Stimulatory and inhibitory actions of excitatory amino acids on inositol phospholipid metabolism in rat cerebral cortex

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Lysosphingomyelin prevents behavioral aberrations and hippocampal neuron loss induced by the metabotropic glutamate receptor agonist quisqualate

1. Excessive excitation of brain neurons by the excitatory neurotransmitter, glutamate, induces a cascade of events leading to increased intracellular Ca++, neuronal degeneration and death. 2. Recent in vitro research has demonstrated that a natural cationic amphiphile in the brain, lysosphingomyelin, may be able to prevent neuronal degeneration by repressing phosphosinositidase-C overactivation induced by excessive excitation of the metabotropic glutamate receptor. 3. This research tested the latter finding in vivo in a rat model of glutamate excitotoxicity. Intracerebroventricular (i.c.v.) administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist, quisqualate, produced seizures, akinesia, destruction of hippocampal pyramidal cell dendritic microtubule-associated protein-2, and major loss of hippocampal CA sector neurons. 4. Prophylactic i.c.v. infusion of lysosphingomyelin powerfully attenuates these quisqualate-induced behaviors and prevents neuronal degeneration. 5. Lysosphingomyelin may be of clinical use in allaying progressive Group 1 mGluR-induced hippocampal cognitive and motor disorders including Alzheimer's disease, brain seizure, and stroke.

Effects of NMDA on carbachol-stimulated phosphatidylinositol resynthesis in rat brain cortical slices

N-methyl-D-aspartate (NMDA) inhibits carbachol-stimulated phosphoinositide breakdown in rat brain cortical slices but not in isolated membranes (1). To gain insight into the mechanisms, we examined the effects of NMDA on carbachol-stimulated [3H]inositol phosphate and intermediates of phosphatidylinositol cycle accumulation in rat cortical slices. The inhibition is primarily on the synthesis of inositol phospholipids subsequent to activation of muscarinic cholinergic receptors. In the absence of lithium, NMDA inhibited carbachol-stimulated [32P]PtdIns but not [32P]PtdOH synthesis. Carbachol-stimulated CDP-DAG formation required trace amount of Ca2+ and the response was inhibited by NMDA at low but not high extracellular Ca2+ concentrations. The inhibition due to NMDA was only seen at millimolar extracellular Mg2+. The inhibition of carbachol-stimulated CDP-DAG formation was not affected by adding tetrodotoxin or cobalt chloride suggesting the inhibitory effect was not due to releasing of neurotransmitters. The inhibitory effects of NMDA could be abolished by MK-801, the specific NMDA receptor associated channel antagonist. When cortical slices were preincubated with ligands and lithium to allow the build up of CDP-DAG, carbachol stimulated the incorporation of [3H]PtdIns. However, this response was not inhibited by NMDA. These results suggest that CDP-DAG synthesis is the primary site of regulation by NMDA. Because CDP-DAG cytidyltransferase requires Mg2+ as cofactor and is sensitive to Ca2+ it is possible that NMDA inhibits ligand-stimulated PtdIns breakdown by blocking the replenish of agonist-sensitive PtdIns pool through changes of divalent cation homeostasis.

Novel neuritic clusters with accumulations of amyloid precursor protein and amyloid precursor-like protein 2 immunoreactivity in brain regions damaged by thiamine deficiency

Experimental thiamine deficiency (TD) is a classical model of a nutritional deficit associated with a generalized impairment of oxidative metabolism and selective cell loss in the brain. In rats, TD-induced cell degeneration is accompanied by an accumulation of amyloid precursor protein (APP)/amyloid precursor-like protein 2 (APLP2) immunoreactivity in abnormal neurites and perikarya along the periphery of, or scattered within, the lesion. Prompted by these data and our previous findings of a genetic variation in the development of TD symptoms, we extended our studies to mice. C57BL/6, ApoE knockout, and APP YAC transgenic mice received thiamine-deficient diet and pyrithiamine injections. Unlike rats, APP/APLP2-immunoreactive neurites in all strains of mice were sparsely scattered within damaged areas and did not delimit the thalamic lesion. In addition, abnormal clusters of intensely immunoreactive neurites occurred only in areas of damage including the thalamus, mammillary body, and inferior colliculus. The clusters appeared as either irregular clumps or round or oval rosettes that strikingly resembled the neuritic component of Alzheimer amyloid plaques. However, immunostaining using various antisera to synthetic amyloid beta-protein (A beta 1-40) and thioflavine S histochemistry failed to show evidence of a component of A beta Neither APP/APLP2-immunoreactive clusters nor amyloid plaques were observed in the brain from patients with Wernicke-Korsakoff syndrome, the clinical manifestation of TD in man. Our results demonstrate species (i.e., genetic) differences in the response to TD-induced damage and support a role for APP and APLP2 in the response to brain injury. This is the first report that chronic oxidative deficits can lead to this novel pathology.

Inhibition by NMDA of carbachol-stimulated inositol tetrakisphosphate accumulation in rat brain cortical slices

The present studies examined the effect of NMDA on carbachol-stimulated accumulation of inositol polyphosphates, with emphasis on the accumulation of inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4), at short time periods in rat brain cortical slices. There was a stimulatory effect of NMDA on accumulation of labeled inositol mono-, bis- and trisphosphates but not on labeled inositol tetrakisphosphates. In the presence of carbachol Ins 1,3,4,5-P4 accumulation was preferentially inhibited by NMDA at early time periods (within 30 seconds after NMDA addition). Subsequently, total phosphoinositide breakdown was inhibited by NMDA. NMDA did not stimulate accumulation of total Ins 1,3,4,5-P4 but immediately inhibited carbachol stimulated accumulation of Ins 1,3,4,5-P4. The inhibitory effect of NMDA (1 mM) was not mimicked by increasing K+ in the medium from 10 to 30 mM. However 30 mM K+ reversed the inhibitory effect of 1 mM NMDA on carbachol-stimulated Ins 1,3,4,5-P4. Parallel experiments with veratridine (a sodium channel activator) suggest that the early inhibitory effects of NMDA on Ins 1,3,4,5-P4 accumulation are not due to decreases in ATP availability or elevations in intracellular Na+. These data indicate that NMDA increases inositol mono-, bis- and trisphosphate accumulation while blocking muscarinic cholinergic stimulated accumulation of Ins 1,3,4,5-P4.

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P3], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P3. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent. A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P3 generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive. The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10(-5)M. While 10(-3)M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P3 levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P3 levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect. Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P3 levels in the tissues.

Differential responsiveness of metabotropic glutamate receptors coupled to phosphoinositide hydrolysis to agonists in various brain areas of the adult rat

The effects of metabotropic glutamate receptor (mGluR) agonists on inositol phosphates (IP) accumulation were investigated in slices of the cerebral cortex, hippocampus, striatum and cerebellum of adult Sprague-Dawley rats. EC(50) values for 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) did not differ significantly between various brain areas (range 10(-5) M), quisqualate was the most potent in all the brain areas (range 10(-7) - 10(-6) M), except the cerebellum (10(-5) M), ibotenate was the most potent in the striatum (range 10(-6) M) and the least potent in the cerebral cortex and hippocampus (range 10(-4) M). The efficacy in the four brain areas showed the following trend of ranking order for ACPD and quisqualate: hippocampus > striatum > cerebral cortex > cerebellum, and for ibotenate: hippocampus > cerebral cortex > striatum > cerebellum, although the observed differences reached the level of statistical significance only in the case of ACPD (hippocampus and striatum vs cerebellum) and ibotenate (hippocampus vs cerebellum). Co-incubation of the agonists at maximally effective concentrations in any pairwise combination resulted in no substantial additivity of IP accumulation. D,L-1-amino-3-phosphonopropionic acid (AP3) and D,L-2-amino-4-phosphonobutyric acid (AP4) at 0.5 mM concentration antagonized ACPD-induced IP accumulation by about 70 and 45 percent, respectively, without differences between brain areas. On the other hand, the antagonistic effects of L-serine-o-phosphate (SOP) at 1 mM concentration were the highest in the hippocampus (75 percent) and the lowest in the cerebellum (25 percent). The comparative data indicate considerable regional receptor heterogeneity, in terms of different ratios of response to the agonists (but not antagonists, except SOP). There is a robust responsiveness of mGluRs not only in the hippocampus and cerebral cortex, but also in the striatum which exhibits the highest affinity to both quisqualate and ibotenate.

