Inflammatory Mediators Enhance the Excitability of Chronically Compressed Dorsal Root Ganglion Neurons

Electroacupuncture alleviates neuropathic pain in a rat model of CCD via suppressing P2X3 expression in dorsal root ganglia

BACKGROUND

Sciatica and low back pain are prevalent clinical types of neuropathic pain that significantly impair patients' quality of life. Conventional therapies often lack effectiveness, making these conditions challenging to treat. Electroacupuncture (EA) is an effective physiotherapy for pain relief. Prior research has demonstrated a relationship between the frequency of neuropathic pain and the analgesic impact of EA stimulation. This work aimed to assess the analgesic effects of EA in a rat model of chronic compression of the dorsal root ganglion (CCD) and to understand the underlying processes.

METHODS

We established a rat CCD model to simulate sciatica and low back pain. EA was applied to rats with CCD at various frequencies (2 Hz, 100 Hz, and 2/100 Hz). The paw withdrawal threshold (PWT) was measured to assess analgesic effects. Additionally, protein levels of the purinergic receptor P2X3 (P2X3) and the expression of nociceptive neuronal markers were analyzed using immunohistochemistry and western blot (WB) techniques. The study also measured levels of proinflammatory cytokines TNF-α and IL-1β in the dorsal root ganglion (DRG). The involvement of P2X3 receptors was further investigated using the P2X3 agonist, α,β-methylene ATP (α,β-meATP).

RESULTS

CCD rats developed pronounced mechanical allodynia. EA stimulation at all tested frequencies produced analgesic effects, with 2/100 Hz showing superior efficacy compared to 2 Hz and 100 Hz. The expression of P2X3 was increased in ipsilateral DRG of CCD model rats. P2X3 were co-labeled with isolectin B4 (IB4) and transient receptor potential vanilloid (TRPV1), indicating their role in nociception. 2/100 Hz EA treatment significantly reduced mechanical allodynia and inhibited the overexpression of P2X3, TRPV1, substance P (SP), and calcitonin gene-related peptide (CGRP) in the ipsilateral DRG of CCD model rats. Additionally, EA reduced the levels of proinflammatory cytokines TNF-α and IL-1β in the ipsilateral DRG, indicating an anti-inflammatory effect. The P2X3 agonist α,β-me ATP attenuated the analgesic effect of 2/100 Hz EA in CCD rats. The WB and immunofluorescence results consistently demonstrated P2X3 inhibition contributed to the analgesic effects of 2/100 Hz EA on CCD-induced neuropathic pain.

CONCLUSIONS

Our findings suggest that 2/100 Hz EA alleviates neuropathic pain in rats by inhibiting the upregulation of P2X3 receptors in the ipsilateral DRG. This study backs up EA as a viable treatment option for sciatica and low back pain in clinical settings.

In vitro models for neuropathic pain phenotypic screening in brain therapeutics

The discovery of brain therapeutics faces a significant challenge due to the low translatability of preclinical results into clinical success. To address this gap, several efforts have been made to obtain more translatable neuronal models for phenotypic screening. These models allow the selection of active compounds without predetermined knowledge of drug targets. In this review, we present an overview of various existing models within the field, examining their strengths and limitations, particularly in the context of neuropathic pain research. We illustrate the usefulness of these models through a comparative review in three crucial areas: i) the development of novel phenotypic screening strategies specifically for neuropathic pain, ii) the validation of the models for both primary and secondary screening assays, and iii) the use of the models in target deconvolution processes.

The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Neuropathic pain is a severe and chronic condition widely found in the general population. The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients. During the processing of pain, the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation. Furthermore, the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies. Here, we will review the complex interplay between cells (satellite glial cells and inflammatory cells) and factors (cytokines, neurotrophic factors and genetic factors) that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain. More importantly, we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Abnormal Spontaneous Discharges of Primary Sensory Neurons and Pain Behavior in a Rat Model of Vascular Dementia

Patients with vascular dementia experience more pain than healthy elders, potentially due to the presence of central neuropathic pain. However, the mechanisms underlying neuropathic pain in vascular dementia remain poorly understood, and there is currently a lack of effective treatment available. In this study, a rat model of vascular dementia was induced by permanently occluding the common carotid arteries bilaterally (2-VO). The cognitive impairments in the 2-VO rats were evaluated using the Morris Water Maze test, while HE and LBF staining were employed to assess brain tissue lesions in the hippocampal, cerebral cortex, and white matter regions known to be associated with severe memory and learning deficits. Furthermore, pain-related behavioral tests, including mechanical and thermal stimuli assessments, were conducted, and in vivo electrophysiological recordings of primary sensory neurons were performed. Compared to sham-operated and pre-operative rats, rats with vascular dementia exhibited mechanical allodynia and thermal hyperalgesia 30 days after surgery. Furthermore, in vivo electrophysiology revealed a significant increase in the occurrence of spontaneous activity of Aβ- and C-fiber sensory neurons in the rat model of vascular dementia. These results indicate that neuropathic pain behaviors developed in the rat model of vascular dementia, and abnormal spontaneous discharges of primary sensory neurons may play a crucial role in the development of pain after vascular dementia.

Correlation between leg pain at rest and spinal nerve edema in symptomatic lumbar foraminal stenosis

BACKGROUND

Previous studies reported that spinal nerve edema on magnetic resonance myelography (MRM) and leg pain at rest were specifically observed in symptomatic lumbar foraminal stenosis patients. However, the correlation between leg pain at rest and spinal nerve edema in symptomatic foraminal stenosis has not been reported.

HYPOTHESIS

The purpose of this prospective study is to reveal a correlation between leg pain at rest and spinal nerve edema focusing on the pathophysiology of symptomatic foraminal stenosis.

PATIENTS AND METHODS

Clinical findings and MRM findings were surveyed among 30 patients with symptomatic foraminal stenosis diagnosed by MR imaging (MRI) and selective nerve root block. Comparisons of patient characteristics and clinical findings between the prevalence and absence groups of spinal nerve edema on MRM were analyzed. A correlation between the visual analogue scale (VAS) for leg pain at rest and the spinal edema ratio calculated as maximum intensity value of the affected spinal nerve/maximum intensity value of the asymptomatic side from region of interest (ROI) made on MRM was evaluated.

RESULTS

Twenty symptomatic foraminal stenosis cases (67%) showed the affected spinal nerve edema on MRM. The prevalence and VAS of leg pain at rest were significantly higher in the presence of spinal nerve edema group (95% and 67.0±36.4, respectively). The correlation coefficient between the VAS for leg pain at rest (53.0±33.6) and the spinal nerve edema ratio (1.3±0.3) was 0.647 (p<0.01).

DISCUSSION

Our study revealed the substantial correlation found between the spinal nerve edema ratio on MRM and the VAS for leg pain at rest in symptomatic foraminal stenosis. The correlation between spinal nerve edema and leg pain at rest has potential to clarify the pathology of symptomatic foraminal stenosis.

LEVEL OF EVIDENCE

IV.

Comprehensive phenotyping of cutaneous afferents reveals early-onset alterations in nociceptor response properties, release of CGRP, and hindpaw edema following spinal cord injury

Spinal cord injury (SCI) is a complex syndrome that has profound effects on patient well-being, including the development of medically-resistant chronic pain. The mechanisms underlying SCI pain have been the subject of thorough investigation but remain poorly understood. While the majority of the research has focused on changes occurring within and surrounding the site of injury in the spinal cord, there is now a consensus that alterations within the peripheral nervous system, namely sensitization of nociceptors, contribute to the development and maintenance of chronic SCI pain. Using an ex vivo skin/nerve/DRG/spinal cord preparation to characterize afferent response properties following SCI, we found that SCI increased mechanical and thermal responding, as well as the incidence of spontaneous activity (SA) and afterdischarge (AD), in below-level C-fiber nociceptors 24 hr following injury relative to naïve controls. Interestingly, the distribution of nociceptors that exhibit SA and AD are not identical, and the development of SA was observed more frequently in nociceptors with low heat thresholds, while AD was found more frequently in nociceptors with high heat thresholds. We also found that SCI resulted in hindpaw edema and elevated cutaneous calcitonin gene-related peptide (CGRP) concentration that were not observed in naïve mice. These results suggest that SCI causes a rapidly developing nociceptor sensitization and peripheral inflammation that may contribute to the early emergence and persistence of chronic SCI pain.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

Sensory Neuron TLR4 mediates the development of nerve-injury induced mechanical hypersensitivity in female mice

Recent studies have brought to light the necessity to discern sex-specific differences in various pain states and different cell-types that mediate these differences. These studies have uncovered the role of neuroimmune interactions to mediate pain states in a sex-specific fashion. While investigating immune function in pain development, we discovered that females utilize immune components of sensory neurons to mediate neuropathic pain development. We utilized two novel transgenic mouse models that eitherrestore expression of toll-like receptor (TLR) 4 inNa_v1.8 nociceptors on a TLR4-null background (TLR4^LoxTB) or remove TLR4 specifically from Na_v1.8 nociceptors (TLR4^fl/fl). After spared nerve injury (SNI), a model of neuropathic injury, we observed a robust female-specific onset of mechanical hypersensitivity in our transgenic animals. Female Na_v1.8-TLR4^fl/fl knockout animals were less mechanically sensitive than cre-negative TLR4^fl/fl littermates. Conversely, female Na_v1.8-TLR4^LoxTB reactivated animals were as mechanically sensitive as their wild-type counterparts. These sex and cell-specific effects were not recapitulated in male animals of either strain. Additionally, we find the danger associated molecular pattern, high mobility group box-1 (HGMB1), a potent TLR4 agonist, localization and ATF3 expression in females is dependent on TLR4 expression in dorsal root ganglia (DRG) populations following SNI. These experiments provide novel evidence toward sensory neuron specific modulation of pain in a sex-dependent manner.

