The effects of excitatory amino acids on isolated gut segments of the rat

The role of NMDA receptors in fish stress response: Assessments based on physiology of the caudal neurosecretory system and defensive behavior

Stress strongly influences the physiology and behavior of animals, and leads into a pathological condition and disease. NMDA receptors (NMDARs) play a crucial role in the modulation of neural activity. To understand the role of NMDARs in fish stress response, we used NMDARs agonist aspartate to test the functional role of its input on the Dahlgren cell population in the caudal neurosecretory system (CNSS) of the olive flounder. In addition, the effect of the NMDARs antagonist D-AP5 on the expression of genes of the main secretory products of the CNSS after stress was investigated by using qPCR technology and the effect of the NMDARs antagonist D-AP5 on post-stress behavior was explored by behavioral methods. Ex vivo electrophysiological experiments showed that the NMDARs agonist aspartate enhanced the firing frequency of Dahlgren cells. Additionally, aspartate treatment increased the incidence of cells exhibiting bursting firing pattern, this result is corroborated by the observed upregulation in the expression of ion channels and major hormone genes in the CNSS. Furthermore, the excitatory influence of aspartate was effectively counteracted by NMDARs antagonist D-AP5. Interestingly, NMDARs antagonist D-AP5 treatment also significantly decreased the plasma cortisol levels and the expression of CRH, UI, and UII in CNSS after acute stress. Treatment with D-AP5 effectively attenuated the stress response, as evidenced by alterations in respiratory metabolism, sand-burying behavior, swimming distance, simulated capture, and escape response. In conclusion, modulation of Dahlgren cell excitability in the CNSS by NMDARs contributes to the regulation of the stress response, NMDARs antagonist D-AP5 can effectively suppress stress response in flounder by regulating the stress hormone expression and secretion. CLINICAL TRIAL REGISTRATION: Project code SHOU-DW-2022-032.

Investigating the L-Glu-NMDA receptor-H2S-NMDA receptor pathway that regulates gastric function in rats' nucleus ambiguus

Background

In previous investigations, we explored the regulation of gastric function by hydrogen sulfide (H2S) and L-glutamate (L-Glu) injections in the nucleus ambiguus (NA). We also determined that both H2S and L-Glu have roles to play in the physiological activities of the body, and that NA is an important nucleus for receiving visceral sensations. The purpose of this study was to explore the potential pathway link between L-Glu and H2S, resulting in the regulation of gastric function.

Methods

Physiological saline (PS), L-glutamate (L-Glu, 2 nmol), NaHS (2 nmol), D-2-amino-5-phopho-novalerate (D-AP5, 2 nmol) + L-Glu (2 nmol), aminooxyacetic acid (AOAA, 2 nmol) + L-Glu (2 nmol), D-AP5 (2 nmol) + NaHS (2 nmol) were injected into the NA. A balloon was inserted into the stomach to observe gastric pressure and for recording the changes of gastric smooth muscle contraction curve. The gastric fluid was collected by esophageal perfusion and for recording the change of gastric pH value.

Results

Injecting L-Glu in NA was found to significantly inhibit gastric motility and promote gastric acid secretion in rats (p < 0.01). On the other hand, injecting the PS, pre-injection N-methyl-D-aspartate (NMDA) receptor blocker D-AP5, cystathionine beta-synthase (CBS) inhibitor AOAA and re-injection L-Glu did not result in significant changes (p > 0.05). The same injection NaHS significantly inhibit gastric motility and promote gastric acid secretion in rats (p < 0.01), but is eliminated by injection D-AP5 (p > 0.05).

Conclusion

The results indicate that both exogenous L-Glu and H2S injected in NA regulate gastric motility and gastric acid secretion through NMDA receptors. This suggests that NA has an L-Glu-NMDA receptor-CBS-H2S pathway that regulates gastric function.

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.

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.

