Cholesterol-dependent LXR transcription factor activity represses pronociceptive effects of estrogen in sensory neurons and pain induced by myelin basic protein fragments

Background

A bioactive myelin basic protein (MBP) fragment, comprising MBP84-104, is released in sciatic nerve after chronic constriction injury (CCI). Intraneural injection (IN) of MBP84-104 in an intact sciatic nerve is sufficient to induce persistent neuropathic pain-like behavior via robust transcriptional remodeling at the injection site and ipsilateral dorsal root ganglia (DRG) and spinal cord. The sex (female)-specific pronociceptive activity of MBP84-104 associates with sex-specific changes in cholesterol metabolism and activation of estrogen receptor (ESR)1 signaling.

Methods

In male and female normal and post-CCI rat sciatic nerves, we assessed: (i) cholesterol precursor and metabolite levels by lipidomics; (ii) MBP84-104 interactors by mass spectrometry of MBP84-104 pull-down; and (iii) liver X receptor (LXR)α protein expression by immunoblotting. To test the effect of LXRα stimulation on IN MBP84-104-induced mechanical hypersensitivity, the LXRα expression was confirmed along the segmental neuraxis, in DRG and spinal cord, followed by von Frey testing of the effect of intrathecally administered synthetic LXR agonist, GW3965. In cultured male and female rat DRGs exposed to MBP84-104 and/or estrogen treatments, transcriptional effect of LXR stimulation by GW3965 was assessed on downstream cholesterol transporter Abc, interleukin (IL)-6, and pronociceptive Cacna2d1 gene expression.

Results

CCI regulated LXRα ligand and receptor levels in nerves of both sexes, with cholesterol precursors, desmosterol and 7-DHC, and oxysterol elevated in females relative to males. MBP84-104 interacted with nuclear receptor coactivator (Ncoa)1, known to activate LXRα, injury-specific in nerves of both sexes. LXR stimulation suppressed ESR1-induced IL-6 and Cacna2d1 expression in cultured DRGs of both sexes and attenuated MBP84-104-induced pain in females.

Conclusion

The injury-released bioactive MBP fragments induce pronociceptive changes by selective inactivation of nuclear transcription factors, including LXRα. By Ncoa1 sequestration, bioactive MBP fragments render LXRα function to counteract pronociceptive activity of estrogen/ESR1 in sensory neurons. This effect of MBP fragments is prevalent in females due to high circulating estrogen levels in females relative to males. Restoring LXR activity presents a promising therapeutic strategy in management of neuropathic pain induced by bioactive MBP.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

12/15-Lipoxygenases mediate neuropathic-like pain hypersensitivity in female mice

It is estimated that chronic neuropathic pain conditions exhibit up to 10% prevalence in the general population, with increased incidence in females. However, nonsteroidal inflammatory drugs (NSAIDs) are ineffective, and currently indicated prescription treatments such as opioids, anticonvulsants, and antidepressants provide only limited therapeutic benefit. In the current work, we extended previous studies in male rats utilizing a paradigm of central Toll-like receptor 4 (TLR4)-dependent, NSAID-unresponsive neuropathic-like pain hypersensitivity to male and female C57BL/6N mice, uncovering an unexpected hyperalgesic phenotype in female mice following intrathecal (IT) LPS. In contrast to previous reports in female C57BL/6J mice, female C57BL/6N mice displayed tactile and cold allodynia, grip force deficits, and locomotor hyperactivity in response to IT LPS. Congruent with our previous observations in male rats, systemic inhibition of 12/15-Lipoxygenases (12/15-LOX) in female B6N mice with selective inhibitors - ML355 (targeting 12-LOX-p) and ML351 (targeting 15-LOX-1) - completely reversed allodynia and grip force deficits. We demonstrate here that 12/15-LOX enzymes also are expressed in mouse spinal cord and that 12/15-LOX metabolites produce tactile allodynia when administered spinally (IT) or peripherally (intraplantar in the paw, IPLT) in a hyperalgesic priming model, similar to others observations with the cyclooxygenase (COX) metabolite Prostaglandin E 2 (PGE 2 ). Surprisingly, we did not detect hyperalgesic priming following IT administration of LPS, indicating that this phenomenon likely requires peripheral activation of nociceptors. Collectively, these data suggest that 12/15-LOX enzymes contribute to neuropathic-like pain hypersensitivity in rodents, with potential translatability as druggable targets across sexes and species using multiple reflexive and non-reflexive outcome measures.

Pain mechanisms in the transgender individual: a review

Specific Aim

Provide an overview of the literature addressing major areas pertinent to pain in transgender persons and to identify areas of primary relevance for future research.

Methods

A team of scholars that have previously published on different areas of related research met periodically though zoom conferencing between April 2021 and February 2023 to discuss relevant literature with the goal of providing an overview on the incidence, phenotype, and mechanisms of pain in transgender patients. Review sections were written after gathering information from systematic literature searches of published or publicly available electronic literature to be compiled for publication as part of a topical series on gender and pain in the Frontiers in Pain Research.

Results

While transgender individuals represent a significant and increasingly visible component of the population, many researchers and clinicians are not well informed about the diversity in gender identity, physiology, hormonal status, and gender-affirming medical procedures utilized by transgender and other gender diverse patients. Transgender and cisgender people present with many of the same medical concerns, but research and treatment of these medical needs must reflect an appreciation of how differences in sex, gender, gender-affirming medical procedures, and minoritized status impact pain.

Conclusions

While significant advances have occurred in our appreciation of pain, the review indicates the need to support more targeted research on treatment and prevention of pain in transgender individuals. This is particularly relevant both for gender-affirming medical interventions and related medical care. Of particular importance is the need for large long-term follow-up studies to ascertain best practices for such procedures. A multi-disciplinary approach with personalized interventions is of particular importance to move forward.

Catheter Tip-Associated Mass With Continuous Infusion of Sufentanil for Persistent Spinal Pain Syndrome Type 2: A Case Report Including Histopathologic Examination and Review of the Associated Basic and Clinical Research

OBJECTIVES

Intrathecal opioids delivered by implanted pumps are used to treat malignant or nonmalignant chronic pain. In this study, we 1) review a case in which intrathecal infusions of sufentanil along with other adjuvants were used and after an extended period led to an intrathecal mass and 2) compared and contrasted the potential mechanisms for these phenomena.

MATERIALS AND METHODS

A woman aged 66 years with a history of scoliosis and multiple spine surgeries was treated with an implantable drug delivery system for treating persistent pain after laminectomy. The patient received intrathecal medication comprising sufentanil, bupivacaine, and clonidine.

RESULTS

Intrathecal therapy over approximately ten years served to reduce pain and improve function over the treatment period. After the extended treatment interval, the patient developed an intrathecal mass that was associated with impairment. The mass was surgically removed. Systematic histopathology revealed the space-occupying mass to largely comprise fibroblasts and some inflammatory cells embedded in a collagen mass located proximally to the catheter tip.

CONCLUSIONS

To our knowledge, this is the first published case report of sufentanil causing this complication. The science and mechanism of intrathecal catheter tip-associated mass formation and associated clinical research correlates are reviewed in detail, and explanations for this phenomenon are proposed based on histochemical analysis of the patient's pathology findings.

