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Background and Purpose Experimental Approach Key Results Conclusion and Implications Tolerance to the analgesic effects of opioids and resultant dose escalation is associated with worsening of side effects and greater addiction risk. Here, we compare the development of tolerance to the conventional opioid fentanyl with a novel pH‐sensitive μ‐opioid receptor (MOR) agonist, (±)‐N‐(3‐fluoro‐1‐phenethylpiperidine‐4‐yl)‐N‐phenyl propionamide (NFEPP) that is active only in acidic inflammatory microenvironments.An opioid tolerance model was developed in male C57BL/6 mice, with and without dextran sulphate sodium colitis, using increasing doses of either fentanyl or NFEPP over 5 days. Visceral nociception was assessed in vivo by measuring visceromotor responses (VMRs) to noxious colorectal distensions and in vitro measuring colonic afferent nerve activity of mesenteric nerves and performing patch‐clamp recordings from isolated dorsal root ganglia neurons. Somatic thermal nociception was tested using a tail immersion assay. Cardiorespiratory effects were analysed by pulse oximeter experiments.VMRs and tail immersion tests demonstrated tolerance to fentanyl, but not to NFEPP in colitis mice. Cross‐tolerance also occurred to fentanyl, but not to NFEPP. The MOR agonist DAMGO inhibited colonic afferent nerve activity in colitis mice exposed to chronic NFEPP, but not those from fentanyl‐treated mice. Similarly, in patch‐clamp recordings from isolated dorsal root ganglia neurons, DAMGO inhibited neurons from NFEPP‐, but not fentanyl‐treated mice.NFEPP did not exhibit tolerance in an inflammatory pain model, unlike fentanyl. Consequently, dose escalation to maintain analgesia during an evolving inflammation could be avoided, mitigating the potential risk of side effects. [ABSTRACT FROM AUTHOR]

The nucleus raphe magnus (RMg) is a key structure of the endogenous antinociceptive system, the activity of which is regulated by serotonin 5-HT1A receptors. A recipient of the RMg descending projections is the caudal ventrolateral medulla (cVLM)—the first supraspinal center for processing visceral and somatic pain signals. Intestinal pathology is known to cause persistent functional alterations in the RMg, which are associated with the development of visceral and somatic hyperalgesia. Presumably, a consequence of the alterations may be changes in the RMg modulating effects on cVLM nociceptive activity. However, the specific neuronal and molecular mechanisms underlying such influence in normal conditions, as well as their changes in pathology remain unexplored. The aim of our neurophysiological experiments performed in anesthetized adult male Wistar rats was to compare the effects of RMg electrical stimulation on the activity of cVLM neurons evoked by visceral (colorectal distension, CRD) and somatic (tail squeezing) pain stimulations that occur in normal conditions and after intestinal inflammation (colitis), with an assessment of the contribution to these processes of the supraspinal 5-HT1A receptor activation with intracerebroventricular buspirone. It has been shown that RMg can exert an inhibitory effect on both non-selective and differential responses of the cVLM neurons to diverse pain stimuli, causing a weakening of excitatory neuronal reactions and an increase in inhibitory responses to CRD while inhibiting both types of reactions to tail squeezing. The RMg-evoked suppression of nociceptive excitation in the caudal medullary neurons is enhanced under activation of supraspinal 5-HT1A receptors by buspirone. It has been established that in postcolitis period the RMg inhibitory action on different populations of cVLM neurons are significantly diminished, indicating an impairment of the nucleus' antinociceptive function. In these conditions, the RMg descending influence loses its 5-HT1A receptor-dependent component. The changes described may contribute to the supraspinal mechanisms underlying pathogenesis of post-inflammatory abdominal pain and comorbid somatic hyperalgesia. [ABSTRACT FROM AUTHOR]

Thymoquinone (TQ) is the main biologically active substance of Nigella sativa (black seeds). It has anti-cancer, anti-inflammatory, anti-diabetic, anti-oxidative and anti-nociceptive properties. This study was aimed to explore the effect of TQ on acetic acid-induced visceral nociception. The central mechanisms of the effect of TQ were investigated using cannabinergic (AM251) and a2-adrenergic (yohimbine [Yoh]) antagonists. The lateral ventricle of the brain was cannulated for intracerebroventricular (ICV) injections. Visceral nociception was induced by intra-peritoneal (IP) injection of acetic acid (1.00% in a volume of 1.00 mL). Measuring the latency time to the first writhing appearance and counting the number of writhing in 5-min intervals for a period of 60 min were performed. Locomotor activity was determined using an open-field test. Oral administration (PO) of 2.50 and 10.00 mg kg-1 TQ increased the latency time to the first writhing appearance and decreased the number of writhing. The AM251 (5.00 µg per rat; ICV) and Yoh (5.00 µg per rat; ICV) partially prevented TQ (10.00 mg kg-1; PO)-induced anti-nociception. Locomotor activity was not altered by these treatments. The results of the present study showed that TQ had the ability to reduce visceral nociception caused by IP injection of acetic acid. The central mechanisms of this action of TQ might be partially mediated by cannabinergic and a2-adrenegic receptors. [ABSTRACT FROM AUTHOR]

The cerebellum and its deep nuclei contribute to the regulation of important functions including motor coordination and pain. Histamine modulates some functions of the fastigial nucleus (FN) such as motor coordination. In this study, by application of histamine and activation of its H1 and H2 receptors, the FN processing of visceral pain, general locomotor activity and motor coordination were targeted. The possible mechanism of action was followed by the inhibition of opioid receptors. The right and left sides of the FN were surgically implanted with guide cannulas. Immediately after an intraperitoneal injection of acetic acid (1.00 mL, 1.00%), the first writhing onset latency and the writhing number over 60 min were recorded. Open-field and rotarod tests were applied for general locomotor and motor coordination assessment, respectively. Histamine and dimaprit (H2 receptor agonist) increased first writhing onset latency, decreased the writhing number and increased falling time from the rod. These effects were prevented by ranitidine (H2 receptor antagonist) pre-treatment. Significant alterations were not observed by histamine H1 receptor agonist (2-pyridylethylamine) and antagonist (mepyramine). Naloxone, with no effect on falling time from the rod, inhibited the antinociceptive effects of histamine and dimaprit. Beam break number was not affected by the above-mentioned treatments. Based on the results, it can be suggested that histamine H2, but not H1 receptors at the FN might have had an inhibitory role on acetic acid-induced visceral pain and improved motor coordination. The antinociception, but not motor coordination might be mediated by FN opioid receptors. [ABSTRACT FROM AUTHOR]

CaV2.3 channels are subthreshold voltage-gated calcium channels that play crucial roles in neurotransmitter release and regulation of membrane excitability, yet modulation of these channels with endogenous molecules and their role in pain processing is not well studied. Here, we hypothesized that an endogenous amino acid l-cysteine could be a modulator of these channels and may affect pain processing in mice. To test this hypothesis, we employed conventional patch-clamp technique in the whole-cell configuration using recombinant CaV2.3 subunit stably expressed in human embryonic kidney (HEK-293) cells. We found in our in vitro experiments that l-cysteine facilitated gating and increased the amplitudes of recombinant CaV2.3 currents likely by chelating trace metals that tonically inhibit the channel. In addition, we took advantage of mouse genetics in vivo using the acetic acid visceral pain model that was performed on wildtype and homozygous Cacna1e knockout male littermates. In ensuing in vivo experiments, we found that l-cysteine administered both subcutaneously and intraperitoneally evoked more prominent pain responses in the wildtype mice, while the effect was completely abolished in knockout mice. Conversely, intrathecal administration of l-cysteine lowered visceral pain response in the wildtype mice, and again the effect was completely abolished in the knockout mice. Our study strongly suggests that l-cysteine-mediated modulation of CaV2.3 channels plays an important role in visceral pain processing. Furthermore, our data are consistent with the contrasting roles of CaV2.3 channels in mediating visceral nociception in the peripheral and central pain pathways. [ABSTRACT FROM AUTHOR]

