Axotomy results in major changes in BDNF expression by dorsal root ganglion cells: BDNF expression in large trkB and trkC cells, in pericellular baskets, and in projections to deep dorsal horn and dorsal column nuclei

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

BDNF in Neuropathic Pain; the Culprit that Cannot be Apprehended

In males but not in females, brain derived neurotrophic factor (BDNF) plays an obligatory role in the onset and maintenance of neuropathic pain. Afferent terminals of injured peripheral nerves release colony stimulating factor (CSF-1) and other mediators into the dorsal horn. These transform the phenotype of dorsal horn microglia such that they express P2X4 purinoceptors. Activation of these receptors by neuron-derived ATP promotes BDNF release. This microglial-derived BDNF increases synaptic activation of excitatory dorsal horn neurons and decreases that of inhibitory neurons. It also alters the neuronal chloride gradient such the normal inhibitory effect of GABA is converted to excitation. By as yet undefined processes, this attenuated inhibition increases NMDA receptor function. BDNF also promotes the release of pro-inflammatory cytokines from astrocytes. All of these actions culminate in the increase dorsal horn excitability that underlies many forms of neuropathic pain. Peripheral nerve injury also alters excitability of structures in the thalamus, cortex and mesolimbic system that are responsible for pain perception and for the generation of co-morbidities such as anxiety and depression. The weight of evidence from male rodents suggests that this preferential modulation of excitably of supra-spinal pain processing structures also involves the action of microglial-derived BDNF. Possible mechanisms promoting the preferential release of BDNF in pain signaling structures are discussed. In females, invading T-lymphocytes increase dorsal horn excitability but it remains to be determined whether similar processes operate in supra-spinal structures. Despite its ubiquitous role in pain aetiology neither BDNF nor TrkB receptors represent potential therapeutic targets.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Transcranial direct current stimulation attenuates chronic pain in knee osteoarthritis by modulating BDNF/TrkB signaling in the descending pain modulation system

Knee osteoarthritis (KOA) is the most common cause of chronic pain, but its pain mechanisms are complex and may be closely related to the descending pain modulation system. Transcranial direct current stimulation (tDCS) is used for relieving pain, but its analgesic mechanisms are still being explored. The purpose of this study was to investigate the role of BDNF/TrkB signaling in chronic pain in KOA and to investigate whether this signaling is related to the analgesic effect of tDCS. Rats were injected with monosodium iodoacetate (MIA) into the left knee joint to establish a chronic pain model and then received 20 min of tDCS for 8 days. Rats were respectively administered the TrkB inhibitor ANA-12 after MIA modeling and exogenous BDNF after tDCS treatment. Behaviors testing was assessed by hot plate and von Frey hairs using the up-down method. In addition, the expression levels of BDNF and TrkB on the periaqueductal gray (PAG)-the rostral ventromedial medulla (RVM)-the spinal dorsal horn (SDH) axis were detected by Western blot and Immunohistochemistry staining. Behavioral results show that tDCS treatment and ANA-12 injection reversed MIA-induced allodynia while reducing BDNF and TrkB expression levels. Furthermore, injection of exogenous BDNF reversed the therapeutic effect of tDCS on pain. These results indicate that upregulation of the BDNF/TrkB signaling in the descending pain modulation system may play an important role in KOA-induced chronic pain in rats, and tDCS may reduce KOA-induced chronic pain by inhibiting the BDNF/TrkB signaling in the descending pain modulation system.

Ultrastructure of dorsal root ganglia

Dorsal root ganglia (DRG) contains thousands of sensory neurons that transmit information about our external and internal environment to the central nervous system. This includes signals related to proprioception, temperature, and nociception. Our understanding of DRG has increased tremendously over the last 50 years and has established the DRG as an active participant in peripheral processes. This includes interactions between neurons and non-neuronal cells such as satellite glia cells and macrophages that contribute to an increasingly complex cellular environment that modulates neuronal function. Early ultrastructural investigations of the DRG have described subtypes of sensory neurons based on differences in the arrangement of organelles such as the Golgi apparatus and the endoplasmic reticulum. The neuron-satellite cell complex and the composition of the axon hillock in DRG have also been investigated, but, apart from basic descriptions of Schwann cells, ultrastructural investigations of other cell types in DRG are limited. Furthermore, detailed descriptions of key components of DRG, such as blood vessels and the capsule that sits at the intersection of the meninges and the connective tissue covering the peripheral nervous system, are lacking to date. With rising interest in DRG as potential therapeutic targets for aberrant signalling associated with chronic pain conditions, gaining further insights into DRG ultrastructure will be fundamental to understanding cell-cell interactions that modulate DRG function. In this review, we aim to provide a synopsis of the current state of knowledge on the ultrastructure of the DRG and its components, as well as to identify areas of interest for future studies.

Molecular-Scale Dynamics of Long Range Retrograde Brain-Derived Neurotrophic Factor Transport Shaped by Cellular Spatial Context

Retrograde neurotrophin (NT) transport is a specialized form of signal transduction used to conduct information from axons to the cell bodies of central and peripheral nervous system neurons. It is activated upon NT-Trk receptor binding, NT-Trk internalization into signaling endosomes, and their motion along the axon toward the cell body. Brain-derived neurotrophic factor (BDNF) is an abundant NT that modulates key brain and spinal cord functions, and defects in BDNF trafficking are associated with neuronal death, neurodegenerative diseases and in nerve injury. Decades of study have yielded impressive progress in elucidating NT retrograde transport; however, much information remains unclear. For example, while it is known that NT function is dependent on tight control of NT-receptor intracellular trafficking, data describing the precise spatiotemporal molecular dynamics of their axonal to somatic transport are lacking. In past work, we showed the use of discrete, photo-bleaching-resistant quantum dot (QD)-BNDF probes to activate and track BDNF-TrkB receptor internalization; this revealed a rich diversity of molecular motions that intracellular BDNF signaling endosomes undergo within the soma of nodose ganglia sensory neurons. Here, we used combined techniques of discrete QD-BDNF tracking with compartmented microfluidic chambers to characterize retrograde BDNF-TrkB transport over long-ranging distances of primary dorsal root ganglion sensory neuronal axons. Our new findings show that axonal retrograde motion is comprised of heterogeneous mixtures of diffusive behaviors, pauses, and variations in net molecular-motor-dependent transport speeds. Notably, specific molecular dynamic features such as NT speed were dependent on spatial context that could be categorized in distance from distal axons and proximity to the soma and were not entirely dictated by active motor transport speed. The important implication is recognition that NT-receptor retrograde transport is comprised of molecular dynamics, which change over the course of long-range trafficking to shape overall transport and possibly signaling.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

Calcium in Neuronal and Glial Response to Axotomy

Neurotrauma assumes an instant or delayed disconnection of axons (axotomy), which affects not only neurons, but surrounding glia as well. Not only mechanically injured glia near the site of disconnection, especially transection, is subjected to the damage, but also glia that is remote from the lesion site. Glial cells, which surround the neuronal body, in turn, support neuron survival, so there is a mutual protection between neuron and glia. Calcium signaling is a central mediator of all post-axotomy events, both in neuron and glia, playing a critical role in their survival/regeneration or death/degeneration. The involvement of calcium in post-axotomy survival of the remote, mechanically intact glia is poorly studied. The purpose of this review is to sum up the calcium-involving mechanisms in responses of neurons and glial cells to axotomy to show their importance and to give some suggestions for future research of remote glia in this context.