Regional concentrations of cyclic nucleotides after experimental brain injury

Regional concentrations of lactate, glucose, cAMP, and cGMP were measured after lateral fluid percussion brain injury in rats. At 5 min after injury, while tissue concentrations of lactate were elevated in the cortices and hippocampi of both the ipsilateral and contralateral hemispheres, those of glucose were decreased in these brain regions. By 20 min after injury, increases of lactate concentrations and decreases of glucose concentrations were observed only in the cortices and in the hippocampus of the ipsilateral hemisphere. Whereas the cAMP concentrations were unchanged in the cortices and hippocampi of the ipsilateral and contralateral hemispheres at 5 min after injury, decreases were found in the injured cortex and ipsilateral hippocampus at 20 min after injury. The tissue concentrations of cGMP were found to be elevated only in the ipsilateral hippocampus at 5 min after injury. The present observation that tissue glucose decreases in the injured cortex and the ipsilateral hippocampus are consistent with the published findings of increased hyperglycolysis and oxidative metabolism in brain immediately after injury. The present findings that the concentrations of cAMP and cGMP change in the cortex and hippocampus provide biochemical evidence for the neurotransmitter's surge after brain injury.

Activation of 5-HT1A receptors inhibits carbachol-stimulated inositol 1,4,5-trisphosphate mass accumulation in the rodent hippocampus

We have previously demonstrated that 5-HT1A receptor agonists partially prevent the stimulation by carbachol of [3H]-phosphoinositide hydrolysis in immature rat hippocampal slices. This negative modulation has been investigated further by measuring, using a radioreceptor assay, the mass accumulation of IP3. In hippocampal slices from developing rats and in hippocampal neurons, carbachol enhanced the accumulation of IP3 and this response was partially inhibited by 8-OH-DPAT with a potency compatible with the affinity of this agonist for 5-HT1A receptors. The inhibition of the carbachol response by 8-OH-DPAT was non-competitive in nature and 8-OH-DPAT did not affect the inhibitory potency of pirenzepine. The inhibitory effect of 8-OH-DPAT was maintained after washing the slices preincubated with this compound but was not observed on the carbachol-stimulated PIP2 hydrolysis in hippocampal membranes, suggesting that this compound induces long lasting changes of muscarinic receptors and/or their effector mechanism by an indirect action.

Inhibition by veratridine of carbachol-stimulated inositol tetrakisphosphate accumulation in rat brain cortical slices

The present studies examined the inhibitory effect of veratridine (a Na+ channel activator) on carbachol (a cholinergic agonist) stimulated inositol 1,3,4,5-tetrakisphosphate accumulation in rat brain cortical slices. Veratridine inhibited carbachol stimulation of inositol 1,3,4,5-tetrakisphosphate formation (after a delay of about 30 seconds) at 60 or 120 seconds when there was little inhibition of inositol 1,4,5 trisphosphate accumulation. The inhibitory effect of veratridine on carbachol stimulated inositol 1,3,4,5-tetrakisphosphate accumulation was abolished in the presence of ouabain or tetrodotoxin but was unaffected in low calcium conditions. Veratridine reduced the total ATP content and this effect was abolished by tetrodotoxin. The inhibitory effect of 10 but not 30 microM veratridine on inositol 1,3,4,5-tetrakisphosphate accumulation in the presence of carbachol was reversed by the presence of exogenous 8-bromo cyclic AMP or forskolin which activates adenylyl cyclase. However, the decrease in brain slice ATP seen in the presence of veratridine was unaffected by forskolin. Our results are compatible with the hypothesis that veratridine inhibition of carbachol-stimulated inositol 1,3,4,5-tetrakisphosphate formation is due to depletion of ATP at the site of Ins 1,3,4,5-P4 formation from Ins 1,4,5-P3.

AMPA receptor activation regulates the glutamate metabotropic receptor stimulated phosphatidylinositol turnover in human cerebral cortex slices

The effect of excitatory amino acids (EAA) on phosphatidylinositol (PI) turnover in human cerebral cortical slices was investigated. Trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) increased inositol phosphate (IP) formation in the 1-1000 microM range. Quisqualic acid (QA) was maximally effective at 10-100 microM, showing an inverse correlation between concentration and effect in the 100-1000 microM range. The glutamate metabotropic receptor antagonist 2-amino-3-phosphonopropionic acid (AP3), the ionotropic non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the NMDA channel blocker dizolcipine (MK-801) failed to prevent the PI response to ACPD (1000 microM). However, CNQX (100 microM) modified the concentration-response curve of QA reducing the effect of QA 10 microM by approx. 50% and enhancing that of QA 1000 microM by 2-fold. In addition, CNQX (100 microM) together with MK-801 (100 microM) unmasked the ability of L-glutamate (L-GLU) 3000 microM to stimulate PI turnover. The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on the EAA-induced PI turnover was also studied. AMPA (0.1-1 microM) potentiated the response to submaximal (30 microM) ACPD and (1 microM) QA concentrations. However, higher AMPA concentrations (10 microM) failed to synergize with ACPD 30 microM and, in addition, inhibited the PI turnover maximally stimulated by QA 10 microM. These results further support the presence of the glutamate metabotropic receptor in the human neocortex. In addition, they show the occurrence of a concentration-related dual interaction between AMPA and glutamate metabotropic receptor activation in the IP formation in this brain area.

Carbachol-induced accumulation of inositol phosphates and its modulation by excitatory amino acids in cortical slices of young and aged rats with down-regulation of muscarinic M-1 receptors

The effects of a subacute intoxication with diisopropyl fluorophosphate (DPF) on total muscarinic acetylcholine receptor sites (mAChRs) and M-1 AChRs were evaluated in the cerebral cortex of young (2-4 months) and aged (22-24 months) Fischer 344 rats. Since M-1 AChRs are coupled to the metabolism of phosphoinositides, carbachol-induced accumulation of inositol phosphates (IP) and its inhibition by glutamate and NMDA was also measured in the cortical slices. DFP treatment caused about 75% inhibition of cholinesterase and 35% down-regulation of mAChRs (measured as [3H]quinuclidinyl benzylate binding) in both young and aged rats. The down-regulation of M-1-ACHRs (measured as [3H]pirenzepine binding) was more pronounced in aged (30%) than in young (17%) DFP-treated rats. There was a significant increase in carbachol-induced IP accumulation in aged, with respect to young, untreated rats. DFP treatment caused a considerable decrease in such IP accumulation in aged but not in young rats. Glutamate and NMDA antagonized carbachol-induced IP accumulation in untreated young and aged rats (and the effects of NMDA were reversed by carboxy-piperazinyl-propyl phosphonic acid). In DFP-treated rats such antagonism was somewhat less pronounced. The data appear of interest in relation to the use of anticholinesterase compounds in the therapy of senile dementia of Alzheimer's type. They suggest that beside their primary action (increasing brain ACh levels) such compounds also act on post-receptor mechanisms and on the interactions between cholinergic and glutamatergic neurotransmitter systems.