Role of TRPV4-P2X7 Pathway in Neuropathic Pain in Rats with Chronic Compression of the Dorsal Root Ganglion

Transient receptor potential vanilloid 4 (TRPV4) is a Ca2+-permeable non-selective cation channel that is involved in the development of neuropathic pain. P2X7 receptor (P2X7) belongs to a class of ATP-gated nonselective cation channels that plays an important role in neuropathic pain. Nevertheless, little is known about the interaction between them for neuropathic pain. In this paper, we investigated role of TRPV4-P2X7 pathway in neuropathic pain. We evaluated the effect of TRPV4-P2X7 pathway on neuropathic pain in a chronic compression of the dorsal root ganglion (DRG) (hereafter termed CCD) model. We analyzed the effect of P2X7 on mechanical and thermal hyperalgesia mediated by TRPV4 in CCD. Furthermore, we assessed the effect of TRPV4 on the expression of P2X7 and the release of IL-1β and IL-6 in DRG after CCD. We found that intraperitoneal injection of TRPV4 agonist GSK-1016790A led to a significant increase of mechanical and thermal hyperalgesia in CCD, which was partially suppressed by P2X7 blockade with antagonist Brilliant Blue G (BBG). Then, we further noticed that GSK-1016790A injection increased the P2X7 expression of CCD, which was decreased by TRPV4 blockade with antagonist RN-1734 and HC-067047. Furthermore, we also discovered that the expressions of IL-1β and IL-6 were upregulated by GSK-1016790A injection but reduced by RN-1734 and HC-067047. Our results provide evidence that P2X7 contributes to development of neuropathic pain mediated by TRPV4 in the CCD model, which may be the basis for treatment of neuropathic pain relief.

A high-content platform for physiological profiling and unbiased classification of individual neurons

High-throughput physiological assays lose single-cell resolution, precluding subtype-specific analyses of activation mechanism and drug effects. We demonstrate APPOINT (automated physiological phenotyping of individual neuronal types), a physiological assay platform combining calcium imaging, robotic liquid handling, and automated analysis to generate physiological activation profiles of single neurons at large scale. Using unbiased techniques, we quantify responses to sequential stimuli, enabling subgroup identification by physiology and probing of distinct mechanisms of neuronal activation within subgroups. Using APPOINT, we quantify primary sensory neuron activation by metabotropic receptor agonists and identify potential contributors to pain signaling. We expand the role of neuroimmune interactions by showing that human serum directly activates sensory neurons, elucidating a new potential pain mechanism. Finally, we apply APPOINT to develop a high-throughput, all-optical approach for quantification of activation threshold and pharmacologically validate contributions of ion channel families to optical activation.

The mechanism of non-blocking inhibition of sodium channels revealed by conformation-selective photolabeling

BACKGROUND AND PURPOSE

Sodium channel inhibitors can be used to treat hyperexcitability-related diseases, including epilepsies, pain syndromes, neuromuscular disorders and cardiac arrhythmias. The applicability of these drugs is limited by their nonspecific effect on physiological function. They act mainly by sodium channel block and in addition by modulation of channel kinetics. While channel block inhibits healthy and pathological tissue equally, modulation can preferentially inhibit pathological activity. An ideal drug designed to target the sodium channels of pathological tissue would act predominantly by modulation. Thus far, no such drug has been described.

EXPERIMENTAL APPROACH

Patch-clamp experiments with ultra-fast solution exchange and photolabeling-coupled electrophysiology were applied to describe the unique mechanism of riluzole on Nav1.4 sodium channels. In silico docking experiments were used to study the molecular details of binding.

KEY RESULTS

We present evidence that riluzole acts predominantly by non-blocking modulation. We propose that, being a relatively small molecule, riluzole is able to stay bound to the binding site, but nonetheless stay off the conduction pathway, by residing in one of the fenestrations. We demonstrate how this mechanism can be recognized.

CONCLUSIONS AND IMPLICATIONS

Our results identify riluzole as the prototype of this new class of sodium channel inhibitors. Drugs of this class are expected to selectively prevent hyperexcitability, while having minimal effect on cells firing at a normal rate from a normal resting potential.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

Inflammation Induced Sensory Nerve Growth and Pain Hypersensitivity Requires the N-Type Calcium Channel Cav2.2

Voltage-gated calcium channels (VGCCs) are important mediators of pain hypersensitivity during inflammatory states, but their role in sensory nerve growth remains underexplored. Here, we assess the role of the N-type calcium channel Cav2.2 in the complete Freund’s adjuvant (CFA) model of inflammatory pain. We demonstrate with in situ hybridization and immunoblotting, an increase in Cav2.2 expression after hind paw CFA injection in sensory neurons that respond to thermal stimuli, but not in two different mechanosensitive neuronal populations. Further, Cav2.2 upregulation post-CFA correlates with thermal but not mechanical hyperalgesia in behaving mice, and this hypersensitivity is blocked with a specific Cav2.2 inhibitor. Voltage clamp recordings reveal a significant increase in Cav2.2 currents post-CFA, while current clamp analyses demonstrate a significant increase in action potential frequency. Moreover, CFA-induced sensory nerve growth, which involves the extracellular signal-related kinase (ERK1/2) signaling pathway and likely contributes to inflammation-induced hyperalgesia, was blocked with the Cav2.2 inhibitor. Together, this work uncovers a role for Cav2.2 during inflammation, demonstrating that VGCC activity can promote thermal hyperalgesia through both changes in firing rates of sensory neurons as well as promotion of new neurite outgrowth.

Adjacent intact nociceptive neurons drive the acute outburst of pain following peripheral axotomy

Injury of peripheral nerves may quickly induce severe pain, but the mechanism remains obscure. We observed a rapid onset of spontaneous pain and evoked pain hypersensitivity after acute transection of the L5 spinal nerve (SNT) in awake rats. The outburst of pain was associated with a rapid development of spontaneous activities and hyperexcitability of nociceptive neurons in the adjacent uninjured L4 dorsal root ganglion (DRG), as revealed by both in vivo electrophysiological recording and high-throughput calcium imaging in vivo. Transection of the L4 dorsal root or intrathecal infusion of aminobutyrate aminotransferase inhibitor attenuated the spontaneous activity, suggesting that retrograde signals from the spinal cord may contribute to the sensitization of L4 DRG neurons after L5 SNT. Electrical stimulation of low-threshold afferents proximal to the axotomized L5 spinal nerve attenuated the spontaneous activities in L4 DRG and pain behavior. These findings suggest that peripheral axotomy may quickly induce hyperexcitability of uninjured nociceptors in the adjacent DRG that drives an outburst of pain.

Cyclic nucleotide signaling in sensory neuron hyperexcitability and chronic pain after nerve injury

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Activation of cAMP-PKA and cGMP-PKG pathways contributes to injury-induced sensory neuron hyperexcitability.

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Activation of cAMP and cGMP contributes to the development of bone cancer pain.

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PAR2 activation mediates injury-induced cAMP-dependent sensory neuron hyperexcitability.

The cyclic nucleotide signaling, including cAMP-PKA and cGMP-PKG pathways, has been well known to play critical roles in regulating cellular growth, metabolism and many other intracellular processes. In recent years, more and more studies have uncovered the roles of cAMP and cGMP in the nervous system. The cAMP and cGMP signaling mediates chronic pain induced by different forms of injury and stress. Here we summarize the roles of cAMP-PKA and cGMP-PKG signaling pathways in the pathogenesis of chronic pain after nerve injury. In addition, acute dissociation and chronic compression of the dorsal root ganglion (DRG) neurons, respectively, leads to neural hyperexcitability possibly through PAR2 activation-dependent activation of cAMP-PKA pathway. Clinically, radiotherapy can effectively alleviate bone cancer pain at least partly through inhibiting the cancer cell-induced activation of cAMP-PKA pathway. Roles of cyclic nucleotide signaling in neuropathic and inflammatory pain are also seen in many other animal models and are involved in many pro-nociceptive mechanisms including the activation of hyperpolarization-activated cyclic nucleotide (HCN)-modulated ion channels and the exchange proteins directly activated by cAMP (EPAC). Further understanding the roles of cAMP and cGMP signaling in the pathogenesis of chronic pain is theoretically significant and clinically valuable for treatment of chronic pain.

