Modulation of spinal visceral nociceptive transmission by NMDA receptor activation in the rat

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

Downregulation of the spinal NMDA receptor NR2B subunit during electro-acupuncture relief of chronic visceral hyperalgesia

The involvement of spinal NR2B, a N-methyl-D-aspartate (NMDA) receptor subunit, in the therapeutic effect of electro-acupuncture (EA) on chronic visceral hyperalgesia was investigated. Chronic visceral hyperalgesia was induced using an irritable bowel syndrome (IBS) model in rats. Graded colorectal distention (CRD) stimuli at strengths of 20, 40, 60 and 80 mmHg were applied, and behavioral tests were performed to measure the abdominal withdrawal reflex (AWR) in response to the CRD stimuli and assess the severity of the visceral hyperalgesia. Rats were randomly divided into four groups: normal intact (control) group, IBS model (model) group, EA-treated IBS rats (EA) group and sham EA-treated IBS rats (sham EA) group. For the EA treatment, electric stimuli were applied through needles inserted into two acupoints [Zu-san-li (ST-36) and Shang-ju-xu (ST-37)] in both hind limbs, while the sham EA treatment consisted of only the insertion of needles into these same acupoints without an application of electric stimuli. Our results showed that AWR scores of the model group responding to CRD stimuli of 20, 40, 60 and 80 mmHg were significantly increased. These increased scores subsequently decreased following EA treatment (P < 0.05) compared with those for the other groups. The expression of NR2B in the superficial laminae (SDH, laminae I and II), nucleus proprius (NP, laminae III and IV), neck of the dorsal horn (NECK, laminae V and VI) and central canal region (lamina X) at thoracolumbar (T13-L2) and lumbosacral (L6-S2) segmental level significantly increased in the model group versus the control group (P < 0.05) and significantly decreased after EA treatment (P < 0.05). There were no significant changes in neither AWR scores nor expression of the NR2B subunit in these spinal regions after the sham EA treatment. These results confirm that EA can relieve chronic visceral hyperalgesia in IBS model rats and suggest that such an effect is possibly mediated through the downregulation of the NR2B subunits of NMDA at the spinal level.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Induction of thermal hyperalgesia and synaptic long-term potentiation in the spinal cord lamina I by TNF-α and IL-1β is mediated by glial cells

Long-term potentiation (LTP) of synaptic strength in nociceptive pathways is a cellular model of hyperalgesia. The emerging literature suggests a role for cytokines released by spinal glial cells for both LTP and hyperalgesia. However, the underlying mechanisms are still not fully understood. In rat lumbar spinal cord slices, we now demonstrate that conditioning high-frequency stimulation of primary afferents activated spinal microglia within <30 min and spinal astrocytes within ~2 s. Activation of spinal glia was indispensible for LTP induction at C-fiber synapses with spinal lamina I neurons. The cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), which are both released by activated glial cells, were individually sufficient and necessary for LTP induction via redundant pathways. They differentially amplified 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)-propanoic acid receptor-mediated and N-methyl-D-aspartic acid receptor-mediated synaptic currents in lamina I neurons. Unexpectedly, the synaptic effects by IL-1β and TNF-α were not mediated directly via activation of neuronal cytokine receptors, but rather, indirectly via IL-1 receptors and TNF receptors being expressed on glial cells in superficial spinal dorsal horn. Bath application of IL-1β or TNF-α led to the release profiles of pro-inflammatory and anti-inflammatory cytokines, chemokines, and growth factors, which overlapped only partially. Heat hyperalgesia induced by spinal application of either IL-1β or TNF-α in naive animals also required activation of spinal glial cells. These results reveal a novel, decisive role of spinal glial cells for the synaptic effects of IL-1β and TNF-α and for some forms of hyperalgesia.

Sex differences in spinal processing of transient and inflammatory colorectal stimuli in the rat

Sex differences in the spinal processing of somatic and visceral stimuli contribute to greater female sensitivity in many pain disorders. The present study examined spinal mechanisms that contribute to sex differences in visceral sensitivity. The visceromotor response to colorectal distention (CRD) was more robust in normal female rats and after intracolonic mustard oil compared with that in male rats. No sex difference was observed in the CRD-evoked response of lumbosacral (LS) and thoracolumbar (TL) colonic afferents in normal and mustard oil-treated rats, but there was a sex difference in spontaneous activity that was exacerbated by intracolonic mustard oil. The response of visceroceptive dorsal horn neurons to CRD was greater in normal female rats in the LS and TL spinal segments. The effect of intracolonic mustard oil on the CRD-evoked response of different phenotypes of visceroceptive dorsal horn neurons was dependent on sex and segment. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response in normal rats with greater effect in male rats. Correspondingly, there was greater cell membrane expression of the GluN1 subunit in dorsal horn extracts in female rats. After intracolonic mustard oil, there was no longer a sex difference in the effect of APV nor GluN1 expression in LS segments, but greater female expression in TL segments. These data document a sex difference in spinal processing of nociceptive visceral stimuli from the normal and inflamed colon. Differences in dorsal horn neuronal activity and NMDA receptor expression contribute to the sex differences in the visceral sensitivity observed in awake rats.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Expression of vesicular glutamate transporters type 1 and 2 in sensory and autonomic neurons innervating the mouse colorectum

