Colonic inflammation induces fos expression in the thoracolumbar spinal cord increasing activity in the spinoparabrachial pathway

Urothelial bladder afferents selectively project to L6/S1 levels and are more peptidergic than those projecting to the T13/L1 levels in female rats

This neuroanatomical study in four, adult, Sprague-Dawley female rats quantified the number of Urothelial (labeled by intravesical DiI dye administration) and Non-Urothelial (labeled by intraparenchymal injection of Fast blue dye) bladder primary afferent neurons (bPANs) located in the T13, L1, L6 and S1 dorsal root ganglia. Additional immunohistochemical labeling using antibodies to detect either Substance P or CGRP further characterized the bPAN samples as peptidergic or non-peptidergic. Cell counts indicated that Urothelial bPANs were more common at the L6/S1 levels and more likely to be identified as peptidergic when compared with bPANs characterized at T13/L1 levels and with Non-Urothelial bPANs. These studies provide additional evidence that at least two distinct neuronal populations, with differing localization of sensory terminals, differing peptide content, and differing projections to the central nervous system, are responsible for bladder sensation.

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.

The role of peripheral nerve signaling in endometriosis

A hallmark of endometriosis - a chronic debilitating condition whose causes are poorly understood - is neuronal innervation of lesions. Recent evidence demonstrates that the peripheral nervous system plays an important role in the pathophysiology of this disease. Sensory nerves, which surround and innervate endometriotic lesions, not only drive the chronic and debilitating pain associated with endometriosis but also contribute to a pro-growth phenotype by secreting neurotrophic factors and interacting with surrounding immune cells. The diverse array of contributions that neurons play in endometriosis indicate that it should be considered as a nerve-centric disease. This review is focused on the emerging field of exoneural biology and how it applies to the field of endometriosis, in particular the role that peripheral nerves play in driving and maintaining endometriotic lesions. A better understanding of the mechanisms of neuronal contribution to endometriosis, as well as their interactions with accompanying stromal and immune cells, will unearth novel disease-relevant pathways and targets, providing additional, more selective therapeutic horizons.

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.

Supraspinal Mechanisms of Intestinal Hypersensitivity

Gut inflammation or injury causes intestinal hypersensitivity (IHS) and hyperalgesia, which can persist after the initiating pathology resolves, are often referred to somatic regions and exacerbated by psychological stress, anxiety or depression, suggesting the involvement of both the spinal cord and the brain. The supraspinal mechanisms of IHS remain to be fully elucidated, however, over the last decades the series of intestinal pathology-associated neuroplastic changes in the brain has been revealed, being potentially responsible for the phenomenon. This paper reviews current clinical and experimental data, including the authors' own findings, on these functional, structural, and neurochemical/molecular changes within cortical, subcortical and brainstem regions processing and modulating sensory signals from the gut. As concluded in the review, IHS can develop and maintain due to the bowel inflammation/injury-induced persistent hyperexcitability of viscerosensory brainstem and thalamic nuclei and sensitization of hypothalamic, amygdala, hippocampal, anterior insular, and anterior cingulate cortical areas implicated in the neuroendocrine, emotional and cognitive modulation of visceral sensation and pain. An additional contribution may come from the pathology-triggered dysfunction of the brainstem structures inhibiting nociception. The mechanism underlying IHS-associated regional hyperexcitability is enhanced NMDA-, AMPA- and group I metabotropic receptor-mediated glutamatergic neurotransmission in association with altered neuropeptide Y, corticotropin-releasing factor, and cannabinoid 1 receptor signaling. These alterations are at least partially mediated by brain microglia and local production of cytokines, especially tumor necrosis factor α. Studying the IHS-related brain neuroplasticity in greater depth may enable the development of new therapeutic approaches against chronic abdominal pain in inflammatory bowel disease.

Pain matrix shift in the rat brain following persistent colonic inflammation revealed by voxel-based statistical analysis

Inflammatory bowel disease (IBD), mainly comprising Crohn’s disease and ulcerative colitis, is characterized by chronic inflammation in the digestive tract. Approximately 60% of the patients experience abdominal pain during acute IBD episodes, which severely impairs their quality of life. Both peripheral and central mechanisms are thought to be involved in such abdominal pain in IBD. Although much attention has been paid to peripheral mechanisms of abdominal pain in IBD pathophysiology, the involvement of supraspinal mechanisms remains poorly understood. To address this issue, we investigated regional brain activity in response to colorectal distension in normal and IBD model rats using voxel-based statistical analysis of 2-deoxy-2-[18F]fluoro-_D-glucose positron emission tomography imaging. The rat IBD model was generated by colorectal administration of 2,4,6-trinitrobenzene sulfonic acid, a chemical compound widely used to generate colitis. Tissue damage and inflammation were induced and dynamically changed with time after 2,4,6-trinitrobenzene sulfonic acid injection, while colorectal distension-induced visceromotor response showed corresponding temporal changes. We found that characteristic brain activations were observed in response to visceral innocuous and noxious colorectal distension and supraspinal nociception shared some physiological sensory pathway. Moreover, widespread brain regions were activated, and the functional coupling between the central medial thalamic nucleus and anterior cingulate cortex was enhanced after noxious colorectal distension in IBD model of rats. Increased brain activity in the anterior insular cortex and anterior cingulate cortex was positively correlated with noxious colorectal distension-induced pain severity in normal and IBD rats, respectively. These findings suggest that the pain matrix was shifted following persistent colonic inflammation, and thalamocortical sensitization in the pathway from the central medial thalamic nucleus to anterior cingulate cortex might be a central mechanism of the visceral hyperalgesia in IBD pathophysiology.

Intracolonic Mustard Oil Induces Visceral Pain in Mice by TRPA1-Dependent and -Independent Mechanisms: Role of Tissue Injury and P2X Receptors

Both TRPA1 and purinergic P2X receptors have been proposed as potential targets for the treatment of visceral pain. We found that the intracolonic administration of a low dose mustard oil (0.5%), a well-known TRPA1 agonist, produced nociceptive responses and abdominal wall referred mechanical hyperalgesia, without inducing apparent tissue damage. Both nociceptive responses and referred hyperalgesia were abolished by the ablation of TRPV1-expressing neurons (and the consequent ablation of TRPA1+ nociceptors) by resiniferatoxin (RTX) treatment, and by the TRPA1 antagonist AP18. However, a higher dose of mustard oil (2.5%) damaged the colonic epithelium and induced pERK activation in the spinal cord, and these processes were clearly independent of TRPV1-expressing neurons ablated by RTX. This higher dose of mustard oil induced nociceptive responses and referred mechanical hyperalgesia which were insensitive or only slightly sensitive to resiniferatoxin or AP18, but were markedly reduced by the P2X antagonist TNP-ATP, which is known to inhibit nociceptive actions induced by ATP released from injured tissues. In conclusion, whereas a low dose of intracolonic mustard oil induces visceral pain in a manner fully dependent on TRPA1 actions, when a high dose of this chemical irritant is used, visceral pain becomes mostly independent of TRPA1 activation but clearly enhanced by ATP purportedly released by the damaged colonic epithelium. Therefore, TRPA1 inhibition is not sufficient to substantially decrease visceral pain during tissue injury, whereas purinergic antagonism appears to be a more effective strategy.

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.

