Visceral pain

Desensitizing the autonomic nervous system to mitigate anti-GD2 monoclonal antibody side effects

Background

Anti-GD2 monoclonal antibodies (mAbs) have shown to improve the overall survival of patients with high-risk neuroblastoma (HR-NB). Serious adverse events (AEs), including pain, within hours of antibody infusion, have limited the development of these therapies. In this study, we provide evidence of Autonomic Nervous System (ANS) activation as the mechanism to explain the main side effects of anti-GD2 mAbs.

Methods

Through confocal microscopy and computational super-resolution microscopy experiments we explored GD2 expression in postnatal nerves of infants. In patients we assessed the ANS using the Sympathetic Skin Response (SSR) test. To exploit tachyphylaxis, a novel infusion protocol (the Step-Up) was mathematically modelled and tested.

Results

Through confocal microscopy, GD2 expression is clearly visible in the perineurium surrounding the nuclei of nerve cells. By computational super-resolution microscopy experiments we showed the selective expression of GD2 on the cell membranes of human Schwann cells in peripheral nerves (PNs) significantly lower than on NB. In patients, changes in the SSR were observed 4 minutes into the anti-GD2 mAb naxitamab infusion. SSR latency quickly shortened followed by gradual decrease in the amplitude before disappearance. SSR response did not recover for 24 hours consistent with tachyphylaxis and absence of side effects in the clinic. The Step-Up protocol dissociated on-target off-tumor side effects while maintaining serum drug exposure.

Conclusion

We provide first evidence of the ANS as the principal non-tumor target of anti-GD2 mAbs in humans. We describe the development and modeling of the Step-Up protocol exploiting the tachyphylaxis phenomenon we demonstrate in patients using the SSR test.

Tramadol for chronic pain in adults: protocol for a systematic review with meta-analysis and trial sequential analysis of randomised clinical trials

BACKGROUND

Chronic pain in adults is a frequent clinical symptom with a significant impact on patient well-being. Therefore, sufficient pain management is of utmost importance. While tramadol is a commonly used pain medication, the quality of evidence supporting its use has been questioned considering the observed adverse events. Our objective will be to assess the benefits and harms of tramadol compared with placebo or no intervention for chronic pain.

METHODS/DESIGN

We will conduct a systematic review of randomised clinical trials with meta-analysis and trial sequential analysis to assess the beneficial and harmful effects of tramadol in any dose, formulation, or duration. We will accept placebo or no intervention as control interventions. We will include adult participants with any type of chronic pain, including cancer-related pain. We will systematically search the Cochrane Library, MEDLINE, EMBASE, Science Citation Index, and BIOSIS for relevant literature. We will follow the recommendations by Cochrane and the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement. The risk of systematic errors ('bias') and random errors ('play of chance') will be assessed. The certainty of evidence will be evaluated using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach.

DISCUSSION

Although tramadol is often being used to manage chronic pain conditions, the beneficial and harmful effects of this intervention are unknown. The present review will systematically assess the current evidence on the benefits and harms of tramadol versus placebo or no intervention to inform clinical practice and future research.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42019140334.

The minimum effective concentration (MEC90) of ropivacaine for ultrasound-guided quadratus lumborum block for analgesia after cesarean delivery: a dose finding study

BACKGROUND

Quadratus lumborum block was recently proposed as an alternative technique for post-cesarean delivery analgesia. However, there is not a definite optimum concentration of local anesthetics. A biased coin design up-and-down method was used to explore the minimum effective concentration of ropivacaine in quadratus lumborum block for satisfactory analgesia after cesarean delivery.

METHODS

Fifty-six patients weighing 60-80 kg after cesarean section and with ages between 18 and 40 years were recruited. For the posterior quadratus lumborum block, a volume of 25 ml of the assigned concentration of ropivacaine was injected bilaterally. The concentration administered to each patient depended on the response to the previous dose. The first patient received 0.25%. If a successful block was observed, the next patient was randomized to receive the same ropivacaine concentration (with a probability of 0.89) or 0.025% less (with a probability of 0.11). After any block failure, the concentration was always increased by 0.025% for the next. The study ended when 45 successful blocks were obtained. We defined effective quadratus lumborum block as a resting visual analog score ≤ 3 and the absence of a need for rescue anesthetics.

