Ionic mechanisms underlying inflammatory mediator-induced sensitization of dural afferents

Electroacupuncture promotes macrophage/microglial M2 polarization and suppresses inflammatory pain through AMPK

Inflammatory pain, the most prevalent disease globally, remains challenging to manage. Electroacupuncture emerges as an effective therapy, yet its underlying mechanisms are not fully understood. This study investigates whether adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK)-regulated silent information regulator 1 (SIRT1) contributes to electroacupuncture's antinociceptive effects by modulating macrophage/microglial polarization in the spinal dorsal horn of a mouse model of inflammatory pain. In this study, mice, introduced to inflammatory pain through subcutaneous injections of complete freund's adjuvant (CFA) in the plantar area, underwent electroacupuncture therapy every alternate day for 30-min sessions. The assessment of mechanical allodynia and thermal hyperalgesia in these subjects was carried out using paw withdrawal frequency and paw withdrawal latency measurements, respectively. Western blot analysis measured levels of AMPK, phosphorylation-adenosine 5'-monophosphate (AMP)-activated protein kinase, SIRT1, inducible nitric oxide synthase, cluster of differentiation 86, arginase 1, and interleukin 10. In contrast to the group treated solely with CFA, the cohort receiving both CFA and electroacupuncture demonstrated notable decreases in both thermal hyperalgesia and mechanical allodynia. This was accompanied by a marked enhancement in AMPK phosphorylation levels. AMPK knockdown reversed electroacupuncture's analgesic effects and reduced M2 macrophage/microglial polarization enhancement. Additionally, AMPK knockdown significantly weakened electroacupuncture-induced SIRT1 upregulation, and EX-527 injection attenuated electroacupuncture's facilitation of M2 macrophage/microglial polarization without affecting AMPK phosphorylation levels. Furthermore, combining electroacupuncture with SRT1720 enhanced the analgesic effect of SRT1720. Our findings suggest that AMPK regulation of SIRT1 plays a critical role in electroacupuncture's antinociceptive effect through the promotion of M2 macrophage/microglial polarization.

Inhibition of persistent sodium current reduces spreading depression-evoked allodynia in a mouse model of migraine with aura

ABSTRACT

We investigated the efficacy of inhibiting persistent Na + currents (I NaP ) in acute rodent models of migraine with aura. Cortical spreading depression (SD) is a slow wave of neuronal and glial depolarization that underlies the migraine aura. Minimally invasive optogenetic SD (opto-SD) causes periorbital mechanical allodynia in mice, suggesting SD activates trigeminal nociceptors. Persistent Na + currents contribute to neuronal intrinsic excitability and have been implicated in peripheral and cortical excitation. We examined a preferential inhibitor of I NaP, GS-458967, on SD-induced periorbital allodynia, SD susceptibility, and formalin-induced peripheral pain. Periorbital mechanical allodynia was tested in male and female Thy1-ChR2-YFP mice after a single opto-SD event using manual von Frey monofilaments. GS-458967 (1 mg/kg, s.c.) or vehicle was dosed immediately after opto-SD induction, and allodynia was tested 1 hour later. The electrical SD threshold and KCl-induced SD frequency were examined in the cortex in male Sprague-Dawley rats after 1 hour pretreatment with GS-458967 (3 mg/kg, s.c.) or vehicle. Effects of GS-458967 (0.5-5 mg/kg, p.o.) on spontaneous formalin hind paw behavior and locomotion were also examined in male CD-1 mice. GS-458967 suppressed opto-SD-induced periorbital allodynia and decreased susceptibility to SD. GS-458967 also diminished early and late phase formalin-induced paw-licking behavior with early phase paw licking responding to lower doses. GS-458967 up to 3 mg/kg had no impact on locomotor activity. These data provide evidence that I NaP inhibition can reduce opto-SD-induced trigeminal pain behavior and support I NaP inhibition as an antinociceptive strategy for both abortive and preventive treatment of migraine.

Inhibiting neuronal AC1 for treating anxiety and headache in the animal model of migraine

Migraines are a common medical condition. From a basic science point of view, the central mechanism for migraine and headache is largely unknown. In the present study, we demonstrate that cortical excitatory transmission is significantly enhanced in the anterior cingulate cortex (ACC)-a brain region which is critical for pain perception. Biochemical studies found that the phosphorylation levels of both the NMDA receptor GluN2B and AMPA receptor GluA1 were enhanced in ACC of migraine rats. Both the presynaptic release of glutamate and postsynaptic responses of AMPA receptors and NMDA receptors were enhanced. Synaptic long-term potentiation (LTP) was occluded. Furthermore, behavioral anxiety and nociceptive responses were increased, which were reversed by application of AC1 inhibitor NB001 within ACC. Our results provide strong evidence that cortical LTPs contribute to migraine-related pain and anxiety. Drugs that inhibit cortical excitation such as NB001 may serve as potential medicines for treating migraine in the future.

Anatomy and Physiology of Headache

Headache disorders can produce recurrent, incapacitating pain. Migraine and cluster headache are notable for their ability to produce significant disability. The anatomy and physiology of headache disorders is fundamental to evolving treatment approaches and research priorities. Key concepts in headache mechanisms include activation and sensitization of trigeminovascular, brainstem, thalamic, and hypothalamic neurons; modulation of cortical brain regions; and activation of descending pain circuits. This review will examine the relevant anatomy of the trigeminal, brainstem, subcortical, and cortical brain regions and concepts related to the pathophysiology of migraine and cluster headache disorders.

Contribution of tetrodotoxin-resistant persistent Na+ currents to the excitability of C-type dural afferent neurons in rats

BACKGROUND

Growing evidence supports the important role of persistent sodium currents (I_NaP) in the neuronal excitability of various central neurons. However, the role of tetrodotoxin-resistant (TTX-R) Na⁺ channel-mediated I_NaP in the neuronal excitability of nociceptive neurons remains poorly understood.

METHODS

We investigated the functional role of TTX-R I_NaP in the excitability of C-type nociceptive dural afferent neurons, which was identified using a fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchloride (DiI), and a whole-cell patch-clamp technique.

RESULTS

TTX-R I_NaP were found in most DiI-positive neurons, but their density was proportional to neuronal size. Although the voltage dependence of TTX-R Na⁺ channels did not differ among DiI-positive neurons, the extent of the onset of slow inactivation, recovery from inactivation, and use-dependent inhibition of these channels was highly correlated with neuronal size and, to a great extent, the density of TTX-R I_NaP. In the presence of TTX, treatment with a specific I_NaP inhibitor, riluzole, substantially decreased the number of action potentials generated by depolarizing current injection, suggesting that TTX-R I_NaP are related to the excitability of dural afferent neurons. In animals treated chronically with inflammatory mediators, the density of TTX-R I_NaP was significantly increased, and it was difficult to inactivate TTX-R Na⁺ channels.

CONCLUSIONS

TTX-R I_NaP apparently contributes to the differential properties of TTX-R Na⁺ channels and neuronal excitability. Consequently, the selective modulation of TTX-R I_NaP could be, at least in part, a new approach for the treatment of migraine headaches.

