Role of gap junctions in chronic pain

Octanol alleviates chronic constriction injury of sciatic nerve-induced peripheral neuropathy by regulating AKT/mTOR signaling

OBJECTIVE

Activation of gap junction channels can induce neuropathic pain. Octanol can limit the conductance of gap junctions containing connexin 43 proteins. Thus, this study focused on the roles of octanol in chronic constriction injury (CCI)-induced peripheral neuropathy in mice and its mechanisms of action.

METHODS

Male mice were assigned into control, sham, CCI, CCI + Octanol-20 mg/kg, CCI + Octanol-40 mg/kg and CCI + Octanol-80 mg/kg groups. CCI was performed by applying three loose ligations to mouse sciatic nerve, and the mice with CCI was administered with 20 mg/kg, 40 mg/kg, or 80 mg/kg octanol. The neuropathic pain development was examined by assessing thermal withdrawal latency, paw withdrawal mechanical threshold, and sciatic functional index. Histopathological changes were evaluated by hematoxylin and eosin staining. The phosphorylation of protein kinase B (Akt) and mammalian target of rapamycin (mTOR) was examined by western blotting. The expression of Akt and mTOR was also evaluated by immunofluorescence staining.

RESULTS

Octanol alleviated the CCI-induced mechanical and thermal hyperalgesia and sciatic functional loss. Additionally, octanol relieved the CCI-induced abnormal histopathological changes. Mechanistically, octanol inactivated the Akt/mTOR pathway in the mice with CCI.

CONCLUSION

In conclusion, octanol can alleviate CCI-induced peripheral neuropathic by regulating the Akt/mTOR pathway and might be a novel pharmacological intervention for neuropathic 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.

Targeting Pannexin-1 Channels: Addressing the 'Gap' in Chronic Pain

Chronic pain complicates many diseases and is notoriously difficult to treat. In search of new therapeutic targets, pannexin-1 (Panx1) channels have sparked intense interest as a key mechanism involved in a variety of chronic pain conditions. Panx1 channels are transmembrane proteins that release ions and small molecules, such as adenosine triphosphate (ATP). They are expressed along important nodes of the pain pathway, modulating activity of diverse cell types implicated in the development and progression of chronic pain caused by injury or pathology. This review highlights advances that have unlocked the core structure and machinery controlling Panx1 function with a focus on understanding and treating chronic pain.

CaMKII and CaV3.2 T-type calcium channel mediate Connexin-43-dependent inflammation by activating astrocytes in vincristine-induced neuropathic pain

Vincristine (VCR), an alkaloid isolated from vinca, is a commonly used chemotherapeutic drug. However, VCR therapy can lead to dose-dependent peripheral neurotoxicity, mainly manifesting as neuropathic pain, which is one of the dominant reasons for limiting its utility. Experimentally, we discovered that VCR-induced neuropathic pain (VINP) was accompanied by astrocyte activation; the upregulation of phospho-CaMKII (p-CaMKII), CaV3.2, and Connexin-43 (Cx43) expression; and the production and release of inflammatory cytokines and chemokines in the spinal cord. Similar situations were also observed in astrocyte cultures. Interestingly, these alterations were all reversed by intrathecal injection of KN-93 (a CaMKII inhibitor) or L-Ascorbic acid (a CaV3.2 inhibitor). In addition, KN-93 and L-Ascorbic acid inhibited the increase in [Ca2+]i associated with astrocyte activation. We also verified that knocking down or inhibiting Cx43 level via intrathecal injection of Cx43 siRNA or Gap27 (a Cx43 mimetic peptide) relieved pain hypersensitivity and reduced the release of inflammatory factors; however, they did not affect astrocyte activation or p-CaMKII and CaV3.2 expression. Besides, the overexpression of Cx43 through the transfection of the Cx43 plasmid did not affect p-CaMKII and CaV3.2 expressions in vitro. Therefore, CaMKII and CaV3.2 may activate astrocytes by increasing [Ca2+]i, thereby mediating Cx43-dependent inflammation in VINP. Moreover, we demonstrated that the CaMKII signalling pathway was involved in VCR-induced inflammation, apoptosis, and mitochondrial damage. Collectively, our findings show a novel mechanism by which CaMKII and CaV3.2 mediate Cx43-dependent inflammation by activating astrocytes in neuropathic pain induced by VCR.

Metabotropic glutamate receptor 5-mediated inhibition of inward-rectifying K+ channel 4.1 contributes to orofacial ectopic mechanical allodynia following inferior alveolar nerve transection in male mice

Inward-rectifying K⁺ channel 4.1 (Kir4.1), which regulates the electrophysiological properties of neurons and glia by affecting K⁺ homeostasis, plays a critical role in neuropathic pain. Metabotropic glutamate receptor 5 (mGluR5) regulates the expression of Kir4.1 in retinal Müller cells. However, the role of Kir4.1 and its expressional regulatory mechanisms underlying orofacial ectopic allodynia remain unclear. This study aimed to investigate the biological roles of Kir4.1 and mGluR5 in the trigeminal ganglion (TG) in orofacial ectopic mechanical allodynia and the role of mGluR5 in Kir4.1 regulation. An animal model of nerve injury was established via inferior alveolar nerve transection (IANX) in male C57BL/6J mice. Behavioral tests indicated that mechanical allodynia in the ipsilateral whisker pad lasted at least 14 days after IANX surgery and was alleviated by the overexpression of Kir4.1 in the TG, as well as intraganglionic injection of an mGluR5 antagonist (MPEP hydrochloride) or a protein kinase C (PKC) inhibitor (chelerythrine chloride); Conditional knockdown of the Kir4.1 gene downregulated mechanical thresholds in the whisker pad. Double immunostaining revealed that Kir4.1 and mGluR5 were co-expressed in satellite glial cells in the TG. IANX downregulated Kir4.1 and upregulated mGluR5 and phosphorylated PKC (p-PKC) in the TG; Inhibition of mGluR5 reversed the changes in Kir4.1 and p-PKC that were induced by IANX; Inhibition of PKC activation reversed the downregulation of Kir4.1 expression caused by IANX (p < .05). In conclusion, activation of mGluR5 in the TG after IANX contributed to orofacial ectopic mechanical allodynia by suppressing Kir4.1 via the PKC signaling pathway.