Immunolocalization of AMPA receptor subunits within the enteric nervous system of the mouse colon and the effect of their activation on spontaneous colonic contractions

BACKGROUND

The appropriate expression of specific neurotransmitter receptors within the cellular networks that compose the enteric nervous system (ENS) is central to the regulation of gastrointestinal (GI) functions. While the ENS expression patterns of the neurotransmitter glutamate have been well documented, the localization of its receptors on ENS neurons remains to be fully characterized. We investigated the expression patterns of glutamate receptor AMPA subunits within ENS neurons of the mouse colon and the consequences of their pharmacological activation on spontaneous colonic contractility.

METHODS

RT-PCR was used to detect individual AMPA receptor (GluR 1-4) subunit expression at the mRNA level in mouse colon tissue. Immunohistochemistry and confocal microscopy was used to localize the expression of the GluR1 and 4 subunits in colon tissue. Brain tissue was used as a positive control. Organ bath preparations were used to determine the effect of AMPA receptors activation on the force and frequency of colonic longitudinal smooth muscle spontaneous contractions.

KEY RESULTS

GluR1, 3, 4 mRNA was detected in the mouse colon. Immunoreactivity for GluR1 and 4 subunits was detected on the somatic and dendritic surfaces of subpopulations of neurochemically defined ENS neurons. The pharmacological activation of AMPA receptors increased the force but not frequency of spontaneous colonic contractions.

CONCLUSIONS & INFERENCES

Molecularly distinct AMPA receptor subtypes are differentially expressed within the neural networks of the mouse colon and have a direct role in motility. These data provide the rationale for the development of AMPA-selective ligands for the therapeutic delivery to the GIT in motility disorders.

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.

A novel role for the metabotropic glutamate receptor-7: modulation of faecal water content and colonic electrolyte transport in the mouse

BACKGROUND AND PURPOSE

Increasing evidence implicates metabotropic glutamate receptor mGlu(7) in the pathophysiology of stress-related disorders such as depression and anxiety. Mood disorders are frequently associated with gastrointestinal (GI) dysfunction; however, the role of mGlu(7) receptors outside the CNS is unknown. This present study investigated the expression and possible functional role of mGlu(7) receptors in the mouse colon.

EXPERIMENTAL APPROACH

Expression of mGlu(7) receptor mRNA and protein was studied in mouse colon by in situ hybridization and Western blotting. Effects of the selective mGlu(7) receptor agonist AMN082 on defecation and faecal parameters were studied in an isolation-induced stress model. AMN082 effects on ion transport and neuronal intracellular signalling were examined via Ussing chambers and calcium imaging.

KEY RESULTS

mGlu(7) receptor mRNA and protein were highly expressed in colon mucosa. Stress-induced faecal output was unaffected by AMN082, although faecal water content was increased. In mucosa/submucosa preparations, 100 nM and 1 microM AMN082 increased bethanechol-induced changes in short-circuit current in the Ussing chamber. This was sensitive to tetrodotoxin. Also, 100 nM AMN082 significantly increased calcium signalling in a subset of submucosal neurons.

CONCLUSIONS AND IMPLICATIONS

Activating mGlu(7) receptors increased colonic secretory function in vivo and ex vivo. In a group of submucosal neurons, AMN082 strongly induced calcium signalling and the presence of submucosal nerves was required for the AMN082-dependent increase in secretion. These data suggest that targeting mGlu(7) receptors may be useful in the treatment of central components of stress disorders and also stress-associated GI dysfunction such as diarrhoea or constipation.

