Regulation of glutamate receptors by cations

Tetanic stimulation-induced changes in [3H]glutamate binding and uptake in rat hippocampus

Tetanus-induced (400 Hz, 200 pulses) long-lasting potentiation of the stratum radiatum-evoked CA1 population spike in hippocampal slices is not accompanied by any change in Na+-independent [3H]glutamate binding sites. Homosynaptic depression that occurs subsequent to either a low frequency tetanus (20 Hz, 600 pulses) or a transient exposure to Cl(-)-free (containing NO3-) medium is associated with an elevation in the amino acid binding. [3H]Glutamate uptake into slices was decreased following a high frequency (400 Hz, 200 pulses) tetanus but in the majority of cases was increased following a low frequency (20 Hz, 600 pulses) tetanus to stratum radiatum. When the high frequency tetanus was given in the absence of extracellular Ca2+, there was a further reduction in [3H]glutamate uptake.

Pathologic concentrations of ammonium ions block L-glutamate uptake

Both K+ and NH4+ are potent inhibitors of Na+-dependent L-glutamate binding to synaptic plasma membranes. Thus, the effects of these ions on Na+-dependent L-glutamate uptake were examined in rat forebrain synaptosome preparations. KCl (2 to 4 mM) stimulated L-glutamate uptake 34% over that in K+-free Krebs bicarbonate buffer; NH4+ was without effect. However, in the presence of 4 mM K+, NH4+ blocked the K+-stimulated component of Na+-dependent L-glutamate uptake. These effects were unrelated to ionic strength of Cl- as added Na+ or tris chloride had no effect on L-glutamate uptake. The results suggest that NH4+ could exert some of its toxic effects by blocking a specific L-glutamate uptake site, thereby elevating L-glutamate in the central nervous system.

Binding sites for L-[3H]glutamate on hippocampal synaptic membranes: three populations differentially affected by chloride and calcium ions

The effects of Cl- and Ca2+ were studied on the specific binding of L-[3H]glutamate to multiple sites on rat hippocampal synaptic membranes. Quisqualate (5 microM) or DL-2-amino-4-phosphonobutyrate (2-APB) (300 microM) was used to discriminate two previously identified classes of binding sites. Saturation isotherms and displacement curves constructed under different ionic conditions suggested that the effects of Cl- and Ca2+ could best be explained by postulating the existence of three major binding site populations in this preparation rather than two. The binding of L-glutamate to Glu A sites exhibits an absolute dependence on Cl-, and Ca2+ markedly increases the maximum density of these sites. Glu A sites bind quisqualate and 2-APB with relatively high affinity. Cl- (47 mM) more than doubles the maximum density of Glu B sites, but Ca2+ appears to have no effect. Glu B sites can be discriminated from the other classes by their relatively low affinity for quisqualate and 2-APB. There is reason to think that the Glu B population is heterogeneous. The novel Glu C population can be virtually selectively labeled by exposing 2-APB-sensitive binding sites to radioligand in Tris-HOAc buffer with Ca2+. Binding of L-[3H]glutamate to these sites is enhanced by both Cl- and Ca2+, but requires neither ion. Ca2+ appears to increase both the affinity of Glu C sites for L-glutamate and their maximum binding site density. In the presence of Ca2+ and Cl-, Glu C sites bind the radioligand with micromolar affinity (KD approximately 2 microM) and high capacity (Bmax approximately 160 pmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of zinc on markers of glutamate and aspartate neurotransmission in rat hippocampus

Receptor binding and synaptosomal uptake of L-[3H]glutamate and L-[3H]aspartate were measured in hippocampus derived from rats maintained on zinc restricted diets from weaning. Despite near lethal zinc deficiency, these markers of excitatory amino acid neurotransmission were unaffected compared to zinc-supplemented controls. However, addition of zinc in vitro markedly inhibited binding of glutamate and aspartate to hippocampal membranes. These data suggest that zinc can modulate the receptor affinities for glutamate and aspartate and may function as a tonic inhibitor of excitatory synapses in vivo.

