Ectopic Discharges Research Articles

Transmural Conduction Is the Predominant Mechanism of Breakthrough During Atrial Fibrillation

Background— Endo-epicardial dissociation (EED) of electric activations resulting in transmural conduction of fibrillation waves (breakthroughs) has been postulated to contribute to the complexity of the substrate of atrial fibrillation (AF). The aim of this study was to elucidate the correlation between EED and incidence of breakthrough and to test the plausibility of transmural conduction versus ectopic focal discharges as sources of breakthrough. Methods and Results— We analyzed high-resolution simultaneous endo-epicardial in vivo mapping data recorded in left atrial free walls of goats with acute AF, 3 weeks and 6 months of AF (all n=7). Waves were analyzed for number, size, and width and categorized according to their origin outside (peripheral wave) or within the mapping area (breakthrough). Breakthrough incidence was lowest (2.1±1.0%) in acute AF, higher (11.4±6.1%) after 3 weeks ( P <0.01 versus acute AF) and highest (14.2±3.8%) after 6 months AF ( P <0.001 versus acute AF) and similar in the epicardium and endocardium. Most of the breakthroughs (86%; n=564) could be explained by transmural conduction, whereas only 13% (n=85) could be explained by ectopic focal discharges. Transmural microreentry did not play a role as source of breakthrough. Conclusions— This is the first study to present simultaneous endo-epicardial in vivo mapping data at sites of breakthrough events. Breakthrough incidence and degree of EED increased with increasing AF substrate complexity. In goat left atrial free walls, most of the breakthroughs can be explained by transmural conduction, whereas ectopic focal discharges play a limited role as source of breakthrough.

Effects of histamine on spontaneous neuropathic pain induced by peripheral axotomy

The present study was designed to investigate the effects of histamine on spontaneous neuropathic pain (NP) induced by peripheral axotomy. Rats and mice were subjected to complete transection of the left sciatic and saphenous nerves to induce spontaneous NP (the neuroma model). Rats were then treated with drugs once daily for 30 days (histidine and loratadine, i.p.) or 21 days (histamine, i.c.v.). Autotomy behavior was scored daily until day 50 post-operation (PO). On days 14 to 21 PO, some rats in the control group were subjected to single-fiber recording. Autotomy behavior was also monitored daily in histidine decarboxylase (the key enzyme for histamine synthesis) knockout (HDC(-/-)) and wild-type mice for 42 days. We found that both histidine (500 mg/kg) (a precursor of histamine that increases histamine levels in the tissues) and histamine (50 μg/5 μL) significantly suppressed autotomy behavior in rats. HDC(-/-) mice lacking endogenous histamine showed higher levels of autotomy than the wild-type. In addition, the analgesic effect of histidine was not antagonized by loratadine (a peripherally-acting H1 receptor antagonist), while loratadine alone significantly suppressed autotomy. Electrophysiological recording showed that ectopic spontaneous discharges from the neuroma were blocked by systemic diphenhydramine (an H1 receptor antagonist). Our results suggest that histamine plays an important role in spontaneous NP. It is likely that histamine in the central nervous system is analgesic, while in the periphery, via H1 receptors, it is algesic. This study justifies the avoidance of a histamine-rich diet and the use of peripherally-acting H1 receptor antagonists as well as agents that improve histamine action in the central nervous system in patients with spontaneous NP.

Phantosmia as a meteorological forecaster

In normosmics, olfactory ability has been found to vary with ambient humidity, barometric pressure, and season. While hallucinated sensations of phantom pain associated with changes in weather have been described, a linkage to chemosensory hallucinations has heretofore not been reported. A 64-year-old white male with Parkinson's disease presents with 5years of phantosmia of a smoky burnt wood which changed to onion-gas and then to a noxious skunk-onion excrement odor. Absent upon waking it increases over the day and persists for hours. When severe, there appears a phantom taste with the same qualities as the odor. It is exacerbated by factors that manipulate intranasal pressure, such as coughing. When eating or sniffing, the actual flavors replace the phantosmia. Since onset, he noted the intensity and frequency of the phantosmia forecasted the weather. Two to 3h before a storm, the phantosmia intensifies from a level 0 to a 7-10, which persists through the entire thunderstorm. Twenty years prior, he reported the ability to forecast the weather, based on pain in a torn meniscus, which vanished after surgical repair. Extensive olfactory testing demonstrates underlying hyposmia. Possible mechanisms for such chemosensory-meteorological linkage includes: air pressure induced synesthesia, disinhibition of spontaneous olfactory discharge, exacerbation of ectopic discharge, affect mediated somatic sensory amplification, and misattribution error with expectation and recall bias. This is the first reported case of weather-induced exacerbation of phantosmia. Further investigation of the connection between chemosensory complaints and ambient weather is warranted.

