Blockade Of N-methyl-D-aspartic Acid Receptors Research Articles

N-Methyl-D-aspartic acid (NMDA) mediated vasodilation in aorta

Although N-Methyl-D-aspartic acid (NMDA) mediated vasodilation has been extensively studied in cerebral circulation, there are limited studies that address the NMDA-mediated vascular responses at the peripheral vascular system. In this study our objective is to address the peripheral vascular responses mediated by NMDA. Vascular responses were obtained via in vitro myography on thoracic aorta segments isolated from C57BL/6 wild type male mice. NMDA at the concentration range of 100 micromolar to 1 millimolar in aortic segments precontracted by 1 micromolar phenylephrine. Maximal vasodilation of 75% in reference to baseline prior to drug application was observed at 3mM NMDA. NMDA receptor blockade by MK-801 did not change NMDA-induced vasodilation (55% vs 58%). Neither vehicle of NMDA, nor corresponding osmolar solutions of mannitol to NMDA concentrations induced vasodilation. Nitric oxide synthase (NOS) inhibition by 100 micromolar m Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME) reduced 3mM NMDA induced vasodilation from 75% to 16%. Removal of endothelial cell layer did not change the vasodilation mediated by NMDA. NMDA at millimolar concentrations relaxes vascular smooth muscle via non-endothelium dependent and NMDA-receptor independent mechanisms. Vasodilatory response to NMDA appears partially mediated by NOS. It is possible that NMDA is exerting its vasodilatory effects via non-NMDA receptors on the vascular smooth muscle. Whether NMDA-induced vasorelaxation is observed in resistance arteries remains warranted. PSC CUNY 66608-00 54. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The 5-HT6R agonist E-6837 and the antagonist SB-271046 reverse the psychotic-like behaviors induced by ketamine.

Schizophrenia is a serious mental disorder that affects 1% of the world's population. Although various therapeutic tools have been developed since the appearance of the first generation of antipsychotics, the effect of these agents does not manage to attenuate a significant part of psychotic symptoms. Ketamine is an anesthetic agent able to produce psychotic-like symptoms through the antagonism of the glutamatergic N-methyl-d-aspartic acid (NMDA) receptors (NMDARs). This drug has been widely used to study new pharmacological tools with potential antipsychotic properties. On the contrary, it is known that the 5-HT6 receptor agonist and antagonist drugs induce procognitive, anxiolytic and antidepressant effects in different preclinical models. Therefore, the aim of this study was to evaluate the behavioral actions of the 5-HT6 receptors' agonist E-6837 and the antagonist SB-271046, in ICR-CD1 mice previously treated with a subchronic ketamine scheme (10 mg/kg i.p. daily for 5 days). Results showed that repeated administration of ketamine induced recognition memory deficit, anxiogenic effects, obsessive-compulsive behaviors and stereotyped movements. The acute administration of both 5-HT6 agents reversed the memory deficit and induced a decrease in anxiety, whereas SB-271046 administration produced a decrease in climbing behavior. The injection of either of these 5-HT6 drugs had no effect in the light-dark test. Surprisingly, when these drugs were injected together with ketamine, anxiogenic actions were produced. Current findings suggest that both agonist and antagonist 5-HT6 drugs play an important role in modulating psychotic-like symptoms induced by the subchronic blockade of NMDAR.

NMDA receptor modulation of glutamate release in activated neutrophils.