Role of NaV1.6 and NaVβ4 Sodium Channel Subunits in a Rat Model of Low Back Pain Induced by Compression of the Dorsal Root Ganglia

Low back pain is a common cause of chronic pain and disability. It is modeled in rodents by chronically compressing the lumbar dorsal root ganglia (DRG) with small metal rods, resulting in ipsilateral mechanical and cold hypersensitivity, and hyperexcitability of sensory neurons. Sodium channels are implicated in this hyperexcitability, but the responsible isoforms are unknown. In this study, we used siRNA-mediated knockdown of the pore-forming Na_V1.6 and regulatory Na_Vβ4 sodium channel isoforms that have been previously implicated in a different model of low back pain caused by locally inflaming the L5 DRG. Knockdown of either subunit markedly reduced spontaneous pain and mechanical and cold hypersensitivity induced by DRG compression, and reduced spontaneous activity and hyperexcitability of sensory neurons with action potentials <1.5 msec (predominately cells with myelinated axons, based on conduction velocities measured in a subset of cells) 4 days after DRG compression. These results were similar to those previously obtained in the DRG inflammation model and some neuropathic pain models, in which sensory neurons other than nociceptors seem to play key roles. The cytokine profiles induced by DRG compression and DRG inflammation were also very similar, with upregulation of several type 1 pro-inflammatory cytokines and downregulation of type 2 anti-inflammatory cytokines. Surprisingly, the cytokine profile was largely unaffected by Na_Vβ4 knockdown in either model. The Na_V1.6 channel, and the Na_Vβ4 subunit that can regulate Na_V1.6 to enhance repetitive firing, play key roles in both models of low back pain; targeting the abnormal spontaneous activity they generate may have therapeutic value.

A New Model of Sensorial Neuron-Like Cells for HTS of Novel Analgesics for Neuropathic Pain

In this study we developed a new translational phenotypic in vitro model for high-throughput screening (HTS) of novel analgesics for treating neuropathic pain, in order to address the poor translation of traditional recombinant models. The immortalized dorsal root ganglia (DRG) neuron-like F11 cell line was selected based on its phenotype after differentiation. The acquisition of neuronal characteristics was evaluated by measuring the expression of TrkA as a DRG neuron marker ( p < 0.01) as well as by measuring the global neurite length ( p < 0.001). The response of F11 cells to ATP and KCl was obtained by measuring intracellular calcium concentration, dynamic mass redistribution, and membrane potential. A KCl-induced increase of intracellular calcium levels was chosen as the readout because of the better signal quality, higher reproducibility, and greater compatibility with HTS assay requirements compared with other methods. The response to KCl differed significantly between differentiated and undifferentiated cells ( p < 0.05), with an EC₅₀ value of 5 mM in differentiated cells. The model was validated by screening the Prestwick Chemical Library. Five hits already proposed for neuropathic-related pain were identified, with IC₅₀ values between 1 and 7 µM. This cell model provides a new tool for screening novel analgesics for the relief of neuropathic pain.

Using dynamic clamp to quantify pathological changes in the excitability of primary somatosensory neurons

KEY POINTS

Primary somatosensory neurons normally respond to somatic depolarization with transient spiking but can switch to repetitive spiking under pathological conditions. This switch in spiking pattern reflects a qualitative change in spike initiation dynamics and contributes to the hyperexcitability associated with chronic pain. Neurons can be converted to repetitive spiking by adding a virtual conductance using dynamic clamp. By titrating the conductance to determine how much must be added to cause repetitive spiking, we found that small cells are more susceptible to switching (i.e. required less added conductance) than medium-large cells. By measuring how much less conductance is required to cause repetitive spiking when dynamic clamp was combined with other pathomimetic manipulations (e.g. application of inflammatory mediators), we measured how much each manipulation facilitated repetitive spiking. Our results suggest that many pathological factors facilitate repetitive spiking but that the switch to repetitive spiking requires the cumulative effect of many co-occurring factors.

ABSTRACT

Primary somatosensory neurons become hyperexcitable in many chronic pain conditions. Hyperexcitability can include a switch from transient to repetitive spiking during sustained somatic depolarization. This switch results from diverse pathological processes that impact ion channel expression or function. Because multiple pathological processes co-occur, isolating how much each contributes to switching the spiking pattern is difficult. Our approach to this challenge involves adding a virtual sodium conductance via dynamic clamp. The magnitude of that conductance was titrated to determine the minimum required to enable rheobasic stimulation to evoke repetitive spiking. The minimum required conductance, termed g¯ Na ∗, was re-measured before and during manipulations designed to model various pathological processes in vitro. The reduction in g¯ Na ∗ caused by each pathomimetic manipulation reflects how much the modelled process contributes to switching the spiking pattern. We found that elevating extracellular potassium or applying inflammatory mediators reduced g¯ Na ∗ whereas direct hyperpolarization had no effect. Inflammatory mediators reduced g¯ Na ∗ more in medium-large (>30 μm diameter) neurons than in small (⩽30 μm diameter) neurons, but had equivalent effects in cutaneous and muscle afferents. The repetitive spiking induced by dynamic clamp was also found to differ between small and medium-large neurons, thus revealing latent differences in adaptation. Our study demonstrates a novel way to determine to what extent individual pathological factors facilitate repetitive spiking. Our results suggest that most factors facilitate but do not cause repetitive spiking on their own, and, therefore, that a switch to repetitive spiking results from the cumulative effect of many co-occurring factors.

Anti-nociceptive effects of bupivacaine-encapsulated PLGA nanoparticles applied to the compressed dorsal root ganglion in mice

Bupivacaine is a commonly used local anesthetic in postoperative pain management. We evaluated the effects of a prolonged, local delivery of bupivacaine on pain behavior accompanying a chronic compression of the dorsal root ganglion (CCD) - an animal model of radicular pain. Poly(lactide-coglycolide) (PLGA) nanoparticles encapsulating bupivacaine were injected unilaterally into the L3 and L4 DRGs of mice just before producing CCD by implanting a stainless-steel rod in the intervertebral foramen of each ganglion. Behavioral sensitivity to punctate mechanical stimuli (Von Frey filaments) of different forces of indentation, delivered to each hind paw, was measured before and on subsequent days of testing after the CCD. Nanoparticles were spherical in morphology and 150 ± 10 nm in diameter. Bupivacaine was steadily released as measured in vitro over 35 days. A dye that was encapsulated in the nanoparticles was found in the intact DRG after 2 weeks. CCD alone or with injection of blank (control) nanoparticles produced a behavioral hypersensitivity to the punctate stimuli on the ipsilateral paw without affecting sensitivity on the contralateral, over a period of 7-14 days. The hypersensitivity was manifested as an increased incidence of paw-withdrawal to indentation forces normally below threshold (allodynia) and an increased shaking to a filament force that always elicited withdrawal prior to CCD (hyperalgesia). In contrast, nanoparticles with bupivacaine prevented any manifestation of allodynia or hyperalgesia on the ipsilateral hind paw while leaving normal nociceptive responses largely intact on both hind paws. CCD induced behavioral hypersensitivity to nociceptive stimuli is known to be associated with a hyperexcitability of sensory neurons originating in the compressed ganglion. We hypothesize that bupivacaine-loaded PLGA nanoparticles may prevent the occurrence of this neuronal hyperexcitability without reducing the nociceptive information normally conducted from the periphery to the central nervous system. The slow, sustained delivery of bupivacaine by nanoparticles may provide a means of preventing the occurrence of postoperative neuronal hyperexcitability that could develop into chronic neuropathic pain.

