Neurofilament immunoreactivity in populations of rat primary afferent neurons: a quantitative study of phosphorylated and non-phosphorylated subunits

The RNA-Binding Protein HuR Is Integral to the Function of Nociceptors in Mice and Humans

HuR is an RNA-binding protein implicated in RNA processing, stability, and translation. Previously, we examined protein synthesis in dorsal root ganglion (DRG) neurons treated with inflammatory mediators using ribosome profiling. We found that the HuR consensus binding element was enriched in transcripts with elevated translation. HuR is expressed in the soma of nociceptors and their axons. Pharmacologic inhibition of HuR with the small molecule CMLD-2 reduced the activity of mouse and human sensory neurons. Peripheral administration of CMLD-2 in the paw or genetic elimination of HuR from sensory neurons diminished behavioral responses associated with NGF- and IL-6-induced allodynia in male and female mice. Genetic disruption of HuR altered the proximity of mRNA decay factors near a key neurotrophic factor (TrkA). Collectively, the data suggest that HuR is required for local control of mRNA stability and reveals a new biological function for a broadly conserved post-transcriptional regulatory factor.SIGNIFICANCE STATEMENT Nociceptors undergo long-lived changes in excitability, which may contribute to chronic pain. Noxious cues that promote pain lead to rapid induction of protein synthesis. The underlying mechanisms that confer specificity to mRNA control in nociceptors are unclear. Here, we identify a conserved RNA-binding protein called HuR as a key regulatory factor in sensory neurons. Using a combination of genetics and pharmacology, we demonstrate that HuR is required for signaling in nociceptors. In doing so, we report an important mechanism of mRNA control in sensory neurons that ensures appropriate nociceptive responses to inflammatory mediators.

Chronic Pain Causes Peripheral and Central Responses in MIA-Induced TMJOA Rats

Chronic pain is the predominant symptom that drives temporomandibular joint osteoarthritis (TMJOA) patients to seek medical care; however, currently used treatment modalities remain less effective. This study aimed to investigate chronic pain and the peripheral and central responses in monoiodoacetate (MIA)-induced TMJOA rats. First, the appropriate dose of MIA was determined based on pain behavior assessment in rats. Alterations of the condylar structure in TMJOA rats were evaluated by histological staining and micro-computed tomography (micro-CT). Second, the period of TMJOA chronic pain was further explored by assessing the numbers of glial fibrillary acidic protein (GFAP)-positive astrocytes and ionized calcium-binding adaptor molecule 1 (IBA-1)-positive microglia in the trigeminal spinal nucleus (TSN) and performing nonsteroidal anti-inflammatory drug (NSAID) efficacy experiments. Finally, the expression of neurofilament 200 (NF200), calcitonin gene-related peptide (CGRP), and isolectin B4 (IB4) in the trigeminal ganglion (TG) and TSN was assessed by immunofluorescence. MIA at 4 mg/kg was considered an appropriate dose. Gradual MIA-induced alterations of the condylar structure were correlated with temporomandibular joint (TMJ) pain. The numbers of GFAP- and IBA-1-positive cells were increased at 2, 3, and 4 weeks after MIA injection. NSAIDs failed to alleviate pain behavior 10 days after MIA injection. CGRP and IB4 levels in the TG and TSN were upregulated at 2 and 4 weeks. These results suggest that TMJOA-related chronic pain emerged 2 weeks after MIA injection. CGRP- and IB4-positive afferents in both the peripheral and central nervous systems may be involved in MIA-induced TMJOA-related chronic pain in rats.

NeuriteNet: A convolutional neural network for assessing morphological parameters of neurite growth

BACKGROUND

During development or regeneration, neurons extend processes (i.e., neurites) via mechanisms that can be readily analyzed in culture. However, defining the impact of a drug or genetic manipulation on such mechanisms can be challenging due to the complex arborization and heterogeneous patterns of neurite growth in vitro. New Method: NeuriteNet is a Convolutional Neural Network (CNN) sorting model that uses a novel adaptation of the XRAI saliency map overlay, which is a region-based attribution method. NeuriteNet compares neuronal populations based on differences in neurite growth patterns, sorts them into respective groups, and overlays a saliency map indicating which areas differentiated the image for the sorting procedure.

RESULTS

In this study, we demonstrate that NeuriteNet effectively sorts images corresponding to dissociated neurons into control and treatment groups according to known morphological differences. Furthermore, the saliency map overlay highlights the distinguishing features of the neuron when sorting the images into treatment groups. NeuriteNet also identifies novel morphological differences in neurons cultured from control and genetically modified mouse strains. Comparison with Existing Methods: Unlike other neurite analysis platforms, NeuriteNet does not require manual manipulations, such as segmentation of neurites prior to analysis, and is more accurate than experienced researchers for categorizing neurons according to their pattern of neurite growth.

CONCLUSIONS

NeuriteNet can be used to effectively screen for morphological differences in a heterogeneous group of neurons and to provide feedback on the key features distinguishing those groups via the saliency map overlay.

Dynamic early clusters of nodal proteins contribute to node of Ranvier assembly during myelination of peripheral neurons

Voltage-gated sodium channels cluster in macromolecular complexes at nodes of Ranvier to promote rapid nerve impulse conduction in vertebrate nerves. Node assembly in peripheral nerves is thought to be initiated at heminodes at the extremities of myelinating Schwann cells, and fusion of heminodes results in the establishment of nodes. Here we show that assembly of 'early clusters' of nodal proteins in the murine axonal membrane precedes heminode formation. The neurofascin (Nfasc) proteins are essential for node assembly, and the formation of early clusters also requires neuronal Nfasc. Early clusters are mobile and their proteins are dynamically recruited by lateral diffusion. They can undergo fusion not only with each other but also with heminodes, thus contributing to the development of nodes in peripheral axons. The formation of early clusters constitutes the earliest stage in peripheral node assembly and expands the repertoire of strategies that have evolved to establish these essential structures.

Cyclic Stretch of Either PNS or CNS Located Nerves Can Stimulate Neurite Outgrowth

The central nervous system (CNS) does not recover from traumatic axonal injury, but the peripheral nervous system (PNS) does. We hypothesize that this fundamental difference in regenerative capacity may be based upon the absence of stimulatory mechanical forces in the CNS due to the protective rigidity of the vertebral column and skull. We developed a bioreactor to apply low-strain cyclic axonal stretch to adult rat dorsal root ganglia (DRG) connected to either the peripheral or central nerves in an explant model for inducing axonal growth. In response, larger diameter DRG neurons, mechanoreceptors and proprioceptors showed enhanced neurite outgrowth as well as increased Activating Transcription Factor 3 (ATF3).

G Protein-Gated Inwardly Rectifying Potassium Channel Subunit 3 is Upregulated in Rat DRGs and Spinal Cord After Peripheral Nerve Injury

Background

G protein-gated inwardly rectifying potassium (GIRK) channels are involved in the regulation of neuronal excitability. Four GIRK subunits (GIRK1-4) are expressed in rat dorsal root ganglia (DRGs). Recently, we have characterized the expression of GIRK1 and −2, and both are downregulated in rat DRGs and spinal cord after a complete sciatic nerve transection (axotomy). Here, we aimed to study the neurochemical characteristics of GIRK3, and its regulation in rat DRGs and spinal cord induced by nerve injury.

Methods

A sciatic nerve axotomy was performed to study the influences of injury on GIRK3 expression in DRGs and spinal cord. A dorsal root rhizotomy and a sciatic nerve crush were employed to study the axonal transport of GIRK3 protein, respectively. Immunohistochemistry analysis was employed for investigating the neurochemical characteristics of GIRK3.

