Spontaneous activity of axotomized afferent neurons after L5 spinal nerve injury in rats

Similar excitability through different sodium channels and implications for the analgesic efficacy of selective drugs

Nociceptive sensory neurons convey pain-related signals to the CNS using action potentials. Loss-of-function mutations in the voltage-gated sodium channel NaV1.7 cause insensitivity to pain (presumably by reducing nociceptor excitability) but clinical trials seeking to treat pain by inhibiting NaV1.7 pharmacologically have struggled. This may reflect the variable contribution of NaV1.7 to nociceptor excitability. Contrary to claims that NaV1.7 is necessary for nociceptors to initiate action potentials, we show that nociceptors can achieve similar excitability using different combinations of NaV1.3, NaV1.7, and NaV1.8. Selectively blocking one of those NaV subtypes reduces nociceptor excitability only if the other subtypes are weakly expressed. For example, excitability relies on NaV1.8 in acutely dissociated nociceptors but responsibility shifts to NaV1.7 and NaV1.3 by the fourth day in culture. A similar shift in NaV dependence occurs in vivo after inflammation, impacting ability of the NaV1.7-selective inhibitor PF-05089771 to reduce pain in behavioral tests. Flexible use of different NaV subtypes exemplifies degeneracy - achieving similar function using different components - and compromises reliable modulation of nociceptor excitability by subtype-selective inhibitors. Identifying the dominant NaV subtype to predict drug efficacy is not trivial. Degeneracy at the cellular level must be considered when choosing drug targets at the molecular level.

Nav1.7 Modulator Bearing a 3-Hydroxyindole Backbone Holds the Potential to Reverse Neuropathic Pain

Chronic pain is a growing global health problem affecting at least 10% of the world's population. However, current chronic pain treatments are inadequate. Voltage-gated sodium channels (Navs) play a pivotal role in regulating neuronal excitability and pain signal transmission and thus are main targets for nonopioid painkiller development, especially those preferentially expressed in dorsal root ganglial (DRG) neurons, such as Nav1.6, Nav1.7, and Nav1.8. In this study, we screened in virtual hits from dihydrobenzofuran and 3-hydroxyoxindole hybrid molecules against Navs via a veratridine (VTD)-based calcium imaging method. The results showed that one of the molecules, 3g, could inhibit VTD-induced neuronal activity significantly. Voltage clamp recordings demonstrated that 3g inhibited the total Na+ currents of DRG neurons in a concentration-dependent manner. Biophysical analysis revealed that 3g slowed the activation, meanwhile enhancing the inactivation of the Navs. Additionally, 3g use-dependently blocked Na+ currents. By combining with selective Nav inhibitors and a heterozygous expression system, we demonstrated that 3g preferentially inhibited the TTX-S Na+ currents, specifically the Nav1.7 current, other than the TTX-R Na+ currents. Molecular docking experiments implicated that 3g binds to a known allosteric site at the voltage-sensing domain IV(VSDIV) of Nav1.7. Finally, intrathecal injection of 3g significantly relieved mechanical pain behavior in the spared nerve injury (SNI) rat model, suggesting that 3g is a promising candidate for treating chronic pain.

Alteration of serotonin release response in the central nucleus of the amygdala to noxious and non-noxious mechanical stimulation in a neuropathic pain model rat

Previously, we found that serotonin (5-HT) release in the central nucleus of the amygdala (CeA) of anesthetized rats decreases in response to innocuous stroking of the skin, irrespective of stimulus laterality, but increases in response to noxious pinching applied to a hindlimb contralateral to the 5-HT measurement site. The aim of the present study was to determine whether intra-CeA 5-HT release responses to cutaneous stimulation were altered in an animal model of neuropathic pain induced by ligation of the left L5 spinal nerve. In anesthetized neuropathic pain model rats, stroking of the left hindlimb increased 5-HT release in the CeA, whereas stroking of the right hindlimb decreased it. Meanwhile, pinching of the left hindlimb increased intra-CeA 5-HT release irrespective of stimulus laterality. In conclusion, the present study demonstrated that intra-CeA 5-HT release responses to cutaneous stimulation are altered in an animal model of neuropathic pain.

Relating Spinal Injury-Induced Neuropathic Pain and Spontaneous Afferent Activity to Sleep and Respiratory Dysfunction

Abstract Spinal cord injury (SCI) can induce dysfunction in a multitude of neural circuits including those that lead to impaired sleep, respiratory dysfunction, and neuropathic pain. We used a lower thoracic rodent contusion SCI model of neuropathic pain that has been shown to associate with increased spontaneous activity in primary afferents and hindlimb mechanosensory stimulus hypersensitivity. Here we paired capture of these variables with chronic capture of three state sleep and respiration to more broadly understand SCI-induced physiological dysfunction and to assess possible interrelations. Noncontact electric field sensors were embedded into home cages to non-invasively capture the temporal evolution of sleep and respiration changes for six weeks after SCI in naturally behaving mice. Hindlimb mechanosensitivity was assessed weekly, and terminal experiments measured primary afferent spontaneous activity in situ from intact lumbar dorsal root ganglia (DRG). We observed that SCI led to increased spontaneous primary afferent activity (both firing rate and the number of spontaneously active DRGs) that correlated with increased respiratory rate variability and measures of sleep fragmentation. This is the first study to measure and link sleep dysfunction and variability in respiratory rate in a SCI model of neuropathic pain, and thereby provide broader insight into the magnitude of overall stress burden initiated by neural circuit dysfunction after SCI.

Network model of nociceptive processing in the superficial spinal dorsal horn reveals mechanisms of hyperalgesia, allodynia, and spinal cord stimulation

The spinal dorsal horn (DH) processes sensory information and plays a key role in transmitting nociception to supraspinal centers. Loss of DH inhibition during neuropathic pain unmasks a pathway from nonnociceptive Aβ-afferent inputs to superficial dorsal horn (SDH) nociceptive-specific (NS) projection neurons, and this change may contribute to hyperalgesia and allodynia. We developed and validated a computational model of SDH neuronal circuitry that links nonnociceptive Aβ-afferent inputs in lamina II/III to a NS projection neuron in lamina I via a network of excitatory interneurons. The excitatory pathway and the NS projection neuron were in turn gated by inhibitory interneurons with connections based on prior patch-clamp recordings. Changing synaptic weights in the computational model to replicate neuropathic pain states unmasked a low-threshold excitatory pathway to NS neurons similar to experimental recordings. Spinal cord stimulation (SCS) is an effective therapy for neuropathic pain, and accumulating experimental evidence indicates that NS neurons in the SDH also respond to SCS. Accounting for these responses may inform therapeutic improvements, and we quantified responses to SCS in the SDH network model and examined the role of different modes of inhibitory control in modulating NS neuron responses to SCS. We combined the SDH network model with a previously published model of the deep dorsal horn (DDH) and identified optimal stimulation frequencies across different neuropathic pain conditions. Finally, we found that SCS-generated inhibition did not completely suppress model NS activity during simulated pinch inputs, providing an explanation of why SCS does not eliminate acute pain.NEW & NOTEWORTHY Chronic pain is a severe public health problem that reduces the quality of life for those affected and exacts an enormous socio-economic burden worldwide. Spinal cord stimulation (SCS) is an effective treatment for chronic pain, but SCS efficacy has not significantly improved over time, in part because the mechanisms of action remain unclear. Most preclinical studies investigating pain and SCS mechanisms have focused on the responses of deep dorsal horn (DDH) neurons, but neural networks in the superficial dorsal horn (SDH) are also important for processing nociceptive information. This work synthesizes heterogeneous experimental recordings from the SDH into a computational model that replicates experimental responses and that can be used to quantify neuronal responses to SCS under neuropathic pain conditions.

