Regulation of glutamate receptors by cations

Revisiting the calpain hypothesis of learning and memory 40 years later

In 1984, Gary Lynch and Michel Baudry published in Science a novel biochemical hypothesis for learning and memory, in which they postulated that the calcium-dependent protease, calpain, played a critical role in regulating synaptic properties and the distribution of glutamate receptors, thereby participating in memory formation in hippocampus. Over the following 40 years, much work has been done to refine this hypothesis and to provide convincing arguments supporting what was viewed at the time as a simplistic view of synaptic biochemistry. We have now demonstrated that the two major calpain isoforms in the brain, calpain-1 and calpain-2, execute opposite functions in both synaptic plasticity/learning and memory and in neuroprotection/neurodegeneration. Thus, calpain-1 activation is required for triggering long-term potentiation (LTP) of synaptic transmission and learning of episodic memory, while calpain-2 activation limits the magnitude of LTP and the extent of learning. On the other hand, calpain-1 is neuroprotective while calpain-2 is neurodegenerative, and its prolonged activation following various types of brain insults leads to neurodegeneration. The signaling pathways responsible for these functions have been identified and involve local protein synthesis, cytoskeletal regulation, and regulation of glutamate receptors. Human families with mutations in calpain-1 have been reported to have impairment in motor and cognitive functions. Selective calpain-2 inhibitors have been synthesized and clinical studies to test their potential use to treat disorders associated with acute neuronal damage, such as traumatic brain injury, are being planned. This review will illustrate the long and difficult journey to validate a bold hypothesis.

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

GDH-Dependent Glutamate Oxidation in the Brain Dictates Peripheral Energy Substrate Distribution

Glucose, the main energy substrate used in the CNS, is continuously supplied by the periphery. Glutamate, the major excitatory neurotransmitter, is foreseen as a complementary energy contributor in the brain. In particular, astrocytes actively take up glutamate and may use it through oxidative glutamate dehydrogenase (GDH) activity. Here, we investigated the significance of glutamate as energy substrate for the brain. Upon glutamate exposure, astrocytes generated ATP in a GDH-dependent way. The observed lack of glutamate oxidation in brain-specific GDH null CnsGlud1(-/-) mice resulted in a central energy-deprivation state with increased ADP/ATP ratios and phospho-AMPK in the hypothalamus. This induced changes in the autonomous nervous system balance, with increased sympathetic activity promoting hepatic glucose production and mobilization of substrates reshaping peripheral energy stores. Our data reveal the importance of glutamate as necessary energy substrate for the brain and the role of central GDH in the regulation of whole-body energy homeostasis.

NMDA receptor activation and calpain contribute to disruption of dendritic spines by the stress neuropeptide CRH

The complex effects of stress on learning and memory are mediated, in part, by stress-induced changes in the composition and structure of excitatory synapses. In the hippocampus, the effects of stress involve several factors including glucocorticoids and the stress-released neuropeptide corticotropin-releasing hormone (CRH), which influence the integrity of dendritic spines and the structure and function of the excitatory synapses they carry. CRH, at nanomolar, presumed-stress levels, rapidly abolishes short-term synaptic plasticity and destroys dendritic spines, yet the mechanisms for these effects are not fully understood. Here we tested the hypothesis that glutamate receptor-mediated processes, which shape synaptic structure and function, are engaged by CRH and contribute to spine destabilization. In cultured rat hippocampal neurons, CRH application reduced dendritic spine density in a time- and dose-dependent manner, and this action depended on the CRH receptor type 1. CRH-mediated spine loss required network activity and the activation of NMDA, but not of AMPA receptors; indeed GluR1-containing dendritic spines were resistant to CRH. Downstream of NMDA receptors, the calcium-dependent enzyme, calpain, was recruited, resulting in the breakdown of spine actin-interacting proteins including spectrin. Pharmacological approaches demonstrated that calpain recruitment contributed critically to CRH-induced spine loss. In conclusion, the stress hormone CRH co-opts mechanisms that contribute to the plasticity and integrity of excitatory synapses, leading to selective loss of dendritic spines. This spine loss might function as an adaptive mechanism preventing the consequences of adverse memories associated with severe stress.

The biochemistry of memory: The 26year journey of a 'new and specific hypothesis'

This Special Issue of Neurobiology of Learning and Memory dedicated to Dr. Richard Thompson to celebrate his 80th birthday and his numerous contributions to the field of learning and memory gave us the opportunity to revisit the hypothesis we proposed more than 25years ago regarding the biochemistry of learning and memory. This review summarizes our early 1980s hypothesis and then describes how it was tested and modified over the years following its introduction. We then discuss the current status of the hypothesis and provide some examples of how it has led to unexpected insights into the memory problems that accompany a broad range of neuropsychiatric disorders.

Tissue specificity of mitochondrial glutamate pathways and the control of metabolic homeostasis

Glutamate is implicated in numerous metabolic and signalling functions that vary according to specific tissues. Glutamate metabolism is tightly controlled by activities of mitochondrial enzymes and transmembrane carriers, in particular glutamate dehydrogenase and mitochondrial glutamate carriers that have been identified in recent years. It is remarkable that, although glutamate-specific enzymes and transporters share similar properties in most tissues, their regulation varies greatly according to particular organs in order to achieve tissue specific functions. This is illustrated in this review when comparing glutamate handling in liver, brain, and pancreatic beta-cells. We describe the main cellular glutamate pathways and their specific functions in different tissues, ultimately contributing to the control of metabolic homeostasis at the organism level.

