L-Aspartate binding sites in rat cerebellum: a comparison of the binding of L-[3H]aspartate and L-[3H]glutamate to synaptic membranes

Potential Mechanism of Cellular Uptake of the Excitotoxin Quinolinic Acid in Primary Human Neurons

In Alzheimer's disease (AD), excessive amounts of quinolinic acid (QUIN) accumulate within the brain parenchyma and dystrophic neurons. QUIN also regulates glutamate uptake into neurons, which may be due to modulation of Na⁺-dependent excitatory amino acid transporters (EAATs). To determine the biological relationships between QUIN and glutamate dysfunction, we first quantified the functionality and kinetics of [³H]QUIN uptake in primary human neurons using liquid scintillation. We then measured changes in the protein expression of the glutamate transporter EAAT3 and EAAT1b in primary neurons treated with QUIN and the EAAT inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (2,4-PDC) using western blotting and immunohistochemistry. Immunohistochemistry was further used to elucidate intracellular transport of exogenous QUIN and the lysosomal-associated membrane protein 2 (LAMP2). Structural insights into the binding between QUIN and EAAT3 were further investigated using molecular docking techniques. We report significant temperature-dependent high-affinity transport leading to neuronal uptake of [³H]QUIN with a Km of 42.2 μM, and a V_max of 9.492 pmol/2 min/mg protein, comparable with the uptake of glutamate. We also found that QUIN increases expression of the EAAT3 monomer while decreasing the functional trimer. QUIN uptake into primary neurons was shown to involve EAAT3 as uptake was significantly attenuated following EAAT inhibition. We also demonstrated that QUIN increases the expression of aberrant EAAT1b protein in neurons further implicating QUIN-induced glutamate dysfunction. Furthermore, we demonstrated that QUIN is metabolised exclusively in lysosomes. The involvement of EAAT3 as a modulator for QUIN uptake was further confirmed using molecular docking. This study is the first to characterise a mechanism for QUIN uptake into primary human neurons involving EAAT3, opening potential targets to attenuate QUIN-induced excitotoxicity in neuroinflammatory diseases.

BDNF rapidly induces aspartate release from cultured CNS neurons

We investigated the effects of brain-derived neurotrophic factor (BDNF) on aspartate release from cultured cerebellar neurons. This release occurred within 1 min after the addition of 100 ng/ml BDNF. The amount of aspartate released was less than that of glutamate. Aspartate release induced by BDNF was rapid and transient, as in the case of glutamate. Although high potassium evoked the release of both excitatory (glutamate and aspartate) and inhibitory (GABA and glycine) amino acid transmitters, BDNF only induced glutamate and aspartate release. BDNF-induced aspartate release was completely blocked by pretreatment with K252a or TrkB-IgG. The aspartate release induced by BDNF was not dependent on the extracellular Ca(2+), but required intracellular Ca(2+) mobilization. These results showed that BDNF may be involved in excitatory transmission using aspartate as well as glutamate through TrkB-mediated signaling in cerebellum.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Autoradiography of L-[3H]aspartate binding sites

The distribution, pharmacology and binding properties of L-[3H]aspartate were determined in sections from rat brain. No binding was detected in the absence of sodium ions. With the addition of sodium ions to the incubation medium, binding was found to be NMDA, AMPA and CNQX insensitive, but was potently inhibited by threo-beta-hydroxyaspartate, D-aspartate and L-2,4 trans-pyrrolidine dicarboxylate; compounds which have been shown to be specific inhibitors of the sodium-dependent EAA transporter. Autoradiography of L-[3H]aspartate closely resembled the pattern of sodium-dependent D-[3H]aspartate binding. Cerebellar binding showed higher affinity and maximal levels of binding than forebrain, consistent with reports of heterogeneous populations of sodium-dependent EAA binding sites. These results suggest that under these conditions, L-[3H]aspartate specifically labels the sodium-dependent EAA transporter.

A metabotropic l-Glutamate receptor agonist: Pharmacological difference between rat central neurones and crayfish neuromuscular junctions

1. 2S,3S,4S-2-(carboxycyclopropyl)glycine (L-CCG-I), a conformationally restricted glutamate analogue, is a potent metabotropic L-glutamate receptor agonist in the mammalian central nervous system. 2. Depolarizing actions of L-CCG-I and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) in the newborn rat spinal motoneurone are temperature-sensitive, and are not depressed by 3-[(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP) and/or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. L-CCG-I and trans-ACPD induced oscillatory responses in Xenopus oocytes injected with rat brain mRNA. Oocytes with oscillatory responses to L-CCG-I and trans-ACPD showed reversal potential of about -20 mV, which was very close to the equilibrium potential of chloride ions. 4. In rat hippocampal synaptoneurosomes, L-CCG-I stimulated phosphoinositide hydrolysis in a concentration dependent manner. L-CCG-I was less potent than quisqualate but more potent than trans-ACPD. 5. At low concentrations, L-CCG-I did not cause any depolarization of newborn rat spinal motoneurones, but reduced substantially amplitudes of monosynaptic reflexes. 6. At the crayfish neuromuscular junction L-CCG-I, acting presynaptically, reduced the amplitude of excitatory junctional potentials. This action was prevented by application of picrotoxin but not pertussis toxin. The actions of trans-ACPD differ from those of either L-CCG-I or ibotenate at the crayfish neuromuscular junction. 7. L-CCG-I has a potential to provide further useful information on metabotropic L-glutamate receptor function.

