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

Opposing effects of 5-HT1A receptor agonist buspirone on supraspinal abdominal pain transmission in normal and visceral hypersensitive rats

The serotonergic 5-HT1A receptors are implicated in the central mechanisms of visceral pain, but their role in these processes is controversial. Considering existing evidences for organic inflammation-triggered neuroplastic changes in the brain serotonergic circuitry, the ambiguous contribution of 5-HT1A receptors to supraspinal control of visceral pain in normal and post-inflammatory conditions can be assumed. In this study performed on male Wistar rats, we used microelectrode recording of the caudal ventrolateral medulla (CVLM) neuron responses to colorectal distension (CRD) and electromyography recording of CRD-evoked visceromotor reactions (VMRs) to evaluate post-colitis changes in the effects of 5-HT1A agonist buspirone on supraspinal visceral nociceptive transmission. In rats recovered from trinitrobenzene sulfonic acid colitis, the CRD-induced CVLM neuronal excitation and VMRs were increased compared with those in healthy animals, revealing post-inflammatory intestinal hypersensitivity. Intravenous buspirone (2 and 4 mg/kg) under urethane anesthesia dose-dependently suppressed CVLM excitatory neuron responses to noxious CRD in healthy rats, but caused dose-independent increase in the already enhanced nociceptive activation of CVLM neurons in post-colitis animals, losing also its normally occurring faciliatory effect on CRD-evoked inhibitory medullary neurotransmission and suppressive action on hemodynamic reactions to CRD. In line with this, subcutaneous injection of buspirone (2 mg/kg) in conscious rats, which attenuated CRD-induced VMRs in controls, further increased VMRs in hypersensitive animals. The data obtained indicate a shift from anti- to pronociceptive contribution of 5-HT1A-dependent mechanisms to supraspinal transmission of visceral nociception in intestinal hypersensitivity conditions, arguing for the disutility of buspirone and possibly other 5-HT1A agonists for relieving post-inflammatory abdominal pain.

Somatosensory assessments in patients with inflammatory bowel disease: a cross-sectional study examining pain processing pathways and the role of multiple patient factors

BACKGROUND

Pain affects the majority of patients with inflammatory bowel disease (IBD), where pain experiences may be influenced by multiple patient factors and changes within central pain processing pathways, termed central sensitization. The current study aimed to investigate pain processing pathways in patients with IBD through somatosensory testing and associations with multiple patient factors.

METHODS

A cross-sectional study of adults with IBD. Assessments included: somatosensory tests [i.e. pressure pain thresholds (PPT), temporal summation (TS), conditioned pain modulation (CPM)], and patient factors (i.e. demographics, comorbidity, sleep quality, psychological, pain severity and interference, and IBD features). Multiple regression analyses explored associations between somatosensory tests and multiple patient factors.

RESULTS

Decreased CPM in participants (N = 51) was associated with worse abdominal pain severity and use of biologic therapies (R2 = 0.30, F(5,44) = 5.18, P = 0.001). Increased TS was associated with biologic use (R2 = 0.11, F(1,49) = 6.13, P = 0.017). Decreased PPT at the low back (R2 = 0.29, F(2,48) = 11.21, P < 0.001) and Tibialis anterior (R2 = 0.41, F(2,48) = 18.26, P < 0.001) were associated with female sex and the absence of a stoma.

CONCLUSION

Study results demonstrated associations between multiple patient factors and somatosensory tests in patients with IBD. The absence of a stoma and female sex was associated with greater sensitivity to pressure in two remote body regions, suggestive of widespread hyperalgesia. Worse abdominal pain severity and biologic use were associated with decreased pain inhibition, and biologic use was also associated with increased pain facilitation. These findings suggest the presence of altered pain processing and mechanisms of central sensitization in patients with IBD.