Prolonged PGE2 treatment increased TTX-sensitive but not TTX-resistant sodium current in trigeminal ganglionic neurons

Prostaglandin E₂ (PGE₂) is an important inflammatory mediator for the initiation and maintenance of inflammatory and neuropathic pain. The acute effect of PGE₂ on sodium currents has been widely characterized in sensory neurons; however, the prolonged effect of PGE₂ remains to be determined. Here, we performed patch clamp recordings to evaluate the acute and prolonged effects of PGE₂ on sodium currents in trigeminal ganglionic (TG) neurons from male Sprague-Dawley rats. We found that 24-h treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 31% in a voltage-dependent manner and shifted the activation curve in a hyperpolarized direction but did not affect steady-state inactivation. Furthermore, treatment with PGE₂ for 24 h increased the current density of tetrodotoxin-sensitive (TTX-S) but not TTX-resistant (TTX-R) channels significantly. Interestingly, TTX-S current was increased mostly in medium-sized, but not in small-sized, neurons after 24 h of treatment with PGE₂. Moreover, the mRNA level of TTX-S Nav1.1 but not TTX-R Nav1.8 or Nav1.9 was significantly increased after 24 h of treatment with PGE_2. In contrast, 5-min treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 29% and increased TTX-R sodium currents, but not TTX-S currents, in both small- and medium-sized TG neurons. Our results presented a differential regulation of subtypes of sodium channels by acute and prolonged treatments of PGE₂, which may help to better understand the mechanism of PGE₂-mediated orofacial pain development.

Cortical mechanisms in migraine

Migraine is the second most prevalent disorder in the world; yet, its underlying mechanisms are still poorly understood. Cumulative studies have revealed pivotal roles of cerebral cortex in the initiation, propagation, and termination of migraine attacks as well as the interictal phase. Investigation of basic mechanisms of the cortex in migraine not only brings insight into the underlying pathophysiology but also provides the basis for designing novel treatments. We aim to summarize the current research literatures and give a brief overview of the cortex and its role in migraine, including the basic structure and function; structural, functional, and biochemical neuroimaging; migraine-related genes; and theories related to cortex in migraine pathophysiology. We propose that long-term plasticity of synaptic transmission in the cortex encodes migraine.

Modeling a Nociceptive Neuro-Immune Synapse Activated by ATP and 5-HT in Meninges: Novel Clues on Transduction of Chemical Signals Into Persistent or Rhythmic Neuronal Firing

Extracellular ATP and serotonin (5-HT) are powerful triggers of nociceptive firing in the meninges, a process supporting headache and whose cellular mechanisms are incompletely understood. The current study aimed to develop, with the neurosimulator NEURON, a novel approach to explore in silico the molecular determinants of the long-lasting, pulsatile nature of migraine attacks. The present model included ATP and 5-HT release, ATP diffusion and hydrolysis, 5-HT uptake, differential activation of ATP P2X or 5-HT3 receptors, and receptor subtype-specific desensitization. The model also tested the role of branched meningeal fibers with multiple release sites. Spike generation and propagation were simulated using variable contribution by potassium and sodium channels in a multi-compartment fiber environment. Multiple factors appeared important to ensure prolonged nociceptive firing potentially relevant to long-lasting pain. Crucial roles were observed in: (i) co-expression of ATP P2X2 and P2X3 receptor subunits; (ii) intrinsic activation/inactivation properties of sodium Nav1.8 channels; and (iii) temporal and spatial distribution of ATP/5-HT release sites along the branches of trigeminal nerve fibers. Based on these factors we could obtain either persistent activation of nociceptive firing or its periodic bursting mimicking the pulsating nature of pain. In summary, our model proposes a novel tool for the exploration of peripheral nociception to test the contribution of clinically relevant factors to headache including migraine pain.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

Animal models of migraine and experimental techniques used to examine trigeminal sensory processing

Background

Migraine is a common debilitating condition whose main attributes are severe recurrent headaches with accompanying sensitivity to light and sound, nausea and vomiting. Migraine-related pain is a major cause of its accompanying disability and can encumber almost every aspect of daily life.

Main body

Advancements in our understanding of the neurobiology of migraine headache have come in large from basic science research utilizing small animal models of migraine-related pain. In this current review, we aim to describe several commonly utilized preclinical models of migraine. We will discuss the diverse array of methodologies for triggering and measuring migraine-related pain phenotypes and highlight briefly specific advantages and limitations therein. Finally, we will address potential future challenges/opportunities to refine existing and develop novel preclinical models of migraine that move beyond migraine-related pain and expand into alternate migraine-related phenotypes.

Conclusion

Several well validated animal models of pain relevant for headache exist, the researcher should consider the advantages and limitations of each model before selecting the most appropriate to answer the specific research question. Further, we should continually strive to refine existing and generate new animal and non-animal models that have the ability to advance our understanding of head pain as well as non-pain symptoms of primary headache disorders.

From Mechanism to Cure: Renewing the Goal to Eliminate the Disease of Pain

Objective

Persistent pain causes untold misery worldwide and is a leading cause of disability. Despite its astonishing prevalence, pain is undertreated, at least in part because existing therapeutics are ineffective or cause intolerable side effects. In this review, we cover new findings about the neurobiology of pain and argue that all but the most transient forms of pain needed to avoid tissue damage should be approached as a disease where a cure can be the goal of all treatment plans, even if attaining this goal is not yet always possible.

Design

We reviewed the literature to highlight recent advances in the area of the neurobiology of pain.

Results

We discuss barriers that are currently hindering the achievement of this goal, as well as the development of new therapeutic strategies. We also discuss innovations in the field that are creating new opportunities to treat and even reverse persistent pain, some of which are in late-phase clinical trials.

Conclusion

We conclude that the confluence of new basic science discoveries and development of new technologies are creating a path toward pain therapeutics that should offer significant hope of a cure for patients and practitioners alike. Classification of Evidence. Our review points to new areas of inquiry for the pain field to advance the goal of developing new therapeutics to treat chronic pain.

Islet-cell autoantigen 69 mediates the antihyperalgesic effects of electroacupuncture on inflammatory pain by regulating spinal glutamate receptor subunit 2 phosphorylation through protein interacting with C-kinase 1 in mice

Supplemental Digital Content is Available in the Text.

A clear role of ICA69 in mediating the antihyperalgesic effects of electroacupuncture was confirmed, and the ICA69-PICK1-GluR2 molecular mechanism to explain these effects is proposed.