Ontogeny of binding sites for [3H] kainic acid in chick and rat cerebellar membranes: a comparative study

We have analyzed the developmental properties of kainate receptors in cerebellar membranes prepared from chick and rat, two vertebrate species with contrasting patterns of functional maturation. Single populations of binding sites have been characterized in the avian and rodent membranes with apparent dissociation constants (Kd) in the 210-280 nM and 40-55 nM ranges, respectively; the number of binding sites (Bmax) increases with age in both species, reaching a maximum of 187 pmol/mg in the case of 10-day chicks vs. 1.28 pmol/mg in 75-day rats. The ontogenetic profiles of kainate receptors in chick and rat cerebella are in consonance with the patent differences in motor development at birth.

Beta-N-oxalylamino-L-alanine action on glutamate receptors

beta-N-Oxalylamino-L-alanine (L-BOAA) is a non-protein excitatory amino acid present in the seed of Lathyrus sativus L. This excitotoxin has been characterized as the causative agent of human neurolathyrism, an upper motor neuron disease producing corticospinal dysfunction from excessive consumption of the lathyrus pea. Previous behavioral, tissue-culture, and in vitro receptor binding investigations revealed that L-BOAA might mediate acute neurotoxicity through quisqualate (QA)-preferring glutamate receptors. The present study demonstrates the stereospecific action of L-BOAA on glutamate receptor binding in whole mouse brain synaptic membranes. L-BOAA was most active in displacing thiocyanate (KSCN)-sensitive specific tritiated (RS)-alpha-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) binding (i.e., QA receptor) (Ki = 0.76 microM) with a rank-order potency of QA greater than kainate greater than N-methyl-D-aspartate (NMDA). By contrast, the nonneurotoxic D-BOAA isomer (100 microM) was essentially inactive in displacing radioligands for glutamate receptors, except the NMDA site, where it was equipotent with L-BOAA. Scatchard analysis of L-BOAA displacement of specific [3H]AMPA binding indicated competitive antagonism (KD: control, 135 nM; L-BOAA, 265 nM) without a significant change in QA-receptor density, and Hill plots yielded coefficients approaching unity. Differential L-BOAA concentration-dependent decreases in specific [3H]AMPA binding were observed in synaptic membranes, indicating that the neurotoxin was more potent in displacing specific binding from frontal cortex membranes, followed by that for corpus striatum, hippocampus, cerebellum, and spinal cord. (ABSTRACT TRUNCATED AT 250 WORDS)

Na+-independent L-aspartate binding sites in chick brain

1. One binding component with a Kd value of 200 x 10(-9) M and half-life of the ligand binding component of 30 min was found. 2. Chloride ions produced a significant increase of L-[3H]aspartate and L-[3H]glutamate binding. 3. L-Glutamate, L-ibotenate, L-quisqualate, and DL-homocysteic acid were potent inhibitors of L-[3H]aspartate binding. 4. In all brain regions major increases of binding were observed during the third week of the in ovo period of life.

Multiple Cl(-)-independent binding sites for the excitatory amino acids: glutamate, aspartate and cysteine sulfinate in rat brain membranes

As we have recently reported that Cl(-)-dependent glutamate (GLU) binding reflects GLU accumulation into membrane vesicles, the characteristics, kinetics and pharmacological specificities of L-[3H]glutamate (L-[3H]GLU) binding to crude rat brain synaptic membranes, were investigated in Cl(-)-free medium. L-[3H]GLU binding was systematically compared to that of L-[3H]cysteine sulfinate (L-[3H]CSA) and L-[3H]ASP), two other putative excitatory amino acids. A high affinity site was determined for each of these radioactive ligands (L-[3H]GLU: Kd = 0.14 microM, Bm = 3.4 pmol/mg protein; L-[3H]CSA: Kd = 0.07 microM, Bm = 2.2 pmol/mg protein; L-[3H]ASP: Kd = 5.8 microM, Bm = 31.2 pmol/mg protein). The pharmacological specificity of these Cl(-)-independent binding sites indicate the existence of at least 3 distinct high affinity sites, all different from the Cl(-)-dependent GLU binding 'site': one having a similar affinity for GLU and CSA, a second one preferring CSA, and a third one preferring ASP. Among the large quantity of structural analogs of the neuroexcitatory amino acids tested, only endogenous compounds (GLU, ASP and CSA) (except hydroxylamine-o-sulfate) were able to interact efficiently. No inhibition by classical agonists and antagonists (such as N-methyl-D-aspartate, quisqualate, kainate, 2-amino-4-phosphonobutyrate, or 2-amino-5-phosphonovalerate) was found. In addition to their high specificity, these Cl(-)-independent sites possess most other biochemical characteristics of receptor proteins.