Role of Ca2+ in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated polyphosphoinositide turnover in primary neuronal cultures

Excitatory amino acid (EAA) receptors and EAA-mediated stimulation of polyphosphoinositide (poly-PI) turnover were studied in cultured neurons at different days in vitro (DIV). Six main observations have emerged from these studies: (a) Neurons increased their sensitivity to EAAs as a function of time in culture, indicated by increasing EAA-mediated poly-PI turnover. (b) Extracellular Ca2+ concentration played an important role in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated poly-PI turnover in cells at 4 DIV, whereas poly-PI turnover mediated by L-glutamate and trans-1-amino-cyclopentane-1,3-dicarboxylic acid was not Ca(2+)-dependent. (c) A marked stimulation of poly-PI turnover by AMPA was seen in the cultured neurons at 4 DIV, but not at 17 DIV, suggesting that a distinct EAA receptor sensitive to AMPA is transiently expressed. (d) The Ca2+ ionophore A23187 increased poly-PI turnover in cultured neurons, suggesting that Ca2+ entry is involved in stimulating poly-PI turnover. (e) Stimulation of poly-PI turnover by carbachol was greater in neurons at 17 DIV as compared with 4 DIV, and appeared to be Ca(2+)-dependent across DIV. (f) 6-Cyano-7-nitroquinoxaline-2,3-dione, an antagonist for non-N-methyl-D-aspartate ionotropic EAA receptors, inhibited 100% and 35% of AMPA- and quisqualate-induced poly-PI turnover, respectively, suggesting an involvement of ionotropic AMPA/quisqualate receptors in stimulating poly-PI turnover.

Modulatory effects of NMDA on phosphoinositide responses evoked by the metabotropic glutamate receptor agonist 1S,3R-ACPD in neonatal rat cerebral cortex

1. The effect of NMDA-receptor stimulation on phosphoinositide signalling in response to the metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) has been examined in neonatal rat cerebral cortex slices. 2. Total [3H]-inositol phosphate ([3H]-InsPx) accumulation, in the presence of 5 mM LiCl, in [3H]-inositol pre-labelled slices was concentration-dependently increased by 1S,3R-ACPD (EC50 16.6 microM) and, at a maximally effective concentration, 1S,3R-ACPD (300 microM) increased [3H]-InsPx accumulation by 12.8 fold over basal values. 3. [3H]-InsPx accumulation stimulated by 1S,1R-ACPD was enhanced by low concentrations of NMDA (3-30 microM), but not by higher concentrations (> 30 microM). [3H]-InsPx accumulations stimulated by 1S,3R-ACPD in the absence or presence of 10 microM NMDA were linear with time, at least over the 15 min period examined; however, in the presence of 100 microM NMDA the initial enhancement of 1S,3R-ACPD-stimulated phosphoinositide hydrolysis progressively decreased with time. 4. In the presence of a maximal enhancing concentration of NMDA (10 microM), the response to 1S,3R-ACPD (300 microM) was increased 1.9 fold and the EC50 for agonist-stimulated [3H]-InsPx accumulation decreased about 4 fold. The enhanced response to the metabotropic agonist was concentration-dependently inhibited by competitive and uncompetitive antagonists of NMDA-receptor activation. 5. 1S,3R-ACPD also stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation with an initial peak response (5-6 fold over basal) at 15 s decaying to a smaller (2 fold), but persistent elevated accumulation (1-10 min). 6. Co-addition of 10 or 100 MicroM NMDA enhanced the initial peak Ins(1,4,5)P3 response to 1S,3RACPD.However, the enhancing effect was only maintained over 10 min in the presence of 1O Micro MNMDA, whilst in contrast, 100 MicroM NMDA ceased to cause a significant enhancement of the metabotropic response by 5 min and completely suppressed lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation at 10 min.7. Both basal and 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulations were reduced when slices were incubated in nominally Ca2"-free medium. Under these conditions only a concentration-dependent enhancement of the response was observed (EC50 for NMDA facilitation of lS,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation of 32 MicroM).8. These experiments have revealed that at low concentrations, NMDA can dramatically potentiate1S,3R-ACPD-stimulated phosphoinositide hydrolysis, probably by a Ca2"-dependent facilitation of agonist-stimulated phosphoinositide-specific phospholipase C activity. Higher concentrations of NMDA result in time-dependent inhibition of the metabotropic agonist-stimulated response. We believe the former effect could be fundamental in glutamate receptor 'cross-talk', whereas the latter may reflect a Ca2+-dependent neurotoxic effect of NMDA on the neonatal cerebral cortex slices.

The glutamate inhibition of carbachol stimulated inositol phosphate production in rat cortical cells is mediated through an ionotropic NMDA receptor

Carbachol (0.1 mM) stimulated accumulation of inositol monophosphate (IP1) (3-4 fold of basal, P < 0.001) in fetal rat cortical cells is attenuated by glutamate (at 0.1 mM, 40-70% of carbachol alone, P < 0.001). This inhibition by glutamate was reduced by 2-amino-5-phosphonopentanoic acid (AP5), but not by gamma-D-glutamylaminomethyl sulphonic acid (GAMS) [corrected] or 2-amino-3-phosphonopropionic acid (AP3). The metabotropic receptor agonist (1S,3R)-1-aminocyclopentane-1-3-dicarboxylic acid [(1S,3R)-ACPD] (up to 0.1 mM) had no effect upon carbachol stimulated IP1. Staurosporine and quinacrine were unable to prevent the inhibition of carbachol stimulated IP1 by glutamate. These data suggest that the inhibition of carbachol-stimulated IP1, by glutamate in rat cortical cells is mediated through an NMDA ionotropic receptor.

Excitatory amino acids (EAAs) stimulate phosphatidylinositol turnover in adult rat striatal slices: interaction between NMDA and EAA metabotropic receptors

The effect of excitatory amino acids (EAAs) on phosphatidylinositol (PI) turnover in adult rat striatal slices was investigated. Quisqualic acid (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainic acid (KA), ibotenic acid (IBO) and N-methyl-D-aspartic acid (NMDA) maximally increased inositol phosphate (IP) formation at 10 microM while trans-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was maximally effective at 100 microM. The NMDA channel blocker dizolcipine (MK-801) counteracted the effect of NMDA 10 microM and IBO 10 microM while it potentiated that of IBO 100 microM and IBO 1000 microM. Conversely, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) prevented the effect of AMPA and KA and reduced that of QA (all at 10 microM). Lowering extracellular Ca2+ concentrations ([Ca2+]0) differentially affected the PI response to EAAs. The ACPD 30 microM effect was unchanged at low [Ca2+]0 (but abolished when EGTA 2 mM was added), while that of ACPD 100 microM was halved in 0.1 mM and almost abolished in a nominally free Ca2+ medium. NMDA 10 microM and AMPA 10 microM were ineffective at low [Ca2+]0 while NMDA 100 microM, ineffective in a 1.2 mM Ca2+ medium, strongly stimulated IP formation in 0.1 mM Ca2+ but not in a nominally free Ca2+ medium. The effect of NMDA on EAA metabotropic receptor agonist stimulated PI turnover was also studied. NMDA 10 microM potentiated the effect of ACPD 30 microM. This positive cooperation persisted at low [Ca2+]0 but not in the presence of EGTA. Conversely, NMDA 100 microM prevented the effect of ACPD 100 microM. This negative interference was reversed when Ca2+ was omitted from the medium. This study shows that in the adult rat striatum both EAA metabotropic and ionotropic receptor activation increases IP formation. A positive and negative interaction between NMDA and metabotropic receptor activation was also found to regulate PI turnover. The role of [Ca2+]0 in subserving the PI response to EAAs was made evident.

