Glutamate-preferring receptors regulate the release of D-[3H]aspartate from rat hippocampal slices

Efficacy of prophylactic versus therapeutic administration of the NMDA receptor antagonist MK-801 on the acute neurochemical response to a concussion in a rat model combining force and rotation

OBJECTIVE

Alterations in amino acid concentrations are a major contributor to the persistent neurological and behavioral effects induced by concussions and mild traumatic brain injuries (TBIs). Glutamate, the most abundant excitatory amino acid in the CNS, has a major role in the pathophysiological process of concussion. The indiscriminate liberation of glutamate immediately after a concussion triggers an excitotoxic response that leads to cell death, neuronal damage, and the dysfunction of surviving neurons, largely by overactivation of N-methyl-d-aspartate (NMDA) glutamatergic receptors. The aim of the present study was to investigate the efficacy of prophylactic versus therapeutic administration of MK-801, a promising NMDA receptor antagonist, on the acute changes in amino acid extracellular concentrations involved in excitotoxicity resulting from a concussive trauma.

METHODS

The immediate neurochemical response to a concussion cannot be characterized in humans. Therefore, the authors used their previously validated combination of a weight-drop concussion rat model and in vivo cerebral microdialysis. The microdialysis probe was inserted inside the hippocampus and left inserted at impact to allow uninterrupted sampling of amino acids of interest immediately after concussion. The primary outcome included amino acid concentrations and the secondary outcome included righting time. Samples were taken in 10-minute increments for 60 minutes before, during, and 60 minutes after impact, and analyzed for glutamate, gamma-aminobutyric acid, taurine, glycine, glutamine, and serine using high-performance liquid chromatography. Righting time was acquired as a neurological restoration indicator. Physiological saline or 10 mg/kg MK-801 was administrated intraperitoneally 60 minutes before or immediately following induction of sham injury or concussion.

RESULTS

Following induction of concussion, glutamate, taurine, and glycine levels as well as righting times in cases from the MK-801 treatment group were comparable to those of vehicle-treated animals. In contrast, righting times and amino acid concentrations observed within the first 10 minutes after induction of concussion in cases assigned to the MK-801 prophylaxis group were comparable to those of sham-injured animals.

CONCLUSIONS

These results suggest that presynaptic actions and peak availability of MK-801 following prophylactic administration significantly inhibit the immediate and indiscriminate release of glutamate, taurine, and glycine in extracellular fluid after a concussion.

Kainic acid: insights into excitatory mechanisms causing selective neuronal degeneration

Kainic acid, an acidic pyrolidine isolated from the seaweed Digenea simplex, is the most potent of the commonly used exogenous excitotoxins. The neurotoxic threshold of kainic acid is nearly two magnitudes lower than that of the other receptor-specific agonists, N-methyl-D-aspartic acid and quisqualic acid. Neurophysiological and ligand-binding studies indicate that the neurotoxic action of kainic acid is mediated by a specific receptor which exhibits a remarkably broad phylogenetic distribution in the nervous system of vertebrates and invertebrates. The mechanism of neurotoxicity of kainic acid appears to be indirect and requires the functional integrity of excitatory afferents to vulnerable neurons. Consistent with the excitotoxin hypothesis, kainic acid depletes high-energy phosphates and glucose at sites of neurotoxic action; nevertheless, the proximate cause of neurotoxicity may involve increases in intraneuronal calcium levels and the activation of calcium-dependent proteases. Kainic acid neurotoxicity provides a useful animal model for selective neuronal vulnerability that may shed light on the pathophysiology of a number of neurodegenerative disorders, including Huntington's disease and temporal lobe epilepsy.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Presynaptic mGlu1 type receptors potentiate transmitter output in the rat cortex

In the present study we used freely moving rats with a microdialysis probe placed in their parietal cortex to study the effects of local application of agonists and antagonists of metabotropic glutamate (mGlu) receptors on glutamate release. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 0.1-1 mM), a non-selective agonist of metabotropic glutamate (mGlu) receptors, increased glutamate concentration in the dialysate up to 3-fold. A significant increase in glutamate output in cortical dialysates was also obtained with (RS)-3,5-dihydroxyphenylglycine (DHPG; 0.5-1 mM), a group 1-selective mGlu receptor agonist, suggesting the involvement of group 1 mGlu receptors in 1S,3R-ACPD effects. S-4-carboxyphenylglycine (S-4CPG; 0.3 microM), a mGlu1 receptor antagonist with a mild agonist action on mGlu2 receptors, antagonised, in a surmountable manner, the effects of 1S,3 R-ACPD. Similarly, 1-aminoindan-1,5-dicarboxylic acid (AIDA; 0.03-1 mM) a selective group 1 antagonist with a preferential action on mGlu1 type receptors, antagonised the effects of 1S,3R-ACPD. Finally, (S)-(+)-2-(3'-Carboxybicyclo[1.1.1]pentyl)-glycine (UPF596; 30-300 microM), a potent mGlu1 antagonist with modest agonist activity on mGlu5, antagonised 1S,3R-ACPD-induced glutamate release. In conclusion, our data showed that 1S,3R-ACPD-induced glutamate release in the parietal cortex is mediated by mGlu1 receptors and that, under basal conditions, these receptors are not tonically activated.

Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

The role of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in controlling the neuronal release of excitatory amino acids has been investigated. Stimulation of presynaptic AMPA receptors by the endogenous agonist L-glutamate, or by (R,S)-AMPA, dose-dependently enhanced the Ca(2+)-dependent, tetrodotoxin-insensitive, electrically-stimulated release of [3H]D-aspartate from rat forebrain slices. This AMPA receptor-mediated response showed marked stereoselectivity with the activity residing solely in the (S)-isomer. (R)-AMPA was inactive in this respect. AMPA-evoked responses were significantly enhanced in the presence of the AMPA receptor desensitization inhibitor, cyclothiazide (10 microM). Moreover, responses to both AMPA and glutamate were inhibited by competitive (NBQX) and non-competitive (GYKI 52466) AMPA receptor-selective antagonists in a dose-dependent manner. These results provide strong support for the existence of presynaptic AMPA receptors acting to enhance the synaptic release of excitatory amino acids in the mammalian forebrain. Such a positive feedback system may play an important functional role in physiological (e.g., long-term potentiation) and/or pathological (e.g., epileptogenesis) processes in the mammalian central nervous system. AMPA-type autoreceptors may provide new targets for drug action.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Glutamate and theta-rhythm stimulation selectively enhance NMDA component of EPSC in CA1 neurons of young rats

Using in situ whole-cell patch clamp of hippocampal CA1 pyramidal neurons we demonstrate that glutamate initiates processes resulting in an increase in the amplitude of the excitatory post-synaptic current (EPSC). In adult animals both, NMDA and non-NMDA components of the EPSC increase in parallel. In young animals only the NMDA component is increased. A similar enhancement of the EPSC can be achieved by the stimulation of excitatory synaptic inputs to CA1 neurons with the frequency of the theta-rhythm. EPSCs remain enhanced for more than 60 min. The selective enhancement of the NMDA component in young animals is inhibited by preincubation of slices with the NO-synthase blocker, N omega-nitro-L-arginine (NA) or by the NO-scavenger, hemoglobin.

Long-term potentiation as synaptic dialogue

We have proposed a testable model of the physiological and biochemical events underlying LTP that offers the following novel features. (1) The focus is not on a single mechanism or synaptic site, but rather on the integration and interaction of mechanisms occurring on both sides of the synapse, (2) beta PKC plays a critical presynaptic role in LTP, while gamma PKC functions postsynaptically. (3) These stages can be ordered in a time-delimited sequence of post- then presynaptic molecular events based on the period of effectiveness of inhibitor compounds. (4) The distinction is made between the time when kinase activation occurs and the time when the potentiated response requiring this kinase activation is observed.

Glutamate-induced inhibition of paired pulse facilitation of monosynaptic excitatory post-synaptic potentials in frog spinal motoneurons

To evaluate actions of glutamate on excitatory synaptic transmission in the central nervous system, we examined glutamate-induced changes in the paired pulse facilitation of monosynaptic excitatory post-synaptic potentials evoked by stimulation of the lateral column fibers (LC-EPSPs) on lumbar motoneurons in the frog spinal cord. Glutamate (1 mM) depolarized motoneurons both in the presence and absence of Mg2+. In most cells perfused with Mg(2+)-free or high Ca(2+)-Mg2+ solutions, the glutamate potential was accompanied by a reduction in peak amplitude of EPSPs, although the degree of change varied with the cells. Glutamate enhanced the EPSP amplitude in a few cells with Mg(2+)-free and high Ca(2+)-Mg2+ solutions, and in most cells with high Mg2+ medium. In 3/5 cells tested, the paired pulse facilitation of EPSPs was reduced by glutamate when the EPSP amplitude either increased or decreased. NMDA (50 microM), kainate (50-100 microM), quisqualate (5-50 microM) and L-2-amino-4-phosphonobutyrate (L-AP4, 1 mM) also decreased the facilitation in about half of the cells tested. The glutamate-induced decrease in the facilitation was observed in both the presence and absence of Mg2+ and was not affected by the concomitant application of glutamate and antagonists for non-NMDA or NMDA receptors, such as 6-cyano-7-nitro-quinoxalinediones (CNQX, 60 microM) or 2-amino-5-phosphonovalerate (APV, 250 microM). Glutamate reduced the facilitation of excitatory post-synaptic currents (EPSCs) recorded at a constant membrane potential under voltage clamp, when the EPSC amplitude either increased or decreased and when the input conductance either increased or decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Pretreatment with NMDA antagonists limits release of excitatory amino acids following traumatic brain injury

After central nervous system (CNS) trauma, there are marked elevations in the extracellular levels of excitatory amino acids (EAA), which are believed to contribute to delayed tissue damage. Administration of N-methyl-D-aspartate (NMDA) receptor antagonists reduces injury severity after brain or spinal cord trauma, presumably by blocking the postsynaptic NMDA receptor. In the present studies, levels of extracellular amino acids were monitored by microdialysis during, and after, a moderately severe fluid-percussion brain injury to rats. Pretreatment (15 min prior to injury) with the non-competitive NMDA antagonist dextrorphan or the competitive NMDA antagonist CGS 19755 significantly attenuated the post-traumatic increase in extracellular glutamate. Pretreatment with dextrorphan attenuated the post-traumatic increase in extracellular levels of aspartate; although these differences did not reach significance when examined as absolute values, they were significant when analyzed as percent increase over pre-trauma baseline levels. These results are consistent with recent experiments and suggest that NMDA antagonists may limit the release of glutamate and aspartate after trauma through a presynaptic mechanism.

