Electroacupuncture Alleviates Pain by Suppressing P2Y12R-Dependent Microglial Activation in Monoarthritic Rats

Electroacupuncture (EA) effectively improves arthritis-induced hyperalgesia and allodynia by repressing spinal microglial activation, which plays a crucial role in pain hypersensitivity following tissue inflammation. However, the mechanism by which EA suppresses spinal microglial activation in monoarthritis (MA) remains unclear. In the present study, a rat model of MA was established through unilateral ankle intra-articular injection of complete Freund's adjuvant (CFA). The relationship among P2Y12 receptor (P2Y12R) expression, spinal microglial activation, and EA analgesia was investigated using quantitative real-time PCR (qRT‒PCR), western blotting, immunofluorescence (IF), and behavioral testing. The results found that EA treatment at the ipsilateral "Huantiao" (GB30) and "Yanglingquan" (GB34) acupoints markedly attenuated pain and spinal microglia M1 polarization in MA rats. In particular, P2Y12R expression was significantly increased at the mRNA and protein levels in the spinal dorsal horn in MA rats, whereas EA treatment effectively repressed the MA-induced upregulation of P2Y12R. IF analysis further revealed that most P2Y12R was expressed in microglia in the spinal dorsal horn. Pharmacological inhibition of P2Y12R by its antagonist (AR-C69931MX) decreased MA-induced spinal microglial activation and subsequent proinflammatory cytokine production. Consequently, AR-C69931MX significantly intensified the anti-pain hypersensitive function of EA in MA rats. Taken together, these results demonstrate that EA alleviates MA-induced pain by suppressing P2Y12R-dependent microglial activation.

Estrogen receptor GPR30 in the anterior cingulate cortex mediates exacerbated neuropathic pain in ovariectomized mice

Menopausal women experience neuropathic pain 63% more frequently than men do, which may attribute to the estrogen withdrawal. However, the underlying mechanisms remain unclear. Here, the role of estrogen receptors (ERs) in ovariectomized (OVX) female mice following chronic constriction injury (CCI) was investigated. With 17β-estradiol (E2) supplemented, aggravated mechanical allodynia in OVX mice could be significantly alleviated, particularly after intra-anterior cingulate cortex (ACC) E2 delivery. Pharmacological interventions further demonstrated that the agonist of G-protein-coupled estrogen receptor 30 (GPR30), rather than ERα or ERβ in the ACC, exhibited the similar analgesic effect as E2, whereas antagonist of GPR30 exacerbated allodynia. Furthermore, OVX surgery reduced GPR30 expression in the ACC, which could be restored with estrogen supplementation. Selective downregulation of GPR30 in the ACC of naïve female mice induces mechanical allodynia, whereas GPR30 overexpression in the ACC remarkedly alleviated OVX-exacerbated allodynia. Collectively, estrogen withdrawal could downregulate the ACC GPR30 expression, resulting in exacerbated neuropathic pain. Our findings highlight the importance of GPR30 in the ACC in aggravated neuropathic pain during menopause, and offer a potential therapeutic candidate for neuropathic pain management in menopausal women.

A cilia-bound unconventional secretory pathway for Drosophila odorant receptors

BACKGROUND

Post-translational transport is a vital process which ensures that each protein reaches its site of function. Though most do so via an ordered ER-to-Golgi route, an increasing number of proteins are now shown to bypass this conventional secretory pathway.

RESULTS

In the Drosophila olfactory sensory neurons (OSNs), odorant receptors (ORs) are trafficked from the ER towards the cilia. Here, we show that Or22a, a receptor of various esters and alcoholic compounds, reaches the cilia partially through unconventional means. Or22a frequently present as puncta at the somatic cell body exit and within the dendrite prior to the cilia base. These rarely coincide with markers of either the intermediary ER-Golgi-intermediate-compartment (ERGIC) or Golgi structures. ERGIC and Golgi also displayed axonal localization biases, a further indication that at least some measure of OR transport may occur independently of their involvement. Additionally, neither the loss of several COPII genes involved in anterograde trafficking nor ERGIC itself affected puncta formation or Or22a transport to the cilium. Instead, we observed the consistent colocalization of Or22a puncta with Grasp65, the sole Drosophila homolog of mammalian GRASP55/Grh1, a marker of the unconventional pathway. The numbers of both Or22a and Grasp65-positive puncta were furthermore increased upon nutritional starvation, a condition known to enhance Golgi-bypassing secretory activity.

CONCLUSIONS

Our results demonstrate an alternative route of Or22a transport, thus expanding the repertoire of unconventional secretion mechanisms in neurons.

Transient Receptor Potential Ankyrin 1-dependent Activation of Extracellular Signal-regulated Kinase 2 in the Cerebral Cortices Contributes to Cortical Spreading Depolarization

Extracellular signal-regulated kinase (ERK) are serine/threonine-selective proteins and ERK1/2 can be phosphorylated in peripheral and central brain regions after cortical spreading depolarization (CSD) and calcitonin gene-related peptide; However, it remains unclear about whether and how ERK activity modulates CSD that correlates to migraine aura. Here, we determined the role of ERK in regulating CSD and explored the underlying mechanism involving transient receptor potential ankyrin 1 (TRPA1), a stress-sensing cation channel. CSD was recorded using intrinsic optical imaging in mouse brain slices, and electrophysiology in rats. Phosphorylated ERK (pERK1/2) and interleukin-1β (IL-1β) protein levels were detected using Western blot or enzyme-linked immunosorbent assay, respectively. IL-1β mRNA level was detected using qPCR. The results showed that an ERK inhibitor, SCH77298, markedly prolonged CSD latency and reduced propagation rate in mouse brain slices. Corresponding to this, CSD induction increased levels of cytosolic pERK1/2 in ipsilateral cerebral cortices of rats, the elevation of which correlated to the level of IL-1β mRNA. Mechanistic analysis showed that pre-treatment of an anti-TRPA1 antibody reduced the cytosolic pERK2 level but not pERK1 following CSD in cerebral cortices of rats and this level of pERK2 correlated with that of cerebral cortical IL-1β protein. Furthermore, an ERK activator, AES16-2M, but not its scrambled control, reversed the prolonged CSD latency by a TRPA1 inhibitor, HC-030031, in mouse brain slices. These data revealed a crucial role of ERK activity in regulating CSD, and elevation of pERK and IL-1β production induced by CSD is predominantly TRPA1 channel-dependent, thereby contributing to migraine pathogenesis.

LncRNA MEG8 ameliorates Parkinson's disease neuro-inflammation through miR-485-3p/FBXO45 axis

OBJECTIVE

Studies suggest that LncRNA maternally expressed 8, small nucleolar RNA host gene (MEG8) contributes to inflammatory regulation, while the function and potential mechanisms of MEG8 in Parkinson's disease (PD) are unknown. This study aimed to assess the clinical value and biological function of MEG8 in PD.

METHODS

One hundred and two PD patients, eighty-six AD patients, and eighty healthy controls were enrolled in this study. Lipopolysaccharide (LPS)-induced microglia BV2 constructs an in vitro cell model. RT-qPCR was conducted to quantify the levels of MEG8, miR-485-3p, and FBXO45 in serum and cells. ROC curve was employed to examine the diagnostic value of MEG8 in PD. Serum and cellular pro-inflammatory factor secretion were quantified by ELISA. Dual-luciferase reporter and RIP assay to validate the targeting relationship between miR-485-3p and FBXO45.