Vesicular glutamate transporters (VGLUTs) have been extensively studied in various neuronal systems, but their expression in visceral sensory and autonomic neurons remains to be analyzed in detail. Here we studied VGLUTs type 1 and 2 (VGLUT(1) and VGLUT(2) , respectively) in neurons innervating the mouse colorectum. Lumbosacral and thoracolumbar dorsal root ganglion (DRG), lumbar sympathetic chain (LSC), and major pelvic ganglion (MPG) neurons innervating the colorectum of BALB/C mice were retrogradely traced with Fast Blue, dissected, and processed for immunohistochemistry. Tissue from additional naïve mice was included. Previously characterized antibodies against VGLUT(1) , VGLUT(2) , and calcitonin gene-related peptide (CGRP) were used. Riboprobe in situ hybridization, using probes against VGLUT(1) and VGLUT(2) , was also performed. Most colorectal DRG neurons expressed VGLUT(2) and often colocalized with CGRP. A smaller percentage of neurons expressed VGLUT(1) . VGLUT(2) -immunoreactive (IR) neurons in the MPG were rare. Abundant VGLUT(2) -IR nerves were detected in all layers of the colorectum; VGLUT(1) -IR nerves were sparse. A subpopulation of myenteric plexus neurons expressed VGLUT2 protein and mRNA, but VGLUT1 mRNA was undetectable. In conclusion, we show 1) that most colorectal DRG neurons express VGLUT(2) , and to a lesser extent, VGLUT(1) ; 2) abundance of VGLUT2-IR fibers innervating colorectum; and 3) a subpopulation of myenteric plexus neurons expressing VGLUT(2). Altogether, our data suggests a role for VGLUT(2) in colorectal glutamatergic neurotransmission, potentially influencing colorectal sensitivity and motility.

Neonatal cystitis-induced colonic hypersensitivity in adult rats: a model of viscero-visceral convergence

BACKGROUND

The objective of this study was to determine if neonatal cystitis alters colonic sensitivity later in life and to investigate the role of peripheral mechanisms.

METHODS

Neonatal rats received intravesical zymosan, normal saline, or anesthesia only for three consecutive days [(postnatal (PN) days 14-16)]. The estrous cycle phase was determined prior to recording the visceromotor response (VMR) to colorectal distension (CRD) in adult rats. Eosinophils and mast cells were examined from colon and bladder tissues. CRD- or urinary bladder distension (UBD)-sensitive pelvic nerve afferents (PNAs) were identified and their responses to distension were examined. The relative expression of N-methyl-d-aspartic acid (NMDA)-NR1 subunit in the lumbo-sacral (L6-S1) spinal cord was examined using Western blot.

KEY RESULTS

The VMR to CRD (≥10mmHg) in the neonatal zymosan group was significantly higher than control in both the diestrus, estrus phase and in all phases combined. There was no difference in the total number of eosinophils, mast cells or number of degranulated mast cells between groups. The spontaneous firing of UBD, but not CRD-sensitive PNAs from the zymosan-treated rats was significantly higher than the saline-treated control. However, the mechanosensitive properties of PNAs to CRD or UBD were no different between groups (P>0.05). The expression of spinal NR1 subunit was significantly higher in zymosan-treated rats compared with saline-treated rats (P<0.05).

CONCLUSIONS & INFERENCES

Neonatal cystitis results in colonic hypersensitivity in adult rats without changing tissue histology or the mechanosensitive properties of CRD-sensitive PNAs. Neonatal cystitis does result in overexpression of spinal NR1 subunit in adult rats.

Clinical application and reliability of a post abdominal surgery pain assessment scale (PASPAS) in horses

The aim of this study was to refine a multi-dimensional scale based on physiological and behavioural parameters, known as the post abdominal surgery pain assessment scale (PASPAS), to quantify pain after laparotomy in horses. After a short introduction, eight observers used the scale to assess eight horses at multiple time points after laparotomy. In addition, a single observer was used to test the correlation of each parameter with the total pain index in 34 patients, and the effect of general anaesthesia on PASPAS was investigated in a control group of eight horses. Inter-observer variability was low (coefficient of variation 0.3), which indicated good reliability of PASPAS. The correlation of individual parameters with the total pain index differed between parameters. PASPAS, which was not influenced by general anaesthesia, was a useful tool to evaluate pain in horses after abdominal surgery and may also be useful to investigate analgesic protocols or for teaching purposes.

Visceral pain: the neurophysiological mechanism

The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic-pituitary-adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.

Basic and clinical aspects of gastrointestinal pain

The gastrointestinal (GI) tract is a system of organs within multicellular animals which facilitates the ingestion, digestion, and absorption of food with subsequent defecation of waste. A complex arrangement of nerves and ancillary cells contributes to the sensorimotor apparatus required to subserve such essential functions that are with the exception of the extreme upper and lower ends of the GI tract normally subconscious. However, it also has the potential to provide conscious awareness of injury. Although this function can be protective, when dysregulated, particularly on a chronic basis, the same system can lead to considerable morbidity. The anatomical and molecular basis of gastrointestinal nociception, conditions associated with chronic unexplained visceral pain, and developments in treatment are presented in this review.

NMDA Receptors and Colitis: Basic Science and Clinical Implications

During the last decade, research focusing primarily on alterations in the peripheral and central nervous system has improved our understanding of the pathophysiological mechanisms of chronic visceral pain. These studies have demonstrated significant physiological changes following injury to the viscera in the firing patterns of both primary afferent neurons that transmit nociceptive information from the viscera and in central neurons that process the nociceptive information. A number of receptors, neurotransmitters, cytokines, and second messenger systems in these neurons have been implicated in the enhancement of visceral nociception. N-methyl-d-aspartic acid (NMDA) receptors play an important role in chronic visceral pain and hypersensitivity that is present in the setting of colonic inflammation. NMDA receptors are found in the peripheral nervous system as well as the central terminal of primary afferent neurons and have been shown to play an important role in regulating the release of nociceptive neurotransmitters. Recent work has demonstrated the presence of NMDA receptors in the enteric nervous system. In this article, we will discuss more recent evidence of the role of NMDA receptors in visceral pain associated with colitis.

