Effects Of Glutamate Receptor Antagonists Research Articles

Small-molecule targeting AMPA-mediated excitotoxicity has therapeutic effects in mouse models for multiple sclerosis.

While most research and treatments for multiple sclerosis (MS) focus on autoimmune reactions causing demyelination, it is possible that neurodegeneration precedes the autoimmune response. Hence, glutamate receptor antagonists preventing excitotoxicity showed promise in MS animal models, though blocking glutamate signaling prevents critical neuronal functions. This study reports the discovery of a small molecule that prevents AMPA-mediated excitotoxicity by targeting an allosteric binding site. A machine learning approach was used to screen for small molecules targeting the AMPA receptor GluA2 subunit. The lead candidate has potent effects in restoring neurological function and myelination while reducing the immune response in experimental autoimmune encephalitis and cuprizone MS mouse models without affecting basal neurotransmission or learning and memory. These findings facilitate development of a treatment for MS with a different mechanism of action than current immune modulatory drugs and avoids important off-target effects of glutamate receptor antagonists. This class of MS therapeutics could be useful as an alternative or complementary treatment to existing therapies.

Involvement of the Hypothalamic Glutamatergic System in the Development of Radiation-Induced Pica in Rats.

Since the peripheral serotoninergic pathway is involved in the development of radiation-induced nausea and vomiting, referred to as radiation sickness, serotonin 5-HT3 receptor antagonists are used as a preventive measure, although patients still suffer from these symptoms. Glutamate is known as the excitatory neurotransmitter and is involved in various autonomic symptoms. We investigated the effect of radiation on glutamate release in rats, as measured by in vivo brain microdialysis, and the effects of glutamate receptor antagonists on radiation-induced pica, which can be used as a behavioral assessment of radiation sickness in rats. A microdialysis probe was inserted into the hypothalamus of rats that received 4 Gy total-body irradiation (TBI) with or without pretreatment of 5-HT3 receptor antagonist (granisetron, 0.1 mg/kg, i.p.), and dialysates were collected for 3 h after TBI and subjected to HPLC assay of glutamate. In addition, rats were intracerebroventricularly injected with NMDA receptor antagonist (MK-801: 3 μg/rat) or AMPA receptor antagonist (CNQX: 1 μg/rat) before TBI, and radiation-induced pica was determined. An increase in glutamate release was observed within 1 h postirradiation. The increased glutamate release was suppressed by granisetron. We also found that CNQX, but not MK-801, effectively inhibited radiation-induced pica. These results indicate that the hypothalamic glutamatergic system contributes to radiation-induced pica through the AMPA receptors.

Regulatory effects of glutamate receptor antagonists on the proliferation and migration of WM451LU malignant melanoma cells and their related mechanisms

Objective To evaluate regulatory effects of glutamate receptor antagonists on the proliferation and migration of WM451LU malignant melanoma cells, and to explore their related mechanisms. Methods WM451LU cells at exponential growth phase were classified into 3 groups to be treated with the glutamate receptor antagonist MK-801 at 100 μmol/L(MK-801 group), the glutamate receptor antagonist CPCCOEt at 10 μmol/L(CPCCOEt group), or culture medium(control group). After 24-hour treatment, methyl thiazolyl tetrazolium(MTT)assay was performed to determine cell proliferation rates, scratch assay to evaluate the migration activity of cells, and Western-blot analysis to measure expression levels of proliferating cell nuclear antigen(PCNA), protein kinase Cα(PKCα)both on cell membrane and in cytoplasm, and phosphorylated mitogen-activated protein kinase(p-MAPK). Results After 24-hour treatment, cell proliferation rates were significantly decreased in the MK-801 group and CPCCOEt group compared with the control group(63% ± 3.1% and 60% ± 2.4% vs. 100% ± 1.1%, both P < 0.05). The scratch assay showed that cell-free zones in the control group gradually narrowed over time, and the scratch wound tended to close. However, the cell-free zones in the MK-801 group and CPCCOEt group narrowed more slowly compared with the control group, and were still wide after 24-hour culture with no obvious closure of the scratch. The MK-801 group and CPCCOEt group both showed significantly decreased expressions of PCNA(77.0% ± 5.4% and 72.0% ± 4.2% respectively), PKCα on the cell membrane(0.12 ± 0.02 and 0.14 ± 0.02 respectively), and p-MAPK(0.48 ± 0.03 and 0.36 ± 0.04 respectively)compared with the control group(PCNA: 100.0% ± 1.3%; PKCα: 0.38 ± 0.01; p-MAPK: 1.00 ± 0.02; all P < 0.05). Conclusion In vitro suppression of glutamate receptors can inhibit the proliferation and migration of WM451LU cells, likely through the mediation of the PKCα-MAPK signaling pathway. Key words: Melanoma, experimental; Receptors, glutamate; Cell proliferation; Protein kinase C-alpha; Mitogen-activated protein kinases