Modulation of muscarinic cholinoceptor-stimulated inositol 1,4,5-trisphosphate accumulation by N-methyl-D-aspartate in neonatal rat cerebral cortex

The mechanisms by which N-methyl-D-aspartate (NMDA) receptor activation can modulate muscarinic receptor-stimulated phosphoinositide turnover have been studied in neonatal rat cerebral cortex slices. A maximally effective concentration of carbachol (1 mM) caused a large stimulation of both total [3H]inositol phosphate ([3H]InsPx) accumulation (30-40-fold over basal levels after 15 min in the presence of 5 mM LiCl) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] mass accumulation (consisting of a rapid peak increase of about 8-10-fold within 15 sec followed by a sustained plateau rise of 4-5-fold which persisted for > 10 min). Low concentrations of NMDA enhanced carbachol-stimulated [3H]InsPx and Ins(1,4,5)P3 accumulations with a maximal effect being observed at 10 microM NMDA. However, at higher concentrations of NMDA (30-300 microM) a dramatic inhibition of these indices of phosphoinositide turnover was observed. Time-course studies demonstrated that NMDA (100 microM) caused a significant enhancement of the initial increases in [3H]InsPx and Ins(1,4,5)P3 accumulations stimulated by carbachol, with the profound inhibitory effects becoming evident at longer incubation times. The modulatory effects of NMDA were antagonized by D-2-amino-5-phosphonopentanoate and MK-801. Reducing extracellular calcium concentration ([Ca2+]e) to the low micromolar range decreased basal Ins(1,4,5)P3 accumulation and attenuated the response to carbachol. Under these conditions NMDA (10-100 microM) caused only a potentiation of agonist-stimulated Ins(1,4,5)P3 accumulation. Under control conditions ([Ca2+]e = 1.3 mM), addition of MK-801 (1 microM) 10 min after carbachol + 100 microM NMDA challenge failed to reverse the inhibitory effect of NMDA on carbachol-stimulated [3H]InsPx accumulation. Furthermore, pre-incubation of cerebral cortex slices with 100 microM NMDA for 15 min (followed by extensive washing of slices to remove NMDA) dramatically decreased [3H]inositol incorporation into the cellular inositol phospholipid fraction and decreased basal and carbachol-stimulated Ins(1,4,5)P3 mass accumulations. We conclude that the enhancement of agonist-stimulated phosphoinositide turnover seen at concentrations of NMDA up to 10 microM may be due to Ca2+ entry and Ca2+ facilitation of phosphoinositide-specific phospholipase C activity. In contrast, the inhibitory effect of high concentrations of NMDA on agonist-stimulated phosphoinositide turnover may be due to progressive, irreversible and, at least in part, Ca(2+)-dependent damage to the cell populations in the slice preparation responding to muscarinic-receptor stimulation.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation

The effects of (1S,3R)-ACPD, a selective metabotropic glutamate receptor agonist, on NMDA-induced 45Ca2+ accumulation and delayed neuronal cell death were determined using primary cerebrocortical cultures. Exposure to (1S,3R)-ACPD alone, although causing small increases in 45Ca2+ accumulation, was not neurotoxic. The presence of (1S,3R)-ACPD during exposure to NMDA attenuated the resulting sustained accumulation of 45Ca2+ and delayed neuronal cell death. Reductions in sustained Ca2+ accumulation were associated both with Ca2+ efflux, in the absence of cell death, and inhibition of delayed intracellular Ca2+ accumulation. The protective effects of (1S,3R)-ACPD on NMDA-induced cell death were inhibited by pretreatment of cultures with pertussis toxin. These results suggest that activation of metabotropic glutamate receptors may stimulate intracellular processes capable of limiting sustained elevations in intracellular calcium and the resulting excitotoxic neuronal damage.

Excitatory amino acid receptor-stimulated phosphoinositide turnover in primary cerebrocortical cultures

1. Characterization of excitatory amino acid-induced accumulation of [3H]-phosphoinositides was carried out in primary cerebrocortical cultures isolated from foetal rats. 2. All of the excitatory amino acid receptor agonists examined caused concentration-dependent enhancement of phosphoinositide (PI) formation. The most potent excitatory amino acid receptor agonists were quisqualate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), ibotenate and glutamate with mean EC50 values of 0.9 +/- 0.4 microM, 15 +/- 5 microM, 15 +/- 3 microM and 41 +/- 8 microM respectively. 3. The selective ionotropic receptor antagonists kynurenic acid (1 mM), 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX, 10 microM) and (+/-)-4-(3-phosphonopropyl)-2 piperazinecarboxylic acid (CPP, 100 microM), failed to block responses to quisqualate, (1S,3R)-ACPD or glutamate. D,L-2-Amino-3-phosphonopropionate (D,L-AP3) did not block 1S,3R-ACPD or quisqualate-induced PI turnover, but had an additive effect with quisqualate or (1S,3R)-ACPD. 4. Exposure of cultures to agonists in the absence of added extracellular calcium reduced the maximal quisqualate response by approximately 45%, revealing a two-component concentration-response curve. Concentration-response curves to ibotenate and glutamate became flattened by omission of extracellular calcium, whereas (1S,3R)-ACPD-stimulated PI turnover was unaffected. 5. Pretreatment of cultures with pertussis toxin markedly inhibited PI responses evoked by (1S,3R)-ACPD. 6. These results suggest that excitatory amino acid-stimulated PI turnover in cerebrocortical cultures is independent of ionotropic receptor activation and is mediated via specific G-protein-linked metabotropic receptors. The partial dependence of the responses to quisqualate, ibotenate and glutamate on the presence of extracellular calcium suggests that the effects of these agonists may be mediated by more than one receptor subtype.

Effects of excitatory amino acids on inositol phosphate accumulation in slices of the cerebral cortex of young and aged rats

The effects of glutamate, NMDA and quisqualate on carbachol- and norepinephrine-elicited formation of inositol phosphate (IP) were evaluated in slices prepared from the cerebral cortex of 3- and 24-month Sprague-Dawley rats. Glutamate, NMDA, and quisqualate antagonized the IP response to carbachol in a concentration-dependent fashion. This antagonism was more pronounced in aged than in young rats, both for glutamate (IC5O 0.114 and 0.210 mM) and NMDA (IC5O 0.0029 and 0.127 mM), but not for quisqualate. Glutamate (but not NMDA) also antagonized in a concentration-dependent fashion the IP response to norepinephrine, IC50s were 0.061 and 0.126 mM for aged and young rats, respectively; quisqualate had an inhibitory effect only at 1 mM concentration in the two age-groups, while in aged rats some stimulatory effect was present at 0.1 mM concentration. Glutamate, NMDA and quisqualate (1 mM) did not affect basal IP accumulation in either young or aged rats; quisqualate, however, at 0.1 mM concentration had some stimulatory effect, more pronounced in aged rats. This effect was probably responsible for the biphasic effect of quisqualate in this age-group. The most important finding consists of the demonstration of an age-related increase in the inhibitory effects of NMDA on carbachol-induced IP accumulation. This implies an altered modulation of cholinergic post-receptor mechanisms by glutamatergic mechanisms.

(R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors mediate a calcium-dependent inhibition of the metabotropic glutamate receptor-stimulated formation of inositol 1,4,5-trisphosphate

L-glutamate (3-1,000 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 10-1,000 microM), a selective agonist for the metabotropic glutamate receptor, stimulated the formation of inositol 1,4,5-trisphosphate in a concentration-dependent manner. L-Glutamate was half as efficacious as 1S,3R-ACPD. N-methyl-D-aspartate (NMDA; 1 nM to 1 mM) did not significantly influence the response to a maximally effective concentration of 1S,3R-ACPD (100 microM). On the other hand, coapplication of (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 1-300 nM) produced a concentration- and time-dependent inhibition of the 1S,3R-ACPD effect, with a maximal inhibition (97%) at 100 nM. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the AMPA receptor, blocked the inhibitory effect of AMPA. Reduced extracellular calcium concentration, as well as 10 microM nimodipine, an L-type calcium channel antagonist, inhibited the AMPA influence on the 1S,3R-ACPD response. W-7, a calcium/calmodulin antagonist, prevented the inhibition by AMPA, whereas H-7, an inhibitor of protein kinase C, had no effect. These data suggest that activation of AMPA receptors has an inhibitory influence on inositol 1,4,5-trisphosphate formation mediated by stimulation of the metabotropic glutamate receptor. The mechanism of action involves calcium influx through L-type type calcium channels and possible activation of calcium/calmodulin-dependent enzymes.

