Visceral Pain Responses Research Articles

More than a gut feeling: the microbiota regulates neurodevelopment and behavior.

The realization of the importance of the gut microbiota in health and disease has been one of the most exciting areas in biomedical research over the past 5 years. We are teeming with microrganisms in and on our body, having >150 times more microbial genes (the microbiome) than mammalian genes. Studies are revealing how variations in the composition of the gut microbiota influence all aspects of physiology, including brain function and even behavior. In terms of neuroscience and psychiatric disease the field is still very much in its infancy, but evidence is accumulating that it has a key role. Factors that shape the bacterial landscape include being born by Caesarian delivery, not being breastfed, environment, gestational age, host genetics, exposure to infections (both maternal and infant), and antibiotic usage. Moreover, stress, especially that in early life and prenatally, can have marked effects on microbiota composition (Borre et al, 2014). A variety of strategies have been used to investigate the role of this so-called microbiota–gut–brain axis in health and disease. Germ-free mice (mice that never have been exposed to any bacteria) have been used to show that microbiota is crucial for hypothalamic–pituitary–adrenal axis function. Moreover, these mice have widespread neurodevelopmental changes in the brain, including alterations in monoaminergic neurotransmission and behavioral changes in anxiety (Foster and McVey Neufeld, 2013). More recently, we have shown that these mice also exhibit autism-like traits, such as deficits in sociability, social cognition, and increased repetitive behaviors (Desbonnet et al, 2014). Interestingly, as in autism and other neurodevelopmental disorders, these effects are much more pronounced in males than females. Moreover, studies in germ-free mice can be expanded to enable research on the ‘humanization' of the gut microbiota, that is, transplanting faecal microbiota from specific human conditions or from animal models. Indeed, it has been shown that behavioral traits can be transplanted between strains of mice (Collins et al, 2013). Administration of various potential probiotic strains in rodents or humans has also been shown to have beneficial effects in rodents. Major strain and species differences occur in terms of their ‘psychobiotic' effects. Studies have shown that some Bifidobacteria and Lactobacilli species as well as Bacteroides fragilis can have positive effects on anxiety, depression, cognition, and autism-related behaviors (Hsiao et al, 2013). Prebiotics, nondigestible food ingredients that promotes the growth of beneficial gut microorganisms, are also being shown to affect brain BDNF levels. Human studies of prebiotics and probiotics have lagged behind to date, although there is interesting neuroimaging studies emerging clearly showing that such bacterial-based dietary interventions can affect brain function. Finally, perturbation of the microbiota by administration of antimicrobial drugs can allow for a temporally controlled and more clinically relevant tool to assess the role of the gut microbiota on behavior. Antibiotics in adulthood and early-life can reverse both antipsychotic-induced obesity (Davey et al, 2013) and increase visceral pain (O'Mahony et al, 2014) responses, respectively. Further studies focused on the mechanisms on how these gut microbiota signal to the brain are now warranted. To date, it is clear that the vagus nerve, immune signaling, and the production of bioactive metabolites are strongly implicated in communication across the microbiota–gut–brain axis (Figure 1). Figure 1 Key pathways involved in microbiota–gut–brain signaling. There are many potential direct and indirect pathways exist through which the gut microbiota can modulate the gut–brain axis. They include endocrine (cortisol), immune (cytokines), ...

Early visceral pain predicts chronic pain after laparoscopic cholecystectomy

Chronic pain after laparoscopic cholecystectomy is related to postoperative pain during the first postoperative week, but it is unknown which components of the early pain response is important. In this prospective study, 100 consecutive patients were examined preoperatively, 1week postoperatively, and 3, 6, and 12months postoperatively for pain, psychological factors, and signs of hypersensitivity. Overall pain, incisional pain (somatic pain component), deep abdominal pain (visceral pain component), and shoulder pain (referred pain component) were registered on a 100-mm visual analogue scale during the first postoperative week. Nine patients developed chronic unexplained pain 12months postoperatively. In a multivariate analysis model, cumulated visceral pain during the first week and number of preoperative biliary pain attacks were identified as independent risk factors for unexplained chronic pain 12months postoperatively. There were no consistent signs of hypersensitivity in the referred pain area either pre- or postoperatively. There were no significant associations to any other variables examined. The risk of chronic pain after laparoscopic cholecystectomy is relatively low, but significantly related to the visceral pain response during the first postoperative week.

