Spinal delivery of analgesics in experimental models of pain and analgesia

Astaxanthin has a beneficial influence on pain-related symptoms and opioid-induced hyperalgesia in mice with diabetic neuropathy-evidence from behavioral studies

BACKGROUND

The treatment of painful diabetic neuropathy is still a clinical problem. The aim of this study was to determine whether astaxanthin, a substance that inhibits mitogen-activated protein kinases, activates nuclear factor erythroid 2-related factor 2 and influences N-methyl-D-aspartate receptor, affects nociceptive transmission in mice with diabetic neuropathy.

METHODS

The studies were performed on streptozotocin-induced mouse diabetic neuropathic pain model. Single intrathecal and intraperitoneal administrations of astaxanthin at various doses were conducted in both males and females. Additionally, repeated twice-daily treatment with astaxanthin (25 mg/kg) and morphine (30 mg/kg) were performed. Hypersensitivity was evaluated with von Frey and cold plate tests.

RESULTS

This behavioral study provides the first evidence that in a mouse model of diabetic neuropathy, single injections of astaxanthin similarly reduce tactile and thermal hypersensitivity in both male and female mice, regardless of the route of administration. Moreover, repeated administration of astaxanthin slightly delays the development of morphine tolerance and significantly suppresses the occurrence of opioid-induced hyperalgesia, although it does not affect blood glucose levels, body weight, or motor coordination. Surprisingly, astaxanthin administered repeatedly produces a better analgesic effect when administered alone than in combination with morphine, and its potency becomes even more pronounced over time.

CONCLUSIONS

These behavioral results provide a basis for further evaluation of the potential use of astaxanthin in the clinical treatment of diabetic neuropathy and suggest that the multidirectional action of this substance may have positive effects on relieving neuropathic pain in diabetes.

Expression and distribution of rAAV9 intrathecally administered in juvenile to adolescent mice

Intrathecal (IT) lumbar puncture delivery of recombinant adeno-associated virus serotype 9 (rAAV9) is a gene therapy approach being explored in preclinical studies and ongoing gene therapy clinical trials for neurological diseases. Few studies address IT rAAV9 vector distribution, tropism, and expression with respect to age of administration. Therefore, we IT delivered a rAAV9/GFP vector in mice at ages ranging from early postnatal development through adulthood (P10-P90). Tissues were assessed for transgene expression, cell tropism, and vector distribution. In the CNS, transduction was highest when delivered at post-natal day 10 (P10) and there was an age-dependent decline in transduction. We found higher transduction of astrocytes relative to neurons when rAAV9 was administered at younger ages and a switch to higher neuronal transduction with delivery at older timepoints. Biodistribution analysis of peripheral tissues showed that when delivered at P10, rAAV9 has the greatest distribution to the heart. Conversely, at P90 rAAV9 liver distribution was highest. As rAAV9 IT-delivered gene therapies continue to emerge for neurological diseases, careful consideration of the age of delivery should be taken in relation to the expected distribution and cell expression in animal models, and how this may translate to human studies.

Neuropeptide Y Y2 Receptors in Sensory Neurons Tonically Suppress Nociception and Itch but Facilitate Postsurgical and Neuropathic Pain Hypersensitivity

BACKGROUND

Neuropeptide Y (NPY) Y2 receptor (Y2) antagonist BIIE0246 can both inhibit and facilitate nociception. The authors hypothesized that Y2 function depends on inflammation or nerve injury status.

METHODS

The authors implemented a battery of behavioral tests in mice of both sexes that received (1) no injury; (2) an incision model of postoperative pain; (3) a spared nerve injury model of neuropathic pain; and (4) a latent sensitization model of chronic postsurgical pain. In addition to Y2 gene expression assays, spinal Y2 G-protein coupling was studied with guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assays.

RESULTS

The authors report that intrathecal BIIE0246 increased mechanical and cold hypersensitivity, produced behavioral signs of spontaneous nociception and itch, and produced conditioned place aversion and preference in normal, uninjured mice. BIIE0246 did not change heat hypersensitivity or motor coordination. Conditional (sensory neuron-specific) Y2 deletion prevented BIIE0246-induced mechanical and cold hypersensitivity, nocifensive behaviors, and aversion. Both conditional deletion and pharmacologic blockade of Y2 reduced mechanical and thermal hypersensitivity after incision or nerve injury. SNI did not change the sensitivity of Y2 G-protein coupling with the Y2 agonist peptide YY (3-36) (PYY3-36), but increased the population of Y2 that effectively coupled G-proteins. Intrathecal PYY3-36 failed to reduce spared nerve injury- or incision-induced hypersensitivity in C57BL/6N mice. Incision did not change Npy2r gene expression in dorsal root ganglion.

CONCLUSIONS

The authors conclude that Y2 at central terminals of primary afferent neurons provides tonic inhibition of mechanical and cold nociception and itch. This switches to the promotion of mechanical and thermal hyperalgesia in models of acute and chronic postsurgical and neuropathic pain, perhaps due to an increase in the population of Y2 that effectively couples to G-proteins. These results support the development of Y2 antagonists for the treatment of chronic postsurgical and neuropathic pain.

EDITOR’S PERSPECTIVE

Sinomenine Ameliorated Microglial Activation and Neuropathic Pain After Chronic Constriction Injury Via TGF-β1/ALK5/Smad3 Signalling Pathway

Sinomenine (SIN), a bioactive isoquinoline alkaloid extracted from the roots and stems of Sinomenium acutum, is efficacious against various chronic pain conditions. Inhibition of microglial activation at the spinal level contributes to the analgesic effects of SIN. Microglial activation in the spinal dorsal horn is key to sensitising neuropathic pain. Consequently, this study aimed to investigate whether the antinociceptive effects of SIN in neuropathic pain are induced through microglial inhibition and the underlying mechanisms. In this study, we observed that SIN alleviated chronic constriction injury (CCI)-induced pain hypersensitivity, spinal microglial activation and neuroinflammation. Consistently, SIN evoked the upregulation of transforming growth factor-beta1 (TGF-β1) and phosphorylated Smad3 in the L4-6 ipsilateral spinal dorsal horn of CCI mice. Intrathecal injection of TGF-β1 siRNA and an activin receptor-like receptor (ALK5) inhibitor reversed SIN's antinociceptive and antimicroglial effects on CCI mice. Moreover, targeting Smad3 in vitro with siRNA dampened the inhibitory effect of TGF-β1 on lipopolysaccharide-induced microglial activation. Finally, targeting Smad3 abrogated SIN-induced pain relief and microglial inhibition in CCI mice. These findings indicate that the TGF-β1/ALK5/Smad3 axis plays a key role in the antinociceptive effects of SIN on neuropathic pain, indicating its suppressive ability on microglia.

A Novel Antigen Design Strategy to Isolate Single-Domain Antibodies that Target Human Nav1.7 and Reduce Pain in Animal Models

Genetic studies have identified the voltage-gated sodium channel 1.7 (Nav1.7) as pain target. Due to the ineffectiveness of small molecules and monoclonal antibodies as therapeutics for pain, single-domain antibodies (VHHs) are developed against the human Nav1.7 (hNav1.7) using a novel antigen presentation strategy. A 70 amino-acid peptide from the hNav1.7 protein is identified as a target antigen. A recombinant version of this peptide is grafted into the complementarity determining region 3 (CDR3) loop of an inert VHH in order to maintain the native 3D conformation of the peptide. This antigen is used to isolate one VHH able to i) bind hNav1.7, ii) slow the deactivation of hNav1.7, iii) reduce the ability of eliciting action potentials in nociceptors, and iv) reverse hyperalgesia in in vivo rat and mouse models. This VHH exhibits the potential to be developed as a therapeutic capable of suppressing pain. This novel antigen presentation strategy can be applied to develop biologics against other difficult targets such as ion channels, transporters and GPCRs.

Intrathecal administration of MCRT produced potent antinociception in chronic inflammatory pain models via μ-δ heterodimer with limited side effects

An important goal in the opioid field is to discover effective analgesic drugs with minimal side effects. MCRT demonstrated potent antinociceptive effects with limited side effects, making it a promising candidate. However, its pharmacological properties and how it minimizes side effects remain unknown. Various mouse pain and opioid side effect models were used to evaluate the antinociceptive properties and safety at the spinal level. The targets of MCRT were identified through cAMP measurement, isolated tissue assays, and pharmacological experiments. Immunofluorescence was employed to visualize protein expression. MCRT displayed distinct antinociceptive effects between acute and chronic inflammatory pain models due to its multifunctional properties at the μ opioid receptor (MOR), µ-δ heterodimer (MDOR), and neuropeptide FF receptor 2 (NPFFR2). Activation of NPFFR2 reduced MOR-mediated antinociception, leading to bell-shaped response curves in acute pain models. However, activation of MDOR produced more effective antinociception in chronic inflammatory pain models. MCRT showed limited tolerance and opioid-induced hyperalgesia in both acute and chronic pain models and did not develop cross-tolerance to morphine. Additionally, MCRT did not exhibit addictive properties, gastrointestinal inhibition, and effects on motor coordination. Mechanistically, peripheral chronic inflammation or repeated administration of morphine and MCRT induced an increase in MDOR in the spinal cord. Chronic administration of MCRT had no apparent effect on microglial activation in the spinal cord. These findings suggest that MCRT is a versatile compound that provides potent antinociception with minimal opioid-related side effects. MDOR could be a promising target for managing chronic inflammatory pain and addressing the opioid crisis.

CT myelography by intrathecal injection of contrast medium though percutaneous administration route visualizes compressed cervical spinal cord in a mouse

BACKGROUND

Chronic compressive myelopathy (CCM) is a major cause of spinal cord disorders in the elderly, in which the spinal cord is compressed by bony or soft tissue structures. Although computed tomography myelography (CTM) has been clinically used for the diagnosis of CCM, a method of CTM in rodents remains to be developed.

