Small-diameter Dorsal Root Ganglion Neurons Research Articles

Trpv1-lineage neuron-expressing Kcnq4 channel modulates itch sensation in mice.

Voltage-gated potassium channel subfamily q member 4 (Kcnq4) is predominantly expressed by hair cells and auditory neurons and regulates the neuronal excitability in the auditory pathway. Although it is further detected in myelinated large-diameter dorsal root ganglia (DRG) neurons in the periphery, the expression and function of Kcnq4 channel in nociceptors remains unknown. Here we showed that Kcnq4 is substantially expressed by unmyelinated small-diameter DRG neurons in both human and mouse. In spite of a dispensable role in acute pain and chronic skin inflammation, Kcnq4 is specifically involved in the regulation of scratching behavior through controlling action potential firing properties, evidenced by the increased neuronal excitability in small-diameter DRG neurons isolated from Kcnq4 deficient mice. Moreover, genetic ablation of Kcnq4 in Trpv1-positive neurons exacerbates both acute and chronic itch behavior in mice. Taken together, our results uncover a functional role of Trpv1-lineage neuron-expressing Kcnq4 channel in the modulation of itch-specific neuronal excitation in the periphery.

Nav1.8 in small dorsal root ganglion neurons contributes to vincristine-induced mechanical allodynia.

Vincristine-induced peripheral neuropathy is a common side effect of vincristine treatment, which is accompanied by pain and can be dose-limiting. The molecular mechanisms that underlie vincristine-induced pain are not well understood. We have established an animal model to investigate pathophysiological mechanisms of vincristine-induced pain. Our previous studies have shown that the tetrodotoxin-sensitive voltage-gated sodium channel Nav1.6 in medium-diameter dorsal root ganglion (DRG) neurons contributes to the maintenance of vincristine-induced allodynia. In this study, we investigated the effects of vincristine administration on excitability in small-diameter DRG neurons and whether the tetrodotoxin-resistant (TTX-R) Nav1.8 channels contribute to mechanical allodynia. Current-clamp recordings demonstrated that small DRG neurons become hyper-excitable following vincristine treatment, with both reduced current threshold and increased firing frequency. Using voltage-clamp recordings in small DRG neurons, we now show an increase in TTX-R current density and a -7.3 mV hyperpolarizing shift in the half-maximal potential (V1/2) of activation of Nav1.8 channels in vincristine-treated animals, which likely contributes to the hyperexcitability that we observed in these neurons. Notably, vincristine treatment did not enhance excitability of small DRG neurons from Nav1.8 knockout mice, and the development of mechanical allodynia was delayed but not abrogated in these mice. Together, our data suggest that sodium channel Nav1.8 in small DRG neurons contributes to the development of vincristine-induced mechanical allodynia.

Role of pattern recognition receptors in chemotherapy-induced neuropathic pain.

Progress in the development of effective chemotherapy is producing a growing population of patients with acute and chronic painful chemotherapy-induced peripheral neuropathy (CIPN), a serious treatment-limiting side effect for which there is currently no US Food and Drug Administration-approved treatment. CIPNs induced by diverse classes of chemotherapy drugs have remarkably similar clinical presentations, leading to the suggestion they share underlying mechanisms. Sensory neurons share with immune cells the ability to detect damage associated molecular patterns (DAMPs), molecules produced by diverse cell types in response to cellular stress and injury, including by chemotherapy drugs. DAMPs, in turn, are ligands for pattern recognition receptors (PRRs), several of which are found on sensory neurons, as well as satellite cells, and cells of the immune system. In the present experiments, we evaluated the role of two PRRs, TLR4 and RAGE, present in dorsal root ganglion (DRG), in CIPN. Antisense (AS)-oligodeoxynucleotides (ODN) against TLR4 and RAGE mRNA were administered intrathecally before ('prevention protocol') or 3 days after ('reversal protocol') the last administration of each of three chemotherapy drugs that treat cancer by different mechanisms (oxaliplatin, paclitaxel and bortezomib). TLR4 and RAGE AS-ODN prevented the development of CIPN induced by all three chemotherapy drugs. In the reversal protocol, however, while TLR4 AS-ODN completely reversed oxaliplatin- and paclitaxel-induced CIPN, in rats with bortezomib-induced CIPN it only produced a temporary attenuation. RAGE AS-ODN, in contrast, reversed CIPN induced by all three chemotherapy drugs. When a TLR4 antagonist was administered intradermally to the peripheral nociceptor terminal, it did not affect CIPN induced by any of the chemotherapy drugs. However, when administered intrathecally, to the central terminal, it attenuated hyperalgesia induced by all three chemotherapy drugs, compatible with a role of TLR4 in neurotransmission at the central terminal but not sensory transduction at the peripheral terminal. Finally, since it has been established that cultured DRG neurons can be used to study direct effects of chemotherapy on nociceptors, we also evaluated the role of TLR4 in CIPN at the cellular level, using patch-clamp electrophysiology in DRG neurons cultured from control and chemotherapy-treated rats. We found that increased excitability of small-diameter DRG neurons induced by in vivo and in vitro exposure to oxaliplatin is TLR4-dependent. Our findings suggest that in addition to the established contribution of PRR-dependent neuroimmune mechanisms, PRRs in DRG cells also have an important role in CIPN.

