Binding sites for L-glutamate in the central nervous system of the rat

Changes of pharmacological properties of (1S,3R)-ACPD-sensitive glutamate binding sites in developing mouse cerebellum

Autoradiography of [3H]glutamate binding to mouse cerebellar sections was used to study the distribution of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-((1S,3R)-ACPD) sensitive [3H]glutamate binding sites and the sensitivity of these sites to drugs preferentially acting on one or few types of the metabotropic receptor family. The inhibitory effect of (1S,3R)-ACPD on [3H]glutamate binding and its estimated inhibition constant showed the presence of a different global metabotropic receptor population according to the region considered. During ontogeny, the (1S,3R)-ACPD binding site density increased in the molecular layer (ML), in contrast it decreased in the internal granular layer (IGL) and the deep cerebellar nuclei (DCN). In addition, different sensitivities to (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), (S)-4-carboxyphenylglycine (4-CPG), (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) and L-2-amino-4-phosphonobutyric acid (L-AP4) were demonstrated according to the region and the age. In the DCN, the high (1S,3R)-ACPD binding site density at PND 10 seems to be also sensitive to L-CCG-I but not to MCPG, 4-CPG or L-AP4. In the ML, the MCPG-, the 4-CPG- and the L-AP4-sensitive [3H]glutamate binding sites appeared during ontogeny and the L-CCG-I-sensitive [3H]glutamate binding sites were already present at PND 10. In the IGL, L-CCG-I-sensitive binding sites disappeared in contrast to the L-AP4-sensitive binding sites which appeared during development even if the total (1S,3R)-ACPD binding site density was relatively weak in the adults. These results all reflected the multiplicity of the receptor subtypes included in the cerebellar metabotropic component.

Endogenous gamma-L-glutamyl and beta-L-aspartyl peptides and excitatory aminoacidergic neurotransmission in the brain

The effects of gamma-L-glutamyl- and beta-L-aspartyl di- and tripeptides on glutamatergic neurotransmission were tested in vitro. Of the peptides, gamma-L-glutamylglutamate was the most effective inhibitor, comparable to glutamate, of both Na(+)-independent and Cl-/Ca(2+)-activated binding/transport of glutamate. gamma-L-glutamylglutamate was most effective in the midbrain and hypothalamus and gamma-L-glutamylaspartate in the hippocampus when tested on the Na(+)-independent binding. The Cl-/Ca(2+)-dependent binding/transport of glutamate was affected by gamma-glutamylaspartate most strongly in the hippocampus. gamma-L-glutamylglycine and beta-L-aspartylglycine moderately inhibited the Na(+)-dependent uptake of L-glutamate and D-aspartate while the other peptides were only weak inhibitors. Reduced and oxidized glutathione enhanced the uptake of L-glutamate. The K(+)-stimulated release of L-glutamate was enhanced by gamma-L-glutamylglutamate and -aspartate and the release of D-aspartate also by gamma-L-glutamylglycine. The results indicate that both pre- and postsynaptic events in glutamatergic neurotransmission are modulated by these endogenous acidic oligopeptides.

Interactions of gamma-L-glutamyltaurine with excitatory aminoacidergic neurotransmission

The in vitro effects of gamma-L-glutamyltaurine on different stages of excitatory aminoacidergic neurotransmission were tested with gamma-D-glutamyltaurine as reference. gamma-L-Glutamyltaurine enhanced the K(+)-stimulated release of [3H]glutamate from cerebral cortical slices (25% at 0.1 mM) and slightly inhibited the uptake by crude brain synaptosomal preparations (about 10% at 1 mM). gamma-L-Glutamyltaurine was also a weak displacer of glutamate and its agonists from their binding sites in brain synaptic membrane preparations, being, however, less selective to quisqualate (QA) sites than gamma-D-glutamyltaurine. The basal influx of Ca2+ into cultured cerebellar granular cells was not affected by 1 mM gamma-L-glutamyltaurine, but the glutamate- and its agonist-activated influx was significantly inhibited in low-Mg2+ (0.1 mM) and Mg(2+)-free media. The glutamate-evoked increase in free intracellular Ca2+ and the kainate-activated formation of cGMP in cerebellar slices were both markedly inhibited by 0.1 mM gamma-L-glutamyltaurine. We propose that gamma-L-glutamyltaurine may act as endogenous modulator in excitatory aminoacidergic neurotransmission.