Glutamate Inhibits Adenylate Cyclase Activity in Dispersed Rat Hippocampal Cells Directly via an N-Methyl-d-Aspartate-Like Metabotropic Receptor

Three major subtypes of glutamate receptors that are coupled to cation channels--N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors--are known as ionotropic receptors in the mammalian CNS. Recently, an additional subtype that is coupled to GTP binding proteins and stimulates (or inhibits) metabolism of phosphoinositides has been proposed as a metabotropic receptor. Incubation of dispersed hippocampal cells from adult rats with glutamate or NMDA decreased forskolin-stimulated cyclic AMP (cAMP) accumulation; half-maximal effects were obtained with 5.6 +/- 2.2 and 6.4 +/- 2.3 microM, respectively. Kainate and quisqualate were less potent. The effect of glutamate was antagonized by 2,3-diaminopropionate and 2-amino-5-phosphonovalerate, NMDA/glutamate receptor antagonists, but not by 0.5 microM Joro spider toxin, a specific blocker of the AMPA receptor. The inhibitory effect of glutamate on cAMP formation was not blocked by 2 microM tetrodotoxin or by the absence of Ca2+. In hippocampal membranes, glutamate, similar to carbachol, inhibited adenylate cyclase activity in a GTP-dependent manner. These findings suggest that the glutamate inhibition of adenylate cyclase is direct and is not due to a result of the release of other neurotransmitters. The effect of glutamate on cAMP accumulation was observed in an assay medium containing 0.7 mM MgCl2, which is known to inhibit both ionotropic NMDA receptor/channels in the hippocampus and metabotropic NMDA receptors in the cerebellum. The inhibitory effect of glutamate was abolished by pertussis toxin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnesium-dependent inhibition of agonist-stimulated phosphoinositide breakdown in rat cortical slices by excitatory amino acids

The excitatory amino acid agonists kainate, N-methyl-D-aspartate (NMDA), and quisqualate inhibited ligand-stimulated phosphoinositide hydrolysis in rat cortical slices. The NMDA channel blocker MK-801 antagonized the inhibition by NMDA but had no effect on the inhibition due to kainate or quisqualate. The antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the effects of quisqualate and kainate but not the effect of NMDA. These data indicate that activation of the NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate types of ionotropic receptors has the same effect. In membranes prepared from cortical slices, there was no inhibition of carbachol-stimulated phosphoinositidase C activity by excitatory amino acids, suggesting that excitatory amino acids indirectly affect carbachol-stimulated phosphoinositide hydrolysis. The inhibition by excitatory amino acids of carbachol-stimulated phosphoinositide breakdown was dependent on extracellular Mg2+ and was abolished by procedures that increase intracellular Ca2+. Veratridine inhibition of carbachol-stimulated phosphoinositide hydrolysis was reversed by ouabain but not by other procedures that increase intracellular Ca2+. In contrast to excitatory amino acids, veratridine potentiated carbachol-stimulated phosphoinositide breakdown in the presence of 10 mM extracellular Mg2+. These data suggest that excitatory amino acids inhibit carbachol-stimulated phosphoinositide breakdown in rat cortex by lowering intracellular Ca2+ through a mechanism dependent on extracellular Mg2+.

Cerebral infarction as a remote complication of childhood Haemophilus influenzae meningitis

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Distribution of the mRNA for a metabotropic glutamate receptor (mGluR1) in the central nervous system: an in situ hybridization study in adult and developing rat

Distribution of the mRNA for a metabotropic glutamate receptor (mGluR1), which is linked to phosphoinositide (PI) hydrolysis, was investigated in adult and developing rat central nervous system (CNS) by in situ hybridization. Transcripts of mGluR1 were specifically localized to neurons and widely distributed throughout the adult rat brain. Most intensely labeled neurons were Purkinje cells of the cerebellum, mitral and tufted cells of the olfactory bulb, and neurons in the hippocampus, lateral septum, thalamus, globus pallidus, entopeduncular nucleus, ventral pallidum, magnocellular preoptic nucleus, substantia nigra, and dorsal cochlear nucleus. Moderately labeled neurons were seen in high density in the dentate gyrus, striatum, islands of Calleja, superficial layers of the retrosplenial, cingulate and entorhinal cortices, mammillary nuclei, red nucleus, and superior colliculus. In the developing rat brain, the level of mGluR1 expression gradually increased during early postnatal days in accordance with the maturation of neuronal elements. These results show prominent expression of mGluR1 in the major targets of putative glutamatergic pathways and unique distribution pattern of mGluR1 distinct from those reported for ionotropic subtypes of glutamate receptors, suggesting specific roles of mGluR1 in the glutamatergic system.

Excitatory amino acids modulate phosphoinositide signal transduction in human epileptic neocortex

Stimulation of phosphoinositide (PI) hydrolysis by norepinephrine (NE), carbachol (Carb), and excitatory amino acids (EAAs) was measured in slices prepared from neocortex excised during epilepsy surgery. NE and Carb markedly enhanced PI turnover (EC50: NE, 12 microM; Carb, 661 microM) as reflected by [3H]inositol monophosphate (IP1) accumulation in tissue slices prelabeled with [3H]myoinositol. These effects were dose-dependent, saturable, and five to six times higher than basal IP1 accumulation. A weaker stimulation (twofold) was observed with quisqualate (QUIS; EC50, 1.1 microM) and glutamate (GLU; EC50, greater than 1 mM), while minimal or no stimulation was seen with kainate (KA) and N-methyl-D-aspartate (NMDA). Agonist-stimulated PI turnover was significantly reduced in samples from actively spiking epileptic neocortex versus nonspiking areas as defined by electrocorticography (NE, -23%, p less than 0.05; Carb, -44%, p less than 0.01). Preincubation of slices with various EEAs inhibited Carb-induced IP1 formation. The maximal extent of inhibition (1 mM) was both amino acid-dependent (IC50: NMDA, 5 microM; KA, 3.3 microM; QUIS, 47 microM; GLU, greater than 1 mM). These data suggest that epileptic activity modulates PI metabolism and alters receptor-effector coupling. As important mediators of epileptogenesis, EAAs may interfere++ with the efficiency of this second messenger system.

Agonist-Stimulated Inositol Polyphosphate Formation in Cerebellum

The accumulation of inositol polyphosphates in the cerebellum in response to agonists has not been demonstrated. Guinea pig cerebellar slices prelabeled with [3H]inositol showed the following increases in response to 1 mM serotonin: At 15 s, there was a peak in 3H label in the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], decreasing to a lower level in about 1 min. The level of 3H label in the putative second-messenger inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] increased rapidly up to 60 s and increased slowly thereafter. The accumulation of 3H label in various inositol phosphate isomers at 10 min, when steady state was obtained, showed the following increases due to serotonin: inositol 1,3,4-trisphosphate [Ins(1,3,4)P3], eight-fold; Ins(1,3,4,5)P4, 6.4-fold; Ins(1,4,5)P3, 75%; inositol 1,4-bisphosphate [Ins(1,4)P2], 0%; inositol 3,4-bisphosphate, 100%; inositol 1-phosphate/inositol 3-phosphate, 30%; and inositol 4-phosphate, 40%. [3H]Inositol 1,3-bisphosphate was not detected in controls, but it accounted for 7.2% of the total inositol bisphosphates formed in the serotonin-stimulated samples. The fact that serotonin did not increase the formation of Ins(1,4)P2 could be due to the fact that Ins(1,4)P2 is rapidly degraded or that Ins(1,4,5)P3 is metabolized primarily by Ins(1,4,5)P3-3'kinase to form Ins(1,3,4,5)P4. In the presence of pargyline (10 microM), [3H]Ins(1,3,4,5)P4 and [3H]Ins(1,3,4)P3 levels were increased, even at 1 microM serotonin. Ketanserin (7 microM) completely inhibited the serotonin effect, indicating stimulation of serotonin2 receptors. Quisqualic acid (100 microM) also increased the levels of [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4, and [3H]Ins(1,3,4)P3, but the profile of these increases was different.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical responses mediated by N-methyl-D-aspartate receptors in rat cortical slices are differentially sensitive to magnesium

