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

Peripheral and central neuroplasticity in a mouse model of endometriosis

Chronic pelvic pain (CPP) is the most debilitating symptom of gynaecological disorders such as endometriosis. However, it remains unclear how sensory neurons from pelvic organs affected by endometriosis, such as the female reproductive tract, detect and transmit nociceptive events and how these signals are processed within the central nervous system (CNS). Using a previously characterized mouse model of endometriosis, we investigated whether the increased pain sensitivity occurring in endometriosis could be attributed to (i) changes in mechanosensory properties of sensory afferents innervating the reproductive tract, (ii) alterations in sensory input from reproductive organs to the spinal cord or (iii) neuroinflammation and sensitization of spinal neural circuits. Mechanosensitivity of vagina-innervating primary afferents was examined using an ex vivo single-unit extracellular recording preparation. Nociceptive signalling from the vagina to the spinal cord was quantified by phosphorylated MAP kinase ERK1/2 immunoreactivity. Immunohistochemistry was used to determine glial and neuronal circuit alterations within the spinal cord. We found that sensory afferents innervating the rostral, but not caudal portions of the mouse vagina, developed mechanical hypersensitivity in endometriosis. Nociceptive signalling from the vagina to the spinal cord was significantly enhanced in mice with endometriosis. Moreover, mice with endometriosis developed microgliosis, astrogliosis and enhanced substance P neurokinin-1 receptor immunoreactivity within the spinal cord, suggesting the development of neuroinflammation and sensitization of spinal circuitry in endometriosis. These results demonstrate endometriosis-induced neuroplasticity occurring at both peripheral and central sites of sensory afferent pathways. These findings may help to explain the altered sensitivity to pain in endometriosis and provide a novel platform for targeted pain relief treatments for this debilitating disorder.

The parasympathetic and sensory innervation of the proximal and distal colon in male mice

Introduction

The distributions of extrinsic neurons innervating the colon show differences in experimental animals from humans, including the vagal and spinal parasympathetic innervation to the distal colon. The neuroanatomical tracing to the mouse proximal colon has not been studied in details. This study aimed to trace the locations of extrinsic neurons projecting to the mouse proximal colon compared to the distal colon using dual retrograde tracing.

Methods

The parasympathetic and sensory neurons projecting to colon were assessed using Cholera Toxin subunit B conjugated to Alexa-Fluor 488 or 555 injected in the proximal and distal colon of the same mice.

Results

Retrograde tracing from the proximal and distal colon labeled neurons in the dorsal motor nucleus of the vagus (DMV) and the nodose ganglia, while the tracing from the distal colon did not label the parasympathetic neurons in the lumbosacral spinal cord at L6-S1. Neurons in the pelvic ganglia which were cholinergic projected to the distal colon. There were more neurons in the DMV and nodose ganglia projecting to the proximal than distal colon. The right nodose ganglion had a higher number of neurons than the left ganglion innervating the proximal colon. In the dorsal root ganglia (DRG), the highest number of neurons traced from the distal colon were at L6, and those from the proximal colon at T12. DRG neurons projected closely to the cholinergic neurons in the intermediolateral column of L6 spinal cord. Small percentages of neurons with dual projections to both the proximal and distal colon existed in the DMV, nodose ganglia and DRG. We also observed long projecting neurons traced from the caudal distal colon to the transverse and proximal colon, some of which were calbindin immunoreactive, while there were no retrogradely labeled neurons traced from the proximal to distal colon.

Discussion

These data demonstrated that the vagal motor and motor and sensory neurons innervate both the proximal and distal colon in mice, and the autonomic neurons in the intermediate zone of the lumbosacral spinal cord do not project directly to the mouse colon, which differs from that in humans.

From the organ bath to the whole person: a review of human colonic motility

Motor function of the colon is essential for health. Our current understanding of the mechanisms that underlie colonic motility are based upon a range of experimental techniques, including molecular biology, single cell studies, recordings from muscle strips, analysis of part or whole organ ex vivo through to in vivo human recordings. For the surgeon involved in the clinical management of colonic conditions this amounts to a formidable volume of material. Here, we synthesize the key findings from these various experimental approaches so that surgeons can be better armed to deal with the complexities of the colon.