Inhibitory effects of SKI3246, the rhizome extract of Atractylodes japonica, on visceral hypersensitivity in experimental irritable bowel syndrome rat models.

We evaluated the effect of SKI3246, the 50% ethanol extract of the rhizome of Atractylodes japonica, on visceral hypersensitivity, which is a major characteristic feature of IBS. We used various rat models of visceral hypersensitivity to assess the visceral pain responses to colorectal distension (CRD) in comparison with conventional IBS treatments. Oral administration of SKI3246 dose-dependently and significantly attenuated the abdominal withdrawal reflex (AWR) score in a model of acetic acid-induced visceral hypersensitivity. We also found that it reduced the number of abdominal contractions in response to CRD in a model of 2,4,6-trinitrobenzenesulfonic acid-induced visceral hypersensitivity, which was comparable to ramosetron or alosetron. Furthermore, treatment with SKI3246 also increased the pain threshold and abolished the elevated AWR scores to CRD in a rat model of neonatal maternal separation. We presumed that the modulation of the NK2 receptor is involved in the inhibitory activity of SKI3246 on the basis that it significantly inhibited the contraction of the distal colonic muscle induced by neurokinin A, the NK2 receptor agonist. The present results indicate that SKI3246 has the potential to be an effective therapeutic agent for IBS, especially insofar as it can relieve visceral hypersensitivity.

NMDA and AMPA receptors in the anterior cingulate cortex mediates visceral pain in visceral hypersensitivity rats

Several studies have shown that N-methyl-d-aspartate (NMDA)-receptor activation in anterior cingulate cortex (ACC) neurons plays critical roles in modulating visceral pain responses in visceral hypersensitivity (VH) rats. However, there are few reports about the expressions of NMDA and α-amino-3-hydroxy-5-methyl-4-isox-azolepropionic-acid (AMPA) receptor subtypes in ACC of VH model rats at different time points. The current study was undertaken to investigate NR2A, NR2B and GluR2 expressions in ACC of VH rats that were induced by administration with 5% mustard oil. Our results indicated that NR2B, but not NR2A, was highly expressed in VH model group on day 15, 22, and 36 compared with normal group (p<0.05). GluR2 expression was also higher in VH model group on day 15, 22, and 36 than that of normal group (p<0.05). These findings suggested increased expression of NR2B and GluR2 might be key mechanisms for long-term synaptic plastic changes in VH rats.

Facilitation of Synaptic Transmission in the Anterior Cingulate Cortex in Viscerally Hypersensitive Rats

Electrophysiological studies have shown the enhanced response of anterior cingulate cortex (ACC) to colorectal distension in viscerally hypersensitive (VH) rats, which can be observed up to 7 weeks following colonic anaphylaxis, independent of colon inflammation, suggesting a mechanism for learning and triggering of pain memories in the ACC neuronal circuitry. Activity-dependent plasticity in synaptic strength may serve as a key mechanism that reflects cortical plasticity. However, only a few reports have indicated the synaptic plasticity of ACC in vivo. In the present study, electrophysiological recording showed long-lasting potentiation of local field potential in the medial thalamus (MT)-ACC synapses in VH rats. Theta burst stimulation in the MT reliably induced long-term potentiation in the MT-ACC pathway in normal rats, but was occluded in the VH state. Further, repeated tetanization of MT increased ACC neuronal activity and visceral pain responses of normal rats, mimicking VH rats. In conclusion, we demonstrated for the first time that visceral hypersensitivity is associated with alterations of synaptic plasticity in the ACC. The ACC synaptic strengthening in chronic visceral pain may engage signal transduction pathways that are in common with those activated by electrical stimulation, and serves as an attractive cellular model of functional visceral pain.

