The uptake and release of putative amino acid neurotransmitters

Excitotoxicity: Still Hammering the Ischemic Brain in 2020

Interest in excitotoxicity expanded following its implication in the pathogenesis of ischemic brain injury in the 1980s, but waned subsequent to the failure of N-methyl-D-aspartate (NMDA) antagonists in high profile clinical stroke trials. Nonetheless there has been steady progress in elucidating underlying mechanisms. This review will outline the historical path to current understandings of excitotoxicity in the ischemic brain, and suggest that this knowledge should be leveraged now to develop neuroprotective treatments for stroke.

Na+-dependent high-affinity uptake ofl-glutamate in cultured fibroblasts

Uptake of 1 microM [3H]L-glutamate by cultured 3T3 fibroblasts was strongly dependent on extracellular Na+; it was reduced by elevated concentrations of K+ (60 mM) but it was not influenced by variations in the concentration of Ca2+ (0-9.6 mM). D- and L-Asparate, D- and L-threo-3-hydroxyaspartate DL-threo-3-methylaspartate and a few other glutamate derivatives and analogues inhibited the uptake but several close analogues of L-glutamate (including D-glutamate) had no effect, implying that the uptake system is highly structurally selective. The recently identified inhibitor of glutamate uptake in synaptosomal preparations, L-trans-pyrrolidine-2,4-dicarboxylate, was also among the inhibitors. Apparent Km of the uptake was found to be less than 10 microM. The present observations indicate that Na(+)-dependent 'high-affinity' uptake of L-glutamate may appear in structures which are apparently unrelated to glutamatergic synaptic transmission in the CNS.

Cell lineage of the subcommissural organ secretory ependymocytes: differentiating role of the environment

SCO-ependymocytes have a secretory activity and a neural innervation relating them to neurosecretory nerve cells. To elucidate the cell lineage of the SCO-ependymocytes and emphasize the role of the neural innervation in their differentiation, in particular 5-HT innervation, we analyzed the developmental pattern of expression of several glial and neuronal markers: (1) in the SCO of mammals possessing (rat, cat) or devoid (mouse, rabbit) of 5-HT innervation, (2) in rat 5-HT deafferented SCO, and (3) in rat SCO transplanted in a foreign environment, the fourth ventricle. The ability of SCO-ependymocytes to transiently express GFAP during development and express the glial alpha alpha-enolase confirms the glial lineage of the SCO-ependymocytes. Synthesis of vimentin by SCO-ependymocytes relates them to the classical ependymocytes. The ability of mature SCO-ependymocytes to take up GABA only when they are innervated by 5-HT terminal underlines the role of the neural environment on the differentiation of these ependymocytes and suggests that differential maturation of the SCO according to its innervation, may lead to specific functional specialization of this organ in different species.

Forty years of amino acid transmission in the brain

This article is concerned with the discovery that amino acids, particularly L-glutamate and gamma-aminobutyrate (GABA), are central neurotransmitters. The crucial observations that lead to the conclusion that these two amino acids produce most of the synaptic excitation and inhibition in the central nervous system, were made in late 1950's. The combination of neurochemical knowledge and improved electrophysiological techniques was paramount in making these discoveries possible. In particular, the use of specific antagonists in microiontophoretic experiments provided the most decisive evidence. The relationship is also explored between these early findings and those of the present era characterised by extensive use of techniques of molecular biology and the development of drugs against targets identified 30 to 40 years ago.

Role of excitatory aminoacids in neonatal hypoglycemia

BACKGROUND

In many neurological disorders, injury to neurons may be due in part to overstimulation of the receptors for the excitatory amino acids glutamate and aspartate. The same excitotoxic mechanism and high aspartate levels in experimental studies led to this study of the concentrations of glutamate and aspartate and zinc, copper, and magnesium levels in the cerebrospinal fluid (CSF) of hypoglycemic newborns.

METHODS

Aspartate and glutamate were determined by high-performance liquid chromatography, and magnesium, zinc and copper by atomic absorption spectrophotometer.

RESULTS

The CSF levels of aspartate (3.98 +/- 1.77 mumol/L) and glutamate (1.7 +/- 1.05 mumol/L) in 20 hypoglycemic newborns were significantly higher when compared with the values of aspartate (2.19 +/- 0.6 mumol/L) and glutamate (0.77 +/- 0.34 mumol/L) of 10 control newborns. In the hypoglycemic patients, the concentration of zinc (0.57 +/- 0.13 microgram/mL), but not copper (0.39 +/- 0.40 microgram/mL) was significantly lower when compared with the control values. There was no difference in the magnesium levels between the two groups.

CONCLUSIONS

The higher levels of excitatory amino acids found in the CSF of hypoglycemic infants than in controls were consistent with previous animal studies, which may indicate the role of excitatory amino acids in the late biochemical effects of hypoglycemia in newborn brain metabolism.

Strychnine-dependent allodynia in the urethane-anesthetized rat is segmentally distributed and prevented by intrathecal glycine and betaine

The blockade of spinal glycine receptors with intrathecal strychnine produces a reversible allodynia-like state in the rat. Thus, hair deflection, in the presence of intrathecal strychnine, induces cardiovascular and motor withdrawal responses comparable with those evoked by noxious thermal, mechanical, or chemical stimulation in the absence of strychnine. In the present study, we mapped the cutaneous sites of abnormal sensitivity to hair deflection throughout the strychnine time course to investigate the segmental distribution of strychnine-induced allodynia. The ability of intrathecal glycine and the glycine derivative betaine to reverse strychnine-induced allodynia was also determined using dose-response analysis. Following intrathecal strychnine (40 micrograms), stroking the legs, flanks, lower back, and tail with a cotton-tipped applicator evoked a pronounced increase in mean arterial pressure, tachycardia, and an abrupt motor withdrawal response in urethane-anesthetized rats. These abnormal responses were only evoked by hair deflection at discrete sites, corresponding to the cutaneous dermatomes innervated by spinal segments near the site of strychnine injection. In rats with intrathecal catheters lying laterally in the subarachnoid space, allodynic sites were observed unilaterally on the ipsilateral side of intrathecal strychnine injection. Recovery from strychnine was complete by 30 min in all affected dermatomes. The cardiovascular and motor withdrawal responses to hair deflection were dose dependently inhibited by intrathecal glycine and intrathecal betaine. The ED50 (95% confidence interval) for intrathecal glycine was 609 (429-865) micrograms for the heart rate response, 694 (548-878) micrograms for the pressor response, and 549 (458-658) micrograms for the motor withdrawal response. The corresponding values for intrathecal betaine were 981 (509-1889), 1045 (740-1476), and 1083 (843-1391) micrograms, respectively. There was no difference in the effect of betaine on sensory-evoked cardiovascular and motor responses. Cortical electroencephalographic activity was not affected by intrathecal glycine or betaine, consistent with a spinal locus of action in reversing strychnine-induced allodynia. These results support the hypothesis that removal of spinal glycinergic modulation from low threshold afferent input with intrathecal strychnine results in segmentally localized, tactile-evoked allodynia.