Effects of Ca(II) ions on Mn(II) dynamics in chick glia and rat astrocytes: potential regulation of glutamine synthetase

Previous studies have demonstrated that in glia and astrocytes Mn(II) is distributed with ca. 30-40% in the cytoplasm, 60-70% in mitochondria. Ca(II) ions were observed to alter both the flux rates and distribution of Mn(II) ions in primary cultures of chick glia and rat astrocytes. External (influxing) Ca(II) ions had the greatest effect on Mn(II) uptake and efflux, compared to internal (effluxing) or internal-external equilibrated Ca(II) ions. External (influxing) Ca(II) ions inhibited the net rate and extent of Mn(II) uptake but enhanced Mn(II) efflux from mitochondria. These observations differ from Ca(II)-Mn(II) effects previously reported with "brain" (neuronal) mitochondria. Overall, increased cytoplasmic Ca(II) acts to block Mn(II) uptake and enhance Mn(II) release by mitochondria, which serve to increase the cytoplasmic concentration of free Mn(II). A hypothesis is presented involving external L-glutamate acting through membrane receptors to mobilize cell Ca(II), which in turn causes mitochondrial Mn(II) to be released. Because the concentration of free cytoplasmic Mn(II) is poised near the Kd for Mn(II) with glutamine synthetase, a slight increase in cytoplasmic Mn(II) will directly enhance the activity of glutamine synthetase, which catalyzes removal of neurotoxic glutamate and ammonia.

gamma-Aminobutyric acidA receptor regulation by a chloride-dependent kinase and a sodium-dependent phosphatase

gamma-Aminobutyric acidA (GABAA) receptors are linked to ion channels which mediate many aspects of neural inhibition. Although the effects of phosphorylation on GABAA receptor function have been widely studied, the actual role of phosphorylation in the regulation of these receptors still remains controversial. In recent reports, we have described the effects of phosphorylating/dephosphorylating enzymes on the regulation of GABAA receptors in a rat cortical slice preparation (Shaw et al., Mol. Neuropharmacol., 2 (1992) 297-302; Shaw and Lanius, Dev. Brain Res., 70 (1992) 153-161; Pasqualotto et al., Neuroreport, 4 (1993) 447-450) and predicted that ionic co-factors are involved in mediating the regulation of GABAA receptors by kinases and phosphatases. In the present report, the effects of chloride, sodium, potassium, and calcium were examined alone and in the presence of cAMP-dependent protein kinase (protein kinase A) or alkaline phosphatase. The results showed a decrease in [3H]SR 95531 (GABAA receptor antagonist) binding after incubation with chloride alone; this decrease was further enhanced in the presence of protein kinase A. Both effects could be blocked by a protein kinase A inhibitor. Conversely, an increase in [3H]SR 95531 binding was observed after incubation with sodium alone; this increase was further enhanced in the presence of alkaline phosphatase. In both cases these increases in binding could be blocked by sodium orthovanadate, a phosphatase inhibitor. Potassium was ineffective under all conditions; calcium showed enzyme-independent effects at low concentrations only. These results suggest the existence of a novel chloride-dependent protein kinase which may have significant sequence homology to protein kinase A, and a novel sodium-dependent phosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible kinase and phosphatase regulation of brain amino acid receptors in postnatal development

In a previous study [Shaw, C., Pasqualotto, B. and Lanius, R.A., Mol. Neuropharmacol., in press] we have shown that phosphorylation and dephosphorylation actions of protein kinase and alkaline phosphatase lead to decreases or increases in the number of GABAA and AMPA receptors in adult rat neocortex. Using the same in vitro cortical slice preparation, we have now examined the role of these enzymes in regulating GABAA and AMPA receptors at different stages of postnatal development. GABAA receptors were labelled with [3H]SR95531 [Shaw, C. and Scarth, B.A., Mol. Brain Res., 11 (1991) 273-282]; AMPA receptors were labelled with [3H]CNQX [Lanius, R.A. and Shaw, C., Mol. Brain Res., 15 (1992) 256-262]. At postnatal day 14, GABAA receptors showed a decrease in binding in response to alkaline phosphatase treatment as opposed to an increase in binding observed in response to protein kinase treatment. Similar effects were observed for AMPA receptors at 20 days of age. The direction of regulation following the enzyme treatments were opposite to those observed in the adult cortex for both receptor populations. These fundamental changes in the enzymatic nature of regulation for such key inhibitory and excitatory receptor populations in cortex may signal an important role for age-dependent kinases and phosphatases in the events leading to modifications in neuronal function during postnatal development.

The effect of hypoxia on neurotransmitter phenotype of forebrain cholinergic neurons

The effect of hypoxia on the neurotransmitter phenotype of rat forebrain cholinergic neurons was analyzed using a dissociated fetal rat culture system. The aims of this study were to examine the feasibility of using choline acetyltransferase (ChAT) activity as a measure of cell injury and/or recovery, to measure the time course of hypoxic effects on ChAT activity, to determine how changes in ChAT activity at 48 h post-injury relate to microscopic changes and LDH release into the medium during that time, and finally to explore the possible mechanisms of hypoxic injury in this model. At exposure to 0.5-1.5% O2 there was a time-dependent decrease in ChAT activity when cells were harvested 48 h after exposure. Forty-eight hours after 8-9 h hypoxic exposure ChAT activity was 50-60% that of controls without any alteration in morphology of neurons. An 8 h exposure to hypoxic conditions caused a post-exposure time-dependent decrease in ChAT activity to 20% of control level at 72 h. Thereafter there was spontaneous recovery of phenotype to 60% of control which remained stable between 5 and 7 days post-exposure. Loss of neurotransmitter phenotype was not well correlated with other measures of cytotoxicity including morphological changes and LDH release. The loss of phenotype observed with hypoxia was mimicked by glutamate and kainate but not by NMDA. Consistent with these observations, neither APV nor AP3 significantly altered the effect of hypoxia on forebrain cholinergic neurons, while the addition of APV and CNQX in combination protected the phenotype of these neurons only if there was 50% or less loss of phenotype following hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Taste of glutamate salts in young and elderly subjects: role of inosine 5'-monophosphate and ions

Taste sensitivity to five glutamate salts (sodium glutamate, potassium glutamate, ammonium glutamate, calcium diglutamate, and magnesium diglutamate) were determined in sixteen young (mean age 25.58 years) and eighteen elderly (mean age 86.89 years) subjects. The effect of inosine 5'-monophosphate (IMP) and ions on taste perception of glutamate compounds was also investigated. The detection thresholds for glutamate salts were 5.04 times higher in elderly subjects than in young subjects; the recognition thresholds were 3.84 times higher. For young subjects, 0.1 mM IMP lowered detection and recognition thresholds for all 5 salts. A stronger concentration of IMP (1 mM) had this effect in both young and elderly groups. Elderly subjects perceived suprathreshold concentrations as less intense than young subjects. Chloride and acetate salts of sodium, potassium, and calcium reduced the detection and recognition thresholds of L-glutamic acid but had no effect sodium glutamate thresholds.