Comparative localization of colorectal sensory afferent central projections in the mouse spinal cord dorsal horn and caudal medulla dorsal vagal complex

The distal colon and rectum (colorectum) are innervated by spinal and vagal afferent pathways. The central circuits into which vagal and spinal afferents relay colorectal nociceptive information remain to be comparatively assessed. To address this, regional colorectal retrograde tracing and colorectal distension (CRD)-evoked neuronal activation were used to compare the circuits within the dorsal vagal complex (DVC) and dorsal horn (thoracolumbar [TL] and lumbosacral [LS] spinal levels) into which vagal and spinal colorectal afferents project. Vagal afferent projections were observed in the nucleus tractus solitarius (NTS), area postrema (AP), and dorsal motor nucleus of the vagus (DMV), labeled from the rostral colorectum. In the NTS, projections were opposed to catecholamine and pontine parabrachial nuclei (PbN)-projecting neurons. Spinal afferent projections were labeled from rostral through to caudal aspects of the colorectum. In the dorsal horn, the number of neurons activated by CRD was linked to pressure intensity, unlike in the DVC. In the NTS, 13% ± 0.6% of CRD-activated neurons projected to the PbN. In the dorsal horn, at the TL spinal level, afferent input was associated with PbN-projecting neurons in lamina I (LI), with 63% ± 3.15% of CRD-activated neurons in LI projecting to the PbN. On the other hand, at the LS spinal level, only 18% ± 0.6% of CRD-activated neurons in LI projected to the PbN. The collective data identify differences in the central neuroanatomy that support the disparate roles of vagal and spinal afferent signaling in the facilitation and modulation of colorectal nociceptive responses.

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Involvement of Brn3a-positive spinal dorsal horn neurons in the transmission of visceral pain in inflammatory bowel disease model mice

The spinal dorsal horn plays a crucial role in the transmission and processing of somatosensory information. Although spinal neural circuits that process several distinct types of somatic sensations have been studied extensively, those responsible for visceral pain transmission remain poorly understood. In the present study, we analyzed dextran sodium sulfate (DSS)-induced inflammatory bowel disease (IBD) mouse models to characterize the spinal dorsal horn neurons involved in visceral pain transmission. Immunostaining for c-fos, a marker of neuronal activity, demonstrated that numerous c-fos-positive cells were found bilaterally in the lumbosacral spinal dorsal horn, and their distribution was particularly abundant in the shallow dorsal horn. Characterization of these neurons by several molecular markers revealed that the percentage of the Pit1-Oct1-Unc86 domain (POU domain)-containing transcription factor Brn3a-positive neurons among the c-fos-positive neurons in the shallow dorsal horn was 30%-40% in DSS-treated mice, which was significantly higher than that in the somatic pain model mice. We further demonstrated by neuronal tracing that, within the shallow dorsal horn, Brn3a-positive neurons were more highly represented in spino-solitary projection neurons than in spino-parabrachial projection neurons. These results raise the possibility that Brn3a-positive spinal dorsal horn neurons make a large contribution to visceral pain transmission, part of which is mediated through the spino-solitary pathway.

Gut reactions: emerging mechanisms of abdominal pain from food intake

Abdominal pain, which is a form of visceral pain, is a highly prevalent symptom worldwide frequently occurring following food ingestion. Its pathophysiology is complex, and many factors, including intestinal environmental cues, the immune system, or the molecular composition of foods, can influence the development of postprandial abdominal pain. Because of the poor efficacy of drug treatments, current strategies are often limited to the exclusion of culprit food(s) from the diet. However, there are two important limitations to this approach. First, patients suffering from food-induced abdominal pain usually recognize several food items as the cause of their gastrointestinal symptoms. Second, not all offending foods can always be identified by these patients. Newly identified mechanisms involving neuroimmune interactions and their communication with the intestinal microbiota shed light on the development of new therapeutic strategies. In this Mini-Review, these novel mechanisms and relevance of such findings are highlighted.

