Varicella zoster virus induces neuropathic changes in rat dorsal root ganglia and behavioral reflex sensitisation that is attenuated by gabapentin or sodium channel blocking drugs

Spider and scorpion knottins targeting voltage-gated sodium ion channels in pain signaling

In sensory neurons that transmit pain signals, whether acute or chronic, voltage-gated sodium channels (VGSCs) are crucial for regulating excitability. NaV1.1, NaV1.3, NaV1.6, NaV1.7, NaV1.8, and NaV1.9 have been demonstrated and defined their functional roles in pain signaling based on their biophysical properties and distinct patterns of expression in each subtype of sensory neurons. Scorpions and spiders are traditional Chinese medicinal materials, belonging to the arachnid class. Most of the studied species of them have evolved venom peptides that exhibit a wide variety of knottins specifically targeting VGSCs with subtype selectivity and conformational specificity. This review provides an overview on the exquisite knottins from scorpion and spider venoms targeting pain-related NaV channels, describing the sequences and the structural features as well as molecular determinants that influence their selectivity on special subtype and at particular conformation, with an aim for the development of novel research tools on NaV channels and analgesics with minimal adverse effects.

Exploring viral neuropathic pain: Molecular mechanisms and therapeutic implications

As the Coronavirus Disease 2019 (COVID-19) pandemic continues, there is a growing concern regarding the relationship between viral infections and neuropathic pain. Chronic neuropathic pain resulting from virus-induced neural dysfunction has emerged as a significant issue currently faced. However, the molecular mechanisms underlying this phenomenon remain unclear, and clinical treatment outcomes are often suboptimal. Therefore, delving into the relationship between viral infections and neuropathic pain, exploring the pathophysiological characteristics and molecular mechanisms of different viral pain models, can contribute to the discovery of potential therapeutic targets and methods, thereby enhancing pain relief and improving the quality of life for patients. This review focuses on HIV-related neuropathic pain (HNP), postherpetic neuralgia (PHN), and neuropathic pain caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infections, examining rodent models and relevant cellular molecular pathways. Through elucidating the connection between viral infections and neuropathic pain, it aims to delineate the current limitations and challenges faced by treatments, thereby providing insights and directions for future clinical practice and research.

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.

SARS-CoV-2 airway infection results in the development of somatosensory abnormalities in a hamster model

Although largely confined to the airways, SARS-CoV-2 infection has been associated with sensory abnormalities that manifest in both acute and chronic phenotypes. To gain insight on the molecular basis of these sensory abnormalities, we used the golden hamster model to characterize and compare the effects of infection with SARS-CoV-2 and influenza A virus (IAV) on the sensory nervous system. We detected SARS-CoV-2 transcripts but no infectious material in the cervical and thoracic spinal cord and dorsal root ganglia (DRGs) within the first 24 hours of intranasal virus infection. SARS-CoV-2-infected hamsters exhibited mechanical hypersensitivity that was milder but prolonged compared with that observed in IAV-infected hamsters. RNA sequencing analysis of thoracic DRGs 1 to 4 days after infection suggested perturbations in predominantly neuronal signaling in SARS-CoV-2-infected animals as opposed to type I interferon signaling in IAV-infected animals. Later, 31 days after infection, a neuropathic transcriptome emerged in thoracic DRGs from SARS-CoV-2-infected animals, which coincided with SARS-CoV-2-specific mechanical hypersensitivity. These data revealed potential targets for pain management, including the RNA binding protein ILF3, which was validated in murine pain models. This work elucidates transcriptomic signatures in the DRGs triggered by SARS-CoV-2 that may underlie both short- and long-term sensory abnormalities.

PK/PD analysis of trazodone and gabapentin in neuropathic pain rodent models: Translational PK-PD modeling from nonclinical to clinical development

A pharmacokinetic/pharmacodynamic (PK/PD) model was developed to describe the time course of writhings after intraperitoneal injection of acetic acid in mice. The model was applied to investigate the antinociceptive effect of trazodone and gabapentin alone and in combination. Writhings time course was described by a transit compartment model with the delay due to the transit of the acetic acid being represented by a chain of intermediate compartments. In the drug-treated animals, the number of writhings decreases according to a k₂ factor linking drug concentration and antinociceptive effect. Compounds' potency parameters were 10.9 and 0.0459 L/μmoles/min for trazodone and gabapentin, respectively, indicating a much higher in vivo potency of trazodone in the PD writhing test. The PK/PD parameters were used to simulate the expected writhing counts in mice at combined doses without efficacy alone, assuming pharmacological additivity. Simulation results indicated that, at low dose combinations, experimental data were mostly below the simulated writhings median, suggesting possible synergic effect. Such hypothesis was tested by adding the γ parameter in the PK/PD model to represent the deviation from the assumption of no-interaction, leading to a reduction of the objective function compared to the additive model. On this basis, several simulations were performed to identify possible starting dose combinations of trazodone and gabapentin in humans, by selecting doses yielding systemic exposures close to those being synergic in the mouse. Simulations indicated that doses of 50-100 mg trazodone could enhance gabapentin antinociceptive effect in humans, supporting the development of a low dose combination for optimal analgesia treatment.

Rodent models of postherpetic neuralgia: How far have we reached?

Background

Induced by varicella zoster virus (VZV), postherpetic neuralgia (PHN) is one of the common complications of herpes zoster (HZ) with refractory pain. Animal models play pivotal roles in disclosing the pain mechanisms and developing effective treatments. However, only a few rodent models focus on the VZV-associated pain and PHN.

Objective

To summarize the establishment and characteristics of popular PHN rodent models, thus offer bases for the selection and improvement of PHN models.

Design

In this review, we retrospect two promising PHN rodent models, VZV-induced PHN model and HSV1-induced PHN model in terms of pain-related evaluations, their contributions to PHN pathogenesis and pharmacology.

Results

Significant difference of two PHN models is the probability of virus proliferation; 2) Most commonly used pain evaluation of PHN model is mechanical allodynia, but pain-induced anxiety and other behaviours are worth noting; 3) From current PHN models, pain mechanisms involve changes in virus gene and host gene expression, neuroimmune-glia interactions and ion channels; 4) antiviral drugs and classical analgesics serve more on the acute stage of herpetic pain.

Conclusions

Different PHN models assessed by various pain evaluations combine to fulfil more comprehensive understanding of PHN.

Subgrouping patients with zoster-associated pain according to sensory symptom profiles: A cluster analysis

Background and goal of study

Patients with zoster-associated pain exhibit a variety of sensory symptoms and forms of pain and complain of different pain patterns. The purpose of this study is to subgroup patients with zoster-associated pain who visited a hospital using painDETECT sensory symptom scores, analyze their respective characteristics and pain-related data, and compare similarities and differences among the groups.

Materials and methods

The characteristics of 1,050 patients complaining of zoster-associated pain and pain-related data were reviewed retrospectively. To identify subgroups of patients with zoster-associated pain according to sensory symptom profiles, a hierarchical cluster analysis was performed based on the responses to a painDETECT questionnaire. Demographics and pain-related data were compared among all subgroups.

