Differential gene expression of neonatal and adult DRG neurons correlates with the differential sensitization of TRPV1 responses to nerve growth factor

Current insights and therapeutic strategies for targeting TRPV1 in neuropathic pain management

Neuropathic pain, a common symptom of several disorders, exerts a substantial socioeconomic burden worldwide. Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel predominantly ex-pressed in nociceptive neurons, plays a pivotal role in nociception, by detecting various endogenous and exogenous stimuli, including heat, pro-inflammatory mediators, and physical stressors. Dysregulation of TRPV1 signaling further contributes to the pathophysiology of neuropathic pain. Therefore, targeting TRPV1 is a promising strategy for developing novel analgesics with improved efficacy and safety profiles. Several pharmacological approaches to modulate TRPV1 activity, including agonists, antagonists, and biological TRPV1 RNA interference (RNAi, small interfering RNA [siRNA]) have been explored. Despite preclinical success, the clinical translation of TRPV1-targeted therapies has encountered challenges, including hyperthermia, hypothermia, pungency, and desensitization. Nevertheless, ongoing research efforts aim to refine TRPV1-targeted interventions through structural modifications, development of selective modulators, and discovery of natural, peptide-based drug candidates. Herein, we provide guidance for researchers and clinicians involved in the development of new interventions specifically targeting TRPV1 by reviewing the existing literature and highlighting current research activities. This study further discusses potential future research endeavors for enhancing the efficacy, safety, and tolerability of TRPV1 candidates, and thereby facilitates the translation of these discoveries into effective clinical interventions to alleviate neuropathic pain disorders.

Transient receptor potential vanilloid 1 involved in the analgesic effects of total flavonoids extracted from Longxuejie ()

OBJECTIVE

To evaluate the analgesic effects of total flavonoids of Longxuejie (Resina Dracaenae Cochinchinensis) (TFDB) and explore the possible analgesic mechanism associated with transient receptor potential vanilloid 1 (TRPV1).

METHODS

Whole-cell patch clamp technique was used to observe the effects of TFDB on capsaicin-induced TRPV1 currents. Rat experiments in vivo were used to observe the analgesic effects of TFDB. Western blot and immunofluorescence experiments were used to test the change of TRPV1 expression in DRG neurons induced by TFDB.

RESULTS

Results showed that TFDB inhibited capsaicin-induced TRPV1 receptor currents in acutely isolated dorsal root ganglion (DRG) neurons of rats and the half inhibitory concentration was (16.7 ± 1.6) mg/L. TFDB (2-20 mg/kg) showed analgesic activity in the phase Ⅱ of formalin test and (0.02-2 mg per paw) reduced capsaicin-induced licking times of rats. TFDB (20 mg/kg) was fully efficacious on complete Freund's adjuvant (CFA)-induced inflammatory thermal hyperalgesia and capsaicin could weaken the analgesic effects. The level of TRPV1 expressions of DRG neurons was also decreased in TFDB-treated CFA-inflammatory pain rats.

CONCLUSION

All these results indicated that the analgesic effect of TFDB may contribute to their modulations on both function and expression of TRPV1 channels in DRG neurons.

Identification of compounds that cause axonal dieback without cytotoxicity in dorsal root ganglia explants and intervertebral disc cells with potential to treat pain via denervation

Low back pain, knee osteoarthritis, and cancer patients suffer from chronic pain. Aberrant nerve growth into intervertebral disc, knee, and tumors, are common pathologies that lead to these chronic pain conditions. Axonal dieback induced by capsaicin (Caps) denervation has been FDA-approved to treat painful neuropathies and knee osteoarthritis but with short-term efficacy and discomfort. Herein, we propose to evaluate pyridoxine (Pyr), vincristine sulfate (Vcr) and ionomycin (Imy) as axonal dieback compounds for denervation with potential to alleviate pain. Previous literature suggests Pyr, Vcr, and Imy can cause undesired axonal degeneration, but no previous work has evaluated axonal dieback and cytotoxicity on adult rat dorsal root ganglia (DRG) explants. Thus, we performed axonal dieback screening using adult rat DRG explants in vitro with Caps as a positive control and assessed cytotoxicity. Imy inhibited axonal outgrowth and slowed axonal dieback, while Pyr and Vcr at high concentrations produced significant reduction in axon length and robust axonal dieback within three days. DRGs treated with Caps, Vcr, or Imy had increased DRG cytotoxicity compared to matched controls, but overall cytotoxicity was minimal and at least 88% lower compared to lysed DRGs. Pyr did not lead to any DRG cytotoxicity. Further, neither Pyr nor Vcr triggered intervertebral disc cell death or affected cellular metabolic activity after three days of incubation in vitro. Overall, our findings suggest Pyr and Vcr are not toxic to DRGs and intervertebral disc cells, and there is potential for repurposing these compounds for axonal dieback compounds to cause local denervation and alleviate pain.