Effects of hypomagnesia on transmitter actions in neocortical slices

1. The effects of hypomagnesia on the neuronal responses induced by iontophorectically applied acetylcholine, glutamate, N-methylaspartate (NMDA) and gamma-aminobutyric acid (GABA) were investigated using intracellular recording techniques in in vitro slices of sensorimotor cortex (guinea-pigs). 2. Perfusion with Mg-free media, with or without tetrodotoxin (TTX), induced a small hyperpolarization (approximately 4 mV) and a small decrease (approximately 10%) in the input resistance of neurones. During TTX-blockade of Na-spike genesis, spontaneous depolarizing waves of low frequencies were observed in neurones of slices under Mg-free conditions. 3. The effects of acetylcholine and to a lesser extent, GABA actions, were depressed in a dose-dependent, reversible manner by decreases in the [Mg2+] of the perfusing media. In neurones of slices that had been incubated in Mg-free artificial cerebrospinal fluid to ensure a maximal depletion, the responses to these transmitters were potentiated by each sequentially administered increase in extracellular [Mg2+]. The actions of NMDA were potentiated during perfusion of Mg-free media. However, the responses to glutamate, which may activate receptors for NMDA, were either depressed or unchanged under these conditions. 4. A regulatory role for external Mg cations in the responses of neocortical neurones to the transmitter substances, acetylcholine and GABA, can be inferred from these investigations which simulate hypomagnesemia. The dose-dependent depression of GABA actions by low extracellular [Mg2+] additionally provides a plausible mechanism that may contribute to the neuronal hyperexcitability that is observed during conditions of hypomagnesemia.

Differential modulation by cations of sigma and phencyclidine binding sites in rat brain

The present investigation attempted to differentiate haloperidol-sensitive sigma sites (sigma H) from phencyclidine (PCP) binding sites in rat brain membranes. We studied the effects of several cations at physiologically relevant concentrations on the binding of radioligands selective for sigma H sites ([3H]haloperidol, [3H](+)3-PPP**), and [3H](+)SKF10,047), or for PCP sites ([3H]PCP and [3H]TCP). The PCP sites displayed a markedly greater sensitivity to cations than sigma H sites. This property was reflected by a greater extent of inhibition of the binding of PCP-selective relative to sigma H-selective ligands at a given cation concentration, as well as by lower IC50's and by steeper slopes of the cation dose-response curves. Divalent cations were approximately 100 times more potent than monovalent cations. All cations were inhibitory, except Sr2+ and Ba2+ which, at micromolar concentrations, enhanced PCP binding but not sigma H binding. Thus, PCP-selective sites appeared to be distinct from sigma H sites with regards to several aspects of cation modulation. This is consistent with the view that PCP and sigma H sites are distinct molecular entities. Further, the marked cation sensitivity of the PCP site is consistent with the current hypothesis according to which the PCP site is linked to the N-methyl-D-aspartate (NMDA) receptor-cation channel complex.

The calpains

In recent years interest has increased concerning the characterization of the structural-functional properties and the identification of the physiological role of non-lysosomal intracellular proteinases. Among these, calpain, a calcium-dependent cysteine proteinase ubiquitously present in a variety of tissues and cells, has been most extensively investigated in terms of activation, regulatory mechanisms, specificity and biological function. This review discusses each of these points on the basis of the most recent results concerning the general characteristics of calpain activity, and its preferential site of action within the cell as related to the specific functions of the proteinase in different cell types. As with other proteinases, calpain has to be under a continuous spatial and temporal control, and the structural and functional properties of the natural calpain inhibitor, calpastatin, must also be considered. The calpain-calpastatin system is the functional proteolytic unit that governs the activity of this intracellular proteolytic system, which is tightly correlated to the control of calcium homeostasis and thereby to the biological process of transmembrane signalling.

L-[35S]cysteic acid selectively detects chloride-dependent L-glutamate transporters in synaptic membrane

Na+-independent L-[35S]cysteic acid (CA) accumulation in rat cortical synaptic membrane was examined. In the absence of Cl-, the accumulation was not observed. Addition of Cl- revealed the accumulation in a dose-dependent manner. Br- and NO3- also did. Ca2+-enhanced the Cl- -dependent accumulation, whereas low concentrations of Na+ reduced it. L-[35S]CA accumulation was inhibited by quisqualate, L-glutamate (L-Glu), L-cysteine sulfinate, D,L-homocysteic acid and D,L-2-amino-4-phosphonobutyrate (D,L-APB) potently. L-CA inhibited L-[3H]Glu accumulation in synaptic membrane in the presence of Cl-. The maximal inhibition of L-CA was equal to that of D,L-APB, but L-CA did not inhibit L-[3H]Glu accumulation in the absence of Cl-. These results show that L-[35S]CA selectively detects the Cl- -dependent L-Glu transporters in synaptic membrane.