CD163+ macrophages monitor enhanced permeability at the blood-dorsal root ganglion barrier

In dorsal root ganglia (DRG), macrophages reside close to sensory neurons and have largely been explored in the context of pain, nerve injury, and repair. However, we discovered that most DRG macrophages interact with and monitor the vasculature by sampling macromolecules from the blood. Characterization of the DRG vasculature revealed a specialized endothelial bed that transformed in molecular, structural, and permeability properties along the arteriovenous axis and was covered by macrophage-interacting pericytes and fibroblasts. Macrophage phagocytosis spatially aligned with peak endothelial permeability, a process regulated by enhanced caveolar transcytosis in endothelial cells. Profiling the DRG immune landscape revealed two subsets of perivascular macrophages with distinct transcriptome, turnover, and function. CD163+ macrophages self-maintained locally, specifically participated in vasculature monitoring, displayed distinct responses during peripheral inflammation, and were conserved in mouse and man. Our work provides a molecular explanation for the permeability of the blood-DRG barrier and identifies an unappreciated role of macrophages as integral components of the DRG-neurovascular unit.

Refinement of intrathecal catheter design to enhance neuraxial distribution

BACKGROUND

Delivery of therapeutics via indwelling intrathecal catheters is highly efficacious for targeting of pain, spasticity, neuraxial cancer and neurodegenerative disorders. However, current catheter designs have some major limitations. Given limited CSF flow, fixed intrathecal volume and the large distance of the rostro-caudal spinal axis, current intrathecal delivery routes fail to achieve adequate drug distribution. Additionally open catheter systems are plagued with cellular ingrowth and debris accumulation if used intermittently.

NEW METHOD

RESULTS/COMPARISON WITH EXISTING METHOD(S): High speed imaging showed micro-valve catheters greatly increase fluid exit velocities compared to typical open-ended catheters, which prevents pooling of injectate proximal to the opening. When implanted intrathecally in rats, small injection volumes (7.5 μL) of dye or AAV9-RFP, resulted in an even rostro-caudal distribution along the spinal axis and robust transfection of neurons from cervical to lumbar dorsal root ganglia. In contrast, such injections with an open-ended catheter resulted in localized distribution and transfection proximal to the delivery site. Our poly micro-valve catheter design resulted in equivalent transfection rates of cervical DRG neurons using 100x lower titer of AAV9-RFP. Unlike open port catheters, no debris accumulation was observed in the lumen of implanted catheters, showing potential for long-term intermittent use.

CONCLUSIONS

This catheter platform, suitable for small animal models is easily scalable for human use and addresses many of the problems observed with common catheter systems.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. Unexpectedly, given prior emphasis on descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We also report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings significantly revise current models of the DPMS and establish a novel supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

AIBP regulates TRPV1 activation in chemotherapy-induced peripheral neuropathy by controlling lipid raft dynamics and proximity to TLR4 in dorsal root ganglion neurons

ABSTRACT

Nociceptive afferent signaling evoked by inflammation and nerve injury is mediated by the opening of ligand-gated and voltage-gated receptors or channels localized to cholesterol-rich lipid raft membrane domains. Dorsal root ganglion (DRG) nociceptors express high levels of toll-like receptor 4 (TLR4), which also localize to lipid rafts. Genetic deletion or pharmacologic blocking of TLR4 diminishes pain associated with chemotherapy-induced peripheral neuropathy (CIPN). In DRGs of mice with paclitaxel-induced CIPN, we analyzed DRG neuronal lipid rafts, expression of TLR4, activation of transient receptor potential cation channel subfamily V member 1 (TRPV1), and TLR4-TRPV1 interaction. Using proximity ligation assay, flow cytometry, and whole-mount DRG microscopy, we found that CIPN increased DRG neuronal lipid rafts and TLR4 expression. These effects were reversed by intrathecal injection of apolipoprotein A-I binding protein (AIBP), a protein that binds to TLR4 and specifically targets cholesterol depletion from TLR4-expressing cells. Chemotherapy-induced peripheral neuropathy increased TRPV1 phosphorylation, localization to neuronal lipid rafts, and proximity to TLR4. These effects were also reversed by AIBP treatment. Regulation of TRPV1-TLR4 interactions and their associated lipid rafts by AIBP covaried with the enduring reversal of mechanical allodynia otherwise observed in CIPN. In addition, AIBP reduced intracellular calcium in response to the TRPV1 agonist capsaicin, which was increased in DRG neurons from paclitaxel-treated mice and in the naïve mouse DRG neurons incubated in vitro with paclitaxel. Together, these results suggest that the assembly of nociceptive and inflammatory receptors in the environment of lipid rafts regulates nociceptive signaling in DRG neurons and that AIBP can control lipid raft-associated nociceptive processing.

Neuraxial drug delivery in pain management: An overview of past, present, and future

Activation of neuraxial nociceptive linkages leads to a high level of encoding of the message that is transmitted to the brain and that can initiate a pain state with its attendant emotive covariates. As we review here, the encoding of this message is subject to a profound regulation by pharmacological targeting of dorsal root ganglion and dorsal horn systems. Though first shown with the robust and selective modulation by spinal opiates, subsequent work has revealed the pharmacological and biological complexity of these neuraxial systems and points to several regulatory targets. Novel therapeutic delivery platforms, such as viral transfection, antisense and targeted neurotoxins, point to disease-modifying approaches that can selectively address the acute and chronic pain phenotype. Further developments are called for in delivery devices to enhance local distribution and to minimize concentration gradients, as frequently occurs with the poorly mixed intrathecal space. The field has advanced remarkably since the mid-1970s, but these advances must always address the issues of safety and tolerability of neuraxial therapy.

Corrigendum: Intrathecal drug delivery: Advances and applications in the management of chronic pain patient

[This corrects the article DOI: 10.3389/fpain.2022.900566.].

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.

Junctional instability in neuroepithelium and network hyperexcitability in a focal cortical dysplasia human model

Focal cortical dysplasia is a highly epileptogenic cortical malformation with few treatment options. Here, we generated human cortical organoids from patients with focal cortical dysplasia type II. Using this human model, we mimicked some focal cortical dysplasia hallmarks, such as impaired cell proliferation, the presence of dysmorphic neurons and balloon cells, and neuronal network hyperexcitability. Furthermore, we observed alterations in the adherens junctions zonula occludens-1 and partitioning defective 3, reduced polarization of the actin cytoskeleton, and fewer synaptic puncta. Focal cortical dysplasia cortical organoids showed downregulation of the small GTPase RHOA, a finding that was confirmed in brain tissue resected from these patients. Functionally, both spontaneous and optogenetically-evoked electrical activity revealed hyperexcitability and enhanced network connectivity in focal cortical dysplasia organoids. Taken together, our findings suggest a ventricular zone instability in tissue cohesion of neuroepithelial cells, leading to a maturational arrest of progenitors or newborn neurons, which may predispose to cellular and functional immaturity and compromise the formation of neural networks in focal cortical dysplasia.