Strain Specific Persistent Suppression of Visceral Pain Perception After Recovery From a Bout of Colitis

Visceral pain is often experienced by patients with GI inflammation, and some studies suggest that such pain can persist after resolution of inflammation. Colitis was induced in male Wistar and Sprague‐Dawley (SD) rats via intrarectal administration of dinitrobenzene sulfonic acid (DNBS) in 50% ethanol, controls received saline only. At day 15, under general anesthesia, a balloon was inserted into the colon and distended (colo‐rectal distention; CRD), heart rate was recorded continuously. Cardioautonomic responses to CRD were analyzed and expressed as % change from the pre‐CRD heart rate. Colonic injury and inflammation were examined histologically and by measurement of tissue myeloperoxidase activity. By 15 days following DNBS administration, colitis had resolved. CRD caused a significant visceral pain response (bradycardia) in both strains of control rats. However, Wistar rats showed a significant suppression of pain response to CRD, whereas the response in SD rats remained the same after colitis. There was no difference in colonic compliance between control and DNBS groups or the two strains. Our results provide evidence of a strain‐dependent alteration in the pain response to CRD that persists after resolution of colitis, likely due to changes in autonomic sensory function induced by inflammation. Our results are consistent with the variability in pain perception seen in patients with GI inflammation.

Acute Colitis Induces Neurokinin 1 Receptor Internalization in the Rat Lumbosacral Spinal Cord

Substance P (SP) and its receptor, the neurokinin 1 receptor (NK1R), play important roles in transmitting and regulating somatosensory nociceptive information. However, their roles in visceral nociceptive transmission and regulation remain to be elucidated. In the previous study, moderate SP immunoreactive (SP-ir) terminals and NK1R-ir neurons were observed in the dorsal commissural nucleus (DCN) of the lumbosacral spinal cord. Thus we hypothesized that the SP-NK1R system is involved in visceral pain transmission and control within the DCN. The acute visceral pain behaviors, the colon histological changes and the temporal and spatial changes of NK1R-ir structures and Fos expression in the neurons of the DCN were observed in rats following lower colon instillation with 5% formalin. The formalin instillation induced significant acute colitis as revealed by the histological changes in the colon. NK1R internalization in the DCN was obvious at 8 min. It reached a peak (75.3%) at 30 min, began to decrease at 90 min (58.1%) and finally reached the minimum (19.7%) at 3 h after instillation. Meanwhile, formalin instillation induced a biphasic visceral pain response as well as a strong expression of Fos protein in the nuclei of neurons in the DCN. Finally, intrathecal treatment with the NK1R antagonist L732138 attenuated the NK1R internalization, Fos expression and visceral nociceptive responses. The present results suggest that the visceral nociceptive information arising from inflamed pelvic organs, such as the lower colon, might be mediated by the NK1R-ir neurons in the DCN of the lumbosacral spinal cord.

Central Amygdala Metabotropic Glutamate Receptor 5 in the Modulation of Visceral Pain

Painful bladder syndrome is a debilitating condition that affects 3-6% of women in the United States. Multiple lines of evidence suggest that changes in CNS processing are key to the development of chronic bladder pain conditions but little is known regarding the underlying cellular, molecular, and neuronal mechanisms. Using a mouse model of distention-induced bladder pain, we found that the central nucleus of the amygdala (CeA) is a critical site of neuromodulation for processing of bladder nociception. Furthermore, we demonstrate that metabotropic glutamate receptor 5 (mGluR5) activation in the CeA induces bladder pain sensitization by increasing CeA output. Thus, pharmacological activation of mGluR5 in the CeA is sufficient to increase the response to bladder distention. Additionally, pharmacological blockade or virally mediated conditional deletion of mGluR5 in the CeA reduced responses to bladder distention suggesting that mGluR5 in the CeA is also necessary for these responses. Finally, we used optogenetic activation of the CeA and demonstrated that this caused a robust increase in the visceral pain response. The CeA-localized effects on responses to bladder distention are associated with changes in extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation in the spinal cord. Overall, these data demonstrate that mGluR5 activation leads to increased CeA output that drives bladder pain sensitization.

