Identification and properties of N-methyl-D-aspartate receptors in rat brain synaptic plasma membranes

Examination of Parameters Influencing [3H]MK-801 Binding in Postmortem Human Cortex

[3H]MK-801 binding was used as an index of the glutamate receptor N-methyl-D-aspartate-subtype channel to examine the influence of gender, age, mode of death (agonal status), interval between death and autopsy (postmortem delay), and time in storage at -70 degrees C in well washed homogenate preparations from postmortem human frontal cortex. Basal binding and the modulatory effects of glutamate, glycine, spermidine, and zinc were examined with respect to these variables. Basal binding was sensitive to agonal status, being higher in sudden death cases. The effect of added glutamate and glycine was sensitive to age, with a trend toward lower binding with increasing age. The effect of added spermidine alone was sensitive to storage time at -70 degrees C, the binding being higher with longer storage time. The effect of added zinc was also sensitive to postmortem delay, with zinc causing a greater reduction in binding with shorter postmortem delays. Thus, with the exception of gender, all variables examined influenced [3H]MK-801 binding, highlighting the attention that should be given to these factors in postmortem studies in normal and diseased human subjects.

Iontophoretically administered drugs acting at the N-methyl-D-aspartate receptor modulate burst firing in A9 dopamine neurons in the rat

Extracellular single-unit recording and iontophoresis were used to examine the effect of N-methyl-D-aspartate (NMDA) and the competitive NMDA antagonist (+/-)-4-(3-phosphonopropyl)-2-piperazine carboxylic acid (CPP) on the firing rate and firing pattern of A9 dopamine (DA) neurons in the rat. Administration of NMDA produced a dose-dependent increase in firing rate (up to nearly 300% of baseline at the highest ejection current), which could be blocked by iontophoretic CPP. Low currents (less than 10 nA) were sufficient to induce apparent depolarisation inactivation in some neurons. In addition to this effect on firing rate, NMDA also caused a dramatic increase in burst firing, which was also dose dependent; cells made more bursts, and each burst consisted of more spikes. The only measured aspect of burst morphology that was not affected was the mean burst interspike interval. All nonbursting cells (n = 10) were converted to burst firing by the drug. CPP administered alone was found to reduce burst firing, without affecting the firing rate. These data suggest that a tonically active excitatory amino acid input to A9 DA neurons is responsible for inducing burst firing in vivo and that this input seems to operate via the NMDA receptor, possibly by virtue of its link to a Ca2+ ionophore.

Inhibitory modulation by sodium ions of the N-methyl-D-aspartate recognition site in brain synaptic membranes

Specific binding of radiolabeled L-glutamic acid (Glu) was examined using rat brain synaptic membranes treated with a low concentration of Triton X-100. The binding drastically increased in proportion to increasing concentrations of the detergent used up to 0.1%. Addition of 100 mM sodium acetate significantly potentiated the binding in membranes not treated with Triton X-100, whereas it markedly inhibited the binding in Triton-treated membranes. The binding in Triton-treated membranes was inversely dependent on incubation temperature and reached a plateau within 10 min after the initiation of incubation at 2 degrees C, whereas the time required to attain equilibrium at 30 degrees C was less than 1 min. Sodium acetate invariably inhibited the binding detected at both temperatures independently of the incubation time via decreasing the affinity for the ligand. The binding was significantly displaced by agonists and antagonists for an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors, but not by those for the other subclasses. Inclusion of sodium acetate reduced the potencies of NMDA agonists to displace the binding without virtually affecting those of NMDA antagonists. Moreover, sodium ions inhibited the ability of Glu to potentiate the binding of N-[3H] [1-(2-thienyl)cyclohexyl]piperidine to open NMDA channels in Triton-treated membranes. These results suggest that sodium ions may play an additional modulatory role in the termination process of neurotransmission mediated by excitatory amino acids via facilitating a transformation of the NMDA recognition site from a state with high affinity for agonists to a state with low affinity.