RESULTS

The 90% minimum effective concentration of ropivacaine was 0.335% (95% CI 0.306 to 0.375%), and the 99% minimum effective concentration was 0.371% (95% CI 0.355 to 0.375%). The sufentanil consumption was 11 (11,13) and 24 (22,27) μg at 12 and 24 hours after quadratus lumborum block, respectively.

CONCLUSIONS

The optimum dosage of ropivacaine is a 25 ml volume of 0.335% for quadratus lumborum block after cesarean delivery.

TRIAL REGISTRATION

The study was registered in the Chinese Clinical Trial Registry (No. ChiCTR2000040415 ).

Myofascial Pain Syndrome in Women with Primary Dysmenorrhea: A Case-Control Study

There is limited information on myofascial trigger points (MTrPs) and specific symptoms of chronic pelvic pain and, more specifically, dysmenorrhea. The objective of this study was to determine whether patients suffering from primary dysmenorrhea present alterations in mechanosensitivity and pain patterns, and greater presence of MTrPs in the abdominal and pelvic floor muscles. A case-control study was carried out with a total sample of 84 participants distributed based on primary dysmenorrhea and contraceptive treatment. The sample was divided into four groups each comprising 21 women. Data on pain, quality of life, and productivity and work absenteeism were collected; three assessments were made in different phases of the menstrual cycle, to report data on pressure pain threshold, MTrP presence, and referred pain areas. One-way ANOVA tests showed statistically significant differences (p < 0.01) between the groups, for the Physical Health domain and the total score of the SF-12 questionnaire, and for all the domains of the McGill questionnaire; but no significant differences were found in the data from the WPAI-GH questionnaire. Statistically significant data (p < 0.01) were found for mechanosensitivity in the abdominal area and limbs, but not for the lumbar assessment, within the group, with very few significant intergroup differences. The frequency of active MTrPs is higher in the groups of women with primary dysmenorrhea and during the menstrual phase, with the prevalence of myofascial trigger points of the iliococcygeus muscle being especially high in all examination groups (>50%) and higher than 70% in women with primary dysmenorrhea, in the menstrual phase, and the internal obturator muscle (100%) in the menstrual phase. Referred pain areas of the pelvic floor muscles increase in women with primary dysmenorrhea.

Association of opioid fills with centers for disease control and prevention opioid guidelines and payer coverage policies: physician, insurance and geographic factors

Background The Centers for Disease Control and Prevention (CDC) issued guidelines and certain healthcare payers have made pharmacy coverage changes (PCC) focusing on regulating prescription opioids. Aim We evaluated differences in the rate of first-time opioid fills at doses ≥ 50 morphine milligram equivalents (MME)/day and first-time opioid fills with benzodiazepine fill overlap following the CDC guidelines and following a PCC between provider types, geographic locations, and insurance types. Method We used OptumLabs® Data Warehouse claims data between 2014 and 2018. Subjects were opioid naïve non-cancer care patients, 18 years and older who had an identified chronic pain condition ICD diagnosis within 2 weeks prior to their first-time opioid fill. We used multiple treatment period segmented regression analysis with interaction terms to test the differences between primary care providers (PCPs) and specialist providers (SPs), urban and rural primary care service areas (PCSAs), and Medicare Advantage (MA) and commercially insured patients (CIPs) in their first-time opioid fill patterns. Results Prescribing first-time opioid fills at doses ≥ 50MME/day declined following the CDC guidelines and PCC, the decline was greater among SPs than PCPs and in rural PCSAs than urban PCSAs. Also, following the CDC guidelines, the decline was greater among MA patients however following the PCC the decline was greater among CIPs. There were no differences in rate of first-time opioid fill with benzodiazepine overlap between groups. Conclusion Responses to the CDC opioid guidelines and a PCC differed between PCPs and SPs, urban and rural PCSAs, and when prescribing to MA and CIPs. Understanding these differences is important to help inform future guidelines.