Changes of NMDA receptor binding in spontaneously epileptic rat and parent strains

We measured the binding of [3H]3-[(+/-)2-carboxypiperazin-4-yl] propyl-1-phosphonic acid ([3H]CPP), a competitive ligand for N-methyl-D-aspartate (NMDA) receptors, in double mutant spontaneously epileptic rats (SER: zi/zi, tm/tm) and their parent strains, zitter rats and tremor rats, and WTC rats (control rats from tremor rats derived from Kyoto:Wistar rats) before and after the onset of seizures in tremor rats and SER. Significantly lower [3H]CPP binding receptor density (Bmax) was found in the cortex of SER and zitter rats at 12-15 weeks of age than in that of WTC rats and tremor rats, and at 4 weeks of age the Bmax in zitter rats was lower than that in the other strains. The reduction of Bmax in SER at 12-15 weeks of age may reflect a down regulation of NMDA receptors due to repetitive tonic seizures in SER.

Association of NMDA receptor sites and seizures of El mice

To investigate the possible role of N-methyl-D-aspartate (NMDA) receptors in the seizures of El mice, a genetic animal model of epilepsy, we measured [3H]3-[(+)-2-(carboxypiperazin-4-yl)][1,2-3H]- propyl-1-phosphonic acid (CPP) binding in several brain regions of El and ddY mice. At 22-24 weeks of age, the maximum number of binding sites (Bmax) of [3H]CPP was lower only in the cerebral cortex of both stimulated and unstimulated El mice (El(+) and El(-), respectively) than in that of ddY mice. A reduction in Bmax values of cortical [3H]CPP binding of El mice was detected after the age of 12 weeks. Cortical [3H]CPP binding in El(+) mice decreased further transiently after evoked seizures. No significant change was observed in El(-) mice after postural stimulation. These results suggest that El(+) and El(-) mice share seizure propensity and that activation of NMDA receptors is involved in the seizures of El mice.

NMDA-sensitive L-[3H]glutamate binding in cerebral cortex of El mice

NMDA-sensitive L-[3H]glutamate binding was examined in the brains of El mice, a genetic animal model of epilepsy, and in ddY mice. In whole brain, Scatchard analysis showed that both stimulated and unstimulated El mice had significantly lower Bmax values for binding than did ddY mice. In regional studies, the binding of NMDA-sensitive L-[3H]glutamate was significantly less in the cerebral cortex of both stimulated and unstimulated El mice than in that of ddY mice. These data suggest that NMDA receptors may be involved in the genetic susceptibility of El mice to seizures.

Displacement of excitatory amino acid receptor ligands by acidic oligopeptides

A number of L-glutamyl and L-aspartyl dipeptides, glutathione, gamma-D-glutamylglycine and gamma-D-glutamyltaurine, were tested for their efficacy to displace ligands specific for different subtypes of excitatory amino acid receptors from rat brain synaptic membranes. In general, the L enanthiomorphs of gamma-glutamyl peptides were more potent displacers than gamma-D-glutamylglycine and -taurine but the latter were more specific for the quisqualate type of receptors. gamma-L-glutamyl-L-glutamate was the most effective dipeptide in displacing the binding of glutamate, 2-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) and 2-amino-5-phosphonoheptanoate (APH), whereas gamma-L-glutamyl-L-aspartate was the most effective in the binding of kainate. Both oxidized and reduced glutathione were inhibitory, being most potent in the binding of AMPA. gamma-L-Glutamylaminomethylsulphonate was most effective in the binding of APH. The most potent gamma-L-glutamyl peptides (glutathione, gamma-L-glutamyl-L-glutamate, -L-aspartate, and -glycine) may act as endogenous modulators of excitatory aminoacidergic neurotransmission.

Actions of excitatory amino acids on somatostatin release from cortical neurons in primary cultures

L-Glutamate, N-methyl-D-aspartic acid (NMDA), quisqualate, and kainate were found to increase endogenous somatostatin release from primary cultures of rat cortical neurons in a dose-dependent manner. The rank order of potency calculated from the dose-response curves was quisqualate greater than glutamate = NMDA greater than kainate, with EC50 values of 0.4, 20, and 40 microM, respectively. Alanine, glutamine, and glycine did not modify the release of somatostatin. The stimulation of somatostatin release elicited by L-glutamate was Ca2+ dependent, was decreased by Mg2+, and was blocked by DL-amino-5-phosphonovaleric acid (APV) and thienylphencyclidine (TCP), two specific antagonists of NMDA receptors. The NMDA stimulatory effect was strongly inhibited by APV in a competitive manner (IC50 = 50 microM) and by TCP in a noncompetitive manner (IC50 = 90 nM). The release of somatostatin induced by the excitatory amino acid agonists was not blocked by tetrodotoxin (1 microM), a result suggesting that tetrodotoxin-sensitive, sodium-dependent action potentials are not involved in the effect. Somatostatin release in response to NMDA was potentiated by glycine, but the inhibitory strychnine-sensitive glycine receptor did not appear to be involved. Our data suggest that glutamate exerts its stimulatory action on somatostatin release essentially through an NMDA receptor subtype.