RESULTS

MEG8 and FBXO45 were significantly decreased in the serum of PD patients and LPS-induced bv2, while miR-485-3p was increased (P < 0.05). ROC curve confirmed that serum MEG8 has high sensitivity and specificity to identify PD patients from healthy controls and AD patients, respectively. Elevated MEG8 alleviated LPS-induced inflammatory factor overproduction compared with LPS-induced BV2 (P < 0.05), but this alleviating effect was eliminated by miR-485-3p (P < 0.05). The LPS-induced inflammatory response was suppressed by the low expression of miR-485-3p but significantly reversed by silencing of FBXO45. MEG8 was a sponge for miR-485-3p and inhibited its levels and promoted FBXO45 expression (P < 0.05).

CONCLUSION

Elevated MEG8 is a potential diagnostic biomarker for PD and may mitigate inflammatory damage in PD via the miR-485-3p/FBXO45 axis.

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

Endophilin A2 controls touch and mechanical allodynia via kinesin-mediated Piezo2 trafficking

BACKGROUND

Tactile and mechanical pain are crucial to our interaction with the environment, yet the underpinning molecular mechanism is still elusive. Endophilin A2 (EndoA2) is an evolutionarily conserved protein that is documented in the endocytosis pathway. However, the role of EndoA2 in the regulation of mechanical sensitivity and its underlying mechanisms are currently unclear.

METHODS

Male and female C57BL/6 mice (8-12 weeks) and male cynomolgus monkeys (7-10 years old) were used in our experiments. Nerve injury-, inflammatory-, and chemotherapy-induced pathological pain models were established for this study. Behavioral tests of touch, mechanical pain, heat pain, and cold pain were performed in mice and nonhuman primates. Western blotting, immunostaining, co-immunoprecipitation, proximity ligation and patch-clamp recordings were performed to gain insight into the mechanisms.

RESULTS

The results showed that EndoA2 was primarily distributed in neurofilament-200-positive (NF200+) medium-to-large diameter dorsal root ganglion (DRG) neurons of mice and humans. Loss of EndoA2 in mouse NF200+ DRG neurons selectively impaired the tactile and mechanical allodynia. Furthermore, EndoA2 interacted with the mechanically sensitive ion channel Piezo2 and promoted the membrane trafficking of Piezo2 in DRG neurons. Moreover, as an adaptor protein, EndoA2 also bound to kinesin family member 5B (KIF5B), which was involved in the EndoA2-mediated membrane trafficking process of Piezo2. Loss of EndoA2 in mouse DRG neurons damaged Piezo2-mediated rapidly adapting mechanically activated currents, and re-expression of EndoA2 rescued the MA currents. In addition, interference with EndoA2 also suppressed touch sensitivity and mechanical hypersensitivity in nonhuman primates.

CONCLUSIONS

Our data reveal that the KIF5B/EndoA2/Piezo2 complex is essential for Piezo2 trafficking and for sustaining transmission of touch and mechanical hypersensitivity signals. EndoA2 regulates touch and mechanical allodynia via kinesin-mediated Piezo2 trafficking in sensory neurons. Our findings identify a potential new target for the treatment of mechanical pain.

Post-Traumatic Trigeminal Neuropathy: Neurobiology and Pathophysiology

Painful traumatic trigeminal neuropathy (PTTN) is a chronic neuropathic pain that may develop following injury to the trigeminal nerve. Etiologies include cranio-orofacial trauma that may result from dental, surgical, or anesthetic procedures or physical trauma, such as a motor vehicle accident. Following nerve injury, there are various mechanisms, including peripheral and central, as well as phenotypic changes and genetic predispositions that may contribute to the development of neuropathic pain. In this article, we review current literature pertaining to the cellular processes that occur following traumatic damage to the trigeminal nerve, also called cranial nerve V, that results in chronic neuropathic pain. We examine the neurobiology and pathophysiology based mostly on pre-clinical animal models of neuropathic/trigeminal pain.

Optogenetic Approach in Trigeminal Neuralgia and Potential Concerns: Preclinical Insights

The integration of optogenetics in the trigeminal pain circuitry broadens and reinforces existing pain investigations. Similar to research on spinal neuropathic pain, the exploration of the underlying determinants of orofacial pain is expanding. Optogenetics facilitates more direct, specific, and subtle investigations of the neuronal circuits involved in orofacial pain. One of the most significant concerns of both dentistry and medicine is trigeminal neuralgia (TN) management due to its substantial impact on a patient's quality of life. Our objective is to gather insights from preclinical studies conducted in TN employing an optogenetic paradigm, thereby extending the prospects for in-depth neurobiological research. This review highlights optogenetic research in trigeminal pain circuitry involving TN. We outline the central and peripheral regions associated with pain-that have been investigated using optogenetics in the trigeminal pain circuitry. The study further reports its scope and limitations as well as its potential for future applications from bench to bedside.

Glia-derived adenosine in the ventral hippocampus drives pain-related anxiodepression in a mouse model resembling trigeminal neuralgia

Glial activation and dysregulation of adenosine triphosphate (ATP)/adenosine are involved in the neuropathology of several neuropsychiatric illnesses. The ventral hippocampus (vHPC) has attracted considerable attention in relation to its role in emotional regulation. However, it is not yet clear how vHPC glia and their derived adenosine regulate the anxiodepressive-like consequences of chronic pain. Here, we report that chronic cheek pain elevates vHPC extracellular ATP/adenosine in a mouse model resembling trigeminal neuralgia (rTN), which mediates pain-related anxiodepression, through a mechanism that involves synergistic effects of astrocytes and microglia. We found that rTN resulted in robust activation of astrocytes and microglia in the CA1 area of the vHPC (vCA1). Genetic or pharmacological inhibition of astrocytes and connexin 43, a hemichannel mainly distributed in astrocytes, completely attenuated rTN-induced extracellular ATP/adenosine elevation and anxiodepressive-like behaviors. Moreover, inhibiting microglia and CD39, an enzyme primarily expressed in microglia that degrades ATP into adenosine, significantly suppressed the increase in extracellular adenosine and anxiodepressive-like behaviors. Blockade of the adenosine A2A receptor (A2AR) alleviated rTN-induced anxiodepressive-like behaviors. Furthermore, interleukin (IL)-17A, a pro-inflammatory cytokine probably released by activated microglia, markedly increased intracellular calcium in vCA1 astrocytes and triggered ATP/adenosine release. The astrocytic metabolic inhibitor fluorocitrate and the CD39 inhibitor ARL 67156, attenuated IL-17A-induced increases in extracellular ATP and adenosine, respectively. In addition, astrocytes, microglia, CD39, and A2AR inhibitors all reversed rTN-induced hyperexcitability of pyramidal neurons in the vCA1. Taken together, these findings suggest that activation of astrocytes and microglia in the vCA1 increases extracellular adenosine, which leads to pain-related anxiodepression via A2AR activation. Approaches targeting astrocytes, microglia, and adenosine signaling may serve as novel therapies for pain-related anxiety and depression.

Interleukin-18 in chronic pain: Focus on pathogenic mechanisms and potential therapeutic targets

Chronic pain has been proven to be an independent disease, other than an accompanying symptom of certain diseases. Interleukin-18 (IL-18), a pro-inflammatory cytokine with pleiotropic biological effects, participates in immune modulation, inflammatory response, tumor growth, as well as the process of chronic pain. Compelling evidence suggests that IL-18 is upregulated in the occurrence of chronic pain. Antagonism or inhibition of IL-18 expression can alleviate the occurrence and development of chronic pain. And IL-18 is located in microglia, while IL-18R is mostly located in astrocytes in the spinal cord. This indicates that the interaction between microglia and astrocytes mediated by the IL-18/IL-18R axis is involved in the occurrence of chronic pain. In this review, we described the role and mechanism of IL-18 in different types of chronic pain. This review provides strong evidence that IL-18 is a potential therapeutic target in pain management.