Evidence for heterogenous glycine domains but conserved multiple states of the excitatory amino acid recognition site of the NMDA receptor: regional binding studies with [3H]glycine and [3H]L-glutamate

The possible heterogeneity of the agonist and glycine sites of the N-methyl-D-aspartate (NMDA) receptor-complex was examined using receptor binding techniques. Binding of [3H]L-glutamate [( 3H]GLU) and [3H]glycine to synaptic membranes of cerebral and cerebellar cortices, and membranes of a granule cell preparation of rat cerebellum, was characterized. [3H]Glycine always labelled a single population of sites; densities of binding sites (Bmax) in cortical, cerebellar and "granule" membranes were 3.1, 0.87 and 3.6 pmol/mg protein, respectively. Dissociation constants (Kd) in the same three preparations were 0.13, 0.31 and 1.9 microM, respectively. In competition studies, D-cycloserine, but not D-serine and 7-chlorokynurenate, showed varying potency between the membrane preparations, and analysis of variance (ANOVA) revealed a significant interaction between ligands and membrane fractions. Binding of [3H]GLU was saturable and to a single population of sites: Kd 0.5-0.9 microM and Bmax 3.2-3.6 pmol/mg protein. In all three membrane preparations the rank order of potency of NMDA agonists as inhibitors of the binding of [3H]GLU was always L-aspartate greater than L-cysteate greater than L-cysteinesulphinate greater than L-serine-O-sulphate greater than ibotenate greater than L-homocysteate. NMDA, quinolinate and competitive NMDA antagonists were only weak inhibitors of the binding of [3H]GLU and never fully inhibited specific binding. Other subtype-selective excitatory amino acids were very weak or ineffective inhibitors of binding. Binding of NMDA agonists was better described by a two site model whereby the proportion of high affinity sites did not vary significantly across the three membrane preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycine-like modulation of N-methyl-D-aspartate receptors by a monoclonal antibody that enhances long-term potentiation

We have identified a monoclonal antibody, B6B21, that significantly elevates long-term potentiation when applied to CA1 pyramidal cell apical dendrites in rat hippocampal slices and characterized its binding to N-methyl-D-aspartate-receptor complexes using extensively washed hippocampal membranes. Five micrograms of affinity-purified B6B21 per 100 micrograms of membranes gave a two- to threefold elevation in N-[1-(2-thienyl)cyclohexyl]-3,4-[3H]piperidine ([3H]TCP) binding. When [3H]TCP binding was stimulated by the combined addition of maximal concentrations of glutamate, glycine, and magnesium, B6B21 no longer stimulated [3H]TCP binding. Like glycine, B6B21 enhanced the effect of N-methyl-D-aspartate and glutamate in stimulating [3H]TCP binding. Moreover, B6B21 reversed 7-chlorokynurenic acid inhibition of [3H]TCP binding, but it had no effect on the inhibition of [3H]TCP binding by D-(-)-2-amino-5-phosphonovaleric acid. B6B21 increased the rate of association and dissociation of [3H]TCP, but had no effect on equilibrium binding. Glutamate, but not glycine, however, increased B6B21-enhancement of [3H]TCP association and dissociation. B6B21 binding at strychnine-insensitive glycine sites was confirmed by direct measurement of [3H]glycine binding. These results suggest that B6B21 binds directly to N-methyl-D-aspartate receptors and displays properties similar to glycine.

Mechanisms of synaptic plasticity. Changes in postsynaptic densities and glutamate receptors in chicken forebrain during maturation

We have shown that the synapse maturation phase of synaptogenesis is a model for synaptic plasticity that can be particularly well-studied in chicken forebrain because for most forebrain synapses, the maturation changes occur slowly and are temporally well-separated from the synapse formation phase. We have used the synapse maturation phase of neuronal development in chicken forebrain to investigate the possible link between changes in the morphology and biochemical composition of the postsynaptic density (PSD) and the functional properties of glutamate receptors overlying the PSD. Morphometric studies of PSDs in forebrains and superior cervical ganglia of chickens and rats have shown that the morphological features of synapse maturation are characteristic of a synaptic type, but that the rate at which these changes occur can vary between types of synapses within one animal and between synapses of the same type in different species. We have investigated, during maturation in the chicken forebrain, the properties of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptors, which are concentrated in the junctional membranes overlying thick PSDs in the adult. There was no change in the number of NMDA receptors during maturation, but there was an increase in the rate of NMDA-stimulated uptake of 45Ca2+ into brain prisms. This functional change was not seen with the other ionotropic subtypes of the glutamate receptor and was NMDA receptor-mediated. The functional change also correlated with the increase in thickness of the PSD during maturation that has previously been shown to be due to an increase in the amount of PSD associated Ca(2+)-calmodulin stimulated protein kinase II (CaM-PK II). Our results provide strong circumstantial evidence for the regulation of NMDA receptors by the PSD and implicate changing local concentrations of CaM-PK II in this process. The results also indicate some of the ways in which properties of existing synapses can be modified by changes at the molecular level.