Introduction: The world population is ageing, resulting in increased prevalence of age-related comorbidities and healthcare costs. Limited data are available on intestinal health in elderly populations. Structural and functional changes, including altered visceroperception, may lead to altered bowel habits and abdominal symptoms in healthy individuals and irritable bowel syndrome (IBS) patients. Our aim was to explore age-related changes in gastrointestinal symptoms and underlying mechanisms. Methods: In total, 780 subjects (IBS patients n = 463, healthy subjects n = 317) from two separate studies were included. Subjects were divided into different age groups ranging from young adult to elderly. Demographics and gastrointestinal symptom scores were collected from all participants using validated questionnaires. A subset of 233 IBS patients and 103 controls underwent a rectal barostat procedure to assess visceral hypersensitivity. Sigmoid biopsies were obtained from 10 healthy young adults and 10 healthy elderly. Expression of the visceral pain-associated receptors transient receptor potential (TRP) Ankyrin 1 (TRPA1) and Vanilloid 1 (TRPV1) genes were investigated by quantitative RT-PCR and immunofluorescence. Results: Both elderly IBS and healthy individuals showed significantly lower scores for abdominal pain (p < 0.001) and indigestion (p < 0.05) as compared to respective young adults. Visceral hypersensitivity was less common in elderly than young IBS patients (p < 0.001). Relative TRPA1 gene transcription, as well as TRPA1 and TRPV1 immunoreactivity were significantly lower in healthy elderly versus healthy young adults (p < 0.05). Conclusions: Our findings show an age-related decrease in abdominal pain perception. This may in part be related to decreased TRPA1 and/or TRPV1 receptor expression. Further studies are needed to reveal precise underlying mechanisms and the associations with intestinal health. [ABSTRACT FROM AUTHOR]

Introduction: The world population is ageing, resulting in increased prevalence of age-related comorbidities and healthcare costs. Limited data are available on intestinal health in elderly populations. Structural and functional changes, including altered visceroperception, may lead to altered bowel habits and abdominal symptoms in healthy individuals and irritable bowel syndrome (IBS) patients. Our aim was to explore age-related changes in gastrointestinal symptoms and underlying mechanisms.Methods: In total, 780 subjects (IBS patients n = 463, healthy subjects n = 317) from two separate studies were included. Subjects were divided into different age groups ranging from young adult to elderly. Demographics and gastrointestinal symptom scores were collected from all participants using validated questionnaires. A subset of 233 IBS patients and 103 controls underwent a rectal barostat procedure to assess visceral hypersensitivity. Sigmoid biopsies were obtained from 10 healthy young adults and 10 healthy elderly. Expression of the visceral pain-associated receptors transient receptor potential (TRP) Ankyrin 1 (TRPA1) and Vanilloid 1 (TRPV1) genes were investigated by quantitative RT-PCR and immunofluorescence.Results: Both elderly IBS and healthy individuals showed significantly lower scores for abdominal pain (p < 0.001) and indigestion (p < 0.05) as compared to respective young adults. Visceral hypersensitivity was less common in elderly than young IBS patients (p < 0.001). Relative TRPA1 gene transcription, as well as TRPA1 and TRPV1 immunoreactivity were significantly lower in healthy elderly versus healthy young adults (p < 0.05).Conclusions: Our findings show an age-related decrease in abdominal pain perception. This may in part be related to decreased TRPA1 and/or TRPV1 receptor expression. Further studies are needed to reveal precise underlying mechanisms and the associations with intestinal health.

Thymoquinone (TQ) is the main biologically active substance of Nigella sativa (black seeds). It has anti-cancer, anti-inflammatory, anti-diabetic, anti-oxidative and anti-nociceptive properties. This study was aimed to explore the effect of TQ on acetic acid-induced visceral nociception. The central mechanisms of the effect of TQ were investigated using cannabinergic (AM251) and α 2 -adrenergic (yohimbine [Yoh]) antagonists. The lateral ventricle of the brain was cannulated for intracerebroventricular (ICV) injections. Visceral nociception was induced by intra-peritoneal (IP) injection of acetic acid (1.00% in a volume of 1.00 mL). Measuring the latency time to the first writhing appearance and counting the number of writhing in 5-min intervals for a period of 60 min were performed. Locomotor activity was determined using an open-field test. Oral administration (PO) of 2.50 and 10.00 mg kg -1 TQ increased the latency time to the first writhing appearance and decreased the number of writhing. The AM251 (5.00 µg per rat; ICV) and Yoh (5.00 µg per rat; ICV) partially prevented TQ (10.00 mg kg -1 ; PO)-induced anti-nociception. Locomotor activity was not altered by these treatments. The results of the present study showed that TQ had the ability to reduce visceral nociception caused by IP injection of acetic acid. The central mechanisms of this action of TQ might be partially mediated by cannabinergic and α 2 -adrenegic receptors., Competing Interests: No financial or other conflicts of interest are declared by the authors., (© 2024 Urmia University. All rights reserved.)

Metabotropic glutamate receptor 5 (mGluR5) and transient receptor potential vanilloid subtype 1 (TRPV1) have been shown to play critical roles in the transduction and modulation of cutaneous nociception in the central nervous system. However, little is known regarding the possible involvement of mGluR5 and TRPV1 in regulating visceral nociception from the uterine cervix. In this study, we used a rat model of uterine cervical distension to examine the effects of noxious stimuli to the uterine cervix on expression of spinal mGluR5 and TRPV1. Our findings included the following: (1) uterine cervical distension resulted in a stimulus-dependent increase in electromyographic, spinal c-Fos signal, and expression of mGluR5 and TRPV1 in the spinal cord; (2) intrathecal administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyri-dine significantly reduced the increased TRPV1 and c-Fos expression induced by uterine cervical distension; (3) the TRPV1 inhibitor SB-366791 inhibited increased spinal c-Fos expression but had no effect on the expression of mGluR5 in response to uterine cervical distension. Our findings indicate that the spinal mGluR5-TRPV1 pathway modulates nociceptive transmission in uterine cervical distension-induced pathological visceral pain. [ABSTRACT FROM AUTHOR]

Peripheral sensitization of nociceptors during disease has long been recognized as a leading cause of inflammatory pain. However, a growing body of data generated over the last decade has led to the increased understanding that peripheral sensitization is also an important mechanism driving abdominal pain in highly prevalent functional bowel disorders, in particular, irritable bowel syndrome (IBS). As such, the development of drugs that target pain-sensing nerves innervating the bowel has the potential to be a successful analgesic strategy for the treatment of abdominal pain in both organic and functional gastrointestinal diseases. Despite the success of recent peripherally restricted approaches for the treatment of IBS, not all drugs that have shown efficacy in animal models of visceral pain have reduced pain end points in clinical trials of IBS patients, suggesting innate differences in the mechanisms of pain processing between rodents and humans and, in particular, how we model disease states. To address this gap in our understanding of peripheral nociception from the viscera and the body in general, several groups have developed experimental systems to study nociception in isolated human tissue and neurons, the findings of which we discuss in this review. Studies of human tissue identify a repertoire of human primary afferent subtypes comparable to rodent models including a nociceptor population, the targeting of which will shape future analgesic development efforts. Detailed mechanistic studies in human sensory neurons combined with unbiased RNAsequencing approaches have revealed fundamental differences in not only receptor/channel expression but also peripheral pain pathways. [ABSTRACT FROM AUTHOR]