L-aspartate and L-glutamate binding sites in developing normal and 'nervous' mutant mouse cerebellum

This study concerns the ontogeny and the cellular localization of L-aspartate and L-glutamate binding sites in normal and 'nervous' mutant mouse cerebellar membranes. The binding kinetics revealed for L-aspartate a single binding system (Kd = 750 nM) and for L-glutamate also a single binding component of higher affinity (Kd = 344 nM). The pharmacological study, using various amino acid analogues, revealed a differential specificity for the binding sites of the two amino acids. The developmental study showed that the binding sites of both amino acids appear mainly during the second and third week of life, a period when parallel and climbing fiber synaptogenesis occurs, but they follow a slightly different developmental pattern. The study using 'nervous', mutant mouse cerebellum showed an age-dependent decrease of L-aspartate and L-glutamate binding, which coincides in time with the Purkinje cell degeneration in this mutant, indicating a cellular localization of these binding sites on the Purkinje cell membranes. These results suggest that L-aspartate and L-glutamate binding sites may be respectively associated with the postsynaptic target of climbing and parallel fibers on the Purkinje cell dendrites. However, the decrease of specific binding in 'nervous' mutant mouse cerebellum was about 50% for L-aspartate and 60% for L-glutamate, implying that a significant number of L-aspartate and L-glutamate binding sites are located on cerebellar elements other than the Purkinje cell membranes.

Binding sites for [3H]glutamate and [3H]aspartate in human cerebellum

The binding of [3H]aspartate and [3H]glutamate to membranes prepared from frozen human cerebellar cortex was studied. The binding sites differed in their relative proportions, their inhibition by amino acids and analogues, and by the effects of cations. A proportion (about 30%) of [3H]glutamate binding was to sites similar to those labelled by [3H]aspartate. An additional component of [3H]glutamate binding (about 50%) was displaced by quisqualate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and may represent a "quisqualate-preferring" receptor. Neither N-methyl-D-aspartic acid-sensitive nor DL-2-amino-4-phosphonobutyric acid-sensitive [3H]glutamate binding was detected.

Chemoresponsiveness of intracellular nuclei neurones to L-aspartate, L-glutamate and related derivatives in rat cerebellar slices maintained in vitro

The sensitivity of intracerebellar nuclei neurones to pulse applications of L-aspartate, L-glutamate, N-methyl-D,L-aspartate and quisqualate was tested in rat cerebellar slices maintained in vitro. The responses of the nuclear neurones to the four agonists consisted of a transient and dose-dependent increase in their firing of simple spikes. When suprathreshold currents were used, quisqualate induced the highest increase in the spike discharge frequency of the cells. Quisqualate mediated responses were unaffected by steady applications of 2-amino-5-phosphonovalerate, whereas the sensitivity of the responses induced by the three other agonists was in the order N-methyl-D,L-aspartate, L-aspartate, L-glutamate. When the superfusing solution was devoid of Mg2+ ions, N-methyl-D,L-aspartate and L-aspartate mediated responses were much potentiated, while quisqualate induced responses were not enhanced. In such a medium, L-glutamate elicited responses were more or less potentiated depending on cells. These results suggest that rat intracerebellar nuclei neurones bear both N-methyl-D-aspartate and non-N-methyl-D-aspartate, probably quisqualate, receptors, and that L-aspartate and L-glutamate have a mixed action upon both types. L-Aspartate preferentially activates N-methyl-D-aspartate receptors, whereas L-glutamate predominantly acts via non-N-methyl-D-aspartate receptors. Furthermore, the potency of L-glutamate in activating N-methyl-D-aspartate receptors appears to vary as a function of the cells.

The binding of acidic amino acids to snail, Helix aspersa, periesophagic ring membranes reveals a single high-affinity glutamate/kainate site

The characterization of specific acidic amino acid binding sites to snail, Helix aspersa, ganglia membranes has been assayed using tritiated glutamate (L-[3H]Glu), aspartate (L-[3H]Asp), cysteine sulfinate (L-[3H]CSA) and kainate. At 2 degrees C, only L-[3H]Glu and [3H]kainate specific binding could be measured using a filtration procedure to separate bound from free ligand. The analysis of L-[3H]Glu specific binding reveals the presence of one class of high-affinity binding sites with Kd = 0.12 microM and Bmax = 30 pmol/mg protein. This L-[3H]Glu binding was specific, reversible and saturable. The order of potency of different substances, agonists or antagonists of the rat brain excitatory amino acid receptors, has been determined. Kainate was the best displacing agent, followed by ibotenate = L-Glu greater than L-alpha-aminoadipate (L-alpha-AA) greater than homocysteate (HCA). Using 10 nM [3H]kainate, a single class of binding site was detected. Its pharmacological properties indicate that it is likely identical to the L-[3H]Glu binding site. This L-Glu-kainate site possesses most of the properties expected for a specific receptor. However, whereas L-[3H]Glu binding could be detected on purified neuronal membranes, the major component of specifically bound L-[3H]Glu appeared to be located on the sheaths surrounding neuronal cell bodies. These findings suggest that Glu or another endogenous acidic amino acid may function as a transmitter at neuromuscular junctions in Helix periesophagic ring, acting at a receptor distinct from those on nerve cells.