Results

In control DRGs, ~18% of neuron profiles (NPs) were GIRK3-positive (⁺), and ~41%, ~48% and ~45% of GIRK3⁺ NPs were CGRP⁺, IB4⁺ and NF200⁺, respectively. GIRK3-like immunoreactivity was observed in glabrous skin of hind paws and axons originating from DRG neurons. Fourteen days after axotomy, more than one-third of DRG NPs were GIRK3⁺, and among these ~51% and ~56% coexpressed galanin and neuropeptide Y, respectively. In control animals, a small group of interneurons found in the dorsal horn was GIRK3⁺. In addition, GIRK3⁺ processes could be observed in superficial laminae of spinal dorsal horn. After nerve injury, the intensity of GIRK3-like immunoreactivity in the superficial layers was increased. Evidence based on rhizotomy and sciatic nerve crush indicated both anterograde and retrograde transport of GIRK3.

Conclusion

Our study demonstrates that GIRK3 is expressed in sensory neurons and spinal cord. GIRK3 has both anterograde and retrograde axonal transport. GIRK3 expression can be regulated by peripheral nerve injury.

Nociceptor subtypes and their incidence in rat lumbar dorsal root ganglia (DRGs): focussing on C-polymodal nociceptors, Aβ-nociceptors, moderate pressure receptors and their receptive field depths

A recent study with Ca++-sensitive-dyes in neurons in whole DRGs (Table 5) found that much lower percentages of nociceptors were polymodal-nociceptors (PMNs) (Emery et al., 2016), than the 50-80% values in many electrophysiological fiber studies. This conflict highlighted the lack of knowledge about percentages of nociceptor-subtypes in the DRG. This was analysed from intracellularly-recorded neurons in rat lumbar DRGs stimulated from outside the skin. Polymodal nociceptors (PMNs) were 11% of all neurons and 19% of all nociceptors. Most PMNs had C-fibers (CPMNs). Percentages of C-nociceptors that were CPMNs varied with receptive field (RF) depths, whether superficial (∼80%), dermal (25%), deep (0%) or cutaneous (superficial + dermal) (40%). This explains CPMN percentages 40-90%, being highest, in electrophysiological studies using cutaneous nerves, and lowest in studies that also include deep RFs, including ours, and the recent Ca++-imaging studies in whole DRGs. Despite having been originally described in 1967 (Burgess and Perl), both Aβ-nociceptors and Aβ-moderate pressure receptors (MPRs) remain overlooked. Most A-fiber nociceptors in rodents have Aβ-fibers. Of rat lumbar Aβ-nociceptors with superficial RFs, 50% were MPRs with variable medium-low trkA-expression. Despite having conduction velocities at the two extremes for nociceptors, both CPMNs and MPRs have relatively low thresholds, superficial/epidermal RFs and low trkA-expression. For abbreviations used see Table 5.

TRESK K+ Channel Activity Regulates Trigeminal Nociception and Headache

Although TWIK-related spinal cord K⁺ (TRESK) channel is expressed in all primary afferent neurons in trigeminal ganglia (TG) and dorsal root ganglia (DRG), whether TRESK activity regulates trigeminal pain processing is still not established. Dominant-negative TRESK mutations are associated with migraine but not with other types of pain in humans, suggesting that genetic TRESK dysfunction preferentially affects the generation of trigeminal pain, especially headache. Using TRESK global knock-out mice as a model system, we found that loss of TRESK in all TG neurons selectively increased the intrinsic excitability of small-diameter nociceptors, especially those that do not bind to isolectin B4 (IB4^-). Similarly, loss of TRESK resulted in hyper-excitation of the small IB4^- dural afferent neurons but not those that bind to IB4 (IB4⁺). Compared with wild-type littermates, both male and female TRESK knock-out mice exhibited more robust trigeminal nociceptive behaviors, including headache-related behaviors, whereas their body and visceral pain responses were normal. Interestingly, neither the total persistent outward current nor the intrinsic excitability was altered in adult TRESK knock-out DRG neurons, which may explain why genetic TRESK dysfunction is not associated with body and/or visceral pain in humans. We reveal for the first time that, among all primary afferent neurons, TG nociceptors are the most vulnerable to the genetic loss of TRESK. Our findings indicate that endogenous TRESK activity regulates trigeminal nociception, likely through controlling the intrinsic excitability of TG nociceptors. Importantly, we provide evidence that genetic loss of TRESK significantly increases the likelihood of developing headache.

Intraarticular injection of processed lipoaspirate cells has anti-inflammatory and analgesic effects but does not improve degenerative changes in murine monoiodoacetate-induced osteoarthritis

BACKGROUND

Previous basic research and clinical studies examined the effects of mesenchymal stem cells (MSCs) on regeneration and maintenance of articular cartilage. However, our pilot study suggested that MSCs are more effective at suppressing inflammation and pain rather than promoting cartilage regeneration in osteoarthritis. Adipose tissue is considered a useful source of MSCs; it can be harvested easily in larger quantities compared with the bone marrow. The present study was designed to evaluate the anti-inflammatory, analgesic, and regenerative effects of intra-articularly injected processed lipoaspirate (PLA) cells (containing adipose-derived MSCs) on degenerative cartilage in a rat osteoarthritis model.

METHODS

PLA cells were isolated from subcutaneous adipose tissue of 12-week-old female Sprague-Dawley rats. Osteoarthritis was induced by injection of monoiodoacetate (MIA). Each rat received 1 × 10⁶ MSCs into the joint at day 7 (early injection group) and day 14 (late injection group) post-MIA injection. At 7, 14, 21 days after MIA administration, pain was assessed by immunostaining and western blotting of dorsal root ganglion (DRG). Cartilage quality was assessed macroscopically and by safranin-O and H&E staining, and joint inflammation was assessed by western blotting of the synovium.

RESULTS

The early injection group showed less cartilage degradation, whereas the late injection group showed cartilage damage similar to untreated OA group. The relative expression level of CGRP protein in DRG neurons was significantly lower in the two treatment groups, compared with the untreated group.

CONCLUSIONS

Intra-articular injection of PLA cells prevented degenerative changes in the early injection group, but had little effect in promoting cartilage repair in the late injection group. Interestingly, intra-articular injection of PLA cells resulted in suppression of inflammation and pain in both OA groups. Further studies are needed to determine the long-term effects of intra-articular injection of PLA cells in osteoarthritis.

Selective tracking of FFAR3-expressing neurons supports receptor coupling to N-type calcium channels in mouse sympathetic neurons

Activation of short-chain free fatty acid receptors 3 (FFAR3) has been suggested to promote sympathetic outflow in postganglionic sympathetic neurons or hamper it by a negative coupling to N-type calcium (Ca_V2.2) channels. Heterogeneity of FFAR3 expression in sympathetic neurons, however, renders single neurons studies extremely time-consuming in wild-type mice. Previous studies demonstrated large variability of the degree of Ca_V2.2 channel inhibition by FFAR3 in a global population of rat sympathetic neurons. Therefore, we focused on a small subpopulation of mouse sympathetic neurons using an FFAR3 antibody and an Ffar3 reporter mouse to perform immunofluorescent and electrophysiological studies. Whole-cell patch-clamp recordings of identified FFAR3-expressing neurons from reporter mice revealed a 2.5-fold decrease in the Ca_V2.2-FFAR3 inhibitory coupling variability and 1.5-fold increase in the mean I_Ca²⁺ inhibition, when compared with unlabeled neurons from wild-type mice. Further, we found that the ablation of Ffar3 gene expression in two knockout mouse models led to a complete loss-of-function. Subpopulations of sympathetic neurons are associated with discrete functional pathways. However, little is known about the neural pathways of the FFAR3-expressing subpopulation. Our data indicate that FFAR3 is expressed primarily in neurons with a vasoconstrictor phenotype. Thus, fine-tuning of chemically-coded neurotransmitters may accomplish an adequate outcome.