Pharmacological dissociation of memory: anisomycin, a protein synthesis inhibitor, and leupeptin, a protease inhibitor, block different learning tasks

Inhibition of protein synthesis by anisomycin for a short duration impairs memory of a one-trial inhibitory avoidance task in rats. Memory of escape conditioning involving eight trials is disrupted only if the duration of protein synthesis is prolonged by repeated injections. In marked contrast, olfactory memory of rats trained on two odor discriminations is not affected by anisomycin even if the duration of inhibition is prolonged and the number of trials is reduced to a minimum. In previous work, leupeptin, a thiol proteinase inhibitor, was shown to impair olfactory discrimination learning, but left inhibitory and avoidance conditioning intact. Together, these results provide a pharmacological double dissociation of memory, and suggest that the same chemistries, or mixtures of chemistries, may not be involved in all types of memory.

Cations differentially affect subpopulations of L-glutamate receptors in rat synaptic plasma membranes

Several cations were examined for their ability to specifically affect one of the 3 L-glutamate (L-Glu) binding sites in rat forebrain synaptic plasma membranes (i.e. Na+-dependent, Cl--dependent and Cl--independent). Na+-dependent binding was potently inhibited by K+ and NH4+ ions. Other monovalent cations tested (Cs+, Li+, triethylammonium) had no effect on this binding site. Polyvalent cations (Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Cr3+) also had little effect on the Na+-dependent L-Glu binding site. Cl--dependent L-Glu binding was potently inhibited by Na+ ions but was not affected by other monovalent ions. All of the divalent cations were potent inhibitors of both Cl--dependent and -independent binding. The results show that these binding sites of L-Glu can be distinguished by their response to cations and suggest possible novel modes of regulation in vivo.

Regional distribution of soluble calcium activated proteinase activity in neonatal and adult rat brain

Calcium dependent proteolytic activity in the soluble fraction of various rat brain regions was assayed using 14C-radiolabelled denatured casein as a substrate. Two forms of activity, distinguishable by their calcium requirement for half maximal activation (5 and 80 microM), were found; both were blocked by sulfhydryl alkylating agents and thiol proteinase inhibitors. Preincubation at 58 degrees C for 10 min also eliminated the high threshold activity. These characteristics are identical to those reported for calcium activated neutral proteinases ('calpains') found in other tissues. Calpain activity varied markedly across brain regions. The greatest values for the high threshold enzyme were found in pons-medulla followed by cerebellum/mesencephalon and finally the telencephalon. The low threshold enzyme had low levels of activity throughout the brainstem and diencephalon and was barely detectable in telencephalic structures. In contrast, a previously described endogenous inhibitor of calpain, 'calpastatin', was found not to vary in its activity across brain regions. Calpain activity was high in the prenatal brain, but while the hindbrain maintained high levels of activity into adulthood, the activity in the forebrain dropped 80% during the early postnatal period. The differences between forebrain and hindbrain activity levels were evident during the first 5 days of the neonatal period, suggesting that glial cell differentiation is not responsible for the regional variations found in the adult. These results are discussed with regard to the possibility that the turnover of anatomical structures differs between brain regions.

Regulation of glutamate receptor binding by the cytoskeletal protein fodrin

The erythrocyte cytoskeleton, which consists primarily of a meshwork of spectrin and actin, controls cell shape and the disposition of proteins within the membrane. Proteins similar to spectrin have recently been found in diverse cells and tissues, and it is possible that they mediate the capping of cell-surface receptors, although this has not been demonstrated directly. In neurones, the spectrin-like protein fodrin lines the cortical cytoplasm and may link actin filaments to the membrane. Fodrin has been hypothesized to regulate the number of receptor binding sites on neuronal membranes for the putative neurotransmitter L-glutamate. Micromolar calcium concentrations activate the thiol protease calpain I, induce fodrin degradation and more than double the density of glutamate binding sites; these effects are all blocked by thiol protease inhibitors. We have now used specific antibodies to examine further the role of fodrin proteolysis in regulating glutamate receptors. We report that fodrin antibodies block the fodrin degradation and increase in glutamate binding normally induced by calcium, and so provide direct evidence for control of membrane receptors by a non-erythroid spectrin.