The pharmacological specificity of N-methyl-D-aspartate receptors in rat cerebral cortex: correspondence between radioligand binding and electrophysiological measurements

1. The pharmacological specificity of N-methyl-D-aspartate (NMDA) receptors has been investigated in the rat cerebral cortex by use of radioligand binding and electrophysiological techniques. 2. A comparison was made between a functional assay (NMDA-induced depolarizations in a rat cortical slice preparation) and NMDA-sensitive L-[3H]-glutamate binding in the same brain region and species, to provide accurate affinity values for agonists and antagonists at the NMDA recognition site. 3. In a preparation of crude postsynaptic densities (PSD) from rat cortex, L-[3H]-glutamate bound with high affinity to an NMDA-sensitive population of sites with KD (geometric mean (-s.e.mean. + s.e. mean) = 120 (114, 126) nM, Bmax (mean +/- s.e.mean) = 11.4 +/- 0.8 pmol mg-1 protein and Hill coefficient (mean +/- s.e.mean) = 1.2 +/- 0.17 (n = 3). 4. There was a good agreement between the relative affinities in radioligand binding and electrophysiological assays for the receptor agonists NMDA, N-methyl-L-aspartate, quinolinate and trans-2,3-piperidine dicarboxylate, which are poor substrates of acidic amino acid transport systems. However, agonists which are good substrates for high affinity uptake systems (L- and D-glutamate, L- and D-aspartate, D-aspartate-beta-hydroxamate and L-glutamate-gamma-hydroxamate) were much weaker in the electrophysiological experiments. 5 Schild analysis of the antagonism of NMDA responses in the rat cortical slice by DL-3(2- carboxypiperazin-4-yl)propyl-1-phosphonate, D- and DL-2-amino-5-phosphonovalerate, D- and DL-2- amino- 7-phosphonoheptanoate, D-beta-aspartylaminomethylphosphonate, D-gamma-glutamylglycine and D-Ofaminoadipate (D-AA) indicated a competitive interaction with respective pA2 values of 6.17, 5.62, 5.24, 5.28, 5.20, 5.00, 4.43 and 3.97. 6 In the radioligand binding experiments the same antagonists inhibited only the NMDA-sensitive component of L-[3H]-glutamate binding. IC50 values showed a good correlation with the pA2 values (correlation coefficient = 0.96), with the exception of D-AA which was more potent than anticipated in the binding experiments (IC50 = 9.8 microM).7 These results confirm that NMDA-sensitive L-[3H]-glutamate binding sites represent the NMDA recognition site of the NMDA receptor and provide affinity values for both agonists and antagonists in the rat cerebral cortex, agreeing well with previous estimates in this and other tissues.

Regional variations in [3H]MK801 binding to rat brain N-methyl-D-aspartate receptors

This study examined (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine maleate [( 3H]MK801) binding to the N-methyl-D-aspartate (NMDA) receptor in membranes prepared from six regions of rat brain. Highest levels of binding were found in hippocampus and cortex, whereas much lower densities were found in brainstem and cerebellum. NMDA receptors in cerebellum exhibited a significantly lower affinity for [3H]MK801 than cortical NMDA receptors. To determine whether forebrain and hindbrain NMDA receptors were distinct, the actions of glutamate, NMDA, ibotenate, quinolinate, glycine, and spermine were investigated. These agents increased [3H]MK801 binding in all brain regions examined. However, agonists were uniformly less efficacious in hindbrain compared to forebrain regions. NMDA mimetics and spermine were less potent in cerebellum compared to cortex whereas glycine was equipotent. Antagonists that act at the various modulatory sites on the NMDA receptor were also examined. DL-Amino-phosphonopentanoic acid and 7-chlorokynurenate were approximately equipotent in cortex and cerebellum. However, antagonists that are believed to act inside the NMDA-operated ion channel, including Mg2+ and phencyclidine, were approximately threefold less potent in cerebellum. The diminished regulation of [3H]MK801 binding by glutamate and glycine in the cerebellum was associated with a smaller effect of these agonists on the dissociation of [3H]MK801 from its binding site. The levels of glutamate, aspartate, glycine, serine, and glutamine in the membrane preparations were determined. However, variations in the levels of endogenous amino acids were not sufficient to account for the regional differences in [3H]MK801 binding. These results do not support the hypothesis that a distinct NMDA receptor exists in hindbrian regions of the rat CNS.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of memory dysfunction following experimental brain injury using the Morris water maze