Electroacupuncture (EA) is widely used in clinical settings to reduce inflammatory pain. Islet-cell autoantigen 69 (ICA69) has been reported to regulate long-lasting hyperalgesia in mice. ICA69 knockout led to reduced protein interacting with C-kinase 1 (PICK1) expression and increased glutamate receptor subunit 2 (GluR2) phosphorylation at Ser880 in spinal dorsal horn. In this study, we evaluated the role of ICA69 in the antihyperalgesic effects of EA and the underlying mechanism through regulation of GluR2 and PICK1 in spinal dorsal horn. Hyperalgesia was induced in mice with subcutaneous plantar injection of complete Freund adjuvant (CFA) to cause inflammatory pain. Electroacupuncture was then applied for 30 minutes every other day after CFA injection. When compared with CFA group, paw withdrawal frequency of CFA+EA group was significantly decreased. Remarkable increases in Ica1 mRNA expression and ICA69 protein levels on the ipsilateral side were detected in the CFA+EA group. ICA69 expression reached the peak value around day 3. More importantly, ICA69 deletion impaired the antihyperalgesic effects of EA on GluR2-p, but PICK1 deletion could not. Injecting ICA69 peptide into the intrathecal space of ICA69-knockout mice mimicked the effects of EA analgesic and inhibited GluR2-p. Electroacupuncture had no effects on the total protein of PICK1 and GluR2. And, EA could increase the formation of ICA69-PICK1 complexes and decrease the amount of PICK1-GluR2 complexes. Our findings indicate that ICA69 mediates the antihyperalgesic effects of EA on CFA-induced inflammatory pain by regulating spinal GluR2 through PICK1 in mice.

Characterization of dural afferent neurons innervating cranial blood vessels within the dura in rats

Dural afferent neurons are implicated in primary headaches including migraine. Although a significant portion of primary afferent neurons innervating the dura are myelinated A-type neurons, previous electrophysiological studies have primarily characterized the functional properties of small-sized C-type sensory neurons. Here we show the functional characterization of dural afferent neurons identified with the fluorescent dye DiI. DiI-positive neurons were divided into three types: small-, medium-, and large-sized neurons, based on their diameter, area, and membrane capacitance. The immunoreactivity of NF200, a marker of A-type myelinated neurons, was detected in most large-sized, but it was also present in a limited number of small- and medium-sized DiI-positive neurons. Capsaicin, a transient receptor potential vanilloid 1 agonist, induced the membrane currents in most small- and medium-sized neurons, but not in large-sized DiI-positive neurons. Tetrodotoxin-resistant Na⁺ channels were expressed in almost all types of DiI-positive neurons. Mechanosensitive currents were detected from a majority of large-sized, and to a lesser extent, small- and medium-sized DiI-positive neurons. The results suggest that most dural afferent neurons are nociceptive, e.g., polymodal C-type for small- and medium-sized neurons, and high-threshold nociceptive A-type mechanoreceptors for large-sized neurons. We also found that DiI-positive neurons differed with respect to passive and active membrane properties, and that sumatriptan, a representative drug used for the acute treatment of migraine attack, inhibited voltage-gated Ca²⁺ currents in all types of DiI-positive neurons. The present results showing the nociceptive properties of dural afferent neurons would contribute to understand the pathophysiology of primary headaches.

Depolarizing Effectors of Bradykinin Signaling in Nociceptor Excitation in Pain Perception

Inflammation is one of the main causes of pathologic pain. Knowledge of the molecular links between inflammatory signals and pain-mediating neuronal signals is essential for understanding the mechanisms behind pain exacerbation. Some inflammatory mediators directly modulate the excitability of pain-mediating neurons by contacting the receptor molecules expressed in those neurons. For decades, many discoveries have accumulated regarding intraneuronal signals from receptor activation through electrical depolarization for bradykinin, a major inflammatory mediator that is able to both excite and sensitize pain-mediating nociceptor neurons. Here, we focus on the final effectors of depolarization, the neuronal ion channels, whose functionalities are specifically affected by bradykinin stimulation. Particular G-protein coupled signaling cascades specialized for each specific depolarizer ion channels are summarized. Some of these ion channels not only serve as downstream effectors but also play critical roles in relaying specific pain modalities such as thermal or mechanical pain. Accordingly, specific pain phenotypes altered by bradykinin stimulation are also discussed. Some members of the effector ion channels are both activated and sensitized by bradykinin-induced neuronal signaling, while others only sensitized or inhibited, which are also introduced. The present overview of the effect of bradykinin on nociceptor neuronal excitability at the molecular level may contribute to better understanding of an important aspect of inflammatory pain and help future design of further research on the components involved and pain modulating strategies.

Action Potential Broadening in Capsaicin-Sensitive DRG Neurons from Frequency-Dependent Reduction of Kv3 Current

Action potential (AP) shape is a key determinant of cellular electrophysiological behavior. We found that in small-diameter, capsaicin-sensitive dorsal root ganglia neurons corresponding to nociceptors (from rats of either sex), stimulation at frequencies as low as 1 Hz produced progressive broadening of the APs. Stimulation at 10 Hz for 3 s resulted in an increase in AP width by an average of 76 ± 7% at 22°C and by 38 ± 3% at 35°C. AP clamp experiments showed that spike broadening results from frequency-dependent reduction of potassium current during spike repolarization. The major current responsible for frequency-dependent reduction of overall spike-repolarizing potassium current was identified as Kv3 current by its sensitivity to low concentrations of 4-aminopyridine (IC₅₀ <100 μm) and block by the peptide inhibitor blood depressing substance I (BDS-I). There was a small component of Kv1-mediated current during AP repolarization, but this current did not show frequency-dependent reduction. In a small fraction of cells, there was a component of calcium-dependent potassium current that showed frequency-dependent reduction, but the contribution to overall potassium current reduction was almost always much smaller than that of Kv3-mediated current. These results show that Kv3 channels make a major contribution to spike repolarization in small-diameter DRG neurons and undergo frequency-dependent reduction, leading to spike broadening at moderate firing frequencies. Spike broadening from frequency-dependent reduction in Kv3 current could mitigate the frequency-dependent decreases in conduction velocity typical of C-fiber axons.SIGNIFICANCE STATEMENT Small-diameter dorsal root ganglia (DRG) neurons mediating nociception and other sensory modalities express many types of potassium channels, but how they combine to control firing patterns and conduction is not well understood. We found that action potentials of small-diameter rat DRG neurons showed spike broadening at frequencies as low as 1 Hz and that spike broadening resulted predominantly from frequency-dependent inactivation of Kv3 channels. Spike width helps to control transmitter release, conduction velocity, and firing patterns and understanding the role of particular potassium channels can help to guide new pharmacological strategies for targeting pain-sensing neurons selectively.