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

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

Release of amino acids into regional superfusates of the spinal cord by mechano-stimulation of the reproductive tract

Based on pharmacological evidence that inhibitory amino acids mediate vaginocervical mechano-stimulation produced analgesia (VSPA), we hypothesized that inhibitory amino acids would be released endogenously in the spinal cord in response to vaginocervical mechano-stimulation (VS). This hypothesis was tested by HPLC analysis of the amino acid content of 5-min superfusates of the spinal cord before, during and after VS (400 g force applied against the cervix) in urethane-anesthetized rats. Utilizing an in vivo push-pull superfusion method, artificial cerebrospinal fluid was continuously superfused over the spinal cord through the intrathecal space surrounding the sacral-lower thoracic region. In addition, concentrations of amino acids in the superfusate were measured in response to KCl stimulation (increasing the superfusion medium from 3.4 to 40.0 mM KCl to produce non-specific depolarization), and noxious hind paw mechano-stimulation (pinching the hind paw to produce a sustained flexor response in ipsilateral hind leg). There was a significant increase in the concentration of Gly, Tau, Asp, Glu and Lys in the superfusate in response to VS (n = 8) and to KCl (n = 8), but not to hind paw stimulation (n = 5). Also, GABA concentrations increased in response to KCl, and the concentration of Ala, Ser, Gln, Thr, Arg and Phe increased in response to VS, however, GABA levels were sometimes below the limits of detection. In contrast, there was no significant change in any amino acid concentration in response to hind paw pinch stimulation, and VS did not significantly affect the concentrations of Tyr, His, Ile, Leu, Met, Trp or Val. The present findings support our hypothesis that VS releases inhibitory amino acids in the spinal cord. Moreover, other amino acids, including 'excitatory' amino acids, are released into the superfusate. The profile of amino acid release in response to VS differs from that in response to paw pinch or KCl administration.

Localization and transport of N-acetylaspartylglutamate in cells of whole murine brain in primary culture

N-Acetylaspartylglutamate (NAAG) is the most abundant neuropeptide in the mammalian nervous system. Considerable data support the hypothesis that NAAG is synaptically released in a manner consistent with neurotransmission. Primary murine brain cultures containing neurons and glia expressed 1.2-3.5 nmol of NAAG/mg of protein. In contrast to conclusions drawn from immunohistochemistry, pure glial cultures also expressed high levels of NAAG (0.6-2.11 nmol/mg of protein). These data suggest that although a subpopulation of neurons contains very high NAAG levels, micromolar concentrations of the peptide also are present in glia. Both culture types demonstrated robust extracellular peptidase activity when incubated with NAAG, as well as peptide transport. Uptake of [3H]NAAG was both temperature and sodium dependent, yet relatively insensitive to the presence of extracellular glutamate. These results indicate that synaptically released NAAG, as well as that which may be released from glia, is removed from the extracellular space by direct uptake as well as the robust enzymatic degradation of the peptide. A kinetic analysis of this NAAG transport (estimated Km = 1.8 microM) suggests a high-affinity NAAG transport system. The balance of the two processes of direct peptide uptake and peptide hydrolysis would markedly influence the sequence of receptor-mediated events that follow NAAG release.

Heterogeneity of high affinity uptake of L-glutamate and L-aspartate in the mammalian central nervous system

Characteristics of high affinity uptake of L-glutamate are examined in order to evaluate the possible use of the uptake of [3H]L-glutamate, [3H]L-aspartate or any other suitable [3H]-labelled substrate as a marker for glutamatergic and aspartergic synapses in autoradiographic studies in the mammalian brain. Review of data on substrate specificity indicates the presence of at least two high affinity uptake systems specific for acidic amino acids in the central nervous tissue; one which takes up L-glutamate and L-aspartate and the other which is selective for L-glutamate only. Studies on ionic requirements, too, point to the existence of at least two distinct uptake systems with high affinity for L-glutamate. The Na(+)-dependent uptake system(s) handle(s) both L-glutamate and L-aspartate whereas the Na(+)-independent uptake system(s) show(s) selectivity for L-glutamate only. Available data do not favour the Na(+)-dependent binding of [3H]D-aspartate to thaw-mounted sections of frozen brain tissue as a suitable marker for glutamatergic/aspartergic synaptic nerve endings. However, there are reasons--such as the results of lesion studies and the existence of uptake sites which have a higher affinity for L-aspartate than for D-aspartate--to suggest that Na(+)-dependent binding of [3H]L-aspartate, rather than that of [3H]D-aspartate, should be further investigated as a possible marker for the glutamatergic/aspartergic synapses in the autoradiographic studies using sections of frozen brain.

Effect of depolarization on striatal amino acid efflux in perinatal rats: an in vivo microdialysis study

We used in vivo microdialysis to determine if infusion of depolarizing concentrations of potassium stimulated striatal excitatory amino acid (EAA) efflux in post-natal day (PND) 7 rats. Dialysis probes were perfused with 100 mM KCl for 60 min (n = 6); EAA efflux was unaffected until 40-60 min after onset of the infusion, when a trend towards increased EAA efflux was observed (glutamate 284 +/- 56% of baseline). In animals exposed to 8% oxygen (n = 7) before a more prolonged (100 min) KCl infusion, again over the first 40 min of KCl there were no changes in EAA efflux; subsequently, glutamate, aspartate and taurine efflux increased (peak values 682 +/- 187%, 228 +/- 32%, and 1208 +/- 437% of baseline). These data suggest that in PND 7 rats a substantial contribution to basal striatal EAA efflux may be derived from non-neurotransmitter pools.