Identification of a Cl-/Ca2+-dependent glutamate (quisqualate) binding site in bovine pineal organ

The presence of a high concentration of glutamic acid, a transmitter shown to have excitatory action in the pineal organ, prompted us to search for and to characterize glutamate receptor site in the bovine pineal organ. By using 10 nM- 100 microM of labeled and unlabeled L-glutamate and by employing the LIGAND computer program, we found a glutamate binding site with a dissociation equilibrium constant (KD) of 0.534 microM and a receptor density (Bmax) of 4.84 pmol/mg protein. This pH- and temperature-dependent binding site showed stereospecificity, was activated by Ca2+, and displayed affinity for both glutamate agonists and antagonists. The IC50 values for L-glutamate, L-aspartate, L-cysteate, L-cysteine sulfinate, quisqualate, and (+/-) ibotenate were 0.5, 2, 12, 16, 25, and 30 microM, respectively, whereas those for D-aspartate, L-alpha-aminoadipate, L-homocysteate, and DL(+/-) 2-amino-4-phosphonobutyrate were greater than 100 microM. Kainate, N-methyl-D-aspartate, and L-glutamic acid diethyl ester were inactive. Based on these results, the presence of a quisqualate-type, Cl-/Ca2+-dependent glutamate binding site in the pineal organ is suggested, and a possible neuroexcitatory role for glutamic acid, aspartic acid, and certain sulfur-containing amino acids is also implied. The precise nature of this excitatory effect in modulating the function(s) of the pineal organ and the synthesis of its hormone(s) remains to be elucidated.

Excitatory amino acids: studies on the biochemical and chemical stability of ibotenic acid and related compounds

The complex pharmacological profile (excitation/inhibition) of ibotenic acid on single neurons in the mammalian CNS prompted studies on the stability of ibotenic acid and a number of structurally related excitatory amino acids under different in vitro conditions in the presence or absence of enzymes. Ibotenic acid, (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-7-carboxylic acid (7-HPCA), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and (RS)-alpha-amino-3-hydroxy-4-bromo-5-isoxazolepropionic acid (4-Br-homoibotenic acid) were all inhibitors of (S)-glutamic acid decarboxylase (GAD) in mouse brain homogenates, but only ibotenic acid was shown to undergo decarboxylation during incubation with brain homogenates. The formation of the decarboxylated product, muscimol, which primarily occurred in a synaptosomal fraction, was dependent on the presence of pyridoxal-5-phosphate (PALP) and was inhibited by (S)-glutamic acid, 3-mercaptopropionic acid (3MPA), aminooxyacetic acid (AOAA), and allyglycine, suggesting that ibotenic acid is a substrate for GAD. The overall decomposition rate for ibotenic acid (8.7 nmol min-1 mg-1 of protein), which apparently embraces other reactions in addition to decarboxylation to muscimol, was higher than the rate of decarboxylation of (S)-glutamic acid (3.2 nmol min-1 mg-1 of protein). At pH 7.4 and 37 degrees C, but in the absence of enzymes, none of the excitatory amino acids under study underwent any detectable decomposition, whereas ibotenic acid and 7-HPCA, but not AMPA and 4-Br-homoibotenic acid, decomposed, partially by decarboxylation, at 100 degrees C in a pH-dependent manner. In the presence of liver homogenates, ibotenic acid was also shown to decompose.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor types mediating the excitatory actions of exogenous L-aspartate and L-glutamate in rat olfactory cortex