Anatomical distribution of CGRP-containing lumbosacral spinal afferent neurons in the mouse uterine horn

Sensory stimuli from the uterus are detected by spinal afferent neurons whose cell bodies arise from thoracolumbar and lumbosacral dorsal root ganglia (DRG). Using an in vivo survival surgical technique developed in our laboratory to remove select DRG from live mice, we recently quantified the topographical distribution of thoracolumbar spinal afferents innervating the mouse uterine horn, revealed by loss of immunoreactivity to calcitonin gene-related peptide (CGRP). Here, we used the same technique to investigate the distribution of lumbosacral uterine spinal afferents, in which L5-S1 DRG were unilaterally removed from adult female C57BL/6J mice (N = 6). Following 10–12 days recovery, CGRP immunoreactivity was quantified along the length of uterine horns using fluorescence immunohistochemistry. Relative to myometrial thickness, overall CGRP density in uterine tissues ipsilateral to L5-S1 DRG removal was reduced compared to the DRG-intact, contralateral side (P = 0.0265). Regionally, however, myometrial CGRP density was unchanged in the cranial, mid, and caudal portions. Similarly, CGRP-expressing nerve fiber counts, network lengths, junctions, and the proportion of area occupied by CGRP immunoreactivity were unaffected by DRG removal (P ≥ 0.2438). Retrograde neuronal tracing from the caudal uterine horn revealed fewer spinal afferents here arise from lumbosacral than thoracolumbar DRG (P = 0.0442) (N = 4). These data indicate that, unlike thoracolumbar DRG, lumbosacral spinal afferent nerves supply relatively modest sensory innervation across the mouse uterine horn, with no regional specificity. We conclude most sensory information between the mouse uterine horn and central nervous system is likely relayed via thoracolumbar spinal afferents.

Disengaging spinal afferent nerve communication with the brain in live mice

Our understanding of how abdominal organs (like the gut) communicate with the brain, via sensory nerves, has been limited by a lack of techniques to selectively activate or inhibit populations of spinal primary afferent neurons within dorsal root ganglia (DRG), of live animals. We report a survival surgery technique in mice, where select DRG are surgically removed (unilaterally or bilaterally), without interfering with other sensory or motor nerves. Using this approach, pain responses evoked by rectal distension were abolished by bilateral lumbosacral L5-S1 DRG removal, but not thoracolumbar T13-L1 DRG removal. However, animals lacking T13-L1 or L5-S1 DRG both showed reduced pain sensitivity to distal colonic distension. Removal of DRG led to selective loss of peripheral CGRP-expressing spinal afferent axons innervating visceral organs, arising from discrete spinal segments. This method thus allows spinal segment-specific determination of sensory pathway functions in conscious, free-to-move animals, without genetic modification.

A surgical method in mice can selectively remove dorsal root ganglia (DRG) at specific spinal levels without interfering with other nerves, providing insight on thoracolumbar vs. lumbosacral DRG contributions to pain signalling and behaviour.

Guanylate cyclase-C agonists as peripherally acting treatments of chronic visceral pain

Irritable bowel syndrome (IBS) is a chronic gastrointestinal disorder characterized by abdominal pain and altered bowel habit that affects ~11% of the global population. Over the past decade, preclinical and clinical studies have revealed a variety of novel mechanisms relating to the visceral analgesic effects of guanylate cyclase-C (GC-C) agonists. Here we discuss the mechanisms by which GC-C agonists target the GC-C/cyclic guanosine-3',5'-monophosphate (cGMP) pathway, resulting in visceral analgesia as well as clinically relevant relief of abdominal pain and other sensations in IBS patients. Due to the preponderance of evidence we focus on linaclotide, a 14-amino acid GC-C agonist with very low oral bioavailability that acts within the gut. Collectively, the weight of experimental and clinical evidence supports the concept that GC-C agonists act as peripherally acting visceral analgesics.