Optically Guided Single Cell Mass Spectrometry of Rat Dorsal Root Ganglia to Profile Lipids, Peptides and Proteins

The mammalian dorsal root ganglia (DRG) are located on the dorsal roots of the spinal nerves and contain cell bodies of primary sensory neurons. DRG cells have been classified into subpopulations based on their size, morphology, intracellular markers, response to stimuli, and neuropeptides. To understand the connections between DRG chemical heterogeneity and cellular function, we performed optically guided, high-throughput single cell profiling using sequential matrix-assisted laser desorption/ionization mass spectrometry (MS) to detect lipids, peptides, and several proteins in individual DRG cells. Statistical analysis of the resulting mass spectra allows stratification of the DRG population according to cellular morphology and, presumably, major cell types. A subpopulation of small cells contained myelin proteins, which are abundant in Schwann cells, and mass spectra of several larger cells contained peaks matching neurofilament, vimentin, myelin basic protein S, and thymosin beta proteins. Of the over 1000 cells analyzed, approximately 78 % produced putative peptide-rich spectra, allowing the population to be classified into three distinct cell types. Two signals with m/z 4404 and 5487 were exclusively observed in a cell type, but could not be matched to results of our previous liquid chromatography-MS analyses.

Non-myelinated C-fibers, but not myelinated A-fibers, elongate into the epidermis in dry skin with itch

Chronic skin diseases with itch and dry skin show increased peripheral nerve fiber elongation into the epidermis. However, the characteristics of the elongated nerve fibers remain unclear. Therefore, we investigated the characteristics of the elongated nerve fibers using a dry skin mouse model with itch. In this mouse model, prepared via repetitive treatments with an acetone/ether mixture and water, the stratum corneum water content was decreased, whereas spontaneous scratching and epidermal hyperplasia were increased. In addition, the number of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers (C-fibers) was increased in the epidermis of treated mice compared to that in non-treated control mice. However, neurofilament 200-immunoreactive nerve fibers (A-fibers) were not detected in the epidermis of treated mice. These results suggest that the elongated epidermal peripheral nerve fibers comprise SP/CGRP-containing C-fibers but not A-fibers. Thus, these fibers may be involved in the induction of dry skin pruritus.

Distribution and neurochemistry of porcine urinary bladder-projecting sensory neurons in subdomains of the dorsal root ganglia: A quantitative analysis

The aim of the present study has been to verify the inter- and intraganglionic distribution pattern of porcine urinary bladder-projecting (UBP) neurons localized in the sacral dorsal root ganglia (DRGs). The morphology and chemical phenotype of these cells have also been investigated. These neurons were visualized using the fluorescent tracer Fast Blue (FB) which was injected bilaterally into the urinary bladder wall of five juvenile female pigs. The intraganglionic distribution showed that small- and medium-sized FB+ perikarya were mainly located in the central (S₃-S₄) and periphero-central (S₂) region of the ganglia, while large cells were heterogeneously distributed. Immunohistochemistry revealed that the most frequently observed markers in small and medium-sized UBP perikarya were: neurofilament 200, lectin from Bandeiraea simplicifolia (Griffonia simplicifolia) isolectin B4, substance P, calcitonin gene-related peptide, pituitary adenylate cyclase-activating polypeptide and transient receptor potential vanilloid 1. Moreover, UBP neurons containing these substances were also mainly observed in the central and periphero-central region of the ganglion. Differences in the percentage of traced cells and their neuropeptide content were observed between the S₂, S₃ and S₄ DRGs. In conclusion, the present study, for the first time, describes the arrangement of UBP DRGs neurons within particular subdomains of sacral ganglia, taking into account their size and chemical phenotype.

Quantitative Analysis of Mouse Dural Afferent Neurons Expressing TRPM8, VGLUT3, and NF200

OBJECTIVE

To quantify the abundance of dural afferent neurons expressing transient receptor potential channel melastatin 8 (TRPM8), vesicular glutamate transporter 3 (VGLUT3), and neurofilament 200 (NF200) in adult mice.

BACKGROUND

With the increasing use of mice as a model system to study headache mechanisms, it is important to understand the composition of dural afferent neurons in mice. In a previous study, we have measured the abundance of mouse dural afferent neurons that express neuropeptide calcitonin gene-related peptide as well as two TRP channels TRPV1 and TRPA1, respectively. Here, we conducted quantitative analysis of three other dural afferent subpopulations in adult mice.

METHODS

We used the fluorescent tracer Fluoro-Gold to retrogradely label dural afferent neurons in adult mice expressing enhanced green fluorescent protein in discrete subpopulations of trigeminal ganglion (TG) neurons. Mechanoreceptors with myelinated fibers were identified by NF200 immunoreactivity. We also conducted Ca²⁺ -imaging experiments to test the overlap between TRPM8 and VGLUT3 expression in mouse primary afferent neurons (PANs).

RESULTS

The abundance of TRPM8-expressing neurons in dural afferent neurons was significantly lower than that in total TG neurons. The percentages of dural afferent neurons expressing VGLUT3 and NF200 were comparable to those of total TG neurons, respectively. TRPM8 agonist menthol evoked Ca²⁺ influx in less than 7% VGLUT3-expressing PANs in adult mice.

CONCLUSIONS

TG neurons expressing TRPM8, VGLUT3, and NF200 all innervate adult mouse dura. TRPM8 and VGLUT3 are expressed in distinct subpopulations of PANs in adult mice. These results provide an anatomical basis to investigate headache mechanisms in mouse models.

Nociceptive DRG neurons express muscle lim protein upon axonal injury

Muscle lim protein (MLP) has long been regarded as a cytosolic and nuclear muscular protein. Here, we show that MLP is also expressed in a subpopulation of adult rat dorsal root ganglia (DRG) neurons in response to axonal injury, while the protein was not detectable in naïve cells. Detailed immunohistochemical analysis of L4/L5 DRG revealed ~3% of MLP-positive neurons 2 days after complete sciatic nerve crush and maximum ~10% after 4–14 days. Similarly, in mixed cultures from cervical, thoracic, lumbar and sacral DRG ~6% of neurons were MLP-positive after 2 days and maximal 17% after 3 days. In both, histological sections and cell cultures, the protein was detected in the cytosol and axons of small diameter cells, while the nucleus remained devoid. Moreover, the vast majority could not be assigned to any of the well characterized canonical DRG subpopulations at 7 days after nerve injury. However, further analysis in cell culture revealed that the largest population of MLP expressing cells originated from non-peptidergic IB4-positive nociceptive neurons, which lose their ability to bind the lectin upon axotomy. Thus, MLP is mostly expressed in a subset of axotomized nociceptive neurons and can be used as a novel marker for this population of cells.