BackgroundNeutrophil depletion improves neurologic outcomes in experimental sepsis/brain injury. We hypothesized that neutrophils may exacerbate neuronal injury through the release of neurotoxic quantities of the neurotransmitter glutamate.MethodsReal-time glutamate release by primary human neutrophils was determined using enzymatic biosensors. Bacterial and direct protein-kinase C (Phorbol 12-myristate 13-acetate; PMA) activation of neutrophils in human whole blood, isolated neutrophils or human cell lines were compared in the presence/absence of N-Methyl-d-aspartic acid receptor (NMDAR) antagonists. Bacterial and direct activation of neutrophils from wild-type and transgenic murine neutrophils deficient in NMDAR-scaffolding proteins were compared using flow cytometry (phagocytosis, reactive oxygen species (ROS) generation) and real-time respirometry (oxygen consumption).FindingsBoth glutamate and the NMDAR co-agonist d-serine are rapidly released by neutrophils in response to bacterial and PMA-induced activation. Pharmacological NMDAR blockade reduced both the autocrine release of glutamate, d-serine and the respiratory burst by activated primary human neutrophils. A highly specific small-molecule inhibitor ZL006 that limits NMDAR-mediated neuronal injury also reduced ROS by activated neutrophils in a murine model of peritonitis, via uncoupling of the NMDAR GluN2B subunit from its' scaffolding protein, postsynaptic density protein-95 (PSD-95). Genetic ablation of PSD-95 reduced ROS production by activated murine neutrophils. Pharmacological blockade of the NMDAR GluN2B subunit reduced primary human neutrophil activation induced by Pseudomonas fluorescens, a glutamate-secreting Gram-negative bacillus closely related to pathogens that cause hospital-acquired infections.InterpretationThese data suggest that release of glutamate by activated neutrophils augments ROS production in an autocrine manner via actions on NMDAR expressed by these cells.FundGLA: Academy Medical Sciences/Health Foundation Clinician Scientist. AVG is a Wellcome Trust Senior Research Fellow.

Pharmacological characterization of N-methyl-d-aspartic acid (NMDA)-like receptors in the single-celled organismParamecium primaurelia

Paramecium primaurelia is a unicellular eukaryote that moves in freshwater by ciliary beating and responds to environmental stimuli by altering motile behaviour. The movements of the cilia are controlled by the electrical changes of the cell membrane: when the intraciliary Ca(2+) concentration associated with plasma membrane depolarization increases, the ciliary beating reverses its direction, and consequently the swimming direction changes. The ciliary reversal duration is correlated with the amount of Ca(2+) influx. Here, we evaluated the effects due to the activation or blockade of N-methyl-d-aspartic acid (NMDA) receptors on swimming behaviour in Paramecium. Paramecia normally swim forward, drawing almost linear tracks. We observed that the simultaneous administration of NMDA and glycine induced a partial ciliary reversal (PaCR) leading to a continuous spiral-like swim. Furthermore, the duration of continuous ciliary reversal (CCR), triggered by high external KCl concentrations, was longer in NMDA+glycine-treated cells. NMDA action required the presence of Ca(2+), as the normal forward swimming was restored when the ion was omitted from the extracellular milieu. The PaCR and the enhancement of CCR duration significantly decreased when the antagonists of the glutamate site D-AP5 or CGS19755, the NMDA channel blocker MK-801 or the glycine site antagonist DCKA was added. The action of NMDA+glycine was also abolished by Zn(2+) or ifenprodil, the GluN2A and the GluN2B NMDA-containing subunit blockers, respectively. Searches of the Paramecium genome database currently available indicate that the NMDA-like receptor with ligand-binding characteristics of an NMDA receptor-like complex, purified from rat brain synaptic membranes and found in some metazoan genomes, is also present in Paramecium. These results provide evidence that functional NMDA receptors similar to those typical of mammalian neuronal cells are present in the single-celled organism Paramecium and thus suggest that the glutamatergic NMDA system is a phylogenetically old behaviour-controlling mechanism.

The uncompetitive NMDA receptor antagonists ketamine and memantine preferentially increase the choice for a small, immediate reward in low-impulsive rats.