Transcriptional Profiling of TGF-β Superfamily Members in Lumbar DRGs of Rats Following Sciatic Nerve Axotomy and Activin C Inhibits Neuropathic Pain.

Neuroinflammation and cytokines play critical roles in neuropathic pain and axon degeneration/regeneration. Cytokines of transforming growth factor-β superfamily have implications in pain and injured nerve repair processing. However, the transcriptional profiles of the transforming growth factor-β superfamily members in dorsal root ganglia under neuropathic pain and axon degeneration/regeneration conditions remain elusive. We aimed to plot the transcriptional profiles of transforming growth factor-β superfamily components in lumbar dorsal root ganglia of sciatic nerve-axotomized rats and to further verify the profiles by testing the analgesic effect of activin C, a representative cytokine, on neuropathic pain. Adult male rats were axotomized in sciatic nerves, and lumbar dorsal root ganglia were isolated for total RNA extraction or section. A custom microarray was developed and employed to plot the gene expression profiles of transforming growth factor-β superfamily components. Realtime RT-PCR was used to confirm changes in the expression of activin/inhibin family genes, and then in situ hybridization was performed to determine the cellular locations of inhibin α, activin βC, BMP-5 and GDF-9 mRNAs. The rat spared nerve injury model was performed, and a pain test was employed to determine the effect of activin C on neuropathic pain. The expression of transforming growth factor-β superfamily cytokines and their signaling, including some receptors and signaling adaptors, were robustly upregulated. Activin βC subunit mRNAs were expressed in the small-diameter dorsal root ganglion neurons and upregulated after axotomy. Single intrathecal injection of activin C inhibited neuropathic pain in spared nerve injury model. This is the first report to investigate the transcriptional profiles of members of transforming growth factor-β superfamily in axotomized dorsal root ganglia. The distinct cytokine profiles observed here might provide clues toward further study of the role of transforming growth factor-β superfamily in the pathogenesis of neuropathic pain and axon degeneration/regeneration after peripheral nerve injury.

Regulation of Eukaryotic Elongation Factor 2 Kinase in Mouse Sensory Neurons and Anti-nociceptive Effects of a Selective Inhibitor

Eukaryotic elongation factor 2 (eEF2) is an essential component of the translation machinery that catalyzes translocation of the ribosome along the mRNA transcript. Phosphorylation of eEF2 causes a transient suspension of ongoing translation. eEF2 kinase, the only enzyme known to phosphorylate eEF2, is activated by calcium/calmodulin and AMP-activated kinase (AMP kinase), a key sensor of ATP availability, among other mechanisms. Using an antibody selective for phosphorylated eEF2 (peEF2), we examined eEF2 regulation in sensory neurons of the mouse dorsal root ganglion (DRG). Immunohistochemistry for peEF2 revealed tonically high levels of peEF2 in a subset of primarily small-diameter DRG neurons, particularly putative nociceptors positive for either TRPV1 or IB4. In dissociated DRG neurons, application of 50 mM K+ for 1 or 10 minutes induced intense peEF2 staining in virtually all neurons; this staining remained elevated above baseline 60 minutes after stimulation and was prevented by a selective eEF2 kinase inhibitor, A484954 (30 micromolar). Both peEF2 and eEF2K levels in DRG increased in response to hindpaw inflammatory insult (complete Freud's adjuvant injection), as measured by western blot. Therefore, A484954 bioavailability and pharmacokinetics were evaluated by mass spectrometry in plasma and brain 30, 60 and 90 minutes after oral administration of 1 or 7.5 mg/kg by gavage. Peak plasma and brain concentrations were observed 60 minutes after administration. Daily oral administration of A484954 in the carrageenan model of inflammatory pain significantly reduced hyperalgesia to radiant heat throughout the behavioral response to carrageenan, suggesting that suppression of eEF2 phosphorylation is anti-nociceptive. L3-5 DRG, lumbar spinal dorsal horn and whole brain were analyzed to confirm inhibition of eEF2 phosphorylation by oral A484954. These results indicate that peEF2 is rapidly induced in response to sensory neuron activation and sustained during inflammatory hyperalgesia. Suppression of eEF2 phosphorylation is anti-nociceptive, possibly through a direct action on DRG nociceptors. Grant support from NIH NIGMS R01102346 and P20GM103643.

