α-Conotoxin Vc1.1 inhibits human dorsal root ganglion neuroexcitability and mouse colonic nociception via GABAB receptors

Experimental colitis-induced visceral hypersensitivity is attenuated by GABA treatment in mice

Ulcerative colitis (UC) is linked with inflammation of the large intestine due to an overactive response of the colon-immune system. UC is associated with weight loss, rectal bleeding, diarrhea, and abdominal pain. Given that γ-amino butyric acid (GABA) suppresses immune cell activity and the excitability of colonic afferents, and that there is a decrease in colonic GABA during UC, we hypothesized that UC pain is due to a decrease in the inhibition of colonic afferents. Thus, restoring GABA in the colon will attenuate inflammatory hypersensitivity. We tested this hypothesis in a mouse model of colitis. Colon inflammation was induced with seven days of dextran sodium sulfate (DSS, 3%) in the drinking water. GABA (40 mg/kg) was administered orally for the same period as DSS, and body weight, colon length, colon permeability, clinical progression of colitis (disease activity index or DAI), and colon histological score (HS) were assessed to determine the effects of GABA on colitis. A day after the end of GABA treatment, visceral sensitivity was assessed with balloon distention (of the colon)-evoked visceromotor response and colon samples were collected for the measurement of GABA and cytokines. Treatment with GABA reduced the DSS-induced increase in the colon permeability, DAI, HS, and decrease in body weight and colon length. Furthermore, GABA inhibited the DSS-induced increase in the proinflammatory cytokines tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-12 (IL-12), and increased the expression of the anti-inflammatory cytokine IL-10 in the colon tissue. Importantly, GABA reduced DSS-induced visceral hypersensitivity. These data suggest that increasing gastrointestinal levels of GABA may be useful for the treatment of colitis.NEW & NOTEWORTHY GABA treatment reduces the severity of colitis and inflammation and produces inhibition of visceral hypersensitivity in colon-inflamed mice. These results raise the promising possibility that GABA treatment may be an effective therapeutic strategy for the management of symptoms associated with colitis. However, clinical studies are required to corroborate whether this mouse-model data translates to human colon.

Role of Diet-Microbiome Interaction in Gastrointestinal Disorders and Strategies to Modulate Them with Microbiome-Targeted Therapies

Diet is an important determinant of health and consequently is often implicated in the development of disease, particularly gastrointestinal (GI) diseases, given the high prevalence of meal-related symptoms. The mechanisms underlying diet-driven pathophysiology are not well understood, but recent studies suggest that gut microbiota may mediate the effect of diet on GI physiology. In this review, we focus primarily on two distinct GI diseases where the role of diet has been best studied: irritable bowel syndrome and inflammatory bowel disease. We discuss how the concurrent and sequential utilization of dietary nutrients by the host and gut microbiota determines the eventual bioactive metabolite profiles in the gut and the biological effect of these metabolites on GI physiology. We highlight several concepts that can be gleaned from these findings, such as how distinct effects of an individual metabolite can influence diverse GI diseases, the effect of similar dietary interventions on multiple disease states, and the need for extensive phenotyping and data collection to help make personalized diet recommendations.

DRG afferents that mediate physiologic and pathologic mechanosensation from the distal colon

The properties of dorsal root ganglia (DRG) neurons that innervate the distal colon are poorly defined, hindering our understanding of their roles in normal physiology and gastrointestinal (GI) disease. Here, we report genetically defined subsets of colon-innervating DRG neurons with diverse morphologic and physiologic properties. Four colon-innervating DRG neuron populations are mechanosensitive and exhibit distinct force thresholds to colon distension. The highest threshold population, selectively labeled using Bmpr1b genetic tools, is necessary and sufficient for behavioral responses to high colon distension, which is partly mediated by the mechanosensory ion channel Piezo2. This Aδ-HTMR population mediates behavioral over-reactivity to colon distension caused by inflammation in a model of inflammatory bowel disease. Thus, like cutaneous DRG mechanoreceptor populations, colon-innervating mechanoreceptors exhibit distinct anatomical and physiological properties and tile force threshold space, and genetically defined colon-innervating HTMRs mediate pathophysiological responses to colon distension, revealing a target population for therapeutic intervention.

Intestinal neuropod cell GUCY2C regulates visceral pain

Visceral pain (VP) is a global problem with complex etiologies and limited therapeutic options. Guanylyl cyclase C (GUCY2C), an intestinal receptor producing cyclic GMP(cGMP), which regulates luminal fluid secretion, has emerged as a therapeutic target for VP. Indeed, FDA-approved GUCY2C agonists ameliorate VP in patients with chronic constipation syndromes, although analgesic mechanisms remain obscure. Here, we revealed that intestinal GUCY2C was selectively enriched in neuropod cells, a type of enteroendocrine cell that synapses with submucosal neurons in mice and humans. GUCY2Chi neuropod cells associated with cocultured dorsal root ganglia neurons and induced hyperexcitability, reducing the rheobase and increasing the resulting number of evoked action potentials. Conversely, the GUCY2C agonist linaclotide eliminated neuronal hyperexcitability produced by GUCY2C-sufficient - but not GUCY2C-deficient - neuropod cells, an effect independent of bulk epithelial cells or extracellular cGMP. Genetic elimination of intestinal GUCY2C amplified nociceptive signaling in VP that was comparable with chemically induced VP but refractory to linaclotide. Importantly, eliminating GUCY2C selectively in neuropod cells also increased nociceptive signaling and VP that was refractory to linaclotide. In the context of loss of GUCY2C hormones in patients with VP, these observations suggest a specific role for neuropod GUCY2C signaling in the pathophysiology and treatment of these pain syndromes.

Multi-omics for biomarker approaches in the diagnostic evaluation and management of abdominal pain and irritable bowel syndrome: what lies ahead

Reliable biomarkers for common disorders of gut-brain interaction characterized by abdominal pain, including irritable bowel syndrome (IBS), are critically needed to enhance care and develop individualized therapies. The dynamic and heterogeneous nature of the pathophysiological mechanisms that underlie visceral hypersensitivity have challenged successful biomarker development. Consequently, effective therapies for pain in IBS are lacking. However, recent advances in modern omics technologies offer new opportunities to acquire deep biological insights into mechanisms of pain and nociception. Newer methods for large-scale data integration of complementary omics approaches have further expanded our ability to build a holistic understanding of complex biological networks and their co-contributions to abdominal pain. Here, we review the mechanisms of visceral hypersensitivity, focusing on IBS. We discuss candidate biomarkers for pain in IBS identified through single omics studies and summarize emerging multi-omics approaches for developing novel biomarkers that may transform clinical care for patients with IBS and abdominal pain.

TGR5 agonists induce peripheral and central hypersensitivity to bladder distension

The mechanisms underlying chronic bladder conditions such as interstitial cystitis/bladder pain syndrome (IC/BPS) and overactive bladder syndrome (OAB) are incompletely understood. However, targeting specific receptors mediating neuronal sensitivity to specific stimuli is an emerging treatment strategy. Recently, irritant-sensing receptors including the bile acid receptor TGR5, have been identified within the viscera and are thought to play a key role in neuronal hypersensitivity. Here, in mice, we identify mRNA expression of TGR5 (Gpbar1) in all layers of the bladder as well as in the lumbosacral dorsal root ganglia (DRG) and in isolated bladder-innervating DRG neurons. In bladder-innervating DRG neurons Gpbar1 mRNA was 100% co-expressed with Trpv1 and 30% co-expressed with Trpa1. In vitro live-cell calcium imaging of bladder-innervating DRG neurons showed direct activation of a sub-population of bladder-innervating DRG neurons with the synthetic TGR5 agonist CCDC, which was diminished in Trpv1-/- but not Trpa1-/- DRG neurons. CCDC also activated a small percentage of non-neuronal cells. Using an ex vivo mouse bladder afferent recording preparation we show intravesical application of endogenous (5α-pregnan-3β-ol-20-one sulphate, Pg5α) and synthetic (CCDC) TGR5 agonists enhanced afferent mechanosensitivity to bladder distension. Correspondingly, in vivo intravesical administration of CCDC increased the number of spinal dorsal horn neurons that were activated by bladder distension. The enhanced mechanosensitivity induced by CCDC ex vivo and in vivo was absent using Gpbar1-/- mice. Together, these results indicate a role for the TGR5 receptor in mediating bladder afferent hypersensitivity to distension and thus may be important to the symptoms associated with IC/BPS and OAB.