Memory dysfunction, a common clinical feature of traumatic brain injury (TBI), is thought to be related to secondary damage of key anatomic structures in the brain, including the hippocampus. In the present study, we have characterized and evaluated a novel experimental paradigm using the Morris water maze (MWM) technique, to measure post-TBI memory retention after lateral (parasagittal) fluid percussion (FP) brain injury in rats. Male Sprague-Dawley rats (n = 37) received a total of 20 training trials over 2 days in the MWM. Two and a half hours after the last training trial, the animals received FP brain injury of moderate severity (2.3 atmospheres, n = 12), high severity (2.6 atm, n = 13), or no injury (n = 12). Forty-two hours after FP brain injury, we observed a highly sufficient memory dysfunction in animals from both injury groups compared to the uninjured group (p less than 0.001). The degree of memory dysfunction was found to be directly related to the severity of injury, with the high severity group scoring significantly worse than the moderately injured group (p = 0.15). In addition, hippocampal cell loss was observed after brain injury, but only unilaterally. These data suggest that lateral FP brain injury causes memory dysfunction possibly related to concurrent hippocampal cell loss and that posttraumatic memory deficits may be sensitively quantitated using the memory testing paradigm described.

Rapid, sensitive, and simple method for quantification of both neurotoxic and neurotrophic effects of NMDA on cultured cerebellar granule cells

A simple and sensitive method adapted from the staining of living cells with fluorescein diacetate was developed to rapidly estimate the number of living cells remaining in a culture dish 24 hr after a few min of NMDA treatment of cerebellar neurons. This method consists of the measurement, after cell lysis, of the total amount of fluorescein produced from fluorescein diacetate by the living granule cells present in each culture dish. We show that this method can also be used to quantify the survival effect of chronic exposure of granule cells to either K+ or NMDA. In both cases, the fluorescence measured was found to be proportional to the number of fluorescein-labelled cells counted under a fluorescence microscope, indicating that the present method can be used to quantify both toxic and trophic effects of NMDA on cerebellar granule cells. This study confirms that these two NMDA effects occur at the same NMDA concentration, and both are inhibited by MK 801 in the same concentration range. We showed, moreover, that granule neurons developed in the presence of NMDA are much less sensitive to NMDA toxicity than neurons developed in K(+)-enriched medium.

Mechanistic distinctions between excitotoxic and acidotic hippocampal damage in an in vitro model of ischemia

Excitotoxicity is believed to underlie the selective loss of vulnerable neurons after transient ischemia, while lactic acidosis seems to be the principal feature and probable cause of tissue infarcts. Primary hippocampal cultures containing both neurons and astrocytes derived from fetal rats were used to examine the relative contributions of and interactions between excitotoxic and acidotic cell injury. Hypoxia-induced damage was energy dependent and involved the N-methyl-D-aspartate (NMDA) receptor. Glucose above 1 mM could completely protect against hypoxia-induced injury in a pH range of 7.4-6.5, while the NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (500 microM) during the posthypoxic period provided only partial protection in the absence of glucose. Astrocyte cultures were undamaged by ischemic-like treatment in this pH range, suggesting that hypoxia-induced cell death in mixed cultures was restricted to neurons. Lowering the extracellular pH to 7.0 and 6.5 caused no neuronal damage in normoxic controls, but in each case provided significant protection against hypoxic neuronal injury. In contrast, a second type of neurotoxicity was observed after a 6-h exposure to pH 6.0, while exposure to pH 5.5 was required to kill astrocytes. This acidotic damage appeared to be energy independent and did not involve the NMDA receptor. These results suggest that excitotoxic neuron death has an energetic component and that acidosis may produce both protective and damaging effects in the hippocampus during ischemic insults.