Association of 3 CDC opioid prescription guidelines for chronic pain and 2 payer pharmacy coverage changes on opioid initiation practices

BACKGROUND: Due to the US opioid epidemic, in March of 2016, the Centers for Disease Control and Prevention (CDC) published new guidelines for primary care providers on opioid prescribing for chronic pain. Payer coverage changes were also implemented to help modify opioid prescribing behavior. Whether these initiatives were associated with changes in opioid initiation patterns is unknown. OBJECTIVE: To assess the association between 3 of the 2016 CDC guidelines and 2 subsequent payer pharmacy coverage changes with changes in opioid initiation behavior across different provider specialties. METHODS: We conducted a real-world evidence study using claims data from OptumLabs Data Warehouse between January of 2014 and December of 2018. Subjects were continuously enrolled opioid naive patients, aged at least 18 years, who had at least 1 chronic pain diagnosis within 2 weeks before their first (first-time) opioid prescription. The study used multiple treatment period segmented regression analysis to evaluate the association, across different provider specialties, between the CDC guideline release and the payer pharmacy coverage changes with immediate change in level and overall change in the rate of first-time extended-release opioid prescriptions, firsttime opioid prescriptions at doses of at least 50 MME (morphine milligram equivalent) per day, and first-time opioid prescriptions with overlapping benzodiazepine prescription. RESULTS: The CDC guidelines were not associated with any change in the rate of first-time prescriptions of extended-release opioids. However, a January 2017 payer pharmacy coverage change was associated with a reduction over time in first-time extended-release opioid prescription rates by 22.15 in every 100,000 prescriptions (CI = -40.04 to -2.92, P = 0.013). The CDC guidelines were associated with an immediate decline in level of first-time opioid prescription at doses of at least 50 MME per day by 74.00 in every 10,000 prescriptions (CI = -124.86 to -23.13, P = 0.004) and an increased rate of decline over time by 13.64 in every 10,000 prescriptions (CI = -17.07 to -10.21, P < 0.001). These associations varied across provider types and specialties. The March 2018, payer coverage change was associated with an immediate reduction in level of first-time opioid prescriptions at doses of at least 50 MME per day across all specialties and an increased reduction over time among surgeons. The CDC guidelines were associated, respectively, with a reduction in the rate of overlapping first-time opioid prescriptions with benzodiazepines among family medicine, internal medicine, surgeons, emergency medicine providers, and providers with unknown specialty by 6.11, 5.10, 2.89, 11.43, and 9.11 in every 10,000 prescriptions monthly (CI = -9.48 to -2.73, -9.86 to -0.35, -5.40 to -0.38, -17.26 to -5.61 and -11.96 to -6.25, respectively, P < 0.001, P = 0.035, P = 0.024, P < 0.001 and P < 0.001). CONCLUSIONS: Some specialist providers also adopted the CDC guidelines, and the response to the guidelines differed across various provider specialties. Some CDC guidelines were associated with a reduction in high-risk first-time opioid prescriptions. Payer pharmacy coverage changes reinforced the guidelines both in situations where the CDC guidelines did and did not show any association. DISCLOSURE: This research was funded by Agency for Healthcare Research and Quality (R01 HS025164; PI: Karaca-Mandic). Karaca-Mandic reports grants from the American Cancer Society and Sempre Health, along with fees from Tactile Medical and Precision Health Economics, unrelated to this study. The other authors have nothing to disclose.