Intrathecal Drug Delivery: Advances and Applications in the Management of Chronic Pain Patient

Advances in our understanding of the biology of spinal systems in organizing and defining the content of exteroceptive information upon which higher centers define the state of the organism and its role in the regulation of somatic and automatic output, defining the motor response of the organism, along with the unique biology and spatial organization of this space, have resulted in an increased focus on therapeutics targeted at this extracranial neuraxial space. Intrathecal (IT) drug delivery systems (IDDS) are well-established as an effective therapeutic approach to patients with chronic non-malignant or malignant pain and as a tool for management of patients with severe spasticity and to deliver therapeutics that address a myriad of spinal pathologies. The risk to benefit ratio of IDD makes it a useful interventional approach. While not without risks, this approach has a significant therapeutic safety margin when employed using drugs with a validated safety profile and by skilled practioners. The present review addresses current advances in our understanding of the biology and dynamics of the intrathecal space, therapeutic platforms, novel therapeutics, delivery technology, issues of safety and rational implementation of its therapy, with a particular emphasis upon the management of pain.

Sex-Specific B Cell and Anti-Myelin Autoantibody Response After Peripheral Nerve Injury

Immunotherapy holds promise as a non-addictive treatment of refractory chronic pain states. Increasingly, sex is recognized to impact immune regulation of pain states, including mechanical allodynia (pain from non-painful stimulation) that follows peripheral nerve trauma. This study aims to assess the role of B cells in sex-specific responses to peripheral nerve trauma. Using a rat model of sciatic nerve chronic constriction injury (CCI), we analyzed sex differences in (i) the release of the immunodominant neural epitopes of myelin basic protein (MBP); (ii) the levels of serum immunoglobulin M (IgM)/immunoglobulin G (IgG) autoantibodies against the MBP epitopes; (iii) endoneurial B cell/CD20 levels; and (iv) mechanical sensitivity behavior after B cell/CD20 targeting with intravenous (IV) Rituximab (RTX) and control, IV immunoglobulin (IVIG), therapy. The persistent MBP epitope release in CCI nerves of both sexes was accompanied by the serum anti-MBP IgM autoantibody in female CCI rats alone. IV RTX therapy during CD20-reactive cell infiltration of nerves of both sexes reduced mechanical allodynia in females but not in males. IVIG and vehicle treatments had no effect in either sex. These findings provide strong evidence for sexual dimorphism in B-cell function after peripheral nervous system (PNS) trauma and autoimmune pathogenesis of neuropathic pain, potentially amenable to immunotherapeutic intervention, particularly in females. A myelin-targeted serum autoantibody may serve as a biomarker of such painful states. This insight into the biological basis of sex-specific response to neuraxial injury will help personalize regenerative and analgesic therapies.

DRGquant: A new modular AI-based pipeline for 3D analysis of the DRG

BACKGROUND

The dorsal root ganglion (DRG) is structurally complex and pivotal to systems processing nociception. Whole mount analysis allows examination of intricate microarchitectural and cellular relationships of the DRG in three-dimensional (3D) space.

NEW METHOD

We present DRGquant a set of tools and techniques optimized as a pipeline for automated image analysis and reconstruction of cells/structures within the DRG. We have developed an open source software pipeline that utilizes machine learning to identify substructures within the DRG and reliably classify and quantify them.

RESULTS

Our methods were sufficiently sensitive to isolate, analyze, and classify individual DRG substructures including macrophages. The activation of macrophages was visualized and quantified in the DRG following intrathecal injection of lipopolysaccharide, and in a model of chemotherapy induced peripheral neuropathy. The percent volume of infiltrating macrophages was similar to a commercial source in quantification. Circulating fluorescent dextran was visualized within DRG macrophages using whole mount preparations, which enabled 3D reconstruction of the DRG and DRGquant demonstrated subcellular spatial resolution within individual macrophages.

COMPARISON WITH EXISTING METHOD(S)

Here we describe a reliable and efficient methodologic pipeline to prepare cleared and whole mount DRG tissue. DRGquant allows automated image analysis without tedious manual gating to reduce bias. The quantitation of DRG macrophages was superior to commercial solutions.

CONCLUSIONS

Using machine learning to separate signal from noise and identify individual cells, DRGquant enabled us to isolate individual structures or areas of interest within the DRG for a more granular and fine-tuned analysis. Using these 3D techniques, we are better able to appreciate the biology of the DRG under experimental inflammatory conditions.

Evaluation of neurotoxicity and long-term function and behavior following intrathecal 1 % 2-chloroprocaine in juvenile rats

Spinally-administered local anesthetics provide effective perioperative anesthesia and/or analgesia for children of all ages. New preparations and drugs require preclinical safety testing in developmental models. We evaluated age-dependent efficacy and safety following 1 % preservative-free 2-chloroprocaine (2-CP) in juvenile Sprague-Dawley rats. Percutaneous lumbar intrathecal 2-CP was administered at postnatal day (P)7, 14 or 21. Mechanical withdrawal threshold pre- and post-injection evaluated the degree and duration of sensory block, compared to intrathecal saline and naive controls. Tissue analyses one- or seven-days following injection included histopathology of spinal cord, cauda equina and brain sections, and quantification of neuronal apoptosis and glial reactivity in lumbar spinal cord. Following intrathecal 2-CP or saline at P7, outcomes assessed between P30 and P72 included: spinal reflex sensitivity (hindlimb thermal latency, mechanical threshold); social approach (novel rat versus object); locomotor activity and anxiety (open field with brightly-lit center); exploratory behavior (rearings, holepoking); sensorimotor gating (acoustic startle, prepulse inhibition); and learning (Morris Water Maze). Maximum tolerated doses of intrathecal 2-CP varied with age (1.0 μL/g at P7, 0.75 μL/g at P14, 0.5 μL/g at P21) and produced motor and sensory block for 10-15 min. Tissue analyses found no significant differences across intrathecal 2-CP, saline or naïve groups. Adult behavioral measures showed expected sex-dependent differences, that did not differ between 2-CP and saline groups. Single maximum tolerated in vivo doses of intrathecal 2-CP produced reversible spinal anesthesia in juvenile rodents without detectable evidence of developmental neurotoxicity. Current results cannot be extrapolated to repeated dosing or prolonged infusion.

Severe altered mentation due to cervicothoracic intrathecal pump after correction of cervical stenosis: a case report

BACKGROUND

Cerebral spinal fluid (CSF) dynamics are complex and changes in spinal anatomy may influence the rostrocaudal movement of intrathecal medications. We present the first reported case demonstrating that acute cervical spinal stenosis may impede the distribution of adjacent intrathecal medications, and that correction of such stenosis and the resulting changes in CSF flow may necessitate significant adjustments in the intrathecal infusates.

CASE PRESENTATION

We present a case of a 60-year-old male patient with a cervicothoracic intrathecal pump (ITP) infusing morphine, bupivacaine, and baclofen for chronic neck pain. The alert and oriented patient had a recent fall resulting in an acute severe cervical stenosis and cord compression which required urgent surgical decompression. Postoperatively, after the cervical decompression, the patient had significant altered mental status requiring a naloxone infusion. Multiple attempts to reduce the naloxone infusion were initially not successful due to worsened somnolence. The previously tolerated ITP medications were continuously reduced over the next 14 days, allowing concomitant decrease and eventual cessation of the naloxone infusion while maintaining patient mental status. The only opioids the patient received during this period were from the ITP.

CONCLUSIONS

This case presents clinical evidence that severe spinal stenosis may impede the rostral CSF distribution of intrathecal medications. Intrathecal medications previously tolerated by patients prior to decompression may need to be significantly reduced in the postoperative period.