Transient Receptor Potential Vanilloid 4 Mediates Hypersensitivity in Chronic Sickle Cell Disease Pain

Sickle cell disease (SCD) pain is debilitating and severely diminishes patients' quality of life. A significant cause of SCD pain is vaso-oclusion, however, pain in SCD can also arise from chronic tissue inflammation and neuropathy. Our lab is interested in identifying novel peripheral targets for treating chronic SCD pain. Transient receptor potential (TRP) channels are among these targets, as they have known somatosensory functions. One of these, TRPV4, is a nonselective cation channel involved in naïve somatosensation, as well as inflammatory and neuropathic pain. TRPV4 is expressed in dorsal root ganglia (DRG) neurons, the primary detectors and transducers of somatosensory information, and in keratinocytes, which compose the majority of the epidermis and are located proximal to afferent terminals in the skin. Further, inflammatory factors that interact with TRPV4, including neutrophil elastase and endothelin-1, are prevalent in SCD. Thus, TRPV4 is a likely contributor to SCD pain. In this study we investigated the role of TRPV4 in persistent SCD somatic hypersensitivity. First, we demonstrated that punctate mechanical allodynia observed in SCD mice was completely and dose-dependently blocked by a TRPV4 antagonist. Functional analysis of small-diameter DRG neurons (many of which are nociceptors in rodents) from SCD mice revealed a moderate TRPV4-mediated increase of calcium flux, and blocking TRPV4 receptors reduced the proportion of mechanically-sensitive neurons from SCD mice. In contrast to these modest DRG data, we found keratinocytes from SCD mice display robust TRPV4 agonist-induced calcium flux compared to WT controls. Given keratinocytes are profuse throughout the skin and are positioned alongside sensory nerve terminals, this evidence suggests keratinocytes play a pivotal role in TRPV4-mediated SCD hypersensitivity. Together, these data suggest TRPV4 plays an integral role in SCD evoked behavioral hypersensitivity, an effect that is likely due to sensitized TRPV4-mediated keratinocyte, and to a lesser extent DRG, activity in these mice. Grant support from R01 NS070711 NIH/NINDS.

Neuronal allodynic mechanisms of Slc7a5 (LAT1) in the spared nerve injury rodent model of neuropathic pain.

High-impact chronic pain is suffered by 1 in 5 patients in the USA and globally. Effective, non-addictive, non-opioid therapeutics are urgently needed for the treatment of chronic pain. Slc7a5 (Lat1), also known as system L-neutral amino acid transporter, is involved in a number of physiological processes related to inflammation. Transcriptomics studies have shown that Slc7a5 and its binding partner Slc3a2 are expressed in neurons of the dorsal root ganglia (DRG) and spinal dorsal horn, which are critical to the initiation and maintenance of nociception and pathophysiology of chronic pain. In addition, Slc7a5 is a transporter for the first-line anti-allodynic gabapentinoid drugs and binds to ion channels implicated in nociception and chronic pain including the voltage-gated sodium channel Nav1.7 and the voltage-gated potassium channels Kv1.1 and Kv1.2. We found that blocking Slc7a5 with intrathecal administration of the drug JPH203 alleviated allodynia in the spared nerve injury (SNI) rodent model of neuropathic pain. Western blot and immunohistochemistry studies revealed an increase in Slc7a5 protein levels in the spinal cord and DRGs of SNI mice compared to control mice. Using whole-cell current-clamp electrophysiology, we observed that JPH203 treatment reduced excitability of small-diameter (< 30µm) DRG neurons from SNI mice, in agreement with its behavioral effects. Voltage-clamp recordings from JPH203-treated naïve rat DRGs identified an effect on tetrodotoxin-resistant (TTX-R) sodium currents. Altogether, these results demonstrate that Slc7a5 is dysregulated in chronic neuropathic pain and can be targeted to provide relief of hypersensitivity.

Protein kinase G signaling pathway is involved in sympathetically maintained pain by modulating ATP-sensitive potassium channels

BackgroundSympathetically maintained pain (SMP) involves an increased excitability of dorsal root ganglion (DRG) neurons to sympathetic nerve stimulation and circulating norepinephrine. The current treatment of SMP has limited efficacy, and...