Therapeutic use of GABA B receptor agonists for conditions like chronic abdominal pain, overactive bladder (OAB) and gastroesophageal reflux disease (GERD) is severely affected by poor blood-brain barrier permeability and potential side effects. ADX71441 is a novel positive allosteric modulator (PAM) of the GABA B receptor that has shown encouraging results in pre-clinical models of anxiety, pain, OAB and alcohol addiction. The present study investigates the analgesic effect of ADX71441 to noxious stimulation of the urinary bladder and colon in rats. In female Sprague-Dawley rats, systemic (i.p), but not intrathecal (i.t), administration of ADX71441 produced a dose-dependent decrease in viscero-motor response (VMR) to graded urinary bladder distension (UBD) and colorectal distension (CRD). Additionally, intra-cerebroventricular (i.c.v.) administration of ADX71441 significantly decreased the VMRs to noxious UBD. In electrophysiology experiments, the drug did not attenuate the responses of UBD-sensitive pelvic nerve afferent (PNA) fibers to UBD. In contrast, ADX71441 significantly decreased the responses of UBD-responsive lumbosacral (LS) spinal neurons in spinal intact rats. However, ADX71441 did not attenuate these LS neurons in cervical (C1-C2) spinal transected rats. During cystometrogram (CMG) recordings, ADX71441 (i.p.) significantly decreased the VMR to slow infusion without affecting the number of voiding contraction. These results indicate that ADX71441 modulate bladder nociception via its effect at the supra-spinal sites without affecting the normal bladder motility and micturition reflex in naïve adult rats. [ABSTRACT FROM AUTHOR]

Key points Voltage-gated sodium channels play a fundamental role in determining neuronal excitability., Specifically, voltage-gated sodium channel subtype NaV1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown., Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for NaV1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling., These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality-specific manner and help to direct drug discovery efforts towards novel visceral analgesics., Abstract Voltage-gated sodium channel NaV1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of NaV1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor-specific NaV1.7 knockout mouse (NaV1.7Nav1.8) and selective small-molecule NaV1.7 antagonist PF-5198007. NaV1.7Nav1.8 mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both NaV1.7Nav1.8 and littermate controls. Loss, or blockade, of NaV1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve-gut preparations in mouse, or following antagonism of NaV1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage-gated sodium channel α subunits revealed NaV1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of NaV1.7 (in NaV1.8-expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective NaV1.7 antagonist PF-5198007. Our data demonstrate that NaV1.7 (in NaV1.8-expressing neurons) contributes to defined pain pathways in a modality-dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of NaV1.7 alone in the viscera may be insufficient in targeting chronic visceral pain. [ABSTRACT FROM AUTHOR]

Objective Little is understood regarding how disease progression alters immune and sensory nerve function in colitis. We investigated how acute colitis chronically alters immune recruitment and the impact this has on re-activated colitis. To understand the impact of disease progress on sensory systems we investigated the mechanisms underlying altered colonic neuro-immune interactions after acute colitis. Design Inflammation was compared in mouse models of health, acute tri-nitrobenzene sulphonic acid (TNBS) colitis, Remission and Reactivated colitis. Cytokine concentrations were compared by ELISA in-situ and in explanted colon tissue. Colonic infiltration by CD11b/F4-80 macrophage, CD4 T HELPER (T H ) and CD8 T CYTOTOXIC (T C ) and α 4 β 7 expression on mesenteric lymph node (MLN) T H and T C was determined by flow cytometry. Cytokine and effector receptor mRNA expression was determined on colo-rectal afferent neurons and the mechanisms underlying cytokinergic effects on high-threshold colo-rectal afferent function were investigated using electrophysiology. Results Colonic damage, MPO activity, macrophage infiltration, IL-1β and IL-6 concentrations were lower in Reactivated compared to Acute colitis. T H infiltration and α 4 β 7 expression on T H MLN was increased in Remission but not Acute colitis. IFN-γ concentrations, T H infiltration and α 4 β 7 expression on T H and T C MLN increased in Reactivated compared to Acute colitis. Reactivated explants secreted more IL-1β and IL-6 than Acute explants. IL-6 and TNF-α inhibited colo-rectal afferent mechanosensitivity in Remission mice via a BK Ca dependent mechanism. Conclusions Acute colitis persistently alters immune responses and afferent nerve signalling pathways to successive episodes of colitis. These findings highlight the complexity of viscero-sensory neuro-immune interactions in painful remitting and relapsing diseases. [ABSTRACT FROM AUTHOR]

The cerebellum and its deep nuclei contribute to the regulation of important functions including motor coordination and pain. Histamine modulates some functions of the fastigial nucleus (FN) such as motor coordination. In this study, by application of histamine and activation of its H 1 and H 2 receptors, the FN processing of visceral pain, general locomotor activity and motor coordination were targeted. The possible mechanism of action was followed by the inhibition of opioid receptors. The right and left sides of the FN were surgically implanted with guide cannulas. Immediately after an intraperitoneal injection of acetic acid (1.00 mL, 1.00%), the first writhing onset latency and the writhing number over 60 min were recorded. Open-field and rotarod tests were applied for general locomotor and motor coordination assessment, respectively. Histamine and dimaprit (H 2 receptor agonist) increased first writhing onset latency, decreased the writhing number and increased falling time from the rod. These effects were prevented by ranitidine (H 2 receptor antagonist) pre-treatment. Significant alterations were not observed by histamine H 1 receptor agonist (2-pyridylethylamine) and antagonist (mepyramine). Naloxone, with no effect on falling time from the rod, inhibited the antinociceptive effects of histamine and dimaprit. Beam break number was not affected by the above-mentioned treatments. Based on the results, it can be suggested that histamine H 2 , but not H 1 receptors at the FN might have had an inhibitory role on acetic acid-induced visceral pain and improved motor coordination. The antinociception, but not motor coordination might be mediated by FN opioid receptors., Competing Interests: No financial or other conflicts of interest are declared by the authors., (© 2023 Urmia University. All rights reserved.)

Background We previously reported estrogen modulates spinal N-methyl- d-aspartate ( NMDA) receptor processing of colorectal pain through changes in spinal GluN1 subunit phosphorylation/expression. The purpose of this study was to investigate whether spinal GluN2B containing NMDA receptors are involved in estrogen modulation of visceral pain processing. Methods Behavioral, molecular, and immunocytochemical techniques were used to determine spinal GluN2B expression/phosphorylation and function 48 h following subcutaneous injection of estradiol (E2) or vehicle (safflower oil, Saff oil) in ovariectomized rats in the absence or presence of colonic inflammation induced by mustard oil. Key Results E2 increased the magnitude of the visceromotor response ( VMR) to colorectal distention compared to Saff oil in non-inflamed rats. Intrathecal injection of the GluN2B subunit antagonist, Ro 25-6981, had no effect on the VMR in non-inflamed E2 or Saff oil rats. Colonic inflammation induced visceral hyperalgesia in E2, but not Saff oil rats. Visceral hyperalgesia in E2 rats was blocked by intrathecal GluN2B subunit selective antagonists. In inflamed rats, E2 increased GluN2B protein and gene expression in the thoracolumbar ( TL), but not lumbosacral ( LS), dorsal spinal cord. Immunocytochemical labeling showed a significant increase in GluN2B subunit in the superficial dorsal horn of E2 rats compared to Saff oil rats. Conclusions & Inferences These data support the hypothesis that estrogen increases spinal processing of colonic inflammation-induced visceral hyperalgesia by increasing NMDA receptor activity. Specifically, an increase in the activity of GluN2B containing NMDA receptors in the TL spinal cord by estrogen underlies visceral hypersensitivity in the presence of colonic inflammation. [ABSTRACT FROM AUTHOR]