Comparative actions of GABA and acetylcholine on the Xenopus laevis lateral line

The effects of GABA, acetylcholine and carbachol on the spontaneous activity of afferent nerve fibers in the lateral line of Xenopus laevis are characterized. Atropine and bicuculline were also tested on drug- and water motion-evoked activity. GABA (0.019-1.25 mM) suppressed and both acetylcholine (1.25-80 microM) and carbachol (1.25-40 microM) increased spontaneous activity. These actions were blocked by bicuculline (100 microM) and atropine (4 microM) respectively. Atropine (20 microM) and bicuculline (100 microM) had no effect on water motion-evoked activity. The results characterize actions of GABA and acetylcholine not previously described and provide evidence that does not support the hypothesis that GABA or acetylcholine are the afferent transmitter.

Ionic regulation of the binding of DL-[3H]2-amino-4-phosphonobutyrate to L-glutamate-sensitive sites on rat brain membranes

The effects of ions on the binding of the excitatory amino acid analogue DL-[3H]2-amino-4-phosphonobutyrate to L-glutamate-sensitive sites on rat brain synaptic membranes was investigated. The divalent cations manganese, magnesium, strontium, and particularly calcium, produced a marked enhancement in specific binding. However, this effect was manifest only in the presence of added chloride, or to a lesser extent, with bromide ions. Application of saturation analysis revealed that both chloride and calcium acted to increase the binding site density in a concentration-dependent manner, without affecting the dissociation constant. The only other ionic species found to have a significant effect on 2-amino-4-phosphonobutyrate binding was sodium, which produced an apparent reduction in site affinity, without modifying the binding site density. Although the significance of these striking ionic effects is as yet unknown, it seems feasible that chloride (and possibly also calcium) ions may serve a role in regulating the interaction of excitatory amino acids with their physiological receptors.

The biochemistry of memory: a new and specific hypothesis

Recent studies have uncovered a synaptic process with properties required for an intermediate step in memory storage. Calcium rapidly and irreversibly increases the number of receptors for glutamate (a probable neurotransmitter) in forebrain synaptic membranes by activating a proteinase (calpain) that degrades fodrin, a spectrin-like protein. This process provides a means through which physiological activity could produce long-lasting changes in synaptic chemistry and ultrastructure. Since the process is only poorly represented in the brain stem, it is hypothesized to be responsible for those forms of memory localized in the telencephalon.

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.

Long-lasting potentiation in hippocampus is not due to an increase in glutamate receptors

In transversely sectioned rat hippocampal slices, the effects of low (20 Hz, 600 pulses) and high (400 Hz, 200 pulses) frequency tetani of Schaffer collaterals were examined on the CA1 population spike as well as on the binding of 3H-glutamate. The population spike was suppressed while 3H-glutamate binding greatly enhanced following a low frequency tetanus. Verapamil (1 micron), which does not block long-lasting potentiation (LLP), counteracted the depression of the population spike as well as the associated increase in 3H-glutamate binding. The high frequency tetanus induced LLP of the population spike but caused no change in the amino acid binding. These results indicate that the increase in the number of glutamate receptors is not a requirement for LLP.

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

The regional distribution of 2-amino-4-phosphonobutyrate (APB)/chloride-insensitive L-[3H]glutamate binding sites in the rat central nervous system was compared with that of APB/chloride-sensitive and with sodium-dependent binding sites. The distribution of APB-sensitive and APB-insensitive sites was not correlated, but the latter was identical to that of the sodium-dependent sites. The pharmacological specificity of the APB-insensitive sites was not consistent with that of an N-methylaspartate-preferring receptor, and was also different from the specificity determined for the sodium-dependent sites. The APB-insensitive sites appear to be unrelated to any other previously described excitatory amino acid binding site.