Results and discussion

Patients with zoster-associated pain were classified into 5 subgroups according to the distribution of sensory profiles, with each subgroup exhibiting distinct differences in the expression of sensory symptoms. Patients in cluster 1 complained of burning sensations, allodynia, and thermal sensitivity, but felt numbness less strongly. Cluster 2 and 3 patients complained of burning sensations and electric shock-like pain, respectively. Cluster 4 patients complained of most sensory symptoms at similar intensities and reported relatively strong prickling pain. Cluster 5 patients suffered from both burning and shock-like pains. Patient ages and the prevalence of cardiovascular disease were significantly lower in cluster 1. Patients in clusters 1 and 4 reported longer pain duration compared with those in clusters 2 and 3. However, no significant differences were found with respect to sex, body mass index, diabetes mellitus, mental health problems, and sleep disturbance. Pain scores, distribution of dermatomes and gabapentinoid use were also similar among the groups.

Conclusions

Five different subgroups of patients with zoster-associated pain were identified on the basis of sensory symptoms. A subgroup of younger patients with longer pain duration showed specific and distinct symptoms, such as burning sensations and allodynia. Unlike patients with acute or subacute pain, patients with chronic pain were associated with diverse sensory symptom profiles.

Decrypting the cellular and molecular intricacies associated with COVID-19-induced chronic pain

Pain is one of the clinical manifestations that can vary from mild to severe symptoms in COVID-19 patients. Pain symptoms can be initiated by direct viral damage to the tissue or by indirect tissue injury followed by nociceptor sensitization. The most common types of pain that are reported to occur in COVID-19 patients are headache, myalgia, and chest pain. With more and more cases coming in the hospitals, many new and unique symptoms of pain are being reported. Testicular and abdominal pain are rare cases of pain that are also being reported and are associated with COVID-19. The SARS-CoV-2 virus has a high affinity for angiotensin-converting enzyme-2 receptor (ACE-2) which acts as an entry point for the virus. ACE-2/ Ang II/AT 1 receptor also participates directly in the transmission of pain signals from the dorsal horn of the spinal cord. It induces a series of complicated responses in the human body. Among which the cytokinetic storm and hypercoagulation are the most prominent pathways that mediate the sensitization of sensory neurons facilitating pain. The elevated immune response is also responsible for the activation of inflammatory lipid mediators such as COX-1 and COX-2 enzymes for the synthesis of prostaglandins (PGs). PG molecules especially PGE2 and PGD2 are involved in the pain transmission and are found to be elevated in COVID-19 patients. Though arachidonic acid pathway is one of the lesser discussed topics in COVID-19 pathophysiology, still it can be useful for explaining the unique and rarer symptoms of pain seen in COVID-19 patients. Understanding different pain pathways is very crucial for the management of pain and can help healthcare systems to end the current pandemic situation. We herein review the role of various molecules involved in the pain pathology of COVID-19.

A Guide to Preclinical Models of Zoster-Associated Pain and Postherpetic Neuralgia

Reactivation of latent varicella-zoster virus (VZV) causes herpes zoster (HZ), which is commonly accompanied by acute pain and pruritus over the time course of a zosteriform rash. Although the rash and associated pain are self-limiting, a considerable fraction of HZ cases will subsequently develop debilitating chronic pain states termed postherpetic neuralgia (PHN). How VZV causes acute pain and the mechanisms underlying the transition to PHN are far from clear. The human-specific nature of VZV has made in vivo modeling of pain following reactivation difficult to study because no single animal can reproduce reactivated VZV disease as observed in the clinic. Investigations of VZV pathogenesis following primary infection have benefited greatly from human tissues harbored in immune-deficient mice, but modeling of acute and chronic pain requires an intact nervous system with the capability of transmitting ascending and descending sensory signals. Several groups have found that subcutaneous VZV inoculation of the rat induces prolonged and measurable changes in nociceptive behavior, indicating sensitivity that partially mimics the development of mechanical allodynia and thermal hyperalgesia seen in HZ and PHN patients. Although it is not a model of reactivation, the rat is beginning to inform how VZV infection can evoke a pain response and induce long-lasting alterations to nociception. In this review, we will summarize the rat pain models from a practical perspective and discuss avenues that have opened for testing of novel treatments for both zoster-associated pain and chronic PHN conditions, which remain in critical need of effective therapies.

SARS-CoV-2 Airway Infection Results in Time-dependent Sensory Abnormalities in a Hamster Model

Despite being largely confined to the airways, SARS-CoV-2 infection has been associated with sensory abnormalities that manifest in both acute and long-lasting phenotypes. To gain insight on the molecular basis of these sensory abnormalities, we used the golden hamster infection model to characterize the effects of SARS-CoV-2 versus Influenza A virus (IAV) infection on the sensory nervous system. Efforts to detect the presence of virus in the cervical/thoracic spinal cord and dorsal root ganglia (DRGs) demonstrated detectable levels of SARS-CoV-2 by quantitative PCR and RNAscope uniquely within the first 24 hours of infection. SARS-CoV-2-infected hamsters demonstrated mechanical hypersensitivity during acute infection; intriguingly, this hypersensitivity was milder, but prolonged when compared to IAV-infected hamsters. RNA sequencing (RNA-seq) of thoracic DRGs from acute infection revealed predominantly neuron-biased signaling perturbations in SARS-CoV-2-infected animals as opposed to type I interferon signaling in tissue derived from IAV-infected animals. RNA-seq of 31dpi thoracic DRGs from SARS-CoV-2-infected animals highlighted a uniquely neuropathic transcriptomic landscape, which was consistent with substantial SARS-CoV-2-specific mechanical hypersensitivity at 28dpi. Ontology analysis of 1, 4, and 30dpi RNA-seq revealed novel targets for pain management, such as ILF3. Meta-analysis of all SARS-CoV-2 RNA-seq timepoints against preclinical pain model datasets highlighted both conserved and unique pro-nociceptive gene expression changes following infection. Overall, this work elucidates novel transcriptomic signatures triggered by SARS-CoV-2 that may underlie both short- and long-term sensory abnormalities while also highlighting several therapeutic targets for alleviation of infection-induced hypersensitivity.

One Sentence Summary

SARS-CoV-2 infection results in an interferon-associated transcriptional response in sensory tissues underlying time-dependent hypersensitivity.

Comparing the efficacy and safety of short-term spinal cord stimulation and pulsed radiofrequency for zoster-related pain: A systematic review and meta-analysis

BACKGROUND

Pulsed radiofrequency (PRF) is a commonly used method for the treatment of zoster-related pain in the clinic. However, PRF therapy has a high recurrence rate and many adverse reactions. Recent studies have shown that short-term spinal cord stimulation (stSCS) can effectively alleviate zoster-related pain. Due to the lack of evidence, it is unclear whether stSCS is superior to PRF in the efficacy of treating zoster-related pain.

OBJECTIVE

This study aimed to compare the efficacy and safety of stSCS and PRF for zoster-related pain.

METHODS

We searched seven electronic databases from the establishment of the database to January 2021. Related randomized controlled trials were included in this meta-analysis. After extracting the data and evaluating the methodological quality of the included trials, the outcome indicators were statistically analyzed by using RevManV.5.3.