P2X4 receptor participates in autophagy regulation in Parkinson's disease

Dysfunctional autophagy often occurs during the development of neurodegenerative diseases, such as Parkinson's disease, Huntington's disease, and Alzheimer's disease. The purinergic P2X4 receptor is an ATP-gated ion channel that is widely expressed in the microglia, astrocytes, and neurons of the central and peripheral nervous systems. P2X4R is involved in the regulation of cellular excitability, synaptic transmission, and neuroinflammation. However, the role played by P2X4R in Parkinson's disease remains poorly understood. Rat models of Parkinson's disease were established by injecting 6-hydroxydopamine into the substantia nigra pars compacta. P2X4R-targeted small interfering RNA (siRNA) was injected into the same area 1 week before injury induction to inhibit the expression of the P2X4 receptor. The results showed that the inhibition of P2X4 receptor expression in Parkinson's disease model rats reduced the rotation behavior induced by apomorphine treatment, increased the latency on the rotarod test, and upregulated the expression of tyrosine hydroxylase, brain-derived neurotrophic factor, LC3-II/LC3-I, Beclin-1, and phosphorylated tropomyosin receptor kinase B (TrkB) in brain tissue, while simultaneously reducing p62 levels. These findings suggest that P2X4 receptor activation might inhibit neuronal autophagy through the regulation of the brain-derived neurotrophic factor/TrkB signaling pathway, leading to dopaminergic neuron damage in the substantia nigra and the further inhibition of P2X4 receptor-mediated autophagy. These results indicate that P2X4 receptor might serve as a potential novel target for the treatment of Parkinson's disease. This study was approved by the Animal Ethics Committee of Affiliated Hospital of Qingdao University (approval No. QYFYWZLL26119) on April 12, 2016.

Rapamycin Accelerates Axon Regeneration Through Schwann Cell-mediated Autophagy Following Inferior Alveolar Nerve Transection in Rats

Sensory disturbance in the orofacial region owing to trigeminal nerve injury is caused by dental treatment or accident. Commercially available therapeutics are ineffective for the treatment of sensory disturbance. Additionally, the therapeutic effects of rapamycin, an allosteric inhibitor of mammalian target of rapamycin (mTOR), which negatively regulates autophagy, on the sensory disturbance are not fully investigated. Thus, we investigated the therapeutic effects of rapamycin on the sensory disturbance in the mandibular region caused by inferior alveolar nerve (IAN) transection (IANX) in rats. The expression levels of the phosphorylated p70S6K, a downstream molecule of mTOR, in the proximal and distal stumps of the transected IAN were significantly reduced by rapamycin administration to the injured site. Conversely, the increments of both Beclin 1 and microtubule-associated protein-1 light chain 3-II protein levels in the proximal and distal stumps of the transected IAN was induced by rapamycin administration. Immunohistochemical analyses revealed that Beclin 1 was located in Schwann cells in the proximal stump of the IAN. Accumulation of myelin protein zero and myelin basic protein in the proximal and distal stumps of the IAN was significantly reduced by rapamycin administration. Rapamycin administration facilitated axon regeneration after IANX and increased the number of brain-derived neurotrophic factor positive neurons in the trigeminal ganglion. Thus, recovery from sensory disturbance in the lower lip caused by IANX was markedly facilitated by rapamycin. These findings suggest that rapamycin administration is a promising treatment for the sensory disturbance caused by IANX.

Rapid elevation of brain-derived neurotrophic factor production in the bilateral trigeminal ganglia by unilateral transection of the mental nerve in mice

OBJECTIVES

Previous spinal nerve injury studies have reported brain-derived neurotrophic factor (BDNF) mRNA upregulation in either the ipsilateral dorsal root ganglion (DRG) neurons or both the contralateral and ipsilateral DRG neurons from early period after peripheral nerve injury. This BDNF elevation induces hyperalgesia in the injured and/or uninjured sites, but this detailed mechanism remains unknown. This study aimed to investigate the BDNF mRNA expression in bilateral DRG neurons caused by unilateral nerve injury and to explore the possible mechanisms by which nitric oxide (NO) mediates BDNF production in the DRG, resulting in contralateral hyperalgesia.

METHODS

Early changes in BDNF mRNA expression in the bilateral trigeminal ganglia, within 1 day after mental nerve transection, were examined. Additionally, the effects on BDNF production of the NO synthase inhibitor N(ω)-nitro-l-arginine methyl ester (L-NAME) were investigated in the bilateral trigeminal ganglia. The relationship between injured neurons and BDNF production in the trigeminal ganglia was then assessed using immunohistochemical and retrograde tracing methods.

RESULTS

Reverse transcription-PCR analysis demonstrated that unilateral transection of the mental nerve induced a rapid elevation of BDNF mRNA expression, which was inhibited by the intracerebroventricular administration of L-NAME prior to nerve transection. This effect was observed in both the ipsilateral and contralateral sides to the nerve transection. BDNF immunostaining combined with FluoroGold retrograde tracing revealed two types of BDNF-reactive neurons, FluoroGold-labelled and non-FluoroGold-labelled neurons, in the ipsilateral and contralateral sides of the trigeminal ganglia. BDNF-positive cells were also observed in the trigeminal ganglia of other trigeminal nerve branches.

CONCLUSIONS

Unilateral nerve injury upregulates BDNF production in the bilateral trigeminal ganglia by NO-mediated and/or indirect activation of afferent neurons, resulting in contralateral hyperalgesia.

Peripheral brain-derived neurotrophic factor contributes to chronic osteoarthritis joint pain

Brain-derived neurotrophic factor (BDNF) and the high-affinity receptor tropomyosin receptor kinase B (TrkB) have important roles in neuronal survival and in spinal sensitization mechanisms associated with chronic pain. Recent clinical evidence also supports a peripheral role of BDNF in osteoarthritis (OA), with synovial expression of TrkB associated with higher OA pain. The aim of this study was to use clinical samples and animal models to explore the potential contribution of knee joint BDNF/TrkB signalling to chronic OA pain. Brain-derived neurotrophic factor and TrkB mRNA and protein were present in knee synovia from OA patients (16 women, 14 men, median age 67 years [interquartile range: 61-73]). There was a significant positive correlation between mRNA expression of NTRK2 (TrkB) and the proinflammatory chemokine fractalkine in the OA synovia. Using the surgical medial meniscal transection (MNX) model and the chemical monosodium iodoacetate (MIA) model of OA pain in male rats, the effects of peripheral BDNF injection, vs sequestering endogenous BDNF with TrkB-Fc chimera, on established pain behaviour were determined. Intra-articular injection of BDNF augmented established OA pain behaviour in MIA rats, but had no effect in controls. Intra-articular injection of the TrkB-Fc chimera acutely reversed pain behaviour to a similar extent in both models of OA pain (weight-bearing asymmetry MIA: -11 ± 4%, MNX: -12 ± 4%), compared to vehicle treatment. Our data suggesting a contribution of peripheral knee joint BDNF/TrkB signalling in the maintenance of chronic OA joint pain support further investigation of the therapeutic potential of this target.