Neurochemical, pharmacological, and developmental studies on cerebellar receptors for dicarboxylic amino acids

Specific binding of L-[3H]glutamate ([3H]Glu) and L[3H]Asp) to cerebellar membranes represented a time-, temperature-, pH- and protein-dependent interaction which was both saturable and reversible. Binding sites for both radioligands appeared maximally enriched in synaptosomal fractions isolated by gradient centrifugation. Kinetically derived dissociation constant (K off/K on = Kd) for [3H]Glu binding to this fraction indicated high-affinity (433 nM). Competition experiments employing analogs of excitatory amino acids, including new antagonists, helped identify binding sites for [3H]Glu and [3H]Asp as receptors with differential pharmacological specificities. Membrane freezing reduced numbers of both receptor types, but binding activity could be recovered partially by incubation at 37 degrees C. Glu receptors exhibited a pronounced deleterious sensitivity to thiol modifying reagents and L-Glu (50-1000 microM) provided protection against these compounds during co-incubation with cerebellar membranes. It is suggested that cold storage may induce partially reversible receptor inactivation by promoting sulfhydryl group/bond modification. Rat cerebellar glutamatergic function (endogenous Glu content, Glu uptake and receptor sites) exhibited an apparent ontogenetic peak between days 8-12 postpartum with a plateauing profile from day 30 to adulthood. The accelerated development (days 8-12) coincides with the first demonstrable Glu release and kainic acid neurotoxicity, as described previously.

A comparative study of L[3H]-glutamate and L[3H]-cysteine sulfinate binding sites in subcellular fractions of rat brain

A comparative study of the binding of L-cysteine sulfinic acid (CSA) and L-glutamic acid (GLU) to various subcellular fractions of membranes from rat brain was made. Kinetic parameters were determined in all fractions for both types of binding. The effects of membrane preincubation, freezing, and thawing were also examined. The GLU and CSA specific binding levels increased in medium-density (C) and high-density (D) synaptic membranes as compared to the crude mitochondrial/synaptosomal membranes (wP2). Freezing and thawing reduced CSA binding in all tested subcellular fractions. GLU binding is reduced in wP2, C, and D. Binding to the "light" synaptic membranes (B) was not significantly affected, suggesting the presence of two GLU sites. Kinetics of the GLU binding indicated that the temperature-sensitive and -insensitive sites have Kd of 600 nM and 1,100/nM, respectively. Preincubation of fresh membranes conversely affected CSA and GLU binding to the various subcellular fractions, increasing CSA binding in wP2, B, C and decreasing it in D suggesting the existence of distinct sites for GLU and CSA. Preincubation of previously frozen membranes similarly modified CSA and GLU binding except in B fractions. CSA and GLU binding exhibited different pH sensitivities in both fresh and frozen membranes. These results indicate that multiple acid amino acid binding sites exist in membranes and that they can be differentiated according to their sensitivity to temperature. They also suggest the existence of distinct sites for CSA and GLU in fresh membranes, giving further support to the hypothesis that CSA may also serve a neurotransmitter role in the rat central nervous system.

Intracellular injections of EGTA block induction of hippocampal long-term potentiation

Hippocampal long-term potentiation (LTP) is a remarkably stable facilitation of synaptic responses resulting from very brief trains of high-frequency stimulation. Because of its persistence and modest induction conditions, LTP represents a promising candidate for a substrate of memory. Some progress has been made in localizing the changes responsible for the effect; for example, it has been shown that LTP is not accompanied by changes in the fibre volleys of the test afferents or by generalized alterations of the dendrites of their target cells. However, it is unknown whether the potentiation is due to pre- or postsynaptic changes and there is evidence in favour of each (for example, see refs 5, 6). We now report that intracellular injections of the calcium chelator EGTA block the development of LTP. These results strongly suggest that LTP is caused by a modification of the postsynaptic neurone and that its induction depends on the level of free calcium.