NEW METHOD

A 50 μl Hamilton syringe attached to a disposable needle was percutaneously inserted into the subarachnoid space (cisterna magna) between the occipital bone and C1 lamina in an anesthetized adult mouse, followed by the injection of contrast medium and CT imaging.

RESULTS

CTM clearly visualized the shape of the spinal cord of intact mice and tiptoe-walking Yoshimura (Twy) mice without any health issues.

COMPARISON WITH EXISTING METHOD(S)

Unlike histology, the current method functions in live mice, directly depicts the compressed spinal cord, and provides clinically related image information. Furthermore, the intrathecal administration of contrast medium through the percutaneous route makes CTM less invasive and takes less time than a conventional intrathecal injection method.

CONCLUSIONS

The CTM method used in the present study enables clear visualization of the shape of the dural sac and spinal cord and is useful when conducting experiments on CCM and other spinal diseases in rodents.

BMP10 accelerated spinal astrocytic activation in neuropathic pain via ALK2/smad1/5/8 signaling

Background

Astrocytic activation in the spinal dorsal horn contributes to the central sensitization of neuropathic pain. Bone morphogenetic protein (BMP) 10, one of the BMPs highly expressed in the central nervous system, has been demonstrated to have an accelerated effect on astrocytic activation. This study aimed to investigate the functional effects of BMP10 on the activation of astrocytes in the spinal dorsal horn of animal model of neuropathic pain and to explore potential mechanisms involved in this process.

Methods

A neuropathic pain mice model was established using the spared nerve injury (SNI). Western blot analysis was performed to detect the expressional levels of BMP10, activin receptor-like receptor 2 (ALK2), Smad1/5/8, phosphorylated Smad1/5/8, and glial fibrillary acidic protein (GFAP). Immunofluorescence staining was used to detect BMP10, ALK2, and GFAP distribution and expression. The behavioral changes in mice were evaluated using paw withdrawal threshold (PWT), thermal withdrawal latency (TWL), and open field test (OFT). The BMP10 siRNA, Smad1 siRNA, BMP10 peptide, and ALK2-IN-2 (ALK2 inhibitor) were intrathecally administrated to mice. A model of lipopolysaccharide (LPS)-stimulated astrocytes was established to investigate the effect of Smad1. The transfection efficiency of siRNAs was detected by western blot and qRT-PCR analysis.

Results

BMP10 levels were increased in the L4-6 ipsilateral spinal dorsal horn of SNI mice and particularly elevated in astrocytes. Consistently, GFAP and phosphorylated Smad1/5/8 were upregulated in the L4-6 ipsilateral spinal dorsal horn after SNI, indicating the activation of astrocytes and Smad1/5/8 signaling. An intrathecal injection of BMP10 siRNA abrogated pain hypersensitivity and astrocytic activation in SNI mice. In addition, intrathecal administration of BMP10 peptide evoked pain hypersensitivity and astrocytic activation in normal mice, and this action was reversed by inhibiting the ALK2. Furthermore, targeting Smad1 in vitro with the help of siRNA inhibited the activation of astrocytes induced by LPS. Finally, targeting Smad1 abrogated BMP10-induced hypersensitivity and activation of astrocytes.

Conclusion

These findings indicate that the BMP10/ALK2/Smad1/5/8 axis plays a key role in pain hypersensitivity after peripheral nerve injury, which indicates its stimulative ability toward astrocytes.

Substance P-Botulinum Mediates Long-term Silencing of Pain Pathways that can be Re-instated with a Second Injection of the Construct in Mice

Chronic pain presents an enormous personal and economic burden and there is an urgent need for effective treatments. In a mouse model of chronic neuropathic pain, selective silencing of key neurons in spinal pain signalling networks with botulinum constructs resulted in a reduction of pain behaviours associated with the peripheral nerve. However, to establish clinical relevance it was important to know how long this silencing period lasted. Now, we show that neuronal silencing and the concomitant reduction of neuropathic mechanical and thermal hypersensitivity lasts for up to 120d following a single injection of botulinum construct. Crucially, we show that silencing and analgesia can then be reinstated with a second injection of the botulinum conjugate. Here we demonstrate that single doses of botulinum-toxin conjugates are a powerful new way of providing long-term neuronal silencing and pain relief. PERSPECTIVE: This research demonstrates that botulinum-toxin conjugates are a powerful new way of providing long-term neuronal silencing without toxicity following a single injection of the conjugate and have the potential for repeated dosing when silencing reverses.

Treponema pallidum-induced prostaglandin E2 secretion in skin fibroblasts leads to neuronal hyperpolarization: A cause of painless ulcers

BACKGROUND

Primary syphilis is characterized by painless ulcerative lesions in the genitalia, the aetiology of painless remains elusive.

OBJECTIVES

To investigate the role of Treponema pallidum in painless ulcer of primary syphilis, and the mechanisms underlying painless ulcers caused by T. pallidum.

METHODS

An experimental rabbit model of primary syphilis was established to investigate its effects on peripheral nerve tissues. Human skin fibroblasts were used to examine the role of T. pallidum in modulating neurotransmitters associated with pain and to explore the signalling pathways related to neurotransmitter secretion by T. pallidum in vitro.

RESULTS

Treponema pallidum infection did not directly lead to neuronal damage or interfere with the neuronal resting potential. Instead, it facilitated the secretion of prostaglandin E2 (PGE2) through endoplasmic reticulum stress in both rabbit and human skin fibroblasts, and upregulation of PGE2 induced the hyperpolarization of neurones. Moreover, the IRE1α/COX-2 signalling pathway was identified as the underlying mechanism by which T. pallidum induced the production of PGE2 in human skin fibroblasts.

CONCLUSION

Treponema pallidum promotes PGE2 secretion in skin fibroblasts, leading to the excitation of neuronal hyperpolarization and potentially contributing to the pathogenesis of painless ulcers in syphilis.

C-176 reduces inflammation-induced pain by blocking the cGAS-STING pathway in microglia

OBJECTIVE

Inflammatory pain, is caused by lesions or diseases of the somatosensory tissue, is a prevalent chronic condition that profoundly impacts the quality of life. However, clinical treatment for this type of pain remains limited. Traditionally, the stimulation of microglia and subsequent inflammatory reactions are considered crucial elements to promote the worsening of inflammatory pain. Recent research has shown the crucial importance of the cGAS-STING pathway in promoting inflammation. It is still uncertain if the cGAS-STING pathway plays the role in the fundamental cause of inflammatory pain. We aim to explore the treatment of inflammatory pain by interfering with cGAS-STING signaling pathway.

METHODS

In this study, we established an inflammatory pain model by CFA into the plantar of mice. Activation of microglia, various inflammatory factors and cGAS-STING protein in the spinal dorsal horn were evaluated. Immunofluorescence staining was used to observe the cellular localization of cGAS and STING. The cGAS-STING pathway proteins expression and mRNA expression of indicated microglial M1/M2 phenotypic markers in the BV2 microglia were detected. STING inhibitor C-176 was intrathecal injected into mice with inflammatory pain, and the pain behavior and microglia were observed.

RESULTS

This research showed that injecting CFA into the left hind paw of mice caused mechanical allodynia and increased inflammation in the spine. Our research results suggested that the cGAS-STING pathway had a function in the inflammation mediated by microglia in the spinal cord dorsal horn. Blocking the cGAS-STING pathway using STING antagonists (C-176) led to reduced release of inflammatory factors and prevented M1 polarization of BV2 microglia in a laboratory setting. Additionally, intrathecal administration of C-176 reduced the allodynia in CFA treated mice.

CONCLUSION

Our results suggest that inhibiting microglial polarization through the cGAS-STING pathway represents a potential novel therapeutic strategy for inflammatory pain.

Comparative dose effectiveness of intravenous and intrathecal AAV9.CB7.hIDS, RGX-121, in mucopolysaccharidosis type II mice

Mucopolysaccharidosis type II (MPS II) is an X-linked recessive lysosomal disease caused by iduronate-2-sulfatase (IDS) deficiency, leading to accumulation of glycosaminoglycans (GAGs) and the emergence of progressive disease. Enzyme replacement therapy is the only currently approved treatment, but it leaves neurological disease unaddressed. Cerebrospinal fluid (CSF)-directed administration of AAV9.CB7.hIDS (RGX-121) is an alternative treatment strategy, but it is unknown if this approach will affect both neurologic and systemic manifestations. We compared the effectiveness of intrathecal (i.t.) and intravenous (i.v.) routes of administration (ROAs) at a range of vector doses in a mouse model of MPS II. While lower doses were completely ineffective, a total dose of 1 × 109 gc resulted in appreciable IDS activity levels in plasma but not tissues. Total doses of 1 × 1010 and 1 × 1011 gc by either ROA resulted in supraphysiological plasma IDS activity, substantial IDS activity levels and GAG reduction in nearly all tissues, and normalized zygomatic arch diameter. In the brain, a dose of 1 × 1011 gc i.t. achieved the highest IDS activity levels and the greatest reduction in GAG content, and it prevented neurocognitive deficiency. We conclude that a dose of 1 × 1010 gc normalized metabolic and skeletal outcomes, while neurologic improvement required a dose of 1 × 1011 gc, thereby suggesting the prospect of a similar direct benefit in humans.

Inhibition of aquaporin-4 and its subcellular localization attenuates below-level central neuropathic pain by regulating astrocyte activation in a rat spinal cord injury model

The mechanisms of central neuropathic pain (CNP) caused by spinal cord injury have not been sufficiently studied. We have found that the upregulation of astrocytic aquaporin-4 (AQP4) aggravated peripheral neuropathic pain after spinal nerve ligation in rats. Using a T13 spinal cord hemisection model, we showed that spinal AQP4 was markedly upregulated after SCI and mainly expressed in astrocytes in the spinal dorsal horn (SDH). Inhibition of AQP4 with TGN020 suppressed astrocyte activation, attenuated the development and maintenance of below-level CNP and promoted motor function recovery in vivo. In primary astrocyte cultures, TGN020 also changed cell morphology, diminished cell proliferation and suppressed astrocyte activation. Moreover, T13 spinal cord hemisection induced cell-surface abundance of the AQP4 channel and perivascular localization in the SDH. Targeted inhibition of AQP4 subcellular localization with trifluoperazine effectively diminished astrocyte activation in vitro and further ablated astrocyte activation, attenuated the development and maintenance of below-level CNP, and accelerated functional recovery in vivo. Together, these results provide mechanistic insights into the roles of AQP4 in the development and maintenance of below-level CNP. Intervening with AQP4, including targeting AQP4 subcellular localization, might emerge as a promising agent to prevent chronic CNP after SCI.