Impulsive behavior is categorically differentiated between impulsive action, the inability to withhold from acting out a response, and impulsive choice, the greater preference for an immediate and smaller reward over a delayed but more advantageous reward. While the effects of N-methyl-D-aspartic acid (NMDA) receptor antagonists on impulsive action have been extensively characterized, there are very few and conflicting reports on the effects of this class of drugs on impulsive choice. Using a modified adjusting delay task, we investigated the effects of uncompetitive and competitive blockade of NMDA receptors on impulsive choice. Male Wistar rats were trained in a modified adjusting delay task, which involved repeated choice between a low reinforcing solution delivered immediately and a highly reinforcing solution delivered after a variable delay. Rats were then administered either the NMDA receptor uncompetitive antagonists ketamine or memantine, or the competitive antagonists D-AP-5 or CGS 19755. Ketamine treatment dose-dependently increased impulsive choice, and this effect was selective for low-impulsive but not high-impulsive rats. Similarly, memantine treatment dose-dependently increased impulsive choice with a preferential effect for low-impulsive rats. While D-AP-5 treatment did not affect impulsive choice, CGS 19755 increased impulsivity, however, at the same doses at which it caused a marked response inhibition. NMDA receptor uncompetitive, but not competitive, antagonists significantly increased impulsive choice, preferentially in low-impulsive rats. These findings demonstrate that the effects of NMDA receptor blockade on impulsive choice are not generalizable and depend on the specific mechanism of action of the antagonist used.

Tetramethylenedisulfotetramine Alters Ca2+ Dynamics in Cultured Hippocampal Neurons: Mitigation by NMDA Receptor Blockade and GABAA Receptor-Positive Modulation

Tetramethylenedisulfotetramine (TETS) is a potent convulsant that is considered a chemical threat agent. We characterized TETS as an activator of spontaneous Ca²⁺ oscillations and electrical burst discharges in mouse hippocampal neuronal cultures at 13-17 days in vitro using FLIPR Fluo-4 fluorescence measurements and extracellular microelectrode array recording. Acute exposure to TETS (≥ 2 µM) reversibly altered the pattern of spontaneous neuronal discharges, producing clustered burst firing and an overall increase in discharge frequency. TETS also dramatically affected Ca²⁺ dynamics causing an immediate but transient elevation of neuronal intracellular Ca²⁺ followed by decreased frequency of Ca²⁺ oscillations but greater peak amplitude. The effect on Ca²⁺ dynamics was similar to that elicited by picrotoxin and bicuculline, supporting the view that TETS acts by inhibiting type A gamma-aminobutyric acid (GABA(A)) receptor function. The effect of TETS on Ca²⁺ dynamics requires activation of N-methyl-D-aspartic acid (NMDA) receptors, because the changes induced by TETS were prevented by MK-801 block of NMDA receptors, but not nifedipine block of L-type Ca²⁺ channels. Pretreatment with the GABA(A) receptor-positive modulators diazepam and allopregnanolone partially mitigated TETS-induced changes in Ca²⁺ dynamics. Moreover, low, minimally effective concentrations of diazepam (0.1 µM) and allopregnanolone (0.1 µM), when administered together, were highly effective in suppressing TETS-induced alterations in Ca²⁺ dynamics, suggesting that the combination of positive modulators of synaptic and extrasynaptic GABA(A) receptors may have therapeutic potential. These rapid throughput in vitro assays may assist in the identification of single agents or combinations that have utility in the treatment of TETS intoxication.

Prolonged exposure to NMDAR antagonist induces cell-type specific changes of glutamatergic receptors in rat prefrontal cortex