Neurochemical characteristics of a postsynaptic density fraction isolated from adult canine hippocampus

Postsynaptic density and synaptic membrane fractions isolated from hippocampal tissue have been compared to those previously isolated from cerebellum and cerebral cortex. In all respects examined, the isolated hippocampal preparations are similar to the cerebral cortex fractions. The morphology of the postsynaptic density (PSD) preparation is the same and the protein composition is similar, but with higher concentrations of the 51-kDa major protein and of calmodulin, and lower concentrations of actin, in the hippocampal PSD fraction. The binding characteristics for glutamate and GABA are also similar between the two fractions, but with higher Bmax and KD glutamate values and lower Bmax and higher KD GABA values for the hippocampal PSD preparation. Both preparations contain GABAA and GABAB receptors. The PSD fraction contains, as does the cerebral cortex fraction, a calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, as well as a cAMP-dependent and a Ca2+/calmodulin-dependent protein kinase, with the same respective substrates. The value of the hippocampal fractions for studies on long-term potentiation and on kindling in the hippocampus is discussed.

Identification and characterization of an N-methyl-D-aspartate-specific L-[3H]glutamate recognition site in synaptic plasma membranes

Conditions have been developed for an L-[3H]glutamate binding assay in which 85-95% of the specific binding is to a site that corresponds to the N-methyl-D-aspartate subclass of acidic amino acid receptors. Incubation of synaptic plasma membranes with L-[3H]glutamate in 50 mM Tris/acetate, pH 7.4, for 2-20 min at 2 degrees C results in binding with pharmacological characteristics of the electrophysiologically defined N-methyl-D-aspartate receptor. The fraction of glutamate binding to this subclass of receptors, relative to the total, decreases with both increased time and temperature. This binding is reversible, is concentrated in the synaptic plasma membrane fraction, has a pH optimum of 7.0-7.4, and is linear with respect to tissue protein concentration. The binding is unaffected by 1 mM concentrations of the anions sulfate, chloride, bromide, thiocyanate, phosphate, acetate, nitrate, or carbonate and the monovalent cations potassium or ammonium. However sodium and the divalent cations copper, cobalt, zinc, cadmium, and manganese decrease binding to this N-methyl-D-aspartate site.

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

The excitatory amino acid receptors selectively activated by N-methyl-D-aspartate (N-Me-D-Asp) (also known as NMDA) are a major determinant of central nervous system neuronal excitability. We report here that rat brain synaptic plasma membranes contain a distinct population of L-[3H]glutamate binding sites with pharmacological properties indicative of the N-Me-D-Asp receptor. The N-Me-D-Asp sites are readily distinguished from other L-[3H]glutamate binding and uptake sites by their sharp pH optimum, more rapid association rate, preferential localization in synaptic structures, and lack of dependence on temperature and inorganic ions. As with other receptor systems, ligand binding at the N-Me-D-Asp site is reduced by guanine nucleotides but not by adenosine nucleotides. Binding is insensitive to ketamine and cyclazocine, indicating that sigma opiates inhibit N-Me-D-Asp excitation at a site different from that of the N-Me-D-Asp binding site. The quantitative pharmacological properties of N-Me-D-Asp-sensitive L-[3H]glutamate binding sites determined in a well-defined dendritic field (stratum radiatum of CA1) by quantitative autoradiography closely correlate to those of both the electrophysiologically identified N-Me-D-Asp receptors in the same dendritic field and the N-Me-D-Asp sites studied in membrane preparations. Under conditions that selectively reveal N-Me-D-Asp receptors, these sites are found to exhibit considerable anatomical specificity as evidenced by variations within cortical, striatal, and thalamic regions. Autoradiography also showed that regions in rodent and primate brain that are especially sensitive to anoxic and excitotoxic neuronal damage (e.g., Sommer's sector or CA1) have a high level of N-Me-D-Asp sites. Since N-Me-D-Asp receptors are known to contribute to these causes of neuronal loss, their selective distribution partially accounts for the pattern of selective damage seen in these pathological conditions.