Adeno-associated virus-mediated gene transfer of arginine decarboxylase to the central nervous system prevents opioid analgesic tolerance

Agmatine, a decarboxylated form of L-arginine, prevents opioid analgesic tolerance, dependence, and self-administration when given by both central and systemic routes of administration. Endogenous agmatine has been previously detected in the central nervous system. The presence of a biochemical pathway for agmatine synthesis offers the opportunity for site-specific overexpression of the presumptive synthetic enzyme for local therapeutic effects. In the present study, we evaluated the development of opioid analgesic tolerance in ICR-CD1 mice pre-treated with either vehicle control or intrathecally delivered adeno-associated viral vectors (AAV) carrying the gene for human arginine decarboxylase (hADC). Vehicle-treated or AAV-hADC-treated mice were each further divided into two groups which received repeated delivery over three days of either saline or systemically-delivered morphine intended to induce opioid analgesic tolerance. Morphine analgesic dose-response curves were constructed in all subjects on day four using the warm water tail flick assay as the dependent measure. We observed that pre-treatment with AAV-hADC prevented the development of analgesic tolerance to morphine. Peripheral and central nervous system tissues were collected and analyzed for presence of hADC mRNA. In a similar experiment, AAV-hADC pre-treatment prevented the development of analgesic tolerance to a high dose of the opioid neuropeptide endomorphin-2. Intrathecal delivery of anti-agmatine IgG (but not normal IgG) reversed the inhibition of endomorphin-2 analgesic tolerance in AAV-hADC-treated mice. To summarize, we report here the effects of AAV-mediated gene transfer of human ADC (hADC) in models of opioid-induced analgesic tolerance. This study suggests that gene therapy may contribute to reducing opioid analgesic tolerance.

Biodistribution of Radioactively Labeled Splice Modulating Antisense Oligonucleotides After Intracerebroventricular and Intrathecal Injection in Mice

Antisense oligonucleotides (AONs) are promising therapeutic candidates, especially for neurological diseases. Intracerebroventricular (ICV) injection is the predominant route of administration in mouse studies, while in clinical trials, intrathecal (IT) administration is mostly used. There is little knowledge on the differences in distribution of these injection methods within the same species over time. In this study, we compared the distribution of splice-switching AONs targeting exon 15 of amyloid precursor protein pre-mRNA injected via the ICV and IT route in mice. The AON was labeled with radioactive indium-111 and mice were imaged using single-photon emission computed tomography (SPECT) 0, 4, 24, 48, 72, and 96 h after injection. In vivo SPECT imaging showed 111In-AON activity diffused throughout the central nervous system (CNS) in the first hours after injection. The 111In-AON activity in the CNS persisted over the course of 4 days, while signal in the kidneys rapidly decreased. Postmortem counting in different organs and tissues showed very similar distribution of 111In-AON activity throughout the body, while the signal in the different brain regions was higher with ICV injection. Overall, IT and ICV injection have very similar distribution patterns in the mouse, but ICV injection is much more effective in reaching the brain.

miR-99b-3p/Mmp13 axis regulates NLRP3 inflammasome-dependent microglial pyroptosis and alleviates neuropathic pain via the promotion of autophagy

BACKGROUND

Neuropathic pain significantly impairs quality of life, and effective interventions are limited. NOD-like receptor thermal protein domain associated protein 3 (NLRP3)-mediated microglial pyroptosis and the subsequent proinflammatory cytokine production are critical in exacerbating pain. Considering microglial pyroptosis as a potential target for developing specific analgesic interventions for neuropathic pain, our study investigated the pathogenesis and therapeutic targets in this condition.

METHODS

In vitro experiments involved the co-culture of the immortalized BV-2 microglia cell line with lipopolysaccharide (LPS) to induce microglial pyroptosis. Differentially expressed microRNAs (miRNAs) were identified using high-throughput sequencing analysis. The downstream target genes of these miRNAs were determined through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, and the downstream target genes, combined with miRNAs, were predicted and verified through dual luciferase reporter gene assays. In vivo experiments were conducted to construct a chronic constriction injury (CCI) neuropathic pain model in rats and evaluate the analgesic effects of intrathecal injection of an adeno-associated virus vector (AAV) carrying miR-99b-3p. Gene expression was modulated through mimic or siRNA transfection. Western blot analysis assessed the expression of microglial pyroptosis and autophagy-related proteins, whereas RT-qPCR measured changes in proinflammatory cytokines expression.

RESULTS

LPS-stimulated up-regulation of proinflammatory cytokines in microglia, accompanied by NLRP3-dependent pyroptosis, including increased NLRP3, GSDMD-N, Caspase1-p20, and mature-IL-1β expression. High-throughput sequencing analysis revealed 16 upregulated and 10 downregulated miRNAs in LPS-stimulated microglia, with miR-99b-3p being the most downregulated. KEGG analysis revealed that the target genes of these miRNAs are primarily enriched in calcium, FoxO, and mitogen-activated protein kinase (MAPK) signal pathways. Furthermore, overexpression of miR-99b-3p through mimic transfection significantly inhibited the inflammatory response and NLRP3-mediated pyroptosis by promoting autophagy levels in activated microglia. In addition, we predicted that the 3' untranslated region (UTR) of matrix metalloproteinase-13 (Mmp13) could bind to miR-99b-3p, and knockdown of Mmp13 expression through siRNA transfection similarly ameliorated enhanced proinflammatory cytokines expression and microglial pyroptosis by enhancing autophagy. In vivo, Mmp13 was co-localized with spinal dorsal horn microglia and was suppressed by intrathecal injection of the AAV-miR-99b-3p vector. Moreover, overpressed miR-99b-3p alleviated CCI-induced mechanical allodynia and neuroinflammation while suppressing pyroptosis by enhancing autophagy in the spinal cord of CCI rats.

CONCLUSION

miR-99b-3p exerts analgesic effects on neuropathic pain by targeting Mmp13. These antinociceptive effects are, at least in part, attributed to the promotion of autophagy, thereby inhibiting neuroinflammation and NLRP3-mediated pyroptosis in activated microglia.

Direct inhibition of microglial activation by a μ receptor selective agonist alleviates inflammatory-induced pain hypersensitivity

Opioids are widely used in the treatment of moderate and severe pain. Nociceptive stimulation has been reported to potentially promote microglial activation and neuroinflammation, which also causes chronic pain sensitization. The aim of this study was to demonstrate whether the novel μ receptor agonist MEL-0614 could inhibit activated microglia directly and the associated signaling pathway. Mice were administered lipopolysaccharide and formalin to induce allodynia. Von Frey test was used to detect the anti-allodynia effect of MEL-0614 before and after LPS and formalin injection. In the spinal cord, the levels of proinflammatory cytokines and microglial activation were determined after MEL-0614 administration. BV2 and primary microglia were cultured to further explore the effect of MEL-0614 on LPS-induced microglial activation and key signaling pathways involved. MEL-0614 partially prevented and reversed allodynia induced by LPS and formalin in vivo, which was not inhibited by the μ receptor antagonist CTAP. Minocycline was effective in reversing the established allodynia. MEL-0614 also downregulated the activation of microglia and related proinflammatory cytokines in the spinal cord. Additionally, in BV2 and primary microglia, MEL-0614 inhibited the LPS-induced upregulation of proinflammatory factors, which was unaffected by CTAP. The NLR family pyrin domain containing 3 (NLRP3) related signaling pathway may be involved in the interaction between MEL-0614 and microglia. The opioid agonist MEL-0614 inhibited the activation of microglia and the subsequent upregulation of proinflammatory factors both in vivo and in vitro. Notably, this effect is partially mediated by the μ receptor.

A novel endomorphin-2/salmon calcitonin hybrid peptide with enhancing anti-allodynic and anti-anxiety effects

Pain, a worldwide problem with a high incidence and complex pathogenesis, has attracted the attention of pharmaceutical enterprises for the development of safer and more effective drugs. Extensive experimental and clinical evidence has demonstrated the analgesic effects of two endogenous peptides: endomorphin-2 (EM-2) and salmon calcitonin (sCT). However, EM-2 has limitations, such as poor ability to cross the blood-brain barrier (BBB) and little therapeutic effect in chronic pain due to rapid in vivo proteolysis. Herein, we propose the design of a novel hybrid peptide TEM2CT by combining EM-2, sCT16-21, and the cell-penetrating peptide HIV-1 trans-activator protein (TAT) with the aim of enhancing their analgesic effects. TEM2CT treatment attenuated nociceptive behavior in both acute and chronic pain mouse models, exhibiting increased anti-allodynic and anti-anxiety effects compared to sCT treatment. Furthermore, TEM2CT also regulated the excitability of pyramidal neurons in the anterior cingulate cortex (ACC) in spared nerve injury (SNI) model mice. The improved efficacy of this hybrid peptide provides a promising strategy for developing analgesic drugs.

Repurposing fluphenazine as an autophagy modulator for treating liver cancer

Hepatocellular carcinoma (HCC) is a common malignant tumor of the digestive system with a low early diagnosis rate. Owing to the side effects, tolerance, and patient contraindications of existing therapies, effective drug treatments for HCC remain a major clinical challenge. However, using approved or investigational drugs not initially intended for cancer therapy is a promising strategy for resolving this problem because their safety have been tested in clinic. Therefore, this study evaluated differentially expressed genes between liver cancer and normal tissues in a cohort of patients with HCC from The Cancer Genome Atlas and applied them to query a connectivity map to identify candidate anti-HCC drugs. As a result, fluphenazine was identified as a candidate for anti-HCC therapy in vitro and in vivo. Fluphenazine suppressed HCC cell proliferation and migration and induced cell cycle arrest and apoptosis, possibly owing to disrupted lysosomal function, blocking autophagy flux. Additionally, in vivo studies demonstrated that fluphenazine suppresses HCC subcutaneous xenografts growth without causing severe side effects. Strikingly, fluphenazine could be used as an analgesic to alleviate oxaliplatin-induced pain as well as pain related anxiety-like behavior. Therefore, fluphenazine could be a novel liver cancer treatment candidate.