The effects of intrathecal administration of the toxins Phα 1 β and ω-conotoxin MVIIA were investigated in visceral nociception induced by an intraperitoneal injection of acetic acid and an intracolonic application of capsaicin. The pretreatments for 2 h with the toxins reduced the number of writhes or nociceptive behaviors compared with the control mice. Phα1β administration resulted in an Imax of 84 ± 6 and an ID 50 of 12 (5–27), and ω-conotoxin MVIIA resulted in an Imax of 82 ± 9 and an ID 50 of 11 (4–35) in the contortions induced by the intraperitoneal injection of acetic acid. The administration of Phα1β resulted in an Imax of 64 ± 4 and an ID 50 of 18 (9–38), and ω-conotoxin MVIIA resulted in an Imax of 71 ± 9 and an ID 50 of 9 (1–83) in the contortions induced by intracolonic capsaicin administration. Phα1β (100/site) or ω-conotoxin MVIIA (30 pmol/site) pretreatments caused a reduction in CSF glutamate release in mice intraperitoneally injected with acetic acid or treated with intracolonic capsaicin. The toxin pretreatments reduced the ROS levels induced by intraperitoneal acetic acid injection. Phα1β, but not ω-conotoxin MVIIA, reduced significantly the ROS levels induced by intracolonic capsaicin administration. Perspective: Phα1β is a ω-toxin with high therapeutic index and a broader action on calcium channels. It shows analgesic effect in several rodents' models of pain, including visceral pain, suggesting that this toxin has the potential to be used in clinical setting as a drug in the control of persistent pathological pain. [ABSTRACT FROM AUTHOR]

Prostaglandin E2 (PGE2) plays a major role in pain processing and hypersensitivity. This study investigated whether PGE2 levels are increased in the esophageal mucosa after acid infusion and whether increases in PGE2 are associated with heartburn. Furthermore, expression of the PGE2 receptor EP1 was investigated in human esophageal mucosa. Fourteen healthy male volunteers were randomized to 30-min lower esophageal acid (1% HCl) or saline perfusion. Before and after acid perfusion, endoscopic biopsies were taken from the distal esophagus. PGE2 concentration (pg/mg protein) and EP1 mRNA and protein in biopsy samples were measured by ELISA, RT-PCR, and Western blotting. Symptom status of heartburn was evaluated with a validated categorical rating scale with a higher values corresponding to increasing intensity. PGE2 levels in the esophageal mucosa significantly increased after acid infusion (before vs. after acid infusion: 23.2 ± 8.6 vs. 68.6 ± 18.3, P < 0.05), but not after saline infusion (before vs. after saline infusion: 9.3 ± 2.5 vs. 9.0 ± 3.2, NS). Time to first sensation (min) after acid infusion was less than after saline (saline vs. acid infusion: 22.1 ± 4.1 vs. 5.4 ± 1.5, P < 0.05). Intensity of heartburn in the acid-infusion group was also significantly greater compared with saline (saline vs. acid infusion: 54.3 ± 13.1 vs. 178.5 ± 22.8, P < 0.01). Changes in PGE2 levels in the esophagus correlated with symptom intensity score (r = 0.80, P = 0.029). EP1 mRNA and protein expression were observed in the normal human esophageal mucosa. Esophageal PGE2 expression is associated with mucosal acid exposure and heartburn. [ABSTRACT FROM AUTHOR]

Rationale: Peroxisome proliferator-activated receptors (PPARs) participate in the control of chronic neuropathic and inflammatory pain, and these receptors could play a role on acute pain. Objectives: We used null (PPAR-α −/−) and wild-type female mice and the PPAR-α blocker GW6471 to evaluate (1) the role of PPAR-α on neuropathic pain, (2) the involvement of PPAR-α on visceral and acute thermal nociception, and (3) tissue levels of pro-inflammatory factors. Methods: Neuropathic pain was induced by sciatic nerve ligature. Acute thermal nociception was evaluated through hot-plate, tail-immersion, and writhing tests. The pro-inflammatory factors nitric oxide, TNF-α, and interleukins-1β and -3 were measured. Results: Regarding neuropathic pain, higher sensitivity to thermal and mechanical non-noxious and noxious stimuli was observed in mice lacking PPAR-α. Cold and mechanical allodynia and heat hyperalgesia were augmented in null mice. With respect to visceral nociception, writhes after acetic acid were enhanced in mutant mice. Although basal thermal sensitivity was enhanced in PPAR-α −/− mice, cutaneous thermal nociception did not differ between genotypes. Blockade of PPAR-α was devoid of effects on acute thermal and writhing tests. Finally, nerve ligature enhanced pro-inflammatory factors in plantar tissue, levels being higher in null mice. No changes in pro-inflammatory factors were observed in the hot-plate test. Conclusions: Genetic ablation of PPAR-α is involved in neuropathic and visceral nociception. Lack of PPAR-α is not involved in acute thermal pain, but it is involved in basal thermal reaction. Changes are biological adaptations to receptor deletion because blockade of PPAR-α does not affect inflammatory pain or thermal reactions. [ABSTRACT FROM AUTHOR]

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder, characterized by recurrent abdominal pain or discomfort in combination with disturbed bowel habits in the absence of identifiable organic cause. Visceral hypersensitivity has emerged as a key hypothesis in explaining the painful symptoms in IBS and has been proposed as a "biological hallmark" for the condition. Current techniques of assessing visceral perception include the computerized barostat using rectal distensions, registering responses induced by sensory stimuli including the flexor reflex and cerebral evoked potentials, as well as brain imaging modalities such as functional magnetic resonance imaging and positron emission tomography. These methods have provided further insight into alterations in pain processing in IBS, although the most optimal method and condition remain to be established. In an attempt to give an overview of these methods, a literature search in the electronic databases PubMed and MEDLINE was executed using the search terms "assessment of visceral pain/visceral nociception/visceral hypersensitivity" and "irritable bowel syndrome." Both original articles and review articles were considered for data extraction. This review aims to discuss currently used modalities in assessing visceral perception, along with advantages and limitations, and aims also to define future directions for methodological aspects in visceral pain research. Although novel paradigms such as brain imaging and neurophysiological recordings have been introduced in the study of visceral pain, confirmative studies are warranted to establish their robustness and clinical relevance. Therefore, subjective verbal reporting following rectal distension currently remains the best-validated technique in assessing visceral perception in IBS. [ABSTRACT FROM AUTHOR]

Abstract: Pro-nociceptive ON-cells in the rostral ventromedial medulla (RVM) facilitate nociceptive processing and contribute to descending serotonergic controls. We use RVM injections of neurotoxic dermorphin-saporin (Derm-SAP) in rats to evaluate the role of putative ON-cells, or μ-opioid receptor-expressing (MOR) neurones, in visceral pain processing. Our immunohistochemistry shows that intra-RVM Derm-SAP locally ablates a substantial proportion of MOR and serotonergic cells. Given the co-localization of these neuronal markers, some RVM ON-cells are serotonergic. We measure visceromotor responses in the colorectal distension (CRD) model in control and Derm-SAP rats, and using the 5-HT3 receptor antagonist ondansetron, we demonstrate pro-nociceptive serotonergic modulation of visceral nociception and a facilitatory drive from RVM MOR cells. The α2δ calcium channel ligand pregabalin produces state-dependent analgesia in neuropathy and osteoarthritis models relating to injury-specific interactions with serotonergic facilitations from RVM MOR cells. Although RVM MOR cells mediate noxious mechanical visceral input, we show that their presence is not a permissive factor for pregabalin analgesia in acute visceral pain. [Copyright &y& Elsevier]