Interleukin-10 down-regulates voltage gated sodium channels in rat dorsal root ganglion neurons

The over-expression of voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) neurons following peripheral nerve injury contributes to neuropathic pain by generation of the ectopic discharges of action potentials. However, mechanisms underlying the change in VGSCs' expression are poorly understood. Our previous work has demonstrated that the pro-inflammatory cytokine TNF-α up-regulates VGSCs. In the present work we tested if anti-inflammatory cytokine IL-10, which had been proven to be effective for treating neuropathic pain, had the opposite effect. Western blot and immunofluorescence results showed that IL-10 receptor was localized in DRG neurons. Recombinant rat IL-10 (200pg/ml) not only reduced the densities of TTX-sensitive and Nav1.8 currents in control DRG neurons, but also reversed the increase of the sodium currents induced by rat recombinant TNF-α (100pg/ml), as revealed by patch-clamp recordings. Consistent with the electrophysiological results, real-time PCR and western blot revealed that IL-10 (200pg/ml) down-regulated VGSCs in both mRNA and protein levels and reversed the up-regulation of VGSCs by TNF-α. Moreover, repetitive intrathecal administration of rrIL-10 for 3days (4 times per day) attenuated mechanical allodynia in L5 spinal nerve ligation model and profoundly inhibited the excitability of DRG neurons. These results suggested that the down-regulation of the sodium channels in DRG neurons might contribute to the therapeutic effect of IL-10 on neuropathic pain.

Methylcobalamin: A Potential Vitamin of Pain Killer

Methylcobalamin (MeCbl), the activated form of vitamin B12, has been used to treat some nutritional diseases and other diseases in clinic, such as Alzheimer's disease and rheumatoid arthritis. As an auxiliary agent, it exerts neuronal protection by promoting regeneration of injured nerves and antagonizing glutamate-induced neurotoxicity. Recently several lines of evidence demonstrated that MeCbl may have potential analgesic effects in experimental and clinical studies. For example, MeCbl alleviated pain behaviors in diabetic neuropathy, low back pain and neuralgia. MeCbl improved nerve conduction, promoted the regeneration of injured nerves, and inhibited ectopic spontaneous discharges of injured primary sensory neurons. This review aims to summarize the analgesic effect and mechanisms of MeCbl at the present.

Long-Term Changes in Trigeminal Ganglionic and Thalamic Neuronal Activities following Inferior Alveolar Nerve Transection in Behaving Rats

The transection of the inferior alveolar nerve (IANx) produces allodynia in the whisker pad (V2 division) of rats. Ectopic discharges from injured trigeminal ganglion (TG) neurons and thalamocortical reorganization are possible contributors to the sensitization of uninjured V2 primary and CNS neurons. To test which factor is more important, TG and ventroposterior medial nucleus (VPM) neurons were longitudinally followed before, during, and after IANx for up to 80 d. Spontaneous discharges and mechanical stimulation-evoked responses were recorded in conscious and in anesthetized states. Results show (1) a sequential increase in spontaneous activities, first in the injured TG neurons of the IAN (2-30 d), followed by uninjured V2 ganglion neurons (6-30 d), and then VPM V2 neurons (7-30 d) after IANx; (2) ectopic discharges included burst and regular firing patterns in the IAN and V2 branches of the TG neurons; and (3) the receptive field expanded, the modality shifted, and long-lasting after-discharges occurred only in VPM V2 neurons. All of these changes appeared in the late or maintenance phase (7-30 d) and disappeared during the recovery phase (40-60 d). These observations suggest that ectopic barrages in the injured IAN contribute more to the development of sensitization, whereas the modality shift and evoked after-discharges in the VPM thalamic neurons contribute more to the maintenance phase of allodynia by redirecting tactile information to the cortex as nociceptive.