KATP Channel Prodrugs Reduce Inflammatory and Neuropathic Hypersensitivity, Morphine-Induced Hypersensitivity, and Precipitated Withdrawal in Mice

Previous studies show ATP-sensitive potassium (KATP) channel openers can reduce hypersensitivity associated with chronic pain models in rodents, and reduce morphine tolerance. Many agonists of KATP channels are not soluble in physiologically relevant vehicles, requiring adaptation for clinical use. This study compared the antinociceptive activity of novel KATP channel targeting prodrugs, CKLP1, CKLP2, and CF3-CKLP. These prodrugs are activated by endogenous alkaline phosphatase enzymes present in the peripheral and central nervous systems. Analgesic capabilities of intrathecally injected prodrugs were tested in rodent models of spinal nerve ligation (SNL) and complete Freund's adjuvant (CFA) as models for neuropathic and inflammatory pain, respectively. CKLP1 and CKLP2 significantly increased mechanical paw withdrawal thresholds 1-2 hours after intrathecal administration in the SNL model, but all three prodrugs were able to attenuate hypersensitivity up to 7 days after CFA treatment. The reduction of opioid tolerance and opioid-induced hypersensitivity in mice treated chronically with morphine was significantly reduced in CKLP1 and CKLP2 treated animals. Prodrug cleavage was confirmed in mouse spinal cords using liquid chromatography. These studies may aid in the further development of KATP channel prodrugs for use in treatments of chronic pain, opioid tolerance, and withdrawal. SIGNIFICANCE STATEMENT: The cromakalim prodrugs, CKLP1, CKLP2, and CF3-CKLP1 reduced hypersensitivity in inflammatory and neuropathic pain models in male and female mice. CKLP1 and CKLP2 also reduced morphine-induced hypersensitivity in a mouse model of chronic morphine exposure. CKLP2 reduced jumping and rearing behaviors after naloxone-induced precipitated morphine withdrawal. Taken together, CKLP2 demonstrates the potential for development as a non-opioid analgesic drug.

Loss of ATP-sensitive channel expression and function decreases opioid sensitivity in a mouse model of type 2 diabetes

During diabetes, β-cell dysfunction due to loss of potassium channels sensitive to ATP, known as KATP channels occurs progressively over time contributing to hyperglycemia. KATP channels are additionally present in the central and peripheral nervous systems and are downstream targets of opioid receptor signaling. The aim of this study is to investigate if KATP channel expression or activity in the nervous system changes in diabetic mice and if morphine antinociception changes in mice fed a high fat diet (HFD) for 16 weeks compared to controls. Mechanical thresholds were also monitored before and after administration of glyburide or nateglinide, KATP channel antagonists, for four weeks. HFD mice have decreased antinociception to systemic morphine, which is exacerbated after systemic treatment with glyburide or nateglinide. HFD mice also have lower rotarod scores, decreased mobility in an open field test, and lower burrowing behavior compared to their control diet counterparts, which is unaffected by KATP channel antagonist delivery. Expression of KATP channel subunits, Kcnj11 (Kir6.2) and Abcc8 (SUR1), were decreased in the peripheral and central nervous system in HFD mice, which is significantly correlated with baseline paw withdrawal thresholds. Upregulation of SUR1 through an adenovirus delivered intrathecally increased morphine antinociception in HFD mice, whereas Kir6.2 upregulation improved morphine antinociception only marginally. Perspective: This article presents the potential link between KATP channel function and neuropathy during diabetes. There is a need for increased knowledge in how diabetes affects structural and molecular changes in the nervous system to lead to the progression of chronic pain and sensory issues.

Electroacupuncture alleviates postoperative pain through inhibiting neuroinflammation via stimulator of interferon genes/type-1 interferon pathway

OBJECTIVE

This work explores the impact of electroacupuncture (EA) on acute postoperative pain (APP) and the role of stimulator of interferon genes/type-1 interferon (STING/IFN-1) signaling pathway modulation in the analgesic effect of EA in APP rats.

METHODS

The APP rat model was initiated through abdominal surgery and the animals received two 30 min sessions of EA at bilateral ST36 (Zusanli) and SP6 (Sanyinjiao) acupoints. Mechanical, thermal and cold sensitivity tests were performed to measure the pain threshold, and electroencephalograms were recorded in the primary somatosensory cortex to identify the effects of EA treatment on APP. Western blotting and immunofluorescence were used to examine the expression and distribution of proteins in the STING/IFN-1 pathway as well as neuroinflammation. A STING inhibitor (C-176) was administered intrathecally to verify its role in EA.

RESULTS

APP rats displayed mechanical and thermal hypersensitivities compared to the control group (P < 0.05). APP significantly reduced the amplitude of θ, α and γ oscillations compared to their baseline values (P < 0.05). Interestingly, expression levels of proteins in the STING/IFN-1 pathway were downregulated after inducing APP (P < 0.05). Further, APP increased pro-inflammatory factors, including interleukin-6, tumor necrosis factor-α and inducible nitric oxide synthase, and downregulated anti-inflammatory factors, including interleukin-10 and arginase-1 (P < 0.05). EA effectively attenuated APP-induced painful hypersensitivities (P < 0.05) and restored the θ, α and γ power in APP rats (P < 0.05). Meanwhile, EA distinctly activated the STING/IFN-1 pathway and mitigated the neuroinflammatory response (P < 0.05). Furthermore, STING/IFN-1 was predominantly expressed in isolectin-B4- or calcitonin-gene-related-peptide-labeled dorsal root ganglion neurons and superficial laminae of the spinal dorsal horn. Inhibition of the STING/IFN-1 pathway by intrathecal injection of C-176 weakened the analgesic and anti-inflammatory effects of EA on APP (P < 0.05).

CONCLUSION

EA can generate robust analgesic and anti-inflammatory effects on APP, and these effects may be linked to activating the STING/IFN-1 pathway, suggesting that STING/IFN-1 may be a target for relieving APP. Please cite this article as: Ding YY, Xu F, Wang YF, Han LL, Huang SQ, Zhao S, Ma LL, Zhang TH, Zhao WJ, Chen XD. Electroacupuncture alleviates postoperative pain through inhibiting neuroinflammation via stimulator of interferon genes/type-1 interferon pathway. J Integr Med. 2023; 21(5): 496-508.

The Antinociceptive Effect of Sympathetic Block is Mediated by Transforming Growth Factor β in a Mouse Model of Radiculopathy

Although sympathetic blockade is clinically used to treat pain, the underlying mechanisms remain unclear. We developed a localized microsympathectomy (mSYMPX), by cutting the grey rami entering the spinal nerves near the rodent lumbar dorsal root ganglia (DRG). In a chemotherapy-induced peripheral neuropathy model, mSYMPX attenuated pain behaviors via DRG macrophages and the anti-inflammatory actions of transforming growth factor-β (TGF-β) and its receptor TGF-βR1. Here, we examined the role of TGF-β in sympathetic-mediated radiculopathy produced by local inflammation of the DRG (LID). Mice showed mechanical hypersensitivity and transcriptional and protein upregulation of TGF-β1 and TGF-βR1 three days after LID. Microsympathectomy prevented mechanical hypersensitivity and further upregulated Tgfb1 and Tgfbr1. Intrathecal delivery of TGF-β1 rapidly relieved the LID-induced mechanical hypersensitivity, and TGF-βR1 antagonists rapidly unmasked the mechanical hypersensitivity after LID+mSYMPX. In situ hybridization showed that Tgfb1 was largely expressed in DRG macrophages, and Tgfbr1 in neurons. We suggest that TGF-β signaling is a general underlying mechanism of local sympathetic blockade.

Analgesic Effects of Fisetin, Peimine, Astaxanthin, Artemisinin, Bardoxolone Methyl and 740 Y-P and Their Influence on Opioid Analgesia in a Mouse Model of Neuropathic Pain

Treatment of neuropathic pain remains a challenge for modern medicine due to the insufficiently understood molecular mechanisms of its development and maintenance. One of the most important cascades that modulate the nociceptive response is the family of mitogen-activated protein (MAP) kinases and phosphatidylinositol-3-kinase (PI3K), as well as nuclear factor erythroid 2-related factor 2 (Nrf2). The aim of this study was to determine the effect of nonselective modulators of MAP kinases-fisetin (ERK1/2 and NFκB inhibitor, PI3K activator), peimine (MAPK inhibitor), astaxanthin (MAPK inhibitor, Nrf2 activator) and artemisinin (MAPK inhibitor, NFκB activator), as well as bardoxolone methyl (selective activator of Nrf2) and 740 Y-P (selective activator of PI3K)-in mice with peripheral neuropathy and to compare their antinociceptive potency and examine their effect on analgesia induced by opioids. The study was performed using albino Swiss male mice that were exposed to chronic constriction injury of the sciatic nerve (CCI model). Tactile and thermal hypersensitivity was measured using von Frey and cold plate tests, respectively. Single doses of substances were administered intrathecally on day 7 after CCI. Among the tested substances, fisetin, peimine, and astaxanthin effectively diminished tactile and thermal hypersensitivity in mice after CCI, while artemisinin did not exhibit analgesic potency in this model of neuropathic pain. Additionally, both of the activators tested, bardoxolone methyl and 740 Y-P, also showed analgesic effects after intrathecal administration in mice exposed to CCI. In the case of astaxanthin and bardoxolone methyl, an increase in analgesia after combined administration with morphine, buprenorphine, and/or oxycodone was observed. Fisetin and peimine induced a similar effect on tactile hypersensitivity, where analgesia was enhanced after administration of morphine or oxycodone. In the case of 740 Y-P, the effects of combined administration with each opioid were observed only in the case of thermal hypersensitivity. The results of our research clearly indicate that substances that inhibit all three MAPKs provide pain relief and improve opioid effectiveness, especially if they additionally block NF-κB, such as peimine, inhibit NF-κB and activate PI3K, such as fisetin, or activate Nrf2, such as astaxanthin. In light of our research, Nrf2 activation appears to be particularly beneficial. The abovementioned substances bring promising results, and further research on them will broaden our knowledge regarding the mechanisms of neuropathy and perhaps contribute to the development of more effective therapy in the future.