Sex differences in neuroimmune and glial mechanisms of pain

ABSTRACT

Pain is the primary motivation for seeking medical care. Although pain may subside as inflammation resolves or an injury heals, it is increasingly evident that persistency of the pain state can occur with significant regularity. Chronic pain requires aggressive management to minimize its physiological consequences and diminish its impact on quality of life. Although opioids commonly are prescribed for intractable pain, concerns regarding reduced efficacy, as well as risks of tolerance and dependence, misuse, diversion, and overdose mortality rates limit their utility. Advances in development of nonopioid interventions hinge on our appreciation of underlying mechanisms of pain hypersensitivity. For instance, the contributory role of immunity and the associated presence of autoimmune syndromes has become of particular interest. Males and females exhibit fundamental differences in innate and adaptive immune responses, some of which are present throughout life, whereas others manifest with reproductive maturation. In general, the incidence of chronic pain conditions, particularly those with likely autoimmune covariates, is significantly higher in women. Accordingly, evidence is now accruing in support of neuroimmune interactions driving sex differences in the development and maintenance of pain hypersensitivity and chronicity. This review highlights known sexual dimorphisms of neuroimmune signaling in pain states modeled in rodents, which may yield potential high-value sex-specific targets to inform future analgesic drug discovery efforts.

Normalization of cholesterol metabolism in spinal microglia alleviates neuropathic pain

Neuroinflammation is a major component in the transition to and perpetuation of neuropathic pain states. Spinal neuroinflammation involves activation of TLR4, localized to enlarged, cholesterol-enriched lipid rafts, designated here as inflammarafts. Conditional deletion of cholesterol transporters ABCA1 and ABCG1 in microglia, leading to inflammaraft formation, induced tactile allodynia in naive mice. The apoA-I binding protein (AIBP) facilitated cholesterol depletion from inflammarafts and reversed neuropathic pain in a model of chemotherapy-induced peripheral neuropathy (CIPN) in wild-type mice, but AIBP failed to reverse allodynia in mice with ABCA1/ABCG1–deficient microglia, suggesting a cholesterol-dependent mechanism. An AIBP mutant lacking the TLR4-binding domain did not bind microglia or reverse CIPN allodynia. The long-lasting therapeutic effect of a single AIBP dose in CIPN was associated with anti-inflammatory and cholesterol metabolism reprogramming and reduced accumulation of lipid droplets in microglia. These results suggest a cholesterol-driven mechanism of regulation of neuropathic pain by controlling the TLR4 inflammarafts and gene expression program in microglia and blocking the perpetuation of neuroinflammation.

Long-lasting analgesia via targeted in situ repression of NaV1.7 in mice

Current treatments for chronic pain rely largely on opioids despite their substantial side effects and risk of addiction. Genetic studies have identified in humans key targets pivotal to nociceptive processing. In particular, a hereditary loss-of-function mutation in NaV1.7, a sodium channel protein associated with signaling in nociceptive sensory afferents, leads to insensitivity to pain without other neurodevelopmental alterations. However, the high sequence and structural similarity between NaV subtypes has frustrated efforts to develop selective inhibitors. Here, we investigated targeted epigenetic repression of NaV1.7 in primary afferents via epigenome engineering approaches based on clustered regularly interspaced short palindromic repeats (CRISPR)-dCas9 and zinc finger proteins at the spinal level as a potential treatment for chronic pain. Toward this end, we first optimized the efficiency of NaV1.7 repression in vitro in Neuro2A cells and then, by the lumbar intrathecal route, delivered both epigenome engineering platforms via adeno-associated viruses (AAVs) to assess their effects in three mouse models of pain: carrageenan-induced inflammatory pain, paclitaxel-induced neuropathic pain, and BzATP-induced pain. Our results show effective repression of NaV1.7 in lumbar dorsal root ganglia, reduced thermal hyperalgesia in the inflammatory state, decreased tactile allodynia in the neuropathic state, and no changes in normal motor function in mice. We anticipate that this long-lasting analgesia via targeted in vivo epigenetic repression of NaV1.7 methodology we dub pain LATER, might have therapeutic potential in management of persistent pain states.

Sexual Dimorphism in the Expression of Pain Phenotype in Preclinical Models of Rheumatoid Arthritis

Rheumatoid arthritis is one of most frequent rheumatic diseases, affecting around 1% of the population worldwide. Pain impacting the quality of life for the patient with rheumatoid arthritis, is often the primary factor leading them to seek medical care. Although sex-related differences in humans and animal models of rheumatoid arthritis are described, the correlation between pain and sex in rheumatoid arthritis has only recently been directly examined. Here we review the literature and explore the mechanisms underlying the expression of the pain phenotype in females and males in preclinical models of rheumatoid arthritis.

A myelin basic protein fragment induces sexually dimorphic transcriptome signatures of neuropathic pain in mice

In the peripheral nerve, mechanosensitive axons are insulated by myelin, a multilamellar membrane formed by Schwann cells. Here, we offer first evidence that a myelin degradation product induces mechanical hypersensitivity and global transcriptomics changes in a sex-specific manner. Focusing on downstream signaling events of the functionally active 84-104 myelin basic protein (MBP(84-104)) fragment released after nerve injury, we demonstrate that exposing the sciatic nerve to MBP(84-104) via endoneurial injection produces robust mechanical hypersensitivity in female, but not in male, mice. RNA-seq and systems biology analysis revealed a striking sexual dimorphism in molecular signatures of the dorsal root ganglia (DRG) and spinal cord response, not observed at the nerve injection site. Mechanistically, intra-sciatic MBP(84-104) induced phospholipase C (PLC)-driven (females) and phosphoinositide 3-kinase-driven (males) phospholipid metabolism (tier 1). PLC/inositol trisphosphate receptor (IP3R) and estrogen receptor co-regulation in spinal cord yielded Ca²⁺-dependent nociceptive signaling induction in females that was suppressed in males (tier 2). IP3R inactivation by intrathecal xestospongin C attenuated the female-specific hypersensitivity induced by MBP(84-104). According to sustained sensitization in tiers 1 and 2, T cell-related signaling spreads to the DRG and spinal cord in females, but remains localized to the sciatic nerve in males (tier 3). These results are consistent with our previous finding that MBP(84-104)-induced pain is T cell-dependent. In summary, an autoantigenic peptide endogenously released in nerve injury triggers multisite, sex-specific transcriptome changes, leading to neuropathic pain only in female mice. MBP(84-104) acts through sustained co-activation of metabolic, estrogen receptor-mediated nociceptive, and autoimmune signaling programs.