Colitis-induced alterations in response properties of visceral nociceptive neurons in the rat caudal medulla oblongata and their modulation by 5-HT3 receptor blockade

There is considerable clinical and experimental evidence that intestinal inflammation is associated with altered visceral nociceptive processing in the spinal cord and brain, but the underlying neuronal mechanisms, especially acting at the supraspinal level, remain unclear. Considering that the caudal ventrolateral medulla (CVLM) and the nucleus tractus solitarius (NTS) are the first sites for supraspinal processing of visceral pain signals, in the present study we evaluated the experimental colitis-induced changes in response properties of CVLM and NTS medullary neurons to noxious colorectal distension (CRD) in urethane-anesthetized adult male Wistar rats. To determine if gut inflammation alters the 5-HT₃ receptor-dependent modulation of visceral pain-related CVLM and NTS cells, we examined the effects of intravenously administered selective 5-HT₃ antagonist granisetron on ongoing and CRD-evoked activity of CVLM and NTS neurons in healthy control and colitic animals. In the absence of colonic pathology, the CVLM neurons were more excited by noxious CRD that the NTS cells, which demonstrated a greater tendency to be inhibited by the stimulation. The difference was eliminated after the development of colitis due to the increase in the proportion of CRD-excited neurons in both medullary regions associated with enhanced magnitude of the neuronal nociceptive responses. Intravenous granisetron (1 or 2 mg/kg) produced the dose-dependent suppression of the ongoing and evoked firing of CRD-excited cells within both the CVLM and NTS in normal conditions as well as was able to substantially reduce excitability of the caudal medullary neurons in the presence of colonic inflammation, arguing for the potential efficacy of the 5-HT₃ receptor blockade with granisetron against both acute and inflammatory abdominal pain. Taken together, the data obtained can contribute to a deeper understanding of supraspinal serotonergic mechanisms responsible for the persistence of visceral hypersensitivity and hyperalgesia triggered by colonic inflammation.

Analgesic Effects of Duloxetine on Formalin-Induced Hyperalgesia and Its Underlying Mechanisms in the CeA

In rodents, the amygdala has been proposed to serve as a key center for the nociceptive perception. Previous studies have shown that extracellular signal-regulated kinase (ERK) signaling cascade in the central nucleus of amygdala (CeA) played a functional role in inflammation-induced peripheral hypersensitivity. Duloxetine (DUL), a serotonin and noradrenaline reuptake inhibitor, produced analgesia on formalin-induced spontaneous pain behaviors. However, it is still unclear whether single DUL pretreatment influences formalin-induced hypersensitivity and what is the underlying mechanism. In the current study, we revealed that systemic pretreatment with DUL not only dose-dependently suppressed the spontaneous pain behaviors, but also relieved mechanical and thermal hypersensitivity induced by formalin hindpaw injection. Consistent with the analgesic effects of DUL on the pain behaviors, the expressions of Fos and pERK that were used to check the neuronal activities in the spinal cord and CeA were also dose-dependently reduced following DUL pretreatment. Meanwhile, no emotional aversive behaviors were observed at 24 h after formalin injection. The concentration of 5-HT in the CeA was correlated with the dose of DUL in a positive manner at 24 h after formalin injection. Direct injecting 5-HT into the CeA suppressed both the spontaneous pain behaviors and hyperalgesia induced by formalin injection. However, DUL did not have protective effects on the formalin-induced edema of hindpaw. In sum, the activation of CeA neurons may account for the transition from acute pain to long-term hyperalgesia after formalin injection. DUL may produce potent analgesic effects on the hyperalgesia and decrease the expressions of p-ERK through increasing the concentration of serotonin in the CeA.

Administration of Wasabia koreana Ameliorates Irritable Bowel Syndrome-Like Symptoms in a Zymosan-Induced Mouse Model

Irritable bowel syndrome (IBS) is a functional gastrointestinal disease with complex pathophysiology involving the brain-gut axis. To assess the effects of Wasabia koreana (WK) on IBS, we employed a mouse model of colonic zymosan injection presenting with diarrhea-predominant IBS-like symptoms. Oral WK administration significantly diminished stool score, suppressed colon length and weight change, and minimized body weight loss without affecting food intake. In WK-treated mice, the submucosal thickening and epithelial lining of the colon were inhibited and were similar to those of naïve mice. Infiltration of mast cells into the colon and serum tumor necrosis factor-α levels were markedly suppressed. These effects were comparable to those of sulfasalazine, an anti-inflammatory drug. Furthermore, the number of visceral pain-related behaviors was significantly decreased, and locomotion activities measured in the elevated plus maze and open field tests were significantly increased by WK in a dose-dependent manner compared with amitriptyline, an antidepressant. These changes were accompanied by reduced FosB2 expression in the brain. Taken together, these data suggest that WK may have potential as a medicinal food for IBS by acting on inflammatory diarrhea and neural activity.

Stress and the Microbiota-Gut-Brain Axis in Visceral Pain: Relevance to Irritable Bowel Syndrome

Visceral pain is a global term used to describe pain originating from the internal organs of the body, which affects a significant proportion of the population and is a common feature of functional gastrointestinal disorders (FGIDs) such as irritable bowel syndrome (IBS). While IBS is multifactorial, with no single etiology to completely explain the disorder, many patients also experience comorbid behavioral disorders, such as anxiety or depression; thus, IBS is described as a disorder of the gut-brain axis. Stress is implicated in the development and exacerbation of visceral pain disorders. Chronic stress can modify central pain circuitry, as well as change motility and permeability throughout the gastrointestinal (GI) tract. More recently, the role of the gut microbiota in the bidirectional communication along the gut-brain axis, and subsequent changes in behavior, has emerged. Thus, stress and the gut microbiota can interact through complementary or opposing factors to influence visceral nociceptive behaviors. This review will highlight the evidence by which stress and the gut microbiota interact in the regulation of visceral nociception. We will focus on the influence of stress on the microbiota and the mechanisms by which microbiota can affect the stress response and behavioral outcomes with an emphasis on visceral pain.

Mechanisms contributing to visceral hypersensitivity: focus on splanchnic afferent nerve signaling

BACKGROUND

Visceral hypersensitivity is a main characteristic of functional bowel disorders and is mediated by both peripheral and central factors. We investigated whether enhanced splanchnic afferent signaling in vitro is associated with visceral hypersensitivity in vivo in an acute and postinflammatory rat model of colitis.

METHODS

Trinitrobenzene sulfonic acid (TNBS)-colitis was monitored individually by colonoscopy to confirm colitis and follow convalescence and endoscopic healing in each rat. Experiments were performed in controls, rats with acute colitis and in postcolitis rats. Colonic afferent mechanosensitivity was assessed in vivo by quantifying visceromotor responses (VMRs), and by making extracellular afferent recordings from splanchnic nerve bundles in vitro. Multiunit afferent activity was classified into single units identified as low threshold (LT), wide dynamic range (WDR), high threshold (HT), and mechanically insensitive afferents (MIA).

KEY RESULTS

During acute TNBS-colitis, VMRs were significantly increased and splanchnic nerve recordings showed proportionally less MIA and increased WDR and HT afferents. Acute colitis gave rise to an enhanced spontaneous activity of both LT and MIA and augmented afferent mechanosensitivity in LT, WDR and HT afferents. Postcolitis, VMRs remained significantly increased, whereas splanchnic nerve recordings showed that the proportion of LT, WDR, HT and MIA had normalized to control values. However, LT and MIA continued to show increased spontaneous activity and WDR and HT remained sensitized to colorectal distension.