Enhanced SCN7A/Nax expression contributes to bone cancer pain by increasing excitability of neurons in dorsal root ganglion

Bone pain is one of the most common complications in cancer patients with bone metastases, and has the most significant impact on quality of life for patients. Patients with bone cancer pain may be difficult to treat due to the poor understanding of the mechanisms; therefore, the mechanisms of bone cancer pain required elucidation for developing new therapeutics. Recent studies show that SCN7A/Nax channel serves as a sodium-level sensor of the body fluid that controls the Na-intake behavior by changing the excitability of neurons. In the current study, the expression of SCN7A/Nax and the excitability of primary sensory neurons in bone cancer pain rats were examined. The analgesic effects of knockdown SCN7A/Nax channel using RNAi lentivirus intrathecal treatment were evaluated with a behavioral test. The results showed that implantation of sarcoma induced ongoing and movement-evoked pain behaviors, whereas SCN7A/Nax knockdown prevented the onset of these hyperalgesia. Immunohistochemistry showed that SCN7A/Nax was located in the medium- to large-sized neurons in dorsal root ganglions (DRGs). The proportion of SCN7A/Nax-positive cells was significantly increased in DRGs ipsilateral to sarcoma implantation. Immunostaining results were further confirmed by Western blot and real time-polymerase chain reaction (RT-PCR) analyses. Recording from primary sensory neurons in excised rat dorsal root ganglias, we found that most of SCN7A/Nax-positive neurons exhibited subthreshold oscillations, depolarized resting membrane potential and more negative threshold of action potential. These electrophysiological changes of neurons increased ectopic spike discharge which was thought to be an important generator of chronic pain, however, the hyperexcitability was completely reversed by SCN7A/Nax knockdown. These results demonstrate that enhanced expression of SCN7A/Nax channel within distinct subpopulation of DRG neurons contributes to bone cancer pain by increasing the excitability of these neurons. These findings may lead to novel strategies for the treatment of bone cancer pain.

Ectopic and reentrant activation patterns in the posterior left atrium during stretch-related atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans and is predicted to dramatically increase its prevalence in the future. There is experimental evidence that increasing stretch increases the dominance of the pulmonary veins (PVs) during AF in isolated hearts and ectopic activity in the isolated PVs, but the ionic mechanisms underlying such effects are not clear and the ability of the PVs to favorably host functional reentry during stretch cannot be excluded. We used a combination of endocardial–epicardial optical mapping with phase and spectral analysis to study stretch-related AF (SRAF) in normal isolated sheep hearts. We have found rapid AF sources in the posterior left atrium (PLA) and PV region and their activation frequency and level of organization correlated with intra-atrial pressure. Analysis of the surfaces' optical mapping data in the phase domain reveals that activation of the PLA consisted of alternating patterns of breakthroughs, reentries and relatively simple waves swiping across the mapped field. The patterns on the endocardial and epicardial PLA surface at any given moment of time of the SRAF could be either identical or not identical, and the activity in the thickness of the PLA wall is hypothesized to conform to either ectopic discharge or scroll waves, but a definite evidence for the presence of such mechanisms is currently lacking. Thus the understanding of the manner by which the mechano-electric feedback effects in the PLA, including the PVs, become important in the initiation and maintenance of AF requires further detailed investigation.

