Inhibition of muscarinic-coupled phosphoinositide hydrolysis by N-methyl-D-aspartate is dependent on depolarization via channel activation

Expression of functional metabotropic glutamate receptors in primary cultured rat osteoblasts. Cross-talk with N-methyl-D-aspartate receptors

Osteoblasts and osteoclasts express functional N-methyl-d-aspartate (NMDA) receptors, which participate in regulation of bone matrix. In rat femoral osteoblasts held in whole cell clamp there is a robust NMDA current but little if any response to l-glutamate. We have investigated expression of metabotropic glutamate receptors (mGluRs) in these cells. By reverse transcription polymerase chain reaction, we have detected expression of mGluR1b (but not mGluR1a, 2, 3, 4, 5, or 6). Blockade of mGluRs with (+/-)-alpha-methyl-carboxyphenyl-glycine resulted in an enlarged l-glutamate-induced current that resembled the response to NMDA. Conversely, prior stimulation of mGluRs with trans-(+/-)-1-amino-1, 3-cyclopentanedicarboxylic acid (1S,3R-ACPD; mGluR agonist) reduced the NMDA-induced current by 77%. Monitoring of [Ca(2+)](i) showed that NMDA induced a sustained elevation of [Ca(2+)](i), which was dependent upon [Ca(2+)](o). Treatment with 1S,3R-ACPD generated an initial transient that was independent of [Ca(2+)](o), followed by a sustained, [Ca(2+)](o)-dependent phase, a response consistent with phospholipase C-mediated mobilization of stored Ca(2+). Investigations of the interaction between the two receptors confirmed inhibitory modulation of the NMDA receptor-induced rise in [Ca(2+)](i) by mGluRs. Parathyroid hormone, which also activates phospholipase C in osteoblasts, had a similar inhibitory effect on the NMDA receptor-induced [Ca(2+)](i) response. Elevation of [Ca(2+)](i) mediated by mGluR activation was reduced by subsequent stimulation of NMDA receptors. This is the first description of mGluRs in bone and shows that complex glutamatergic signaling can occur in this tissue.

Contrasting calcium dependencies of SAPK and ERK activations by glutamate in cultured striatal neurons

Stress-activated protein kinase (SAPK) and extracellular signal-regulated kinase (ERK), both members of the mitogen-activated protein kinase (MAPK) family, may in some circumstances serve opposing functions with respect to cell survival. However, SAPK and ERK can also be coordinately activated in neurons in response to glutamate stimulation of NMDA receptors. To explore the mechanisms of these MAPK activations, we compared the ionic mechanisms mediating SAPK and ERK activations by glutamate. In primary cultures of striatal neurons, glutamatergic activation of ERK and one of its transcription factor targets, CREB, showed a calcium dependence typical of NMDA receptor-mediated responses. In contrast, extracellular calcium was not required for glutamatergic, NMDA receptor-mediated activation of SAPK and phosphorylation of its substrate, c-Jun. Increasing extracellular calcium enhanced ERK activation but reversed SAPK activation, further distinguishing the calcium dependencies of these two NMDA receptor-mediated effects. Finally, reducing extracellular sodium prevented the glutamatergic activation of SAPK but only partially blocked that of ERK. These contrasting ionic dependencies suggest a mechanism by which NMDA receptor activation may, under distinct conditions, differentially regulate neuronal MAPKs and their divergent functions.

Measurement of NMDA receptor protein subunits in discrete hippocampal regions of kindled animals

Kindling refers to a phenomenon in which repeated application of initially subconvulsive electrical stimulations produces limbic and clonic motor seizures of progressively increasing severity. Once established, the increased excitability is lifelong. A diversity of studies demonstrate that kindling results in long lasting (28 days) alterations of the functional and pharmacologic properties of NMDA receptors, indicating that kindling may cause changes intrinsic to the NMDA receptor itself. Our previous studies disclosed no differences in NMDA receptor subunit gene or splice isoform mRNA expression between control and kindled animals 28 days after the last kindled seizure. Here, we extend those earlier studies by measuring levels of subunit protein for NMDAR1, NR2A, and NR2B in the hippocampus of control and kindled animals, 28 days after the last kindled seizure. We report that kindling does not effect long-lasting changes in the levels of NMDA receptor subunit protein. Together these findings support the idea that alterations in NMDA receptor protein expression do not contribute to the novel properties of NMDA receptors induced by kindling.

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.

Regulation of alternative splicing of NMDAR1 in the kindling model

Kindling refers to a phenomenon in which repeated application of initially subconvulsive electrical stimulations produces limbic and clonic motor seizures of progressively increasing severity. Once established, the increased excitability is lifelong. Several lines of investigation suggest that the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor participates in the expression of the increased neuronal excitability of the kindled brain. Many studies demonstrate that kindling results in altered NMDA receptor functional and pharmacological properties, indicating that kindling may cause changes intrinsic to the NMDA receptor itself. It is possible that altered expression of NMDA receptor subunit genes and splice isoforms of genes leads to subunit combinations resulting in the novel NMDA receptor properties identified in the hippocampus of kindled animals. To begin to address this possibility, we previously examined the hippocampal expression of known NMDA receptor genes and found no differences in expression between control and kindled animals either 24 h or 28 days after the last kindled seizure. Here, we extend that earlier study by examining the expression of NMDAR1 splice isoforms in the hippocampus of control and kindled animals. We report that kindling induces the transient reduction of specific splice isoforms of NMDAR1 containing the first of the carboxy-terminal splice cassettes (exon 21). We discuss the potential significance of this regulation in terms of its relevance to previous findings in the kindling model and possible effects on NMDA receptor function.

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.

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.

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.