Purification from synaptosomal plasma membranes of calpain I, a thiol protease activated by micromolar calcium concentrations

Synaptosomal plasma membranes (SPMs) were prepared from whole rat brain and assayed for calcium-stimulated proteolytic activity. Addition of calcium to SPMs caused a dose-dependent increase in trichloroacetic acid-soluble protein. Two peaks of protease activity directed against a casein substrate were detectable when SPMs were incubated with low-ionic-strength buffer and the extract was fractionated on DEAE-cellulose. The enzyme in peak 1 required less than 1/10 the calcium concentration for activation as the peak 2 protease (Kact1 = 35 microM; Kact2 = 500 microM). The specific thiol-protease inhibitors leupeptin and antipain and the alkylator iodoacetate blocked enzyme activity. The low-sensitivity protease was converted to a high-sensitivity enzyme (Kact = 20 microM) by substrate affinity chromatography in the presence of calcium. This protease was purified 550-fold from SPMs. The high- and low-sensitivity membrane-associated calcium-dependent proteases are part of a family of enzymes, the calpains, previously reported in cytosolic fractions of several tissues.

Freezing eliminates a specific population of L-glutamate receptors in synaptic membranes

The binding of L-[3H]glutamate (L-Glu) to freeze-thawed synaptic membranes (SPMs) exhibited saturation kinetics, with Kd 507 nM and Bmax 6.99 pmol/mg protein. The effects of ions, the susceptibility to Triton X-100 and the pharmacological properties of the binding indicated that those sites detected in freeze-thawed SPMs were only of the Cl-/Ca2+-independent type. The Cl-/Ca2+-dependent (2-amino-4-phosphonobutyrate-sensitive) L-Glu binding sites which are additionally present in fresh SPMs are abolished by freezing.

Modulation of GABA receptor binding by Ca2+

In frozen-thawed repeatedly washed rat cortical synaptic membranes, Ca2+ (1-5 mM) decreased the binding of [3H]muscimol whereas it increased the binding of [3H]gamma-aminobutyric acid (GABA). However, the binding of [3H]GABA was decreased by the same extent as the binding of [3H]muscimol when the membranes were incubated with baclofen (a selective ligand for the GABAB binding site) and Ca2+. Scatchard analysis of [3H]muscimol binding revealed that Ca2+ reduced the density of GABA binding sites without affecting the dissociation constant. Ca2+ was more potent than Ba2+, Mg2+ was ineffective, and the Ca2+ antagonist La3+ stimulated [3H]muscimol binding. The inhibition of [3H]muscimol binding by Ca2+ was not influenced by calmodulin (50 micrograms/ml), trifluoperazine (10(-5) M), verapamil (10(-6) M), quinacrine (10(-4) M), cordycepin (0.1 mM), leupeptin (20 microM), or soybean trypsin inhibitor (0.1 mg/ml). Moreover, the effect of Ca2+ was additive to that of GABA-modulin. These results indicate that Ca2+ decreases the number of GABAA binding sites while unveiling GABAB binding sites.

Irreversibility and time course of calcium stimulated [3H]glutamate binding to rat hippocampal membranes

Previous studies have indicated that calcium ions induce an irreversible increase of [3H]glutamate binding to rat hippocampal membranes. The present study was intended to provide an estimate of the time-course for this effect. The calcium chelator EGTA and the proteinase inhibitor leupeptin were introduced at various times during the incubation of hippocampal membranes with calcium and the binding of [3H]glutamate subsequently assayed. Both compounds totally blocked the calcium-induced increase in binding when added at the beginning of the incubation period but produced progressively less inhibition when added after longer delays. This method indicates that calcium produces its maximal effect between 5 and 10 min of incubation, a time-course compatible with the hypothesis that calcium stimulates a membrane-bound proteinase which results in the unmasking of [3H]glutamate binding sites.