RESULTS

This meta-analysis included 6 trials with a total of 509 patients. Compared with PRF group, stSCS group showed lower pain intensity (standardized mean difference=-0.83, 95%CI [-1.37, -0.30], P=.002), better sleep quality (mean difference=-1.43, 95%CI [-2.29, -0.57], P=.001), lower pain rating index scores, and less incidence of adverse events (RR=0.32, 95%CI [0.12, 0.83], P<.05). However, the efficacies of PRF and stSCS for treating postherpetic neuralgia were consistent in the response rate (RR= 1.10, 95% CI [0.82, 1.48], P=.51) and the complete remission rate (RR=1.05, 95% CI [0.66, 1.68], P=.84).

CONCLUSIONS

In this study, stSCS showed a better analgesic effect and higher safety than PRF. Our meta-analysis results suggested that stSCS may be a feasible and safe invasive treatment for zoster-related pain. However, high-quality, randomized controlled trials with large sample sizes are needed to further verify our conclusions.

A New Gal in Town: A Systematic Review of the Role of Galanin and Its Receptors in Experimental Pain

Galanin is a neuropeptide expressed in a small percentage of sensory neurons of the dorsal root ganglia and the superficial lamina of the dorsal horn of the spinal cord. In this work, we systematically reviewed the literature regarding the role of galanin and its receptors in nociception at the spinal and supraspinal levels, as well as in chronic pain conditions. The literature search was performed in PubMed, Web of Science, Scopus, ScienceDirect, OVID, TRIP, and EMBASE using "Galanin" AND "pain" as keywords. Of the 1379 papers that were retrieved in the initial search, we included a total of 141 papers in this review. Using the ARRIVE guidelines, we verified that 89.1% of the works were of good or moderate quality. Galanin shows a differential role in pain, depending on the pain state, site of action, and concentration. Under normal settings, galanin can modulate nociceptive processing through both a pro- and anti-nociceptive action, in a dose-dependent manner. This peptide also plays a key role in chronic pain conditions and its antinociceptive action at both a spinal and supraspinal level is enhanced, reducing animals' hypersensitivity to both mechanical and thermal stimulation. Our results highlight galanin and its receptors as potential therapeutic targets in pain conditions.

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.

Fosphenytoin alleviates herpes simplex virus infection-induced provoked and spontaneous pain-like behaviors in mice

In this study, we investigated the effects of fosphenytoin (fPHT) a water-soluble prodrug of phenytoin, on the pain responses of a mouse herpes zoster (HZ) pain model. Transdermal herpes simplex virus type 1 (HSV-1) inoculation induced mechanical allodynia and hyperalgesia of the hind paw and spontaneous pain-like behaviors, such as licking the affected skin. Intravenous injection of fPHT (15 and 30 mg/kg) alleviated HSV-1-induced provoked pain (allodynia and hyperalgesia). The suppressive effects of fPHT on provoked pain were weaker than those of diclofenac and pregabalin which were used as positive controls. fPHT, diclofenac, and pregabalin significantly suppressed HSV-1-induced spontaneous pain-like behaviors. Among them, high-dose fPHT (30 mg/kg) showed the strongest suppression. Intravenous fPHT may become a viable option for an acute HZ pain, especially for spontaneous pain.

Repurposing Psychotropic Agents for Viral Disorders: Beyond Covid

Recent reports have highlighted the possible role of the antipsychotic chlorpromazine and the antidepressant fluvoxamine as anti-coronavirus disease 2019 (COVID-19) agents. The objective of this narrative review is to explore what is known about the activity of psychotropic medications against viruses in addition to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). PubMed was queried for "drug repurposing, antiviral activity," and for "antiviral activity" with "psychotropic drugs" and individual agents, through November 2020. Of more than 100 psychotropic agents, 37 drugs, including 27 with a history of pediatric use were identified, which had been studied in the preclinical setting and found to have activity against viruses which are human pathogens. Effects were evaluated by type of virus and by category of psychotropic agent. Activity was identified both against viruses known to cause epidemics such as SARS-CoV-2 and Ebola and against those that are the cause of rare disorders such as Human Papillomatosis Virus-related respiratory papillomatosis. Individual drugs and classes of psychotropics often had activity against multiple viruses, with promiscuity explained by shared viral or cellular targets. Safety profiles of psychotropics may be more tolerable in this context than when they are used long-term in the setting of psychiatric illness. Nonetheless, translation of in vitro results to the clinical arena has been slow. Psychotropic medications as a class deserve further study, including in clinical trials for repurposing as antiviral drugs for children and adults.

Varicella-zoster virus early infection but not complete replication is required for the induction of chronic hypersensitivity in rat models of postherpetic neuralgia

Herpes zoster, the result of varicella-zoster virus (VZV) reactivation, is frequently complicated by difficult-to-treat chronic pain states termed postherpetic neuralgia (PHN). While there are no animal models of VZV-induced pain following viral reactivation, subcutaneous VZV inoculation of the rat causes long-term nocifensive behaviors indicative of mechanical and thermal hypersensitivity. Previous studies using UV-inactivated VZV in the rat model suggest viral gene expression is required for the development of pain behaviors. However, it remains unclear if complete infection processes are needed for VZV to induce hypersensitivity in this host. To further assess how gene expression and replication contribute, we developed and characterized three replication-conditional VZV using a protein degron system to achieve drug-dependent stability of essential viral proteins. Each virus was then assessed for induction of hypersensitivity in rats under replication permissive and nonpermissive conditions. VZV with a degron fused to ORF9p, a late structural protein that is required for virion assembly, induced nocifensive behaviors under both replication permissive and nonpermissive conditions, indicating that complete VZV replication is dispensable for the induction of hypersensitivity. This conclusion was confirmed by showing that a genetic deletion recombinant VZV lacking DNA packaging protein ORF54p still induced prolonged hypersensitivities in the rat. In contrast, VZV with a degron fused to the essential IE4 or IE63 proteins, which are involved in early gene regulation of expression, induced nocifensive behaviors only under replication permissive conditions, indicating importance of early gene expression events for induction of hypersensitivity. These data establish that while early viral gene expression is required for the development of nocifensive behaviors in the rat, complete replication is dispensable. We postulate this model reflects events leading to clinical PHN, in which a population of ganglionic neurons become abortively infected with VZV during reactivation and survive, but host signaling becomes altered in order to transmit ongoing pain.

Synergistic interaction between trazodone and gabapentin in rodent models of neuropathic pain

Neuropathic pain is a chronic debilitating condition caused by injury or disease of the nerves of the somatosensory system. Although several therapeutic approaches are recommended, none has emerged as an optimal treatment leaving a need for developing more effective therapies. Given the small number of approved drugs and their limited clinical efficacy, combining drugs with different mechanisms of action is frequently used to yield greater efficacy. We demonstrate that the combination of trazodone, a multifunctional drug for the treatment of major depressive disorders, and gabapentin, a GABA analogue approved for neuropathic pain relief, results in a synergistic antinociceptive effect in the mice writhing test. To explore the potential relevance of this finding in chronic neuropathic pain, pharmacodynamic interactions between low doses of trazodone (0.3 mg/kg) and gabapentin (3 mg/kg) were evaluated in the chronic constriction injury (CCI) rat model, measuring the effects of the two drugs both on evoked and spontaneous nociception and on general well being components. Two innate behaviors, burrowing and nest building, were used to assess these aspects. Besides exerting a significant antinociceptive effect on hyperalgesia and on spontaneous pain, combined inactive doses of trazodone and gabapentin restored in CCI rats innate behaviors that are strongly reduced or even abolished during persistent nociception, suggesting that the combination may have an impact also on pain components different from somatosensory perception. Our results support the development of a trazodone and gabapentin low doses combination product for optimal multimodal analgesia treatment.