Presynaptic Glutamate Receptors Regulate Noradrenaline Release from Isolated Nerve Terminals

The wide-ranging neuronal actions of excitatory amino acids, such as glutamate, are thought to be mediated mainly by postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. We now report the existence of presynaptic glutamate receptors in isolated nerve terminals (synaptosomes) prepared from hippocampus, olfactory bulb, and cerebral cortex. Activation of these receptors by NMDA or non-NMDA agonists, in a concentration-dependent manner, resulted in Ca(2+)-dependent release of noradrenaline from vesicular transmitter stores. The NMDA-stimulated release was potentiated by glycine and was blocked by Mg2+ and selective NMDA antagonists. In contrast, release stimulated by selective non-NMDA agonists was blocked by 6-cyano-7-nitroquinoxaline-2,3- dione, but not by Mg2+ or NMDA antagonists. Our data suggest that the presynaptic glutamate receptors can be classified pharmacologically as both the NMDA and non-NMDA types. These receptors, localized on nerve terminals of the locus ceruleus noradrenergic neurons, may play an important role in interactions between noradrenaline and glutamate.

Terminal excitability of the corticostriatal pathway. II. Regulation by glutamate receptor stimulation

The influence of impulse activity and glutamate receptor stimulation on the electrical excitability of the corticostriatal terminal field was explored. Antidromic responses were recorded from prefrontal cortical neurons the electrical stimulation of their terminal field in the contralateral striatum. Terminal excitability was assessed by determining the percentage of subthreshold current stimulus presentations eliciting an antidromic response. Terminal excitability was found to be positively correlated with variations in spontaneous firing rate: increases and decreases in firing rate were accompanied by corresponding changes in the percentage of antidromic responses elicited by a subthreshold stimulus. Drugs were applied to the striatal stimulation site in a volume of 312 nl delivered over 5 min. Striatal administration of either the competitive NMDA antagonist D-alpha-aminoadipate (DAA) or D-2-amino-7-phosphonoheptanoate (AP-7) or the competitive non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) blocked the correlation between excitability and firing rate. Further examination revealed that the terminal field was rendered more excitable for a period of 20-80 ms following the arrival of an action potential. This post-impulse facilitation of terminal excitability was attenuated after local application of AP-7 (10 microM) or CNQX (20 microM). At half these doses, AP-7 or CNQX produced a non-significant effect, however when administered simultaneously a significant attenuation was observed. The participation of interneurons in these excitability effects was ruled out since they were still seen following kainic acid lesions. We propose that this impulse-dependent enhancement in terminal excitability results from the release of glutamate induced by the action potential in the terminal field and the subsequent stimulation of glutamate autoreceptors on the terminals.

Autoreceptor regulation of glutamate and aspartate release from slices of the hippocampal CA1 area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.

N-methyl-D-aspartate releases excitatory amino acids in rat corpus striatum in vivo

There is a considerable amount of conflicting evidence from several studies as to the action of applied N-methyl-D-aspartate (NMDA) on the release of glutamate and aspartate in the brain. In the present study the effect of NMDA on extracellular levels of endogenous amino acids was investigated in conscious, unrestrained rats using intracerebral microdialysis. NMDA caused dose-related increases in extracellular levels of glutamate and aspartate; threonine and glutamine were unaffected. The NMDA-evoked release of glutamate and aspartate was significantly decreased by the specific NMDA receptor antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid. In addition, increasing the perfusate concentration (and therefore the extracellular concentration) of Ca2+ significantly enhanced the NMDA-evoked release of glutamate and aspartate, whereas removal of Ca2+ and addition of a high Mg2+ concentration to the perfusate caused a significant reduction in their NMDA-evoked release. Moreover, the NMDA-evoked release of glutamate and aspartate was reduced in decorticate animals. These results demonstrate that, in the striatum in vivo, NMDA causes selective release of endogenous glutamate and aspartate from neurone terminals and that this action occurs through an NMDA receptor-mediated mechanism. The ability of NMDA receptor activation to induce release of glutamate and aspartate, perhaps by a positive feedback mechanism, may be relevant to the pathologies underlying epilepsy and ischaemic and hypoglycaemic brain damage.

Excitatory amino acids evoke taurine release from cerebral cortex slices from adult and developing mice

Glutamate, aspartate and the agonists of the excitatory amino acid receptors, N-methyl-D-aspartate, kainate and quisqualate, evoked more release of both endogenous and preloaded exogenous taurine from cerebral cortical slices from three-day-old than from specimens from adult mice. The N-methyl-D-aspartate- and quisqualate-evoked release was blocked by D-2-amino-5-phosphonovalerate and glutamatediethylester, respectively, in three-day-old mice but not in the adults. The kainate-evoked release was not affected by gamma-D-glutamyltaurine and gamma-D-glutamylglycine in either age group. Exposure of the slices to excitatory amino acids and their agonists caused intracellular swelling of the slices, which was directly proportional to the increase in taurine release in adult mice. In three-day-old mice the correlation between the swelling and taurine release was less pronounced. The excitatory amino acid receptors seem to modify more effectively the release of taurine in the developing than the adult brain. In the adults the evoked release of taurine may be related to cell volume regulation in the context of the excitation-coupled ionic and water movements across plasma membranes.

Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors

In order to investigate the localization and the type(s) of excitatory amino acid receptors in the frog vestibular system, the exogenous amino acid agonists Quisqualic acid, Kainic acid and N-methyl-D-aspartic acid were tested on the sensory organ of semicircular canals. Intracellular recordings of the resting discharge from single afferents showed that these agonists exerted a complex excitatory action consisting in a rapid and brief increase in frequency of both EPSPs and spikes, followed by a slower and longer lasting membrane depolarization. The progressive impairment of natural transmitter release achieved by adding Mg2+ or Co2+ in the bath caused a dose-dependent decrease of the agonist-induced afferent discharge, without substantially affecting axonal depolarization. These results suggest that the exogenous amino acid agonists act both pre- and postsynaptically on the vestibular organs. Quisqualic acid and kainic acid were much more potent than N-methyl-D-aspartic acid in inducing excitatory effects, suggesting that the amino acid receptors located on both hair cells and afferent endings are mainly of the non-NMDA type. The present findings, while not excluding that an excitatory amino acid may be the afferent transmitter, highlight its possible function as a presynaptic modulator of the afferent transmission in the frog vestibular system.