The effects of magnesium on the inhibition of phosphoinositide (PI) hydrolysis and the stimulation of [3H]norepinephrine release by N-methyl-D-aspartate (NMDA) in rat cortical slices were investigated. Removal of the magnesium from the buffer resulted in a small reduction of the inhibitory effect of 100 microM NMDA (34% inhibition in the absence of magnesium, compared with 51% for the control) when slices were coincubated with NMDA and carbachol. Addition of 10 mM Mg2+ also allowed the inhibitory effect of 100 microM NMDA on carbachol-stimulated PI hydrolysis to be expressed (44% inhibition) under these conditions. Concentration-effect curve analysis for the NMDA-induced inhibition of carbachol-stimulated PI hydrolysis indicated that the IC50 for NMDA was decreased from 14.9 microM for the control to 4.2 microM in the absence of magnesium. The absence of magnesium also had small effects on the concentration-effect curve for (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate reversal of the inhibitory effects of NMDA on carbachol-stimulated PI hydrolysis. The absence of magnesium also shifted slightly downward and flattened the NMDA concentration-effect curve if the cortical slices were pretreated with NMDA in the presence or absence of magnesium followed by removal of the NMDA and subsequent stimulation with carbachol. In contrast, cortical slices that had been prepared and treated similarly to the slices used in the PI experiments were very sensitive to the inhibitory effects of magnesium when using the NMDA stimulation of [3H]norepinephrine release assay in the presence or absence of carbachol.(ABSTRACT TRUNCATED AT 250 WORDS)

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: II. Calcium dependency, cadmium inhibition

In this article, we demonstrate that an increase in intracellular Ca2+ concentration may represent a specific common step(s) in the mechanism(s) of action of glutamate (Glu) and depolarizing agents on formation of inositol phosphates (IPs) in 8-day-old rat forebrain synaptoneurosomes. In fact, A23187, a Ca2+ ionophore, induces a dose-dependent accumulation of IPs, which is not additive with that evoked by Glu and K+ but is slightly synergistic with that induced by carbachol. In addition, Glu and K+ augment the intracellular Ca2+ concentration in synaptoneurosome preparations as measured by the fura-2 assay. The absence of external Ca2+ decreases basal and Glu-, and K(+)-stimulated formation of IPs. Cd2+ (100 microM) fully inhibits both Glu- and K(+)-evoked formation of IPs without affecting the carbachol-elicited response of IPs. Zn2+ inhibits Glu- and K(+)-stimulated accumulation of IPs (IC50 approximately 0.4 mM) but with a lower affinity than Cd2+ (IC50 approximately 0.035 mM). The organic Ca2+ channel blockers verapamil (10 microM), nifedipine (10 microM), omega-conotoxin (2 microM), and amiloride (10 microM) as well as the inorganic blockers Co2+ (100 microM) and La3+ (100 microM) block neither Glu- nor K(+)-evoked formation of IPs, a result suggesting that the opening of the L-, T-, N-, or P-type Ca2+ channels does not participate in these responses. All these data suggest that an increase in intracellular Ca2+ concentration resulting from an influx of Ca2+, sensitive to Cd2+ but not to other classical Ca2+ antagonists, may play a key role in the transduction mechanism activated by Glu or depolarizing agents.

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: I. External Na+ requirement

The characteristics of the transduction mechanism(s) activated by glutamate (Glu) via the quisqualate metabotropic receptor, as well as by depolarizing agents, to trigger formation of inositol phosphates (IPs) were investigated in 8-day-old rat forebrain synaptoneurosomes. The replacement of external Na+ by various compounds (Li+, Tris+, N-methyl-D-glucamine+, and sucrose) induces an increase in basal accumulation of IPs and depolarizes synaptoneurosome membranes. Under these conditions, Glu- and K(+)-induced accumulations of IPs are inhibited, whereas the carbachol (Carb)-elicited response of IPs parallels the basal one. Agents increasing Na+ influx, such as veratridine and monensin, depolarize synaptoneurosomes and stimulate formation of IPs. These stimulations are not additive with responses of IPs elicited by Glu or K+. These data suggest that (a) Glu activates phosphoinositide metabolism via a specific mechanism (distinct from that of cholinergic agonists), (b) depolarizing agents and Glu share at least one common intermediate step in their mechanisms of activation of the metabolism of IPs, and (c) the depolarization may correspond to this common step. In addition, Na+ seems to be required for Glu stimulation of metabolism of IPs. The depolarization associated with the action of Glu on formation of IPs results neither from an influx via tetrodotoxin-sensitive voltage-dependent Na+ channels nor from an entry via the classically characterized Na+/Ca2+ or Na+/H+ exchangers. In fact, tetrodotoxin (2 microM) has no effect on the Glu- or K(+)-elicited response of IPs. Amiloride (greater than 50 microM) and some of its derivatives similarly inhibit not only Glu- and K(+)- but also Carb-evoked formation of IPs.

Do polyamines regulate the NMDA inhibition of muscarinic receptor-induced phosphoinositide turnover in guinea pig brain?

The N-methyl-D-aspartate (NMDA) receptor-induced inhibition of muscarinic receptor-stimulated phosphoinositide turnover in guinea pig cerebral cortical slices was investigated to determine whether polyamines regulate the function of NMDA in this system. None of the polyamines tested produced significant stimulation of phosphoinositide turnover. Neither spermine nor the substituted polyamine philanthotoxin-343 altered the inhibitory effect of NMDA. The simpler substituted polyamines N-(4-hydroxyphenylpropanoyl)-spermine and N-(4-hydroxyphenylacetyl)-spermine, however, antaganised the NMDA receptor inhibition of the muscarinic response with low affinity (Ki greater than or equal to 100 microM). Diethylenetriamine, a purported polyamine antagonist of the NMDA receptor, was without effect on the NMDA inhibitory response, as was ifenprodil, a structurally unrelated compound with reported antagonist capacity. In summary, therefore, we fail to observe polyamine regulation of the NMDA receptor mediating an inhibition of the muscarinic phosphoinositide turnover response in guinea pig cerebral cortical slices.

Excitatory Actions of the Metabotropic Excitatory Amino Acid Receptor Agonist, trans-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate (t-ACPD), on Rat Thalamic Neurons In Vivo

The metabotropic excitatory amino acid receptor agonist trans-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate (t-ACPD) was applied to rat ventrobasal thalamic neurons by iontophoresis. This agonist typically evoked an excitatory response which was slower in onset and of longer duration than responses to the other excitatory amino acid agonists, N-methyl-aspartate, kainate or (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. Responses to t-ACPD were resistant to the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and kynurenate. These results suggest that t-ACPD may exert its effects via the so-called 'metabotropic' excitatory amino acid receptor. The putative antagonists at this receptor, d-2-amino-4-phosphono-butyrate (d-AP4), l-2-amino-4-phosphono-butyrate (l-AP4) and l-2-amino-3-phosphono-propionate (l-AP3), were able to reduce responses to t-ACPD under certain circumstances. However, such antagonism was always accompanied by similar reductions in excitatory responses to other agonists. These non-selective effects would appear to limit the usefulness of AP4 and AP3 as antagonists of t-ACPD.

Stimulatory and inhibitory effects of N-methyl-D-aspartate on 3H-inositol polyphosphate accumulation in rat cortical slices

The actions of the excitatory amino acid N-methyl-D-aspartate (NMDA) on the accumulation of 3H-inositol polyphosphate isomers in rat cerebral cortex slices have been examined over short (less than 5 min) incubation periods. NMDA caused the dose-dependent accumulation of only [3H]inositol monophosphate and [3H]inositol bisphosphate (maximal effect between 0.3 and 1 mM), with no increase in [3H]inositol trisphosphate ([3H]InsP3) and [3H]inositol tetrakisphosphate ([3H]InsP4). HPLC analysis confirmed this, showing no increases in the breakdown products of [3H]Ins(1,3,4,5)P4. When present with the muscarinic agonist carbachol (1 mM), high concentrations of NMDA (1 mM) could almost totally inhibit carbachol-induced accumulation of 3H-inositol polyphosphates. In contrast, at lower concentrations of NMDA (10 microM), the inhibitory effect was replaced with a synergistic accumulation of inositol polyphosphates, especially [3H]InsP4 and [3H]InsP3. The inhibitory effects of NMDA were only apparent when extracellular Ca2+ was present, although incubation in media with no added Ca2+ resulted in somewhat reduced stimulatory responses to NMDA alone, but suppressed totally the inhibitory effects of 1 mM NMDA and reduced the synergistic effects of 10 microM NMDA on carbachol responses. These studies, therefore, reveal Ca(2+)-dependent effects of NMDA indicative of indirect mechanisms of action and show that care must be made in interpreting the effects of NMDA on phosphoinositide metabolism unless the inositol polyphosphate composition has been fully characterised.