Fibroblast growth factor 13 stabilizes microtubules to promote Na+ channel function in nociceptive DRG neurons and modulates inflammatory pain

IntroductionFibroblast growth factor homologous factors (FHFs), among other fibroblast growth factors, are increasingly found to be important regulators of ion channel functions. Although FHFs have been link to several neuronal diseases and arrhythmia, its role in inflammatory pain still remains unclear. ObjectivesThis study aimed to investigate the role and mechanism of FGF13 in inflammatory pain. MethodsFgf13 conditional knockout mice were generated and CFA-induced chronic inflammatory pain model was established to measure the pain threshold. Immunostaining, western blot and quantitative real-time reverse transcription PCR (qRT-PCR) were performed to detect the expression of FGF13 in CFA-induced inflammatory pain. Whole-cell patch clamp recording was used to record the action potential firing properties and sodium currents of DRG neurons. ResultsConditional knockout of Fgf13 in dorsal root ganglion (DRG) neurons (Fgf13-/Y) led to attenuated pain responses induced by complete Freund's adjuvant (CFA). FGF13 was expressed predominantly in small-diameter DRG neurons. CFA treatment resulted in an increased expression of FGF13 proteins as well as an increased excitability in nociceptive DRG neurons which was inhibited when FGF13 was absent. The role of FGF13 in neuronal excitability of DRG was linked to its modulation of voltage-gated Na+ channels mediated by microtubules. Overexpression of FGF13, but not FGF13 mutant which lacks the ability to bind and stabilize microtubules, rescued the decreased neuronal excitability and Na+ current density in DRG neurons of Fgf13-/Y mice. ConclusionThis study revealed that FGF13 could stabilize microtubules to modulate sodium channel function in DRG neurons and modulate inflammatory pain. This study provides a novel mechanism for FGF13 modulation of sodium channel function and suggests that FGF13 might be a novel target for inflammatory pain treatment.

Mechanisms Mediating High-Molecular-Weight Hyaluronan-Induced Antihyperalgesia.

We evaluated the mechanism by which high-molecular-weight hyaluronan (HMWH) attenuates nociceptor sensitization, in the setting of inflammation. HMWH attenuated mechanical hyperalgesia induced by the inflammatory mediator prostaglandin E2 (PGE2) in male and female rats. Intrathecal administration of an oligodeoxynucleotide antisense (AS-ODN) to mRNA for cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and intradermal administration of A5G27, a CD44 receptor antagonist, both attenuated antihyperalgesia induced by HMWH. In male rats, HMWH also signals via Toll-like receptor 4 (TLR4), and AS-ODN for TLR4 mRNA administered intrathecally, attenuated HMWH-induced antihyperalgesia. Since HMWH signaling is dependent on CD44 clustering in lipid rafts, we pretreated animals with methyl-β-cyclodextrin (MβCD), which disrupts lipid rafts. MβCD markedly attenuated HMWH-induced antihyperalgesia. Inhibitors for components of intracellular signaling pathways activated by CD44, including phospholipase C and phosphoinositide 3-kinase (PI3K), also attenuated HMWH-induced antihyperalgesia. Furthermore, in vitro application of HMWH attenuated PGE2-induced sensitization of tetrodotoxin-resistant sodium current, in small-diameter dorsal root ganglion neurons, an effect that was attenuated by a PI3K inhibitor. Our results indicate a central role of CD44 signaling in HMWH-induced antihyperalgesia and suggest novel therapeutic targets, downstream of CD44, for the treatment of pain generated by nociceptor sensitization.SIGNIFICANCE STATEMENT High-molecular-weight-hyaluronan (HMWH) is used to treat osteoarthritis and other pain syndromes. In this study we demonstrate that attenuation of inflammatory hyperalgesia by HMWH is mediated by its action at cluster of differentiation 44 (CD44) and activation of its downstream signaling pathways, including RhoGTPases (RhoA and Rac1), phospholipases (phospholipases Cε and Cγ1), and phosphoinositide 3-kinase, in nociceptors. These findings contribute to our understanding of the antihyperalgesic effect of HMWH and support the hypothesis that CD44 and its downstream signaling pathways represent novel therapeutic targets for the treatment of inflammatory pain.

Alterations in SUMOylation of the hyperpolarization-activated cyclic nucleotide-gated ion channel 2 during persistent inflammation.