Efficacy of hyaluronic acid or steroid injections for the treatment of a rat model of rotator cuff injury

This study evaluated dorsal root ganglia from C3-C7, analyzed gait, and compared the expression of calcitonin gene-related peptide (CGRP) which was a marker of inflammatory pain in a rat rotator cuff tear model in which the supraspinatus and infraspinatus tendons were detached; comparisons were made to a sham group in which only the tendons were exposed. Fluorogold was injected into the glenohumeral joint 21 days after surgery in both groups, and saline, steroids, or hyaluronic acid was injected into the glenohumeral joint in the rotator cuff tear group 26 days after surgery. The proportions of CGRP-immunoreactive neurons were higher and the gait parameters were impaired in the rotator cuff tear group compared to in the sham group. However, the CGRP expression was reduced and the gait was improved with steroid or hyaluronic acid injection compared to saline, suggesting that both hyaluronic acid and steroid injections suppressed of inflammation which thought to be provided pain relief. While there were no significant differences, the suppression of CGRP expression and the improved gait after hyaluronic acid and steroid injections suggested that both methods were effective for rat rotator cuff tear model.

The potential of metabolomic analysis techniques for the characterisation of α1-adrenergic receptors in cultured N1E-115 mouse neuroblastoma cells

Several studies of neuropathic pain have linked abnormal adrenergic signalling to the development and maintenance of pain, although the mechanisms underlying this are not yet fully understood. Metabolomic analysis is a technique that can be used to give a snapshot of biochemical status, and can aid in the identification of the mechanisms behind pathological changes identified in cells, tissues and biological fluids. This study aimed to use gas chromatography-mass spectrometry-based metabolomic profiling in combination with reverse transcriptase-polymerase chain reaction and immunocytochemistry to identify functional α1-adrenergic receptors on cultured N1E-115 mouse neuroblastoma cells. The study was able to confirm the presence of mRNA for the α1D subtype, as well as protein expression of the α1-adrenergic receptor. Furthermore, metabolomic data revealed changes to the metabolite profile of cells when exposed to adrenergic pharmacological intervention. Agonist treatment with phenylephrine hydrochloride (10 µM) resulted in altered levels of several metabolites including myo-inositol, glucose, fructose, alanine, leucine, phenylalanine, valine, and n-acetylglutamic acid. Many of the changes observed in N1E-115 cells by agonist treatment were modulated by additional antagonist treatment (prazosin hydrochloride, 100 µM). A number of these changes reflected what is known about the biochemistry of α1-adrenergic receptor activation. This preliminary study therefore demonstrates the potential of metabolomic profiling to confirm the presence of functional receptors on cultured cells.

G protein-gated inwardly rectifying potassium channel subunits 1 and 2 are down-regulated in rat dorsal root ganglion neurons and spinal cord after peripheral axotomy

Background

Increased nociceptive neuronal excitability underlies chronic pain conditions. Various ion channels, including sodium, calcium and potassium channels have pivotal roles in the control of neuronal excitability. The members of the family of G protein-gated inwardly rectifying potassium (GIRK) channels, GIRK1–4, have been implicated in modulating excitability. Here, we investigated the expression and distribution of GIRK1 and GIRK2 in normal and injured dorsal root ganglia (DRGs) and spinal cord of rats.

Results

We found that ~70% of the DRG neurons expressed GIRK1, while only <10% expressed GIRK2. The neurochemical profiles of GIRK1- and GIRK2-immunoreactive neurons were characterized using the neuronal markers calcitonin gene-related peptide, isolectin-B4 and neurofilament-200, and the calcium-binding proteins calbindin D28k, calretinin, parvalbumin and secretagogin. Both GIRK subunits were expressed in DRG neurons with nociceptive characteristics. However, while GIRK1 was widely expressed in several sensory neuronal subtypes, GIRK2 was detected mainly in a group of small C-fiber neurons. In the spinal dorsal horn, GIRK1- and -2-positive cell bodies and processes were mainly observed in lamina II, but also in superficial and deeper layers. Abundant GIRK1-, but not GIRK2-like immunoreactivity, was found in the ventral horn (laminae VI–X). Fourteen days after axotomy, GIRK1 and GIRK2 were down-regulated in DRG neurons at the mRNA and protein levels. Both after axotomy and rhizotomy there was a reduction of GIRK1- and -2-positive processes in the dorsal horn, suggesting a presynaptic localization of these potassium channels. Furthermore, nerve ligation caused accumulation of both subunits on both sides of the lesion, providing evidence for anterograde and retrograde fast axonal transport.

Conclusions

Our data support the hypothesis that reduced GIRK function is associated with increased neuronal excitability and causes sensory disturbances in post-injury conditions, including neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0044-z) contains supplementary material, which is available to authorized users.

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

Piezo2 expression in corneal afferent neurons

Recently, a novel class of mechanically sensitive channels has been identified and have been called Piezo channels. In this study, we explored Piezo channel expression in sensory neurons supplying the guinea pig corneal epithelium, which have well-defined modalities in this species. We hypothesized that a proportion of corneal afferent neurons express Piezo2, and that these neurons are neurochemically distinct from corneal polymodal nociceptors or cold-sensing neurons. We used a combination of retrograde tracing to identify corneal afferent neurons and double label in situ hybridization and/or immunohistochemistry to determine their molecular and/or neurochemical profile. We found that Piezo2 expression occurs in ∼26% of trigeminal ganglion neurons and 30% of corneal afferent neurons. Piezo2 corneal afferent neurons are almost exclusively non-calcitonin gene-related peptide (CGRP)-immunoreactive (-IR), medium- to large-sized neurons that are NF200-IR, suggesting they are not corneal polymodal nociceptors. There was no coexpression of Piezo2 and transient receptor potential cation channel subfamily M member 8 (TRPM8) transcripts in any corneal afferent neurons, further suggesting that Piezo2 is not expressed in corneal cold-sensing neurons. We also noted that TRPM8-IR or CGRP-IR corneal afferent neurons are almost entirely small and lack NF200-IR. Piezo2 expression occurs in a neurochemically distinct subpopulation of corneal afferent neurons that are not polymodal nociceptors or cold-sensing neurons, and is likely confined to a subpopulation of pure mechano-nociceptors in the cornea. This provides the first evidence in an in vivo system that Piezo2 is a strong candidate for a channel that transduces noxious mechanical stimuli.

Differential upregulation in DRG neurons of an α2δ-1 splice variant with a lower affinity for gabapentin after peripheral sensory nerve injury

The α2δ-1 protein is an auxiliary subunit of voltage-gated calcium channels, critical for neurotransmitter release. It is upregulated in dorsal root ganglion (DRG) neurons following sensory nerve injury, and is also the therapeutic target of the gabapentinoid drugs, which are efficacious in both experimental and human neuropathic pain conditions. α2δ-1 has 3 spliced regions: A, B, and C. A and C are cassette exons, whereas B is introduced via an alternative 3' splice acceptor site. Here we have examined the presence of α2δ-1 splice variants in DRG neurons, and have found that although the main α2δ-1 splice variant in DRG is the same as that in brain (α2δ-1 ΔA+B+C), there is also another α2δ-1 splice variant (ΔA+BΔC), which is expressed in DRG neurons and is differentially upregulated compared to the main DRG splice variant α2δ-1 ΔA+B+C following spinal nerve ligation. Furthermore, this differential upregulation occurs preferentially in a small nonmyelinated DRG neuron fraction, obtained by density gradient separation. The α2δ-1 ΔA+BΔC splice variant supports CaV2 calcium currents with unaltered properties compared to α2δ-1 ΔA+B+C, but shows a significantly reduced affinity for gabapentin. This variant could therefore play a role in determining the efficacy of gabapentin in neuropathic pain.

Dichotomizing sensory nerve fibers innervating both the lumbar vertebral body and the area surrounding the iliac crest: a possible mechanism of referred lateral back pain from lumbar vertebral body

STUDY DESIGN

Animal study.

OBJECTIVE

To determine the existence of dichotomizing sensory nerve fibers innervating both the lumbar vertebral body and the area surrounding the iliac crest (ASIC).