Lipid rafts in glial cells: role in neuroinflammation and pain processing

Activation of microglia and astrocytes secondary to inflammatory processes contributes to the development and perpetuation of pain with a neuropathic phenotype. This pain state presents as a chronic debilitating condition and affects a large population of patients with conditions like rheumatoid arthritis and diabetes, or after surgery, trauma, or chemotherapy. Here, we review the regulation of lipid rafts in glial cells and the role they play as a key component of neuroinflammatory sensitization of central pain signaling pathways. In this context, we introduce the concept of an inflammaraft (i-raft), enlarged lipid rafts harboring activated receptors and adaptor molecules and serving as an organizing platform to initiate inflammatory signaling and the cellular response. Characteristics of the inflammaraft include increased relative abundance of lipid rafts in inflammatory cells, increased content of cholesterol per raft, and increased levels of inflammatory receptors, such as toll-like receptor (TLR)4, adaptor molecules, ion channels, and enzymes in lipid rafts. This inflammaraft motif serves an important role in the membrane assembly of protein complexes, for example, TLR4 dimerization. Operating within this framework, we demonstrate the involvement of inflammatory receptors, redox molecules, and ion channels in the inflammaraft formation and the regulation of cholesterol and sphingolipid metabolism in the inflammaraft maintenance and disruption. Strategies for targeting inflammarafts, without affecting the integrity of lipid rafts in noninflammatory cells, may lead to developing novel therapies for neuropathic pain states and other neuroinflammatory conditions.

Unilateral Epidural Targeting of Resiniferatoxin Induces Bilateral Neurolysis of Spinal Nociceptive Afferents

OBJECTIVE

This study modeled image-guided epidural drug delivery to test whether intraprocedural distribution of pre-injected contrast reliably predicts the neuroanatomical reach of resiniferatoxin-mediated nociceptive neurolysis.

METHODS

Swine (N = 12) received unilateral L4-S2 computed tomography fluoroscopy injections by a blinded neuroradiologist; 0.25 mL of contrast was pre-injected to confirm dorsal periganglionic targeting, followed by a 0.5-mL injection of 5 µg of resiniferatoxin/Tween80 or vehicle control. Epidural contrast distribution was graded according to maximum medial excursion. Spinal cord substance P immunostaining quantified the magnitude and anatomical range of resiniferatoxin activity.

RESULTS

Periganglionic injection was well tolerated by all animals without development of neurological deficits or other complications. Swine were a suitable model of human clinical spinal intervention. The transforaminal approach was used at all L4 and 50% of L5 segments; the remaining segments were approached by the interlaminar route. All injections were successful with unilateral contrast distribution for all resiniferatoxin injections (N = 28). Immunohistochemistry showed bilateral ablation of substance P+ fibers entering the spinal cord of all resiniferatoxin-treated segments. The intensity of substance P immunostaining in treated segments fell below the lower 99% confidence interval of controls, defining the knockout phenotype. Substance P knockout occurred over a narrow range and was uncorrelated to the anatomical distribution of pre-injected contrast.

CONCLUSIONS

Periganglionic resiniferatoxin/Tween80 induced bilateral ablation of spinal cord substance P despite exclusively unilateral targeting. These data suggest that the location of pre-injected contrast is an imperfect surrogate for the neuroanatomical range of drugs delivered to the dorsal epidural compartment that may fail to predict contralateral drug effects.

Neuraxial Cytokines in Pain States

A high-intensity potentially tissue-injuring stimulus generates a homotopic response to escape the stimulus and is associated with an affective phenotype considered to represent pain. In the face of tissue or nerve injury, the afferent encoding systems display robust changes in the input–output function, leading to an ongoing sensation reported as painful and sensitization of the nociceptors such that an enhanced pain state is reported for a given somatic or visceral stimulus. Our understanding of the mechanisms underlying this non-linear processing of nociceptive stimuli has led to our appreciation of the role played by the functional interactions of neural and immune signaling systems in pain phenotypes. In pathological states, neural systems interact with the immune system through the actions of a variety of soluble mediators, including cytokines. Cytokines are recognized as important mediators of inflammatory and neuropathic pain, supporting system sensitization and the development of a persistent pathologic pain. Cytokines can induce a facilitation of nociceptive processing at all levels of the neuraxis including supraspinal centers where nociceptive input evokes an affective component of the pain state. We review here several key proinflammatory and anti-inflammatory cytokines/chemokines and explore their underlying actions at four levels of neuronal organization: (1) peripheral nociceptor termini; (2) dorsal root ganglia; (3) spinal cord; and (4) supraspinal areas. Thus, current thinking suggests that cytokines by this action throughout the neuraxis play key roles in the induction of pain and the maintenance of the facilitated states of pain behavior generated by tissue injury/inflammation and nerve injury.

Inhibition of Neuroinflammation by AIBP: Spinal Effects upon Facilitated Pain States

Apolipoprotein A-I binding protein (AIBP) reduces lipid raft abundance by augmenting the removal of excess cholesterol from the plasma membrane. Here, we report that AIBP prevents and reverses processes associated with neuroinflammatory-mediated spinal nociceptive processing. The mechanism involves AIBP binding to Toll-like receptor-4 (TLR4) and increased binding of AIBP to activated microglia, which mediates selective regulation of lipid rafts in inflammatory cells. AIBP-mediated lipid raft reductions downregulate LPS-induced TLR4 dimerization, inflammatory signaling, and expression of cytokines in microglia. In mice, intrathecal injections of AIBP reduce spinal myeloid cell lipid rafts, TLR4 dimerization, neuroinflammation, and glial activation. Intrathecal AIBP reverses established allodynia in mice in which pain states were induced by the chemotherapeutic cisplatin, intraplantar formalin, or intrathecal LPS, all of which are pro-nociceptive interventions known to be regulated by TLR4 signaling. These findings demonstrate a mechanism by which AIBP regulates neuroinflammation and suggest the therapeutic potential of AIBP in treating preexisting pain states.

Characterization of Analgesic Actions of the Chronic Intrathecal Infusion of H-Dmt-D-Arg-Phe-Lys-NH2 in Rat

OBJECTIVES

DMT-DALDA (H-Dmt-D-Arg-Phe-Lys-NH2; Dmt = 2',6'-dimethyltyrosine) is a selective mu opioid agonist. We sought to characterize efficacy, tolerance, dependence and side-effect profile when given by continuous intrathecal infusion.

MATERIALS AND METHODS

Adult male Sprague Dawley rats were prepared with chronic intrathecal catheters and osmotic mini-pumps to deliver vehicle (saline), DMT-DALDA or morphine. Hind paw thermal escape latencies were assessed. In addition, effects upon intraplantar formalin-evoked flinching and withdrawal after 14 days of infusion were examined. The flare response after intradermal delivery was examined in the canine model.

RESULTS

1) Intrathecal infusion of 0.3 to 30 pmol/μL/hour of DMT-DALDA or 37.5 nmol/μL/hour of morphine more than 7 or 14 days resulted in a dose-dependent increase in thermal escape latency. The maximum antinociceptive effect was observed between 1 and 4 days after start of infusion with preserved cornea, blink, placing and stepping. By days 12 to 14, response latencies were below baseline. 2) On days 2 to 4 of DMT-DALDA infusion, the pan opioid receptor antagonist naloxone (Nx), but not the delta-preferring antagonist naltrindole, antagonized the analgesic effects. 3) Assessment of formalin flinching on day 1 following IT DMT-DALDA Infusion showed significant analgesia in phases 1 and 2. On day 6 of infusion there was minimal effect, while on day 13, there was an increase in flinching. 4) On days 7 and 14 of infusion Nx resulted in prominent withdrawal signs indicating dependence and withdrawal. 5) Intradermal morphine and DMT-DALDA both yield a naltrexone-insensitive, cromolyn-sensitive flare in the canine model at similar concentrations.