Sympathetic-Sensory Coupling as a Potential Mechanism for Acupoints Sensitization

A series of studies have demonstrated acupoint sensitization, in which acupoints can be activated in combination with sensory hypersensitivity and functional plasticity during visceral disorders. However, the mechanisms of acupoint sensitization remain unclear. Neuroanatomy evidence showed nociceptors innervated in acupoints contribute to the mechanism of acupoint sensitization. Increasing studies suggested sympathetic nerve plays a key role in modulating sensory transmission by sprouting or coupling with sensory neuron/nociceptor in the peripheral, forming the functional structure of the sympathetic-sensory coupling. Notably, the sensory inputs of the disease-induced sensitized acupoint contribute to the homeostatic regulation and also involve in delivering therapeutic information under acupuncture, hence, the role of sprouted sympathetic in acupoint function should be given attention. We herein reviewed the current knowledge of sympathetic and its sprouting in pain modulation, then discussed and highlighted the potential value of sympathetic-sensory coupling in acupoint functional plasticity.

Differences and Similarities in Spontaneous Activity Between Animal Models of Cancer-Induced Pain and Neuropathic Pain

Background

Clinical data on cancer-induced pain (CIP) demonstrate widespread changes in sensory function. It is characterized in humans not only by stimulus-invoked pain, but also by spontaneous pain. In our previous studies in an animal model of CIP, we observed changes in intrinsic membrane properties and excitability of dorsal root ganglion (DRG) sensory neurons corresponding to mechanical allodynia and hyperalgesia, of which abnormal activities of Aβ-fiber sensory neurons are consistent in a rat model of peripheral neuropathic pain (NEP).

Objective

To investigate whether there are related peripheral neural mechanisms between the CIP and NEP models of spontaneous pain, we compared the electrophysiological properties of DRG sensory neurons at 2–3 weeks after CIP and NEP model induction.

Methods

CIP models were induced with metastasis tumour-1 rat breast cancer cells implanted into the distal epiphysis of the femur. NEP models were induced with a polyethylene cuff implanted around the sciatic nerve. Spontaneous pain in animals is measured by spontaneous foot lifting (SFL). After measurement of SFL, the animals were prepared for electrophysiological recordings of spontaneous activity (SA) in DRG neurons in vivo.

Results

Our data showed that SFL and SA occurred in both models. The proportion of SFL and SA of C-fiber sensory neurons in CIP was more significantly increased than in NEP models. There was no difference in duration of SFL and the rate of SA between the two models. The duration of SFL is related to the rate of SA in C-fiber in both models.

Conclusion

Thus, SFL may result from SA activity in C-fiber neurons in CIP and NEP rats. The differences and similarities in spontaneous pain between CIP and NEP rats is related to the proportion and rate of SA in C-fibers, respectively.

Effect of sympathetic sprouting on the excitability of dorsal root ganglion neurons and afferents in a rat model of neuropathic pain

Increased sympathetic nerve excitability has been reported to aggravate a variety of chronic pain conditions, and an increase in the number of sympathetic nerve fibers in the dorsal root ganglion (DRG) has been found in neuropathic pain (NP) models. However, the mechanism of the neurotransmitter norepinephrine (NE) released by sympathetic nerve fiber endings on the excitability of DRG neurons is still controversial, and the adrenergic receptor subtypes involved in this biological process are also controversial. In our study, we have two objectives: (1) To determine the effect of the neurotransmitter NE on the excitability of different neurons in DRG; (2) To determine which adrenergic receptors are involved in the excitability of DRG neurons by NE released by sprouting sympathetic fibers. In this experiment, a unique field potential recording method of spinal cord dorsal horn was innovatively adopted, which can be used for electrophysiological study in vivo. The results showed that： Forty days after SNI, patch clamp and field potential recording methods confirmed that NE enhanced the excitability of ipsilateral DRG large neurons, and then our in vivo electrophysiological results showed that the α₂ receptor blocker Yohimbine could block the excitatory effect of NE on A-fiber and the inhibitory effect on C-fiber, while the α_2A-adrenergic receptor agonist guanfacine (100 μM) had the same biological effect as NE. Finally, we concluded that NE from sympathetic fiber endings is involved in the regulation of pain signaling by acting on α_2A-adrenergic receptors in DRG.

Dorsal root ganglion stimulation of injured sensory neurons in rats rapidly eliminates their spontaneous activity and relieves spontaneous pain

ABSTRACT

Dorsal root ganglion field stimulation (GFS) relieves evoked and spontaneous neuropathic pain by use-dependent blockade of impulse trains through the sensory neuron T-junction, which becomes complete within less than 1 minute for C-type units, also with partial blockade of Aδ units. We used this tool in the spinal nerve ligation (SNL) rat model to selectively block sensory neuron spontaneous activity (SA) of axotomized neurons at the fifth lumbar (L5) level vs blockade of units at the L4 level that remain uninjured but exposed to inflammation. In vivo dorsal root single-unit recordings after SNL showed increased SA in L5 units but not L4 units. Ganglion field stimulation blocked this SA. Ganglion field stimulation delivered at the L5 dorsal root ganglion blocked mechanical hyperalgesia behavior, mechanical allodynia, and ongoing spontaneous pain indicated by conditioned place preference, whereas GFS at L4 blocked evoked pain behavior but not spontaneous pain. In vivo single-unit recordings of spinal cord dorsal horn (DH) wide-dynamic-range neurons showed elevated SA after SNL, which was reduced by GFS at the L5 level but not by GFS at the L4 level. In addition, L5 GFS, but not L4 GFS, increased mechanical threshold of DH units during cutaneous mechanical stimulation, while L5 GFS exceeded L4 GFS in reducing evoked firing rates. Our results indicate that SA in injured neurons supports increased firing of DH wide-dynamic-range neurons, contributing to hyperalgesia, allodynia, and ongoing pain. Ganglion field stimulation analgesic effects after nerve injury are at least partly attributable to blocking propagation of this SA.

Transcriptional Reprogramming of Distinct Peripheral Sensory Neuron Subtypes after Axonal Injury

Primary somatosensory neurons are specialized to transmit specific types of sensory information through differences in cell size, myelination, and the expression of distinct receptors and ion channels, which together define their transcriptional and functional identity. By profiling sensory ganglia at single-cell resolution, we find that all somatosensory neuronal subtypes undergo a similar transcriptional response to peripheral nerve injury that both promotes axonal regeneration and suppresses cell identity. This transcriptional reprogramming, which is not observed in non-neuronal cells, resolves over a similar time course as target reinnervation and is associated with the restoration of original cell identity. Injury-induced transcriptional reprogramming requires ATF3, a transcription factor that is induced rapidly after injury and necessary for axonal regeneration and functional recovery. Our findings suggest that transcription factors induced early after peripheral nerve injury confer the cellular plasticity required for sensory neurons to transform into a regenerative state.

Effects of alkaloids on peripheral neuropathic pain: a review

Neuropathic pain is a debilitating pathological pain condition with a great therapeutic challenge in clinical practice. Currently used analgesics produce deleterious side effects. Therefore, it is necessary to investigate alternative medicines for neuropathic pain. Chinese herbal medicines have been widely used in treating intractable pain. Compelling evidence revealed that the bioactive alkaloids of Chinese herbal medicines stand out in developing novel drugs for neuropathic pain due to multiple targets and satisfactory efficacy. In this review, we summarize the recent progress in the research of analgesic effects of 20 alkaloids components for peripheral neuropathic pain and highlight the potential underlying molecular mechanisms. We also point out the opportunities and challenges of the current studies and shed light on further in-depth pharmacological and toxicological studies of these bioactive alkaloids. In conclusion, the alkaloids hold broad prospects and have the potentials to be novel drugs for treating neuropathic pain. This review provides a theoretical basis for further applying some alkaloids in clinical trials and developing new drugs of neuropathic pain.

