Altered sodium channel expression in second-order spinal sensory neurons contributes to pain after peripheral nerve injury

The Role of the Thalamus in Nociception: Important but Forgotten

Pain is a complex response to noxious stimuli. Upon detection of the nociceptive stimulus by first-order neurons or nociceptors, an action potential ascends to the spinal dorsal horn, a crucial site for synapsing with second-order neurons. These second-order neurons carry the nociceptive stimulus to supraspinal regions, notably the thalamus. Although extensive research has focused on spinal-level nociceptive mechanisms (e.g., neurotransmitters, receptors, and glial cells), the thalamus is still poorly elucidated. The role of the thalamus in relaying sensory and motor responses to the cortex is well known. However, a comprehensive understanding of the mechanisms in the synapse between the second-order and third-order neurons that transmit this impulse to the somatosensory cortex, where the response is processed and interpreted as pain, is still lacking. Thus, this review investigated the thalamus's role in transmitting nociceptive impulses. Current evidence indicates the involvement of the neurotransmitters glutamate and serotonin, along with NMDA, P2X4, TLR4, FGR, and NLRP3 receptors, as well as signaling pathways including ERK, P38, NF-κB, cytokines, and glial cells at nociceptive synapses within the thalamus.

Pain in Parkinson's disease: a neuroanatomy-based approach

Parkinson's disease is a progressive neurodegenerative disorder characterized by the deposition of misfolded alpha-synuclein in different regions of the central and peripheral nervous system. Motor impairment represents the signature clinical expression of Parkinson's disease. Nevertheless, non-motor symptoms are invariably present at different stages of the disease and constitute an important therapeutic challenge with a high impact for the patients' quality of life. Among non-motor symptoms, pain is frequently experienced by patients, being present in a range of 24-85% of Parkinson's disease population. Moreover, in more than 5% of patients, pain represents the first clinical manifestation, preceding by decades the exordium of motor symptoms. Pain implies a complex biopsychosocial experience with a downstream complex anatomical network involved in pain perception, modulation, and processing. Interestingly, all the anatomical areas involved in pain network can be affected by a-synuclein pathology, suggesting that pathophysiology of pain in Parkinson's disease encompasses a 'pain spectrum', involving different anatomical and neurochemical substrates. Here the various anatomical sites recruited in pain perception, modulation and processing are discussed, highlighting the consequences of their possible degeneration in course of Parkinson's disease. Starting from peripheral small fibres neuropathy and pathological alterations at the level of the posterior laminae of the spinal cord, we then describe the multifaceted role of noradrenaline and dopamine loss in driving dysregulated pain perception. Finally, we focus on the possible role of the intertwined circuits between amygdala, nucleus accumbens and habenula in determining the psycho-emotional, autonomic and cognitive experience of pain in Parkinson's disease. This narrative review provides the first anatomically driven comprehension of pain in Parkinson's disease, aiming at fostering new insights for personalized clinical diagnosis and therapeutic interventions.

SIRT1 mediates the excitability of spinal CaMKIIα-positive neurons and participates in neuropathic pain by controlling Nav1.3

AIMS

Neuropathic pain is a common chronic pain disorder, which is largely attributed to spinal central sensitization. Calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) activation in the spinal dorsal horn (SDH) is a major contributor to spinal sensitization. However, the exact way that CaMKIIα-positive (CaMKIIα+) neurons in the SDH induce neuropathic pain is still unclear. This study aimed to explore the role of spinal CaMKIIα+ neurons in neuropathic pain caused by chronic constriction injury (CCI) and investigate the potential epigenetic mechanisms involved in CaMKIIα+ neuron activation.

METHODS

CCI-induced neuropathic pain mice model, Sirt1loxP/loxP mice, and chemogenetic virus were used to investigate whether the activation of spinal CaMKIIα+ neurons is involved in neuropathic pain and its involved mechanism. Transcriptome sequence, western blotting, qRT-PCR, and immunofluorescence analysis were performed to assay the expression of related molecules and activation of neurons. Co-immunoprecipitation was used to observe the binding relationship of protein. Chromatin immunoprecipitation (ChIP)-PCR was applied to analyze the acetylation of histone H3 in the Scn3a promoter region.

RESULTS

The expression of sodium channel Nav1.3 was increased and the expression of SIRT1 was decreased in the spinal CaMKIIα+ neurons of CCI mice. CaMKIIα neurons became overactive after CCI, and inhibiting their activation relieved CCI-induced pain. Overexpression of SIRT1 reversed the increase of Nav1.3 and alleviated pain, while knockdown of SIRT1 or overexpression of Nav1.3 promoted CaMKIIα+ neuron activation and induced pain. By knocking down spinal SIRT1, the acetylation of histone H3 in the Scn3a (encoding Nav1.3) promoter region was increased, leading to an increased expression of Nav1.3.

CONCLUSION

The findings suggest that an aberrant reduction of spinal SIRT1 after nerve injury epigenetically increases Nav1.3, subsequently activating CaMKIIα+ neurons and causing neuropathic pain.

A Comprehensive Narrative Review of Neuropathic Pain: From Pathophysiology to Surgical Treatment

Neuropathic pain is a challenging condition. Despite the immense progress made in the pathophysiology and treatment of such conditions, so much work still has to be done. New frontiers previously unexplored are now objects of study with exciting results, mainly regarding neuromodulation and optogenetics. This review explores the already known pathophysiology and the clinical and surgical treatment in the light of evidence-based medicine. Additionally, new concepts and insights are discussed, presenting the hope for the development of new paradigms in the treatment of neuropathic pain.

Structure-function and rational design of a spider toxin Ssp1a at human voltage-gated sodium channel subtypes

The structure-function and optimization studies of NaV-inhibiting spider toxins have focused on developing selective inhibitors for peripheral pain-sensing NaV1.7. With several NaV subtypes emerging as potential therapeutic targets, structure-function analysis of NaV-inhibiting spider toxins at such subtypes is warranted. Using the recently discovered spider toxin Ssp1a, this study extends the structure-function relationships of NaV-inhibiting spider toxins beyond NaV1.7 to include the epilepsy target NaV1.2 and the pain target NaV1.3. Based on these results and docking studies, we designed analogues for improved potency and/or subtype-selectivity, with S7R-E18K-rSsp1a and N14D-P27R-rSsp1a identified as promising leads. S7R-E18K-rSsp1a increased the rSsp1a potency at these three NaV subtypes, especially at NaV1.3 (∼10-fold), while N14D-P27R-rSsp1a enhanced NaV1.2/1.7 selectivity over NaV1.3. This study highlights the challenge of developing subtype-selective spider toxin inhibitors across multiple NaV subtypes that might offer a more effective therapeutic approach. The findings of this study provide a basis for further rational design of Ssp1a and related NaSpTx1 homologs targeting NaV1.2, NaV1.3 and/or NaV1.7 as research tools and therapeutic leads.

A Review on Autophagy in Orofacial Neuropathic Pain

Orofacial neuropathic pain indicates pain caused by a lesion or diseases of the somatosensory nervous system. It is challenging for the clinician to diagnose and manage orofacial neuropathic pain conditions due to the considerable variability between individual clinical presentations and a lack of understanding of the mechanisms underlying the etiology and pathogenesis. In the last few decades, researchers have developed diagnostic criteria, questionnaires, and clinical assessment methods for the diagnosis of orofacial neuropathic pain. Recently, researchers have observed the role of autophagy in neuronal dysfunction as well as in the modulation of neuropathic pain. On this basis, in the present review, we highlight the characteristics, classification, and clinical assessment of orofacial neuropathic pain. Additionally, we introduce autophagy and its potential role in the modulation of orofacial neuropathic pain, along with a brief overview of the pathogenesis, which in future may reveal new possible targets for treating this condition.