Nociceptive neuronal Fc-gamma receptor I is involved in IgG immune complex induced pain in the rat

Antigen-specific immune diseases such as rheumatoid arthritis are often accompanied by pain and hyperalgesia. Our previous studies have demonstrated that Fc-gamma-receptor type I (FcγRI) is expressed in a subpopulation of rat dorsal root ganglion (DRG) neurons and can be directly activated by IgG immune complex (IgG-IC). In this study we investigated whether neuronal FcγRI contributes to antigen-specific pain in the naïve and rheumatoid arthritis model rats. In vitro calcium imaging and whole-cell patch clamp recordings in dissociated DRG neurons revealed that only the small-, but not medium- or large-sized DRG neurons responded to IgG-IC. Accordingly, in vivo electrophysiological recordings showed that intradermal injection of IgG-IC into the peripheral receptive field could sensitize only the C- (but not A-) type sensory neurons and evoke action potential discharges. Pain-related behavioral tests showed that intradermal injection of IgG-IC dose-dependently produced mechanical and thermal hyperalgesia in the hindpaw of rats. These behavioral effects could be alleviated by localized administration of non-specific IgG or an FcγRI antibody, but not by mast cell stabilizer or histamine antagonist. In a rat model of antigen-induced arthritis (AIA) produced by methylated bovine serum albumin, FcγRI were found upregulated exclusively in the small-sized DRG neurons. In vitro calcium imaging revealed that significantly more small-sized DRG neurons responded to IgG-IC in the AIA rats, although there was no significant difference between the AIA and control rats in the magnitude of calcium changes in the DRG neurons. Moreover, in vivo electrophysiological recordings showed that C-nociceptive neurons in the AIA rats exhibited a greater incidence of action potential discharges and stronger responses to mechanical stimuli after IgG-IC was injected to the receptive fields. These results suggest that FcγRI expressed in the peripheral nociceptors might be directly activated by IgG-IC and contribute to antigen-specific pain in pathological conditions.

Brain network alterations in the inflammatory soup animal model of migraine

Advances in our understanding of the human pain experience have shifted much of the focus of pain research from the periphery to the brain. Current hypotheses suggest that the progression of migraine depends on abnormal functioning of neurons in multiple brain regions. Accordingly, we sought to capture functional brain changes induced by the application of an inflammatory cocktail known as inflammatory soup (IS), to the dura mater across multiple brain networks. Specifically, we aimed to determine whether IS alters additional neural networks indirectly related to the primary nociceptive pathways via the spinal cord to the thalamus and cortex. IS comprises an acidic combination of bradykinin, serotonin, histamine and prostaglandin PGE2 and was introduced to basic pain research as a tool to activate and sensitize peripheral nociceptors when studying pathological pain conditions associated with allodynia and hyperalgesia. Using this model of intracranial pain, we found that dural application of IS in awake, fully conscious, rats enhanced thalamic, hypothalamic, hippocampal and somatosensory cortex responses to mechanical stimulation of the face (compared to sham synthetic interstitial fluid administration). Furthermore, resting state MRI data revealed altered functional connectivity in a number of networks previously identified in clinical chronic pain populations. These included the default mode, sensorimotor, interoceptive (Salience) and autonomic networks. The findings suggest that activation and sensitization of meningeal nociceptors by IS can enhance the extent to which the brain processes nociceptive signaling, define new level of modulation of affective and cognitive responses to pain; set new tone for hypothalamic regulation of autonomic outflow to the cranium; and change cerebellar functions.

MAPK Pathways Are Involved in Neuropathic Pain in Rats with Chronic Compression of the Dorsal Root Ganglion

The aim of the present study was to investigate whether the MAPK pathways were involved in the mechanism of neuropathic pain in rats with chronic compression of the dorsal root ganglion. We determined the paw withdrawal mechanical threshold (PWMT) of rats before and after CCD surgery and then after p38, JNK, or ERK inhibitors administration. Western blotting, RT-PCR, and immunofluorescence of dorsal root ganglia were performed to investigate the protein and mRNA level of MAPKs and also the alternation in distributions of positive neurons in dorsal root ganglia. Intrathecal administration of MAPKs inhibitors, SB203580 (p38 inhibitor), SP600125 (JNK inhibitor), and U0126 (ERK inhibitor), resulted in a partial reduction in CCD-induced mechanical allodynia. The reduction of allodynia was associated with significant depression in the level of both MAPKs mRNA and protein expression in CCD rats and also associated with the decreased ratios of large size MAPKs positive neurons in dorsal root ganglia. In conclusion, the specific inhibitors of MAPKs contributed to the attenuation of mechanical allodynia in CCD rats and the large size MAPKs positive neurons in dorsal root ganglia were crucial.

Effects of serum immunoglobulins from patients with complex regional pain syndrome (CRPS) on depolarisation-induced calcium transients in isolated dorsal root ganglion (DRG) neurons

Complex regional pain syndrome (CRPS) is thought to have an auto-immune component. One such target recently proposed from the effects of auto-immune IgGs on Ca(2+) transients in cardiac myocytes and cell lines is the α1-adrenoceptor. We have tested whether such IgGs exerted comparable effects on nociceptive sensory neurons isolated from rat dorsal root ganglia. Depolarisation-induced [Ca(2+)]i transients were generated by applying 30 mM KCl for 2 min and monitored by Fura-2 fluorescence imaging. No IgGs tested (including 3 from CRPS patients) had any significant effect on these [Ca(2+)]i transients. However, IgG from one CRPS patient consistently and significantly reduced the K(+)-induced response of cells that had been pre-incubated for 24h with a mixture of inflammatory mediators (1 μM histamine, 5-hydroxytryptamine, bradykinin and PGE2). Since this pre-incubation also appeared to induce a comparable inhibitory response to the α1-agonist phenylephrine, this is compatible with the α1-adrenoceptor as a target for CRPS auto-immunity. A mechanism whereby this might enhance pain is suggested.

Modulation of Spinal GABAergic Inhibition and Mechanical Hypersensitivity following Chronic Compression of Dorsal Root Ganglion in the Rat

Chronic compression of dorsal root ganglion (CCD) results in neuropathic pain. We investigated the role of spinal GABA in CCD-induced pain using rats with unilateral CCD. A stereological analysis revealed that the proportion of GABA-immunoreactive neurons to total neurons at L4/5 laminae I-III on the injured side decreased in the early phase of CCD (post-CCD week 1) and then returned to the sham-control level in the late phase (post-CCD week 18). In the early phase, the rats showed an increase in both mechanical sensitivity of the hind paw and spinal WDR neuronal excitability on the injured side, and such increase was suppressed by spinally applied muscimol (GABA-A agonist, 5 nmol) and baclofen (GABA-B agonist, 25 nmol), indicating the reduced spinal GABAergic inhibition involved. In the late phase, the CCD-induced increase in mechanical sensitivity and neuronal excitability returned to pre-CCD levels, and such recovered responses were enhanced by spinally applied bicuculline (GABA-A antagonist, 15 nmol) and CGP52432 (GABA-B antagonist, 15 nmol), indicating the regained spinal GABAergic inhibition involved. In conclusion, the alteration of spinal GABAergic inhibition following CCD and leading to a gradual reduction over time of CCD-induced mechanical hypersensitivity is most likely due to changes in GABA content in spinal GABA neurons.

New insights of nociceptor sensitization in bone cancer pain

INTRODUCTION

Numerous studies have shown that an intact CNS is required for the conscious perception of cancer-induced bone pain (CIBP) and that changes in the CNS are clearly evident. Accordingly, the blockage of nociceptive stimulus into the CNS can effectively relieve or markedly attenuate CIBP, revealing the clinical implication of the blockage of ongoing peripheral inputs for the control of CIBP.

AREAS COVERED

In this review, the heterogeneity and excitability of nociceptors in bone are covered. Furthermore, their role in initiating and maintaining CIBP is also described.

EXPERT OPINION

Developing mechanistic therapies to treat CIBP is a challenge, but they have the potential to fundamentally change our ability to effectively block/relieve CIBP and increase the functional status and quality of life of patients with bone metastasis. Further studies are desperately needed at both the preclinical and clinical levels to determine whether the targets as mentioned in this review are viable and feasible for patient populations.

Sphingosine 1-phosphate to p38 signaling via S1P1 receptor and Gαi/o evokes augmentation of capsaicin-induced ionic currents in mouse sensory neurons

The perception of painful thermal stimuli by sensory neurons is largely mediated by TRPV1. Upon tissue injury or inflammation, S1P is secreted by thrombocytes as part of an inflammatory cocktail, which sensitizes nociceptive neurons towards thermal stimuli. S1P acts on G-protein coupled receptors that are expressed in sensory neurons and sensitize TRPV1 channels towards thermal stimuli. In this study, the S1P mediated signaling pathway required for sensitization of TRPV1 channels was explored.The capsaicin induced peak inward current (ICAPS) of sensory neurons was significantly increased after S1P stimulation within minutes after application. The potentiation of ICAPS resulted from activation of Gαi through G-protein coupled receptors for S1P. Consequently, Gαi led to a signaling cascade, involving phosphoinositide-3-kinase (PI3K) and protein kinase C, which augmented ICAPS in nociceptive neurons. The S1P1 receptor agonist SEW2871 resulted in activation of the same signaling pathway and potentiation of ICAPS. Furthermore, the mitogen-activated protein kinase p38 was phosphorylated after S1P stimulation and inhibition of p38 signaling by SB203580 prevented the S1P-induced ICAPS potentiation. The current data suggest that S1P sensitized ICAPS through G-protein coupled S1P1 receptor activation of Gαi-PI3K-PKC-p38 signaling pathway in sensory neurons.

cGMP and cGMP-dependent protein kinase I pathway in dorsal root ganglia contributes to bone cancer pain in rats

STUDY DESIGN

A prospective, randomized experimental research.