Post-inflammatory Abdominal Pain in Patients with Inflammatory Bowel Disease During Remission: A Comprehensive Review

Patients with inflammatory bowel disease often experience ongoing pain even after achieving mucosal healing (i.e., post-inflammatory pain). Factors related to the brain-gut axis, such as peripheral and central sensitization, altered sympatho-vagal balance, hypothalamic-pituitary-adrenal axis activation, and psychosocial factors, play a significant role in the development of post-inflammatory pain. A comprehensive study investigating the interaction between multiple predisposing factors, including clinical psycho-physiological phenotypes, molecular mechanisms, and multi-omics data, is still needed to fully understand the complex mechanism of post-inflammatory pain. Furthermore, current treatment options are limited and new treatments consistent with the underlying pathophysiology are needed to improve clinical outcomes.

Review article: The aetiology of fatigue in inflammatory bowel disease and potential therapeutic management strategies

BACKGROUND

Fatigue is the inability to achieve or maintain an expected work output resulting from central or peripheral mechanisms. The prevalence of inflammatory bowel disease (IBD) fatigue can reach 86% in active disease, persisting in 50%-52% of patients with mild to inactive disease. Fatigue is the commonest reason for work absence in IBD, and patients often report fatigue burden to be greater than that of primary disease symptoms. Relatively few evidence-based treatment options exist, and the aetiology is poorly understood.

AIM

To review the available data and suggest a possible aetiology of IBD fatigue and to consider the efficacy of existing management strategies and highlight potential future interventions.

METHODS

We reviewed fatigue-related literature in IBD using PubMed database.

RESULTS

Disease related factors such as inflammation and pharmacological treatments negatively impact skeletal muscle and brain physiology, likely contributing to fatigue symptoms. Secondary factors such as malnutrition, anaemia, sleep disturbance and psychological comorbidity are potential determinants. Immune profile, faecal microbiota composition and physical fitness differ significantly between fatigued and non-fatigued patients, suggesting these may be aetiological factors. Solution-focused therapy, high-dosage thiamine supplementation and biological therapy may reduce fatigue perception in IBD. The effect of physical activity interventions is inconclusive.

CONCLUSIONS

A multimodal approach is likely required to treat IBD fatigue. Established reversible factors like anaemia, micronutrient deficiencies and active disease should initially be resolved. Psychosocial intervention shows potential efficacy in reducing fatigue perception in quiescent disease. Restoring physical deconditioning by exercise training intervention may further improve fatigue burden.

Criterion validity and discriminatory ability of the central sensitization inventory short form in individuals with inflammatory bowel diseases

OBJECTIVES

Increased symptoms related to central sensitization have previously been reported in inflammatory bowel disease (IBD) patients, identified by the original central sensitization inventory (CSI-25). However, the recently developed CSI short form (CSI-9) may be more clinically useful. The aim of the present study was to evaluate the performance of CSI-9 compared to the original CSI-25 in individuals with IBD. Study objectives were to investigate the criterion validity of the CSI-9 to the CSI-25, assess individual association of the CSI measures with clinical features of IBD and pain presentations, and to establish disease-specific CSI-9 and CSI-25 cut-off scores for discriminating the presence of self-reported pain in individuals with IBD.

METHODS

Cross-sectional online survey was performed on adults with IBD exploring self-reported demographics, comorbidity, and clinical IBD and pain features. Criterion validity of the CSI-9 was investigated using intraclass correlation coefficient (ICC)_3,1. Area under the receiver operating characteristic curve (AUC-ROC) analysis was conducted to investigate the discriminative ability of both versions of CSI.

RESULTS

Of the 320 participants, 260 reported the presence of abdominal and/or musculoskeletal pain. CSI-9 and CSI-25 demonstrated substantial agreement (ICC_3,1=0.64, 95% CI [0.58, 0.69]). AUC (95% CI) indicated that CSI-9 (0.788 (0.725, 0.851), p<0.001) and CSI-25 (0.808 (0.750, 0.867), p<0.001) were able to adequately discriminate the presence of pain using cut-offs scores of ≥17 (CSI-9) and ≥40 (CSI-25). Abdominal pain severity was the only feature to differ in significant association to CSI-25 (p=0.002) compared to CSI-9 (p=0.236). All other features demonstrated significant associations to both CSI versions, except age (p=0.291 and 0.643) and IBD subtype (p=0.115 and 0.675).