Modern venomics-Current insights, novel methods, and future perspectives in biological and applied animal venom research

Venoms have evolved >100 times in all major animal groups, and their components, known as toxins, have been fine-tuned over millions of years into highly effective biochemical weapons. There are many outstanding questions on the evolution of toxin arsenals, such as how venom genes originate, how venom contributes to the fitness of venomous species, and which modifications at the genomic, transcriptomic, and protein level drive their evolution. These questions have received particularly little attention outside of snakes, cone snails, spiders, and scorpions. Venom compounds have further become a source of inspiration for translational research using their diverse bioactivities for various applications. We highlight here recent advances and new strategies in modern venomics and discuss how recent technological innovations and multi-omic methods dramatically improve research on venomous animals. The study of genomes and their modifications through CRISPR and knockdown technologies will increase our understanding of how toxins evolve and which functions they have in the different ontogenetic stages during the development of venomous animals. Mass spectrometry imaging combined with spatial transcriptomics, in situ hybridization techniques, and modern computer tomography gives us further insights into the spatial distribution of toxins in the venom system and the function of the venom apparatus. All these evolutionary and biological insights contribute to more efficiently identify venom compounds, which can then be synthesized or produced in adapted expression systems to test their bioactivity. Finally, we critically discuss recent agrochemical, pharmaceutical, therapeutic, and diagnostic (so-called translational) aspects of venoms from which humans benefit.

On the Utility of Chemical Strategies to Improve Peptide Gut Stability

Inherent susceptibility of peptides to enzymatic degradation in the gastrointestinal tract is a key bottleneck in oral peptide drug development. Here, we present a systematic analysis of (i) the gut stability of disulfide-rich peptide scaffolds, orally administered peptide therapeutics, and well-known neuropeptides and (ii) medicinal chemistry strategies to improve peptide gut stability. Among a broad range of studied peptides, cyclotides were the only scaffold class to resist gastrointestinal degradation, even when grafted with non-native sequences. Backbone cyclization, a frequently applied strategy, failed to improve stability in intestinal fluid, but several site-specific alterations proved efficient. This work furthermore highlights the importance of standardized gut stability test conditions and suggests defined protocols to facilitate cross-study comparison. Together, our results provide a comparative overview and framework for the chemical engineering of gut-stable peptides, which should be valuable for the development of orally administered peptide therapeutics and molecular probes targeting receptors within the gastrointestinal tract.

Biomedical Potential of the Neglected Molluscivorous and Vermivorous Conus Species

Within the Conidae family, the piscivorous Conus species have been a hotspot target for drug discovery. Here, we assess the relevance of Conus and their other feeding habits, and thus under distinctive evolutionary constraints, to highlight the potential of neglected molluscivorous and vermivorous species in biomedical research and pharmaceutical industry. By singling out the areas with inadequate Conus disquisition, such as the Tamil Nadu Coast and the Andaman Islands, research resources can be expanded and better protected through awareness. In this study, 728 Conus species and 190 species from three other genera (1 from Californiconus, 159 from Conasprella and 30 from Profundiconus) in the Conidae family are assessed. The phylogenetic relationships of the Conidae species are determined and their known feeding habits superimposed. The worm-hunting species appeared first, and later the mollusc- and fish-hunting species were derived independently in the Neogene period (around 23 million years ago). Interestingly, many Conus species in the warm and shallow waters become polyphagous, allowing them to hunt both fish and worms, given the opportunities. Such newly gained trait is multi originated. This is controversial, given the traditional idea that most Conus species are specialized to hunt certain prey categories. However, it shows the functional complexity and great potential of conopeptides from some worm-eating species. Pharmaceutical attempts and relevant omics data have been differentially obtained. Indeed, data from the fish-hunting species receive strong preference over the worm-hunting ones. Expectedly, conopeptides from the fish-hunting species are believed to include the most potential candidates for biomedical research. Our work revisits major findings throughout the Conus evolution and emphasizes the importance of increasing omics surveys complemented with further behavior observation studies. Hence, we claim that Conus species and their feeding habits are equally important, highlighting many places left for Conus exploration worldwide. We also discuss the Conotoxin drug discovery potentials and the urgency of protecting the bioresources of Conus species. In particular, some vermivorous species have demonstrated great potential in malaria therapy, while other conotoxins from several worm- and mollusc-eating species exhibited explicit correlation with SARS-CoV-2. Reclaiming idle data with new perspectives could also promote interdisciplinary studies in both virological and toxicological fields.

Olorinab (APD371), a peripherally acting, highly selective, full agonist of the cannabinoid receptor 2, reduces colitis-induced acute and chronic visceral hypersensitivity in rodents

ABSTRACT

Abdominal pain is a key symptom of inflammatory bowel disease and irritable bowel syndrome, for which there are inadequate therapeutic options. We tested whether olorinab-a highly selective, full agonist of the cannabinoid receptor 2 (CB2)-reduced visceral hypersensitivity in models of colitis and chronic visceral hypersensitivity (CVH). In rodents, colitis was induced by intrarectal administration of nitrobenzene sulfonic acid derivatives. Control or colitis animals were administered vehicle or olorinab (3 or 30 mg/kg) twice daily by oral gavage for 5 days, starting 1 day before colitis induction. Chronic visceral hypersensitivity mice were administered olorinab (1, 3, 10, or 30 mg/kg) twice daily by oral gavage for 5 days, starting 24 days after colitis induction. Visceral mechanosensitivity was assessed in vivo by quantifying visceromotor responses (VMRs) to colorectal distension. Ex vivo afferent recordings determined colonic nociceptor firing evoked by mechanical stimuli. Colitis and CVH animals displayed significantly elevated VMRs to colorectal distension and colonic nociceptor hypersensitivity. Olorinab treatment significantly reduced VMRs to control levels in colitis and CVH animals. In addition, olorinab reduced nociceptor hypersensitivity in colitis and CVH states in a concentration- and CB2-dependent manner. By contrast, olorinab did not alter VMRs nor nociceptor responsiveness in control animals. Cannabinoid receptor 2 mRNA was detected in colonic tissue, particularly within epithelial cells, and dorsal root ganglia, with no significant differences between healthy, colitis, and CVH states. These results demonstrate that olorinab reduces visceral hypersensitivity through CB2 agonism in animal models, suggesting that olorinab may provide a novel therapy for inflammatory bowel disease- and irritable bowel syndrome-associated abdominal pain.

GABAB Receptors and Pain

A substantial fraction of the human population suffers from chronic pain states, which often cannot be sufficiently treated with existing drugs. This calls for alternative targets and strategies for the development of novel analgesics. There is substantial evidence that the G protein-coupled GABA_B receptor is involved in the processing of pain signals and thus has long been considered a valuable target for the generation of analgesics to treat chronic pain. In this review, the contribution of GABA_B receptors to the generation and modulation of pain signals, their involvement in chronic pain states as well as their target suitability for the development of novel analgesics is discussed.

From Crystal Structures of RgIA4 in Complex with Ac-AChBP to Molecular Determinants of Its High Potency of α9α10 nAChR

α9-containing nicotinic acetylcholine receptors (nAChRs) have been shown to play critical roles in neuropathic pain. The α-conotoxin (α-CTx) RgIA and its analog RgIA4 were identified as the most selective inhibitor of α9α10 nAChR. However, the mechanism of their selectivity toward α9α10 nAChR remains elusive. Here, we reported the co-crystal structure of RgIA and RgIA4 in complex with Aplysia californica acetylcholine binding protein (Ac-AChBP) at resolution of 2.6 Å, respectively. Based on the structure of the complexes, together with molecular dynamic simulation (MD-simulation), we suggested the key residues of α9α10 nAChR in determining its high affinity for RgIA/RgIA4. This is the first time the complex between pain-related conotoxins and Ac-AChBP was reported and the complementary side of α9 subunit in binding of the antagonists shown. These results provide realistic template for the design of new therapeutic in neuropathic pain.