N-methyl-d-aspartic acid (NMDA) receptors are critical for both normal brain functions and the pathogenesis of schizophrenia. We investigated the functional changes of glutamatergic receptors in the pyramidal cells and fast-spiking (FS) interneurons in the adolescent rat prefrontal cortex in MK-801 model of schizophrenia. We found that although both pyramidal cells and FS interneurons were affected by in vivo subchronic blockade of NMDA receptors, MK-801 induced distinct changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptors in the FS interneurons compared with pyramidal cells. Specifically, the amplitude, but not the frequency, of AMPA-mediated miniature excitatory postsynaptic currents (mEPSCs) in FS interneurons was significantly decreased whereas both the frequency and amplitude in pyramidal neurons were increased. In addition, MK-801-induced new presynaptic NMDA receptors were detected in the glutamatergic terminals targeting pyramidal neurons but not FS interneurons. MK-801 also induced distinct alterations in FS interneurons but not in pyramidal neurons, including significantly decreased rectification index and increased calcium permeability. These data suggest a distinct cell-type specific and homeostatic synaptic scaling and redistribution of AMPA and NMDA receptors in response to the subchronic blockade of NMDA receptors and thus provide a direct mechanistic explanation for the NMDA hypofunction hypothesis that have long been proposed for the schizophrenia pathophysiology.

Postnatal BDNF Expression Profiles in Prefrontal Cortex and Hippocampus of a Rat Schizophrenia Model Induced by MK-801 Administration

Neonatal blockade of N-methyl-D-aspartic acid (NMDA) receptors represents one of experimental animal models for schizophrenia. This study is to investigate the long-term brain-derived neurotrophic factor (BDNF) expression profiles in different regions and correlation with “schizophrenia-like” behaviors in the adolescence and adult of this rat model. The NMDA receptor antagonist MK801 was administered to female Sprague-Dawley rats on postnatal days (PND) 5 through 14. Open-field test was performed on PND 42, and PND 77 to examine the validity of the current model. BDNF protein levels in hippocampus and prefrontal cortex (PFC) were analyzed on PND 15, PND 42, and PND 77. Results showed that neonatal challenge with MK-801 persistently elevated locomotor activity as well as BDNF expression; the alterations in BDNF expression varied at different developing stages and among brain regions. However, these findings provide neurochemical evidence that the blockade of NMDA receptors during brain development results in long-lasting alterations in BDNF expression and might contribute to neurobehavioral pathology of the present animal model for schizophrenia. Further study in the mechanisms and roles of the BDNF may lead to better understanding of the pathophysiology of schizophrenia.

Regulation of extinction-related plasticity by opioid receptors in the ventrolateral periaqueductal gray matter

Recent work has led to a better understanding of the neural mechanisms underlying the extinction of Pavlovian fear conditioning. Long-term synaptic changes in the medial prefrontal cortex (mPFC) are critical for extinction learning, but very little is currently known about how the mPFC and other brain areas interact during extinction. The current study examined the effect of drugs that impair the extinction of fear conditioning on the activation of the extracellular-related kinase/mitogen-activated protein kinase (ERK/MAPK) in brain regions that likely participate in the consolidation of extinction learning. Inhibitors of opioid and N-methyl-d-aspartic acid (NMDA) receptors were applied to the ventrolateral periaqueductal gray matter (vlPAG) and amygdala shortly before extinction training. Results from these experiments show that blocking opioid receptors in the vlPAG prevented the formation of extinction memory, whereas NMDA receptor blockade had no effect. Conversely, blocking NMDA receptors in the amygdala disrupted the formation of fear extinction memory, but opioid receptor blockade in the same brain area did not. Subsequent experiments tested the effect of these drug treatments on the activation of the ERK/MAPK signaling pathway in various brain regions following extinction training. Only opioid receptor blockade in the vlPAG disrupted ERK phosphorylation in the mPFC and amygdala. These data support the idea that opiodergic signaling derived from the vlPAG affects plasticity across the brain circuit responsible for the formation of extinction memory.