Effect of glutamate receptor antagonists on migrating neural progenitor cells

Neurotransmitters such as glutamate are potential regulators of neurogenesis. Interference with defined glutamate receptor subtypes affects proliferation, migration and differentiation of neural progenitor cells. The cellular targets for the actions of different glutamate receptor ligands are less well known. In this study we have combined calcium imaging, measurement of membrane potential, time-lapse imaging and immunocytochemistry to obtain a spatial overview of migrating mouse embryonic neural progenitor cell-derived cells responding to glutamate receptor agonists and antagonists. Responses via metabotropic glutamate receptor 5 correlated with radial glial cells and dominated in the inner migration zones close to the neurosphere. Block of metabotropic glutamate receptor 5 resulted in shorter radial glial processes, a transient increase in neuron-like cells emerging from the neurosphere and increased motility of neuron-like cells. α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors are present on the majority of migrating neuronal cells, which with time accumulate at the outer edge of the migration zone. Blocking these receptors leads to an enhanced extension of radial glial processes and a reduced motility of neuron-like cells. Our results indicate that functional glutamate receptors have profound effects on the motility of neural progenitor cells. The main target for metabotropic glutamate receptor 5 appears to be radial glial cells while AMPA/kainate receptors are mainly expressed in newborn neuronal cells and regulate the migratory progress of these cells. The results suggest that both metabotropic glutamate receptor 5 and AMPA/kainate receptors are of importance for the guidance of migrating embryonic progenitor cells.

Effect of glutamate receptor antagonists microinjected into the nucleus accumbens on place aversion induced by naloxone in single-dose, morphine-treated rats

It is well established that acute morphine withdrawal can be observed following opioid receptor antagonism in rodents. Glutamate receptor antagonists can attenuate the conditioning place aversion (CPA) induced by naloxone in single-dose, morphine-treated rats. Anatomically, the nucleus accumbens appears to be involved in opiate dependence. In the present study, we examined the effects of various glutamate receptor antagonists in the nucleus accumbens on naloxone-induced CPA in rats. MK-801 (an NMDA receptor antagonist), GYKI52466 (an AMPA receptor antagonist), and MCPG (a metabotropic glutamate receptor antagonist) significantly attenuated naloxone-induced CPA following microinjection into the accumbens. In contrast, none of the agents showed place conditioning ability on their own in either morphine-exposed or naïve rats. The present study suggests that glutamate receptors in the nucleus accumbens play a key role in the motivational component of withdrawal during acute morphine dependence.

Involvement of Dopaminergic Receptor Signaling in the Effects of Glutamatergic Receptor Antagonists on Conditioned Place Aversion Induced by Naloxone in Single-Dose Morphine-Treated Rats

