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

Altered development of glutamatergic synapses in layer V pyramidal neurons in NR3A knockout mice

Expression of the NMDA receptor (NMDAR) subunit NR3A reaches its highest level in layer V of the developing rodent cortex during the second postnatal week, a peak period of synaptogenesis. Incorporation of NR3A leads to the formation of non-canonical, Mg2+-insensitive NMDARs, but it is not known whether they participate in synaptic transmission and maturation. Here we show that in the second postnatal week, layer V pyramidal neurons in the somatosensory cortex of wild type (WT) mice exhibited evoked excitatory postsynaptic currents (eEPSCs) with 3- to 6-fold lower Mg2+ sensitivity than NR3A knockout (KO) mice and their reversal potential was approximately 2 mV more negative compared to KO mice consistent with decreased P(Ca) of NMDARs. Surprisingly, ablation of NR3A also led to a 20-fold reduction of the ratio of AMPAR- to NMDAR-mediated eEPSC amplitudes in KO mice. Insertion of AMPARs at the synapses of layer V pyramidal neurons appears to be facilitated by the expression of Mg2+-insensitive NMDARs. The data indicate that NR3A plays a significant role in the development of excitatory synapses in layer V of the developing neocortex.

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.

N-methyl-D-aspartate receptors modulate neurotransmitter release and peristalsis in the guinea pig isolated colon

To assess the role of NMDA receptors in modulating neurotransmitter release in the myenteric plexus, we studied the effects of L-glutamic acid and NMDA on endogenous acetylcholine and noradrenaline overflow (assayed by HPLC) from the guinea pig isolated distal colon. L-Glutamic acid and NMDA enhanced electrically evoked acetylcholine and noradrenaline overflow and these effects were reversed by selective NMDA receptor antagonists. The possible functional significance of these findings was studied by measuring the efficiency of the colonic peristaltic reflex in the presence of NMDA receptor agonists. NMDA inhibited propulsion velocity at all concentrations tested, this effect being antagonized by (+/-)-2-amino-5-phosphonopentanoic acid and virtually abolished in sympathetically denervated animals. In conclusion, the inhibitory effect of NMDA on peristalsis, being almost entirely dependent on the integrity of sympathetic pathways, could be, at least in part, due to NMDA-induced noradrenaline release.

Presynaptic control of dopamine synthesis and release by excitatory amino acids in rat striatal synaptosomes

Purified striatal synaptosomes were continuously superfused with L,3,5[3H]tyrosine in order to estimate the synthesis ([3H]water) and release of newly formed [3H]dopamine. In the presence of magnesium, L-glutamate, D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate, but not N-methyl-D-aspartate (NMDA) and 1-aminocyclopentane-1S,3R-dicarboxylate (t-ACPD), stimulated the release of [3H]dopamine, in a dose-dependent manner. When magnesium was omitted or in the presence of AMPA, NMDA also increased the release of [3H]dopamine. The effects of AMPA and kainate were competitively inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitro-quinoxaline-2,3-dione (DNQX), whereas those of NMDA were reduced by 2-amino-5-phosphonovalerate (APV) or (+)-5-methyl-10,11-dihydro-5-H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK801). The stimulation of [3H]dopamine release by a high concentration of glutamate resulted from the concomitant activation of AMPA and NMDA receptors since this effect was potentiated by glycine and reduced by 2-amino-5-phosphonovalerate or MK801. This reduction was almost complete in the combined presence of DNQX and MK801. Surprisingly, glutamate and NMDA (in the absence of magnesium) reduced the efflux of [3H]water. The reduction of [3H]dopamine synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Neither AMPA nor kainate affected dopamine synthesis. The inhibition of [3H]dopamine synthesis resulting from the stimulation of NMDA receptors was prevented when synaptosomes were continuously superfused with adenosine deaminase and quinpirole, a combined treatment known to markedly reduce the phosphorylation of tyrosine hydroxylase by cAMP-dependent protein kinase. The opposite effects of a high concentration of glutamate on [3H]dopamine synthesis and release were mimicked by ionomycin. As a working hypothesis, it is proposed that the NMDA-triggered calcium influx could lead to a reduction of tyrosine hydroxylase phosphorylation, possibly through an activation of calcineurin.

Regulation of [3H]norepinephrine release by N-methyl-D-aspartate receptors in minislices from the dentate gyrus and the CA1-CA3 area of the rat hippocampus

It has been reported previously that N-methyl-D-aspartic acid induces a significant release of [3H]norepinephrine preaccumulated in slices from the hippocampus. In the present study, we investigated whether there are regional differences in the hippocampus regarding this N-methyl-D-aspartate effect. In the absence of Mg2+, N-methyl-D-aspartate (10-200 microM) induced the release of [3H]norepinephrine from superfused minislices containing the dentate gyrus area or the CA1-CA3 region of the hippocampus. Such N-methyl-D-aspartate effects on [3H]norepinephrine release were significantly higher in the dentate gyrus than in the CA1-CA3 area. The N-methyl-D-aspartate effects in both hippocampal areas were also reduced significantly by D-2-amino-5-phosphonovaleric acid (50 microM), an antagonist of the N-methyl-D-aspartate receptor, and by tetrodotoxin, a blocker of the voltage-dependent Na+ channels. The extent of this reduction was the same in the dentate gyrus and the CA1-CA3 area. Further experiments, conducted in the presence of Mg2+, demonstrated that N-methyl-D-aspartic acid increased K(+)-induced release of [3H]norepinephrine from dentate gyrus minislices but not from the CA1-CA3 area. The results are consistent with the existence of a higher density and/or different subtypes of N-methyl-D-aspartate receptors modulating [3H]norepinephrine release in the dentate gyrus as compared with the CA1-CA3 hippocampal area.