Revisiting PNS Plasticity: How Uninjured Sensory Afferents Promote Neuropathic Pain

Despite the widespread study of how injured nerves contribute to chronic pain, there are still major gaps in our understanding of pain mechanisms. This is particularly true of pain resulting from nerve injury, or neuropathic pain, wherein tactile or thermal stimuli cause painful responses that are particularly difficult to treat with existing therapies. Curiously, this stimulus-driven pain relies upon intact, uninjured sensory neurons that transmit the signals that are ultimately sensed as painful. Studies that interrogate uninjured neurons in search of cell-specific mechanisms have shown that nerve injury alters intact, uninjured neurons resulting in an activity that drives stimulus-evoked pain. This review of neuropathic pain mechanisms summarizes cell-type-specific pathology of uninjured sensory neurons and the sensory ganglia that house their cell bodies. Uninjured neurons have demonstrated a wide range of molecular and neurophysiologic changes, many of which are distinct from those detected in injured neurons. These intriguing findings include expression of pain-associated molecules, neurophysiological changes that underlie increased excitability, and evidence that intercellular signaling within sensory ganglia alters uninjured neurons. In addition to well-supported findings, this review also discusses potential mechanisms that remain poorly understood in the context of nerve injury. This review highlights key questions that will advance our understanding of the plasticity of sensory neuron subpopulations and clarify the role of uninjured neurons in developing anti-pain therapies.

In vitro affinity optimization of an anti-BDNF monoclonal antibody translates to improved potency in targeting chronic pain states in vivo

The role of brain-derived neurotrophic factor (BDNF) signaling in chronic pain has been well documented. Given the important central role of BDNF in long term plasticity and memory, we sought to engineer a high affinity, peripherally-restricted monoclonal antibody against BDNF to modulate pain. BDNF shares 100% sequence homology across human and rodents; thus, we selected chickens as an alternative immune host for initial antibody generation. Here, we describe the affinity optimization of complementarity-determining region-grafted, chicken-derived R3bH01, an anti-BDNF antibody specifically blocking the TrkB receptor interaction. Antibody optimization led to the identification of B30, which has a > 300-fold improvement in affinity based on BIAcore, an 800-fold improvement in potency in a cell-based pERK assay and demonstrates exquisite selectivity over related neurotrophins. Affinity improvements measured in vitro translated to in vivo pharmacological activity, with B30 demonstrating a 30-fold improvement in potency over parental R3bH01 in a peripheral nerve injury model. We further demonstrate that peripheral BDNF plays a role in maintaining the plasticity of sensory neurons following nerve damage, with B30 reversing neuron hyperexcitability associated with heat and mechanical stimuli in a dose-dependent fashion. In summary, our data demonstrate that effective sequestration of BDNF via a high affinity neutralizing antibody has potential utility in modulating the pathophysiological mechanisms that drive chronic pain states.

The Role of BDNF in Peripheral Nerve Regeneration: Activity-Dependent Treatments and Val66Met

Despite the ability of peripheral nerves to spontaneously regenerate after injury, recovery is generally very poor. The neurotrophins have emerged as an important modulator of axon regeneration, particularly brain derived neurotrophic factor (BDNF). BDNF regulation and signaling, as well as its role in activity-dependent treatments including electrical stimulation, exercise, and optogenetic stimulation are discussed here. The importance of a single nucleotide polymorphism in the BDNF gene, Val66Met, which is present in 30% of the human population and may hinder the efficacy of these treatments in enhancing regeneration after injury is considered. Preliminary data are presented on the effectiveness of one such activity-dependent treatment, electrical stimulation, in enhancing axon regeneration in mice expressing the met allele of the Val66Met polymorphism.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

The reduction of intraepidermal P2X3 nerve fiber density correlates with behavioral hyperalgesia in a rat model of nerve injury-induced pain

Skin biopsies from patients with neuropathic pain often show changes in epidermal innervation, although it remains to be elucidated to what extent such changes can be linked to a particular subgroup of nerve fibers and how these changes are correlated with pain intensity. Here, we investigated to what extent behavioral signs of hyperalgesia are correlated with immunohistochemical changes of peptidergic and non-peptidergic epidermal nerve fibers in a rat model of nerve injury-induced pain. Rats subjected to unilateral partial ligation of the sciatic nerve developed significant mechanical and thermal hyperalgesia as tested by the withdrawal responses of the ipsilateral footpad to von Frey hairs and hotplate stimulation. At day 14, epidermal nerve fiber density and total epidermal nerve fiber length/mm² were significantly and consistently reduced compared to the contralateral side, following testing and re-testing by two blinded observers. The expression of calcitonin gene-related peptide, a marker for peptidergic nerve fibers, was not significantly changed on the ipsilateral side. In contrast, the expression of the P2X₃ receptor, a marker for non-peptidergic nerve fibers, was not only significantly reduced but could also be correlated with behavioral hyperalgesia. When labeling both peptidergic and non-peptidergic nerve fibers with the pan-neuronal marker PGP9.5, the expression was significantly reduced, albeit without a significant correlation with behavioral hyperalgesia. In conjunction, our data suggest that the pathology of the P2X₃ epidermal nerve fibers can be selectively linked to neuropathy, highlighting the possibility that it is the degeneration of these fibers that drives hyperalgesia.