A better understanding of the neurochemical mechanisms mediating the aversive consequences of drug withdrawal is important for understanding drug addiction. We previously demonstrated that the inhibitory effect of glutamate receptor antagonists on the conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal after a single morphine exposure could be blocked by dopamine receptor antagonists. Thus, a glutamatergic-dopaminergic interaction may participate in this phenomenon. The current study was undertaken to further characterize this interaction by employing both D(1) (SCH 23390) and D(2) (raclopride and eticlopride) dopamine receptor antagonists. The influence of these antagonists on the attenuation of CPA by MK-801 (NMDA receptor antagonist), GYKI 52466 (AMPA receptor antagonist), and MCPG (metabotropic glutamate receptor antagonist) was determined in rats receiving a single dose of morphine. The dopamine antagonists showed either a significant reversal or a tendency to reverse the effects of MK-801 on CPA. The effect of GYKI 52466 was also attenuated by the blockade of either D(1) or D(2) receptors. The effect of MCPG, however, was only blocked by D(2) antagonists and not by the D(1) antagonist SCH 23390. These results add evidence to the hypothesis that a glutamatergic-dopaminergic interaction may be involved in the CPA induced by naloxone-precipitated withdrawal following a single morphine exposure and suggest that both D(1) and D(2) dopamine receptor signaling mechanisms play a role in mediating the aversive aspects of acute dependence.

Glutamatergic calcium dynamics and deregulation of rat retinal ganglion cells

A rise in intracellular calcium levels ([Ca(2+)](i)) is a key trigger for the lethal effects of the excitatory neurotransmitter glutamate in various central neurons, but a consensus has not been reached on the pathways that mediate glutamate-dependent increases of [Ca(2+)](i) in retinal ganglion cells (RGCs). Using Ca(2+) imaging techniques we demonstrated that, in the absence of external Mg(2+), the Ca(2+) signal evoked by glutamate was predominantly mediated by NMDA-type glutamate receptors (NMDA-Rs) in immunopanned RGCs isolated from neonatal or adult rats. Voltage-gated Ca(2+) channels and AMPA/kainate-Rs contributed a smaller portion of the Ca(2+) response at saturating concentrations of glutamate. Consistent with NMDA-R involvement, extracellular Mg(2+) inhibited RGC glutamate responses, while glycine had a potentiating effect. With Mg(2+) present externally, the effect of AMPA/kainate-R antagonists was enhanced and both NMDA- and AMPA/kainate-R antagonists greatly reduced the glutamate-induced increases of RGC [Ca(2+)](i). This finding indicates that the primary contribution of AMPA/kainate-Rs to RGC glutamatergic Ca(2+) dynamics is through the depolarization-dependent relief of the Mg(2+) block of NMDA-R channels. The effect of glutamate receptor antagonists on glutamatergic Ca(2+) signals from RGCs in adult rat retinal wholemounts yielded results similar to those obtained using immunopanned RGCs. Additional experiments on isolated RGCs revealed that during a 1 h glutamate (10-1000 microm) exposure, 18-28% of RGCs exhibited delayed Ca(2+) deregulation (DCD) and the RGCs that underwent DCD were positive for the death marker annexin V. RGCs with larger glutamate-evoked Ca(2+) signals were more likely to undergo DCD, and NMDA-R blockade significantly reduced the occurrence of DCD. Identifying the mechanisms underlying RGC excitotoxicity aids in our understanding of the pathophysiology of retinal ischaemia, and this work establishes a major role for NMDA-R-mediated increases in [Ca(2+)](i) in glutamate-related RGC death.

Calcium‐dependent production of reactive oxygen species is involved in neuronal damage induced during glycolysis inhibition in cultured hippocampal neurons

Neuronal damage associated with in vivo hypoglycemia has been suggested to be excitotoxic due to the release of excitatory amino acids and the protective effect of glutamate receptor antagonists. The production of reactive oxygen species (ROS) has been also implicated in hypoglycemic damage. Excitotoxicity involves oxidative stress, insofar as the influx of calcium through N-methyl-D-aspartate (NMDA) receptors stimulates ROS production. We have studied the participation of NMDA receptors and intracellular calcium in ROS production and cell death triggered during moderate and severe glycolysis inhibition in cultured hippocampal neurons. Iodoacetate (IOA), an inhibitor of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), dose dependently reduces ATP levels and cell survival and increases the intracellular concentration of calcium. During mild glycolysis inhibition, the increases in intracellular calcium, ROS production, and cell death are dependent on NMDA receptor activation. In contrast, during severe glycolysis impairment, these processes are not inhibited by NMDA receptor blockade. BAPTA-AM and vitamin E efficiently reduce ROS generation and cell death under both conditions. Results suggest that calcium influx through NMDA receptors is involved in ROS production and neuronal damage resulting from moderate energy depletion, whereas intracellular calcium increase and ROS generation during severe glycolysis inhibition are more related to energy depletion.