Management of Neuropathic Pain with Methylprednisolone at the Site of Nerve Injury

Peripheral nerve blocks with methylprednisolone may provide effective pain therapy by decreasing ectopic neuronal discharge and the release of local inflammatory mediators at the site of nerve injury. In this study, we aimed to compare the efficacy of lidocaine alone with a combination of depo-methylprednisolone plus lidocaine in the management of neuropathic pain due to peripheral nerve damage. Randomized, double-blind comparator trial. Group control (N = 44) received 0.5% lidocaine and group methylprednisolone (N = 44) received 80 mg depo-methylprednisolone + 0.5% lidocaine proximal to the site of nerve injury with a total amount of 10-20 mL solution according to the type of peripheral nerve block with nerve stimulator. Demographic data, preblock numerical rating scales (NRSs), the Leeds assessment of neuropathic symptoms and signs (LANSS(0) ) score, accompanying symptoms, and analgesic requirements were recorded. Postblock NRS scores were noted following peripheral nerve block and after 3 months. LANSS(1) , accompanying symptoms, and analgesic requirements were also reevaluated 3 months after the injection. Demographic data, preblock NRS (8 ± 1.5 and 8.1 ± 1.2, respectively), postblock NRS (2.1 ± 1.2 and 2.4 ± 1.4, respectively), LANSS(0) (18.4 ± 2.2 and 18.2 ± 2.1, respectively), and accompanying symptoms were comparable between groups. Scores for the methylprednisolone group were significantly improved at 3-month postblock for NRS (2 ± 1.4 vs 5.2 ± 1.7) and LANSS(1) scores (4.14 ± 2.7 vs 14.1 ± 2.8), accompanying symptoms, and analgesic requirements (P < 0.0001). Our results suggest that peripheral nerve block with 80 mg depo-methylprednisolone plus 0.5% lidocaine provides effective management in the treatment of neuropathic pain due to peripheral nerve damage.

Effects of Regional and Whole-body Hypothermic Treatment before and after Median Nerve Injury on Neuropathic Pain and Glial Activation in Rat Cuneate Nucleus

Neuroprotective effects of hypothermia on peripheral nerve injury remain uncertain. This study investigated the efficacy of hypothermia in attenuating neuropathic pain and glial activation in the cuneate nucleus in a median nerve chronic constriction injury (CCI) model. Sprague-Dawley rats (n = 246) that underwent median nerve ligature at the elbow received various degrees of regional and whole-body hypothermia 15 min before CCI and 5 h, 1, 3, and 5 days after CCI. Hypothermia was maintained for 4 h. Seven days after CCI, behavioral and electrophysiological testings were conducted. Immunohistochemistry, immunoblotting, and enzyme-linked immunosorbent assay were used for qualitative and quantitative analysis of glial activation and measuring pro-inflammatory cytokines, respectively. Mild (32°C) and deep (28°C) regional hypothermia administered preinjury and 5 h postinjury attenuated neuropathic pain and glial activation. Application of whole-body hypothermia preinjury and 5 h postinjury provided a similar therapeutic effect. However, whole-body hypothermia, but not regional hypothermia, applied 1, 3, and 5 days postinjury attenuated glial activation and neuropathic pain. Similarly, on days 1, 3, and 5 postinjury, only whole-body hypothermia was effective in decreasing proinflammatory cytokine levels. The increase in injury discharge observed after CCI could be suppressed by regional or whole-body hypothermia at different stages of nerve injury. At the early stage following nerve injury, regional and whole-body hypothermia suppresses ectopic discharges, and consequently inhibits glial activation and neuropathic pain. At the later stage, pain processing is mediated mainly by cytokines released from activated microglia; therefore, only whole-body hypothermia is effective in modulating pain.

Dorsal Root Ganglion Application of Muscimol Prevents Hyperalgesia and Stimulates Myelin Protein Expression After Sciatic Nerve Injury in Rats