The Influence of Donor Age, Nerve Growth Factor, and Cografting with Drosophila Cells on Survival of Peripherally Grafted Embryonic or Fetal Rat Dorsal Root Ganglia

Previous studies have shown that embryonic rat and human dorsal root ganglion (DRG) cells survive grafting to the cavity of extirpated adult rat DRG. Furthermore, grafted human embryonic neurons were shown to send axons peripherally and into the spinal cord, where they establish functional synaptic connections. This study analyzed the survival of orthotopically allografted rat DRG cells from embryonic stages 15 (E15) and 20 (E20), and the influence on their survival of nerve growth factor (NGF). NGF was delivered to the DRG transplants either by pump infusion or by cotransplantation of cells from Drosophila melanogaster, transgenic for human NGF. Lumbar DRGs of adult rats were removed and a collection of E15 or E20 DRGs placed in the cavity. One month after grafting the total number of DRG cells in the grafts was counted. Differentiation of subpopulations of DRG cells was estimated by counting cells immunostained for calcitonin gene-related peptide (CGRP), Griffonia simplicifolia agglutinin isolectin B4 (GSA), or heavy neurofilament protein (antibody RT97). The results show: i) similar survival of E15 and E20 grafts, with great variability in the survival of different subpopulations in E15 transplants, but a more consistent distribution of different phenotypes in E20 transplants; ii) infusion of NGF for 2 weeks increases the survival of E15 transplants, but has a negative effect on E20 transplants; iii) Drosophila cells transfected with human NGF gene survive peripheral xenografting and have a positive effect on the survival of the GSA- and CGRP-positive populations in E15 and E20 transplants; iv) Drosophila cells without the human NGF gene increase cell survival in E20 transplants. These data suggest that i) the effect of NGF is dependent on the embryonic stage of the transplants, ii) age-dependent sensitivity to NGF influences graft survival, and iii) transgenic Drosophila cells can be cotransplanted with embryonic neural tissue to the mammalian peripheral nervous system with a positive effect on the survival of neural grafts.

Spinal mechanisms of neuropathic pain: Is there a P2X4-BDNF controversy?

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This viewpoint article compares the evidence supporting a biological relevance of the P₂X₄ and BDNF system in neuropathic pain with recent data which question such importance.

More than a decade ago the novel concept that glial cells are major players in the modulation of pain mechanisms in the spinal cord has started a prolific series of work addressing the modalities of neuron-glia communication. Mike Salter with Kazuhide Inoue laboratories introduced ATP as pivotal mediator for such communication via activation of P₂X₄ receptors expressed by microglia in the dorsal horn ipsilateral to a peripheral nerve injury. Activation of P₂X₄ receptors result in release of the neurotrophin BDNF, which, through the activation of neuronal TrkB receptors, alters neuronal excitability and this effect is associated with behavioural ipsilateral allodynia. This viewpoint article compares the evidence supporting a biological relevance of the P₂X₄ and BDNF system in neuropathic pain with recent data which question such importance. Having read this article, readers will be able to formulate their own opinion on such controversy.

Medicinal Chemistry, Pharmacology, and Clinical Implications of TRPV1 Receptor Antagonists

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed on sensory neurons triggering an influx of cations. TRPV1 receptors function as homotetramers responsive to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Its phosphorylation increases sensitivity to both chemical and thermal stimuli, while desensitization involves a calcium-dependent mechanism resulting in receptor dephosphorylation. TRPV1 functions as a sensor of noxious stimuli and may represent a target to avoid pain and injury. TRPV1 activation has been associated to chronic inflammatory pain and peripheral neuropathy. Its expression is also detected in nonneuronal areas such as bladder, lungs, and cochlea where TRPV1 activation is responsible for pathology development of cystitis, asthma, and hearing loss. This review offers a comprehensive overview about TRPV1 receptor in the pathophysiology of chronic pain, epilepsy, cough, bladder disorders, diabetes, obesity, and hearing loss, highlighting how drug development targeting this channel could have a clinical therapeutic potential. Furthermore, it summarizes the advances of medicinal chemistry research leading to the identification of highly selective TRPV1 antagonists and their analysis of structure-activity relationships (SARs) focusing on new strategies to target this channel.

Orthopedic surgery modulates neuropeptides and BDNF expression at the spinal and hippocampal levels

Pain is a critical component hindering recovery and regaining of function after surgery, particularly in the elderly. Understanding the role of pain signaling after surgery may lead to novel interventions for common complications such as delirium and postoperative cognitive dysfunction. Using a model of tibial fracture with intramedullary pinning in male mice, associated with cognitive deficits, we characterized the effects on the primary somatosensory system. Here we show that tibial fracture with pinning triggers cold allodynia and up-regulates nerve injury and inflammatory markers in dorsal root ganglia (DRGs) and spinal cord up to 2 wk after intervention. At 72 h after surgery, there is an increase in activating transcription factor 3 (ATF3), the neuropeptides galanin and neuropeptide Y (NPY), brain-derived neurotrophic factor (BDNF), as well as neuroinflammatory markers including ionized calcium-binding adaptor molecule 1 (Iba1), glial fibrillary acidic protein (GFAP), and the fractalkine receptor CX3CR1 in DRGs. Using an established model of complete transection of the sciatic nerve for comparison, we observed similar but more pronounced changes in these markers. However, protein levels of BDNF remained elevated for a longer period after fracture. In the hippocampus, BDNF protein levels were increased, yet there were no changes in Bdnf mRNA in the parent granule cell bodies. Further, c-Fos was down-regulated in the hippocampus, together with a reduction in neurogenesis in the subgranular zone. Taken together, our results suggest that attenuated BDNF release and signaling in the dentate gyrus may account for cognitive and mental deficits sometimes observed after surgery.

Effects of peripheral nerve injury on parvalbumin expression in adult rat dorsal root ganglion neurons

Background

Parvalbumin (PV) is a calcium binding protein that identifies a subpopulation of proprioceptive dorsal root ganglion (DRG) neurons. Calcitonin gene-related peptide (CGRP) is also expressed in a high proportion of muscle afferents but its relationship to PV is unclear. Little is known of the phenotypic responses of muscle afferents to nerve injury. Sciatic nerve axotomy or L5 spinal nerve ligation and section (SNL) lesions were used to explore these issues in adult rats using immunocytochemistry.

Results

In naive animals, the mean PV expression was 25 % of L4 or L5 dorsal root ganglion (DRG) neurons, and this was unchanged 2 weeks after sciatic nerve axotomy. Colocalization studies with the injury marker activating transcription factor 3 (ATF3) showed that approximately 24 % of PV neurons expressed ATF3 after sciatic nerve axotomy suggesting that PV may show a phenotypic switch from injured to uninjured neurons. This possibility was further assessed using the spinal nerve ligation (SNL) injury model where injured and uninjured neurons are located in different DRGs. Two weeks after L5 SNL there was no change in total PV staining and essentially all L5 PV neurons expressed ATF3. Additionally, there was no increase in PV-ir in the adjacent uninjured L4 DRG cells. Co-labelling of DRG neurons revealed that less than 2 % of PV neurons normally expressed CGRP and no colocalization was seen after injury.

Conclusion

These experiments clearly show that axotomy does not produce down regulation of PV protein in the DRG. Moreover, this lack of change is not due to a phenotypic switch in PV immunoreactive (ir) neurons, or de novo expression of PV-ir in uninjured neurons after nerve injury. These results further illustrate differences that occur when muscle afferents are injured as compared to cutaneous afferents.