The effects of glutamate receptor antagonists on cerebellar granule cell survival and development

N-Methyl- d-aspartate (NMDA) receptor stimulation promotes neuronal survival and differentiation under both in vitro and in vivo conditions. We studied the effects of various NMDA receptor antagonists acting at different NMDA receptor binding sites and non-NMDA receptor antagonists on the development and survival of cerebellar granule cell (CGC) culture. Only three of the drugs tested induced neurotoxicity—MK-801 (non-competitive NMDA channel blocking antagonist), ifenprodil (an antagonist of the NR2B site and polyamine site of the NMDA receptor) and L-701.324 (full antagonist at glycine site), while CGP-37849 (a competitive NMDA antagonist), (+)-HA-966 (a partial agonist of the glycine site of the NMDA receptor), and NBQX (a competitively acting AMPA receptor antagonist) were not toxic at any concentration (1–100 μM) used. Among these drugs, only MK-801 was toxic for the immature CGC on second day in vitro (2DIV), and toxicity was diminished parallel to the neuronal maturation. In more mature neurons (7DIV), MK-801 demonstrated some neuroprotection, which diminished spontaneously occurring neuronal death in culture. Neither NMDA nor glutamate were able to prevent the neurotoxic effect of MK-801 at 2DIV. MK-801, ifenprodil and L-701.324 induced DNA fragmentation on 2DIV in CGC culture measured by the TUNEL method. The BOC-D-FMK, the universal caspase inhibitor, completely reversed MK-801-induced DNA fragmentation, suggesting an apoptotic pathway of MK-801-induced cell death. Neurite outgrowth as a characteristic feature of the development of CGC was diminished after treatment with MK-801, ifenprodil and L-701.324. In conclusion, the results of the present study demonstrate that only nonselective channel blocker MK-801 decreases cell viability, induces apoptosis and inhibits neurite outgrowth of CGC in a development-dependent manner.

Mechanisms of cell death of neural progenitor cells caused by trophic support deprivation

Cell death of neural progenitor cells is the primary problem limiting the value of neural progenitor cell-based therapy for central nervous system disorders. However, little is known about the mechanism of cell death of neural progenitor cells. In this study, we investigated the mechanisms of cell death of a multipotent cell line, MEB5, caused by deprivation of epidermal growth factor (EGF). When EGF was removed from the culture medium, the total number of viable MEB5 cells reduced, and nuclear condensation and elevation of caspase-3-like enzyme activity were observed in MEB5 cells. Treatment with a broad-range caspase inhibitor reduced cell death in a concentration-dependent manner, indicating that MEB5 cells undergo caspase-mediated apoptotic cell death caused by EGF deprivation. We also investigated the effects of glutamate receptor antagonists, antioxidants and nitric oxide synthase inhibitor on EGF deprivation-induced cell death. N-methyl- d-aspartate (NMDA) glutamate receptor antagonists, α-amino-3-hydrozy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptor antagonist and nitric oxide synthase inhibitor failed to reduce cell death. In contrast, two antioxidants with different chemical structures reduced cell death in a concentration-dependent manner. The production of reactive oxygen species was detected in MEB5 cells after EGF deprivation by monitoring dichlorodihydrofluorescein fluorescence as a marker of reactive oxygen species-related radicals. Our results suggest that oxidative stress triggers caspase-mediated apoptosis of neural progenitor cells by trophic support deprivation.