BackgroundUnilateral injection of Complete Freund's Adjuvant (CFA) into the intra‐plantar surface of the rodent hindpaw elicits chronic inflammation and hyperalgesia in the ipsilateral hindlimb. Mechanisms contributing to this hyperalgesia may act over multiple time courses and can include changes in ion channel expression and post‐translational SUMOylation. Hyperpolarization‐activated, cyclic nucleotide‐gated (HCN) channels mediate the hyperpolarization‐activated current, Ih. An HCN2‐mediated increase in C‐nociceptor Ih contributes to mechanical hyperalgesia in the CFA model of inflammatory pain. Changes in HCN2 post‐translational SUMOylation and protein expression have not been systematically documented for a given dorsal root ganglia (DRG) throughout the time course of inflammation.MethodsThis study examined HCN2 protein expression and post‐translational SUMOylation in a rat model of CFA‐induced hindpaw inflammation. L5 DRG cryosections were used in immunohistochemistry experiments and proximity ligation assays to investigate HCN2 expression and SUMOylation, respectively, on days 1 and 3 post‐CFA.ResultsUnilateral CFA injection elicited a significant bilateral increase in HCN2 staining intensity in small diameter DRG neurons on day 1 post‐CFA, and a significant bilateral increase in the number of small neurons expressing HCN2 but not staining intensity on day 3 post‐CFA. HCN2 channels were hyper‐SUMOylated in small diameter neurons of ipsilateral relative to contralateral DRG on days 1 and 3 post‐CFA.ConclusionsUnilateral CFA injection elicits unilateral mechanical hyperalgesia, a bilateral increase in HCN2 expression and a unilateral increase in post‐translational SUMOylation. This suggests that enhanced HCN2 expression in L5 DRG is not sufficient for mechanical hyperalgesia in the early stages of inflammation and that hyper‐SUMOylation of HCN2 channels may also be necessary.SignificanceNociceptor HCN2 channels mediate an increase in Ih that is necessary for mechanical hyperalgesia in a CFA model of chronic pain, but the mechanisms producing the increase in nociceptor Ih have not been resolved. The data presented here suggest that the increase in Ih during the early stages of inflammation may be mediated by an increase in HCN2 protein expression and post‐translational SUMOylation.

Tetrodotoxin-Sensitive Sodium Channels Mediate Action Potential Firing and Excitability in Menthol-Sensitive Vglut3-Lineage Sensory Neurons.

Small-diameter vesicular glutamate transporter 3-lineage (Vglut3lineage) dorsal root ganglion (DRG) neurons play an important role in mechanosensation and thermal hypersensitivity; however, little is known about their intrinsic electrical properties. We therefore set out to investigate mechanisms of excitability within this population. Calcium microfluorimetry analysis of male and female mouse DRG neurons demonstrated that the cooling compound menthol selectively activates a subset of Vglut3lineage neurons. Whole-cell recordings showed that small-diameter Vglut3lineage DRG neurons fire menthol-evoked action potentials and exhibited robust, transient receptor potential melastatin 8 (TRPM8)-dependent discharges at room temperature. This heightened excitability was confirmed by current-clamp and action potential phase-plot analyses, which showed menthol-sensitive Vglut3lineage neurons to have more depolarized membrane potentials, lower firing thresholds, and higher evoked firing frequencies compared with menthol-insensitive Vglut3lineage neurons. A biophysical analysis revealed voltage-gated sodium channel (NaV) currents in menthol-sensitive Vglut3lineage neurons were resistant to entry into slow inactivation compared with menthol-insensitive neurons. Multiplex in situ hybridization showed similar distributions of tetrodotoxin (TTX)-sensitive NaV transcripts between TRPM8-positive and -negative Vglut3lineage neurons; however, NaV1.8 transcripts, which encode TTX-resistant channels, were more prevalent in TRPM8-negative neurons. Conversely, pharmacological analyses identified distinct functional contributions of NaV subunits, with NaV1.1 driving firing in menthol-sensitive neurons, whereas other small-diameter Vglut3lineage neurons rely primarily on TTX-resistant NaV channels. Additionally, when NaV1.1 channels were blocked, the remaining NaV current readily entered into slow inactivation in menthol-sensitive Vglut3lineage neurons. Thus, these data demonstrate that TTX-sensitive NaVs drive action potential firing in menthol-sensitive sensory neurons and contribute to their heightened excitability.SIGNIFICANCE STATEMENT Somatosensory neurons encode various sensory modalities including thermoreception, mechanoreception, nociception, and itch. This report identifies a previously unknown requirement for tetrodotoxin-sensitive sodium channels in action potential firing in a discrete subpopulation of small-diameter sensory neurons that are activated by the cooling agent menthol. Together, our results provide a mechanistic understanding of factors that control intrinsic excitability in functionally distinct subsets of peripheral neurons. Furthermore, as menthol has been used for centuries as an analgesic and anti-pruritic, these findings support the viability of NaV1.1 as a therapeutic target for sensory disorders.