Injury-associated PACAP expression in rat sensory and motor neurons is induced by endogenous BDNF

Peripheral nerve injury results in dramatic upregulation in pituitary adenylate cyclase activating polypeptide (PACAP) expression in adult rat dorsal root ganglia and spinal motor neurons mirroring that described for the neurotrophin brain derived neurotrophic factor (BDNF). Thus, we posited that injury-associated alterations in BDNF expression regulate the changes in PACAP expression observed in the injured neurons. The role of endogenous BDNF in induction and/or maintenance of PACAP mRNA expression in injured adult rat motor and sensory neurons was examined by intrathecally infusing or intraperitoneally injecting BDNF-specific antibodies or control IgGs immediately at the time of L4-L6 spinal nerve injury, or in a delayed fashion one week later for 3 days followed by analysis of impact on PACAP expression. PACAP mRNA in injured lumbar sensory and motor neurons was detected using in situ hybridization, allowing quantification of relative changes between experimental groups, with ATF-3 immunofluorescence serving to identify the injured subpopulation of motor neurons. Both the incidence and level of PACAP mRNA expression were dramatically reduced in injured sensory and motor neurons in response to immediate intrathecal anti-BDNF treatment. In contrast, neither intraperitoneal injections nor delayed intrathecal infusions of anti-BDNF had any discernible impact on PACAP expression. This impact on PACAP expression in response to BDNF immunoneutralization in DRG was confirmed using qRT-PCR or by using BDNF selective siRNAs to reduce neuronal BDNF expression. Collectively, our findings support that endogenous injury-associated BDNF expression is critically involved in induction, but not maintenance, of injury-associated PACAP expression in sensory and motor neurons.

Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Peripheral nerve injury disrupts the normal functions of sensory and motor neurons by damaging the integrity of axons and Schwann cells. In contrast to the central nervous system, the peripheral nervous system possesses a considerable capacity for regrowth, but regeneration is far from complete and functional recovery rarely returns to pre-injury levels. During development, the peripheral nervous system strongly depends upon trophic stimulation for neuronal differentiation, growth and maturation. The perhaps most important group of trophic substances in this context is the neurotrophins (NGF, BDNF, NT-3 and NT-4/5), which signal in a complex spatial and timely manner via the two structurally unrelated p75(NTR) and tropomyosin receptor kinase (TrkA, Trk-B and Trk-C) receptors. Damage to the adult peripheral nerves induces cellular mechanisms resembling those active during development, resulting in a rapid and robust increase in the synthesis of neurotrophins in neurons and Schwann cells, guiding and supporting regeneration. Furthermore, the injury induces neurotrophin-mediated changes in the dorsal root ganglia and in the spinal cord, which affect the modulation of afferent sensory signaling and eventually may contribute to the development of neuropathic pain. The focus of this review is on the expression patterns of neurotrophins and their receptors in neurons and glial cells of the peripheral nervous system and the spinal cord. Furthermore, injury-induced changes of expression patterns and the functional consequences in relation to axonal growth and remyelination as well as to neuropathic pain development will be reviewed.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

Kv2 dysfunction after peripheral axotomy enhances sensory neuron responsiveness to sustained input

Peripheral nerve injuries caused by trauma are associated with increased sensory neuron excitability and debilitating chronic pain symptoms. Axotomy-induced alterations in the function of ion channels are thought to largely underlie the pathophysiology of these phenotypes. Here, we characterise the mRNA distribution of Kv2 family members in rat dorsal root ganglia (DRG) and describe a link between Kv2 function and modulation of sensory neuron excitability. Kv2.1 and Kv2.2 were amply expressed in cells of all sizes, being particularly abundant in medium-large neurons also immunoreactive for neurofilament-200. Peripheral axotomy led to a rapid, robust and long-lasting transcriptional Kv2 downregulation in the DRG, correlated with the onset of mechanical and thermal hypersensitivity. The consequences of Kv2 loss-of-function were subsequently investigated in myelinated neurons using intracellular recordings on ex vivo DRG preparations. In naïve neurons, pharmacological Kv2.1/Kv2.2 inhibition by stromatoxin-1 (ScTx) resulted in shortening of action potential (AP) after-hyperpolarization (AHP). In contrast, ScTx application on axotomized neurons did not alter AHP duration, consistent with the injury-induced Kv2 downregulation. In accordance with a shortened AHP, ScTx treatment also reduced the refractory period and improved AP conduction to the cell soma during high frequency stimulation. These results suggest that Kv2 downregulation following traumatic nerve lesion facilitates greater fidelity of repetitive firing during prolonged input and thus normal Kv2 function is postulated to limit neuronal excitability. In summary, we have profiled Kv2 expression in sensory neurons and provide evidence for the contribution of Kv2 dysfunction in the generation of hyperexcitable phenotypes encountered in chronic pain states.

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Kv2.1 and Kv2.2 are expressed in rat dorsal root ganglion neurons.

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Kv2 subunits are most abundant in myelinated sensory neurons.

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Kv2.1 and Kv.2 subunits are downregulated in a traumatic nerve injury pain model.

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Kv2 inhibition ex vivo allows higher firing rates during sustained stimulation.

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We conclude that Kv2 channels contribute to limiting peripheral neuron excitability.

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

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

An anatomical and immunohistochemical characterization of afferents innervating the C6-C7 facet joint after painful joint loading in the rat

STUDY DESIGN

This study used retrograde neuronal tracing and immunohistochemistry to identify neurons innervating the C6-C7 facet joint and those expressing calcitonin gene-related peptide (CGRP) in the dorsal root ganglion (DRG) of rats after painful cervical facet joint injury.

OBJECTIVE

The objective of this study was to characterize the innervation of the C6-C7 facet joint after painful joint injury in the rat.

SUMMARY OF BACKGROUND DATA

The cervical facet joint is a source of neck pain, and its loading can initiate persistent pain. CGRP is a nociceptive neurotransmitter; peptidergic afferents have been identified in the facet joint's capsule. Although studies suggest that facet joint injury alters CGRP expression in joint afferents, the distribution of neurons innervating the C6-C7 facet joint and their expression of CGRP after a painful joint injury have not been investigated.

METHODS

Holtzman rats (Harlan Sprague-Dawley, Indianapolis, IN) received an intra-articular injection of cholera toxin subunit B in the C6-C7 facet joints. After injection, subgroups underwent either a painful joint distraction or sham procedure. Mechanical sensitivity was assessed, and immunohistochemical techniques were used to quantify CGRP expression and cholera toxin subunit B labeling in the C5-C8 DRGs.

RESULTS

Facet joint distraction-induced (P ≤ 0.0002) hypersensitivity. Neurons labeled by the joint injection were identified in the C5-C8 DRGs. Significantly, more (P ≤ 0.0001) cholera toxin subunit B-positive neurons were identified in the C7 DRG than any other level. At C7, 54.4% ± 15.3% of those neurons were also CGRP-positive, whereas only 41.5% ± 5.4% of all neurons were CGRP-positive; this difference was significant (P = 0.0084).