Release of glutamate and of free fatty acids in vasogenic brain edema

The pathophysiological potential of mediator substances in manifestations of secondary brain damage is attracting increased attention. This is particularly true of the excitatory transmitters glutamate and arachidonic acid. Noxious properties of these compounds in central nervous tissue have been demonstrated. The current study was performed to determine whether glutamate and arachidonate are released in brain tissue secondary to focal trauma. For this purpose, a cold injury of exposed cerebral cortex was induced in cats. Marked accumulation of glutamate was observed in interstitially drained edema fluid, reaching 10 to 15 times the level that was assessed in normal cerebrospinal fluid (CSF) prior to trauma. The extracellular release of glutamate was further dramatically enhanced by a critical decrease of the cerebral perfusion pressure due to a malignant increase of intracranial pressure. Under these conditions, glutamate concentrations 1000 to 1500 times normal levels accumulated in vasogenic edema fluid, demonstrating a relationship between the extent of the release of glutamate in damaged brain and the severity of the insult. Although under normal conditions glutamate concentrations in plasma were considerably higher than in the interstitial fluid, the pronounced increase of glutamate in this compartment due to trauma cannot be explained by transport of the compound together with the plasma-like edema from the intravascular space. Corresponding findings were obtained for free fatty acid concentrations in edema fluid. Almost all fatty acids that were studied had a significantly higher concentration in edema fluid than in normal CSF obtained as a control prior to trauma. However, contrary to the findings for glutamate, fatty acid concentrations in edema fluid were lower than in plasma. Accumulation of fatty acids in vasogenic edema fluid might, therefore, have resulted from uptake of the material together with edema fluid through the breached blood-brain barrier. Arachidonic acid was an exception. Its concentrations were significantly higher in edema fluid than in plasma, suggesting that it was released from cerebral parenchyma as the underlying mechanism of its extracellular accumulation. The current observations provide further support for a mediator function of glutamate and arachidonic acid in acute traumatic lesions of the brain. Quantitative assessment of the release of highly active mediator substances in brain tissue may facilitate analysis of the therapeutic efficiency of specific treatment aimed at interfering with the release or pathological function of mediators of secondary brain damage.

L-[3H]glutamate binding to a membrane preparation from crayfish muscle

The binding of L-[3H]glutamate to an isolated membrane preparation from crayfish tail muscle has been studied. The muscle homogenate was osmotically shocked, frozen and thawed, and thoroughly washed before incubation with L-[3H]glutamate. The preparation showed high specific binding of L-glutamate with a KD of 0.12 microM and Bmax of 4.7 pmol/mg protein measured in Tris/HCl pH 7.3 and at 4 degrees C. Nonspecific binding was 5-10% of total binding. The glutamate binding was highly stereospecific [K0.5 (D-glutamate), 270 microM] and showed a high degree of discrimination between L-glutamate and L-aspartate [K0.5 (L-aspartate), 54 microM]. In mammalian CNS preparations potent agonists of L-glutamate such as kainate and N-methyl-D-aspartate had no effect at 1 mM, and quisqualate was a weak inhibitor of L-glutamate binding [K0.5 (quisqualate), 162 microM]. Ibotenate was the most potent inhibitor [K0.5 (ibotenate), 0.27 microM], and various esters of L-glutamate were of intermediate potency as displacers of L-[3H]glutamate binding (K0.5 values from 6 to 60 microM). The glutamate binding site from crayfish muscle is clearly different from any of the subclasses of glutamate receptors in mammalian CNS. A possible physiological function of the binding site is a postsynaptic receptor for glutamate, either an extra-junctional or a junctional receptor.

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

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

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

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

Chronic administration of a thiol-proteinase inhibitor blocks long-term potentiation of synaptic responses

It has been proposed that activation of a calcium-sensitive protease (calpain) is a crucial step in the induction of long-term potentiation (LTP). To test this hypothesis, we used chronic recording techniques to measure the effects of intraventricular infusion of leupeptin, a calpain inhibitor, on LTP in the hippocampus. Rats implanted bilaterally with stimulating electrodes in the Schaffer-commissural system and one recording electrode in the apical dendrites of field CA1 were fitted with osmotic mini-pumps delivering either leupeptin (20 mg/ml) or saline at a rate of 0.5 microliter/h into the lateral ventricle. Short bursts of high-frequency stimulation with the bursts delivered at 5/s were used to induce LTP in those animals which had stable responses for several days. Rats in the saline group (n = 11) exhibited an immediate LTP effect that remained in place over successive days of testing, while only 3 of 13 leupeptin treated animals showed evidence of LTP 24 h after high-frequency stimulation, and in only one of those was a sizeable effect recorded over several days. The average change in responses at the 24-h test point was +33% for the controls and +4% for the leupeptin group (P less than 0.01). The block of LTP induction was reversible, since high-frequency stimulation applied after disconnecting the pumps led to a robust LTP effect that lasted for several days in 6 of 7 animals tested. There were no detectable differences in baseline responses in the presence and absence of leupeptin.

The ultrastructural localization of calcium-activated protease "calpain" in rat brain

Calpain I, a calcium-activated neutral protease which degrades a number of cytoskeletal proteins, has been implicated in the rapid turnover of structural proteins that may participate in synaptic plasticity. In the present study, an antibody raised against purified erythrocyte calpain I was biochemically characterized and demonstrated to specifically bind the Mr = 80,000 subunit of both rat erythrocyte and brain calpain I. This antibody was used to examine the cellular distribution of calpain I at the electron microscopic level in rat brain and spinal cord using the avidin-biotin immunocytochemical technique. Reaction product was observed throughout neuronal perikarya, within both axonal and dendritic processes, and within spine heads and necks. Postsynaptic densities in both shaft and spine synapses were also immunoreactive. Glial cell bodies and processes were densely stained. In both neurons and glia, the reaction product was deposited along cytoskeletal elements. The localization of calpain I immunoreactivity to glial processes suggests this degradative enzyme may play a role in the glial hypertrophy and process retraction seen in brain. The presence of the enzyme in spines and postsynaptic densities is consistent with the hypothesis that it is involved in the turnover of synaptic cytoskeleton, thus providing a means through which transient physiological events effect lasting changes in the chemistry and morphology of spines.

L-arginine methylester reduces Ca2+/Cl(-)-dependent L-[3H]glutamate binding and Ca2+-activated neutral protease activity in rat hippocampal membranes

Specific binding of L-[3H]glutamate was measured in Tris-HCl buffer in rat hippocampal membranes. In these experimental conditions 1 mM CaCl2 induced an increase in binding due to an increase in Bmax. L-Arginine methylester did not modify the Cl(-)-dependent binding of L-[3H]glutamate, but it decreased Ca2+/Cl(-)-stimulated binding in a dose-dependent manner, decreasing Bmax without changing KD. L-Arginine methylester reduced calcium-activated neutral protease activity in a dose-dependent manner. Serine protease inhibitors (aprotinin and di-isopropylfluorophosphate) did not affect L-[3H]glutamate binding, whereas leupeptin reduced it in a dose-dependent manner. L-Arginine did not mimic the effect of L-arginine methylester in either model.