Somatosensory changes in the referred pain area before and after cholecystectomy in patients with uncomplicated gallstone disease

OBJECTIVE

It is estimated that 25-40% of patients have continuing symptoms after cholecystectomy and that 5-10% have pain. The pain may be related to central neuroplastic changes of sensory pathways induced by the gallstone disease. Such neuronal hyperexcitability can be reflected in the somatic referred pain area sharing central pathways with the gallbladder. The aim of this study was to examine somatosensory changes in the referred pain area evoked by painful gallstone attacks before and after cholecystectomy in patients with uncomplicated gallstone disease.

MATERIAL AND METHODS

Thirty-seven patients with uncomplicated gallstone disease were included in the study. The sensations and pain thresholds to pinprick, pinching, pressure, thermal and electrical stimulation were studied before and 4-12 weeks after surgery in the area where the pain was referred to during the previous gallstone attacks. An area on the contralateral side of the abdomen served as the control.

RESULTS

Somatosensory hyperalgesia in the referred pain area was observed in 84% of the patients before surgery. After elective cholecystectomy, none of the patients had pain complaints, and the sensibility in the referred area was normalized.

CONCLUSIONS

Uncomplicated gallstone disease leads to significant hyperalgesia in the somatic referred pain area. At the time of the postoperative investigation none of the patients suffered from pain, which was reflected in the normal sensory findings in the previous referred pain area.

An experimental study of viscero-visceral hyperalgesia using an ultrasound-based multimodal sensory testing approach

Widespread visceral hypersensitivity and the overlap of symptom complexes observed in functional gastrointestinal disorders may be related to central sensitization and neuroplastic changes. A multimodal and multi-segmental model was developed to evaluate viscero-visceral hyperalgesia induced by experimental esophageal sensitization in healthy volunteers. Twelve healthy subjects were studied using a double-blinded, placebo-controlled design. The sensitivity to mechanical and heat stimulations was assessed in the proximal esophagus, duodenum and rectum before and after perfusion of the distal esophagus with acid or saline. A special-designed probe was used allowing cross-sectional ultrasound imaging during mechanical and heat stimulation of the esophagus and duodenum. Another probe was used for mechanical stimulation of the rectum. The referred somatic pain areas to gut pain stimulations were also assessed. Following acid perfusion 11 of 12 volunteers showed increased sensitivity to one or more stimulation modalities. An overall increased sensitivity to mechanical stretch in the three gut segments was seen (P=0.0001). Posthoc analysis showed that this was mainly due to increased sensitivity in the rectum (P<0.001). No changes were seen to thermal stimulations (all P-values>0.4). The somatic referred pain area to duodenal stimulations increased (P=0.04), while it was unaffected to esophageal and rectal stimulations (P>0.3). The present method demonstrated a new approach to assess multimodal sensitivity to experimental sensitization of the esophagus and related viscero-visceral hyperalgesia. Central mechanisms can explain the remote hyperalgesia to mechanical visceral stimulation and the increase in referred pain areas. The present method may be used to explore pathophysiology and pharmacological interventions in patients with visceral hypersensitivity.

Differential effects of glutamate receptor antagonists on dorsal horn neurons responding to colorectal distension in a neonatal colon irritation rat model

AIM: To investigate and compare the effects of spinal D-(-)-2-amino-7-phosphonoheptanoic acid (AP-7) and 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX), two glutamate receptor antagonists, on the responses of dorsal horn neurons to colorectal distension (CRD) in adult rats exposed to neonatal colon irritation (CI).

METHODS: Hypersensitive SD rats were generated by CI during postnatal days 8, 10 and 12. Experiments on adult rats were performed using extracellular single-unit recording. The effects of spinal application of AP-7 (0.001, 0.01, 0.1, 1 mmoL) were tested on the CRD-evoked neuronal responses in 16 controls and 17 CI rats. The effects of CNQX (0.2, 2, 5, 10 μmoL) were also tested on the CRD-evoked responses of 17 controls and 18 CI neurons.

RESULTS: (1) The average responses of lumbosacral neurons to all intensities of CRD in CI rats were significantly higher than those in control rats; (2) In control rats, AP-7（0.01 mmoL）had no significant effect on the neuronal response to all intensities of CRD (20, 40, 60, 80 mmHg); while AP-7 (0.1 mmoL) inhibited the neuronal response to 80-mmHg CRD. By contrast, in CI rats, AP-7 (0.01-1 mmoL) attenuated the CRD-evoked neuronal responses to all distention pressures in a dose-dependent manner; (3) In control rats, CNQX (2 μmoL) had no significantly effect on the neuronal response to all intensities of CRD; however, CNQX (5 μmoL) significantly attenuated the responses to CRD in the 40-80 mmHg range. By contrast, CNQX (2-10 μmoL) significantly decreased the neuronal responses in CI rats to non-noxious and noxious CRD in a dose-dependent manner.

CONCLUSION: Our results suggest that spinal N-methyl-D-aspartate (NMDA) and non-NMDA receptors may contribute to the processing of central sensitivity in a neonatal CI rat model, but they may play different roles in it.

Differential processing of noxious colonic input by thoracolumbar and lumbosacral dorsal horn neurons in the rat

Previous studies suggest the lumbosacral (LS) spinal cord processes acute colorectal stimuli whereas the thoracolumbar (TL) and LS spinal segments process inflammatory stimuli. In this study, the effects of colorectal distention (CRD) on TL and LS dorsal horn neuronal activity were recorded in Nembutal-anesthetized male rats both with and without colonic inflammation. Both single cells (before and after inflammation) and populations (multiple cells from noninflamed or inflamed rats) were studied. CRD-responsive neurons had excitatory Abrupt (on-off with stimulus) or Sustained (prolonged after discharge) responses or were Inhibited by CRD. In noninflamed rats, a significantly greater percentage of LS neurons (63% Abrupt, 27% Sustained) were excited by CRD than TL neurons (61% Abrupt, 3% Sustained). The remaining cells were Inhibited (10% LS, 36% TL). LS Abrupt neurons had lower thresholds and greater response magnitudes to CRD compared with TL Abrupt neurons. After colonic inflammation, TL neurons became more excitable: the percentage of Inhibited neurons decreased, the response magnitude of Abrupt neurons increased, and the threshold decreased. In contrast, in single-cell recordings, the response of LS Sustained neurons increased, whereas LS Abrupt neurons decreased. These data suggest that in noninflamed rats, the net response to CRD of TL visceroceptive spinal sensory neurons is less than that of LS neurons. Colonic inflammation increases the net response of TL neurons and differentially modulates the response of LS neurons. These differences may contribute to the functional dichotomy between the TL and LS spinal segments in processing acute and inflammatory colorectal pain.