CONCLUSIONS & INFERENCES

Visceral hypersensitivity in vivo is associated with sensitized splanchnic afferent responses both during acute colitis and in the postinflammatory phase. However, splanchnic afferent subpopulations are affected differentially at both time points.

Stress-induced visceral pain: toward animal models of irritable-bowel syndrome and associated comorbidities

Visceral pain is a global term used to describe pain originating from the internal organs, which is distinct from somatic pain. It is a hallmark of functional gastrointestinal disorders such as irritable-bowel syndrome (IBS). Currently, the treatment strategies targeting visceral pain are unsatisfactory, with development of novel therapeutics hindered by a lack of detailed knowledge of the underlying mechanisms. Stress has long been implicated in the pathophysiology of visceral pain in both preclinical and clinical studies. Here, we discuss the complex etiology of visceral pain reviewing our current understanding in the context of the role of stress, gender, gut microbiota alterations, and immune functioning. Furthermore, we review the role of glutamate, GABA, and epigenetic mechanisms as possible therapeutic strategies for the treatment of visceral pain for which there is an unmet medical need. Moreover, we discuss the most widely described rodent models used to model visceral pain in the preclinical setting. The theory behind, and application of, animal models is key for both the understanding of underlying mechanisms and design of future therapeutic interventions. Taken together, it is apparent that stress-induced visceral pain and its psychiatric comorbidities, as typified by IBS, has a multifaceted etiology. Moreover, treatment strategies still lag far behind when compared to other pain modalities. The development of novel, effective, and specific therapeutics for the treatment of visceral pain has never been more pertinent.

Neuroplasticity and dysfunction after gastrointestinal inflammation

The gastrointestinal tract is innervated by several distinct populations of neurons, whose cell bodies either reside within (intrinsic) or outside (extrinsic) the gastrointestinal wall. Normally, most individuals are unaware of the continuous, complicated functions of these neurons. However, for patients with gastrointestinal disorders, such as IBD and IBS, altered gastrointestinal motility, discomfort and pain are common, debilitating symptoms. Although bouts of intestinal inflammation underlie the symptoms associated with IBD, increasing preclinical and clinical evidence indicates that infection and inflammation are also key risk factors for the development of other gastrointestinal disorders. Notably, a strong correlation exists between prior exposure to gut infection and symptom occurrence in IBS. This Review discusses the evidence for neuroplasticity (structural, synaptic or intrinsic changes that alter neuronal function) affecting gastrointestinal function. Such changes are evident during inflammation and, in many cases, long after healing of the damaged tissues, when the nervous system fails to reset back to normal. Neuroplasticity within distinct populations of neurons has a fundamental role in the aberrant motility, secretion and sensation associated with common clinical gastrointestinal disorders. To find appropriate therapeutic treatments for these disorders, the extent and time course of neuroplasticity must be fully appreciated.

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

Effects of electroacupuncture on expression of c-fos protein in the spinal dorsal horn of rats with chronic visceral hyperalgesia

OBJECTIVE

Acupuncture is widely used in clinics to suppress chronic visceral pain in patients with irritable bowel syndrome (IBS); however, the exact neurobiological mechanisms for its therapeutic effects need further exploration. The aim of this study was to investigate the possible involvement of spinal neurons in the effects of electroacupuncture (EA) in relieving chronic visceral hyperalgesia in a rat model of IBS.

METHODS

Colon mechanical irritation was applied to male neonatal Sprague-Dawley rats to establish the IBS model. Behavioral test of the abdominal withdraw reflex (AWR) response to colorectal distention stimuli was conducted to judge the degree of colorectal sensitivity. EA at acupoints Zusanli (ST36) and Shangjuxu (ST37) was applied bilaterally in a total of four times every other day, while sham-EA at similar acupoints was done by inserting needles without electrical stimulation. Immunohistochemical methods were used to display the expression of proto-oncogene protein c-fos in the spinal dorsal horn.

RESULTS

It was found that AWR scores were significantly increased in the IBS model rats (P<0.01), accompanied with significant increase in the expression of c-fos protein in the superficial laminae (SDH, laminae I and II) and nucleus proprius (NP, laminae III and IV), the neck of the dorsal horn (NECK, laminae V and VI) at lumbosacral (L6-S2) spinal level, and in NECK at thoracolumbar (T13-L2) spinal level, when compared with normal rats (P<0.05). After EA treatment, AWR scores and the expression of c-fos protein in SDH, NP and NECK at similar spinal levels were significantly decreased in the IBS model rats (P<0.05). No such effects on either AWR scores or the expression of c-fos protein were observed in IBS model rats after sham-EA treatment.

CONCLUSION

The abnormally high neuronal excitability in the spinal dorsal horn may be an important reason underlying the visceral hyperalgesia in IBS model rats. EA treatment can relieve the chronic visceral hyperalgesia in IBS rats by suppressing the abnormal neuronal excitability in the spinal dorsal horn.

Peripheral contribution of NGF and ASIC1a to colonic hypersensitivity in a rat model of irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder associated with idiopathic colonic hypersensitivity (CHS). However, recent studies suggest that low-grade inflammation could underlie CHS in IBS. The pro-inflammatory mediator nerve growth factor (NGF) plays a key role in the sensitization of peripheral pain pathways and several studies have reported its contribution to visceral pain development. NGF modulates the expression of Acid-Sensing Ion Channels (ASICs), which are proton sensors involved in sensory neurons sensitization. This study examined the peripheral contribution of NGF and ASICs to IBS-like CHS induced by butyrate enemas in the rat colon.

METHODS

Colorectal distension and immunohistochemical staining of sensory neurons were used to evaluate NGF and ASICs contribution to the development of butyrate-induced CHS.

KEY RESULTS

Systemic injection of anti-NGF antibodies or the ASICs inhibitor amiloride prevented the development of butyrate-induced CHS. A significant increase in NGF and ASIC1a protein expression levels was observed in sensory neurons of rats displaying butyrate-induced CHS. This increase was specific of small- and medium-diameter L1 + S1 sensory neurons, where ASIC1a was co-expressed with NGF or trkA in CGRP-immunoreactive somas. ASIC1a was also overexpressed in retrogradely labeled colon sensory neurons. Interestingly, anti-NGF antibody administration prevented ASIC1a overexpression in sensory neurons of butyrate-treated rats.

CONCLUSIONS & INFERENCES

Our data suggest that peripheral NGF and ASIC1a concomitantly contribute to the development of butyrate-induced CHS NGF-ASIC1a interplay may have a pivotal role in the sensitization of colonic sensory neurons and as such, could be considered as a potential new therapeutic target for IBS treatment.

Sprouting of colonic afferent central terminals and increased spinal mitogen-activated protein kinase expression in a mouse model of chronic visceral hypersensitivity

Visceral pain following infection or inflammation is a major clinical problem. Although we have knowledge of how peripheral endings of colonic afferents change in disease, their central projections have been overlooked. With neuroanatomical tracing and colorectal distension (CRD), we sought to identify colonic afferent central terminals (CACTs), the dorsal horn (DH) neurons activated by colonic stimuli in the thoracolumbar (T10-L1) DH, and determine how they are altered by postinflammatory chronic colonic mechanical hypersensitivity. Retrograde tracing from the colon identified CACTs in the DH, whereas immunohistochemistry for phosphorylated MAP kinase ERK 1/2 (pERK) identified DH neurons activated by CRD (80 mmHg). In healthy mice, CACTs were located primarily in DH laminae I (LI) and V (LV) and projected down middle and lateral DH collateral pathways. CRD evoked pERK immunoreactivity in DH neurons, the majority of which were located in LI and LV, the same regions as CACTs. In postinflammatory mice, CACTs were significantly increased in T12-L1 compared with healthy mice. Although CACTs remained abundant in LI, they were more widespread and were now present in deeper laminae. After CRD, significantly more DH neurons were pERK-IR postinflammation (T12-L1), with abundant expression in LI and deeper laminae. In both healthy and postinflammatory mice, many pERK neurons were in close apposition to CACTs, suggesting that colonic afferents can stimulate specific DH neurons in response to noxious CRD. Overall, we demonstrate that CACT density and the number of responsive DH neurons in the spinal cord increase postinflammation, which may facilitate aberrant central representation of colonic nociceptive signaling following chronic peripheral hypersensitivity.