Regulation of glutamate and aspartate release from the Schaffer collaterals and other projections of CA3 hippocampal pyramidal cells

Excitatory synaptic transmission in the CNS can be modulated by endogenous substances and metabolic states that alter release of the transmitter, usually glutamate and/or aspartate. To explore this issue, we have studied the release of endogenous glutamate and aspartate from synaptic terminals of the CA3-derived Schaffer collateral, commissural and ipsilateral associational fibers in slices of hippocampal area CA1. These terminals release glutamate and aspartate in about a 5:1 ratio. The release process is modulated by adenosine, by the transmitters themselves and by nerve terminal metabolism. Adenosine inhibits the release of both amino acids by acting upon an A1 receptor. The transmitters, once released, can regulate their further release by acting upon both an NMDA and a non-NMDA (quisqualate/kainate) receptor. Activation of the NMDA receptor enhances the release of both glutamate and aspartate, whereas activation of the non-NMDA receptor depresses the release of aspartate only. Superfusion of CA1 slices with a glucose-deficient medium increases the release of both amino acids and reduces the glutamate/aspartate ratio. These results have implications for the regulation of excitatory synaptic transmission in the CA1 area and for the mechanism of hypoglycemic damage to CA1 pyramidal cells.

Dendritic excitation by glutamate in CA1 hippocampal cells

In order to reveal properties and effects of glutamate excitation, CA1 pyramidal cells in rat hippocampal slices were impaled and responses to iontophoresis of glutamate onto sensitive spots in the dendrites were analyzed. The glutamate-elicited response consisted of a steady depolarization; its amplitude was dose-dependent. The cellular response to repeated applications of glutamate showed a striking degree of stability. Both dendritic and somatic depolarization, induced by glutamate and current, respectively, elicited similar discharge patterns. The sensitivity to glutamate was highly localized, corresponding to the dendritic tree of a given cell. Short, repeated glutamate pulses did not interfere with an orthodromic test response, whereas longer glutamate ejections often depressed the EPSP. Combined temporal and spatial pairing of glutamate and orthodromic activation was followed by a lasting increase in synaptic efficiency, similar to LTP.

Effects of an N-methyl-D-aspartate receptor agonist and its antagonist CPP on the levels of dopamine and serotonin metabolites in rat striatum collected in vivo by using a brain dialysis technique

3-((+-)-2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) is an antagonist at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In the present study, levels of dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) were measured after intracerebroventricular injection of NMDA, CPP or both in rat striatum using a brain dialysis method. The injection of NMDA produced a significant increase in DOPAC level. HVA level was also increased by NMDA injection. The level of 5-HIAA was not affected by NMDA injection. The injection of CPP had no effect on DOPAC, HVA and 5-HIAA levels. The injection of CPP restrained the increase of DOPAC and HVA levels induced by NMDA injection. The results suggest that intracerebral injection of NMDA may increase dopamine release from rat striatum, but have no effect on serotonin release. Furthermore, CPP inhibits NMDA induced release of dopamine.

A microtest plate assay of functional excitatory amino acid receptors

A microtest plate assay of functional excitatory amino acid receptors present on cultured chick embryo retinal cells has been developed. It is based on measurements of excitatory amino acid-mediated increase in 22Na+ influx into retinal cells adhering to each of the 96 wells of microtest plates. Dose-dependent responses to L-glutamate, kainate and N-methyl-D-aspartate but not to quisqualate can be measured. These responses are selectively inhibited by antagonists of excitatory amino acids. The assay is reliable, fast to perform and parsimonious in terms of the volume and thus of amount of the drug applied. It allows a single investigator to perform, in one day, measurements of the effects of known or putative glutamatergic ligands in more than 100 different conditions.

Long-term potentiation: studies in the hippocampal slice

Long-term potentiation (LTP) is an example of activity-dependent plasticity that was discovered in the hippocampal formation. There is growing evidence that LTP not only is a useful model for mnemonic processes, but also may represent the cellular substrate for at least some kinds of learning and memory. The hippocampal slice preparation has proven exceptionally useful in pharmacological studies of possible mechanisms of LTP. A slice remains viable and stable for several hours, and known concentrations of drugs in the bathing medium can be added and then washed out. Drugs can also be applied under visual guidance from micropipettes to discrete neuronal regions, an accomplishment that is aided by the lamellar organization of the hippocampus. Electrical stimulation of the perforant path (PP) in the molecular layer of the dentate gyrus produces a monosynaptic excitatory postsynaptic potential (EPSP) and action potential, which can be recorded extracellularly as a population EPSP and population spike, respectively. Presentation of a high-frequency train (HFT; 100 Hz X 1 s) to the PP results in a long-lasting (greater than 30 min) potentiation of the maximal EPSP slope and of the population spike amplitude. Similarly, exposure of the slice to norepinephrine (e.g. 20 microM for 30 min) results in a long-lasting potentiation (LLP) of both EPSP and population spike (Stanton and Sarvey (1987) Brain Res. Bull., 18: 115). No such LLP was seen in field CA1 following NE application (Stanton and Sarvey (1985) Brain Res., 361: 276). beta-Adrenergic antagonists, such as propranolol, inhibit both LTP and NE-induced LLP in dentate (Stanton and Sarvey, J. Neurosci., 5: 2169 (1985); Stanton and Sarvey (1985) Brain Res., 361: 276). Cyclic AMP levels are increased by either an HFT or NE (Stanton and Sarvey (1985) Brain Res., 358: 343). Thus, NE, acting through a beta-receptor, appears to be both necessary and sufficient to produce long-lasting enhancement of synaptic responses. Finally, inhibitors of protein synthesis, such as emetine, also block both LTP and NE-induced LLP (Stanton and Sarvey, J. Neurosci., (1984) 4: 3080; Stanton and Sarvey (1985) Brain Res., 361: 276). The N-methyl-D-aspartate (NMDA) excitatory amino acid receptor subtype appears to play a role in a number of forms of neuronal plasticity. Bath-application of a 1 microM concentration of the NMDA antagonists D-2-amino-5-phosphonavaleric acid (AVP) or 3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked both LTP and NE-induced LLP in the dentate gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