SUMMARY OF BACKGROUND DATA

Elderly patients with osteoporosis sometimes experience lumbar vertebral fracture and may feel diffuse nonlocalized pain in the back, the lateral portion of the trunk, and the ASIC. The pattern of sensory innervation of vertebral bodies remains unclear. DRG neurons with dichotomizing axons have been reported and are thought to be related to referred pain. The purpose of this study was to investigate the existence of dichotomizing axons to the lumbar vertebral bodies and the ASIC in rats.

METHODS

Two kinds of neurotracers (1,1´-dioctadecyl-3,3,3´,3´-tetramethylindocarbocyanine perchlorate [DiI] and Fluoro-Gold [FG]) were used. DiI crystals were placed in the left ASIC, and FG was applied into the L2 vertebral body in 10 rats. Four weeks later, left DRGs from L1 to L6 were resected, sectioned, and observed under a fluorescence microscope.

RESULTS

DiI-labeled DRG neurons innervating the ASIC and FG-labeled DRG neurons innervating the vertebral L2 body were distributed from L1 to L6. The ratio of total double-labeled per total DiI-labeled DRG neurons was 10.2%, and that of total double-labeled per total FG-labeled DRG neurons was 14.7%. These double-labeled DRG neurons innervating the L2 vertebral body had other axons that extended to the ASIC.

CONCLUSION

This finding provides a possible neuroanatomical explanation for referred pain in the ASIC from vertebral bodies.

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

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

Differentiation of nerve fibers storing CGRP and CGRP receptors in the peripheral trigeminovascular system

Primary headaches such as migraine are postulated to involve the activation of sensory trigeminal pain neurons that innervate intracranial blood vessels and the dura mater. It is suggested that local activation of these sensory nerves may involve dural mast cells as one factor in local inflammation, causing sensitization of meningeal nociceptors. Immunofluorescence was used to study the detailed distribution of calcitonin gene-related peptide (CGRP) and its receptor components calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) in whole-mount rat dura mater and in human dural vessels. The relative distributions of CGRP, CLR, and RAMP1 were evaluated with respect to each other and in relationship to mast cells, myelin, substance P, neuronal nitric oxide synthase, pituitary adenylate cyclase-activating polypeptide, and vasoactive intestinal peptide. CGRP expression was found in thin unmyelinated fibers, whereas CLR and RAMP1 were expressed in thicker myelinated fibers coexpressed with an A-fiber marker. CLR and RAMP1 immunoreactivity colocalized with mast cell tryptase in rodent; however, expression of both receptor components was not observed in human mast cells. Immunoreactive substance P fibers coexpressed CGRP, although neuronal nitric oxide synthase and vasoactive intestinal peptide expression was very limited, and these fibers were distinct from the CGRP-positive fibers. Few pituitary adenylate cyclase-activating polypeptide immunoreactive fibers occurred and some colocalized with CGRP.

PERSPECTIVE

This study demonstrates the detailed distribution of CGRP and its receptor in the dura mater. These data suggest that CGRP is expressed in C-fibers and may act on A-fibers, rodent mast cells, and vascular smooth muscle cells that express the CGRP receptor. These sites represent potential pathophysiological targets of novel antimigraine agents such as the newly developed CGRP receptor antagonists.

The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K⁺ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.

GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

Histamine modulation of acute nociception involves regulation of Nav 1.8 in primary afferent neurons in mice

AIMS

To explore the role of histamine in acute pain perception and its possible mechanisms.

METHODS

Pain-like behaviors induced by four types of noxious stimuli (hot-plate, tail-pressure, acetic acid, and formalin) were accessed in mice. Nav 1.8 expression and functions in primary afferent neurons were compared between histidine decarboxylase knockout (HDC(-/-) ) mice and their wild-types.

RESULTS

HDC(-/-) mice, lacking in endogenous histamine, showed elevated sensitivity to all these noxious stimuli, as compared with the wild-types. In addition, a depletion of endogenous histamine with α-fluoromethylhistidine (α-FMH), a specific HDC inhibitor, or feeding mice a low-histamine diet also enhanced nociception in the wild-types. Nav 1.8 expression in primary afferent neurons was increased both in HDC(-/-) and in α-FMH-treated wild-type mice. A higher Nav 1.8 current density, a lower action potential (AP) threshold, and a higher firing rate in response to suprathreshold stimulation were observed in nociception-related small DRG neurons of HDC(-/-) mice. Nav 1.8 inhibitor A-803467, but not TTX, diminished the hyperexcitability and blocked repetitive AP firing of these neurons.

CONCLUSION

Our results indicate that histamine participates in acute pain modulation in a dose-related manner. The regulation of Nav 1.8 expression and the excitability of nociceptive primary afferent neurons may be involved in the underlying mechanisms.

Expression of NPR-B in neurons of the dorsal root ganglia of the rat

C-type natriuretic peptide (CNP) is an abundant neuropeptide in the central nervous system, which exerts its physiological effects through natriuretic peptide receptor B (NPR-B). Recently, the CNP/NPR-B system has been recognized as an important regulator for the development of sensory axons. The dorsal root ganglion (DRG) contains neurons transmitting several kinds of spinal sensory stimuli to the central nervous system. In this study, we characterized NPR-B receptor expression in the rat DRG, using reverse transcription-polymerase chain reaction, Western blotting and immunohistochemistry. Immunostaining revealed that NPR-B was expressed in neuronal cell bodies and processes of the DRG, with NPR-B immunoreactivity mainly prominent in small and medium-sized DRG neurons. Double-immunolabeling showed that NPR-B was expressed in calcitonin gene-related peptide- and isolectin B4-positive neurons. Furthermore, NPR-B expression was co-localized with calcitonin gene-related peptide in the dorsal horn of the spinal cord. Together, our data suggest that the natriuretic peptides may perform several biological actions on sensory neurons via their binding to NPR-B in the DRG.

Sensory innervation of rat contracture shoulder model

BACKGROUND

To date, few studies have investigated the cause of pain experienced by patients with frozen shoulder. The purposes of this study were to establish a rat contracture model and clarify the innervation pattern of the glenohumeral (GH) joint and subacromial bursa (SAB) using immunohistochemistry in the dorsal root ganglion (DRG) neurons.

MATERIALS AND METHODS

The rat contracture models were made by tying the animal's humerus and scapula with No. 2-0 FiberWire (Arthrex, Naples, FL, USA). Contracture was confirmed on x-ray images taken 8 weeks after the operation. Subsequently, two kinds of neurotracers, Fluoro-Gold (FG) (Fluorochrome, Denver, CO, USA) and 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) (Molecular Probes, Eugene, OR, USA), were used to detect the GH joints and SAB separately. FG tracers were injected into GH joints, and DiI tracers were injected into the SAB. At 7 days after injection, DRGs were harvested between C1 and T1. Immunohistochemistry by use of calcitonin gene-related peptide (CGRP) was performed. CGRP is thought to be one of the causes of pain sensation in joint disease. We evaluated the percentages of FG-labeled CGRP-immunoreactive (CGRP-ir) neurons in the total number of FG-labeled neurons and of DiI-labeled CGRP-ir neurons in the total number of DiI-labeled neurons.

RESULTS

Abduction and total arc of the rotation were statistically significantly decreased in the contracture group. Furthermore, the percentage of CGRP-ir DRG neurons was significantly higher in the contracture group in both the GH joint and SAB.

CONCLUSION

These results show that pain sensation in rat shoulder contracture may be induced by the up-regulation of CGRP expression in DRG neurons.