Microglia Are Indispensable for Synaptic Plasticity in the Spinal Dorsal Horn and Chronic Pain

Spinal long-term potentiation (LTP) at C-fiber synapses is hypothesized to underlie chronic pain. However, a causal link between spinal LTP and chronic pain is still lacking. Here, we report that high-frequency stimulation (HFS; 100 Hz, 10 V) of the mouse sciatic nerve reliably induces spinal LTP without causing nerve injury. LTP-inducible stimulation triggers chronic pain lasting for more than 35 days and increases the number of calcitonin gene-related peptide (CGRP) terminals in the spinal dorsal horn. The behavioral and morphological changes can be prevented by blocking NMDA receptors, ablating spinal microglia, or conditionally deleting microglial brain-derived neurotrophic factor (BDNF). HFS-induced spinal LTP, microglial activation, and upregulation of BDNF are inhibited by antibodies against colony-stimulating factor 1 (CSF-1). Together, our results show that microglial CSF1 and BDNF signaling are indispensable for spinal LTP and chronic pain. The microglia-dependent transition of synaptic potentiation to structural alterations in pain pathways may underlie pain chronicity.

Oral application of bulleyaconitine A attenuates morphine tolerance in neuropathic rats by inhibiting long-term potentiation at C-fiber synapses and protein kinase C gamma in spinal dorsal horn

Morphine is frequently used for the treatment of chronic pain, while long-term use of the drug leads to analgesic tolerance. At present, the prevention of the side effect remains a big challenge. Bulleyaconitine A, a diterpenoid alkaloid from Aconitum bulleyanum plants, has been used to treat chronic pain in China for more than 30 years. In the present study, we tested the effect of bulleyaconitine A on analgesic tolerance induced by morphine injections (10 mg/kg s.c., b.i.d.) in the lumbar 5 spinal nerve ligation model of neuropathic pain. We found that intragastrical application of bulleyaconitine A (0.4 mg/kg) 30 min before each morphine injection substantially inhibited the decrease in morphine’s inhibitory effect on mechanical allodynia and thermal hyperalgesia. Mechanistically, morphine injections further potentiated the lumbar 5 spinal nerve ligation induced long-term potentiation at C-fiber synapses in the spinal dorsal horn, a synaptic model of chronic pain. This effect was completely blocked by intragastrical bulleyaconitine A. It has been well established that activation of protein kinase C gamma and of glial cells in the spinal dorsal horn are critical for the development of opioid tolerance and neuropathic pain. We found that morphine injections exacerbated the upregulation of phospho-protein kinase C gamma (an active form of protein kinase C gamma), and the activation of microglia and astrocytes in the spinal dorsal horn induced by lumbar 5 spinal nerve ligation, and the effects were considerably prohibited by intragastrical bulleyaconitine A. Thus, spinal long-term potentiation at C-fiber synapses may underlie morphine tolerance. Oral administration of bulleyaconitine A may be a novel and simple approach for treating of opioid tolerance.

The role of NaV channels in synaptic transmission after axotomy in a microfluidic culture platform

Voltage gated sodium channels are key players in aberrant pain signaling and sensitization of nociceptors after peripheral nerve injury. The extent to which sodium channel activity after injury contributes to synaptic transmission at the first pain synapse however remains unclear. To investigate the effect of axotomy on synaptic transmission between dorsal root ganglia neurons and dorsal horn neurons, we reconstructed the first pain synapse in a novel microfluidic based compartmentalized cell culture system, which recapitulates the connectivity of peripheral pain signaling. We show that following axotomy of the distal axons, inhibition of Na_V1.7 and Na_V1.8 sodium channels in incoming presynaptic DRG axons is no longer sufficient to block activation of these axons and the resulting synaptic transmission to dorsal horn neurons. We found that blockade of Na_V1.6 activity is highly effective in reducing activation of incoming axons contributing to synaptic transmission after axotomy of DRG neurons. The microfluidic culture system described here offers an in vitro platform to recapitulate and study the first pain synapse.

Spinal glial cell line-derived neurotrophic factor infusion reverses reduction of Kv4.1-mediated A-type potassium currents of injured myelinated primary afferent neurons in a neuropathic pain model

High frequency spontaneous activity in injured primary afferents has been proposed as a pathological mechanism of neuropathic pain following nerve injury. Although spinal infusion of glial cell line-derived neurotrophic factor reduces the activity of injured myelinated A-fiber neurons after fifth lumbar (L5) spinal nerve ligation in rats, the implicated molecular mechanism remains undetermined. The fast-inactivating transient A-type potassium current (I_A) is an important determinant of neuronal excitability, and five voltage-gated potassium channel (Kv) alpha-subunits, Kv1.4, Kv3.4, Kv4.1, Kv4.2, and Kv4.3, display I_A in heterologous expression systems. Here, we examined the effect of spinal glial cell line-derived neurotrophic factor infusion on I_A and the expression of these five Kv mRNAs in injured A-fiber neurons using the in vitro patch clamp technique and in situ hybridization histochemistry. Glial cell line-derived neurotrophic factor infusion reversed axotomy-induced reduction of the rheobase, elongation of first spike duration, and depolarization of the resting membrane potential. L5 spinal nerve ligation significantly reduced the current density of I_A and glial cell line-derived neurotrophic factor treatment reversed the reduction. Among the examined Kv mRNAs, only the change in Kv4.1-expression was parallel with the change in I_A after spinal nerve ligation and glial cell line-derived neurotrophic factor treatment. These findings suggest that glial cell line-derived neurotrophic factor should reduce the hyperexcitability of injured A-fiber primary afferents by I_A recurrence. Among the five I_A-related Kv channels, Kv4.1 should be a key channel, which account for this I_A recurrence.

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.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Mechanisms for therapeutic effect of bulleyaconitine A on chronic pain

Bulleyaconitine A, a diterpenoid alkaloid isolated from Aconitum bulleyanum plants, has been used for the treatment of chronic pain in China since 1985. Clinical studies show that the oral administration of bulleyaconitine A is effective for treating different kinds of chronic pain, including back pain, joint pain, and neuropathic pain with minimal side effect in human patients. The experimental studies have revealed that bulleyaconitine A at therapeutic doses potently inhibits the peripheral sensitization and central sensitization that underlie chronic pain and has no effect on acute pain. Bulleyaconitine A preferably blocks tetrodotoxin-sensitive voltage-gated sodium channels in dorsal root ganglion neurons by inhibition of protein kinase C, and the effect is around 600 times more potent in neuropathic animals than in naïve ones. Bulleyaconitine A at 5 nM inhibits the hypersensitivity of dorsal root ganglion neurons in neuropathic rats but has no effect on excitability of dorsal root ganglion neurons in sham group. Bulleyaconitine A inhibits long-term potentiation at C-fiber synapses in spinal dorsal horn, a synaptic model of pathological pain, preferably in neuropathic pain rats over naïve rats. The following mechanisms may underlie the selective effect of bulleyaconitine A on chronic pain. (1) In neuropathic conditions, protein kinase C and voltage-gated sodium channels in dorsal root ganglion neurons are upregulated, which enhances bulleyaconitine A's effect. (2) Bulleyaconitine A use-dependently blocks voltage-gated sodium channels and therefore inhibits the ectopic discharges that are important for neuropathic pain. (3) Bulleyaconitine A is shown to inhibit neuropathic pain by the modulation of spinal microglia, which are involved in the chronic pain but not in acute (nociceptive) pain. Moreover, bulleyaconitine A facilitates the anesthetic effect of morphine and inhibits morphine tolerance in rats. Together, bulleyaconitine A is able to inhibit chronic pain by targeting at multiple molecules. Further clinical and experimental studies are needed for evaluating the efficacy of bulleyaconitine A in different forms of chronic pain in patients and for exploring the underlying mechanisms.