Changes in potential between the pial and cut surfaces of rat olfactory cortex slices evoked by N-methyl-D-aspartate (NMDA), quisqualate, kainate, L-glutamate and L-aspartate and also by gamma-aminobutyric acid (GABA) have been monitored using extracellular electrodes. All agonists produced a pial-negative potential response when superfused onto the pial surface, GABA, L-aspartate and L-glutamate being less potent than the others. Repeated applications of NMDA, but not of the other agonists, led to a progressive reduction in response to approximately 30% of the initial depolarization. The responses to NMDA (100 microM) were selectively abolished by (+/-)2-amino-5-phosphonopentanoic acid (APP; 100 microM) while depolarizations evoked by L-glutamate and L-aspartate (both at 10 mM) were only antagonized by 21 +/- 2 (n = 12) and 36 +/- 3 (n = 12) percent respectively (means +/- S.E.M.). gamma-D-Glutamylglycine (gamma-DGG; 1 mM) and (+/-)cis-2,3-piperidine dicarboxylate (cis-PDA; 2 and 5 mM), in addition to antagonizing responses to NMDA, also partially blocked quisqualate- and kainate-evoked depolarizations. When a mixture of APP (100 microM), gamma-DGG (1 mM) and cis-PDA (5 mM) was applied to preparations, although NMDA receptors were completely blocked and responses to both quisqualate and kainate antagonized by approximately 80%, L-glutamate and L-aspartate evoked depolarizations were only reduced by 51 +/- 7 (n = 4) and 49 +/- 4 (n = 4) percent respectively (means +/- S.E.M.). The results are discussed in terms of the contributions made by NMDA, quisqualate and kainate receptors to the composite responses evoked by L-aspartate and L-glutamate.

Selective retrograde labelling of the rat olivocerebellar climbing fiber system with D-[3H]aspartate

Selective retrograde labelling of the olivocerebellar climbing fiber system with D-[3H]aspartate has been observed in the rat, and the results have implications for the identification of a transmitter candidate as well as the neuroanatomical understanding of these cerebellar afferents. Microinjections of D-[3H]aspartate (50 nl, ca 10-2 M) were made into various parts of cerebellar cortex. Survival times were 6, 12 or 24 h. Pronounced diffusion of the tracer resulted in large injection sites. Within the zone of injection, glial elements were labelled over background. Most granule cells exposed to the tracer were unlabelled; the small numbers demonstrating labelling were believed to have been injured by the micropipette penetration. Beneath injection sites, large numbers of well-labelled nerve fibers appeared in the white matter and could be followed through the brainstem to the contralateral inferior olive, where labelled perikarya were found. After the inferior olivary neurons had been effectively destroyed with 3-acetylpyridine, evidence of cerebellar afferent labelling with D-[3H]aspartate was missing. Retrograde labelling of the olivocerebellar system was also observed after superfusion of the vermis with D-[3H]aspartate at concentrations in the range of Km for high affinity uptake (10(-5) or 10(-4) M, for 2 h). Mossy fiber or monoaminergic afferents to the cerebellum were never labelled with D-[3H]aspartate. The distribution of labelled cells in the olivary subnuclei after injections in different cerebellar areas was in line with the olivocerebellar organization previously described in the cat. Moreover, it was demonstrated that fibers from the different subnuclei follow different routes through the brainstem towards the cerebellum. Labelling of climbing fiber collaterals in uninjected parts of cerebellum indicated that some of the retrogradely migrating D-[3H]aspartate was directed in anterograde direction at axonal branching points. Collaterals were demonstrated in all deep cerebellar and Deiters' nuclei, and the results of intranuclear injections suggested that virtually every olivary neuron sends collaterals to these nuclei. Intracortical collaterals were organized in sagittal zones. Midline injections into the anterior lobe and VI lobule labelled collaterals in several zones of the posterior lobe spinal area and uninjected parts of the anterior lobe vermis. Hemispheral injection into copula pyramidis labelled collaterals in two prominent bundles in the anterior lobe.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and characterisation of L-[3H]aspartate binding sites on rat spinal cord synaptic membranes

The binding of L-[3H] aspartate to extensively-washed rat spinal cord synaptic membranes was investigated. Specific binding was enriched in synaptic membranes and was optimal under physiological conditions of temperature and pH. Equilibrium binding was established relatively slowly over a period of 30 min, and was totally reversible within 40 min. Saturation analysis revealed complex binding patterns. Two sites were clearly demonstrable, only one of which was shown to be saturable over the ligand concentration range employed in the study (0.1-10 microM). There was also some indication of the presence of a higher affinity site, although this was not investigated in any detail. Saturable binding demonstrated a KD = 1.4 microM and Bmax = 105 pmole/mg protein. Structure-activity studies with a range of amino acid analogues indicated that binding was stereospecific and was inhibited by a very restricted range of compounds. The most potent inhibitors of binding were L-glutamate and L-aspartate. There was no evidence for the involvement of NMDA receptors. Effects of possible endogenous modulators, including ions and guanosine nucleotides were investigated, and the chemical nature of the binding site probed with a number of protein-modifying agents.