Sex differences in spinal processing of transient and inflammatory colorectal stimuli in the rat

Sex differences in the spinal processing of somatic and visceral stimuli contribute to greater female sensitivity in many pain disorders. The present study examined spinal mechanisms that contribute to sex differences in visceral sensitivity. The visceromotor response to colorectal distention (CRD) was more robust in normal female rats and after intracolonic mustard oil compared with that in male rats. No sex difference was observed in the CRD-evoked response of lumbosacral (LS) and thoracolumbar (TL) colonic afferents in normal and mustard oil-treated rats, but there was a sex difference in spontaneous activity that was exacerbated by intracolonic mustard oil. The response of visceroceptive dorsal horn neurons to CRD was greater in normal female rats in the LS and TL spinal segments. The effect of intracolonic mustard oil on the CRD-evoked response of different phenotypes of visceroceptive dorsal horn neurons was dependent on sex and segment. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response in normal rats with greater effect in male rats. Correspondingly, there was greater cell membrane expression of the GluN1 subunit in dorsal horn extracts in female rats. After intracolonic mustard oil, there was no longer a sex difference in the effect of APV nor GluN1 expression in LS segments, but greater female expression in TL segments. These data document a sex difference in spinal processing of nociceptive visceral stimuli from the normal and inflamed colon. Differences in dorsal horn neuronal activity and NMDA receptor expression contribute to the sex differences in the visceral sensitivity observed in awake rats.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Spinal cord plasticity and acid-sensing ion channels involvement in a rodent model of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common functional gastro-intestinal disorder characterized by intractable chronic abdominal pain. In this study, we examined the possible spinal mechanisms underlying colonic hypersensitivity (CHS) using a non-inflammatory rat model of IBS induced by rectal enemas of butyrate, a short-chain fatty acid. We hypothesized that spinal plasticity could be responsible for CHS and that ASIC channels, which are known to support pain-elicited currents in the spinal cord, could contribute to central sensitization in our model of IBS. First, in order to determine if visceral pain relies on changes in spinal activity, we analyzed Fos expression in the spinal cord of rats treated with butyrate following a challenge with repetitive noxious colorectal distension. We found that Fos immunoreactivity was increased in thoracic T10-11-12, lumbar L1-2-6 and sacral S1 spinal segments. In control rats treated with saline, noxious repetitive colorectal distensions evoked Fos expression only in L1-2-6 and S1 spinal segments. Secondly, intrathecal injection of PcTx1, a specific ASIC1A antagonist, in the lumbar spinal cord completely prevented the development of CHS induced by butyrate. ASIC1 and 2 mRNAs, especially ASIC1A, were upregulated in the lumbar spinal cord. ASIC1A could thus contribute to spinal sensitization in our model of IBS, as it is supported by spinal colocalization of ASIC1A and Fos proteins. The whole data pinpoint a potential critical role of thoracic spinal cord in non-inflammatory pain states such as IBS and suggest that ASIC channels are part of the molecular effectors of central sensitization leading to visceral pain.

Stress-related alterations of visceral sensation: animal models for irritable bowel syndrome study

Stressors of different psychological, physical or immune origin play a critical role in the pathophysiology of irritable bowel syndrome participating in symptoms onset, clinical presentation as well as treatment outcome. Experimental stress models applying a variety of acute and chronic exteroceptive or interoceptive stressors have been developed to target different periods throughout the lifespan of animals to assess the vulnerability, the trigger and perpetuating factors determining stress influence on visceral sensitivity and interactions within the brain-gut axis. Recent evidence points towards adequate construct and face validity of experimental models developed with respect to animals' age, sex, strain differences and specific methodological aspects such as non-invasive monitoring of visceromotor response to colorectal distension as being essential in successful identification and evaluation of novel therapeutic targets aimed at reducing stress-related alterations in visceral sensitivity. Underlying mechanisms of stress-induced modulation of visceral pain involve a combination of peripheral, spinal and supraspinal sensitization based on the nature of the stressors and dysregulation of descending pathways that modulate nociceptive transmission or stress-related analgesic response.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Reactive oxygen species are involved in group I mGluR-mediated facilitation of nociceptive processing in amygdala neurons

Recent biochemical and behavioral data implicate reactive oxygen species (ROS) in peripheral and spinal pain mechanisms. However, pain-related functions of ROS in the brain and mechanisms of pain-related ROS activation remain to be determined. Our previous studies showed that the amygdala plays a key role in emotional-affective pain responses and pain modulation. Hyperactivity of amygdala neurons in an animal pain model depends on group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5), but their signaling pathway remains to be determined. Here we tested the hypothesis that activation of group I mGluRs increases nociceptive processing in amygdala neurons through a mechanism that involves ROS. Extracellular single-unit recordings were made from neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC) in anesthetized adult male rats. Administration of a group I mGluR agonist (DHPG) into the CeLC by microdialysis increased the responses to innocuous and noxious somatosensory (knee joint compression) and visceral (colorectal distention [CRD]) stimuli. A ROS scavenger (PBN) and a superoxide dismutase mimetic (TEMPOL) reversed the facilitatory effects of DHPG. An mGluR5 antagonist (MPEP) also inhibited the effects of DHPG on the responses to innocuous and noxious somatosensory and visceral stimuli, whereas an mGluR1 antagonist (LY367385) decreased only the responses to visceral stimulation. The results show for the first time that ROS mediate group I mGluR-induced facilitation of nociceptive processing in amygdala neurons. The antagonist data may suggest differential contributions of subtypes mGluR1 and mGluR5 to the processing of somatosensory and visceral nociceptive information in the amygdala.

NMDA receptor subunit expression and PAR2 receptor activation in colospinal afferent neurons (CANs) during inflammation induced visceral hypersensitivity

BACKGROUND

Visceral hypersensitivity is a clinical observation made when diagnosing patients with functional bowel disorders. The cause of visceral hypersensitivity is unknown but is thought to be attributed to inflammation. Previously we demonstrated that a unique set of enteric neurons, colospinal afferent neurons (CANs), co-localize with the NR1 and NR2D subunits of the NMDA receptor as well as with the PAR2 receptor. The aim of this study was to determine if NMDA and PAR2 receptors expressed on CANs contribute to visceral hypersensitivity following inflammation. Recently, work has suggested that dorsal root ganglion (DRG) neurons expressing the transient receptor potential vanilloid-1 (TRPV1) receptor mediate inflammation induced visceral hypersensitivity. Therefore, in order to study CAN involvement in visceral hypersensitivity, DRG neurons expressing the TRPV1 receptor were lesioned with resiniferatoxin (RTX) prior to inflammation and behavioural testing.

