Behavioral and molecular processing of visceral pain in the brain of mice: impact of colitis and psychological stress

Sex Differences in Visceral Pain and Comorbidities: Clinical Outcomes, Preclinical Models, and Cellular and Molecular Mechanisms

Sexual dimorphism of visceral pain has been documented in clinics and experimental animal models. Aside from hormones, emerging evidence suggests the sex-differential intrinsic neural regulation of pain generation and maintenance. According to the International Association for the Study of Pain (IASP) and the American College of Gastroenterology (ACG), up to 25% of the population have visceral pain at any one time, and in the United States 10-15 percent of adults suffer from irritable bowel syndrome (IBS). Here we examine the preclinical and clinical evidence of sex differences in visceral pain focusing on IBS, other forms of bowel dysfunction and IBS-associated comorbidities. We summarize preclinical animal models that provide a means to investigate the underlying molecular mechanisms in the sexual dimorphism of visceral pain. Neurons and nonneuronal cells (glia and immune cells) in the peripheral and central nervous systems, and the communication of gut microbiota and neural systems all contribute to sex-dependent nociception and nociplasticity in visceral painful signal processing. Emotion is another factor in pain perception and appears to have sexual dimorphism.

Rodent Animal Models of Endometriosis-Associated Pain: Unmet Needs and Resources Available for Improving Translational Research in Endometriosis

Chronic pain induced by endometriosis is a maladaptive pain experienced by half of women with this disease. The lack of pharmacological treatments suitable for the long-term relief of endometriosis-associated pain, without an impact on fertility, remains an urgent unmet need. Progress has been slowed by the absence of a reproducible rodent endometriosis model that fully replicates human physiopathological characteristics, including pain symptoms. Although pain assessment in rodents is a complicated task requiring qualified researchers, the choice of the behavioral test is no less important, since selecting inappropriate tests can cause erroneous data. Pain is usually measured with reflex tests in which hypersensitivity is evaluated by applying a noxious stimulus, yet this ignores the associated emotional component that could be evaluated via non-reflex tests. We conducted a systematic review of endometriosis models used in rodents and the number of them that studied pain. The type of behavioral test used was also analyzed and classified according to reflex and non-reflex tests. Finally, we determined the most used reflex tests for the study of endometriosis-induced pain and the main non-reflex behavioral tests utilized in visceral pain that can be extrapolated to the study of endometriosis and complement traditional reflex tests.

Sex differences in pain-related behaviors and clinical progression of disease in mouse models of colonic pain

ABSTRACT

Previous studies have reported sex differences in patients with irritable bowel syndrome and inflammatory bowel disease, including differences in visceral pain perception. Despite this, sex differences in behavioral manifestations of visceral pain and underlying pathology of the gastrointestinal tract have been largely understudied in preclinical research. In this study, we evaluated potential sex differences in spontaneous nociceptive responses, referred abdominal hypersensitivity, disease progression, and bowel pathology in mouse models of acute and persistent colon inflammation. Our experiments show that females exhibit more nociceptive responses and referred abdominal hypersensitivity than males in the context of acute but not persistent colon inflammation. We further demonstrate that, after acute and persistent colon inflammation, pain-related behavioral responses in females and males are distinct, with increases in licking of the abdomen only observed in females and increases in abdominal contractions only seen in males. During persistent colon inflammation, males exhibit worse disease progression than females, which is manifested as worse physical appearance and higher weight loss. However, no measurable sex differences were observed in persistent inflammation-induced bowel pathology, stool consistency, or fecal blood. Overall, our findings demonstrate sex differences in pain-related behaviors and disease progression in the context of acute and persistent colon inflammation, highlighting the importance of considering sex as a biological variable in future mechanistic studies of visceral pain as well as in the development of diagnostics and therapeutic options for chronic gastrointestinal diseases.

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.

Acute gut inflammation reduces neural activity and spine maturity in hippocampus but not basolateral amygdala

Gastrointestinal tract (gut) inflammation increases stress and threat-coping behaviors, which are associated with altered activity in fear-related neural circuits, such as the basolateral amygdala and hippocampus. It remains to be determined whether inflammation from the gut affects neural activity by altering dendritic spines. We hypothesized that acute inflammation alters dendritic spines in a brain region-specific manner. Here we show that acute gut inflammation (colitis) evoked by dextran sodium sulfate (DSS) did not affect the overall spine density in the CA1 region of hippocampus, but increased the relative proportion of immature spines to mature spines on basal dendrites of pyramidal neurons. In contrast, in animals with colitis, no changes in spine density or composition on dendrites of pyramidal cells was observed in the basolateral amygdala. Rather, we observed decreased spine density on dendrites of stellate neurons, but not the relative proportions of mature vs immature spines. We used cFos expression evoked by the forced swim task as a measure of neural activity during stress and found no effect of DSS on the density of cFos immunoreactive neurons in basolateral amygdala. In contrast, fewer CA1 neurons expressed cFos in mice with colitis, relative to controls. Furthermore, CA1 cFos expression negatively correlated with active stress-coping in the swim task and was negatively correlated with gut inflammation. These data reveal that the effects of acute gut inflammation on synaptic remodeling depend on brain region, neuronal phenotype, and dendrite location. In the hippocampus, a shift to immature spines and hypoactivity are more strongly related to colitis-evoked behavioral changes than is remodeling in basolateral amygdala.