Engineering Sensory Ganglion Multicellular System to Model Tissue Nerve Ingrowth

Discogenic pain is associated with deep nerve ingrowth in annulus fibrosus tissue (AF) of intervertebral disc (IVD). To model AF nerve ingrowth, primary bovine dorsal root ganglion (DRG) micro-scale tissue units are spatially organised around an AF explant by mild hydrodynamic forces within a collagen matrix. This results in a densely packed multicellular system mimicking the native DRG tissue morphology and a controlled AF-neuron distance. Such a multicellular organisation is essential to evolve populational-level cellular functions and in vivo-like morphologies. Pro-inflammatory cytokine-primed AF demonstrates its neurotrophic and neurotropic effects on nociceptor axons. Both effects are dependent on the AF-neuron distance underpinning the role of recapitulating inter-tissue/organ anatomical proximity when investigating their crosstalk. This is the first in vitro model studying AF nerve ingrowth by engineering mature and large animal tissues in a morphologically and physiologically relevant environment. The new approach can be used to biofabricate multi-tissue/organ models for untangling pathophysiological conditions and develop novel therapies.

Type I collagen concentration affects neurite outgrowth of adult rat DRG explants by altering mechanical properties of hydrogels

Type I collagen is a predominant fibrous protein that makes up the extracellular matrix. Collagen enhances cell attachment and is commonly used in three-dimensional culture systems, to mimic the native extracellular environment, for primary sensory neurons such as dorsal root ganglia (DRG). However, the effects of collagen concentration on adult rat DRG neurite growth have not been assessed in a physiologically relevant, three-dimensional culture. This study focuses on the effects of type I collagen used in a methacrylated hyaluronic acid (MAHA)-laminin-collagen gel (triple gel) on primary adult rat DRG explants in vitro. DRGs were cultured in triple gels, and the neurite lengths and number of support cells were quantified. Increased collagen concentration significantly reduced neurite length but did not affect support cell counts. Mechanical properties, fiber diameter, diffusivity, and mesh size of the triple gels with varying collagen concentration were characterized to further understand the effects of type I collagen on hydrogel property that may affect adult rat DRG explants. Gel stiffness significantly increased as collagen concentration increased and is correlated to DRG neurite length. Collagen concentration also significantly impacted fiber diameter but there was no correlation with DRG neurite length. Increasing collagen concentration had no significant effect on mesh size and diffusivity of the hydrogel. These data suggest that increasing type I collagen minimizes adult rat DRG explant growth in vitro while raising gel stiffness. This knowledge can help develop more robust 3D culture platforms to study sensory neuron growth and design biomaterials for nerve regeneration applications.

Cutaneous neurogenic inflammation mediated by TRPV1-NGF-TRKA pathway activation in rosacea is exacerbated by the presence of Demodex mites

BACKGROUND

Rosacea is a common chronic inflammatory skin condition that is often refractory to treatment, with frequent relapses. Alterations in the skin immunological response and Demodex mite infestation are the primary aetiologic factors targeted for treatment. Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a nociceptive cation channel that plays a role in cutaneous neurogenic pain and can be activated by various rosacea triggers.

OBJECTIVES

We investigated the effects of TRPV1 modulation in rosacea, focussing on Demodex mite colonization and cutaneous neurogenic inflammation.

METHODS

We examined mRNA expression levels according to Demodex population counts. An in vitro study using capsazepine as a TRPV1 antagonist was performed to assess the influence of TRPV1 in keratinocytes. A rosacea-like mouse model was generated by the injection of the 37-amino acid C-terminal cathelicidin peptide (LL37), and changes in the skin, dorsal root ganglion (DRG) and ears were examined.

RESULTS

Increased Demodex mite population counts were associated with increased expression levels of TRPV1, tropomyosin receptor kinase A (TrkA) and nerve growth factor (NGF), and these levels could be reduced by capsazepine treatment in keratinocytes. In an in vivo study, the downstream effects of TRPV1 activation were investigated in the skin, DRG and ears of the rosacea-like mouse model.

CONCLUSIONS

The findings of this study are instrumental for understanding the underlying causes of rosacea and could potentially lead to the development of new treatments targeting the NGF-TrkA-TRPV1 pathway. The identification of this pathway as a therapeutic target could represent a major breakthrough for rosacea research, potentially resulting in more effective and targeted rosacea treatments. This study contributes to an improved understanding of rosacea pathophysiology, which may lead to the development of more effective treatments in the future.

Antagonism of TRPV4 channels partially reduces mechanotransduction in rat skeletal muscle afferents