Nociceptive transmitter release in the dorsal spinal cord by capsaicin-sensitive fibers after noxious gastric stimulation

Little is known about transmitters that encode noxious gastric stimuli in the spinal cord. The release of glutamate, substance P, and CGRP from the spinal cord was therefore investigated in response to acid injury of the gastric mucosa. Dorsal halves of the caudal thoracic spinal cord (T7-T13) were removed 6 h after oral application of 0.5 M HCl or saline, transferred to a superfusion chamber, and the basal and capsaicin-stimulated (3.3 microM) transmitter release was determined. After acid injury, basal glutamate release increased 134% as compared to saline-treated animals. Capsaicin-stimulated release of CGRP and SP was 48% and 58% lower in acid- than in saline-treated animals, indicating that capsaicin-sensitive fibers in the dorsal spinal cord were already partially depleted by acid treatment. Capsaicin denervation reduced basal glutamate release by 33% after acid injury as compared to non-denervated acid-treated animals. Gastric origin and capsaicin sensitivity of glutamatergic, CGRP- and SP-containing primary afferents in thoracic dorsal root ganglia were then determined by retrograde tracing with True Blue and immunohistochemical labeling with the vanilloid receptor TRPV1. About 65% of True Blue-labeled cells were glutamatergic and more than 73% of this population expressed the TRPV1 receptor. Nearly all True Blue/CGRP (85%)- and True Blue/SP-positive cells (97%) coexpressed TRPV1. We conclude that noxious gastric stimulation with acid induces release of glutamate, SP, and CGRP from capsaicin-sensitive sensory afferents in the dorsal horn of the spinal cord where they may play an important role in gastric nociception and hyperalgesia.

Acute nociceptive somatic stimulus sensitizes neurones in the spinal cord to colonic distension in the rat

The common co-existence of fibromyalgia and chronic abdominal pain could be due to sensitization of spinal neurones (SNs), as a result of viscero-somatic convergence. The objective of this study is to explore the influence of acute nociceptive somatic stimulation in the form of acid injections, into the ipsilateral somatic receptive field of neurones responsive to colorectal distension (CRD), and the potential role of ionotropic glutamate receptors on sensitization. Action potentials of CRD-sensitive SNs were recorded extracellularly from the lumbar (L(2)-L(5)) spinal cord. Stimulus-response functions (SRFs) to graded CRD (10-80 mmHg, 30 s) were constructed before and 30 min after ipsilateral injection of low pH (4.0, 100 microl) saline into the somatic receptive fields. In some experiments, cervical (C(1)-C(2)) spinalization was performed to eliminate supraspinal influence. The selective NMDA receptor antagonist CGS 19755 and AMPA receptor antagonist NBQX were injected (25 micromol kg(-1), i.v.) to examine their influence on sensitization. Three types of neurones were characterized as short-latency abrupt (SLA, n = 24), short latency sustained (SLS, n = 12), and long-latency (LL, n = 6) to CRD. Ipsilateral injection of low pH (4.0) in the somatic receptive field, but not the contralateral gastrocnemius (GN) or front leg muscles, sensitized responses of these neurones to CRD. Spinalization had no influence on the development of low pH-induced sensitization. Both CGS 19755 and NBQX significantly attenuated the sensitized response to CRD in intact and spinalized animals. Acute nociceptive somatic stimulus sensitizes CRD-sensitive SNs receiving viscero-somatic convergence. The sensitization occurs at the spinal level and is independent of supraspinal influence. Ionotropic glutamate receptors in the spinal cord are involved in sensitization.

Response properties of antral mechanosensitive afferent fibers and effects of ionotropic glutamate receptor antagonists

The ionotropic glutamate receptors N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are present peripherally in the primary sensory afferent neurons innervating the viscera. Multiple studies have reported roles of glutamate receptors in gastric functions. However, no study has previously shown the direct influence of ionotropic glutamate receptor antagonist on vagal sensory neurons. The objective of this study was to investigate the effects of NMDA and AMPA receptor antagonists on mechanotransduction properties of vagal afferent fibers innervating the rat stomach. Action potentials were recorded from the hyponodal vagus nerve innervating the antrum of the Long-Evans rats. For antral distension (AD), a small latex balloon was inserted into the stomach and positioned in the antrum. The antral contractions were recorded with solid-state probe inserted into the water-filled balloon. Antral units were identified to isovolumic (0.2-1 ml) or isobaric AD (5-60 mm Hg). NMDA and AMPA receptor antagonists were injected in a cumulative fashion (1-100 micromol/kg, i.v.). After the conclusion of experiment, the abdomen was opened and receptive field was mapped by probing the serosa of the stomach. Thirty-two fibers were identified to AD. The receptive fields of 26 fibers were located in the posterior part of the antrum. All fibers exhibited spontaneous firing (mean: 7.00+/-0.97 impulses/s). Twenty fibers exhibited a rhythmic firing that was in phase with antral contractions, whereas 12 fibers exhibited non-rhythmic spontaneous firing unrelated to spontaneous antral contraction. Both groups of fibers exhibited a linear increase in responses to graded isovolumic or isobaric distensions. NMDA (memantine HCl and dizocilpine (MK-801)) and AMPA/kainate (6-cyano-7-nitroquinoxaline 2,3-dione; CNQX) receptor antagonists dose-dependently attenuated the mechanotransduction properties of these fibers to AD. However, competitive NMDA antagonist dl-2-amino-5 phosphopentanoic acid (AP-5) had no effect. The study documents that glutamate receptor antagonists can attenuate responses of gastric vagal sensory afferent fibers innervating the distal stomach, offering insight to potential pharmacological agents in the treatment of gastric disorders.