Genetic Variants of Fatty Acid Amide Hydrolase Modulate Acute Inflammatory Responses to Colitis in Adult Male Mice

Cannabinoids, including cannabis derived phytocannabinoids and endogenous cannabinoids (endocannabinoids), are typically considered anti-inflammatory. One such endocannabinoid is N-arachidonoylethanolamine (anandamide, AEA), which is metabolized by fatty acid amide hydrolase (FAAH). In humans, there is a loss of function single nucleotide polymorphism (SNP) in the FAAH gene (C385A, rs324420), that leads to increases in the levels of AEA. Using a mouse model with this SNP, we investigated how this SNP affects inflammation in a model of inflammatory bowel disease. We administered 2,4,6-trinitrobenzene sulfonic acid (TNBS) intracolonically, to adult male FAAH SNP mice and examined colonic macroscopic tissue damage and myeloperoxidase activity, as well as levels of plasma and amygdalar cytokines and chemokines 3 days after administration, at the peak of colitis. We found that mice possessing the loss of function alleles (AC and AA), displayed no differences in colonic damage or myeloperoxidase activity compared to mice with wild type alleles (CC). In contrast, in plasma, colitis-induced increases in interleukin (IL)-2, leukemia inhibitory factor (LIF), monocyte chemoattractant protein (MCP)-1, and tumor necrosis factor (TNF) were reduced in animals with an A allele. A similar pattern was observed in the amygdala for granulocyte colony stimulating factor (G-CSF) and MCP-1. In the amygdala, the mutant A allele led to lower levels of IL-1α, IL-9, macrophage inflammatory protein (MIP)-1β, and MIP-2 independent of colitis-providing additional understanding of how FAAH may serve as a regulator of inflammatory responses in the brain. Together, these data provide insights into how FAAH regulates inflammatory processes in disease.

The Alopecia Areata Phenotype Is Induced by the Water Avoidance Stress Test In cchcr1-Deficient Mice

We recently discovered a nonsynonymous variant in the coiled-coil alpha-helical rod protein 1 (CCHCR1) gene within the alopecia areata (AA) risk haplotype. We also reported that the engineered mice with this risk allele exhibited. To investigate more about the involvement of the CCHCR1 gene in AA pathogenesis, we developed an AA model using C57BL/6N cchcr1 gene knockout mice. In this study, mice (6–8 weeks) were divided into two groups: cchcr1^−/− mice and wild-type (WT) littermates. Both groups were subjected to a water avoidance stress (WAS) test. Eight weeks after the WAS test, 25% of cchcr1^−/− mice exhibited non-inflammatory foci of alopecia on the dorsal skin. On the other hand, none of wild-type littermates cause hair loss. The foci resembled human AA in terms of gross morphology, trichoscopic findings and histological findings. Additionally, gene expression microarray analysis of cchcr1^−/− mice revealed abnormalities of hair related genes compared to the control. Our results strongly suggest that CCHCR1 is associated with AA pathogenesis and that cchcr1^−/− mice are a good model for investigating AA.

Spontaneous, Voluntary, and Affective Behaviours in Rat Models of Pathological Pain

In pain patients affective and motivational reactions as well as impairment of daily life activities dominate the clinical picture. In contrast, many rodent pain models have been established on the basis of mechanical hypersensitivity testing. Up to today most rodent studies on pain still rely on reflexive withdrawal responses only. This discrepancy has likely contributed to the low predictive power of preclinical pain models for novel therapies. Here, we used a behavioural test array for rats to behaviourally evaluate five aetiologically distinct pain models consisting of inflammatory-, postsurgical-, cephalic-, neuropathic- and chemotherapy-induced pain. We assessed paralleling clinical expressions and comorbidities of chronic pain with an array of behavioural tests to assess anxiety, social interaction, distress, depression, and voluntary/spontaneous behaviours. Pharmacological treatment of the distinct pain conditions was performed with pathology-specific and clinically efficacious analgesics as gabapentin, sumatriptan, naproxen, and codeine. We found that rats differed in their manifestation of symptoms depending on the pain model and that pathology-specific analgesics also reduced the associated behavioural parameters. Based on all behavioural test performed, we screened for tests that can discriminate experimental groups on the basis of reflexive as well as non-sensory, affective parameters. Together, we propose a set of non-evoked behaviours with a comparable predictive power to mechanical threshold testing for each pain model.

Functional disruption of cortical cingulate activity attenuates visceral hypersensitivity and anxiety induced by acute experimental colitis

Visceral pain is a highly complex experience and is the most common pathological feature in patients suffering from inflammatory gastrointestinal disorders. Whilst it is increasingly recognized that aberrant neural processing within the gut-brain axis plays a key role in development of neurological symptoms, the underlying mechanisms remain largely unknown. Here, we investigated the cortical activation patterns and effects of non-invasive chemogenetic suppression of cortical activity on visceral hypersensitivity and anxiety-related phenotypes in a well-characterized mouse model of acute colitis induced by dextran sulfate sodium (DSS). We found that within the widespread cortical network, the mid-cingulate cortex (MCC) was consistently highly activated in response to innocuous and noxious mechanical stimulation of the colon. Furthermore, during acute experimental colitis, impairing the activity of the MCC successfully alleviated visceral hypersensitivity, anxiety-like behaviors and visceromotor responses to colorectal distensions (CRDs) via downregulating the excitability of the posterior insula (PI), somatosensory and the rostral anterior cingulate cortices (rACC), but not the prefrontal or anterior insula cortices. These results provide a mechanistic insight into the central cortical circuits underlying painful visceral manifestations and implicate MCC plasticity as a putative target in cingulate-mediated therapies for bowel disorders.