Analgesic α-conotoxins modulate native and recombinant GIRK1/2 channels via activation of GABAB receptors and reduce neuroexcitability

BACKGROUND AND PURPOSE

Activation of GIRK channels via G protein-coupled GABA_B receptors has been shown to attenuate nociceptive transmission. The analgesic α-conotoxin Vc1.1 activates GABA_B receptors resulting in inhibition of Ca_v 2.2 and Ca_v 2.3 channels in mammalian primary afferent neurons. Here, we investigated the effects of analgesic α-conotoxins on recombinant and native GIRK-mediated K⁺ currents and on neuronal excitability.

EXPERIMENTAL APPROACH

The effects of analgesic α-conotoxins, Vc1.1, RgIA, and PeIA, were investigated on inwardly-rectifying K⁺ currents in HEK293T cells recombinantly co-expressing either heteromeric human GIRK1/2 or homomeric GIRK2 subunits, with GABA_B receptors. The effects of α-conotoxin Vc1.1 and baclofen were studied on GIRK-mediated K⁺ currents and the passive and active electrical properties of adult mouse dorsal root ganglion neurons.

KEY RESULTS

Analgesic α-conotoxins Vc1.1, RgIA, and PeIA potentiate inwardly-rectifying K⁺ currents in HEK293T cells recombinantly expressing human GIRK1/2 channels and GABA_B receptors. GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen occurs via a pertussis toxin-sensitive G protein and is inhibited by the selective GABA_B receptor antagonist CGP 55845. In adult mouse dorsal root ganglion neurons, GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen hyperpolarizes the cell membrane potential and reduces excitability.

CONCLUSIONS AND IMPLICATIONS

This is the first report of GIRK channel potentiation via allosteric α-conotoxin Vc1.1-GABA_B receptor agonism, leading to decreased neuronal excitability. Such action potentially contributes to the analgesic effects of Vc1.1 and baclofen observed in vivo.

Pharmacological Inhibition of the Voltage-Gated Sodium Channel NaV1.7 Alleviates Chronic Visceral Pain in a Rodent Model of Irritable Bowel Syndrome

The human nociceptor-specific voltage-gated sodium channel 1.7 (hNa_V1.7) is critical for sensing various types of somatic pain, but it appears not to play a primary role in acute visceral pain. However, its role in chronic visceral pain remains to be determined. We used assay-guided fractionation to isolate a novel hNa_V1.7 inhibitor, Tsp1a, from tarantula venom. Tsp1a is 28-residue peptide that potently inhibits hNa_V1.7 (IC₅₀ = 10 nM), with greater than 100-fold selectivity over hNa_V1.3-hNa_V1.6, 45-fold selectivity over hNa_V1.1, and 24-fold selectivity over hNa_V1.2. Tsp1a is a gating modifier that inhibits Na_V1.7 by inducing a hyperpolarizing shift in the voltage-dependence of channel inactivation and slowing recovery from fast inactivation. NMR studies revealed that Tsp1a adopts a classical knottin fold, and like many knottin peptides, it is exceptionally stable in human serum. Remarkably, intracolonic administration of Tsp1a completely reversed chronic visceral hypersensitivity in a mouse model of irritable bowel syndrome. The ability of Tsp1a to reduce visceral hypersensitivity in a model of irritable bowel syndrome suggests that pharmacological inhibition of hNa_V1.7 at peripheral sensory nerve endings might be a viable approach for eliciting analgesia in patients suffering from chronic visceral pain.

Vitamin B-6-Induced Neuropathy: Exploring the Mechanisms of Pyridoxine Toxicity

Vitamin B-6 in the form of pyridoxine (PN) is commonly used by the general population. The use of PN-containing supplements has gained lots of attention over the past years as they have been related to the development of peripheral neuropathy. In light of this, the number of reported cases of adverse health effects due to the use of vitamin B-6 have increased. Despite a long history of study, the pathogenic mechanisms associated with PN toxicity remain elusive. Therefore, the present review is focused on investigating the mechanistic link between PN supplementation and sensory peripheral neuropathy. Excessive PN intake induces neuropathy through the preferential injury of sensory neurons. Recent reports on hereditary neuropathy due to pyridoxal kinase (PDXK) mutations may provide some insight into the mechanism, as genetic deficiencies in PDXK lead to the development of axonal sensory neuropathy. High circulating concentrations of PN may lead to a similar condition via the inhibition of PDXK. The mechanism behind PDXK-induced neuropathy is unknown; however, there is reason to believe that it may be related to γ-aminobutyric acid (GABA) neurotransmission. Compounds that inhibit PDXK lead to convulsions and reductions in GABA biosynthesis. The absence of central nervous system-related symptoms in PDXK deficiency could be due to differences in the regulation of PDXK, where PDXK activity is preserved in the brain but not in peripheral tissues. As PN is relatively impermeable to the blood-brain barrier, PDXK inhibition would similarly be confined to the peripheries and, as a result, GABA signaling may be perturbed within peripheral tissues, such as sensory neurons. Perturbed GABA signaling within sensory neurons may lead to excitotoxicity, neurodegeneration, and ultimately, the development of peripheral neuropathy. For several reasons, we conclude that PDXK inhibition and consequently disrupted GABA neurotransmission is the most plausible mechanism of toxicity.

Activation of MrgprA3 and MrgprC11 on Bladder-Innervating Afferents Induces Peripheral and Central Hypersensitivity to Bladder Distension

Understanding the sensory mechanisms innervating the bladder is paramount to developing efficacious treatments for chronic bladder hypersensitivity conditions. The contribution of Mas-gene-related G protein-coupled receptors (Mrgpr) to bladder signaling is currently unknown. Using male and female mice, we show with single-cell RT-PCR that subpopulations of DRG neurons innervating the mouse bladder express MrgprA3 (14%) and MrgprC11 (38%), either individually or in combination, with high levels of coexpression with Trpv1 (81%-89%). Calcium imaging studies demonstrated MrgprA3 and MrgprC11 agonists (chloroquine, BAM8-22, and neuropeptide FF) activated subpopulations of bladder-innervating DRG neurons, showing functional evidence of coexpression between MrgprA3, MrgprC11, and TRPV1. In ex vivo bladder-nerve preparations, chloroquine, BAM8-22, and neuropeptide FF all evoked mechanical hypersensitivity in subpopulations (20%-41%) of bladder afferents. These effects were absent in recordings from Mrgpr-clusterΔ^-/- mice. In vitro whole-cell patch-clamp recordings showed that application of an MrgprA3/C11 agonist mixture induced neuronal hyperexcitability in 44% of bladder-innervating DRG neurons. Finally, in vivo instillation of an MrgprA3/C11 agonist mixture into the bladder of WT mice induced a significant activation of dorsal horn neurons within the lumbosacral spinal cord, as quantified by pERK immunoreactivity. This MrgprA3/C11 agonist-induced activation was particularly apparent within the superficial dorsal horn and the sacral parasympathetic nuclei of WT, but not Mrgpr-clusterΔ^-/- mice. This study demonstrates, for the first time, functional expression of MrgprA3 and MrgprC11 in bladder afferents. Activation of these receptors triggers hypersensitivity to distension, a critically valuable factor for therapeutic target development.SIGNIFICANCE STATEMENT Determining how bladder afferents become sensitized is the first step in finding effective treatments for common urological disorders such as overactive bladder and interstitial cystitis/bladder pain syndrome. Here we show that two of the key receptors, MrgprA3 and MrgprC11, that mediate itch from the skin are also expressed on afferents innervating the bladder. Activation of these receptors results in sensitization of bladder afferents, resulting in sensory signals being sent into the spinal cord that prematurely indicate bladder fullness. Targeting bladder afferents expressing MrgprA3 or MrgprC11 and preventing their sensitization may provide a novel approach for treating overactive bladder and interstitial cystitis/bladder pain syndrome.