RESULTS

CANs do not express the TRPV1 receptor; therefore, they survive following RTX injection. RTX treatment resulted in a significant decrease in TRPV1 expressing neurons in the colon and immunohistochemical analysis revealed no change in peptide or receptor expression in CANs following RTX lesioning as compared to control data. Behavioral studies determined that both inflamed non-RTX and RTX animals showed a decrease in balloon pressure threshold as compared to controls. Immunohistochemical analysis demonstrated that the NR1 cassettes, N1 and C1, of the NMDA receptor on CANs were up-regulated following inflammation. Furthermore, inflammation resulted in the activation of the PAR2 receptors expressed on CANs.

CONCLUSION

Our data show that inflammation causes an up-regulation of the NMDA receptor and the activation of the PAR2 receptor expressed on CANs. These changes are associated with a decrease in balloon pressure in response to colorectal distension in non-RTX and RTX lesioned animals. Therefore, these data suggest that CANs contribute to visceral hypersensitivity during inflammation.

NMDA receptor subunit expression and PAR2 receptor activation in colospinal afferent neurons (CANs) during inflammation induced visceral hypersensitivity

Background

Visceral hypersensitivity is a clinical observation made when diagnosing patients with functional bowel disorders. The cause of visceral hypersensitivity is unknown but is thought to be attributed to inflammation. Previously we demonstrated that a unique set of enteric neurons, colospinal afferent neurons (CANs), co-localize with the NR1 and NR2D subunits of the NMDA receptor as well as with the PAR2 receptor. The aim of this study was to determine if NMDA and PAR2 receptors expressed on CANs contribute to visceral hypersensitivity following inflammation. Recently, work has suggested that dorsal root ganglion (DRG) neurons expressing the transient receptor potential vanilloid-1 (TRPV1) receptor mediate inflammation induced visceral hypersensitivity. Therefore, in order to study CAN involvement in visceral hypersensitivity, DRG neurons expressing the TRPV1 receptor were lesioned with resiniferatoxin (RTX) prior to inflammation and behavioural testing.

Results

CANs do not express the TRPV1 receptor; therefore, they survive following RTX injection. RTX treatment resulted in a significant decrease in TRPV1 expressing neurons in the colon and immunohistochemical analysis revealed no change in peptide or receptor expression in CANs following RTX lesioning as compared to control data. Behavioral studies determined that both inflamed non-RTX and RTX animals showed a decrease in balloon pressure threshold as compared to controls. Immunohistochemical analysis demonstrated that the NR1 cassettes, N1 and C1, of the NMDA receptor on CANs were up-regulated following inflammation. Furthermore, inflammation resulted in the activation of the PAR2 receptors expressed on CANs.

Conclusion

Our data show that inflammation causes an up-regulation of the NMDA receptor and the activation of the PAR2 receptor expressed on CANs. These changes are associated with a decrease in balloon pressure in response to colorectal distension in non-RTX and RTX lesioned animals. Therefore, these data suggest that CANs contribute to visceral hypersensitivity during inflammation.

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-alpha (ERalpha) and ERbeta are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERalpha and ERbeta with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.

Sex differences in the activation of the spinoparabrachial circuit by visceral pain

Women are more sensitive to most noxious visceral stimuli, both in terms of intensity and frequency. The spinoparabrachial (spino-PBn) pathway is an essential neural circuit for the central relay of viscerosensitive information, but studies characterizing the anatomical and physiological characteristics of this pathway have only been conducted in males. Sex differences in the anatomical and/or physiological organization of the spino-PBn may contribute to the sexually dimorphic incidence rate for visceral pain syndromes. Retrograde labeling and colorectal distention (CRD) induced Fos expression was used to delineate the spino-PBn circuit in male and cycling female Sprague-Dawley rats. The ability of morphine to suppress CRD evoked responses was also examined. Neurons retrogradely labeled from the PBn were localized primarily within the superficial dorsal horn and sacral parasympathetic nucleus of the L5-S1 spinal cord. While no sex differences were noted in either the distribution of spino-PBn neurons or in CRD-induced Fos expression, significantly greater Fos expression was noted specifically in spino-PBn neurons in males compared to females. Morphine selectively attenuated Fos expression in spino-PBn neurons in males, but not females. Subsequent anatomical studies showed significantly reduced mu opioid receptor protein levels and radioligand binding within the PBn of females in comparison to males. Together, these data indicate that there are profound sex differences in how a noxious visceral stimulus and opiates engage the spino-PBn pathway, which may account for the observed clinical differences in visceral pain sensitivity and morphine antinociception.

Development, plasticity and modulation of visceral afferents

Visceral pain is the most common reason for doctor visits in the US. Like somatic pain, virtually all visceral pain sensations begin with the activation of primary sensory neurons innervating the viscera and/or the blood vessels associated with these structures. Visceral afferents also play a central role in tissue homeostasis. Recent studies show that in addition to monitoring the state of the viscera, they perform efferent functions through the release of small molecules (e.g. peptides like CGRP) that can drive inflammation, thereby contributing to the development of visceral pathologies (e.g. diabetes Razavi, R., Chan, Y., Afifiyan, F.N., Liu, X.J., Wan, X., Yantha, J., Tsui, H., Tang, L., Tsai, S., Santamaria, P., Driver, J.P., Serreze, D., Salter, M.W., Dosch, H.M., 2006. TRPV1+ sensory neurons control beta cell stress and islet inflammation in autoimmune diabetes, Cell 127 1123-1135). Visceral afferents are heterogeneous with respect to their anatomy, neurochemistry and function. They are also highly plastic in that their cellular environment continuously influences their response properties. This plasticity makes them susceptible to long-term changes that may contribute significantly to the development of persistent pain states such as those associated with irritable bowel syndrome, pancreatitis, and visceral cancers. This review examines recent insights into visceral afferent anatomy and neurochemistry and how neonatal insults can affect the function of these neurons in the adult. New approaches to the treatment of visceral pain, which focus on primary afferents, will also be discussed.

A rat model of chronic postinflammatory visceral pain induced by deoxycholic acid

BACKGROUND & AIMS

Chronic visceral hyperalgesia is considered an important pathophysiologic symptom in irritable bowel syndrome (IBS); previous gastrointestinal inflammation is a potent etiologic factor for developing IBS. Although there are several animal models of adult visceral hypersensitivity after neonatal perturbation or acute colonic irritation/inflammation, current models of postinflammatory chronic visceral hyperalgesia are unsatisfactory. The aim of this study was to establish a model of chronic visceral hyperalgesia after colonic inflammation in the rat.

METHODS

Deoxycholic acid (DCA) was instilled into the rat colon daily for 3 days and animals were tested for up to 4 weeks.

RESULTS

DCA induced mild, transient colonic inflammation within 3 days that resolved within 3 weeks. An exaggerated visceromotor response, referred pain to mechanical stimulation, increased spinal Fos expression, and colonic afferent and dorsal horn neuron activity were apparent by 1 week and persisted for at least 4 weeks, indicating chronic dorsal horn hyperexcitability and visceral hyperalgesia. There was no spontaneous pain, based on open field behavior. There was a significant increase in opioid-receptor activity.

CONCLUSIONS

DCA induces mild, transient colitis, resulting in persistent visceral hyperalgesia and referred pain in rats, modeling some aspects of postinflammatory IBS.