CONCLUSIONS

This is the first study to explore and validate the use of CSI-9 in IBD patients. Results demonstrated concurrent validity of the CSI-9 to CSI-25, with similar significant association to multiple patient features, and a suggested cut-off value of 17 on CSI-9 to screen for individuals with pain experiences. Study findings suggest that CSI-9 is suitable to use as a brief tool in IBD patients.

Behavior and electrophysiology studies of the peripheral neuropathy induced by individual and co-administration of paclitaxel and oxaliplatin in rat

AIMS

Antitumor agents, as taxanes and platinum compounds, induce peripheral neuropathies which can hamper their use for cancer treatment. The study of chemotherapy-induced neuropathies in humans is difficult because of ethical reasons, differences among administration protocols and intrinsic characteristics of patients. The aim of the present study is to compare the neuropathic signs induced by individual or combined administration of paclitaxel and oxaliplatin.

MAIN METHODS

Oxaliplatin and paclitaxel were administered individually and combined to induce peripheral neuropathy in rats, sensory neuropathic signs were assessed in the hind limbs and orofacial area. The in vitro skin-saphenous nerve preparation was used to record the axonal activity of Aδ sensory neurons.

KEY FINDINGS

Animals treated with the combination developed mechanical allodynia in the paws and muscular hyperalgesia in the orofacial area, which was similar to that in animals treated with monotherapy, the latter also developed cold allodynia in the paws. Aδ-fibers of the rats treated with the combination were hyperexcited and presented hypersensitivity to pressure stimulation of the innervated skin, also similar to that recorded in the fibers of the animals treated with monotherapy.

SIGNIFICANCE

Our work objectively demonstrates that the combination of a platinum compound with a taxane does not worsen the development of sensorial neuropathies in rats, which is an interesting data to take into account when the combination of antitumor drugs is necessary. Co-administration of antitumor drugs is more effective in cancer treatment without increasing the risk of the disabling neuropathic side effects.

Symptoms of central sensitization in patients with inflammatory bowel diseases: a case-control study examining the role of musculoskeletal pain and psychological factors

OBJECTIVES

Musculoskeletal (MSK) pain is a common complaint in patients with inflammatory bowel diseases (IBD). MSK pain in IBD has previously demonstrated association with symptoms of central sensitization; however it is uncertain whether these symptoms are influenced simply by the presence of MSK pain and/or IBD. Primary aim of this study was to investigate whether symptoms of central sensitization differed across three groups: IBD patients with and without MSK pain and healthy controls. Secondary aim was to investigate between-group differences for measures of somatosensory functioning.

METHODS

Cross-sectional study was performed on adults with IBD. Assessments included: central sensitization inventory (CSI), pressure pain threshold, temporal summation, conditioned pain modulation, perceived stress, affect style, anxiety, depression, and pain catastrophizing. One-way analyses of variance and covariance were used to investigate between-group differences for measures of central sensitization and potential confounding by psychological factors.

RESULTS

Study participants (n=66) were age/gender matched across three study groups. Between-group differences were solely demonstrated for CSI scores [F(2,63)=19.835, p<0.001, r=0.62], with IBD patients with MSK pain demonstrating the highest CSI scores and healthy controls the lowest. After controlling for individual psychological features, post hoc comparisons indicated that CSI scores were significantly different between-groups (p≤0.025) after controlling for most psychological variables, with the exception of perceived stress (p=0.063) and pain catastrophizing (p=0.593).

CONCLUSIONS

IBD patients as a whole demonstrated significantly greater symptoms of central sensitization compared to healthy controls. However, IBD patients with persistent MSK pain demonstrated the greatest symptoms of central sensitization compared to patients without MSK pain and healthy controls. Between-group differences for CSI in IBD patients with MSK were not confounded by psychological features.