The pharmacological specificity of N-methyl-D-aspartate receptors in rat cerebral cortex: correspondence between radioligand binding and electrophysiological measurements

1. The pharmacological specificity of N-methyl-D-aspartate (NMDA) receptors has been investigated in the rat cerebral cortex by use of radioligand binding and electrophysiological techniques. 2. A comparison was made between a functional assay (NMDA-induced depolarizations in a rat cortical slice preparation) and NMDA-sensitive L-[3H]-glutamate binding in the same brain region and species, to provide accurate affinity values for agonists and antagonists at the NMDA recognition site. 3. In a preparation of crude postsynaptic densities (PSD) from rat cortex, L-[3H]-glutamate bound with high affinity to an NMDA-sensitive population of sites with KD (geometric mean (-s.e.mean. + s.e. mean) = 120 (114, 126) nM, Bmax (mean +/- s.e.mean) = 11.4 +/- 0.8 pmol mg-1 protein and Hill coefficient (mean +/- s.e.mean) = 1.2 +/- 0.17 (n = 3). 4. There was a good agreement between the relative affinities in radioligand binding and electrophysiological assays for the receptor agonists NMDA, N-methyl-L-aspartate, quinolinate and trans-2,3-piperidine dicarboxylate, which are poor substrates of acidic amino acid transport systems. However, agonists which are good substrates for high affinity uptake systems (L- and D-glutamate, L- and D-aspartate, D-aspartate-beta-hydroxamate and L-glutamate-gamma-hydroxamate) were much weaker in the electrophysiological experiments. 5 Schild analysis of the antagonism of NMDA responses in the rat cortical slice by DL-3(2- carboxypiperazin-4-yl)propyl-1-phosphonate, D- and DL-2-amino-5-phosphonovalerate, D- and DL-2- amino- 7-phosphonoheptanoate, D-beta-aspartylaminomethylphosphonate, D-gamma-glutamylglycine and D-Ofaminoadipate (D-AA) indicated a competitive interaction with respective pA2 values of 6.17, 5.62, 5.24, 5.28, 5.20, 5.00, 4.43 and 3.97. 6 In the radioligand binding experiments the same antagonists inhibited only the NMDA-sensitive component of L-[3H]-glutamate binding. IC50 values showed a good correlation with the pA2 values (correlation coefficient = 0.96), with the exception of D-AA which was more potent than anticipated in the binding experiments (IC50 = 9.8 microM).7 These results confirm that NMDA-sensitive L-[3H]-glutamate binding sites represent the NMDA recognition site of the NMDA receptor and provide affinity values for both agonists and antagonists in the rat cerebral cortex, agreeing well with previous estimates in this and other tissues.

Amino acid-mediated EPSPs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord

1. Fast excitatory postsynaptic potentials (EPSPs) evoked by stimulation of A delta and C fibres were examined by intracellular recording from substantia gelatinosa (SG) neurones in a transverse slice preparation of adult rat spinal cord. 2. Single low-intensity stimuli applied to the dorsal root activated A delta fibres and evoked monosynaptic EPSPs in 70% of SG neurones. In 5% of SG neurones, increasing the intensity and duration of stimulation evoked solely C fibre-mediated EPSPs. About 20% of neurones received both A delta and C fibre input from primary afferents. 3. Low concentrations of tetrodotoxin (TTX, approximately 50 nM) blocked EPSPs evoked by stimulation of A delta fibres without affecting those evoked by C fibre stimulation. Higher concentrations of TTX (500 nM) also blocked C fibre-evoked responses. 4. EPSPs evoked by A delta and C fibre stimulation reversed in polarity at membrane potentials near 0 mV, similar to the reversal potential of spontaneous EPSPs and of the potential change evoked by exogenous glutamate. 5. A delta and C fibre-evoked EPSPs were depressed by kynurenate and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); C fibre-evoked EPSPs appeared to be less sensitive. 6. In the presence of TTX, only 50% of SG neurones were depolarized by L-glutamate. However, neurones which exhibited no direct response to L-glutamate received afferent-evoked EPSPs which were sensitive to CNQX. In sensitive neurones, the depolarization evoked by L-glutamate was depressed by only approximately 15% in the presence of CNQX, whereas afferent-evoked EPSPs recorded from the same neurone were almost completely suppressed. Combined application of DL-2-amino-5-phosphonovaleric acid (APV) and CNQX depressed the response to L-glutamate by only approximately 25%. 7. These findings suggest that A delta and C fibres use L-glutamate or a related amino acid as a transmitter at synapses with substantia gelatinosa neurones. The postsynaptic actions of this transmitter are mediated predominantly by non N-methyl-D-aspartic acid (NMDA) receptors. The failure of CNQX and APV to completely block the L-glutamate-evoked depolarization of substantia gelatinosa neurones raises the possibility that exogenously applied L-glutamate activates a non-NMDA receptor distinct from that which mediates the actions of the synaptically released afferent transmitter.

Effects of serotonin on the hypothalamic-pituitary GABAergic system

The effects of serotonin (5-HT) and its precursor, 5-hydroxytryptophan (5-HTP) on the GABAergic system in the mediobasal hypothalamus (MBH) and the anterior pituitary were studied. The IP administration of 5-HTP produced a transient increase (only at 45 min after the injection) in glutamate decarboxylase activity (GAD) of MBH and in GABA concentration in anterior pituitary. Besides, 5-HTP administration increased the in vitro evoked GABA release from MBH. The administration of 5-HTP to hypophysectomized rats partially reversed the inhibitory effects of hypophysectomy on GABA concentration in MBH. We also examined the direct effect of 5-HT on some parameters on the hypothalamic GABAergic system. The presence of 5-HT in the incubation medium increased GAD activity and evoked GABA release from MBH. These observations indicate that the serotoninergic stimulation of the hypothalamic GABAergic system could be a direct effect which may, at least partially, be independent of the feedback mechanism induced by prolactin on the GABAergic neurons. The serotoninergic increase of prolactin secretion could be accomplished through stimulation of the hypothalamic GABAergic transmission.