Normalization of Neuroinflammation: A New Strategy for Treatment of Persistent Pain and Memory/Emotional Deficits in Chronic Pain

Chronic pain, which affects around 1/3 of the world population and is often comorbid with memory deficit and mood depression, is a leading source of suffering and disability. Studies in past decades have shown that hyperexcitability of primary sensory neurons resulting from abnormal expression of ion channels and central sensitization mediated pathological synaptic plasticity, such as long-term potentiation in spinal dorsal horn, underlie the persistent pain. The memory/emotional deficits are associated with impaired synaptic connectivity in hippocampus. Dysregulation of numerous endogenous proteins including receptors and intracellular signaling molecules is involved in the pathological processes. However, increasing knowledge contributes little to clinical treatment. Emerging evidence has demonstrated that the neuroinflammation, characterized by overproduction of pro-inflammatory cytokines and glial activation, is reliably detected in humans and animals with chronic pain, and is sufficient to induce persistent pain and memory/emotional deficits. The abnormal expression of ion channels and pathological synaptic plasticity in spinal dorsal horn and in hippocampus are resulting from neuroinflammation. The neuroinflammation is initiated and maintained by the interactions of circulating monocytes, glial cells and neurons. Obviously, unlike infectious diseases and cancer, which are caused by pathogens or malignant cells, chronic pain is resulting from alterations of cells and molecules which have numerous physiological functions. Therefore, normalization (counterbalance) but not simple inhibition of the neuroinflammation is the right strategy for treating neuronal disorders. Currently, no such agent is available in clinic. While experimental studies have demonstrated that intracellular Mg²⁺ deficiency is a common feature of chronic pain in animal models and supplement Mg²⁺ are capable of normalizing the neuroinflammation, activation of upregulated proteins that promote recovery, such as translocator protein (18k Da) or liver X receptors, has a similar effect. In this article, relevant experimental and clinical evidence is reviewed and discussed.

Ocular pain in ocular graft-versus-host disease patients with neurotrophic keratopathy

PURPOSE

Neurotrophic keratopathy (NK) is a degenerative disorder of the cornea characterized by decreased sensory innervation, epitheliopathy, and impaired epithelial healing. In this study, we assessed ocular pain and quality-of-life-related parameters in ocular graft-versus-host disease (oGVHD) patients with and without NK.

METHODS

We included 213 oGVHD patients in this retrospective study, including 29 patients with NK assessed by the Cochet-Bonnet esthesiometer. We evaluated their records for ocular pain assessment survey (OPAS) scores and clinical parameters, including corneal sensation, corneal fluorescein staining (CFS) score, Schirmer's test, tear break-up time (TBUT), and ocular surface disease index (OSDI) score.

RESULTS

oGVHD patients with NK had lower corneal sensation (3.4 ± 1.4 vs. 5.9 ± 0.3; p < 0.0001), higher CFS scores (6.4 ± 4.2 vs. 4.7 ± 4.0; p = 0.01), and lower TBUT scores (1.2 ± 2.1 vs. 2.2 ± 3.1; p = 0.08) compared to oGVHD patients without NK and additionally had significantly higher ocular pain intensity scores (OPAS 24-h average eye pain intensity: 2.0 ± 2.8 vs. 1.1 ± 1.9; p = 0.03). Patients with NK more commonly reported burning (0.2 ± 0.3 vs. 0.3 ± 0.4; p = 0.021) and sensitivity to light (0.2 ± 0.3 vs. 0.3 ± 0.4; p = 0.049) as compared to patients without NK.

CONCLUSION

Clinical signs of ocular surface disease are worse in oGVHD patients with NK compared to oGVHD patients without NK. These patients additionally experience higher intensity ocular pain and lower quality-of-life-related parameters.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

Systemic DNA/RNA heteroduplex oligonucleotide administration for regulating the gene expression of dorsal root ganglion and sciatic nerve

Neuropathic pain, a heterogeneous condition, affects 7%–10% of the general population. To date, efficacious and safe therapeutic approaches remain limited. Antisense oligonucleotide (ASO) therapy has opened the door to treat spinal muscular atrophy, with many ongoing clinical studies determining its therapeutic utility. ASO therapy for neuropathic pain and peripheral nerve disease requires efficient gene delivery and knockdown in both the dorsal root ganglion (DRG) and sciatic nerve, key tissues for pain signaling. We previously developed a new DNA/RNA heteroduplex oligonucleotide (HDO) technology that achieves highly efficient gene knockdown in the liver. Here, we demonstrated that intravenous injection of HDO, comprising an ASO and its complementary RNA conjugated to α-tocopherol, silences endogenous gene expression more than 2-fold in the DRG, and sciatic nerve with higher potency, efficacy, and broader distribution than ASO alone. Of note, we observed drastic target suppression in all sizes of neuronal DRG populations by in situ hybridization. Our findings establish HDO delivery as an investigative and potentially therapeutic platform for neuropathic pain and peripheral nerve disease.

Voltage-Gated Sodium Channel Modulation by a New Spider Toxin Ssp1a Isolated From an Australian Theraphosid

Given the important role of voltage-gated sodium (NaV) channel-modulating spider toxins in elucidating the function, pharmacology, and mechanism of action of therapeutically relevant NaV channels, we screened the venom from Australian theraphosid species against the human pain target hNaV1.7. Using assay-guided fractionation, we isolated a 33-residue inhibitor cystine knot (ICK) peptide (Ssp1a) belonging to the NaSpTx1 family. Recombinant Ssp1a (rSsp1a) inhibited neuronal hNaV subtypes with a rank order of potency hNaV1.7 > 1.6 > 1.2 > 1.3 > 1.1. rSsp1a inhibited hNaV1.7, hNaV1.2 and hNaV1.3 without significantly altering the voltage-dependence of activation, inactivation, or delay in recovery from inactivation. However, rSsp1a demonstrated voltage-dependent inhibition at hNaV1.7 and rSsp1a-bound hNaV1.7 opened at extreme depolarizations, suggesting rSsp1a likely interacted with voltage-sensing domain II (VSD II) of hNaV1.7 to trap the channel in its resting state. Nuclear magnetic resonance spectroscopy revealed key structural features of Ssp1a, including an amphipathic surface with hydrophobic and charged patches shown by docking studies to comprise the interacting surface. This study provides the basis for future structure-function studies to guide the development of subtype selective inhibitors.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Neuropharmacological basis for multimodal analgesia in chronic pain

Managing chronic pain remains a major unmet clinical challenge. Patients can be treated with a range of interventions, but pharmacotherapy is the most common. These include opioids, antidepressants, calcium channel modulators, sodium channel blockers, and nonsteroidal anti-inflammatory drugs. Many of these drugs target a particular mechanism; however, chronic pain in many diseases is multifactorial and induces plasticity throughout the sensory neuroaxis. Furthermore, comorbidities such as depression, anxiety, and sleep disturbances worsen quality of life. Given the complexity of mechanisms and symptoms in patients, it is unsurprising that many fail to achieve adequate pain relief from a single agent. The efforts to develop novel drug classes with better efficacy have not always proved successful; a multimodal or combination approach to analgesia is an important strategy in pain control. Many patients frequently take more than one medication, but high-quality evidence to support various combinations is often sparse. Ideally, combining drugs would produce synergistic action to maximize analgesia and reduce side effects, although sub-additive and additive analgesia is still advantageous if additive side-effects can be avoided. In this review, we discuss pain mechanisms, drug actions, and the rationale for mechanism-led treatment selection.Abbreviations: COX - cyclooxygenase, CGRP - calcitonin gene-related peptide, CPM - conditioned pain modulation, NGF - nerve growth factor, NNT - number needed to treat, NMDA - N-methyl-d-aspartate, NSAID - nonsteroidal anti-inflammatory drugs, TCA - tricyclic antidepressant, SNRI - serotonin-noradrenaline reuptake inhibitor, QST - quantitative sensory testing.