IMPLICATIONS

Study results indicate that persistent MSK pain in IBD represents patients with greater central sensitization symptomology. This increased symptomology is suggestive of underlying mechanisms related to central sensitization, highlighting patient potentially at risk for worse pain experiences.

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.

Antidepressants in inflammatory bowel disease

Gut-brain dysregulation has been recognized by the scientific community as being crucial to the understanding of chronic gastrointestinal conditions, and this has translated into the practice of a newly established discipline, psychogastroenterology. Along with psychotherapy, antidepressants (a subtype of central neuromodulators) have been proposed as treatments for gut-brain disorders that might benefit both psychological and gastrointestinal health. Antidepressants have been found to be effective for the treatment of comorbid anxiety and depression, pain and impaired sleep. Although the efficacy of antidepressants is well established in disorders of gut-brain interaction (DGBI), evidence is only now emerging in IBD. This Perspective discusses the use of antidepressants in DGBI and IBD, focusing on how what we have learnt about the role of antidepressants in DGBI could be applied to help optimize the management of IBD.

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.

Central hypersensitivity - A model for persistent musculoskeletal pain in inflammatory bowel diseases

Pain is reported to affect over 70% of individuals with inflammatory bowel diseases (IBD), with abdominal and musculoskeletal (MSK) pain representing the most common complaints. MSK pain is typically considered within the narrow framework of inflammatory extraintestinal manifestations of IBD, resulting in a limited scope for the nature and underlying mechanisms participating in MSK pain experiences in this population. Symptoms related to central sensitization have recently demonstrated association with active IBD and worse MSK pain experiences, suggesting a potential roll for central mechanisms in MSK-related pain. Current literature exploring persistent pain in chronic inflammatory and MSK populations propose complex pain models comprised of dynamic nervous system relationships influenced by primary disease features and concomitant pain states, as well as affective and cognitive components. Nervous system contributions in the development and maintenance of persistent pain are postulated to include mechanisms of peripheral and central sensitization, changes in descending central modulation, as well as structural brain changes. These models go beyond current MSK pain models described in IBD literature, highlighting the need for new frameworks for considering MSK-related pain in IBD. Consequently, this paper proposes a broader theoretical model whereby central mechanisms, such as central sensitization and grey matter changes, as well as psychological and disease factors are suggested to modulate pain experiences in this population. Exploration of relationships within the proposed framework may provide not only a deeper understanding of the generation and maintenance of persistent MSK pain in IBD, but also highlight the need for new targeted management pathways in this population. This paper hypothesizes that exploration of central sensitization in IBD patients will demonstrate altered somatosensory functioning in patients with MSK pain, and that IBD activity and psychological factors will be associated with altered somatosensory functioning and worse pain experiences.

Natural products as a perspective for cancer pain management: A systematic review

BACKGROUND

Cancer is the leading cause of death in the world and one of the main symptoms affecting these individuals is chronic pain, which must be evaluated and treated in its various components. Several drugs are currently used, but beyond the high cost, they have harmful side effects to patients or are transitorily effective. Ergo, there is a need to look for new options for cancer pain relief. Natural products (NPs) present themselves as strong candidates for the development of new drugs for the treatment of chronic pain, such as cancer pain.

PURPOSE

This systematic review aimed to summarize current knowledge about the analgesic profile of NPs in cancer pain.

METHODS

The search included PubMed, Scopus and Web of Science (from inception to June 2018) sought to summarize the articles studying new proposals with NPs for the management of oncological pain. Two independent reviewers extracted data on study characteristics, methods and outcomes.

RESULTS

After an extensive survey, 21 articles were selected, which described the analgesic potential of 15 natural compounds to relieve cancer pain. After analyzing the data, it can be suggested that these NPs, which have targets in central and peripheral mechanisms, are interesting candidates for the treatment of cancer pain for addressing different pharmacological mechanisms (even innovative), but ensuring the safety of these compounds is still a challenge. Likewise, the cannabinoids compounds leave the front as the most promising compounds for direct applicability due to the clinical studies that have already been developed and the background already established about these effects on chronic pain.