Skin-Nerve Co-Culture Systems for Disease Modeling and Drug Discovery

Prominent clinical problems related to the skin-nerve interface include barrier dysfunction and erythema, but it is the symptoms of pain and itch that most often lead patients to seek medical treatment. Tissue-engineered innervated skin models provide an excellent solution for studying the mechanisms underlying neurocutaneous disorders for drug screening, and cutaneous device development. Innervated skin substitutes provide solutions beyond traditional monolayer cultures and have advantages that make them preferable to in vivo animal studies for certain applications, such as measuring somatosensory transduction. The tissue-engineered innervated skin models replicate the complex stratified epidermis that provides barrier function in native skin, a feature that is lacking in monolayer co-cultures, while allowing for a level of detail in measurement of nerve morphology and function that cannot be achieved in animal models. In this review, the advantages and disadvantages of different cell sources and scaffold materials will be discussed and a presentation of the current state of the field is reviewed. Impact statement A review of the current state of innervated skin substitutes and the considerations that need to be addressed when developing these models. Tissue-engineered skin substitutes are customizable and provide barrier function allowing for screening of topical drugs and for studying nerve function.

A spider-venom peptide with multitarget activity on sodium and calcium channels alleviates chronic visceral pain in a model of irritable bowel syndrome

ABSTRACT

Chronic pain is a serious debilitating condition that affects ∼20% of the world's population. Currently available drugs fail to produce effective pain relief in many patients and have dose-limiting side effects. Several voltage-gated sodium (NaV) and calcium (CaV) channels are implicated in the etiology of chronic pain, particularly NaV1.1, NaV1.3, NaV1.7-NaV1.9, CaV2.2, and CaV3.2. Numerous NaV and CaV modulators have been described, but with few exceptions, they display poor potency and/or selectivity for pain-related channel subtypes. Here, we report the discovery and characterization of 2 novel tarantula-venom peptides (Tap1a and Tap2a) isolated from Theraphosa apophysis venom that modulate the activity of both NaV and CaV3 channels. Tap1a and Tap2a inhibited on-target NaV and CaV3 channels at nanomolar to micromolar concentrations and displayed moderate off-target selectivity for NaV1.6 and weak affinity for NaV1.4 and NaV1.5. The most potent inhibitor, Tap1a, nearly ablated neuronal mechanosensitivity in afferent fibers innervating the colon and the bladder, with in vivo intracolonic administration reversing colonic mechanical hypersensitivity in a mouse model of irritable bowel syndrome. These findings suggest that targeting a specific combination of NaV and CaV3 subtypes provides a novel route for treatment of chronic visceral pain.

The effects of Kctd12, an auxiliary subunit of GABAB receptor in dentate gyrus on behavioral response to chronic social defeat stress in mice

Adaptive responses to stress are critical to enhance physical and mental well-being, but excessive or prolonged stress may cause inadaptability and increase the risks of psychiatric disorders, such as depression. GABABR signaling is fundamental to brain function and has been identified in neuropsychiatric disorders. KCTD12 is a critical auxiliary subunit in GABABR signaling, but its role in mental disorders, such as depression is unclear. In the present study, we used a well-validated mice model, chronic social defeat stress (CSDS) to investigate behavioral responses to stress and explore the role of Kctd12 in stress response, as well as the relevant mechanisms. We found that CSDS increased the expression of Kctd12 in the dentate gyrus (DG), a subregion of hippocampus. Overexpression of Kctd12 in DG induced higher responsiveness to acute stress and increased vulnerability to social stress in mice, whereas knock-down of Kctd12 in DG prevented the social avoidance. Furthermore, an increased expression of GABAB receptor 2 (GB2) in the DG of CSDS-treated mice was observed, and CGP35348, an antagonist of GABABR, improved the stress-induced behavior responses along with suppressing the excess expression of Kctd12. In addition, Kctd12 regulated the excitability of granule cell in DG, and the stimulation of neuronal activity by silencing Kctd12 contributed to the antidepressant-like effect of fluoxetine. These findings identify that the Kctd12 in DG works as a critical mediator of stress responses, providing a promising therapeutic target in stress-related psychiatric disorders, including depression.

Pharmacological modulation of voltage-gated sodium (NaV) channels alters nociception arising from the female reproductive tract

Dyspareunia, also known as vaginal hyperalgesia, is a prevalent and debilitating symptom of gynaecological disorders such as endometriosis and vulvodynia. Despite this, the sensory pathways transmitting nociceptive information from female reproductive organs remain poorly characterised. As such, the development of specific treatments for pain associated with dyspareunia is currently lacking. Here, we examined, for the first time, (1) the mechanosensory properties of pelvic afferent nerves innervating the mouse vagina; (2) the expression profile of voltage-gated sodium (NaV) channels within these afferents; and (3) how pharmacological modulation of these channels alters vaginal nociceptive signalling ex vivo, in vitro, and in vivo. We developed a novel afferent recording preparation and characterised responses of pelvic afferents innervating the mouse vagina to different mechanical stimuli. Single-cell reverse transcription-polymerase chain reaction determined mRNA expression of NaV channels within vagina-innervating dorsal root ganglia neurons. Vagina-innervating dorsal root ganglia neuroexcitability was measured using whole-cell patch-clamp electrophysiology. Nociception evoked by vaginal distension was assessed by dorsal horn neuron activation within the spinal cord and quantification of visceromotor responses. We found that pelvic afferents innervating the vagina are tuned to detect various mechanical stimuli, with NaV channels abundantly expressed within these neurons. Pharmacological modulation of NaV channels (with veratridine or tetrodotoxin) correspondingly alters the excitability and mechanosensitivity of vagina-innervating afferents, as well as dorsal horn neuron activation and visceromotor responses evoked by vaginal distension. This study identifies potential molecular targets that can be used to modulate vaginal nociceptive signalling and aid in the development of approaches to manage endometriosis and vulvodynia-related dyspareunia.

Molecular and Functional Characterization of Neurogenin-2 Induced Human Sensory Neurons

Sensory perception is fundamental to everyday life, yet understanding of human sensory physiology at the molecular level is hindered due to constraints on tissue availability. Emerging strategies to study and characterize peripheral neuropathies in vitro involve the use of human pluripotent stem cells (hPSCs) differentiated into dorsal root ganglion (DRG) sensory neurons. However, neuronal functionality and maturity are limited and underexplored. A recent and promising approach for directing hPSC differentiation towards functionally mature neurons involves the exogenous expression of Neurogenin-2 (NGN2). The optimized protocol described here generates sensory neurons from hPSC-derived neural crest (NC) progenitors through virally induced NGN2 expression. NC cells were derived from hPSCs via a small molecule inhibitor approach and enriched for migrating NC cells (66% SOX10+ cells). At the protein and transcript level, the resulting NGN2 induced sensory neurons (_NGN2iSNs) express sensory neuron markers such as BRN3A (82% BRN3A+ cells), ISLET1 (91% ISLET1+ cells), TRKA, TRKB, and TRKC. Importantly, _NGN2iSNs repetitively fire action potentials (APs) supported by voltage-gated sodium, potassium, and calcium conductances. In-depth analysis of the molecular basis of _NGN2iSN excitability revealed functional expression of ion channels associated with the excitability of primary afferent neurons, such as Nav1.7, Nav1.8, Kv1.2, Kv2.1, BK, Cav2.1, Cav2.2, Cav3.2, ASICs and HCN among other ion channels, for which we provide functional and transcriptional evidence. Our characterization of stem cell-derived sensory neurons sheds light on the molecular basis of human sensory physiology and highlights the suitability of using hPSC-derived sensory neurons for modeling human DRG development and their potential in the study of human peripheral neuropathies and drug therapies.