Paclitaxel, but Not Cisplatin, Affects Satellite Glial Cells in Dorsal Root Ganglia of Rats with Chemotherapy-Induced Peripheral Neurotoxicity

Chemotherapy-induced peripheral neurotoxicity is one of the most common dose-limiting toxicities of several widely used anticancer drugs such as platinum derivatives (cisplatin) and taxanes (paclitaxel). Several molecular mechanisms related to the onset of neurotoxicity have already been proposed, most of them having the sensory neurons of the dorsal root ganglia (DRG) and the peripheral nerve fibers as principal targets. In this study we explore chemotherapy-induced peripheral neurotoxicity beyond the neuronocentric view, investigating the changes induced by paclitaxel (PTX) and cisplatin (CDDP) on satellite glial cells (SGC) in the DRG and their crosstalk. Rats were chronically treated with PTX (10 mg/Kg, 1qwx4) or CDDP (2 mg/Kg 2qwx4) or respective vehicles. Morpho-functional analyses were performed to verify the features of drug-induced peripheral neurotoxicity. Qualitative and quantitative immunohistochemistry, 3D immunofluorescence, immunoblotting, and transmission electron microscopy analyses were also performed to detect alterations in SGCs and their interconnections. We demonstrated that PTX, but not CDDP, produces a strong activation of SGCs in the DRG, by altering their interconnections and their physical contact with sensory neurons. SGCs may act as principal actors in PTX-induced peripheral neurotoxicity, paving the way for the identification of new druggable targets for the treatment and prevention of chemotherapy-induced peripheral neurotoxicity.

Pathophysiology of Post-Traumatic Trigeminal Neuropathic Pain

Trigeminal nerve injury is one of the causes of chronic orofacial pain. Patients suffering from this condition have a significantly reduced quality of life. The currently available management modalities are associated with limited success. This article reviews some of the common causes and clinical features associated with post-traumatic trigeminal neuropathic pain (PTNP). A cascade of events in the peripheral and central nervous system function is involved in the pathophysiology of pain following nerve injuries. Central and peripheral processes occur in tandem and may often be co-dependent. Due to the complexity of central mechanisms, only peripheral events contributing to the pathophysiology have been reviewed in this article. Future investigations will hopefully help gain insight into trigeminal-specific events in the pathophysiology of the development and maintenance of neuropathic pain secondary to nerve injury and enable the development of new therapeutic modalities.

Revisiting APP secretases: an overview on the holistic effects of retinoic acid receptor stimulation in APP processing

Alzheimer's disease (AD) is the leading cause of dementia worldwide and is characterized by the accumulation of the β-amyloid peptide (Aβ) in the brain, along with profound alterations in phosphorylation-related events and regulatory pathways. The production of the neurotoxic Aβ peptide via amyloid precursor protein (APP) proteolysis is a crucial step in AD development. APP is highly expressed in the brain and is complexly metabolized by a series of sequential secretases, commonly denoted the α-, β-, and γ-cleavages. The toxicity of resulting fragments is a direct consequence of the first cleaving event. β-secretase (BACE1) induces amyloidogenic cleavages, while α-secretases (ADAM10 and ADAM17) result in less pathological peptides. Hence this first cleavage event is a prime therapeutic target for preventing or reverting initial biochemical events involved in AD. The subsequent cleavage by γ-secretase has a reduced impact on Aβ formation but affects the peptides' aggregating capacity. An array of therapeutic strategies are being explored, among them targeting Retinoic Acid (RA) signalling, which has long been associated with neuronal health. Additionally, several studies have described altered RA levels in AD patients, reinforcing RA Receptor (RAR) signalling as a promising therapeutic strategy. In this review we provide a holistic approach focussing on the effects of isoform-specific RAR modulation with respect to APP secretases and discuss its advantages and drawbacks in subcellular AD related events.

Applying the NIA Health Disparities Research Framework to Identify Needs and Opportunities in Chronic Musculoskeletal Pain Research

Disparities in the experience of chronic musculoskeletal pain in the United States stem from a confluence of a broad array of factors. Organized within the National Institute on Aging Health Disparity Research Framework, a literature review was completed to evaluate what is known and what is needed to move chronic musculoskeletal pain research forward specific to disproportionately affected populations. Peer-reviewed studies published in English, on human adults, from 2000 to 2019, and conducted in the United States were extracted from PubMed and Web of Science. Articles were reviewed for key words that focused on underrepresented ethnic/race groups with chronic musculoskeletal pain applying health factor terms identified in the NIAHealth Disparity Research Framework four levels of analysis: 1) environmental, 2) sociocultural, 3) behavioral, and 4) biological. A total of 52 articles met inclusion criteria. There were limited publications specific to underrepresented ethnic/race groups with chronic musculoskeletal pain across all levels with particular research gaps under sociocultural and biological categories. Current limitations in evidence may be supplemented by a foundation of findings specific to the broader topic of "chronic pain" which provides guidance for future investigations. Study designs including a focus on protective factors and multiple levels of analyses would be particularly meritorious. PERSPECTIVE: Chronic musculoskeletal pain unequally burdens underrepresented ethnic/race groups. In order to move research forward and to systematically investigate the complex array of factors contributing toward health disparities, an organized approach is necessary. Applying the NIA Health Disparities Research Framework, an overview of the current state of evidence specific to chronic musculoskeletal pain and underrepresented ethnic/race groups is provided with future directions identified.

Astrocytes in the rostral ventromedial medulla contribute to the maintenance of oro-facial hyperalgesia induced by late removal of dental occlusal interference

BACKGROUND

Astrocytes in the rostral ventromedial medulla (RVM) contribute to descending pain modulation, but their role in oro-facial pain induced by persistent experimental dental occlusal interference (PEOI) or following EOI removal (REOI) is unknown.

OBJECTIVE

To explore the involvement of RVM astrocytes in PEOI-induced oro-facial hyperalgesia or its maintenance following REOI.

METHODS

Male rats were randomly assigned into five groups: sham-EOI, postoperative day 6 and 14 of PEOI (PEOI 6 d and PEOI 14 d), postoperative day 6 following REOI on day 3 (REOI 3 d) and postoperative day 14 following REOI on day 8 (REOI 8 d). The nociceptive head withdrawal threshold (HWT) and activities of RVM ON- or OFF-cells were recorded before and after intra-RVM astrocyte gap junction blocker carbenoxolone (CBX) microinjection. RVM astrocytes were labelled immunohistochemically with glial fibrillary acidic protein (GFAP) and analysed semi-quantitatively.

RESULTS

Persistent experimental dental occlusal interference-induced oro-facial hyperalgesia, as reflected in decreased HWTs, was partially inhibited by REOI at day 3 but not at day 8 after EOI placement. Increased GFAP-staining area occurred only in REOI 8 d group in which CBX could inhibit the maintained hyperalgesia; CBX was ineffective in inhibiting hyperalgesia in PEOI 14 d group. OFF-cell activities showed no change, but the spontaneous activity and responses of ON-cells were significantly enhanced that could be suppressed by CBX in REOI 8 d group.