Electrophysiology of glutamate and sodium co-transport in a glial cell of the salamander retina

1. Müller cells were isolated from salamander retinas and their membrane voltage was controlled with a whole-cell voltage clamp. External D-aspartate, L-aspartate and L-glutamate each induced a membrane current. D-Glutamate, kainate, quisqualate and N-methyl-D-aspartate were more than 100x less effective than L-aspartate. Kynurenic acid had no effect on the current produced by L-glutamate, L-aspartate or D-aspartate. 2. The current induced by an acidic amino acid (AAA) was completely dependent on the presence of external Na+. Neither Li+, Cs+, choline nor TEA+ were able to substitute for Na+. The relationship between external Na+ concentration and current amplitude can be explained if the binding of three Na+ ions enabled transport. The apparent affinity constant for Na+ binding was 41 mM. Altering K+, H+ and Cl- concentrations demonstrated that these ions are not required for transport. 3. The shape of the current-voltage relation did not depend on the external amino acid concentration. The relationship between D-aspartate concentration and current amplitude can be described by the binding of D-aspartate to a single site with an apparent affinity constant of 20 microM. 4. Influx and efflux of AAA were not symmetric. Although influx was electrogenic, efflux did not produce a current. Moreover, influx stimulated efflux; but efflux inhibited influx. 5. Removing external Na+ demonstrated that Na+ carried a current in the absence of an AAA. Li+ was a very poor substitute for Na+. This current may be due to the uncoupled movement of Na+ through the transporter. The relationship between the external Na+ concentration and the amplitude of the uncoupled current can be explained if the binding of two or three Na+ ions enabled the translocation of Na+ in the absence of an AAA. The apparent affinity constant for Na+ binding was approximately 90 mM. 6. The temperature dependence of the AAA-induced current had a Q10 between 8 and 18 degrees C of 1.95. The Q10 is consistent with a rate constant for influx of 10(4) s-1 (at -70 mV and 20 degrees C). The maximum rate of influx was measured following a concentration jump produced by the photolysis of 'caged' L-glutamate. The onset of the observed current was limited by the 1.3 ms resolution of the recording system. Hence, the rate constant for influx must be faster than 10(3) s-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemical pharmacology of isolated neuronal growth cones: implications for synaptogenesis

The neuronal growth cone is critical to the establishment of neuronal polarity through its motile, pathfinding and target recognition properties exhibited during synaptogenesis. Subcellular fractionation procedures yielding milligram quantities of isolated growth cones has allowed for biochemical and pharmacological investigation of intrinsic growth cone components that are likely to be involved in regulation of growth cone function in neuronal development. These 'mapping' studies of growth cone components are prerequisites to elucidating the mechanisms by which extracellular factors influence the motility, adhesion and directed growth of the growth cone. For example, neurotransmitters and polypeptide growth factors which have been shown in other systems to modulate growth cone behavior are presumed to act through receptors on the growth cone, inducing second-messenger molecule formation and consequent modification and regulation of proteins effecting the response(s) of the growth cone (i.e. proteins involved in motility, adhesion and membrane turnover). In a relatively short period of time, work with the isolated growth cone preparation has identified, in independent studies, many of the elements involved in this proposed scheme of events, including transmitter receptors, second-messenger cascades, and second-messenger post-translational modifications. An obvious future goal will be to analyze in more detail the intracellular events, and the relationships between them, in the growth cone and how they transmit extracellular signals into responses such as motility and adhesivity which underly the growth cone's synaptogenic properties. It is to be expected that much of this information will come forth from experimentation with the isolated growth cone preparation.

Induction of locomotion in spinal tadpoles by excitatory amino acids and their agonists

Bath application of the excitatory amino acids L-aspartate and/or L-glutamate or their agonists N-methyl-D,L-aspartate and/or kainate elicited swimming movements in spinal tadpoles. Swimming cycles induced by the amino acids were in the frequency range of natural movements, and could be evoked after sectioning all dorsal roots in the exposed spinal segments. Locomotion was only elicited by L-aspartate or L-glutamate at low concentrations when the bath medium was rapidly circulated over the exposed surface of the spinal cord, and was of much shorter duration than the agonist-induced movements. These results indicate some differences between the actions of L-aspartate and L-glutamate and their agonists on the tadpole spinal cord.

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.

Release of [3H]L-glutamate and [3H]L-glutamine in rat cerebellum slices: a comparison of the effect of veratridine and electrical stimulation

Depolarization-elicited release of neurotransmitter glutamate was studied in rat cerebellar slices previously loaded with either [3H]L-glutamate or [3H]L-glutamine. Both depolarization conditions used (e.g. long-lasting tonic depolarization elicited by veratridine, or short repetitive electrical pulses) increased 6 to 8 folds the release of labelled glutamate and of another compound, presumably alpha-ketoglutarate, without modifying the release of labeled glutamine. Because of the position of the label in the precursor radioactive molecules, GABA was weakly labeled and aspartate was unlabeled. The properties of the evoked glutamate release from cerebellar slices were those of a neurotransmitter since it was inhibited by tetrodotoxin and was Ca2+-dependent. Alpha-ketoglutarate is either coreleased from nerve terminals or is released from astrocytes and could participate in glutamate recycling. The data confirm the generally accepted model implying the presence of two neurotransmitter glutamate pools, a neuronal pool of newly synthesized glutamate and an astrocytic storage pool, but in addition indicate that the former is in rapid isotopic equilibrium with the extracellular compartment. Our present results also indicate that the glutamate/glutamine cycle is not activated in depolarizing conditions.

Potassium-induced release of endogenous glutamate and two as yet unidentified substances from the lateral line of Xenopus laevis

The release of endogenous glutamate and other primary amines from the lateral-line of Xenopus laevis was studied using an in vitro superfusion technique and high performance liquid chromatography. Potassium stimulation (50 mM KCl) applied to 60 or 120 lateral-line organs dissected from the skin and pooled in a perfusion chamber induced the release of glutamate and aspartate. The release of aspartate was smaller than that of glutamate and more variable. A variable release of two, as yet, unidentified substances was also detected. In low calcium (0.1 mM CaCl2), high magnesium (10 mM MgCl2) solution, 50 mM potassium failed to induce an increase in glutamate, aspartate and the two unknowns, suggesting they are released in a transmitter-like manner. The technique presents a new and simple method for studying transmitters in hair-cell systems. Although other interpretations are possible, the results are consistent with the hypothesis that glutamate is a hair-cell transmitter and suggest a potential role for other substances in the transduction process, perhaps as neuromodulators.

Neuronal control of [3H]GABA uptake in the ependymocytes of the subcommissural organ: an in vivo model of neuron-glia interaction

The rat subcommissural organ (SCO) is a particular but adequate paradigm for the approach, in vivo, to some aspects of neuron-glia interaction in gamma-aminobutyric acid (GABA) uptake. The rat SCO ependymocytes (the main component of this structure lying at the junction of the aqueduct and the third ventricle) accumulate [3H]GABA by a highly specific uptake mechanism and receive a serotoninergic input forming typical synaptic contacts. It seems that there is a correlation between the capacity of the rat SCO ependymocytes to take up [3H]GABA and the presence of a serotonin (5-HT) innervation. Indeed, in the newborn rat, no uptake of [3H]GABA was observed before the onset of this innervation and the increased [3H]GABA accumulation in the SCO was correlated with the appearance of the 5-HT terminals in the SCO. Moreover, in the mouse, whose SCO is devoid of a 5-HT innervation, no accumulation of [3H]GABA was observed in the SCO ependymocytes. Thus, the 5-HT innervation could be involved directly or indirectly in the onset of the GABA uptake carriers. On the other hand, in adult rats parachlorophenylalanine (pCPA) treatment decreased the 5-HT content of the SCO, and increased [3H]GABA accumulation; such an augmentation was not observed when rats were treated with pCPA plus 5-hydroxytryptophan to restore the 5-HT content. However, an increase in 5-HT content of the SCO by pargyline treatment appeared to have no effect on [3H]GABA uptake. Control of GABA uptake activity by 5-HT in the SCO ependymocytes could be an interesting model for the study of a possible interaction between amino-acids and other neurotransmitters by terminating their action in the extracellular space.