Comparing Gene Expression in the Parabrachial and Amygdala of Diestrus and Proestrus Female Rats after Orofacial Varicella Zoster Injection

The orofacial pain pathway projects to the parabrachial and amygdala, and sex steroids have been shown to affect neuronal activity in these regions. GABA positive cells in the amygdala are influenced by sex steroid metabolites to affect pain, and sex steroids have been shown to alter the expression of genes in the parabrachial, changing neuronal excitability. Mechanisms by which sex steroids affect amygdala and parabrachial signaling are unclear. The expression of genes in the parabrachial and amygdala in diestrus (low estradiol) and proestrus (high estradiol) female rats were evaluated in this study. First, varicella zoster virus was injected into the whisker pad of female rats to induce a pain response. Second, gene expression was quantitated using RNA-seq one week after injection. Genes that had the greatest change in expression and known to function in pain signaling were selected for the quantitation of protein content. Protein expression of four genes in the parabrachial and seven genes in the amygdala were quantitated by ELISA. In the parabrachial, neurexin 3 (Nrnx3) was elevated at proestrus. Nrnx3 has a role in AMPA receptor and GABA signaling. Neuronatin (Nnat) and protein phosphatase, Mg²⁺/Mn²⁺ dependent 1E (Ppm1e) were elevated in the parabrachial of diestrus animals both genes having a role in pain signaling. Epoxide hydroxylase (Ephx2) was elevated in the parabrachial at proestrus and the vitamin D receptor (Vdr) was elevated in the amygdala. Ephx2 antagonists and vitamin D have been used to treat neuropathic pain. In conclusion, sex steroids regulate genes in the parabrachial and amygdala that might result in the greater pain response observed during diestrus.

Multi-targeting sodium and calcium channels using venom peptides for the treatment of complex ion channels-related diseases

Venom peptides are amongst the most exquisite group of bioactive molecules able to alter the normal physiology of organisms. These bioactive peptides penetrate tissues and blood vessels to encounter a number of receptors and ion channels to which they bind with high affinity and execute modulatory activities. Arachnid is the most diverse class of venomous animals often rich in peptides modulating voltage-gated sodium (Na_V), calcium (Ca_V), and potassium (K_V) channels. Spider venoms, in particular, contain potent and selective peptides targeting these channels, with a few displaying interesting multi-target properties for Na_V and Ca_V channels underlying disease mechanisms such as in neuropathic pain, motor neuron disease and cancer. The elucidation of the pharmacology and structure-function properties of these venom peptides are invaluable for the development of effective drugs targeting Na_V and Ca_V channels. This perspective discusses spider venom peptides displaying multi-target properties to modulate Na_V and Ca_V channels in regard to their pharmacological features, structure-function relationships and potential to become the next generation of effective drugs to treat neurological disorders and other multi-ion channels related diseases.

Determinants of neurological syndromes caused by varicella zoster virus (VZV)

Varicella zoster virus (VZV) is a pathogenic human herpes virus which causes varicella as a primary infection, following which it becomes latent in peripheral autonomic, sensory, and cranial nerve ganglionic neurons from where it may reactivate after decades to cause herpes zoster. VZV reactivation may also cause a wide spectrum of neurological syndromes, in particular, acute encephalitis and vasculopathy. While there is potentially a large number of coding viral mutations that might predispose certain individuals to VZV infections, in practice, a variety of host factors are the main determinants of VZV infection, both disseminated and specifically affecting the nervous system. Host factors include increasing age with diminished cell-mediated immunity to VZV, several primary immunodeficiency syndromes, secondary immunodeficiency syndromes, and drug-induced immunosuppression. In some cases, the molecular immunological basis underlying the increased risk of VZV infections has been defined, in particular, the role of POL III mutations, but in other cases, the mechanisms have yet to be determined. The role of immunization in immunosuppressed individuals as well as its possible efficacy in preventing both generalized and CNS-specific infections will require further investigation to clarify in such patients.

Postherpetic Neuralgia: Current Evidence on the Topical Film-Forming Spray with Bupivacaine Hydrochloride and a Review of Available Treatment Strategies

PURPOSE OF REVIEW

This is a comprehensive review of the literature about the use of bupivacaine hydrochloride for the treatment of post-herpetic neuralgia (PHN). It briefly reviews the background, biology, diagnosis and conventional treatment for PHN, and then introduces and compares the recent evidence for the use of topical bupivacaine.

RECENT FINDINGS

PHN is defined by pain lasting 90 days or more after the initial presentation of herpes zoster ("Shingles", HZ) rash and is the most common complication of this disease. A product of re-activation of the Varicella-Zoster virus (VZV), HZ is diagnosed more than 1 million times annually in the United States. Approximately 20% of patients with HZ will experience PHN and will continue to suffer intermittent neuropathic symptoms, including itching and pain, that is sharp, stabbing, throbbing or burning, with the pain localized to the site of their original rash. This long-lasting pain compares with the severity of long-standing rheumatics and osteo-arthritis and is accompanied by severe allodynia causing significant suffering, and a financial burden that is manifested in both healthcare costs and loss of quality-adjusted life years. Prevention of PHN may be achieved with the Zoster vaccine, although there is still a large segment of unvaccinated population. Moreover, the Zoster vaccine is not always effective for prevention. Current treatment includes medical (systemic tricyclic antidepressants, anticonvulsants and opioids, topical lidocaine and capsaicin) and interventional (subcutaneous Botox injections, nerve blocks and nerve stimulation) therapies. These therapies are not always effective, and each carries their own profile of side effects and risks. Moreover, up to 50% of patients with PHN are refractory to management. Recent evidence is emerging to support the use of topical local anesthetics for the treatment of PHN. Two small studies recently found topical lidocaine spray to be effective in treating paroxysmal pain attacks associated with PHN. Bupivacaine is a longer-lasting local anesthetic, and a film-forming formulation allows easy and durable application to the affected skin. Recent studies show that topical film-forming bupivacaine is safe and as effective as lidocaine for the treatment of PHN. PHN is an important though common complication of HZ and can cause long-lasting pain and disability. Current treatment for PNH is limited by efficacy and safety profiles of individual therapies. Recent evidence points to topical local anesthetics as an effective and safe alternative to conventional therapy. Film-forming bupivacaine may offer a durable and safe option for this otherwise difficult to treat syndrome.

Inhibitory effect of amenamevir on acute herpetic pain and postherpetic neuralgia in mice infected with herpes simplex virus-1

BACKGROUND

Amenamevir (AMNV) is a helicase-primase inhibitor with antiviral activity against herpesviruses [herpes simplex viruses (HSV)-1 and -2, and varicella-zoster virus], which are associated with the development of acute herpetic pain (AHP) and postherpetic neuralgia. However, the inhibitory effects of helicase-primase inhibitors on AHP and postherpetic neuralgia remain incompletely understood.

OBJECTIVE

In this study, we investigated the effects of AMNV on AHP and postherpetic pain (PHP) in HSV-1-infected mice accompanied by zosteriform-like skin lesions.