Abstract: Luminal hydrogen sulfide (H2S) causes colonic pain and referred hyperalgesia in mice through activation of T-type Ca2+ channels. To test a hypothesis that H2S might chelate and remove endogenous Zn2+ that inhibits the Cav3.2 isoform of T-type Ca2+ channels, facilitating visceral nociception, we asked if intracolonic (i.col.) administration of Zn2+ chelators mimics H2S-induced visceral nociception. Visceral nociceptive behavior and referred abdominal allodynia/hyperalgesia were determined after i.col. administration of NaHS, a donor for H2S, or Zn2+ chelators in mice. Phospholylation of extracellular signal-regulated protein kinase (ERK) in the spinal cord was analyzed by immunohistochemistry. The visceral nociceptive behavior and referred abdominal allodynia/hyperalgesia caused by i.col. NaHS in mice were abolished by i.col. preadministration of zinc chloride (ZnCl2), known to selectively inhibit Cav3.2, but not Cav3.1 or Cav3.3, isoforms of T-type Ca2+ channels, and by i.p. preadministration of mibefradil, a pan-T-type Ca2+ channel blocker. Two distinct Zn2+ chelators, N,N,N'',N''-tetrakis(2-pyridylmethyl)ehylenediamine (TPEN) and dipicolinic acid, when administered i.col., mimicked the NaHS-evoked visceral nociceptive behavior and referred abdominal allodynia/hyperalgesia, which were inhibited by mibefradil and by NNC 55-0396, another T-type Ca2+ channel blocker. Like i.col. NaHS, i.col. TPEN caused prompt phosphorylation of ERK in the spinal dorsal horn, an effect blocked by mibefradil. Removal of luminal Zn2+ by H2S and other Zn2+ chelators thus produces colonic pain through activation of T-type Ca2+ channels, most probably of the Cav3.2 isoform. Hence, endogenous Zn2+ is considered to play a critical role in modulating visceral pain. [Copyright &y& Elsevier]

Abstract: The study examined the antiinflammatory and antinociceptive effects of the sesquiterpene (−)-α-bisabolol (BISA). The antiinflammatory effect was evaluated on acute models of dermatitis induced by Croton oil, arachidonic acid, phenol and capsaicin, respectively, in mouse ear. BISA inhibited the dermatitis induced by all noxious agents, except capsaicin. BISA was assessed in two established mouse models of visceral nociception. Mice were pretreated orally with BISA, and the pain-related behavioral responses to intraperitoneal cyclophosphamide or to intracolonic mustard oil were analyzed. BISA showed a dose-unrelated significant antinociception. Collectively, the results suggest that BISA may be an topical antiinflammatory and visceral antinociceptive agent. [Copyright &y& Elsevier]

Background The high prevalence of functional bowel disorders among the general population contrasts with the limited number of pharmacological treatment options for this condition. This has led to an interest for alternative therapeutic approaches. Padma® Lax is an herbal laxative on the basis of Tibetan formulas. Our aim is to examine the effect of Padma Lax (A) on visceral nociception in vivo and (B) on contractile activity of longitudinal smooth muscle of the lower gut in vitro and ex vivo. Methods (A) Visceral sensory function in response to colorectal distension was assessed by abdominal wall electromyography in male Wistar rats pretreated with Padma Lax. (B) Effects of Padma Lax on contractility of gut smooth muscles were studied both in vitro with superfusion of the agent and ex vivo following oral administration of the preparation. Activities were measured as area under the curve. Key Results (A) For visceral sensitivity, no differences were observed between the Padma Lax and the control group. (B) Proximal colon muscle strips of the Padma Lax pretreated group showed significantly lower spontaneous contractility ex vivo than controls. Cholinergic procontractile stimulation was reduced in Padma Lax pretreated group and in colon strips of naive rats when Padma Lax was superfused in vitro (all P < 0.05). Conclusion & Inferences Cholinergic mechanisms appear to be important in the modulation of rat proximal colon contractility of orally and directly applied Padma Lax. These findings help elucidate a potential mechanism of action of this herbal remedy which has undergone clinical testing in patients with constipation predominant irritable bowel syndrome. [ABSTRACT FROM AUTHOR]

Objective: This study was designed to investigate the role of brain histamine and H1 and H2 receptors in mediating the central perception of visceral pain in rats. Materials and Methods: In conscious rats implanted with a lateral brain ventricle cannula, the effect of intracerebroventricular (i.c.v.) injection of histamine (2.5, 10, and 40 μg), and chlorpheniramine and ranitidine at the same doses of 5, 20, and 80 μg were investigated on visceral pain. Visceral nociception induced by intraperitoneal (i.p.) injection of acetic acid (1 mL, 1%), and the number of complete abdominal wall muscle contractions accompanied with stretching of hind limbs (writhes) were counted for 1 h. Results: Histamine at doses of 10 and 40 μg and chlorpheniramine and ranitidine at the same doses of 20 and 80 μg, significantly decreased the numbers of writhes (P < 0.05). Pretreatment with chlorpheniramine and ranitidine at the same dose of 80 μg, significantly prevented histamine (40 μg)-induced antinociception (P < 0.05). Conclusion: The results of this study suggest that brain histamine may be involved in modulation of visceral antinociception through both central H1 and H2 receptors. [ABSTRACT FROM AUTHOR]

Abstract: Neonatal bladder inflammation has been demonstrated to produce hypersensitivity to bladder re-inflammation as an adult. The purpose of this study was to investigate the effects of neonatal urinary bladder inflammation on adult bladder function and structure. Female Sprague-Dawley rats were treated on postnatal days 14 to 16 with intravesical zymosan or anesthesia alone. At 12 to 16 weeks of age, micturition frequency and cystometrograms were measured. Similarly treated rats had their bladders removed for measurement of plasma extravasation after intravesical mustard oil, for neuropeptide analysis (calcitonin gene-related peptide or Substance P) or for detailed histological examination. Rats treated with zymosan as neonates exhibited increased micturition frequency, reduced micturition volume thresholds, greater extravasation of Evans blue after intravesical mustard oil administration, and greater total bladder content of calcitonin gene-related peptide and Substance P. In contrast, there were no quantitative histological changes in the thickness, fibrosis, or mast cells of bladder tissue due to neonatal zymosan treatments. Functional changes in urologic systems observed in adulthood, coupled with the increased neuropeptide content and neurogenic plasma extravasation in adult bladders, suggest that the neonatal bladder inflammation treatment enhanced the number, function, and/or neurochemical content of primary afferent neurons. These data support the hypothesis that insults to the urologic system in infancy may contribute to the development of adult bladder hypersensitivity. Perspective: Inflammation of the bladder early in life in the rat has multiple sequelae, including laboratory measures that suggest an alteration of the neurophysiological substrates related to the bladder. Some painful bladder syndromes in humans have similar characteristics and so may be due to similar mechanisms. [Copyright &y& Elsevier]

Protease-activated receptors (PARs) are a family of G-protein-coupled receptors with a widespread distribution that are involved in various physiological functions including inflammation and nociception. In a recent study in Neurogastroenterology and Motility, Augé et al. describe for the first time the presence of PAR4 on visceral primary afferent neurons and its role in modulating colonic nociceptive responses, colonic hypersensitivity and primary afferent responses to PAR2 and Transient Receptor Potential Vanilloid-4 (TRPV4). Using the model of visceromotor response (VMR) to colorectal distension (CRD), they show that a PAR4 agonist delivered into the colon lumen decreases basal visceral response to CRD and reduces the exacerbated VMR to CRD induced by treatment with PAR2 or TRPV4 agonists. In isolated sensory neurons, they show that a PAR4 agonist inhibits calcium mobilization induced by PAR2 or TRPV4 agonists. Finally, they describe increased pain behaviour evoked by luminal application of mustard oil in PAR4 deficient mice compared to wild type controls. The newly discovered role of PAR4 in modulating visceral pain adds to our growing understanding of the contribution of colonic proteases and PARs to the mechanisms involved in colonic hypersensitivity and their potential role as therapeutic targets for irritable bowel syndrome. [ABSTRACT FROM AUTHOR]