High-frequency ultrasound-guided intrathecal injections in a young mouse model: Targeting the central nervous system in drug delivery

BACKGROUND

Intrathecal injections provide important access to the central nervous system for delivery of anesthetic, analgesic or chemotherapeutic drugs that do not otherwise cross the blood-brain barrier. The administration of drugs via this route in animal models is challenging due to an inability to visualize the small target space during injection. Successful drug delivery therefore requires expertise in indirectly assessing vertebral and spinal cord anatomy and gaining advanced procedural skills. These factors are especially compounded in small animals such as mice (the most common mammalian model) and in investigations modeling pediatric drug delivery, where the animal is even smaller.

NEW METHOD

To address these issues, we have developed a method in which high-frequency ultrasound imaging is used to visualize and target the lumbar intrathecal space for injections. The technique is demonstrated in mice as young as postnatal day 16. To evaluate the method, a gadolinium-based magnetic resonance imaging (MRI) contrast agent was injected intrathecally, and subsequent brain delivery was verified post-injection by MRI.

RESULTS

Successful intrathecal injections of the MRI contrast agent showed distribution to the brain. In this study, we achieved a targeting success rate of 80% in 20 animals.

COMPARISON WITH EXISTING METHODS AND CONCLUSION

We expect that the new method will be convenient for drug delivery to the central nervous system in rodent research and provide higher reliability than unguided approaches, an essential contribution that will enable intrathecal delivery in pediatric mouse models.

Effect of pharmacological modulation of the kynurenine pathway on pain-related behavior and opioid analgesia in a mouse model of neuropathic pain

Dysfunction of the central nervous system are accompanied by changes in tryptophan metabolism, with the kynurenine pathway (KP) being the main route of its catabolism. Recently, KP metabolites, which are collectively called kynurenines, have become an area of intense research due to their ability to directly and indirectly affect a variety of classic neurotransmitter systems. However, the significance of KP in neuropathic pain is still poorly understood. Therefore, we designed several experiments to verify changes in the mRNA levels of KP enzymes in parallel with other factors related to this metabolic route after chronic constriction injury of the sciatic nerve (CCI model) in mice. The analysis revealed an increase in, Kmo, Kynu and Haoo mRNA levels in the spinal cord on the 7th day after CCI, while Kat1, Kat2, Tdo2, Ido2 and Qprt mRNA levels remain unchanged. Subsequent pharmacological studies provided evidence that modulation of KP by single intrathecal administration of 1-D-MT, UPF468 or L-kynurenine attenuates mechanical and thermal hypersensitivity and increases the effectiveness of selected opioids in mice as measured on day 7 after CCI. Moreover, our results provide the first evidence that the injection of L-kynurenine preceded by UPF468 (KMO inhibitor) is more effective at reducing hypersensitivity in animals with neuropathic pain. Importantly, L-kynurenine also exerts an analgesic effect after intravenous injections, which is enhanced by the administration of minocycline, an inhibitor of microglial activation. Additionally, L-kynurenine administered intrathecally and intravenously enhances analgesia evoked by all tested opioids (morphine, buprenorphine and oxycodone). Overall, our results indicate that the modulation of KP at different levels might be a new pharmacological tool in neuropathy management.

Mesenchymal stem cell-derived extracellular vesicles carrying miR-99b-3p restrain microglial activation and neuropathic pain by stimulating autophagy

Neuropathic pain is a complex condition that seriously affects human quality of life. This study aimed to investigate the therapeutic mechanism of mesenchymal stem cell-derived extracellular vesicles (MSC-EVs) and try to discover new targets for alleviating neuropathic pain. Extracellular vesicles were isolated and identified via ultracentrifugation. BV-2 microglial cells were stimulated with lipopolysaccharide (LPS) in the presence or absence of MSC-EVs. Further, microglial activation and neuroinflammation were evaluated by flow cytometry, RT-qPCR, and ELISA. High-throughput sequencing analysis was performed to reveal the differentially expressed (DE) miRNAs in BV-2 microglia. Autophagy-related regulators were assessed by Western blotting and Immunofluorescence staining. Chronic constriction injury (CCI) model was used to induce neuropathic pain in rats, and the mechanical withdrawal threshold (MWT) was measured. High-throughput sequencing analysis identified 17 DE miRNAs, which were mainly enriched in PI3K-AKT and mTOR signaling pathways. MSC-EVs inhibited the activation of PI3K/AKT/mTOR signaling pathway in LPS-stimulated microglia. Moreover, MSC-EVs treatment enhanced the autophagy level in activated microglia, whereas autophagy inhibitor 3-MA reversed the suppressing effects of MSC-EVs on microglial activation and neuroinflammation. The MSC-EV-mediated transfer of miR-99b-3p was verified to promote microglial autophagy, and miR-99b-3p overexpression suppressed the expression of pro-inflammatory factors in activated microglia. During in vivo studies, intrathecal injection of MSC-EVs significantly up-regulated the expression of miR-99b-3p, and alleviated mechanical allodynia caused by activated microglia in the spinal cord dorsal horn of CCI rats. Moreover, MSC-EVs treatment repaired CCI-induced autophagic impairment by stimulating autophagy in the spinal cord. Collectively, our findings demonstrated that MSC-EVs had an analgesic effect on neuropathic pain via promoting autophagy, and these antinociceptive effects were at least in part caused by MSC-EV-mediated transfer of miR-99b-3p, thereby inhibiting microglial activation and pro-inflammatory cytokines expression.

Huc-MSCs-derived exosomes attenuate neuropathic pain by inhibiting activation of the TLR2/MyD88/NF-κB signaling pathway in the spinal microglia by targeting Rsad2

BACKGROUND

Mesenchymal stem cells (MSCs)-derived exosomes have shown promise as a cell-free therapeutic strategy for neuropathic pain. This study was conducted to explore the potential mechanisms underlying the analgesic effects of MSC-derived exosomes in treating neuropathic pain.

METHODS

Human umbilical cord MSCs (huc-MSCs)-derived exosomes were isolated and identified. BV-2 microglia were stimulated with lipopolysaccharide (LPS) in the presence or absence of exosomes. Differentially expressed proteins were identified by tandem mass tag (TMT)-based proteomic analysis. The analgesic effects of huc-MSCs-derived exosomes were evaluated in a rat model of chronic constriction injury (CCI). The underlying mechanism was investigated by flow cytometry, RT-qPCR, Western blotting, immunofluorescent staining, and small interfering RNA transfection.

RESULTS

In vitro, huc-MSCs-derived exosomes suppressed LPS-induced microglial activation and inhibited activation of the TLR2/MyD88/NF-κB signaling pathway. Based on the proteomic analysis, Rsad2 was identified and confirmed to be down-regulated by huc-MSCs-derived exosomes. Importantly, knockdown of Rsad2 also inhibited microglial activation and restrained activation of the TLR2/MyD88/NF-κB signaling pathway. In vivo, intrathecal injection of exosomes ameliorated CCI-induced mechanical allodynia, down-regulated Rsad2 expression and restrained TLR2/MyD88/NF-κB signaling activation in the spinal microglia.

CONCLUSION

Huc-MSCs-derived exosomes exerted analgesic effects on neuropathic pain by inhibiting activation of the TLR2/MyD88/NF-κB signaling pathway in the spinal microglia. The mechanism underlying these antinociceptive effects involved exosome-mediated interference with Rsad2 expression, thereby inhibiting microglial activation.

Exploring pharmacological inhibition of Gq/11 as an analgesic strategy

BACKGROUND AND PURPOSE

Misuse of opioids has greatly affected our society. One potential solution is to develop analgesics that act at targets other than opioid receptors. These can be used either as stand-alone therapeutics or to improve the safety profile of opioid drugs. Previous research showed that activation of G_q/11 proteins by G-protein coupled receptors has pro-nociceptive properties, suggesting that blockade of G_q/11 signalling could be beneficial for pain control. The aim of this study was to test this hypothesis pharmacologically by using potent and selective G_q/11 inhibitor YM-254890.

EXPERIMENTAL APPROACH

We used a series of behavioural assays to evaluate the acute responses of mice to painful thermal stimulation while administering YM-254890 alone and in combination with morphine. We then used electrophysiological recordings to evaluate the effects of YM-254890 on the excitability of dorsal root ganglion (DRG) nociceptor neurons.

KEY RESULTS

We found that systemic administration of YM-254890 produced anti-nociceptive effects and also augmented morphine analgesia in both hotplate and tail flick paradigms. However, it also caused substantial inhibition of locomotion, which may limit its therapeutic utility. To circumvent these issues, we explored the local administration of YM-254890. Intrathecal injections of YM-254890 produced lasting analgesia in a tail flick test and greatly augmented the anti-nociceptive effects of morphine without any significant effects on locomotor behaviour. Electrophysiological studies showed that YM-254890 reduced the excitability of DRG nociceptors and augmented their opioid-induced inhibition.

CONCLUSION AND IMPLICATIONS

These findings indicate that pharmacological inhibition of G_q/11 could be explored as an analgesic strategy.