CONCLUSIONS

These data suggest that DMT-DALDA is a potent, spinally active agonist with a propensity to produce tolerance dependence and mast cell degranulation. While it was equiactive to morphine in producing mast cell degranulation, it was >1000 fold more potent in producing analgesia, suggesting a possible lower risk in producing a spinal mass at equianalgesic doses.

Characterization of the antinociceptive effects of intrathecal DALDA peptides following bolus intrathecal delivery

Background and aims We systematically characterized the potency and side effect profile of a series of small opioid peptides with high affinity for the mu opioid receptor. Methods Male Sprague Dawley rats were prepared with intrathecal (IT) catheters, assessed with hind paw thermal escape and evaluated for side effects including Straub tail, truncal rigidity, and pinnae and corneal reflexes. In these studies, DMT-DALDA (dDAL) (H-Dmt-D-Arg-Phe-Lys-NH2 MW=981), dDALc (H-Dmt-Cit-Phe-Lys-NH2 MW=868), dDALcn (H-Dmt-D-Cit-Phe-Nle-NH2 MW=739), TAPP (H-Tyr-D-Ala-Phe-Phe-NH2 MW=659), dDAL-TICP ([Dmt1]DALDA-(CH2)2-NH-TICP[psi]; MW=1519), and dDAL-TIPP (H-Dmt-D-Arg-Phe-Lys(Nε-TIPP)-NH2 were examined. In separate studies, the effects of approximately equiactive doses of IT DMT DALDA (10 pmol), morphine (30 nmol) and fentanyl (1 nmol) were examined on formalin-induced flinching at different pretreatment intervals (15 min - 24 h). Results (1) All agents resulted in a dose-dependent reversible effect upon motor function (Straub Tail>Truncal rigidity). (2) The ordering of analgesic activity (%MPE) at the highest dose lacking reliable motor signs after bolus delivery was: DMT-DALDA (80%±6/3 pmol); dDALc (75%±8/1 pmol); dDALcn (84%±10/300 pmol); TAPP (56%±12/10 nmol); dDAL-TICP (52%±27/300 pmol). (3) All analgesic effects were reversed by systemic (IP) naloxone (1 mg/kg). Naltrindole (3 mg/kg, IP) had no significant effect upon the maximum usable peptide dose. (4) Tolerance and cross-tolerance development after 5 daily boluses of DMT-DALDA (3 pmol) and morphine (30 nmol) revealed that both agents displayed a progressive decline over 5 days. (5) Cross-tolerance assessed at day 5 revealed a reduction in response to morphine in DMT-DALDA treated animal but not DMT-DALDA in the morphine treated animal, indicating an asymmetric cross-tolerance. (6) IT DMT-DALDA, morphine and fentanyl resulted in significant reductions in phase 1 and phase 2 flinching. With a 15 min pretreatment all drugs resulted in comparable reductions in flinching. However, at 6 h, the reduction in flinching after DMT-DALDA and morphine were comparably reduced while fentanyl was not different from vehicle. All effects on flinching were lost by 24 h. Conclusions These results emphasize the potent mu agonist properties of the DALDA peptidic structure series, their persistence similar to morphine and their propensity to produce tolerance. The asymmetric cross-tolerance between equiactive doses may reflect the relative intrinsic activity of morphine and DMT-DALDA. Implications These results suggest that the DALDA peptides with their potency and duration of action after intrathecal delivery suggest their potential utility for their further development as a spinal therapeutic to manage pain.

Mast Cell Degranulation and Fibroblast Activation in the Morphine-induced Spinal Mass: Role of Mas-related G Protein-coupled Receptor Signaling

BACKGROUND

As the meningeally derived, fibroblast-rich, mass-produced by intrathecal morphine infusion is not produced by all opiates, but reduced by mast cell stabilizers, the authors hypothesized a role for meningeal mast cell/fibroblast activation. Using the guinea pig, the authors asked: (1) Are intrathecal morphine masses blocked by opiate antagonism?; (2) Do opioid agonists not producing mast cell degranulation or fibroblast activation produce masses?; and (3) Do masses covary with Mas-related G protein-coupled receptor signaling thought to mediate mast cell degranulation?

METHODS

In adult male guinea pigs (N = 66), lumbar intrathecal catheters connected to osmotic minipumps (14 days; 0.5 µl/h) were placed to deliver saline or equianalgesic concentrations of morphine sulfate (33 nmol/h), 2',6'-dimethyl tyrosine-(Tyr-D-Arg-Phe-Lys-NH2) (abbreviated as DMT-DALDA; 10 pmol/h; μ agonist) or PZM21 (27 nmol/h; biased μ agonist). A second pump delivered subcutaneous naltrexone (25 µg/h) in some animals. After 14 to 16 days, animals were anesthetized and perfusion-fixed. Drug effects on degranulation of human cultured mast cells, mouse embryonic fibroblast activation/migration/collagen formation, and Mas-related G protein-coupled receptor activation (PRESTO-Tango assays) were determined.

RESULTS

Intrathecal infusion of morphine, DMT-DALDA or PZM21, but not saline, comparably increased thermal thresholds for 7 days. Spinal masses proximal to catheter tip, composed of fibroblast/collagen type I (median: interquartile range, 0 to 4 scale), were produced by morphine (2.3: 2.0 to 3.5) and morphine plus naltrexone (2.5: 1.4 to 3.1), but not vehicle (1.2: 1.1 to 1.5), DMT-DALDA (1.0: 0.6 to 1.3), or PZM21 (0.5: 0.4 to 0.8). Morphine in a naloxone-insensitive fashion, but not PZM21 or DMT-DALDA, resulted in mast cell degranulation and fibroblast proliferation/collagen formation. Morphine-induced fibroblast proliferation, as mast cell degranulation, is blocked by cromolyn. Mas-related G protein-coupled receptor activation was produced by morphine and TAN67 (∂-opioid agonist), but not by PZM21, TRV130 (mu biased ligand), or DMT-DALDA.

CONCLUSIONS

Opiates that activate Mas-related G protein-coupled receptor will degranulate mast cells, activate fibroblasts, and result in intrathecal mass formation. Results suggest a mechanistically rational path forward to safer intrathecal opioid therapeutics.

Characterization of Effect of Repeated Bolus or Continuous Intrathecal Infusion of Morphine on Spinal Mass Formation in the Dog

BACKGROUND

We determined whether intrathecally delivering the same daily dose of morphine (MS) at a fixed concentration of 25 mg/mL by periodic boluses versus continuous infusion would reduce intrathecal mass (IMs) formation in dogs.

METHODS

Adult dogs (hound cross, n = 32) were implanted with intrathecal catheters connected to SynchroMed II infusion pumps. Animals were randomly assigned to receive infusion of 0.48 mL/day of saline or MS dosing (12 mg/day at 25 mg/mL) as boluses: x1 (q24hour), x2 (q12hour), x4 (q6hour), or x8 (q3hour) given at the rate of 1000 μL/hour, or as a continuous infusion (25 mg/mL/20 μL/hour).