Muscimol, AP5, or scopolamine infused into perirhinal cortex impairs two-choice visual discrimination learning in rats

The perirhinal cortex (PRh) has been strongly implicated in object recognition memory and visual stimulus representation. Studies of object recognition have revealed evidence for the involvement of several neurotransmitter subsystems, including those involving NMDA ( N-methyl- d-aspartic acid) and muscarinic cholinergic receptors. In the present study, we assessed the possible involvement of PRh and related receptor subsystems in two-choice visual discrimination learning by Lister Hooded rats tested in touchscreen-equipped operant boxes. In Experiment 1, daily pre-training inactivation of PRh with the GABA A receptor agonist muscimol (0.5 μg/hemisphere) significantly impaired acquisition of the two-choice visual discrimination. In Experiment 2, daily pre-training blockade of either NMDA or muscarinic receptors in PRh with AP5 (5.9 μg/hemisphere) or scopolamine (10 μg/hemisphere), respectively, impaired task acquisition. These results parallel the findings from object recognition studies and suggest a generality of neurotransmitter receptor involvement underlying the role of PRh in both object recognition memory and visual discrimination learning. The involvement of PRh in both types of tasks may be related to its role in complex visual stimulus representation.

NMDA receptor-mediated long-term alterations in epileptiform activity in experimental chronic epilepsy

When epileptiform activity is acutely induced in vitro, transient partial blockade of N-methyl- d-aspartic acid (NMDA) receptor-mediated calcium influx leads to selective long-term depotentiation of the synapses involved in the epileptic activity as well as a reduction in the probability of further epileptiform activity. If such selective depotentiation occurred within foci of epileptic activity in vivo, the corresponding long-term reduction in seizure probability could form the basis for a novel treatment of epilepsy. Continuous radiotelemetric EEG monitoring demonstrated modest acute anticonvulsant effects but no long-term reductions in the probability of spontaneous seizures after transient partial blockade of NMDA receptors (NMDAR) during ictal and interictal activity in the kainate animal model of chronic epilepsy. In vitro, depotentiation was induced when NMDAR were partially blocked during epileptiform activity in hippocampal slices from control animals, but not in slices from chronically epileptic rats. However in slices from epileptic animals, depotentiation during epileptiform activity was induced by partial block of NMDAR using NR2B- but not NR2A-selective antagonists. These results suggest that chronic epileptic activity is associated with changes in NMDA receptor-mediated signaling that is reflected in the pharmacology of activity- and NMDA receptor-dependent depotentiation.

Blockade of NMDA receptors in the prefrontal cortex increases dopamine and acetylcholine release in the nucleus accumbens and motor activity

The present study investigates the effects of injections of a specific N-methyl-D-aspartic acid (NMDA) antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phophonic acid (CPP) into the prefrontal cortex (PFC) on the extracellular concentrations of dopamine and acetylcholine in the nucleus accumbens (NAc) and on motor activity in the freely moving rat. Sprague-Dawley male rats were implanted with guide cannulas into the medial PFC and NAc to perform bilateral microinjections and microdialysis experiments. Spontaneous motor activity was monitored in the open field. Injections of CPP (1 microg/0.5 microL) into the PFC produced a significant increase of the baseline extracellular concentrations of dopamine (up to 130%), dihydroxyphenylacetic acid (DOPAC; up to 120%), homovanillic acid (HVA; up to 130%), and acetylcholine (up to 190%) in the NAc as well as motor hyperactivity. In the NAc, perfusion of the NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate antagonists CPP (50 microM)+6,7-dinitroquinoxaline-2,3-dione (DNQX; 50 microM) through the microdialysis probe blocked acetylcholine release, but not DOPAC and HVA increases produced by CPP injections into the PFC. Also, increases in motor activity produced by prefrontal injections of CPP were significantly reduced by bilateral injections into the NAc of a mixed D1/D2 antagonist, flupenthixol (5 and 25 microg/0.5 microL). Injections into the NAc of the muscarinic antagonist scopolamine (1 and 10 microg/0.5 microL) further increased, and of the nicotinic antagonist mecamylamine (1 and 10 microg/0.5 microL) did not change, the increases in motor activity produced by prefrontal CPP injections. These results suggest that the dysfunction of NMDA receptors in the PFC could be a key factor in the neurochemical and motor effects associated with corticolimbic hyperactivity.