Calcium dependent aspects of synaptic plasticity, excitatory amino acid neurotransmission, brain aging and schizophrenia: a unifying hypothesis

(1) The functional and structural reorganization of dendritic spines by calcium activated proteases is postulated to play a causal role in the production of the phenomenology of brain aging and in particular in the development of pathology and degeneration. Excitatory neurotransmission appears to be essential for the development of irreversible synaptic changes. (2) One of the genes modified in schizophrenia is postulated to be directly or indirectly linked to the control of excitatory neurotransmission; possibly the normal switching on of the expression of the adult form of the NMDA receptor is altered, resulting in an inappropriate functioning of this receptor. This genetic characteristic might explain the apparent resistance of schizophrenic brains to aging.

Brain spectrin, calpain and long-term changes in synaptic efficacy

This chapter discusses the possibility that proteolytic digestion of cytoskeletal proteins, in particular spectrin, is part of the mechanisms through which physiological activity elicits structural and chemical changes in brain synapses. Recent work from several laboratories has produced a description of the initial events that trigger the long-term potentiation (LTP) of synaptic responses that appears in hippocampus after brief episodes of high frequency electrical stimulation. A likely sequence is as follows: suppression of IPSPs, prolongation of EPSPs, activation of N-methyl-D-aspartate (NMDA) receptors, influx of calcium into target cells. After briefly describing the evidence for this triggering sequence, the review takes up the question of what types of calcium sensitive chemistries are available to synaptic region that could produce functional changes lasting for weeks (i.e., for LTP). It is argued that the partial degradation of spectrin by a calcium-activated protease (calpain) provides a mechanism of this type. Spectrin is a substrate for calpain and both it and a breakdown product comparable to that produced by calpain are found in postsynaptic densities. Moreover, there is substantial evidence that spectrin regulates the surface chemistry and morphology of cells and thus its partial degradation would be expected to produce pronounced and persistent modifications in synapses. To reinforce this point, the review discusses recent findings suggesting that calpain mediated proteolysis of spectrin and other cytoskeletal proteins produces substantial changes in the shape of blood-borne cells and the distribution of their surface receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of [3H]glutamate binding by N-methyl-D-aspartic acid in rat adrenal

Some neurochemical characteristics of [3H]L-glutamic acid binding sites were studied using membranous homogenate preparations obtained from the rat adrenal. It was found that the binding was inhibited by the addition (10(-7)-10(-3) M) of L-isomers of structure-related compounds in a concentration-dependent manner. A significant inhibition of the binding was induced by L-glutamic acid diethylester, but not by alpha-aminoadipic acid. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 0.19 +/- 0.05 microM and a Bmax of 4.11 +/- 0.71 pmol/mg protein, respectively. In vitro addition of sodium acetate (1-100 mM) elicited a stimulatory action on the binding at 2 degrees C, while inducing a significant attenuation of the binding at 30 degrees C. The binding reached a plateau within 30 min of incubation followed by a gradual decline up to 60 min in the presence of 100 mM sodium acetate at 30 degrees C, whereas the binding continued to increase up to 60 min in the absence of sodium acetate. Addition (0.1-10 mM) of N-methyl-D-aspartic acid (NMDA), one of the agonists for central excitatory amino acid neurotransmitter receptors, exerted a significant augmentation of the adrenal binding independently of the incubation temperature in a concentration-dependent manner. The latter facilitation of the binding, however, was not affected by the classical antagonists for central NMDA receptors such as 2-amino-5-phosphonovaleric acid and 2-amino-7-phosphonoheptanoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Are Ca2+-dependent proteases really responsible for Cl(-)-dependent and Ca2+-stimulated binding of [3H]glutamate in rat brain?

The role of Ca2+ ions in [3H]glutamate binding was re-examined using synaptic membranous preparations obtained from the rat brain. In vitro addition (0.1-5 mM) of calcium chloride exhibited a profound enhancement of the binding in a temperature-dependent manner, whereas that of calcium acetate had no significant effect on the binding independently of the incubation temperature. Calcium acetate elicited a significantly additional stimulation of the Cl(-)-induced and temperature-dependent facilitation of the binding. The augmentation by these two ions was invariably eliminated by the addition of an antagonist for the anion channels including picrotoxinin as well as of inhibitors of anion transport such as ethacrynic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. L-Aspartate exerted a more potent inhibitory action on the Cl(-)-dependent binding and Cl(-)-dependent and Ca2+-stimulated binding, than D-aspartate. The latter two bindings were selectively abolished by an agonist (quisqualic acid) and an antagonist (DL-2-amino-4-phosphonobutyric acid) for central glutamate receptors, respectively. It was also found that pretreatment of the membranes with calcium acetate resulted in a complete abolishment of the Ca2+-stimulated binding with a concomitant stimulation of the Cl(-)-dependent binding, which invariably occurred independently of the preincubation temperature (2 or 30 degrees C). No significant alteration was detected in the basal binding following the latter pretreatment. None of various protease inhibitors such as leupeptin, antipain, chymostatin and pepstatin induced a significant alteration in the basal, Cl(-)-dependent, Ca2+-stimulated and Na+-dependent bindings of [3H]glutamate, respectively. These results suggest that Ca2+ ions may elicit their stimulatory action on the Cl(-)-dependent binding of [3H]glutamate even in the absence of Cl- ions added through the temperature-independent and apparently irreversible interaction with the anion transport carriers rather than the direct action on the binding sites of the ligand. The evidence presented here also suggests that the widely held view that Ca2+-dependent proteases are responsible for the exhibition of Cl(-)-dependent and Ca2+-stimulated binding of [3H]glutamate may need to be re-evaluated.