METHODS

HSV-1 was percutaneously infected on the femoral region of mice. AMNV was orally administered twice a day for 5 days. Pain-related response in the hind paw was evaluated using a paintbrush. The infiltration of inflammatory cells in dorsal root ganglion (DRG) and spinal cord (SC) was evaluated by hematoxylin and eosin staining. The viral load in DRG and the expression of pain-related genes in SC were measured by real-time PCR.

RESULTS

Pain response was begun to be observed from day 3 post-infection (pi) in HSV-1-infected mice. AMNV administered repeatedly from day 3 pi or day 4 pi, but not day 5 pi, showed an inhibitory effect on the development of AHP and the transition to PHP. Repeated AMNV administration inhibited inflammatory cell infiltration and increases in the viral load and the expression of pain-related genes (ATF-3, TNF-α, COX-2).

CONCLUSION

These results demonstrate that AMNV potently suppresses the development of AHP and the transition to PHP as a consequence of decreased viral load in DRG and reduced expression of pain-related genes in SC.

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.

γ-Aminobutyric Acid and Derivatives Reduce the Incidence of Acute Pain after Herpes Zoster - A Systematic Review and Meta-analysis

Herpes zoster (HZ) causes considerable pain and distress, and γ-Aminobutyric acid (GABA) and its derivatives are assumed to control this, but the available data are inconsistent. This meta-analysis and systematic review aimed to assess the effectiveness of GABA derivatives in the prevention of acute herpetic pain. The metaanalysis was conducted following the PRISMA guidelines using PICO format, registered in PROSPERO number CRD42018095758. PubMed, Web of Science, Ovid, Scopus, and EMBASE databases were searched. Records were included if they were randomized controlled trials of patients undergoing HZ infection, investigating the effect of GABA derivatives versus placebo in the treatment of HZ pain. Eligible trials were evaluated for the risk of bias. Then data were extracted and analysed. The number of patients with observed presence of pain after treatment was used to calculate odds ratio in a random effect model with the DerSimonian-Laird estimator. The I2 statistic was analysed for heterogeneity. The potential risk of bias was measured using Egger's regression test. The meta-analysis included three randomized controlled trials with a total of 297 patients. The incidence of acute HZ pain events for GABA group was significantly lower compared to placebo group,18/148 vs 44/149, respectively (OR = 0.36; 95% CI = 0.14 to 0.93; Z = 2.11; P = 0.035), Egger's test yielded P = 0.308. In conclusion, the present meta-analysis demonstrates that GABA derivatives reduce the incidence of acute herpetic pain. However, additional, well-designed randomized clinical trials are needed to determine their dose- and time-dependency regarding this symptom.

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.

VGF: a biomarker and potential target for the treatment of neuropathic pain?

Supplemental Digital Content is Available in the Text.

Neuropathic pain (NP) remains an area of considerable unmet medical need. A persistent challenge in the management of NP is to target the specific mechanisms leading to a change from normal to abnormal sensory perception while ensuring that the defensive pain perception remains intact. Targeting VGF-derived neuropeptides may offer this opportunity. VGF was first identified in 1985 and is highly expressed after nerve injury and inflammation in neurons of both the peripheral and central nervous system. Subsequent studies implicate the vgf gene and its products in pain pathways. This narrative review was supported by a systematic search to identify, select, and critically appraise all relevant research investigating the role of VGF-derived neuropeptides in pain pathways. It predominantly focuses on in vivo investigations of the role of VGF in the initiation and maintenance of NP. VGF expression levels are very low under normal physiological conditions and nerve injury results in rapid and robust upregulation, increasing mechanical and thermal hypersensitivity. The identification of the 2 complement receptors with which VGF neuropeptides interact suggests a novel interplay of neuronal and immune signalling mediators. The understanding of the molecular mechanisms and signalling events by which VGF-derived active neuropeptides exert their physiological actions is in its infancy. Future work should aim to improve understanding of the downstream consequences of VGF neuropeptides thereby providing novel insights into pain mechanisms potentially leading to the identification of novel therapeutic targets.

Antinociceptive effect of PnTx4(5-5), a peptide from Phoneutria nigriventer spider venom, in rat models and the involvement of glutamatergic system

Background:

The venom of Phoneutria nigriventer spider is a source of numerous bioactive substances, including some toxins active in insects. An example is PnTx4(5-5) that shows a high insecticidal activity and no apparent toxicity to mice, although it inhibited NMDA-evoked currents in rat hippocampal neurons. In this work the analgesic activity of PnTx4(5-5) (renamed Γ-ctenitoxin-Pn1a) was investigated.

Methods:

The antinociceptive activity was evaluated using the paw pressure test in rats, after hyperalgesia induction with intraplantar injection of carrageenan or prostaglandin E₂ (PGE₂).

Results:

PnTx4(5-5), subcutaneously injected, was able to reduce the hyperalgesia induced by PGE₂ in rat paw, demonstrating a systemic effect. PnTx4(5-5) administered in the plantar surface of the paw caused a peripheral and dose-dependent antinociceptive effect on hyperalgesia induced by carrageenan or PGE₂. The hyperalgesic effect observed in these two pain models was completely reversed with 5 µg of PnTx4(5-5). Intraplantar administration of L-glutamate induced hyperalgesic effect that was significantly reverted by 5 μg of PnTx4(5-5) injection in rat paw.

Conclusion:

The antinociceptive effect for PnTx4(5-5) was demonstrated against different rat pain models, i.e. induced by PGE₂, carrageenan or glutamate. We suggest that the antinociceptive effect of PnTx4(5-5) may be related to an inhibitory activity on the glutamatergic system.

Modeling Varicella Zoster Virus Persistence and Reactivation - Closer to Resolving a Perplexing Persistent State

The latent state of the human herpesvirus varicella zoster virus (VZV) has remained enigmatic and controversial. While it is well substantiated that VZV persistence is established in neurons after the primary infection (varicella or chickenpox), we know little of the types of neurons harboring latent virus genomes, if all can potentially reactivate, what exactly drives the reactivation process, and the role of immunity in the control of latency. Viral gene expression during latency has been particularly difficult to resolve, although very recent advances indicate that it is more restrictive than was once thought. We do not yet understand how genes expressed in latency function in the maintenance and reactivation processes. Model systems of latency are needed to pursue these questions. This has been especially challenging for VZV because the development of in vivo models of VZV infection has proven difficult. Given that up to one third of the population will clinically reactivate VZV to develop herpes zoster (shingles) and suffer from its common long term problematic sequelae, there is still a need for both in vivo and in vitro model systems. This review will summarize the evolution of models of VZV persistence and address insights that have arisen from the establishment of new in vitro human neuron culture systems that not only harbor a latent state, but permit experimental reactivation and renewed virus production. These models will be discussed in light of the recent data gleaned from the study of VZV latency in human cadaver ganglia.

Efficacy of CT Guided Pulsed Radiofrequency Treatment for Trigeminal Postherpetic Neuralgia

Objectives

Trigeminal postherpetic neuralgia (TPHN) often presents with moderate to severe pain, hyperalgesia, and allodynia. Conventional analgesic treatments are poorly effective, which seriously affects the quality of life. This retrospective study aimed to evaluate the efficacy of pulsed radiofrequency (PRF) for the treatment of TPHN.