NMDA receptor expression and activity in osteoarthritic human articular chondrocytes

Classical neuronal signalling molecules such as substance P and glutamate have been identified in cartilage and have roles in regulation of chondrocyte function. This study looks at expression and activity of the ionotropic glutamate NMDA (N-methyl-D-aspartic acid) receptor (NMDAR) in human osteoarthritic (OA) chondrocytes. Chondrocytes were obtained from human knee joint arthroplasty specimens. NMDAR subunits and PSD-95 (postsynaptic density protein 95) expression were analysed by reverse transcription-polymerase chain reaction and Western blotting. Activity of NMDAR was assayed by radioactive calcium(45) uptake and changes in membrane potential in the presence and absence of NMDA and NMDAR antagonists and blockade of cell membrane ion channels. NMDAR 1, 2A, 2B and PSD-95 were detected in human OA chondrocytes whereas NR2B was absent from normal chondrocytes. NMDA induced calcium flux into OA chondrocytes and cell membrane depolarisation. These responses were blocked by NMDAR antagonists, removal of extracellular calcium, inhibition of nNOS (neuronal nitric oxide synthase) activity and uncoupling of NMDAR from PSD-95. Blockade of sodium channels by tetrodotoxin resulted in NMDA-induced membrane hyperpolarisation which was, in turn inhibited by apamin, a blocker of SK channels. NMDA-induced changes in cell membrane potential were not affected by l-type and stretch activated calcium channel inhibitors. Human OA and normal articular chondrocytes differ in the expression of NMDAR subunits. In OA chondrocytes NMDAR signalling requires extracellular calcium, association with PSD-95, and nNOS activity. Downstream signalling results in activation of tetrodotoxin sensitive sodium channels and SK channels, a response that differs from that of normal chondrocytes suggesting altered activity of NMDAR in OA.

Non-vagal apnea evoked by intracommon carotid artery injection of N-methyl-D-aspartic acid (NMDA) in anesthetized rats

Respiratory effects of an intra-common carotid artery injection of N-methyl-D-aspartic acid (NMDA) were investigated in anesthetized spontaneously breathing rats, using three experimental paradigms: (1) midcervical vagotomy followed by supranodosal vagotomy, (2) midcervical vagotomy followed by section of the carotid sinus nerves (CSNs), and (3) midcervical vagotomy followed by pharmacological blockade of NMDA receptors. The intra-common carotid artery injection of NMDA (4 mg/kg) induced transient expiratory apnea followed by a brief and variably occurring period of breathing at reduced tidal volume. There were no consistent changes in respiratory rate in rats subjected to midcervical vagotomy alone. Supranodose vagotomy exerted no effect on NMDA-induced respiratory arrest, whereas CSNs' section or blockade of NMDA receptors with AP-7 abolished the apnea. These results indicate that the apnea induced by intraarterial NMDA challenge is due to activation of peripheral NMDA receptors and is mediated via carotid body afferents.

Is NMDA Receptor Activation Essential for the Production of Locomotor-Like Activity in the Neonatal Rat Spinal Cord?