Effect of glutamate receptor antagonists and antirheumatic drugs on proliferation of synoviocytes in vitro

One of the most striking features of inflammatory arthritis is the hyperplasia of synovial fibroblasts. It is not known whether the massive synovial hyperplasia characteristic of rheumatoid arthritis is due to the proliferation of synovial fibroblasts or to defective apoptosis. It has been found that glutamate receptor antagonists inhibit proliferation of different human tumour cells and the anticancer potential of glutamate receptor antagonists was suggested. Here, we investigated the effect of glutamate receptor antagonists and selected antirheumatic drugs on proliferation of synoviocytes in vitro. Experiments were conducted on rabbit synoviocytes cell line HIG-82 obtained from American Type Culture Collection (Menassas, VA, USA). Cell proliferation was assessed by means of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The IC 50 value (the concentration of drug necessary to induce 50% inhibition) together with confidence limits was calculated. Glutamate receptor antagonists, 1-(4-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4 H-2,3-benzodiazepin-4-one (CFM-2), riluzole, memantine, 1-4-aminophenyl-methyl-7,8-methylenedioxy-5 H-2,3-benzodiazepine (GYKI 52466), dizocilpine, ketamine and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo( f)quinoxaline (NBQX), inhibited proliferation of synoviocytes with the following IC 50 values (in mM): 0.014, 0.017, 0.065, 0.102, 0.15, 0.435 and 1.16, respectively. Antirheumatic drugs, celecoxib, diclofenac, nimesulide, sulfasalazine, naproxen and methotrexate, inhibited proliferation of synoviocytes with the following IC 50 values (in mM): 0.0043, 0.034, 0.044, 0.096, 0.385 and 1.123, respectively. Thus, the antiproliferative potential of glutamate receptor antagonists is comparable to that of antirheumatic drugs.

Role of glutamate receptors in the ventromedial hypothalamus in the regulation of female rat sexual behaviors: II. Behavioral effects of selective glutamate receptor antagonists AP-5, CNQX, and DNQX

Bilateral infusions of glutamate or its selective ionotrophic receptor agonists to the ventromedial hypothalamus (VMH) produce a rapid inhibition of both appetitive and consummatory sexual behavior in hormone-primed female rats. The present study examined whether infusion of selective ionotrophic glutamate receptor antagonists to the VMH can facilitate female sexual behavior in females treated with estradiol benzoate (EB) and progesterone (P), or EB alone. Ovariectomized, sexually experienced female rats were implanted bilaterally with guide cannulae aimed at the ventrolateral VMH. After recovery from surgery, females were primed either with EB + P or EB alone, and infused with saline, or one of two doses each of AP-5 (to target NMDA receptors), CNQX, or DNQX (to target AMPA/kainate receptors), immediately before tests with sexually vigorous male rats in bilevel chambers. In general, the drug infusions had a more powerful effect in females primed with EB alone compared to females primed with EB + P. AP-5 increased lordosis in females primed with EB alone. CNQX had a similar facilitative effect on lordosis, and also increased solicitations. DNQX increased solicitations in both hormone-priming conditions, increased lordosis quotients and magnitudes, and decreased pacing and defensive responses in the EB-alone condition. These results indicate that antagonism of glutamate receptors in the VMH resembles the effect of P, and that the addition of P to an EB baseline eliminates most of the effects of glutamate receptor antagonists. These data support the notion that glutamate receptors in the VMH contribute a strong inhibitory influence in the control of female sexual behavior.

Metabotropic glutamate receptor antagonists but not NMDA antagonists affect conditioned taste aversion acquisition in the parabrachial nucleus of rats

The effect of glutamate receptor antagonists on conditioned taste aversion (CTA) was studied in rats. The association of the short-term memory of a gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in the CTA paradigm takes place in the parabrachial nuclei (PBN) of the brainstem. The first direct evidence of participation of glutamatergic neurotransmission in the PBN during CTA demonstrated that the extracellular level of glutamate rises during saccharin drinking (Bielavska et al. in Brain Res 887:413-417, 2000). Our results show an effect of microdialysis administration of selective GluR antagonists into the PBN on the formation of CTA engram. We used four glutamate receptor (GluR) antagonists of different types (D-AP5, MK-801 as antagonists of ionotropic GluR and L-AP3, MSPG as antagonists of metabotropic GluR). The disruptive effect of MK-801 on CTA formation in the PBN is concentration-dependent, with the greatest inhibition under the higher concentrations eliciting significant disruption. The application of D-AP5 (0.1, 1, 5 mM) did not elicit a statistically significant blockade of CTA acquisition. This indicates that the association of the US-CS in the PBN is not dependent on NMDA receptors. On the contrary, application of L-AP3 (0.1, 1, 5 mM) blocked the CS-US association.