Metabolic fate and function of dietary glutamate in the gut

Glutamate is a main constituent of dietary protein and is also consumed in many prepared foods as an additive in the form of monosodium glutamate. Evidence from human and animal studies indicates that glutamate is a major oxidative fuel for the gut and that dietary glutamate is extensively metabolized in first pass by the intestine. Glutamate also is an important precursor for bioactive molecules, including glutathione, and functions as a key neurotransmitter. The dominant role of glutamate as an oxidative fuel may have therapeutic potential for improving function of the infant gut, which exhibits a high rate of epithelial cell turnover. Our recent studies in infant pigs show that when glutamate is fed at higher (4-fold) than normal dietary quantities, most glutamate molecules are either oxidized or metabolized by the mucosa into other nonessential amino acids. Glutamate is not considered to be a dietary essential, but recent studies suggest that the level of glutamate in the diet can affect the oxidation of some essential amino acids, namely leucine. Given that substantial oxidation of leucine occurs in the gut, ongoing studies are investigating whether dietary glutamate affects the oxidation of leucine in the intestinal epithelial cells. Our studies also suggest that at high dietary intakes, free glutamate may be absorbed by the stomach as well as the small intestine, thus implicating the gastric mucosa in the metabolism of dietary glutamate. Glutamate is a key excitatory amino acid, and metabolism and neural sensing of dietary glutamate in the developing gastric mucosa, which is poorly developed in premature infants, may play a functional role in gastric emptying. These and other recent reports raise the question as to the metabolic role of glutamate in gastric function. The physiologic significance of glutamate as an oxidative fuel and its potential role in gastric function during infancy are discussed.

Effects of exogenous glutamate and kainate on electric field-stimulated contractions of isolated human ureter

OBJECTIVES

A neurotransmitter role for glutamate in the autonomous nervous system was recently demonstrated in the gastrointestinal tract, and its stimulatory effect on spontaneous motility of human ureter was shown. The aim of our study was to investigate the effects of glutamate on the release of neurotransmitters from intramural nerves of the human ureter.

METHODS

The effects of exogenous glutamate were tested on electric field-stimulated contractions of isolated human ureter, taken from 16 adult patients after nephrectomy. The longitudinal tension and intraluminal pressure of the isolated ureter were recorded simultaneously. The electric field stimulation was done with square wave pulses (20 V through electrodes, 400 mA, duration 1 ms, frequency 16 Hz). The pulse trains lasted for 30 s, a with 30-s pause.

RESULTS

Glutamate (7.9 x 10(-6) M/L to 10.6 x 10(-3) M/L) and kainic acid (6.3 x 10(-8) M/L to 2.2 x 10(-5) M/L) produced a concentration-dependent decrease in the electric field-stimulated activity of the isolated preparations. However, N-methyl-D-aspartic acid (9.1 x 10(-8) M/L to 3.1 x 10(-5) M/L), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (7.2 x 10(-8) M/L to 3.2 x 10(-6) M/L) and (+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (7.7 x 10(-8) M/L to 6.5 x 10(-5) M/L) were ineffective. The electric field-stimulated contractions of isolated ureter were also inhibited by lidocaine (3.70 x 10(-4)M/L) and atropine (1.00 x 10(-6)M/L).

CONCLUSIONS

The results of our study suggest that glutamate inhibits electric field-stimulated release of acetylcholine in the human ureter through activation of kainate ionotropic receptors, located on the intramural nerve fibers.

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.

Effect of exogenous glutamate and N-Methyl-D-aspartic acid on spontaneous activity of isolated human ureter

OBJECTIVES

While the neurotransmitter role of glutamate in the gastrointestinal tract has been shown, its effects on smooth muscle of the human ureter have not previously been investigated. In our study we have investigated the effects of exogenous glutamate on the spontaneous activity of isolated human ureter, taken from 14 adult patients after nephrectomy.

METHODS

The segment of ureter, excised 3 cm distal from the pyeloureteral junction, was isolated in an organ bath. Both longitudinal tension and intraluminal pressure of the segment were recorded simultaneously.