Kv7.5 is the primary Kv7 subunit expressed in C-fibers

Kv7 (KCNQ) potassium channel openers (enhancers) decrease neuropathic pain in experimental models. Here we show that C-fibers, and their associated small-diameter neurons in the dorsal root ganglia (both IB4- and TrkA-positive), expressed Kv7.5. In contrast, C-fibers did not express detectable levels of Kv7.2 or Kv7.3, which are instead localized to nodes of Ranvier and the cell bodies of large sensory neurons. These data suggest that Kv7.5 provides the primary M current in nociceptive neurons.

Spinal nerve ligation decreases γ-aminobutyric acidB receptors on specific populations of immunohistochemically identified neurons in L5 dorsal root ganglion of the rat

This study examined the distribution of γ-aminobutyric acid (GABA)(B) receptors on immunohistochemically identified neurons, and levels of GABA(B(1)) and GABA(B(2)) mRNA, in the L4 and L5 dorsal root ganglia (DRG) of the rat in the absence of injury and 2 weeks after L5 spinal nerve ligation. In uninjured DRG, GABA(B(1)) immunoreactivity colocalized exclusively with the neuronal marker (NeuN) and did not colocalize with the satellite cell marker S-100. The GABA(B(1)) subunit colocalized to >97% of DRG neurons immunoreactive (IR) for neurofilament 200 (N52) or calcitonin gene-related peptide (CGRP), or labeled by isolectin B4 (IB4). Immunoreactivity for GABA(B(2)) was not detectable. L5 spinal nerve ligation did not alter the number of GABA(B(1)) -IR neurons or its colocalization pattern in the L4 DRG. However, ligation reduced the number of GABA(B(1)) -IR neurons in the L5 DRG by ≈38% compared with sham-operated and naïve rats. Specifically, ligation decreased the number of CGRP-IR neurons in the L5 DRG by 75%, but did not decrease the percent colocalization of GABA(B(1)) in those that remained. In the few IB4-positive neurons that remained in the L5 DRG, colocalization of GABA(B(1)) -IR decreased to 75%. Ligation also decreased levels of GABA(B(1)) and GABA(B(2)) mRNA in the L5, but not the L4 DRG compared with sham-operated or naïve rats. These findings indicate that the GABA(B) receptor is positioned to presynaptically modulate afferent transmission by myelinated, unmyelinated, and peptidergic afferents in the dorsal horn. Loss of GABA(B) receptors on primary afferent neurons may contribute to the development of mechanical allodynia after L5 spinal nerve ligation.

Cannabinoid type-1 receptor reduces pain and neurotoxicity produced by chemotherapy

Painful peripheral neuropathy is a dose-limiting complication of chemotherapy. Cisplatin produces a cumulative toxic effect on peripheral nerves, and 30-40% of cancer patients receiving this agent experience pain. By modeling cisplatin-induced hyperalgesia in mice with daily injections of cisplatin (1 mg/kg, i.p.) for 7 d, we investigated the anti-hyperalgesic effects of anandamide (AEA) and cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB597), an inhibitor of AEA hydrolysis. Cisplatin-induced mechanical and heat hyperalgesia were accompanied by a decrease in the level of AEA in plantar paw skin. No changes in motor activity were observed after seven injections of cisplatin. Intraplantar injection of AEA (10 μg/10 μl) or URB597 (9 μg/10 μl) transiently attenuated hyperalgesia through activation of peripheral CB₁ receptors. Co-injections of URB597 (0.3 mg/kg daily, i.p.) with cisplatin decreased and delayed the development of mechanical and heat hyperalgesia. The effect of URB597 was mediated by CB₁ receptors since AM281 (0.33 mg/kg daily, i.p.) blocked the effect of URB597. Co-injection of URB597 also normalized the cisplatin-induced decrease in conduction velocity of Aα/Aβ-fibers and reduced the increase of ATF-3 and TRPV1 immunoreactivity in dorsal root ganglion (DRG) neurons. Since DRGs are a primary site of toxicity by cisplatin, effects of cisplatin were studied on cultured DRG neurons. Incubation of DRG neurons with cisplatin (4 μg/ml) for 24 h decreased the total length of neurites. URB597 (100 nM) attenuated these changes through activation of CB₁ receptors. Collectively, these results suggest that pharmacological facilitation of AEA signaling is a promising strategy for attenuating cisplatin-associated sensory neuropathy.

Sensory and autonomic innervation of the cervical intervertebral disc in rats: the pathomechanics of chronic discogenic neck pain

STUDY DESIGN

An immunohistological analysis of the cervical intervertebral disc (IVD).

OBJECTIVE

To investigate sensory and autonomic innervation of the rat cervical IVD.

SUMMARY OF BACKGROUND DATA

Many clinicians are challenged with treating wide-ranging chronic neck pain. Several authors have reported that sympathetic nerves participate in chronic pain, and various sympathectomy procedures can effectively treat chronic pain.

METHODS

The neuro-tracer Fluoro-gold (FG) was applied to the anterior surfaces of C5-C6 IVDs from 10 Sprague-Dawley rats to label the neurons of the innervating dorsal root ganglion (DRG), stellate ganglion (SG; sympathetic ganglion), and nodose ganglion (NG; parasympathetic ganglion). Seven days postsurgery, DRGs from level C1-C8, SG, and NG neurons were harvested, sectioned, and immunostained for calcitonin gene-related peptide (CGRP; a marker for peptide-containing neurons) and isolectin B4 (IB4; a marker for nonpeptide-containing neurons). The proportion of FG-labeled DRG neurons that were CGRP-immunoreactive (CGRP-IR), IB4-binding, and non-CGRP-IR and IB4-binding, and the proportion of FG-labeled SG neurons and NG neurons were calculated.

RESULTS

FG-labeled neurons innervating the C5-C6 IVD were distributed throughout the C2-C8 DRGs. The proportions of FG-labeled DRG neurons that were CGRP-IR, IB4-binding, non-CGRP-IR and IB4-binding, as well as SG neurons, and NG neurons were 20.6%, 3.3%, 55.7%, 8.9%, and 11.5%, respectively. The proportion of CGRP-IR FG-labeled DRG neurons was significantly higher than the proportion of IB4-binding FG-labeled DRG neurons at each level (P < 0.05).

CONCLUSION

The C5-C6 IVD was innervated multisegmentally from neurons of the C2-C8 DRG, SG, and NG. Overall, 79.6% of the nerve fibers innervating the IVD were sensory nerves and 20.4% were autonomic nerves. Furthermore, 23.9% of the nerve fibers innervating the IVD were afferent sensory pain-related nerves, 8.9% were efferent sympathetic nerves, and 11.5% were efferent parasympathetic nerves. These findings may explain the wide-ranging and chronic discogenic pain that occurs via the somatosensory and autonomic nervous system.