Concurrent ocular pain in patients with neurotrophic keratopathy

PURPOSE

To illustrate that ocular pain may occur in patients with neurotrophic keratopathy (NK) that typically are thought to lack symptoms of discomfort, and that aa subset of these patients may also present with neuropathic corneal pain (NCP).

METHOD

Retrospective Case series of 7 stage 1 NK patients who presented with concurrent ocular pain, as confirmed by clinical examination, proparacaine challenge test, and in vivo corneal confocal microscopy (IVCM). Records were assessed for results of ocular surface disease index (OSDI), pain on visual analog scale (VAS), ocular pain assessment survey (OPAS), best-corrected visual acuity (BCVA), corneal fluorescein staining (CFS) score, and IVCM findings. IVCM findings were compared to that of 20 healthy reference controls.

RESULTS

Mean age of patients was 63.7 ± 11.6 (range 44-76) years and 56.9 ± 8.6 (range 42-74) years in reference controls (p = 0.11). At presentation, ocular discomfort was 8.0 ± 1.3 (range 7-10) on VAS and mean OSDI scores were 72.26 ± 6.81 (range 62.50-79.54). Mean BCVA was 20/40, and mean CFS scores were 3.43 ± 0.79 (range 2-4) on the Oxford scale. IVCM analysis showed significant decrease in mean total, main and branch nerve densities in ranges consistent with NK as compared to normal controls (p < 0.001 for all), increased dendritiform cell density in three patients (p < 0.001), and the presence of microneuromas in six of the patients.

CONCLUSION

Patients with NK are thought to present with hypoesthesia. However, nerve damage and inflammation, which play a role in the development of NK may result in the development of chronic ocular pain, such as NCP, resulting in potential underdiagnosis of either disease.

Depression and Pain: use of antidepressant

Background:

Emotional disorders are common comorbid affectations that exacerbate the severity and persistence of chronic pain. Specifically, depressive symptoms can lead to an excessive duration and intensity of pain. Clinical and preclinical studies have been focused on the underlying mechanisms of chronic pain and depression comorbidity and the use of antidepressants to reduce pain.

Aim:

This review provides an overview of the comorbid relationship of chronic pain and depression, the clinical and pre-clinical studies performed on the neurobiological aspects of pain and depression, and the use of antidepressants as analgesics.

Methods:

A systematic search of literature databases was conducted according to pre-defined criteria. The authors independently conducted a focused analysis of the full-text articles.

Results:

Studies suggest that pain and depression are highly intertwined and may co-exacerbate physical and psychological symptoms. One important biochemical basis for pain and depression focuses on the serotonergic and norepinephrine system, which have been shown to play an important role in this comorbidity. Brain structures that codify pain are also involved in mood. It is evident that using serotonergic and norepinephrine antidepressants are strategies commonly employed to mitigate pain

Conclusion:

Literature indicates that pain and depression impact each other and play a prominent role in the development and maintenance of other chronic symptoms. Antidepressants continue to be a major therapeutic tool for managing chronic pain. Tricyclic antidepressants (TCAs) are more effective in reducing pain than Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Noradrenaline Reuptake Inhibitors (SNRIs).

The effect of IL-6/Piezo2 on the trigeminal neuropathic pain

The nature of trigeminal neuropathic pain (TN) attacks is regarded as the ignition of ectopic action potentials from the trigeminal root following vascular compression, which seemed to be related to transmembrane proteins and inflammation factors. This study focused on the mechanosensitive channel Piezo2 and cytokine IL-6. The chronic constriction injury of infraorbital nerve in SD rats was used to establish the TN model. The trigeminal ganglion was then achieved to perform immunocytochemistry studies. A significant upregulation of Piezo2 and IL-6 was showed in the TN model rats. The Piezo2 positive accounted for 72.3±9.5% in those IL-6 positive neurons. The Piezo2 co-localized with CGRP, IB4 and NF-200 but not with GFAP, which implied that it was expressed in both the C-type and the A-type neurons. After administration of GsMTx4 or anti-rat IL-6 antibody in the TN model, the dynamic allodynia and pinprick hyperalgesia scores as well as the mechanical threshold changed significantly. In the sham-operation rates, with local administration of IL-6, an upregulation of Piezo2 was also exhibited. Our study demonstrated that the up-regulation of Piezo2 in the pain afferent neurons following trigeminal nerve injury may play a role in the development of the neuralgia. Meanwhile, the expression of Piezo2 may be modulated by inflammatory cytokines, such as IL-6.

Lipopolysaccharides induce a RAGE-mediated sensitization of sensory neurons and fluid hypersecretion in the upper airways

Thoracic dorsal root ganglia (tDRG) contribute to fluid secretion in the upper airways. Inflammation potentiates DRG responses, but the mechanisms remain under investigation. The receptor for advanced glycation end-products (RAGE) underlies potentiation of DRG responses in pain pathologies; however, its role in other sensory modalities is less understood. We hypothesize that RAGE contributes to electrophysiological and biochemical changes in tDRGs during inflammation. We used tDRGs and tracheas from wild types (WT), RAGE knock-out (RAGE-KO), and with the RAGE antagonist FPS-ZM1, and exposed them to lipopolysaccharides (LPS). We studied: capsaicin (CAP)-evoked currents and action potentials (AP), tracheal submucosal gland secretion, RAGE expression and downstream pathways. In WT neurons, LPS increased CAP-evoked currents and AP generation, and it caused submucosal gland hypersecretion in tracheas from WT mice exposed to LPS. In contrast, LPS had no effect on tDRG excitability or gland secretion in RAGE-KO mice or mice treated with FPS-ZM1. LPS upregulated full-length RAGE (encoded by Tv1-RAGE) and downregulated a soluble (sRAGE) splice variant (encoded by MmusRAGEv4) in tDRG neurons. These data suggest that sensitization of tDRG neurons contributes to hypersecretion in the upper airways during inflammation. And at least two RAGE variants may be involved in these effects of LPS.

Intercellular communication and ion channels in neuropathic pain chronicization

BACKGROUND

Neuropathic pain is caused by primary lesion or dysfunction of either peripheral or central nervous system. Due to its complex pathogenesis, often related to a number of comorbidities, such as cancer, neurodegenerative and neurovascular diseases, neuropathic pain still represents an unmet clinical need, lacking long-term effective treatment and complex case-by-case approach.