CONCLUSION

The greatest number of afferents from the C6-C7 facet joint has cell bodies in the C7 DRG, implicating this level as the most relevant for pain from this joint. In addition, peptidergic afferents seem to have an important role in facet joint-mediated pain.

Epigenetic regulation of BDNF expression in the primary sensory neurons after peripheral nerve injury: implications in the development of neuropathic pain

Brain-derived neurotrophic factor (BDNF) is known to be up-regulated in the dorsal root ganglion (DRG) after peripheral nerve injury, and to contribute to neuropathic pain. Here, we found that thermal hyperalgesia and mechanical allodynia at day 7 post-injury were inhibited only when anti-BDNF antibody was intrathecally administrated at day 2 post-injury. Consistent with behavioral results, Western blot analysis showed that the expression levels of BDNF protein in the spinal dorsal horn were markedly induced during early stage post-injury. Moreover, the maximal increase in BDNF mRNA expression in the DRG was observed at day 1 post-injury, and significantly elevated levels were sustained for at least 14 days. Four of five BDNF mRNA transcripts were up-regulated after nerve injury, and the most inducible transcript was exon I. Using a chromatin immunoprecipitation (ChIP) assay, we found that nerve injury promotes histone H3 and H4 acetylation, transcriptionally active modifications, at BDNF promoter I at day 1 post-injury, and the levels of histone acetylation remain elevated for at least 7 days. Taken together, our findings suggest that an initial increase in BDNF exon I expression controlled by epigenetic mechanisms might have a crucial role in the development of neuropathic pain.

Sensory neuron downregulation of the Kv9.1 potassium channel subunit mediates neuropathic pain following nerve injury

Chronic neuropathic pain affects millions of individuals worldwide, is typically long-lasting, and remains poorly treated with existing therapies. Neuropathic pain arising from peripheral nerve lesions is known to be dependent on the emergence of spontaneous and evoked hyperexcitability in damaged nerves. Here, we report that the potassium channel subunit Kv9.1 is expressed in myelinated sensory neurons, but is absent from small unmyelinated neurons. Kv9.1 expression was strongly and rapidly downregulated following axotomy, with a time course that matches the development of spontaneous activity and pain hypersensitivity in animal models. Interestingly, siRNA-mediated knock-down of Kv9.1 in naive rats led to neuropathic pain behaviors. Diminished Kv9.1 function also augmented myelinated sensory neuron excitability, manifested as spontaneous firing, hyper-responsiveness to stimulation, and persistent after-discharge. Intracellular recordings from ex vivo dorsal root ganglion preparations revealed that Kv9.1 knock-down was linked to lowered firing thresholds and increased firing rates under physiologically relevant conditions of extracellular potassium accumulation during prolonged activity. Similar neurophysiological changes were detected in animals subjected to traumatic nerve injury and provide an explanation for neuropathic pain symptoms, including poorly understood conditions such as hyperpathia and paresthesias. In summary, our results demonstrate that Kv9.1 dysfunction leads to spontaneous and evoked neuronal hyperexcitability in myelinated fibers, coupled with development of neuropathic pain behaviors.

Dose-dependent expression of neuronal injury markers during experimental osteoarthritis induced by monoiodoacetate in the rat

Background

It was recently reported that the mono-iodoacetate (MIA) experimental model of osteoarthritis (OA) courses with changes of neurons innervating the affected joints that are commonly interpreted as a neuronal response to axonal injury. To better characterize these changes, we evaluated the expression of two markers of neuronal damage, ATF-3 and NPY, and the growth associated protein GAP-43, in primary afferent neurons of OA animals injected with three different doses of MIA (0.3, 1 or 2 mg). Measurements were performed at days 3, 7, 14, 21 and 31 post-MIA injection.

Results

OA animals showed the characteristic histopathological changes of the joints and the accompanying nociceptive behaviour, evaluated by the Knee-Bed and CatWalk tests. An increase of ATF-3 expression was detected in the DRG of OA animals as early as 3 days after the injection of 1 or 2 mg of MIA and 7 days after the injection of 0.3 mg. NPY expression was increased in animals injected with 1 or 2 mg of MIA, at day 3 or in all time-points, respectively. From day 7 onwards there was a massive increase of GAP-43 expression in ATF-3 cells.

Conclusions

The expression of the neuronal injury markers ATF-3 and NPY as well as an up-regulation of GAP-43 expression, indicative of peripheral fibre regeneration, suggests that axonal injury and a regeneration response may be happening in this model of OA. This opens new perspectives in the unravelling of the physiopathology of the human disease.

Gene expression in skin, muscle, and dorsal root ganglion after plantar incision in the rat

BACKGROUND

Treating postoperative pain remains a significant challenge for perioperative medicine. Recent studies have shown that nerve growth factor is up-regulated and contributes to incisional pain. To date, few studies have examined expression of other neurotrophin-related mediators that may contribute to the development and/or maintenance of incisional pain.

METHODS

Male Sprague-Dawley rats underwent a plantar incision, and pain behaviors were examined (n = 6). In a separate group of rats, expression of neurotrophic factors were studied. At various times after incision (n = 4) or sham surgery (n = 4), the skin, muscle, and dorsal root ganglia were harvested and total RNA isolated. Real-time reverse transcription polymerase chain reaction was performed and the fold change in gene expression was analyzed using significance analysis of microarrays.

RESULTS

Several genes were changed (P < 0.05) as early as 1 h after incision. Expression of artemin and nerve growth factor were increased in both incised skin and muscle. Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-5 were all down-regulated in the skin but up-regulated in the muscle 48 h after incision. Few genes changed in the dorsal root ganglion. Most changes in expression occurred in the first 48 h after incision, a timeframe when pain behavior was the greatest.

CONCLUSION

Surgical incision is associated with pain-related gene expression changes in skin, muscle, and, to a lesser extent, dorsal root ganglion. The gene expression profile provides clues as to mediators that are involved in peripheral sensitization and pain transmission after surgical incision and also suggest mechanisms for resolution of postoperative pain when more persistent pain syndromes like neuropathic pain continue.

Changes in functional properties of A-type but not C-type sensory neurons in vivo in a rat model of peripheral neuropathy

Background

The aim of this study was to compare primary sensory neurons in controls and in an animal neuropathic pain model in order to understand which types of neurons undergo changes associated with peripheral neuropathy. On the basis of intracellular recordings in vivo from somata, L4 sensory dorsal root ganglion neurons were categorized according to action potential configuration, conduction velocity, and receptive field properties to mechanical stimuli.

Methods

Intracellular recordings were made from functionally identified dorsal root ganglion neurons in vivo in the Mosconi and Kruger animal model of peripheral neuropathic pain.