Methods

A total of 90 TPHN patients were selected between January 2014 and December 2016 in the Department of Pain Management, Shengjing Hospital, China Medical University. Patients were randomly divided into two groups according to the order of enrollment (n = 45 per group): group A, peripheral nerve (supraorbital nerve, infraorbital nerve and mental nerve) PRF; group B, gasserian ganglion PRF. Follow-up assessments of visual analogue scale (VAS) pain assessment, SF-36 health status questionnaire, total efficiency rate, and drug dosage of anticonvulsants and opioid analgesics were performed at time points of 1 week, 1 month, 3 months, 6 months, and 1 year after surgery.

Results

At each postsurgery time point, the VAS decreased, SF-36 (physical and mental components) increased, and drug dosage of anticonvulsants and opioids analgesics decreased in both treatment groups; values at each time point were significantly different from presurgery values (P < 0.05). Compared with group A, VAS decreased, SF-36 increased, and dosage of anticonvulsants and opioids analgesics decreased significantly in group B (P < 0.05). The total efficiency rates one year after surgery in group A and group B were 68.9 and 86.7%, respectively. The total efficiency rate of group B was statistically higher than that of group A (P < 0.05).

Conclusion

PRF relieved TPHN, and gasserian ganglion PRF was more effective than peripheral nerve PRF. The method was effective and improved the quality of life of the patients. PRF is recommended as a treatment for TPHN.

Efficacy and Safety of Computed Tomography-Guided Pulsed Radiofrequency Modulation of Thoracic Dorsal Root Ganglion on Herpes Zoster Neuralgia

OBJECTIVE

Pulsed radiofrequency (PRF) can relieve postherpetic neuralgia (PHN) caused by herpes zoster (HZ) infection. Nevertheless, its curative effect can vary and may be related to the duration of treatment period. The following study investigates the efficacy and safety of CT-guided PRF modulation on HZ neuralgia over different periods and different time points.

MATERIALS AND METHODS

A total of 150 patients with HZ/PHN were enrolled at the Pain Department, Shengjing Hospital of China Medical University between January 2013 and December 2016. According to the course of disease, the patients were randomly divided into group A, which included patients with acute stage (n = 50; course <1 m); group B, which included patients with subacute stage (n = 50; 1 m <course <3 m); and group C, which included patients with chronic stage (n = 50; course >3 m). The PRF therapy was performed in all patients by targeting thoracic dorsal root ganglion (DRG). The visual analogue scale (VAS), SF-36, total effective rate of treatment, and dosage of antiepileptic analgesic drugs were observed at different time points, before and after the surgery.

RESULTS

Compared to preoperative time, decreased VAS, improved SF-36, and the decreased dosage of antiepileptic analgesic drugs were observed at all time points, and in all groups after surgery (p < 0.05). In group A, pain relief lasted longer, and it further decreased over time. In addition, significantly lower VAS, higher SF-36, and lower dosage of antiepileptic analgesic drugs were found in group A compared to group B, and in group B compared to group C (all p < 0.05). Furthermore, the total effective rates in groups A, B, and C were 88, 72, and 52%, respectively.

CONCLUSIONS

CT-guided PRF targeting thoracic DRG for modulation of HZ neuralgia in different periods is safe and effective. It is recommended to perform early intervention therapy at the acute phase of HZ.

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.

Advances in Experimental Medicine and Biology: Intrafascicular Local Anesthetic Injection Damages Peripheral Nerve-Induced Neuropathic Pain

Peripheral nerve blockade (PNB) is advantageous for patients undergoing surgery to decrease the perioperative opioid consumptions and enhance recovery after surgery.Inadvertent local anesthetic (LA) administration into nerve fiber intrafascicularly easily results in unrecognized nerve injury. Using nerve block guidance either by ultrasound, electrical nerve stimulator, or using pressure devices does not prevent nerve damage, even though most of the nerve injury is transiently. The incidence of neurologic symptoms or neuropathy is in the range of 0.02-2.2%, and no significant difference of postoperative neurologic symptoms is found as compared with using ultrasound or guided nerve stimulator technique. However, intrafascicular lidocaine brought about macrophage migration into the damaged fascicle, Schwann cell proliferation, increased intensity of myelin basic protein, and shorten withdrawal time to mechanical stimuli. In dorsal root ganglion (DRG), intrafascicular LA injection increased the activated transcriptional factor 3 (ATF-3) and downregulated Nav1.8 (Nav1.8). In spinal dorsal horn (SDH), the microglia and astrocytes located in SDH were activated and proliferated after intrafascicular LA injection and returned to baseline gradually at the end of the month. This is a kind of neuropathic pain, so low injection pressure should be maintained, the correct needle bevel used, nerve stimulator or ultrasound guidance applied, and careful and deliberately slow injection employed as important parts of the injection technique to prevent intrafascicular LA administration-induced neuropathic pain.

Post-herpetic neuralgia - a review of current management and future directions

Post-herpetic neuralgia (PHN) is common and treatment is often suboptimal with less than half of patients achieving adequate 50% pain relief. As an area of unmet clinical need and as an archetype of neuropathic pain, it deserves the attention of clinicians and researchers. Areas covered: This review summarises the epidemiology, pathophysiology, risk factors and clinical features of varicella infection. It describes the current and possible future management strategies for preventing varicella infection and reactivation and for treating PHN. Expert opinion: A highly successful Varicella Zoster (VZV) vaccine has not been universally adopted due to fears that it may increase Herpes Zoster (HZ) incidence - and thus PHN - in older, unvaccinated generations. This is a controversial theory but advances in the efficacy of vaccines against HZ may allay these fears and encourage more widespread adoption of the VZV vaccine. Treatment of PHN, as for any neuropathic pain, must be multidisciplinary and multimodal. Advances in sensory phenotyping technology and genomics may allow more individualised treatment. Traditional research methodologies are ill-suited to assess the kind of complex interventions that are necessary to achieve better clinical outcomes in this challenging field.

Role for the Ventral Posterior Medial/Posterior Lateral Thalamus and Anterior Cingulate Cortex in Affective/Motivation Pain Induced by Varicella Zoster Virus

Varicella zoster virus (VZV) infects the face and can result in chronic, debilitating pain. The mechanism for this pain is unknown and current treatment is often not effective, thus investigations into the pain pathway become vital. Pain itself is multidimensional, consisting of sensory and affective experiences. One of the primary brain substrates for transmitting sensory signals in the face is the ventral posterior medial/posterior lateral thalamus (VPM/VPL). In addition, the anterior cingulate cortex (ACC) has been shown to be vital in the affective experience of pain, so investigating both of these areas in freely behaving animals was completed to address the role of the brain in VZV-induced pain. Our lab has developed a place escape avoidance paradigm (PEAP) to measure VZV-induced affective pain in the orofacial region of the rat. Using this assay as a measure of the affective pain experience a significant response was observed after VZV injection into the whisker pad and after VZV infusion into the trigeminal ganglion. Local field potentials (LFPs) are the summed electrical current from a group of neurons. LFP in both the VPM/VPL and ACC was attenuated in VZV injected rats after inhibition of neuronal activity. This inhibition of VPM/VPL neurons was accomplished using a designer receptor exclusively activated by a designer drug (DREADD). Immunostaining showed that cells within the VPM/VPL expressed thalamic glutamatergic vesicle transporter-2, NeuN and DREADD suggesting inhibition occurred primarily in excitable neurons. From these results we conclude: (1) that VZV associated pain does not involve a mechanism exclusive to the peripheral nerve terminals, and (2) can be controlled, in part, by excitatory neurons within the VPM/VPL that potentially modulate the affective experience by altering activity in the ACC.