Release of gamma-[3H]aminobutyric acid from rat olfactory bulb and substantia nigra: differential modulation by glutamic acid

We have studied the glutamate modulation of gamma-[3H]aminobutyric acid ([3H]GABA) release from GABAergic dendrites of the external plexiform layer of the olfactory bulb and from GABAergic axons of the substantia nigra. In the olfactory bulb, [3H]GABA release was induced by high K+ and kainate, and not by aspartate and glutamate alone. However, when the tissue was conditioned by a previous K+ depolarization, glutamate and aspartate caused [3H]GABA release. The effect of glutamate was significantly enhanced when the GABA uptake mechanism was blocked by nipecotic acid. N-Methyl-D-aspartate and quisqualate did not cause [3H]GABA release under the same conditions. The acidic amino acid receptor antagonist 2-amino-4-phosphonobutyric acid and the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid significantly inhibited the K+-glutamate- and the kainate-induced [3H]GABA release. Mg2+ (5 mM), which blocks the N-methyl-D-aspartate receptors, significantly inhibited the K+-glutamate-induced but not the kainic acid-induced [3H]GABA release. The K+-glutamate-stimulated release, but not the K+-stimulated [3H]GABA release, was strongly inhibited by Na+-free solutions or by 300 nM tetrodotoxin. Apparently the glutamate-induced release of [3H]GABA occurs through an interneuron because it is dependent on the presence of nerve conduction. In the substantia nigra no [3H]GABA release was elicited by any of the glutamate agonists tested. The present results clearly differentiate between the effects of glutamate on the release of [3H]GABA from the substantia nigra and from the olfactory bulb.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous amino acid release from cultured cerebellar neuronal cells: effect of tetanus toxin on glutamate release

Endogenous amino acid release was measured in developing cerebellar neuronal cells in primary culture. In the presence of 25 mM K+ added to the culture medium, cerebellar cells survived more than 3 weeks and showed a high level of differentiation. These cultures are highly enriched in neurons, and electron-microscopic observation of these cells after 12 days in vitro (DIV) confirmed the presence of a very large proportion of cells with the morphological characteristics of granule cells, making synapses containing many synaptic vesicles. Synaptogenesis was also confirmed by immunostaining the cells with antisera against synapsin I and synaptophysin, two proteins associated with synaptic vesicles. From these cultures, endogenous glutamate release stimulated by 56 mM K+ was already detected after only a few days in culture, the maximal release value (1,579% increase over basal release) being reached after 10 DIV. In addition to that of glutamate, the release of aspartate, asparagine, alanine, and, particularly, gamma-aminobutyric acid (GABA) was stimulated by 56 mM K+ after 14 DIV, but to a lesser extent. No increase in serine, glutamine, taurine, or tyrosine release was observed during K+ depolarization. The effect of K+ on amino acid release was strictly Ca2+-dependent. Stimulation of the cells with veratridine resulted in a qualitatively similar effect on endogenous amino acid release. In the absence of Ca2+, 30% of the veratridine effect persisted. The Ca2+-dependent release was quantitatively similar after stimulation by veratridine and K+. Treatment of cerebellar cells with tetanus toxin (5 micrograms/ml) for 24 h resulted in a total inhibition of the Ca2+-dependent component of the glutamate release evoked by K+ or veratridine. It is concluded that glutamate is the main amino acid neurotransmitter of cerebellar cells developed in primary culture under the present conditions and that glutamate is probably mainly released through the exocytosis of synaptic vesicles.

Inhibition by excitatory sulphur amino acids of the high-affinity L-glutamate transporter in synaptosomes and in primary cultures of cortical astrocytes and cerebellar neurons

A detailed kinetic study of the inhibitory effects of L- and D-enantiomers of cysteate, cysteine sulphinate, homocysteine sulphinate, homocysteate, and S-sulpho-cysteine on the neuronal, astroglial and synaptosomal high-affinity glutamate transport system was undertaken. D-[3H] Aspartate was used as the transport substrate. Kinetic characterisation of uptake in the absence of sulphur compounds confirmed the high-affinity nature of the transport systems, the Michaelis constant (Km) for D-aspartate uptake being 6 microM, 21 microM and 84 microM, respectively, in rat brain cortical synaptosomes and primary cultures of mouse cerebellar granule cells and cortical astrocytes. In those cases where significant effects could be demonstrated, the nature of the inhibition was competitive irrespective of the neuronal versus glial systems. The rank order of inhibition was essentially similar in synaptosomes, neurons and astrocytes. Potent inhibition (Ki approximately Km) of transport in each system was exhibited by L-cysteate, and L- and D-cysteine sulphinate whereas substantially weaker inhibitory effects (Ki greater than 10-1000 times the appropriate Km value) were exhibited by the remaining sulphur amino acids. In general, inhibition: (i) was markedly stereospecific in favor of the L-enantiomers (except for cysteine sulphinate) and (ii) was found to decrease with increasing chain length. Computer-assisted molecular modelling studies, in which volume contour maps of the sulphur compounds were superimposed on those of D-aspartate and L-glutamate, demonstrated an order of inhibitory potency which was, qualitatively, in agreement with that obtained quantitatively by in vitro kinetic studies.