Medicinal chemistry, pharmacology, and therapeutic potential of α-conotoxins antagonizing the α9α10 nicotinic acetylcholine receptor

α-Conotoxins are disulfide-rich and well-structured peptides, most of which can block nicotinic acetylcholine receptors (nAChRs) with exquisite selectivity and potency. There are various nAChR subtypes, of which the α9α10 nAChR functions as a heteromeric ionotropic receptor in the mammalian cochlea and mediates postsynaptic transmission from the medial olivocochlear. The α9α10 nAChR subtype has also been proposed as a target for the treatment of neuropathic pain and the suppression of breast cancer cell proliferation. Therefore, α-conotoxins targeting the α9α10 nAChR are potentially useful in the development of specific therapeutic drugs and pharmacological tools. Despite dissimilarities in their amino acid sequence and structures, these conopeptides are potent antagonists of the α9α10 nAChR subtype. Consequently, the activity and stability of these peptides have been subjected to chemical modifications. The resulting synthetic analogues have not only functioned as molecular probes to explore ligand binding sites of the α9α10 nAChR, but also have the potential to become candidates for drug development. From the perspectives of medicinal chemistry and pharmacology, we highlight the structure and function of the α9α10 nAChR and review studies of α-conotoxins targeting it, including their three-dimensional structures, structure optimization strategies, and binding modes at the α9α10 nAChR, as well as their therapeutic potential.

α-Conotoxin Peptidomimetics: Probing the Minimal Binding Motif for Effective Analgesia

Several analgesic α-conotoxins have been isolated from marine cone snails. Structural modification of native peptides has provided potent and selective analogues for two of its known biological targets-nicotinic acetylcholine and γ-aminobutyric acid (GABA) G protein-coupled (GABAB) receptors. Both of these molecular targets are implicated in pain pathways. Despite their small size, an incomplete understanding of the structure-activity relationship of α-conotoxins at each of these targets has hampered the development of therapeutic leads. This review scrutinises the N-terminal domain of the α-conotoxin family of peptides, a region defined by an invariant disulfide bridge, a turn-inducing proline residue and multiple polar sidechain residues, and focusses on structural features that provide analgesia through inhibition of high-voltage-activated Ca2+ channels. Elucidating the bioactive conformation of this region of these peptides may hold the key to discovering potent drugs for the unmet management of debilitating chronic pain associated with a wide range of medical conditions.

Histamine induces peripheral and central hypersensitivity to bladder distension via the histamine H1 receptor and TRPV1

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a common chronic pelvic disorder with sensory symptoms of urinary urgency, frequency, and pain, indicating a key role for hypersensitivity of bladder-innervating sensory neurons. The inflammatory mast cell mediator histamine has long been implicated in IC/BPS, yet the direct interactions between histamine and bladder afferents remain unclear. In the present study, we show, using a mouse ex vivo bladder afferent preparation, that intravesical histamine enhanced the mechanosensitivity of subpopulations of afferents to bladder distension. Histamine also recruited “silent afferents” that were previously unresponsive to bladder distension. Furthermore, in vivo intravesical histamine enhanced activation of dorsal horn neurons within the lumbosacral spinal cord, indicating increased afferent signaling in the central nervous system. Quantitative RT-PCR revealed significant expression of histamine receptor subtypes ( Hrh1– Hrh3) in mouse lumbosacral dorsal root ganglia (DRG), bladder detrusor smooth muscle, mucosa, and isolated urothelial cells. In DRG, Hrh1 was the most abundantly expressed. Acute histamine exposure evoked Ca2+ influx in select populations of DRG neurons but did not elicit calcium transients in isolated primary urothelial cells. Histamine-induced mechanical hypersensitivity ex vivo was abolished in the presence of the histamine H1 receptor antagonist pyrilamine and was not present in preparations from mice lacking transient receptor potential vanilloid 1 (TRPV1). Together, these results indicate that histamine enhances the sensitivity of bladder afferents to distension via interactions with histamine H1 receptor and TRPV1. This hypersensitivity translates to increased sensory input and activation in the spinal cord, which may underlie the symptoms of bladder hypersensitivity and pain experienced in IC/BPS.

RgIA4 Accelerates Recovery from Paclitaxel-Induced Neuropathic Pain in Rats

Chemotherapeutic drugs are widely utilized in the treatment of human cancers. Painful chemotherapy-induced neuropathy is a common, debilitating, and dose-limiting side effect for which there is currently no effective treatment. Previous studies have demonstrated the potential utility of peptides from the marine snail from the genus Conus for the treatment of neuropathic pain. α-Conotoxin RgIA and a potent analog, RgIA4, have previously been shown to prevent the development of neuropathy resulting from the administration of oxaliplatin, a platinum-based antineoplastic drug. Here, we have examined its efficacy against paclitaxel, a chemotherapeutic drug that works by a mechanism of action distinct from that of oxaliplatin. Paclitaxel was administered at 2 mg/kg (intraperitoneally (IP)) every other day for a total of 8 mg/kg. Sprague Dawley rats that were co-administered RgIA4 at 80 µg/kg (subcutaneously (SC)) once daily, five times per week, for three weeks showed significant recovery from mechanical allodynia by day 31. Notably, the therapeutic effects reached significance 12 days after the last administration of RgIA4, which is suggestive of a rescue mechanism. These findings support the effects of RgIA4 in multiple chemotherapeutic models and the investigation of α9α10 nicotinic acetylcholine receptors (nAChRs) as a non-opioid target in the treatment of chronic pain.

Activation of pruritogenic TGR5, MrgprA3, and MrgprC11 on colon-innervating afferents induces visceral hypersensitivity

Itch induces scratching that removes irritants from the skin, whereas pain initiates withdrawal or avoidance of tissue damage. While pain arises from both the skin and viscera, we investigated whether pruritogenic irritant mechanisms also function within visceral pathways. We show that subsets of colon-innervating sensory neurons in mice express, either individually or in combination, the pruritogenic receptors Tgr5 and the Mas-gene-related GPCRs Mrgpra3 and Mrgprc11. Agonists of these receptors activated subsets of colonic sensory neurons and evoked colonic afferent mechanical hypersensitivity via a TRPA1-dependent mechanism. In vivo intracolonic administration of individual TGR5, MrgprA3, or MrgprC11 agonists induced pronounced visceral hypersensitivity to colorectal distension. Coadministration of these agonists as an "itch cocktail" augmented hypersensitivity to colorectal distension and changed mouse behavior. These irritant mechanisms were maintained and enhanced in a model of chronic visceral hypersensitivity relevant to irritable bowel syndrome. Neurons from human dorsal root ganglia also expressed TGR5, as well as the human ortholog MrgprX1, and showed increased responsiveness to pruritogenic agonists in pathological states. These data support the existence of an irritant-sensing system in the colon that is a visceral representation of the itch pathways found in skin, thereby contributing to sensory disturbances accompanying common intestinal disorders.

Colonic afferent input and dorsal horn neuron activation differs between the thoracolumbar and lumbosacral spinal cord

The distal colon is innervated by the splanchnic and pelvic nerves, which relay into the thoracolumbar and lumbosacral spinal cord, respectively. Although the peripheral properties of the colonic afferent nerves within these pathways are well studied, their input into the spinal cord remain ill defined. The use of dual retrograde tracing from the colon wall and lumen, in conjunction with in vivo colorectal distension and spinal neuronal activation labeling with phosphorylated MAPK ERK 1/2 (pERK), allowed us to identify thoracolumbar and lumbosacral spinal cord circuits processing colonic afferent input. In the thoracolumbar dorsal horn, central projections of colonic afferents were primarily labeled from the wall of the colon and localized in laminae I and V. In contrast, lumbosacral projections were identified from both lumen and wall tracing, present within various dorsal horn laminae, collateral tracts, and the dorsal gray commissure. Nonnoxious in vivo colorectal distension evoked significant neuronal activation (pERK-immunoreactivity) within the lumbosacral dorsal horn but not in thoracolumbar regions. However, noxious in vivo colorectal distension evoked significant neuronal activation in both the thoracolumbar and lumbosacral dorsal horn, with the distribution of activated neurons correlating to the pattern of traced projections. Dorsal horn neurons activated by colorectal distension were identified as possible populations of projection neurons or excitatory and inhibitory interneurons based on their neurochemistry. Our findings demonstrate how colonic afferents in splanchnic and pelvic pathways differentially relay mechanosensory information into the spinal cord and contribute to the recruitment of spinal cord pathways processing non-noxious and noxious stimuli.NEW & NOTEWORTHY In mice, retrograde tracing from the colon wall and lumen was used to identify unique populations of afferent neurons and central projections within the spinal cord dorsal horn. We show that there are pronounced differences between the spinal cord regions in the distribution pattern of colonic afferent central projections and the pattern of dorsal horn neuron activation evoked by colorectal distension. These findings demonstrate how colonic afferent input influences spinal processing of colonic mechanosensation.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Human Dorsal Root Ganglia