New Insights on Extrinsic Innervation of the Enteric Nervous System and Non-neuronal Cell Types That Influence Colon Function

The enteric nervous system not only innervates the colon to execute various functions in a semi-autonomous manner but also receives neural input from three extrinsic sources, (1) vagal, (2) thoracolumbar (splanchnic), and (3) lumbosacral (pelvic) pathways, that permit bidirectional communication between the colon and central nervous system. Extrinsic pathways signal sensory input via afferent fibers, as well as motor autonomic output via parasympathetic or sympathetic efferent fibers, but the shared and unique roles for each pathway in executing sensory-motor control of colon function have not been well understood. Here, we describe the recently developed approaches that have provided new insights into the diverse mechanisms utilized by extrinsic pathways to influence colon functions related to visceral sensation, motility, and inflammation. Based on the cumulative results from anatomical, molecular, and functional studies, we propose pathway-specific functions for vagal, thoracolumbar, and lumbosacral innervation of the colon.

Gut-innervating TRPV1+ Neurons Drive Chronic Visceral Pain via Microglial P2Y12 Receptor

BACKGROUND & AIMS

Chronic abdominal pain is a common symptom of inflammatory bowel diseases (IBDs). Peripheral and central mechanisms contribute to the transition from acute to chronic pain during active disease and clinical remission. Lower mechanical threshold and hyperexcitability of visceral afferents induce gliosis in central pain circuits, leading to persistent visceral hypersensitivity (VHS). In the spinal cord, microglia, the immune sentinels of the central nervous system, undergo activation in multiple models of VHS. Here, we investigated the mechanisms of microglia activation to identify centrally acting analgesics for chronic IBD pain.

METHODS

Using Designer Receptors Exclusively Activated by Designer Drugs (DREADD) expressed in transient receptor potential vanilloid member 1-expressing visceral neurons that sense colonic inflammation, we tested whether neuronal activity was indispensable to control microglia activation and VHS. We then investigated the neuron-microglia signaling system involved in visceral pain chronification.

RESULTS

We found that chemogenetic inhibition of transient receptor potential vanilloid member 1⁺ visceral afferents prevents microglial activation in the spinal cord and subsequent VHS in colitis mice. In contrast, chemogenetic activation, in the absence of colitis, enhanced microglial activation associated with VHS. We identified a purinergic signaling mechanism mediated by neuronal adenosine triphosphate (ATP) and microglial P2Y12 receptor, triggering VHS in colitis. Inhibition of P2RY12 prevented microglial reactivity and chronic VHS post-colitis.

CONCLUSIONS

Overall, these data provide novel insights into the central mechanisms of chronic visceral pain and suggest that targeting microglial P2RY12 signaling could be harnessed to relieve pain in patients with IBD who are in remission.

Differential Activation of Colonic Afferents and Dorsal Horn Neurons Underlie Stress-Induced and Comorbid Visceral Hypersensitivity in Female Rats

Chronic Overlapping Pain Conditions, including irritable bowel syndrome (IBS) and temporomandibular disorder (TMD), represent a group of idiopathic pain conditions that likely have peripheral and central mechanisms contributing to their pathology, but are poorly understood. These conditions are exacerbated by stress and have a female predominance. The presence of one condition predicts the presence or development of additional conditions, making this a significant pain management problem. The current study was designed to determine if the duration and magnitude of peripheral sensitization and spinal central sensitization differs between restraint stress-induced visceral hypersensitivity (SIH) and chronic comorbid pain hypersensitivity (CPH; stress during pre-existing orofacial pain). SIH in female rats, as determined by the visceromotor response, persisted at least four but resolved by seven weeks. In contrast, CPH persisted at least seven weeks. Surprisingly, colonic afferents in both SIH and CPH rats were sensitized at seven weeks. CPH rats also had referred pain through seven weeks, but locally anesthetizing the colon only attenuated the referred pain through four weeks, suggesting a transition to colonic afferent independent central sensitization. Different phenotypes of dorsal horn neurons were sensitized in the CPH rats seven weeks post stress compared to four weeks or SIH rats. The current study suggests differential processing of colonic afferent input to the lumbosacral spinal cord contributes to visceral hypersensitivity during comorbid chronic pain conditions. PERSPECTIVE: Chronic Overlapping Pain Conditions represent a unique challenge in pain management. The diverse nature of peripheral organs hinders a clear understanding of underlying mechanisms accounting for the comorbidity. This study highlights a mismatch between the condition-dependent behavior and peripheral and spinal mechanisms that contribute to visceral pain hypersensitivity.