Puerarin Relieves Paclitaxel-Induced Neuropathic Pain: The Role of Nav1.8 β1 Subunit of Sensory Neurons

Currently there is no effective treatment available for clinical patients suffering from neuropathic pain induced by chemotherapy paclitaxel. Puerarin is a major isoflavonoid extracted from the Chinese medical herb kudzu root, which has been used for treatment of cardiovascular disorders and brain injury. Here, we found that puerarin dose-dependently alleviated paclitaxel-induced neuropathic pain. At the same time, puerarin preferentially reduced the excitability and blocked the voltage-gated sodium (Na_v) channels of dorsal root ganglion (DRG) neurons from paclitaxel-induced neuropathic pain rats. Furthermore, puerarin was a more potent blocker of tetrodotoxin-resistant (TTX-R) Na_v channels than of tetrodotoxin-sensitive (TTX-S) Na_v channels in chronic pain rats’ DRG neurons. In addition, puerarin had a stronger blocking effect on Na_v1.8 channels in DRG neurons of neuropathic pain rats and β1 subunit siRNA can abolish this selective blocking effect on Na_v1.8. Together, these results suggested that puerarin may preferentially block β1 subunit of Na_v1.8 in sensory neurons contributed to its anti-paclitaxel induced neuropathic pain effect.

Bulleyaconitine A attenuates hyperexcitability of dorsal root ganglion neurons induced by spared nerve injury: The role of preferably blocking Nav1.7 and Nav1.3 channels

Background

Oral administration of Bulleyaconitine A, an extracted diterpenoid alkaloid from Aconitum bulleyanum plants, is effective for treating chronic pain in rats and in human patients, but the underlying mechanisms are poorly understood.

Results

As the hyperexcitability of dorsal root ganglion neurons resulting from the upregulation of voltage-gated sodium (Nav) channels has been proved critical for development of chronic pain, we tested the effects of Bulleyaconitine A on Nav channels in rat spared nerve injury model of neuropathic pain. We found that Bulleyaconitine A at 5 nM increased the threshold of action potentials and reduced the firing rate of dorsal root ganglion neurons in spared nerve injury rats but not in sham rats. Bulleyaconitine A preferably blocked tetrodotoxin-sensitive Nav channels over tetrodotoxin-resistant ones in dorsal root ganglion neurons of spared nerve injury rats. Bulleyaconitine A was more potent for blocking Nav1.3 and Nav1.7 than Nav1.8 in cell lines. The half maximal inhibitory concentration (IC₅₀) values for resting Nav1.3, Nav1.7, and Nav1.8 were 995.6 ± 139.1 nM, 125.7 ± 18.6 nM, and 151.2 ± 15.4 μM, respectively, which were much higher than those for inactivated Nav1.3 (20.3 ± 3.4 pM), Nav1.7 (132.9 ± 25.5 pM), and Nav1.8 (18.0 ± 2.5 μM). The most profound use-dependent blocking effect of Bulleyaconitine A was observed on Nav1.7, less on Nav1.3, and least on Nav1.8 at IC₅₀ concentrations. Bulleyaconitine A facilitated the inactivation of Nav channels in each subtype.

Conclusions

Preferably blocking tetrodotoxin-sensitive Nav1.7 and Nav1.3 in dorsal root ganglion neurons may contribute to Bulleyaconitine A’s antineuropathic pain effect.

Neuritis and vinblastine-induced axonal transport disruption lead to signs of altered dorsal horn excitability

BACKGROUND

Many patients with neuropathic pain present without signs of nerve injury on routine clinical examination. Some of these patients may have inflamed peripheral nerves (neuritis). In this study, we have examined whether neuritis causes changes within the dorsal horn that may contribute to a central pain mechanism. Comparisons have been made to a model of axonal transport disruption induced using vinblastine, since neuritis disrupts such processes.

RESULTS

At the peak of cutaneous hypersensitivities, recordings from wide dynamic range neurons revealed increases in wind-up following neuritis but not vinblastine treatment. Ongoing activity from these neurons was unchanged. Vinblastine treatment caused a reduction in the responses of wide dynamic range neurons to noxious mechanical stimulation of the receptive field. The response of neurons to innocuous mechanical stimulation was also reduced in wide dynamic range neurons that were at a depth ≥550 µm following vinblastine treatment. An examination of the superficial dorsal horn revealed an increase in c-Fos-positive neurons in both groups following electrical stimulation of the sciatic nerve. The area of dorsal horn expressing substance P was also decreased following vinblastine treatment.

CONCLUSION

These findings indicate that a minor nerve insult, such as neuritis, can lead to changes within the dorsal horn that are consistent with a central neuropathic pain mechanism.

Hyperpolarization-activated cyclic nucleotide-gated channels contribute to spontaneous activity in L4 C-fiber nociceptors, but not Aβ-non-nociceptors, after axotomy of L5-spinal nerve in the rat in vivo

Peripheral neuropathic pain associated with partial nerve injury is believed to be driven partly by aberrant spontaneous activity (SA) in both injured and uninjured dorsal root ganglion (DRG) neurons. The underlying ionic mechanisms are not fully understood, but hyperpolarization-activated cyclic nucleotide-gated (HCN) channels which underlie the excitatory Ih current have been implicated in SA generation in axotomized A-fiber neurons after L5-spinal nerve ligation/axotomy (SNL/SNA). Here, using a modified model of SNA (mSNA) which involves, in addition to L5-SNA, loose ligation of the L4-spinal nerve with neuroinflammation-inducing chromic gut, we examined whether HCN channels also contribute to SA in the adjacent L4-neurons. Intracellular recordings from L4-DRG neurons in control rats, and L4-DRG neurons in mSNA rats were made using in vivo voltage- and current-clamp techniques. Compared with control, L4 C-nociceptors and Aβ-low-threshold mechanoreceptors (LTMs) exhibited SA 7 days after mSNA. This was accompanied, in C-nociceptors, by a significant increase in Ih amplitude, the percentage of Ih-expressing neurons, and Ih activation rate. Hyperpolarization-activated cyclic nucleotide-gated channel blockade with ZD7288 (10 mg/kg, intravenously) suppressed SA in C-nociceptors, but not Aβ-LTMs, and caused in C-nociceptors, membrane hyperpolarization and a decrease in Ih activation rate. Furthermore, intraplantar injection of ZD7288 (100 μM) was found to be as effective as gabapentin (positive control) in attenuating cold hypersensitivity in mSNA rats. These findings suggest that HCN channels contribute to nerve injury-induced SA in L4 C-nociceptors, but not Aβ-LTMs, and that ZD7288 exerts its analgesic effects by altering Ih activation properties and/or causing membrane hyperpolarization in L4 C-nociceptors.

Upregulation of N-type calcium channels in the soma of uninjured dorsal root ganglion neurons contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury

N-type voltage-gated calcium (Cav2.2) channels are expressed in the central terminals of dorsal root ganglion (DRG) neurons, and are critical for neurotransmitter release. Cav2.2 channels are also expressed in the soma of DRG neurons, where their function remains largely unknown. Here, we showed that Cav2.2 was upregulated in the soma of uninjured L4 DRG neurons, but downregulated in those of injured L5 DRG neurons following L5 spinal nerve ligation (L5-SNL). Local application of specific Cav2.2 blockers (ω-conotoxin GVIA, 1-100 μM or ZC88, 10-1000 μM) onto L4 and 6 DRGs on the operated side, but not the contralateral side, dose-dependently reversed mechanical allodynia induced by L5-SNL. Patch clamp recordings revealed that both ω-conotoxin GVIA (1 μM) and ZC88 (10 μM) depressed hyperexcitability in L4 but not in L5 DRG neurons of L5-SNL rats. Consistent with this, knockdown of Cav2.2 in L4 DRG neurons with AAV-Cav2.2 shRNA substantially prevented L5-SNL-induced mechanical allodynia and hyperexcitability of L4 DRG neurons. Furthermore, in L5-SNL rats, interleukin-1 beta (IL-1β) and IL-10 were upregulated in L4 DRGs and L5 DRGs, respectively. Intrathecal injection of IL-1β induced mechanical allodynia and Cav2.2 upregulation in bilateral L4-6 DRGs of naïve rats, whereas injection of IL-10 substantially prevented mechanical allodynia and Cav2.2 upregulation in L4 DRGs in L5-SNL rats. Finally, in cultured DRG neurons, Cav2.2 was dose-dependently upregulated by IL-1β and downregulated by IL-10. These data indicate that the upregulation of Cav2.2 in uninjured DRG neurons via IL-1β over-production contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury.