Sensory neurons with cell bodies situated in dorsal root ganglia convey information from external or internal sites of the body such as actual or potential harm, temperature or muscle length to the central nervous system. In recent years, large investigative efforts have worked toward an understanding of different types of DRG neurons at transcriptional, translational, and functional levels. These studies most commonly rely on data obtained from laboratory animals. Human DRG, however, have received far less investigative focus over the last 30 years. Nevertheless, knowledge about human sensory neurons is critical for a translational research approach and future therapeutic development. This review aims to summarize both historical and emerging information about the size and location of human DRG, and highlight advances in the understanding of the neurochemical characteristics of human DRG neurons, in particular nociceptive neurons.

Peripheral GABA receptors regulate colonic afferent excitability and visceral nociception

KEY POINTS

While the presence of GABA receptors on primary afferents has been well described, most functional analyses have focused on the regulation of transmitter release from central terminals and/or signalling in the sensory neuron cell body. Evidence that GABA receptors are transported to peripheral terminals and that there are several sources of GABA in the colon raise the possibility that GABA signalling in the periphery may influence colonic afferent excitability. GABA_A and GABA_B are present and functional in the colon, where exogenous agonists decrease the excitability of colonic afferents and suppress visceral nociception. Endogenous GABA release within the colon is sufficient to establish the resting excitability of colonic afferents as well as the behavioural response to noxious stimulation of the colon, primarily via GABA_A receptors. Peripheral GABA receptors may serve as a viable target for the treatment of visceral pain.

ABSTRACT

It is well established that GABA receptors at the central terminals of primary afferent fibres regulate afferent input to the superficial dorsal horn. However, the extent to which peripheral GABA signalling may also regulate afferent input remains to be determined. The colon was used to explore this issue because of the numerous endogenous sources of GABA that have been described in this tissue. The influence of GABA signalling on colonic afferent excitability was assessed in an ex vivo mouse colorectum pelvic nerve preparation where test compounds were applied to the receptive field. The visceromotor response (VMR) evoked by noxious colorectal distension was used to assess the impact of GABA signalling on visceral nociception, where test compounds were applied directly to the colon. Application of either GABA_A or GABA_B receptor agonists attenuated the colonic afferent response to colon stretch. Conversely, GABA_A and GABA_B receptor antagonists increased the stretch response. However, while the noxious distension-induced VMR was attenuated in the presence of GABA_A and GABA_B receptor agonists, the VMR was only consistently increased by GABA_A receptor antagonists. These results suggest that GABA receptors are present and functional in the peripheral terminals of colonic afferents and activation of these receptors via endogenous GABA release contributes to the establishment of colonic afferent excitability and visceral nociception. These results suggest that increasing peripheral GABA receptor signalling could be used to treat visceral pain.

NaV 1.6 regulates excitability of mechanosensitive sensory neurons

KEY POINTS

Voltage-gated sodium channels are critical for peripheral sensory neuron transduction and have been implicated in a number of painful and painless disorders. The β-scorpion toxin, Cn2, is selective for Na_V 1.6 in dorsal root ganglion neurons. Na_V 1.6 plays an essential role in peripheral sensory neurons, specifically at the distal terminals of mechanosensing fibres innervating the skin and colon. Na_V 1.6 activation also leads to enhanced response to mechanical stimulus in vivo. This works highlights the use of toxins in elucidating pain pathways moreover the importance of non-peripherally restricted Na_V isoforms in pain generation.

ABSTRACT

Peripheral sensory neurons express multiple voltage-gated sodium channels (Na_V ) critical for the initiation and propagation of action potentials and transmission of sensory input. Three pore-forming sodium channel isoforms are primarily expressed in the peripheral nervous system (PNS): Na_V 1.7, Na_V 1.8 and Na_V 1.9. These sodium channels have been implicated in painful and painless channelopathies and there has been intense interest in them as potential therapeutic targets in human pain. Emerging evidence suggests Na_V 1.6 channels are an important isoform in pain sensing. This study aimed to assess, using pharmacological approaches, the function of Na_V 1.6 channels in peripheral sensory neurons. The potent and Na_V 1.6 selective β-scorpion toxin Cn2 was used to assess the effect of Na_V 1.6 channel activation in the PNS. The multidisciplinary approach included Ca²⁺ imaging, whole-cell patch-clamp recordings, skin-nerve and gut-nerve preparations and in vivo behavioural assessment of pain. Cn2 facilitates Na_V 1.6 early channel opening, and increased persistent and resurgent currents in large-diameter dorsal root ganglion (DRG) neurons. This promotes enhanced excitatory drive and tonic action potential firing in these neurons. In addition, Na_V 1.6 channel activation in the skin and gut leads to increased response to mechanical stimuli. Finally, intra-plantar injection of Cn2 causes mechanical but not thermal allodynia. This study confirms selectivity of Cn2 on Na_V 1.6 channels in sensory neurons. Activation of Na_V 1.6 channels, in terminals of the skin and viscera, leads to profound changes in neuronal responses to mechanical stimuli. In conclusion, sensory neurons expressing Na_V 1.6 are important for the transduction of mechanical information in sensory afferents innervating the skin and viscera.

Antibodies and venom peptides: new modalities for ion channels

Ion channels play fundamental roles in both excitable and non-excitable tissues and therefore constitute attractive drug targets for myriad neurological, cardiovascular and metabolic diseases as well as for cancer and immunomodulation. However, achieving selectivity for specific ion channel subtypes with small-molecule drugs has been challenging, and there currently is a growing trend to target ion channels with biologics. One approach is to improve the pharmacokinetics of existing or novel venom-derived peptides. In parallel, after initial studies with polyclonal antibodies demonstrated the technical feasibility of inhibiting channel function with antibodies, multiple preclinical programmes are now using the full spectrum of available technologies to generate conventional monoclonal and engineered antibodies or nanobodies against extracellular loops of ion channels. After a summary of the current state of ion channel drug discovery, this Review discusses recent developments using the purinergic receptor channel P2X purinoceptor 7 (P2X7), the voltage-gated potassium channel KV1.3 and the voltage-gated sodium channel NaV1.7 as examples of targeting ion channels with biologics.

Visceral Pain

Most of us live blissfully unaware of the orchestrated function that our internal organs conduct. When this peace is interrupted, it is often by routine sensations of hunger and urge. However, for >20% of the global population, chronic visceral pain is an unpleasant and often excruciating reminder of the existence of our internal organs. In many cases, there is no obvious underlying pathological cause of the pain. Accordingly, chronic visceral pain is debilitating, reduces the quality of life of sufferers, and has large concomitant socioeconomic costs. In this review, we highlight key mechanisms underlying chronic abdominal and pelvic pain associated with functional and inflammatory disorders of the gastrointestinal and urinary tracts. This includes how the colon and bladder are innervated by specialized subclasses of spinal afferents, how these afferents become sensitized in highly dynamic signaling environments, and the subsequent development of neuroplasticity within visceral pain pathways. We also highlight key contributing factors, including alterations in commensal bacteria, altered mucosal permeability, epithelial interactions with afferent nerves, alterations in immune or stress responses, and cross talk between these two adjacent organs.