RESULTS

Glutamate administered in the lumen of the isolated ureteral segments (7.8 x 10(-7) M/L-3.5 x 10(-2) M/L) was ineffective. When added to the isolated organ bath from the serous side of the ureteral segment, glutamate (7.9 x 10(-6) M/L-10.6 x 10(-3) M/L) and N-Methyl-D-aspartic acid (NMDA) (9.1 x 10(-8) M/L-3.1 x 10(-5) M/L) produced a concentration-dependent increase in spontaneous activity of the isolated preparations, while kainic acid (6.3 x 10(-8) M/L-10.5 x 10(-5) M/L) and (+/-)-trans-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) (7.7 x 10(-8) M/L -6.5 x 10(-5) M/L) were ineffective.

CONCLUSIONS

The results of our study suggest that an excitatory neurotransmitter glutamate stimulates spontaneous activity of the human ureter through activation of NMDA ionotropic receptors, located on smooth muscle cells or intramural nerve fibers.

M2 and M3 muscarinic receptors are involved in enteric nerve-mediated contraction of the mouse ileum: Findings obtained with muscarinic-receptor knockout mouse

The involvement of muscarinic receptors in neurogenic responses of the ileum was studied in wild-type and muscarinic-receptor (M-receptor) knockout (KO) mice. Electrical field stimulation to the wild-type mouse ileum induced a biphasic response, a phasic and sustained contraction that was abolished by tetrodotoxin. The sustained contraction was prolonged for an extended period after the termination of electrical field stimulation. The phasic contraction was completely inhibited by atropine. In contrast, the sustained contraction was enhanced by atropine. Ileal strips prepared from M2-receptor KO mice exhibited a phasic contraction similar to that seen in wild-type mice and a sustained contraction that was larger than that in wild-type mice. In M3-receptor KO mice, the phasic contraction was smaller than that observed in wild-type mice. Acetylcholine exogenously administrated induced concentration-dependent contractions in strips isolated from wild-type, M2- and M3-receptor KO mice. However, contractions in M3-receptor KO mice shifted to the right. The sustained contraction was inhibited by capsaicin and neurokinin NK2 receptor antagonist, suggesting that it is mediated by substance P (SP). SP-induced contraction of M2-receptor KO mice did not differ from that of wild-type mice. SP immunoreactivity was located in enteric neurons, colocalized with M2 receptor immunoreactivity. These results suggest that atropine-sensitive phasic contraction is mainly mediated via the M3 receptor, and SP-mediated sustained contraction is negatively regulated by the M2 receptor at a presynaptic level.

Enhancement of rat bladder contraction by artificial sweeteners via increased extracellular Ca2+ influx

INTRODUCTION

Consumption of carbonated soft drinks has been shown to be independently associated with the development of overactive bladder symptoms (OR 1.62, 95% CI 1.18, 2.22) [Dallosso, H.M., McGrother, C.W., Matthews, R.J., Donaldson, M.M.K., 2003. The association of diet and other lifestyle factors with overactive bladder and stress incontinence: a longitudinal study in women. BJU Int. 92, 69-77]. We evaluated the effects of three artificial sweeteners, acesulfame K, aspartame and sodium saccharin, on the contractile response of isolated rat detrusor muscle strips.

METHODS

Strips of detrusor muscle were placed in an organ bath and stimulated with electrical field stimulation (EFS) in the absence and presence of atropine, and with alpha,beta methylene ATP, potassium, calcium and carbachol.

RESULTS

Sweeteners 10(-7) M to 10(-2) M enhanced the contractile response to 10 Hz EFS compared to control (p<0.01). The atropine-resistant response to EFS was marginally increased by acesulfame K 10(-6) M, aspartame 10(-7) M and sodium saccharin 10(-7) M. Acesulfame K 10(-6) M increased the maximum contractile response to alpha,beta methylene ATP by 35% (+/-9.6%) (p<0.05) and to KCl by 12% (+/-3.1%) (p<0.01). Sodium saccharin also increased the response to KCl by 37% (+/-15.2%) (p<0.05). These sweeteners shifted the calcium concentration-response curves to the left. Acesulfame K 10(-6) M increased the log EC(50) from -2.79 (+/-0.037) to -3.03 (+/-0.048, p<0.01) and sodium saccharin 10(-7) M from -2.74 (+/-0.03) to 2.86 (+/-0.031, p<0.05). The sweeteners had no significant effect on the contractile response to carbachol but they did increase the amplitude of spontaneous bladder contractions.