Effect of septal kindling on glutamate binding and calcium/calmodulin-dependent phosphorylation in a postsynaptic density fraction isolated from rat cerebral cortex

Postsynaptic density (PSD) fractions were isolated from the cerebral cortices of control and kindled rats and assayed for glutamate and gamma-aminobutyric acid-binding capacities and for the Ca2+/calmodulin-dependent protein kinase. Glutamate binding was found to be increased by approximately 50% in the PSDs isolated from kindled rats as compared to controls; this increase was almost completely from an increase in Bmax; Kd decreased only slightly. Studies with inhibitors indicate that the receptors involved were of the N-methyl-D-aspartate and quisqualate types. PSDs isolated from control and kindled rats did not differ in gamma-aminobutyric acid or flunitrazepam binding. The in vitro autophosphorylation of the Ca2+/calmodulin-dependent protein kinase was depressed by 45-76% in PSDs isolated from kindled rats as compared to controls, with little change in amount of the kinase. Therefore, we infer that (i) the kindled state is associated with an increase in glutamate activation of postsynaptic sites, allowing Ca2+ to enter dendritic spines, (ii) a change has occurred in activity of the protein kinase, which is the major cerebral cortex PSD protein, and (iii) perhaps major alterations in the PSD are a concomitant to the long-lasting nature of the kindled state.

Distribution of quinolinic acid phosphoribosyltransferase in the human hippocampal formation and parahippocampal gyrus

The morphological distribution of quinolinic acid phosphoribosyltransferase (QPRT), the degradative enzyme of the endogenous excitotoxin quinolinic acid, was studied in the human hippocampal formation and parahippocampal gyrus by immunohistochemical techniques. In seven neurologically normal human brains obtained at autopsy, QPRT-immunoreactivity (QPRT-i) was found in both glial cells and neurons. Glial cells exhibiting QPRT-immunoreactivity morphological features of astrocytes, were observed in all hippocampal subfields. The polymorphic layer of the dentate gyrus contained the highest density of QPRT-i glial cells. Numerous QPRT-i glial cells were also found along both sides of the fused hippocampal fissure and in the white matter including the alveus of Ammon's horn, whereas only a few were observed in the granule cell layer and the stratum pyramidale. Neurons containing QPRT-i were found mainly in the subiculum and in the strata oriens and pyramidale of CA1. They were mostly small and polymorphic or fusiform, thus indicating that they may belong to a subpopulation of interneurons. Moderate numbers of QPRT-i glial cells and neurons were also observed throughout layers II-VI of parahippocampal cortex. The localization of QPRT-i in selected glial cells and neurons suggests that in the regions examined these cellular elements might play specific roles in the regulation of quinolinic acid function.

Glycine potentiates the stimulation of inositol phospholipid hydrolysis by excitatory amino acids in primary cultures of cerebellar neurons

Glycine potentiates stimulation of inositol phospholipid hydrolysis by glutamate and N-methyl-D-aspartate, but not by quisqualate or carbamylcholine, in primary cultures of cerebellar granule cells. This potentiation occurs in the absence of extracellular Mg2+, but is more evident when stimulation of inositol phospholipid hydrolysis by N-methyl-D-aspartate is measured in the presence of 1 mM Mg2+. The action of glycine is not antagonized by strychnine. These results suggest that glycine acts as a positive modulator of signal transduction at a specific class of N-methyl-D-aspartate-sensitive glutamate receptors coupled to inositol phospholipid hydrolysis in cerebellar granule cells.

Glycine binding to rat cortex and spinal cord: binding characteristics and pharmacology reveal distinct populations of sites