Previous work has established that in vitro bath application of N-methyl-D-aspartic acid (NMDA) promotes locomotor activity in a variety of vertebrate preparations including the neonatal rat spinal cord. In addition, NMDA receptor activation gives rise to active membrane properties that are postulated to contribute to the generation or stabilization of locomotor rhythm. However, earlier studies yielded conflicting evidence as to whether NMDA receptors are essential in this role. Therefore in this study, we examined the effect of NMDA receptor blockade, using D-2-amino-5-phosphono-valeric acid (AP5), on locomotor-like activity in the in vitro neonatal rat spinal cord. Locomotor-like activity was induced using 5-hydroxytryptamine (5-HT), acetylcholine, combined 5-HT and NMDA receptor activation, increased K(+) concentration, or electrical stimulation of the brain stem and monitored using suction electrode recordings of left and right lumbar ventral root discharge. We also studied the effect on locomotor capacity of selectively suppressing NMDA receptor-mediated active membrane properties; this was achieved by removing Mg(2+) ions from the bath, which in turn abolishes voltage-sensitive blockade of the NMDA receptor channel. The results show that, although NMDA receptor activation may seem essential for locomotor network operation under some experimental conditions, locomotor-like rhythms can nevertheless be generated in the presence of AP5 if spinal cord circuitry is exposed to appropriate levels of non-NMDA receptor-dependent excitation. Therefore neither NMDA receptor-mediated nonlinear membrane properties nor NMDA receptor activation in general is universally essential for locomotor network activation in the in vitro neonatal rat spinal cord.

Respiratory responses evoked by blockades of ionotropic glutamate receptors within the Bötzinger complex and the pre‐Bötzinger complex of the rabbit

The respiratory role of excitatory amino acid (EAA) receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (pre-BötC) was investigated in alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nL) of EAA receptor antagonists. Blockade of both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors by 50 mM kynurenic acid (KYN) within the BötC induced a pattern of breathing characterized by low-amplitude, high-frequency irregular oscillations superimposed on tonic phrenic activity and successively the disappearance of respiratory rhythmicity in the presence of intense tonic inspiratory discharges (tonic apnea). KYN microinjections into the pre-BötC caused similar respiratory responses that, however, never led to tonic apnea. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1, 10 and 20 mM) within the BötC induced increases in respiratory frequency and decreases in peak phrenic amplitude; the highest concentrations caused tonic apnea insensitive to chemical stimuli. Blockade of non-NMDA receptors by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 and 20 mM) within the BötC produced only less pronounced increases in respiratory frequency. Responses to D-AP5 in the pre-BötC were similar, although less pronounced than those elicited in the BötC and never characterized by tonic apnea. In the same region, CNQX provoked increases in respiratory frequency similar to those elicited in the BötC, associated with slight reductions in peak phrenic activity. The results show that EAA receptors within the investigated medullary subregions mediate a potent control on both the intensity and frequency of inspiratory activity, with a major role played by NMDA receptors.

Carotid baroreflex in the rat: role of glutamate receptors in the medial subnucleus of the solitary tract

Experiments were done in urethane-anesthetized adult male Wistar rats to investigate the role of glutamate receptors in the medial subnucleus of the solitary tract (mNTS) in mediating the carotid sinus baroreflex responses. The carotid sinus on one side was isolated from the general circulation and perfused with a warm perfusion fluid (37 °C; pH 7.4) saturated with 100% oxygen. The carotid sinus was then connected to an apparatus that permitted application of pressure increments (20–100 mm Hg) to stimulate specifically baroreceptors. The mNTS ipsilateral to the isolated carotid sinus was identified by microinjections (100 nL) of l-glutamate (5 mM). The stereotaxic coordinates for mNTS were: 0.5–0.6 mm rostral to the calamus scriptorius, 0.5–0.6 mm lateral to the midline, and 0.5–0.6 mm deep from the dorsal medullary surface. Microinjections of either d(−)-2-amino-7-phosphono-heptanoic acid, which is an N-methyl- d-aspartic acid (NMDA) receptor antagonist (5 mM) or 2,3-dioxo-6-nitro-1,2,3,4-tetrahydro-benzo[f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist; 2 mM) significantly attenuated the depressor responses elicited by carotid baroreceptor stimulation. Simultaneous blockade of NMDA and non-NMDA receptors in the ipsilateral mNTS completely abolished the depressor responses to carotid baroceptor stimulation. Microinjections of either ( RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 50 mM) or ( RS)-α-cyclopropyl-4-phosphono-phenyl-glycine (CPPG; 80 mM) did not alter baroreflex responses. AIDA blocked group I and III while CPPG blocked all three groups of metabotropic glutamate receptors (mGLURs). These results suggest that ionotropic glutamate receptors, but not mGLURs, in the mNTS mediate the reflex depressor responses to carotid baroreceptor stimulation in the rat.