Effect of glutamate receptor antagonists on place aversion induced by naloxone in single‐dose morphine‐treated rats

The neurobiological mechanism underlying the negative motivational component of withdrawal from acute opiate dependence is far from understood. Our objectives were to determine whether the glutamatergic system is involved in the motivational component of morphine withdrawal in acutely dependent rats and such an involvement is associated with dopaminergic neurotransmission. We examined the effects of various kinds of glutamate receptor antagonists on conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal from a single morphine exposure 24 h before. Furthermore, the influence of pretreatment with the dopamine receptor antagonist haloperidol on those effects of glutamate receptor antagonists was also investigated. CPA was attenuated in a dose-dependent manner by all glutamate receptor antagonists examined including the NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK-801) and phencyclidine hydrochloride (PCP), AMPA receptor antagonist 1-(4-aminophenyl)4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), and metabotropic receptor antagonists (+/-)-2-amino-3-phosphonopropionic acid (AP-3) and (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG). The effects of MK-801, GYKI 52466 and MCPG were blocked by haloperidol. These results suggest that the glutamatergic system involving multiple classes of receptors plays a role in the motivational component of withdrawal from acute morphine dependence, and the function of the glutamatergic system would be closely associated with dopaminergic neurotransmission.

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

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). 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 micromoL) were also tested on the CRD-evoked responses of 17 controls and 18 CI neurons. (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 micromoL) had no significantly effect on the neuronal response to all intensities of CRD; however, CNQX (5 micromoL) significantly attenuated the responses to CRD in the 40-80 mmHg range. By contrast, CNQX (2-10 micromoL) significantly decreased the neuronal responses in CI rats to non-noxious and noxious CRD in a dose-dependent manner. 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.

Glutamate and Schizophrenia

Schizophrenia is a complex and chronic disease that affects multiple aspects of cognition and behavior, including attention, perception, thought processes, emotion and volition. However, the pathophysiology of schizophrenia has not yet been fully clarified. For more than a decade, the dopamine hypothesis has been the most influential hypothesis in schizophrenia research. It postulates that schizophrenia is a manifestation of a hyperdopaminergic state in some regions of the brain. However, several studies have reported results that are inconsistent with the dopamine theory of schizophrenia and it is now believed that other neurotransmitter systems are probably involved. Interest in the role of excitatory amino acids in schizophrenia is rapidly increasing. Excitatory amino acids and their receptors are thought to play a role in the pathology of schizophrenia, based on several lines of evidence, such as the psychomimetic effects of glutamate receptor antagonists in non-schizophrenic subjects and patients with schizophrenia, postmortem studies of reporting changes in glutamate receptors in schizophrenia, and studies that have treated schizophrenic patients with glutamate receptor agonists. This review will focus on clinical evidence contributing to the “glutamate hypothesis of schizophrenia”. Keywords: glutamate, schizophrenia, cognition, hyperdopaminergic, excitatory amino acids, glutamate receptor antagonists, glutamate hypothesis

AMPA/kainate-type glutamate receptor antagonist reduces progressive inner hair cell loss after transient cochlear ischemia

We investigated the effect of glutamate receptor antagonists on progressive inner hair cell (IHC) loss following transient cochlear ischemia in gerbils. Transient cochlear ischemia was induced by 15-min bilateral vertebral artery occlusion. An alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate-type glutamate receptor antagonist, 6-7-dinitroquinoxaline-2,3-dione (DNQX), or an N-methyl- d-aspartate (NMDA)-type receptor antagonist, MK-801, was administered 10 min before the ischemic insult. Hearing was assessed by sequentially recording compound action potentials (CAPs) before, during, and after the ischemia. The degree of hair cell loss in the organ of Corti was evaluated in specimens stained with rhodamine–phalloidin and Hoechst 33342. On the seventh day after ischemia, the increases in the CAP threshold and the progressive IHC loss were significantly reduced in cochleae treated with DNQX, while MK-801 was ineffective. These results suggest that the AMPA receptor plays a critical role in the development of the progressive IHC loss induced by ischemia/reperfusion injury in the cochlea.