SIP30 involvement in vesicle exocytosis from PC12 cells

SNAP25 (synaptosome-associated protein of 25 kDa) is a core SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) protein; and the interaction between SNAP25 and other SNARE proteins is essential for synaptic vesicle exocytosis. Identified as a SNAP25 interacting protein, SIP30 (SNAP25 interacting protein at 30 kDa) has been shown to modulate neuropathic pain behavior, and is potentially involved in the cellular process of vesicle exocytosis. Previous study demonstrated that using a vesicle secretion assay in PC12 cells, anti-SIP30 siRNA reduced vesicle exocytosis. We investigated vesicle exocytosis from PC12 cells with FM1-43 fluorescence dye, and demonstrated that anti-SIP30 siRNA reduced the pool of releasable vesicles and the rate of vesicle exocytosis, without affecting the endocytosis and recycling of the exocytosed vesicles. The results show that SIP30 is involved in vesicle exocytosis, suggesting a potential mechanism of SIP30 modulation of neuropathic pain.

Phylogenetic analysis of mammalian SIP30 sequences indicating accelerated adaptation of functional domain in primates

SIP30, characterized by a coiled-coil functional domain, plays a key role in regulating synaptic vesicle exocytosis and is implicated in neuropathic pain resulting from peripheral nerve injury. Because neuropathic pain is studied in primates (including human), domesticated animals, and rodents, we conducted a phylogenetic analysis of SIP30 in selected species of these three groups of mammals. SIP30 exhibits a high degree of sequence divergence in comparison to its protein binding partners SNAP25 and ZW10, which show broad sequence conservation. Notably, we observed an increased rate of change in the highly conserved coiled-coil domain in the SIP30 protein, specifically within primates. This observation suggests an accelerated adaptation of this functional domain in primate species.

P2X7 receptors: a bibliometric review from 2002 to 2023

For many years, there has been ongoing research on the P2X7 receptor (P2X7R). A comprehensive, systematic, and objective evaluation of the scientific output and status of P2X7R will be instrumental in guiding future research directions. This study aims to present the status and trends of P2X7R research from 2002 to 2023. Publications related to P2X7R were retrieved from the Web of Science Core Collection database. Quantitative analysis and visualization tools were Microsoft Excel, VOSviewer, and CiteSpace software. The analysis content included publication trends, literature co-citation, and keywords. 3282 records were included in total, with the majority of papers published within the last 10 years. Based on literature co-citation and keyword analysis, neuroinflammation, neuropathic pain, gastrointestinal diseases, tumor microenvironment, rheumatoid arthritis, age-related macular degeneration, and P2X7R antagonists were considered to be the hotspots and frontiers of P2X7R research. Researchers will get a more intuitive understanding of the status and trends of P2X7R research from this study.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88, a primary cilium-specific intraflagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E2, and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift52. Ift52 siRNA results in loss of Ift52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.

Neural Circuitry Polarization in the Spinal Dorsal Horn (SDH): A Novel Form of Dysregulated Circuitry Plasticity during Pain Pathogenesis

Pathological pain emerges from nociceptive system dysfunction, resulting in heightened pain circuit activity. Various forms of circuitry plasticity, such as central sensitization, synaptic plasticity, homeostatic plasticity, and excitation/inhibition balance, contribute to the malfunction of neural circuits during pain pathogenesis. Recently, a new form of plasticity in the spinal dorsal horn (SDH), named neural circuit polarization (NCP), was discovered in pain models induced by HIV-1 gp120 and chronic morphine administration. NCP manifests as an increase in excitatory postsynaptic currents (EPSCs) in excitatory neurons and a decrease in EPSCs in inhibitory neurons, presumably facilitating hyperactivation of pain circuits. The expression of NCP is associated with astrogliosis. Ablation of reactive astrocytes or suppression of astrogliosis blocks NCP and, concomitantly, the development of gp120- or morphine-induced pain. In this review, we aim to compare and integrate NCP with other forms of plasticity in pain circuits to improve the understanding of the pathogenic contribution of NCP and its cooperation with other forms of circuitry plasticity during the development of pathological pain.

Machine learning-based evaluation of spontaneous pain and analgesics from cellular calcium signals in the mouse primary somatosensory cortex using explainable features

Introduction

Pain that arises spontaneously is considered more clinically relevant than pain evoked by external stimuli. However, measuring spontaneous pain in animal models in preclinical studies is challenging due to methodological limitations. To address this issue, recently we developed a deep learning (DL) model to assess spontaneous pain using cellular calcium signals of the primary somatosensory cortex (S1) in awake head-fixed mice. However, DL operate like a "black box", where their decision-making process is not transparent and is difficult to understand, which is especially evident when our DL model classifies different states of pain based on cellular calcium signals. In this study, we introduce a novel machine learning (ML) model that utilizes features that were manually extracted from S1 calcium signals, including the dynamic changes in calcium levels and the cell-to-cell activity correlations.

Method

We focused on observing neural activity patterns in the primary somatosensory cortex (S1) of mice using two-photon calcium imaging after injecting a calcium indicator (GCaMP6s) into the S1 cortex neurons. We extracted features related to the ratio of up and down-regulated cells in calcium activity and the correlation level of activity between cells as input data for the ML model. The ML model was validated using a Leave-One-Subject-Out Cross-Validation approach to distinguish between non-pain, pain, and drug-induced analgesic states.

Results and discussion

The ML model was designed to classify data into three distinct categories: non-pain, pain, and drug-induced analgesic states. Its versatility was demonstrated by successfully classifying different states across various pain models, including inflammatory and neuropathic pain, as well as confirming its utility in identifying the analgesic effects of drugs like ketoprofen, morphine, and the efficacy of magnolin, a candidate analgesic compound. In conclusion, our ML model surpasses the limitations of previous DL approaches by leveraging manually extracted features. This not only clarifies the decision-making process of the ML model but also yields insights into neuronal activity patterns associated with pain, facilitating preclinical studies of analgesics with higher potential for clinical translation.

GDF15 Contributes to the Regulation of the Mechanosensitive Responses of PdL Fibroblasts through the Modulation of IL-37

During orthodontic tooth movement (OTM), areas of compressive and tensile forces are generated in the periodontal ligament (PdL), a mechanoreactive connective tissue between the teeth and alveolar bone. Mechanically stimulated PdL fibroblasts (PdLFs), the main cell type of PdL, express significantly increased levels of growth differentiation factor 15 (GDF15). In compressed PdL areas, GDF15 plays a fundamental role in modulating relevant OTM processes, including inflammation and osteoclast activation. However, the specific function of this factor in tensile areas has not yet been investigated. Thus, the aim of this study was to investigate the role of GDF15 in the mechanoresponse of human PdLFs (hPdLFs) that were exposed to biaxial tensile forces in vitro. Using siRNA-mediated knockdown experiments, we demonstrated that GDF15 had no impact on the anti-inflammatory force response of elongated hPdLFs. Although the anti-inflammatory markers IL1RN and IL10, as well as the activation of immune cells remained unaffected, we demonstrated an inhibitory role of GDF15 for the IL-37 expression. By analyzing osteogenic markers, including ALPL and RUNX2, along with an assessment of alkaline phosphatase activation, we further showed that the regulation of IL-37 by GDF15 modulates the osteogenic differentiation potential of hPdLFs. Despite bone resorption in tensile areas being rather limited, GDF15 was also found to positively modulate osteoclast activation in those areas, potentially by adjusting the IL-37 levels. In light of our new findings, we hypothesize that GDF15 modulates force-induced processes in tissue and bone remodeling through its various intra- and extracellular signaling pathways as well as interaction partners. Potentially acting as a master regulator, the modulation of GDF15 levels may hold relevance for clinical implications.