Membrane potential oscillations are not essential for spontaneous firing generation in L4 Aβ-afferent neurons after L5 spinal nerve axotomy and are not mediated by HCN channels

NEW FINDINGS

What is the central question of this study? Is spontaneous activity (SA) in L4 dorsal root ganglion (DRG) neurons induced by L5 spinal nerve axotomy associated with membrane potential oscillations in these neurons, and if so, are these membrane oscillations mediated by HCN channels? What is the main finding and its importance? Unlike injured L5 DRG neurons, which have been shown to be incapable of firing spontaneously without membrane potential oscillations, membrane potential oscillations are not essential for SA generation in conducting 'uninjured' L4 neurons, and they are not mediated by HCN channels. These findings suggest that the underlying cellular mechanisms of SA in injured and 'uninjured' DRG neurons induced by spinal nerve injury are distinct.

ABSTRACT

The underlying cellular and molecular mechanisms of peripheral neuropathic pain are not fully understood. However, preclinical studies using animal models suggest that this debilitating condition is driven partly by aberrant spontaneous activity (SA) in injured and uninjured dorsal root ganglion (DRG) neurons, and that SA in injured DRG neurons is triggered by subthreshold membrane potential oscillations (SMPOs). Here, using in vivo intracellular recording from control L4-DRG neurons, and ipsilateral L4-DRG neurons in female Wistar rats that had previously undergone L5 spinal nerve axotomy (SNA), we examined whether conducting 'uninjured' L4-DRG neurons in SNA rats exhibit SMPOs, and if so, whether such SMPOs are associated with SA in those L4 neurons, and whether they are mediated by hyperpolarization-activated cyclic nucleotide gated (HCN) channels. We found that 7 days after SNA: (a) none of the control A- or C-fibre DRG neurons showed SMPOs or SA, but 50%, 43% and 0% of spontaneously active cutaneous L4 Aβ-low threshold mechanoreceptors, Aβ-nociceptors and C-nociceptors exhibited SMPOs, respectively, in SNA rats with established neuropathic pain behaviors; (b) neither SMPOs nor SA in L4 Aβ-neurons was suppressed by blocking HCN channels with ZD7288 (10 mg kg^-1 , i.v.); and (c) there is a tendency for female rats to show greater pain hypersensitivity than male rats. These results suggest that SMPOs are linked to SA only in some of the conducting L4 Aβ-neurons, that such oscillations are not a prerequisite for SA generation in those L4 A- or C-fibre neurons, and that HCN channels are not involved in their electrogenesis.

Neuropathic pain-induced enhancement of spontaneous and pain-evoked neuronal activity in the periaqueductal gray that is attenuated by gabapentin

Neuropathic pain is a debilitating pathological condition that is poorly understood. Recent evidence suggests that abnormal central processing occurs during the development of neuropathic pain induced by the cancer chemotherapeutic agent, paclitaxel. Yet, it is unclear what role neurons in supraspinal pain network sites, such as the periaqueductal gray, play in altered behavioral sensitivity seen during chronic pain conditions. To elucidate these mechanisms, we studied the spontaneous and thermally evoked firing patterns of ventrolateral periaqueductal gray (vlPAG) neurons in awake-behaving rats treated with paclitaxel to induce neuropathic pain. In the present study, vlPAG neurons in naive rats exhibited either excitatory, inhibitory, or neutral responses to noxious thermal stimuli, as previously observed. However, after development of behavioral hypersensitivity induced by the chemotherapeutic agent, paclitaxel, vlPAG neurons displayed increased neuronal activity and changes in thermal pain-evoked neuronal activity. This involved elevated levels of spontaneous firing and heightened responsiveness to nonnoxious stimuli (allodynia) as well as noxious thermal stimuli (hyperalgesia) as compared with controls. Furthermore, after paclitaxel treatment, only excitatory neuronal responses were observed for both nonnoxious and noxious thermal stimuli. Systemic administration of gabapentin, a nonopioid analgesic, induced significant dose-dependent decreases in the elevated spontaneous and thermally evoked vlPAG neuronal firing to both nonnoxious and noxious thermal stimuli in rats exhibiting neuropathic pain, but not in naive rats. Thus, these results show a strong correlation between behavioral hypersensitivity to thermal stimuli and increased firing of vlPAG neurons in allodynia and hyperalgesia that occur in this neuropathic pain model.

Activation of Astrocytes and Microglial Cells and CCL2/CCR2 Upregulation in the Dorsolateral and Ventrolateral Nuclei of Periaqueductal Gray and Rostral Ventromedial Medulla Following Different Types of Sciatic Nerve Injury

Peripheral nerve injuries (PNIs) may result in cellular and molecular changes in supraspinal structures possibly involved in neuropathic pain (NPP) maintenance. Activated glial cells in specific supraspinal subregions may affect the facilitatory role of descending pathways. Sterile chronic compression injury (sCCI) and complete sciatic nerve transection (CSNT) in rats were used as NPP models to study the activation of glial cells in the subregions of periaqueductal gray (PAG) and rostral ventromedial medulla (RVM). Molecular markers for activated astrocytes (glial fibrillary acidic protein, GFAP) and microglial cells (OX42) were assessed by quantitative immunohistochemistry and western blotting. The cellular distribution of CCL2/CCR2 was monitored using immunofluorescence. sCCI induced both mechanical and thermal hypersensitivity from day 1 up to 3 weeks post-injury. Unilateral sCCI or CSNT for 3 weeks induced significant activation of astrocytes bilaterally in both dorsolateral (dlPAG) and ventrolateral PAG (vlPAG) compared to naïve or sham-operated rats. More extensive astrocyte activation by CSNT compared to sCCI was induced bilaterally in dlPAG and ipsilaterally in vlPAG. Significantly more extensive activation of astrocytes was also found in RVM after CSNT than sCCI. The CD11b immunopositive region, indicating activated microglial cells, was remarkably larger in dlPAG and vlPAG of both sides from sCCI- and CSNT-operated rats compared to naïve or sham-operated controls. No significant differences in microglial activation were detected in dlPAG or vlPAG after CSNT compared to sCCI. Both nerve injury models induced no significant differences in microglial activation in the RVM. Neurons and activated GFAP+ astrocytes displayed CCL2-immunoreaction, while activated OX42+ microglial cells were CCR2-immunopositive in both PAG and RVM after sCCI and CSNT. Overall, while CSNT induced robust astrogliosis in both PAG and RVM, microglial cell activation was similar in the supraspinal structures in both injury nerve models. Activated astrocytes in PAG and RVM may sustain facilitation of the descending system maintaining NPP, while microglial activation may be associated with a reaction to long-lasting peripheral injury. Microglial activation via CCR2 may be due to neuronal and astrocytal release of CCL2 in PAG and RVM following injury.