Anatomical basis of fascial plane blocks

Fascial plane blocks (FPBs) are regional anesthesia techniques in which the space ("plane") between two discrete fascial layers is the target of needle insertion and injection. Analgesia is primarily achieved by local anesthetic spread to nerves traveling within this plane and adjacent tissues. This narrative review discusses key fundamental anatomical concepts relevant to FPBs, with a focus on blocks of the torso. Fascia, in this context, refers to any sheet of connective tissue that encloses or separates muscles and internal organs. The basic composition of fascia is a latticework of collagen fibers filled with a hydrated glycosaminoglycan matrix and infiltrated by adipocytes and fibroblasts; fluid can cross this by diffusion but not bulk flow. The plane between fascial layers is filled with a similar fat-glycosaminoglycan matric and provides gliding and cushioning between structures, as well as a pathway for nerves and vessels. The planes between the various muscle layers of the thorax, abdomen, and paraspinal area close to the thoracic paravertebral space and vertebral canal, are popular targets for ultrasound-guided local anesthetic injection. The pertinent musculofascial anatomy of these regions, together with the nerves involved in somatic and visceral innervation, are summarized. This knowledge will aid not only sonographic identification of landmarks and block performance, but also understanding of the potential pathways and barriers for spread of local anesthetic. It is also critical as the basis for further exploration and refinement of FPBs, with an emphasis on improving their clinical utility, efficacy, and safety.

Mechanisms of action of fascial plane blocks: a narrative review

BACKGROUND

Fascial plane blocks (FPBs) target the space between two fasciae, rather than discrete peripheral nerves. Despite their popularity, their mechanisms of action remain controversial, particularly for erector spinae plane and quadratus lumborum blocks.

OBJECTIVES

This narrative review describes the scientific evidence underpinning proposed mechanisms of action, highlights existing knowledge gaps, and discusses implications for clinical practice and research.

FINDINGS

There are currently two plausible mechanisms of analgesia. The first is a local effect on nociceptors and neurons within the plane itself or within adjacent muscle and tissue compartments. Dispersion of local anesthetic occurs through bulk flow and diffusion, and the resulting conduction block is dictated by the mass of local anesthetic reaching these targets. The extent of spread, analgesia, and cutaneous sensory loss is variable and imperfectly correlated. Explanations include anatomical variation, factors governing fluid dispersion, and local anesthetic pharmacodynamics. The second is vascular absorption of local anesthetic and a systemic analgesic effect at distant sites. Direct evidence is presently lacking but preliminary data indicate that FPBs can produce transient elevations in plasma concentrations similar to intravenous lidocaine infusion. The relative contributions of these local and systemic effects remain uncertain.

CONCLUSION

Our current understanding of FPB mechanisms supports their demonstrated analgesic efficacy, but also highlights the unpredictability and variability that result from myriad factors at play. Potential strategies to improve efficacy include accurate deposition close to targets of interest, injections of sufficient volume to encourage physical spread by bulk flow, and manipulation of concentration to promote diffusion.

Brain Responses to Noxious Stimuli in Patients With Chronic Pain: A Systematic Review and Meta-analysis

IMPORTANCE

Functional neuroimaging is a valuable tool for understanding how patients with chronic pain respond to painful stimuli. However, past studies have reported heterogenous results, highlighting opportunities for a quantitative meta-analysis to integrate existing data and delineate consistent associations across studies.

OBJECTIVE

To identify differential brain responses to noxious stimuli in patients with chronic pain using functional magnetic resonance imaging (fMRI) while adhering to current best practices for neuroimaging meta-analyses.

DATA SOURCES

All fMRI experiments published from January 1, 1990, to May 28, 2019, were identified in a literature search of PubMed/MEDLINE, EMBASE, Web of Science, Cochrane Library, PsycINFO, and SCOPUS.

STUDY SELECTION

Experiments comparing brain responses to noxious stimuli in fMRI between patients and controls were selected if they reported whole-brain results, included at least 10 patients and 10 healthy control participants, and used adequate statistical thresholding (voxel-height P < .001 or cluster-corrected P < .05). Two independent reviewers evaluated titles and abstracts returned by the search. In total, 3682 abstracts were screened, and 1129 full-text articles were evaluated.

DATA EXTRACTION AND SYNTHESIS

Thirty-seven experiments from 29 articles met inclusion criteria for meta-analysis. Coordinates reporting significant activation differences between patients with chronic pain and healthy controls were extracted. These data were meta-analyzed using activation likelihood estimation. Data were analyzed from December 2019 to February 2020.