Electroacupuncture Alleviates Hyperalgesia and Anxiety-Like Behaviors in Pain Memory Model Rats Through Activation of GABAergic Neurons and GABA Receptor in the Rostral Anterior Cingulate Cortex

Recent studies have confirmed that pain memory is often accompanied by negative emotions. Electroacupuncture (EA) can block the retrieval of painful memories, thereby alleviating the associated negative behaviors. However, the underlying mechanism is poorly understood. This study revealed that the effect of EA on pain memory-induced negative behaviors is related to the mediation of GABAergic neuron activity and GABA receptor expression in the rostral anterior cingulate cortex (rACC). Previous studies have shown that the rACC is a crucial area for regulating nociceptive behaviors and negative emotions in pain memory models. The GABAergic neurons and receptors in the rACC are largely involved in pain sensation and related effects. However, the relationships among pain memory, GABAergic neurons and receptors in the rACC have not been investigated. In this study, we established a pain memory model via secondary plantar cross-injection of carrageenan and EA treatment. Using chemogenetic methods and behavioral assessments of pain and negative emotion, we found that early excitation of GABAergic neurons in the rACC blocked the recall of pain memories and reduced anxiety-like behaviors in pain memory model rats. Furthermore, pharmacological methods revealed that excitation of GABAA and GABAB receptors in the rACC blocks hyperpathia associated with pain memory and pain-induced anxiety-like behaviors, while inhibition of GABAA and GABAB receptors reverses these effects. These results suggest that EA may alleviate pain and associated anxiety-like behaviors related to pain memories through the activation of GABAergic neurons and excitation of GABAA and GABAB receptors in the rACC.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in Caenorhabditis elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signalling effector that controls diverse cellular functions.

Corticotropin-releasing hormone neurons control trigeminal neuralgia-induced anxiodepression via a hippocampus-to-prefrontal circuit

Anxiety and depression are frequently observed in patients suffering from trigeminal neuralgia (TN), but neural circuits and mechanisms underlying this association are poorly understood. Here, we identified a dedicated neural circuit from the ventral hippocampus (vHPC) to the medial prefrontal cortex (mPFC) that mediates TN-related anxiodepression. We found that TN caused an increase in excitatory synaptic transmission from vHPCCaMK2A neurons to mPFC inhibitory neurons marked by the expression of corticotropin-releasing hormone (CRH). Activation of CRH+ neurons subsequently led to feed-forward inhibition of layer V pyramidal neurons in the mPFC via activation of the CRH receptor 1 (CRHR1). Inhibition of the vHPCCaMK2A-mPFCCRH circuit ameliorated TN-induced anxiodepression, whereas activating this pathway sufficiently produced anxiodepressive-like behaviors. Thus, our studies identified a neural pathway driving pain-related anxiodepression and a molecular target for treating pain-related psychiatric disorders.

Glial cell alterations in diabetes-induced neurodegeneration

Type 2 diabetes mellitus is a global epidemic that due to its increasing prevalence worldwide will likely become the most common debilitating health condition. Even if diabetes is primarily a metabolic disorder, it is now well established that key aspects of the pathogenesis of diabetes are associated with nervous system alterations, including deleterious chronic inflammation of neural tissues, referred here as neuroinflammation, along with different detrimental glial cell responses to stress conditions and neurodegenerative features. Moreover, diabetes resembles accelerated aging, further increasing the risk of developing age-linked neurodegenerative disorders. As such, the most common and disabling diabetic comorbidities, namely diabetic retinopathy, peripheral neuropathy, and cognitive decline, are intimately associated with neurodegeneration. As described in aging and other neurological disorders, glial cell alterations such as microglial, astrocyte, and Müller cell increased reactivity and dysfunctionality, myelin loss and Schwann cell alterations have been broadly described in diabetes in both human and animal models, where they are key contributors to chronic noxious inflammation of neural tissues within the PNS and CNS. In this review, we aim to describe in-depth the common and unique aspects underlying glial cell changes observed across the three main diabetic complications, with the goal of uncovering shared glial cells alterations and common pathological mechanisms that will enable the discovery of potential targets to limit neuroinflammation and prevent neurodegeneration in all three diabetic complications. Diabetes and its complications are already a public health concern due to its rapidly increasing incidence, and thus its health and economic impact. Hence, understanding the key role that glial cells play in the pathogenesis underlying peripheral neuropathy, retinopathy, and cognitive decline in diabetes will provide us with novel therapeutic approaches to tackle diabetic-associated neurodegeneration.

Exercise, Spinal Microglia and Neuropathic Pain: Potential Molecular Mechanisms

As one of the most common neuropathic disorders, neuropathic pain often has a negative impact on patients with persistent pain, mood disorders and sleep disturbances. Currently, neuropathic pain is not treated with any specific drug, instead, drugs for other diseases are used as replacements in clinics, but most have adverse effects. In recent years, the role of spinal cord microglia in the pathogenesis of neuropathic pain has been widely recognized, and they are being explored as potential therapeutic targets. Spinal microglia are known to be involved in the pathogenic mechanisms of neuropathic pain through purine signaling, fractalkine signaling, and p38 MAPK signaling. Exercise is a safe and effective treatment, and numerous studies have demonstrated its effectiveness in improving neurological symptoms. Nevertheless, it remains unclear what the exact molecular mechanism is. This review summarized the specific molecular mechanisms of exercise in alleviating neuropathic pain by mediating the activity of spinal microglia and maintaining the phenotypic homeostasis of spinal microglia through purine signaling, fractalkine signaling and p38 MAPK signaling. In addition, it has been proposed that different intensities and types of exercise affect the regulation of the above-mentioned signaling pathways differently, providing a theoretical basis for the improvement of neuropathic pain through exercise.

Sex differences in stress-induced hyperalgesia and its mechanisms

Chronic stress induces a variety of physiological and/or psychological abnormalities, including hyperalgesia. Researchers have discovered sex differences in the prevalence of stress-induced hyperalgesia (SIH) in recent years. Sex differences may be one of the reasons for the heterogeneity of susceptibility to stress-related diseases. In this review, the potential mechanisms of sex differences in SIH are discussed, such as hypothalamus-pituitary-adrenal axis responses, regulation of sex hormones, and immune system responses.

Direct paraventricular thalamus-basolateral amygdala circuit modulates neuropathic pain and emotional anxiety

The comorbidity of chronic pain and mental dysfunctions such as anxiety disorders has long been recognized, but the underlying mechanisms remained poorly understood. Here, using a mouse model of neuropathic pain, we demonstrated that the thalamic paraventricular nucleus (PVT) played a critical role in chronic pain-induced anxiety-like behavioral abnormalities. Fiber photometry and electrophysiology demonstrated that chronic pain increased the activities in PVT glutamatergic neurons. Chemogenetic manipulation revealed that suppression of PVT glutamatergic neurons relieved pain-like behavior and anxiety-like behaviors. Conversely, selective activation of PVT glutamatergic neurons showed algesic and anxiogenic effects. Furthermore, the elevated excitability of PVT glutamatergic neurons resulted in increased excitatory inputs to the basolateral complex (BLA) neurons. Optogenetic manipulation of the PVT-BLA pathway bilaterally modulates both the pain-like behavior and anxiety-like phenotypes. These findings shed light on how the PVT-BLA pathway contributed to the processing of pain-like behavior and maladaptive anxiety, and targeting this pathway might be a straightforward therapeutic strategy to both alleviate nociceptive hypersensitivity and rescue anxiety behaviors in chronic pain conditions.