Mechanical distortion of working skeletal muscle induces sympathoexcitation via thin fibre afferents, a reflex response known as the skeletal muscle mechanoreflex. However, to date, the receptor ion channels responsible for mechanotransduction in skeletal muscle remain largely undetermined. Transient receptor potential vanilloid 4 (TRPV4) is known to sense mechanical stimuli such as shear stress or osmotic pressure in various organs. It is hypothesized that TRPV4 in thin-fibre primary afferents innervating skeletal muscle is involved in mechanotransduction. Fluorescence immunostaining revealed that 20.1 ± 10.1% of TRPV4 positive neurons were small dorsal root ganglion (DRG) neurons that were DiI-labelled, and among them 9.5 ± 6.1% of TRPV4 co-localized with the C-fibre marker peripherin. In vitro whole-cell patch clamp recordings from cultured rat DRG neurons demonstrated that mechanically activated current amplitude was significantly attenuated after the application of the TRPV4 antagonist HC067047 compared to control (P = 0.004). Such reductions were also observed in single-fibre recordings from a muscle-nerve ex vivo preparation where HC067047 significantly decreased afferent discharge to mechanical stimulation (P = 0.007). Likewise, in an in vivo decerebrate rat preparation, the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to passive stretch of hindlimb muscle were significantly reduced by intra-arterial injection of HC067047 (ΔRSNA: P = 0.019, ΔMAP: P = 0.002). The findings suggest that TRPV4 plays an important role in mechanotransduction contributing to the cardiovascular responses evoked by the skeletal muscle mechanoreflex during exercise. KEY POINTS: Although a mechanical stimulus to skeletal muscle reflexively activates the sympathetic nervous system, the receptors responsible for mechanotransduction in skeletal muscle thin fibre afferents have not been fully identified. Evidence suggests that TRPV4 is a mechanosensitive channel that plays an important role in mechanotransduction within various organs. Immunocytochemical staining demonstrates that TRPV4 is expressed in group IV skeletal muscle afferents. In addition, we show that the TRPV4 antagonist HC067047 decreases the responsiveness of thin fibre afferents to mechanical stimulation at the muscle tissue level as well as at the level of dorsal root ganglion neurons. Moreover, we demonstrate that intra-arterial HC067047 injection attenuates the sympathetic and pressor responses to passive muscle stretch in decerebrate rats. These data suggest that antagonism of TRPV4 attenuates mechanotransduction in skeletal muscle afferents. The present study demonstrates a probable physiological role for TRPV4 in the regulation of mechanical sensation in somatosensory thin fibre muscle afferents.

Peripheral ablation of type Ⅲ adenylyl cyclase induces hyperalgesia and eliminates KOR-mediated analgesia in mice

Ca2+/calmodulin-stimulated group Ⅰ adenylyl cyclase (AC) isoforms AC1 and AC8 have been involved in nociceptive processing and morphine responses. However, whether AC3, another member of group I ACs, is involved in nociceptive transmission and regulates opioid receptor signaling remain elusive. Here we report that conditional knockout of AC3 (AC3CKO) in L3 and L4 DRGs robustly facilitates the mouse nociceptive responses, decreases voltage-gated potassium (Kv) channel currents and increases neuronal excitability. Also, AC3CKO eliminates the analgesic effect of κ opioid receptor (KOR) agonist and its inhibition on Kv channel by classical Gαi/o signaling or nonclassical direct interaction of KOR and AC3 proteins. Interestingly, significantly upregulated AC1 level and cAMP concentration are detected in AC3 deficient DRGs. Inhibition of AC1 completely reversed cAMP upregulation, neuronal excitability enhancement and nociceptive behavioral hypersensitivity in AC3CKO mice. Our findings suggest a crucial role of peripheral AC3 in nociceptive modulation and KOR opioid analgesia.

RSV infection potentiates TRPV1-mediated calcium transport in bronchial epithelium of asthmatic children

The transient receptor potential vanilloid 1 (TRPV₁) channel is expressed in human bronchial epithelium (HBE), where it transduces Ca²⁺ in response to airborne irritants. TRPV₁ activation results in bronchoconstriction, cough, and mucus production, and may therefore contribute to the pathophysiology of obstructive airway disease. Since children with asthma face the greatest risk of developing virus-induced airway obstruction, we hypothesized that changes in TRPV₁ expression, localization, and function in the airway epithelium may play a role in bronchiolitis and asthma in childhood. We sought to measure TRPV₁ protein expression, localization, and function in HBE cells from children with versus without asthma, both at baseline and after RSV infection. We determined changes in TRPV₁ protein expression, subcellular localization, and function both at baseline and after RSV infection in primary HBE cells from normal children and children with asthma. Basal TRPV₁ protein expression was higher in HBE from children with versus without asthma and primarily localized to plasma membranes (PMs). During RSV infection, TRPV₁ protein increased more in the PM of asthmatic HBE as compared with nonasthmatic cells. TRPV₁-mediated increase in intracellular Ca²⁺ was greater in RSV-infected asthmatic cells, but this increase was attenuated when extracellular Ca²⁺ was removed. Nerve growth factor (NGF) recapitulated the effect of RSV on TRPV₁ activation in HBE cells. Our data suggest that children with asthma have intrinsically hyperreactive airways due in part to higher TRPV₁-mediated Ca²⁺ influx across epithelial membranes, and this abnormality is further exacerbated by NGF overexpression during RSV infection driving additional Ca²⁺ from intracellular stores.

Oncostatin M induces hyperalgesic priming and amplifies signaling of cAMP to ERK by RapGEF2 and PKA

Hyperalgesic priming is characterized by enhanced nociceptor sensitization by pronociceptive mediators, prototypically PGE₂ . Priming has gained interest as a mechanism underlying the transition to chronic pain. Which stimuli induce priming and what cellular mechanisms are employed remains incompletely understood. In adult male rats, we present the cytokine Oncostatin M (OSM), a member of the IL-6 family, as an inducer of priming by a novel mechanism. We used a high content microscopy based approach to quantify the activation of endogenous PKA-II and ERK of thousands sensory neurons in culture. Incubation with OSM increased and prolonged ERK activation by agents that increase cAMP production such as PGE₂ , forskolin, and cAMP analogs. These changes were specific to IB4/CaMKIIα positive neurons, required protein translation, and increased cAMP-to-ERK signaling. In both, control and OSM-treated neurons, cAMP/ERK signaling involved RapGEF2 and PKA but not Epac. Similar enhancement of cAMP-to-ERK signaling could be induced by GDNF, which acts mostly on IB4/CaMKIIα-positive neurons, but not by NGF, which acts mostly on IB4/CaMKIIα-negative neurons. In vitro, OSM pretreatment rendered baseline TTX-R currents ERK-dependent and switched forskolin-increased currents from partial to full ERK-dependence in small/medium sized neurons. In summary, priming induced by OSM uses a novel mechanism to enhance and prolong coupling of cAMP/PKA to ERK1/2 signaling without changing the overall pathway structure.