STING/NF-κB/IL-6-Mediated Inflammation in Microglia Contributes to Spared Nerve Injury (SNI)-Induced Pain Initiation

Innate immune response acts as the first line of host defense against damage and is initiated following the recognition of pathogen-associated molecular patterns (PAMPs). For double-stranded DNA (dsDNA) sensing, interferon gene stimulator (STING) was discovered to be an integral sensor and could mediate the immune and inflammatory response. Selective STING antagonist C-176 was administered and pain behaviors were assessed following spared nerve injury (SNI)-induced neuropathic pain. The level of serum dsDNA following neuropathic pain was assessed using Elisa analysis. STING signaling pathway, microglia activation, and proinflammatory cytokines were assessed by qPCR, western blots, Elisa, and immunofluorescence staining. STING agonist DMXAA was introduced into BV-2 cells to assess the inflammatory response in microglial cells. dsDNA was significantly increased following SNI and STING/TANK-binding kinase 1 (TBK1)/nuclear factor-kappa B (NF-κB) pathway was activated in vivo and vitro. Early but not the late intrathecal injection of C-176 attenuated SNI-induced pain hypersensitivity, microglia activation, proinflammatory factors, and phosphorylated JAK2/STAT3 in the spinal cord dorsal horn, and the analgesic effect of C-176 was greatly abolished by recombinant IL-6 following SNI. We provided evidence clarifying dsDNA mediated activation of microglia STING signaling pathway, after which promoting expression of proinflammatory cytokines that are required for hyperalgesia initiation in the spinal cord dorsal horn of SNI model. Further analysis showed that microglial STING/TBK1/NF-κB may contribute to pain initiation via IL-6 signaling. Pharmacological blockade of STING may be a promising target in the treatment of initiation of neuropathic pain.

Reduced activity of adenylyl cyclase 1 attenuates morphine induced hyperalgesia and inflammatory pain in mice

Opioid tolerance, opioid-induced hyperalgesia during repeated opioid administration, and chronic pain are associated with upregulation of adenylyl cyclase activity. The objective of this study was to test the hypothesis that a reduction in adenylyl cyclase 1 (AC1) activity or expression would attenuate morphine tolerance and hypersensitivity, and inflammatory pain using murine models. To investigate opioid tolerance and opioid-induced hyperalgesia, mice were subjected to twice daily treatments of saline or morphine using either a static (15 mg/kg, 5 days) or an escalating tolerance paradigm (10–40 mg/kg, 4 days). Systemic treatment with an AC1 inhibitor, ST03437 (2.5–10 mg/kg, IP), reduced morphine-induced hyperalgesia in mice. Lumbar intrathecal administration of a viral vector incorporating a short-hairpin RNA targeting Adcy1 reduced morphine-induced hypersensitivity compared to control mice. In contrast, acute morphine antinociception, along with thermal paw withdrawal latencies, motor performance, exploration in an open field test, and burrowing behaviors were not affected by intrathecal Adcy1 knockdown. Knockdown of Adcy1 by intrathecal injection also decreased inflammatory mechanical hyperalgesia and increased burrowing and nesting activity after intraplantar administration of Complete Freund’s Adjuvant (CFA) one-week post-injection.

Targeting the somatosensory system with AAV9 and AAV2retro viral vectors

Adeno-associated viral (AAV) vectors allow for site-specific and time-dependent genetic manipulation of neurons. However, for successful implementation of AAV vectors, major consideration must be given to the selection of viral serotype and route of delivery for efficient gene transfer into the cell type being investigated. Here we compare the transduction pattern of neurons in the somatosensory system following injection of AAV9 or AAV2retro in the parabrachial complex of the midbrain, the spinal cord dorsal horn, the intrathecal space, and the colon. Transduction was evaluated based on Cre-dependent expression of tdTomato in transgenic reporter mice, following delivery of AAV9 or AAV2retro carrying identical constructs that drive the expression of Cre/GFP. The pattern of distribution of tdTomato expression indicated notable differences in the access of the two AAV serotypes to primary afferent neurons via peripheral delivery in the colon and to spinal projections neurons via intracranial delivery within the parabrachial complex. Additionally, our results highlight the superior sensitivity of detection of neuronal transduction based on reporter expression relative to expression of viral products.

Akt/Aquaporin-4 Signaling Aggravates Neuropathic Pain by Activating Astrocytes after Spinal Nerve Ligation in Rats

Aquaporins (AQPs) play critical physiological roles in water balance in the central nervous system (CNS). Aquaporin-4 (AQP4), the principal aquaporin expressed in the CNS, has been implicated in the processing of sensory and pain transmission. Akt signaling is also involved in pain mediation, such as neuroinflammatory pain and bone cancer pain. Previously, we found that expression of AQP4 and p-Akt was altered in the rat spinal cord after spinal nerve ligation (SNL). Here, we further investigated the effects of the AQP4 and Akt pathways in the spinal dorsal horn (SDH) on the pathogenesis of neuropathic pain (NP). Spinal AQP4 was significantly upregulated after SNL and was primarily expressed in astrocytes in the SDH. Inhibition of AQP4 with TGN-020 attenuated the development and maintenance of NP by inhibiting glial activation and anti-neuroinflammatory mechanisms. Moreover, inhibition of AQP4 suppressed astrocyte activation both in the SDH and in primary cultures. Similar to AQP4, we found that p-Akt was also significantly elevated after SNL. Inhibition of Akt with MK2206 suppressed AQP4 upregulation and astrocyte activation both in vivo and in vitro. Furthermore, Akt blockade with MK2206 alleviated NP in the early and late phases after SNL. These results elucidate the mechanisms involved in the roles of Akt/AQP4 signaling in the development and maintenance of NP. AQP4 is likely to be a novel therapeutic target for NP management.

Comparative Effectiveness of Intracerebroventricular, Intrathecal, and Intranasal Routes of AAV9 Vector Administration for Genetic Therapy of Neurologic Disease in Murine Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I (MPS I) is an inherited metabolic disorder caused by deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). The two current treatments [hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT)], are insufficiently effective in addressing neurologic disease, in part due to the inability of lysosomal enzyme to cross the blood brain barrier. With a goal to more effectively treat neurologic disease, we have investigated the effectiveness of AAV-mediated IDUA gene delivery to the brain using several different routes of administration. Animals were treated by either direct intracerebroventricular (ICV) injection, by intrathecal (IT) infusion into the cerebrospinal fluid, or by intranasal (IN) instillation of AAV9-IDUA vector. AAV9-IDUA was administered to IDUA-deficient mice that were either immunosuppressed with cyclophosphamide (CP), or immunotolerized at birth by weekly injections of human iduronidase. In animals treated by ICV or IT administration, levels of IDUA enzyme ranged from 3- to 1000-fold that of wild type levels in all parts of the microdissected brain. In animals administered vector intranasally, enzyme levels were 100-fold that of wild type in the olfactory bulb, but enzyme expression was close to wild type levels in other parts of the brain. Glycosaminoglycan levels were reduced to normal in ICV and IT treated mice, and in IN treated mice they were normalized in the olfactory bulb, or reduced in other parts of the brain. Immunohistochemical analysis showed extensive IDUA expression in all parts of the brain of ICV treated mice, while IT treated animals showed transduction that was primarily restricted to the hind brain with some sporadic labeling seen in the mid- and fore brain. At 6 months of age, animals were tested for spatial navigation, memory, and neurocognitive function in the Barnes maze; all treated animals were indistinguishable from normal heterozygous control animals, while untreated IDUA deficient animals exhibited significant learning and spatial navigation deficits. We conclude that IT and IN routes are acceptable and alternate routes of administration, respectively, of AAV vector delivery to the brain with effective IDUA expression, while all three routes of administration prevent the emergence of neurocognitive deficiency in a mouse MPS I model.

The µ-δ opioid heteromer masks latent pain sensitization in neuropathic and inflammatory pain in male and female mice

The episodic nature of chronic pain can be studied in the rodent model of latent pain sensitization. After remission, central sensitization is opposed by activation of opioid receptors. At the behavioral level, latent pain sensitization is unmasked when pain hypersensitivity is reinstated by opioid receptor (OR) antagonism. Previous studies have focused on inflammatory pain and male rodents. Whether latent pain sensitization occurs in models of chemotherapy-induced neuropathic pain in female and male mice is unknown. The first aim of this study was to investigate whether μ- and δ-OR suppress latent pain sensitization in our model of chemotherapy-induced neuropathic pain in both sexes. Mounting evidence suggests that μ-and δ-ORs form a heteromer and that the heteromer modulates pain sensitivity. Potential implications of the μ-δ OR heteromer in latent pain sensitization have not been fully explored due to a lack of tools to effectively modulate the heteromer. To specifically target the μ-δ OR heteromer, we used a specific interfering peptide blocking the heteromerization. The second aim of this study was to investigate whether disruption of the μ-δOR heteromer, after remission, reinstates pain hypersensitivity. After remission from cisplatin-induced neuropathic pain, antagonism of µ-OR and δOR reinstates pain hypersensitivity in both sexes. After remission from cisplatin-induced neuropathic pain and postoperative pain, disruption of the μ-δOR heteromer reinstates pain hypersensitivity in both sexes. Taken together our findings suggest that the μ-δOR heteromer plays a crucial role in remission in various pain models and may represent a novel therapeutic target to prevent the relapse to pain and the transition to chronic pain.

Metabolically stable neurotensin analogs exert potent and long-acting analgesia without hypothermia

The endogenous tridecapeptide neurotensin (NT) has emerged as an important inhibitory modulator of pain transmission, exerting its analgesic action through the activation of the G protein-coupled receptors, NTS1 and NTS2. Whereas both NT receptors mediate the analgesic effects of NT, NTS1 activation also produces hypotension and hypothermia, which may represent obstacles for the development of new pain medications. In the present study, we implemented various chemical strategies to improve the metabolic stability of the biologically active fragment NT(8-13) and assessed their NTS1/NTS2 relative binding affinities. We then determined their ability to reduce the nociceptive behaviors in acute, tonic, and chronic pain models and to modulate blood pressure and body temperature. To this end, we synthesized a series of NT(8-13) analogs carrying a reduced amide bond at Lys⁸-Lys⁹ and harboring site-selective modifications with unnatural amino acids, such as silaproline (Sip) and trimethylsilylalanine (TMSAla). Incorporation of Sip and TMSAla respectively in positions 10 and 13 of NT(8-13) combined with the Lys⁸-Lys⁹ reduced amine bond (JMV5296) greatly prolonged the plasma half-life time over 20 h. These modifications also led to a 25-fold peptide selectivity toward NTS2. More importantly, central delivery of JMV5296 was able to induce a strong antinociceptive effect in acute (tail-flick), tonic (formalin), and chronic inflammatory (CFA) pain models without inducing hypothermia. Altogether, these results demonstrate that the chemically-modified NT(8-13) analog JMV5296 exhibits a better therapeutic profile and may thus represent a promising avenue to guide the development of new stable NT agonists and improve pain management.