MAIN OUTCOMES AND MEASURES

A whole-brain meta-analysis evaluated whether reported differences in brain activation in response to noxious stimuli between patients and healthy controls were spatially convergent. Follow-up analyses examined the directionality of any differences. Finally, an exploratory (nonpreregistered) region-of-interest analysis examined differences within the pain network.

RESULTS

The 37 experiments from 29 unique articles included a total of 511 patients and 433 controls (944 participants). Whole-brain meta-analyses did not reveal significant differences between patients and controls in brain responses to noxious stimuli at the preregistered statistical threshold. However, exploratory analyses restricted to the pain network revealed aberrant activity in patients.

CONCLUSIONS AND RELEVANCE

In this systematic review and meta-analysis, preregistered, whole-brain analyses did not reveal aberrant fMRI activity in patients with chronic pain. Exploratory analyses suggested that subtle, spatially diffuse differences may exist within the pain network. Future work on chronic pain biomarkers may benefit from focus on this core set of pain-responsive areas.

Animal models of pain: Diversity and benefits

Chronic pain is a maladaptive neurological disease that remains a major health problem. A deepening of our knowledge on mechanisms that cause pain is a prerequisite to developing novel treatments. A large variety of animal models of pain has been developed that recapitulate the diverse symptoms of different pain pathologies. These models reproduce different pain phenotypes and remain necessary to examine the multidimensional aspects of pain and understand the cellular and molecular basis underlying pain conditions. In this review, we propose an overview of animal models, from simple organisms to rodents and non-human primates and the specific traits of pain pathologies they model. We present the main behavioral tests for assessing pain and investing the underpinning mechanisms of chronic pathological pain. The validity of animal models is analysed based on their ability to mimic human clinical diseases and to predict treatment outcomes. Refine characterization of pathological phenotypes also requires to consider pain globally using specific procedures dedicated to study emotional comorbidities of pain. We discuss the limitations of pain models when research findings fail to be translated from animal models to human clinics. But we also point to some recent successes in analgesic drug development that highlight strategies for improving the predictive validity of animal models of pain. Finally, we emphasize the importance of using assortments of preclinical pain models to identify pain subtype mechanisms, and to foster the development of better analgesics.

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Identification of a Sacral, Visceral Sensory Transcriptome in Embryonic and Adult Mice

Visceral sensory neurons encode distinct sensations from healthy organs and initiate pain states that are resistant to common analgesics. Transcriptome analysis is transforming our understanding of sensory neuron subtypes but has generally focused on somatic sensory neurons or the total population of neurons in which visceral neurons form the minority. Our aim was to define transcripts specifically expressed by sacral visceral sensory neurons, as a step towards understanding the unique biology of these neurons and potentially leading to identification of new analgesic targets for pelvic visceral pain. Our strategy was to identify genes differentially expressed between sacral dorsal root ganglia (DRG) that include somatic neurons and sacral visceral neurons, and adjacent lumbar DRG that comprise exclusively of somatic sensory neurons. This was performed in adult and E18.5 male and female mice. By developing a method to restrict analyses to nociceptive Trpv1 neurons, a larger group of genes were detected as differentially expressed between spinal levels. We identified many novel genes that had not previously been associated with pelvic visceral sensation or nociception. Limited sex differences were detected across the transcriptome of sensory ganglia, but more were revealed in sacral levels and especially in Trpv1 nociceptive neurons. These data will facilitate development of new tools to modify mature and developing sensory neurons and nociceptive pathways.

Cannabinoids versus placebo or no intervention for pain: protocol for a systematic review with meta-analysis and trial sequential analysis

INTRODUCTION

Pain is a frequent clinical symptom with significant impact on the patient's well-being. Therefore, adequate pain management is of utmost importance. While cannabinoids have become a more popular alternative to traditional types of pain medication among patients, the quality of evidence supporting the use of cannabinoids has been questioned. The beneficial and harmful effects of cannabinoids in patients with pain is unknown. Accordingly, we aim to assess the efficacy, tolerability and safety of cannabinoids (herbal, plant-derived extracts and synthetic) compared with placebo or no intervention for any type of pain.