CONCLUSION

Rostral ventromedial medulla astrocytes may not participate in PEOI-induced oro-facial hyperalgesia or hyperalgesia inhibition by early REOI but are involved in the maintenance of oro-facial hyperalgesia by late REOI.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

Regulation of the KATP-JNK gap junction signaling pathway by immunomodulator astragaloside IV attenuates neuropathic pain

BACKGROUND AND OBJECTIVES

Gap junctions play a pivotal role in contributing to the formation of astroglial networks and in chronic pain. However, the mechanisms underlying the dysfunction of astroglial gap junctions in chronic pain have not been fully elucidated.

METHODS

Chronic constriction injury (CCI) of the sciatic nerve was used to establish rat neuropathic pain model. C6 cells were used to perform experiments in vitro. Von Frey hairs and Hargreave's method were used to determine the withdrawal threshold of rats. Protein expression was detected by immunofluorescence and western blotting.

RESULTS

Astragaloside IV (AST IV) significantly attenuated neuropathic pain and suppressed the excitation of spinal astrocytes in rats with CCI. The antinociceptive effect of AST IV was reversed by the gap junction decoupler carbenoxolone (CBX). AST IV inhibited the high expression of phosphorylated connexin 43 (p-Cx43) and p-c-Jun N-terminal kinase (p-JNK) in spinal cord of rats with CCI. JNK inhibitor alleviated neuropathic pain, which was reversed by CBX. JNK inhibitor decreased the high expression of p-Cx43 in both rats with CCI and tumor necrosis factor-alpha (TNF-α)-treated C6 cells. Additionally, the analgesic effect of AST IV was reversed by the adenosine triphosphate-sensitive potassium (K_ATP) channel blocker, glibenclamide (Glib). Glib abolished the inhibitory effects of AST IV on p-JNK and p-Cx43 both in vivo and in vitro. K_ATP channel opener (KCO) mimicked the inhibitory effects of AST IV on p-JNK and p-Cx43 in TNF-α-treated C6 cells.

CONCLUSION

Our results indicate that the sciatic nerve CCI induces the dysfunction of gap junctions in the spinal cord by activating K_ATP/JNK signaling to contribute to neuropathic pain. AST IV attenuates neuropathic pain via regulating the K_ATP-JNK gap junction axis.

Molecular mechanisms of painful traumatic trigeminal neuropathy-Evidence from animal research and clinical correlates

Painful traumatic trigeminal neuropathy (PTTN) may occur following major craniofacial or oral trauma, or may be subsequent to relatively minor dental interventions. Following injury, pain may originate from a peripheral nerve, a ganglion, or from the central nervous system. In this review, we focus on molecular mechanisms of pain resulting from injury to the peripheral branch of the trigeminal nerve. This syndrome has been termed painful traumatic trigeminal neuropathy (PTTN) by the International Headache Society and replaces previous terms including atypical odontalgia, deafferentation pain, traumatic neuropathy and phantom toothache. We emphasize the scientific evidence supporting the events purported to lead to PTTN by reviewing the pathophysiology of PTTN based on relevant animal models. Additionally, we briefly overview clinical correlates and pathophysiological manifestations of PTTN.

Cytoarchitectural analysis of the neuron-to-glia association in the dorsal root ganglia of normal and diabetic mice

Dorsal root ganglia (DRGs) host the somata of sensory neurons which convey information from the periphery to the central nervous system. These neurons have heterogeneous size and neurochemistry, and those of small-to-medium size, which play an important role in nociception, form two distinct subpopulations based on the presence (peptidergic) or absence (non-peptidergic) of transmitter neuropeptides. Few investigations have so far addressed the spatial relationship between neurochemically different subpopulations of DRG neurons and glia. We used a whole-mount mouse lumbar DRG preparation, confocal microscopy and computer-aided 3D analysis to unveil that IB4⁺ non-peptidergic neurons form small clusters of 4.7 ± 0.26 cells, differently from CGRP⁺ peptidergic neurons that are, for the most, isolated (1.89 ± 0.11 cells). Both subpopulations of neurons are ensheathed by a thin layer of satellite glial cells (SGCs) that can be observed after immunolabeling with the specific marker glutamine synthetase (GS). Notably, at the ultrastructural level we observed that this glial layer was discontinuous, as there were patches of direct contact between the membranes of two adjacent IB4⁺ neurons. To test whether this cytoarchitectonic organization was modified in the diabetic neuropathy, one of the most devastating sensory pathologies, mice were made diabetic by streptozotocin (STZ). In diabetic animals, cluster organization of the IB4⁺ non-peptidergic neurons was maintained, but the neuro-glial relationship was altered, as STZ treatment caused a statistically significant increase of GS staining around CGRP⁺ neurons but a reduction around IB4⁺ neurons. Ultrastructural analysis unveiled that SGC coverage was increased at the interface between IB4⁺ cluster-forming neurons in diabetic mice, with a 50% reduction in the points of direct contacts between cells. These observations demonstrate the existence of a structural plasticity of the DRG cytoarchitecture in response to STZ.

Intercellular communication and ion channels in neuropathic pain chronicization

BACKGROUND

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

AIM AND METHODS

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

CONCLUSION

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

The gap junction inhibitor INI-0602 attenuates mechanical allodynia and depression-like behaviors induced by spared nerve injury in rats

Gap junctions (GJs) are novel molecular targets for pain therapeutics due to their pain-promoting function. INI-0602, a new GJ inhibitor, exerts a neuroprotective role, while its role in neuropathic pain is unclear. The objective was to investigate the analgesic role and mechanisms of INI-0602 in neuropathic pain induced by spared nerve injury (SNI), and whether INI-0602 attenuated pain-induced depression-like behaviors. Rats were randomly assigned to saline treatment groups (sham+NS and SNI+NS) or INI-0602 treatment groups (sham+INI-0602 and SNI+INI-0602). The von Frey test was used to assess pain behavior, and the sucrose preference test, the forced swimming test, and the tail suspension test were used to assess depression-like behaviors. Gap junction intercellular communication (GJIC) was measured by parachute assay. Western blots were used to determine the protein expression. In vitro, INI-0602 significantly suppressed GJIC by decreasing connexin43 and connexin32 expression. In vivo, INI-0602 significantly suppressed mechanical allodynia during initiation (7 days after SNI) and the maintenance phase (21 days after SNI) and simultaneously attenuated accompanying depression-like behaviors. Furthermore, INI-0602 markedly suppressed the activation of astrocytes and microglia on days 7 and 21 by reducing GJIC. Finally, INI-0602 reversed the changes in the brain-derived neurotrophic factor and Nr2b subunits of the N-methyl-D-aspartate receptor in SNI rats, suggesting that these effects of INI-0602 were related to its analgesic effect. Our findings demonstrated that blocking GJs with INI-0602 attenuated mechanical pain hypersensitivity and related depression-like behaviors in SNI rats by reducing glial activation.