Label-free optical detection of bioelectric potentials using electrochromic thin films

Understanding how a network of interconnected neurons receives, stores, and processes information in the human brain is one of the outstanding scientific challenges of our time. The ability to reliably detect neuroelectric activities is essential to addressing this challenge. Optical recording using voltage-sensitive fluorescent probes has provided unprecedented flexibility for choosing regions of interest in recording neuronal activities. However, when recording at a high frame rate such as 500 to 1,000 Hz, fluorescence-based voltage sensors often suffer from photobleaching and phototoxicity, which limit the recording duration. Here, we report an approach called electrochromic optical recording (ECORE) that achieves label-free optical recording of spontaneous neuroelectrical activities. ECORE utilizes the electrochromism of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films, whose optical absorption can be modulated by an applied voltage. Being based on optical reflection instead of fluorescence, ECORE offers the flexibility of an optical probe without suffering from photobleaching or phototoxicity. Using ECORE, we optically recorded spontaneous action potentials in cardiomyocytes, cultured hippocampal and dorsal root ganglion neurons, and brain slices. With minimal perturbation to cells, ECORE allows long-term optical recording over multiple days.

Endometriosis-related chronic pelvic pain: A mini review on pathophysiology and impact on mental health

Endometriosis-related chronic pelvic pain is a major component of the disease that affects quality of life in women of reproductive age suffering from endometriosis. The present review summarizes current evidence upon pathophysiology and its impact on mental health. It seems that endometriosis-related chronic pelvic pain is the result of chronic stress on the central nervous system as a consequence of chronic pelvic inflammation. Mental health issues may rise as a result of central nervous system derangement and further aggravate pain perception and therefore quality of life. Further properly designed studies are needed in order to elucidate the connection between mental disorders and endometriosis-related chronic pelvic pain.

Injury-Induced Effectors of Neuropathic Pain

Injuries typically result in the development of neuropathic pain, which decreases in parallel with wound healing. However, the pain may remain after the injury appears to have healed, which is generally associated with an ongoing underlying pro-inflammatory state. Injury induces many cells to release factors that contribute to the development of a pro-inflammatory state, which is considered an essential first step towards wound healing. However, pain elimination requires a transition of the injury site from pro- to anti-inflammatory. Therefore, developing techniques that eliminate chronic pain require an understanding of the cells resident at and recruited to injury sites, the factors they release, that promote a pro-inflammatory state, and promote the subsequent transition of that site to be anti-inflammatory. Although a relatively large number of cells, factors, and gene expression changes are involved in these processes, it may be possible to control a relatively small number of them leading to the reduction and elimination of chronic neuropathic pain. This first of two papers examines the roles of the most salient cells and mediators associated with the development and maintenance of chronic neuropathic pain. The following paper examines the cells and mediators involved in reducing and eliminating chronic neuropathic pain.

Emerging neurotechnology for antinoceptive mechanisms and therapeutics discovery

The tolerance, abuse, and potential exacerbation associated with classical chronic pain medications such as opioids creates a need for alternative therapeutics. Phenotypic screening provides a complementary approach to traditional target-based drug discovery. Profiling cellular phenotypes enables quantification of physiologically relevant traits central to a disease pathology without prior identification of a specific drug target. For complex disorders such as chronic pain, which likely involves many molecular targets, this approach may identify novel treatments. Sensory neurons, termed nociceptors, are derived from dorsal root ganglia (DRG) and can undergo changes in membrane excitability during chronic pain. In this review, we describe phenotypic screening paradigms that make use of nociceptor electrophysiology. The purpose of this paper is to review the bioelectrical behavior of DRG neurons, signaling complexity in sensory neurons, various sensory neuron models, assays for bioelectrical behavior, and emerging efforts to leverage microfabrication and microfluidics for assay development. We discuss limitations and advantages of these various approaches and offer perspectives on opportunities for future development.

Constitutive activity of transient receptor potential vanilloid type 1 triggers spontaneous firing in nerve growth factor-treated dorsal root ganglion neurons of rats

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We examined the role of TRPV1 in the generation of spontaneous APs in NGF-treated cultured DRG neurons of rats.

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Spontaneous firing in the on-cell configuration was abolished by TRPV1 antagonists capsazepine and BCTC.

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Chronic treatment with NGF induced capsazepine- and BCTC-sensitive cation conductance.

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NGF-induced cation conductance through TRPV1 causes spontaneous firing.