Voltage-gated Na+ currents in human dorsal root ganglion neurons

Available evidence indicates voltage-gated Na⁺ channels (VGSCs) in peripheral sensory neurons are essential for the pain and hypersensitivity associated with tissue injury. However, our understanding of the biophysical and pharmacological properties of the channels in sensory neurons is largely based on the study of heterologous systems or rodent tissue, despite evidence that both expression systems and species differences influence these properties. Therefore, we sought to determine the extent to which the biophysical and pharmacological properties of VGSCs were comparable in rat and human sensory neurons. Whole cell patch clamp techniques were used to study Na⁺ currents in acutely dissociated neurons from human and rat. Our results indicate that while the two major current types, generally referred to as tetrodotoxin (TTX)-sensitive and TTX-resistant were qualitatively similar in neurons from rats and humans, there were several differences that have important implications for drug development as well as our understanding of pain mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.23235.001

A PTEN-Regulated Checkpoint Controls Surface Delivery of δ Opioid Receptors

The δ opioid receptor (δR) is a promising alternate target for pain management because δR agonists show decreased abuse potential compared with current opioid analgesics that target the μ opioid receptor. A critical limitation in developing δR as an analgesic target, however, is that δR agonists show relatively low efficacy in vivo, requiring the use of high doses that often cause adverse effects, such as convulsions. Here we tested whether intracellular retention of δR in sensory neurons contributes to this low δR agonist efficacy in vivo by limiting surface δR expression. Using direct visualization of δR trafficking and localization, we define a phosphatase and tensin homolog (PTEN)-regulated checkpoint that retains δR in the Golgi and decreases surface delivery in rat and mice sensory neurons. PTEN inhibition releases δR from this checkpoint and stimulates delivery of exogenous and endogenous δR to the neuronal surface both in vitro and in vivo PTEN inhibition in vivo increases the percentage of TG neurons expressing δR on the surface and allows efficient δR-mediated antihyperalgesia in mice. Together, we define a critical role for PTEN in regulating the surface delivery and bioavailability of the δR, explain the low efficacy of δR agonists in vivo, and provide evidence that active δR relocation is a viable strategy to increase δR antinociception.SIGNIFICANCE STATEMENT Opioid analgesics, such as morphine, which target the μ opioid receptor (μR), have been the mainstay of pain management, but their use is highly limited by adverse effects and their variable efficacy in chronic pain. Identifying alternate analgesic targets is therefore of great significance. Although the δ opioid receptor (δR) is an attractive option, a critical limiting factor in developing δR as a target has been the low efficacy of δR agonists. Why δR agonists show low efficacy is still under debate. This study provides mechanistic and functional data that intracellular localization of δR in neurons is a key factor that contributes to low agonist efficacy, and presents a proof of mechanism that relocating δR improves efficacy.

Paclitaxel-induced increase in mitochondrial volume mediates dysregulation of intracellular Ca2+ in putative nociceptive glabrous skin neurons from the rat

We have recently demonstrated that in a rat model of chemotherapy-induced peripheral neuropathy (CIPN), there is a significant decrease in the duration of the depolarization-evoked Ca²⁺ transient in small diameter, IB4+, and capsaicin-responsive neurons innervating the glabrous skin of the hindpaw. This change was specific to the transient duration and significantly smaller if not undetectable in neurons innervating the dorsal skin of the hindpaw or the skin of the inner thigh. Given the importance of mitochondria in intracellular Ca²⁺ regulation and the findings of chemotherapy-associated increase in mitotoxicity along the sensory neuron axons, we hypothesized that CIPN is due to both increases and decreases in mitochondria function, with changes manifest in distinct subpopulations of afferents. To begin to test this hypothesis, we used confocal microscopy and Ca²⁺ imaging in combination with pharmacological manipulations to study paclitaxel-induced changes in retrograde tracer-labeled neurons from naïve, vehicle-treated, and paclitaxel-treated rats. Paclitaxel treatment was not associated with decreased mitochondrial membrane potential or increased superoxide levels in the somata of putative nociceptive glabrous skin neurons. However, it was associated with significant increases in the relative contribution of mitochondria to the control of the evoked Ca²⁺ transient duration in putative nociceptive glabrous skin neurons, as well as increases in mitotracker and Tom20 staining which reflected an increase in mitochondrial volume. Furthermore, the relative contribution of the sarco-endoplasmic reticulum Ca²⁺ ATPase to the regulation of the duration of the depolarization evoked Ca²⁺ transient was also increased in this subpopulation of neurons from paclitaxel treated rats. Our results indicate that the paclitaxel-induced decrease in the duration of the evoked Ca²⁺ transient is due to both direct and indirect influences of mitochondria. It remains to be determined if and how these changes contribute to the manifestation of CIPN.

Diet-induced obesity alters dural CGRP release and potentiates TRPA1-mediated trigeminovascular responses

Background Clinical studies suggest a link between obesity and the primary headache disorder migraine. In our study we aimed to reveal the effect of obesity on meningeal nociceptor function in rats receiving a high-fat, high-sucrose diet. Methods Transient receptor potential ankyrin 1 (TRPA1) receptor activation-induced changes in meningeal blood flow, release of calcitonin gene-related peptide (CGRP) from trigeminal afferents and TRPA1 protein expression in the trigeminal ganglia were measured in control and obese rats. Metabolic parameters of the animals were assessed by measuring glucose and insulin homeostasis as well as plasma cytokine concentrations. Results The present experiments revealed an enhanced basal and TRPA1 receptor agonist-induced CGRP release from meningeal afferents of obese insulin-resistant rats and an attenuated CGRP release to potassium chloride. Obesity was also associated with an augmented vasodilatation in meningeal arteries after dural application of the TRPA1 agonist acrolein, a reduction in TRPA1 protein expression in the trigeminal ganglia and elevations in circulating proinflammatory cytokines IL-1β and IL-6 in addition to increased fasting blood glucose and insulin concentrations. Conclusions Our results suggest trigeminal sensitisation as a mechanism for enhanced headache susceptibility in obese individuals after chemical exposure of trigeminal nociceptors.

Sensory neuron subpopulation-specific dysregulation of intracellular calcium in a rat model of chemotherapy-induced peripheral neuropathy

The purpose of the present study was to test the prediction that the unique manifestation of chemotherapeutic-induced peripheral neuropathy (CIPN) would be reflected in a specific pattern of changes in the regulation of the intracellular Ca(2+) concentration ([Ca(2+)]i) in subpopulations of cutaneous neurons. To test this prediction, we characterized the pattern of changes in mechanical nociceptive threshold associated with paclitaxel administration (2mg/kg, iv, every other day for four days), as well as the impact of target of innervation and paclitaxel treatment on the regulation of [Ca(2+)]i in subpopulations of putative nociceptive and non-nociceptive neurons. Neurons innervating the glabrous and hairy skin of the hindpaw as well as the thigh were identified with retrograde tracers, and fura-2 was used to assess changes in [Ca(2+)]i. Paclitaxel was associated with a persistent decrease in mechanical nociceptive threshold in response to stimuli applied to the glabrous skin of the hindpaw, but not the hairy skin of the hindpaw or the thigh. However, in both putative nociceptive and non-nociceptive neurons, resting [Ca(2+)]i was significantly lower in neurons innervating the thigh after treatment. The magnitude of the depolarization-evoked Ca(2+) transient was also lower in putative non-nociceptive thigh neurons. More interestingly, while paclitaxel had no detectable influence on either resting or depolarization-evoked Ca(2+) transients in putative non-nociceptive neurons, in putative nociceptive neurons there was a subpopulation-specific decrease in the duration of the evoked Ca(2+) transient that was largely restricted to neurons innervating the glabrous skin. These results suggest that peripheral nerve length alone, does not account for the selective distribution of CIPN symptoms. Rather, they suggest the symptoms of CIPN reflect an interaction between the toxic actions of the therapeutic and unique properties of the neurons deleteriously impacted.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

The properties, distribution and function of Na(+)-Ca(2+) exchanger isoforms in rat cutaneous sensory neurons