Regulation by calcium ions of glutamate receptor binding in hippocampal slices

Hippocampal slices were incubated in a Krebs-bicarbonate buffer with various concentrations of calcium and [3H]glutamate receptor binding was measured in crude synaptic membranes derived from these slices. Increasing the calcium concentration from 0 to 2.5 mM resulted in a 2.2-fold increase in the maximal number of the Na-independent [3H]glutamate binding sites without changes in their affinity for [3H]glutamate. This effect was totally blocked by the addition of the protease inhibitor leupeptin (50 microM) to the slice incubation medium. No effect was observed on the Na-dependent [3H]glutamate binding nor on the Na-independent [ 3H ]glutamate binding measured in the presence of a concentration of calcium of 250 microM. Increasing the calcium concentration also resulted in an increased proteolytic activity which was inhibited by about 70% by the addition of leupeptin. Finally, increasing the calcium concentration induced the degradation of high-molecular weight proteins, the microtubule-associated proteins (MAPs) and the 220 000 dalton doublet protein corresponding to fodrin. Both effects were partially prevented by the addition of leupeptin in the slice incubation medium. These results indicate that the same calcium-dependent processes which were previously shown to regulate [ 3H ]glutamate receptor binding to hippocampal membranes occur in the hippocampal slice preparation, and they suggest a mechanism by which fluctuations in calcium levels can activate a calcium-dependent proteinase, the degradation of cytoskeletal-associated proteins and the unmasking of additional glutamate receptors. The participation of such processes in various forms of plasticity is discussed.

Evidence for interactions between [3H]glutamate and [3H]kainic acid binding sites in rat striatal membranes. Possible relevance for kainic acid neurotoxicity

In vitro studies have shown that kainic acid (10(-6) M) but not N-methyl-D-aspartate (NMDA) in the same concentration reduces the number of striatal [3H]glutamate binding sites and increases their affinity in striatal membranes. In vitro studies also show that L-glutamate (10(-8) M) but not NMDA (10(-6) M) increases the number of [3H]kainic acid binding sites and reduces their affinity in striatal membranes. Ibotenic acid (10(-6) M) can also reduce the affinity of [3H]kainic acid binding sites in striatal membranes. These results give indications for the existence of bidirectional receptor-receptor interactions between two receptors for excitatory amino acids in local striatal circuits. These interactions could partly explain the involvement of glutamate in kainate neurotoxicity.

Effects of cations and temperature on the binding of [3H]spiperone to sheep caudate nucleus

The specific [3H]spiperone binding by sheep caudate nucleus homogenate is increased by divalent cations. The effect of Ca2+ or Mn2+ (5 mM) is temperature-dependent, and it is optimal at about 37 degrees, but is relatively low below 15 degrees and above 50 degrees. In the absence of added Ca2+ or Mn2+, the maximal specific [3H]spiperone binding is observed at about 25 degrees, and the cations shift the optimum to about 37 degrees. Under the experimental conditions used, the KD is about 0.6 nM and is not influenced by Ca2+ or Mn2+, or by temperature (25 and 37 degrees). In addition to Ca2+ and Mn2+, Mg2+ and Zn2+ also increase the specific [3H]spiperone binding, but to a smaller extent. At the concentrations of Ca2+, Mn2+, Mg2+ and Zn2+ which produce a maximal increase in the [3H]spiperone binding, the membranes are nearly saturated with the cations which bind about 100 nmoles of Ca2+ or Mg2+/mg of protein, 170 nmoles Zn2+/mg of protein and at least 300 nmoles Mn2+/mg of protein. It is suggested that the cations increase the [3H]spiperone binding by either exposing more binding sites, by preventing denaturation or by increasing the solubility of [3H]spiperone in the membrane phase, or by a combination of these processes.

Effects of excitatory amino acids, and of their agonists and antagonists on the release of neurotransmitters from the chick retina

Effects of glutamic, aspartic, and cysteic acid, and of kainic acid and N-methyl aspartate on the release of labeled GABA, glycine, and taurine were examined in isolated, perfused chick retina. Glutamic acid (0.5-2 mM), increased the release of 3H-GABA by more than four times and that of 14C-glycine by about two times. The release of GABA decreased 50% and that of glycine 95% in the presence of the antagonist of glutamic acid receptors, glutamate diethyl ester (300 microM). N-methyl aspartate, used as an agonist of aspartic acid receptors, preferentially increased the release of GABA (seven times) over that of glycine (three times). The stimulatory effect of N-methyl aspartate was antagonized by D-alpha-aminoadipate and by Mg. Kainic acid (10 microM) induced the release of glycine but not that of GABA. Cysteic acid failed to modify the release of any of the amino acids examined. The efflux of labeled taurine was practically unaffected by all the compounds utilized. The release of GABA by the excitatory amino acids and agonists was Ca-independent but Na-dependent, whereas the release of glycine was markedly Ca-dependent. The evidence presented here suggests that experimental conditions activating receptors of excitatory amino acids differently affect the release of inhibitory amino acids.