Inhibitory effect of NMDA receptor activation on quisqualate-stimulated phosphatidylinositol turnover in the human cerebral cortex

The effect of excitatory amino acids (EAA) on the phosphatidylinositol (PI) turnover in human cerebral cortical slices was investigated. Quisqualic acid (QA) and, to lesser extent, ibotenic acid (IBO) at 10(-5)-10(-3) M increased inositol phosphate (IP) accumulation. L-Glutamic acid (L-glu), kainic acid (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartic acid (NMDA) were ineffective. NMDA dose-dependently antagonized the QA facilitatory effect. Such inhibition was prevented by the NMDA receptor complex antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine (MK-801) and by 3[+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid. The effect of IBO (but not that of QA) was greatly potentiated by MK-801. These data suggest that the EAA metabotropic receptor described in the rodent brain is also present in human cerebral cortex and is negatively modulated by the NMDA receptor.

The inhibition of agonist- or depolarisation-evoked formation of inositol phosphate by excitatory amino acids in rat cerebral cortex is due to the neurotoxic action of this class of neurotransmitter and is mediated by sodium influx

Previous work has shown that excitatory amino acids inhibit agonist or depolarisation evoked formation of inositol phosphate in brain. In this paper, possible mechanisms by which this may be occurring have been investigated. The inhibition of carbachol-stimulated formation of inositol phosphate by kainic acid (KA) was abolished if the tissue was incubated in a sodium-free medium. The sodium channel activator, veratridine (10 microM) and the sodium ionophore, monensin (3 microM), also inhibited the response of inositol phosphate to carbachol; tetrodotoxin (300 nM) reversed the effect of veratridine but not monensin or KA. Incubation with cadmium (0.3 mM) or removal of extracellular calcium did not alter the effects of KA, monensin or veratridine. The effects of KA were significantly reduced with the Na+/K(+)-ATPase inhibitor, ouabain (10-100 microM). Inhibition by KA was still observed in tissue that had been prestimulated with KA and then washed to remove the agonist. Incorporation of [3H]inositol into inositol lipids was significantly reduced by KA, in the absence or presence of carbachol. It is suggested that the inhibition of the turnover of stimulated phosphoinositide, by excitatory amino acids, is related to the neurotoxic actions of these transmitters and is mediated by Na+ influx, with a consequent activation of Na+/K(+)-ATPase, depletion of cellular ATP and reduction in synthesis of inositol lipid.

Modulation of carbachol-stimulated inositol phospholipid breakdown in rat cerebral cortical miniprisms by excitatory amino acids and by BAY K-8644 is dependent upon the assay calcium and potassium concentrations used

The calcium and potassium ion dependency of the inositol phospholipid breakdown response to stimulatory agents has been investigated in rat cerebral cortical miniprisms. The calcium channel agonist BAY K-8644 (10 microM) potentiated the response to carbachol at 6 mM K+ when Ca2(+)-free, but not when 2.52 mM Ca2+ assay buffer was used. In Ca2(+)-free buffer, verapamil (10 microM) inhibited the response to carbachol at both 6 and 18 mM K+ but higher concentrations (30-300 microM) were needed when 2.52 mM Ca2+ was used. At these higher concentrations, however, verapamil inhibited the binding of 2 nM [3H]pirenzepine to muscarinic recognition sites. N-Methyl-D-Aspartate (NMDA, 100 microM) significantly reduced the basal phosphoinositide breakdown rate at 18 mM K+ at 1.3 mM Ca2+, but was without effect on the basal rate at other K+ and Ca2+ concentrations. In the presence of NMDA (100 microM) or quisqualate (100 microM), the responses to carbachol were reduced, the degree of reduction showing a complex dependency upon the assay K+ and Ca2+ concentrations used. These results indicate that the inositol phospholipid breakdown response to carbachol in cerebral cortical miniprisms can be modulated in a manner dependent upon the extracellular calcium and potassium concentrations used.

The glycine antagonist 7-chlorokynurenic acid blocks the effects of N-methyl-D-aspartate on agonist-stimulated phosphoinositide hydrolysis in guinea-pig brain slices

Despite having no effect on basal phosphoinositide hydrolysis. N-methyl-D-aspartate (NMDA) inhibited carbachol-stimulated accumulation of 3H-inositol phosphates and enhanced that due to noradrenaline in guinea-pig cerebral cortex slices. The glycine antagonist 7-chlorokynurenic acid inhibited the effects of NMDA and this inhibition was reversed by glycine. The action of 7-chlorokynurenic acid was not mimicked by strychnine or HA 966 (1-hydroxy-3-aminopyrrolid-2-one). L-Glutamate also inhibited carbachol-stimulated accumulation of 3H-inositol phosphates, but this inhibition was not blocked by 7-chlorokynurenic acid. The data are consistent with glycine maintaining tonic control over NMDA receptor activity in guinea-pig brain.

Maitotoxin induces phosphoinositide turnover and modulates glutamatergic and muscarinic cholinergic receptor function in cultured cerebellar neurons

Maitotoxin (MTX) stimulated inositol phosphate (IP) formation in primary cultures of rat cerebellar granule cells. MTX-induced IP production was dependent on extracellular Ca2+ but independent of extracellular Na+. The stimulation of IP formation elicited by MTX was unaffected by pretreatment of cells with phorbol dibutyrate, pertussis toxin, and a variety of Ca2+ entry blockers, such as nimodipine, nisoldipine, Co2+, and Mn2+. The presence of MTX markedly attenuated IP production induced by carbachol and glutamate, with no apparent effect on the responses to norepinephrine (NE), histamine, 5-hydroxytryptamine (5-HT), and endothelin-1. The inhibition of the carbachol- and glutamate-induced responses by MTX was dose dependent with IC50 values of 1.2 and 0.5 ng/ml, respectively. Pretreatment of cells with a lower concentration of MTX (0.3 ng/ml) also attenuated carbachol- and glutamate-induced IP formation, in a time-dependent manner, with a decrease observed after 30 min prestimulation, but failed to affect NE-, histamine-, 5-HT-, endothelin-1, and sarafotoxin S6b-induced responses. Thus, MTX elicited a marked Ca2(+)-dependent phosphoinositide (PI) turnover in cerebellar granule cells and selectively inhibited carbachol- and glutamate-induced PI hydrolysis. Possible mechanisms underlying these selective modulations are discussed.

Generation of inositol phosphates, cytosolic Ca2+, and secretion of noradrenaline in PC12 cells treated with glutamate

Glutamate transiently stimulated rat pheochromocytoma PC12 cells and caused an inositol trisphosphate formation and an increase in levels of Ca+ in the cytosol. The rank order of potency of glutamate greater than N-methyl-D-aspartate (NMDA) much greater than kainate = quisqualate is characteristic of an interaction with NMDA receptors. The effect of glutamate on inositol trisphosphate formation disappeared in a low Mg2+ buffer and was not blocked by DL-2-amino-5-phosphonovalerate, an antagonist for NMDA receptors coupled to ion channels. Although glutamate failed to stimulate noradrenaline secretion, glutamate enhanced the effect of bradykinin, but not of Ca ionophore A23187, or KCl. These results suggest the existence of metabotropic glutamate receptors, different from previously reported receptors, in PC12 cells.

