Effect of Isonicotinic Acid Hydrazide on Extracellular Amino Acids and Convulsions in the Rat: Reversal of Neurochemical and Behavioural Deficits by Sodium Valproate

Abstract: We have studied the effect of isonicotinic acid hydrazide (INH), a convulsant agent, on the extracellular levels of amino acids in the hippocampus, and the effect of sodium valproate (VPA) administration in INH-treated rats. INH (250 mg/kg) caused a rapid and sustained decrease in basal levels of GABA, and during this period convulsions of increasing severity were observed. Basal levels of glutamine, taurine, aspartate, and glutamate were unchanged by INH. When VPA was coadministered with INH, basal GABA levels were increased and no convulsions were observed. When transmitter release was evoked using 100 m M K⁺, the increase in dialysate GABA observed in INH-treated animals was less than that seen in controls and convulsions increased in frequency. K⁺-evoked release of glutamate and aspartate tended to be higher following INH treatment, and in the case of aspartate, this increase was significant. VPA reversed the changes in evoked release of glutamate and aspartate, and release of GABA was considerably greater than that seen in control or INH-treated rats. No drug effect on evoked changes in taurine or glutamine level was seen. These are the first data to show decreased extracellular GABA in conjunction with convulsions in freely moving animals in vivo.     

Endogenous Dopamine Increases Extracellular Concentrations of Glutamate and GABA in Striatum of the Freely Moving Rat: Involvement of D1 and D2 Dopamine Receptors

Interactions between endogenous dopamine, glutamate, GABA, and taurine were investigated in striatum of the freely moving rat by using microdialysis. Intrastriatal infusions of the selective dopamine uptake inhibitor nomifensine (NMF) were used to increase the endogenous extracellular dopamine. NMF produced a dose-related increase in extracellular dopamine and also increased extracellular concentrations of glutamate, GABA, and taurine. Extracellular increases of dopamine were significantly correlated with extracellular increases of glutamate and GABA, but not taurine. To investigate whether the increased extracelular dopamine produced by NMF was responsible for the concomitant increase of glutamate and GABA, D1, and D2 receptor antagonists were used. Dopamine receptor antagonists D1 (SCH23390) and D2 (sulpiride) significantly attenuated the increases of glutamate and GABA produced by NMF. These data suggest that endogenous dopamine, through both D1 and D2 dopamine receptors, plays a role in releasing glutamate and GABA in striatum of the freely moving rat.     

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.     

In vivo release patterns and cardiovascular properties of inhibitory and excitatory amino acids in the hypothalamus

Summary The posterior hypothalamus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of amino acids was determined in the superfusate. Under basal conditions, the release rates of taurine, GABA and glutamate fluctuated according to ultradian rhythms with different frequencies. Hypothalamic superfusion with veratridine or high concentrations of potassium choride enhanced the release rates of taurine, GABA and glutamate in a concentration-dependent way. Tetrodotoxin decreased the basal release rates of the three amino acids. The release of arginine was not influenced significantly by these compounds. A fall of blood pressure elicited by intravenous infusion of nitroprusside decreased the release rates of GABA and taurine and enhanced the release of glutamate. Infusion of noradrenaline increased blood pressure and release rates of GABA and taurine, while the release of glutamate was not influenced. Neither the pressor, nor the depressor responses to drugs influenced the release of arginine in the hypothalamus. It is concluded that the inhibitory amino acids taurine and GABA released from hypothalamic neurons possess a tonic hypotensive function. The excitatory amino acid glutamate, released from glutamatergic neurons of the hypothalamus, seems to possess a hypertensive function in counteracting a fall of blood pressure.This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung. These results were presented at the Third International Congress on Amino Acids, Vienna, August 1993     

The use of microdialysis for studying the regional effects of pharmacological manipulation on extracellular levels of amino acids--some methodological aspects.

The purpose of this study was to examine and validate the use of microdialysis for sampling and pharmacologically manipulating extracellular amino acids in the brain. Repeated use of microdialysis probes in acute intracerebral experiments did not significantly alter the relative recovery in vitro for the amino acids quantitated (GABA, aspartate, glutamate, glycine, taurine, and alanine). Regional differences in basal levels of some of the amino acids were detected in dialysates collected from the dorsomedial hypothalamus, striatum, and frontal cortex. The percent in vitro recoveries for the amino acids from the probes used in the three regions were not significantly different suggesting that the regional differences in basal levels of amino acids were functionally derived and not a consequence of variations in probe recovery. Perfusion with nipecotic acid, an inhibitor of GABA uptake, resulted in selective elevations in extracellular GABA in the three regions studied. Conversely, perfusion with high-potassium, a depolarizing agent, resulted in significant elevations in not only extracellular GABA but also aspartate, glutamate, and taurine. Thus, microdialysis is a method which can be employed to assess and to pharmacologically manipulate extracellular amino acids in the rat brain.     