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.

Transient receptor potential ankyrin 1 contributes to somatic pain hypersensitivity in experimental colitis

Pain evoked by visceral inflammation is often ‘referred’ to the somatic level. Transient receptor potential ankyrin 1 (TRPA1) has been reported to contribute to visceral pain-like behavior in dextran sulfate sodium (DSS)-evoked colitis. However, the role of TRPA1 in somatic component of hypersensitivity due to visceral inflammation is unknown. The present study investigated the role of TRPA1 in colitis-evoked mechanical hypersensitivity at the somatic level. Colitis was induced in mice by adding DSS to drinking water for one week. Control and DSS-treated mice were tested for various parameters of colitis as well as mechanical pain sensitivity in abdominal and facial regions. DSS treatment caused mechanical hypersensitivity in the abdominal and facial skin. Pharmacological blockade or genetic deletion of TRPA1 prevented the colitis-associated mechanical hypersensitivity in the abdominal and facial skin areas although the severity of colitis remained unaltered. DSS treatment increased expression of TRPA1 mRNA in cultured dorsal root ganglion (DRG) neurons, but not trigeminal ganglion neurons, and selectively enhanced currents evoked by the TRPA1 agonist, allyl isothiocyanate, in cultured DRG neurons. Our findings indicate that the TRPA1 channel contributes to colitis-associated mechanical hypersensitivity in somatic tissues, an effect associated with upregulation of TRPA1 expression and responsiveness in DRG nociceptors.

Colitis-Induced Microbial Perturbation Promotes Postinflammatory Visceral Hypersensitivity

BACKGROUND & AIMS

Despite achieving endoscopic remission, more than 20% of inflammatory bowel disease patients experience chronic abdominal pain. These patients have increased rectal transient receptor potential vanilloid-1 receptor (TRPV1) expression, a key transducer of inflammatory pain. Because inflammatory bowel disease patients in remission exhibit dysbiosis and microbial manipulation alters TRPV1 function, our goal was to examine whether microbial perturbation modulated transient receptor potential function in a mouse model.

METHODS

Mice were given dextran sodium sulfate (DSS) to induce colitis and were allowed to recover. The microbiome was perturbed by using antibiotics as well as fecal microbial transplant (FMT). Visceral and somatic sensitivity were assessed by recording visceromotor responses to colorectal distention and using hot plate/automated Von Frey tests, respectively. Calcium imaging of isolated dorsal root ganglia neurons was used as an in vitro correlate of nociception. The microbiome composition was evaluated via 16S rRNA gene variable region V4 amplicon sequencing, whereas fecal short-chain fatty acids (SCFAs) were assessed by using targeted mass spectrometry.

RESULTS

Postinflammatory DSS mice developed visceral and somatic hyperalgesia. Antibiotic administration during DSS recovery induced visceral, but not somatic, hyperalgesia independent of inflammation. FMT of postinflammatory DSS stool into antibiotic-treated mice increased visceral hypersensitivity, whereas FMT of control stool reversed antibiotics' sensitizing effects. Postinflammatory mice exhibited both increased SCFA-producing species and fecal acetate/butyrate content compared with controls. Capsaicin-evoked calcium responses were increased in naive dorsal root ganglion neurons incubated with both sodium butyrate/propionate alone and with colonic supernatants derived from postinflammatory mice.

CONCLUSIONS

The microbiome plays a central role in postinflammatory visceral hypersensitivity. Microbial-derived SCFAs can sensitize nociceptive neurons and may contribute to the pathogenesis of postinflammatory visceral pain.

Effect of N-palmitoylethanolamine-oxazoline on comorbid neuropsychiatric disturbance associated with inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic disorder characterized by inflammation of the gastrointestinal (GI) tract, and it is associated with different neurological disorders. Recent evidence has demonstrated that the gut-brain-axis has a central function in the perpetuation of IBS, and for this reason, it can be considered a possible therapeutic target. N-Palmitoylethanolamine-oxazoline (PEA-OXA) possesses anti-inflammatory and potent neuroprotective effects. Although recent studies have explained the neuroprotective properties of PEA-OXA, nothing is known about its effects on the gut-brain axis during colitis. The aim of this study is to explore the mechanism and the effect of PEA-OXA on the gut-brain axis in rats subjected to experimental colitis induced by oral administration of dextran sulfate sodium (DSS). Daily oral administration of PEA-OXA (10 mg/kg daily o.s.) was able to decrease the body weight loss, macroscopic damage, colon length, histological alteration, and inflammation after DSS induction. Additionally, PEA-OXA administration enhanced neurotrophic growth factor release and decreased the astroglial and microglial activation induced by DSS. Moreover, PEA-OXA restored intestinal permeability and tight junctions (TJs) as well as reduced apoptosis in the colon and brain. In our work, we demonstrated, for the first time, the action of PEA-OXA on the gut-brain axis in a model of DSS-induced colitis and its implication on the "secondary" effects associated with colonic disturbance.