Glycine is the principal inhibitory neurotransmitter in posterior regions of the brain. In addition, glycine serves as an allosteric regulator of excitatory neurotransmission mediated by the N-methyl-D-aspartate (NMDA) acidic amino acid receptor subtype. The studies presented here characterize [3H]glycine binding to washed membranes prepared from rat spinal cord and cortex, areas enriched in glycine inhibitory and NMDA receptors, respectively, in an attempt to define the glycine recognition sites on the two classes of receptors. Specific binding for [3H]glycine was seen in both cortex and spinal cord. Saturation analyses in cortex were best fitted by a two-site model with respective equilibrium dissociation constants (KD values) of 0.24 and 5.6 microM and respective maximal binding constants (Bmax values) of 3.4 and 26.7 pmol/mg of protein. Similar analyses in spinal cord were best fitted by a one-site model with a KD of 5.8 microM and Bmax of 20.2 pmol/mg of protein. Na+ had no effect on [3H]glycine binding to cortical membranes but increased the binding to spinal cord membranes by greater than 15-fold. This Na+-dependent binding may reflect glycine binding to the recognition site of the high-affinity, Na+-dependent glycine uptake system. Several short-chain, neutral amino acids displaced [3H]glycine binding from both cortical and spinal cord membranes. The most potent displacers of [3H]glycine binding to cortical membranes were D-serine and D-alanine, followed by the L-isomers of serine and alanine and beta-alanine. In contrast, D-serine and D-alanine were similar in potency to L-serine in spinal cord membranes. Compounds active at receptors for the acidic amino acids had disparate effects on the binding of [3H]glycine. At 10 microM, NMDA resulted in a 25% increase, whereas D- and L-2-amino-5-phosphonovaleric acid at 100 microM resulted in a 30% decrease, in [3H]glycine binding to cortical membranes. Kynurenic acid was the most potent of the acidic amino acid-related compounds at displacing [3H]glycine binding. In cortical membranes, kynurenic acid displacement was resolved into a high- and a low-affinity component; the high-affinity component displaced the high-affinity component of [3H]glycine binding.(ABSTRACT TRUNCATED AT 400 WORDS)

A glycine site associated with N-methyl-D-aspartic acid receptors: characterization and identification of a new class of antagonists

Membranes from rat telencephalon contain a single class of strychnine-insensitive glycine sites. That these sites are associated with N-methyl-D-aspartic acid (NMDA) receptors is indicated by the observations that [3H]glycine binding is selectively modulated by NMDA receptor ligands and, conversely, that several amino acids interacting with the glycine sites increase [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the phencyclidine site of the NMDA receptor. The endogenous compound kynurenate and several related quinoline and quinoxaline derivatives inhibit glycine binding with affinities that are much higher than their affinities for glutamate binding sites. In contrast to glycine, kynurenate-type compounds inhibit [3H]TCP binding and thus are suggested to form a novel class of antagonists of the NMDA receptor acting through the glycine site. These results suggest the existence of a dual and opposite modulation of NMDA receptors by endogenous ligands.

Modulation of dendritic release of dopamine by N-methyl-D-aspartate receptors in rat substantia nigra

A superfusion system was used to study the effects of excitatory amino acids (EAA) on release of [3H]dopamine ([3H]DA) previously taken up by rat substantia nigra (SN) slices. The EAA tested (20-250 microM), with the exception of quisqualate and kainate, markedly evoked [3H]DA release from nigral slices when Mg2+ ions were omitted from the superfusion medium. The EAA receptor agonists exhibited the following relative potency in stimulating [3H]DA release: L-glutamate (L-Glu) greater than N-methyl-D-aspartate (NMDA) greater than NM(D,L)A greater than D-Glu much greater than quisqualate = kainate. D-2-Amino-5-phosphonovalerate (100-200 microM), an antagonist for NMDA receptors, substantially reduced [3H]DA release evoked by L-Glu or NMDA. In contrast, L-Glu diethyl ester (100-200 microM) produced a lesser blocking effect on [3H]DA release evoked by the EAA. Further experiments showed that the NMDA-mediated release of [3H]DA was totally suppressed by the omission of Ca2+ or by the addition of tetrodotoxin (0.1 microM) to the superfusion medium. In addition, strychnine, an antagonist for glycine (Gly) receptors, significantly decreased NMDA (100 microM)-evoked as well as glycine (100 microM)-evoked release of [3H]DA from nigral slices. The results shown support the idea that activation of NMDA subtype receptors in SN may trigger a Ca2+-dependent release of DA from dendrites of nigro-striatal DA-containing neurons. Furthermore, a transsynaptic mechanism that may partially involve Gly-containing interneurons is proposed to account for some of the events mediating NMDA receptor activation and DA release in SN.