Ischemia and reoxygenation induced amino acid release release and tissue damage in the slices of rat corpus striatum

Summary. Ischemic incubation significantly increased amino acid release from rat striatal slices. Reoxygenation (REO) of the ischemic slices, however, enhanced only taurine and citrulline levels in the medium. Ischemia-induced increases in glutamate, taurine and GABA outputs were accompanied with a similar amount of decline in their tissue levels. Tissue final aspartic acid level, however, was doubled by ischemia. Lactate dehydrogenase (LDH) leakage was not altered by ischemia, but enhanced during REO. Presence of tetrodotoxine (TTX) during ischemic period caused significant decline in ischemia-induced glutamate output, but not altered REO-induced LDH leakage. Although omission of extracellular calcium ions from the medium during ischemic period protected the slices against REO-induced LDH leakage, this treatment failed to alter ischemia-induced glutamate and GABA outputs. The release of other amino acids, however, declined 50% in calcium-free medium. Blockade of the glutamate uptake transporter by L-trans-PDC, on the other hand, doubled ischemia induced glutamate and aspartic acid outputs. These results indicate that more than one mechanisms probably support the ischemia-evoked accumulation of glutamate and other amino acids in the extracellular space. Although LDH leakage enhanced during REO, processes involved in this increment were found to be dependent on extracellular calcium ions during ischemia but not REO period.     

Extracellular levels of brain aspartate, glutamate and GABA during an inhibitory avoidance response in the rat

The extracellular levels of aspartate, glutamate and GABA were measured by microdialysis, coupled with an HPLC method, in rat prefrontal cortex (mPFC) and ventral hippocampus (VH) before and during the performance of a step-down inhibitory task. The basal levels of glutamate were about 50% higher than those of aspartate, and GABA levels were about 20-folds smaller than those of the excitatory amino acids. There were no significant differences in the basal levels of any of the three amino acids between the two brain regions. The extracellular levels of aspartate increased during acquisition and recall trials in both VH and mPFC, whereas those of glutamate increased in the VH during acquisition only. A significant increase in GABA levels was also detected during acquisition but only in the mPFC. The neuronal origin of the increased extracellular levels of aspartate, glutamate and GABA was demonstrated by administering tetrodotoxin directly into the mPFC or VH by reverse dialysis. These findings, together with previous evidence from our and other laboratories, indicate a differential release of aspartate and glutamate from excitatory neurons during the performance of behavioral responses, and therefore, distinct roles for the two excitatory amino acids should be envisaged.     

Release of Endogenous Glutamate,Aspartate, GABA,and Taurine from Hippocampal Slices from Adult and Developing Mice under Cell-Damaging Conditions

The releases of endogenous glutamate, aspartate, GABA and taurine from hippocampal slices from 7-day-, 3-, 12-, and 18-month-old mice were investigated under cell-damaging conditions using a superfusion system. The slices were superfused under hypoxic conditions in the presence and absence of glucose and exposed to hydrogen peroxide. In the adult hippocampus under normal conditions the basal release of taurine was highest, with a response only about 2-fold to potassium stimulation (50 mM). The low basal releases of glutamate, aspartate, and GABA were markedly potentiated by K⁺ ions. In general, the release of the four amino acids was enhanced under all above cell-damaging conditions. In hypoxia and ischemia (i.e., hypoxia in the absence of glucose) the release of glutamate, aspartate and GABA increased relatively more than that of taurine, and membrane depolarization by K⁺ markedly potentiated the release processes. Taurine release was doubled in hypoxia and tripled in ischemia but K⁺ stimulation was abolished. In both the mature and immature hippocampus the release of glutamate and aspartate was greatly enhanced in the presence of H₂O₂, that of aspartate particularly in developing mice. In the immature hippocampus the increase in taurine release was 10-fold in hypoxia and 30-fold in ischemia, and potassium stimulation was partly preserved. The release processes of the four amino acids in ischemia were all partially Ca²⁺-dependent. High concentrations of excitatory amino acids released under cell-damaging conditions are neurotoxic and contribute to neuronal death during ischemia. The substantial amounts of the inhibitory amino acids GABA and taurine released simultaneously may constitute an important protective mechanism against excitatory amino acids in excess, counteracting their harmful effects. In the immature hippocampus in particular, the massive release of taurine under cell-damaging conditions may have a significant function in protecting neural cells and aiding in preserving their viability.     

Acute metabolic effects of taurine on the enzymes metabolizing glutamate and gaba

100 mg of taurine per kg body weight had been administered intraperitoneally and 30 min after the administration the animals were sacrificed. Glutamate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, glutaminase, glutamine synthetase, glutamate decarboxylase and GABA aminotransferase along with the content of glutamate and GABA in cerebral cortex, cerebellum and brain stem were studied and compared with the same obtained in the rats treated with normal saline in place of taurine. The results indicated a significant decrease in the activity of glutamate dehydrogenase in cerebral cortex and cerebellum and a significant increase in brain stem. Glutaminase and glutamine synthetase were found to increase significantly both in cerebral cortex and cerebellum. The activities of glutamate decarboxylase was found to increase in all the three regions along with a significant decrease in GABA aminotransferase while the content of glutamate showed a decrease in all the three brain regions, the content of GABA was observed to increase significantly. The above effects of taurine on the metabolism of glutamate and GABA are discussed in relation to the functional role of GABA and glutamate. The results indicate that taurine administration would result in a state of inhibition in brain.     