Spinal Afferent Innervation of the Colon and Rectum

Despite their seemingly elementary roles, the colon and rectum undertake a variety of key processes to ensure our overall wellbeing. Such processes are coordinated by the transmission of sensory signals from the periphery to the central nervous system, allowing communication from the gut to the brain via the "gut-brain axis". These signals are transmitted from the peripheral terminals of extrinsic sensory nerve fibers, located within the wall of the colon or rectum, and via their axons within the spinal splanchnic and pelvic nerves to the spinal cord. Recent studies utilizing electrophysiological, anatomical and gene expression techniques indicate a surprisingly diverse set of distinct afferent subclasses, which innervate all layers of the colon and rectum. Combined these afferent sub-types allow the detection of luminal contents, low- and high-intensity stretch or contraction, in addition to the detection of inflammatory, immune, and microbial mediators. To add further complexity, the proportions of these afferents vary within splanchnic and pelvic pathways, whilst the density of the splanchnic and pelvic innervation also varies along the colon and rectum. In this review we traverse this complicated landscape to elucidate afferent function, structure, and nomenclature to provide insights into how the extrinsic sensory afferent innervation of the colon and rectum gives rise to physiological defecatory reflexes and sensations of discomfort, bloating, urgency, and pain.

Discovery Methodology of Novel Conotoxins from Conus Species

Cone snail venoms provide an ideal resource for neuropharmacological tools and drug candidates discovery, which have become a research hotspot in neuroscience and new drug development. More than 1,000,000 natural peptides are produced by cone snails, but less than 0.1% of the estimated conotoxins has been characterized to date. Hence, the discovery of novel conotoxins from the huge conotoxin resources with high-throughput and sensitive methods becomes a crucial key for the conotoxin-based drug development. In this review, we introduce the discovery methodology of new conotoxins from various Conus species. It focuses on obtaining full N- to C-terminal sequences, regardless of disulfide bond connectivity through crude venom purification, conotoxin precusor gene cloning, venom duct transcriptomics, venom proteomics and multi-omic methods. The protocols, advantages, disadvantages, and developments of different approaches during the last decade are summarized and the promising prospects are discussed as well.

Etiology related irritable bowel syndrome animal models

Irritable bowel syndrome (IBS) is a common chronic functional disease of the gastrointestinal tract. Its incidence is increasing worldwide. However, its etiology and pathogenesis are not clear yet, although some factors, such as visceral hypersensitivity, intestinal infection, mental state, gastrointestinal hormones, intestinal flora, and genetic factors, are widely accepted. Great progress has been made in the study of animal models related to the etiology and pathogenesis of IBS. This article summarizes the domestic and international etiology related animal models of IBS, in order to provide reference for choosing appropriate animal models in the basic research of IBS.

Chronic linaclotide treatment reduces colitis-induced neuroplasticity and reverses persistent bladder dysfunction

Irritable bowel syndrome (IBS) patients suffer from chronic abdominal pain and extraintestinal comorbidities, including overactive bladder (OAB) and interstitial cystitis/painful bladder syndrome (IC-PBS). Mechanistic understanding of the cause and time course of these comorbid symptoms is lacking, as are clinical treatments. Here, we report that colitis triggers hypersensitivity of colonic afferents, neuroplasticity of spinal cord circuits, and chronic abdominal pain, which persists after inflammation. Subsequently, and in the absence of bladder pathology, colonic hypersensitivity induces persistent hypersensitivity of bladder afferent pathways, resulting in bladder-voiding dysfunction, indicative of OAB/IC-PBS. Daily administration of linaclotide, a guanylate cyclase-C (GC-C) agonist that is restricted to and acts within the gastrointestinal tract, reverses colonic afferent hypersensitivity, reverses neuroplasticity-induced alterations in spinal circuitry, and alleviates chronic abdominal pain in mice. Intriguingly, daily linaclotide administration also reverses persistent bladder afferent hypersensitivity to mechanical and chemical stimuli and restores normal bladder voiding. Linaclotide itself does not inhibit bladder afferents, rather normalization of bladder function by daily linaclotide treatment occurs via indirect inhibition of bladder afferents via reduced nociceptive signaling from the colon. These data support the concepts that cross-organ sensitization underlies the development and maintenance of visceral comorbidities, while pharmaceutical treatments that inhibit colonic afferents may also improve urological symptoms through common sensory pathways.

Human visceral nociception: findings from translational studies in human tissue

Peripheral sensitization of nociceptors during disease has long been recognized as a leading cause of inflammatory pain. However, a growing body of data generated over the last decade has led to the increased understanding that peripheral sensitization is also an important mechanism driving abdominal pain in highly prevalent functional bowel disorders, in particular, irritable bowel syndrome (IBS). As such, the development of drugs that target pain-sensing nerves innervating the bowel has the potential to be a successful analgesic strategy for the treatment of abdominal pain in both organic and functional gastrointestinal diseases. Despite the success of recent peripherally restricted approaches for the treatment of IBS, not all drugs that have shown efficacy in animal models of visceral pain have reduced pain end points in clinical trials of IBS patients, suggesting innate differences in the mechanisms of pain processing between rodents and humans and, in particular, how we model disease states. To address this gap in our understanding of peripheral nociception from the viscera and the body in general, several groups have developed experimental systems to study nociception in isolated human tissue and neurons, the findings of which we discuss in this review. Studies of human tissue identify a repertoire of human primary afferent subtypes comparable to rodent models including a nociceptor population, the targeting of which will shape future analgesic development efforts. Detailed mechanistic studies in human sensory neurons combined with unbiased RNA-sequencing approaches have revealed fundamental differences in not only receptor/channel expression but also peripheral pain pathways.

Upregulation of N-type calcium channels in the soma of uninjured dorsal root ganglion neurons contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury

N-type voltage-gated calcium (Cav2.2) channels are expressed in the central terminals of dorsal root ganglion (DRG) neurons, and are critical for neurotransmitter release. Cav2.2 channels are also expressed in the soma of DRG neurons, where their function remains largely unknown. Here, we showed that Cav2.2 was upregulated in the soma of uninjured L4 DRG neurons, but downregulated in those of injured L5 DRG neurons following L5 spinal nerve ligation (L5-SNL). Local application of specific Cav2.2 blockers (ω-conotoxin GVIA, 1-100 μM or ZC88, 10-1000 μM) onto L4 and 6 DRGs on the operated side, but not the contralateral side, dose-dependently reversed mechanical allodynia induced by L5-SNL. Patch clamp recordings revealed that both ω-conotoxin GVIA (1 μM) and ZC88 (10 μM) depressed hyperexcitability in L4 but not in L5 DRG neurons of L5-SNL rats. Consistent with this, knockdown of Cav2.2 in L4 DRG neurons with AAV-Cav2.2 shRNA substantially prevented L5-SNL-induced mechanical allodynia and hyperexcitability of L4 DRG neurons. Furthermore, in L5-SNL rats, interleukin-1 beta (IL-1β) and IL-10 were upregulated in L4 DRGs and L5 DRGs, respectively. Intrathecal injection of IL-1β induced mechanical allodynia and Cav2.2 upregulation in bilateral L4-6 DRGs of naïve rats, whereas injection of IL-10 substantially prevented mechanical allodynia and Cav2.2 upregulation in L4 DRGs in L5-SNL rats. Finally, in cultured DRG neurons, Cav2.2 was dose-dependently upregulated by IL-1β and downregulated by IL-10. These data indicate that the upregulation of Cav2.2 in uninjured DRG neurons via IL-1β over-production contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury.