Enhanced histamine-induced itch in diacylglycerol kinase iota knockout mice.

Subsets of small-diameter dorsal root ganglia (DRG) neurons detect pruritogenic (itch-causing) and algogenic (pain-causing) stimuli and can be activated or sensitized by chemical mediators. Many of these chemical mediators activate receptors that are coupled to lipid hydrolysis and diacylglycerol (DAG) production. Diacylglycerol kinase iota (DGKI) can phosphorylate DAG and is expressed at high levels in small-diameter mouse DRG neurons. Given the importance of these neurons in sensing pruritogenic and algogenic chemicals, we sought to determine if loss of DGKI impaired responses to itch- or pain-producing stimuli. Using male and female Dgki-knockout mice, we found that in vivo sensitivity to histamine—but not other pruritogens—was enhanced. In contrast, baseline pain sensitivity and pain sensitization following inflammatory or neuropathic injury were equivalent between wild type and Dgki-/- mice. In vitro calcium responses in DRG neurons to histamine was enhanced, while responses to algogenic ligands were unaffected by Dgki deletion. These data suggest Dgki regulates sensory neuron and behavioral responses to histamine, without affecting responses to other pruritogenic or algogenic agents.

CCL2/CCR2 signaling elicits itch- and pain-like behavior in a murine model of allergic contact dermatitis

Spontaneous itch and pain are the most common symptoms in various skin diseases, including allergic contact dermatitis (ACD). The chemokine (C-C motif) ligand 2 (CCL2, also referred to as monocyte chemoattractant protein 1 (MCP-1)) and its receptor CCR2 are involved in the pathophysiology of ACD, but little is known of the role of CCL2/CCR2 for the itch- and pain-behaviors accompanying the murine model of this disorder, termed contact hypersensitivity (CHS). C57BL/6 mice previously sensitized to the hapten, squaric acid dibutyl ester, applied to the abdomen were subsequently challenged twice with the hapten delivered to either the cheek or to the hairy skin of the hind paw resulting in CHS at that site. By 24 h after the 2nd challenge to the hind paw CCL2 and CCR2 mRNA, protein, and signaling activity were upregulated in the dorsal root ganglion (DRG). Calcium imaging and whole-cell current-clamp recordings revealed that CCL2 directly acted on its neuronal receptor, CCR2 to activate a subset of small-diameter, nociceptive-like DRG neurons retrogradely labeled from the CHS site. Intradermal injection of CCL2 into the site of CHS on the cheek evoked site-directed itch- and pain-like behaviors which could be attenuated by prior delivery of an antagonist of CCR2. In contrast, CCL2 failed to elicit either type of behavior in control mice. Results are consistent with the hypothesis that CHS upregulates CCL2/CCR2 signaling in a subpopulation of cutaneous small diameter DRG neurons and that CCL2 can activate these neurons through neuronal CCR2 to elicit itch- and pain-behavior. Targeting the CCL2/CCR2 signaling might be beneficial for the treatment of the itch and pain sensations accompanying ACD in humans.

Differential use-dependent inactivation of Nav1.8 in the subpopulation of cultured dorsal root ganglion

Sodium channel Nav1.8 is expressed preferentially in members of the peripheral nervous system such as nociceptive dorsal root ganglion (DRG) neurons. Using immunocytochemistry and eletrophysiological recording, we found that a subpopulation of small DRG neurons, grouped by isolectin B4 (IB4) immunoreactivity, showed a different use-dependent inactivation of Nav1.8 current, and this situation changed over time during in vitro culture. The IB 4 - immunoreactivity of DRG neurons was not changed during in vitro culture with the exception of this small population of IB4-negative small-diameter DRG neurons. The Nav1.8 channel in IB 4 + neurons underwent a level of use-dependent inactivation that was significantly stronger than that seen in IB 4 - neurons at 1 and 2 days-in-vitro (DIV). The use-dependent inactivation of the Nav1.8 channel in IB 4 + neurons at 1 DIV was significantly attenuated at 2 DIV. The values for voltage dependency of activation and steady-state inactivation of Nav1.8 were similar in all subpopulations of DRG neurons and did not change over time. The time constant for entry into slow inactivation of Nav1.8 in IB 4 + neurons was significantly faster than in IB 4 - neurons at 1 and 2 DIV, while the rate of recovery from slow inactivation of Nav1.8 in IB 4 + neurons was slower than that seen in IB 4 - neurons. Moreover, the time constant for entry into the slow inactivation of Nav1.8 in IB 4 + neurons after 1 DIV was significantly faster than at 2 DIV, and the rate of recovery from the slow inactivation of Nav1.8 in IB 4 + neurons at 1 DIV was slower than that at 2 DIV, which indicated that the strong use-dependent inactivation in IB 4 + neurons at 1 DIV was the result of a greater preference for the slow inactivation state than at 2 DIV. Our data suggest that the time-dependent change of the use-dependent inactivation of the Nav1.8 channel in DRG neurons cultured in vitro would contribute to the excitability of a subpopulation of DRG neurons and could play an important role in the development of inflammatory and neuropathic pain.