Respiratory responses to ionotropic glutamate receptor antagonists in the ventral respiratory group of the rabbit.

Selective excitatory amino acid receptor antagonists acting on either N-methyl-D-aspartic acid (NMDA) or non-NMDA receptors were microinjected (30-50 nl) bilaterally into different subregions of the ventral respiratory group (VRG) of alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1 or 10 mM) within the inspiratory portion of the VRG (iVRG) dose-dependently decreased the peak amplitude and rate of rise of phrenic nerve activity, without significant changes in respiratory timing. Decreases in respiratory frequency and peak phrenic amplitude up to apnoea were evoked by 20 mM D-AP5; phrenic nerve activity was restored transiently by hypoxic or hypercapnic stimulation during D-AP5-induced apnoea. Microinjections of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 or 20 mM) into the iVRG provoked less intense depressant respiratory effects. No significant respiratory responses were evoked by microinjections of these antagonists into more caudal VRG subregions. The results suggest that ionotropic glutamate receptors within the iVRG are involved mainly in the control of the intensity of inspiratory activity, with a major role played by NMDA receptors. Glutamate receptor antagonism in the iVRG does not seem to impair the basic mechanisms underlying respiratory rhythm generation.

The lateral parabrachial nucleus is involved in mediation of rostral ventrolateral medulla cholinergic inputs in rats

These studies examined the role of spinal N-methyl-D-aspartic acid (NMDA) receptors in mediating Sympathoexcitation evoked by stimulation of neurons in the rostral ventrolateral medulla (RVLM). In urethane-anesthetized rats, blood pressure, heart rate, and splanchnic sympathetic nerve activity (SNA) were recorded. The NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid (D-AP7) was administered to the spinal cord via intrathecal (IT) catheter. Blockade of spinal NMDA receptors reduced arterial blood pressure, heart rate, and SNA. Spinal administration of D-AP7 markedly attenuated the pressor and sympathoexcitatory responses evoked by L-glutamate stimulation of the RVLM. The small increases in heart rate evoked by stimulation of the RVLM were not affected by IT administration of D-AP7. These results indicate that NMDA receptors in the spinal cord mediate the pressor and sympathoexcitatory responses evoked by activation of a bulbospinal pathway originating from the RVLM. Moreover, these data suggest that excitatory amino acid neurotransmitters and NMDA receptors in the spinal cord play an important role in the maintenance and regulation of SNA and cardiovascular function.

Ascending noradrenergic pathways from the nucleus of the solitary tract to the subfornical organ in rats

These studies examined the role of spinal N-methyl-D-aspartic acid (NMDA) receptors in mediating Sympathoexcitation evoked by stimulation of neurons in the rostral ventrolateral medulla (RVLM). In urethane-anesthetized rats, blood pressure, heart rate, and splanchnic sympathetic nerve activity (SNA) were recorded. The NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid (D-AP7) was administered to the spinal cord via intrathecal (IT) catheter. Blockade of spinal NMDA receptors reduced arterial blood pressure, heart rate, and SNA. Spinal administration of D-AP7 markedly attenuated the pressor and sympathoexcitatory responses evoked by L-glutamate stimulation of the RVLM. The small increases in heart rate evoked by stimulation of the RVLM were not affected by IT administration of D-AP7. These results indicate that NMDA receptors in the spinal cord mediate the pressor and sympathoexcitatory responses evoked by activation of a bulbospinal pathway originating from the RVLM. Moreover, these data suggest that excitatory amino acid neurotransmitters and NMDA receptors in the spinal cord play an important role in the maintenance and regulation of SNA and cardiovascular function.