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

Chronic pain, including inflammatory, neuropathic pain, is a serious clinical issue. There are increasing numbers of patients with chronic pain due to the growing number of elderly and it is estimated that about 25% of the global population will develop chronic pain. Chronic pain patients are refractory to medications used to treat acute pain such as opioids and non-steroidal anti-inflammatory drugs. Furthermore, the complexity and diversity of chronic pain mechanisms hinder the development of new analgesics. Thus, a better understanding of the mechanism of chronic pain is needed, which would facilitate the development of novel analgesics based on novel mechanisms. With this goal, connexins (Cxs) could be targeted for the development of new analgesics. Connexins are proteins with 20 subtypes, and function as channels, gap junctions between cells, and hemichannels that sample the extracellular space and release molecules such as neurotransmitters. Furthermore, Cxs could have functions independent of channel activity. Recent studies have shown that Cxs could be crucial in the induction and maintenance of chronic pain, and modulation of the activity or the expression of Cxs ameliorates nociceptive hypersensitivity in multiple chronic pain models. This review will cite novel findings on the role of of Cxs in the nociceptive transduction pathway under the chronic pain state and antinociceptive effects of various molecules modulating activity or expression of Cxs. Also, the potential of Cx modulation as a therapeutic strategy for intractable chronic pain will be discussed.

RARβ Agonist Drug (C286) Demonstrates Efficacy in a Pre-clinical Neuropathic Pain Model Restoring Multiple Pathways via DNA Repair Mechanisms

Neuropathic pain (NP) is associated with profound gene expression alterations within the nociceptive system. DNA mechanisms, such as epigenetic remodeling and repair pathways have been implicated in NP. Here we have used a rat model of peripheral nerve injury to study the effect of a recently developed RARβ agonist, C286, currently under clinical research, in NP. A 4-week treatment initiated 2 days after the injury normalized pain sensation. Genome-wide and pathway enrichment analysis showed that multiple mechanisms persistently altered in the spinal cord were restored to preinjury levels by the agonist. Concomitant upregulation of DNA repair proteins, ATM and BRCA1, the latter being required for C286-mediated pain modulation, suggests that early DNA repair may be important to prevent phenotypic epigenetic imprints in NP. Thus, C286 is a promising drug candidate for neuropathic pain and DNA repair mechanisms may be useful therapeutic targets to explore.

The effects of sympathetic nerve damage on satellite glial cells in the mouse superior cervical ganglion

Neurons in sensory, sympathetic, and parasympathetic ganglia are surrounded by satellite glial cell (SGCs). There is little information on the effects of nerve damage on SGCs in autonomic ganglia. We studied the consequences of damage to sympathetic nerve terminals by 6-hydroxydopamine (6-OHDA) on SGCs in the mouse superior cervical ganglia (Sup-CG). Immunostaining revealed that at 1-30 d post-6-OHDA injection, SGCs in Sup-CG were activated, as assayed by upregulation of glial fibrillary acidic protein. Intracellular labeling showed that dye coupling between SGCs around different neurons increased 4-6-fold 1-14 d after 6-OHDA injection. Behavioral testing 1-7 d post-6-OHDA showed that withdrawal threshold to tactile stimulation of the hind paws was reduced by 65-85%, consistent with hypersensitivity. A single intraperitoneal injection of the gap junction blocker carbenoxolone restored normal tactile thresholds in 6-OHDA-treated mice, suggesting a contribution of SGC gap junctions to pain. Using calcium imaging we found that after 6-OHDA treatment responses of SGCs to ATP were increased by about 30% compared with controls, but responses to ACh were reduced by 48%. The same experiments for SGCs in trigeminal ganglia from 6-OHDA injected mice showed no difference from controls, confirming that 6-OHDA acted selectively on sympathetic nerves. However, systemic inflammation induced by lipopolysaccharide did not affect SGCs of Sup-CG, but did influence SGCs in trigeminal ganglia in the same manner as 6-OHDA did on SGCs in Sup-CG. We conclude that even though SGCs in sympathetic and sensory ganglia are morphologically similar, they are quite different functionally, particularly after damage.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Neurons and satellite glial cells in adult rat lumbar dorsal root ganglia express connexin 36

Previous studies have shown that following peripheral nerve injury there was a downregulation of the gap junction protein connexin 36 (Cx36) in the spinal cord; however, it is not known whether Cx36 protein is expressed in the dorsal root ganglia (DRGs), nor if its levels are altered following peripheral nerve injuries. Here we address these aspects in the adult rat lumbar DRG. Cx36 mRNA was detected using qRT-PCR, and Cx36 protein was identified in DRG sections using immunohistochemistry (IHC) and immunofluorescence (IF). Double staining revealed that Cx36 co-localizes with both anti-β-III tubulin, a neuronal marker, and anti-glutamine synthetase, a satellite glial cell (SGC) marker. In neurons, Cx36 staining was mostly uniform in somata and fibers of all sizes and its intensity increased at the cell membranes. This labeling pattern was in contrast with Cx36 IF dots mainly found at junctional membranes in islet beta cells used as a control tissue. Co-staining with anti-Cx43 and anti-Cx36 showed that whereas mostly uniform staining of Cx36 was found throughout neurons and SGCs, Cx43 IF puncta were localized to SGCs. Cx36 mRNA was expressed in normal lumbar DRG, and it was significantly down-regulated in L4 DRG of rats that underwent sciatic nerve injury resulting in persistent hypersensitivity. Collectively, these findings demonstrated that neurons and SGCs express Cx36 protein in normal DRG, and suggested that perturbation of Cx36 levels may contribute to chronic neuropathic pain resulting from a peripheral nerve injury.

Illuminating the Gap: Neuronal Cross-Talk within Sensory Ganglia and Persistent Pain

How primary sensory neurons contribute to persistent pain remains unclear. A novel imaging technique introduced here by Kim et al. (2016) in this issue of Neuron to view the activities of large numbers of ganglion neurons simultaneously analyzes the importance of neuronal cross-talk in pain transmission.