RETRACTED: Vagus nerve stimulation modulates visceral pain-related affective memory

Within a biopsychosocial model of pain, pain is seen as a conscious experience modulated by mental, emotional and sensory mechanisms. Recently, using a rodent visceral pain assay that combines the colorectal distension (CRD) model with the conditioned place avoidance (CPA) paradigms, we measured a learned behavior that directly reflects the affective component of visceral pain, and showed that perigenual anterior cingulate cortex (pACC) activation is critical for memory processing involved in long-term visceral affective state and prediction of aversive stimuli by contextual cue. Electrical vagus nerve stimulation (VNS) has become an established therapy for treatment-resistant epilepsy. VNS has also been shown to enhance memory performance in rats and humans. High-intensity VNS (400 μA) immediately following conditional training significantly increases the CRD-induced CPA scores, and enhanced the pain affective memory retention. In contrast, VNS (400 μA) had no effect on CPA induced by non-nociceptive aversive stimulus (U69,593). Low-intensity VNS (40 μA) had no effect on CRD-induced CPA. Electrophysiological recording showed that VNS (400 μA) had no effect on basal and CRD-induced ACC neuronal firing. Further, VNS did not alter CRD-induced visceral pain responses suggesting high intensity VNS facilitates visceral pain aversive memory independent of sensory discriminative aspects of visceral pain processing. The findings that vagus nerve stimulation facilities visceral pain-related affective memory underscore the importance of memory in visceral pain perception, and support the theory that postprandial factors may act on vagal afferents to modulate ongoing nature of visceral pain-induced affective disorder observed in the clinic, such as irritable bowel syndrome.

Visceral analgesia induced by acute and repeated water avoidance stress in rats: sex difference in opioid involvement

Chronic psychological stress-induced alterations in visceral sensitivity have been predominantly assessed in male rodents. We investigated the effect of acute and repeated water avoidance stress (WAS) on the visceromotor response (VMR) to colorectal distension (CRD) and the role of opioids in male and cycling female Wistar rats using a novel non-invasive manometric technique. After a baseline VMR (1st CRD, day 0), rats were exposed to WAS (1 h day(-1) ) either once or for four consecutive days, without injection or with naloxone (1 mg kg(-1) ) or saline injected subcutaneously before each WAS session. The VMR to CRD recorded on day 1 or 4 immediately after the last WAS was reduced in both females and males. The visceral analgesia was mainly naloxone-dependent in females, but naloxone-independent in males. In non-injected animals, on days 2 and 5, VMR was not significantly different from baseline in males whereas females exhibited a significant VMR increase at 60 mmHg on day 5. Basal CRD and CRD on days 1, 2, and 5 in both sexes without WAS induced similar VMR. When monitored non-invasively, psychological stress induces an immediate poststress visceral analgesia mediated by an opiate signaling system in females while naloxone-independent in males, and hyperalgesia at 24 h after repeated stress only in females. These data highlight the importance of sex-specific interventions to modulate visceral pain response to stress.

273 Gastric Electrical Stimulation Decreases Gastric Distension-Induced Visceral Pain and Central Response Through Direct Action on Primary Afferents

273 Gastric Electrical Stimulation Decreases Gastric Distension-Induced Visceral Pain and Central Response Through Direct Action on Primary Afferents

Retracted: Phosphorylated CaMKII post‐synaptic binding to NR2B subunits in the anterior cingulate cortex mediates visceral pain in visceral hypersensitive rats

The NR2B subunit of NMDA receptor in the anterior cingulate cortex (ACC) is up-regulated in viscerally hypersensitive (VH) rats induced by colonic anaphylaxis. It plays a critical role in modulation of ACC sensitization and visceral pain responses. Given the key role of calcium/calmodulin-dependent protein kinase II (CaMKII) in synaptic plasticity and behavior learning and memory, we hypothesize that phosphorylation of CaMKII binding to NR2B mediates visceral pain in VH states. We performed in vivo electroporation of CaMKII siRNA produced inhibition of colorectal distension-induced visceromotor response in the VH rats. The NR2B, CaMKII and P-CaMKII-Thr²⁸⁶ protein levels were increased in 180%, 220% and 304% fold in the post-synaptic density (PSD) fraction in VH rats separately. Western blotting following co-immunoprecipitation showed that P-CaMKII-Thr²⁸⁶ bound to NR2B in the PSD, which was increased to 267% of control in VH rats. Administration of CaMKII antagonist Antennapedia-CaMKIINtide suppressed visceromotor response in VH rats in parallel with decrease of NR2B levels and reduction of the NR2B-P-CaMKII-Thr²⁸⁶ protein complex in PSD. In conclusion, CaMKII is a critical signaling molecule in the ACC glutamatergic synaptic transmission and phosphorylation of CaMKII at Thr286, which binds to NR2B subunit at post-synaptic site, modulates visceral pain in viscerally hypersensitive state.