Objective. Tricyclic antidepressants (TCAs) are well established in the treatment of patients with irritable bowel syndrome (IBS). The effects are believed to be linked to serotoninergic antinociceptive properties, but data on the antinociceptive effects of various TCAs with variable serotoninergic and non-serotoninergic properties have not been investigated. The aim of this study was to compare the antinociceptive effects of different TCAs. Material and methods. Colorectal distension (CRD) using a barostat device was carried out in rats and the visceromotor response (VMR) to CRD was quantified by abdominal wall electromyography. Prior to CRD, saline (control), amitriptyline (AM), desipramine (DES), reserpine (RES) or a combination of TCAs and RES (AM + RES or DES + RES) was applied intraperitoneally. Serum 5-HT levels were determined using high-performance liquid chromatography (HPLC). RES was used to antagonize the serotoninergic actions of TCAs in order to discriminate between these effects and others. Results. Both TCAs decreased the VMR compared to placebo. After RES application without TCAs, the VMR was increased compared to controls (6403 µV±1772 µV). Co-administration of AM and RES resulted in a modest decrease in VMR (5774 µV±1953 µV), while in rats treated with RES and DES the VMR again was significantly lower (3446 µV (±1347 µV; p <0.05)). 5-HT levels were higher in TCA pretreated rats than those in controls and significantly lower 5-HT levels were found in all rats pretreated with RES. Conclusions. AM and DES have antinociceptive properties while RES is pro-nociceptive. The antinociceptive effects of DES are not abolished by RES pretreatment, while AM only attenuates the pro-nociceptive effects of RES. The non-serotoninergic properties of TCAs substantially contribute to the differences in the antinococeptive effects of various TCAs. [ABSTRACT FROM AUTHOR]

Abstract: Unpleasant sensory and emotional experiences originating from the respiratory system, such as dyspnea, breathlessness, unsatisfied inspiration or air hunger are not immediately associated with nociceptive neurons, although they may be detected by the same class of C-fibre afferents. Nevertheless, the conceptual parallel between pain research, nociception and dyspnea is emerging as a paradigm that has the potential to identify novel molecular mechanisms contributing to dyspnea. Recent advances in pain biology such as the cloning of the transient receptor potential vanilloid 1 (TRPV1) ion channel, exploration of the molecular mechanisms leading to TRPV1 activation and the production of a TRPV1 mutant mouse have defined its role as an integrator of noxious thermal and chemical stimuli. These observations, coupled with the biology of lung C-fibre afferents and their reflex effects, suggest that TRPV1 may contribute to the multifactorial nature of dyspnea. The present review summarizes the properties of the TRPV1 ion channel known to be expressed in lung C-fibre afferents, the possible role of TRPV1 in respiratory sensation and neurogenic inflammation, as well as the limitations and opportunities associated with current TRPV1 antagonists. [Copyright &y& Elsevier]

Abstract: Pain symptoms in several chronic pain disorders in women, including irritable bowel syndrome, fluctuate with the menstrual cycle suggesting a gonadal hormone component. In female rats, estrogens modulate visceral sensitivity although the underlying mechanism(s) are unknown. In the present study the effects of 17-β estradiol on N-methyl-d-aspartate (NMDA) receptor signaling of colorectal nociceptive processing in the spinal cord were examined. Estrogen receptor alpha and the NR1 subunit of the NMDA receptor are co-expressed in dorsal horn neurons, supporting a direct action of estradiol on NMDA receptors. Intrathecal administration of the NMDA receptor antagonist d(−)-2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response with greater potency in ovariectomized (OVx) rats compared to OVx with estradiol replacement (E2) rats. Estradiol significantly increased protein expression of NR1 in the lumbosacral spinal cord compared to OVx rats. Colorectal distention significantly increased phosphorylation of NR1ser-897, a PKA phosphorylation site on the NR1 subunit in E2, but not OVx rats. Intrathecal administration of a PKA inhibitor significantly attenuated the visceromotor response, decreased NR1 phosphorylation and increased the potency of APV to attenuate the visceromotor response compared to vehicle-treated E2 rats. These data suggest that estradiol increases spinal processing of visceral nociception by increasing NMDA receptor NR1 subunit expression and increasing site-specific receptor phosphorylation on the NR1 subunit contributing to an increase in NMDA receptor activity. [Copyright &y& Elsevier]

Visceral hypersensitivity has been implicated as an important pathophysiological mechanism in functional gastrointestinal disorders. In this study, we investigated whether the sustained visceral hyperalgesia induced by repeated psychological stress in rats involves the activation of CRF1 signaling system using two different antagonists. Male Wistar rats were exposed to 10 consecutive days of water avoidance stress (WAS) or sham stress for 1 h/day, and the visceromotor response to phasic colorectal distension (CRD) was assessed before and after the stress period. Animals were injected subcutaneously with the brain penetrant CRF1 antagonist, CP-154,526, acutely (30 min before the final CRD) or chronically (via osmotic minipump implanted subcutaneously, during stress) or with the peripherally restricted, nonselective CRF1 and CRF2 antagonist, astressin, chronically (15 min before each stress session). Repeated WAS induced visceral hypersensitivity to CRD at 40 and 60 mmHg. CP-154,526 injected acutely significantly reduced stress-induced visceral hyperalgesia at 40 mmHg but not at 60 mmHg. Chronic subcutaneous delivery of astressin reduced the stress-induced visceral hyperalgesia to baseline at all distension pressures. Interestingly, chronically administered CP-154,526 eliminated hyperalgesia and produced responses below baseline at 40 mmHg and 60 mmHg, indicating a hypoalgesic effect of the compound. These data support a major role for CRF1 in both the development and maintenance of visceral hyperalgesia induced by repeated stress and indicate a possible role of peripheral CRF receptors in such mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: A mathematical model of visceral perception was constructed, comprising primary sensory, motor, intestinofugal and principal neurons, interstitial cells of Cajal and smooth muscle elements that are arranged in a functional circuit through chemical synapses. The mathematical description of constructive elements was based on detailed morphological, anatomical, electrophysiological and neuropharmacological characteristics of cells and chemical processes of electrochemical coupling. Emphasis was given to signal transduction mechanisms that involved multiple neurotransmitters and receptor polymodality. The role of co-transmission by acetylcholine (ACh), serotonin (5-HT), noradrenalin (NA), N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and their corresponding receptors—muscarinic and nicotinic type ACh receptors, β-adrenoceptors, 5-HT3/4 type serotonergic receptors, NMDA and AMPA receptors in pathogenesis of nociception was studied numerically. Results of computer simulations reproduced patterns of electrical activity of neurons and mechanical responses of the smooth muscle similar to those observed in in vivo and in vitro experiments when ACh, 5-HT, NA, NMDA and AMPA were acting either alone or co-jointly. The results provide neurochemical bases for explanation of pathophysiological mechanisms of visceral nociception, which cannot be elucidated by existing experimental methods. Care should be taken though when extrapolating the numerical results onto the actual system because of limiting assumptions of the model. [Copyright &y& Elsevier]

The purpose of the present study was to determine whether an intraspinal nociceptive pathway from the lungs modulated activity of spinal neurons that also received afferent input from the colon. Extracellular potentials of single lumbosacral (L6-S2) spinal neurons were recorded in pentobarbital-anesthetized, paralyzed, and ventilated male rats. The lower airways and lungs were irritated by injecting ammonia vapor over a 30% NH4OH solution into the inspiratory line of the ventilator (0.5 ml, 20 s). Graded colorectal distension (CRD; 20-60 mmHg, 20 s) was produced by air inflation of a balloon. Inhaled ammonia (IA) altered activity of 31/5 1 (61%) lumbosacral spinal neurons responding to noxious CRD (60 mmHg, 20 s). In contrast, IA changed activity of 3⁄30 (10%) spinal neurons with somatic fields that did not respond to colorectal inputs. IA decreased activity of 16/31 (52%) spinal neurons and increased activity of the other 15 neurons with colorectal input. Multiple patterns of viscerovisceral convergent spinal neurons with excitatory and inhibitory responses to CRD and IA were observed; 87% (27/31) of the viscerovisceral convergent neurons also responded to innocuous and/or noxious stimuli of somatic fields. Bilateral cervical vagotomy abolished responses to IA in 2/8 tested neurons, indicating that the remaining 6 neurons had input originating from sympathetic afferent fibers. Rostral Cl spinal transection did not abolish inhibitory responses to IA in 4/4 neurons, but L2 transection eliminated inhibitory responses to IA in 3/3 neurons. These results indicated that irritation of the lower airways modulated activity of lumbosacral spinal neurons with colorectal input. It might contribute to intraspinal cross talk between the colon and lungs. [ABSTRACT FROM AUTHOR]