Behavioral adaptations in a relapsing mouse model of colitis

Inflammatory bowel disease (IBD) is characterized by relapsing periods of gut inflammation, and is comorbid with depression, anxiety, and cognitive deficits. Animal models of IBD that explore the behavioral consequences almost exclusively use acute models of gut inflammation, which fails to recapitulate the cyclic, chronic nature of IBD. This study sought to identify behavioral differences in digging, memory, and stress-coping strategies in mice exposed to one (acute) or three (chronic) cycles of gut inflammation, using the dextran sodium sulfate (DSS) model of colitis. Similar levels of gut pathology were observed between acute and chronically exposed mice, although mice in the chronic treatment had significantly shorter colons, suggesting more severe disease. Behavioral measures revealed an unexpected pattern in which chronic treatment evoked fewer deficits than acute treatment. Specifically, acutely-treated mice showed alterations in measures of object burying, novel object recognition, object location memory, and stress-coping (forced swim task). Chronically-treated animals, however, showed similar alterations in object burying, but not the other measures. These data suggest an adaptive or tolerizing effect of repeated cycles of peripheral gut inflammation on mnemonic function and stress-coping, whereas some other behaviors continue to be affected by gut inflammation. We speculate that the normalization of some functions may involve the reversion to the baseline state of the hypothalamic-pituitary-adrenal axis and/or levels of neuroinflammation, which are both activated by the first exposure to the colitic agent.

Moxibustion Eases Chronic Inflammatory Visceral Pain In Rats Via MAPK Signaling Pathway In The Spinal Cord

Purpose

The purpose of this study was to explore the central analgesia mechanism of moxibustion for chronic inflammatory visceral pain (CIVP).

Methods

A CIVP rat model was established by 2,4,6-trinitrobenzene sulfonic acid (TNBS) plus 50% ethanol via enema. The analgesic effect of moxibustion was evaluated using the abdominal withdrawal reflex (AWR), mechanical withdrawal threshold (MWT), and thermal withdrawal latency (TWL). The expression profile of phosphorylated proteins of the mitogen-activated protein kinase (MAPK) signaling pathway in the spinal cord was assayed by protein microarray. The differentially expressed proteins were examined by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for functional clusters and corresponding signaling pathways.

Results

Moxibustion exerted a significant analgesic effect for CIVP rats, mainly presenting as a decrease in the AWR score (all P<0.01) under different levels of distending pressure and an increase in MWT and TWL thresholds (all P<0.05). Compared with the normal group, 76 proteins were upregulated while 15 were downregulated, and MAPK signaling pathway was activated in the model group. Compared with the model group, there were 53 downregulated and 38 upregulated proteins in the moxibustion group, and MAPK signaling pathway was inhibited. Fold change (FC)>1.3 or <0.77 was taken as the screening standard to define the differentially expressed proteins. Fifteen differentially expressed proteins upregulated in the model group were downregulated in the moxibustion group. GO analysis showed that the differentially expressed proteins mainly controlled cellular metabolism regulation, transportation, and stress reactions. KEGG analysis revealed that these differentially expressed proteins were mostly involved in the ERK, JNK, and p38 pathways, and the ERK pathway was predominant.

Conclusion

Moxibustion mitigates CIVP in rats and inhibits the phosphorylation of proteins in the spinal MAPK signaling pathway. The analgesic effect of moxibustion may be associated with the regulation of the spinal MAPK signaling pathway.

Differential responses of neurons in the rat caudal ventrolateral medulla to visceral and somatic noxious stimuli and their alterations in colitis

Visceral and somatic types of pain have been reported to manifest crucial differences not only in the experience, but also in their peripheral and central processing. However, the precise neuronal mechanisms that responsible for the modality-specific transmission of pain signals, especially at the supraspinal level, remain unclear. Very little is known also about the potential involvement of such mechanisms in the development of viscero-somatic hyperalgesia. Therefore, in the present study performed on urethane-anesthetized adult male Wistar rats we examined responses of neurons in the caudal ventrolateral medulla (CVLM)-the first site for supraspinal processing of both internal and external pain signals-to visceral (colorectal distension, CRD) and somatic (squeezing of the tail) noxious stimulations and evaluated alterations in response properties of these cells after the induction of colitis. It has been found out that the CVLM of healthy control rats, along with harboring of cells excited by both stimulations (23.7%), contained neurons that were activated by either visceral (31.9%) or somatic noxious stimuli (44.4%). In inflamed animals, the percentages of the visceral and somatic nociceptive cells were decreased (to 18.3% and 34.3%, correspondingly) and the number of bimodal neurons was increased (up to 47.4%); these alterations were associated with substantially enhanced responses of both the modality-specific and convergent CVLM neurons not only to CRD, but also to squeezing of the tail. Under these conditions, visceral and somatic pain stimuli induced similar changes in arterial blood pressure and respiratory rate, whereas in the absence of intestinal inflammation noxious CRD and tail stimulation evoked predominantly divergent autonomic reactions. The data obtained can benefit to a deeper understanding of the neuronal mechanisms that underlie differential supraspinal processing of visceral and somatic noxious stimuli and can potentially contribute to the realization of specific cardiovascular and respiratory accompaniments inherent to a particular type of pain. Therewith, results of the study elucidate colitis-induced alterations in these mechanisms, which may be responsible for the combined development of visceral hypersensitivity and somatic hyperalgesia.