Peripheral nerve injuries may result in debilitating pain that is poorly responsive to conventional treatment. Neuropathic pain induced by peripheral nerve injury is caused, in part, by ectopic discharges from the injury site or the dorsal root ganglia (DRG) resulting in enhanced central input and central hyperexcitability. A heterogeneous family of γ-aminobutyric acid (GABA)(A) channels is important in quieting neuronal excitability. We have recently reported that in vivo modulation of GABAergic neurons in DRG can alter the course of neuropathic pain development after peripheral nerve injury. It seems that direct application of a potent GABA(A) agonist, muscimol, to the ipsilateral DRG prevents the development of hyperalgesia in rats subjected to a sciatic nerve crush injury. In addition to potentially curtailing hyperexcitability, GABAergic stimulation upregulated expression of peripheral myelin protein 22 (PMP22), a key component of the basal lamina. PMP22 expression correlates with peripheral myelin formation and nerve regeneration. Because of the importance of PMP22 for the formation and stability of myelin, and the fact that PMP22 expression could be GABAergically modulated, we examined whether direct DRG application of muscimol can restore PMP22 protein expression and the integrity of nerve fibers after crush injury of a sciatic nerve. Using adult female rats and a crush injury model, we found that GABAergic modulation in the ipsilateral DRG restores PMP22 protein expression in the distal segment of the sciatic nerve and improves myelin stability in the basal membrane of nerve fibers, thus giving the morphological appearance of lessened nerve injury or faster nerve fiber regeneration. Both the enhanced PMP22 protein expression and morphological improvements coincide with the abolishment of thermal and mechanical hypersensitivity. The DRG could be a promising therapeutic target in nerve regeneration and pain alleviation after crush injury of a myelinated peripheral nerve.

Emerging role of microglial kinin B1 receptor in diabetic pain neuropathy

Nowadays diabetes mellitus has reached epidemic level and is considered as the primary cause of foot amputation and pain neuropathy. The classical theories explaining the development of diabetic pain neuropathy include the imbalance of neuronal biochemical pathways (Polyol pathway, Na+/K+ ATPase pump, AGE, ROS) and microangiopathy which promote nerve fibers depolarization, sensitization, ectopic discharges, demyelization and ultimately neuronal death. However, the current pharmacotherapy targeting those pathways brings variable, not always satisfactory and temporal relief in patients experiencing diabetic pain neuropathy. Interestingly, recent research in animal models yielded compelling evidence that glial cells, mainly microglia, play a critical role in the mediation of diabetic pain facilitation mechanisms. Preventing microglia activation could therefore be considered as a potential therapeutic target. The lack of specific agents inhibiting microglia activity remains, however, a major obstacle for further treatment in diabetic patients. An alternative and new strategy would be the targeting of key mediators involved in microglia activation, migration and the subsequent release of pro-inflammatory substances contributing to neuronal hyperexitability. The present review summarizes recent evidence that the kinin B1 receptor (B1R) may represent such a target of potential value for new medicines in the treatment of chronic pain. A few selective B1R antagonists have been fully characterized in animal models although small molecules orally active are urgently needed for targeting human B1R on CNS microglia. Thus far, the pharmacological blockade of kinin B1R in various animal paradigms or its genetic deletion in B1R knock-out mice failed to cause unwanted side effects, making this approach feasible. This is consistent with the highly inducible feature of this atypical G-protein coupled receptor whose expression can be seen as the alarming signature of immune and inflammatory diseases, notably diabetes mellitus.

Microneurographic identification of spontaneous activity in C-nociceptors in neuropathic pain states in humans and rats

C-nociceptors do not normally fire action potentials unless challenged by adequate noxious stimuli. However, in pathological states nociceptors may become hyperexcitable and may generate spontaneous ectopic discharges. The aim of this study was to compare rat neuropathic pain models and to assess their suitability to model the spontaneous C-nociceptor activity found in neuropathic pain patients. Studies were performed in normal rats (n = 40), healthy human subjects (n = 15), peripheral neuropathic pain patients (n = 20), and in five rat neuropathic pain models: nerve crush (n = 24), suture (n = 14), chronic constriction injury (n = 12), STZ-induced diabetic neuropathy (n = 56), and ddC-induced neuropathy (n = 15). Microneurographic recordings were combined with electrical stimulation to monitor activity in multiple C fibers. Stimulation at 0.25 Hz allowed spontaneous impulses to be identified by fluctuations in baseline latency. Abnormal latency fluctuations could be produced by several mechanisms, and spontaneous activity was most reliably identified by the presence of unexplained latency increases corresponding to two or more additional action potentials. Spontaneous activity was present in a proportion of mechano-insensitive C-nociceptors in the patients and all rat models. The three focal traumatic nerve injury models provided the highest proportion (59.5%), whereas the two polyneuropathy models had fewer (18.6%), and the patients had an intermediate proportion (33.3%). Spontaneously active mechano-sensitive C-nociceptors were not recorded. Microneurographic recordings of spontaneous activity in diseased C-nociceptors may be useful for both short- and long-term drug studies, both in animals and in humans.