Dorsal root ganglion (DRG) neurons cultured in the presence of nerve growth factor (NGF, 100 ng/ml) often show a spontaneous action potential. Underlying mechanisms of this spontaneous firing were examined using the patch clamp technique. The spontaneous firing in the on-cell configuration was abolished by a decrease in the Na⁺ concentration and by the TRPV1 antagonists capsazepine (10 μM) and BCTC (1 μM). These responses were accompanied by hyperpolarization of the resting potential. The holding current observed in neurons voltage clamped at –60 mV in the whole-cell configuration was significantly larger in the neurons that fired spontaneously, indicating that these neurons had an additional cation conductance that caused depolarization and triggered action potentials. The holding current in the firing neurons was decreased by extracellular Na⁺ reduction, capsazepine and BCTC. The amplitudes of the capsazepine- or BCTC-sensitive component of the holding current in the spontaneously firing neurons were ten times as large as those recorded in the other neurons showing no spontaneous firing. However, the amplitudes of the current responses to capsaicin (1 μM) were not different regardless of the presence of spontaneous firing or treatment with NGF. These results indicate that chronic NGF treatment of cultured DRG neurons in rats induces a constitutively active cation conductance through TRPV1, which depolarizes the neurons and triggers spontaneous action potentials in the absence of any stimuli. Since NGF in the DRG is reported to increase after nerve injury, this NGF-mediated regulation of TRPV1 may be a cause of the pathogenesis of neuropathic pain.

Adult mouse sensory neurons on microelectrode arrays exhibit increased spontaneous and stimulus-evoked activity in the presence of interleukin-6

Following inflammation or injury, sensory neurons located in the dorsal root ganglia (DRG) may exhibit increased spontaneous and/or stimulus-evoked activity, contributing to chronic pain. Current treatment options for peripherally mediated chronic pain are highly limited, driving the development of cell- or tissue-based phenotypic (function-based) screening assays for peripheral analgesic and mechanistic lead discovery. Extant assays are often limited by throughput, content, use of tumorigenic cell lines, or tissue sources from immature developmental stages (i.e., embryonic or postnatal). Here, we describe a protocol for culturing adult mouse DRG neurons on substrate-integrated multiwell microelectrode arrays (MEAs). This approach enables multiplexed measurements of spontaneous as well as stimulus-evoked extracellular action potentials from large populations of cells. The DRG cultures exhibit stable spontaneous activity from 9 to 21 days in vitro. Activity is readily evoked by known chemical and physical agonists of sensory neuron activity such as capsaicin, bradykinin, PGE₂, heat, and electrical field stimulation. Most importantly, we demonstrate that both spontaneous and stimulus-evoked activity may be potentiated by incubation with the inflammatory cytokine interleukin-6 (IL-6). Acute responsiveness to IL-6 is inhibited by treatment with a MAPK-interacting kinase 1/2 inhibitor, cercosporamide. In total, these findings suggest that adult mouse DRG neurons on multiwell MEAs are applicable to ongoing efforts to discover peripheral analgesic and their mechanisms of action. NEW & NOTEWORTHY This work describes methodologies for culturing spontaneously active adult mouse dorsal root ganglia (DRG) sensory neurons on microelectrode arrays. We characterize spontaneous and stimulus-evoked adult DRG activity over durations consistent with pharmacological interventions. Furthermore, persistent hyperexcitability could be induced by incubation with inflammatory cytokine IL-6 and attenuated with cercosporamide, an inhibitor of the IL-6 sensitization pathway. This constitutes a more physiologically relevant, moderate-throughput in vitro model for peripheral analgesic screening as well as mechanistic lead discovery.

Crosstalk from cAMP to ERK1/2 emerges during postnatal maturation of nociceptive neurons and is maintained during aging

Maturation of nociceptive neurons depends on changes in transcription factors, ion channels and neuropeptides. Mature nociceptors initiate pain in part by drastically reducing the activation threshold via intracellular sensitization signaling. Whether sensitization signaling also changes during development and aging remains so far unknown. Using a novel automated microscopy approach, we quantified changes in intracellular signaling protein expression and in their signaling dynamics, as well as changes in intracellular signaling cascade wiring, in sensory neurons from newborn to senescent (24 months of age) rats. We found that nociceptive subgroups defined by the signaling components protein kinase A (PKA)-RIIβ (also known as PRKAR2B) and CaMKIIα (also known as CAMK2A) developed at around postnatal day 10, the time of nociceptor maturation. The integrative nociceptor marker, PKA-RIIβ, allowed subgroup segregation earlier than could be achieved by assessing the classical markers TRPV1 and Na_v1.8 (also known as SCN10A). Signaling kinetics remained constant over lifetime despite in part strong changes in the expression levels. Strikingly, we found a mechanism important for neuronal memory - i.e. the crosstalk from cAMP and PKA to ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1, respectively) - to emerge postnatally. Thus, maturation of nociceptors is closely accompanied by altered expression, activation and connectivity of signaling pathways known to be central for pain sensitization and neuronal memory formation.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Development of a spontaneously active dorsal root ganglia assay using multiwell multielectrode arrays