CONCLUSION

Regarding these findings, it can be concluded that the variability of possible biological sites of action is strategic for new perspectives in the development of therapeutic proposals different from those available in the current market.

Electroacupuncture Alleviated Referral Hindpaw Hyperalgesia via Suppressing Spinal Long-Term Potentiation (LTP) in TNBS-Induced Colitis Rats

Although referred pain or hypersensitivity has been repeatedly reported in irritable bowel syndrome (IBS) patients and experimental colitis rodents, little is known about the neural mechanisms. Spinal long-term potentiation (LTP) of nociceptive synaptic transmission plays a critical role in the development of somatic hyperalgesia in chronic pain conditions. Herein, we sought to determine whether spinal LTP contributes to the referral hyperalgesia in colitis rats and particularly whether electroacupuncture (EA) is effective to alleviate somatic hyperalgesia via suppressing spinal LTP. Rats in the colitis group (induced by colonic infusion of 2,4,6-trinitrobenzenesulfonic acid, TNBS), instead of the control and vehicle groups, displayed evident focal inflammatory destruction of the distal colon accompanied not only with the sensitized visceromotor response (VMR) to noxious colorectal distension (CRD) but also with referral hindpaw hyperalgesia indicated by reduced mechanical and thermal withdrawal latencies. EA at Zusanli (ST36) and Shangjuxu (ST37) attenuated the severity of colonic inflammation, as well as the visceral hypersensitivity and referral hindpaw hyperalgesia in colitis rats. Intriguingly, the threshold of C-fiber-evoked field potentials (CFEFP) was significantly reduced and the spinal LTP was exaggerated in the colitis group, both of which were restored by EA treatment. Taken together, visceral hypersensitivity and referral hindpaw hyperalgesia coexist in TNBS-induced colitis rats, which might be attributed to the enhanced LTP of nociceptive synaptic transmission in the spinal dorsal horn. EA at ST36 and ST37 could relieve visceral hypersensitivity and, in particular, attenuate referral hindpaw hyperalgesia by suppressing the enhanced spinal LTP.

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

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.

Effects of Carvacrol, Thymol and essential oils containing such monoterpenes on wound healing: a systematic review

Objectives

The treatment of wounds accounts for a considerable fraction of health expenses as well as serious socioeconomic problems. The use of natural substances stands out as a source of new therapeutic discoveries for the wound healing. Thus, this review compiled scientific findings on the applicability of carvacrol and thymol, or essential oils containing at least one of these compounds, for the treatment of wounds.

Methods

This review was performed at PubMed, SCOPUS, Web of Science databases using keywords as wound healing, thymol/carvacrol and essential oils. Thirteen studies were selected for discussion.

Key findings

Thymol/carvacrol was able to act in the three phases of wound healing. In the first phase, they showed modulatory effect of the inflammatory cytokines, oxidative stress and antimicrobial power. In the second phase, they promoted re-epithelialization, angiogenesis and development of granulation tissue. Finally, in the third phase, they improve the collagen deposition and modulated the growth of fibroblasts and keratinocytes.

Conclusions

These compounds present a high potential for the development of new therapeutic for wound repair. However, dose, efficacy and safety of these compounds for the treatment of wounds, as well as the mechanisms by which those effects can be observed, are challenges for future studies.

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.