Chronic colitis-induced visceral pain is associated with increased anxiety during quiescent phase

Inflammatory bowel diseases (IBD) are characterized by repetition of flares and remission periods leading to chronic postinflammatory sequelae. Among postinflammatory sequelae, one-third of patients with IBD are suffering from functional symptoms or psychological comorbidities that persist during remission. The aim of our study was to assess functional and behavioral sequelae of chronic colitis in rats with quiescent intestinal inflammation. Chronic colitis was induced by a weekly intrarectal injection of increasing concentrations of trinitrobenzene sulfonic acid (TNBS) for 3 wk (15-45 mg of TNBS) in 30 rats, whereas the control rats (n = 24) received the vehicle. At 50 days post-TNBS, visceral sensitivity was assessed by visceromotor response to colorectal distension, and transient receptor potential vanilloid type 1 (TRPV1) expression was also quantified in the colon and dorsal root ganglia. Barrier function and inflammatory response were assessed by studying intestinal permeability, tight junction protein, myeloperoxidase activity, histological score, and cytokine production (IL-6, IL-10, and TNF-α). Anxiety behavioral tests were performed from 50 to 64 days after the last TNBS injection. Chronic TNBS induced 1) a visceral hypersensitivity (P = 0.03), 2) an increased colon weight-to-length ratio (P = 0.01), 3) higher inflammatory and fibrosis scores (P = 0.0390 and P = 0.0016, respectively), and 4) a higher colonic IL-6 and IL-10 production (P = 0.008 and P = 0.005, respectively) compared with control rats. Intestinal permeability, colonic production of TNF-α, myeloperoxidase activity, and TRPV1 expression did not differ among groups. Chronic TNBS increased anxiety-related behavior in the open-field test and in the acoustic stress test. In conclusion, chronic colitis induced functional sequelae such as visceral hypersensitivity and increased anxiety with a low-grade intestinal inflammation. Development of a representative animal model will allow defining novel therapeutic approaches to achieve a better management of IBD-related sequelae. NEW & NOTEWORTHY Patients with inflammatory bowel diseases have impaired quality of life. Therapeutic progress to control mucosal inflammation provides us an opportunity to develop novel approaches to understand mechanisms behind postinflammatory sequelae. We used a chronic colitis model to study long-term sequelae on visceral pain, gut barrier function, and psychological impact. Chronic colitis induced functional symptoms and increased anxiety in the remission period. It might define novel therapeutic approaches to achieve a better inflammatory bowel disease-related sequelae management.

Fluid supplementation accelerates epithelial repair during chemical colitis

The dextran sulfate sodium (DSS) model of colitis is a common animal model of inflammatory bowel disease that causes pain and distress. In this study, we aimed to determine whether fluid supplementation can be used as a welfare-based intervention to minimize animal suffering. C57Bl/6 females undergoing acute colitis by administration of 3% DSS in drinking water were supplemented with 1 mL intraperitoneal injections of NaCl and compared to non-supplemented control mice. Mouse behavior and locomotive activity were assessed on days 5–6 after DSS initiation by means of tail suspension, novel object recognition and open field activity tests. Mice were euthanized after either the acute (day 7) or the recovery phase (day 12) of colitis and inflammation, epithelial proliferation, and differentiation were assessed by means of histology, immunohistochemistry, quantitative PCR, and western blot. We found that fluid-supplemented mice had reduced signs of colitis with no alterations in behavior or locomotive activity. Furthermore, we observed an accelerated epithelial repair response after fluid hydration during the acute phase of colitis, characterized by increased crypt proliferation, activation of ERK1/2, and modulation of TGF-β1 expression. Consistent with these findings, fluid-supplemented mice had increased numbers of goblet cells, upregulated expression of differentiation markers for absorptive enterocytes, and reduced inflammation during the recovery phase. Our results show that fluid hydration does not reduce stress in DSS-treated mice but alters colitis evolution by reducing clinical signs and accelerating epithelial repair. These results argue against the routine use of fluid supplementation in DSS-treated mice.

Performance of behavioral assays: the Rat Grimace Scale, burrowing activity and a composite behavior score to identify visceral pain in an acute and chronic colitis model

Supplemental Digital Content is Available in the Text.

The ability of spontaneous behavioral assays to assess chronic pain is limited. In colitis, RGS identified acute and chronic pain, burrowing identified acute pain.

Introduction:

The Rat Grimace Scale (RGS), a facial expression scale, quantifies the affective component of pain in rats. The RGS was developed to identify acute and inflammatory pain, and applicability in acute and chronic visceral pain is unknown. The dextran sulfate sodium (DSS) colitis model is commonly used in rats, but pain is rarely assessed, instead, disease progression is monitored with the Disease Activity Index (DAI; assessing fecal blood, stool consistency, and weight loss).

Objectives:

The aim of this study was to assess whether the RGS and 2 additional behavioral tools (composite behavior score [CBS] and burrowing) could identify pain in an acute and chronic DSS colitis model.

Methods:

Male and female Sprague-Dawley rats were block randomized to (1) acute colitis (4 days DSS in drinking water); (2) chronic colitis (4 days DSS, 7 days water, and 3 days DSS); or (3) control (14 days water). Disease Activity Index, RGS, CBS, and burrowing assessments were performed daily.

Results:

Rat Grimace Scale scores increased as DAI scores increased during both acute and chronic phases. Burrowing only decreased during the acute phase. By contrast, CBS scores did not increase significantly during either colitis phase.

Conclusions:

These data show that the RGS and burrowing did not decrease in a sustained manner during chronic phase visceral pain, and that variables assessed in the DAI are indicative of pain. This suggests that the RGS can be applied to a wider range of pain types and chronicity than originally suggested. These findings increase the application of the RGS as a pain scale and welfare improvement tool.

Tadalafil improves bladder dysfunction and object recognition in rats with pelvic venous congestion

OBJECTIVES

To examine the effects of tadalafil on bladder function and object recognition ability in rats with alterations in urinary frequency and locomotor activity as a result of pelvic venous congestion.