Adenylate cyclase 3: a potential genetic link between obesity and major depressive disorder

Obesity and major depressive disorder (MDD) are both significant health issues that have been increasing in prevalence and are associated with multiple comorbidities. Obesity and MDD have been shown to be bidirectionally associated, and they are both influenced by genetics and environmental factors. However, the molecular mechanisms that link these two diseases are not yet fully understood. It is possible that these diseases are connected through the actions of the cAMP/protein kinase A (PKA) pathway. Within this pathway, adenylate cyclase 3 (Adcy3) has emerged as a key player in both obesity and MDD. Numerous genetic variants in Adcy3 have been identified in humans in association with obesity. Rodent knockout studies have also validated the importance of this gene for energy homeostasis. Furthermore, Adcy3 has been identified as a top candidate gene and even a potential blood biomarker for MDD. Adcy3 and the cAMP/PKA pathway may therefore serve as an important genetic and functional link between these two diseases. In this mini-review, we discuss the role of both Adcy3 and the cAMP/PKA pathway, including specific genetic mutations, in both diseases. Understanding the role that Adcy3 mutations play in obesity and MDD could open the door for precision medicine approaches and treatments for both diseases that target this gene.

Exploring altered oscillatory activity in the anterior cingulate cortex after nerve injury: Insights into mechanisms of neuropathic allodynia

While neural oscillations play a critical role in sensory perception, it remains unclear how these rhythms function under conditions of neuropathic allodynia. Recent studies demonstrated that the anterior cingulate cortex (ACC) is associated with the affective-aversive component of pain, and plasticity changes in this region are closely linked to abnormal allodynic sensations. Here, to study the mechanisms of allodynia, we recorded local field potentials (LFPs) in the bilateral ACC of awake-behaving rats and compared the spectral power and center frequency of brain oscillations between healthy and CCI (chronic constriction injury) induced neuropathic pain conditions. Our results indicated that activation of the ACC occurs bilaterally in the presence of neuropathic pain, similar to the healthy condition. Furthermore, CCI affects both spontaneous and stimulus-induced activity of ACC neurons. Specifically, we observed an increase in spontaneous beta activity after nerve injury compared to the healthy condition. By stimulating operated or unoperated paws, we found more intense event-related desynchronization (ERD) responses in the theta, alpha, and beta frequency bands and faster alpha center frequency after CCI compared to before CCI. Although the behavioral manifestation of allodynia was more pronounced in the operated paw than the unoperated paw following CCI, there was no significant difference in the center frequency and ERD responses observed in the ACC between stimulation of the operated and unoperated limbs. Our findings offer evidence supporting the notion that aberrancies in ACC oscillations may contribute to the maintenance and development of neuropathic allodynia.

Neuroproteomics: Unveiling the Molecular Insights of Psychiatric Disorders with a Focus on Anxiety Disorder and Depression

Anxiety and depression are two of the most common mental disorders worldwide, with a lifetime prevalence of up to 30%. These disorders are complex and have a variety of overlapping factors, including genetic, environmental, and behavioral factors. Current pharmacological treatments for anxiety and depression are not perfect. Many patients do not respond to treatment, and those who do often experience side effects. Animal models are crucial for understanding the complex pathophysiology of both disorders. These models have been used to identify potential targets for new treatments, and they have also been used to study the effects of environmental factors on these disorders. Recent proteomic methods and technologies are providing new insights into the molecular mechanisms of anxiety disorder and depression. These methods have been used to identify proteins that are altered in these disorders, and they have also been used to study the effects of pharmacological treatments on protein expression. Together, behavioral and proteomic research will help elucidate the factors involved in anxiety disorder and depression. This knowledge will improve preventive strategies and lead to the development of novel treatments.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88 , a primary cilium-specific intra-flagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E 2 , and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift 52. Ift 52 siRNA results in loss of Ift 52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.

Interplay between hippocampal TACR3 and systemic testosterone in regulating anxiety-associated synaptic plasticity

Tachykinin receptor 3 (TACR3) is a member of the tachykinin receptor family and falls within the rhodopsin subfamily. As a G protein-coupled receptor, it responds to neurokinin B (NKB), its high-affinity ligand. Dysfunctional TACR3 has been associated with pubertal failure and anxiety, yet the mechanisms underlying this remain unclear. Hence, we have investigated the relationship between TACR3 expression, anxiety, sex hormones, and synaptic plasticity in a rat model, which indicated that severe anxiety is linked to dampened TACR3 expression in the ventral hippocampus. TACR3 expression in female rats fluctuates during the estrous cycle, reflecting sensitivity to sex hormones. Indeed, in males, sexual development is associated with a substantial increase in hippocampal TACR3 expression, coinciding with elevated serum testosterone and a significant reduction in anxiety. TACR3 is predominantly expressed in the cell membrane, including the presynaptic compartment, and its modulation significantly influences synaptic activity. Inhibition of TACR3 activity provokes hyperactivation of CaMKII and enhanced AMPA receptor phosphorylation, associated with an increase in spine density. Using a multielectrode array, stronger cross-correlation of firing was evident among neurons following TACR3 inhibition, indicating enhanced connectivity. Deficient TACR3 activity in rats led to lower serum testosterone levels, as well as increased spine density and impaired long-term potentiation (LTP) in the dentate gyrus. Remarkably, aberrant expression of functional TACR3 in spines results in spine shrinkage and pruning, while expression of defective TACR3 increases spine density, size, and the magnitude of cross-correlation. The firing pattern in response to LTP induction was inadequate in neurons expressing defective TACR3, which could be rectified by treatment with testosterone. In conclusion, our study provides valuable insights into the intricate interplay between TACR3, sex hormones, anxiety, and synaptic plasticity. These findings highlight potential targets for therapeutic interventions to alleviate anxiety in individuals with TACR3 dysfunction and the implications of TACR3 in anxiety-related neural changes provide an avenue for future research in the field.

Role of the P2 × 7 receptor in neurodegenerative diseases and its pharmacological properties

Neurodegenerative diseases seriously affect patients' physical and mental health, reduce their quality of life, and impose a heavy burden on society. However, their treatment remains challenging. Therefore, exploring factors potentially related to the pathogenesis of neurodegenerative diseases and improving their diagnosis and treatment are urgently needed. Recent studies have shown that P2 × 7R plays a crucial role in regulating neurodegenerative diseases caused by neuroinflammation. P2 × 7R is an adenosine 5'-triphosphate ligand-gated cation channel receptor present in most tissues of the human body. An increase in P2 × 7R levels can affect the progression of neurodegenerative diseases, and the inhibition of P2 × 7R can alleviate neurodegenerative diseases. In this review, we comprehensively describe the biological characteristics (structure, distribution, and function) of this gene, focusing on its potential association with neurodegenerative diseases, and we discuss the pharmacological effects of drugs (P2 × 7R inhibitors) used to treat neurodegenerative diseases.

Role of spinal astrocytes through the perisynaptic astrocytic process in pathological pain

Pathological pain is caused by abnormal activity in the neural circuit that transmits nociceptive stimuli. Beyond homeostatic functions, astrocytes actively participate in regulating synaptic transmission as members of tripartite synapses. The perisynaptic astrocytic process (PAP) is the key structure that allows astrocytes to play these roles and not only physically supports synapse formation through cell adhesion molecules (CAMs) but also regulates the efficiency of chemical signaling. Accumulating evidence has revealed that spinal astrocytes are involved in pathological pain by modulating the efficacy of neurotransmitters such as glutamate and GABA through transporters located in the PAP and by directly regulating synaptic transmission through various gliotransmitters. Although various CAMs contribute to pathological pain, insufficient evidence is available as to whether astrocytic CAMs also have this role. Therefore, more in-depth research is needed on how pathological pain is induced and maintained by astrocytes, especially in the PAP surrounding the synapse, and this will subsequently increase our understanding and treatment of pathological pain.

A solid lipid particle formulation of long pepper extract reduces pain and astrocyte activation in a rat model of neuropathic pain

OBJECTIVES

To investigate the effects of solid lipid microparticle (SLM) creams containing a long pepper extract (LPE) or piperine on neuropathy-related pain and the expression of glial fibrillary acidic protein (GFAP) as a measure of astrogliosis.