N-Methyl-D-aspartate and L-aspartate activate distinct receptors in piriform cortex

The effects of ionophoretically applied N-methyl-DL-aspartate (NMDA) and aspartate on identified pyramidal neurons in rat piriform cortex were examined in isolated, submerged, and perfused brain slices. NMDA was more potent than aspartate in eliciting neuronal discharge. Perfusion of the acidic amino acid antagonists, DL-2-amino-5-phosphonovalerate (APV), 10(-6) or 10(-5) M, DL-2-amino-7-phosphonoheptanoate (APH), 10(-5) M, and gamma-D-glutamylglycine (gamma DGG), 10(-5) M, selectively blocked the response to NMDA without effect on the response to aspartate. At higher concentrations which blocked responses to both NMDA and aspartate, gamma DGG blocked kainate responses and depressed glutamate and quisqualate responses. These results suggest that in piriform neurons NMDA and aspartate act at distinct receptor sites, not a common receptor site, and that both of these sites are distinct from those that mediate responses to glutamate, quisqualate, and kainate.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Changes in the status of neurotransmitter receptors in a rabbit model of hepatic encephalopathy

It has previously been shown in an animal model of hepatic encephalopathy (HE) that the number of receptors for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), increases and that the number of receptors for the excitatory neurotransmitter, glutamate, decreases. To determine the functional status of other neurotransmitter systems in HE, measurements were made of the specific binding of other neurotransmitters to synaptic membranes prepared from the brains of normal rabbits and rabbits in HE due to galactosamine-induced acute liver failure. The development of HE was associated with: (i) a decrease in the density (Bmax) of receptors for the two excitatory amino acid neurotransmitters, aspartate and kainic acid; (ii) an increase in the Bmax of both the low and high affinity binding site for strychnine, a marker for the inhibitory neurotransmitter glycine; (iii) a decrease in the affinity (Kd) of receptors for dopamine, and (iv) no appreciable change in either the specific binding of [3H]D-ala2-methionine enkephalinamide or [3H]naloxone, markers for opiate receptors, or in the Bmax or the Kd of receptors for acetylcholine. If it is assumed that the sensitivity of the brain to neurotransmitters varies directly with the density of neurotransmitter receptors, HE may be associated with increased sensitivity to inhibitory amino acid neurotransmitters and decreased sensitivity to excitatory amino acid neurotransmitters. Thus, the observed changes in neurotransmitter receptors in HE afford a feasible pathophysiological basis for the mediation of the neural inhibition of HE.

L-glutamate has higher affinity than other amino acids for [3H]-D-AP5 binding sites in rat brain membranes

Electrophysiological studies indicate the existence of several types of receptors for excitatory amino acids. Thus, responses induced by N-methyl-D-aspartate (NMDA) are potently and selectively blocked by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), while responses induced by such agonists as kainate and quisqualate are relatively resistant to this antagonist. Evidence is mounting that excitatory amino acid receptors are involved in synaptic excitation in many regions of the central nervous system (see refs 1 and 4 for reviews). Although the identity of the transmitter(s) acting at these receptors remains uncertain, L-aspartate has been considered the most likely transmitter at NMDA receptors and L-glutamate at kainate/quisqualate receptors. Other endogenous acidic amino acids proposed as possible transmitters include a range of sulphur-containing amino acids and the tryptophan metabolite, quinolinic acid. Ligand-binding studies offer a means not only of assessing receptor densities in different brain regions but also of comparing affinities of transmitter candidates for these receptors. However, to avoid difficulties of interpretation arising from the use of ligands which bind to more than one type of receptor, such as [3H]-L-glutamate and [3H]-L-aspartate (for example, refs 8-12), ligands with high receptor selectivity are required. Here, we report that [3H]-D-AP5 binds specifically to rat brain membranes, that the hippocampus and cerebral cortex are enriched in these sites relative to other brain areas and that L-glutamate has higher affinity for these receptors than have all other transmitter candidates tested.

L-[3H]Glutamate binding to hippocampal synaptic membranes: two binding sites discriminated by their differing affinities for quisqualate

The excitatory glutamate analogs quisqualate and ibotenate were employed to distinguish multiple binding sites for L-[3H]glutamate on freshly prepared hippocampal synaptic membranes. The fraction of bound radioligand that was displaceable by 5 microM quisqualate was termed GLU A binding. That which persisted in the presence of 5 microM quisqualate, but was displaceable by 100 microM ibotenate, was termed GLU B binding. GLU A binding equilibrated within 5 min and remained unchanged for up to 80 min. GLU B binding appeared to equilibrate at least as rapidly, but incubation with ligand unmasked latent binding sites. Saturation binding curves were best fitted by single exponentials, which yielded KD values of about 200 nM (GLU A) and 1 microM (GLU B). On the average, GLU B binding sites were about twice as abundant in these membranes as were GLU A sites. Rapid freezing of the membranes, followed by storage at -26 degrees C and rapid thawing markedly diminished GLU A binding, but nearly tripled GLU B binding. Both site bound L-glutamate with 10-30 times the affinity of D-glutamate. The GLU A site also bound L-glutamate with about 10 times the affinity of L-aspartate and discriminated poorly between L- and D-aspartate. In contrast, the GLU B site bound L-aspartate with an affinity similar to that for L-glutamate, and with an order-of-magnitude greater affinity than D-aspartate. The structural specificities of the GLU A and GLU B binding sites suggest that these sites may correspond to receptors on hippocampal pyramidal cell dendrites that are activated by iontophoretically applied L-glutamate.