Results

In this peripheral neuropathy model, a specific population of Aβ-fiber low threshold mechanoreceptor neurons, which respond normally to innocuous mechanical stimuli, exhibited differences in action potential configuration and conduction velocity when compared with control animals. No abnormal conduction velocity, action potential shapes, or tactile sensitivity of C-fiber neurons were encountered.

Conclusion

This study provides evidence for defining a potential role of Aβ-fiber low threshold mechanoreceptor neurons that might contribute to peripheral neuropathic pain.

Sox11 modulates brain-derived neurotrophic factor expression in an exon promoter-specific manner

Sox11 is a high-mobility group (HMG)-containing transcription factor that is significantly elevated in peripheral neurons in response to nerve injury. In vitro and in vivo studies support a central role for Sox11 in adult neuron growth and survival following injury. Brain-derived neurotrophic factor (BDNF) is a pleiotropic growth factor that has effects on neuronal survival, differentiation, synaptic plasticity, and regeneration. BDNF transcription is elevated in the dorsal root ganglia (DRG) following nerve injury in parallel with Sox11, allowing for the possible regulation by Sox11. To begin to assess the possible influence of Sox11, we used reverse transcriptase PCR assays to determine the relative expression of the nine (I-IXa) noncoding exons and one coding exon (exon IX) of the BDNF gene after sciatic nerve axotomy in the mouse. Exons with upstream promoter regions containing the Sox binding motif 5'-AACAAAG-3' (I, IV, VII, and VIII) were increased at 1 or 3 days following axotomy. Exons 1 and IV showed the greatest increase, and only exon 1 remained elevated at 3 days. Luciferase assays showed that Sox11 could activate the most highly regulated exons, I and IV, and that this activation was reduced by mutation of putative Sox binding sites. Exon expression in injured DRG neurons had some overlap with Neuro2a cells that overexpress Sox11, showing elevation in exon IV and VII transcripts. These findings indicate cell type and contextual specificity of Sox11 in modulation of BDNF transcription.

Prostaglandin E2 contributes to the synthesis of brain-derived neurotrophic factor in primary sensory neuron in ganglion explant cultures and in a neuropathic pain model

Brain-derived neurotrophic factor (BDNF) exists in small to medium size neurons in adult rat dorsal root ganglion (DRG) and serves as a modulator at the first synapse of the pain transmission pathway in the spinal dorsal horn. Peripheral nerve injury increases BDNF expression in DRG neurons, an event involved in the genesis of neuropathic pain. In the present study, we tested the hypothesis that prostaglandin E2 (PGE2) over-produced in injured nerves contributes to the up-regulation of BDNF in DRG neurons. Two weeks after partial sciatic nerve ligation (PSNL), BDNF levels in the ipsilateral L4-L6 DRG of injured rats were significantly increased compared to the contralateral side. Perineural injection of a selective cyclooxygenase (COX2) inhibitor or a PGE2 EP4 receptor antagonist not only dose-dependently relieved PSNL elicited mechanical hypersensitivity, but also suppressed the increased BDNF levels in DRG neurons. PSNL shifted BDNF expression in the ipsilateral DRG from small to medium and larger size injured neurons. BDNF is mainly co-expressed with the EP1 and EP4 while moderately with the EP2 and EP3 receptor subtypes in naïve and PSNL rats. PSNL also shifted the expression of EP1-4 receptors to a larger size population of DRG neurons. In DRG explant cultures, a stabilized PGE2 analog 16,16 dimethyl PGE2 (dmPGE2) or the agonists of EP1 and EP4 receptors significantly increased BDNF levels and the phosphorylated protein kinase A (PKA), extracellular signal-regulated kinase (ERK)/mitogen activated protein kinase (MAPK) and cAMP response element binding protein (CREB). The EP1 and EP4 antagonists, a sequester of nerve growth factor (NGF), the inhibitors of PKA and MEK as well as CREB small interfering RNA suppressed dmPGE2-induced BDNF. Taken together, EP1 and EP4 receptor subtypes, PKA, ERK/MAPK and CREB signaling pathways as well as NGF are involved in PGE2-induced BDNF synthesis in DRG neurons. Injured nerve derived-PGE2 contributes to BDNF up-regulation in DRG neurons following nerve injury. Facilitating the synthesis of BDNF in primary sensory neurons is a novel mechanism underlying the role of PGE2 in the genesis of neuropathic pain.

Neuropathy-induced spinal GAP-43 expression is not a main player in the onset of mechanical pain hypersensitivity

Structural plasticity within the spinal nociceptive network may be fundamental to the chronic nature of neuropathic pain. In the present study, the spatiotemporal expression of growth-associated protein-43 (GAP-43), a protein which has been traditionally implicated in nerve fiber growth and sprouting, was investigated in relation to mechanical pain hypersensitivity. An L5 spinal nerve transection model was validated by the presence of mechanical pain hypersensitivity and an increase in the early neuronal activation marker cFos within the superficial spinal dorsal horn upon innocuous hindpaw stimulation. Spinal GAP-43 was found to be upregulated in the superficial L5 dorsal horn from 5 up to 10 days after injury. GAP-43 was co-localized with calcitonin-gene related peptide (CGRP), but not vesicular glutamate transporter-1 (VGLUT-1), IB4, or protein kinase-γ (PKC-γ), suggesting the regulation of GAP-43 in peptidergic nociceptive afferents. These GAP-43/CGRP fibers may be indicative of sprouting peptidergic fibers. Fiber sprouting largely depends on growth factors, which are typically associated with neuro-inflammatory processes. The putative role of neuropathy-induced GAP-43 expression in the development of mechanical pain hypersensitivity was investigated using the immune modulator propentofylline. Propentofylline treatment strongly attenuated the development of mechanical pain hypersensitivity and glial responses to nerve injury as measured by microglial and astroglial markers, but did not affect neuropathy-induced levels of spinal GAP-43 or GAP-43 regulation in CGRP fibers. We conclude that nerve injury induces structural plasticity in fibers expressing CGRP, which is regarded as a main player in central sensitization. Our data do not, however, support a major role of these structural changes in the onset of mechanical pain hypersensitivity.

Role of prostaglandin E2 in the synthesis of the pro-inflammatory cytokine interleukin-6 in primary sensory neurons: an in vivo and in vitro study

Following various types of nerve injury, cyclooxygenase 2 and prostaglandin E2 (PGE2) are universally and chronically up-regulated in injured nerves and contribute to the genesis of neuropathic pain. Persistent high levels of PGE2 likely exert chronic effects on nociceptive dorsal root ganglion (DRG) neurons. In the present study, we tested the hypothesis that injured nerve-derived PGE2 contributes to the up-regulation of the pro-inflammatory cytokine interleukin-6 (IL-6) in DRG neurons following partial sciatic nerve ligation. In naive adult rats, IL-6 was expressed in only a few small size DRG neurons which all co-expressed EP4 receptors. Partial sciatic nerve ligation increased and shifted IL-6 expression from small to medium and large size damaged DRG neurons. Perineural injection of a selective cyclooxygenase 2 inhibitor or a selective EP4 receptor antagonist significantly suppressed the up-regulation of IL-6 in DRG, suggesting that injured nerve derived PGE2 contributes to the de novo synthesis of IL-6 in DRG neurons through EP4 receptors. In cultured sensory ganglion explants, a stabilized PGE2 analog increased IL-6 mRNA and protein levels through the activation of EP4, protein kinase A, protein kinase C, extracellular regulated protein kinase/MAPK, cAMP response element binding protein and NFκB signalling pathways. Taken together, these data indicate that facilitating the de novo synthesis of pain-related cytokines in injured medium and large size DRG neurons is a novel mechanism underlying the role of injured nerve derived PGE2 in the genesis of neuropathic pain.