The effect of a monoclonal antibody to calcitonin-gene related peptide (CGRP) on injury-induced ectopic discharge following lingual nerve injury

The development of ectopic neural discharge at a site of peripheral nerve injury is thought to contribute to the initiation of sensory disturbances and pain. We have previously shown that this discharge can be initiated or increased by the neuropeptide calcitonin gene-related peptide (CGRP). We have now studied a potential therapeutic approach to reducing the discharge by evaluating the effect of a systemically administered monoclonal antibody to CGRP on injury-induced activity in the lingual nerve. In 16 anaesthetised adult ferrets the left lingual nerve was sectioned. One day after the injury, the animals received a subcutaneous injection of either a monoclonal antibody to CGRP or a vehicle control. Three days after the injury, under a second anaesthetic, single-unit electrophysiological recordings were made from central to the injury site (469 and 391 units were analysed in antibody and vehicle groups, respectively), and the proportion of units that were spontaneously active was determined. In the vehicle-treated animals 6.4±2.7 [SEM]% of the units were spontaneously active, with conduction velocities of 8.8–40.8m/s and discharge frequencies of 0.03–2.7Hz. In the monoclonal antibody-treated animals 5.7±2.0% of the units were spontaneously active, with conduction velocities of 13.9–38.8m/s and discharge frequencies of 0.07–1.8Hz. There was no significant difference between these two groups (for spontaneous activity and conduction velocity: p>0.05, Student's t-test; for discharge frequency: p>0.05, Mann–Whitney test), suggesting that the spontaneous activity initiated by a nerve injury cannot be modulated by administration of a monoclonal antibody to CGRP.

F250 MANAGEMENT OF NEUROPATHIC PAIN WITH METHYLPREDNISOLONE AT THE SITE OF NERVE INJURY

Objective. Peripheral nerve blocks with methylprednisolone may provide effective pain therapy by decreasing ectopic neuronal discharge and the release of local inflammatory mediators at the site of nerve injury. In this study, we aimed to compare the efficacy of lidocaine alone with a combination of depo-methylprednisolone plus lidocaine in the management of neuropathic pain due to peripheral nerve damage. Design. Randomized, double-blind comparator trial Setting. Group control (N = 44) received 0.5% lidocaine and group methylprednisolone (N = 44) received 80 mg depo-methylprednisolone + 0.5% lidocaine proximal to the site of nerve injury with a total amount of 10–20 mL solution according to the type of peripheral nerve block with nerve stimulator. Outcome Measures. Demographic data, preblock numerical rating scales (NRSs), the Leeds assessment of neuropathic symptoms and signs (LANSS0) score, accompanying symptoms, and analgesic requirements were recorded. Postblock NRS scores were noted following peripheral nerve block and after 3 months. LANSS1, accompanying symptoms, and analgesic requirements were also reevaluated 3 months after the injection. Results. Demographic data, preblock NRS (8 1.5 and 8.1 1.2, respectively), postblock NRS (2.1 1.2 and 2.4 1.4, respectively), LANSS0 (18.4 2.2 and 18.2 2.1, respectively), and accompanying symptoms were comparable between groups. Scores for the methylprednisolone group were significantly improved at 3-month postblock for NRS (2 1.4 vs 5.2 1.7) and LANSS1 scores (4.14 2.7 vs 14.1 2.8), accompanying symptoms, and analgesic requirements (P < 0.0001). Conclusions. Our results suggest that peripheral nerve block with 80 mg depo-methylprednisolone plus 0.5% lidocaine provides effective management in the treatment of neuropathic pain due to peripheral nerve damage.