METHODS AND ANALYSES

We will conduct a systematic review of randomised clinical trials with meta-analysis and Trial Sequential Analysis to assess the beneficial and harmful effects of cannabinoids in any dose, formulation and duration. We will accept placebo or no treatment as control interventions. We will include participants with any type of pain (acute and chronic pain, cancer-related pain, headache, neuropathic pain or any other types of pain). We will systematically search The Cochrane Library, MEDLINE, Embase, Science Citation Index and BIOSIS for relevant literature. We will follow the recommendations by Cochrane and the Preferred Reporting Items for Systematic Review and Meta-Analysis statement. The risk of systematic errors (bias) and random errors (play of chance) will be assessed. The overall certainty of evidence will be evaluated using the Grading of Recommendations Assessment, Development and Evaluation approach.

ETHICS AND DISSEMINATION

Ethical approval is not a requirement since no primary data will be collected. The findings of this systematic review will be submitted for peer-reviewed publication and disseminated in national and international conferences.

DISCUSSION

Although cannabinoids are now being used to manage different pain conditions, the evidence for the clinical effects are unclear. The present review will systematically assess the current evidence for the benefits and harms of cannabinoids to inform practice and future research.

Targeting epigenetic mechanisms for chronic visceral pain: A valid approach for the development of novel therapeutics

BACKGROUND

Chronic visceral pain is persistent pain emanating from thoracic, pelvic, or abdominal origin that is poorly localized with regard to the specific organ affected. The prevalence can range up to 25% in the adult population as chronic visceral pain is a common feature of many visceral disorders, which may or may not be accompanied by distinct structural or histological abnormalities within the visceral organs. Mounting evidence suggests that changes in epigenetic mechanisms are involved in the top-down or bottom-up sensitization of pain pathways and the development of chronic pain. Epigenetic changes can lead to long-term alterations in gene expression profiles of neurons and consequently alter functionality of peripheral neurons, dorsal root ganglia, spinal cord, and brain neurons. However, epigenetic modifications are dynamic, and thus, detrimental changes may be reversible. Hence, external factors/therapeutic interventions may be capable of modulating the epigenome and restore normal gene expression for extended periods of time.

PURPOSE

The goal of this review is to highlight the latest discoveries made toward understanding the epigenetic mechanisms that are involved in the development or maintenance of chronic visceral pain. Furthermore, this review will provide evidence supporting that targeting these epigenetic mechanisms may represent a novel approach to treat chronic visceral pain.

Do MicroRNAs Modulate Visceral Pain?

Visceral pain, a common characteristic of multiple diseases relative to viscera, impacts millions of people worldwide. Although hundreds of studies have explored mechanisms underlying visceral pain, it is still poorly managed. Over the past decade, strong evidence emerged suggesting that microRNAs (miRNAs) play a significant role in visceral nociception through altering neurotransmitters, receptors and other genes at the posttranscriptional level. Under pathological conditions, one kind of miRNA may have several target mRNAs and several kinds of miRNAs may act on one target, suggesting complex interactions and mechanisms between miRNAs and target genes lead to pathological states. In this review we report on recent progress in examining miRNAs responsible for visceral sensitization and provide miRNA-based therapeutic targets for the management of visceral pain.

Regulation of transient receptor potential cation channel subfamily V1 protein synthesis by the phosphoinositide 3-kinase/Akt pathway in colonic hypersensitivity