Attenuation of mechanical pain hypersensitivity by treatment with Peptide5, a connexin-43 mimetic peptide, involves inhibition of NLRP3 inflammasome in nerve-injured mice

Connexin43 (Cx43) hemichannels in spinal cord astrocytes are implicated in the maintenance of neuropathic pain following peripheral nerve injury. Peptide5 is a Cx43 mimetic peptide that blocks hemichannels. In this study, we investigated the effects of spinal delivery of Peptide5 on mechanical pain hypersensitivity in two mouse models of neuropathic pain, peripheral nerve injury and chemotherapy-induced peripheral neuropathy (CIPN). We demonstrated that 10days following a chronic constriction injury (CCI) of the sciatic nerve, Cx43 expression, co-localised predominantly with astrocytes, was increased in the ipsilateral L3-L5 lumbar spinal cord. An intrathecal injection of Peptide5 into nerve-injured mice, on day 10 when pain was well-established, caused significant improvement in mechanical pain hypersensitivity 8h after injection. Peptide5 treatment resulted in significantly reduced Cx43, and microglial and astrocyte activity in the dorsal horn of the spinal cord, as compared to control saline-treated CCI mice. Further in vitro investigations on primary astrocyte cultures showed that 1h pre-treatment with Peptide5 significantly reduced adenosine triphosphate (ATP) release in response to extracellular calcium depletion. Since ATP is a known activator of the NOD-like receptor protein 3 (NLRP3) inflammasome complex, a key mediator of neuroinflammation, we examined the effects of Peptide5 treatment on NLRP3 inflammasome expression. We found that NLRP3, its adaptor apoptosis-associated spec-like protein (ASC) and caspase-1 protein were increased in the ipsilateral spinal cord of CCI mice and reduced to naïve levels following Peptide5 treatment. In the models of oxaliplatin- and paclitaxel-induced peripheral neuropathy, treatment with Peptide5 had no effect on mechanical pain hypersensitivity. Interestingly, in these CIPN models, although spinal Cx43 expression was significantly increased at day 13 following chemotherapy, NLRP3 expression was not altered. These results suggest that the analgesic effect of Peptide5 is specifically achieved by reducing NLRP3 expression. Together, our findings demonstrate that blocking Cx43 hemichannels with Peptide5 after nerve injury attenuates mechanical pain hypersensitivity by specifically targeting the NLRP3 inflammasome in the spinal cord.

Gap junctions, pannexins and pain

Enhanced expression and function of gap junctions and pannexin (Panx) channels have been associated with both peripheral and central mechanisms of pain sensitization. At the level of the sensory ganglia, evidence includes augmented gap junction and pannexin1 expression in glial cells and neurons in inflammatory and neuropathic pain models and increased synchrony and enhanced cross-excitation among sensory neurons by gap junction-mediated coupling. In spinal cord and in suprapinal areas, evidence is largely limited to increased expression of relevant proteins, although in several rodent pain models, hypersensitivity is reduced by treatment with gap junction/Panx1 channel blocking compounds. Moreover, targeted modulation of Cx43 expression was shown to modulate pain thresholds, albeit in somewhat contradictory ways, and mice lacking Panx1 expression globally or in specific cell types show depressed hyperalgesia. We here review the evidence for involvement of gap junctions and Panx channels in a variety of animal pain studies and then discuss ways in which gap junctions and Panx channels may mediate their action in pain processing. This discussion focusses on spread of signals among satellite glial cells, in particular intercellular Ca²⁺ waves, which are propagated through both gap junction and Panx1-dependent routes and have been associated with the phenomenon of spreading depression and the malady of migraine headache with aura.

The effects of endothelin-1 on satellite glial cells in peripheral ganglia

Endothelins (ET) are a family of highly active neuropeptides with manifold influences via ET receptors (ETR) in both the peripheral and central nervous systems. We have shown previously that satellite glial cells (SGCs) in mouse trigeminal ganglia (TG) are extremely sensitive to ET-1 in evoking [Ca²⁺]_in increase, apparently via ET_BR activation, but there is no functional information on ETR in SGCs of other peripheral ganglia. Here we tested the effects of ET-1 on SGCs in nodose ganglia (NG), which is sensory, and superior cervical ganglia (Sup-CG), which is part of the sympathetic nervous system, and further investigated the influence of ET-1 on SGCs in TG. Using calcium imaging we found that SGCs in intact, freshly isolated NG and Sup-CG are highly sensitive to ET-1, with threshold concentration at 0.1nM. Our results showed that [Ca²⁺]_in elevation in response to ET-1 was partially due to Ca²⁺ influx from the extracellular space and partially to Ca²⁺ release from intracellular stores. Using receptor selective ETR agonists and antagonists, we found that the responses were mediated by mixed ET_AR/ET_BR in SGCs of NG and predominantly by ET_BR in SGCs of Sup-CG. By employing intracellular dye injection we examined coupling among SGCs around different neurons in the presence of 5nM ET-1 and observed coupling inhibition in all the three ganglion types. In summary, our work showed that SGCs in mouse sensory and sympathetic ganglia are highly sensitive to ET-1 and that this peptide markedly reduces SGCs coupling. We conclude that ET-1, which may participate in neuron-glia communications, has similar functions in wide range of peripheral ganglia.

The Dorsal Root Ganglion as a Therapeutic Target for Chronic Pain

Chronic neuropathic pain is a widespread problem with negative personal and societal consequences. Despite considerable clinical neuroscience research, the goal of developing effective, reliable, and durable treatments has remained elusive. The critical role played by the dorsal root ganglion (DRG) in the induction and maintenance of chronic pain has been largely overlooked in these efforts, however. It may be that, by targeting this site, robust new options for pain management will be revealed. This review summarizes recent advances in the knowledge base for DRG-targeted treatments for neuropathic pain:• Pharmacological options including the chemical targeting of voltage-dependent calcium channels, transient receptor potential channels, neurotrophin production, potentiation of opioid transduction pathways, and excitatory glutamate receptors.• Ablation or modulation of the DRG via continuous thermal radiofrequency and pulsed radiofrequency treatments.• Implanted electrical neurostimulator technologies.• Interventions involving the modification of DRG cellular function at the genetic level by using viral vectors and gene silencing methods.

Reversal of TRESK Downregulation Alleviates Neuropathic Pain by Inhibiting Activation of Gliocytes in the Spinal Cord

Despite the consensus that activation of TWIK-related spinal cord K⁺ (TRESK) might contribute to the pathogenesis of chronic pain, the specific mechanisms underlying the transfer and development of pain signals still remain obscure. In the present study, we validated that TRESK was expressed in neurons instead of glial cells. Furthermore, in the SNI model of neuropathic pain (NP), downregulation of TRESK in spinal cord neurons resulted in upregulation of connexin 36 (Cx36) and connexin 43 (Cx43), both being subtypes of gap junctions in the spinal cord, with gliocytes in the spinal cord activated ultimately. Compared with SNI rats, intrathecal injection of TRESK gene recombinant adenovirus significantly downregulated the expression levels of Cx36 and Cx43 and suppressed the activation of gliocytes in the spinal cord, with hyperalgesia significantly reduced. In conclusion, TRESK contributes to the pathogenesis of NP by upregulation of synaptic transmission and activation of gliocytes.