The Na(+)-Ca(2+) exchanger (NCX) appears to play an important role in the regulation of the high K(+)-evoked Ca(2+) transient in putative nociceptive dorsal root ganglion (DRG) neurons. The purpose of the present study was to (1) characterize the properties of NCX activity in subpopulations of DRG neurons, (2) identify the isoform(s) underlying NCX activity, and (3) begin to assess the function of the isoform(s) in vivo. In retrogradely labelled neurons from the glabrous skin of adult male Sprague-Dawley rats, NCX activity, as assessed with fura-2-based microfluorimetry, was only detected in putative nociceptive IB4+ neurons. There were two modes of NCX activity: one was evoked in response to relatively large and long lasting (∼325 nm for >12 s) increases in the concentration of intracellular Ca(2+) ([Ca(2+)]i), and a second was active at resting [Ca(2+)]i > ∼150 nm. There also were two modes of evoked activity: one that decayed relatively rapidly (<5 min) and a second that persisted (>10 min). Whereas mRNA encoding all three NCX isoforms (NCX1-3) was detected in putative nociceptive cutaneous neurons with single cell PCR, pharmacological analysis and small interfering RNA (siRNA) knockdown of each isoform in vivo suggested that NCX2 and 3 were responsible for NCX activity. Western blot analyses suggested that NCX isoforms were differentially distributed within sensory neurons. Functional assays of excitability, action potential propagation, and nociceptive behaviour suggest NCX activity has little influence on excitability per se, but instead influences axonal conduction velocity, resting membrane potential, and nociceptive threshold. Together these results indicate that the function of NCX in the regulation of [Ca(2+)]i in putative nociceptive neurons may be unique relative to other cells in which these exchanger isoforms have been characterized and it has the potential to influence sensory neuron properties at multiple levels.

Spinal presynaptic inhibition in pain control

The gate control theory proposed that the nociceptive sensory information transmitted to the brain relies on an interplay between the inputs from nociceptive and non-nociceptive primary afferent fibers. Both inputs are normally under strong inhibitory control in the spinal cord. Under healthy conditions, presynaptic inhibition activated by non-nociceptive fibers modulates the afferent input from nociceptive fibers onto spinal cord neurons, while postsynaptic inhibition controls the excitability of dorsal horn neurons, and silences the non-nociceptive information flow to nociceptive-specific (NS) projection neurons. However, under pathological conditions, this spinal inhibition may be altered and lead to chronic pain. This review summarizes our knowledge of presynaptic inhibition in pain control, with particular focus on how its alteration after nerve or tissue injury contributes to neuropathic or inflammatory pain syndromes, respectively.

Activation of TRPC channels contributes to OA-NO2-induced responses in guinea-pig dorsal root ganglion neurons

Effects of nitro-oleic acid (OA-NO2) on TRP channels were examined in guinea-pig dissociated dorsal root ganglia (DRG) neurons using calcium imaging and patch clamp techniques. OA-NO2 increased intracellular Ca(2+) in 60-80% DRG neurons. 1-Oleoyl-2acetyl-sn-glycerol (OAG), a TRPC agonist, elicited responses in 36% of OA-NO2-sensitive neurons while capsaicin (TRPV1 agonist) or allyl-isothiocyanate (AITC, TRPA1 agonist) elicited responses in only 16% and 10%, respectively, of these neurons. A TRPV1 antagonist (diarylpiperazine, 5 μm) in combination with a TRPA1 antagonist (HC-030031, 30 μm) did not change the amplitude of the Ca(2+) transients or percentage of neurons responding to OA-NO2; however, a reducing agent DTT (50 mm) or La(3+) (50 μm) completely abolished OA-NO2 responses. OA-NO2 also induced a transient inward current associated with a membrane depolarization followed by a prolonged outward current and hyperpolarization in 80% of neurons. The reversal potentials of inward and outward currents were approximately -20 mV and -60 mV, respectively. Inward current was reduced when extracellular Na(+) was absent, but unchanged by niflumic acid (100 μm), a Cl(-) channel blocker. Outward current was abolished in the absence of extracellular Ca(2+) or a combination of two Ca(2+)-activated K(+) channel blockers (iberiotoxin, 100 nm and apamin, 1 μm). BTP2 (1 or 10 μm), a broad spectrum TRPC antagonist, or La(3+) (50 μm) completely abolished OA-NO2 currents. RT-PCR performed on mRNA extracted from DRGs revealed the expression of all seven subtypes of TRPC channels. These results support the hypothesis that OA-NO2 activates TRPC channels other than the TRPV1 and TRPA1 channels already known to be targets in rat and mouse sensory neurons and challenge the prevailing view that electrophilic compounds act specifically on TRPA1 or TRPV1 channels. The modulation of sensory neuron excitability via actions on multiple TRP channels can contribute to the anti-inflammatory effect of OA-NO2.

Ion channels and migraine

Migraine is one of the most common neurological disorders. Despite its prevalence, the basic physiology of the molecules and mechanisms that contribute to migraine headache is still poorly understood, making the discovery of more effective treatments extremely difficult. The consistent presence of head-specific pain during migraine suggests an important role for activation of the peripheral nociceptors localized to the head. Accordingly, this review will cover the current understanding of the biological mechanisms leading to episodic activation and sensitization of the trigeminovascular pain pathway, focusing on recent advances regarding activation and modulation of ion channels.

Myelinated Ah-type trigeminal ganglion neurons in female rats: neuroexcitability, chemosensitivity to histamine, and potential clinical impact

Migraine is a chronic neurological disorder characterized by recurrent moderate-to-severe headaches often associated with numerous autonomic nervous system symptoms, and it is more prevalent in women. To fully understand the underlying mechanism, standard electrophysiology was performed with trigeminal ganglion neurons (TGNs) isolated from adult rats of both genders using the whole-cell patch clamp technique to test the distribution, neuroexcitability, and chemosensitivity to histamine. In addition to traditionally classified A- and C-type TGNs, myelinated Ah-type TGNs were also observed in females. The electrophysiological features showed low firing threshold and the capability to fire repetitively upon stimulation. Ah-type neurons also functionally expressed persistent TTX-R Na(+) channels with more hyperpolarized activating voltage. Iberiotoxin and NS11021 significantly altered the discharge profiles of Ah-type TGNs. Finally, Ah-type TGNs showed a more potent reaction to histamine, with relatively larger inward currents and membrane depolarization compared with C-types. These data provide evidence of the gender-specific distribution of myelinated Ah-type TGNs in adult female rats, characterized by a low threshold and high frequency of firing that are at least partially attributable to persistent TTX-R Na(+) and BK-KCa channel expression and potent chemosensitivity to histamine, suggesting that Ah-type TGNs may play a key role in gender differences in migraine.