Lateral thalamic control of nociceptive response after whisker pad injection of varicella zoster virus

Pain is a common complication of herpes zoster (HZ) infection which results from reactivation of a latent varicella zoster virus (VZV). A third of HZ patients' progress to a chronic pain state known as post herpetic neuralgia (PHN), and about a quarter of these patients' have orofacial pain. The mechanisms controlling the pain responses are not understood. Studies suggest central pathways involving the thalamus could control pain related to HZ, and studies in our lab suggest (VGAT) in the lateral thalamus influences orofacial pain. We hypothesized that thalamic VGAT functions, in part, to reduce pain, particularly orofacial pain, associated with VZV. To address this hypothesis VZV was injected into the whisker pad. Affective and motivational aspects of pain were measured using the Place Escape/Avoidance Paradigm. Thalamic neuronal activity was modulated after injecting an adeno-associated virus (AAV) expressing an engineered acetylcholine Gi-protein-coupled receptor. This receptor inhibits neuronal firing when bound by clozapine-n-oxide (CNO). VGAT expression was attenuated in the thalamus by injecting an AAV construct that expressed a VGAT silencing shRNA. VZV-induced nociception was significantly decreased after administering CNO in male rats. Nociception significantly increased concomitant with increased thalamic c-fos expression after attenuating thalamic VGAT expression. These data establish that the lateral thalamus (posterior, ventral posteromedial, ventral posterolateral and/or reticular thalamic nucleus) controls VZV-induced nociception in the orofacial region, and that GABA in this region appears to reduce the response to VZV-induced nociception possibly by gating facial pain input.

Sex differences underlying orofacial varicella zoster associated pain in rats

BACKGROUND

Most people are initially infected with varicella zoster virus (VZV) at a young age and this infection results in chickenpox. VZV then becomes latent and reactivates later in life resulting in herpes zoster (HZ) or "shingles". Often VZV infects neurons of the trigeminal ganglia to cause ocular problems, orofacial disease and occasionally a chronic pain condition termed post-herpetic neuralgia (PHN). To date, no model has been developed to study orofacial pain related to varicella zoster. Importantly, the incidence of zoster associated pain and PHN is known to be higher in women, although reasons for this sex difference remain unclear. Prior to this work, no animal model was available to study these sex-differences. Our goal was to develop an orofacial animal model for zoster associated pain which could be utilized to study the mechanisms contributing to this sex difference.

METHODS

To develop this model VZV was injected into the whisker pad of rats resulting in IE62 protein expression in the trigeminal ganglia; IE62 is an immediate early gene in the VZV replication program.

RESULTS

Similar to PHN patients, rats showed retraction of neurites after VZV infection. Treatment of rats with gabapentin, an agent often used to combat PHN, ameliorated the pain response after whisker pad injection. Aversive behavior was significantly greater for up to 7 weeks in VZV injected rats over control inoculated rats. Sex differences were also seen such that ovariectomized and intact female rats given the lower dose of VZV showed a longer affective response than male rats. The phase of the estrous cycle also affected the aversive response suggesting a role for sex steroids in modulating VZV pain.

CONCLUSIONS

These results suggest that this rat model can be utilized to study the mechanisms of 1) orofacial zoster associated pain and 2) the sex differences underlying zoster associated pain.

Postherpetic Neuralgia: Practical Experiences Return to Traditional Chinese Medicine

Post-herpetic neuralgia (PHN) is a complication of herpes zoster that can cause different types of pain in the affected area. It often occurs mainly in severe cases of herpes zoster. The problem is defined as a persisting pain for 90-120 days after relieving of acute phase of herpes lesions. This complication causes suffering in patients and reduces the quality of life. In western medicine's viewpoints, PHN is due to disturbance in local and dermatomal nerves. There are several topical and systemic drugs that are used to manage the pain relief. In traditional medicine (TM), PHN is mostly due to incomplete heat and damp clearing in liver and spleen meridians, qi and toxic pathogens stagnation, accumulation of yin (blood stagnation in microcapillary), internal fire, and heat and obstruction of meridians. Acupuncture works based on the eradication of wind, clearing of heat, and destroying of damp by regulating qi and blood movement. In clinics, several methods of TM are used to relief PHN, such as simultaneous needling, surrounding needling, acupuncture, electro acupuncture, moxibustion, wet cupping or hijamat, and herbal medicine. In this review, we discussed all these methods and their role in reducing PHN and pain.

Altered homotopic connectivity in postherpetic neuralgia: a resting state fMRI study

Objective

The aim of this study was to explore interhemispheric intrinsic connectivity in patients with postherpetic neuralgia (PHN).

Methods

We obtained resting-state functional magnetic resonance imaging data from 18 right-handed PHN patients (11 males, 7 females; mean age, 59.67±8.41 years) and 18 well-matched healthy controls (11 males, 7 females; mean age, 38.50±7.51 years). Interhemispheric connectivity was examined using voxel-mirrored homotopic connectivity (VMHC), and seed-based functional connectivity analysis was performed.

Results

Compared with the healthy controls, the patients with PHN showed abnormally decreased homotopic connectivity in the dorsolateral prefrontal cortex and the precuneus and posterior cingulate cortex (PCUN/PCC). The decreased VMHC in the PCUN/PCC was positively correlated with the visual analog scale of PHN in the PHN patient group (ρ=0.651; P=0.006). Receiver operating characteristic (ROC) analysis revealed that the areas under the curves for the two brain regions were 0.898 for the prefrontal cortex and 0.923 for the PCUN/PCC, which indicated that the VMHC could be used to discriminate PHN patients from healthy controls. A subsequent seed-based functional connectivity analysis revealed widely disrupted intrinsic connectivity between the regions that showed local homotopic connectivity deficits and the areas subserving the default-mode network.

Conclusion

Our results indicated reduced interhemispheric functional connectivity in patients with PHN, which seems to be an important new avenue to investigate to better understand the nature of disconnection of the functional architecture in patients with PHN.

Structure and function of μ-conotoxins, peptide-based sodium channel blockers with analgesic activity

μ-Conotoxins block voltage-gated sodium channels (VGSCs) and compete with tetrodotoxin for binding to the sodium conductance pore. Early efforts identified µ-conotoxins that preferentially blocked the skeletal muscle subtype (NaV1.4). However, the last decade witnessed a significant increase in the number of µ-conotoxins and the range of VGSC subtypes inhibited (NaV1.2, NaV1.3 or NaV1.7). Twenty µ-conotoxin sequences have been identified to date and structure-activity relationship studies of several of these identified key residues responsible for interactions with VGSC subtypes. Efforts to engineer-in subtype specificity are driven by in vivo analgesic and neuromuscular blocking activities. This review summarizes structural and pharmacological studies of µ-conotoxins, which show promise for development of selective blockers of NaV1.2, and perhaps also NaV1.1,1.3 or 1.7.