The spinal anti-allodynic effects of endomorphin analogs with C-terminal hydrazide modification in neuropathic pain model

The present study was undertaken to further investigate the spinal anti-allodynic effects of endomorphins (EMs) and their C-terminal hydrazide modified analogs EM-1-NHNH₂ and EM-2-NHNH₂ in the spared nerve injury (SNI) model of neuropathic pain in mice. Our results demonstrated that intrathecal (i.t.) administration of endomorphin-1 (EM-1), endomorphin-2 (EM-2), EM-1-NHNH₂ and EM-2-NHNH₂ produced potent anti-allodynic effects ipsilaterally in neuropathic pain model. Judging from the area under the curve (AUC) values, these two analogs exhibited higher antinociception than their parent peptides. Moreover, they also displayed significant antinociceptive effects in the contralateral paw administered intrathecally. Interestingly, EM-1 and its analog EM-1-NHNH₂ displayed their antinociception probably by μ₂-opioid receptor subtype since the μ₁-opioid receptor antagonist naloxonazine didn't significantly block the anti-allodynia of EM-1 and EM-1-NHNH₂, which implied a same opioid mechanism. However, the anti-allodynia induced by EM-2, but not EM-2-NHNH₂ was significantly reduced by both μ₁-opioid antagonist, naloxonazine and κ-antagonist, nor-binaltorphamine (nor-BNI), indicating multiple opioid receptors were involved in the anti-allodynic effects of EM-2. Most importantly, EM-1-NHNH₂ decreased the antinociceptive tolerance, and EM-2-NHNH₂ displayed non-tolerance-forming antinociception. Therefore, C-terminal amide to hydrazide conversion changed the spinal antinociceptive profiles of EMs in neuropathic pain. The present investigation is of great value in the development of novel opioid therapeutics against neuropathic pain.

Novel hybrid compounds, opioid agonist+melanocortin 4 receptor antagonist, as efficient analgesics in mouse chronic constriction injury model of neuropathic pain

When the nerve tissue is injured, endogenous agonist of melanocortin type 4 (MC4) receptor, α-MSH, exerts tonic pronociceptive action in the central nervous system, contributing to sustaining the neuropathic pain state and counteracting the analgesic effects of exogenous opioids. With the intent of enhancing opioid analgesia in neuropathy by blocking the MC4 activation, so-called parent compounds (opioid agonist, MC4 antagonist) were joined together using various linkers to create novel bifunctional hybrid compounds. Analgesic action of four hybrids was tested after intrathecal (i.t.) administration in mouse models of acute and neuropathic pain (chronic constriction injury model, CCI). Under nerve injury conditions, one of the hybrids, UW3, induced analgesia in 1500 times lower i.t. dose than the opioid parent (ED50: 0.0002 nmol for the hybrid, 0.3 nmol for the opioid parent) and in an over 16000 times lower dose than the MC4 parent (ED50: 3.33 nmol) as measured by the von Frey test. Two selected hybrids were tested for analgesic properties in CCI mice after intravenous (i.v.) and intraperitoneal (i.p.) administration. Opioid receptor antagonists and MC4 receptor agonists diminished the analgesic action of these two hybrids studied, though the extent of this effect differed between the hybrids; this suggests that linker is of key importance here. Further results indicate a significant advantage of hybrid compounds over the physical mixture of individual pharmacophores in their analgesic effect. All this evidence justifies the idea of synthesizing a bifunctional opioid agonist-linker-MC4 antagonist compound, as such structure may bring important benefits in neuropathic pain treatment.

Sensory satellite glial Gq-GPCR activation alleviates inflammatory pain via peripheral adenosine 1 receptor activation

Glial fibrillary acidic protein expressing (GFAP+) glia modulate nociceptive neuronal activity in both the peripheral nervous system (PNS) and the central nervous system (CNS). Resident GFAP+ glia in dorsal root ganglia (DRG) known as satellite glial cells (SGCs) potentiate neuronal activity by releasing pro-inflammatory cytokines and neuroactive compounds. In this study, we tested the hypothesis that SGC Gq-coupled receptor (Gq-GPCR) signaling modulates pain sensitivity in vivo using Gfap-hM3Dq mice. Complete Freund's adjuvant (CFA) was used to induce inflammatory pain, and mechanical sensitivity and thermal sensitivity were used to assess the neuromodulatory effect of glial Gq-GPCR activation in awake mice. Pharmacogenetic activation of Gq-GPCR signaling in sensory SGCs decreased heat-induced nociceptive responses and reversed inflammation-induced mechanical allodynia via peripheral adenosine A1 receptor activation. These data reveal a previously unexplored role of sensory SGCs in decreasing afferent excitability. The identified molecular mechanism underlying the analgesic role of SGCs offers new approaches for reversing peripheral nociceptive sensitization.

Blockade of CCR4 Diminishes Hypersensitivity and Enhances Opioid Analgesia - Evidence from a Mouse Model of Diabetic Neuropathy

Chemokine signaling has been implicated in the pathogenesis of diabetic neuropathy; however, the role of chemokine CC motif receptor 4 (CCR4) remains unknown. The goal was to examine the function of CCR4 in hypersensitivity development and opioid effectiveness in diabetic neuropathy. Streptozotocin (STZ; 200 mg/kg, intraperitoneally administered)-induced mouse model of diabetic neuropathy were used. An analysis of the mRNA/protein expression of CCR4 and its ligands was performed by qRT-PCR, microarray and/or Western blot methods. C021 (CCR4 antagonist), morphine and buprenorphine were injected intrathecally or intraperitoneally, and pain-related behavior was evaluated by the von Frey, cold plate and rotarod tests. We observed that on day 7 after STZ administration, the blood glucose level was increased, and as a consequence, hypersensitivity to tactile and thermal stimuli developed. In addition, we observed an increase in the mRNA level of CCL2 but not CCL17/CCL22. The microarray technique showed that the CCL2 protein level was also upregulated. In naive mice, the pronociceptive effect of intrathecally injected CCL2 was blocked by C021, suggesting that this chemokine acts through CCR4. Importantly, our results provide the first evidence that in a mouse model of diabetic neuropathy, single intrathecal and intraperitoneal injections of C021 diminished neuropathic pain-related behavior in a dose-dependent manner and improved motor functions. Moreover, both single intrathecal and intraperitoneal injections of C021 enhanced morphine and buprenorphine effectiveness. These results reveal that pharmacological modulation of CCR4 may be a good potential therapeutic target for the treatment of diabetic neuropathy and may enhance the effectiveness of opioids.

CCR4 Antagonist (C021) Administration Diminishes Hypersensitivity and Enhances the Analgesic Potency of Morphine and Buprenorphine in a Mouse Model of Neuropathic Pain

Neuropathic pain is a chronic condition that remains a major clinical problem owing to high resistance to available therapy. Recent studies have indicated that chemokine signaling pathways are crucial in the development of painful neuropathy; however, the involvement of CC chemokine receptor 4 (CCR4) has not been fully elucidated thus far. Therefore, the aim of our research was to investigate the role of CCR4 in the development of tactile and thermal hypersensitivity, the effectiveness of morphine/buprenorphine, and opioid-induced tolerance in mice exposed to chronic constriction injury (CCI) of the sciatic nerve. The results of our research demonstrated that a single intrathecal or intraperitoneal administration of C021, a CCR4 antagonist, dose dependently diminished neuropathic pain-related behaviors in CCI-exposed mice. After sciatic nerve injury, the spinal expression of CCL17 and CCL22 remained unchanged in contrast to that of CCL2, which was significantly upregulated until day 14 after CCI. Importantly, our results provide evidence that in naive mice, CCL2 may evoke pain-related behaviors through CCR4 because its pronociceptive effects are diminished by C021. In CCI-exposed mice, the pharmacological blockade of CCR4 enhanced the analgesic properties of morphine/buprenorphine and delayed the development of morphine-induced tolerance, which was associated with the silencing of IBA-1 activation in cells and decrease in CCL2 production. The obtained data suggest that the pharmacological blockade of CCR4 may be a new potential therapeutic target for neuropathic pain polytherapy.

Cervical Epidural and Subarachnoid Catheter Placement in Standing Adult Horses

Horses underwent either cervical epidural space (CES) catheterization or subarachnoid space (SAS) catheterization while restrained in stocks, under deep sedation (detomidine and morphine) and local anesthesia (mepivacaine 2%) block. Catheters were placed under ultrasound guidance with visualization of the dura, SAS, and spinal cord between the first (C1) and second (C2) cervical vertebrae. Following sedation and sterile skin preparation, operator 1 placed under ultrasound guidance, a 6- or 8-inch Tuohy needle with the bevel oriented caudally. For CES, a 6-inch Touhy needle was used with the hanging drop technique to detect negative pressure, and operator 2 then passed the epidural catheter into the CES. For SAS, following puncture of the dura, cerebrospinal fluid (CSF) was aspirated prior to placement of the epidural catheter. Placement into either CES or SAS was confirmed with plain and contrast radiography. Catheters were wrapped for the duration of the study. CSF cytology was assessed up to every 24 h for the study period. Horses were assessed daily for signs of discomfort, neck pain, catheter insertion site swelling, or changes in behavior. A complete postmortem assessment of the spinal tissues was performed at the end of the study period (72 h). Two horses had CES catheters and five horses had SAS catheters placed successfully. All horses tolerated the catheter well for the duration of the study with no signs of discomfort. Ultrasound was essential to assist placement, and radiography confirmed the anatomical location of the catheters. CSF parameters did not change over the study period (P > 0.9). There was evidence of mild meningeal acute inflammation in one horse and hemorrhage in another consistent with mechanical trauma. Placement of an indwelling CES or SAS catheter appears to be safe, technically simple, and well tolerated in standing sedated normal horses.