Interaction of [3H]MK-801 with multiple states of the N-methyl-D-aspartate receptor complex of rat brain

N-Methyl-D-aspartate (N-Me-D-Asp) and phencyclidine receptors interactively mediate central nervous system processes including psychotomimetic effects of drugs as well as neurodegenerative, cognitive, and developmental events. To elucidate the mechanism of this interaction, effects of N-Me-D-Asp agonists and antagonists and of glycine-like agents upon binding of the radiolabeled phencyclidine receptor ligand [3H]MK-801 were determined in rat brain. Scatchard analysis revealed two discrete components of [3H]MK-801 binding after 4 hr of incubation. Incubation in the presence of L-glutamate led to an increase in apparent densities but not in affinities of both components of [3H]MK-801 binding as well as conversion of sites from apparent low to high affinity. Incubation in the presence of combined D-serine and L-glutamate led to an increase in the apparent density of high-affinity [3H]MK-801 binding compared with incubation in the presence of either L-glutamate or D-serine alone. These data support a model in which phencyclidine receptor ligands bind differentially to closed as well as open conformations of the N-Me-D-Asp receptor complex and in which glycine-like agents permit or factilitate agonist-induced conversion of N-Me-D-Asp receptors from closed to open conformations.

Effects of nicotinic agonists and antagonists on N-methyl-D-aspartate-induced 3H-norepinephrine release and 3H-(1-[1-(2-thienyl)cyclohexyl]-piperidine) binding in rat hippocampus

The nicotinic agonists dimethylphenylpiperazinium iodide (DMPP) and carbachol (CARB) as well as (-)nicotine [-)NIC) were tested alone and in combination with N-methyl-D-aspartate (NMDA) for their abilities to enhance the efflux of 3H-norepinephrine (NE) from slices of rat hippocampus. CARB and (-)NIC produced small, transient increases in NE efflux, while DMPP produced larger, longlasting increases. Inasmuch as the nicotinic antagonists mecamylamine (MECA) and hexamethonium (C6) did not consistently inhibit the increases in NE efflux produced by these agonists, the role of a nicotinic receptor in mediating these responses is uncertain. CARB and DMPP enhanced the ability of NMDA to stimulate NE release, while (-)NIC did not. MECA, but not C6, was found to selectively antagonize NMDA-stimulated NE release that did not appear to involve a nicotinic receptor. Binding studies indicated that MECA and the related nicotinic antagonist pempidine produced an inhibition of NMDA-stimulated NE release by an action at the PCP receptor that is known to be linked to the NMDA receptor-ionophore complex. These data suggest that the actions of these ganglionic blocking agents on excitatory responses in the hippocampus involve inhibition of excitatory amino acid as well as nicotinic receptors.

Two classes of N-methyl-D-aspartate recognition sites: differential distribution and differential regulation by glycine

The N-methyl-D-aspartate (NMDA) receptor, a subtype of excitatory amino acid receptor, mediates synaptic responses in many regions of the central nervous system. This receptor plays a critical role in the mechanisms of both synaptic plasticity and excitotoxicity. Although these receptors were generally thought to be a single homogeneous receptor population, we report observations indicating that two anatomically distinct forms of the NMDA-receptor complex exist. (i) The distribution of NMDA receptors, as labeled by the NMDA agonist L-[3H]glutamate, differs from that obtained with the radiolabeled antagonist 3H-labeled 3-[(+/-)2-carboxypiperazine-4-yl]propyl-1-phosphonic acid [( 3H]CPP). Relative to L-[3H]glutamate, [3H]CPP binding is low in the striatum and septum and high in the thalamus and inner cerebral cortex. (ii) NMDA antagonists are relatively more potent than agonists at displacing L-[3H]glutamate binding in the thalamus and cerebral cortex; agonists are relatively more potent in the striatum and cerebellum. (iii) Glycine, which potentiates NMDA-receptor responses to glutamate, causes a greater percentage increase in L-[3H]glutamate binding to NMDA receptors in the thalamus and cerebral cortex than in the striatum, septum, and cerebellum. Radiolabeled NMDA-antagonist binding, in contrast, is inhibited by glycine. Thus, as observed for gamma-aminobutyric acid type A receptors, NMDA receptors have an agonist-preferring binding-site population and an antagonist-preferring binding site population. These may represent two distinct receptors and/or two interconverting forms. It could be of significant clinical importance if these two sites differ in their response to NMDA.