β-Adrenoceptor blockade enhances the anticonvulsant effect of glutamate receptor antagonists against maximal electroshock

In this study, we evaluated whether β-adrenoceptor antagonists may modify the protective efficacy of dizocilpine (MK-801), a NMDA receptor antagonist, and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), a non-NMDA (AMPA/kainate) receptor antagonist, against maximal electroshock-induced seizures in mice. Propranolol, acebutolol, metoprolol and atenolol were used in doses that did not alter the electroconvulsive threshold. Propranolol potentiated the anticonvulsant activity of MK-801 and GYKI 52466, significantly lowering their ED 50 values from 0.38 and 15.0 to 0.15 ( P<0.001) and 8.4 mg/kg ( P<0.001), respectively. Similarly, metoprolol lowered the ED 50 of MK-801 and GYKI 52466 from 0.38 and 15.0 to 0.17 ( P<0.05) and 11.2 mg/kg ( P<0.05). Acebutolol enhanced the protective action of GYKI 52466, lowering its ED 50 value from 15.0 to 12.2 mg/kg ( P<0.05), but not that of MK-801. Atenolol, not penetrating the blood–brain barrier, did not affect the anticonvulsive efficacy of MK-801 and GYKI 52466. In conclusion, β-adrenoceptor antagonists may act synergistically with excitatory amino acid receptor antagonists to inhibit generalised tonic-clonic seizures.

Glutaminergic vagal afferents may mediate both retching and gastric adaptive relaxation in dogs

We previously reported that intra-4th-ventricular (i.4th.v.) administration of a non-NMDA receptor antagonist, NBQX, abolished vagally induced retching. This study was undertaken to ascertain whether or not the neuronal response in the solitary tract nucleus (NTS) to vagal stimulation and the vago-vagal gastric reflexes induced by non-emetic stimulation are also abolished by NBQX with a similar latency as in the case of retching. Ketamine and thiopental- or chloralose-anesthetized dogs were decerebrated, and the dorsal surface of the medulla was exposed. This study consisted of two series of experiments. In the first series, extracellular neuronal responses in the NTS to pulse-train vagal stimulation were recorded. Effects of NBQX on the neural response and vagally induced fictive retching were observed. In the second series, effects of glutamate receptor antagonists on gastric corpus responses to esophageal or gastric antral distension were observed. Retching was abolished 5–15 min after an i.4th.v. application of NBQX, and the neuronal responses disappeared within 14 min after application in nine of 10 NTS neurons. On the other hand, corpus contractility was inhibited by esophageal distension, and inhibited and/or enhanced by antral distension. While the inhibitory responses disappeared within 17 min after NBQX, the enhanced response remained even after NBQX and vagotomy, but was abolished by i.v. administration of hexamethonium. These results suggest that adaptive relaxation in the corpus, as well as retching, may be mediated by glutaminergic vagal afferents and non-NMDA receptors in the NTS.

Dual role of glutamatergic neurotransmission on amyloid β 1–42 aggregation and neurotoxicity in embryonic avian retina

The effects of glutamate receptor antagonists on the toxicity of the β-amyloid peptide (Aβ 1–42) in embryonic chick retina were investigated. When used alone or in combination, the N-methyl- d-asparate antagonist, MK-801, the (±)-αamino-3-hydroxyl-5-methylisoxazole-4-propinic acid/kainate antagonist, DNQX, and the metabotropic receptor antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid, blocked the neurotoxicity of Aβ 1–42. Aggregation of Aβ 1–42 was significantly increased in the presence of acidic glutamate solutions, but not in the presence of other neurotransmitters. These results point to a dual role of glutamatergic transmission in Alzheimer's disease (AD): (i) Aβ neurotoxicity requires activation of glutamate receptors and its blockade prevents cell death; (ii) high concentrations of glutamate in the synaptic cleft indirectly enhance Aβ aggregation through acidification of the medium, resulting in increased amounts of neurotoxic amyloid fibrils. These results suggest that glutamatergic neurotransmission may represent a novel target for therapeutic approaches in AD.