OBJECTIVE

To demonstrate the role of cGMP (cyclic guanosine monophosphate)-cGKI (cGMP-dependent protein kinase I) pathway in dorsal root ganglia (DRG) in bone cancer pain.

SUMMARY OF BACKGROUND DATA

Treating bone cancer pain continues to possess a major clinical challenge because the specific cellular and molecular mechanisms underlying bone cancer pain remain elusive. cGMP and cGMP-dependent protein kinases pathway in DRG plays important role in nerve injury-induced hyperexcitability of DRG neurons, as well as neuropathic pain, however, whether this pathway participates in bone cancer pain is unknown.

METHODS

The rat model of bone cancer pain was produced by intramedullary injection of rat breast cancer cells (Walker 256) into right tibia. Thermal hyperalgesia and mechanical allodynia were measured before and after administration of inhibitor of cGMP-cGKs pathway (Rp-8-pCPT-cGMPS). Immunofluorescence and reverse transcription-polymerase chain reaction were used to reflect expression of cGKI in DRG neurons, whereas the concentration of cGMP in DRG was tested using enzyme-linked immunosorbent assay method. Whole-cell patch clamp was used to record the hyperexcitability of small neurons in DRG with or without cGKs inhibitor after tumor cell implantation (TCI).

RESULTS

TCI treatment significantly increased the concentration of cGMP in DRG and activity of cGKs in DRG and the spinal cord. TCI treatment also induced upregulation of cGKI messenger ribonucleic acid and protein in DRG, as well as enhanced hyperexcitability in DRG neurons. Spinal administration of Rp-8-pCPT-cGMPS, cGMP-cGKs inhibitor, significantly suppressed TCI-induced activation of cGMP-cGKI signaling, and hyperexcitability of DRG neurons. Meanwhile, in vivo intrathecal delivery of the Rp-8-pCPT-cGMPS significantly prevented and suppressed TCI-induced hyperalgesia and allodynia.

CONCLUSION

From these results, we confirm that TCI treatment activates cGMP-cGKI signaling pathway and continuing activation of this pathway in DRG is required for hyperalgesia and/or hyperalgesia and allodynia after TCI treatment.

LEVEL OF EVIDENCE

N/A.

Enhanced excitability of primary sensory neurons and altered gene expression of neuronal ion channels in dorsal root ganglion in paclitaxel-induced peripheral neuropathy

BACKGROUND

The mechanism of chemotherapy-induced peripheral neuropathy after paclitaxel treatment is not well understood. Given the poor penetration of paclitaxel into central nervous system, peripheral nervous system is most at risk.

METHODS

Intrinsic membrane properties of dorsal root ganglion neurons were studied by intracellular recordings. Multiple-gene real-time polymerase chain reaction array was used to investigate gene expression of dorsal root ganglion neuronal ion channels.

RESULTS

Paclitaxel increased the incidence of spontaneous activity from 4.8 to 27.1% in large-sized and from 0 to 33.3% in medium-sized neurons. Paclitaxel decreased the rheobase (nA) from 1.6 ± 0.1 to 0.8 ± 0.1 in large-sized, from 1.5 ± 0.2 to 0.6 ± 0.1 in medium-sized, and from 1.6 ± 0.2 to 1.0 ± 0.1 in small-sized neurons. After paclitaxel treatment, other characteristics of membrane properties in each group remained the same except that Aδ neurons showed shorter action potential fall time (ms) (1.0 ± 0.2, n = 10 vs. 1.8 ± 0.3, n = 9, paclitaxel vs. vehicle). Meanwhile, real-time polymerase chain reaction array revealed an alteration in expression of some neuronal ion channel genes including up-regulation of hyperpolarization-activated cyclic nucleotide-gated channel 1 (fold change 1.76 ± 0.06) and Nav1.7 (1.26 ± 0.02) and down-regulation of Kir channels (Kir1.1, 0.73 ± 0.05, Kir3.4, 0.66 ± 0.06) in paclitaxel-treated animals.

CONCLUSION

The increased neuronal excitability and the changes in gene expression of some neuronal ion channels in dorsal root ganglion may provide insight into the molecular and cellular basis of paclitaxel-induced neuropathy, which may lead to novel therapeutic strategies.

Organization of sensory input to the nociceptive-specific cutaneous trunk muscle reflex in rat, an effective experimental system for examining nociception and plasticity

Detailed characterization of neural circuitries furthers our understanding of how nervous systems perform specific functions and allows the use of those systems to test hypotheses. We have characterized the sensory input to the cutaneous trunk muscle (CTM; also cutaneus trunci [rat] or cutaneus maximus [mouse]) reflex (CTMR), which manifests as a puckering of the dorsal thoracolumbar skin and is selectively driven by noxious stimuli. CTM electromyography and neurogram recordings in naïve rats revealed that CTMR responses were elicited by natural stimuli and electrical stimulation of all segments from C4 to L6, a much greater extent of segmental drive to the CTMR than previously described. Stimulation of some subcutaneous paraspinal tissue can also elicit this reflex. Using a selective neurotoxin, we also demonstrate differential drive of the CTMR by trkA-expressing and nonexpressing small-diameter afferents. These observations highlight aspects of the organization of the CTMR system that make it attractive for studies of nociception and anesthesiology and plasticity of primary afferents, motoneurons, and the propriospinal system. We use the CTMR system to demonstrate qualitatively and quantitatively that experimental pharmacological treatments can be compared with controls applied either to the contralateral side or to another segment, with the remaining segments providing controls for systemic or other treatment effects. These data indicate the potential for using the CTMR system as both an invasive and a noninvasive quantitative assessment tool providing improved statistical power and reduced animal use.

Anoctamin 1 contributes to inflammatory and nerve-injury induced hypersensitivity

BACKGROUND

Various pathological conditions such as inflammation or injury can evoke pain hypersensitivity. That represents the response to innocuous stimuli or exaggerated response to noxious stimuli. The molecular mechanism based on the pain hypersensitivity is associated with changes in many of ion channels in dorsal-root ganglion (DRG) neurons. Anoctamin 1 (ANO1/TMEM16A), a Ca2+ activated chloride channel is highly visible in small DRG neurons and responds to heat. Mice with an abolished function of ANO1 in DRG neurons demonstrated attenuated pain-like behaviors when exposed to noxious heat, suggesting a role in acute thermal nociception. In this study, we further examined the function of ANO1 in mediating inflammation- or injury-induced hyperalgesia or allodynia.

RESULTS

Using Advillin/Ano1fl/fl (Adv/Ano1fl/fl) mice that have a functional ablation of Ano1 mainly in DRG neurons, we were able to determine its role in mediating thermal hyperalgesia and mechanical allodynia induced by inflammation or nerve injury. The thermal hyperalgesia and mechanical allodynia induced by carrageenan injection and spared-nerve injury were significantly reduced in Adv/Ano1fl/fl mice. In addition, flinching or licking behavior after bradykinin or formalin injection was also significantly reduced in Adv/Ano1fl/fl mice. Since pathological conditions augment nociceptive behaviors, we expected ANO1's contribution to the excitability of DRG neurons. Indeed, the application of inflammatory mediators reduced the threshold for action potential (rheobase) or time for induction of the first action potential in DRG neurons isolated from control (Ano1fl/fl) mice. These parameters for neuronal excitability induced by inflammatory mediators were not changed in Adv/Ano1fl/fl mice, suggesting an active contribution of ANO1 in augmenting the neuronal excitability.

CONCLUSIONS

In addition to ANO1's role in mediating acute thermal pain as a heat sensor, ANO1 is also capable of augmenting the excitability of DRG neurons under inflammatory or neuropathic conditions and thereby aggravates inflammation- or tissue injury-induced pathological pain.