Light-evoked somatosensory perception of transgenic rats that express channelrhodopsin-2 in dorsal root ganglion cells

In vertebrate somatosensory systems, each mode of touch-pressure, temperature or pain is sensed by sensory endings of different dorsal root ganglion (DRG) neurons, which conducted to the specific cortical loci as nerve impulses. Therefore, direct electrical stimulation of the peripheral nerve endings causes an erroneous sensation to be conducted by the nerve. We have recently generated several transgenic lines of rat in which channelrhodopsin-2 (ChR2) transgene is driven by the Thy-1.2 promoter. In one of them, W-TChR2V4, some neurons were endowed with photosensitivity by the introduction of the ChR2 gene, coding an algal photoreceptor molecule. The DRG neurons expressing ChR2 were immunohistochemically identified using specific antibodies to the markers of mechanoreceptive or nociceptive neurons. Their peripheral nerve endings in the plantar skin as well as the central endings in the spinal cord were also examined. We identified that ChR2 is expressed in a certain population of large neurons in the DRG of W-TChR2V4. On the basis of their morphology and molecular markers, these neurons were classified as mechanoreceptive but not nociceptive. ChR2 was also distributed in their peripheral sensory nerve endings, some of which were closely associated with CK20-positive cells to form Merkel cell-neurite complexes or with S-100-positive cells to form structures like Meissner's corpuscles. These nerve endings are thus suggested to be involved in the sensing of touch. Each W-TChR2V4 rat showed a sensory-evoked behavior in response to blue LED flashes on the plantar skin. It is thus suggested that each rat acquired an unusual sensory modality of sensing blue light through the skin as touch-pressure. This light-evoked somatosensory perception should facilitate study of how the complex tactile sense emerges in the brain.

Up-regulation of Cavβ3 subunit in primary sensory neurons increases voltage-activated Ca2+ channel activity and nociceptive input in neuropathic pain

High voltage-activated calcium channels (HVACCs) are essential for synaptic and nociceptive transmission. Although blocking HVACCs can effectively reduce pain, this treatment strategy is associated with intolerable adverse effects. Neuronal HVACCs are typically composed of α(1), β (Cavβ), and α(2)δ subunits. The Cavβ subunit plays a crucial role in the membrane expression and gating properties of the pore-forming α(1) subunit. However, little is known about how nerve injury affects the expression and function of Cavβ subunits in primary sensory neurons. In this study, we found that Cavβ(3) and Cavβ(4) are the most prominent subtypes expressed in the rat dorsal root ganglion (DRG) and dorsal spinal cord. Spinal nerve ligation (SNL) in rats significantly increased mRNA and protein levels of the Cavβ(3), but not Cavβ(4), subunit in the DRG. SNL also significantly increased HVACC currents in small DRG neurons and monosynaptic excitatory postsynaptic currents of spinal dorsal horn neurons evoked from the dorsal root. Intrathecal injection of Cavβ(3)-specific siRNA significantly reduced HVACC currents in small DRG neurons and the amplitude of monosynaptic excitatory postsynaptic currents of dorsal horn neurons in SNL rats. Furthermore, intrathecal treatment with Cavβ(3)-specific siRNA normalized mechanical hyperalgesia and tactile allodynia caused by SNL but had no significant effect on the normal nociceptive threshold. Our findings provide novel evidence that increased expression of the Cavβ(3) subunit augments HVACC activity in primary sensory neurons and nociceptive input to dorsal horn neurons in neuropathic pain. Targeting the Cavβ(3) subunit at the spinal level represents an effective strategy for treating neuropathic pain.

Angiotensin II-NADPH oxidase-derived superoxide mediates diabetes-attenuated cell excitability of aortic baroreceptor neurons

Overactivation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is involved in diabetes-depressed excitability of aortic baroreceptor neurons in nodose ganglia. This involvement links to the autonomic dysfunction associated with high morbidity and mortality in diabetic patients. The present study examined the effects of an angiotensin II type I receptor (AT(1)R) antagonist (losartan), a NADPH oxidase inhibitor (apocynin), and a superoxide dismutase mimetic (tempol) on the enhanced HCN currents and attenuated cell excitability in diabetic nodose neurons. In sham and streptozotocin-induced type 1 diabetic rats, HCN currents and cell excitability of aortic baroreceptor neurons were recorded by the whole cell patch-clamp technique. The angiotensin II level in nodose ganglia from diabetic rats was higher than that from sham rats (101.6 ± 4.8 vs. 38.9 ± 4.2 pg/mg protein, P < 0.05). Single-cell RT-PCR, Western blot, immunofluorescence staining, and chemiluminescence data showed that mRNA and protein expression of AT(1)R, protein expression of NADPH oxidase components, and superoxide production in nodose neurons were increased in diabetic rats compared with those from sham rats. HCN current density was higher and cell excitability was lower in aortic baroreceptor neurons from diabetic rats than that from sham rats. Losartan (1 μM), apocynin (100 μM), and tempol (1 mM) normalized the enhanced HCN current density and increased the cell excitability in the aortic baroreceptor neurons of diabetic rats. These findings suggest that endogenous angiotensin II-NADPH oxidase-superoxide signaling contributes to the enhanced HCN currents and the depressed cell excitation in the aortic baroreceptor neurons of diabetic rats.

Genotype-selective phenotypic switch in primary afferent neurons contributes to neuropathic pain

Pain is normally mediated by nociceptive Aδ and C fibers, while Aβ fibers signal touch. However, after nerve injury, Aβ fibers may signal pain. Using a genetic model, we tested the hypothesis that phenotypic switching in neurotransmitters expressed by Aβ afferents might account for heritable differences in neuropathic pain behavior. The study examined selection-line rats in which one line, high autotomy (HA), shows higher levels of spontaneous pain in the neuroma neuropathy model, and of tactile allodynia in the spinal nerve ligation (SNL) model, than the companion low autotomy (LA) line. Changes in calcitonin gene-related peptide (CGRP) and Substance P expression were evaluated immunohistochemically in L4 and L5 dorsal root ganglia 7 days after SNL surgery. Expression of CGRP was decreased in axotomized small- and medium-diameter neurons in both rat lines. However, in HA but not in LA rats, there was a tenfold increase in CGRP immunoreactivity (CGRP-IR) in large-diameter neurons. Corresponding changes in CGRP-IR in axon terminals in the nucleus gracilis were also seen. Finally, there were indications of enhanced CGRP neurotransmission in deep laminae of the dorsal horn. Substance P immunoreactivity was also upregulated in large-diameter neurons, but this change was similar in the 2 lines. Our findings suggest that phenotypic switching contributes to the heritable difference in pain behavior in HA vs LA rats. Specifically, we propose that in HA rats, but less so in LA rats, injured, spontaneously active Aβ afferents both directly drive CGRP-sensitive central nervous system pain-signaling neurons and also trigger and maintain central sensitization, hence generating spontaneous pain and tactile allodynia.

Pain-related sensory innervation in monoiodoacetate-induced osteoarthritis in rat knees that gradually develops neuronal injury in addition to inflammatory pain

Background

The exact mechanism of knee osteoarthritis (OA)-associated pain is unclear, whereas mixed evidence of inflammatory pain and neuropathic pain has been noted. We aimed to investigate pain-related sensory innervation in a monoiodoacetate (MIA)-induced model of OA.

Methods

Sixty of seventy female Sprague Dawley rats of six week-old underwent intra-articular MIA and fluorogold (FG) retrograde neurotracer injection into their right (ipsilateral) knee, while their left knees were treated with FG in saline as a control (contralateral knee). Other rats were treated with FG only bilaterally, and used as controls. Rats were evaluated for tactile allodynia using von Frey hairs. Proinflammatory mediators in the knee soft tissues, including tumor necrosis factor (TNF)-α, interleukin (IL)-6, and nerve growth factor (NGF), were quantified using ELISAs to evaluate inflammation in the knee after 1, 4, 7,14,21, and 28 days post injection:. Dorsal root ganglia (DRG) were immunostained for three molecules after 7,14,21, and 28 days post injection: calcitonin gene-related peptide (CGRP), a marker of inflammatory pain; and activating transcription factor-3 (ATF3) and growth associated protein-43 (GAP43), as markers for nerve injury and regenerating axons. The distribution of microglia in the spinal cord were also evaluated, because they have been reported to increase in neuropathic pain states. These evaluations were performed up to 28 days postinjection. P < 0.05 was considered significant.