METHODS

A total of 48 female rats were divided into three groups (sham, pelvic venous congestion and pelvic venous congestion/tadalafil groups). In the pelvic venous congestion and pelvic venous congestion/tadalafil groups, the bilateral common iliac veins and uterine veins were ligated under anesthesia. Rats in the pelvic venous congestion/tadalafil group received a diet containing tadalafil, and the other rats were fed a normal diet. After 4 weeks, rats underwent analysis of voiding behavior, locomotor activity, a novel object recognition test, continuous cystometry, measurement of plasma monoamines, and measurement of plasma and urinary nitric oxide metabolites. Expression of nitric oxide synthase messenger ribonucleic acid in the bladder wall was also assessed, along with histological examination of the bladder.

RESULTS

Rats with pelvic venous congestion showed a higher urinary frequency, lower locomotor activity, and lower plasma and urinary nitric oxide levels than sham rats. The bladder wall endothelial nitric oxide synthase messenger ribonucleic acid level was low and object recognition was impaired. Pelvic venous congestion/tadalafil rats showed improvement in locomotor activity, bladder function and object recognition compared with pelvic venous congestion rats, as well as elevation of plasma and urinary nitric oxide, plasma monoamines, and bladder neuronal nitric oxide synthase messenger ribonucleic acid expression. Bladder wall vascularity was greater in pelvic venous congestion/tadalafil rats compared with sham rats.

CONCLUSIONS

In rats with pelvic venous congestion, tadalafil might improve bladder function and the general condition by increasing blood flow to the bladder and brain, and by increasing dopamine levels.

Investigating Gut Permeability in Animal Models of Disease

A growing number of investigations report the association between gut permeability and intestinal or extra-intestinal disorders under the basis that translocation of gut luminal contents could affect tissue function, either directly or indirectly. Still, in many cases it is unknown whether disruption of the gut barrier is a causative agent or a consequence of these conditions. Adequate experimental models are therefore required to further understand the pathophysiology of health disorders associated to gut barrier disruption and to develop and test pharmacological treatments. Here, we review the current animal models that display enhanced intestinal permeability, and discuss (1) their suitability to address mechanistic questions, such as the association between gut barrier alterations and disease and (2) their validity to test potential treatments for pathologies that are characterized by enhanced intestinal permeability.

Saxagliptin but Not Sitagliptin Inhibits CaMKII and PKC via DPP9 Inhibition in Cardiomyocytes

Some oral anti-hyperglycemic drugs, including gliptins that inhibit dipeptidyl peptidase 4 (DPP4), have been linked to the increased risk of heart failure (HF) in type-2 diabetic patients. While the cardiovascular safety trial, TECOS, revealed no link between sitagliptin and the risk of HF, a substantial 27% increase in the hospitalization for HF was observed in type-2 diabetic patients treated with saxagliptin within the SAVOR-TIMI 53 trial. A previous in vitro study revealed that saxagliptin impairs the Ca2+/calmodulin-dependent protein kinase II (CaMKII)-phospholamban (PLB)-sarcoplasmic reticulum Ca2+-ATPase 2a axis and protein kinase C (PKC) activity in cardiomyocytes leading to impaired cardiac contractility and electrophysiological function. However, the link between saxagliptin and its target proteins (CaMKII and PKC) remains to be explored. Since DPP8 and DPP9 (but not DPP4) are expressed by cardiomyocytes and saxagliptin is internalized by cardiomyocytes, we investigated whether DPP8/9 contribute to saxagliptin-mediated inhibition of CaMKII and PKC activity. Structural analysis revealed that the DPP4-saxagliptin interaction motif (S630, Y547) for the cyanopyrrolidine group is conserved in DPP8 (S755, Y669) and DPP9 (S730, Y644). Conversely, F357 that facilitates binding of the anchor lock domain of sitagliptin in the S2 extensive subsite of DPP4 is not conserved in DPP8/9. In parallel, unlike saxagliptin, sitagliptin did not affect phosphorylation of CaMKII/PLB or activity of PKC in HL-1 cardiomyocytes. These findings were recapitulated by pharmacological inhibition (TC-E-5007, a DPP8/9 antagonist) and knock-down of DPP9 (but not DPP8). In primary mouse ventricular cardiomyocytes, saxagliptin (but not sitagliptin) impaired Ca2+ transient relaxation and prolonged action potential duration (APD). These results suggest that saxagliptin-DPP9 interaction impairs the CaMKII-PLB and PKC signaling in cardiomyocytes. We reveal a novel and potential role of DPP9 in cardiac signaling. The interaction of saxagliptin with DPP9 may represent an underlying mechanism for the link between saxagliptin and HF. Elucidation of saxagliptin-DPP9 interaction and downstream events may foster a better understanding of the role of gliptins as modulators of cardiac signaling.