Involvement of N-methyl-d-aspartate (NMDA) receptors in a rat model of visceral hypersensitivity

N-Methyl-d-aspartate (NMDA) is known to be involved in the transmission of nociceptive information. In the present study, we investigated the effect of peripheral and central NMDA receptor antagonist MK-801 in visceral hypersensitivity. In an animal model of colorectal distension (CRD), administration of both intrathecal MK-801 (1.5 nmol) and intraperitoneal MK-801 (0.15 mg/kg) completely abolished the CRD-induced visceral hypersensitivity of noxious and innocuous stimuli. Thus, the results from this experiment demonstrate the efficacy of MK-801 in blocking the visceral hypersensitivity mediated by central and peripheral mechanisms.

The development and maintenance of human visceral pain hypersensitivity is dependent on the N-methyl-D-aspartate receptor

BACKGROUND & AIMS

Visceral hypersensitivity is a common feature of functional gastrointestinal disorders. One speculated mechanism is an activity-dependent increase in spinal cord neuronal excitability (central sensitization), which is dependent on activation of the N-methyl-D-aspartate (NMDA) receptor. Our aims were to determine whether the development and maintenance of human visceral hypersensitivity is NMDA receptor mediated.

METHODS

Healthy subjects were studied using a randomized, double-blind, placebo-controlled, crossover design. Pain thresholds to electrical stimulation were determined both in the proximal esophagus and in the foot (control) before and after a 30-minute distal esophageal infusion of 0.15 mol/L HCl acid. Ketamine (NMDA receptor antagonist) or saline (vehicle) was given intravenously either prior to or following acid infusion, and pain thresholds were measured for the following 120 minutes. Protocol 1: In 6 subjects, the effect of ketamine in the esophagus was assessed without acid infusion. Protocol 2: In 14 subjects, ketamine was given prior to esophageal acid. Protocol 3: In 12 subjects, ketamine was given after esophageal acid.

RESULTS

Protocol 1: In the absence of esophageal acid, ketamine had no effect on either esophageal or foot pain thresholds (area-under-the-curve, [AUC] P = 0.36 esophagus, P = 0.34 foot, ANOVA) within 30 minutes of cessation of the infusion. Protocol 2: Acid-induced esophageal hypersensitivity was prevented by ketamine (AUC, P < 0.0001, ANOVA) without affecting foot pain thresholds (AUC, P = 0.06, ANOVA). Protocol 3: Ketamine delivered after acid reversed the induction of esophageal hypersensitivity induced by acid (AUC, P < 0.0001, ANOVA).

CONCLUSIONS

The induction and maintenance of acid-induced esophageal hypersensitivity is prevented and reversed by ketamine. This finding strongly indicates that central sensitization is a mechanism of visceral hypersensitivity.

Quantitative assessment and characterization of visceral nociception and hyperalgesia in mice

Colorectal distension (CRD) is a well-characterized model of visceral nociception, which we adapted to the mouse. CRD reproducibly evoked contractions of the abdominal musculature [visceromotor response (VMR)], which was graded to stimulus intensity. The magnitude of VMR was greater in male C57BL6 and female 129S6 mice than in male 129S6 and B6.129 mice. In 129S6, C57BL6, and B6.129 mice strains, VMR was reduced dose dependently by morphine (1-10 mg/kg) and by the kappa-opioid agonist U-69593 (0.2-2 mg/kg), although U-69593 was significantly less potent in C57BL6 mice. In additional experiments, the VMR was recorded from adult male 129S6 mice before and after intracolonic administration of various irritants. Only 30% ethanol significantly enhanced responses to CRD. The colon hyperalgesia persisted for 14 days and was associated with a significant shift of the morphine dose-response function to the left. We believe this will be a useful model for study of visceral nociception and hyperalgesia, including studies of transgenic mice with mutations relevant to pain.

NMDA receptor antagonists attenuate noxious and nonnoxious colorectal distention-induced Fos expression in the spinal cord and the visceromotor reflex

In the present three-part study, the effects of intrathecally administered N-methyl-D-aspartate (NMDA) receptor antagonists on responses to noxious and innocuous colorectal distention (CRD) were examined. In the first part, a passive-avoidance paradigm was used to confirm that 80 mm Hg CRD is a noxious stimulus since it produced avoidance behavior. Acquisition of this behavior was blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonopetanoic acid (APV, 60 nmol, intrathecal). In contrast, 20 mm Hg CRD is an innocuous stimulus since there was no difference in the behavior of these animals compared to nondistended controls. In the second part, the effects of the NMDA receptor antagonist dizocilpine maleate (MK-801, 0-100 nmol, intrathecal) on CRD-induced Fos expression in the lumbosacral spinal cord were examined. Noxious and innocuous CRD induced 98+/-4 and 50+/-2 Fos labeled cells per section per side of the spinal cord, respectively. MK-801 dose-dependently attenuated noxious CRD-induced Fos. Compared to saline, the peak attenuation was 55%. Innocuous CRD-induced Fos was attenuated by 36% following 100 nmol MK-801. In the third part, the effects of APV (0-240 nmol, intrathecal) on the visceromotor reflex were examined. APV dose-dependently attenuated the visceromotor reflex to graded intensities of CRD that went from the innocuous into the noxious range. In separate animals that only received innocuous stimulation, APV dose-dependently attenuated the visceromotor reflex. The magnitude of attenuation was similar for both stimulus paradigms. These data expand upon our previous dorsal horn neuronal recordings which showed that spinal NMDA receptors partially mediate the processing of both noxious and innocuous colorectal stimuli. They further underscore a difference from somatic tissue in the role of NMDA receptors in processing acute or transient visceral stimuli in the absence of tissue injury.