Release of endogenous N-acetylaspartylglutamate (NAAG) and uptake of [3H]NAAG in guinea pig cerebellar slices

For the purpose of obtaining chemical information about the physiological role of N-acetylaspartylglutamate (NAAG), the release of endogenous NAAG from and the uptake of [3H]NAAG by Guinea pig cerebellar slices were investigated in comparison with L-aspartate (Asp) and L-glutamate (Glu). Although endogenous NAAG was found to be released spontaneously from the slices as is endogenous Asp and Glu, high-K+-induced facilitation of release occurred only for endogenous Asp and Glu in a Ca2+-dependent manner, but not for NAAG. It was confirmed that [3H]NAAG itself was taken up in a Na+-dependent manner by the slices by two low-affinity processes with small Vmax values, and labeled Glu and glutamine were detected as the metabolites of [3H]NAAG in the slices. The [3H]NAAG uptake was slower than that of labeled Glu and was significantly depressed by NAAG, Asp, Glu and D-aspartate, but not affected by gamma-aminobutyrate, suggesting that NAAG may share a common uptake carrier with excitatory amino acids. These results suggest that endogenous NAAG may act extracellularly, but the amount of endogenous NAAG released from nerve terminals by presynaptic depolarization may be very small if any, and also that spontaneously liberated NAAG can be inactivated by low-affinity uptake systems, at least, in the Guinea pig cerebellum.

Compartmental distribution of endogenous amino acids in the substantia nigra of the rat

Using a push-pull cannula we have monitored the spontaneous efflux of 8 endogenous amino acids into perfusates of the substantia nigra of the rat. The extracellular concentrations of the amino acids were estimated and compared to their respective tissue levels. High intra-/extracellular concentration ratios were found for gamma-aminobutyric acid (GABA), aspartate, glutamate and taurine. Much lower values were found for glycine, alanine, serine and glutamine. These results provide evidence for possible neurotransmitter roles for aspartate, glutamate and taurine in the substantia nigra in addition to that, long recognized, for GABA.

Expression of neurotransmitter transport from rat brain mRNA in Xenopus laevis oocytes

To permit a molecular characterization of neurotransmitter transporter proteins, we have studied uptake activities induced in Xenopus laevis oocytes after injection of adult rat forebrain, cerebellum, brainstem, and spinal cord poly(A)+ RNA. L-Glutamate uptake could be observed as early as 24 hr after injection, was linearly related to the quantity of mRNA injected, and could be induced after injection of as little as 1 ng of cerebellar mRNA. Transport of radiolabeled L-glutamate, gamma-aminobutyric acid, glycine, dopamine, serotonin, and choline could be measured in single microinjected oocytes with a regional profile consistent with the anatomical distribution of particular neurotransmitter synthesizing soma. Forebrain L-glutamate and dopamine uptake, as well as cerebellar L-glutamate transport, were found to be Na+-dependent. Cerebellar mRNA-induced L-glutamate transport was both time and temperature-dependent, was saturable by substrate, suggesting a single activity with an apparent transport Km of 14.2 microM and a Vmax of 15.2 pmol/hr per oocyte, and was sensitive to inhibitors of brain L-glutamate transport. Thus, the oocyte L-glutamate transport induced by injection of adult rat cerebellar mRNA appears essentially identical to the high-affinity, Na+-dependent L-glutamate uptake found in brain slices and nerve terminals. Experiments with size-fractionated cerebellar mRNA reveal single, comigrating peaks for cerebellar L-glutamate and gamma-aminobutyric acid transport, with peak activity obtained in fractions of approximately 2.7 kilobases, suggesting the presence of single or similarly sized mRNAs encoding each of these activities.

Depolarization-induced release of glycine and beta-alanine from plasma membrane vesicles derived from rat brain synaptosomes

Glycine and beta-alanine actively loaded into brain synaptic plasma membrane vesicles were released into the external medium by using the classical depolarization agents high K+ and veratridine. This release occurs via a Ca2+-independent process. Measurements of membrane depolarization using tetraphenylphosphonium uptake show a close correlation between changes in the membrane potential and stimulation of the efflux process. Results shown herein and previously reported by our group (Aragón, M.C. and Giménez, C. (1986) Biochim. Biophys. Acta 855, 257-264; Agulló, L., Jiménez, B., Aragón, M.C. and Giménez, C. (1986) Eur. J. Biochem. 159, 611-617), suggest that the glycine and beta-alanine transport systems in synaptic plasma membranes are susceptible of modulation by changes in ionic fluxes and hence in the membrane potential, similar to those occurring during depolarization and repolarization.

Colocalization of glycine-like and GABA-like immunoreactivities in Golgi cell terminals in the rat cerebellum: a postembedding light and electron microscopic study

Consecutive sections of rat cerebella were incubated with antisera raised against glycine or gamma-aminobutyric acid (GABA) conjugated to protein by glutaraldehyde. The sections were subsequently processed according to the peroxidase-antiperoxidase technique (semithin sections) or treated with a secondary antibody coupled to colloidal gold particles (ultrathin sections). Corroborating previous light microscopic observations based on pre-embedding immunocytochemistry, a major proportion (about 70%) of the Golgi cell bodies showed immunoreactivity for both glycine and GABA. Analyses of semithin sections further suggested that the two immunoreactivities were colocalized in the same glomeruli and even in the same Golgi cell terminals. This was confirmed by electron microscopy. Quantification of the immunogold labelling for glycine (which is assumed to play metabolic roles in addition to its presumed role as a transmitter) showed that the net gold particle density was an order of magnitude higher over Golgi cell terminals than over the other constituents of the cerebellar glomeruli (mossy fibre terminals and granule cell dendrites). The total particle density over the latter was only slightly higher than the background level (over empty resin), suggesting that the concentration of 'metabolic' glycine is generally low compared to the concentration of glycine in Golgi cells. The stellate and basket cell terminals (which similarly to the Golgi cells are thought to release GABA as transmitter) were immunoreactive for GABA, but (with very few exceptions) virtually unlabelled for glycine, suggesting that our results were not confounded by any crossreactivity of the glycine antiserum with fixed GABA. Direct evidence that the sera reacted selectively with fixed glycine or GABA under the conditions used was obtained by incubating the tissue sections together with test sections containing a series of different amino acid-glutaraldehyde-brain macromolecule conjugates. Adsorption tests with soluble amino acid-glutaraldehyde complexes similarly suggested that the double-labelling of the Golgi terminals indeed reflected a colocalization of glycine and GABA. The results show that two 'classical' transmitters, both being inhibitory and acting on Cl- channels, may coexist in the same nerve terminals.