Chloride and calcium ions separate L-glutamate receptor populations in synaptic membranes

Cl-/Ca2+-dependent and Cl-/Ca2+-independent L-[3H]glutamate binding sites in rat brain synaptic membranes showed marked differences in their pharmacological properties. One site resembled L-2-amino-4-phosphonobutyrate (L-APB)-sensitive receptors and the other N-methyl-D-aspartate (NMDA) receptors. Inhibition studies demonstrated that L-aspartate was more potent at Cl-/Ca2+-independent than at Cl-/Ca2+-dependent sites although L-glutamate was of similar potency at both sites; the D-isomers of aspartate, glutamate and alpha-aminoadipate exhibited the opposite trend. Quisqualate and ibotenate showed high and low affinity inhibition components in the presence of Cl- and Ca2+, and only low affinity inhibition at Cl-/Ca2+-independent sites. For a series of alpha-amino-omega-phosphono carboxylic acids (propionate-heptanoate), peaks of inhibitory activity in the presence of Cl- and Ca2+ were shifted to l-carbon shorter homologues than in the absence of these ions. These data indicate that the ionic environment is of critical importance for the activity of different physiological receptor populations in vitro.

Receptors for the excitatory amino acids in the mammalian central nervous system

On the basis largely of neuropharmacological analysis, three different receptors mediating neuronal excitation can be identified. The first is activated by quisqualate and other "flexible" molecules including L-glutamate and appears to bind its ligands in a folded configuration. The second is excited by NMDA and has a more extended conformation, the spacing between the amino and the omega-carboxylate groups being the determinant of specificity. The third type accepts kainate and appears to possess a reactive site for the unsaturated side chain which is essential to the operation of this receptor. All three classes appear to be implicated in synaptic events [although some kainate receptors at least are certainly extra-synaptic (Watkins et al., 1981)] and each appears to activate different ionophores in neuronal membranes. Of the endogenous amino acids which may function as synaptic transmitters, L-glutamate and L-cysteate seem to react preferentially with quisqualate receptors (McLennan and Lodge, 1979), while L-aspartate is more of a mixed agonist capable of reaction both with quisqualate and with the NMDA types. Whether folate has a physiological role involving kainate receptors is unknown; and the same is true of any action possessed by quinolinate. The fact that there are amino acid excitants which are pharmacologically distinct from those reacting with any of the three best known receptors suggests that at least one more class of receptor may also exist, but no further information is available at the present time. Other sites with which the pharmacologically active acidic amino acids react are identifiable neurochemically in membrane preparations derived from tissues of the central nervous system. Kinetic studies and analysis of inhibition of sodium-independent binding indicate that there are sites which accept glutamate, others binding aspartate and a third which binds kainate. However, the first does not correspond completely to the quisqualate excitatory receptor, and NMDA does not react with any of the binding sites. It is difficult to conclude then that any of these binding sites can be fully identified with the excitatory receptors. Finally, there are a number of systems which in their patterns of activity again appear completely distinct, but which presumably mediate uptake of amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for cysteine sulfinate as a neurotransmitter

The Na+-independent binding of L-[3H]cysteine sulfinate and L-[3H]cysteine sulfinate uptake were investigated in rat brain membranes and vesicles. Specific binding of L-[3H]cysteine sulfinate was saturable and occurred by a single high affinity process with a Kb of 100 nM +/- 9 and a capacity (Bmax) of 2.4 +/- 0.22 pmol/mg protein. Sodium ions were found to have a biphasic effect; low concentrations (in the range of 0.1-3 mM) induced a marked inhibition of the binding whereas higher concentrations (10-300 mM) resulted in a dose-dependent stimulation of binding. The inhibition potency, expressed as the Ki values of a wide range of compounds with known pharmacological activities was tested. L-Cysteine sulfinate was the most potent inhibitor being 3-fold more potent than L-glutamate and 80 times more potent than L-aspartate. The regional distribution of the binding of L-[3H]cysteine sulfinate in the brain was found to be heterogeneous. These results provide the first evidence for an interaction of cysteine sulfinate with specific receptor sites on the synaptic membrane. The rate of L-[3H]cysteine sulfinate uptake shows a biphasic dependence on the concentration of L-cysteine sulfinate, corresponding to a high affinity (27.2 microM) and a low affinity (398 microM) transport system. The maximum L-[3H]cysteine sulfinate uptake is reached at 2 min. The reversibility of this transport was demonstrated. The L-[3H]cysteine sulfinate uptake increases as a function of the sodium concentration. Chloride and potassium ions stimulate the uptake. The decrease or increase in the electrical membrane potential (delta psi) caused by replacing the chloride ions by the sulfate or sulfocyanate ions respectively leads to a decrease or increase in the rate of uptake. Increase in the extravesicular osmolarity leads to a decrease in the extent of L-[3H]cysteine sulfinate accumulation. Amino acids with an acidic group in position omega were found to be potent inhibitors (the most potent being L-aspartate). The length of the carbon chain also has a bearing on the inhibitory effect. The regional distribution of L-[3H]cysteine sulfinate uptake in the brain was heterogeneous. These results demonstrate the existence of a high affinity system which may correspond to the transmitter inactivation. Binding and uptake sites are distinguishable as evidenced by the affinity constants, the ionic and pharmacological effects and the different regional distributions in the brain. Finally, these results give further evidence for a neurotransmitter role of L-cysteine sulfinate.