TRPV1, TRPA1, and CB1 in the isolated vagus nerve – Axonal chemosensitivity and control of neuropeptide release

Vagal sensory afferents innervating airways and abdominal tissues express TRPV1 and TRPA1, two depolarizing calcium permeable ion channels playing a major role in sensing environmental irritants and endogenous metabolites which cause neuropeptide release and neurogenic inflammation. Here we have studied axonal chemosensitivity and control of neuropeptide release from the isolated rat and mouse vagus nerve by using prototypical agonists of these transduction channels - capsaicin, mustard oil and the specific endogenous activators, anandamide (methyl arachidonyl ethanolamide, mAEA), and acrolein, respectively. Capsaicin evoked iCGRP release from the rat vagus nerve with an EC₅₀ of 0.12 μM. Co-application of mAEA had a dual effect: nanomolar concentrations of mAEA (0.01 μM) significantly reduced capsaicin-evoked iCGRP release while concentrations ≥ 1 μM mAEA had sensitizing effects. Only 100 μM mAEA directly augmented iCGRP release by itself. In the mouse, 310 μM mAEA increased release in wildtype and TRPA1-/- mice which could be inhibited by capsazepine (10 μM) and was completely absent in TRPV1-/- mice. CB1-/- and CB1/CB2 double -/- mice equally displayed increased sensitivity to mAEA (100 μM) and a sensitizing effect to capsaicin, in contrast to wildtypes. Acrolein and mustard oil (MO)--at μM concentrations--induced a TRPA1-dependent iCGRP release; however, millimolar concentrations of mustard oil (>1mM) evoked iCGRP release by activating TRPV1, confirming recent evidence for TRPV1 agonism of high mustard oil concentrations. Taken together, we present evidence for functional expression of excitatory TRPV1, TRPA1, and inhibitory CB1 receptors along the sensory fibers of the vagus nerve which lend pathophysiological relevance to the axonal membrane and the control of neuropeptide release that may become important in cases of inflammation or neuropathy. Sensitization and possible ectopic discharge may contribute to the development of autonomic dysregulation in visceral tissues that are innervated by the vagus nerve.

Fasciculations and their F-response revisited: High-density surface EMG in ALS and benign fasciculations

Objective To compare the prevalence of fasciculation potentials (FPs) with F-responses between patients with amyotrophic lateral sclerosis (ALS) and patients with benign fasciculations. Methods In seven patients with ALS and seven patients with benign fasciculations, high-density surface EMG was recorded for 15 min from the gastrocnemius muscle. Template matching was used to search for pairs of FPs with a repetition within 10–110 ms. Results Interspike interval (ISI) histograms were constructed from 282 pairs of benign fasciculations and from 337 FP pairs in ALS. Peaks attributable to F-waves were found at latencies of 32 ms (benign) and 35 ms (ALS). Five patients with benign fasciculations and four patients with ALS had FPs with F-waves. Conclusions F-waves of FPs occur in both conditions – therefore they are not diagnostically helpful. Significance F-waves confirm the distal origin of FPs for an individual axon. The occurrence of these FPs in a benign condition suggests that the generation of ectopic discharges in the distal axons is not specific to progressive neurodegeneration.

Complex symptoms of demyelination and nerve damage explained by nonlinear dynamical analysis of conductance-based models