Quinolinic acid effects on amino acid release from the rat cerebral cortex in vitro and in vivo

1. The effect of quinolinic acid, N-methyl-D,L-aspartate (NMDLA) and kainate on the release of endogenous and exogenous amino acids from the rat cerebral cortex in vitro and in vivo was studied. 2. Neither quinolinic acid nor NMDLA had any effect on the basal or potassium-evoked release of [3H]-D-aspartate from slices of rat cerebral cortex either in the presence or absence of magnesium. Kainic acid failed to modify the basal efflux of [3H]-D-aspartate but significantly inhibited (by 34.4% +/- 0.04%, P less than 0.05) the potassium-evoked release. 3. Neither quinolinate nor NMDLA had any effect on the basal efflux of endogenous amino acids from rat cortical slices either in the presence or absence of magnesium ions at concentrations between 10 microM and 5 mM. 4. Both NMDLA (1 mM) and quinolinate (5 mM) produced an efflux of endogenous aspartate (371.4% +/- 11.6%; 389.3% +/- 12.1%) and glutamate (405.4% +/- 13.6%; 430.1 +/- 8.7%) respectively from the rat cerebral cortex in vivo (P less than 0.01). The quinolinic acid-evoked efflux was abolished by the NMDLA antagonist, 2-amino-5-phosphonovaleric acid (200 microM). 5. Kainic acid also caused an efflux of endogenous amino acids from the rat cerebral cortex in vivo. However, the profile of this release was different from that produced by quinolinate and NMDLA. 6. The results add further support to the suggestion that quinolinic acid acts at the NMDLA-preferring receptor and may also explain the requirement for intact afferent projections for the neurotoxic effects of quinolinate to be manifested.

Retrograde transport of [3H]-D-aspartate label by cochlear and vestibular efferent neurons

[3H]-D-aspartic acid was injected into the inner ear of rats. After a six hour survival time, labeled cells were found at all locations known to contain efferent cochlear or vestibular neurons. Most labeled neurons were found in the ipsilateral lateral superior olivary nucleus (LSO), although both ventral nuclei of the trapezoid body (VTB), group E, and the caudal pontine reticular nucleus (CPR) just adjacent to the ascending limb of the facial nerve also contained labeled cells. Because not all efferent neurons in the rat could be previously shown to be cholinergic, aspartate and glutamate are efferent transmitter candidates.

Effect of ethanol on GABA uptake and release from hypothalamic fragments

A study was performed on the effect of ethanol on the basal and K+-evoked efflux of endogenous GABA from rat hypothalamic fragments. The amount of GABA present in the medium and in the tissue was measured by radioreceptor assay. In vitro addition of ethanol (50 and 100 mM) enhanced the K+-evoked efflux of GABA in a Ca++-dependent manner, and increased tissue GABA content. Since K+-evoked outflow induced by ethanol was not affected by the presence of nipecotic acid, ethanol appears to alter the uptake of endogenous GABA. An inhibitory effect of ethanol on 3H-GABA uptake was observed under K+ depolarization. On the other hand, acute ethanol administration produced a decrease in basal and K+-evoked efflux from hypothalamic fragments and in tissue GABA concentration. Changes in GABA efflux may lie behind some of the neuropharmacological effects of ethanol.

Neurotransmitters of the cerebellar glomeruli: uptake and release of labeled gamma-aminobutyric acid, glycine, serotonin and choline in a purified glomerulus fraction and in granular layer slices

We have studied some properties of the uptake and release of labeled gamma-aminobutyric acid (GABA), glycine, serotonin and choline in a purified fraction of glomeruli and in slices of the granular layer of the rat cerebellum. The uptake of both GABA and glycine into the glomerulus particles was dependent on the presence of Na+ in the medium. In contrast, the uptake of both serotonin and choline was Na+-independent. In slices of the granular layer also a slight Na+-dependence was observed for both serotonin and choline uptake; imipramine and hemicholinium partially inhibited the uptake of serotonin and choline, respectively. Choline uptake into the glomerulus particles showed two components, with apparent Km values of 16.8 and 102 microM. GABA release was stimulated by K+-depolarization about 100% (peak stimulation) and this value was reduced to 50% when Ca2+ was omitted. The release of glycine was stimulated more rapidly and notably than GABA (200%) and this stimulation was completely abolished in the absence of Ca2+. Serotonin release from the glomerulus particles was only slightly stimulated by depolarization, but this stimulation was strictly Ca2+-dependent. In slices of the granular layer, this stimulation was considerably larger (about 40%) and it was also almost totally dependent on Ca2+. In contrast, after loading with labeled choline the release of radioactivity from both the glomerulus particles and the cerebellar slices was not stimulated at all by K+-depolarization, either in the presence or in the absence of Ca2+. Most of the radioactivity released spontaneously corresponded to choline, and only a small proportion (8-14%) to acetylcholine. From the results of the release experiments and taking into account the pertinent data from the literature, it is concluded that GABA and glycine are probably the transmitters of different populations of Golgi axon terminals, whereas serotonin might be the transmitter of at least a certain population of the mossy fiber giant terminals, in the rat cerebellar glomeruli. In contrast, acetylcholine does not seem to have any transmitter role in the synaptic structures of the glomeruli.

Ouabain and the membrane transport of amino acids and amines

Evidence is discussed that ouabain has a direct inhibiting effect on the sodium-dependent uptake of amino acids and amines from the extracellular space of the mammalian central nervous system rather than the inhibition being a consequence of raised intracellular sodium levels.

Role of sialic acid in synaptosomal transport of amino acid transmitters

Active, high-affinity, sodium-dependent uptake of gamma-aminobutyric acid and of the acidic amino acid D-aspartate was inhibited by pretreatment of synaptosomes with neuraminidase from Vibrio cholerae. Inhibition was of a noncompetitive type and was related to the amount of sialic acid released. The maximum accumulation ratios of both amino acids (intracellular [amino acid]/extracellular [amino acid]) remained largely unaltered. Treatment with neuraminidase affected neither the synaptosomal energy levels nor the concentration of internal potassium. It is suggested that the gamma-aminobutyric acid and acidic amino acid transporters are glycosylated and that sialic acid is involved in the operation of the carrier proteins directly and not through modification of driving forces responsible for amino acid uptake.

Selective immunocytochemical staining of mitral cells in rat olfactory bulb with affinity purified antibodies against N-acetyl-aspartyl-glutamate

Polyclonal antibodies were raised in rabbits against N-acetyl-aspartyl-glutamate (NAAG) coupled to bovine serum albumin (BSA) with carbodiimide and were purified by affinity chromatography sequentially over BSA-agarose and NAAG-agarose resins. Solid-phase RIA revealed a distinct pattern of specificity of the antibodies for N-acetylated acidic peptides, with highest signal obtained for NAAG, and essentially no immunoreactivity demonstrable for aspartate or glutamate. Coronal sections through carbodiimide-fixed rat olfactory bulb were incubated with the purified antiserum and antigen localization visualized by the avidin-biotin peroxidase techniques. Immunoreactivity was restricted to the mitral cells, the major excitatory projection neurons of the lateral olfactory tract, a putative glutamatergic pathway. Immunoreactivity was selectively blocked by preincubation of the antibody with 1 microgram/ml of NAAG-BSA. These results demonstrate a specific neuronal localization of NAAG-like immunoreactivity and support the candidacy of NAAG as a neurotransmitter of the lateral olfactory tract.