Molecular distinction of three N-methyl-D-aspartate-receptor subtypes in situ and developmental receptor maturation demonstrated with the photoaffinity ligand 125I-labeled CGP 55802A

Activation of N-methyl-D-aspartate (NMDA) receptors is essential for synaptic plasticity in the central nervous system and contributes to neuronal death under various pathological conditions. Although several subunits have been cloned, the structure of NMDA receptors in situ is unresolved. By using a photoreactive antagonist with nanomolar affinity to the NMDA-binding site, three types of receptors were differentiated by their pattern of photoaffinity-labeled subunits. In adult brain, a protein of 175-kDa was photoreactive that displayed a profile of ligand binding and autoradiographical distribution corresponding to NMDA receptors. In contrast, in early postnatal brain, proteins of both 175 kDa and 115 kDa were photolabeled. This labeling pattern is switched to that of adult brain around postnatal day 10, pointing to a structural maturation of NMDA receptors. A third type of receptor could be identified in cerebellar granule cell cultures, where NMDA receptors mediate trophic effects and photolabeling was exclusively targeted to a 115-kDa protein. To identify the proteins labeled in situ, recombinant receptors were subjected to photolabeling. When the NR1 subunit was coexpressed with either the NR2A, NR2B, or NR2C subunit, only the combination of NR1/NR2A was photoreactive. Both the NR1 and NR2A subunits were photolabeled, corresponding in size to the proteins labeled in situ. However, the lack of subunit-selectivity in photolabeling the NR1/NR2A combination suggests the presence of additional receptor components in situ to explain the subunit-selective photoreactivity in adult brain (175 kDa) and in cerebellar granule cells (115 kDa). The subunit combination NR1/NR2A by itself appears insufficient to describe a major population of NMDA receptors, in particular, in adult brain.

Novel ion specificity of a carboxylate cluster Mg(II) binding site: strong charge selectivity and weak size selectivity

Carboxylate cluster Mg(II) binding sites consist of a cluster of side-chain carboxylates, typically 3-4 in number, partially buried in a shallow cleft on the surface of a Mg(II) binding protein. Such clusters are often found in the active sites of enzymes catalyzing phosphochemistry. An example is the phospho-signaling protein CheY of the Escherichia coli chemotaxis pathway, which binds Mg(II) via a cluster of three carboxylates at its phosphorylation site. The present study quantitates both the ion charge and size specificity of the CheY site by measuring the dissociation constants of metal ions from groups Ia, IIa, IIIa, and the lanthanides; these spherical cations provide a range of substrates with incrementally varying charge and radius. The site binds divalent and trivalent cations, but it effectively excludes monovalent cations, including the physiological ions Na(I) and K(I). This charge specificity is in contrast to the site's remarkable lack of size specificity: divalent and trivalent cations exhibit affinities which are essentially independent of radius. It is revealing to compare the ion specificity of the Mg(II) site with the previously characterized specificity of the EF-hand class of Ca(II) sites commonly found in Ca(II) signaling proteins. The Mg(II) and Ca(II) sites exhibit similar charge selectivity, but the Ca(II) site is highly size-selective, preferring divalent and trivalent ions with radii similar to that of Ca(II).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular diversity of glutamate receptors and implications for brain function

The glutamate receptors mediate excitatory neurotransmission in the brain and are important in memory acquisition, learning, and some neurodegenerative disorders. This receptor family is classified in three groups: the N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate, and metabotropic receptors. Recent molecular studies have shown that many receptor subtypes exist in all three groups of the receptors and exhibit heterogeneity in function and expression patterns. This article reviews the molecular and functional diversity of the glutamate receptors and discusses their implications for integrative brain function.

GABAB receptor activation partially inhibits N-methyl-D-aspartate-mediated tyrosine hydroxylase stimulation in rat striatal slices

The effect of the GABAB agonist, (+/-)-baclofen, on the N-methyl-D-aspartate (NMDA)-mediated stimulation of tyrosine hydroxylase activity was investigated in slices of rat striatum. Tyrosine hydroxylase activity was stimulated by NMDA in a concentration-dependent manner (EC50, 1.3 +/- 0.3 microM; maximum stimulation 194 +/- 7% of basal activity). The action of NMDA was reversed by the NMDA antagonist, 2-amino-5-phosphonovalerate (AP-5). (+/-)-Baclofen (100 microM) decreased the maximum effect of NMDA by 24 +/- 2% without significantly modifying its EC50. The IC50 for the inhibitory action of (+/-)-baclofen was 4.2 +/- 1.2 microM. These results show that GABAB receptor activation modulates NMDA-stimulated tyrosine hydroxylase activity, further supporting the possibility of a role of GABA in the regulation of striatal dopaminergic neurotransmission.

Heteromeric NMDA Receptors: Molecular and Functional Distinction of Subtypes

The N-methyl D-aspartate (NMDA) receptor subtype of glutamate-gated ion channels possesses high calcium permeability and unique voltage-dependent sensitivity to magnesium and is modulated by glycine. Molecular cloning identified three complementary DNA species of rat brain, encoding NMDA receptor subunits NMDAR2A (NR2A), NR2B, and NR2C, which are 55 to 70% identical in sequence. These are structurally related, with less than 20% sequence identity, to other excitatory amino acid receptor subunits, including the NMDA receptor subunit NMDAR1 (NR1). Upon expression in cultured cells, the new subunits yielded prominent, typical glutamate- and NMDA-activated currents only when they were in heteromeric configurations with NR1. NR1-NR2A and NR1-NR2C channels differed in gating behavior and magnesium sensitivity. Such heteromeric NMDA receptor subtypes may exist in neurons, since NR1 messenger RNA is synthesized throughout the mature rat brain, while NR2 messenger RNA show a differential distribution.