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)

Modulation of long-term potentiation: effects of adrenergic and neuroleptic drugs

A variety of drugs which either mimic or antagonize the effects of norepinephrine and dopamine were tested for their ability to modulate long-term potentiation (LTP) in the rat hippocampus in vitro. Neither administration of norepinephrine, amphetamine or adrenergic antagonists, nor pretreatment with reserpine or DSP4 (which selectively destroys noradrenergic afferents to the hippocampus) had any significant effect on the magnitude of LTP. Isoproterenol, a beta-adrenergic receptor agonist, was able to partially block LTP, but this did not appear to be due to a direct action of isoproterenol on LTP. Neuroleptic drugs such as trifluoperazine were able to block LTP almost completely; however, this action was not stereospecific. Other dopamine antagonists such as sulpiride had no effect on LTP. The ability of neuroleptics to antagonize LTP was more closely related to their ability to block calmodulin than to their relative potencies as dopamine antagonists. It would appear that neither norepinephrine nor adrenergic antagonists influence the amount of LTP elicited by repetitive stimulation; however, drugs which have been shown to interfere with calmodulin-mediated cellular processes do antagonize this phenomenon.

The effect of Ca2+ on the pharmacological specificity of [3H]glutamate binding sites in rat hippocampus

The inhibition of [3H]glutamate binding to rat hippocampal membranes by various compounds was measured. L-Glutamic, L-2-amino-4-phosphonobutyric and 4-fluoroglutamic acids had similar IC50 values (2.12 microM, 4.98 microM and 3.90 microM respectively). D-Glutamate and D-2-amino-4-phosphonobutyrate were less potent than L-glutamate (IC50S 18.9 microM and 46.5 microM respectively) and L-glutamate diethylester (IC50 519 microM) was the weakest inhibitor tested. Addition of 10 mM CaCl2 caused a 2.27-fold increase in displaceable binding but did not alter the IC50 values of any of the inhibitors tested.

Effects of high-frequency synaptic stimulation on glumate receptor binding studied with a modified in vitro hippocampal slice preparation

Slices of the field CA1 minus the stratum moleculare were prepared from the rat hippocampus and maintained in an in vitro recording chamber. The physiological properties of these "minislices" were similar to those reported for non-dissected hippocampal slices. Slices receiving various patterns of electrical stimulation through multiple electrodes were subsequently homogenized and crude membrane fractions prepared; the binding of [3H]glutamate was measured by a rapid filtration assay. Binding in membranes prepared from control slices exhibited kinetic properties and sensitivities to pharmacological and ionic manipulations which were comparable to those found in previous studies using conventional fractionation and assay techniques. Brief bursts of high-frequency stimulation increased [3H]glutamate binding compared to non-stimulated controls in 3 separate experiments. Stimulation at low frequency or at high frequency in low calcium medium did not produce this effect. In addition to suggesting that glutamate binding sites are regulated by patterns of afferent activity, these findings indicate that the minislice preparation should be of general utility in relating synaptic physiology to synaptic chemistry.

Chloride ions enhance L-glutamate binding to rat brain synaptic membranes

Chloride ions increased L-glutamate (L-Glu) binding to synaptic membranes. The binding was saturable and resulted in a 2.5-fold enhancement at concentrations of 20--40 mM chloride. Sodium and potassium ions inhibited only chloride stimulated L-Glu binding. Calcium ions also increased L-Glu binding but this was observed only in the presence of chloride. The anion selectivity of the enhancement of L-Glu binding was similar to that reported for the membrane chloride channel, suggesting that some L-Glu binding sites may be associated with this channel.

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.