AIM AND METHODS

We analyzed the recent literature on the role of selective voltage-sensitive sodium, calcium and potassium permeable channels and non-selective gap junctions (GJs) and hemichannels (HCs) in establishing and maintaining chronic neuropathic conditions. We finally focussed our review on the role of extracellular microenvironment modifications induced by resident glial cells and on the recent advances in cell-to-cell and cell-to-extracellular environment communication in chronic neuropathies.

CONCLUSION

In this review, we provide an update on the current knowledge of neuropathy chronicization processes with a focus on both neuronal and glial ion channels, as well as on channel-mediated intercellular communication.

Exploring the Promise of Flavonoids to Combat Neuropathic Pain: From Molecular Mechanisms to Therapeutic Implications

Neuropathic pain (NP) is the result of irregular processing in the central or peripheral nervous system, which is generally caused by neuronal injury. The management of NP represents a great challenge owing to its heterogeneous profile and the significant undesirable side effects of the frequently prescribed psychoactive agents, including benzodiazepines (BDZ). Currently, several established drugs including antidepressants, anticonvulsants, topical lidocaine, and opioids are used to treat NP, but they exert a wide range of adverse effects. To reduce the burden of adverse effects, we need to investigate alternative therapeutics for the management of NP. Flavonoids are the most common secondary metabolites of plants used in folkloric medicine as tranquilizers, and have been claimed to have a selective affinity to the BDZ binding site. Several studies in animal models have reported that flavonoids can reduce NP. In this paper, we emphasize the potentiality of flavonoids for the management of NP.

Modulation of Voltage-Gated Sodium Channel Activity in Human Dorsal Root Ganglion Neurons by Herpesvirus Quiescent Infection

The molecular mechanisms of pain associated with alphaherpesvirus latency are not clear. We hypothesize that the voltage-gated sodium channels (VGSC) on the dorsal root ganglion (DRG) neurons controlling electrical impulses may have abnormal activity during latent viral infection and reactivation. We used herpes simplex virus 1 (HSV-1) to infect the human DRG-derived neuronal cell line HD10.6 in order to study the establishment and maintenance of viral latency, viral reactivation, and changes in the functional expression of VGSCs. Differentiated cells exhibited robust tetrodotoxin (TTX)-sensitive sodium currents, and acute infection significantly reduced the functional expression of VGSCs within 24 h and completely abolished VGSC activity within 3 days. A quiescent state of infection mimicking latency can be achieved in the presence of acyclovir (ACV) for 7 days followed by 5 days of ACV washout, and then the viruses can remain dormant for another 3 weeks. It was noted that during the establishment of HSV-1 latency, the loss of VGSC activity caused by HSV-1 infection could not be blocked by ACV treatment. However, neurons with continued ACV treatment for another 4 days showed a gradual recovery of VGSC functional expression. Furthermore, the latently infected neurons exhibited higher VGSC activity than controls. The overall regulation of VGSCs by HSV-1 during quiescent infection was proved by increased transcription and possible translation of Nav1.7. Together, these observations demonstrated a very complex pattern of electrophysiological changes during HSV infection of DRG neurons, which may have implications for understanding of the mechanisms of virus-mediated pain linked to latency and reactivation.IMPORTANCE The reactivation of herpesviruses, most commonly varicella-zoster virus (VZV) and pseudorabies virus (PRV), may cause cranial nerve disorder and unbearable pain. Clinical studies have also reported that HSV-1 causes postherpetic neuralgia and chronic occipital neuralgia in humans. The current work meticulously studies the functional expression profile changes of VGSCs during the processes of HSV-1 latency establishment and reactivation using human dorsal root ganglion-derived neuronal HD10.6 cells as an in vitro model. Our results indicated that VGSC activity was eliminated upon infection but steadily recovered during latency establishment and that latent neurons exhibited even higher VGSC activity. This finding advances our knowledge of how ganglion neurons generate uncharacteristic electrical impulses due to abnormal VGSC functional expression influenced by the latent virus.

Treating Persistent Pain After Breast Cancer Surgery

Breast cancer is one of the most commonly diagnosed cancers among women, and since the prognosis of breast cancer has substantially improved in past decades, complications of management are becoming increasingly apparent. Persistent pain lasting greater than 3 months after breast cancer surgery is unfortunately a common complication affecting approximately 30% of patients after tumour resection. Persistent breast cancer pain has neuropathic features and is typically mild-to-moderate in intensity, with approximately 10% suffering from severe pain. There is an increasing need to prevent persistent pain through the use of transitional pain programmes and perioperative interventions, and to identify novel treatment modalities to reduce suffering in those who unfortunately develop persistent pain. This review serves to provide an overview on persistent pain after breast cancer surgery, its pathophysiology, and current management strategies.

Injury-Induced Effectors of Neuropathic Pain

Injuries typically result in the development of neuropathic pain, which decreases in parallel with wound healing. However, the pain may remain after the injury appears to have healed, which is generally associated with an ongoing underlying pro-inflammatory state. Injury induces many cells to release factors that contribute to the development of a pro-inflammatory state, which is considered an essential first step towards wound healing. However, pain elimination requires a transition of the injury site from pro- to anti-inflammatory. Therefore, developing techniques that eliminate chronic pain require an understanding of the cells resident at and recruited to injury sites, the factors they release, that promote a pro-inflammatory state, and promote the subsequent transition of that site to be anti-inflammatory. Although a relatively large number of cells, factors, and gene expression changes are involved in these processes, it may be possible to control a relatively small number of them leading to the reduction and elimination of chronic neuropathic pain. This first of two papers examines the roles of the most salient cells and mediators associated with the development and maintenance of chronic neuropathic pain. The following paper examines the cells and mediators involved in reducing and eliminating chronic neuropathic pain.

Spider Knottin Pharmacology at Voltage-Gated Sodium Channels and Their Potential to Modulate Pain Pathways

Voltage-gated sodium channels (Na_Vs) are a key determinant of neuronal signalling. Neurotoxins from diverse taxa that selectively activate or inhibit Na_V channels have helped unravel the role of Na_V channels in diseases, including chronic pain. Spider venoms contain the most diverse array of inhibitor cystine knot (ICK) toxins (knottins). This review provides an overview on how spider knottins modulate Na_V channels and describes the structural features and molecular determinants that influence their affinity and subtype selectivity. Genetic and functional evidence support a major involvement of Na_V subtypes in various chronic pain conditions. The exquisite inhibitory properties of spider knottins over key Na_V subtypes make them the best lead molecules for the development of novel analgesics to treat chronic pain.

Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Cutaneous somatosensory neurons convey innocuous and noxious mechanical, thermal, and chemical stimuli from peripheral tissues to the CNS. Among these are nociceptive neurons that express calcitonin gene-related peptide-α (CGRPα). The role of peripheral CGRPα neurons (CANs) in acute and injury-induced pain has been studied using diphtheria toxin ablation, but their functional roles remain controversial. Because ablation permanently deletes a neuronal population, compensatory changes may ensue that mask the physiological or pathophysiological roles of CANs, particularly for injuries that occur after ablation. Therefore, we sought to define the role of intact CANs in vivo under baseline and injury conditions by using noninvasive transient optogenetic inhibition. We assessed pain behavior longitudinally from acute to chronic time points. We generated adult male and female mice that selectively express the outward rectifying proton pump archaerhodopsin-3 (Arch) in CANs, and inhibited their peripheral cutaneous terminals in models of neuropathic (spared nerve injury) and inflammatory (skin-muscle incision) pain using transdermal light activation of Arch. After nerve injury, brief activation of Arch reversed the chronic mechanical, cold, and heat hypersensitivity, alleviated the spontaneous pain, and reversed the sensitized mechanical currents in primary afferent somata. In contrast, Arch inhibition of CANs did not alter incision-induced hypersensitivity. Instead, incision-induced mechanical and heat hypersensitivity was alleviated by peripheral blockade of CGRPα peptide-receptor signaling. These results reveal that CANs have distinct roles in the time course of pain during neuropathic and incisional injuries and suggest that targeting peripheral CANs or CGRPα peptide-receptor signaling could selectively treat neuropathic or postoperative pain, respectively.SIGNIFICANCE STATEMENT The contribution of sensory afferent CGRPα neurons (CANs) to neuropathic and inflammatory pain is controversial. Here, we left CANs intact during neuropathic and perioperative incision injury by using transient transdermal optogenetic inhibition of CANs. We found that peripheral CANs are required for neuropathic mechanical, cold, and heat hypersensitivity, spontaneous pain, and sensitization of mechanical currents in afferent somata. However, they are dispensable for incisional pain transmission. In contrast, peripheral pharmacological inhibition of CGRPα peptide-receptor signaling alleviated the incisional mechanical and heat hypersensitivity, but had no effect on neuropathic pain. These results show that CANs have distinct roles in neuropathic and incisional pain and suggest that their targeting via novel peripheral treatments may selectively alleviate neuropathic versus incisional pain.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Targeting Neuropathic Pain: Pathobiology, Current Treatment and Peptidomimetics as a New Therapeutic Opportunity

There is a huge need for pharmaceutical agents for the treatment of chronic Neuropathic Pain (NP), a complex condition where patients can suffer from either hyperalgesia or allodynia originating from central or peripheral nerve injuries. To date, the therapeutic guidelines include the use of tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors and anticonvulsants, beside the use of natural compounds and non-pharmacological options. Unfortunately, these drugs suffer from limited efficacy and serious dose-dependent adverse effects. In the last decades, the heptapeptide SP1-7, the major bioactive metabolite produced by Substance P (SP) cleavage, has been extensively investigated as a potential target for the development of novel peptidomimetic molecules to treat NP. Although the physiological effects of this SP fragment have been studied in detail, the mechanism behind its action is not fully clarified and the target for SP1-7 has not been identified yet. Nevertheless, specific binding sites for the heptapeptide have been found in brain and spinal cord of both mouse and rats. Several Structure-Affinity Relationship (SAR) studies on SP1-7 and some of its synthetic analogues have been carried out aiming to developing more metabolically stable and effective small molecule SP1-7-related amides that could be used as research tools for a better understanding of the SP1-7 system and, in a longer perspective, as potential therapeutic agents for future treatment of NP.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Antisense oligonucleotides selectively suppress target RNA in nociceptive neurons of the pain system and can ameliorate mechanical pain

There is an urgent need for better treatments for chronic pain, which affects more than 1 billion people worldwide. Antisense oligonucleotides (ASOs) have proven successful in treating children with spinal muscular atrophy, a severe infantile neurological disorder, and several ASOs are currently being tested in clinical trials for various neurological disorders. Here, we characterize the pharmacodynamic activity of ASOs in spinal cord and dorsal root ganglia (DRG), key tissues for pain signaling. We demonstrate that activity of ASOs lasts up to 2 months after a single intrathecal bolus dose. Interestingly, comparison of subcutaneous, intracerebroventricular, and intrathecal administration shows that DRGs are targetable by systemic and central delivery of ASOs, while target reduction in the spinal cord is achieved only after direct central delivery. Upon detailed characterization of ASO activity in individual cell populations in DRG, we observe robust target suppression in all neuronal populations, thereby establishing that ASOs are effective in the cell populations involved in pain propagation. Furthermore, we confirm that ASOs are selective and do not modulate basal pain sensation. We also demonstrate that ASOs targeting the sodium channel Nav1.7 induce sustained analgesia up to 4 weeks. Taken together, our findings support the idea that ASOs possess the required pharmacodynamic properties, along with a long duration of action beneficial for treating pain.

Psychopharmacology of chronic pain

Chronic pain is a frequent condition that affects an estimated 20% of people worldwide, accounting for 15%-20% of doctors' appointments (Treede et al., 2015). It lacks the acute warning function of physiologic nociception, and instead involves the activation of multiple neurophysiologic mechanisms in the somatosensory system, a complex neuronal network under the control of powerful autoregulatory loops and able to undergo rapid neuroplastic alteration (Verdu et al., 2008). There is a growing body of research suggesting that some such pathways are shared by major psychologic disorders such as depression and anxiety, opening new avenues in co-treatment strategies. In particular, besides anticonvulsants, which are today used as analgesics, other psychopharmaceuticals, such as the tricyclic antidepressants, are displaying efficacy in the treatment of neuropathic and nociceptive chronic pain. The state of the art regarding the mechanisms of nociception and the pharmacology of both the neurotransmitters involved and the wide range of psychoactive compounds that may be useful in the treatment of chronic pain are discussed.

Neuropathy following spinal nerve injury shares features with the irritable nociceptor phenotype: A back-translational study of oxcarbazepine

BACKGROUND

The term 'irritable nociceptor' was coined to describe neuropathic patients characterized by evoked hypersensitivity and preservation of primary afferent fibres. Oxcarbazepine is largely ineffectual in an overall patient population, but has clear efficacy in a subgroup with the irritable nociceptor profile. We examine whether neuropathy in rats induced by spinal nerve injury shares overlapping pharmacological sensitivity with the irritable nociceptor phenotype using drugs that target sodium channels.

METHODS

In vivo electrophysiology was performed in anaesthetized spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range (WDR) neurones in the ventral posterolateral thalamus (VPL) and dorsal horn.

RESULTS

In neuropathic rats, spontaneous activity in the VPL was substantially attenuated by spinal lidocaine, an effect that was absent in sham rats. The former measure was in part dependent on ongoing peripheral activity as intraplantar lidocaine also reduced aberrant spontaneous thalamic firing. Systemic oxcarbazepine had no effect on wind-up of dorsal horn neurones in sham and SNL rats. However, in SNL rats, oxcarbazepine markedly inhibited punctate mechanical-, dynamic brush- and cold-evoked neuronal responses in the VPL and dorsal horn, with minimal effects on heat-evoked responses. In addition, oxcarbazepine inhibited spontaneous activity in the VPL. Intraplantar injection of the active metabolite licarbazepine replicated the effects of systemic oxcarbazepine, supporting a peripheral locus of action.

CONCLUSIONS

We provide evidence that ongoing activity in primary afferent fibres drives spontaneous thalamic firing after spinal nerve injury and that oxcarbazepine through a peripheral mechanism exhibits modality-selective inhibitory effects on sensory neuronal processing.

SIGNIFICANCE

The inhibitory effects of lidocaine and oxcarbazepine in this rat model of neuropathy resemble the clinical observations in the irritable nociceptor patient subgroup and support a mechanism-based rationale for bench-to-bedside translation when screening novel drugs.