In vitro phenotypic assays of sensory neuron activity are important tools for identifying potential analgesic compounds. These assays are typically characterized by hyperexcitable and/or abnormally, spontaneously active cells. Whereas manual electrophysiology experiments provide high-resolution biophysical data to characterize both in vitro models and potential therapeutic modalities (e.g., action potential characteristics, the role of specific ion channels, and receptors), these techniques are hampered by their low throughput. We have established a spontaneously active dorsal root ganglia (DRG) platform using multiwell multielectrode arrays (MEAs) that greatly increase the ability to evaluate the effects of multiple compounds and conditions on DRG excitability within the context of a cellular network. We show that spontaneous DRG firing can be attenuated with selective Na(+) and Ca(2+) channel blockers, as well as enhanced with K(+) channel blockers. In addition, spontaneous activity can be augmented with both the transient receptor potential cation channel subfamily V member 1 agonist capsaicin and the peptide bradykinin and completely blocked with neurokinin receptor antagonists. Finally, we validated the use of this assay by demonstrating that commonly used neuropathic pain therapeutics suppress DRG spontaneous activity. Overall, we have optimized primary rat DRG cells on a multiwell MEA platform to generate and characterize spontaneously active cultures that have the potential to be used as an in vitro phenotypic assay to evaluate potential therapeutics in rodent models of pain.

Persistent Nociception Triggered by Nerve Growth Factor (NGF) Is Mediated by TRPV1 and Oxidative Mechanisms

Nerve growth factor (NGF) is elevated in certain chronic pain conditions and is a sufficient stimulus to cause lasting pain in humans, but the actual mechanisms underlying the persistent effects of NGF remain incompletely understood. We developed a rat model of NGF-induced persistent thermal hyperalgesia and mechanical allodynia to determine the role of transient receptor potential vanilloid 1 (TRPV1) and oxidative mechanisms in the persistent effects of NGF. Persistent thermal hypersensitivity and mechanical allodynia require de novo protein translation and are mediated by TRPV1 and oxidative mechanisms. By comparing effects after systemic (subcutaneous), spinal (intrathecal) or hindpaw (intraplantar) injections of test compounds, we determined that TRPV1 and oxidation mediate persistent thermal hypersensitivity via peripheral and spinal sites of action and mechanical allodynia via only a spinal site of action. Therefore, NGF-evoked thermal and mechanical allodynia are mediated by spatially distinct mechanisms. NGF treatment evoked sustained increases in peripheral and central TRPV1 activity, as demonstrated by increased capsaicin-evoked nocifensive responses, increased calcitonin gene-related peptide release from hindpaw skin biopsies, and increased capsaicin-evoked inward current and membrane expression of TRPV1 protein in dorsal root ganglia neurons. Finally, we showed that NGF treatment increased concentrations of linoleic and arachidonic-acid-derived oxidized TRPV1 agonists in spinal cord and skin biopsies. Furthermore, increases in oxidized TRPV1-active lipids were reduced by peripheral and spinal injections of compounds that completely blocked persistent nociception. Collectively, these data indicate that NGF evokes a persistent nociceptive state mediated by increased TRPV1 activity and oxidative mechanisms, including increased production of oxidized lipid TRPV1 agonists.

The role of adenylyl cyclase isoform 6 in β-adrenoceptor signalling in murine airways

BACKGROUND AND PURPOSE

Adenylyl cyclase (AC) is a key signalling enzyme for many GPCRs and catalyses the conversion of ATP to cAMP which, in turn, is a crucial determinant of many biological responses. β-Adrenoceptor agonists are prescribed as bronchodilators for asthma and chronic obstructive pulmonary disease, and it is commonly assumed that they elicit their actions via AC-dependent production of cAMP. However, empirical evidence in support of this is lacking and the exact mechanism by which these drugs acts remains elusive. This is partly due to the existence of at least 10 different isoforms of AC and the absence of any truly selective pharmacological inhibitors. Here, we have used genetically modified mice and model systems to establish the role of AC isoforms in the airway responses to β-adrenoceptor agonists.

EXPERIMENTAL APPROACH

Receptors mediating responses to β-adrenoceptor agonists in airway smooth muscle (ASM) and sensory nerve were identified in isolated tissue systems. Expression of mRNA for the AC isoforms in ASM and neurones was determined by qPCR. Functional responses were assessed in AC isoform KO mice and wild-type controls.

KEY RESULTS

Airway and vagal tissue expressed mRNA for various isoforms of AC. AC6 was the most prominent isoform. Responses to β-adrenoceptor agonists in tissues from AC6 KO mice were virtually abolished.

CONCLUSIONS AND IMPLICATIONS

AC6 played a critical role in relaxation of ASM to β1 -adrenoceptor agonists and in modulation of sensory nerves by β1-3 -adrenoceptor agonists. These results further unravel the signalling pathway of this extensively prescribed class of medicine.