Evidence for the involvement of the spinoparabrachial pathway, but not the spinothalamic tract or post-synaptic dorsal column, in acute bone nociception

We have previously reported that acute noxious mechanical stimulation of bone activates neurons throughout the dorsal horn of the lumbar spinal cord, and argued that the spinal mechanisms that mediate bone nociception are different to those that mediate cutaneous and visceral nociception. In the present study, we provide evidence that the ascending spinal pathways that mediate acute bone nociception also differ to those that mediate acute cutaneous and visceral nociception. Injections of a retrograde tracer (Fluorogold) were made into the thalamus, gracile nucleus or lateral parabrachial nucleus to identify spinothalamic, post-synaptic dorsal column or spinoparabrachial projection neurons respectively (n=4 in each group). Spinal dorsal horn neurons activated by acute noxious mechanical stimulation of bone (bone drilling) were identified in these animals using Fos immunohistochemistry. Fluorogold and Fos-like immunoreactivity was not colocalized in any dorsal horn neurons projecting to the thalamus or gracile nucleus. In contrast, a total of 12.2+/-1.1% (mean+/-S.E.M.) of the spinoparabrachial projection neurons contained Fos-like immunoreactive nuclei following bone drilling and this was significantly greater than the percentage (3.4+/-0.5%) in animals of a sham surgery group (n=4) that were not exposed to bone drilling (Mann-Whitney; p<0.05). These data provide evidence for the involvement of the spinoparabrachial pathway, but not the spinothalamic or post-synaptic dorsal column pathways, in the relay of information regarding acute noxious mechanical stimuli applied to bone, and suggest that spinal pathways that mediate acute bone nociception may be different to those that mediate acute nociception of cutaneous and visceral origin.

Membrane estrogen receptors acting through metabotropic glutamate receptors: an emerging mechanism of estrogen action in brain

It has been over 60 years since the first studies have been published describing the effects of steroid hormones on brain function. For over 30 years, estrogen has been presumed to directly affect gene expression and protein synthesis through a specific receptor. More than 20 years ago, the first estrogen receptor was cloned and identified as a transcription factor. Yet, throughout their course of study, estrogens have also been observed to affect nervous system function via mechanisms independent of intracellular receptor regulation of gene expression. Up until recently, the membrane estrogen receptors responsible for these rapid actions have remained elusive. Recent studies have demonstrated that a large number of these rapid, membrane-initiated actions of estradiol are due to surface expression of classical estrogen receptors. This review focuses on the importance of membrane estrogen receptor interactions with metabotropic glutamate receptors for understanding rapid estradiol signaling mechanisms and downstream effectors, as well as their significance in a variety of physiological processes.

Transient receptor potential vanilloid 4 mediates protease activated receptor 2-induced sensitization of colonic afferent nerves and visceral hyperalgesia

Protease-activated receptor (PAR(2)) is expressed by nociceptive neurons and activated during inflammation by proteases from mast cells, the intestinal lumen, and the circulation. Agonists of PAR(2) cause hyperexcitability of intestinal sensory neurons and hyperalgesia to distensive stimuli by unknown mechanisms. We evaluated the role of the transient receptor potential vanilloid 4 (TRPV4) in PAR(2)-induced mechanical hyperalgesia of the mouse colon. Colonic sensory neurons, identified by retrograde tracing, expressed immunoreactive TRPV4, PAR(2), and calcitonin gene-related peptide and are thus implicated in nociception. To assess nociception, visceromotor responses (VMR) to colorectal distension (CRD) were measured by electromyography of abdominal muscles. In TRPV4(+/+) mice, intraluminal PAR(2) activating peptide (PAR(2)-AP) exacerbated VMR to graded CRD from 6-24 h, indicative of mechanical hyperalgesia. PAR(2)-induced hyperalgesia was not observed in TRPV4(-/-) mice. PAR(2)-AP evoked discharge of action potentials from colonic afferent neurons in TRPV4(+/+) mice, but not from TRPV4(-/-) mice. The TRPV4 agonists 5',6'-epoxyeicosatrienoic acid and 4alpha-phorbol 12,13-didecanoate stimulated discharge of action potentials in colonic afferent fibers and enhanced current responses recorded from retrogradely labeled colonic dorsal root ganglia neurons, confirming expression of functional TRPV4. PAR(2)-AP enhanced these responses, indicating sensitization of TRPV4. Thus TRPV4 is expressed by primary spinal afferent neurons innervating the colon. Activation of PAR(2) increases currents in these neurons, evokes discharge of action potentials from colonic afferent fibers, and induces mechanical hyperalgesia. These responses require the presence of functional TRPV4. Therefore, TRPV4 is required for PAR(2)-induced mechanical hyperalgesia and excitation of colonic afferent neurons.

The pattern of Fos expression in the spinal dorsal horn following acute noxious mechanical stimulation of bone

Little is known of the spinal mechanisms that mediate bone nociception. The aim of this study was to determine the pattern of neuronal activation in the spinal dorsal horn following acute noxious mechanical stimulation of bone. This was achieved by examining Fos expression in the spinal dorsal horn following acute, noxious mechanical stimulation of the rat tibia. Noxious mechanical stimuli were applied by bone drilling and raising tibial intra-osseous pressure. Control experiments consisted of surgery to expose the tibia. There was a significant increase in the number of Fos-like immunoreactive (Fos-LI) nuclei in the superficial, ipsilateral dorsal horn of animals in the bone drilling and pressure groups relative to animals of the control group at spinal cord segments L3 and L4 (P<0.05). The number of Fos-LI nuclei in the deep dorsal horn was always lower than the number in the superficial dorsal horn (significant at L3 but not L4; P<0.05). Whilst there appeared to be a small increase in the number of Fos-LI nuclei in the ipsilateral deep dorsal horn of bone drilling and pressure groups relative to the ipsilateral deep dorsal horn control group at both L3 and L4 segments, no significant effect was observed (P>0.05). The present study implicates the superficial dorsal horn of the spinal cord as a region of interest in studies of acute bone pain, and highlights the notion that spinal mechanisms that mediate bone nociception may be different to those that mediate nociception of cutaneous and visceral origin.

Pelvic nerve input mediates descending modulation of homovisceral processing in the thoracolumbar spinal cord of the rat

BACKGROUND & AIMS

Colonic afferents project to the lumbosacral and thoracolumbar spinal cord via the pelvic and hypogastric/lumbar colonic nerves, respectively. Both spinal regions process inflammatory colonic stimuli. The role of thoracolumbar segments in processing acute colorectal pain is questionable, however, because the lumbosacral spinal cord appears sufficient to process reflex responses to acute pain. Here, we show that activity in pelvic nerve colonic afferents actively modulates thoracolumbar dorsal horn neuron processing of the same colonic stimulus through a supraspinal loop: homovisceral descending modulation.

METHODS

Dorsal horn neurons were recorded in the rat thoracolumbar spinal cord after acute or chronic pelvic neurectomy and cervical cold block.

RESULTS

Acute pelvic neurectomy or lidocaine inhibition of lumbosacral dorsal roots facilitated the excitatory response of thoracolumbar dorsal horn neurons to colorectal distention (CRD) and decreased the percentage of neurons inhibited by CRD, suggesting colonic input over the pelvic nerve inhibits thoracolumbar processing of the same stimulus. Ectopic activity developed in the proximal pelvic nerve after chronic neurectomy reactivating the inhibitory circuit, inhibiting thoracolumbar neurons. Cervical cold block alleviated the inhibition in intact or chronic neurectomized rats. However, the facilitated response after acute pelvic neurectomy was inhibited by cervical cold block, exposing an underlying descending facilitation. Inhibiting pelvic nerve input after cervical cold block had minimal effect.