Engraftment of nonintegrating neural stem cells differentially perturbs cortical activity in a dose-dependent manner

Neural stem cell (NSC) therapy represents a potentially powerful approach for gene transfer in the diseased central nervous system. However, transplanted primary, embryonic stem cell- and induced pluripotent stem cell-derived NSCs generate largely undifferentiated progeny. Understanding how physiologically immature cells influence host activity is critical to evaluating the therapeutic utility of NSCs. Earlier inquiries were limited to single-cell recordings and did not address the emergent properties of neuronal ensembles. To interrogate cortical networks post-transplant, we used voltage sensitive dye imaging in mouse neocortical brain slices, which permits high temporal resolution analysis of neural activity. Although moderate NSC engraftment largely preserved host physiology, subtle defects in the activation properties of synaptic inputs were induced. High-density engraftment severely dampened cortical excitability, markedly reducing the amplitude, spatial extent, and velocity of propagating synaptic potentials in layers 2-6. These global effects may be mediated by specific disruptions in excitatory network structure in deep layers. We propose that depletion of endogenous cells in engrafted neocortex contributes to circuit alterations. Our data provide the first evidence that nonintegrating cells cause differential host impairment as a function of engrafted load. Moreover, they emphasize the necessity for efficient differentiation methods and proper controls for engraftment effects that interfere with the benefits of NSC therapy.

Preclinical studies of low back pain

Chronic low back pain is a major cause of disability and health care costs. Current treatments are inadequate for many patients. A number of preclinical models have been developed that attempt to mimic aspects of clinical conditions that contribute to low back pain. These involve application of nucleus pulposus material near the lumbar dorsal root ganglia (DRG), chronic compression of the DRG, or localized inflammation of the DRG. These models, which are primarily implemented in rats, have many common features including behavioral hypersensitivity of the hindpaw, enhanced excitability and spontaneous activity of sensory neurons, and locally elevated levels of inflammatory mediators including cytokines. Clinically, epidural injection of steroids (glucocorticoids) is commonly used when more conservative treatments fail, but clinical trials evaluating these treatments have yielded mixed results. There are relatively few preclinical studies of steroid effects in low back pain models. One preclinical study suggests that the mineralocorticoid receptor, also present in the DRG, may have pro-inflammatory effects that oppose the activation of the glucocorticoid receptor. Although the glucocorticoid receptor is the target of anti-inflammatory steroids, many clinically used steroids activate both receptors. This could be one explanation for the limited effects of epidural steroids in some patients. Additional preclinical research is needed to address other possible reasons for limited efficacy of steroids, such as central sensitization or presence of an ongoing inflammatory stimulus in some forms of low back pain.

Dorsal root ganglion compression as an animal model of sciatica and low back pain

As sciatica and low back pain are among the most common medical complaints, many studies have duplicated these conditions in animals. Chronic compression of the dorsal root ganglion (CCD) is one of these models. The surgery is simple: after exposing the L4/L5 intervertebral foramina, stainless steel rods are implanted unilaterally, one rod for each vertebra, to chronically compress the lumbar dorsal root ganglion (DRG). Then, CCD can be used to simulate the clinical conditions caused by stenosis, such as a laterally herniated disc or foraminal stenosis. As the intraforaminal implantation of a rod results in neuronal somal hyperexcitability and spontaneous action potentials associated with hyperalgesia, spontaneous pain, and mechanical allodynia, CCD provides an animal model that mimics radicular pain in humans. This review concerns the mechanisms of neuronal hyperexcitability, focusing on various patterns of spontaneous discharge including one possible pain signal for mechanical allodynia - evoked bursting. Also, new data regarding its significant property of maintaining peripheral input are also discussed. Investigations using this animal model will enhance our understanding of the neural mechanisms for low back pain and sciatica. Furthermore, the peripheral location of the DRG facilitates its use as a locus for controlling pain with minimal central effects, in the hope of ultimately uncovering analgesics that block neuropathic pain without influencing physiological pain.

Transient receptor potential canonical 3 (TRPC3) is required for IgG immune complex-induced excitation of the rat dorsal root ganglion neurons

Chronic pain may accompany immune-related disorders with an elevated level of serum IgG immune complex (IgG-IC), but the underlying mechanisms are obscure. We previously demonstrated that IgG-IC directly excited a subpopulation of dorsal root ganglion (DRG) neurons through the neuronal Fc-gamma receptor I (FcγRI). This might be a mechanism linking IgG-IC to pain and hyperalgesia. The purpose of this study was to investigate the signaling pathways and transduction channels activated downstream of IgG-IC and FcγRI. In whole-cell recordings, IgG-IC induced a nonselective cation current (I(IC)) in the rat DRG neurons, carried by Ca(2+) and Na(+). The I(IC) was potentiated or attenuated by, respectively, lowering or increasing the intracellular Ca(2+) buffering capacity, suggesting that this current was regulated by intracellular calcium. Single-cell RT-PCR revealed that transient receptor potential canonical 3 (TRPC3) mRNA was always coexpressed with FcγRI mRNA in the same DRG neuron. Moreover, ruthenium red (a general TRP channel blocker), BTP2 (a general TRPC channel inhibitor), and pyrazole-3 (a selective TRPC3 blocker) each potently inhibited the I(IC). Specific knockdown of TRPC3 using small interfering RNA attenuated the IgG-IC-induced Ca(2+) response and the I(IC). Additionally, the I(IC) was blocked by the tyrosine kinase Syk inhibitor OXSI-2, the phospholipase C (PLC) inhibitor neomycin, and either the inositol triphosphate (IP(3)) receptor antagonist 2-aminoethyldiphenylborinate or heparin. These results indicated that the activation of neuronal FcγRI triggers TRPC channels through the Syk-PLC-IP(3) pathway and that TRPC3 is a key molecular target for the excitatory effect of IgG-IC on DRG neurons.

Spontaneous excitation patterns computed for axons with injury-like impairments of sodium channels and Na/K pumps

In injured neurons, “leaky” voltage-gated sodium channels (Nav) underlie dysfunctional excitability that ranges from spontaneous subthreshold oscillations (STO), to ectopic (sometimes paroxysmal) excitation, to depolarizing block. In recombinant systems, mechanical injury to Nav1.6-rich membranes causes cytoplasmic Na⁺-loading and “Nav-CLS”, i.e., coupled left-(hyperpolarizing)-shift of Nav activation and availability. Metabolic injury of hippocampal neurons (epileptic discharge) results in comparable impairment: left-shifted activation and availability and hence left-shifted I_Na-window. A recent computation study revealed that CLS-based I_Na-window left-shift dissipates ion gradients and impairs excitability. Here, via dynamical analyses, we focus on sustained excitability patterns in mildly damaged nodes, in particular with more realistic Gaussian-distributed Nav-CLS to mimic “smeared” injury intensity. Since our interest is axons that might survive injury, pumps (sine qua non for live axons) are included. In some simulations, pump efficacy and system volumes are varied. Impacts of current noise inputs are also characterized. The diverse modes of spontaneous rhythmic activity evident in these scenarios are studied using bifurcation analysis. For “mild CLS injury”, a prominent feature is slow pump/leak-mediated E_Ion oscillations. These slow oscillations yield dynamic firing thresholds that underlie complex voltage STO and bursting behaviors. Thus, Nav-CLS, a biophysically justified mode of injury, in parallel with functioning pumps, robustly engenders an emergent slow process that triggers a plethora of pathological excitability patterns. This minimalist “device” could have physiological analogs. At first nodes of Ranvier and at nociceptors, e.g., localized lipid-tuning that modulated Nav midpoints could produce Nav-CLS, as could co-expression of appropriately differing Nav isoforms.

Nerve cells damaged by trauma, stroke, epilepsy, inflammatory conditions etc, have chronically leaky sodium channels that eventually kill. The usual job of sodium channels is to make brief voltage signals –action potentials– for long distance propagation. After sodium channels open to generate action potentials, sodium pumps work harder to re-establish the intracellular/extracellular sodium imbalance that is, literally, the neuron's battery for firing action potentials. Wherever tissue damage renders membranes overly fluid, we hypothesize, sodium channels become chronically leaky. Our experimental findings justify this. In fluidized membranes, sodium channel voltage sensors respond too easily, letting channels spend too much time open. Channels leak, pumps respond. By mathematical modeling, we show that in damaged channel-rich membranes the continual pump/leak counterplay would trigger the kinds of bizarre intermittent action potential bursts typical of injured neurons. Arising ectopically from injury regions, such neuropathic firing is unrelated to events in the external world. Drugs that can silence these deleterious electrical barrages without blocking healthy action potentials are needed. If fluidized membranes house the problematic leaky sodium channels, then drug side effects could be diminished by using drugs that accumulate most avidly into fluidized membranes, and that bind their targets with highest affinity there.