Two N-methyl-D-aspartate receptors in rat dorsal root ganglia with different subunit composition and localization

N-methyl-D-aspartate (NMDA) receptors in sensory afferents participate in chronic pain by mediating peripheral and central sensitization. We studied the presence of NMDA receptor subunits in different types of primary afferents. Western blots indicated that rat dorsal root ganglia (DRG) contain NR1, NR2B, NR2C, and NR2D but not NR2A. Real-time RT-PCR showed that NR2B and NR2D were expressed at higher levels than NR2A and NR2C in DRG. Immunofluorescence with an antibody that recognized NR1 and another that recognized NR2A and NR2B showed that NR1 and NR2B colocalized in 90% of DRG neurons, including most A-fibers (identified by the presence of neurofilament 200 kDa). In contrast, an antibody recognizing NR2C and NR2D labeled only neurofilament-negative DRG profiles. This antibody stained practically all DRG cells that contained calcitonin gene-related peptide and neurokinins and those that bound isolectin B4. The percentage of cells immunoreactive for NR1, NR2A/NR2B, and NR2C/NR2D were the same in the T9, T12, L4, and L6 DRG. The intracellular distribution of the NR2 subunits was strikingly different: Whereas NR2A/NR2B immunoreactivity was found in the Golgi apparatus and occasionally at the plasma membrane, NR2C/NR2D immunoreactivity was found in the cytoplasm but not in the Golgi. The NR1 subunit was present throughout the cytoplasm and was more intense in the Golgi. These findings indicate that DRG neurons have two different NMDA receptors, one containing the NR1, NR2D, and possibly the NR2C subunits, found only in C-fibers, and the diheteromer NR1/NR2B, present in the Golgi apparatus of both A- and C-fibers.

The role of CNS NMDA receptors and nitric oxide in visceral hyperalgesia

The studies summarized here document the role of NMDA receptors and nitric oxide in the lumbosacral spinal cord and rostral ventromedial medulla in the maintenance of visceral hyperalgesia. Experiments were conducted in rats in which drugs were administered into either the lumbosacral intrathecal space or directly into the rostral ventromedial medulla. The visceral stimulus was noxious colorectal distension, administered before and 3 h after intracolonic instillation of either saline or 25% zymosan. The visceromotor response to colonic distension was quantified and found to be significantly enhanced in rats in which the colon had previously been treated with zymosan. Enhanced responses to distension were attenuated dose-dependently by intrathecal administration of the NMDA receptor channel blocker MK-801 and by inhibition of the neuronal isoform of nitric oxide synthase (nNOS). In corresponding studies wherein drugs were administered directly into the rostral ventromedial medulla, NMDA receptor antagonism and NOS inhibition dose-dependently attenuated exaggerated responses to colonic distension. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced both at the level of the spinal cord and rostral ventromedial medulla, and that descending facilitatory influences from the rostral ventromedial medulla are important to the maintenance of visceral hyperalgesia.

Spinal NMDA receptors contribute to neuronal processing of acute noxious and nonnoxious colorectal stimulation in the rat

The present study investigated the role of NMDA receptors in the spinal processing of acute noxious and nonnoxious colorectal stimulation using extracellular single-unit recording in the rat. Fifty-three neurons in the L6-S2 dorsal horn of the spinal cord were studied. Neurons were identified using touch and light pinch of the ipsilateral perianal/scrotal area and colorectal distention (CRD). All neurons had excitatory responses to CRD. Thirty neurons were studied using a search stimulus of 80-mmHg CRD. The effects of a systemically administered N-methyl-D-aspartate (NMDA) receptor channel blocker, dizocilpine maleate (MK-801) (0.1, 0.5, 1.0, and 5.0 mg/kg), were tested on the CRD-evoked responses of 13 neurons. The lowest dose had no effect on the neuronal responses to CRD, while greater doses lowered the CRD-evoked responses at all distention pressures tested (20, 40, 60, and 80 mmHg). Similarly, spinal application of MK-801 (20, 50, 100, and 200 nmol) attenuated CRD-evoked activity (n = 9). In addition, a spinally administered competitive NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV) (30, 60, 120, and 240 nmol), dose-dependently attenuated the CRD-evoked response at all distention pressures (n = 5). Systemically administered APV did not affect neuronal responses to CRD (n = 3). Twenty-three neurons were studied in animals that never received distention pressures exceeding 30 mmHg; the search stimulus ranged between 20- and 30-mmHg CRD. These neurons were tested using 20-mmHg CRD. Systemically administered MK-801 facilitated the response to 20-mmHg CRD in three neurons and inhibited the response in five neurons, and the response of five neurons was not affected. Spinally administered MK-801 had no effect on neuronal responses to 20-mmHg CRD in six neurons. However, spinally administered APV dose-dependently decreased the response to 20-mmHg CRD in four neurons. These results are consistent with our previous observations that used Fos expression as the index, suggesting that spinal NMDA receptors contribute to processing of both noxious and nonnoxious CRD.