Increase of close homolog of cell adhesion molecule L1 in primary afferent by nerve injury and the contribution to neuropathic pain

The L1 family of cell adhesion molecules (L1-CAMs) is known to be involved in various neuronal functions such as cell adhesion, axon guidance, and synaptic plasticity. We investigated the detailed expression/changes of a close homolog of the L1 cell adhesion molecule (CHL1) after nerve injury and the possible role on neuropathic pain using the rat spared nerve injury (SNI) model. SNI induced the expression of CHL1 in L4/5 DRG neurons, particularly in small-size injured neurons and in satellite cells. In the spinal cord, CHL1 immunoreactivity increased mainly in laminae I-II of the dorsal horn on the side ipsilateral to the nerve injury. Ultrastructural study clarified the fine localization of CHL1 in axons of primary afferents in the dorsal horn. CHL1 immunoreactivities were localized in the adherence such as axon-axon, axon-dorsal horn neurons (dendrite, soma), and axon-glial cells (astrocyte and microglia). Experimental inhibition of CHL1 adhesion by intrathecal administration of the antibody for CHL1 extracellular domain significantly prevented and reversed SNI-induced mechanical allodynia. Thus, alterations of CHL1 may be involved in the structural plasticity after peripheral nerve injury and have important roles in neuropathic pain.

Gene expression profile in rat dorsal root ganglion following sciatic nerve injury and systemic neurotrophin-3 administration

Following sciatic nerve transection in adult rats, a proportion of injured dorsal root ganglion (DRG) neurons die, through apoptosis, over the following 6 months. Previous studies showed that axotomy and neurotrophin-3 administration may have effects on expression of neurotrophins and their receptors in DRG. In the current study, the fourth and fifth lumbar DRGs of rats were examined 2 weeks after right sciatic nerve transection and ligation. The effects of axotomy and systemic NT-3 treatment on neuronal genes were investigated by microarray. The results demonstrated that bone morphogenetic protein (BMP) and Janus protein tyrosine kinase signaling pathways are induced in axotomized DRG, and PI-3 kinase and BMP pathways and genes controlling various cellular functions were induced after axotomy and NT-3 administration.

Brain-derived neurotrophic factor modulates antiretroviral-induced mechanical allodynia in the mouse

Nucleoside reverse transcriptase inhibitors (NRTIs) are key components of HIV/AIDS treatment to reduce viral load. However, these drugs can induce chronic neuropathic pain, leading to increased morbidity in HIV patients. This study examines the role of brain-derived neurotrophic factor (BDNF) in the spinal dorsal horn (SDH) in development of mechanical allodynia in male C57BL/6J mice treated with the NRTI stavudine (d4T). After d4T administration, mice developed increased neuronal activity and BDNF expression in the SDH and hind paw mechanical allodynia that was exacerbated by intrathecal BDNF administration. Intrathecal BDNF alone also increased neuronal activity and caused mechanical allodynia. Because excess BDNF amplified d4T-induced mechanical allodynia and neuronal activity, the impact of decreasing BDNF in the SDH was investigated. After d4T, BDNF heterozygous mice were less allodynic than wild-type littermates, which was negated by intrathecal BDNF administration. Finally, pretreatment with intrathecal trkB-Fc chimera prior to d4T or administration of the tyrosine kinase inhibitor K252a 3 days after d4T blocked BDNF-mediated signaling, significantly attenuated the development of mechanical allodynia (trkB-Fc), and decreased neuronal activity (trkB-Fc and K252a). Taken together, these findings provide evidence that BDNF in the SDH contributes to the development of NRTI-induced painful peripheral neuropathy and may represent a new therapeutic opportunity.

Decoy strategy targeting the brain-derived neurotrophic factor exon I to attenuate tactile allodynia in the neuropathic pain model of rats

The mechanism underlying neuropathic pain is still largely unclear. Recently, much attention has been focused on the role of brain-derived neurotrophic factor (BDNF) as a neuromodulator in the spinal cord. We previously reported that the expression of Bdnf exon I mRNA was remarkably up-regulated in the dorsal root ganglion (DRG) neurons with the rat L5 spinal nerve ligation (SNL) model. In the present study, we investigated whether neuropathic pain response would be reduced by the inhibition of the Bdnf exon I in the rat SNL model. We identified the promoter region of exon I and synthesized the decoy ODNs targeting the region. Reverse transcription-polymerase chain reaction analysis confirmed that the decoy ODN treatment reduced SNL-induced Bdnf exon I mRNA up-regulation in ipsilateral L4 and L5 DRGs. Furthermore, post-treatment with the decoy ODNs significantly attenuated SNL-induced tactile allodynia. This study suggested that decoy ODNs targeting the Bdnf exon I might provide a novel analgesic strategy for the treatment of neuropathic pain.

Cellular and molecular insights into neuropathy-induced pain hypersensitivity for mechanism-based treatment approaches

Neuropathic pain is currently being treated by a range of therapeutic interventions that above all act to lower neuronal activity in the somatosensory system (e.g. using local anesthetics, calcium channel blockers, and opioids). The present review highlights novel and often still largely experimental treatment approaches based on insights into pathological mechanisms, which impact on the spinal nociceptive network, thereby opening the 'gate' to higher brain centers involved in the perception of pain. Cellular and molecular mechanisms such as ectopia, sensitization of nociceptors, phenotypic switching, structural plasticity, disinhibition, and neuroinflammation are discussed in relation to their involvement in pain hypersensitivity following either peripheral neuropathies or spinal cord injury. A mechanism-based treatment approach may prove to be successful in effective treatment of neuropathic pain, but requires more detailed insights into the persistence of cellular and molecular pain mechanisms which renders neuropathic pain unremitting. Subsequently, identification of the therapeutic window-of-opportunities for each specific intervention in the particular peripheral and/or central neuropathy is essential for successful clinical trials. Most of the cellular and molecular pain mechanisms described in the present review suggest pharmacological interference for neuropathic pain management. However, also more invasive treatment approaches belong to current and/or future options such as neuromodulatory interventions (including spinal cord stimulation) and cell or gene therapies, respectively.

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.