Effects of β-(1,3–1,6)-d-glucan on irritable bowel syndrome-related colonic hypersensitivity

Irritable bowel syndrome (IBS) is a gastrointestinal disorder characterized by chronic abdominal pain associated with altered bowel habits. Since the prevalence of IBS is very high and thus, involves elevated health-care costs, treatment of this condition by methods other than prescribed medicines could be beneficial. β-(1,3)-d-glucan with β-(1,6) branches (β-glucan) has been used as a nutritional supplement for many years. In this study, we examined the effect of β-glucan on fecal pellet output and visceral pain response in animal models of IBS. Oral administration of β-glucan suppressed the restraint stress- or drug-induced fecal pellet output. β-Glucan also suppressed the visceral pain response to colorectal distension. These results suggest that β-glucan could be beneficial for the treatment and prevention of IBS.

Stress-induced visceral analgesia assessed non-invasively in rats is enhanced by prebiotic diet.

To investigate the influence of repeated water avoidance stress (rWAS) on the visceromotor response (VMR) to colorectal distension (CRD) and the modulation of the response by a prebiotic diet in rats using a novel surgery-free method of solid-state manometry. Male Wistar rats fed a standard diet with or without 4% enzyme-treated rice fiber (ERF) for 5 wk were subjected to rWAS (1 h daily x 10 d) or no stress. The VMR to graded phasic CRD was assessed by intraluminal colonic pressure recording on days 0 (baseline), 1 and 10 (45 min) and 11 (24 h) after rWAS and expressed as percentage change from baseline. Cecal content of short chain fatty acids and distal colonic histology were assessed on day 11. WAS on day 1 reduced the VMR to CRD at 40 and 60 mmHg similarly by 28.9% ± 6.6% in both diet groups. On day 10, rWAS-induced reduction of VMR occurred only at 40 mmHg in the standard diet group (36.2% ± 17.8%) while in the ERF group VMR was lowered at 20, 40 and 60 mmHg by 64.9% ± 20.9%, 49.3% ± 11.6% and 38.9% ± 7.3% respectively. The visceral analgesia was still observed on day 11 in ERF- but not in standard diet-fed rats. By contrast the non-stressed groups (standard or ERF diet) exhibited no changes in VMR to CRD. In standard diet-fed rats, rWAS induced mild colonic histological changes that were absent in ERF-fed rats exposed to stress compared to non-stressed rats. The reduction of cecal content of isobutyrate and total butyrate, but not butyrate alone, was correlated with lower visceral pain response. Additionally, ERF diet increased rWAS-induced defecation by 26% and 75% during the first 0-15 min and last 15-60 min, respectively, compared to standard diet, and reduced rats' body weight gain by 1.3 fold independently of their stress status. These data provide the first evidence of psychological stress-related visceral analgesia in rats that was enhanced by chronic intake of ERF prebiotic.