The clinical utility of repetitive transcranial magnetic stimulation in reducing the risks of transitioning from acute to chronic pain in traumatically injured patients

Pain is a multifaceted condition and a major ongoing challenge for healthcare professionals having to treat patients in whom pain put them at risk of developing other conditions. Significant efforts have been invested in both clinical and research settings in an attempt to demystify the mechanisms at stake and develop optimal treatments as well as to reduce individual and societal costs. It is now universally accepted that neuroinflammation and central sensitization are two key underlying factors causing pain chronification as they result from maladaptive central nervous system plasticity. Recent research has shown that the mechanisms of action of repetitive transcranial magnetic stimulation (rTMS) make it a particularly promising avenue in treating various pain conditions. This review will first discuss the contribution of neuroinflammation and central sensitization in the transition from acute to chronic pain in traumatically injured patients. A detailed discussion on how rTMS may allow the restoration from maladaptive plasticity in addition to breaking down the chain of events leading to pain chronification will follow. Lastly, this review will provide a theoretical framework of what might constitute optimal rTMS modalities in dealing with pain symptoms in traumatically injured patients based on an integrated perspective of the physiopathological mechanisms underlying pain.

Spinal Cord Stimulation for Pain Treatment After Spinal Cord Injury

In addition to restoration of bladder, bowel, and motor functions, alleviating the accompanying debilitating pain is equally important for improving the quality of life of patients with spinal cord injury (SCI). Currently, however, the treatment of chronic pain after SCI remains a largely unmet need. Electrical spinal cord stimulation (SCS) has been used to manage a variety of chronic pain conditions that are refractory to pharmacotherapy. Yet, its efficacy, benefit profiles, and mechanisms of action in SCI pain remain elusive, due to limited research, methodological weaknesses in previous clinical studies, and a lack of mechanistic exploration of SCS for SCI pain control. We aim to review recent studies and outline the therapeutic potential of different SCS paradigms for traumatic SCI pain. We begin with an overview of its manifestations, classification, potential underlying etiology, and current challenges for its treatment. The clinical evidence for using SCS in SCI pain is then reviewed. Finally, future perspectives of pre-clinical research and clinical study of SCS for SCI pain treatment are discussed.

Addressing the Issue of Tetrodotoxin Targeting

This review is devoted to the medical application of tetrodotoxin (TTX), a potent non-protein specific blocker of voltage-gated sodium (NaV) channels. The selectivity of action, lack of affinity with the heart muscle NaV channels, and the inability to penetrate the blood–brain barrier make this toxin an attractive candidate for anesthetic and analgesic drug design. The efficacy of TTX was shown in neuropathic, acute and inflammatory pain models. The main emphasis of the review is on studies focused on the improvement of TTX efficacy and safety in conjunction with additional substances and drug delivery systems. A significant improvement in the effectiveness of the toxin was demonstrated when used in tandem with vasoconstrictors, local anesthetics and chemical permeation enhancers, with the best results obtained with the encapsulation of TTX in microparticles and liposomes conjugated to gold nanorods.

Sodium channels and pain: from toxins to therapies

Voltage-gated sodium channels (NaV channels) are essential for the initiation and propagation of action potentials that critically influence our ability to respond to a diverse range of stimuli. Physiological and pharmacological studies have linked abnormal function of NaV channels to many human disorders, including chronic neuropathic pain. These findings, along with the description of the functional properties and expression pattern of NaV channel subtypes, are helping to uncover subtype specific roles in acute and chronic pain and revealing potential opportunities to target these with selective inhibitors. High-throughput screens and automated electrophysiology platforms have identified natural toxins as a promising group of molecules for the development of target-specific analgesics. In this review, the role of toxins in defining the contribution of NaV channels in acute and chronic pain states and their potential to be used as analgesic therapies are discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Corneal sensitivity and subjective complaints of ocular pain in patients with fibromyalgia

PurposeFibromyalgia (FM) is a chronic pain disorder associated with pain and hypersensitivity in various parts of the body. The aim of this study was to understand whether changes in corneal sensitivity were associated with the subjective complaints of these patients.Patients and methodsIn this study, we included 36 patients with FM (30 female, 6 male, mean age: 46.7±9.2 years) and 39 healthy control subjects (33 females, 6 males, mean age: 44.3±7.6 years). We performed a detailed ophthalmological examination, measured tear film breakup time (TBUT), and performed Schirmer I test without anesthetic (SIT). Only patients that did not have any eye disease were included in this study. Subjective complaints of the patients were evaluated with ocular surface disease index (OSDI) scores and the corneal sensation was evaluated with Cochet-Bonnet esthesiometer.ResultsThere was no significant difference between the groups for age, sex, and visual acuity. Both SIT (16.1 mm vs 15.3 mm, P=0.36) and TBUT results (17.8 s vs 18.8 s, P=0.40) were similar in FM group and the controls subjects. However, corneal sensations in central (60.0 mm vs 55.0 mm, P=0.03), superior (57.5 mm vs 50.0 mm, P=0.005), and inferior (53.89 mm vs 46.03 mm, P<0.001) regions were significantly increased in FM patients. There was a significant positive correlation between corneal sensation and OSDI scores.ConclusionIn this study, we have demonstrated that the patients with FM have increased corneal sensitivity and have related ocular surface complaints similar to dry eye disease in the absence of it.

Therapeutic potential of Pak1 inhibition for pain associated with cutaneous burn injury

Painful burn injuries are among the most debilitating form of trauma, globally ranking in the top 15 leading causes of chronic disease burden. Despite its prevalence, however, chronic pain after burn injury is under-studied. We previously demonstrated the contribution of the Rac1-signaling pathway in several models of neuropathic pain, including burn injury. However, Rac1 belongs to a class of GTPases with low therapeutic utility due to their complex intracellular dynamics. To further understand the mechanistic underpinnings of burn-induced neuropathic pain, we performed a longitudinal study to address the hypothesis that inhibition of the downstream effector of Rac1, Pak1, will improve pain outcome following a second-degree burn injury. Substantial evidence has identified Pak1 as promising a clinical target in cognitive dysfunction and is required for dendritic spine dysgenesis associated with many neurological diseases. In our burn injury model, mice exhibited significant tactile allodynia and heat hyperalgesia and dendritic spine dysgenesis in the dorsal horn. Activity-dependent expression of c-fos also increased in dorsal horn neurons, an indicator of elevated central nociceptive activity. To inhibit Pak1, we repurposed an FDA-approved inhibitor, romidepsin. Treatment with romidepsin decreased dendritic spine dysgenesis, reduced c-fos expression, and rescued pain thresholds. Drug discontinuation resulted in a relapse of cellular correlates of pain and in lower pain thresholds in behavioral tests. Taken together, our findings identify Pak1 signaling as a potential molecular target for therapeutic intervention in traumatic burn-induced neuropathic pain.