Properties of miniature excitatory junctional currents at the locust nerve-muscle junction

1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na(+) (i.e. in normal medium, Ca(2+)-free medium and Cl(-)-free medium). M.e.j.c.s were also examined in isotonic Ca(2+) where the inward post-synaptic current was carried mainly by Ca(2+).2. In normal medium, mean m.e.j.c. amplitude = 2.34+/-0.05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) was tau(m.e.j.c.) = 2.62+/-0.11 msec (V(m) = -80 mV, T = 22 degrees C). Decay-time was not markedly changed in Ca(2+)-free or Cl(-)-free medium. tau(m.e.j.c.) approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. tau(m.e.j.c.) decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182.+/-24.8 mV change in V(m).5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca(2+)-free and Cl(-)-free solutions.6. The kinetics of junctional channels are altered in isotonic Ca(2+). M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j.c. = 0.74+/-0.03 nA. The decay time becomes markedly briefer, tau(m.e.j.c.) = 1.01+/-0.08 msec, indicating a reduction in mean channel life-time (V(m) = -80 mV, T = 22 degrees C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca(2+), thus producing a prolongation in mean tau(m.e.j.c.). This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of tau(m.e.j.c.) of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca(2+). The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

Selective potentiation of retinal horizontal cell responses to L-glutamate by D-aspartate

1. L-Glutamate and L-aspartate depolarize type H1 horizontal cells in the isolated retina of goldfish, but only at millimolar concentrations. 2. When applied in the presence of D-aspartate, L-glutamate depolarizes H1 cells at concentrations nearly 15-fold lower than when it is applied alone. The effects of L-aspartate were not potentiated by either D-aspartate or D-glutamate. 3. Since D-aspartate seems also to enhance the effect of the transmitter released by cone photoreceptors, these results are consistent with the possibility that L-glutamate is a neurotransmitter of cones.

The role of calcium ion in the L-glutamate-induced depolarization in the frog spinal cord

1. The role of Ca2+ in L-glutamate-induced depolarization was investigated in the isolated frog spinal cord. 2. The size of a depolarization induced by L-glutamate (3 mM) was inversely related to the extracellular Ca2+ concentration, but was reduced in a Ca2+-free medium containing EGTA (0.3 mM). 3. L-Glutamate caused a marked depolarization in both ventral and dorsal roots, even in a NaCl-deficient medium (Ca2+, 2.0 mM). The size of the depolarization was attenuated by a prolonged or repeated application of L-glutamate. Ca2+ can be replaced by Sr2+ or Mg2+. 4. Concanavalin A (1 microM) prevents the development of desensitization to L-glutamate. 5. Present results suggest that Ca2+ plays the role of a charge carrier for L-glutamate-induced depolarization and of a regulator of modulator for L-glutamate-receptor sensitivity. The roles are exaggerated in NaCl-free medium.

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.

Glutamate-stimulated phosphorylation of a specific protein in P2 fractions of rat cerebral cortex

Incubation of P2 fractions from rat cerebral cortex with 32Pi in the presence of L-glutamate caused an increased phosphorylation of a protein with apparent molecular weight of 43,000 (P43) as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. This glutamate-stimulated phosphorylation of P43 was already detectable 10 s after the addition of glutamate and was dependent on the concentrations of glutamate in the incubation medium. Other excitatory amino acids such as D-glutamate, L-aspartate, D,L-cysteic acid, L-cysteinesulfinic acid, and D,L-alpha-aminoadipic acid did not stimulate the phosphorylation of P43. In contrast, alpha-ketoglutarate and succinate stimulated the phosphorylation of this protein. Glutamate-stimulated phosphorylation of P43 seemed not to be mediated by either cAMP or cGMP and was inhibited by the presence of Ca2+ in the incubation medium. Experiments performed with metabolic inhibitors indicated that glutamate-stimulated protein phosphorylation is localized in mitochondria. This conclusion is supported by the occurrence of glutamate-stimulated phosphorylation of P43 in mitochondrial fractions from several peripheral tissues. The present results are consistent with the hypothesis that P43 is a component of the pyruvate dehydrogenase complex of mitochondria.