Properties, regulation, pharmacology, and functions of the K₂p channel, TRESK

TWIK-related spinal cord K(+) channel (TRESK) is the gene product of KCNK18, the last discovered leak potassium K2P channel gene. Under resting conditions, TRESK is constitutively phosphorylated at two regulatory regions. Protein kinase A (PKA) and microtubule affinity-regulating (MARK) kinases can be applied in experiments to phosphorylate these sites of TRESK expressed in Xenopus oocytes, respectively. Upon generation of a calcium signal, TRESK is dephosphorylated and thereby activated by calcineurin. In this process, the binding of calcineurin to the channel by non-catalytic interacting sites is essential. The phosphorylation/dephosphorylation regulatory process is modified by 14-3-3 proteins. Human, but not murine TRESK is also activated by protein kinase C. TRESK is expressed most abundantly in sensory neurons of the dorsal root ganglia (DRG) and trigeminal ganglia, and the channel modifies certain forms of nociceptive afferentation. In a large pedigree, a dominant negative mutant TRESK allele was found to co-segregate perfectly with migraine phenotype. While this genetic defect may be responsible only for a very small fraction of migraine cases, specific TRESK activation is expected to exert beneficial effect in common forms of the disease.

Neurotrophins and spinal circuit function

Work early in the last century emphasized the stereotyped activity of spinal circuits based on studies of reflexes. However, the last several decades have focused on the plasticity of these spinal circuits. These considerations began with studies of the effects of monoamines on descending and reflex circuits. In recent years new classes of compounds called growth factors that are found in peripheral nerves and the spinal cord have been shown to affect circuit behavior in the spinal cord. In this review we will focus on the effects of neurotrophins, particularly nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), on spinal circuits. We also discuss evidence that these molecules can modify functions including nociceptive behavior, motor reflexes and stepping behavior. Since these substances and their receptors are normally present in the spinal cord, they could potentially be useful in improving function in disease states and after injury. Here we review recent findings relevant to these translational issues.

Nerve growth factor improves functional recovery by inhibiting endoplasmic reticulum stress-induced neuronal apoptosis in rats with spinal cord injury

Background

Endoplasmic reticulum (ER) stress-induced apoptosis plays a major role in various diseases, including spinal cord injury (SCI). Nerve growth factor (NGF) show neuroprotective effect and improve the recovery of SCI, but the relations of ER stress-induced apoptosis and the NGF therapeutic effect in SCI still unclear.

Methods

Young adult female Sprague-Dawley rats’s vertebral column was exposed and a laminectomy was done at T9 vertebrae and moderate contusion injuries were performed using a vascular clip. NGF stock solution was diluted with 0.9% NaCl and administered intravenously at a dose of 20 μg/kg/day after SCI and then once per day until they were executed. Subsequently, the rats were executed at 1d, 3 d, 7d and 14d. The locomotor activities of SCI model rats were tested by the 21-point Basso-Beattie-Bresnahan (BBB) locomotion scale, inclined plane test and footprint analysis. In addition, Western blot analysis was performed to identify the expression of ER-stress related proteins including CHOP, GRP78 and caspase-12 both in vivo and in vitro. The level of cell apoptosis was determined by TUNEL in vivo and Flow cytometry in vitro. Relative downstream signals Akt/GSK-3β and ERK1/2were also analyzed with or without inhibitors in vitro.

Results

Our results demonstrated that ER stress-induced apoptosis was involved in the injury of SCI model rats. NGF administration improved the motor function recovery and increased the neurons survival in the spinal cord lesions of the model rats. NGF decreases neuron apoptosis which measured by TUNEL and inhibits the activation of caspase-3 cascade. The ER stress-induced apoptosis response proteins CHOP, GRP78 and caspase-12 are inhibited by NGF treatment. Meanwhile, NGF administration also increased expression of growth-associated protein 43 (GAP43). The administration of NGF activated downstream signals Akt/GSK-3β and ERK1/2 in ER stress cell model in vitro.

Conclusion

The neuroprotective role of NGF in the recovery of SCI is related to the inhibition of ER stress-induced cell death via the activation of downstream signals, also suggested a new trend of NGF translational drug development in the central neural system injuries which involved in the regulation of chronic ER stress.

Expression of transient receptor potential vanilloid 1 and anoctamin 1 in rat trigeminal ganglion neurons innervating the tongue

Transient receptor potential vanilloid 1 (TRPV1) is a polymodal sensor that is activated by heat (>43 °C), acid, or capsaicin, the pungent ingredient of hot peppers. Reports that mice lacking TRPV1 display heat avoidance behaviors and TRPV1-negative neurons respond to heat suggest that an additional heat sensor is present. Anoctamin 1 (ANO1; also known as transmembrane protein 16A [TMEM16A]), is a component of Ca(2+)-activated chloride channels (CaCCs), and has been recently identified as a heat sensor, activated by temperatures over 44 °C. ANO1 is highly co-localized with TRPV1 in small-diameter dorsal root ganglion (DRG) neurons. The aim of the present study was to investigate co-expression of ANO1 and TRPV1 in rat trigeminal ganglion (TG) neurons innervating the tongue by using retrograde labeling and immunohistochemical techniques. Fluoro-gold (FG) retrograde labeling was used to identify the TG neurons innervating the anterior two thirds of the tongue; as expected, most labeling was detected in the mandibular division of the TGs. The FG-labeled TG neurons showed TRPV1 immunoreactivity (17.9%) and ANO1 immunoreactivity (13.7%), indicating that TRPV1- and ANO1-expressing neurons were present in the mandibular division of the TGs. Seventy-six percent of the ANO1-immunoreactive TG neurons were also immunoreactive for TRPV1; this co-expression was mainly detected in small- to medium-diameter TG neurons. The high degree of co-expression of TRPV1 and ANO1 suggests that cooperation between ANO1 and TRPV1 plays a role in the signaling pathways of nociceptive TG neurons.