DISCUSSION

These results suggest that low concentrations of artificial sweeteners enhanced detrusor muscle contraction via modulation of L-type Ca(+2) channels.

Phosphorylation of NMDA NR1 subunits in the myenteric plexus during TNBS induced colitis

N-Methyl-d-aspartic acid (NMDA) receptors are known to function in the mediation of pain and have a significant role in the development of hyperalgesia following inflammation. Serine phosphorylation regulation of NMDA receptor function occurs in a variety of conditions. No studies have demonstrated a change in phosphorylation of enteric NMDA receptors following colonic inflammation. We examined the levels of NMDA NR1 phosphorylation in trinitrobenzene sulfonic acid (TNBS) induced colitis in rats and compared it to protein translation and the development of visceral hypersensitivity. We have previously, demonstrated an increase in the C1 cassette of NR1 mRNA expression at 14, 21, and 28 days following TNBS administration. In this study, we examined the NR1 serine phosphorylation at 14 days following TNBS injection. Male Sprague-Dawley rats (200-250 g) were treated with TNBS (20mg per rat) diluted in 50% ethanol (n=3) and vehicle controls of 50% ethanol (n=3). TNBS and vehicle controls were administered with a 24 gauge catheter inserted into the lumen of the rat colon. The animals were sacrificed at 14 days after induction of the colitis and their distal colon was retrieved for two-dimensional (2D) western blot analysis. Serine phosphorylation of the NR1 subunit with C1 cassette appears at 14 days after TNBS injection. In contrast, there was no NR1-C1 expression in the vehicle controls and untreated normal controls. These results suggest a role for colonic-NMDA receptor phosphorylation in the development of neuronal plasticity following colonic inflammation. Phosphorylation of NR1 may partially explain visceral hypersensitivity present during colonic inflammation.

Selective up-regulation of NMDA-NR1 receptor expression in myenteric plexus after TNBS induced colitis in rats

Background

N-methyl-D-aspartic acid (NMDA) spinal cord receptors play an important role in the development of hyperalgesia following inflammation. It is unclear, however, if changes in NMDA subunit receptor gene expression in the colonic myenteric plexus are associated with colonic inflammation. We investigated regulation of NMDA-NR1 receptor gene expression in TNBS induced colitis in rats. Male Sprague-Dawley rats (150 g–250 g) were treated with 20 mg trinitrobenzene sulfonic acid (TNBS) diluted in 50% ethanol. The agents were delivered with a 24 gauge catheter inserted into the lumen of the colon. The animals were sacrificed at 2, 7, 14, 21, and 28 days after induction of the colitis, their descending colon was retrieved for reverse transcription-polymerase chain reaction; a subset of animals' distal colon was used for two-dimensional (2-D) western analysis and immunocytochemistry.

Results

NR1-exon 5 (N1) and NR1-exon 21 (C1) appeared 14, 21 and 28 days after TNBS treatment. NR1 pan mRNA was up-regulated at 14, 21, and 28 days. The NR1-exon 22 (C2) mRNA did not show significant changes. Using 2-D western analysis, untreated control rats were found to express only NR1₀₀₁ whereas TNBS treated rats expressed NR1₀₀₁, NR1_011, and NR1₁₁₁. Immunocytochemistry demonstrated NR1-N1 and NR1-C1 to be present in the myenteric plexus of TNBS treated rats.