The role of Cx36 and Cx43 in 4-aminopyridine-induced rhythmic activity in the spinal nociceptive dorsal horn: an electrophysiological study in vitro

Connexin (Cx) proteins and gap junctions support the formation of neuronal and glial syncytia that are linked to different forms of rhythmic firing and oscillatory activity in the CNS. In this study, quantitative reverse transcription polymerase chain reaction (RT-qPCR) was used to profile developmental expression of two specific Cx proteins, namely glial Cx43 and neuronal Cx36, in postnatal lumbar spinal cord aged 4, 7, and 14 days. Extracellular electrophysiology was used to determine the contribution of Cx36 and Cx43 to a previously described form of 4-aminopyridine (4-AP)-induced 4-12 Hz rhythmic activity within substantia gelatinosa (SG) of rat neonatal dorsal horn (DH) in vitro. The involvement of Cx36 and Cx43 was probed pharmacologically using quinine, a specific uncoupler of Cx36 and the mimetic peptide blocker Gap 26 which targets Cx43. After establishment of 4-12 Hz rhythmic activity by 4-AP (25 μmol/L), coapplication of quinine (250 μmol/L) reduced 4-AP-induced 4-12 Hz rhythmic activity (P < 0.05). Preincubation of spinal cord slices with Gap 26 (100 μmol/L), compromised the level of 4-AP-induced 4-12 Hz rhythmic activity in comparison with control slices preincubated in ACSF alone (P < 0.05). Conversely, the nonselective gap junction "opener" trimethylamine (TMA) enhanced 4-12 Hz rhythmic behavior (P < 0.05), further supporting a role for Cx proteins and gap junctions. These data have defined a physiological role for Cx36 and Cx43 in rhythmic firing in SG, a key nociceptive processing area of DH. The significance of these data in the context of pain and Cx proteins as a future analgesic drug target requires further study.

An improved method for in vitro morphofunctional analysis of mouse dorsal root ganglia

Sensory neurons in dorsal root ganglia (DRGs) are the first-order neurons along the pathway conveying sensory information from the periphery to the central nervous system. The analysis of the morphological and physiological features of these neurons and their alterations in pathology is the necessary prerequisite to understand pain encoding mechanisms. Here, we describe an in vitro procedure for combined morphofunctional analysis of mouse DRGs. Freshly excised DRGs obtained from adult mice were incubated in collagenase to dissolve the ensheathing connective capsule. The degradation of the connective tissue facilitates both access to the neurons by classical recording glass pipettes and the penetration of primary antibodies for immunohistochemical procedures. The entire DRGs were then imaged using a confocal microscope obtaining a fine 3D representation of their cytoarchitecture without requiring tissue sectioning. Thus, our proposed whole-mount preparation represents a flexible in vitro approach for both functional and phenotypic analysis of DRG neurons by at the same time preserving their neuroanatomical relationships.

Spinal Gap Junction Channels in Neuropathic Pain

Damage to peripheral nerves or the spinal cord is often accompanied by neuropathic pain, which is a complex, chronic pain state. Increasing evidence indicates that alterations in the expression and activity of gap junction channels in the spinal cord are involved in the development of neuropathic pain. Thus, this review briefly summarizes evidence that regulation of the expression, coupling, and activity of spinal gap junction channels modulates pain signals in neuropathic pain states induced by peripheral nerve or spinal cord injury. We particularly focus on connexin 43 and pannexin 1 because their regulation vastly attenuates symptoms of neuropathic pain. We hope that the study of gap junction channels eventually leads to the development of a suitable treatment tool for patients with neuropathic pain.

Cancer Pain Management

Safe, effective, and evidence-based management of cancer-related pain is a cornerstone of comprehensive cancer care. Despite increasing interest in and efforts to improve its management, pain remains poorly controlled in nearly half of all patients with cancer, with little change in the past 20 years. Limited training in pain assessment and management, overestimation of providers' own skills to treat pain, and failure to refer patients to pain specialists can result in suboptimal pain management with devastating effects on quality of life, physical functioning, and increased psychological distress. From a thorough assessment of cancer-related pain to appropriate treatments that may include opiates, adjuvant medications, nerve blocks, and nondrug interventions, this article is intended as a brief overview of the mechanisms and types of pain as well as a review of current, new, and promising approaches to its management.

Downregulation of connexin36 in mouse spinal dorsal horn neurons leads to mechanical allodynia

Connexin36 (Cx36), a component of neuronal gap junctions, is crucial for interneuronal communication and regulation. Gap junction dysfunction underlies neurological disorders, including chronic pain. Following a peripheral nerve injury, Cx36 expression in the ipsilateral spinal dorsal horn was markedly decreased over time, which paralleled the time course of hind paw tactile allodynia. Intrathecal (i.t.) injection of Cx36 siRNA (1 and 5 pg) significantly reduced the expression of Cx36 protein in the lumbar spinal cord, peaking 3 days after the injection, which corresponded with the onset of hind paw tactile allodynia. It is possible that some of the tactile allodynia resulting from Cx36 downregulation could be mediated through excitatory neuromodulators, such as glutamate and substance P. The Cx36 knockdown-evoked tactile allodynia was significantly attenuated by i.t. treatment with the N-methyl-D-aspartate glutamate receptor antagonist MK-801 but not the substance P receptor antagonist CP96345. Immunohistochemistry showed that Cx36 was colocalized with glycine transporter-2, a marker for inhibitory glycinergic spinal interneurons, but not with glutamate decarboxylase 67, a marker for inhibitory GABAergic spinal interneurons. The results indicate that spinal inhibition through glycinergic interneurons is reduced, leading to increased glutamatergic neurotransmission, as a result of Cx36 downregulation. The current data suggest that gap junction dysfunction underlies neuropathic pain and further suggest a novel target for the development of analgesics.

Gap junction proteins and their role in spinal cord injury

Gap junctions are specialized intercellular communication channels that are formed by two hexameric connexin hemichannels, one provided by each of the two adjacent cells. Gap junctions and hemichannels play an important role in regulating cellular metabolism, signaling, and functions in both normal and pathological conditions. Following spinal cord injury (SCI), there is damage and disturbance to the neuronal elements of the spinal cord including severing of axon tracts and rapid cell death. The initial mechanical disruption is followed by multiple secondary cascades that cause further tissue loss and dysfunction. Recent studies have implicated connexin proteins as playing a critical role in the secondary phase of SCI by propagating death signals through extensive glial networks. In this review, we bring together past and current studies to outline the distribution, changes and roles of various connexins found in neurons and glial cells, before and in response to SCI. We discuss the contribution of pathologically activated connexin proteins, in particular connexin 43, to functional recovery and neuropathic pain, as well as providing an update on potential connexin specific pharmacological agents to treat SCI.