Running in the wheel: Defining individual severity levels in mice

The fine-scale grading of the severity experienced by animals used in research constitutes a key element of the 3Rs (replace, reduce, and refine) principles and a legal requirement in the European Union Directive 2010/63/EU. Particularly, the exact assessment of all signs of pain, suffering, and distress experienced by laboratory animals represents a prerequisite to develop refinement strategies. However, minimal and noninvasive methods for an evidence-based severity assessment are scarce. Therefore, we investigated whether voluntary wheel running (VWR) provides an observer-independent behaviour-centred approach to grade severity experienced by C57BL/6J mice undergoing various treatments. In a mouse model of chemically induced acute colitis, VWR behaviour was directly related to colitis severity, whereas clinical scoring did not sensitively reflect severity but rather indicated marginal signs of compromised welfare. Unsupervised k-means algorithm–based cluster analysis of body weight and VWR data enabled the discrimination of cluster borders and distinct levels of severity. The validity of the cluster analysis was affirmed in a mouse model of acute restraint stress. This method was also applicable to uncover and grade the impact of serial blood sampling on the animal’s welfare, underlined by increased histological scores in the colitis model. To reflect the entirety of severity in a multidimensional model, the presented approach may have to be calibrated and validated in other animal models requiring the integration of further parameters. In this experimental set up, however, the automated assessment of an emotional/motivational driven behaviour and subsequent integration of the data into a mathematical model enabled unbiased individual severity grading in laboratory mice, thereby providing an essential contribution to the 3Rs principles.

Animal-based biomedical research is often accompanied by experience of discomfort or pain by the animal. Recognition of disturbed animal welfare is mandatory, and the classification and assessment of its severity is a crucial part of the legislative framework in the European Union (EU). In the present study, we analysed voluntary wheel running (VWR) behaviour as a measure of compromised welfare in a mouse colitis model. Unsupervised mathematical clustering of clinical and VWR data enabled us to allocate and classify severity levels. This cluster model was verified using VWR data from a restraint stress model and allowed us to uncover the impact of routine experimental procedures on these mice. We propose that clustering of VWR behaviour provides a useful method for assessing the severity level of experimental procedures conducted on mice.

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.

Altered Brain Excitability and Increased Anxiety in Mice With Experimental Colitis: Consideration of Hyperalgesia and Sex Differences

Crohn’s disease (CD) and ulcerative colitis (UC) are incurable lifelong inflammatory bowel diseases (IBD) with a rising worldwide incidence. IBD is characterized by diarrhea, rectal bleeding, severe cramping and weight loss. However, there is a growing evidence that IBD is also associated with anxiety- and depression-related disorders, which further increase the societal burden of these diseases. Given the limited knowledge of central nervous system (CNS) changes in IBD, we investigated CNS-related comorbidities in a mouse model of experimental colitis induced by dextran sulfate sodium (DSS) administration in drinking water for 5 days. In male and female C57BL6J mice, DSS treatment caused increased brain excitability, revealed by a decrease in seizure onset times after intraperitoneal administration of kainic acid. Moreover, both sexes showed increased anxiety-related behavior in the elevated plus-maze (EPM) and open field (OF) paradigms. We assessed somatic pain levels, because they may influence behavioral responses. Only male mice were hyperalgesic when tested with calibrated von Frey hairs and on the hotplate for mechanical and thermal pain sensitivity respectively. Administration of diazepam (DZP; ip, 1 mg/kg) 30 min before EPM rescued the anxious phenotype and improved locomotion, even though it significantly increased thermal sensitivity in both sexes. This indicates that the altered behavioral response is unlikely attributable to an interference with movement due to somatic pain in females. We show that experimental colitis increases CNS excitability in response to administration of kainic acid, and increases anxiety-related behavior as revealed using the EPM and OF tests.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Visceral Inflammation and Immune Activation Stress the Brain

Stress refers to a dynamic process in which the homeostasis of an organism is challenged, the outcome depending on the type, severity, and duration of stressors involved, the stress responses triggered, and the stress resilience of the organism. Importantly, the relationship between stress and the immune system is bidirectional, as not only stressors have an impact on immune function, but alterations in immune function themselves can elicit stress responses. Such bidirectional interactions have been prominently identified to occur in the gastrointestinal tract in which there is a close cross-talk between the gut microbiota and the local immune system, governed by the permeability of the intestinal mucosa. External stressors disturb the homeostasis between microbiota and gut, these disturbances being signaled to the brain via multiple communication pathways constituting the gut-brain axis, ultimately eliciting stress responses and perturbations of brain function. In view of these relationships, the present article sets out to highlight some of the interactions between peripheral immune activation, especially in the visceral system, and brain function, behavior, and stress coping. These issues are exemplified by the way through which the intestinal microbiota as well as microbe-associated molecular patterns including lipopolysaccharide communicate with the immune system and brain, and the mechanisms whereby overt inflammation in the GI tract impacts on emotional-affective behavior, pain sensitivity, and stress coping. The interactions between the peripheral immune system and the brain take place along the gut-brain axis, the major communication pathways of which comprise microbial metabolites, gut hormones, immune mediators, and sensory neurons. Through these signaling systems, several transmitter and neuropeptide systems within the brain are altered under conditions of peripheral immune stress, enabling adaptive processes related to stress coping and resilience to take place. These aspects of the impact of immune stress on molecular and behavioral processes in the brain have a bearing on several disturbances of mental health and highlight novel opportunities of therapeutic intervention.

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.