Potential of Endocannabinoids to Control Bladder Pain

Bladder-related pain is one of the most common forms of visceral pain, and visceral pain is among the most common complaints for which patients seek physician consultation. Despite extensive studies of visceral innervation and treatment of visceral pain, opioids remain a mainstay for management of bladder pain. Side effects associated with opioid therapy can profoundly diminish quality of life, and improved options for treatment of bladder pain remain a high priority. Endocannabinoids, primarily anandamide (AEA) and 2-arachidonoylglycerol (2-AG), are endogenously-produced fatty acid ethanolamides with that induce analgesia. Animal experiments have demonstrated that inhibition of enzymes that degrade AEA or 2-AG have the potential to prevent development of visceral and somatic pain. Although experimental results in animal models have been promising, clinical application of this approach has proven difficult. In addition to fatty acid amide hydrolase (FAAH; degrades AEA) and monacylglycerol lipase (MAGL; degrades 2-AG), cyclooxygenase (COX) acts to metabolize endocannabinoids. Another potential limitation of this strategy is that AEA activates pro-nociceptive transient receptor potential vanilloid 1 (TRPV1) channels. Dual inhibitors of FAAH and TRPV1 or FAAH and COX have been synthesized and are currently undergoing preclinical testing for efficacy in providing analgesia. Local inhibition of FAAH or MAGL within the bladder may be viable options to reduce pain associated with cystitis with fewer systemic side effects, but this has not been explored. Further investigation is required before manipulation of the endocannabinoid system can be proven as an efficacious alternative for management of bladder pain.

Systematic review: psychosocial factors associated with pain in inflammatory bowel disease

BACKGROUND

Pain is a frequently reported symptom of inflammatory bowel disease (IBD) experienced by patients in active disease and remission. Psychological factors play a significant role in pain, but have not been systematically reviewed in IBD.

AIM

To review psychosocial factors associated with pain in adults diagnosed with IBD.

METHODS

Electronic (PsycInfo, MEDLINE, EMBASE, Cochrane Library, CINAHL, Web of Science), and hand-searching were conducted February-May 2017. Two authors carried out screening and data extraction.

RESULTS

Fifteen studies including 5539 IBD patients were identified. Emotional, cognitive-behavioural and personality factors were associated with IBD-pain. Depression and anxiety were the most commonly explored constructs, followed by perceived stress and pain catastrophising, all of which were positively associated with greater pain. Greater abdominal pain was associated with a concurrent mood disorder over fivefold (OR 5.76, 95% CI 1.39, 23.89). Coping strategies and pain fear avoidance correlated with pain levels. Perceived social support (r = .26) and internal locus of control (r = .33) correlated with less pain. Patients reporting pain in IBD remission more frequently had an existing diagnosis of a mood disorder, a chronic pain disorder and irritable bowel syndrome. Six studies controlled for disease activity, of which 4 found that psychosocial factors significantly predicted pain. The majority of studies (n = 10) were of high quality.

CONCLUSION

Psychosocial factors appear to play a significant role in IBD-pain. Further research is required to explore psychosocial constructs in relation to IBD-pain, with use of validated pain measures, large sample sizes and clearer characterisation of disease activity.

Electrical Stimulation of Dorsal Root Ganglion in the Context of Pain: A Systematic Review of In Vitro and In Vivo Animal Model Studies

OBJECTIVE

Dorsal root ganglion (DRG) has recently emerged as an attractive target for neuromodulation therapy since primary sensory neurons and their soma in DRGs are important sites for pathophysiologic changes that lead to neuropathic pain. Our aim was to create evidence synthesis about the effects of electrical stimulation of DRG in the context of pain from in vitro and in vivo animal models, analyze methodology and quality of studies in the field.

METHODS

For conducting systematic review we searched three data bases: MEDLINE, Embase and Web of Science. The quality of included studies was assessed with the Systematic Review Centre for Laboratory Animal Experimentation risk of bias tool for animal studies. The study was registered in the Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies data base.

RESULTS

We included six in vitro and eight in vivo animal studies. All included in vitro studies combined neurostimulation with substances or drugs and reported an improvement in pain-related parameters due to neurostimulation. Among in vivo studies, six used pulsed radiofrequency, while two used electrical field stimulation. All in vivo studies reported improvement in pain-related behavior following stimulation. Meta-analysis was not possible because of heterogeneity and missing data. The quality of included studies was suboptimal since all had an unclear risk of bias in multiple domains.