Low-concentration N-acetylaspartylglutamate suppresses the climbing fiber response of Purkinje cells in guinea pig cerebellar slices and the responses to excitatory amino acids of Xenopus laevis oocytes injected with cerebellar mRNA

Whether N-acetylaspartylglutamate (NAAG) at micromolar concentrations shows a modulatory action on the synaptic transmission mediated by excitatory amino acids was investigated using Guinea pig cerebellar slices and the Xenopus laevis oocytes injected with mRNA from Guinea pig cerebellum. The climbing fiber response consisted of an excitatory postsynaptic potential (EPSP) and a plateau potential intracellularly recorded from Purkinje cell dendrite was depressed by 30 microM NAAG; the EPSP was decreased by about 21% in amplitude and the plateau potential was depressed by about 42% in duration. The depolarization induced by L-aspartate, L-glutamate, N-methyl-D-aspartate and quisqualate in mRNA-injected Xenopus oocytes were non-selectively antagonized by 0.5 microM-5 microM NAAG, the mean % blockade by 5 microM NAAG being about 38%. Higher concentrations (greater than 100 microM) of NAAG alone by 33% on average by 10 microM NAAG. These results suggest the possibility that micromolar concentrations of NAAG may attenuate the synaptic transmission mediated by glutamate receptors not only by blocking postsynaptic receptors but also by facilitating the high-affinity re-uptake of transmitter amino acids.

Ibotenic acid stimulates D-[3H]aspartate release from cultured cerebellar granule cells

The release of D-[3H]aspartate from cultured cerebellar granule cells can be evoked by potassium depolarization or by superfusion with the excitatory amino acids glutamic and aspartic acids. The latter phenomenon is not mediated by N-methyl-D-aspartate (NMDA)-, kainate- or quisqualate-preferring receptors, but is stimulated by ibotenic acid. Excitatory amino acid-stimulated D-[3H]aspartate release is antagonised by L-serine-O-phosphate and riluzole. These compounds did not block potassium-stimulated D-[3H]aspartate release. This pharmacology resembles an excitatory amino acid receptor coupled to inositol phosphate (PI) turnover.

Evidence for excitatory amino acid transmission between mesencephalic nucleus of V afferents and jaw-closer motoneurons in the guinea pig

Previous studies have suggested that monosynaptic transmission between spinal primary afferent fibers and motoneurons is mediated by an excitatory amino acid, most likely glutamate or aspartate. No such comparable studies have been carried out in the trigeminal system. In an attempt to elucidate the neurotransmitter(s) mediating monosynaptic transmission between mesencephalic of V nucleus afferents (Mes V) and trigeminal jaw-closer motoneurons, the effect of iontophoretic application of excitatory amino acid antagonists on the Mes V-induced field potential, recorded in the trigeminal motor nucleus (Mot V), was examined. Application of DL-2-amino-4-phosphonobutyrate (APB) and the broad spectrum amino acid antagonists, kynurenic acid (KYN) and gamma-D-glutamylglycine (DGG), for 3-4 min reversibly reduced the amplitude of the Mes V induced field potential. The effect of APB was much greater than any of the other compounds tested. On the other hand, the specific N-methyl-D-aspartate (NMDA) receptor blocker DL-2-amino-5-phosphonovaleric acid (APV), was without effect on the field potential. Based on current-response curves for each antagonist tested, the order of potency was determined to be APB greater than KYN greater than DGG greater than APV. These antagonists were also compared with respect to their efficacy in blocking individual jaw-closer motoneuron activity induced by iontophoretic application of amino acid receptor excitants glutamate (Glut), aspartate (Asp), kainate (K), and quisqualate (Q). NMDA application was without effects on these motoneurons. The profile of activity of these antagonists on these amino acid excitants was similar to that found in other areas of the CNS by other investigators. KYN and DGG both significantly reduced responses induced by all excitants tested, whereas APB had more modest effects on K and Q excitation and was without effect on Glut and Asp excitations in most cells tested. The data suggest that an excitatory amino acid, activating non-NMDA receptors, mediates some component of synaptic transmission between Mes V afferents and jaw-closer motoneurons. The data is also consistent with the proposal made in other systems that APB blocks synaptic transmission by a mechanism other than postsynaptic receptor blockade.