Visceral hyperalgesia caused by peptide YY deletion and Y2 receptor antagonism

Altered levels of colonic peptide YY (PYY) have been reported in patients suffering from functional and inflammatory bowel disorders. While the involvement of neuropeptide Y (NPY) and Y receptors in the regulation of nociception is well established, the physiological role of PYY in somatic and visceral pain is poorly understood. In this work, the role of PYY in pain sensitivity was evaluated using PYY knockout (PYY^(−/−)) mice and Y2 receptor ligands. PYY^(−/−) mice were more sensitive to somatic thermal pain compared to wild type (WT) mice. Visceral pain was assessed by evaluating pain-related behaviors, mouse grimace scale (MGS) and referred hyperalgesia after intrarectal administration of allyl isothiocyanate (AITC, 1 or 2%) or its vehicle, peanut oil. The pain-related behaviors induced by AITC were significantly exaggerated by PYY deletion, whereas the MGS readout and the referred hyperalgesia were not significantly affected. The Y2 receptor antagonist, BII0246, increased pain-related behaviors in response to intrarectal AITC compared to vehicle treatment while the Y2 receptor agonist, PYY(3–36), did not have a significant effect. These results indicate that endogenous PYY has a hypoalgesic effect on somatic thermal and visceral chemical pain. The effect on visceral pain seems to be mediated by peripheral Y2 receptors.

Mu and delta opioid receptor knockout mice show increased colonic sensitivity

BACKGROUND

Opiates act through opioid receptors to diminish pain. Here, we investigated whether mu (MOR) and delta (DOR) receptor endogenous activity assessed in the whole mouse body or in particular at peripheral receptors on primary nociceptive neurons, control colonic pain.

METHODS

We compared global MOR and DOR receptor knockout (KO) mice, mice with a conditional deletion of MOR and DOR in Nav1.8-positive nociceptive primary afferent neurons (cKO), and control floxed mice of both genders for visceral sensitivity. Visceromotor responses to colorectal distension (CRD) and macroscopic colon scores were recorded on naïve mice and mice with acute colitis induced by 3% dextran sodium sulphate (DSS) for 5 days. Transcript expression for opioid genes and cytokines was measured by quantitative RT-PCR.

RESULTS

Naïve MOR and DOR global KO mice show increased visceral sensitivity that was not observed in cKO mice. MOR and preproenkephalin (Penk) were the most expressed opioid genes in colon. MOR KO mice had augmented kappa opioid receptor and Tumour-Necrosis-Factor-α and diminished Penk transcript levels while DOR, preprodynorphin and Interleukin-1β were unchanged. Global MOR KO females had a thicker colon than floxed females. No alteration was detected in DOR mutant animals. A 5-day DSS treatment led to comparable hypersensitivity in the different mouse lines.

CONCLUSION

Our results suggest that mu and delta opioid receptor global endogenous activity but not activity at the peripheral Nav1.8 neurons contribute to visceral sensitivity in naïve mice, and that endogenous MOR and DOR tones were insufficient to elicit analgesia after 5-day DSS-induced colitis.

SIGNIFICANCE

Knockout mice for mu and delta opioid receptor have augmented colon sensitivity in the CRD assay. It shows endogenous mu and delta opioid analgesia that may be explored as potential targets for alleviating chronic intestinal pain.

Comparative effects of α2δ-1 ligands in mouse models of colonic hypersensitivity

AIM: To investigate anti-hypersensitive effects of α₂δ-1 ligands in non-inflammatory and inflammation-associated colonic hypersensitivity (CHS) mouse models.

METHODS: To induce an inflammation-associated CHS, 1% dextran sulfate sodium (DSS) was administered to C57Bl/6J male mice, in drinking water, for 14 d. Regarding the non-inflammatory neonatal maternal separation (NMS) -induced CHS model, wild-type C57BI/6J pups were isolated from their mother from day 2 to day 14 (P2 to P14), three hours per day (from 9:00 a.m. to 12:00 p.m.). Colorectal distension was performed by inflating distension probe from 20 μL to 100 μL by 20 μL increment step every 10 s. After a first colorectal distension (CRD), drugs were administered subcutaneously, in a cumulative manner, (Gabapentin at 30 mg/kg and 100 mg/kg; Pregabalin at 10 mg/kg and 30 mg/kg; Carbamazepine at 10 mg/kg and 30 mg/kg) and a second CRD was performed one hour after each injection.

RESULTS: The visceromotor response (VMR) to CRD was increased by our NMS paradigm protocol in comparison to non-handled (NH) mice, considering the highest distension volumes (80 μL: 0.783 ± 0.056 mV/s vs 0.531 ± 0.034 mV/s, P < 0.05 and 100 μL: 1.087 ± 0.056 mV/s vs 0.634 ± 0.038 mV/s, P < 0.05 for NMS and NH mice, respectively). In the inflammation-associated CHS, DSS-treated mice showed a dramatic and significant increase in VMR at 60 and 80 μL distension volumes when compared to control mice (60 μL: 0.920 ± 0.079 mV/s vs 0.426 ± 0.100 mV/s P < 0.05 and 80 μL: 1.193 ± 0.097 mV/s vs 0.681 ± 0.094 mV/s P < 0.05 for DSS- and Water-treated mice, respectively). Carbamazepine failed to significantly reduce CHS in both models. Gabapentin significantly reduced CHS in the DSS-induced model for both subcutaneous injections at 30 or 100 mg/kg. Pregabalin significantly reduced VMR to CRD in the non-inflammatory NMS-induced CHS model for the acute subcutaneous administration of the highest cumulative dose (30 mg/kg) and significantly reduced CHS in low-dose DSS-treated mice in a dose-dependent manner. Finally, the percent decrease of AUC induced by acute GBP or Pregabalin treatment were higher in the inflammatory DSS-induced CHS model in comparison to the non-inflammatory NMS-induced CHS model.

CONCLUSION: This preclinical study demonstrates α₂δ-1 ligands efficacy on inflammation-associated CHS, highlighting their potential clinical interest in patients with chronic abdominal pain and moderate intestinal inflammation.