Conclusion

These results suggest a role for colonic myenteric plexus NMDA receptors in the development of neuronal plasticity and visceral hypersensitivity in the colon. Up-regulation of NMDA receptor subunits may reflect part of the basis for chronic visceral hypersensitivity in conditions such as post-infectious irritable bowel syndrome.

A new anti-obesity drug treatment: first clinical evidence that, antagonising glutamate-gated Ca2+ ion channels with memantine normalises binge-eating disorders

The regulation of appetite relies on complex hypothalamic neurocircuitry of which the arcuate nucleus, and the hormone leptin play important roles. Arcuate nucleus neurones are essential for the regulation of eating behaviour, but they can be intoxicated by elevated serum levels of the amino acid glutamate (GLU). Neurotoxic effects of GLU are mediated by the N-methyl-D-aspartate receptor (NMDA-R). But the neurotoxic effects of GLU can be prevented. Concurrent administration of dizocilpine maleate (MK-801), a selective and highly potent non-competitive NMDA-R antagonist, antagonises GLU-gated Ca2+ ion channels and completely prevents the adverse effects of GLU. Also the non-competitive NMDA-R antagonist memantine displays neuroprotective properties. In view of a previously published hypothesis that human obesity results from chronic over-consumption of GLU, we performed a therapeutic trial in five obese, but otherwise healthy women. Memantine treatment markedly decreased appetite within few hours and complete suppressed the binge-eating disorder within 24 h. Body weight decreased markedly within a few days. The findings strongly support the hypothesis that elevated levels of nutritional GLU play an important role in the pathomechanism of human obesity. We suggest to treat human obesity by protecting the hypothalamic signalling cascade of leptin action with low to moderate affinity, non-competitive NMDA-R antagonists that selectively block the GLU-gated Ca2+ ion channels.

Function of GABAergic and glutamatergic neurons in the stomach

Gamma-aminobutyric acid (GABA) and L-glutamic acid (L-Glu) are transmitters of GABAergic and glutamatergic neurons in the enteric interneurons, targeting excitatory or inhibitory GABA receptors or glutamate receptors that modulate gastric motility and mucosal function. GABAergic and glutamatergic neuron immunoreactivity have been found in cholinergic enteric neurons in the stomach. GABA and L-Glu may also subserve hormonal and paracrine signaling. Disruption in gastrointestinal function following perturbation of enteric GABA receptors and glutamate receptors presents potential new target sites for drug development.

Effect of the exogenous glutamate and the NMDA on electric field-stimulated contractions of isolated rat ileum

While neurotransmitter role of glutamate in gastrointestinal intrinsic nervous system was shown, its effects in various segments of gastrointestinal tract were not yet understood completely. In our study, we have investigated effects of exogenous glutamate on electric field-stimulated contractions of isolated rat ileum. The ilea from forty Wistar rats of both sexes were isolated in an organ bath, according to the Magnus mounting method, and exposed to field stimulation with square wave pulses (20 V over the electrodes, 400 mA, 1 ms duration, frequency 16 Hz, pulse trains for 30 s with 30 s pause). The stimulation produced tonic contractions and relaxations recorded with an isometric system. Atropine (2.3 x 10(-6) M/l) completely abolished contractions of isolated rat ileum produced by the field stimulation, while relaxations remained unaffected. Glutamate (from 7.8 x 10(-6) to 3.5 x 10(-3) M/l) and N-Methyl-D-aspartic acid (NMDA) (from 9.1 x 10(-9) to 1.3 x 10(-5) M/l) caused concentration-dependent inhibition of the field-stimulated contractions of isolated rat ileum (EC50 was 90.17 +/- 1.81 x 10(-5) and 53.51 +/- 3.68 x 10(-9) M/l, respectively) while not affecting the field-stimulated relaxations. On the other hand, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (from 6.0 x 10(-9) to 9.3 x 10(-6) M/l), kainic acid (from 5.8 x 10(-9) to 8.3 x 10(-6) M/l) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid ((+/-)-trans-ACPD) (from 7.0 x 10(-9) to 1.1 x 10(-5) M/l) did not influence both the field-stimulated contractions and the field-stimulated relaxations of isolated rat ileum. The results of our study suggest inhibitory effect of glutamate on acetylcholine release from intrinsic neurons of rat ileum, mediated through NMDA receptors.