DRG Voltage-Gated Sodium Channel 1.7 Is Upregulated in Paclitaxel-Induced Neuropathy in Rats and in Humans with Neuropathic Pain

Chemotherapy-induced peripheral neuropathy (CIPN) is a common adverse effect experienced by cancer patients receiving treatment with paclitaxel. The voltage-gated sodium channel 1.7 (Na_v1.7) plays an important role in multiple preclinical models of neuropathic pain and in inherited human pain phenotypes, and its gene expression is increased in dorsal root ganglia (DRGs) of paclitaxel-treated rats. Hence, the potential of change in the expression and function of Na_v1.7 protein in DRGs from male rats with paclitaxel-related CIPN and from male and female humans with cancer-related neuropathic pain was tested here. Double immunofluorescence in CIPN rats showed that Na_v1.7 was upregulated in small DRG neuron somata, especially those also expressing calcitonin gene-related peptide (CGRP), and in central processes of these cells in the superficial spinal dorsal horn. Whole-cell patch-clamp recordings in rat DRG neurons revealed that paclitaxel induced an enhancement of ProTx II (a selective Na_v1.7 channel blocker)-sensitive sodium currents. Bath-applied ProTx II suppressed spontaneous action potentials in DRG neurons occurring in rats with CIPN, while intrathecal injection of ProTx II significantly attenuated behavioral signs of CIPN. Complementarily, DRG neurons isolated from segments where patients had a history of neuropathic pain also showed electrophysiological and immunofluorescence results indicating an increased expression of Na_v1.7 associated with spontaneous activity. Na_v1.7 was also colocalized in human cells expressing transient receptor potential vanilloid 1 and CGRP. Furthermore, ProTx II decreased firing frequency in human DRGs with spontaneous action potentials. This study suggests that Na_v1.7 may provide a potential new target for the treatment of neuropathic pain, including chemotherapy (paclitaxel)-induced neuropathic pain.SIGNIFICANCE STATEMENT This work demonstrates that the expression and function of the voltage-gated sodium channel Na_v1.7 are increased in a preclinical model of chemotherapy-induced peripheral neuropathy (CIPN), the most common treatment-limiting side effect of all the most common anticancer therapies. This is key as gain-of-function mutations in human Na_v1.7 recapitulate both the distribution and pain percept as shown by CIPN patients. This work also shows that Na_v1.7 is increased in human DRG neurons only in dermatomes where patients are experiencing acquired neuropathic pain symptoms. This work therefore has major translational impact, indicating an important novel therapeutic avenue for neuropathic pain as a class.

Neuropathic Corneal Pain: Approaches for Management

Neuropathic pain is caused by a primary lesion or dysfunction of the nervous system and can occur in the cornea. However, neuropathic corneal pain (NCP) is currently an ill-defined disease. Patients with NCP are extremely challenging to manage, and evidence-based clinical recommendations for the management of patients with NCP are scarce. The objectives of this review are to provide guidelines for diagnosis and treatment of patients with NCP and to summarize current evidence-based literature in this area. We performed a systematic literature search of all relevant publications between 1966 and 2017. Treatment recommendations are, in part, based on methodologically sound randomized controlled trials (RCTs), demonstrating superiority to placebo or relevant control treatments, and on the consistency of evidence, degree of efficacy, and safety. In addition, the recommendations include our own extensive experience in the management of these patients over the past decade. A comprehensive algorithm, based on clinical evaluation and complementary tests, is presented for diagnosis and subcategorization of patients with NCP. Recommended first-line topical treatments include neuroregenerative and anti-inflammatory agents, and first-line systemic pharmacotherapy includes tricyclic antidepressants and an anticonvulsant. Second-line oral treatments recommended include an opioid-antagonist and opiate analgesics. Complementary and alternative treatments, such as cardiovascular exercise, acupuncture, omega-3 fatty acid supplementation, and gluten-free diet, may have additional benefits, as do potential noninvasive and invasive procedures in recalcitrant cases. Medication selection should be tailored on an individual basis, considering side effects, comorbidities, and levels of peripheral and centralized pain. Nevertheless, there is an urgent need for long-term studies and RCTs assessing the efficacy of treatments for NCP.

µ-Conotoxins Modulating Sodium Currents in Pain Perception and Transmission: A Therapeutic Potential

The Conus genus includes around 500 species of marine mollusks with a peculiar production of venomous peptides known as conotoxins (CTX). Each species is able to produce up to 200 different biological active peptides. Common structure of CTX is the low number of amino acids stabilized by disulfide bridges and post-translational modifications that give rise to different isoforms. µ and µO-CTX are two isoforms that specifically target voltage-gated sodium channels. These, by inducing the entrance of sodium ions in the cell, modulate the neuronal excitability by depolarizing plasma membrane and propagating the action potential. Hyperexcitability and mutations of sodium channels are responsible for perception and transmission of inflammatory and neuropathic pain states. In this review, we describe the current knowledge of µ-CTX interacting with the different sodium channels subtypes, the mechanism of action and their potential therapeutic use as analgesic compounds in the clinical management of pain conditions.

Old Friends With New Faces: Are Sodium Channel Blockers the Future of Adjunct Pain Medication Management?

Providers are being asked to decrease the emphasis and overutilization of long-term opioid therapy, but many are left without proper guidance on appropriate utilization of nonopioid therapies. Furthermore, therapeutic options are quite limited and many providers lack confidence in distinguishing available alternatives. When first-line therapy has failed in a patient, there is an apparent lack of knowledge on how to proceed with choosing subsequent therapy. To choose among alternative agents, an understanding of pharmacology, pharmacokinetics, and efficacy in targeting various pain conditions is necessary. This article focuses on the use of the carboxamide class of sodium channel blockers (carbamazepine, oxcarbazepine, eslicarbazepine) for adjunct pain medication management including research updates in pharmacology, pharmacokinetics, and current evidence for pain along with promising areas of research. It is an evidence update for clinical use of sodium channel blockers, clarifies misconceptions regarding their use, and highlights emerging research for improved pain targets that justifies additional study. We performed a complete review of the literature using the search terms, "oxcarbazepine," "carbamazepine," and "eslicarbazepine" in conjunction with "pharmacokinetics," "adverse effects," "pharmacology," "voltage-gated sodium channel subtype," "neuropathic pain," "inflammatory pain," "metabolism," "epoxide metabolite formation," "drug interactions," "CYP450 interactions," "pain phenotype," and "chronic pain management." Databases searched included PubMed and Google Scholar. Package inserts were used for drug structure illustration, adverse reactions, and bioavailability. Pharmacology and pharmacokinetic data were taken from randomized controlled trials evaluating this area as well as in vitro published results. For validity, only peer-reviewed literature was included. Evidence for sodium channel blockers in chronic pain management was limited. This review focuses on highlighting the data available for the use of sodium channel blockers for certain pain syndromes as well as underutilized potential. Emerging literature on sodium channel subtypes and their connection to neuropathic, inflammatory, and mechanical pain transmission is elucidated. The authors also scrutinize literature surrounding the pharmacokinetics of oxcarbazepine and eslicarbazepine to provide clearer guidance to the significance of any drug interactions and refute assumptions made on the basis of structural similarity to carbamazepine and its known undesirable drug interactions. Side effect profiles are outlined and compared, emphasizing the differences between agents. Sodium channel blocker doses used in certain pain syndromes are outlined with a call for further research to better understand their place in chronic pain management. Identification of sodium channel subtypes with links to specific pain conditions and the ability to target them hints at the potential for truly individualized therapy. Sodium channel inhibitors are underutilized on the basis of available evidence, and emerging research has identified this area as promising for additional clinical trials to better guide clinical practice.

PERSPECTIVE

This article provides a review of the pharmacology, evidence for pain management, and pharmacokinetics of oxcarbazepine, carbamazepine, and eslicarbazepine. There is a disparity in evidence using sodium channel blockers for pain and this article highlights the potential that is currently underutilized. The authors believe this will catalyze interest for further studies.

