horn following compression, with only astrocytic activation paralleling allodynia patterns.

CONCLUSIONS

Results imply a force threshold exists less than 10 gf for persistent pain symptoms following transient cervical nerve root compression. Findings also suggest that spinal glial activation may be related to behavioral sensitivity and may modulate cervical nerve root mediated pain.

Pharmacological strategies in relieving neuropathic pain

The pharmacological treatment of neuropathic pain relies, to a large extent, on drugs belonging to a small number of defined classes. Opioids, tricyclic antidepressants, antiepileptic drugs and membrane stabilisers form the current basis of treatment. Varying levels of evidence support the use of individual members of these classes and overall show no indication that one class of drug, or individual drug has universal effectiveness. More refined knowledge of the modes of action of these agents used to treat neuropathic pain should lead to a more logical approach to the management of this difficult series of conditions. A number of drugs currently licensed for a different indication have recently had an analgesic effect in neuropathic pain attributed to them. In addition, a number of novel compounds are undergoing investigation and provide hope of dicovering more efficacious treatment options in the future.

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Differential blockade of nerve injury–induced thermal and tactile hypersensitivity by systemically administered brain-penetrating and peripherally restricted local anesthetics

Systemic administration of local anesthetics has been shown to transiently reverse thermal and tactile hypersensitivity induced by peripheral nerve injury, effects that have been taken as suggesting direct actions on the peripheral nerves. The present study sought to determine whether a central site of action could contribute to, or account for, the effects of lidocaine on nerve injury-induced thermal and tactile hypersensitivity. Systemic lidocaine and its peripherally restricted analogues, QX-314 or QX-222, effectively reversed thermal hypersensitivity after spinal nerve ligation injury. Nerve injury-induced tactile hypersensitivity, however, was reversed by systemic lidocaine but not QX-314 or QX-222. Microinjection of either lidocaine or QX-314 into the rostral ventromedial medulla fully reversed spinal nerve ligation-induced thermal and tactile hypersensitivity. The data strongly suggest that nerve injury-induced thermal and tactile hypersensitivity are mediated through different mechanisms. In addition, the present study supports a prominent contribution of the central nervous system in the activity of systemically given lidocaine against nerve injury-induced tactile and thermal hypersensitivity. Thus, lidocaine might reverse tactile hypersensitivity mainly through its actions within the central nervous system, whereas its reversal of thermal hypersensitivity might occur through either central or peripheral sites.

PERSPECTIVE

Nerve injury-induced neuropathic pain has proved remarkably difficult to treat. Systemic administration of ion channel blockers such as lidocaine has been explored for the management of chronic pain. This work indicates that systemic rather than local administration of lidocaine would be more effective in treating tactile allodynia.

Unilateral nerve injury produces bilateral loss of distal innervation

There are no known anatomical connections between neurons that innervate homologous right and left body parts. Nevertheless, some patients develop bilateral abnormalities after unilateral injury, a phenomenon often unrecognized and not yet characterized. Therefore, we examined in rats the effects of ligating and cutting one tibial nerve on sensory function and on density of innervation in hind paws contralaterally as well as ipsilaterally to the injury, at times between 1 day and 5 months after surgery. Punches removed from tibial- or sural-innervated planter paw skin were immunolabeled to quantitate epidermal nerve endings. Naive and sham-operated rats provided controls. Axotomized rats had near-total loss of PGP9.5(+) innervation within ipsilateral tibial-innervated skin at all time-points. Adjacent ipsilateral sural-innervated skin had persistent hyperalgesia without denervation, and robust axonal sprouting at 5 months after surgery. Contralesional hind paws lost 54% of innervation in tibial-innervated epidermis starting 1 week after surgery and persisting throughout. Contralesional sural-innervated skin had neither neurite loss nor sprouting. These results imply that unilateral nerve injury can cause profound, long lasting, nerve-branch-specific loss of distal innervation contralaterally as well as ipsilaterally. They discredit the practice of using tissues contralateral to an injury to provide normative controls and suggest the possibility of rapid, transmedian postinjury signals between homologous mirror-image neurons.