Structure-Activity Studies Reveal the Molecular Basis for GABAB-Receptor Mediated Inhibition of High Voltage-Activated Calcium Channels by α-Conotoxin Vc1.1

α-Conotoxins are disulfide-bonded peptides from cone snail venoms and are characterized by their affinity for nicotinic acetylcholine receptors (nAChR). Several α-conotoxins with distinct selectivity for nAChR subtypes have been identified as potent analgesics in animal models of chronic pain. However, a number of α-conotoxins have been shown to inhibit N-type calcium channel currents in rodent dissociated dorsal root ganglion (DRG) neurons via activation of G protein-coupled GABA_B receptors (GABA_BR). Therefore, it is unclear whether activation of GABA_BR or inhibition of α9α10 nAChRs is the analgesic mechanism. To investigate the mechanisms by which α-conotoxins provide analgesia, we synthesized a suite of Vc1.1 analogues where all residues, except the conserved cysteines, in Vc1.1 were individually replaced by alanine (A), lysine (K), and aspartic acid (D). Our results show that the amino acids in the first loop play an important role in binding of the peptide to the receptor, whereas those in the second loop play an important role for the selectivity of the peptide for the GABA_BR over α9α10 nAChRs. We designed a cVc1.1 analogue that is >8000-fold selective for GABA_BR-mediated inhibition of high voltage-activated (HVA) calcium channels over α9α10 nAChRs and show that it is analgesic in a mouse model of chronic visceral hypersensitivity (CVH). cVc1.1[D11A,E14A] caused dose-dependent inhibition of colonic nociceptors with greater efficacy in ex vivo CVH colonic nociceptors relative to healthy colonic nociceptors. These findings suggest that selectively targeting GABA_BR-mediated HVA calcium channel inhibition by α-conotoxins could be effective for the treatment of chronic visceral pain.

NKA enhances bladder-afferent mechanosensitivity via urothelial and detrusor activation

Tachykinins are expressed within bladder-innervating sensory afferents and have been shown to generate detrusor contraction and trigger micturition. The release of tachykinins from these sensory afferents may also activate tachykinin receptors on the urothelium or sensory afferents directly. Here, we investigated the direct and indirect influence of tachykinins on mechanosensation by recording sensory signaling from the bladder during distension, urothelial transmitter release ex vivo, and direct responses to neurokinin A (NKA) on isolated mouse urothelial cells and bladder-innervating DRG neurons. Bath application of NKA induced concentration-dependent increases in bladder-afferent firing and intravesical pressure that were attenuated by nifedipine and by the NK2 receptor antagonist GR159897 (100 nM). Intravesical NKA significantly decreased bladder compliance but had no direct effect on mechanosensitivity to bladder distension (30 µl/min). GR159897 alone enhanced bladder compliance but had no effect on mechanosensation. Intravesical NKA enhanced both the amplitude and frequency of bladder micromotions during distension, which induced significant transient increases in afferent firing, and were abolished by GR159897. NKA increased intracellular calcium levels in primary urothelial cells but not bladder-innervating DRG neurons. Urothelial ATP release during bladder distention was unchanged in the presence of NKA, whereas acetylcholine levels were reduced. NKA-mediated activation of urothelial cells and enhancement of bladder micromotions are novel mechanisms for NK2 receptor-mediated modulation of bladder mechanosensation. These results suggest that NKA influences bladder afferent activity indirectly via changes in detrusor contraction and urothelial mediator release. Direct actions on sensory nerves are unlikely to contribute to the effects of NKA.

NaV1.1 inhibition can reduce visceral hypersensitivity

Functional bowel disorder patients can suffer from chronic abdominal pain, likely due to visceral hypersensitivity to mechanical stimuli. As there is only a limited understanding of the basis of chronic visceral hypersensitivity (CVH), drug-based management strategies are ill defined, vary considerably, and include NSAIDs, opioids, and even anticonvulsants. We previously reported that the 1.1 subtype of the voltage-gated sodium (NaV; NaV1.1) channel family regulates the excitability of sensory nerve fibers that transmit a mechanical pain message to the spinal cord. Herein, we investigated whether this channel subtype also underlies the abdominal pain that occurs with CVH. We demonstrate that NaV1.1 is functionally upregulated under CVH conditions and that inhibiting channel function reduces mechanical pain in 3 mechanistically distinct mouse models of chronic pain. In particular, we use a small molecule to show that selective NaV1.1 inhibition (a) decreases sodium currents in colon-innervating dorsal root ganglion neurons, (b) reduces colonic nociceptor mechanical responses, and (c) normalizes the enhanced visceromotor response to distension observed in 2 mouse models of irritable bowel syndrome. These results provide support for a relationship between NaV1.1 and chronic abdominal pain associated with functional bowel disorders.

Voltage-gated sodium channels: (NaV )igating the field to determine their contribution to visceral nociception

Chronic visceral pain, altered motility and bladder dysfunction are common, yet poorly managed symptoms of functional and inflammatory disorders of the gastrointestinal and urinary tracts. Recently, numerous human channelopathies of the voltage-gated sodium (NaV ) channel family have been identified, which induce either painful neuropathies, an insensitivity to pain, or alterations in smooth muscle function. The identification of these disorders, in addition to the recent utilisation of genetically modified NaV mice and specific NaV channel modulators, has shed new light on how NaV channels contribute to the function of neuronal and non-neuronal tissues within the gastrointestinal tract and bladder. Here we review the current pre-clinical and clinical evidence to reveal how the nine NaV channel family members (NaV 1.1-NaV 1.9) contribute to abdominal visceral function in normal and disease states.

Cross-organ sensitization between the colon and bladder: to pee or not to pee?

Chronic abdominal and pelvic pain are common debilitating clinical conditions experienced by millions of patients around the globe. The origin of such pain commonly arises from the intestine and bladder, which share common primary roles (the collection, storage, and expulsion of waste). These visceral organs are located in close proximity to one another and also share common innervation from spinal afferent pathways. Chronic abdominal pain, constipation, or diarrhea are primary symptoms for patients with irritable bowel syndrome or inflammatory bowel disease. Chronic pelvic pain and urinary urgency and frequency are primary symptoms experienced by patients with lower urinary tract disorders such as interstitial cystitis/painful bladder syndrome. It is becoming clear that these symptoms and clinical entities do not occur in isolation, with considerable overlap in symptom profiles across patient cohorts. Here we review recent clinical and experimental evidence documenting the existence of "cross-organ sensitization" between the colon and bladder. In such circumstances, colonic inflammation may result in profound changes to the sensory pathways innervating the bladder, resulting in severe bladder dysfunction.

Cyclic analogues of α-conotoxin Vc1.1 inhibit colonic nociceptors and provide analgesia in a mouse model of chronic abdominal pain

BACKGROUND AND PURPOSE

Patients with irritable bowel syndrome suffer from chronic visceral pain (CVP) and limited analgesic therapeutic options are currently available. We have shown that α-conotoxin Vc1.1 induced activation of GABA_B receptors on the peripheral endings of colonic afferents and reduced nociceptive signalling from the viscera. However, the analgesic efficacy of more stable, cyclized versions of Vc1.1 on CVP remains to be determined.

EXPERIMENTAL APPROACH

Using ex vivo colonic afferent preparations from mice, we determined the inhibitory actions of cyclized Vc1.1 (cVc1.1) and two cVc1.1 analogues on mouse colonic nociceptors in healthy and chronic visceral hypersensitivity (CVH) states. Using whole-cell patch clamp recordings, we also assessed the inhibitory actions of these peptides on the neuronal excitability of colonic innervating dorsal root ganglion neurons. In vivo, the analgesic efficacy of these analogues was assessed by determining the visceromotor response to colorectal distension in healthy and CVH mice.

KEY RESULTS

cVc1.1 and the cVc1.1 analogues, [C2H,C8F]cVc1.1 and [N9W]cVc1.1, all caused concentration-dependent inhibition of colonic nociceptors from healthy mice. Inhibition by these peptides was greater than those evoked by linear Vc1.1 and was substantially greater in colonic nociceptors from CVH mice. cVc1.1 also reduced excitability of colonic dorsal root ganglion neurons, with greater effect in CVH neurons. CVH mice treated with cVc1.1 intra-colonically displayed reduced pain responses to noxious colorectal distension compared with vehicle-treated CVH mice.

CONCLUSIONS AND IMPLICATIONS

Cyclic versions of Vc1.1 evoked significant anti-nociceptive actions in CVH states, suggesting that they could be novel candidates for treatment of CVP.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.