213 NEUROGENIC BLADDER CAUSES MARKED BLADDER REMODELING AND ALTERED PAIN RESPONSE IN MICE WITH EXPERIMENTAL AUTOIMMUNE ENCEPHALOMYELITIS

You have accessJournal of UrologyBladder and Urethra: Anatomy, Physiology and Pharmacology1 Apr 2011213 NEUROGENIC BLADDER CAUSES MARKED BLADDER REMODELING AND ALTERED PAIN RESPONSE IN MICE WITH EXPERIMENTAL AUTOIMMUNE ENCEPHALOMYELITIS Andrew Horowitz, Cengiz Altuntas, Ahmet Ozer, Kenan Izgi, Fuat Bicer, and Firouz Daneshgari Andrew HorowitzAndrew Horowitz Cleveland, OH More articles by this author , Cengiz AltuntasCengiz Altuntas Cleveland, OH More articles by this author , Ahmet OzerAhmet Ozer Cleveland, OH More articles by this author , Kenan IzgiKenan Izgi Cleveland, OH More articles by this author , Fuat BicerFuat Bicer Cleveland, OH More articles by this author , and Firouz DaneshgariFirouz Daneshgari Cleveland, OH More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2011.02.323AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Multiple sclerosis (MS) is an inflammatory disease of the central nervous system which causes neurogenic bladder (NGB). Experimental autoimmune encephalomyelitis (EAE) mice have been widely used in MS research as they demonstrate similar T cell mediated demyelination and neurodegeneration. We have previously reported the evidence of NGB in EAE mice. The aim of this study was to examine the stages of neurologic deficit in relation to bladder remodeling, and examine the EAE mice responses to visceral pain. METHODS Female SJL mice (n=118) were immunized with mycobacteria tuberculosis along with Freund's adjuvant (control), and then of this sample, 100 mice were injected with myelin protolipid protein (PLP 139–151) to induce EAE. On days 0, 3, and 7 all mice received intraperitoneal injection of purified Bordetella pertussis toxin. Daily weights, and neurologic clinical scores (CS) were assessed for signs of neurodegeneration and graded as 1–5. Then, the animals were divided into two groups. The first group (n=42) was euthanized at 70 days, bladders were harvested and weighed, and then prepared with routine hematoxylin-eosin and Masson's trichrome staining. Digital imaging analysis was used to quantify bladder cross-sectional areas, and smooth muscle, urothelium, and collagen (Image-Pro Plus). The second group (n=76) was assessed after short and long term (day 70) for signs of neurodegeneration (CS 1–5), along with visceral pain responses from perpendicular pressure with calibrated von Frey hair monofilaments to the hind paw and supra-pubic region. RESULTS Significant increases in selective bladder areas of control compared to EAE (CS4) for total wall, urothelium, smooth muscle, collagen, and inner lumen (2.31 vs. 3.50, P=0.014; 0.3078 vs. 0.62, P=0.001; 1.49 vs. 2.00, P=0.036; 0.50 vs. 0.87, P=0.021; 0.22 vs. 0.71, P=0.001, respectively) were seen. Also noted were statistically significant increases from the outer perimeter (P=0.004), and percent of urothelium and collagen (P=0.0008 and P=0.004, respectively). Along with CS increase , the threshold response (>50%) to pain stimuli dramatically increased and visceral pain response decreased with short term supra-pubic area of 2.83g vs. 3.84g; and hind paw 3.61g vs. >4.56g (control vs. CS3, respectively). CONCLUSIONS EAE causes neurologic deficit in mice and contributes to marked bladder remodeling and a dramatic decrease in visceral pain response, which proportionally worsens as the neurodegeneration progresses. The EAE mouse could be used as a robust tool for studies of NGB. © 2011 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 185Issue 4SApril 2011Page: e88 Peer Review Report Advertisement Copyright & Permissions© 2011 by American Urological Association Education and Research, Inc.MetricsAuthor Information Andrew Horowitz Cleveland, OH More articles by this author Cengiz Altuntas Cleveland, OH More articles by this author Ahmet Ozer Cleveland, OH More articles by this author Kenan Izgi Cleveland, OH More articles by this author Fuat Bicer Cleveland, OH More articles by this author Firouz Daneshgari Cleveland, OH More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...