Glutathione-induced inhibition of Na+-independent and -dependent bindings of L-[3H]glutamate in rat brain

Reduced glutathione (GSH, 10(-7)-10(-3) M) was found to exert a profound suppressive action on the Na+-independent and -dependent bindings of L-[3H]glutamic acid (Glu) in a temperature-independent manner. Similarly significant reduction of the bindings resulted from the addition of oxidized glutathione (GSSG). Scatchard analysis revealed that GSH as well as GSSG invariably decreased the affinity of the binding sites for [3H]Glu without significantly affecting the number of the binding sites. These results suggest that GSH (GSSG) may in part participate in the synaptic transmission at central Glu neurons through interaction with the receptors and/or the uptake sites for Glu.

Differentiation of the Ca2+-stimulated binding from the Cl- -dependent binding of [3H]glutamate in synaptic membranes from rat brain

The effect of Ca2+ as well as Cl- ions on [3H]glutamate (Glu) binding was re-examined using rat brain synaptic membranes frozen at -80 degrees C in 0.32 M sucrose. The inclusion of 20 mM ammonium chloride or 20 mM ammonium chloride plus 2.5 mM calcium acetate disclosed the Cl- -dependent binding or Ca2+-stimulated binding even at 2 min after the initiation of incubation at 30 degrees C and each binding reached a plateau within 30 min. In contrast, the binding reached its maximal value within 10 min followed by a progressive decline up to 60 min in the presence of 100 mM sodium acetate. Scatchard analysis revealed that Cl- as well as Cl-/Ca2+ ions invariably caused a significant increment of the number of binding sites without altering their affinity, whereas Na+ ions induced a prominent increment of the density of binding sites with a concomitant lowering of their affinity. DL-2-Amino-4-phosphonobutyric acid selectively abolished the Cl- -dependent and Ca2+-stimulated bindings without significantly affecting the basal or Na+-dependent binding. Quisqualic acid induced a profound inhibition of both Cl- -dependent and Ca2+-stimulated bindings, to a significantly greater extent than that of the basal and Na+-dependent bindings. D-Aspartic acid exhibited a potent inhibition of the Na+-dependent binding with a significantly less potent displacement of the basal, Cl- -dependent and Ca2+-stimulated bindings. An inhibitor of anion transport, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), not only eliminated the Cl- -dependent binding, but also completely abolished the Ca2+-stimulated binding. Scatchard analysis revealed that DIDS (0.1 mM) prevented the Cl- - and Cl-/Ca2+-induced increment of the density of binding sites with no significant change of their affinity. Pretreatment of the membranes with hydrophilic SH-reactive agents such as N-ethylmaleimide and 5,5'-dithiobis-(2-nitrobenzoic acid) invariably resulted in a more sensitive inhibition of the Ca2+-stimulated binding than that of the Cl- -dependent binding, while hydrophobic reagent p-chloromercuribenzoic acid produced a similarly potent elimination of the Cl- -dependent and Ca2+-stimulated bindings. Calcium-stimulated binding was also found to be sensitively diminished by dithiothreitol and dithioerythritol as compared with the Cl- -dependent binding. In vitro addition of L-ascorbic acid (10(-6)-10(-3) M) attenuated the Ca2+-stimulated binding to a significantly greater extent than the inhibition of the Cl- -dependent binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of Na+-dependent binding sites of [3H]glutamate in synaptic membranes from rat brain

Some biochemical characteristics of Na+-dependent binding of [3H]L-glutamic acid (Glu) were studied using crude synaptic membrane preparations from the rat brain as compared with Na+-independent binding. In vitro addition of sodium chloride (1-100 mM) exhibited a significant enhancement of [3H]Glu binding to synaptic membranes in a concentration-dependent manner independent of the incubation temperature employed (2 or 30 degrees C). In contrast, sodium acetate elicited a concentration-dependent augmentation of the binding at 2 degrees C to a significantly greater extent than that found at 30 degrees C. It was found that the binding found in the presence of 100 mM sodium acetate reached its maximal value within 10 min of incubation followed by a rapid decline up to 60 min at 30 degrees C, while gradually increasing up to 60 min at 2 degrees C. The Na+-independent basal binding was significantly activated by the alteration of incubation temperature from 2 to 30 degrees C and reached equilibrium within 10 min of incubation at both incubation temperatures. The Na+-dependent binding was more promptly attenuated by the addition of excess of non-radioactive Glu (1 mM) at 30 degrees C than that at 2 degrees C, whereas the Na+-independent binding was greatly suppressed by the addition at 2 degrees C in comparison with that at 30 degrees C. Quisqualic acid induced a considerably less-potent inhibition of the Na+-dependent binding than that of the Na+-independent binding. Neither N-methyl-D-aspartic acid nor kainic acid had such a significant effect on each binding.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Characterization of L-glutamate binding sites in rat spinal cord synaptic membranes: evidence for multiple chloride ion-dependent sites

The effects of various ions on L-glutamate (L-Glu) binding sites (Na+-dependent, Cl(-)-dependent, and Cl(-)-independent) in synaptic plasma membranes (SPM) isolated from rat spinal cord and forebrain were examined. Cl(-)-dependent binding sites were over twofold higher in spinal cord (Bmax = 152 +/- 34 pmol/mg protein) as compared to forebrain SPM (Bmax = 64 +/- 12 pmol/mg protein). Na+-dependent binding, on the other hand, was nearly sixfold less in spinal cord (Bmax = 74 +/- 10 pmol/mg protein) compared to forebrain SPM (408 +/- 26 pmol/mg protein). Uptake of L-Glu (Na+-dependent) was also eightfold less in the P2 fraction from spinal cord relative to forebrain (Vmax of 2.89 and 22.3 pmol/mg protein/min, respectively). The effects of Na+, K+, NH4+, and Ca2+ on L-Glu binding sites were similar in both regions of the CNS. In addition, in spinal cord membranes, Br-, I-, and NO3- were equivalent to Cl- in their capacity to stimulate L-Glu binding, whereas F- and CO3- were less effective. Cl(-)-dependent L-Glu binding in spinal cord membranes consisted of two distinct sites. The predominant site (74% of the total) had characteristics similar to the Cl(-)-dependent binding site in forebrain membranes [i.e., Ki values of 5.7 +/- 1.4 microM and 119 +/- 38 nM for 2-amino-4-phosphonobutyric acid (AP4) and quisqualic acid, (QUIS), respectively]. The other Cl(-)-dependent site was unaffected by AP4 but was blocked by QUIS (Ki = 14.2 +/- 4.8 microM).