Multiple sclerosis and other demyelination diseases are associated with positive (gain of function), negative (loss of function), and paroxysmal (sudden and short duration) symptoms. These symptoms occur in a remarkable diversity of modalities including prolonged or, in the case of paroxysmal symptoms, brief episodes of pain, paresthesia (numbness), dysarthria (difficulty speaking), weakness, muscle spasms, etc. [1]. Using computational modeling, we have recently established the mechanisms underlying four distinct phases of action potential (AP) firing associated with demyelination and secondary compensatory changes [2]. Transitions in axonal excitability caused by varying the ratio of gNa and gL indicate that a single mechanism controlling conductance balance is sufficient to explain normal spiking, failed conduction, afterdischarge (AD) and spontaneous activity, and can thus account for the full range of negative, paroxysmal and tonic positive symptoms, respectively. Positive symptoms are thought to arise from ectopic spike discharge, but the biophysical mechanisms responsible for the abrupt onset and self limiting duration of paroxysmal AD are poorly understood and were thus a particular area of focus. We used two types of complementary models. The first was a Hodgkin-Huxley (HH), multi-compartment model of a focally demyelinated axon created using the NEURON Simulator (http://www.neuron.yale.edu/neuron, [3]) based on a geometrically simplified model to first understand the behaviors of electrical conductances. The second was a modified Morris-Lecar, single compartment model [4] that has been described in detail [5]. We explored this model with phase-plane and bifurcation analysis using XPPAUT in order to rigorously characterize the dynamical mechanisms underlying phenomena identified in the HH model. In both models, the paroxysmal AD can only occur when the system is bi-stable and initiation occurs when a perturbation abruptly switches the system between attractor states. Termination of paroxysms occurs when internal dynamics shift the basins of attraction until the system falls back to its original stable state. Temporal summation and refractoriness can also be simply explained within this framework. Disruption of the oligodendrocytes that form the myelin sheaths may lead to abnormal electrolyte homeostasis in the vicinity of the denuded axon. Our models indicate that the pathological accumulation of Na + or K + can influence the duration of paroxysmal firing by acting as an additional, ultra-slow feedback mechanism in the dynamical system. As such, a naked patch of previously myelinated axonal membrane becomes capable of competing with the same neuron’s soma for the initiation of burst firing patterns and interferes with normal firing patterns. Although not biologically detailed, our modeling nonetheless provides significant insight into how biologically realistic mechanisms can interact to produce neuronal discharges as well as other abnormal firing characteristics in the course of demyelination.

Eugenol reverses mechanical allodynia after peripheral nerve injury by inhibiting hyperpolarization-activated cyclic nucleotide-gated (HCN) channels

Mechanical allodynia is a common symptom found in neuropathic patients. Hyperpolarization-activated cyclic nucleotide-gated channels and their current, I h, have been suggested to play an important role in neuropathic pain, especially in mechanical allodynia and spontaneous pain, by involvement in spontaneous ectopic discharges after peripheral nerve injury. Thus, I h blockers may hold therapeutic potential for the intervention of mechanical allodynia under diverse neuropathic conditions. Here we show that eugenol blocks I h and abolishes mechanical allodynia in the trigeminal system. Eugenol produced robust inhibition of I h with IC 50 of 157 μM in trigeminal ganglion (TG) neurons, which is lower than the dose of eugenol that inhibits voltage-gated Na channels. Eugenol-induced I h inhibition was not mediated by G i/o-protein activation, but was gradually diminished by an increase in intracellular cAMP concentration. Eugenol also inhibited I h from injured TG neurons which were identified by retrograde labeling with DiI and reversed mechanical allodynia in the orofacial area after chronic constriction injury of infraorbital nerve. We propose that eugenol could be potentially useful for reversing mechanical allodynia in neuropathic pain patients.

Sensory Symptom Profiles and Co-Morbidities in Painful Radiculopathy

Painful radiculopathies (RAD) and classical neuropathic pain syndromes (painful diabetic polyneuropathy, postherpetic neuralgia) show differences how the patients express their sensory perceptions. Furthermore, several clinical trials with neuropathic pain medications failed in painful radiculopathy. Epidemiological and clinical data of 2094 patients with painful radiculopathy were collected within a cross sectional survey (painDETECT) to describe demographic data and co-morbidities and to detect characteristic sensory abnormalities in patients with RAD and compare them with other neuropathic pain syndromes. Common co-morbidities in neuropathic pain (depression, sleep disturbance, anxiety) do not differ considerably between the three conditions. Compared to other neuropathic pain syndromes touch-evoked allodynia and thermal hyperalgesia are relatively uncommon in RAD. One distinct sensory symptom pattern (sensory profile), i.e., severe painful attacks and pressure induced pain in combination with mild spontaneous pain, mild mechanical allodynia and thermal hyperalgesia, was found to be characteristic for RAD. Despite similarities in sensory symptoms there are two important differences between RAD and other neuropathic pain disorders: (1) The paucity of mechanical allodynia and thermal hyperalgesia might be explained by the fact that the site of the nerve lesion in RAD is often located proximal to the dorsal root ganglion. (2) The distinct sensory profile found in RAD might be explained by compression-induced ectopic discharges from a dorsal root and not necessarily by nerve damage. These differences in pathogenesis might explain why medications effective in DPN and PHN failed to demonstrate efficacy in RAD.