RESULTS

With IT saline, minimal pathology was noted. In contrast, animals receiving morphine displayed spinally compressing durally derived masses with the maximal cross-sectional area being greatest near the catheter tip. Histopathology showed that IMs consisted of fibroblasts in a collagen (type 1) matrix comprised of newly formed collagen near the catheter and mature collagen on the periphery of the mass. The rank order of median cross-sectional mass area (mm² ) was: Saline: 0.7 mm² ; x2: 1.8 mm² ; x4: 2.7 mm² ; x1: 2.7 mm² ; x8: 4.2 mm² ; Continuous: 8.1 mm² , with statistical difference from saline being seen with continuous (p < 0.0001) and x8 (p < 0.05). Bench studies with a 2D diffusion chamber confirmed an increase in dye distribution and lower peak concentrations after bolus delivery versus continuous infusion of dye.

CONCLUSIONS

Using multiple bolus dosing, IMs were reduced as compared to continuous infusion, suggesting relevance of bolus delivery in yielding reduced intrathecal masses.

Neuraxial TNF and IFN-beta co-modulate persistent allodynia in arthritic mice

In rheumatoid arthritis, joint pain can persist despite resolution of swelling. Similarly, in the murine K/BxN serum transfer model, a persistent tactile allodynia is observed after the resolution of joint inflammation (post-inflammatory pain) in male mice. Here, we found female wild type (WT) mice show inflammatory, but reduced post-inflammatory tactile allodynia. The transition to the post-inflammatory phenotype is dependent on TLR4 signaling. At the spinal level, we found differences in TNF and IFNβ mRNA expression in WT and TLR4 deficient males. In wild type male and female mice, there is differential temporal spinal expression of TNF and IFNβ. In WT males, blockade of TNF or administration of IFNβ was insufficient to affect the persistent allodynia. However, co-administration of intrathecal (IT) IFNβ and anti-TNF antibodies in male WT mice permanently reversed tactile allodynia. IT IFNβ treatment induces expression of anti-inflammatory proteins, contributing to the beneficial effect. Together, these experiments illustrated differences in the transition to chronic tactile allodynia in male and female animals and the complexities of effective pharmacologic interventions.

Role of Toll-like receptor 4 signaling in mast cell-mediated migraine pain pathway

Degranulation of meningeal mast cells leading to the sensitization of trigeminal vascular afferent processing is believed to be one of the mechanisms underlying the migraine pain pathway. Recent work suggests that Toll-like receptor 4 (TLR4) may be involved in signaling states of central sensitization. Using a murine model of light aversion produced by compound 48/80 (2 mg/kg, intraperitoneal) mast cell degranulation, employed as a surrogate marker for photophobia observed in migraineurs, we examined the role of TLR4 in migraine-like behavior and neuronal activation. Using a two-chambered light/dark box, we found that compound 48/80 administration in male and female C57Bl/6 mice produced light aversion lasting up to 2 h, and that pre-treatment with sumatriptan (1 mg/kg, i.p.) reliably prevented this effect. Genetic deletion and pharmacological blockade of TLR4 with TAK-242 (3 mg/kg, i.p.) reversed the light aversive effects of compound 48/80 in males but not in females. Assessing the downstream signaling pathway in mutant mice, we found that the TLR4-mediated, light aversion was dependent upon myeloid differentiation primary response gene 88 but not Toll-interleukin-1 receptor domain-containing adapter-inducing interferon-β signaling. In separate groups, male mice sacrificed at 10 min following compound 48/80 revealed a significant increase in the incidence of evoked p-extracellular signal–regulated kinases (+) neurons in the nucleus caudalis of wild type but not Tlr4−/− mice or in mice pre-treated with sumatriptan. This study thus provides the first evidence for involvement of TLR4 signaling through MyD88 in initiating and maintaining migraine-like behavior and nucleus caudalis neuronal activation in the mouse.

Botulinum toxin blocks mast cells and prevents rosacea like inflammation

BACKGROUND

Rosacea is a chronic inflammatory skin condition whose etiology has been linked to mast cells and the antimicrobial peptide cathelicidin LL-37. Individuals with refractory disease have demonstrated clinical benefit with periodic injections of onabotulinum toxin, but the mechanism of action is unknown.

OBJECTIVES

To investigate the molecular mechanism by which botulinum toxin improves rosacea lesions.

METHODS

Primary human and murine mast cells were pretreated with onabotulinum toxin A or B or control. Mast cell degranulation was evaluated by β-hexosaminidase activity. Expression of botulinum toxin receptor Sv2 was measured by qPCR. The presence of SNAP-25 and VAMP2 was established by immunofluorescence. In vivo rosacea model was established by intradermally injecting LL-37 with or without onabotulinum toxin A pretreatment. Mast cell degranulation was assessed in vivo by histologic counts. Rosacea biomarkers were analyzed by qPCR of mouse skin sections.

RESULTS

Onabotulinum toxin A and B inhibited compound 48/80-induced degranulation of both human and murine mast cells. Expression of Sv2 was established in mouse mast cells. Onabotulinum toxin A and B increased cleaved SNAP-25 and decreased VAMP2 staining in mast cells respectively. In mice, injection of onabotulinum toxin A significantly reduced LL-37-induced skin erythema, mast cell degranulation, and mRNA expression of rosacea biomarkers.

CONCLUSIONS

These findings suggest that onabotulinum toxin reduces rosacea-associated skin inflammation by directly inhibiting mast cell degranulation. Periodic applications of onabotulinum toxin may be an effective therapy for refractory rosacea and deserves further study.

Structural homology of myelin basic protein and muscarinic acetylcholine receptor: Significance in the pathogenesis of complex regional pain syndrome

Complex regional pain syndrome is an extremely painful condition that develops after trauma to a limb. Complex regional pain syndrome exhibits autoimmune features in part mediated by autoantibodies against muscarinic‐2 acetylcholine (M2) receptor. The mechanisms underlying the M2 receptor involvement in complex regional pain syndrome remain obscure. Based on our recent work demonstrating that limb nerve trauma releases a potent proalgesic, immunodominant myelin basic protein fragment, our present sequence database analyses reveal an unexpected and previously undescribed structural homology of the proalgesic myelin basic protein fragment with the M2 receptor. As both complex regional pain syndrome and the proalgesic myelin basic protein activity are prevalent in females, this myelin basic protein/M2 homology presents an inviting hypothesis explaining the mechanisms of autoimmune pathogenesis and sexual dimorphism that underlies vulnerability toward developing complex regional pain syndrome and other pain states with neuropathic features. This hypothesis may aid in the development of novel diagnostic and therapeutic strategies to chronic pain.

Rapid continuous 3D printing of customizable peripheral nerve guidance conduits

Engineered nerve guidance conduits (NGCs) have been demonstrated for repairing peripheral nerve injuries. However, there remains a need for an advanced biofabrication system to build NGCs with complex architectures, tunable material properties, and customizable geometrical control. Here, a rapid continuous 3D-printing platform was developed to print customizable NGCs with unprecedented resolution, speed, flexibility, and scalability. A variety of NGC designs varying in complexity and size were created including a life-size biomimetic branched human facial NGC. In vivo implantation of NGCs with microchannels into complete sciatic nerve transections of mouse models demonstrated the effective directional guidance of regenerating sciatic nerves via branching into the microchannels and extending toward the distal end of the injury site. Histological staining and immunostaining further confirmed the progressive directional nerve regeneration and branching behavior across the entire NGC length. Observational and functional tests, including the von Frey threshold test and thermal test, showed promising recovery of motor function and sensation in the ipsilateral limbs grafted with the 3D-printed NGCs.