The transient receptor potential cation channel subfamily V member 1 (TRPV1), also known as the capsaicin receptor or vanilloid receptor 1 (VR1), is expressed in nociceptive neurons in the dorsal root ganglia (DRG) and participates in the transmission of pain. The present study investigated the underlying molecular mechanisms by which TRPV1 was regulated by nerve growth factor (NGF) signaling pathways in colonic hypersensitivity in response to colitis. We found that during colitis TRPV1 protein levels were significantly increased in specifically labeled colonic afferent neurons in both L1 and S1 DRGs. TRPV1 protein up-regulation in DRG was also enhanced by NGF treatment. We then found that TRPV1 protein up-regulation in DRG was regulated by activation of the phosphoinositide 3-kinase (PI3K)/Akt pathway both in vivo and in vitro. Suppression of endogenous PI3K/Akt activity during colitis or NGF treatment with a specific PI3K inhibitor LY294002 reduced TRPV1 protein production in DRG neurons, and also reduced colitis-evoked TRPV1-mediated visceral hypersensitivity tested by hyper-responsiveness to colorectal distention (CRD) and von Frey filament stimulation of abdomen. Further studies showed that TRPV1 mRNA levels in the DRG were not regulated by either colitis or NGF. We then found that an up-regulation of the protein synthesis pathway was involved by which both colitis and NGF caused a PI3K-dependent increase in the phosphorylation level of eukaryotic translation initiation factor 4E-binding protein (4E-BP)1. These results suggest a novel mechanism in colonic hypersensitivity which involves PI3K/Akt-mediated TRPV1 protein, not mRNA, up-regulation in primary afferent neurons, likely through activation of the protein synthesis pathways.

Relating Chronic Pelvic Pain and Endometriosis to Signs of Sensitization and Myofascial Pain and Dysfunction

Chronic pelvic pain is a frustrating symptom for patients with endometriosis and is frequently refractory to hormonal and surgical management. While these therapies target ectopic endometrial lesions, they do not directly address pain due to central sensitization of the nervous system and myofascial dysfunction, which can continue to generate pain from myofascial trigger points even after traditional treatments are optimized. This article provides a background for understanding how endometriosis facilitates remodeling of neural networks, contributing to sensitization and generation of myofascial trigger points. A framework for evaluating such sensitization and myofascial trigger points in a clinical setting is presented. Treatments that specifically address myofascial pain secondary to spontaneously painful myofascial trigger points and their putative mechanisms of action are also reviewed, including physical therapy, dry needling, anesthetic injections, and botulinum toxin injections.

Intrapericardial capsaicin and bradykinin induce different cardiac-somatic and cardiovascular reflexes in rats

Patients with myocardial infarction experience various types of chest pain and autonomic disturbance symptoms. Studies in rats have shown that pericardial infusions of certain chemicals induce cardiac-related muscle pain and cardiovascular reflexes. In the present study, bradykinin or capsaicin was injected into the pericardial sac and the resulting cardiac-somatic reflexes and blood pressure (BP) alterations were record. We found that the cardiac-somatic reflex induced by bradykinin had a longer latency, shorter duration, and lower firing rate than that induced by capsaicin (p<0.05). We also found that bradykinin induced a hypertensive response (p<0.05), while capsaicin induced a hypotensive response (p<0.05). Bilateral vagotomy had no effect on the cardiac-somatic reflex induced by bradykinin (p>0.05) but reduced the reflex induced by capsaicin (p<0.05). However, vagotomy had no effect on the BP alterations induced by both bradykinin and capsaicin (p>0.05). These results suggest that bradykinin and capsaicin activate different pathways to induce cardiac-somatic and cardiovascular reflexes and that the vagus nerve is involved in TRPV1-related muscle pain modulation.

Preclinical Assessment of Inflammatory Pain

While acute inflammation is a natural physiological response to tissue injury or infection, chronic inflammation is maladaptive and engenders a considerable amount of adverse pain. The chemical mediators responsible for tissue inflammation act on nociceptive nerve endings to lower neuronal excitation threshold and sensitize afferent firing rate leading to the development of allodynia and hyperalgesia, respectively. Animal models have aided in our understanding of the pathophysiological mechanisms responsible for the generation of chronic inflammatory pain and allowed us to identify and validate numerous analgesic drug candidates. Here we review some of the commonly used models of skin, joint, and gut inflammatory pain along with their relative benefits and limitations. In addition, we describe and discuss several behavioral and electrophysiological approaches used to assess the inflammatory pain in these preclinical models. Despite significant advances having been made in this area, a gap still exists between fundamental research and the implementation of these findings into a clinical setting. As such we need to characterize inherent pathophysiological pathways and develop new endpoints in these animal models to improve their predictive value of human inflammatory diseases in order to design safer and more effective analgesics.