Dural fibroblasts play a potential role in headache pathophysiology

Nociceptive signaling from the meninges is proposed to contribute to many forms of headache. However, the events within the meninges that drive afferent activity are not clear. Meningeal fibroblasts are traditionally thought to produce extracellular proteins that constitute the meninges but not to contribute to headache. The purpose of these studies was to determine whether dural fibroblasts release factors that activate/sensitize dural afferents and produce headache-like behavior in rats. Dura mater was removed from male rats and dural fibroblasts were cultured. Fibroblast cultures were stimulated with vehicle or lipopolysaccharide (LPS), washed, and conditioned media was collected. Fibroblast media conditioned with vehicle or LPS was applied to retrogradely labeled rat dural trigeminal ganglion neurons in vitro. Patch-clamp electrophysiology was performed to determine whether conditioned media activated/sensitized dural afferents. A preclinical behavioral model was used where conditioned media was applied directly to the rat dura to determine the presence of cutaneous facial and hind-paw allodynia. Conditioned media was also tested for interleukin-6 (IL-6) content using an enzyme-linked immunosorbent assay. Application of LPS-conditioned fibroblast media to dural afferents produced a significant increase in action potential firing as well as cutaneous facial and hind-paw allodynia when this media was applied to the dura. Finally, stimulation of cultured fibroblasts with LPS increased IL-6 levels in the media. These findings demonstrate that fibroblasts stimulated with LPS release factors capable of activating/sensitizing dural afferents. Further, they suggest that fibroblasts play a potential role in the pathophysiology of headache.

Bradykinin controls pool size of sensory neurons expressing functional δ-opioid receptors

Analgesics targeting the δ-opioid receptor (DOR) may lead to fewer side effects than conventional opioid drugs, which mainly act on μ-opioid receptors (MOR), because of the less abundant expression of DOR in the CNS compared with MOR. Analgesic potential of DOR agonists increases after inflammation, an effect that may be mediated by DOR expressed in the peripheral sensory fibers. However, the expression of functional DOR at the plasma membrane of sensory neurons is controversial. Here we have used patch-clamp recordings and total internal reflection fluorescence microscopy to study the functional expression of DOR in sensory neurons from rat trigeminal (TG) and dorsal root ganglia (DRG). Real-time total internal reflection fluorescence microscopy revealed that treatment of TG and DRG cultures with the inflammatory mediator bradykinin (BK) caused robust trafficking of heterologously expressed GFP-tagged DOR to the plasma membrane. By contrast, treatment of neurons with the DOR agonist [d-Ala(2), d-Leu(5)]-enkephalin (DADLE) caused a decrease in the membrane abundance of DOR, suggesting internalization of the receptor after agonist binding. Patch-clamp experiments revealed that DADLE inhibited voltage-gated Ca(2+) channels (VGCCs) in 23% of small-diameter TG neurons. Pretreatment with BK resulted in more than twice as many DADLE responsive neurons (54%) but did not affect the efficacy of VGCC inhibition by DADLE. Our data suggest that inflammatory mediator-induced membrane insertion of DOR into the plasma membrane of peripheral sensory neurons may underlie increased DOR analgesia in inflamed tissue. Furthermore, the majority of BK-responsive TG neurons may have a potential to become responsive to DOR ligands in inflammatory conditions.

Voltage gated sodium and calcium channel blockers for the treatment of chronic inflammatory pain

The inflammatory response is a natural response of the body that occurs immediately following tissue damage, which may be due to injury, infection or disease. The acute inflammatory response is an essential mechanism that promotes healing and a key aspect is the ensuing pain, which warns the subject to protect the site of injury. Thus, it is common to see a zone of primary sensitization as well as consequential central sensitization that generally, is maintained by a peripheral drive from the zone of tissue injury. Inflammation associated with chronic pain states, such as rheumatoid and osteoarthritis, cancer and migraine etc. is deleterious to health and often debilitating for the patient. Thus there is a large unmet clinical need. The mechanisms underlying both acute and chronic inflammatory pain are extensive and complex, involving a diversity of cell types, receptors and proteins. Among these the contribution of voltage gated sodium and calcium channels on peripheral nociceptors is critical for nociceptive transmission beyond the peripheral transducers and changes in their distribution, accumulation, clustering and functional activities have been linked to both inflammatory and neuropathic pain. The latter has been the main area for trials and use of drugs that modulate ion channels such as carbamazepine and gabapentin, but given the large peripheral drive that follows tissue damage, there is a clear rationale for blocking voltage gated sodium and calcium channels in these pain states. It has been hypothesized that pain of inflammatory origin may evolve into a condition that resembles neuropathic pain, but mixed pains such as low back pain and cancer pain often include elements of both pain states. This review considers the therapeutic potential for sodium and calcium channel blockers for the treatment of chronic inflammatory pain states.

Ezogabine (KCNQ2/3 channel opener) prevents delayed activation of meningeal nociceptors if given before but not after the occurrence of cortical spreading depression

We proposed recently that induction of delayed activation of trigeminovascular neurons by cortical spreading depression (CSD) can explain the delayed onset of headache after the migraine aura ("aura"). This prompted us to search for ways to block the neuronal activation by CSD - a preclinical correlate of an attempt to find a drug that can block the initiation of headache when administered shortly after onset of aura (i.e., preemptively). Because migraine headache and epileptic seizures are comorbid chronic neurological disorders characterized by hyperexcitable brain networks, we began the search for such goal with an M-type potassium channel opener. We opted to use ezogabine, recently approved by the FDA as adjunctive treatment of partial onset seizures in adults, because it is a selective KCNQ2/3 channel opener. When CSD was induced before ezogabine injection (8.25 mg/kg, i.p.), 40% (6/15) of the units doubled their firing rate about 45 min later for about 95 min. Similarly, when CSD was induced before vehicle was injected (4% DMSO, 0.5% methylcellulose), 50% (3/6) of the units doubled their firing rate about 30 min later for about 120 min. When CSD was triggered 1h after ezogabine injection, it activated only 8% of the units. By itself, ezogabine injection resulted in a 30% attenuation of ongoing firing in all 10 control units. Thus, activation of KCNQ2/3 channels during the aura is unlikely to preempt the onset of headache but may reduce the incidence of migraine if given during prodromes that precede the headache by hours. Given the mechanistic similarities between migraine aura and epileptic seizures, it may be worthwhile to determine whether preemptive administration of ezogabine can prevent oncoming seizures in patients whose warning signs precede their seizures by more than an hour.

Modality-specific nociceptor sensitization following UV-B irradiation of human skin

Ultraviolet-B (UV-B) irradiation is a well-established inflammatory pain model inducing mechanical and thermal hyperalgesia, presumably mediated by released mediators that sensitize sensory nerve endings. Here, we used additional electrical stimulation to investigate axonal hyperexcitability. The lower leg of 13 volunteers was irradiated with 3-fold the minimum erythema UV-B dose and sensitization was recorded at days 1, 3, 7, and 14. Maximum heat pain (47°C, 5 seconds) developed at day 1 (visual analog scale [VAS: 0-100]; 59), was reduced at day 3 (VAS 43, P < .002), and was back to normal at day 7 (VAS 18). Mechanical impact pain (8 m/s), pinprick (150 mN), and pressure (100 kPa) hyperalgesia were maximum throughout days 1 to 3 (VAS 16, 8, and 12, respectively, P < .001) and back to normal at day 7. Suprathreshold transcutaneous electrical stimuli (1.5-fold pain threshold) were delivered in trains of 10 pulses at frequencies of 5 to 100 Hz. Electrical pain thresholds (determined at 2 Hz) decreased significantly (P < .002) and suprathreshold electrical pain increased by about 70% at days 1 to 3 after irradiation (VAS 36, P < .002). Electrical hyperalgesia did not correlate with mechanical sensitization but with reduced heat pain threshold and increased tonic heat pain (r = -.46 and .53; P < .05 and < .01), indicating that axonal hyperexcitability might contribute to heat hyperalgesia. Released inflammatory mediators (eg, prostaglandins) might sensitize both heat transducer molecules and axonal ion channels and receptors, which would explain the simultaneous development and close correlation between heat hyperalgesia and axonal hyperexcitability.