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 N-methyl-D-aspartate on quisqualate-stimulated phosphoinositide hydrolysis in slices of kitten striate cortex

Stimulation of phosphoinositide (PI) hydrolysis by excitatory amino acids (EAAs) was studied in coronal slices of kitten visual cortex. Coincubation with N-methyl-D-aspartate (NMDA) markedly reduced the stimulation by quisqualate, however, this inhibition developed with a latency of > 10 min and occurred even when the NMDA exposure preceded, but did not overlap with, incubation in quisqualate. This time-course of NMDA inhibition of EAA-stimulated PI turnover places new constraints on its possible mechanism of inhibition.

Amygdala kindling-induced seizures selectively impair spatial memory. 2. Effects on hippocampal neuronal and glial muscarinic acetylcholine receptor

The muscarinic acetylcholine receptor is linked via hydrolysis of phosphoinositides to the protein kinase C pathway. In a preceding paper (Beldhuis, H. J. A., H. G. J. Everts, E. A. Vander Zee, P. G. M. Luiten, and B. Bohus (1992) Amygdala kindling-induced seizures selectively impair spatial memory. 1. Behavioral characteristics and effects on hippocampal neuronal protein kinase C isoforms. Hippocampus 2:397-410), the role of different isoforms of protein kinase C in neurobiological processes associated with plasticity was studied using both a spatial learning paradigm and amygdala kindling in the rat. This study extended the findings on protein kinase C activity to the level of the muscarinic acetylcholine receptor. Rats were trained in a spatial learning paradigm and kindled simultaneously in the amygdala to develop generalized motor convulsions. Control rats were trained only in the spatial learning paradigm to acquire stable working and reference memory performance. Alteration in the expression of the muscarinic acetylcholine receptor was investigated using a monoclonal antibody to muscarinic acetylcholine receptor proteins. Trained control rats that were exposed repeatedly to the spatial learning paradigm showed an increase in immunoreactivity for the muscarinic acetylcholine receptor located in the same hippocampal regions in which the protein kinase C activity was increased. In fully kindled rats, however, this increase located in principal neurons was absent, whereas expression of muscarinic acetylcholine receptor proteins was increased in hippocampal astrocytes. Moreover, fully kindled rats showed an impairment in reference memory performance as compared to trained control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.

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+.

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)

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.

Galanin reduces carbachol stimulation of phosphoinositide turnover in rat ventral hippocampus by lowering Ca2+ influx through voltage-sensitive Ca2+ channels

The 29-amino-acid peptide galanin (GAL) caused concentration-dependent inhibition of the accumulation of 3H-inositol phosphates (3H-InsPs) induced by the muscarinic agonist carbachol (CARB; 10(-3)-10(-5) M) in the presence of 5 mM lithium, specifically in tissue miniprisms from rat ventral hippocampus. The inhibitory effect of GAL involved the mono-, bis-, tris-, and tetrakisphosphates formed during activation for 2 min of phospholipase C by CARB (1 mM) in the absence of lithium. GAL (1 microM) did not affect alpha-adrenergic or serotonergic type 2 receptor-mediated phosphoinositide (PI) breakdown in the same tissue. GAL by itself neither acted on basal levels of 3H-InsPs nor affected muscarinic receptors in binding studies. Blockade of the T-, N-, and L-types of voltage-sensitive calcium channel (VSCC) with 200 microM Cd2+ reduced muscarinic receptor-mediated PI breakdown by 50% and abolished the inhibitory effect of GAL (1 microM). Reduction of the extracellular Ca2+ concentration from 1.3 mM to 0.49 microM abolished the GAL inhibition of CARB-stimulated PI hydrolysis. Ca2+ influx promoted by 18 mM K+ depolarization or by 1 microM Bay K 8644, a selective agonist of the L-type VSCC, prevented the inhibitory effect of GAL. Blockade of the L-type VSCC with nifedipine (1 microM) potentiated the inhibitory effects of GAL without affecting muscarinic stimulation of PI breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

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.

Reduction of Na+ enhances phosphoinositide hydrolysis and differentiates the stimulatory and inhibitory responses to quisqualate in rat brain slices

The concentration of Na+ in the incubation medium significantly influenced phosphoinositide hydrolysis induced by some, but not all, agonists in rat cerebral cortical slices. Reductions of the Na+ concentration below 120 mM resulted in incremental increases in basal and norepinephrine-stimulated accumulation of [3H]inositol monophosphate in cortical slices that had been prelabelled with [3H]inositol, and maximal responses were obtained with 0 and 5 mM Na+. In contrast, the responses to carbachol and ibotenate were similar in medium containing 120 or 5 mM Na+. In medium with 120 mM Na+, quisqualate has two effects on phosphoinositide hydrolysis in cortical slices, including a relatively weak stimulatory effect and an inhibitory modulation of the stimulation induced by norepinephrine. These two responses to quisqualate were differentially modulated by Na+; in 5 mM compared with 120 mM Na+ the stimulatory response was greatly increased and the inhibitory effect was mostly eliminated. That these were two separate events was confirmed by the use of L-BOAA (beta-N-oxalyl-L-alpha, beta-diaminopropionic acid), which reproduces the inhibitory, but not the stimulatory effect of quisqualate on phosphoinositide hydrolysis. In 5 mM Na+, inhibition by L-BOAA of norepinephrine-stimulated phosphoinositide hydrolysis was completely eliminated. These results demonstrate that a physiological concentration of Na+ maintains phosphoinositide hydrolysis at a submaximal level of sensitivity to some, but not all, agonists. The differential effects of Na+ on the stimulatory and inhibitory effects of quisqualate further substantiate the suggestion that these are two separate processes and indicate that alterations of the Na+ concentration may influence the effects of quisqualate, and other agonists, on phosphoinositide hydrolysis.

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.