Oral S(+)-ketamine does not change visceral perception in health

Application of N-methyl-D-aspartate (NMDA)-receptor antagonists may hold promise for the treatment of visceral pain. In this study we evaluated the effect of oral S(+)-ketamine (sKET), a non-competitive NMDA-receptor antagonist, on visceral sensitivity in healthy volunteers. Eight healthy volunteers (five male, three female) underwent a gastric barostat study following oral administration of placebo, 25 mg sKET, and 50 mg sKET. Studies were performed in a double-blind randomized crossover fashion. Sensations evoked by stepwise isobaric distension (2 mm Hg/2 min) were scored on a 100-mm visual analogue scale. In addition, fasting and postprandial fundic volume were measured at a fixed pressure level (MDP + 2 mm Hg). During gastric distension, sKET did not alter sensation scores for bloating, nausea, satiation, and pain compared to placebo. sKET had also no effects on the thresholds for pain/discomfort, fundic wall compliance, fundic tone, or meal-induced fundic relaxation. sKET does not reduce visceral perception or gastric motility in healthy volunteers. The role of sKET in conditions characterized by visceral hypersensitivity needs to be studied further.

Role of glutamate receptors in the development and maintenance of bladder overactivity after cerebral infarction in the rat

PURPOSE

To investigate the role of glutamate receptors in overactive bladder (OAB) caused by cerebral infarction (CI) we examined the effects of 2 different types of receptors antagonists on OAB induced by left middle cerebral artery (MCA) occlusion.

MATERIALS AND METHODS

Female rats were intravenously injected with dizocilpine, an NMDA (N-methyl-D-aspartate) receptor antagonist, or NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide), an AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor antagonist, before or after MCA occlusion. Awake rats were cystometrically examined for 8 hours. Detrusor strips were evaluated for force development in response to dizocilpine and NBQX.

RESULTS

In CI rats without pretreatment bladder capacity (BC) was significantly decreased after MCA occlusion and remained consistently below half that of pre-occlusion capacity. Dizocilpine (0.5 mg/kg intravenously) administered before MCA occlusion blocked the decrease in BC in awake rats 5 to 8 hours after MCA occlusion. In CI rats pretreated with NBQX (10 or 30 mg/kg intravenously) BC was not different from that in rats without pretreatment. Increasing doses of dizocilpine (0.01 to 10 mg/kg) or NBQX (0.1 to 30 mg/kg) increased rat BC 2 hours after MCA occlusion. NBQX did not change the BC of sham operated rats. No differences in the contractile response to dizocilpine or NBQX of detrusor strips from sham operated and CI rats were observed.

CONCLUSIONS

These results indicate that NMDA receptor has an essential role in the development of OAB after CI. AMPA receptor antagonist cannot block the development of OAB. However, AMPA receptor antagonist temporally inhibits OAB after it is established by CI.

Role of N-methyl-D-aspartate receptors in gastric mucosal blood flow induced by histamine