Involvement of the transient receptor potential vanilloid 1 (TRPV1) in the development of acute visceral hyperalgesia during colorectal distension in rats

Transient receptor potential vanilloid 1 (TRPV1) channels have been implicated in pain mechanisms and, particularly, in the development of hyperalgesia. We used selective TRPV1 antagonists (NGV-1, SB-750364 and JYL 1421) to assess the role of TRPV1 channels in repetitive noxious colorectal distension (CRD)-induced visceral pain responses in rats. Isobaric CRD (80 mmHg) induced a viscerosomatic response, indicative of visceral pain associated to the distension procedure. Repetition (12 consecutive distensions) of the CRD resulted in an increase in the response over time (119 ± 23% increase at distension 12, P < 0.05 vs response during the 1st distension) indicative of acute mechanical sensitization. NGV-1 (0.1, 0.3, 1 or 3 μmol/kg, i.v.) prevented in a dose-related manner the development of sensitization, without inducing hypoalgesic responses. SB-750364 (30 μmol/kg, i.v.) had a transitory effect, partially reducing the sensitization response, while JYL 1421 (4.7 μmol/kg, i.v.) was without effect. In the same conditions, the cannabinoid receptor 1 (CB 1) agonist, WIN55,212-2 (0.1 μmol/kg) reduced pain responses leading to a hypoalgesic state. At 3 μmol/kg, NGV-1, did not affect the pressure–volume relationship during CRD, indicating that TRPV1 channels do not modulate colonic compliance. These observations suggest that TRPV1 channels are involved in the development of acute mechanical colonic hyperalgesia during repetitive noxious CRD in rats. Antagonism of TRPV1 channels might result in antihyperalgesic effects without hypoalgesic activity and might be beneficial in the treatment of visceral pain disorders, such as irritable bowel syndrome. These observations warrant the clinical assessment of TRPV1 antagonists for the treatment of visceral pain.

RETRACTED: Up-Regulation of Anterior Cingulate Cortex NR2B Receptors Contributes to Visceral Pain Responses in Rats

Electrophysiologic and behavioral studies have shown that increased N-methyl-D-aspartate (NMDA)-receptor activation of anterior cingulate cortex (ACC) neurons has a critical role in modulating visceral pain responses in viscerally hypersensitive (VH) rats. This study aimed to identify the NMDA receptor subtypes in perigenual ACC (pACC) neurons involved in the facilitation of visceral nociception. We performed in vivo electrophysiologic recordings of pACC neurons and examined the visceromotor response (VMR) to colorectal distention (CRD) in normal and VH rats induced by colonic anaphylaxis. The NR2A-subtype-receptor antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) and the NR2B-receptor-antagonist Ro25-6981 were microinjected into the pACC. To down-regulate NR2B-receptor gene expression, an NR2B-specific small interfering RNA (siRNA) and a plasmid (pEGFP-N1) that expressed the green fluorescent protein were administered into ACC neurons by electroporation. Reverse microdialysis of NVP-AAM077 had no effect on basal and CRD-induced ACC neuronal firing in VH and control groups. In VH rats, Ro25-6981 (500 micromol/L) inhibited ACC neuronal firing, evoked by 30 and 50 mm Hg CRD, by 98% and 52%, respectively. NVP-AAM077 did not affect the VMR in either group. Ro25-6981 significantly suppressed the VMR in VH but not normal rats. Immunoblot analysis showed increased NR2B-receptor expression in the pACC of VH rats. NR2B siRNA-treated VH rats showed a significant reduction in the VMR, compared with controls. The NR2B subunit of the NMDA receptor has a critical role in the modulation of ACC sensitization and visceral pain responses in VH rats.

Tuning Thalamic Firing Modes via Simultaneous Modulation of T- and L-Type Ca2+Channels Controls Pain Sensory Gating in the Thalamus