Fade of the response to prolonged glutamate application in the rat hippocampal slice

The effect of prolonged glutamate (GLU) application was examined on 60 CA1 pyramidal neurons in the in vitro rat hippocampal slice preparation. Continuous application of L-GLU, either by bath perfusion (0.5-2 mM) of the slices or iontophoresis (200 mM) into the dendritic region of the neurons, elicited a transient depolarization which faded to a mean of 53% of the initial peak amplitude despite continued exposure to the agonist. Membrane depolarization to aspartate (ASP) and the d-isomer of GLU also faded with time. In contrast, the depolarizing response to the excitatory amino acid agonists N-methyl-D,L-aspartate (NMA), quisqualate (QUIS), and kainate (KA) did not fade significantly during continuous application. The fade of the GLU depolarization was not affected by the NMDA antagonist D-2-amino-5-phosphonovalerate (APV) or by blocking synaptic transmission with tetrodotoxin. At the time of maximum fade of the GLU depolarization, there was no change in input resistance or GLU reversal potential. In addition, fade of the response was not a consequence of changes in extracellular potassium concentration, GLU uptake mechanisms, or the electrogenic pump. The most likely explanation for fade is postsynaptic receptor desensitization.

alpha-[3H]Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding to rat striatal membranes: effects of selective brain lesions

The binding of alpha-[3H]amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA), a structural Glu analog, to rat striatal membranes was studied. In the absence of potassium thiocyanate and Cl-/Ca2+, saturation-curve analysis of [3H]AMPA binding suggested that a single class of noninteracting binding sites with a KD value of 340 +/- 27 nM was involved, although AMPA inhibition of [3H]AMPA binding set at a concentration of 100 nM suggested, in contrast, the presence of multiple populations of striatal binding sites. Several other excitatory amino acid receptor agonists and antagonists were tested, and the most potent and selective quisqualic acid (QA) receptor agonists (QA, L-Glu, and AMPA) were found to represent the most potent inhibitors of [3H]AMPA binding. N-Methyl-D-aspartate receptor agonists and antagonists were ineffective as displacers of the [3H]AMPA binding. Lesions of intrastriatal neurons (using kainic acid local injections) and of corticostriatal afferent fibers led 2-3 weeks later to large decreases (63 and 30%, respectively) in striatal [3H]AMPA binding, whereas selective lesion of the nigrostriatal dopaminergic pathway (using nigral injection of 6-hydroxy-dopamine) was without any influence. Taken together, these results suggest that [3H]AMPA binding is primarily associated with postsynaptic intrastriatal neurons. Some [3H]AMPA binding sites may also be located presynaptically on corticostriatal nerve endings. So, in addition to the possibility that [3H]AMPA binding sites may be involved in corticostriatal synaptic transmission, it is interesting that these putative QA-preferring excitatory amino acid receptor sites may also play some role in autoregulatory processes underlying this excitatory synaptic transmission.

Excitatory amino acid antagonists and endogenous aspartate and glutamate release from rat hippocampal slices

1. The effect of excitatory amino acid agonists and antagonists on the efflux of endogenous aspartate and glutamate from the rat hippocampus in vitro was studied. 2. None of the compounds tested had any effect on the basal efflux of endogenous aspartate and glutamate. 3. 2-Amino-5-phosphonovaleric acid (APV), 2-amino-7-phosphonoheptanoic acid (APH) and MK-801 all reduced the potassium-evoked efflux of aspartate and glutamate by between 14.9% and 34.3% (P less than 0.05). 4. The depression of efflux brought about by APV was still observed in the presence of tetrodotoxin. 5. Neither N-methyl-D,L-aspartate nor quinolinic acid had any effect on the potassium-evoked efflux of aspartate and glutamate. 6. These results imply the existence of presynaptic amino acid receptors that are capable of modulating the efflux of endogenous aspartate and glutamate.

Changes in the relative amounts of aspartate and glutamate released and retained in hippocampal slices during stimulation

It has been found previously that the ratio of aspartate to glutamate released and retained by brain slices reversibly changes with changing glucose concentrations in the medium. To find out whether increased neuronal activity also results in changes in the ratio of aspartate to glutamate, in this study electrical-field stimulation was applied for 10 min to hippocampal slices in the presence of 0.2-5 mM glucose. In 5 mM glucose, the ratio of aspartate to glutamate released did not change during stimulation, but the amount of aspartate retained at the end of stimulation was reduced. In contrast, in 1 mM or less glucose, the ratio of aspartate to glutamate released increased progressively and the rate of increase was inversely proportional to the glucose content of the medium. The evoked release of aspartate and glutamate both in low and high glucose was nearly suppressed in low (0.1 mM) Ca2+ or by tetrodotoxin. In low glucose, the ratio of aspartate to glutamate contained in the slices also increased as a result of stimulation. This increase was reduced only a little in low Ca2+, but was nearly eliminated by tetrodotoxin. Results suggest that increased neuronal activity causes a shift in the ratio of aspartate to glutamate released in the presence of glucose concentrations similar to those found in the brain in normoglycemic rats. This shift, due to an increased energy demand, probably originates from terminals which release aspartate and glutamate in different proportions.

Glutamate receptor subtypes in cultured cerebellar neurons: modulation of glutamate and gamma-aminobutyric acid release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.

Effect of excitatory amino acid analogues on the release of D-[3H]aspartate from chick retina

There is good evidence to suggest that L-glutamate (L-Glu) and/or L-aspartate (L-Asp) might function as excitatory neurotransmitters in the vertebrate retina. Postsynaptic receptors for these compounds have been identified in both plexiform layers by means of physiological and biochemical techniques. However, the presence of excitatory amino acid receptors which could regulate the release of these compounds has not previously been demonstrated. We have now shown that the K+-stimulated, Ca2+-dependent release of [3H]D-aspartate from superfused chick retina is inhibited by L-Glu, N-methyl-D-aspartate (NMDA), kainate (KA) and several other neuroactive analogues. While alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and (+/-)2-amino-5-phosphonovaleric acid both exhibited agonist activity when tested alone, the former substance reversed the inhibition observed with L-Glu and KA whereas the latter effectively antagonized the NMDA-induced inhibition of release, possibly acting as partial agonists. The results demonstrate an interaction of NMDA and AMPA with KA probably at receptors in the chick retina that are involved in the regulation of glutamate/aspartate release.