Herpes Zoster and Postherpetic Neuralgia: Practical Consideration for Prevention and Treatment

Herpes zoster (HZ) is a transient disease caused by the reactivation of latent varicella zoster virus (VZV) in spinal or cranial sensory ganglia. It is characterized by a painful rash in the affected dermatome. Postherpetic neuralgia (PHN) is the most troublesome side effect associated with HZ. However, PHN is often resistant to current analgesic treatments such as antidepressants, anticonvulsants, opioids, and topical agents including lidocaine patches and capsaicin cream and can persist for several years. The risk factors for reactivation of HZ include advanced age and compromised cell-mediated immunity (CMI). Early diagnosis and treatment with antiviral agents plus intervention treatments is believed to shorten the duration and severity of acute HZ and reduce the risk of PHN. Prophylactic vaccination against VZV can be the best option to prevent or reduce the incidence of HZ and PHN. This review focuses on the pathophysiology, clinical features, and management of HZ and PHN, as well as the efficacy of the HZ vaccine.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Current gene therapy using viral vectors for chronic pain

The complexity of chronic pain and the challenges of pharmacotherapy highlight the importance of development of new approaches to pain management. Gene therapy approaches may be complementary to pharmacotherapy for several advantages. Gene therapy strategies may target specific chronic pain mechanisms in a tissue-specific manner. The present collection of articles features distinct gene therapy approaches targeting specific mechanisms identified as important in the specific pain conditions. Dr. Fairbanks group describes commonly used gene therapeutics (herpes simplex viral vector (HSV) and adeno-associated viral vector (AAV)), and addresses biodistribution and potential neurotoxicity in pre-clinical models of vector delivery. Dr. Tao group addresses that downregulation of a voltage-gated potassium channel (Kv1.2) contributes to the maintenance of neuropathic pain. Alleviation of chronic pain through restoring Kv1.2 expression in sensory neurons is presented in this review. Drs Goins and Kinchington group describes a strategy to use the replication defective HSV vector to deliver two different gene products (enkephalin and TNF soluble receptor) for the treatment of post-herpetic neuralgia. Dr. Hao group addresses the observation that the pro-inflammatory cytokines are an important shared mechanism underlying both neuropathic pain and the development of opioid analgesic tolerance and withdrawal. The use of gene therapy strategies to enhance expression of the anti-pro-inflammatory cytokines is summarized. Development of multiple gene therapy strategies may have the benefit of targeting specific pathologies associated with distinct chronic pain conditions (by Guest Editors, Drs. C. Fairbanks and S. Hao).

Neuronal changes induced by Varicella Zoster Virus in a rat model of postherpetic neuralgia

A significant fraction of patients with herpes zoster, caused by Varicella Zoster Virus (VZV), experience chronic pain termed postherpetic neuralgia (PHN). VZV-inoculated rats develop prolonged nocifensive behaviors and serve as a model of PHN. We demonstrate that primary rat cultures show a post-entry block for VZV replication, suggesting the rat is not fully permissive. However, footpads of VZV infected animals show reduced peripheral innervation and innervating dorsal root ganglia (DRG) contained VZV DNA and transcripts of candidate immediate early and early genes. The VZV-infected DRG showed changes in host gene expression patterns, with 84 up-regulated and 116 down-regulated genes seen in gene array studies. qRT-PCR validated the modulation of nociception-associated genes Ntrk2, Trpv1, and Calca (CGRP). The data suggests that VZV inoculation of the rat results in a single round, incomplete infection that is sufficient to induce pain behaviors, and this involves infection of and changes induced in neuronal populations.

The effects of glucocorticoids on neuropathic pain: a review with emphasis on intrathecal methylprednisolone acetate delivery

Methylprednisolone acetate (MPA) has a long history of use in the treatment of sciatic pain and other neuropathic pain syndromes. In several of these syndromes, MPA is administered in the epidural space. On a limited basis, MPA has also been injected intrathecally in patients suffering from postherpetic neuralgia and complex regional pain syndrome. The reports on efficacy of intrathecal administration of MPA in neuropathic pain patients are contradictory, and safety is debated. In this review, we broadly consider mechanisms whereby glucocorticoids exert their action on spinal cascades relevant to the pain arising after nerve injury and inflammation. We then focus on the characteristics of the actions of MPA in pharmacokinetics, efficacy, and safety when administered in the intrathecal space.

Improving the translation of analgesic drugs to the clinic: animal models of neuropathic pain

Neuropathic pain remains an area of considerable unmet clinical need. Research based on preclinical animal models has failed to deliver truly novel treatment options, questioning the predictive value of these models. This review addresses the shortcomings of rodent in vivo models commonly used in the field and highlights approaches which could increase their predictivity, including more clinically relevant assays, outcome measures and animal characteristics. The methodological quality of animal studies also needs to be improved. Low internal validity and incomplete reporting lead to a waste of valuable research resources and animal lives, and ultimately prevent an objective assessment of the true predictivity of in vivo models.

Relief of pain induced by varicella-zoster virus in a rat model of post-herpetic neuralgia using a herpes simplex virus vector expressing enkephalin

Acute and chronic pain (post-herpetic neuralgia or PHN) are encountered in patients with herpes zoster that is caused by reactivation of varicella-zoster virus (VZV) from a state of neuronal latency. PHN is often refractory to current treatments, and additional strategies for pain relief are needed. Here we exploited a rat footpad model of PHN to show that herpes simplex virus (HSV) vector-mediated gene delivery of human preproenkephalin (vHPPE) effectively reduced chronic VZV-induced nocifensive indicators of pain. VZV inoculated at the footpad induced prolonged mechanical allodynia and thermal hyperalgesia that did not develop in controls or with ultraviolet light-inactivated VZV. Subsequent footpad administration of vHPPE relieved VZV-induced pain behaviors in a dose-dependent manner for extended periods, and prophylactic vector administration prevented VZV-induced pain from developing. Short-term pain relief following low-dose vHPPE administration could be effectively prolonged by vector re-administration. HPPE transcripts were increased three- to fivefold in ipsilateral ganglia, but not in the contralateral dorsal root ganglia. VZV hypersensitivity and its relief by vHPPE were not affected by peripheral delivery of opioid receptor agonist or antagonist, suggesting that the efficacy was mediated at the ganglion and/or spinal cord level. These results support further development of ganglionic expression of enkephalin as a novel treatment for the pain associated with Zoster.

Animal models and pharmacology of herpetic and postherpetic pain

Varicella-zoster virus (VZV) causes varicella upon primary infection and subsequently becomes latent in the sensory ganglia. Reactivation of latent VZV in the sensory ganglion results in herpes zoster, which usually begins with pain and dysesthesia. Pain that persists long after healing of the rash is termed postherpetic neuralgia. VZV inoculation into rats induces mechanical allodynia and thermal hyperalgesia without causing herpes zoster. As with VZV, herpes simplex virus 1 (HSV1) is an alphaherpesvirus. HSV1 also becomes latent in the sensory ganglia after primary infection, and reactivation of latent HSV1 in the sensory ganglion results in herpes simplex. HSV1 inoculation into mice causes zoster-like skin lesions together with mechanical allodynia and mechanical hyperalgesia. A marked difference between the two rodent models is whether the herpes virus proliferates in the nervous system after inoculation. VZV-inoculated rats are useful for investigating mechanical allodynia induced by latent infection with herpes virus. HSV1-inoculated mice are useful for investigating mechanical allodynia induced by the proliferation of herpes virus in sensory neurons and for assessing the effects of acute herpetic pain on the incidence of postherpetic allodynia.