Abstract: Despite its beneficial effect in IBS patients, the mechanism of action of the 5-HT3 receptor (5-HT3R) antagonist alosetron is still incompletely understood. We aimed to characterize the effect and site(s) of action in a model of stress-induced sensitization of visceral nociception in rats. Adult male Wistar rats were equipped for recording of visceromotor response (VMR) to phasic colorectal distension (CRD; 10–60mmHg). VMR to CRD was recorded 24h after an acute session of water avoidance (WA) stress (post-WA). Baseline and post-WA responses were measured in rats exposed to WA or sham-WA, treated with alosetron at 0.3mg/kg subcutaneously (s.c.) 25nmol intrathecally (i.t.) or vehicle before post-WA CRD. Some rats were treated with capsaicin/vehicle on the cervical vagus nerve and received alosetron (0.3mg/kg, s.c.) 15min before post-WA CRD. WA stress led to visceral hyperalgesia 24h later. Alosetron (0.3mg/kg, s.c.), failed to inhibit WA-induced exacerbation of VMR to CRD. Stress-induced visceral hyperalgesia was abolished when alosetron was injected intrathecally (P <0.05) in intact rats or subcutaneously (0.3mg/kg) in capsaicin-pretreated animals (P <0.05). Capsaicin-pretreatment did not affect the exacerbating effect of stress on visceral sensitivity. Alosetron had no inhibitory effect on normal visceral pain responses when administered subcutaneously or intrathecally. We demonstrated that 5-HT3Rs on central terminals of spinal afferents are engaged in the facilitatory effect of stress on visceral sensory information processing. In addition, we showed that stress-induced sensitization of visceral nociception is independent of 5-HT3R activation on vagal afferents. [Copyright &y& Elsevier]

Abstract: Respiratory symptoms experienced in some patients with cardiac diseases may be due to convergence of noxious cardiac and pulmonary inputs onto neurons of the central nervous system. For example, convergence of cardiac and respiratory inputs onto single solitary tract neurons may be in part responsible for integration of regulatory and defensive reflex control. However, it is unknown whether inputs from the lungs and heart converge onto single neurons of the spinal cord. The present aim was to characterize upper thoracic spinal neurons responding to both noxious stimuli of the heart and lungs in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer bradykinin (BK, 10 μg/ml, 0.2 ml, 1 min) as a noxious cardiac stimulus. The lung irritant, ammonia, obtained as vapor over a 30% solution of NH4OH was injected into the inspiratory line of the ventilator (0.5–1.0 ml over 20 s). Intrapericardial bradykinin (IB) altered activity of 58/65 (89%) spinal neurons that responded to inhaled ammonia (IA). Among those cardiopulmonary convergent neurons, 81% (47/58) were excited by both IA and IB, and the remainder had complex response patterns. Bilateral cervical vagotomy revealed that vagal afferents modulated but did not eliminate responses of individual spinal neurons to IB and IA. The convergence of pulmonary and cardiac nociceptive signaling in the spinal cord may be relevant to situations where a disease process in one organ influences the behavior of the other. [Copyright &y& Elsevier]

Abstract: Visceral noxious stimulation induces central neuronal plasticity changes and suggests that the c-AMP-dependent protein kinase (PKA) signal transduction cascade contributes to long-term changes in nociceptive processing at the spinal cord level. Our previous studies reported the clinical neurosurgical interruption of post synaptic dorsal column neuron (PSDC) pathway by performing midline myelotomy effectively alleviating the intractable visceral pain in patients with severe pain. However, the intracellular cascade in PSDC neurons mediated by PKA nociceptive neurotransmission was not known. In this study, by using multiple experimental approaches, we investigated the role of PKA in nociceptive signaling in the spinal cord and PSDC neurons in a visceral pain model in rats with the intracolonic injection of mustard oil. We found that mustard oil injection elicited visceral pain that significantly changed exploratory behavior activity in rats in terms of decreased numbers of entries, traveled distance, active and rearing time, rearing activity and increased resting time when compared to that of rats receiving mineral oil injection. However, the intrathecal infusion of PKA inhibitor, H89 partially reversed the visceral pain-induced effects. Results from Western blot studies showed that mustard oil injection significantly induced the expression of PKA protein in the lumbosacral spinal cord. Immunofluorescent staining in pre-labeled PSDC neurons showed that mustard oil injection greatly induces the neuronal profile numbers. We also found that the intrathecal infusion of a PKA inhibitor, H89 significantly blocked the visceral pain-induced phosphorylation of c-AMP-responsive element binding (CREB) protein in spinal cord in rats. The results of our study suggest that the PKA signal transduction cascade may contribute to visceral nociceptive changes in spinal PSDC pathways. [Copyright &y& Elsevier]

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to ‘sense’ the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways. [ABSTRACT FROM AUTHOR]

Abstract: The rat thalamic nucleus submedius responds to noxious pressure stimuli in the colon. Some neurons in and near Barrington''s nucleus, a pontine center related to bladder function, also respond to colon distension. We hypothesized that colonic nociception may be relayed via Barrington''s nucleus to the nucleus submedius. Experiments were carried out in 22 urethane-anesthetized male rats. Noxious stimuli were applied to the toes using standardized clips and to the colon by inflation of the balloon to 80mmHg for 30s using a barostat. The brain was exposed to allow recording from the nucleus submedius with a monopolar tungsten electrode and the activity of rectus muscle was assessed via silver wire electrodes. A glass pipette was inserted into Barrington''s nucleus for injection of 5mM CoCl2, a temporary neural blocker. The site of CoCl2 injection was confirmed by the presence of FluoroGold which was incorporated into the CoCl2 solution. We recorded 51 units in submedius that were excited by noxious toe pinch, 4 were inhibited. Colon distension to 80mmHg produced visceromotor responses, excited 23units in submedius and inhibited 13units. Injection of CoCl2 into the region of Barrington''s nucleus blocked the response to colon distension in 10 of 12 Sm units tested, but had no influence on the accompanying visceromotor response. These data point to a previously unrecognized relationship between Barrington''s nucleus and submedius that may subserve colon nociception. [Copyright &y& Elsevier]

Chronic stress plays an important role in the development and exacerbation of symptoms in functional gastrointestinal disorders. To better understand the mechanisms underlying this relationship, we aimed to characterize changes in visceral and somatic nociception, colonic motility, anxiety-related behavior, and mucosal immune activation in rats exposed to 10 days of chronic psychological stress. Male Wistar rats were submitted daily to either 1-h water avoidance (WA) stress or sham WA for 10 consecutive days. The visceromotor response to colorectal distension, thermal somatic nociception, and behavioral responses to an open field test were measured at baseline and after chronic WA. Fecal pellets were counted after each WA stress or sham WA session as a measure of stress-induced colonic motility. Colonic samples were collected from both groups and evaluated for structural changes and neutrophil infiltration, mast cell number by immunohistochemistry, and cytokine expression by quantitative RT-PCR. Rats exposed to chronic WA (but not sham stress) developed persistent visceral hyperalgesia, whereas only transient changes in somatic nociception were observed. Chronically stressed rats also exhibited anxiety-like behaviors, enhanced fecal pellet excretion, and small but significant increases in the mast cell numbers and the expression of IL-1 β and IFN-γ. Visceral hyperalgesia following chronic stress persisted for at least a month. Chronic psychological stress in rats results in a robust and long-lasting alteration of visceral, but not somatic nociception. Visceral hyperalgesia is associated with other behavioral manifestations of stress sensitization but was only associated with minor colonic immune activation arguing against a primary role of mucosal immune activation in the maintenance of this phenomenon. [ABSTRACT FROM AUTHOR]