Gastrodin inhibits allodynia and hyperalgesia in painful diabetic neuropathy rats by decreasing excitability of nociceptive primary sensory neurons

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus and adversely affects the patients' quality of life. Evidence has accumulated that PDN is associated with hyperexcitability of peripheral nociceptive primary sensory neurons. However, the precise cellular mechanism underlying PDN remains elusive. This may result in the lacking of effective therapies for the treatment of PDN. The phenolic glucoside, gastrodin, which is a main constituent of the Chinese herbal medicine Gastrodia elata Blume, has been widely used as an anticonvulsant, sedative, and analgesic since ancient times. However, the cellular mechanisms underlying its analgesic actions are not well understood. By utilizing a combination of behavioral surveys and electrophysiological recordings, the present study investigated the role of gastrodin in an experimental rat model of STZ-induced PDN and to further explore the underlying cellular mechanisms. Intraperitoneal administration of gastrodin effectively attenuated both the mechanical allodynia and thermal hyperalgesia induced by STZ injection. Whole-cell patch clamp recordings were obtained from nociceptive, capsaicin-sensitive small diameter neurons of the intact dorsal root ganglion (DRG). Recordings from diabetic rats revealed that the abnormal hyperexcitability of neurons was greatly abolished by application of GAS. To determine which currents were involved in the antinociceptive action of gastrodin, we examined the effects of gastrodin on transient sodium currents (I(NaT)) and potassium currents in diabetic small DRG neurons. Diabetes caused a prominent enhancement of I(NaT) and a decrease of potassium currents, especially slowly inactivating potassium currents (I(AS)); these effects were completely reversed by GAS in a dose-dependent manner. Furthermore, changes in activation and inactivation kinetics of I(NaT) and total potassium current as well as I(AS) currents induced by STZ were normalized by GAS. This study provides a clear cellular basis for the peripheral analgesic action of gastrodin for the treatment of chronic pain, including PDN.

Chronic compression or acute dissociation of dorsal root ganglion induces cAMP-dependent neuronal hyperexcitability through activation of PAR2

Chronic compression (CCD) or dissociation of dorsal root ganglion (DRG) can induce cyclic adenosine monophosphate (cAMP)-dependent DRG neuronal hyperexcitability and behaviorally expressed hyperalgesia. Here, we report that protease-activated receptor 2 (PAR2) activation after CCD or dissociation mediates the increase of cAMP activity and protein kinase A (PKA) and cAMP-dependent hyperexcitability and hyperalgesia in rats. CCD and dissociation, as well as trypsin (a PAR2 activator) treatment, increased level of cAMP concentration, mRNA, and protein expression for PKA subunits PKA-RII and PKA-c and protein expression of PAR2, in addition to producing neuronal hyperexcitability and, in CCD rats, thermal hyperalgesia. The increased expression of PAR2 was colocalized with PKA-c subunit. A PAR2 antagonistic peptide applied before and/or during the treatment, prevented or largely diminished the increased activity of cAMP and PKA, neuronal hyperexcitability, and thermal hyperalgesia. However, posttreatment with the PAR2 antagonistic peptide failed to alter either hyperexcitability or hyperalgesia. In contrast, an adenylyl cyclase inhibitor, SQ22536, administrated after dissociation or CCD, successfully suppressed hyperexcitability and hyperalgesia, in vitro and/or in vivo. Trypsin-induced increase of the intracellular calcium [Ca(2+)](i) was prevented in CCD or dissociation DRG neurons. These alterations were further confirmed by knockdown of PAR2 with siRNA. In addition, trypsin and PAR2 agonistic peptide-induced increase of cAMP was prevented by inhibition of PKC, but not Gαs. These findings suggest that PAR2 activation is critical to induction of nerve injury-induced neuronal hyperexcitability and cAMP-PKA activation. Inhibiting PAR2 activation may be a potential target for preventing/suppressing development of neuropathic pain.

Enhanced excitability of small dorsal root ganglion neurons in rats with bone cancer pain

Background

Primary and metastatic cancers that affect bone are frequently associated with severe and intractable pain. The mechanisms underlying the development of bone cancer pain are largely unknown. The aim of this study was to determine whether enhanced excitability of primary sensory neurons contributed to peripheral sensitization and tumor-induced hyperalgesia during cancer condition. In this study, using techniques of whole-cell patch-clamp recording associated with immunofluorescent staining, single-cell reverse-transcriptase PCR and behavioral test, we investigated whether the intrinsic membrane properties and the excitability of small-sized dorsal root ganglion (DRG) neurons altered in a rat model of bone cancer pain, and whether suppression of DRG neurons activity inhibited the bone cancer-induced pain.

Results

Our present study showed that implantation of MRMT-1 tumor cells into the tibial canal in rats produced significant mechanical and thermal hyperalgesia in the ipsilateral hind paw. Moreover, implantation of tumor cells provoked spontaneous discharges and tonic excitatory discharges evoked by a depolarizing current pulse in small-sized DRG neurons. In line with these findings, alterations in intrinsic membrane properties that reflect the enhanced neuronal excitability were observed in small DRG neurons in bone cancer rats, of which including: 1) depolarized resting membrane potential (RMP); 2) decreased input resistance (R_in); 3) a marked reduction in current threshold (CT) and voltage threshold (TP) of action potential (AP); 4) a dramatic decrease in amplitude, overshot, and duration of evoked action potentials as well as in amplitude and duration of afterhyperpolarization (AHP); and 5) a significant increase in the firing frequency of evoked action potentials. Here, the decreased AP threshold and increased firing frequency of evoked action potentials implicate the occurrence of hyperexcitability in small-sized DRG neurons in bone cancer rats. In addiotion, immunofluorescent staining and single-cell reverse-transcriptase PCR revealed that in isolated small DRG neurons, most neurons were IB4-positive, or expressed TRPV1 or CGRP, indicating that most recorded small DRG neurons were nociceptive neurons. Finally, using in vivo behavioral test, we found that blockade of DRG neurons activity by TTX inhibited the tumor-evoked mechanical allodynia and thermal hyperalgesia in bone cancer rats, implicating that the enhanced excitability of primary sensory neurons underlied the development of bone cancer pain.

Conclusions

Our present results suggest that implantation of tumor cells into the tibial canal in rats induces an enhanced excitability of small-sized DRG neurons that is probably as results of alterations in intrinsic electrogenic properties of these neurons. Therefore, alterations in intrinsic membrane properties associated with the hyperexcitability of primary sensory neurons likely contribute to the peripheral sensitization and tumor-induced hyperalgesia under cancer condition.

Bilateral mechanical and thermal hyperalgesia and tactile allodynia after chronic compression of dorsal root ganglion in mice

OBJECTIVE Low back pain is one of the most inextricable problems encountered in clinics. Animal models that imitate symptoms in humans are valuable tools for investigating low back pain mechanisms and the possible therapeutic applications. With the development of genetic technology in pain field, the possibility of mutating specific genes in mice has provided a potent tool for investigating the specific mechanisms of pain. The aim of the present study was to develop a mouse model of chronic compression of dorsal root ganglion (CCD), in which gene mutation can be applied to facilitate the studies of chronic pain. METHODS Chronic compression of L4 and L5 dorsal root ganglia was conducted in mice by inserting fine stainless steel rods into the intervertebral foramina, one at L4 and the other at L5. Mechanical allodynia and thermal hyperalgesia were examined with von Frey filaments and radiating heat stimulator, respectively. RESULTS The CCD mice displayed dramatic mechanical and thermal hyperalgesia as well as tactile allodynia in the hindpaw ipsilateral to CCD. In addition, this mechanical and thermal hyperalgesia as well as tactile allodynia was also found to spread to the contralateral hindpaw. CONCLUSION This model, combined with the possible genetic modification, will strengthen our knowledge of the underlying mechanisms of low back pain. It also favors the development of new treatment strategies for pain and hyperalgesia after spinal injury and other disorders which affect the dorsal root ganglion in humans.

Subdiaphragmatic vagotomy increases the sensitivity of lumbar Aδ primary afferent neurons along with voltage-dependent potassium channels in rats

Subdiaphragmatic vagal dysfunction causes chronic pain. To verify whether this chronic pain is accompanied by enhanced peripheral nociceptive sensitivity, we evaluated primary afferent neuronal excitability in subdiaphragmatic vagotomized (SDV) rats. SDV rats showed a decrease in the electrical stimuli-induced hind limb-flexion threshold at 250 Hz, but showed no similar effect at 5 or 2000 Hz, which indicated that lumbar primary afferent Aδ sensitivity was enhanced in SDV rats. The whole-cell patch-clamp technique also revealed the hyper-excitability of acutely dissociated medium-sized lumbar dorsal root ganglion (DRG) neurons isolated from SDV rats. The contribution of changes in voltage-dependent potassium (Kv) channels was assessed, and transient A-type K(+) (I(A) ) current density was apparently decreased. Moreover, Kv4.3 immunoreactivity in medium-sized DRG neurons was significantly reduced in SDV rats compared to sham. These results indicate that SDV causes hyper-excitability of lumbar primary Aδ afferent neurons, which may be induced along with suppressing I(A) currents via the decreased expression of Kv4.3. Thus, peripheral Aδ neuroplasticity may contribute to the chronic lower limb pain caused by SDV.