Lithium amplifies inhibitions of inositol phospholipid hydrolysis in mammalian brain slices

1. We have examined the effects of lithium chloride (LiCl) on inhibitions of inositol phospholipid hydrolysis in guinea-pig and rat brain slices by assessing the accumulation of [3H]-inositol phosphates ([3H]-InsP), in vitro. 2. In guinea-pig and rat cerebral cortex slices the accumulation of total [3H]-inositol phosphates due to the cholinoceptor agonist carbachol was inhibited by the excitatory amino acid L-glutamate, but only when LiCl was present. 3. The effects of LiCl were time and concentration-dependent. Significant inhibitions of the carbachol response by glutamate (in the presence of LiCl) being evident only after 20-30 min of stimulation at LiCl concentrations above 1.2 mM. 4. N-methyl-D-aspartate (NMDA), in the absence of LiCl, enhanced the response to carbachol at low concentrations of the amino acid and inhibited the response at higher concentrations. In the presence of 5 mM LiCl, only the inhibitory phase was observed. 5. In rat cerebral cortex slices, aluminium fluoride inhibited [3H]-InsP accumulation in the presence of carbachol, noradrenaline and a depolarising concentration of KCl and these inhibitions were more marked when LiCl was present. The response to histamine was unaffected. 6. The data presented provide evidence that LiCl amplifies inhibitions of inositol phospholipid hydrolysis due to receptor and non-receptor mediated stimuli, although the mechanism underlying the effect is, as yet, obscure.

gamma-Aminobutyric acid inhibition of histamine-induced inositol phosphate formation in guinea-pig cerebellum: comparison with guinea-pig and rat cerebral cortex

1. gamma-Aminobutyric acid (GABA), 2 mM, inhibited basal accumulation of [3H]-inositol monophosphate ([3H]-IP1) in lithium-treated slices of guinea-pig cerebellum preincubated with [3H]-inositol. In contrast, 2 mM GABA stimulated the accumulation of [3H]-IP1 in rat cerebral cortical slices over a 60 min incubation period, but had no significant effect in slices of guinea-pig cerebral cortex. The estimated IC50 for the inhibitory action of GABA in guinea-pig cerebellar slices was 0.52 +/- 0.12 mM. 2. GABA inhibited histamine-induced [3H]-IP1 accumulation in guinea-pig cerebellar slices in a non-competitive manner. The best-fit value for the maximum level of inhibition was 74 +/- 6%. The estimated IC50 for GABA was 0.77 +/- 0.15 mM and was not significantly different from the IC50 for inhibition of the basal accumulation of [3H]-IP1. The response to histamine in guinea-pig and rat cerebral cortical slices was also inhibited by 2 mM GABA. 3. In guinea-pig cerebellar slices 2 mM GABA potentiated histamine-induced [3H]-inositol bisphosphate ([3H]-IP2) accumulation, whereas in both guinea-pig and rat cerebral cortex the effect was inhibition. 4. Isoguvacine and muscimol, GABAA-selective agonists, and (-)-baclofen, GABA(B)-selective, had no significant effect on basal or histamine-stimulated accumulation of [3H]-IPs in guinea-pig cerebellar slices. (-)-Baclofen had only a weak inhibitory effect on [3H]-IP1 accumulation in guinea-pig-cerebral cortex (16 +/- 6% inhibition with 10 microM (-)-baclofen), whereas in rat cerebral cortex (-)-baclofen mimicked the inhibitory effect of GABA. 5. Nipecotic acid (1 mM) had qualitatively similar effects to those of 2mm GABA in guinea-pig cerebellar slices. 6. The competitive GABA uptake inhibitors SK&F 89976-A, SK&F 100330-A and SK&F 100561-A were potent histamine H,-receptor antagonists, as indicated by the inhibition of [3H]-mepyramine binding to homogenates of guinea-pig cerebellum and cerebral cortex. 7. GABA (2 mM) caused a small inhibition (12 + 3%) of [3H]-inositol incorporation into total inositol phospholipids in guinea-pig cerebellar slices, as in rat cerebral cortical slices, whereas 0.2mm histamine caused a small stimulation (15 + 4%). In the presence of both GABA and histamine, [3H]-inositol incorporation was unchanged from basal (101 + 5%). 8. GABA also inhibited [3H]-IP1 formation induced by endothelin-1 in guinea-pig cerebellar slices and increased, but not significantly, the amount of [3H]-IP2 accumulated. This, taken with the inhibitory effect on basal and histamine-stimulated accumulation, suggests that the action of GABA in guinea-pig cerebellar slices may be non-selective and may not be exerted through a specific GABA receptor.

Discriminatory effects of forskolin and EGTA on the indirect cyclic AMP responses to histamine, noradrenaline, 5-hydroxytryptamine, and glutamate in guinea-pig cerebral cortical slices

The effects of forskolin (1 microM) and EGTA (5 mM) on indirect cyclic AMP responses in slices of guinea-pig cerebral cortex were examined. Forskolin had little effect on the direct 2-chloroadenosine-stimulated cyclic AMP response. However, it completely abolished the glutamate-induced augmentation of this response. In contrast, forskolin had very little effect on the indirect cyclic AMP responses to noradrenaline, 5-hydroxytryptamine, and histamine. Conversely, rapid removal of extracellular calcium with EGTA 2 min before addition of the indirectly acting agent markedly reduced the augmentation responses produced by these latter agonists, but had little effect on the glutamate augmentation. When EGTA was added once a steady level of cyclic AMP had been achieved with the indirect agents, it was without effect on any of the responses. Thus, calcium appears to have a role in the early, but not the later, stages of the noradrenaline, 5-hydroxytryptamine, and histamine responses. A role for protein kinase C in the glutamate augmentation response was suggested, because forskolin inhibited the augmentation of the 2-chloroadenosine response produced by phorbol esters (which mimic the actions of diacylglycerol in activating protein kinase C). We conclude that there is more than one mechanism by which the augmentation of cyclic AMP responses can occur.

NMDA and quisqualate reduce a Ca-dependent K+ current by a protein kinase-mediated mechanism

Stimulation of N-methyl-D-aspartate (NMDA) or quisqualate (Quis) receptors by submicromolar concentrations of NMDA or Quis but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) reduced post-spike train after hyperpolarizations (AHPs) and blocked the underlying Iahp in dentate granule (DG) neurones in vitro. The NMDA but not Quis action was blocked by the NMDA receptor blocker 2-D,L-aminophosphonovaleric acid (APV). Actions of both NMDA and Quis were abolished by isoquinolinesulphonyl-2-methyl-piperazine dihydrochloride (H-7), an inhibitor of several protein kinases. These data suggest that there is a link between excitatory amino acid receptor activation, the protein kinase system, and neuronal excitability.

Stimulatory and inhibitory actions of excitatory amino acids on inositol phospholipid metabolism in rabbit retina. Evidence for a specific quisqualate receptor subtype associated with neurones

The effects of excitatory amino acid agonists on [3H]inositol phosphates (InsPs) levels have been examined in rabbit retinal tissues under basal conditions and after agonist stimulation. Quisqualate (QA) is the most effective excitatory amino acid agonist at stimulating InsPs accumulation with an EC50 value of 0.1 microM. The responses for maximally effective concentrations of QA with either ibotenate or kainate were not additive, which suggested that all the excitatory amino acid agonists which stimulate InsPs accumulation (quisqualate, kainate, NMDA, glutamate, ibotenate, aspartate) have a common site of action. None of the following antagonists: DL-2-amino-5-phosphonovalerate (APV), DL-2-amino-4-phosphonobutyrate (APB) and glutamate dimethyl ester (GDEE), prazosin, ketanserin or atropine influenced the excitatory amino agonist stimulation of InsPs. These data suggest the presence of a specific QA-receptor subtype in the retina. QA, and to a lesser extent other excitatory amino acid agonists, were also effective in stimulating InsPs accumulation and the mobilization of internal calcium levels in 3-5-day-old retinal cultures but not in the older cultures (25-30 days old), which lack neurones but contain Müller cells. The QA receptor subtypes linked to InsPs accumulation in the retina are therefore present on neurones. Kainate and NMDA had a weak inhibitory action on the effect of the carbachol-induced stimulation of InsPs at 50 microM. The NMDA action was abolished by APV, whereas this antagonist had no effect on the action of kainate.(ABSTRACT TRUNCATED AT 250 WORDS)