Inhibition by phencyclidine of excitatory amino acid-stimulated release of neurotransmitter in the nucleus accumbens

The effects of phencyclidine (PCP) on the release of acetylcholine and dopamine, stimulated by excitatory amino acid agonists was examined in slices of nucleus accumbens of the rat. In slices incubated in [3H]choline or [3H]dopamine, the amount of tritium efflux produced by 1 mM N-methyl-D-aspartate (NMDA), kainic acid (KA) or quisqualic acid (QA) was compared with that produced in the presence of varying concentrations of phencyclidine. N-Methyl-D-aspartate stimulated the calcium-dependent release of both ACh and DA, which was completely inhibited by physiological concentrations of magnesium and inhibited by 2-aminophosphonovalerate (2-APV). Kainic acid- and quisqualic acid-stimulated release of ACh and DA was partially inhibited by magnesium or by 2-APV. Phencyclidine inhibited NMDA-stimulated release of ACh and DA with IC50's around 100 nM. Phencyclidine (0.1 microM) also significantly inhibited kainic acid and quisqualic acid-induced release of ACh in magnesium-free but not magnesium-containing buffer, suggesting that the effect of PCP on kainic acid- and possibly quisqualic acid-stimulated release of ACh is on that part of the response which is mediated by NMDA receptors. The results suggest that the inhibition by PCP of the release of ACh and DA in the nucleus accumbens is selective for NMDA-type receptors.

Dicarboxylic amino acids block epileptiform activity in hippocampal slice

Effects of prolonged (5-10 min) continuous perfusion of excitatory amino acids on penicillin (PEN)-evoked epileptiform activity in hippocampal slices were examined with extracellular and intracellular recordings. L-glutamate (GLU), L-aspartate (ASP), quisqualate (QUIS), and N-methyl-D,L-aspartate reversibly depressed multiple (epileptiform) population spikes elicited by PEN (1.7 mM). Intracellularly recorded, PEN-evoked paroxysmal depolarization shifts (PDS) were partially blocked by 1 mM GLU and largely eliminated by 2 mM GLU or ASP. In the presence of PEN, perfusion with both GLU and ASP induced a transient 4 to 6-mV depolarization, usually followed by spontaneous return of membrane potential to control levels. During the amino acid (AA)-induced block of epileptiform activity, there was no significant change in resting membrane potential, input resistance, or the ability to fire action potentials in response to depolarization, indicating that the decreased responsiveness is not a consequence of nonspecific pyramidal cell overdepolarization. The observed depression of epileptiform activity by continued exposure to GLU and its analogues may reflect desensitization or another regulatory mechanism to limit overexcitation.

Effects of excitatory amino acid antagonists on evoked and spontaneous excitatory potentials in guinea-pig hippocampus

Evoked and spontaneous excitatory post-synaptic potentials (e.p.s.p.s) at the mossy fibre input to CA3 pyramidal neurones were recorded intracellularly in slices from the guinea-pig hippocampus. The effects of several amino acid antagonists on these responses were examined. L-2-amino-4-phosphonobutyrate (L-AP4), L-serine-O-phosphate (L-SOP), kynurenate, and N-(p-bromobenzoyl)piperazine-2,3-dicarboxylate (pBB-PzDA) reduced the amplitude of evoked mossy fibre e.p.s.p.s without affecting membrane potential or input resistance. Antagonism of mossy fibre spontaneous miniature e.p.s.p.s (m.e.p.s.p.s) by these compounds fell into two groups. L-AP4 and L-SOP applied at concentrations that blocked evoked e.p.s.p.s did not affect amplitude distributions of spontaneous m.e.p.s.p.s. Kynurenate and pBB-PzDA significantly affected the amplitude distributions and reduced the mean amplitude of spontaneous m.e.p.s.p.s. These results are consistent with a presynaptic site of action for L-AP4 and L-SOP and a post-synaptic site of action for kynurenate and pBB-PzDA as antagonists of e.p.s.p.s at the guinea-pig mossy fibre-CA3 pyramidal neurone synapse.

The presence of N-methyl-D-aspartate-type receptors for glutamic acid in the guinea pig myenteric plexus

The actions of agonists and antagonists of excitatory amino acid receptors were studied in the isolated ileal longitudinal muscle-myenteric plexus preparation of the guinea pig incubated 'in vitro', by recording the contraction of the longitudinal muscle. L-Glutamate, L-aspartate, quinolinate and N-methyl-D-aspartate (NMDA), in concentrations ranging from 10(-6) to 10(-4) M, induced a rapid contraction of this preparation while kainate and quisqualate were not active at a concentration of 10(-4) M. The excitatory amino acid responses were competitively antagonized by 2-amino-5-phosphonovalerate. They were also prevented by Mg2+ ions (0.1-1 mM), by tetrodotoxin 3 X 10(-6) M and by hyoscine 10(-7) M. The last observations suggest that the myenteric cholinergic interneurons are in some way involved in the glutamate-induced ileal contraction. These results demonstrate the existence of receptors for excitatory amino acids (possibly of NMDA type) in the myenteric plexus of the guinea pig.

Polyamines contribute to calcium-stimulated release of aspartate from brain particulate fractions

The calcium-stimulated release of several neurotransmitters has been studied in crude synaptosomal preparations following accumulation of various radioactively labeled neurotransmitters or their precursors. The high affinity uptake of D-aspartate, gamma-aminobutyric acid (GABA) and dopamine was not modulated in the presence of 5 mM of difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase (ODC). Depolarization-induced calcium influx caused release of labeled compounds. In the case of D-aspartate, this process was inhibited by DFMO. This major inhibition was completely reversed in the presence of 0.5 mM putrescine, the product of ODC-mediated catabolism of ornithine. This effect could not be shown for the depolarization-related release of dopamine, GABA or acetylcholine. In our preparation, a direct effect of the depolarizing medium upon levels of synaptosomal ODC could not be demonstrated, and the 50 mM K+-stimulated entry of 45Ca into the synaptosomal fraction was not inhibited by DFMO. These data suggest that polyamines are involved in the regulation of neurotransmitter release within certain classes of neurons.