Peripherally Acting μ-Opioid Receptor Agonists Attenuate Ongoing Pain-associated Behavior and Spontaneous Neuronal Activity after Nerve Injury in Rats.

Ongoing neuropathic pain is difficult to treat. The authors examined whether dermorphin [D-Arg2, Lys4] (1-4) amide, a peripherally acting µ-opioid receptor agonist, attenuates ongoing pain-associated manifestations after nerve injury in rats and mice. Using conditioned place preference assay, the authors tested whether animals show a preference to the environment associated with drug treatment. Wide-dynamic range and dorsal root ganglion neuronal activities were measured by electrophysiology recording and calcium imaging. Nerve-injured animals stayed longer in dermorphin [D-Arg2, Lys4] (1-4) amide-paired chamber after conditioning than during preconditioning (rats: 402.4 ± 61.3 vs. 322.1 ± 45.0 s, 10 mg/kg, n = 9, P = 0.009; mice: 437.8 ± 59.4 vs. 351.3 ± 95.9 s, 2 mg/kg, n = 8, P = 0.047). Topical ganglionic application of dermorphin [D-Arg2, Lys4] (1-4) amide (5 μM, 1 μl, n = 5) reduced the numbers of small-diameter dorsal root ganglion neurons that showed spontaneous activity (1.1 ± 0.4 vs. 1.5 ± 0.3, P = 0.044) and that were activated by test stimulation (15.5 ± 5.5 vs. 28.2 ± 8.2, P = 0.009) after injury. In neuropathic rats, dermorphin [D-Arg2, Lys4] (1-4) amide (10 mg/kg, n = 8) decreased spontaneous firing rates in wide-dynamic range neurons to 53.2 ± 46.6% of predrug level, and methylnaltrexone (5 mg/kg, n = 9) blocked dermorphin [D-Arg2, Lys4] (1-4) amide-induced place preference and inhibition of wide-dynamic range neurons. Dermorphin [D-Arg2, Lys4] (1-4) amide increased paw withdrawal threshold (17.5 ± 2.2 g) from baseline (3.5 ± 0.7 g, 10 mg/kg, n = 8, P = 0.002) in nerve-injured rats, but the effect diminished after repeated administrations. Peripherally acting μ-opioids may attenuate ongoing pain-related behavior and its neurophysiologic correlates. Yet, repeated administrations cause antiallodynic tolerance.

The Distributions of Voltage-Gated K+ current Subtypes in Different Cell Sizes from Adult Mouse Dorsal Root Ganglia.

Voltage-gated K+ (KV) currents play a crucial role in regulating pain by controlling neuronal excitability, and are divided into transient A-type currents (IA) and delayed rectifier currents (IK). The dorsal root ganglion (DRG) neurons are heterogeneous and the subtypes of KV currents display different levels in distinct cell sizes. To observe correlations of the subtypes of KV currents with DRG cell sizes, KV currents were recorded by whole-cell patch clamp in freshly isolated mouse DRG neurons. Results showed that IA occupied a high proportion in KV currents in medium- and large-diameter DRG neurons, whereas IK possessed a larger proportion of KV currents in small-diameter DRG neurons. A lower correlation was found between the proportion of IA or IK in KV currents and cell sizes. These data suggest that IA channels are mainly expressed in medium and large cells and IK channels are predominantly expressed in small cells.

Regulation of Nociceptive Glutamatergic Signaling by Presynaptic Kv3.4 Channels in the Rat Spinal Dorsal Horn.