METHODS

Neuropathic pain in male Spraque Dawley rats was induced by sciatic nerve ligation (SNL) and followed by treatment with LPE-SLM, piperine-SLM, capsaicin or vehicle creams. The pain score was assessed by thermal hyperalgesia test. The GFAP expression in the spinal cord was determined by immunohistochemistry.

RESULTS

Pain scores were significantly increased after SNL and decreased when treated by LPE-SLM. The number of GFAP immunopositive cells was significantly increased in the SNL rats. Treated by LPE-SLM and capsaicin creams resulted in a significant reduction of the number of GFAP immunopositive cells. The LPE-SLM treated rats showed greater effects than the piperine and capsaicin preparations.

CONCLUSIONS

The LPE-SLM cream has a potential effect on pain attenuation via a decrease of spinal astrocyte activation-related mechanism. The LPE in SLM preparation could provide an alternative therapeutic strategy for treating neuropathic pain.

SHED-derived exosomes attenuate trigeminal neuralgia after CCI of the infraorbital nerve in mice via the miR-24-3p/IL-1R1/p-p38 MAPK pathway

BACKGROUND

Microglial activation in the spinal trigeminal nucleus (STN) plays a crucial role in the development of trigeminal neuralgia (TN). The involvement of adenosine monophosphate-activated protein kinase (AMPK) and N-methyl-D-aspartate receptor 1 (NMDAR1, NR1) in TN has been established. Initial evidence suggests that stem cells from human exfoliated deciduous teeth (SHED) have a potential therapeutic effect in attenuating TN. In this study, we propose that SHED-derived exosomes (SHED-Exos) may alleviate TN by inhibiting microglial activation. This study sought to assess the curative effect of SHED-Exos administrated through the tail vein on a unilateral infraorbital nerve chronic constriction injury (CCI-ION) model in mice to reveal the role of SHED-Exos in TN and further clarify the potential mechanism.

RESULTS

Animals subjected to CCI-ION were administered SHED-Exos extracted by differential ultracentrifugation. SHED-Exos significantly alleviated TN in CCI mice (increasing the mechanical threshold and reducing p-NR1) and suppressed microglial activation (indicated by the levels of TNF-α, IL-1β and IBA-1, as well as p-AMPK) in vivo and in vitro. Notably, SHED-Exos worked in a concentration dependent manner. Mechanistically, miR-24-3p-upregulated SHED-Exos exerted a more significant effect, while miR-24-3p-inhibited SHED-Exos had a weakened effect. Bioinformatics analysis and luciferase reporter assays were utilized for target gene prediction and verification between miR-24-3p and IL1R1. Moreover, miR-24-3p targeted the IL1R1/p-p38 MAPK pathway in microglia was increased in CCI mice, and participated in microglial activation in the STN.

CONCLUSIONS

miR-24-3p-encapsulated SHED-Exos attenuated TN by suppressing microglial activation in the STN of CCI mice. Mechanistically, miR-24-3p blocked p-p38 MAPK signaling by targeting IL1R1. Theoretically, targeted delivery of miR-24-3p may offer a potential strategy for TN.

Acute inhibition of acid sensing ion channel 1a after spinal cord injury selectively affects excitatory synaptic transmission, but not intrinsic membrane properties, in deep dorsal horn interneurons

Following a spinal cord injury (SCI), secondary damage mechanisms are triggered that cause inflammation and cell death. A key component of this secondary damage is a reduction in local blood flow that initiates a well-characterised ischemic cascade. Downstream hypoxia and acidosis activate acid sensing ion channel 1a (ASIC1a) to trigger cell death. We recently showed that administration of a potent venom-derived inhibitor of ASIC1a, Hi1a, leads to tissue sparing and improved functional recovery when delivered up to 8 h after ischemic stroke. Here, we use whole-cell patch-clamp electrophysiology in a spinal cord slice preparation to assess the effect of acute ASIC1a inhibition, via a single dose of Hi1a, on intrinsic membrane properties and excitatory synaptic transmission long-term after a spinal cord hemisection injury. We focus on a population of interneurons (INs) in the deep dorsal horn (DDH) that play a key role in relaying sensory information to downstream motoneurons. DDH INs in mice treated with Hi1a 1 h after a spinal cord hemisection showed no change in active or passive intrinsic membrane properties measured 4 weeks after SCI. DDH INs, however, exhibit significant changes in the kinetics of spontaneous excitatory postsynaptic currents after a single dose of Hi1a, when compared to naive animals (unlike SCI mice). Our data suggest that acute ASIC1a inhibition exerts selective effects on excitatory synaptic transmission in DDH INs after SCI via specific ligand-gated receptor channels, and has no effect on other voltage-activated channels long-term after SCI.

Cingulate cGMP-dependent protein kinase I facilitates chronic pain and pain-related anxiety and depression

ABSTRACT

Patients with chronic pain often experience exaggerated pain response and aversive emotion, such as anxiety and depression. Central plasticity in the anterior cingulate cortex (ACC) is assumed to be a critical interface for pain perception and emotion, which has been reported to involve activation of NMDA receptors. Numerous studies have documented the key significance of cGMP-dependent protein kinase I (PKG-I) as a crucial downstream target for the NMDA receptor-NO-cGMP signaling cascade in regulating neuronal plasticity and pain hypersensitivity in specific regions of pain pathway, ie, dorsal root ganglion or spinal dorsal horn. Despite this, whether and how PKG-I in the ACC contributes to cingulate plasticity and comorbidity of chronic pain and aversive emotion has remained elusive. Here, we uncovered a crucial role of cingulate PKG-I in chronic pain and comorbid anxiety and depression. Chronic pain caused by tissue inflammation or nerve injury led to upregulation of PKG-I expression at both mRNA and protein levels in the ACC. Knockdown of ACC-PKG-I relieved pain hypersensitivity as well as pain-associated anxiety and depression. Further mechanistic analysis revealed that PKG-I might act to phosphorylate TRPC3 and TRPC6, leading to enhancement of calcium influx and neuronal hyperexcitability as well as synaptic potentiation, which results in the exaggerated pain response and comorbid anxiety and depression. We believe this study sheds new light on the functional capability of ACC-PKG-I in modulating chronic pain as well as pain-associated anxiety and depression. Hence, cingulate PKG-I may represent a new therapeutic target against chronic pain and pain-related anxiety and depression.

Engineering a multicompartment in vitro model for dorsal root ganglia phenotypic assessment

Despite the significant global prevalence of chronic pain, current methods to identify pain therapeutics often fail translation to the clinic. Phenotypic screening platforms rely on modeling and assessing key pathologies relevant to chronic pain, improving predictive capability. Patients with chronic pain often present with sensitization of primary sensory neurons (that extend from dorsal root ganglia [DRG]). During neuronal sensitization, painful nociceptors display lowered stimulation thresholds. To model neuronal excitability, it is necessary to maintain three key anatomical features of DRGs to have a physiologically relevant platform: (1) isolation between DRG cell bodies and neurons, (2) 3D platform to preserve cell-cell and cell-matrix interactions, and (3) presence of native non-neuronal support cells, including Schwann cells and satellite glial cells. Currently, no culture platforms maintain the three anatomical features of DRGs. Herein, we demonstrate an engineered 3D multicompartment device that isolates DRG cell bodies and neurites and maintains native support cells. We observed neurite growth into isolated compartments from the DRG using two formulations of collagen, hyaluronic acid, and laminin-based hydrogels. Further, we characterized the rheological, gelation and diffusivity properties of the two hydrogel formulations and found the mechanical properties mimic native neuronal tissue. Importantly, we successfully limited fluidic diffusion between the DRG and neurite compartment for up to 72 h, suggesting physiological relevance. Lastly, we developed a platform with the capability of phenotypic assessment of neuronal excitability using calcium imaging. Ultimately, our culture platform can screen neuronal excitability, providing a more translational and predictive system to identify novel pain therapeutics to treat chronic pain.