Pruritogenic mechanisms and gut sensation: putting the "irritant" into irritable bowel syndrome

Chronic abdominal pain is a common clinical condition experienced by patients with irritable bowel syndrome (IBS). A general lack of suitable treatment options for the management of visceral pain is the major contributing factor to the debilitating nature of the disease. Understanding the underlying causes of chronic visceral pain is pivotal to identifying new effective therapies for IBS. This review provides the current evidence, demonstrating that mediators and receptors that induce itch in the skin also act as "gut irritants" in the gastrointestinal tract. Activation of these receptors triggers specific changes in the neuronal excitability of sensory pathways responsible for the transmission of nociceptive information from the periphery to the central nervous system leading to visceral hypersensitivity and visceral pain. Accumulating evidence points to significant roles of irritant mediators and their receptors in visceral hypersensitivity and thus constitutes potential targets for the development of more effective therapeutic options for IBS.

Calcium imaging in population of dorsal root ganglion neurons unravels novel mechanisms of visceral pain sensitization and referred somatic hypersensitivity

ABSTRACT

Mechanisms of visceral pain sensitization and referred somatic hypersensitivity remain unclear. We conducted calcium imaging in Pirt-GCaMP6s mice to gauge responses of dorsal root ganglion (DRG) neurons to visceral and somatic stimulation in vivo. Intracolonic instillation of 2,4,6-trinitrobenzene sulfonic acid (TNBS) induced colonic inflammation and increased the percentage of L6 DRG neurons that responded to colorectal distension above that of controls at day 7. Colorectal distension did not activate L4 DRG neurons. TNBS-treated mice exhibited more Evans blue extravasation than did control mice and developed mechanical hypersensitivity in low-back skin and hind paws, which are innervated by L6 and L4 DRG neurons, respectively, suggesting that colonic inflammation induced mechanical hypersensitivity in both homosegmental and heterosegmental somatic regions. Importantly, the percentage of L4 DRG neurons activated by hind paw pinch and brush stimulation and calcium responses of L6 DRG neurons to low-back brush stimulation were higher at day 7 after TNBS than those in control mice. Visceral irritation from intracolonic capsaicin instillation also increased Evans blue extravasation in hind paws and low-back skin and acutely increased the percentage of L4 DRG neurons responding to hind paw pinch and the response of L6 DRG neurons to low-back brush stimulation. These findings suggest that TNBS-induced colitis and capsaicin-induced visceral irritation may sensitize L6 DRG neurons to colorectal and somatic inputs and also increase the excitability of L4 DRG neurons that do not receive colorectal inputs. These changes may represent a potential peripheral neuronal mechanism for visceral pain sensitization and referred somatic hypersensitivity.

Postoperative ileus-An ongoing conundrum

BACKGROUND

Postoperative ileus is common and is a major clinical problem. It has been widely studied in patients and in experimental models in laboratory animals. A wide variety of treatments have been tested to prevent or modify the course of this disorder.

PURPOSE

This review draws together information on animal studies of ileus with studies on human patients. It summarizes some of the conceptual advances made in understanding the mechanisms that underlie paralytic ileus. The treatments that have been tested in human subjects (both pharmacological and non-pharmacological) and their efficacy are summarized and graded consistent with current clinical guidelines. The review is not intended to provide a comprehensive overview of ileus, but rather a general understanding of the major clinical problems associated with it, how animal models have been useful to elucidate key mechanisms and, finally, some perspectives from both scientists and clinicians as to how we may move forward with this debilitating yet common condition.

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.