Despite differences in cytosolic calcium regulation, lidocaine toxicity is similar in adult and neonatal rat dorsal root ganglia in vitro

BACKGROUND

Neuraxial local anesthetics may have neurological complications thought to be due to neurotoxicity. A primary site of action of local anesthetics is the dorsal root ganglia (DRG) neuron. Physiologic differences have been noted between young and adult DRG neurons; hence, the authors examined whether there were any differences in lidocaine-induced changes in calcium and lidocaine toxicity in neonatal and adult rat DRG neurons.

METHODS

DRG neurons were cultured from postnatal day 7 (P7) and adult rats. Lidocaine-induced changes in cytosolic calcium were examined with the calcium indicator Fluo-4. Cells were incubated with varying concentrations of lidocaine and examined for viability using calcein AM and ethidium homodimer-1 staining. Live imaging of caspase-3/7 activation was performed after incubation with lidocaine.

RESULTS

The mean KCl-induced calcium transient was greater in P7 neurons (P < 0.05), and lidocaine significantly inhibited KCl-induced calcium responses in both ages (P < 0.05). Frequency distribution histograms of KCl-evoked calcium increases were more heterogeneous in P7 than in adult neurons. With lidocaine, KCl-induced calcium transients in both ages became more homogeneous but remained different between the groups. Interestingly, cell viability was decreased by lidocaine in a dose-dependent manner similarly in both ages. Lidocaine treatment also activated caspase-3/7 in a dose- and time-dependent manner similarly in both ages.

CONCLUSIONS

Despite physiological differences in P7 and adult DRG neurons, lidocaine cytotoxicity is similar in P7 and adult DRG neurons in vitro. Differences in lidocaine- and KCl-evoked calcium responses suggest the similarity in lidocaine cytotoxicity involves other actions in addition to lidocaine-evoked effects on cytosolic calcium responses.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

TRPV1 and TRPA1 antagonists prevent the transition of acute to chronic inflammation and pain in chronic pancreatitis

Visceral afferents expressing transient receptor potential (TRP) channels TRPV1 and TRPA1 are thought to be required for neurogenic inflammation and development of inflammatory hyperalgesia. Using a mouse model of chronic pancreatitis (CP) produced by repeated episodes (twice weekly) of caerulein-induced AP (AP), we studied the involvement of these TRP channels in pancreatic inflammation and pain-related behaviors. Antagonists of the two TRP channels were administered at different times to block the neurogenic component of AP. Six bouts of AP (over 3 wks) increased pancreatic inflammation and pain-related behaviors, produced fibrosis and sprouting of pancreatic nerve fibers, and increased TRPV1 and TRPA1 gene transcripts and a nociceptive marker, pERK, in pancreas afferent somata. Treatment with TRP antagonists, when initiated before week 3, decreased pancreatic inflammation and pain-related behaviors and also blocked the development of histopathological changes in the pancreas and upregulation of TRPV1, TRPA1, and pERK in pancreatic afferents. Continued treatment with TRP antagonists blocked the development of CP and pain behaviors even when mice were challenged with seven more weeks of twice weekly caerulein. When started after week 3, however, treatment with TRP antagonists was ineffective in blocking the transition from AP to CP and the emergence of pain behaviors. These results suggest: (1) an important role for neurogenic inflammation in pancreatitis and pain-related behaviors, (2) that there is a transition from AP to CP, after which TRP channel antagonism is ineffective, and thus (3) that early intervention with TRP channel antagonists may attenuate the transition to and development of CP effectively.

Fetal ethanol exposure attenuates aversive oral effects of TrpV1, but not TrpA1 agonists in rats

In humans, fetal ethanol exposure is highly predictive of adolescent ethanol use and abuse. Prior work in our labs indicated that fetal ethanol exposure results in stimulus-induced chemosensory plasticity in the taste and olfactory systems of adolescent rats. In particular, we found that increased ethanol acceptability could be attributed, in part, to an attenuated aversion to ethanol's aversive odor and quinine-like bitter taste quality. Here, we asked whether fetal ethanol exposure also alters the oral trigeminal response of adolescent rats to ethanol. We focused on two excitatory ligand-gated ion channels, TrpV1 and TrpA1, which are expressed in oral trigeminal neurons and mediate the aversive orosensory response to many chemical irritants. To target TrpV1, we used capsaicin, and to target TrpA1, we used allyl isothiocyanate (or mustard oil). We assessed the aversive oral effects of ethanol, together with capsaicin and mustard oil, by measuring short-term licking responses to a range of concentrations of each chemical. Experimental rats were exposed in utero by administering ethanol to dams through a liquid diet. Control rats had ad libitum access to an iso-caloric iso-nutritive liquid diet. We found that fetal ethanol exposure attenuated the oral aversiveness of ethanol and capsaicin, but not mustard oil, in adolescent rats. Moreover, the increased acceptability of ethanol was directly related to the reduced aversiveness of the TrpV1-mediated orosensory input. We propose that fetal ethanol exposure increases ethanol avidity not only by making ethanol smell and taste better, but also by attenuating ethanol's capsaicin-like burning sensations.