Presynaptic voltage-gated K+ (Kv) channels in dorsal root ganglion (DRG) neurons are thought to regulate nociceptive synaptic transmission in the spinal dorsal horn. However, the Kv channel subtypes responsible for this critical role have not been identified. The Kv3.4 channel is particularly important because it is robustly expressed in DRG nociceptors, where it regulates action potential (AP) duration. Furthermore, Kv3.4 dysfunction is implicated in the pathophysiology of neuropathic pain in multiple pain models. We hypothesized that, through their ability to modulate AP repolarization, Kv3.4 channels in DRG nociceptors help to regulate nociceptive synaptic transmission. To test this hypothesis, we investigated Kv3.4 immunoreactivity (IR) in the rat cervical superficial dorsal horn (sDH) in both sexes and implemented an intact spinal cord preparation to investigate glutamatergic synaptic currents from second order neurons in the sDH under conditions that selectively inhibit the Kv3.4 current. We found presynaptic Kv3.4 IR in peptidergic and nonpeptidergic nociceptive fibers of the sDH. The Kv3.4 channel is hypersensitive to 4-aminopyridine and tetraethylammonium (TEA). Accordingly, 50 μm 4-aminopyridine and 500 μm TEA significantly prolong the AP, slow the maximum rate of repolarization in small-diameter DRG neurons, and potentiate monosynaptic excitatory postsynaptic currents (EPSCs) in dorsal horn laminae I and II through a presynaptic mechanism. In contrast, highly specific inhibitors of BK, Kv7, and Kv1 channels are less effective modulators of the AP and have little to no effect on EPSCs. The results strongly suggest that presynaptic Kv3.4 channels are major regulators of nociceptive synaptic transmission in the spinal cord.SIGNIFICANCE STATEMENT Intractable neuropathic pain can result from disease or traumatic injury and many studies have been conducted to determine the underlying pathophysiological changes. Voltage-gated ion channels, including the K+ channel Kv3.4, are dysregulated in multiple pain models. Kv3.4 channels are ubiquitously expressed in the dorsal root ganglion (DRG), where they are major regulators of DRG excitability. However, little is known about the ionic mechanisms that regulate nociceptive synaptic transmission at the level of the first synapse in the spinal cord, which is critical to pain transmission in both intact and pathological states. Here, we show that Kv3.4 channels have a significant impact on glutamatergic synaptic transmission in the dorsal horn, further illuminating its potential as a molecular pain therapeutic target.

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.

Calcineurin Dysregulation Underlies Spinal Cord Injury-Induced K+ Channel Dysfunction in DRG Neurons.

Dysfunction of the fast-inactivating Kv3.4 potassium current in dorsal root ganglion (DRG) neurons contributes to the hyperexcitability associated with persistent pain induced by spinal cord injury (SCI). However, the underlying mechanism is not known. In light of our previous work demonstrating modulation of the Kv3.4 channel by phosphorylation, we investigated the role of the phosphatase calcineurin (CaN) using electrophysiological, molecular, and imaging approaches in adult female Sprague Dawley rats. Pharmacological inhibition of CaN in small-diameter DRG neurons slowed repolarization of the somatic action potential (AP) and attenuated the Kv3.4 current. Attenuated Kv3.4 currents also exhibited slowed inactivation. We observed similar effects on the recombinant Kv3.4 channel heterologously expressed in Chinese hamster ovary cells, supporting our findings in DRG neurons. Elucidating the molecular basis of these effects, mutation of four previously characterized serines within the Kv3.4 N-terminal inactivation domain eliminated the effects of CaN inhibition on the Kv3.4 current. SCI similarly induced concurrent Kv3.4 current attenuation and slowing of inactivation. Although there was little change in CaN expression and localization after injury, SCI induced upregulation of the native regulator of CaN 1 (RCAN1) in the DRG at the transcript and protein levels. Consistent with CaN inhibition resulting from RCAN1 upregulation, overexpression of RCAN1 in naive DRG neurons recapitulated the effects of pharmacological CaN inhibition on the Kv3.4 current and the AP. Overall, these results demonstrate a novel regulatory pathway that links CaN, RCAN1, and Kv3.4 in DRG neurons. Dysregulation of this pathway might underlie a peripheral mechanism of pain sensitization induced by SCI.SIGNIFICANCE STATEMENT Pain sensitization associated with spinal cord injury (SCI) involves poorly understood maladaptive modulation of neuronal excitability. Although central mechanisms have received significant attention, recent studies have identified peripheral nerve hyperexcitability as a driver of persistent pain signaling after SCI. However, the ion channels and signaling molecules responsible for this change in primary sensory neuron excitability are still not well defined. To address this problem, this study used complementary electrophysiological and molecular methods to determine how Kv3.4, a voltage-gated K+ channel robustly expressed in dorsal root ganglion neurons, becomes dysfunctional upon calcineurin (CaN) inhibition. The results strongly suggest that CaN inhibition underlies SCI-induced dysfunction of Kv3.4 and the associated excitability changes through upregulation of the native regulator of CaN 1 (RCAN1).