Applications of quantitative measurements for assessing glutamate neurotoxicity

The role of the N-methyl-D-aspartate receptor channel in glutamate neurotoxicity was investigated in cultured hippocampal neurons of the CA1 region. An equation, the survival function, was developed to quantify the effects of putative modulators of neurotoxicity. 2-Amino-5-phosphonovaleric acid (30 microM) reduced the neuronal sensitivity to glutamate by a factor greater than 20, whereas glycine (1 microM) enhanced it by a factor of 7.5 +/- 2.5. Neurons were protected by increasing Mg2+ concentrations in a predictable way based on the ion's ability to block the N-methyl-D-aspartate channel. These findings provide a quantitative basis for the assessment of various neuroprotective agents and add further support to the hypothesis that the N-methyl-D-aspartate channel is central to glutamate neurotoxicity.

Characterisation of Na+-independent L-[3H]glutamate binding sites in human temporal cortex

The binding of L-[3H]glutamate to membranes from human temporal cortex was studied in the absence of Na+, Ca2+, and Cl- ions. Pharmacological characterisation revealed that approximately 35% of specific binding at 50 nM L-[3H]glutamate was sensitive to a combination of kainate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The remaining approximately 65% of specific binding was to a single population of sites with a KD of 844 nM and a Bmax of 0.92 pmol/mg protein. The pharmacological characteristics were consistent with an interaction at the N-methyl-D-aspartate subclass of excitatory amino acid receptor. The inclusion of Cl- ions revealed additional glutamate binding; this was sensitive to quisqualate and DL-2-amino-4-phosphonobutyrate, but not to kainate, DL-2-amino-7-phosphonoheptanoate, or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid.

Learning and memory: regional changes in N-methyl-D-aspartate receptors in the chick brain after imprinting

An extensive series of experiments has implicated a restricted region of the chick forebrain in the learning process of imprinting. The region is the intermediate and medial part of the hyperstriatum ventrale (IMHV). Previous studies have shown that training is associated with an increase in the area of the postsynaptic density of axospinous synapses in the left but not the right IMHV. The postsynaptic density is a site of high receptor density, and at least some axospinous synapses are excitatory. We found that imprinting is associated with a 59% increase in N-methyl-D-aspartate-sensitive binding of the excitatory amino acid L-[3H]glutamic acid in the left IMHV. The increase is probably due to an increased number of binding sites. The profile of sensitivity of the sites to a series of amino-, phosphono-substituted carboxylic acids (2-amino-3-phosphonopropionate to 2-amino-8-phosphonooctanoate) is characteristic of N-methyl-D-aspartate-type receptors. There were no significant effects of training on binding in the right IMHV. The effect of training on left IMHV binding could not be attributed to light exposure, arousal, or motor activity per se but was a function of how much the chicks learned. The changes in the left IMHV could increase the effectiveness of synaptic transmission in a region crucial for information storage and so form a neural basis for recognition memory.

Phencyclidine is a negative allosteric modulator of signal transduction at two subclasses of excitatory amino acid receptors

Phencyclidine (PCP) and some of its pharmacological congeners inhibit the signal transduction at specific excitatory amino acid receptors of cerebellar granule cells in primary cultures. These drugs do not bind to the transmitter recognition sites, and affinity of this specific binding site is increased by the presence of the transmitter bound to its recognition sites. PCP inhibits phosphatidylinositol phosphate hydrolysis mediated by Mg2+-sensitive glutamate receptors (GP1) but not that mediated by Mg2+-insensitive glutamate receptors (GP2). In addition, PCP inhibits Ca2+ influx and cGMP formation mediated by the activation of Mg2+-sensitive glutamate receptors (GC1) but not that mediated by Mg2+-insensitive glutamate receptors (GC2). In this cell culture the activation of phosphatidylinositol phosphate hydrolysis by muscarinic receptor agonists is not affected by PCP. Since PCP inhibits noncompetitively GP1 and GC1 signal transduction it may act as a negative allosteric modulator of signal transduction at both receptors. The pharmacological profile of PCP and its congeners delimits a class of drugs modulating allosterically the action of the primary transmitter at GP1 and GC1 receptors. These drugs need the presence of the transmitter to act and they cannot be termed inverse agonists because they are devoid of activity in the absence of the transmitter; moreover, they do not bind to the transmitter recognition site nor do they prevent the transmitter binding to its recognition sites.