CONCLUSIONS

These data demonstrate that input over the pelvic nerve modulates the response of thoracolumbar spinal neurons to CRD by a supraspinal loop and that increasing thoracolumbar processing increases visceral hyperalgesia.

Increased visceral sensitivity to capsaicin after DSS-induced colitis in mice: spinal cord c-Fos expression and behavior

During acute and chronic inflammation visceral pain perception is altered. Conflicting data exist, however, on visceral pain perception in the postinflammatory phase. The aim of the present study was to investigate whether visceral pain perception is altered after resolution of dextran sodium sulfate (DSS)-induced inflammation of the colon. Visceral sensory function in mice was assessed by monitoring behavioral responses to intracolonic capsaicin instillation. Two hours later the number of c-Fos-positive neurons in lamina I/II and X of spinal cord segments T(12/13)-S1 was determined as a measure of neuronal activation. DSS colitis was induced by adding 1% of DSS to the drinking water. The course of DSS-induced colitis was assessed by determining the disease activity index score. Animals developed a transient colitis and had recovered at day 49. At this time point, cytokine levels and colon length were similar to control animals. Importantly, after resolution of DSS-induced colitis the behavioral response to intracolonic capsaicin was increased compared with control mice. Moreover, capsaicin-induced spinal cord neuronal c-Fos expression was significantly increased. Interestingly, after colitis animals also exhibited referred somatic hyperalgesia as measured with von Frey hairs on the abdominal wall. We conclude that postinflammatory visceral hyperalgesia occurs after resolution of DSS-induced colitis and that capsaicin-induced behavioral responses and spinal cord neuronal c-Fos activation are effective readouts for determination of visceral pain perception.

Infection-induced viscerosensory signals from the gut enhance anxiety: implications for psychoneuroimmunology

Infection and inflammation lead to changes in mood and cognition. Although the "classic" sickness behavior syndrome, involving fatigue, social withdrawal, and loss of appetites are most familiar, other emotional responses accompany immune activation, including anxiety. Recent studies have shown that gastrointestinal bacterial infections lead to enhanced anxiety-like behavior in mice. The bacteria-induced signal is most likely carried by vagal sensory neurons, and occurs early on (within 6h) during the infection. These signals induce evidence of activation in brain regions that integrate viscerosensory information with mood, and potentiate activation in brain regions established as key players in fear and anxiety. The findings underline the importance of viscerosensory signals arising from the gastrointestinal tract in modulation of behaviors appropriate for coping with threats, and suggest that these signals may contribute to affective symptoms associated with gastrointestinal disorders.

A preliminary study of neck-stomach syndrome

AIM: To determine the expression of c-Fos, caspase-3 and interleukin-1β (IL-1β) in the cervical cord and stomach of rats with cervical spondylosis, to analyze their relationship, and to offer an explanation of one possible cause for functional dyspepsia (FD) and irritable bowel syndrome (IBS) caused by cervical spondylosis.

METHODS: The cervical spondylosis model in rats was established by destroying the stability of cervical posterior column. The cord segments C4-6 and gastric antrum were collected 3 mo and 5 mo after the operation. Rats with the sham operation were used as controls. The expressions of c-Fos, caspase-3 and IL-1β in the cervical cord and gastric antrum were determined by immunohistochemistry and/or Western blot.

RESULTS: Immunohistochemical staining showed a few c-Fos, caspase-3 and IL-1β-positive cells in the cervical cord and antrum in the control. There was a significant increase in c-Fos, caspase-3 and IL-1β expression in model groups compared to the control groups at 3 mo and 5 mo after operation. More importantly, there was a significant (P < 0.05) increase in c-Fos, caspase-3 and IL-1β expression in the model group rats at 3 mo compared to those at 5 mo after the operation (c-Fos: 11.20 ± 2.26 vs 27.68 ± 4.36 in the cervical cord, 11.3 ± 2.3 vs 29.3 ± 4.6 in the gastric antrum; caspase-3: 33.83 ± 3.71 vs 36.32 ± 4.01 in the cervical cord, 13.23 ± 3.21 vs 26.32 ± 4.01 in the gastric antrum; IL-1β: 42.06 ± 2.95 vs 45.91 ± 3.98 in the cervical cord, 26.56 ± 2.65 vs 32.01 ± 2.98 in the gastric antrum). Western blot analysis showed time-dependent changes of caspase-3 and IL-1β protein in the cervical cord and gastric antrum of rats with cervical spondylosis; there was no significant expression of caspase-3 and IL-1β protein in the control group at 3 mo and 5 mo after the sham operation, whereas there was a significant difference in caspase-3 and IL-1β protein levels between the model group rats followed up for 3 mo and those for 5 mo (P < 0.05).

CONCLUSION: There is a significant association of c-Fos, caspase-3 and IL-1β expressions in the gastric antrum with that in the spinal cord in rats with cervical spondylosis, suggesting that the gastrointestinal function may be affected by cervical spondylosis.

Differences in spinal distribution and neurochemical phenotype of colonic afferents in mouse and rat

Visceral pain is a prevalent clinical problem and one of the most common ailments for which patients seek medical attention. Recent studies have described many of the physiological properties of visceral afferents, but not much is known regarding their anatomical characteristics. To determine the spinal distribution and neurochemical phenotype of colonic afferents in rodents, Alexa Fluor-conjugated cholera toxin-beta (CTB) was injected subserosally into the proximal and distal portions of the descending colon in Sprague Dawley rats and C57Bl/6 mice. Dorsal root ganglia (T10-S2) were processed for fluorescent immunohistochemistry and visualized by confocal microscopy. In the mouse, CTB-positive neurons were most numerous in the lumbosacral region (LS; L6-S1), with a smaller contribution in the thoracolumbar ganglia (TL; T13-L1). In contrast, CTB-positive neurons in the rat were most numerous in the TL ganglia, with a smaller contribution in the LS ganglia. The vast majority of CTB-positive neurons in both mouse and rat were positive for TRPV1 and CGRP and most likely unmyelinated, in that most colonic afferents were not positive for neurofilament heavy chain. In the mouse, the TL ganglia had a significantly higher percentage of TRPV1- and CGRP-positive neurons than did the LS ganglia, whereas no differences were observed in the rat. The high incidence of TRPV1-positive colonic afferents in rodents suggests that hypersensitivity from the viscera may be partially a TRPV1-mediated event, thereby providing a suitable target for the treatment of visceral pain.

Modulatory effects of estrogen and progesterone on colorectal hyperalgesia in the rat

The contribution of estrogen and progesterone to colorectal hyperalgesia was examined in female rats. The electromyogram recorded from the abdominal wall (visceromotor response, vmr) and the discharge of lumbosacral dorsal horn neurons to colorectal distention (CRD) were measured in intact female, ovariectomized (OVx) and estradiol replaced OVx (E2; 50mug, 48h) rats with and without colonic inflammation. Colorectal hyperalgesia was transient in intact rats, but persisted at least 4h in E2 and OVx rats. The magnitude of hyperalgesia in E2 rats was greater than OVx which was greater than intact rats. Dorsal horn neurons that responded to CRD with an Abrupt (on and off with stimulus) excitatory discharge showed similar sensitivity to estradiol as the vmr following colonic inflammation. In contrast, inflammation did not increase the magnitude of response of excitatory neurons with sustained afterdischarges in any of the treatment groups. Intact female rats have a comparable plasma estrogen concentration to E2 rats, suggesting the difference in responses may have been due to antinociceptive effects of progesterone. This was tested by administering E2+/- progesterone (1mg) and measuring the vmr. Progesterone reduced the facilitation of the vmr produced by E2 before and following colonic inflammation. The present study suggests that estrogen replacement enhances visceral signal processing following colonic inflammation. Furthermore, progesterone may counteract the effects of estrogen on colorectal sensitivity.