Localization of [3H]glutamate binding sites in rat adrenal medulla

A significant activity of L-[3H]glutamic acid binding was detected in the membranous particulate preparations obtained from the rat adrenal glands. In vitro addition of sodium acetate (100 mM) resulted in a drastic enhancement of the binding to cerebral preparations, while inducing a significant inhibition of the adrenal binding. Quisqualic acid elicited a prominent suppression of the adrenal binding in a concentration-dependent manner to a slightly greater extent than the inhibition of cerebral binding. N-Methyl-D-aspartic acid exerted a significant stimulatory action on the adrenal binding without affecting the cerebral binding. The adrenal medullary part possessed more than 5-fold higher binding activity than that in cortical part. These results suggest the possible existence of [3H]glutamate binding sites in rat adrenal glands which are distinctly different from those in the central structures.

Calcium-induced long-term potentiation in the hippocampal slice: characterization of the time course and conditions

A transient increase in extracellular calcium concentration causes a long-lasting enhancement of radiatum fibers evoked excitatory postsynaptic potential and population spike responses of CA1 pyramidal neurons which resembles long-term potentiation (LTP). The duration of this potentiation is much longer than described previously and is probably limited by the survival of the preparation itself (greater than 8 hr). Therefore, Ca-induced LTP can be used for the investigation of a postulated late phase of LTP. Ca effects were activity-independent, since the subsequently evoked responses were facilitated even when the presynaptic fibers were not concurrently stimulated during or immediately after superfusion with the high Ca medium. In contrast, if too frequent testing of the synaptic input was done during the high Ca pulse, a short lasting depression instead of potentiation was observed. A lower extracellular magnesium concentration in the standard medium (1.3 instead of 2.0 mM MgSO4) prevents the potentiation of the EPSP at least for the first few hours. Presumably, both tetanus- and Ca-induced LTP share some common mechanisms, since an additional tetanization after Ca induction was not followed by an additional LTP. Compared to the potentiation following tetanization, the Ca-induced LTP was, however, not accompanied by a potentiation of the EPSP/spike ratio within the range of the population spike threshold intensity.

Na+-dependent "binding" of D-aspartate in brain membranes is largely due to uptake into membrane-bounded saccules

Na+-dependent "binding" of acidic amino acids in brain plasma membranes was examined by procedures similar to those employed in earlier studies, using the metabolically inert D-[3H]aspartate as a probe. The "binding" showed characteristics similar to those described before in terms of affinity (KD, 400 nM), density of sites (Bmax, 300 pmol/mg protein), sensitivity to D,L-threo-3-hydroxyaspartate, and requirement for Na+. It turned out that the "binding" represents uptake into membrane-bounded saccules (which according to the inulin and H2O spaces constituted 3.4 microliters/mg protein and comprised about 50% of the volume of the sedimented membranes), rather than binding to the transport carrier. This conclusion is based on the observations that the "binding" of D-aspartate was released by osmotic shock; was abolished by thorough washing of membranes in H2O prior to assay, which removed endogenous contents of amino acids, and could be recovered by loading the washed membranes with glutamate; was reduced by prior freezing and thawing; was low on incubation at 0 degree C; had a bell-shaped time course similar to that reported for uptake; and had a slow rate of reversal compared to the apparent KD. True binding would have considerably lower apparent Bmax than the carrier-mediated uptake. This and its likely rapid rate of dissociation would make binding to the carrier difficult to detect by the methods used up to now.

Ionic regulation of the binding of DL-2-amino-7-phosphono-[4,5-3H]heptanoic acid to synaptosome-enriched homogenates of rat cerebral cortex

The ionic requirements of the site labelled by DL-2-amino-7-phosphono-[4,5-3H]heptanoic acid ([3H]DL-2AP7) in synaptosome-enriched homogenates of rat cerebral cortex were examined using radioligand binding methodology. Binding of [3H]DL-2AP7 was increased by calcium and chloride ions by an apparently non-competitive mechanism. The actions of ions on specific binding displayed similarities to the effects of ions on DL-[3H]2-amino-4-phosphonobutyric acid and Cl-/Ca2+-dependent L-[3H]glutamate ([3H]Glu) binding sites but were more consistent with the Glu-C binding site.

13C n.m.r. study of the structure and the metal ion binding sites of neuropeptides composed of L-Asp and L-Glu

13C NMR spectral data are presented for a variety of possible neuropeptides composed of L-Asp, Ac-L-Asp, and L-Glu which contain alpha and beta peptide linkages. The data for the various compounds are compared to the data presented for Ac-Asp-Glu, a known neuropeptide, in order to gain structural information about the related compounds. Indications are that for compounds 1 and 5, the cis peptide bond configuration exists due to the interaction of zwitterionic species. This interaction appears to be eliminated when the beta peptide bonds are formed, as in the case of compounds 3 and 7. Spin-lattice relaxation times were used to confirm the structures. Electron-nuclear relaxation rates are also used to elucidate the metal ion binding sites of these species.

[3H]glutamate binding sites in the rat pituitary

A significant activity of [3H]glutamate binding was detected in homogenate particulate preparations obtained from the rat pituitary, in addition to central structures including the cerebral cortex. In contrast to the cerebral binding, the pituitary binding was significantly inhibited by Na+ ions. It was also found that neurohypophysis possessed more than a two-fold higher binding activity than that found in adenohypophysis. These results suggest a possible significance of glutamate in rodent pituitary.

Purine nucleotides inhibit the binding of DL-[3H] 2-amino-4-phosphonobutyrate (DL-[3H] APB) to L-glutamate-sensitive sites on rat brain membranes

The effects of purine nucleotides on the binding of DL-[3H] 2-amino-4-phosphonobutyrate (DL-[3H] APB) to rat brain membranes were investigated. Certain guanine nucleotides, especially cyclic GMP and GTP, were found to be potent inhibitors of binding. Kinetic studies revealed that both cyclic GMP and GTP acted to decrease receptor affinity without affecting significantly binding site density. These endogenous substances may therefore play an important role in the regulation of excitatory amino acid receptor function.