In vivo determination of extracellular concentration of amino acids in the rat hippocampus. A method based on brain dialysis and computerized analysis

Extracellular (EC) concentrations of amino acids were determined in the rat dentate gyrus by means of non-linear regression analysis of 'in vivo' brain dialysis data, considering a simple model of diffusion through a dialysis membrane. The apparent diffusion constants (K) of several amino acids were also calculated in the 'in vivo' situation. While putative amino acid neurotransmitters (glutamate, aspartate and gamma-aminobutyric acid (GABA) were present in the EC fluid at the low micromolar range (0.8-2.9 microM), glutamine was by far the most prominent (193.4 microM). The values of intra/extracellular concentration ratios formed 3 groups: high (greater than 2000) for putative neurotransmitters; low (less than 100) for serine, glutamine, arginine and alpha-alanine; and intermediate (about 400) for taurine. The 'in vivo' calculated K values proved useful for estimation of both basal and changing EC concentrations of amino acids in relatively brief perfusions. These data were evaluated in terms of the functional significance of absolute EC concentrations and tissue-EC fluid ratios. Present findings indicate the simultaneous existence of both an inhibitory and an excitatory tonus as well as the utility of high intra/extracellular concentration ratios in determination of the possible neurotransmitter role of specific amino acids.

Release of endogenous amino acids from striatal neurons in primary culture

Endogenous amino acid release was examined in highly purified striatal neurons obtained from fetal mouse brain, and differentiated in primary culture. This study aimed to determine which amino acids are released from striatal neurons after a brief depolarization period induced by elevated potassium concentration or veratrine. Amino acids released into the extracellular medium, subsequent to a 3-min exposure of striatal neurons, were subjected to HPLC analysis. At 14 days in vitro potassium (56 mM) depolarization elicited a 25-fold increase in gamma-aminobutyric acid release, 85% of which was calcium-dependent. This effect was small but apparent at 7 days in vitro (two-fold increase) and greatly increased between 11 and 14 days in vitro, subsequent to the appearance of synaptic vesicles in nerve terminals. gamma-Aminobutyric acid release was readily reversible within minutes of return to the resting state. Veratrine induced a quantitatively similar but calcium-independent increase in gamma-aminobutyric acid release. Similar results were observed on aspartate and glutamate release, but the increase was very small even after 14 days in vitro (62.2 and 123.3% increase over basal release, respectively). Taurine and hypotaurine release increased during and after depolarization induced by potassium. This effect remained constant between 11 and 18 days in vitro. BAY K 8644, a dihydropyridine-sensitive calcium channel agonist, augmented the effect of 15 mM potassium on gamma-aminobutyric acid release, but this effect remained very small as compared to the potassium (56 mM) or veratrine effects. In addition, nifedipine inhibited this BAY K 8644-induced release. These results demonstrate the high level of differentiation among striatal neurons containing gamma-aminobutyric acid in this in vitro system.

Extracellular overflow of neuroactive amino acids during severe insulin-induced hypoglycemia: in vivo dialysis of the rat hippocampus

Hypoglycemia-evoked changes in levels of extracellular excitatory and inhibitory amino acids were studied using the microdialysis technique. A newly designed dialysis probe was inserted stereotaxically into the rat hippocampus. Animals were then subjected to insulin-induced hypoglycemia; then blood glucose levels were restored by glucose injections after a 30-min period of isoelectric electroencephalography. Dialysates were collected before, during, and after the isoelectric period. Amino acids in the dialysates were analyzed by liquid chromatography and fluorescence detection following automatic precolumn derivatization with o-phthaldialdehyde. During the isoelectric phase, the concentration of aspartate increased 15-fold, whereas glutamate, gamma-amino-butyric acid, taurine, and phosphoethanolamine levels were elevated three- to sixfold. Smaller increases were observed for nonneuroactive amino acids such as asparagine, alanine, and phenylalanine. In contrast to all other amino acids, the glutamine content was reduced to less than 30% of preisoelectric values. The concentrations of the neuroactive amino acids were restored to normal in the post-isoelectric phase. These data demonstrate that there is an extracellular overflow of neuroactive amino acids, especially aspartate, during severe hypoglycemia.

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)

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.

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

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

The role of putative excitatory amino acid neurotransmitters in the initiation of locomotion in the lamprey spinal cord. II. The effects of amino acid uptake inhibitors

Fictive locomotion can be evoked in an in vitro preparation of the lamprey spinal cord by an activation of N-methyl-D-aspartate (NMDA) or kainate receptors. To obtain further knowledge of the putative transmitters underlying this activation the effects of L-glutamate and L-aspartate were examined. These endogenous amino acids exerted a distinctly different effect as compared to the synthetic amino acids (N-methyl-D,L-aspartate and kainate) previously tested. In a wide dose range L-glutamate and L-aspartate elicited fictive locomotion only when the bathing solution was rapidly circulated over the spinal cord surface. In the absence of fluid circulation the activity rapidly ceased. To test if this effect was due to an uptake of amino acids, two amino acid uptake inhibitors were administered. After exposure to p-chloromercuriphenylsulphonate (pCMS) or dihydrokainate (DHK), L-glutamate and L-aspartate elicited continuous fictive locomotion independently of whether the bathing fluid was circulated or not. This treatment also markedly lowered the threshold doses of L-glutamate and L-aspartate, while the effects of NMA and kainate were barely affected. Fictive locomotion induced by sensory stimulation of the tailfin was also prolonged by dihydrokainate. These findings suggest that a highly effective amino acid uptake system is present in the lamprey spinal cord and furthermore that it takes part in the inactivation of synaptically released acidic amino acid neurotransmitters, which are of importance for the initiation of locomotion.

Inhibition of [3H] glutamate release from rat hippocampal slices by L-phenylisopropyladenosine

Hippocampal slices were labelled with [3H]glutamate. Evoked release, which was calcium-dependent and inhibited by tetrodotoxin, could be reduced concentration-dependently by L-PIA. The effect of L-PIA was blocked by 10 microM 8-phenyltheophylline. The results indicate that there are adenosine receptors modulating the efflux of glutamate in field stimulated hippocampal slices.