The effect of fentanyl, DNQX and MK-801 on dorsal horn neurones responsive to colorectal distension in the anaesthetized rat

Certain dorsal horn neurones respond in a graded manner to noxious colorectal distension (CRD). Morphine inhibits these responses in the spinalized rat, but the role of excitatory amino acids in baseline visceral nociceptive transmission is less clear. This study examines the effect of the mu-opiate receptor agonist fentanyl, and the non-NMDA and NMDA antagonists DNQX and MK-801, respectively, on such responses to CRD in the sodium pentobarbitone-anaesthetized rat. Male rats were prepared for extracellular recording from the lumbosacral spinal cord. 90 neurones responsive to CRD, located throughout the dorsal horn, were classified according to their response duration and latency to 60 mmHg distension, as SL-A (short latency-abrupt; 59%), SL-S (short latency-sustained; 23%), L-L (long-latency; 10%) and Inhib (inhibited; 8%). Convergent cutaneous receptive fields were mapped for 79/90 neurones and classified as LT (low threshold), WDR (wide dynamic range) or HT (high threshold). CRD (20-100 mm Hg) elicited graded responses in most neurones. In 6/6 SL-S neurones, fentanyl (1-8 microg kg-1) dose-dependently inhibited the response to 60 mm Hg CRD, in a naloxone-sensitive manner, with an ID50 value (+/-95% confidence limits) of 2.48 (1.7-3. 7) microg kg-1. In 6/6 SL-A neurones, fentanyl had no significant effect on the response to CRD. DNQX (0.03-3 mg kg-1) produced a dose-dependent inhibition of the response to CRD in 5/5 SL-A neurones, with an ID50 value of 0.32 (0.01-41.1) mg kg-1. MK-801 (0. 03-0.3 mg kg-1) had no significant effect on responses to CRD in 6/6 SL-A neurones. The differential inhibitory effects of fentanyl on two neuronal subtypes may indicate functional differences. In SL-A neurones AMPA/kainate, but not NMDA receptors are involved in mediating baseline nociceptive neurotransmission.

N-methyl-D-aspartate enhancement of the glycine response in the rat sacral dorsal commissural neurons

The effect of N-methyl-D-aspartate (NMDA) on the glycine (Gly) response was examined in neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. The application of 100 microM NMDA to SDCN neurons reversibly potentiated Gly-activated Cl- currents (IGly) without affecting the Gly binding affinity and the reversal potential of IGly. A selective NMDA receptor antagonist, APV (100 microM), blocked the NMDA-induced potentiation of IGly, whereas 50 microM CNQX, a non-NMDA receptor antagonist, did not. The potentiation effect was reduced when NMDA was applied in a Ca2+-free extracellular solution or in the presence of BAPTA AM, and was independent of the activation of voltage-dependent Ca2+ channels. Pretreatment with KN-62, a selective Ca2+-calmodulin-dependent protein kinase II (CaMKII) inhibitor, abolished the NMDA action. Inhibition of calcineurin (CaN) further enhanced the NMDA-induced potentiation of IGly. In addition, the GABAA receptor-mediated currents were suppressed by NMDA receptor activation in the SDCN neurons. The present results show that Ca2+ entry through NMDA receptors modulates the Gly receptor function via coactivation of CaMKII and CaN in the rat SDCN neurons. This interaction may represent one of the important regulatory mechanisms of spinal nociception. The results also suggest that GABAA and Gly receptors may be subject to different intracellular modulatory pathways.

The NMDA receptor antagonist MK-801 attenuates c-Fos expression in the lumbosacral spinal cord following repetitive noxious and non-noxious colorectal distention

The effects of pretreatment with an NMDA receptor antagonist, MK-801, on c-Fos (Fos) expression in the lumbosacral spinal cord following repetitive, noxious (80 mmHg) or non-noxious (20 mmHg) colorectal distention (CRD) was examined immunocytochemically in awake and urethane anesthetized rats. In awake rats, noxious CRD induced Fos expression in the lumbosacral spinal cord. Pretreatment with MK-801 (0.1-1.0 mg/kg, i.p.) produced no change or an increase in noxious CRD induced-Fos expression and caused aversive side effects. In order to examine greater doses of MK-801, further experiments were performed in rats anesthetized with urethane. Both noxious and non-noxious CRD induced Fos in the lumbosacral spinal cord. Pretreatment with MK-801 (0.5, 1.0, 5.0 mg/kg, i.p.) dose-dependently attenuated noxious CRD-induced Fos by 20-40%. Five mg/kg MK-801 attenuated non-noxious CRD-induced Fos by 20%. Lesser doses did not significantly attenuate Fos expression. The laminar distribution of Fos following MK-801 pretreatment revealed a tendency towards the deeper laminae showing the greatest attenuation at the highest dose of MK-801. Protein plasma extravasation in the colon measured with Evan's blue dye showed no difference between rats without balloons, rats with balloons that were not distended and non-noxious CRD. There was significantly more extravasation following noxious CRD. Pretreatment with systemic MK-801 had no effect on plasma extravasation produced by noxious CRD. These data suggest that the induction of Fos in the lumbosacral spinal cord by noxious and non-noxious CRD is partially NMDA receptor mediated. However, NMDA receptor activation contributes significantly more to noxious than non-noxious CRD-induced Fos. Inflammation of the colon following noxious CRD likely contributes to sensitization of colonic afferents which may contribute to the increased NMDA receptor-mediated Fos following the noxious stimulus.