Binding of L-[3H]glutamate to fresh or frozen synaptic membrane and postsynaptic density fractions isolated from cerebral cortex and cerebellum of fresh or frozen canine brain

Synaptic membrane (SPM) and postsynaptic density (PSD) fractions isolated from cerebral cortex (CTX) and cerebellum (CL) of canine brain, either fresh or frozen and isolated from either fresh or frozen tissue, were found to contain L-[3H]glutamate binding sites. It was found that there was a concentration of L-glutamate binding sites in CTX-PSD and CL-PSD over the respective membrane fractions, and the Bmax value of CL-PSD (92.0 pmol/mg protein) was about three times that of CTX-PSD (28.9 pmol/mg). The results, together with those of others, suggest that the thin CL-PSD are probably derived from the excitatory synapses in the molecular layer. The ion dependency of L-glutamate binding to canine CTX-SPM fraction was found to be similar to that reported for a rat brain SPM fraction: (a) Cl- increased the number of L-glutamate binding sites and the effect was enhanced by Ca2+; Ca2+ alone had no significant effect; (b) the Cl-/Ca2+-sensitive binding sites were abolished by 2-amino-4-phosphonobutyrate (APB) or freezing and thawing; (c) the effect of Na+ ion was biphasic; low concentration of Na+ (less than 5 mM) decreased Cl-/Ca2+-dependent L-glutamate binding sites, whereas at higher concentrations of Na+ the binding of glutamate was found to increase either in the presence or absence of Ca2+ and Cl-. In addition, the K+ ion (50 mM) was found to decrease the Na+-independent and Cl-/Ca2+-independent binding of L-glutamate to fresh CTX-SPM by 18%, but it decreased the Na+-dependent and Cl-/Ca2+-independent L-glutamate binding by 93%; in the presence of Cl-/Ca2+, the K+ ion decreased the Na+-dependent binding by 78%. Freezing and thawing of CTX-SPM resulted in a 50% loss of the Na+-dependent L-glutamate binding sites assayed in the absence of Ca2+ and Cl-. The CL-SPM fraction showed similar ion dependency of L-glutamate binding except for the absence of Na+-dependent glutamate binding sites. The CTX-PSD fraction contained neither Na+-dependent nor APB (or Cl-/Ca2+)-sensitive L-glutamate binding sites and its L-glutamate binding was unaffected by freezing and thawing, in agreement with the reported findings using rat brain PSD preparation. L-Glutamate binding to CTX-SPM or CTX-PSD fraction was not affected by pretreatment with 10 mM L-glutamate, nor by simultaneous incubations with calmodulin.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of a micromethod to study the Na+-independent L-[3H]glutamic acid binding to rat striatal membranes. I. Biochemical and pharmacological characterization

A micromethod was developed to measure the Na+-independent L-[3H]glutamic acid (Glu) binding to rat striatal membranes by using slightly purified membranes from very small tissue amounts, ranging from 0.2 to 0.5 mg wet tissue. The specific binding reached equilibrium in about 30 min incubation at 37 degrees C and was shown to be partly reversible. Scatchard's analysis of saturation data suggest the presence on striatal membranes of an apparent single homogeneous population of Na+-independent binding sites with Kd value 1.75 microM and Bmax 3.89 nmol/g protein. Hill's plot of these data was linear with slope not significantly different from unity, indicating the absence of cooperative interactions. Cl- and Ca2+ ions were shown to severely influence the L-[3H]Glu binding to striatal tissue. Maximal activating effects were obtained in the presence of both ions, although Cl- alone was shown to have a powerful stimulating action on the binding. Pharmacological studies suggested, however, the presence of at least two subpopulations of binding sites which bound quisqualic acid as well as ibotenic acid with differential affinities. L-aspartic acid and L-serine-O-sulfate were shown to be potent inhibitors of the L-[3H]Glu binding while DL-2-amino-4-phosphonobutyric acid (DL-APB) and glutamic acid diethylester (GDEE) competed with the binding but only at high concentrations. N-methyl-D-aspartic acid (NMDA), DL-2-amino-5-phosphonovaleric acid (DL-APV), D-alpha-aminoadipate (D-alpha AA) and kainic acid were shown not to significantly influence the binding of Glu to striatal membranes.

Chloride-dependent binding sites for L-[3H]glutamate on dendrodendritic synaptosomal membranes of rat olfactory bulb

Dendrodendritic synapses occur between granule cell dendrites and secondary dendrites of mitral cells within the olfactory bulb and are attainable in a subcellular fraction (DDS). Since the mitral cells are thought to utilize an excitatory amino acid as a neurotransmitter, we determined the pharmacologic specificity of Na+-independent L-[3H]glutamate binding to fresh membranes of DDS in 50 mM Tris-HCl, pH 7.1. Binding of L-glutamate to membranes of DDS was specific, Cl(-)-dependent, and saturable. Scatchard plots were analyzed by nonlinear regression analyses using the computer program LIGAND, and the data was best-fitted to a one-site model with KD of 0.56 +/- 0.04 microM and an apparent Bmax of 48 +/- 5 pmol/mg protein. Hill plots also indicated the presence of one site and no cooperativity (nH = 0.99 +/- 0.03). However, the relative effectiveness of several compounds in inhibiting L-glutamate binding to membranes of DDS clearly demonstrated the presence of more than one site. Electrophysiological studies suggest that 2-amino-4-phosphonobutyrate (APB) is a potent antagonist of evoked responses elicited by stimulation of mitral cell axons and that quisqualate is a potent agonist; both of these compounds were highly effective inhibitors of L-glutamate binding to DDS membranes. APB displaced about 70% of the sites labeled with 200 nM L-glutamate with a KI of 1.6 microM, whereas quisqualate inhibition of L-glutamate binding yielded a line that was curvilinear in the Scatchard plot and was resolved into two sites of relatively high affinity (KI values of 0.02 and 0.65 microM).(ABSTRACT TRUNCATED AT 250 WORDS)