An objective approach to assess colonic pain in mice using colonometry

The present study presents a non-surgical approach to assess colonic mechanical sensitivity in mice using colonometry, a technique in which colonic stretch-reflex contractions are measured by recording intracolonic pressures during saline infusion into the distal colon in a constant rate. Colonometrical recording has been used to assess colonic function in healthy individuals and patients with neurological disorders. Here we found that colonometry can also be implemented in mice, with an optimal saline infusion rate of 1.2 mL/h. Colonometrograms showed intermittent pressure rises that was caused by periodical colonic contractions. In the sceneries of colonic hypersensitivity that was generated post 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colonic inflammation, following chemogenetic activation of primary afferent neurons, or immediately after noxious stimulation of the colon by colorectal distension (CRD), the amplitude of intracolonic pressure (A_ICP) was markedly elevated which was accompanied by a faster pressure rising (ΔP/Δt). Colonic hypersensitivity-associated A_ICP elevation was a result of the enhanced strength of colonic stretch-reflex contraction which reflected the heightened activity of the colonic sensory reflex pathways. The increased value of ΔP/Δt in colonic hypersensitivity indicated a lower threshold of colonic mechanical sensation by which colonic stretch-reflex contraction was elicited by a smaller saline infusion volume during a shorter period of infusion time. Chemogenetic inhibition of primary afferent pathway that was governed by Nav1.8-expressing cells attenuated TNBS-induced up-regulations of A_ICP, ΔP/Δt, and colonic pain behavior in response to CRD. These findings support that colonometrograms can be used for analysis of colonic pain in mice.

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.

Pain in Endometriosis

Endometriosis is a chronic and debilitating condition affecting ∼10% of women. Endometriosis is characterized by infertility and chronic pelvic pain, yet treatment options remain limited. In many respects this is related to an underlying lack of knowledge of the etiology and mechanisms contributing to endometriosis-induced pain. Whilst many studies focus on retrograde menstruation, and the formation and development of lesions in the pathogenesis of endometriosis, the mechanisms underlying the associated pain remain poorly described. Here we review the recent clinical and experimental evidence of the mechanisms contributing to chronic pain in endometriosis. This includes the roles of inflammation, neurogenic inflammation, neuroangiogenesis, peripheral sensitization and central sensitization. As endometriosis patients are also known to have co-morbidities such as irritable bowel syndrome and overactive bladder syndrome, we highlight how common nerve pathways innervating the colon, bladder and female reproductive tract can contribute to co-morbidity via cross-organ sensitization.

Unique Molecular Characteristics of Visceral Afferents Arising from Different Levels of the Neuraxis: Location of Afferent Somata Predicts Function and Stimulus Detection Modalities

Viscera receive innervation from sensory ganglia located adjacent to multiple levels of the brainstem and spinal cord. Here we examined whether molecular profiling could be used to identify functional clusters of colon afferents from thoracolumbar (TL), lumbosacral (LS), and nodose ganglia (NG) in male and female mice. Profiling of TL and LS bladder afferents was also performed. Visceral afferents were back-labeled using retrograde tracers injected into proximal and distal regions of colon or bladder, followed by single-cell qRT-PCR and analysis via an automated hierarchical clustering method. Genes were chosen for assay (32 for bladder; 48 for colon) based on their established role in stimulus detection, regulation of sensitivity/function, or neuroimmune interaction. A total of 132 colon afferents (from NG, TL, and LS ganglia) and 128 bladder afferents (from TL and LS ganglia) were analyzed. Retrograde labeling from the colon showed that NG and TL afferents innervate proximal and distal regions of the colon, whereas 98% of LS afferents only project to distal regions. There were clusters of colon and bladder afferents, defined by mRNA profiling, that localized to either TL or LS ganglia. Mixed TL/LS clustering also was found. In addition, transcriptionally, NG colon afferents were almost completely segregated from colon TL and LS neurons. Furthermore, colon and bladder afferents expressed genes at similar levels, although different gene combinations defined the clusters. These results indicate that genes implicated in both homeostatic regulation and conscious sensations are found at all anatomic levels, suggesting that afferents from different portions of the neuraxis have overlapping functions.SIGNIFICANCE STATEMENT Visceral organs are innervated by sensory neurons whose cell bodies are located in multiple ganglia associated with the brainstem and spinal cord. For the colon, this overlapping innervation is proposed to facilitate visceral sensation and homeostasis, where sensation and pain are mediated by spinal afferents and fear and anxiety (the affective aspects of visceral pain) are the domain of nodose afferents. The transcriptomic analysis performed here reveals that genes implicated in both homeostatic regulation and pain are found in afferents across all ganglia types, suggesting that conscious sensation and homeostatic regulation are the result of convergence, and not segregation, of sensory input.

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.