Systemic inflammation alters satellite glial cell function and structure. A possible contribution to pain

Local peripheral injury activates satellite glial cells (SGCs) in sensory ganglia, which may contribute to chronic pain. We hypothesized that systemic inflammation affects sensory ganglia like local injury. We induced systemic inflammation in mice by injecting lipopolysaccharide (LPS) intraperitoneally, and characterized SGCs and neurons in dorsal root ganglia (DRG), using dye injection, calcium imaging, electron microscopy (EM), immunohistochemistry, and electrical recordings. Several days post-LPS, SGCs were activated, and dye coupling among SGCs increased 3-4.5-fold. EM showed abnormal growth of SGC processes and the formation of new gap junctions. Sensitivity of SGCs to ATP increased twofold, and neuronal excitability was augmented. Blocking gap junctions reduced pain behavior in LPS-treated mice. Thus, changes in DRG due to systemic inflammation are similar to those due to local injury, which may explain the pain in sickness behavior and in other systemic diseases.

Connexins: sensors of epidermal integrity that are therapeutic targets

Gap junction proteins (connexins) are differentially expressed throughout the multiple layers of the epidermis. A variety of skin conditions arise with aberrant connexin expression or function and suggest that maintaining the epidermal gap junction network has many important roles in preserving epidermal integrity and homeostasis. Mutations in a number of connexins lead to epidermal dysplasias giving rise to a range of dermatological disorders of differing severity. 'Gain of function' mutations reveal connexin-mediated roles in calcium signalling within the epidermis. Connexins are involved in epidermal innate immunity, inflammation control and in wound repair. The therapeutic potential of targeting connexins to improve wound healing responses is now clear. This review discusses the role of connexins in epidermal integrity, and examines the emerging evidence that connexins act as epidermal sensors to a variety of mechanical, temperature, pathogen-induced and chemical stimuli. Connexins thus act as an integral component of the skin's protective barrier.

The gap junction blocker carbenoxolone attenuates nociceptive behavior and medullary dorsal horn central sensitization induced by partial infraorbital nerve transection in rats

Glial cells are being increasingly implicated in mechanisms underlying pathological pain, and recent studies suggest glial gap junctions involving astrocytes may contribute. The aim of this study was to examine the effect of a gap junction blocker, carbenoxolone (CBX), on medullary dorsal horn (MDH) nociceptive neuronal properties and facial mechanical nociceptive behavior in a rat trigeminal neuropathic pain model involving partial transection of the infraorbital nerve (p-IONX). p-IONX produced facial mechanical hypersensitivity reflected in significantly reduced head withdrawal thresholds that lasted for more than 3weeks. p-IONX also produced central sensitization in MDH nociceptive neurons that was reflected in significantly increased receptive field size, reduction of mechanical activation threshold, and increases in noxious stimulation-evoked responses. Intrathecal CBX treatment significantly attenuated the p-IONX-induced mechanical hypersensitivity and the MDH central sensitization parameters, compared to intrathecal vehicle treatment. These results provide the first documentation that gap junctions may be critically involved in orofacial neuropathic pain mechanisms.

Spinal astrocyte gap junctions contribute to oxaliplatin-induced mechanical hypersensitivity

Spinal glial cells contribute to the development of many types of inflammatory and neuropathic pain. Here the contribution of spinal astrocytes and astrocyte gap junctions to oxaliplatin-induced mechanical hypersensitivity was explored. The expression of glial fibrillary acidic protein (GFAP) in spinal dorsal horn was significantly increased at day 7 but recovered at day 14 after oxaliplatin treatment, suggesting a transient activation of spinal astrocytes by chemotherapy. Astrocyte-specific gap junction protein connexin 43 (Cx43) was significantly increased in dorsal horn at both day 7 and day 14 following chemotherapy, but neuronal (connexin 36 [Cx36]) and oligodendrocyte (connexin 32 [Cx32]) gap junction proteins did not show any change. Blockade of astrocyte gap junction with carbenoxolone (CBX) prevented oxaliplatin-induced mechanical hypersensitivity in a dose-dependent manner and the increase of spinal GFAP expression, but had no effect once the mechanical hypersensitivity induced by oxaliplatin had fully developed. These results suggest that oxaliplatin chemotherapy induces the activation of spinal astrocytes and this is accompanied by increased expression of astrocyte-astrocyte gap junction connections via Cx43. These alterations in spinal astrocytes appear to contribute to the induction but not the maintenance of oxaliplatin-induced mechanical hypersensitivity. Combined, these results suggest that targeting spinal astrocyte/astrocyte-specific gap junction could be a new therapeutic strategy to prevent oxaliplatin-induced neuropathy.

PERSPECTIVE

Spinal astrocytes but not microglia were recently shown to be recruited in paclitaxel-related chemoneuropathy. Here, spinal astrocyte gap junctions are shown to play an important role in the induction of oxaliplatin neuropathy.

The multitarget opioid ligand LP1's effects in persistent pain and in primary cell neuronal cultures

Persistent pain states, such as those caused by nerve injury or inflammation, are associated with altered sensations, allodynia and hyperalgesia, that are resistant to traditional analgesics. A contribution to development and maintenance in altered pain perception comes from nociceptive processing and descending modulation from supraspinal sites. A multitarget ligand seems to be useful for pain relief with a decreased risk of adverse events and a considerable analgesic efficacy. The multitarget MOR agonist-DOR antagonist LP1, (3-[(2R,6R,11R)-8-hydroxy-6,11-dimethyl-1,4,5,6-tetrahydro-2,6-methano-3-benazocin-3(2H)-yl]-N-phenylpropanamide, is a central acting antinociceptive agent with low potential to induce tolerance. LP1 was tested in models of neuropathic pain - induced by chronic constriction injury (CCI) of the left sciatic nerve - and inflammatory pain - produced by intraplantar injection of carrageenan. In CCI rats, subcutaneous (s.c.) LP1 (3 mg/kg) showed a significant antiallodynic effect, measured with von Frey filaments, and antihyperalgesic effect, evoked in response to a radiant heat stimulus with plantar test. Analogously, LP1 significantly reduced allodynic and hyperalgesic thresholds in a model of inflammatory pain induced by carrageenan. To evaluate the contribution of opioid receptor subtypes in LP1 antinociceptive effects, the multitarget LP1 profile was assessed using selective opioid antagonists. Moreover, functional electrophysiological in vitro assays, using primary cortical and spinal cord networks, allowed to define the "pharmacological fingerprint" of LP1. The EC₅₀ values in this functional screening seem to confirm LP1 as a potent opioid ligand (EC₅₀ = 0.35 fM and EC₅₀ = 44 pM in spinal cord and frontal cortex, respectively). Using a NeuroProof data-base of well characterised reference compounds, a similarity profile of LP1 to opioid and non-opioid drugs involved in pain modulation was detected. Our studies seem to support that multitarget ligand approach should be useful for persistent pain conditions in which mechanical allodynia and thermal hyperalgesia are significant components of the nociceptive response.