Aquaporin 4 beyond a water channel; participation in motor, sensory, cognitive and psychological performances, a comprehensive review

Aquaporin 4 (AQP4) is a protein highly expressed in the central nervous system (CNS) and peripheral nervous system (PNS) as well as various other organs, whose different sites of action indicate its importance in various functions. AQP4 has a variety of essential roles beyond water homeostasis. In this article, we have for the first time summarized different roles of AQP4 in motor and sensory functions, besides cognitive and psychological performances, and most importantly, possible physiological mechanisms by which AQP4 can exert its effects. Furthermore, we demonstrated that AQP4 participates in pathology of different neurological disorders, various effects depending on the disease type. Since neurological diseases involve a spectrum of dysfunctions and due to the difficulty of obtaining a treatment that can simultaneously affect these deficits, it is therefore suggested that future studies consider the role of this protein in different functional impairments related to neurological disorders simultaneously or separately by targeting AQP4 expression and/or polarity modulation.

Endogenous opiates and behavior: 2022

This paper is the forty-fifth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2022 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in C. elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signaling effector that controls diverse cellular functions.

Application and underlying mechanism of acupuncture for the nerve repair after peripheral nerve injury: remodeling of nerve system

Peripheral nerve injury (PNI) is a structural event with harmful consequences worldwide. Due to the limited intrinsic regenerative capacity of the peripheral nerve in adults, neural restoration after PNI is difficult. Neurological remodeling has a crucial effect on the repair of the form and function during the regeneration of the peripheral nerve after the peripheral nerve is injured. Several studies have demonstrated that acupuncture is effective for PNI-induced neurologic deficits, and the potential mechanisms responsible for its effects involve the nervous system remodeling in the process of nerve repair. Moreover, acupuncture promotes neural regeneration and axon sprouting by activating related neurotrophins retrograde transport, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), N-cadherin, and MicroRNAs. Peripheral nerve injury enhances the perceptual response of the central nervous system to pain, causing central sensitization and accelerating neuronal cell apoptosis. Together with this, the remodeling of synaptic transmission function would worsen pain discomfort. Neuroimaging studies have shown remodeling changes in both gray and white matter after peripheral nerve injury. Acupuncture not only reverses the poor remodeling of the nervous system but also stimulates the release of neurotrophic substances such as nerve growth factors in the nervous system to ameliorate pain and promote the regeneration and repair of nerve fibers. In conclusion, the neurological remodeling at the peripheral and central levels in the process of acupuncture treatment accelerates nerve regeneration and repair. These findings provide novel insights enabling the clinical application of acupuncture in the treatment of PNI.

The potent analgesia of intrathecal 2R, 6R-HNK via TRPA1 inhibition in LF-PENS-induced chronic primary pain model

BACKGROUND

Chronic primary pain (CPP) is an intractable pain of unknown cause with significant emotional distress and/or dysfunction that is a leading factor of disability globally. The lack of a suitable animal model that mimic CPP in humans has frustrated efforts to curb disease progression. 2R, 6R-hydroxynorketamine (2R, 6R-HNK) is the major antidepressant metabolite of ketamine and also exerts antinociceptive action. However, the analgesic mechanism and whether it is effective for CPP are still unknown.

METHODS

Based on nociplastic pain is evoked by long-term potentiation (LTP)-inducible high- or low-frequency electrical stimulation (HFS/LFS), we wanted to develop a novel CPP mouse model with mood and cognitive comorbidities by noninvasive low-frequency percutaneous electrical nerve stimulation (LF-PENS). Single/repeated 2R, 6R-HNK or other drug was intraperitoneally (i.p.) or intrathecally (i.t.) injected into naïve or CPP mice to investigate their analgesic effect in CPP model. A variety of behavioral tests were used to detect the changes in pain, mood and memory. Immunofluorescent staining, western blot, reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) and calcium imaging of in cultured dorsal root ganglia (DRG) neurons by Fluo-8-AM were used to elucidate the role and mechanisms of 2R, 6R-HNK in vivo or in vitro.

RESULTS

Intrathecal 2R, 6R-HNK, rather than intraperitoneal 2R, 6R-HNK or intrathecal S-Ketamine, successfully mitigated HFS-induced pain. Importantly, intrathecal 2R, 6R-HNK displayed effective relief of bilateral pain hypersensitivity and depressive and cognitive comorbidities in a dose-dependent manner in LF-PENS-induced CPP model. Mechanically, 2R, 6R-HNK markedly attenuated neuronal hyperexcitability and the upregulation of calcitonin gene-related peptide (CGRP), transient receptor potential ankyrin 1 (TRPA1) or vanilloid-1 (TRPV1), and vesicular glutamate transporter-2 (VGLUT2) in peripheral nociceptive pathway. In addition, 2R, 6R-HNK suppressed calcium responses and CGRP overexpression in cultured DRG neurons elicited by the agonists of TRPA1 or/and TRPV1. Strikingly, the inhibitory effects of 2R, 6R-HNK on these pain-related molecules and mechanical allodynia were substantially occluded by TRPA1 antagonist menthol.

CONCLUSIONS

In the newly designed CPP model, our findings highlighted the potential utility of intrathecal 2R, 6R-HNK for preventing and therapeutic modality of CPP. TRPA1-mediated uprgulation of CGRP and neuronal hyperexcitability in nociceptive pathways may undertake both unique characteristics and solving process of CPP.

Total rupture of Achilles tendon induces inflammatory response and glial activation on the spinal cord of mice

Rupture of Achilles tendon is a common accident affecting professional and recreational athletes. Acute and chronic pain are symptoms commonly observed in patients with rupture. However, few studies have investigated whether Achilles tendon rupture is able to promote disorders in the central nervous system (CNS). Therefore, the current study aimed to evaluate nociceptive alterations and inflammatory response in the L5 lumbar segment of Balb/c mice spinal cord after Achilles tendon rupture. We found increased algesia in the paw of the ruptured group on the 7th and 14th days post-tenotomy compared with the control group. This phenomenon was accompanied by overexpression of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase-2 (NOS-2) as well as hyperactivation of astrocytes and microglia in nociceptive areas of L5 spinal cord as evidenced by intense GFAP and IBA-1 immunostaining, respectively. Biochemical studies also demonstrated increased levels of nitrite in the L5 spinal cord of tenotomized animals compared with the control group. Thus, we have demonstrated for the first time that total rupture of the Achilles tendon induced inflammatory response and nitrergic and glial activation in the CNS in the L5 spinal cord region.

Comparisons of the analgesic effect of different pulsed radiofrequency targets in SNI-induced neuropathic pain

An injury of the peripheral nerve may lead to neuropathic pain, which could be treated with pulsed radiofrequency to the dorsal root ganglion (DRG) or peripheral nerve [the nerve trunk (NT) or proximal to the injury site (NI)]. However, it is not clear whether there is any difference in analgesic effect or maintenance among the three targets. PRF was applied to the ipsilateral L5 DRG, peripheral nerve (NT or NI) 5 days after spared nerve injury (SNI). Triptolide (10 µg/kg) or vehicle was intrathecally administered 5 days after SNI for 3 days. Mechanical withdrawal thresholds were tested after treatment at different time points. Furthermore, microglia and the P2X7 receptor (P2X7R) in the ipsilateral spinal cord were measured with immunofluorescence and western blotting, respectively. PRF + NI exerted a more remarkable analgesic effect than PRF + DRG and PRF + NT at the early stage, but PRF + DRG had a stronger analgesic effect than PRF + NI and PRF + NT at the end of our study. In addition, PRF + DRG showed no significant difference from intrathecal administration of triptolide. Moreover, SNI-induced microglia activation and upregulation of P2X7R in spinal dorsal horn could be markedly inhibited by PRF + DRG. The results suggest that the analgesic effect of PRF + DRG increased with time whereas the other two not and microglia and P2X7R in the ipsilateral spinal dorsal horn may be involved in the process.