PERSPECTIVE

Local inflammation by UV-B irradiation sensitizes not only sensory endings, but also axons. Increased axonal excitability could contribute to inflammatory hyperalgesia by facilitating spike generation and increasing peak discharge frequencies of nociceptors. Thus, axonal channels and receptors crucial for this sensitization need to be identified to provide new therapeutic targets.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Pathophysiology of migraine

Migraine is a collection of perplexing neurological conditions in which the brain and its associated tissues have been implicated as major players during an attack. Once considered exclusively a disorder of blood vessels, compelling evidence has led to the realization that migraine represents a highly choreographed interaction between major inputs from both the peripheral and central nervous systems, with the trigeminovascular system and the cerebral cortex among the main players. Advances in in vivo and in vitro technologies have informed us about the significance to migraine of events such as cortical spreading depression and activation of the trigeminovascular system and its constituent neuropeptides, as well as about the importance of neuronal and glial ion channels and transporters that contribute to the putative cortical excitatory/inhibitory imbalance that renders migraineurs susceptible to an attack. This review focuses on emerging concepts that drive the science of migraine in both a mechanistic direction and a therapeutic direction.

Calcium channels and migraine

Missense mutations in CACNA1A, the gene that encodes the pore-forming α1 subunit of human voltage-gated Ca(V)2.1 (P/Q-type) calcium channels, cause a rare form of migraine with aura (familial hemiplegic migraine type 1: FHM1). Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache that may be preceded by transient neurological aura symptoms. This review, first, briefly summarizes current understanding of the pathophysiological mechanisms that are believed to underlie migraine headache, migraine aura and the onset of a migraine attack, and briefly describes the localization and function of neuronal Ca(V)2.1 channels in the brain regions that have been implicated in migraine pathogenesis. Then, the review describes and discusses i) the functional consequences of FHM1 mutations on the biophysical properties of recombinant human Ca(V)2.1 channels and native Ca(V)2.1 channels in neurons of knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene, and ii) the functional consequences of these mutations on neurophysiological processes in the cerebral cortex and trigeminovascular system thought to be involved in the pathophysiology of migraine, and the insights into migraine mechanisms obtained from the functional analysis of these processes in FHM1 knockin mice. This article is part of a Special Issue entitled: Calcium channels.

Persistent inflammation increases GABA-induced depolarization of rat cutaneous dorsal root ganglion neurons in vitro

Persistent inflammation is associated with a shift in spinal GABA(A) signaling from inhibition to excitation such that GABA(A)-receptor activation contributes to inflammatory hyperalgesia. We tested the hypothesis that the primary afferent is the site of the persistent inflammation-induced shift in GABA(A) signaling which is due to a Na(+)-K(+)-Cl(-)-co-transporter (NKCC1)-dependent depolarization of the GABA(A) current equilibrium potential (E(GABA)). Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion (DRG) neurons from naïve and inflamed (3 days after a subcutaneous injection of complete Freund's adjuvant) adult male rats were studied with Ca(2+) imaging, western blot and gramicidin-perforated patch recording. GABA evoked a Ca(2+) transient in a subpopulation of small- to medium-diameter capsaicin-sensitive cutaneous neurons. Inflammation was associated with a significant increase in the magnitude of GABA-induced depolarization as well as the percentage of neurons in which GABA evoked a Ca(2+) transient. There was no detectable change in NKCC1 protein or phosphoprotein at the whole ganglia level. Furthermore, the increase in excitatory response was comparable in both HEPES- and HCO(3)(-)-buffered solutions, but was only associated with a depolarization of E(GABA) in HCO(3)(-)-based solution. In contrast, under both recording conditions, the excitatory response was associated with an increase in GABA(A) current density, a decrease in low threshold K(+) current density, and resting membrane potential depolarization. Our results suggest that increasing K(+) conductance in afferents innervating a site of persistent inflammation may have greater efficacy in the inhibition of inflammatory hyperalgesia than attempting to drive a hyperpolarizing shift in E(GABA).

Inflammation-induced changes in BK(Ca) currents in cutaneous dorsal root ganglion neurons from the adult rat

BACKGROUND

Inflammation-induced sensitization of primary afferents is associated with a decrease in K(+) current. However, the type of K(+) current and basis for the decrease varies as a function of target of innervation. Because glabrous skin of the rat hindpaw is used often to assess changes in nociception in models of persistent pain, the purpose of the present study was to determine the type and extent to which K(+) currents contribute to the inflammation-induced sensitization of cutaneous afferents. Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion neurons from naïve and inflamed (3 days post complete Freund's adjuvant injection) rats were studied with whole cell and perforated patch techniques.

RESULTS

Inflammation-induced sensitization of small diameter cutaneous neurons was associated with an increase in action potential duration and rate of decay of the afterhyperpolarization. However, no changes in voltage-gated K(+) currents were detected. In contrast, Ca(2+) modulated iberiotoxin sensitive and paxilline sensitive K(+) (BK(Ca)) currents were significantly smaller in small diameter IB4+ neurons. This decrease in current was not associated with a detectable change in total protein levels of the BK(Ca) channel α or β subunits. Single cell PCR analysis revealed a significant change in the pattern of expression of α subunit splice variants and β subunits that were consistent, at least in part, with inflammation-induced changes in the biophysical properties of BK(Ca) currents in cutaneous neurons.

CONCLUSIONS

Results of this study provide additional support for the conclusion that it may be possible, if not necessary to selectively treat pain arising from specific body regions. Because a decrease in BK(Ca) current appears to contribute to the inflammation-induced sensitization of cutaneous afferents, BK(Ca) channel openers may be effective for the treatment of inflammatory pain.

The complex actions of sumatriptan on rat dural afferents

AIM

To test the hypothesis that the clinical efficacy of triptans reflects convergent modulation of ion channels also involved in inflammatory mediator (IM)-induced sensitization of dural afferents.

METHODS

Acutely dissociated retrogradely labeled rat dural afferents were studied with whole cell and perforated patch techniques in the absence and presence of sumatriptan and/or IM (prostaglandin E2, bradykinin, and histamine).