Branching patterns of parabrachial neurons projecting to the central extended amgydala: single axonal reconstructions

Electrophysiological evidence suggests that the spinoparabrachioamygdaloid pathway carries nociceptive information that may be important for the elaboration of physiological and emotional responses to noxious events. The pontine parabrachial nucleus (pPB) sends a massive projection to the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BSTL), both regions belonging to a broader macrostructure, the central extended amygdala (EAc). The aim of this study was to examine whether different EAc components are targeted by a same pPB neuron, by reconstructing single axonal branching patterns after anterograde labelling. Small deposits of biotinylated dextran amine in the region of the external lateral pPB result in dense and specific labelling in the whole EAc. Reconstructed axons innervate either the lateral or the capsular part of the CeA with perisomatic or bushy terminals, respectively. A subset of axons enters the stria terminalis rostrally to follow its trajectory caudally toward the CeA. Individual axons targeting the CeA usually send collaterals to other EAc components, especially those projecting to the lateral CeA, which often coinnervate the BSTL. By contrast, only few branches were found outside the EAc. These results suggest that the noxious information travelling from the pPB to the CeA may also be transmitted to other EAc components. This pPB-EAc pathway, which appears distinct from the parabrachiohypothalamic and parabrachiothalamic projections, would be the anatomical basis through which the EAc elaborates the autonomic, endocrine, and emotional components of pain.

Stimulation of cardiac sympathetic afferents activates glutamatergic neurons in the parabrachial nucleus: relation to neurons containing nNOS

Our previous studies have demonstrated that stimulation of cardiac sympathetic afferents activates neurons in the parabrachial nucleus (PBN), a region known to play a role in central integration of cardiovascular autonomic reflexes. However, phenotypes of these activated neurons have not been well identified. Glutamate, an important excitatory neurotransmitter in the brain, is involved in PBN-mediated cardiovascular responses. Recent identification of vesicular glutamate transporter 3 (VGLUT3) has provided a novel and unique marker to locate distinctive perikarya of neurons that use glutamate as a neurotransmitter. The action of glutamate in the brain is influenced by nitric oxide. Thus, using triple immunofluorescent labeling, the present study examined expression of c-Fos, an immediate early gene, in the neurons containing VGLUT3 and neuronal nitric oxide synthase (nNOS) in the PBN following stimulation of cardiac sympathetic afferents. In anesthetized cats with bilateral barodenervation and cervical vagotomy, topical application of bradykinin (BK, 1-10 microg/ml, 50 microl, n = 6) on the left ventricle was performed six times, every 20 min. Repeated administration of BK elicited consistent increases in blood pressure over a 100 min period while no changes were noted in the animals treated with the vehicle for BK (0.9% saline, n=5). Compared to control cats, c-Fos expression was increased significantly in the cell bodies containing VGLUT3 as well as both VGLUT3 and nNOS in the external lateral PBN (elPBN) in BK-treated animals (all P < 0.01). In addition, using similar triple-staining method, we noted that fibers of activated neurons containing nNOS in the elPBN co-localized with vesicular glutamate transporter 2 following BK stimulation. These data suggest that glutamatergic neurons represent a cell type in the PBN that is activated by stimulation of cardiac sympathetic afferents. Nitric oxide has the potential to influence the action of glutamatergic neurons in regulation of excitatory cardiovascular responses induced by activation of cardiac sympathetic afferents.

GFAP and Fos immunoreactivity in lumbo-sacral spinal cord and medulla oblongata after chronic colonic inflammation in rats

AIM: To investigate the response of astrocytes and neurons in rat lumbo-sacral spinal cord and medulla oblongata induced by chronic colonic inflammation, and the relationship between them.

METHODS: Thirty-three male Sprague-Dawley rats were randomly divided into two groups: experimental group (n = 17), colonic inflammation was induced by intra-luminal administration of trinitrobenzenesulfonic acid (TNBS); control group (n = 16), saline was administered intra-luminally. After 3, 7, 14, and 28 d of administration, the lumbo-sacral spinal cord and medulla oblongata were removed and processed for anti-glial fibrillary acidic protein (GFAP), Fos and GFAP/Fos immunohistochemistry.

RESULTS: Activated astrocytes positive for GFAP were mainly distributed in the superficial laminae (laminae I-II) of dorsal horn, intermediolateral nucleus (laminae V), posterior commissural nucleus (laminae X) and anterolateral nucleus (laminae IX). Fos-IR (Fos-immunoreactive) neurons were mainly distributed in the deeper laminae of the spinal cord (laminae III-IV, V-VI). In the medulla oblongata, both GFAP-IR astrocytes and Fos-IR neurons were mainly distributed in the medullary visceral zone (MVZ). The density of GFAP in the spinal cord of experimental rats was significantly higher after 3, 7, and 14 d of TNBS administration compared with the controls (50.4±16.8, 29.2±6.5, 24.1±5.6, P<0.05). The density of GFAP in MVZ was significantly higher after 3 d of TNBS administration (34.3±2.5, P<0.05). After 28 d of TNBS administration, the density of GFAP in the spinal cord and MVZ decreased and became comparable to that of the controls (18.0±4.9, 14.6±6.4, P>0.05).

CONCLUSION: Astrocytes in spinal cord and medulla oblongata can be activated by colonic inflammation. The activated astrocytes are closely related to Fos-IR neurons. With the recovery of colonic inflammation, the activity of astrocytes in the spinal cord and medulla oblongata is reduced.

Differential processing of noxious colonic input by thoracolumbar and lumbosacral dorsal horn neurons in the rat

Previous studies suggest the lumbosacral (LS) spinal cord processes acute colorectal stimuli whereas the thoracolumbar (TL) and LS spinal segments process inflammatory stimuli. In this study, the effects of colorectal distention (CRD) on TL and LS dorsal horn neuronal activity were recorded in Nembutal-anesthetized male rats both with and without colonic inflammation. Both single cells (before and after inflammation) and populations (multiple cells from noninflamed or inflamed rats) were studied. CRD-responsive neurons had excitatory Abrupt (on-off with stimulus) or Sustained (prolonged after discharge) responses or were Inhibited by CRD. In noninflamed rats, a significantly greater percentage of LS neurons (63% Abrupt, 27% Sustained) were excited by CRD than TL neurons (61% Abrupt, 3% Sustained). The remaining cells were Inhibited (10% LS, 36% TL). LS Abrupt neurons had lower thresholds and greater response magnitudes to CRD compared with TL Abrupt neurons. After colonic inflammation, TL neurons became more excitable: the percentage of Inhibited neurons decreased, the response magnitude of Abrupt neurons increased, and the threshold decreased. In contrast, in single-cell recordings, the response of LS Sustained neurons increased, whereas LS Abrupt neurons decreased. These data suggest that in noninflamed rats, the net response to CRD of TL visceroceptive spinal sensory neurons is less than that of LS neurons. Colonic inflammation increases the net response of TL neurons and differentially modulates the response of LS neurons. These differences may contribute to the functional dichotomy between the TL and LS spinal segments in processing acute and inflammatory colorectal pain.