Two firing modes of thalamocortical (TC) neurons, tonic and burst firings, are thought to reflect the divergent states of sensory signal transmission from the thalamus to the cortex. However, the behavioral consequences of changes in the thalamic firing between the two modes have not been well demonstrated. Moreover, although the firing modes of TC neurons are known to be affected by corticothalamic inputs via thalamic metabotropic glutamate receptor type 1 (mGluR1)-phospholipase C beta4 (PLCbeta4) pathway, its molecular mechanisms have not been well elucidated. We addressed these questions using PLCbeta4-deficient mice, which show decreased visceral pain responses. We demonstrate that burst and tonic firings of TC neurons are concomitantly regulated by PLCbeta4 pathway. Blocking of this pathway by the mutation simultaneously increases bursting and decreases tonic firing of TC neurons through concurrent upregulation of T- and L-type Ca(2+) currents. The mice with increased bursting and decreased tonic firing of TC neurons showed reduced visceral pain responses. Furthermore, we show that modulation of the Ca(2+) channels or protein kinase C (PKC), a downstream molecule of PLCbeta4, altered the firing modes of TC neurons and pain responses in the predicted ways. Our data demonstrate the molecular mechanism and behavioral consequences of altered firing modes of TC neurons in relaying the visceral pain signals. Our study also highlights the thalamic PLCbeta4-PKC pathway as a "molecular switch" for the firing modes of TC neurons and thus for pain sensory gating.

Effects of pregabalin on visceral pain responses and colonic compliance in rats

Pregabalin, which binds to the alpha2-delta subunit of voltage-gated calcium channels, increased the threshold for pain during colorectal distension (CRD) in irritable bowel syndrome (IBS) patients. We tested the effects of oral pregabalin on the visceral pain-related viscerosomatic and autonomic cardiovascular responses to CRD and colonic compliance in rats. The activity of the abdominal musculature (viscerosomatic response), monitored by electromyography and intracolonic manometry, and changes in blood pressure and heart rate, monitored by telemetry, were assessed simultaneously in conscious rats during CRD. Pregabalin (10-200 micromol kg(-1), p.o.) inhibited dose dependently the viscerosomatic response to phasic, noxious CRD (12 distensions at 80 mm Hg). At 200 mumol kg(-1), pregabalin also reduced the increase in blood pressure and heart rate associated with noxious CRD. Moreover, pregabalin (200 micromol kg(-1), p.o.) reduced the visceromotor response to ascending phasic CRD (10-80 mm Hg) and significantly increased the threshold pressure for response. During phasic CRD (2-20 mm Hg), pregabalin (200 micromol kg(-1), p.o.) increased intracolonic volume, resulting in a shift to the left of the pressure-volume relationship curve, indicative of an increase of compliance. Pregabalin reduced the viscerosomatic and autonomic responses associated with CRD-induced visceral pain and increased colonic compliance in rats. These observations confirm the analgesic activity of pregabalin on visceral pain and support the translational value of the CRD model to humans. Ligands for the alpha2-delta subunit might represent interesting compounds for the treatment of visceral pain disorders, such as IBS.

Involvement of κ‐opioid receptors in visceral nociception in mice

It has been shown that the behavioural responses to chemically evoked visceral nociception are increased in transgenic mice lacking the kappa-opioid receptor (KOR). The aim of the present study was to evaluate the contribution of KOR in mechanically evoked visceral pain by performing colorectal distension (CRD) and monitoring the subsequent visceromotor response (VMR) in control mice (KOR(+/+)) and in mice lacking KOR (KOR(-/-)). Pseudo-affective visceral pain responses were evoked in conscious mice using increasing (10-80 mmHg) and repeated (12 x 55 mmHg) phasic CRD paradigms. The resulting VMR was determined by monitoring the electromyographic activity of the abdominal muscle. The increasing and repeated CRD paradigms, respectively, evoked similar responses in both KOR(+/+) and KOR(-/-) mice. The selective KOR-agonists U-69593 (5 and 25 mg kg(-1), s.c.) and asimadoline (25 mg kg(-1), s.c.) significantly decreased the VMR in KOR(+/+) mice, while having no effect in KOR(-/-) mice. In contrast, the selective mu-opioid receptor agonist fentanyl significantly reduced the VMR in both types of mice and appeared more efficacious in KOR(-/-) mice. The opioid receptor antagonist naloxone (0.3-30 mg kg(-1) s.c.) did not affect the response to CRD in C57BL/6 mice at any dose tested. In conclusion, the data confirm that the KOR agonists used in this study inhibit the VMR to CRD in mice by acting via KOR receptors. In addition, the data suggest that the endogenous opioid system is not likely to modulate the VMR to mechanically evoked visceral pain in mice.