Spinal Fbxo3-Dependent Fbxl2 Ubiquitination of Active Zone Protein RIM1α Mediates Neuropathic Allodynia through CaV2.2 Activation

Spinal plasticity, a key process mediating neuropathic pain development, requires ubiquitination-dependent protein turnover. Presynaptic active zone proteins have a crucial role in regulating vesicle exocytosis, which is essential for synaptic plasticity. Nevertheless, the mechanism for ubiquitination-regulated turnover of presynaptic active zone proteins in the progression of spinal plasticity-associated neuropathic pain remains unclear. Here, after research involving Sprague Dawley rats, we reported that spinal nerve ligation (SNL), in addition to causing allodynia, enhances the Rab3-interactive molecule-1α (RIM1α), a major active zone protein presumed to regulate neural plasticity, specifically in the synaptic plasma membranes (SPMs) of the ipsilateral dorsal horn. Spinal RIM1α-associated allodynia was mediated by Fbxo3, which abates Fbxl2-dependent RIM1α ubiquitination. Subsequently, following deubiquitination, enhanced RIM1α directly binds to CaV2.2, resulting in increased CaV2.2 expression in the SPMs of the dorsal horn. While exhibiting no effect on Fbxo3/Fbxl2 signaling, the focal knockdown of spinal RIM1α expression reversed the SNL-induced allodynia and increased spontaneous EPSC (sEPSC) frequency by suppressing RIM1α-facilitated CaV2.2 expression in the dorsal horn. Intrathecal applications of BC-1215 (a Fbxo3 activity inhibitor), Fbxl2 mRNA-targeting small-interfering RNA, and ω-conotoxin GVIA (a CaV2.2 blocker) attenuated RIM1α upregulation, enhanced RIM1α expression, and exhibited no effect on RIM1α expression, respectively. These results confirm the prediction that spinal presynaptic Fbxo3-dependent Fbxl2 ubiquitination promotes the subsequent RIM1α/CaV2.2 cascade in SNL-induced neuropathic pain. Our findings identify a role of the presynaptic active zone protein in pain-associated plasticity. That is, RIM1α-facilitated CaV2.2 expression plays a role in the downstream signaling of Fbxo3-dependent Fbxl2 ubiquitination/degradation to promote spinal plasticity underlying the progression of nociceptive hypersensitivity following neuropathic injury.

SIGNIFICANCE STATEMENT

Ubiquitination is a well known process required for protein degradation. Studies investigating pain pathology have demonstrated that ubiquitination contributes to chronic pain by regulating the turnover of synaptic proteins. Here, we found that the spinal presynaptic active zone protein Rab3-interactive molecule-1α (RIM1α) participates in neuropathic pain development by binding to and upregulating the expression of CaV2.2. In addition, Fbxo3 modifies this pathway by inhibiting Fbxl2-mediated RIM1α ubiquitination, suggesting that presynaptic protein ubiquitination makes a crucial contribution to the development of neuropathic pain. Research in this area, now in its infancy, could potentially provide a novel therapeutic strategy for pain relief.

How diagnostic tests help to disentangle the mechanisms underlying neuropathic pain symptoms in painful neuropathies

Neuropathic pain, ie, pain arising directly from a lesion or disease affecting the somatosensory afferent pathway, manifests with various symptoms, the commonest being ongoing burning pain, electrical shock-like sensations, and dynamic mechanical allodynia. Reliable insights into the mechanisms underlying neuropathic pain symptoms come from diagnostic tests documenting and quantifying somatosensory afferent pathway damage in patients with painful neuropathies. Neurophysiological investigation and skin biopsy studies suggest that ongoing burning pain primarily reflects spontaneous activity in nociceptive-fiber pathways. Electrical shock-like sensations presumably arise from high-frequency ectopic bursts generated in demyelinated, nonnociceptive, Aβ fibers. Although the mechanisms underlying dynamic mechanical allodynia remain debatable, normally innocuous stimuli might cause pain by activating spared and sensitized nociceptive afferents. Extending the mechanistic approach to neuropathic pain symptoms might advance targeted therapy for the individual patient and improve testing for new drugs.

Altered potassium channel distribution and composition in myelinated axons suppresses hyperexcitability following injury

Neuropathic pain following peripheral nerve injury is associated with hyperexcitability in damaged myelinated sensory axons, which begins to normalise over time. We investigated the composition and distribution of shaker-type-potassium channels (Kv1 channels) within the nodal complex of myelinated axons following injury. At the neuroma that forms after damage, expression of Kv1.1 and 1.2 (normally localised to the juxtaparanode) was markedly decreased. In contrast Kv1.4 and 1.6, which were hardly detectable in the naïve state, showed increased expression within juxtaparanodes and paranodes following injury, both in rats and humans. Within the dorsal root (a site remote from injury) we noted a redistribution of Kv1-channels towards the paranode. Blockade of Kv1 channels with α-DTX after injury reinstated hyperexcitability of A-fibre axons and enhanced mechanosensitivity. Changes in the molecular composition and distribution of axonal Kv1 channels, therefore represents a protective mechanism to suppress the hyperexcitability of myelinated sensory axons that follows nerve injury.

De novo expression of Nav1.7 in injured putative proprioceptive afferents: Multiple tetrodotoxin-sensitive sodium channels are retained in the rat dorsal root after spinal nerve ligation

Tetrodotoxin-sensitive (TTX-s) spontaneous activity is recorded from the dorsal roots after peripheral nerve injury. Primary sensory neurons in the dorsal root ganglion (DRG) express multiple TTX-s voltage-gated sodium channel α-subunits (Navs). Since Nav1.3 increases, whereas all other Navs decrease, in the DRG neurons after peripheral nerve lesion, Nav1.3 is proposed to be critical for the generation of these spontaneous discharges and the contributions of other Navs have been ignored. Here, we re-evaluate the changes in expression of three other TTX-s Navs, Nav1.1, Nav1.6 and Nav1.7, in the injured 5th lumbar (L5) primary afferent components following L5 spinal nerve ligation (SNL) using in situ hybridization histochemistry and immunohistochemistry. While the overall signal intensities for these Nav mRNAs decreased, many injured DRG neurons still expressed these transcripts at clearly detectable levels. All these Nav proteins accumulated at the proximal stump of the ligated L5 spinal nerve. The immunostaining patterns of Nav1.6 and Nav1.7 associated with the nodes of Ranvier were maintained in the ipsilateral L5 dorsal root. Interestingly, putative proprioceptive neurons characterized by α3 Na+/K+ ATPase-immunostaining specifically lacked Nav1.7 mRNA in naïve DRG but displayed de novo expression of this transcript following SNL. Nav1.7-immunoreactive fibers were significantly increased in the ipsilateral gracile nucleus where central axonal branches of the injured A-fiber afferents terminated. These data indicate that multiple TTX-s channel subunits could contribute to the generation and propagation of the spontaneous discharges in the injured primary afferents. Specifically, Nav1.7 may cause some functional changes in sensory processing in the gracile nucleus after peripheral nerve injury.