Acid-sensing ion channel 3 mediates peripheral anti-hyperalgesia effects of acupuncture in mice inflammatory pain

Background

Peripheral tissue inflammation initiates hyperalgesia accompanied by tissue acidosis, nociceptor activation, and inflammation mediators. Recent studies have suggested a significantly increased expression of acid-sensing ion channel 3 (ASIC3) in both carrageenan- and complete Freund's adjuvant (CFA)-induced inflammation. This study tested the hypothesis that acupuncture is curative for mechanical hyperalgesia induced by peripheral inflammation.

Methods

Here we used mechanical stimuli to assess behavioral responses in paw and muscle inflammation induced by carrageenan or CFA. We also used immunohistochemistry staining and western blot methodology to evaluate the expression of ASIC3 in dorsal root ganglion (DRG) neurons.

Results

In comparison with the control, the inflammation group showed significant mechanical hyperalgesia with both intraplantar carrageenan and CFA-induced inflammation. Interestingly, both carrageenan- and CFA-induced hyperalgesia were accompanied by ASIC3 up-regulation in DRG neurons. Furthermore, electroacupuncture (EA) at the ST36 rescued mechanical hyperalgesia through down-regulation of ASIC3 overexpression in both carrageenan- and CFA-induced inflammation.

Conclusions

In addition, electrical stimulation at the ST36 acupoint can relieve mechanical hyperalgesia by attenuating ASIC3 overexpression.

Neuronal Fc-gamma receptor I mediated excitatory effects of IgG immune complex on rat dorsal root ganglion neurons

Pain often accompanies antigen-specific immune-related disorders though little is known of the underlying neural mechanisms. A common feature among these disorders is the elevated level of antigen-specific immunoglobulin (Ig) G in the serum and the presence of IgG immune complex (IC) in the affected tissue. We hypothesize that IC may directly activate the Fc-gamma receptor type I (FcγRI) expressed in nociceptive dorsal root ganglion (DRG) neurons and increase neuronal excitability thus potentially contributing to pain. Immunofluorescent labeling indicated that FcγRI, but not FcγRIIB or FcγRIII, was expressed in a subpopulation of rat DRG neurons including those expressing nociceptive markers. Calcium imaging revealed that the IC, but neither of the antibody (IgG) or antigen alone, produced an increase in intracellular calcium. This effect was abolished by the removal of the IgG Fc portion in the IC or the application of an anti-FcγRI antibody, suggesting a key role of the FcγRI receptor. Removal of extracellular calcium or depletion of intracellular calcium stores prevented the IC-induced calcium response. In whole-cell current-clamp recordings, IC depolarized the resting membrane potential, decreased the rheobase, and increased the number of action potentials evoked by a depolarizing current at 2× rheobase. In about half of the responsive neurons, IC evoked action potential discharges. These results suggest that a subpopulation of nociceptive neurons expresses functional FcγRI and that the activation of this receptor by IC increases neuronal excitability.

Does norepinephrine influence pain behavior mediated by dorsal root ganglia?: a pilot study

BACKGROUND

Postganglionic neurons in the sympathetic nervous system reportedly are involved in lumbar radicular pain and release norepinephrine (NE), a neurotransmitter. Increased numbers of sympathetic nerve fibers have been found in dorsal root ganglion (DRG) neurons in a root constriction model. Whether this is a reasonable model for pain, however, is unclear

QUESTIONS/PURPOSES

We asked whether: (1) painful behaviors occurred in the root constriction model; (2) NE enhanced the excitability of DRG neurons in the root constriction model; and (3) which adrenoceptors were related to the mediation of the NE effects.

METHODS

The L5 root was sutured proximal to the DRG as the root constriction model. Behavioral tests were performed until 28 days after surgery. At 10 to 14 days after the root constriction, DRG neurons were quickly excised and digested with collagenase for electrophysiologic studies. Action potentials were recorded from single DRG neurons using a whole-cell patch clamp technique. NE (10 μmol/L) was directly applied to the DRG neurons. The adrenergic sensitivity was examined in combination with antagonists.

RESULTS

The rats with root constriction exhibited painful behavior. NE increased the excitability of DRG neurons in the root constriction model. The effects of NE were inhibited by pretreatment with an α-antagonist and α(2)-antagonist but not an α(1)-antagonist.

CONCLUSIONS

Our observations suggest NE plays an important role in generating lumbar radicular pain mainly via α(2)-adrenoceptors.

CLINICAL RELEVANCE

An α(2)-antagonist may be an appropriate agent for trials to treat lumbar radicular pain.

Molecular mechanisms of mechanotransduction in mammalian sensory neurons

The somatosensory system mediates fundamental physiological functions, including the senses of touch, pain and proprioception. This variety of functions is matched by a diverse array of mechanosensory neurons that respond to force in a specific fashion. Mechanotransduction begins at the sensory nerve endings, which rapidly transform mechanical forces into electrical signals. Progress has been made in establishing the functional properties of mechanoreceptors, but it has been remarkably difficult to characterize mechanotranducer channels at the molecular level. However, in the past few years, new functional assays have provided insights into the basic properties and molecular identity of mechanotransducer channels in mammalian sensory neurons. The recent identification of novel families of proteins as mechanosensing molecules will undoubtedly accelerate our understanding of mechanotransduction mechanisms in mammalian somatosensation.

Expression of inwardly rectifying potassium channels by an inducible adenoviral vector reduced the neuronal hyperexcitability and hyperalgesia produced by chronic compression of the spinal ganglion

Background

A chronic compressed dorsal root ganglion (CCD) in rat produces pain behavior and an enhanced excitability of neurons within the compressed ganglion. Kir2.1 is an inwardly rectifying potassium channel that acts to stabilize the resting potential of certain cell types. We hypothesized that an inducible expression of Kir2.1 channels in CCD neurons might suppress neuronal excitability in the dorsal root ganglion (DRG) and reduce the associated pain behavior.

Results

We delivered, by microinjection into the fourth lumbar (L4) DRG, an adenoviral vector containing a reporter gene encoding the enhanced green fluorescent protein (GFP) and a Kir2.1 channel (AdKir). At the same time the ganglion was compressed by implantation of a rod through the intervertebral foramen (CCD). The in vivo expression of the transferred gene was controlled by an ecdysone analog via an ecdysone-inducible promoter in the viral vector. In comparison with the effects of vehicle or a control vector containing only the GFP gene, AdKir significantly reduced the neuronal hyperexcitability after CCD. Electrophysiological recordings, in vivo, from nociceptive and non-nociceptive DRG neurons expressing the virally produced Kir2.1 channels revealed a hyperpolarized resting membrane potential, an increased rheobase, and lack of spontaneous activity. Inducing the Kir2.1 gene at the beginning of CCD surgery partially prevented the development of mechanical hyperalgesia. However, a delayed induction of the Kir2.1 gene (3 days after CCD surgery) produced no significant effect on the pain behavior.

Conclusions

We found that an inducible expression of Kir2.1 channels in chronically compressed DRG neurons can effectively suppress the neuronal excitability and, if induced at the beginning of CCD injury, prevent the development of hyperalgesia. We hypothesize that a higher level of neuronal hyperexcitability in the DRG is required to initiate than to maintain the hyperalgesia and that the hyperexcitability contributing to neuropathic pain is best inhibited as soon as possible after injury.

Contribution of primary afferent channels to neuropathic pain

Neuropathic pain remains a serious medical problem because of patient morbidity and the absence of effective therapeutic interventions. Recent evidence suggests that this type of pain may be particularly difficult to manage because underlying mechanisms are influenced by a variety of factors, including type of injury, site of injury, and time after injury. This situation is exacerbated by the fact that different mechanisms may contribute to unique aspects of neuropathic pain, including ongoing pain as well as mechanical and thermal hypersensitivity. The different ion channels present in primary afferent neurons implicated in each of these aspects of neuropathic pain are reviewed.

Behavioral models of pain states evoked by physical injury to the peripheral nerve

Physical injury or compression of the root, dorsal root ganglion, or peripheral sensory axon leads to well-defined changes in biology and function. Behaviorally, humans report ongoing painful dysesthesias and aberrations in function, such that an otherwise innocuous stimulus will yield a pain report. These behavioral reports are believed to reflect the underlying changes in nerve function after injury, wherein increased spontaneous activity arises from the neuroma and dorsal root ganglion and spinal changes increase the response of spinal projection neurons. These pain states are distinct from those associated with tissue injury and pose particular problems in management. To provide for developing an understanding of the underlying mechanisms of these pain states and to promote development of therapeutic agents, preclinical models involving section, compression, and constriction of the peripheral nerve or compression of the dorsal root ganglion have been developed. These models give rise to behaviors, which parallel those observed in the human after nerve injury. The present review considers these models and their application.