Electroacupuncture alleviates neuropathic pain caused by SNL by promoting M2 microglia polarization through PD-L1

As a common clinical disease, neuropathic pain is difficult to be cured with drugs. The occurrence and progression of pain is closely related to the response of spinal microglia. Aspartof the regulation of microglialactivity,PD-L1 playsacriticalrole. Loss of PD-L1 promoted the polarization of M1-like microglia. Increased expression of PD-L1 promoted M2-like polarization. Electroacupuncture has a significant analgesic effect in clinical practice, but its specific mechanism remains to be further explored. In this study, we verified the role of PD-L1 in EA analgesia and the underlying molecular mechanism through spinal nerve ligation (SNL) in rats and lipopolysaccharide (LPS)-treated BV2 microglial cells. Forbehavioralstudiesofrats,mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were measured, and spinal cord neuros were examined under transmission electron microscopyto determine changes to their myelin structure. The expression levels of PD-L1 and M1/M2-specific markers in rat spinal cord and BV2 microglial cells were measured by enzyme-linked immunosorbent assay, flow cytometry, immunofluorescence staining and Western blot analysis. Our study showed that EA increased the pain threshold, reduced the destruction of myelin structure, promoted the expression of PD-L1 and PD-1, inhibited the MAPK signaling pathway, and promoted the conversion of microglial polarization from the M1 phenotype to the M2 phenotype in SNL rats. PD-L1 knockdown reversed these effects of EA. In addition, PD-L1 knockdown activated the MAPK signaling pathway, promoted microglial polarization to the M1 phenotype, decreased the expression of anti-inflammatory mediators and increased the expression of proinflammatory factors in LPS-stimulated BV2 microglial cells. Our results showed that EA may regulate the excitability of primary afferent neurons through PD-L1 and then inhibit the MAPK signaling pathway to promote the transformation of activated M1 microglia into M2 microglia, reduce inflammatory reactions, and finally achieve analgesic effects. A therapy targeting PD-L1 may be an effective strategy for treating neuropathic pain.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Janus effect of the anterior cingulate cortex: Pain and emotion

Over the past 20 years, clinical and preclinical studies point to the anterior cingulate cortex (ACC) as a site of interest for several neurological and psychiatric conditions. The ACC plays a critical role in emotion, autonomic regulation, pain processing, attention, memory and decision making. An increasing number of studies have demonstrated the involvement of the ACC in the emotional component of pain and its comorbidity with emotional disorders such as anxiety and depression. Thanks to the development of animal models combined with state-of-the-art technologies, we now have a better mechanistic understanding of the functions of the ACC. Hence, the primary aim of this review is to compile the most recent preclinical studies on the role of ACC in the emotional component and consequences of chronic pain. Herein, we thus thoroughly describe the pain-induced electrophysiological, molecular and anatomical alterations in the ACC and in its related circuits. Finally, we discuss the next steps that are needed to strengthen our understanding of the involvement of the ACC in emotional and pain processing.

Transcriptome Analysis of the Mouse Medial Prefrontal Cortex in a Chronic Constriction Injury Model

The medial prefrontal cortex (mPFC) is critical for both the sensory and emotional/cognitive components of pain. However, the underlying mechanism remains largely unknown. Here, we examined changes in the transcriptomic profiles in the mPFC of mice with chronic pain using RNA sequencing (RNA-seq) technology. A mouse model of peripheral neuropathic pain was established via chronic constriction injury (CCI) of the sciatic nerve. CCI mice developed sustained mechanical allodynia and thermal hyperalgesia, as well as cognitive impairment four weeks after surgery. RNA-seq was conducted 4 weeks after CCI surgery. Compared with contral group, RNA-seq identified a total 309 and 222 differentially expressed genes (DEGs) in the ipsilateral and contralateral mPFC of CCI model mice, respectively. GO analysis indicated that the functions of these genes were mainly enriched in immune- and inflammation-related processes such as interferon-gamma production and cytokine secretion. KEGG analysis further showed the enrichment of genes involved in the neuroactive ligand-receptor interaction signaling pathway and Parkinson disease pathway that have been reported to be importantly involved in chronic neuralgia and cognitive dysfunction. Our study may provide insights into the possible mechanisms underlying neuropathic pain and pain-related comorbidities.

Microglial inflammation modulates opioid analgesic tolerance

As we all know, opioids are the drugs of choice for treating severe pain. However, very often, opioid use leads to tolerance, dependence, and hyperalgesia. Therefore, understanding the mechanisms underlying opioid tolerance and designing strategies for increasing the efficacy of opioids in chronic pain are important areas of research. Microglia are brain macrophages that remove debris and dead cells from the brain and participate in immune defense of the central nervous system during an insult or injury. However, recent studies indicate that microglial activation and generation of proinflammatory molecules (e.g., cytokines, nitric oxide, eicosanoids, etc.) in the brain may contribute to opioid tolerance and other side effects of opioid use. In this review, we will summarize the evidence and possible mechanisms by which proinflammatory molecules produced by activated microglia may antagonize the analgesic effect induced by opioids, and thus, lead to opioid tolerance. We will also delineate specific examples of studies that suggest therapeutic targets to counteract the development of tolerance clinically using suppressors of microglial inflammation.

Activation of the N-methyl-D-aspartate receptor and calcium/calmodulin-dependent protein kinase IIα signal in the rostral anterior cingulate cortex is involved in pain-related aversion in rats with peripheral nerve injury

The rostral anterior cingulate cortex (rACC) of rat brain is associated with pain-related emotions. However, the underlying molecular mechanism remains unclear. Here, we investigated the effects of the N-methyl-D-aspartate (NMDA) receptor and Ca2+/Calmodulin-dependent protein kinase type II (CaMKII)α signal on pain-related aversion in the rACC of a rat model of neuropathic pain (NP). Mechanical and thermal hyperalgesia were examined using von Frey and hot plate tests in a rat model of NP induced by spared nerve injury (SNI) of the unilateral sciatic nerve. Bilateral rACC pretreatment with the CaMKII inhibitor tat-CN21 (derived from the cell-penetrating tat sequence and CaM-KIIN amino acids 43-63) or tat-Ctrl (the tat sequence and the scrambled sequence of CN21) was performed on postoperative days 29-35 in Sham rats or rats with SNI. Spatial memory performance was tested using an eight-arm radial maze on postoperative days 34-35. Pain-related negative emotions (aversions) were evaluated using the place escape/avoidance paradigm on postoperative day 35 following the spatial memory performance test. The percentage of time spent in the light area was used to assess pain-related negative emotions (i.e., aversion). The expression levels of the NMDA receptor GluN2B subunit, CaMKIIα, and CaMKII-Threonine at position 286 (Thr286) phosphorylation in contralateral rACC specimens were detected by Western blot or real time PCR following the aversion test. Our data showed that pretreatment of the rACC with tat-CN21 increased determinate behavior but did not alter hyperalgesia or spatial memory performance in rats with SNI. In addition, tat-CN21 reversed the enhanced CaMKII-Thr286 phosphorylation and had no effect on the upregulated expression of GluN2B, CaMKIIα protein, and mRNA. Our data suggested that activation of the NMDA receptor-CaMKIIα signal in rACC is associated with pain-related aversion in rats with NP. These data may provide a new approach for the development of drugs that modulate cognitive and emotional pain aspects.