CCR4 antagonist (C021) influences the level of nociceptive factors and enhances the analgesic potency of morphine in a rat model of neuropathic pain

Neuropathic pain is a chronic condition which significantly reduces the quality of life and serious clinical issue that is in general resistant to available therapies. Therefore looking for new analgesics is still critical issue. Recent, studies have indicated that chemokine signaling pathways are crucial for the development of neuropathy; however, the role of CC chemokine receptor 4 (CCR4) in this process has not yet been studied. Therefore, the aim of our research was to investigate the influence of C021 (a CCR4 antagonist) and CCR4 CC chemokine ligands 17 and 22 (CCL17 and CCL22) on the development of hypersensitivity and the effectiveness of morphine induced analgesia in naive animals and/or animals exposed to chronic constriction injury (CCI) of the sciatic nerve. Firstly, we demonstrated that the intrathecal administration of CCL17 and CCL22 induced pain-related behavior in naive mice. Secondly, we revealed that the intrathecal injection of C021 significantly reduced CCI-induced hypersensitivity after nerve injury. In parallel, C021 reduced microglia/macrophages activation and the level of some pronociceptive interleukins (IL-1beta; IL-18) in the spinal cord 8 days after CCI. Moreover, C021 not only attenuated mechanical and thermal hypersensitivity but also enhanced the analgesic properties of morphine. Our research indicates that CCR4 ligands might be important factors in the early stages of neuropathy, when we observe intense microglia/macrophages activation. Moreover, pharmacological blockade of CCR4 may serve as a potential new target for better understanding the mechanisms of neuropathic pain development.

Immunomodulatory and anti-oxidative effect of the direct TRPV1 receptor agonist capsaicin on Schwann cells

Background

Only few studies describe the impact of nutritive factors on chronic inflammatory demyelinating polyneuropathy (CIDP), an inflammatory disease of the peripheral nervous system. The active component of chili pepper, capsaicin, is the direct agonist of the transient receptor potential channel vanilloid subfamily member 1. Its anti-inflammatory effect in the animal model experimental autoimmune neuritis (EAN) has been previously demonstrated.

Methods

In the present study, we describe the anti-inflammatory and anti-oxidative influence of capsaicin on Schwann cells (SCs) in an in vitro setting. Hereby, we analyze the effect of capsaicin on Schwann cells’ gene expression pattern, major histocompatibility complex class II (MHC-II) presentation, and H₂O₂-induced oxidative stress. Furthermore, the effect of capsaicin on myelination was examined in a SC-dorsal root ganglia (DRG) coculture by myelin basic protein staining. Finally, in order to investigate the isolated effect of capsaicin on SCs in EAN pathology, we transplant naïve and capsaicin pre-treated SCs intrathecally in EAN immunized rats and analyzed clinical presentation, electrophysiological parameters, and cytokine expression in the sciatic nerve.

Results

In SC monoculture, incubation with capsaicin significantly reduces interferon gamma-induced MHC-II production as well as toll-like receptor 4 and intercellular adhesion molecule 1 mRNA expression. Calcitonin gene-related peptide mRNA production is significantly upregulated after capsaicin treatment. Capsaicin reduces H₂O₂-induced oxidative stress in SC in a preventive, but not therapeutic setting. In a SC-DRG coculture, capsaicin does not affect myelination rate. After intrathecal transplantation of naïve and capsaicin pre-treated SCs in EAN-immunized rats, naïve, but not capsaicin pre-treated intrathecal SCs, ameliorated EAN pathology in rats.

Conclusions

In conclusion, we were able to demonstrate a direct immunomodulatory and anti-oxidative effect of capsaicin in a SC culture by reduced antigen presentation and expression of an anti-inflammatory profile. Furthermore, capsaicin increases the resistance of SCs against oxidative stress. A primary effect of capsaicin on myelination was not proven. These results are in concordance with previous data showing an anti-inflammatory effect of capsaicin, which might be highly relevant for CIDP patients.

Neuropeptide Y tonically inhibits an NMDAR➔AC1➔TRPA1/TRPV1 mechanism of the affective dimension of chronic neuropathic pain

Transection of the sural and common peroneal branches of the sciatic nerve produces cutaneous hypersensitivity at the tibial innervation territory of the mouse hindpaw that resolves within a few weeks. We report that interruption of endogenous neuropeptide Y (NPY) signaling during remission, with either conditional NPY knockdown in NPY^tet/tet mice or intrathecal administration of the Y1 receptor antagonist BIBO3304, reinstated hypersensitivity. These data indicate that nerve injury establishes a long-lasting latent sensitization of spinal nociceptive neurons that is masked by spinal NPY-Y1 neurotransmission. To determine whether this mechanism extends beyond the sensory component of nociception, we used conditioned place aversion and preference assays to evaluate the affective component of pain. We found that BIBO3304 produced place aversion in mice when administered during remission. Furthermore, the analgesic drug gabapentin produced place preference after NPY knockdown in NPY^tet/tet but not control mice. We then used pharmacological agents and deletion mutant mice to investigate the cellular mechanisms of neuropathic latent sensitization. BIBO3304-induced reinstatement of mechanical hypersensitivity and conditioned place aversion could be prevented with intrathecal administration of an N-methyl-d-aspartate receptor antagonist (MK-801) and was absent in adenylyl cyclase type 1 (AC1) deletion mutant mice. BIBO3304-induced reinstatement could also be prevented with intrathecal administration an AC1 inhibitor (NB001) or a TRPV1 channel blocker (AMG9801), but not vehicle. Intrathecal administration of a TRPA1 channel blocker (HC030031) prevented the reinstatement of neuropathic hypersensitivity produced either by BIBO3304, or by NPY knockdown in NPY^tet/tet but not control mice. Our results confirm new mediators of latent sensitization: TRPA1 and TRPV1. We conclude that NPY acts at spinal Y1 to tonically inhibit a molecular NMDAR➔AC1 intracellular signaling pathway in the dorsal horn that is induced by peripheral nerve injury and drives both the sensory and affective components of chronic neuropathic pain.

The Operant Plantar Thermal Assay: A Novel Device for Assessing Thermal Pain Tolerance in Mice

Pain is a multidimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the operant plantar thermal assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision-making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of using the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy.

Perispinal injection of a TNF blocker directed to the brain of rats alleviates the sensory and affective components of chronic constriction injury-induced neuropathic pain

Neuropathic pain is chronic pain that follows nerve injury, mediated in the brain by elevated levels of the inflammatory protein tumor necrosis factor-alpha (TNF). We have shown that peripheral nerve injury increases TNF in the hippocampus/pain perception region, which regulates neuropathic pain symptoms. In this study we assessed pain sensation and perception subsequent to specific targeting of brain-TNF (via TNF antibody) administered through a novel subcutaneous perispinal route. Neuropathic pain was induced in Sprague-Dawley rats via chronic constriction injury (CCI), and thermal hyperalgesia was monitored for 10 days post-surgery. On day 8 following CCI and sensory pain behavior testing, rats were randomized to receive perispinal injection of TNF antibody or control IgG isotype antibody. Pain perception was assessed using conditioned place preference (CPP) to the analgesic, amitriptyline. CCI-rats receiving the perispinal injection of TNF antibody had significantly decreased CCI-induced thermal hyperalgesia the following day, and did not form an amitriptyline-induced CPP, whereas CCI-rats receiving perispinal IgG antibody experienced pain alleviation only in conjunction with i.p. amitriptyline and did form an amitriptyline-induced CPP. The specific targeting of brain TNF via perispinal delivery alleviates thermal hyperalgesia and positively influences the affective component of pain. PERSPECTIVE: This study presents a novel route of drug administration to target central TNF for treatment of neuropathic pain. Targeting central TNF through perispinal drug delivery could potentially be a more efficient and sustained method to treat patients with neuropathic pain.

Experimental Autoimmune Encephalomyelitis Potentiates Mouse Mast Cell Protease 4-Dependent Pressor Responses to Centrally or Systemically Administered Big Endothelin-1

Multiple sclerosis is a neurodegenerative disease affecting predominantly female patients between 20 and 45 years of age. We previously reported the significant contribution of mouse mast cell protease 4 (mMCP-4) in the synthesis of endothelin-1 (ET-1) in healthy mice and in a murine model of experimental autoimmune encephalomyelitis (EAE). In the current study, the cardiovascular effects of ET-1 and big endothelin-1 (big-ET-1) administered systemically or intrathecally were assessed in the early preclinical phase of EAE in telemetry instrumented/conscious mice. Chymase-specific enzymatic activity was also measured in the lung, brain, and mast cell extracts in vitro. Finally, the impact of EAE immunization was studied on the pulmonary and brain mRNA expression of different genes of the endothelin pathway, interleukin-33 (IL-33), and monitoring of immunoreactive tumor necrosis factor-α (TNF-α). Systemically or intrathecally administered big-ET-1 triggered increases in blood pressure in conscious mice. One week post-EAE, the pressor responses to big-ET-1 were potentiated in wild-type (WT) mice but not in mMCP-4 knockout (KO) mice. EAE triggered mMCP-4-specific activity in cerebral homogenates and peritoneal mast cells. Enhanced pulmonary, but not cerebral preproendothelin-1 and IL-33 mRNA were found in KO mice and further increased 1 week post-EAE immunization, but not in WT animals. Finally, TNF-α levels were also increased in serum from mMCP-4 KO mice, but not WT, 1 week post-EAE. Our study suggests that mMCP-4 activity is enhanced both centrally and systemically in a mouse model of EAE.