CONCLUSIONS

Limited data from in vitro and in vivo animal studies indicate that electrical stimulation of DRG has a positive therapeutic effect in the context of pain-related outcomes. Further studies with a standardized methodological approach and outcomes will provide useful information about electrical stimulation of DRG in animal models.

Granulocyte-colony-stimulating factor (G-CSF) signaling in spinal microglia drives visceral sensitization following colitis

Pain is a main symptom of inflammatory diseases and often persists beyond clinical remission. Although we have a good understanding of the mechanisms of sensitization at the periphery during inflammation, little is known about the mediators that drive central sensitization. Recent reports have identified hematopoietic colony-stimulating factors as important regulators of tumor- and nerve injury-associated pain. Using a mouse model of colitis, we identify the proinflammatory cytokine granulocyte-colony-stimulating factor (G-CSF or Csf-3) as a key mediator of visceral sensitization. We report that G-CSF is specifically up-regulated in the thoracolumbar spinal cord of colitis-affected mice. Our results show that resident spinal microglia express the G-CSF receptor and that G-CSF signaling mediates microglial activation following colitis. Furthermore, healthy mice subjected to intrathecal injection of G-CSF exhibit pronounced visceral hypersensitivity, an effect that is abolished by microglial depletion. Mechanistically, we demonstrate that G-CSF injection increases Cathepsin S activity in spinal cord tissues. When cocultured with microglia BV-2 cells exposed to G-CSF, dorsal root ganglion (DRG) nociceptors become hyperexcitable. Blocking CX3CR1 or nitric oxide production during G-CSF treatment reduces excitability and G-CSF-induced visceral pain in vivo. Finally, administration of G-CSF-neutralizing antibody can prevent the establishment of persistent visceral pain postcolitis. Overall, our work uncovers a DRG neuron-microglia interaction that responds to G-CSF by engaging Cathepsin S-CX3CR1-inducible NOS signaling. This interaction represents a central step in visceral sensitization following colonic inflammation, thereby identifying spinal G-CSF as a target for treating chronic abdominal pain.

Analgesia and mouse strain influence neuromuscular plasticity in inflamed intestine

BACKGROUND

Mouse models of inflammatory bowel disease (IBD) identify an impact on the enteric nervous system (ENS) but do not distinguish between Crohn's disease and ulcerative colitis phenotypes. In these models, analgesia is required, but its influence on different strains and disease outcomes is unknown. Therefore, changes to the ENS and intestinal smooth muscle were studied in trinitrobenzene sulfonic acid (TNBS) and dextran sodium sulfate (DSS) induced colitis to identify the effects of analgesia, and compared between two mouse strains.

METHODS

Colitis was induced in CD1 or BALB/c mice receiving analgesia with either buprenorphine or tramadol. Euthanasia was on Day 8 (DSS) or Day 4 (TNBS). Outcomes were Disease Activity Index and cytokine assay, and quantitative histology and immunocytochemistry were used to evaluate effects of inflammation on neurons and smooth muscle.

KEY RESULTS

In BALB/c mice, both models of colitis caused >2-fold increase in smooth muscle cell number. DSS caused axon proliferation without neuron loss while TNBS caused significant neuron loss and axonal damage. Buprenorphine (but not tramadol) was generally anti-inflammatory in both strains, but correlated with lethal outcomes to TNBS in BALB/c mice.

CONCLUSIONS AND INFERENCES

Smooth muscle growth is common to both models of colitis. In contrast, ENS damage in TNBS is correlated with the severe response of a Crohn's disease-like phenotype, while DSS correlates with a milder, ulcerative colitis-like outcome in the deeper tissues. Analgesia with tramadol over buprenorphine is supported for mouse studies of IBD.

Systematic review: interventions for abdominal pain management in inflammatory bowel disease

BACKGROUND

Abdominal pain is frequently reported by people with inflammatory bowel disease (IBD), including in remission. Pain is an under-treated symptom.