An approach to identify microRNAs involved in neuropathic pain following a peripheral nerve injury

Peripheral nerve injury alters the expression of hundreds of proteins in dorsal root ganglia (DRG). Targeting some of these proteins has led to successful treatments for acute pain, but not for sustained post-operative neuropathic pain. The latter may require targeting multiple proteins. Since a single microRNA (miR) can affect the expression of multiple proteins, here, we describe an approach to identify chronic neuropathic pain-relevant miRs. We used two variants of the spared nerve injury (SNI): Sural-SNI and Tibial-SNI and found distinct pain phenotypes between the two. Both models induced strong mechanical allodynia, but only Sural-SNI rats maintained strong mechanical and cold allodynia, as previously reported. In contrast, we found that Tibial-SNI rats recovered from mechanical allodynia and never developed cold allodynia. Since both models involve nerve injury, we increased the probability of identifying differentially regulated miRs that correlated with the quality and magnitude of neuropathic pain and decreased the probability of detecting miRs that are solely involved in neuronal regeneration. We found seven such miRs in L3-L5 DRG. The expression of these miRs increased in Tibial-SNI. These miRs displayed a lower level of expression in Sural-SNI, with four having levels lower than those in sham animals. Bioinformatic analysis of how these miRs could affect the expression of some ion channels supports the view that, following a peripheral nerve injury, the increase of the seven miRs may contribute to the recovery from neuropathic pain while the decrease of four of them may contribute to the development of chronic neuropathic pain. The approach used resulted in the identification of a small number of potentially neuropathic pain relevant miRs. Additional studies are required to investigate whether manipulating the expression of the identified miRs in primary sensory neurons can prevent or ameliorate chronic neuropathic pain following peripheral nerve injuries.

Stimulation-induced ectopicity and propagation windows in model damaged axons

Neural tissue injuries render voltage-gated Na⁺ channels (Nav) leaky, thereby altering excitability, disrupting propagation and causing neuropathic pain related ectopic activity. In both recombinant systems and native excitable membranes, membrane damage causes the kinetically-coupled activation and inactivation processes of Nav channels to undergo hyperpolarizing shifts. This damage-intensity dependent change, called coupled left-shift (CLS), yields a persistent or “subthreshold” Nav window conductance. Nodes of Ranvier simulations involving various degrees of mild CLS showed that, as the system’s channel/pump fluxes attempt to re-establish ion homeostasis, the CLS elicits hyperexcitability, subthreshold oscillations and neuropathic type action potential (AP) bursts. CLS-induced intermittent propagation failure was studied in simulations of stimulated axons, but pump contributions were ignored, leaving open an important question: does mild-injury (small CLS values, pumps functioning well) render propagation-competent but still quiescent axons vulnerable to further impairments as the system attempts to cope with its normal excitatory inputs? We probe this incipient diffuse axonal injury scenario using a 10-node myelinated axon model. Fully restabilized nodes with mild damage can, we show, become ectopic signal generators (“ectopic nodes”) because incoming APs stress Na⁺/K⁺ gradients, thereby altering spike thresholds. Comparable changes could contribute to acquired sodium channelopathies as diverse as epileptic phenomena and to the neuropathic amplification of normally benign sensory inputs. Input spike patterns, we found, propagate with good fidelity through an ectopically firing site only when their frequencies exceed the ectopic frequency. This “propagation window” is a robust phenomenon, occurring despite Gaussian noise, large jitter and the presence of several consecutive ectopic nodes.

Modeling effects of spinal cord stimulation on wide-dynamic range dorsal horn neurons: influence of stimulation frequency and GABAergic inhibition

Spinal cord stimulation (SCS) is a clinical therapy for chronic, neuropathic pain, but an incomplete understanding of the mechanisms underlying SCS contributes to the lack of improvement in SCS efficacy over time. To study the mechanisms underlying SCS, we constructed a biophysically based network model of the dorsal horn circuit consisting of interconnected dorsal horn interneurons and a wide-dynamic range (WDR) projection neuron and representations of both local and surround receptive field inhibition. We validated the network model by reproducing cellular and network responses relevant to pain processing including wind-up, A fiber-mediated inhibition, and surround receptive field inhibition. We then simulated the effects of SCS on the activity of the WDR projection neuron and found that the response of the model WDR neuron to SCS depends on the SCS frequency; SCS frequencies of 30–100 Hz maximally inhibited the model WDR neuron, while frequencies under 30 Hz and over 100 Hz excited the model WDR neuron. We also studied the impacts on the effects of SCS of loss of inhibition due to the loss of either GABA or KCC2 function. Reducing the influence of local and surround GABAergic interneurons by weakening their inputs or their connections to the WDR neuron and shifting the anionic reversal potential of the WDR neurons upward each reduced the range of optimal SCS frequencies and changed the frequency at which SCS had a maximal effect. The results of this study provide insights into the mechanisms of SCS and pave the way for improved SCS parameter selection.

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.

Fifth lumbar spinal nerve injury causes neurochemical changes in corresponding as well as adjacent spinal segments: a possible mechanism underlying neuropathic pain

Previous investigations of the anatomical basis of the neuropathic-like manifestations in the spinal nerve ligation animal model have shown that the central terminations of the unmyelinated primary afferents of L5 spinal nerve are not restricted to the corresponding L5 spinal segment, and rather extend to two spinal segments rostrally and one segment caudally where they intermingle with primary afferents of the adjacent L4 spinal nerve. The aim of the present study was to investigate the neurochemical changes in the dorsal horn of the spinal cord and DRGs after L5 nerve injury in rats. In the first experiment, the right L5 nerve was ligated and sectioned for 14 days, and isolectin B4 (IB4, a tracer for unmyelinated primary afferents) was injected into the left L5 nerve. The results showed that the vasoactive intestinal peptide (VIP) was up-regulated in laminae I-II of L3-L6 spinal segments on the right side in exactly the same areas where IB4 labelled terminals were revealed on the left side. In the second experiment, L5 was ligated and sectioned and the spinal cord and DRGs were stained immunocytochemically with antibodies raised against various peptides known to be involved in pain transmission and hyperalgesia. The results showed that L5 nerve lesion caused down-regulation of substance P, calcitonin-gene related peptide and IB4 binding and up-regulation of neuropeptide Y and neurokinin-1 receptor in the dorsal horn of L4 and L5 spinal segments. Similar neurochemical changes were observed only in the corresponding L5 DRG with minimal effects observed in L3, L4 and L6 DRGs. Although, L5 nerve injury caused an up-regulation in NPY, no change in SP and CGRP immunoreactivity was observed in ipsilateral garcile nucleus. These neuroplastic changes in the dorsal horn of the spinal cord, in the adjacent uninjured territories of the central terminations of the adjacent uninjured nerves, might explain the mechanism of hyperalgesia after peripheral nerve injury.

Dynamic genotype-selective "phenotypic switching" of CGRP expression contributes to differential neuropathic pain phenotype

Using a genetic model we demonstrate the role played by "phenotypic switching" of calcitonin gene related peptide (CGRP) expression in axotomized large Aβ afferents in the development of neuropathic pain behavior in rats. After nerve injury both substance P and CGRP are upregulated in Aβ afferents in the corresponding DRGs. It has been proposed that intraspinal release of these neurotransmitters upon gentle stroking of skin drives ascending pain signaling pathways resulting in tactile allodynia. We reported previously that in rat lines genetically selected for high (HA) vs. low (LA) pain phenotype, SP is upregulated equally in both strains, but that CGRP is upregulated exclusively in the pain prone HA line (Nitzan-Luques et al., 2011). This implicates CGRP as the principal driver of tactile allodynia. Here we confirm this conclusion by showing: 1) that the time of emergence of CGRP-IR in DRG Aβ neurons and their central terminals in HA rats matches that of pain behavior, 2) that following spinal nerve lesion (SNL) selective activation of low threshold afferents indeed drives postsynaptic pain-signaling neurons and induces central sensitization in HA rats, as monitored using c-Fos as a marker. These changes are much less prominent in LA rats, 3) that intrathecal (i.t.) administration of CGRP induces tactile allodynia in naïve rats and 4) that i.t. administration of the CGRP-receptor antagonist BIBN4096BS (Olcegepant) attenuates SNL-evoked tactile allodynia, without blocking baseline nociception. Together, these observations support the hypothesis that genotype-selective phenotypic switching of CGRP expression in Aβ afferents following nerve injury is a fundamental mechanism of neuropathic tactile allodynia.