Astrocytes--multitaskers in chronic pain

Treatment of chronic pain remains a clinical challenge and sufficient pharmacological management is difficult to achieve without concurrent adverse drug effects. Recently the concept of chronic pain as a solely neuron-mediated phenomenon has evolved and it is now appreciated that also glial cells are of critical importance in pain generation and modulation. Astrocytes are macroglial cells that have close structural relationships with neurons; they contact neuronal somata and dendrites and enwrap synapses, where small astrocytic processes have been shown to be highly motile. This organization allows astrocytes to directly influence and coordinate neurons located within their structural domains. Moreover, astrocytes form astroglial networks and calcium wave propagations can spread through neighbouring astrocytes. ATP, which is released from astrocytes in response to elevated intracellular calcium concentrations, can contribute to the central mechanisms in chronic pain via purinergic receptors. In this review we highlight the structural organization and the functionalities of astrocytes that allow them to undertake critical roles in pain processing and we stress the possibility that astrocytes contribute to chronic pain not via a single pathway, but by undertaking various roles depending on the pain condition.

Synergistic effect of chemical penetration enhancer and iontophoresis on transappendageal transport of oligodeoxynucleotides

Gap junction protein connexin43 (Cx43) specific antisense oligodeoxynucleotides (AsODN) have been shown to improve a number of inflammatory conditions and may therefore offer a novel strategy for persistent pain management. However, for such molecules to be clinically effective, delivery challenges owing to the molecules' high molecular weight, negative charge and hydrophilicity have to be overcome. In this study, the effect of various chemical penetration enhancers and cathodal iontophoresis on transdermal delivery was evaluated. Initial skin permeation studies revealed only a slight increase in the passive flux of the model anionic drug sodium fluorescein using limonene/ethanol. Applying cathodal iontophoresis, the amount of the model drug permeated through untreated skin was tripled, while a combination of chemical and physical penetration enhancement resulted in a fourfold increase in the fluorescein amount permeated. However, even the synergistic effect of limonene/ethanol and iontophoresis was insufficient to achieve complete permeation of Cy3-labeled Cx43 AsODN across the entire skin thickness. Instead, molecules were trapped in the epidermis or permeated deeply into the hair follicles. These results suggest that the synergistic effect of chemical and physical penetration enhancement increases intradermal delivery of oligonucleotides but is insufficient to deliver such large molecules across intact skin.

Astrocytic CX43 hemichannels and gap junctions play a crucial role in development of chronic neuropathic pain following spinal cord injury

Chronic neuropathic pain is a frequent consequence of spinal cord injury (SCI). Yet despite recent advances, upstream releasing mechanisms and effective therapeutic options remain elusive. Previous studies have demonstrated that SCI results in excessive ATP release to the peritraumatic regions and that purinergic signaling, among glial cells, likely plays an essential role in facilitating inflammatory responses and nociceptive sensitization. We sought to assess the role of connexin 43 (Cx43) as a mediator of CNS inflammation and chronic pain. To determine the extent of Cx43 involvement in chronic pain, a weight-drop SCI was performed on transgenic mice with Cx43/Cx30 deletions. SCI induced robust and persistent neuropathic pain including heat hyperalgesia and mechanical allodynia in wild-type control mice, which developed after 4 weeks and was maintained after 8 weeks. Notably, SCI-induced heat hyperalgesia and mechanical allodynia were prevented in transgenic mice with Cx43/Cx30 deletions, but fully developed in transgenic mice with only Cx30 deletion. SCI-induced gliosis, detected as upregulation of glial fibrillary acidic protein in the spinal cord astrocytes at different stages of the injury, was also reduced in the knockout mice with Cx43/Cx30 deletions, when compared with littermate controls. In comparison, a standard regimen of post-SCI treatment of minocycline attenuated neuropathic pain to a significantly lesser degree than Cx43 deletion. These findings suggest Cx43 is critically linked to the development of central neuropathic pain following acute SCI. Since Cx43/Cx30 is expressed by astrocytes, these findings also support an important role of astrocytes in the development of chronic pain.

Human Connexin Channel Specificity of Classical and New Gap Junction Inhibitors

Connexins are transmembrane proteins involved in gap junction intercellular communication. They present cell- and tissue-specific expression, with own electric and metabolic coupling specificities. These proteins are involved in numerous physiological processes in the brain and among them neuronal synchronization and trafficking of glucose. Such proteins are also described as being misregulated in various pathologies in the central nervous system. Thus, connexin blockers have been proposed as pharmacological tools to dissect these implications. However, such approaches lack accurate characterization of known inhibitors toward gap junction isoform specificity. In addition, those compounds are limited to few chemical classes and exhibit other activities, for example, an anti-inflammatory effect. The aims of this study were to evaluate the selectivity of described inhibitors and to enrich this pharmacopeia by new chemical classes. In this study, we present the specificity of published inhibitors toward several connexin isoforms expressed in the brain. Furthermore, after a screening of compounds using cellular models, we identified seven new inhibitors, with high functional reversibility and different relative selectivity toward isoforms. They constitute new chemical classes of connexin modulators completing those previously described. These new inhibitors might also provide new insights in understanding numerous pathophysiological processes involving gap junctions.

Gap junction proteins: master regulators of the planarian stem cell response to tissue maintenance and injury

Gap junction (GJ) proteins are crucial mediators of cell-cell communication during embryogenesis, tissue regeneration and disease. GJ proteins form plasma membrane channels that facilitate passage of small molecules across cells and modulate signaling pathways and cellular behavior in different tissues. These properties have been conserved throughout evolution, and in most invertebrates GJ proteins are known as innexins. Despite their critical relevance for physiology and disease, the mechanisms by which GJ proteins modulate cell behavior are poorly understood. This review summarizes findings from recent work that uses planarian flatworms as a paradigm to analyze GJ proteins in the complexity of the whole organism. The planarian model allows access to a large pool of adult somatic stem cells (known as neoblasts) that support physiological cell turnover and tissue regeneration. Innexin proteins are present in planarians and play a fundamental role in controlling neoblast behavior. We discuss the possibility that GJ proteins participate as cellular sensors that inform neoblasts about local and systemic physiological demands. We believe that functional analyses of GJ proteins will bring a complementary perspective to studies that focus on the temporal expression of genes. Finally, integrating functional studies along with molecular genetics and epigenetic approaches would expand our understanding of cellular regulation in vivo and greatly enhance the possibilities for rationally modulating stem cell behavior in their natural environment. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.