The primary afferent depolarizing action of kainate in the rat

Dorsal roots (L3-L7) isolated from immature (1-9 day old) rats were depolarized selectively by kainate (1-100 microM). L-Glutamate (25-100 microM), but not L-aspartate, mimicked the action of kainate. N-methylaspartate had no activity on these preparations and quisqualate was thirty times less active than kainate. Depolarizations evoked by L-glutamate (100-1000 microM) faded rapidly in the presence of L-glutamate. Depolarizations evoked by kainate were depressed during the fade induced by L-glutamate. Certain electrically evoked C-fibre volleys in dorsal roots or leg nerves of rats at any age were selectively depressed or abolished in the presence of kainate. The effect of kainate was more selective than that of gamma-aminobutyric acid or capsaicin. Prolonged treatment of dorsal roots with kainate did not appear to be deleterious to C-fibres. It is suggested that certain primary afferent C-fibres possess kainate receptors which may be activated physiologically by L-glutamate released at their central terminations.

Heterogeneity of sodium-dependent excitatory amino acid uptake mechanisms in rat brain

The pharmacologic and kinetic characteristics of sodium-dependent uptake of [3H]L-glutamate, [3H]D-aspartate, and [3H]L-aspartate into crude synaptosomal preparations of rat corpus striatum and cerebellum have been examined in vitro. In cerebellum the apparent Kts and Vmax for the three excitatory amino acids were identical whereas in striatal synaptosomes, the Vmax for [3H]L-glutamate was 30% greater (P less than or equal to .001) than for [3H]D-aspartate and 50% greater (P less than or equal to .001) than for [3H]L-aspartate. L-Amino adipic acid inhibited the uptake of the three amino acids in both regions of brain was 15- to 20-fold more potent in cerebellum than in striatum. In contrast, dihydrokainic acid inhibited transport processes in the corpus striatum but was without activity in cerebellar preparations. The neurotoxin kainic acid blocked only a portion (60%) of [3H]L-glutamate and [3H]D-aspartate uptake in cerebellum while completely inhibiting amino acid transport in corpus striatum. Three days post kainic acid lesion, [3H]D-aspartate uptake was attenuated more than [3H]L-glutamate uptake in the corpus striatum; destruction of corticostriatal afferents reduced [3H]L-glutamate to a greater extent than [3H]D-aspartate. Various lesions of the cerebellum affected excitatory amino acid transport processes to a similar extent. These results suggest that excitatory amino acid transport systems are pharmacologically distinct in different brain regions and may be heterogeneous within a single region.

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

Structure-activity of primary afferent depolarising action (PAD) mediated by gamma-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for L-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.

Excessive glutamate as an inhibitor of excitatory transmission in rat hippocampal slice

Exposure of rat hippocampal slices to perfusate containing 1-2 mM glutamate (GLU) induces reversible and relatively selective blockade of excitatory transmission. Intracellular recordings from 20 region CA1 hippocampal cells demonstrated only transient and mild effects on resting membrane properties and action potentials. In contrast, in 2 mM GLU excitatory postsynaptic potentials declined to 28% of control (P less than 0.001); inhibitory postsynaptic potentials remained robust at 88% of control. This suggests that excess exposure to GLU may result in a selective 'down-regulation' of excitatory synaptic transmission, while preserving inhibitory pathways. These observations may have practical implications for development of new anticonvulsant drugs.

Presynaptic regulation by ACh of the NE mediated responses in the rat vas deferens

In the isolated electrically stimulated vas deferens preparation the effect of exogenous acetylcholine (ACh) was studied. It was possible to differentiate two separate sites for the action of ACh. A postsynaptic effect (M1) which is revealed as a sudden decrease in the basal tension of the muscular twitch, antagonized competitively by atropine (pA2 = 8.44 +/- 0.79), potentiated at a ratio of 10.24 by neostigmine, and not altered by hexamethonium, yohimbine, clonidine, theophylline or prazosin. Treatment with reserpine or 6-OH-DA induced a supersensitivity of this effect. The second action is a presynaptic effect (M2), which is manifested by a rapid increase in the muscular twitch, is dose-dependent to ACh, and is potentiated significantly by reserpine, neostigmine and clonidine. This latter effect was not modified by atropine, hexamethonium, yohimbine, prazosin or theophylline. The normal efficacy of ACh is 185 times greater in presynaptic over postsynaptic receptors. The increase in the twitch is not considered to be by the release of endogenous ACh or ATP. The evidence presented here indicates that the presynaptic effect of ACh (M2) is due to participation of alpha 2-adrenoceptors in enhancement of the release of norepinephrine (NE) from the nerve terminals.

A role for taurine in the maintenance of homeostasis in the central nervous system during hyperexcitation?

Employing the brain dialysis technique, we demonstrate that the aspartate congener N-methyl-D,L-aspartic acid (NMA) stimulates the release of endogenous taurine in vivo in a Ca2+-dependent manner. Furthermore, exogenous taurine (1-10 mM) inhibits the NMA-induced Ca2+ influx into intracellular compartments. This suggests that the extracellular taurine concentration may control Ca2+ movement and thereby provide a homeostatic mechanism in situations of excessive excitation.