AIM

To systematically review evidence on interventions (excluding disease-modifying interventions) for abdominal pain management in IBD.

METHODS

Databases (MEDLINE, EMBASE, PsycInfo, CINAHL, Scopus, Cochrane Library) were searched (February 2016). Two researchers independently screened references and extracted data.

RESULTS

Fifteen papers were included: 13 intervention studies and two cross-sectional surveys. A variety of psychological, dietary and pharmacological interventions were reported. Four of six studies reported pain reduction with psychological intervention including individualised and group-based relaxation, disease anxiety-related Cognitive Behavioural Therapy and stress management. Both psychologist-led and self-directed stress management in inactive Crohn's disease reduced pain compared with controls (symptom frequency reduction index=-26.7, -11.3 and 17.2 at 6-month follow-up, respectively). Two dietary interventions (alcoholic drinks with high sugar content and fermentable carbohydrate with prebiotic properties) had an effect on abdominal pain. Antibiotics (for patients with bacterial overgrowth) and transdermal nicotine patches reduced abdominal pain. Current and past cannabis users report it relieves pain. One controlled trial of cannabis reduced SF-36 and EQ-5D pain scores (1.84 and 0.7, respectively). These results must be treated with caution: data were derived from predominantly small uncontrolled studies of moderate to low quality.

CONCLUSIONS

Few interventions have been tested for IBD abdominal pain. The limited evidence suggests that relaxation and changing cognitions are promising, possibly with individualised dietary changes. There is a need to develop interventions for abdominal pain management in IBD.

Monosynaptic convergence of somatic and visceral C-fiber afferents on projection and local circuit neurons in lamina I: a substrate for referred pain

Spinal lamina I is the first site in the central nervous system where somatic and visceral pathways monosynaptically converge onto projection and local circuit neurons.

Referred pain is a phenomenon of feeling pain at a site other than the site of the painful stimulus origin. It arises from a pathological mixing of nociceptive processing pathways for visceral and somatic inputs. Despite numerous studies based on unit recordings from spinal and supraspinal neurons, the exact mechanism and site of this mixing within the central nervous system are not known. Here, we selectively recorded from lamina I neurons, using a visually guided patch-clamp technique, in thoracic spinal cord preparation with preserved intercostal (somatic) and splanchnic (visceral) nerves. We show that somatic and visceral C fibers converge monosynaptically onto a group of lamina I neurons, which includes both projection and local circuit neurons. Other groups of lamina I neurons received inputs from either somatic or visceral afferents. We have also identified a population of lamina I local circuit neurons showing overall inhibitory responses upon stimulation of both nerves. Thus, the present data allow us to draw two major conclusions. First, lamina I of the spinal cord is the first site in the central nervous system where somatic and visceral pathways directly converge onto individual projection and local circuit neurons. Second, the mechanism of somatovisceral convergence is complex and based on functional integration of monosynaptic and polysynaptic excitatory as well as inhibitory inputs in specific groups of neurons. This complex pattern of convergence provides a substrate for alterations in the balance between visceral and somatic inputs causing referred pain.

Preclinical assessment of pain: improving models in discovery research

To date, animal models have not sufficiently "filtered" targets for new analgesics, increasing the failure rate and cost of drug development. Preclinical assessment of "pain" has historically relied on measures of evoked behavioral responses to sensory stimuli in animals. Such measures can often be observed in decerebrated animals and therefore may not sufficiently capture affective and motivational aspects of pain, potentially diminishing translation from preclinical studies to the clinical setting. Further, evidence indicates that there are important mechanistic differences between evoked behavioral responses of hypersensitivity and ongoing pain, limiting evaluation of mechanisms that could mediate aspects of clinically relevant pain. The mechanisms underlying ongoing pain in preclinical models are currently being explored and may serve to inform decisions towards the transition from drug discovery to drug development for a given target.