Results

Progressive tactile allodynia and elevated cytokine concentrations were observed in ipsilateral knees. CGRP-immunoreactive (-ir) ipsilateral DRG neurons significantly increased, peaking at 14 days postinjection, while expression of FG-labeled ATF3-ir or ATF3-ir GAP43-ir DRG neurons significantly increased in a time-dependent manner. Significant proliferation of microglia were found with time in the ipsilateral dorsal horn.

Conclusions

Pain-related characteristics in a MIA-induced rat OA model can originate from an inflammatory pain state induced by the local inflammation initiated by inflammatory cytokines, and that state will be followed by gradual initiation of neuronal injury, which may induce the neuropathic pain state.

Phenotypic alterations of neurons that innervate osteoarthritic joints in rats

OBJECTIVE

Pain is a prominent feature of osteoarthritis (OA). To further understand the primary mechanisms of nociception in OA, we studied the expression of the phenotype markers calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), and neurofilament 200 (NF200) in sensory neurons innervating the OA knee joint in rats.

METHODS

OA was induced in rats by intraarticular injection of 2 mg of mono-iodoacetate (MIA) into the knee. Neurons innervating the joint were identified by retrograde labeling with fluorogold in dorsal root ganglia (DRG) and colocalized with neurochemical markers by immunofluorescence. The total number of DRG cells was determined by stereologic methods in Nissl-stained sections.

RESULTS

A 37% decrease in the number of fluorogold-backlabeled cells was observed in rats with OA when compared with control rats, even though no decrease in the total number of cells was observed. However, an increase in the number of medium/large cell bodies and a decrease in the number of the smallest cells were observed, suggesting the occurrence of perikarya hypertrophy. The percentage of CGRP-positive cells increased significantly, predominantly in medium/large cells, suggesting the occurrence of a phenotypic switch. Colocalization of CGRP and NF200 revealed no significant changes in the percentage of double-labeled cells, but an increase in the number of medium/large double-labeled cells was observed. No differences in the expression of either IB4 or NF200 were observed in fluorogold-backlabeled cells.

CONCLUSION

These results indicate that MIA-induced OA causes an up-regulation of CGRP in different subpopulations of primary afferent neurons in DRG due to a phenotypic switch and/or cell hypertrophy which may be functionally relevant in terms of the onset of pain in this pathologic condition.

Comparative study of voltage-gated sodium channel α-subunits in non-overlapping four neuronal populations in the rat dorsal root ganglion

Voltage-gated sodium channel α-subunit (Nav) is the major determinant of neuronal electrophysiological characters. In order to compare the composition of Navs among neurochemically different neurons in the rat dorsal root ganglion (DRG), we examined the expression of Nav transcripts in four non-overlapping neuronal populations, with (+) or without (-) N52 immunoreactivity, a marker of neurons with myelinated axons, and TrkA mRNA identified by in situ hybridization histochemistry. Both N52-/TrkA+ and N52-/TrkA- populations had high levels of signals for Nav1.7, Nav1.8, and Nav1.9 mRNAs, but rarely expressed Nav1.1 or Nav1.6. There was no significant difference in these signals, suggesting that C-fiber peptidergic and non-peptidergic neurons have similar electrophysiological characters with regard to sodium currents. N52+/TrkA+ neurons (putative A-fiber nociceptors) had similar high levels of signals for Nav1.7 and Nav1.8, but a significantly lower level of Nav1.9 signals, as compared to N52- neurons. Although, almost no N52+/TrkA- neurons had Nav1.8 or Nav1.9, half of this population expressed Nav1.7 at similar levels to other three populations and the other half completely lacked this channel. These data suggest that Nav1.8 is a common channel for both C- and A-fiber nociceptors, and Nav1.9 is rather selective for C-fiber nociceptors. Nav1.7 is the most universal channel while some functionally unknown N52+/TrkA- subpopulation selectively lacks it.

Reduced expression and activation of voltage-gated sodium channels contributes to blunted baroreflex sensitivity in heart failure rats

Voltage-gated sodium (Na(v)) channels are responsible for initiation and propagation of action potential in the neurons. To explore the mechanisms of chronic heart failure (CHF)-induced baroreflex dysfunction, we measured the expression and current density of Na(v) channel subunits (Na(v)1.7, Na(v)1.8, and Na(v)1.9) in the aortic baroreceptor neurons and investigated the role of Na(v) channels in aortic baroreceptor neuron excitability and baroreflex sensitivity in sham and CHF rats. CHF was induced by left coronary artery ligation. The development of CHF (6-8 weeks after the coronary ligation) was confirmed by hemodynamic and morphological characteristics. Immunofluorescent data indicated that Na(v)1.7 was expressed in A-type (myelinated) and C-type (unmyelinated) nodose neurons, but Na(v)1.8 and Na(v)1.9 were expressed only in C-type nodose neurons. Real-time RT-PCR and Western blot data showed that CHF reduced mRNA and protein expression levels of Na(v) channels in nodose neurons. In addition, using the whole-cell patch-clamp technique, we found that Na(v) current density and cell excitability of the aortic baroreceptor neurons were lower in CHF rats than that in sham rats. Aortic baroreflex sensitivity was blunted in anesthetized CHF rats, compared with that in sham rats. Furthermore, Na(v) channel activator (rATX II, 100 nM) significantly enhanced Na(v) current density and cell excitability of aortic baroreceptor neurons and improved aortic baroreflex sensitivity in CHF rats. These results suggest that reduced expression and activation of the Na(v) channels are involved in the attenuation of baroreceptor neuron excitability, which subsequently contributes to the impairment of baroreflex in CHF state.

Single-cell analysis of sodium channel expression in dorsal root ganglion neurons

Sensory neurons of the dorsal root ganglia (DRG) express multiple voltage-gated sodium (Na) channels that substantially differ in gating kinetics and pharmacology. Small-diameter (<25 μm) neurons isolated from the rat DRG express a combination of fast tetrodotoxin-sensitive (TTX-S) and slow TTX-resistant (TTX-R) Na currents while large-diameter neurons (>30 μm) predominately express fast TTX-S Na current. Na channel expression was further investigated using single-cell RT-PCR to measure the transcripts present in individually harvested DRG neurons. Consistent with cellular electrophysiology, the small neurons expressed transcripts encoding for both TTX-S (Nav1.1, Nav1.2, Nav1.6, and Nav1.7) and TTX-R (Nav1.8 and Nav1.9) Na channels. Nav1.7, Nav1.8 and Nav1.9 were the predominant Na channels expressed in the small neurons. The large neurons highly expressed TTX-S isoforms (Nav1.1, Nav1.6, and Nav1.7) while TTX-R channels were present at comparatively low levels. A unique subpopulation of the large neurons was identified that expressed TTX-R Na current and high levels of Nav1.8 transcript. DRG neurons also displayed substantial differences in the expression of neurofilaments (NF200, peripherin) and Necl-1, a neuronal adhesion molecule involved in myelination. The preferential expression of NF200 and Necl-1 suggests that large-diameter neurons give rise to thick myelinated axons. Small-diameter neurons expressed peripherin, but reduced levels of NF200 and Necl-1, a pattern more consistent with thin unmyelinated axons. Single-cell analysis of Na channel transcripts indicates that TTX-S and TTX-R Na channels are differentially expressed in large myelinated (Nav1.1, Nav1.6, and Nav1.7) and small unmyelinated (Nav1.7, Nav1.8, and Nav1.9) sensory neurons.