Intra-spinal microstimulation may alleviate chronic pain after spinal cord injury

Chronic pain after spinal cord injury (SCI) is a form of central neuropathic pain that is debilitating and often refractory to current pharmacological treatments. Neurostimulation pain therapies, such as epidural spinal cord stimulation, have only moderate success in reducing SCI pain. The pathogenesis of SCI pain may involve a state of central neuronal hyperexcitability, especially in the spinal cord dorsal horn, that develops after injury. We hypothesize that the neuronal structures near the spinal cord injury site may be an important pain generator, and intraspinal microstimulation (ISMS) may normalize dorsal horn neuronal hyperexcitability and hence alleviate SCI pain. Specifically, ISMS may induce frequency-dependent conduction block on axons of afferent sensory neurons, in the spinothalamic tract and Lissauer's tract. ISMS may also facilitate primary afferent depolarization that elicits presynaptic inhibition of incoming afferent inputs. Together, these actions will reduce abnormal afferent inputs and ascending pain signals before they can reach the brain. Furthermore, ISMS may directly induce inhibitory postsynaptic potentials in dorsal horn neurons, and trigger the release of endogenous inhibitory neurotransmitters, opioids and serotonin to inhibit postsynaptic neurons and restore the compromised segmental pain inhibition after SCI. Finally, ISMS may alter the frequency and pattern of discharge such that the rostrally conducted impulses no longer code pain or activate brain areas concerned with pain signaling. Based on recent progress in understanding spinal learning and plasticity, we also postulate that repetitive or long-term ISMS may help the dorsal horn "reset" neuronal excitability and regain normal pain processing for a prolonged period. By finely tuning the stimulation parameters (e.g., intensity, pulse width, frequency), position, and geometry of ISMS electrode, multiple spinal structures (e.g., dorsal horn, dorsal column, spinothalamic tract) may be modulated to induce synergistic pain inhibition. Our hypothesis can be readily tested in preclinical models of SCI pain by using a combination of in vivo electrophysiological (neuronal activity) and animal behavioral (pain response) approaches. Since ISMS electrodes stimulate the spinal structures directly, we expect that the effective stimulus intensity and energy consumption can be lower than that for epidural spinal cord stimulation. The proposed hypothesis may provide insights and rationales for developing a novel neurostimulation pain therapy by directly inhibiting the pain generators in the spinal cord, and ISMS may be an alternative strategy to treat SCI pain.

MiR-30b Attenuates Neuropathic Pain by Regulating Voltage-Gated Sodium Channel Nav1.3 in Rats

Nav1.3 is a tetrodotoxin-sensitive isoform among voltage-gated sodium channels that are closely associated with neuropathic pain. It can be up-regulated following nerve injury, but its biological function remains uncertain. MicroRNAs (miRNAs) are endogenous non-coding RNAs that can regulate post-transcriptional gene expression by binding with their target mRNAs. Using Target Scan software, we discovered that SCN3A is the major target of miR-30b, and we then determined whether miR-30b regulated the expression of Nav1.3 by transfecting miR-30b agomir through the stimulation of TNF-α or by transfecting miR-30b antagomir in primary dorsal root ganglion (DRG) neurons. The spinal nerve ligation (SNL) model was used to determine the contribution of miR-30b to neuropathic pain, to evaluate changes in Nav1.3 mRNA and protein expression, and to understand the sensitivity of rats to mechanical and thermal stimuli. Our results showed that miR-30b agomir transfection down-regulated Nav1.3 mRNA stimulated with TNF-α in primary DRG neurons. Moreover, miR-30b overexpression significantly attenuated neuropathic pain induced by SNL, with decreases in the expression of Nav1.3 mRNA and protein both in DRG neurons and spinal cord. Activation of Nav1.3 caused by miR-30b antagomir was identified. These data suggest that miR-30b is involved in the development of neuropathic pain, probably by regulating the expression of Nav1.3, and might be a novel therapeutic target for neuropathic pain. Perspective: This study is the first to explore the important role of miR-30b and Nav1.3 in spinal nerve ligation-induced neuropathic pain, and our evidence may provide new insight for improving therapeutic approaches to pain.

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

Cardamonin attenuates hyperalgesia and allodynia in a mouse model of chronic constriction injury-induced neuropathic pain: Possible involvement of the opioid system

Neuropathic pain arises from the injury of nervous system. The condition is extremely difficult to be treated due to the ineffectiveness and presence of various adverse effects of the currently available drugs. In the present study, we investigated the antiallodynic and antihyperlagesic properties of cardamonin, a naturally occurring chalcone in chronic constriction injury (CCI)-induced neuropathic pain mice model. Our findings showed that single and repeated dose of intra-peritoneal administration of cardamonin (3, 10, 30mg/kg) significantly inhibited (P<0.001) the chronic constriction injury-induced neuropathic pain using the Hargreaves plantar test, Randall-Selitto analgesiometer test, dynamic plantar anesthesiometer test and the cold plate test in comparison with the positive control drug used (amitriptyline hydrochloride, 20mg/kg, i.p.). Pre-treatment with naloxone hydrochloride (1mg/kg, i.p.) and naloxone methiodide (1mg/kg, s.c) significantly reversed the antiallodynic and antihyperalgesic effects of cardamonin in dynamic plantar anesthesiometer test and Hargreaves plantar test, respectively. In conclusion, the current findings demonstrated novel antiallodynic and antihyperalgesic effects of cardamonin through the activation of the opioidergic system both peripherally and centrally and may prove to be a potent lead compound for the development of neuropathic pain drugs in the future.

Gene expression profile of sodium channel subunits in the anterior cingulate cortex during experimental paclitaxel-induced neuropathic pain in mice

Paclitaxel, a chemotherapeutic agent, causes neuropathic pain whose supraspinal pathophysiology is not fully understood. Dysregulation of sodium channel expression, studied mainly in the periphery and spinal cord level, contributes to the pathogenesis of neuropathic pain. We examined gene expression of sodium channel (Na_v) subunits by real time polymerase chain reaction (PCR) in the anterior cingulate cortex (ACC) at day 7 post first administration of paclitaxel, when mice had developed paclitaxel-induced thermal hyperalgesia. The ACC was chosen because increased activity in the ACC has been observed during neuropathic pain. In the ACC of vehicle-treated animals the threshold cycle (Ct) values for Na_v1.4, Na_v1.5, Na_v1.7, Na_v1.8 and Na_v1.9 were above 30 and/or not detectable in some samples. Thus, comparison in mRNA expression between untreated control, vehicle-treated and paclitaxel treated animals was done for Na_v1.1, Na_v1.2, Na_v1.3, Na_v1.6, Na_x as well as Na_vβ1–Na_vβ4. There were no differences in the transcript levels of Na_v1.1–Na_v1.3, Na_v1.6, Na_x, Na_vβ1–Na_vβ3 between untreated and vehicle-treated mice, however, vehicle treatment increased Na_vβ4 expression. Paclitaxel treatment significantly increased the mRNA expression of Na_v1.1, Na_v1.2, Na_v1.6 and Na_x, but not Na_v1.3, sodium channel alpha subunits compared to vehicle-treated animals. Treatment with paclitaxel significantly increased the expression of Na_vβ1 and Na_vβ3, but not Na_vβ2 and Na_vβ4, sodium channel beta subunits compared to vehicle-treated animals. These findings suggest that during paclitaxel-induced neuropathic pain (PINP) there is differential upregulation of sodium channels in the ACC, which might contribute to the increased neuronal activity observed in the area during neuropathic pain.