Recent and forgotten aspects of visceral pain

Progress in the field of visceral pain research has been particularly rapid in recent years. Some aspects of the symptom that had previously been neglected for some time have now received a great deal of attention in both clinical and experimental studies. This regards, in particular, phenomena of hyperalgesia: (a) of visceral structures, because of local inflammatory/sensitizing processes (visceral hyperalgesia); (b) of areas of referred pain from viscera (referred somatic hyperalgesia from viscera); and (c) of a visceral structure, because of an algogenic process of another visceral domain (viscero-visceral hyperalgesia). Clinical studies in patients have led to characterisation of subjective and objective symptoms of these phenomena. A number of studies in human volunteers (employing experimental procedures to stimulate and measure pain reactivity in both visceral structures and somatic areas of referral) have further increased the knowledge about modalities of generation of the various forms of hyperalgesia.Animal experiments have improved understanding of pathophysiological mechanisms, mostly those underlying the referred hyperalgesia, with a number of findings supporting the notion of central changes at the basis of the phenomenon. An important aspect of laboratory experiments in recent years has been the setting up of animal models of visceral pain conditions closely mimicking a number of clinical pain states in patients. As a result, the outcome of experimental studies (electrophysiological, pharmacological, etc.) appears more directly applicable to the interpretation of the clinical reality.Finally, in the context of laboratory studies, a novel trend of investigation is represented by genetic experiments, particularly those employing 'knock-out' mice. These experiments, by generating animals lacking specific genes responsible for the production of various receptors implicated in pain transmission, have further contributed to the understanding of the generation of visceral pain symptoms. Although studies in this field are in their early stage, they seem particularly promising for a better understanding of the pathophysiology of visceral pain, and thus the establishment of more satisfying therapies in the future. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Functional properties of ionotropic glutamate receptor channels in rat sacral dorsal commissural neurons

Nystatin perforated patch and conventional whole-cell recording configurations were used to characterize the properties of ionotropic glutamate receptor (GluR) channels in neurons freshly dissociated from the rat sacral dorsal commissural nucleus (SDCN). L-Glutamate (Glu), N-methyl-D-aspartate (NMDA), quisqualate (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleprop ionate (AMPA) and kainate (KA) applied via a Y-tube produced inward currents at -44 mV which increased in a concentration-dependent manner; they desensitized when induced at higher concentrations except for the KA-induced current (IKA). (1S-3R)1-amino-cyclopentane-1,3-dicarboxylate (1S-3R-ACPD) evoked no response. The EC50 and Hill coefficient (nH) values of the GluR responses were 3.3 x 10(-5) M, 0.74 for Glu; 9.0 x 10(-5) M, 0.83 for NMDA; 6.4 x 10(-7) M, 1.30 for QA; 1.3 x 10(-4) M, 1.10 for AMPA and 9.6 x 10(-5) M, 1.30 for KA, respectively. The reversal potentials of the GluR responses were all near 0 mV. The 6-Cyano-7-nitroquinoxaline-2-3-dione (CNQX) and D-2-amino-5-phosphonovalerate (D-APV) suppressed the non-NMDA and NMDA responses in a concentration-dependent manner, respectively. Cyclothiazide strongly potentiated both KA- and AMPA-induced responses while concanavalin A potentiated both the responses to a much lesser degree. NS-102 produced no significant effect on either KA- or AMPA-activated currents, while GYKI 52466 reversibly blocked both the currents. The Ca2+ permeabilities (PCa/PCs) of the NMDA and AMPA receptor channels were 8.33 and 1.23, respectively. In addition, the current-voltage (I-V) relationship of IKA showed little rectification. There was a poor correlation between the Ca2+ permeability and the shape of the I-V curves of IKA. These results suggest that rat SDCN neurons possess NMDA and non-NMDA receptor channels, and express AMPA type receptors with unique properties (slow desensitization to AMPA, high Ca2+ permeability but lack of inward rectification). These ionotropic receptor channels may play important roles in mediating and regulating pelvic visceral information including nociception.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

A psychophysical study of discomfort produced by repeated filling of the urinary bladder

Psychophysical studies were performed in 10 healthy, female volunteers using urinary bladder distension (UBD) as a visceral stimulus. Stimulus methodology was similar to that used clinically for obtaining cystometrograms with a fixed-rate (100 cc/min normal saline) filling of the urinary bladder, occasional pauses and simultaneous measure of bladder pressure using a catheter-transducer assembly. During bladder filling, subjects were asked to report sensations by verbal report and by using an electronic, hand-held, visual-analog-scale device. Sensations evoked by UBD were generally localized to the suprapubic region. UBD produced cardiovascular responses which increased with repeated trials. Sensation intensity increased with repeated UBDs as indicated by global pain ratings. Intravesical pressure and volume correlated with sensation intensity. The volume of distending fluid needed to produce a report of discomfort was highly variable from trial to trial and did not change significantly with repeated UBDs. The intravesical pressure which produced a report of discomfort was less variable and significantly decreased with repeated UBDs. The change in intravesical pressure and volume needed to produce discomfort was inversely correlated with initial intravesical pressure measures. Similar to findings in other organ systems, these findings demonstrate that repeated presentations of a visceral stimulus may lead to an increase in physiological and perceptual responses to pain.

Intracolonic zymosan produces visceral hyperalgesia in the rat that is mediated by spinal NMDA and non-NMDA receptors

The present study examined the effects of colonic inflammation on reflex responses to colorectal distension (CRD) in awake rats. Visceromotor responses (VMR) to CRD were recorded in rats that received either no treatment or intracolonic saline or zymosan. Three hours following zymosan treatment (25 mg/ml; 1 ml) VMR response magnitudes were significantly increased at all intensities of CRD tested (10-80 mmHg). The enhanced responses to CRD were attenuated in a dose-dependent fashion by intrathecal administration of the non-competitive N-methyl-D-aspartate (NMDA) receptor channel blocker MK-801 to 60% of control and by the non-NMDA receptor antagonist DNQX to less than 20% of control. The metabotropic receptor antagonist AP-3 was without effect. Signs of multi-focal colonic inflammation were clearly present 3 h after zymosan treatment, characterized by an ingress of inflammatory cells and damaged crypts in and around these foci. Taken together these findings suggest that tissue inflammation increases the sensitivity of the colon to mechanical stimuli, leading to enhanced responses to CRD. This enhancement involves the activation of spinal NMDA as well as non-NMDA receptors, but not metabotropic receptors.