Reappraising neuropathic pain in humans--how symptoms help disclose mechanisms

Neuropathic pain--that is, pain arising directly from a lesion or disease that affects the somatosensory system--is a common clinical problem, and typically causes patients intense distress. Patients with neuropathic pain have sensory abnormalities on clinical examination and experience pain of diverse types, some spontaneous and others provoked. Spontaneous pain typically manifests as ongoing burning pain or paroxysmal electric shock-like sensations. Provoked pain includes pain induced by various stimuli or even gentle brushing (dynamic mechanical allodynia). Recent clinical and neurophysiological studies suggest that the various pain types arise through distinct pathophysiological mechanisms. Ongoing burning pain primarily reflects spontaneous hyperactivity in nociceptive-fibre pathways, originating from 'irritable' nociceptors, regenerating nerve sprouts or denervated central neurons. Paroxysmal sensations can be caused by several mechanisms; for example, electric shock-like sensations probably arise from high-frequency bursts generated in demyelinated non-nociceptive Aβ fibres. Most human and animal findings suggest that brush-evoked allodynia originates from Aβ fibres projecting onto previously sensitized nociceptive neurons in the dorsal horn, with additional contributions from plastic changes in the brainstem and thalamus. Here, we propose that the emerging mechanism-based approach to the study of neuropathic pain might aid the tailoring of therapy to the individual patient, and could be useful for drug development.

Smad-interacting protein 1 affects acute and tonic, but not chronic pain

BACKGROUND

Smad-interacting protein 1 (also named Zeb2 and Zfhx1b) is a transcription factor that plays an important role in neuronal development and, when mutated, causes Mowat-Wilson syndrome (MWS). A corresponding mouse model carrying a heterozygous Zeb2 deletion was comprehensively analysed in the German Mouse Clinic. The most prominent phenotype was the reduced pain sensitivity. In this study, we investigated the role of Zeb2 in inflammatory and neuropathic pain.

METHODS

For this, we tested mutant Zeb2 animals in different models of inflammatory pain like abdominal constriction, formalin and carrageenan test. Furthermore, we studied the pain reactivity of the mice after peripheral nerve ligation. To examine the nociceptive transmission of primary sensory dorsal root ganglia (DRG) neurons, we determined the neuronal activity in the spinal dorsal horn after the formalin test using staining of c-Fos. Next, we characterized the neuronal cell population in the DRGs and in the sciatic nerve to study the effect of the Zeb2 mutation on peripheral nerve morphology.

RESULTS

The present data show that Zeb2 is involved in the development of primary sensory DRG neurons, especially of C- and Aδ fibres. These alterations contribute to a hypoalgesic phenotype in inflammatory but not in neuropathic pain in these Zeb2(+/-) mice.

CONCLUSION

Our data suggest that the under-reaction to pain observed in MWS patients results from a reduced responsivity to nociceptive stimulation rather than an inability to communicate discomfort.

Hypofunction of glutamatergic neurotransmission in the periaqueductal gray contributes to nerve-injury-induced neuropathic pain

Neuropathic pain, a chronic pain due to neuronal lesion, remains unaltered even after the injury-induced spinal afferent discharges have declined, suggesting an involvement of supraspinal dysfunction. The midbrain ventrolateral periaqueductal gray (vlPAG) is known to be a crucial supraspinal region for initiating descending pain inhibition, but its role in neuropathic pain remains unclear. Therefore, here we examined neuroplastic changes in the vlPAG of midbrain slices isolated from neuropathic rats induced by L5/L6 spinal nerve ligation (SNL) via electrophysiological and neurochemical approaches. Significant mechanical hypersensitivity was induced in rats 2 d after SNL and lasted for >14 d. Compared with the sham-operated group, vlPAG slices from neuropathic rats 3 and 10 days after SNL displayed smaller EPSCs with prolonged latency, less frequent and smaller miniature EPSCs, higher paired-pulse ratio of EPSCs, smaller AMPAR-mediated EPSCs, smaller AMPA currents, greater NMDAR-mediated EPSCs, greater NMDA currents, lower AMPAR-mediated/NMDAR-mediated ratios, and upregulation of the NR1 and NR2B subunits, but not the NR2A, GluR1, or GluR2 subunits, of glutamate receptors. There were no significant differences between day 3 and day 10 neuropathic groups. These results suggest that SNL leads to hypoglutamatergic neurotransmission in the vlPAG resulting from both presynaptic and postsynaptic mechanisms. Upregulation of NMDARs might contribute to hypofunction of AMPARs via subcellular redistribution. Long-term hypoglutamatergic function in the vlPAG may lead to persistent reduction of descending pain inhibition, resulting in chronic neuropathic pain.

Induction of monocyte chemoattractant protein-1 (MCP-1) and its receptor CCR2 in primary sensory neurons contributes to paclitaxel-induced peripheral neuropathy

The use of paclitaxel (Taxol), a microtubule stabilizer, for cancer treatment is often limited by its associated peripheral neuropathy (chemotherapy-induced peripheral neuropathy [CIPN]), which predominantly results in sensory dysfunction, including chronic pain. Here we show that paclitaxel CIPN was associated with induction of chemokine monocyte chemoattractant protein-1 (MCP-1) and its cognate receptor CCR2 in primary sensory neurons of dorsal root ganglia. Immunostaining revealed that MCP-1 was mainly expressed in small nociceptive neurons whereas CCR2 was expressed in large and medium-sized myelinated neurons. Direct application of MCP-1 consistently induced intracellular calcium increases in dorsal root ganglia large and medium-sized neurons but not in small neurons mainly dissociated from paclitaxel-treated but not vehicle-treated animals. Paclitaxel also induced increased expression of MCP-1 in spinal astrocytes, but no CCR2 signal was detected in the spinal cord. Local blockade of MCP-1/CCR2 signaling by anti-MCP-1 antibody or CCR2 antisense oligodeoxynucleotides significantly attenuated paclitaxel CIPN phenotypes including mechanical hypersensitivity and loss of intraepidermal nerve fibers in hindpaw glabrous skin. These results suggest that activation of paracrine MCP-1/CCR2 signaling between dorsal root ganglion neurons plays a critical role in the development of paclitaxel CIPN, and targeting MCP-1/CCR2 signaling could be a novel therapeutic approach.

PERSPECTIVE

CIPN is a severe side effect accompanying paclitaxel chemotherapy and lacks effective treatments. The current study suggests that blocking MCP-1/CCR2 signaling could be a new therapeutic strategy to prevent or reverse paclitaxel CIPN. This preclinical evidence encourages future clinical evaluation of this strategy.

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.

The up-regulation of IL-6 in DRG and spinal dorsal horn contributes to neuropathic pain following L5 ventral root transection

Our previous works have shown that pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-α) plays an important role in neuropathic pain produced by lumber 5 ventral root transection (L5-VRT). In the present work we evaluate the role of interleukin-6 (IL-6), another key inflammatory cytokine, in the L5-VRT model. We found that IL-6 was up-regulated in the ipsilateral L4 and L5 dorsal root ganglian (DRG) neurons and in bilateral lumbar spinal cord following L5-VRT. Double immunofluorescence stainings revealed that in DRGs the increased immunoreactivity (IR) of IL-6 was almost restricted in neuronal cells, while in the spinal dorsal horn IL-6-IR up-regulated in both glial cells (astrocyte and microglia) and neurons. Intrathecal administration of IL-6 neutralizing antibody significantly delayed the induction of mechanical allodynia in bilateral hindpaws after L5-VRT. Furthermore, inhibition of TNF-α synthesis by intraperitoneal thalidomide prevented both mechanical allodynia and the up-regulation of IL-6 in DRGs following L5-VRT. These data suggested that the increased IL-6 in afferent neurons and spinal cord contribute to the development of neuropathic pain following motor fiber injury, and that TNF-α is responsible for the up-regulation of IL-6.