Ionotropic N-methyl-D-aspartate (NMDA) receptor agonists, L-aspartic acid (L-Asp) and NMDA, have been shown to inhibit histamine-stimulated acid secretion, but their effect on gastric mucosal blood flow (GMBF) is largely unknown. The aim of this study was to investigate whether L-Asp and NMDA inhibit histamine-stimulated GMBF and to examine the expression patterns of NMDA receptor subunits NR1, NR2A, and NR2B in rat stomach. Laser Doppler flowmetry was used to measure gastric blood flow in anesthetized rats. The GMBF was assessed during an intravenous infusion of histamine in the presence of tripelennamine. The effects of L-Asp and NMDA on histamine-induced gastric blood flow were examined. In addition, the distribution patterns of NR1-, NR2A-, and NR2B-contaning NMDA receptors in rat stomach were determined immunohistochemically by using specific antibodies against NR1, NR2A, and NR2B. Histamine-induced enhancement of GMBF depended on acid secretion and the activation of H(2)-receptors. Neither L-Asp nor NMDA had an effect on the spontaneous GMBF. However, L-Asp and NMDA reduced the histamine-induced increase in GMBF. DL-2-amino-5-phosphonopentanoic acid (AP-5), an NMDA receptor antagonist; and prazosin, an alpha(1)-receptor antagonist; but not propanolol, a beta(2)-receptor antagonist; or yohimbine, a alpha(2)-receptor antagonist; reversed the inhibitory effect of L-Asp and NMDA on the histamine-induced increase in GMBF. Therefore, L-Asp and NMDA inhibit histamine-induced GMBF via a mechanism involving the activation of NMDA receptors and alpha(1)- adrenoceptors. The fact that NMDA receptor subunits NR1, NR2A, and NR2B were found to be localized in the rat stomach as visualized immunohistochemically with specific antibodies against NR1, NR2A, and NR2B is consistent with this hypothesis.

Effect of the low-affinity, noncompetitive N-methyl-d-aspartate receptor antagonist dextromethorphan on visceral perception in healthy volunteers

BACKGROUND

The use of N-methyl-d-aspartate (NMDA) receptor antagonists may hold promise for the treatment of pain of visceral origin, in particular in conditions characterized by visceral hypersensitivity.

AIM

To study the effect of dextromethorphan, a low affinity, non-competitive NMDA receptor antagonist, on visceral perception in healthy volunteers.

METHODS

Nine healthy volunteers (5 female, median age 22 years) underwent a gastric barostat study after oral administration of placebo, dextromethorphan 10 mg or dextromethorphan 30 mg, on three separate days in a double-blind, randomised order. Sensations induced by step-wise isobaric gastric distension (2 mmHg/2 min) were studied during fasting and 30 min after a meal. In addition, proximal gastric tone was measured during fasting and postprandially.

RESULTS

Compared to placebo, dextromethorphan 30 mg significantly increased the distension-evoked sensation scores for nausea (P=0.004) and satiation (P=0.004) during fasting; and for bloating (P= 0.001), nausea (P=0.000) and satiation (P=0.01) 30 min postprandially. Dextromethorphan did not alter pain scores, proximal gastric tone or gastric compliance.

CONCLUSIONS

Dextromethorphan increases the perception of non-painful sensations during gastric distension, without altering the perception of pain. Therefore, application of dextromethorphan as a visceral analgesic is questionable. Future studies with more specific NMDA receptor antagonist are warranted.

Functional group I metabotropic glutamate receptors in submucous plexus of guinea-pig ileum

Intracellular recording methods and immunostaining revealed the existence of functional group I metabotropic glutamate receptors (mGluRs) in submucous plexus neurons of guinea-pig ileum. Selective group I, but not groups II or III metabotropic glutamate receptor agonists induced concentration-dependent, slowly-activating depolarizing responses. Group I metabotropic glutamate receptor antagonism observed with (S)-4-carboxyphenylglycine (S-4-CPG) (100 - 600 microM) was competitive as determined by Schild analysis (pA(2)=3.81+/-0.02). Neither the group II and III metabotropic nor ionotropic glutamate receptor antagonists altered responses evoked by group I receptor agonists. Immunoreactivities for metabotropic glutamate 1alpha and 5 receptors were found to locate exclusively in neurons in the submucous plexus of guinea-pig ileum with the highest density around the cell bodies. The results suggest that group I metabotropic glutamate receptors are functionally expressed in the submucous plexus of guinea-pig small intestine.