This study tested the hypothesis that the ventrolateral column of the midbrain periaqueductal gray (vlPAG) region mediates the hypotension and bradycardia evoked by visceral nociception. To test this, the local anesthetic lidocaine (2%; 0.5 μl) was microinjected into the vlPAG of halothane-anesthetized rats bilaterally and visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Acetic acid injection caused an abrupt fall in arterial pressure (-12.2±2.1 mm Hg) and heart rate (-37±93 bpm) lasting approximately 15 min. Lidocaine injection into the vlPAG prevented the fall in arterial pressure and heart rate completely. Cobalt chloride (5 mM; 0.2 or 0.5 μl) injection into the vlPAG also prevented nociceptive hypotension but it did not affect the fall in heart rate significantly. Lidocaine pretreatment also inhibited the depressor response caused by intramuscular formalin (5%; 0.2 ml) administration, a model of deep somatic nociception, although it did not prevent the response completely. To determine if opioid receptors mediate the response, selective mu, delta or kappa opioid receptor antagonists were microinjected into the vlPAG 5 min before intraperitoneal (ip) acetic acid administration. Naltrindole, a delta receptor antagonist, inhibited the response significantly but mu and kappa antagonists were completely ineffective. Lidocaine and naltrindole had no effect when injected into the dorsolateral PAG and did not influence cardiovascular function when injected into the vlPAG of saline treated control animals. These data support the hypothesis that the vlPAG mediates the depressor response evoked by visceral nociception and indicate that delta opioid receptors participate in the response. [Copyright &y& Elsevier]

The aim of this study was to characterize thoracic spinal neurons receiving convergent inputs from the esophagus, heart and somatic receptive fields. Extracellular potentials of single T3–T4 spinal neurons were recorded in pentobarbital anesthetized male rats. Thoracic and cervical esophageal distensions (TED, CED) were produced by water inflation of a latex balloon. A catheter was placed in the pericardial sac to administer bradykinin or a mixture of algogenic chemicals. 96/311 (31%) neurons responded to both TED and intrapericardial chemicals (IC) and 48/177 (27%) neurons responded to both CED and IC. Long-lasting excitatory responses were more frequently encountered (P<0.05) in esophagocardiac spinal neurons responding to TED (T-ECSNs, 62/91) than in neurons responding to CED (C-ECSNs, 23/47). Ninety-one percent of T-ECSNs and 98% of C-ECSNs had somatic fields on chest, axilla and upper back areas. Esophagocardiac convergence on thoracic spinal neurons provided a spinal mechanism that might mediate viscerovisceral nociception and reflexes. [Copyright &y& Elsevier]

Recent studies have revealed that noxious visceral inputs travel in the dorsal column pathway, and interactions between colorectal noxious and tactile inputs occur in the ventrobasal thalamus. This investigation was to test whether the somatovisceral interactions also take place at a lower level in the dorsal column nuclei. Extracellular single neuron recordings were carried out in nucleus gracilus of anesthetized rats. Forty-three neurons responsive to colorectal distension (CRD) all had excitatory responses to tactile stimuli, and their tactile responses were predominantly (31/43 units) enhanced by preceding CRD. In contrast, the neuronal responses to CRD were reduced in 22/43 units when preceded by tactile stimulation but in two units there was an enhancement. The similarity and differences in the gracile response features in comparison with the thalamic recordings suggest that somatovisceral interactions take place at multiple levels in the dorsal column-medial lemniscus system. [Copyright &y& Elsevier]

A rat model of endometriosis, in which pieces of uterine horn (versus fat in controls) are autotransplanted into the abdomen where they form cysts, reduces fecundity and produces vaginal hyperalgesia. The cysts gradually enlarge over a 2-month period postsurgically and then plateau. Cysts regress with low estrogen levels and reappear when they rise. Based on the hypothesis that the vaginal hyperalgesia depends upon the cysts, this study tested two predictions: that (1) the hyperalgesia would develop postsurgically in parallel with the cysts, and (2) the hyperalgesia would vary with estrous, being greatest when estrogen levels are high (proestrus) and least when low (estrus). In rats trained to escape vaginal distention, percentage escape responses to different distention volumes were measured across the rat''s 4-day estrous cycle for 2.5 months before and up to 4 months after autotransplantation of uterus (n = 9) or fat (n = 6) in abdominal sites. Vaginal pressures were also measured. In rats with uterine but not fat autotransplants, escape percentages increased postsurgically over a 2-month period and then plateaued. The increase was greatest in proestrus and failed to occur in estrus. Vaginal pressures were unchanged in all groups. These results strongly support the hypothesis that the vaginal hyperalgesia depends upon the cysts. Because the cysts were located in sites remote from the vagina, the hyperalgesia involves viscero-visceral interactions and is likely centrally mediated, whereas the estrous modulation could involve hormonal actions either on the cysts or, more likely, on vaginal afferent fibers, and/or on central neurons. [Copyright &y& Elsevier]

The current study investigates whether visceral nociceptive inputs affect tactile processing in the thalamic ventroposterior lateral nucleus in anesthetized rats by means of extracellular single unit recordings. Twenty out of the 44 neurons had their response to tactile stimulation increased by preceding nociceptive colorectal distension (CRD), and this influence appear more potent than the opposite effect, tactile on CRD response. There was a dynamic change of tactile response along with CRD response and the background activities as a function of repeated colorectal stimulation. The abnormal neuronal discharge in response to tactile stimulation in the condition of visceral nociception could contribute to the development of referred pain and allodynia. [Copyright &y& Elsevier]

The purposes of this study were to examine responses of superficial (depth <300 μm) and deeper thoracic spinal neurons to chemical stimulation of cardiac afferents and effects of descending influences on these neurons. Extracellular potentials of single T3–T4 neurons were recorded in pentobarbital anesthetized, paralyzed and ventilated male rats. A catheter was placed in the pericardial sac to administer 0.2 ml of a mixture of algogenic chemicals that contained adenosine (10−3 M), bradykinin, histamine, serotonin, prostaglandin E2 (10−5 M). Fifteen of 55 (27%) superficial neurons responsive to intrapericardial chemicals were compared to 80/169 (47%) deeper neurons. All 15 superficial neurons that responded to cardiac afferents were excited (E), whereas 66 deeper neurons were excited, ten were inhibited and four showed excitation–inhibition. Spontaneous activity of superficial neurons with short-lasting excitatory responses was significantly lower than that of deeper neurons (P<0.05). Somatic receptive fields on chest, axilla, arm and upper back areas were found for 77/95 (81%) neurons that responded to intrapericardial chemicals. The proportion of somatic field properties and their sizes in superficial neurons were similar to deeper neurons. After cervical spinal transection, both spontaneous activity and responses to chemical stimulation of cardiac afferents significantly increased in six out of six neurons excited by intrapericardial injections. Results showed that chemical stimulation of cardiac afferents excited superficial T3–T4 spinal neurons, whereas deeper neurons exhibited multiple patterns of responses. Some characteristics of subgroups of superficial neurons were quantitatively different from deeper neurons. Thoracic spinal neurons processing cardiac nociceptive information were under tonic descending inhibition. [Copyright &y& Elsevier]