Targeting toll-like receptor-4 (TLR4)-an emerging therapeutic target for persistent pain states

Toll-like receptors (TLRs) are a family of pattern recognition receptors that initiate signaling in innate and adaptive immune pathways. The highly conserved family of transmembrane proteins comprises an extracellular domain that recognizes exogenous and endogenous danger molecules and an ectodomain that activates downstream pathways in response. Recent studies suggest that continuous activation or dysregulation of TLR signaling may contribute to chronic disease states. The receptor is located not only on inflammatory cells (meningeal and peripheral macrophages) but on neuraxial glia (microglia and astrocytes), Schwann cells, fibroblasts, dorsal root ganglia, and dorsal horn neurons. Procedures blocking TLR functionality have shown pronounced effects on pain behavior otherwise observed in models of chronic inflammation and nerve injury. This review addresses the role of TLR4 as an emerging therapeutic target for the evolution of persistent pain and its role in noncanonical signaling, mediating anomalous pro-algesic actions of opiates. Accordingly, molecules targeting inhibition of this receptor have promise as disease-modifying and opioid-sparing alternatives for persistent pain states.

Editor's Highlight: Formulation and Toxicology Evaluation of the Intrathecal AYX1 DNA-Decoy in Sprague Dawley Rats

The longevity of pain after surgery is debilitating and limits the recovery of patients. AYX1 is a double-stranded, unprotected, 23 base-pair oligonucleotide designed to reduce acute post-surgical pain and prevent its chronification with a single intrathecal perioperative dose. AYX1 mimics the DNA sequence normally bound by EGR1 on chromosomes, a transcription factor transiently induced in the dorsal root ganglia-spinal cord network following a noxious input. AYX1 binds to EGR1 and prevents it from launching waves of gene regulation that are necessary to maintain pain over time. A formulation suitable for an intrathecal injection of AYX1 was developed, including a specific ratio of AYX1 and calcium so the ionic homeostasis of the cerebrospinal fluid is maintained and no impact on neuromuscular control is produced upon injection. A GLP toxicology study in naïve Sprague Dawley rats was conducted using 3 dose levels up to the maximum feasible dose. Clinical observations, neurobehavioral observations, clinical pathology and histopathology of the nervous system and peripheral tissues were conducted. An additional nonGLP study was conducted in the spared nerve injury model of chronic neuropathic pain in which EGR1 is induced in the dorsal root ganglia and spinal cord. Similar testing was performed, including a modified Irwin test to assess a potential impact of AYX1 on autonomic nervous system responses, locomotion, activity, arousal, sensorimotor, and neuromuscular function. No AYX1-related adverse events were observed in any of the studies and the no-observed-adverse-effect-level was judged to be the maximum feasible dose.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

Effects of opioid and nonopioid analgesics on canine wheal formation and cultured human mast cell degranulation

Mast cell (MC) degranulation has been implicated in the side effect profile of a variety of clinically useful agents. Thus, after intrathecal delivery, formation of space-occupying, meningeally-derived masses may be related to local MC degranulation. We systematically characterized degranulating effects of opioid and nonopioid analgesics on cutaneous flares in the dog and in primary human MC (hMC) cultures.

METHODS

Dogs were anesthetized with IV propofol and received intradermal (ID) injections (50μL). Flare diameters were measured at 30min. Drugs showing flare responses were tested after intramuscular (IM) cromolyn (10mg/kg), a MC stabilizer. Human primary MCs (human cord blood CD34⁺/CD45⁺ cells) were employed and β-hexosaminidase in cell-free supernatants were measured to assess degranulation.

RESULTS

A significant skin flare for several classes of agents was observed including opioids, ziconotide, ketamine, ST-91, neostigmine, adenosine, bupivacaine, lidocaine, MK-801 and 48/80. Tizanidine, fentanyl, alfentanil, gabapentin and baclofen produced no flare. Flare produced by all ID agents, except adenosine, bupivacaine and lidocaine, was reduced by cromolyn. Naloxone had no effect upon opiate or 48/80 evoked flares. In hMC studies, 48/80 resulted in a concentration-dependent release of β-hexosaminidase. The rank order of drug-induced hMC β-hexosaminidase release was similar to that for flares.

CONCLUSIONS

A variety of therapeutically useful drugs degranulate MCs. This action may account for side effects such as the intrathecal granuloma resulting from spinally-delivered opioids. This degranulating effect may be useful in predicting potential intrathecal toxicity in the development of novel agents.

Toxicology Evaluation of Drugs Administered via Uncommon Routes: Intranasal, Intraocular, Intrathecal/Intraspinal, and Intra-Articular

As the need for nasal, ocular, spinal, and articular therapeutic compounds increases, toxicology assessments of drugs administered via these routes play an important role in human safety. This symposium outlined the local and systemic evaluation to support safety during the development of these drugs in nonclinical models with some case studies. Discussions included selection of appropriate species for the intended route; conducting nonclinical studies that closely mimic the intended use with adequate duration; functional assessment, if deemed necessary; evaluation of local tissues with special histological staining procedure; and evaluations of safety margins based on local and systemic toxicity.

Target engagement and histopathology of neuraxial resiniferatoxin in dog

OBJECTIVE

To evaluate target engagement of intracisternally (IC) delivered TRPV1 agonist, resiniferatoxin (RTX), as measured by primary afferent and dorsal horn substance P immunoreactivity (sP-IR), histopathology and thermal escape latencies in dogs.

STUDY DESIGN

Prospective experimental trial.

ANIMALS

Fourteen adult male Beagle dogs, weighing 10.3-13.2 kg; 11 dogs surviving to scheduled euthanasia.

METHODS

Anesthetized dogs were randomly assigned to be administered IC RTX (3.6 μg, 0.1 mL kg^-1) in a hyperbaric (hRTX, n = 6), normobaric (nRTX, n = 4) vehicle or a hyperbaric vehicle (hVehicle, n = 4). Over 16 days, animals were examined for thoracic and pelvic limb paw thermal withdrawal latencies and neurologic function. Spinal cords, trigeminal ganglia and dorsal root ganglia (DRGs) were assessed for morphologic changes and sP-IR.

RESULTS

IC RTX in anesthetized dogs resulted in a < 1 hour increase in blood pressure. Acute reactions leading to euthanasia within 8 hours occurred in three dogs (two hRTX, one nRTX). All other animals recovered with normal neurologic, bowel and bladder function. Final groups were: vehicle n = 4, hRTX n = 4 and nRTX n = 3. Animals in nRTX and hRTX showed increases in escape latencies in thoracic paws and, to a lesser extent, in pelvic paws, correlating to a loss of sP-IR in cervical cord with smaller reductions in thoracic and lumbar cord. In animals surviving to euthanasia, thickening of the arachnoid membrane (predominantly in the cervical region) was the most consistent change. This change, present in controls, was interpreted to be vehicle related. There was no evidence of structural changes in brain and spinal cord.

CONCLUSIONS AND CLINICAL RELEVANCE

IC RTX produced localized loss of spinal and DRG sP with a corresponding thermal analgesia, absent motor impairment or spinal pathology. Loss of three animals emphasizes the need to refine the use of this promising therapeutic modality in managing companion animal pain.