RESULTS

Sumatriptan dose-dependently suppressed voltage-gated Ca²⁺ currents. Acute (2 min) sumatriptan application increased dural afferent excitability and occluded further IM-induced sensitization. In contrast, pre-incubation (30 min) with sumatriptan had no influence on dural afferent excitability and partially prevented IM-induced sensitization of dural afferents. The sumatriptan-induced suppression of voltage-gated Ca²⁺ currents and acute sensitization and pre-incubation-induced block of IM-induced sensitization were blocked by the 5-HT(1D) antagonist BRL 15572. Pre-incubation with sumatriptan failed to suppress the IM-induced decrease in action potential threshold and overshoot (which results from modulation of voltage-gated Na⁺ currents) and activation of Cl⁻ current, and had no influence on the Cl⁻ reversal potential. However, pre-incubation with sumatriptan caused a dramatic hyperpolarizing shift in the voltage dependence of K⁺ current activation.

DISCUSSION

These results indicate that although the actions of sumatriptan on dural afferents are complex, at least two distinct mechanisms underlie the antinociceptive actions of this compound. One of these mechanisms, the shift in the voltage dependence of K⁺ channel activation, may suggest a novel strategy for future development of anti-migraine agents.

Migraine: a disorder of brain excitatory-inhibitory balance?

Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache and interictal hypersensitivity to sensory stimuli. Migraine arises from a primary brain dysfunction that leads to episodic activation and sensitization of the trigeminovascular pain pathway and as a consequence to headache. Major open issues concern the molecular and cellular mechanisms of the primary brain dysfunction(s) and of migraine pain. We review here our current understanding of these mechanisms, focusing on recent advances regarding migraine genetics, headache mechanisms, and the primary brain dysfunction(s) underlying migraine onset and susceptibility to cortical spreading depression, the neurophysiological correlate of migraine aura. We also discuss insights obtained from the functional analysis of familial hemiplegic migraine mouse models.

Inflammatory mediators potentiate high affinity GABA(A) currents in rat dorsal root ganglion neurons

Following acute tissue injury action potentials may be initiated in afferent processes terminating in the dorsal horn of the spinal cord that are propagated back out to the periphery, a process referred to as a dorsal root reflex (DRR). The DRR is dependent on the activation of GABA(A) receptors. The prevailing hypothesis is that DRR is due to a depolarizing shift in the chloride equilibrium potential (E(Cl)) following an injury-induced activation of the Na(+)-K(+)-Cl(-)-cotransporter. Because inflammatory mediators (IM), such as prostaglandin E(2) are also released in the spinal cord following tissue injury, as well as evidence that E(Cl) is already depolarized in primary afferents, an alternative hypothesis is that an IM-induced increase in GABA(A) receptor mediated current (I(GABA)) could underlie the injury-induced increase in DRR. To test this hypothesis, we explored the impact of IM (prostaglandin E(2) (1 μM), bradykinin (10 μM), and histamine (1 μM)) on I(GABA) in dissociated rat dorsal root ganglion (DRG) neurons with standard whole cell patch clamp techniques. IM potentiated I(GABA) in a subpopulation of medium to large diameter capsaicin insensitive DRG neurons. This effect was dependent on the concentration of GABA, manifest only at low concentrations (<10 μM). THIP evoked current were also potentiated by IM and GABA (1 μM) induced tonic currents enhanced by IM were resistant to gabazine (20 μM). The present data are consistent with the hypothesis that an acute increase in I(GABA) contributes to the emergence of injury-induced DRR.

Whole-cell recording in isolated primary sensory neurons

The isolated sensory neuron in vitro is a powerful model with which to address a number of important neurobiological questions. Isolated neurons are relatively easy to prepare from both neonatal and adult animals and can be studied both acutely and after considerable time on culture. Intracellular recording is one of the most powerful ways to study these neurons. Methods are described for both the preparation of isolated sensory neurons in vitro as well as for recording major classes of ionic currents (Na(+), K(+), and Ca(2+)) from these neurons with whole cell voltage-clamp techniques. Methods are also provided for an initial characterization of active and passive electrophysiological properties of these neurons in current clamp as well as the use of perforated patch recording as a means to mitigate some of the limitations associated with conventional whole cell patch recording. The reader should be aware that the regulation of ion channels in sensory neurons may very subtle, requiring considerably more sophisticated protocols than have been provided here. The reader should also be aware that there is a tremendous heterogeneity among sensory neurons, which is both a curse and a blessing for those who wish to study them. Thus, the methods provided here should only be considered the starting point for a more detailed analysis of sensory neurons.

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

Background

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

Methods

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

Results

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

Conclusions

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

Sex differences in the inflammatory mediator-induced sensitization of dural afferents

Approximately 20% of the adult population suffers from migraine. This debilitating pain disorder is three times more prevalent in women than in men. To begin to evaluate the underlying mechanisms that may contribute to this sex difference, we tested the hypothesis that there is a sex difference in the inflammatory mediator (IM)-induced sensitization of dural afferents. Acutely dissociated retrogradely labeled dural afferents from adult Sprague-Dawley rats were examined with whole cell patch-clamp recordings. Baseline passive and active electrophysiological properties of dural afferents from both sexes were comparable. However, while IM-induced increases in the excitability of dural afferents from male and female rats were also comparable, the proportion of dural afferents from female rats sensitized by IM (~100%) was significantly greater than that of dural afferents from male rats (~50%). This appeared to be due to differences downstream of IM receptors, as tetrodotoxin-resistant sodium current was increased by IM in a majority of male dural afferents (13/14). These data indicate that there are both quantitative and qualitative differences in the IM-induced sensitization of dural afferents that may contribute to the sex difference in the manifestation of migraine.

Role of glia in orofacial pain

Several acute and chronic pain conditions in the face or mouth are very common, and some are unique to the orofacial region. However, the etiology and pathogenesis of most orofacial chronic pain conditions are unresolved, and they are difficult to diagnose and manage. This article provides a brief overview of the neural mechanisms underlying orofacial pain and then highlights recent findings indicating that nonneural cells, specifically satellite cells in the sensory ganglia and astroglia and microglia cells in the central nervous system, are important players in both acute and chronic inflammatory and neuropathic orofacial pain conditions and may offer new targets for management of these conditions.

Tumor necrosis factor-α increases brain-derived neurotrophic factor expression in trigeminal ganglion neurons in an activity-dependent manner

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α (TNFα) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNFα on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNFα increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNFα increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNFα-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNFα on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNFα-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNFα stimulates de novo synthesis of BDNF in these neurons. Thus, TNFα should be considered in mechanisms of BDNF-dependent neuronal plasticity.

Nociceptor sensitization in pain pathogenesis

The incidence of chronic pain is estimated to be 20-25% worldwide. Few patients with chronic pain obtain complete relief from the drugs that are currently available, and more than half report inadequate relief. Underlying the challenge of developing better drugs to manage chronic pain is incomplete understanding of the heterogeneity of mechanisms that contribute to the transition from acute tissue insult to chronic pain and to pain conditions for which the underlying pathology is not apparent. An intact central nervous system (CNS) is required for the conscious perception of pain, and changes in the CNS are clearly evident in chronic pain states. However, the blockage of nociceptive input into the CNS can effectively relieve or markedly attenuate discomfort and pain, revealing the importance of ongoing peripheral input to the maintenance of chronic pain. Accordingly, we focus here on nociceptors: their excitability, their heterogeneity and their role in initiating and maintaining pain.