Specific high-affinity binding of L-[3H]aspartate to rat brain membranes

The binding of L-[3H]aspartate was investigated in washed membranes prepared from whole rat brain. We were able to differentiate two separate binding sites differing in their Na dependence. The Na-independent binding was saturable, reversible, and optimal at 20 degrees C and at pHs in the neutral range. The dissociation constant (Kd) at 20 degrees C was about 200 nM. This binding site seemed to be modulated by magnesium and calcium at physiological concentrations. None of the amino acids tested was a potent competitor for Na-independent L-[3H]aspartate binding. This binding site was unevenly distributed in the rat central nervous system: cerebellum = cerebral cortex greater than pons-medulla greater than spinal cord. Destruction of the intrinsic neurons of the cerebellum by injecting kainic acid 30 days before sacrifice resulted in a 53% reduction in Na-independent binding in this region. The Na-dependent binding of L-[3H]-aspartate (Kd = 4894 nM) was strongly inhibited by D-aspartate, L-glutamate, D,L-aspartate beta-hydroxamate; was unaffected by calcium and magnesium; and showed a different pattern of distribution: cerebral cortex greater than cerebellum = pons-medulla = spinal cord. This binding in cerebellum was unaffected by injections of kainic acid.

Phosphonate analogues of acidic amino acids: inhibition of excitatory amino acid transmitter binding to cerebellar membranes and of the stimulation of cerebellar cyclic GMP levels

An examination was made of the ability of phosphonate analogues of acidic amino acids to interact with excitatory amino acid receptors, employing binding assays for [3H]glutamate and [3H]aspartate and by investigating the inhibitory effects of these compounds on the stimulation of cerebellar cyclic GMP levels. In both systems the phosphonates exhibited inhibitory activity, although the cGMP system corresponded better with electrophysiological data. The higher homologues (e.g. 2-amino-5-phosphonovalerate) showed high selectivity for NMDA-type receptors and this effect was stereospecific, with the (-)-isomers being at least 4-7 times more active than the (+)-forms.

Characterization of [L-3H]aspartate binding to chick retinal subcellular fractions

Binding of [L-3H]aspartate to synaptic receptors was examined in membranes from whole chick retina and subcellular fractions enriched with photoreceptor terminals (P1) or terminals from the inner plexiform layer (P2), Na+-independent, stereospecific, high affinity binding was concentrated in the P1 fraction (Kb = 40 nM). P2 fraction also showed a high affinity binding system (KB = 11.8 nM) with lower capacity than in the P1 fraction. Comparative studies with [L-3H]-aspartate, [L-3H]-glutamate and [H3]-kainate showed that L-aspartate and L-glutamate are the most potent inhibitors of the binding of the three ligands. Aspartate and glutamate binding were effectively displaced by N-methyl-DL-aspartate and alpha-amino adipate, whereas only [3H]-glutamate binding was significantly inhibited by glutamate-diethyl-ester. Kainic acid exhibited negligible affinity for aspartate and glutamate binding sites. Results indicate the presence of different receptors for glutamate and aspartate in both plexiform layers of the retina.

L-glutamate and L-aspartate bind to separate sites in rat brain synaptic membranes

The specific binding of L-glutamate (L-Glu) and L-aspartate (L-Asp) was measured in rat brain synaptic plasma membranes (SPMs). A distinction between the binding sites for these amino acids was made on the basis of the kinetics, ion effects, pharmacology and chemical susceptibility of the binding. The existence of distinct binding sites for L-Glu and L-Asp is consistent with electrophysiological data that mammalian neurons possess separate receptors for these amino acids.

Stimulation of rat cerebellar guanosine 3',5'-cyclic monophosphate (cyclic GMP) levels: effects of amino acid antagonists

1 The ability of glutamate, aspartate and related neuroexcitants to produce large calcium-dependent increases in the levels of guanosine 3',5'-cyclic monophosphate (cyclic GMP) in immature rat cerebellar slices has been demonstrated. 2 These effects were inhibited by selective antagonist compounds, indicating the presence of at least two types of excitatory amino acid receptor mediating the cyclic GMP response. 3 Protoveratrine also produced large increases in cyclic GMP, and this action was antagonized by L-glutamate diethylester suggesting that released endogenous glutamate, subsequently interacting with its postsynaptic receptors, is the predominant mechanism. 4 The kinetic characteristics of several of the inhibitor compounds were investigated.