17beta-estradiol effect on the extracellular concentration of amino acids in the glutamate excitotoxicity model in the rat

Estrogen has demonstrated a neuroprotective role in a rat model of glutamate excitotoxicity and other neurodegenerative disorders. We studied the effect of 17-estradiol on glutamate-induced increases in amino acids levels (aspartate, histidine, taurine and GABA) in the rat cortex. Local perfusion of glutamate produced a transient increase of aspartate, histidine, taurine and GABA in the extracellular fluid. Pretreatment with 17-estradiol significantly reduced the increases of taurine and moderately attenuated that of histidine, whereas aspartate and GABA releases were not modified. The effect of 17-estradiol on histidine release was reversed by the antiestrogen tamoxifen, suggesting a receptor-dependent mechanism. Good correlations between the volumes of the glutamate-induced lesions and the extracellular concentrations of taurine and aspartate were observed. These findings suggest that the attenuation of the glutamate-induced release of taurine by 17-estradiol may participate in the neuroprotective effects of 17-estradiol and that increased levels of aspartate and taurine are markers for the severity of the glutamate-induced cortical lesions.     

Effects of Endogenous Glutamate on Extracellular Concentrations of GABA, Dopamine, and Dopamine Metabolites in the Prefrontal Cortex of the Freely Moving Rat: Involvement of NMDA and AMPA/KA Receptors

Using microdialysis, interactions between endogenous glutamate, dopamine, and GABA were investigated in the medial prefrontal cortex of the freely moving rat. Interactions between glutamate and other neurotransmitters in the prefrontal cortex had already been studied using pharmacological agonists or antagonists of glutamate receptors. This research investigated whether glutamate itself, through the increase of its endogenous extracellular concentration, is able to modulate the extracellular concentrations of GABA and dopamine in the prefrontal cortex. Intracortical infusions of the selective glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) were used to increase the endogenous extracellular glutamate. PDC (0.5, 2, 8, 16 and 32 mM) produced a dose-related increase in dialysate glutamate in a range of 1–36 M. At the dose of 16 mM, PDC increased dialysate glutamate from 1.25 to 28 M. PDC also increased extracellular GABA and taurine, but not dopamine; and decreased extracellular concentrations of the dopamine metabolites DOPAC and HVA. NMDA and AMPA/KA receptor antagonists were used to investigate whether the increases of extracellular glutamate were responsible for the changes in the release of GABA, and dopamine metabolites. The NMDA antagonist had no effect on the increase of extracellular GABA, but blocked the decreases of extracellular DOPAC and HVA, produced by PDC. In contrast, the AMPA/KA antagonist blocked the increases of extracellular GABA without affecting the decreases of extracellular DOPAC and HVA produced by PDC. These results suggest that endogenous glutamate acts preferentially through NMDA receptors to decrease dopamine metabolism, and through AMPA/KA receptors to increase GABAergic activity in the medial prefrontal cortex of the awake rat.     

Glutamate release from cerebellar granule cells differentiating in culture: Modulation of the K+-stimulated release by inhibitory amino acids

The properties of l-[³H]glutamate release with an emphasis on the modulation by inhibitory amino acids of the potassium-induced release were studied with cerebellar granule cells from 7-day-old rats cultured for 7 or 14 days. Spontaneous glutamate release from cells grown for 7 days was fast, being slightly enchanced in Na⁺-free medium. l-Glutamate, kainate and quisqualate stimulated the release whereas N-methyl-d-aspartate and taurine were without any effect. The potassium-evoked glutamate release was Ca²⁺-dependent and potentiated by l-glutamate and quisqualate. Stimulated release was strongly depressed by glutamatediethylester. This inhibition was antagonized by GABA but not by taurine. GABA and its structural analogues taurine, hypotaurine, β-alanine and glycine were all equally effective in depressing stimulated glutamate release. The inhibition by GABA could be blocked by GABA antagonist. Both K⁺-evoked release and the kainate-induced release of glutamate were significantly greater in 14-day-old than in 7-day-old cultures, but the other properties of release were similar. The demonstration of calcium-dependent and potassium-stimulated glutamate release from cerebellar granule cells is consonant with the proposed neurotransmitter role of glutamate in these cells. The release could be modulated by both glutamatergic substances and inhibitory amino acids, the effect of GABA probably being mediated by GABAergic receptors.     

Enhancement of Endogenous Release of Glutamate and γ-Aminobutyric Acid from Hippocampus CA1 Slices After In Vivo Long-Term Potentiation

The effect of long-term potentiation (LTP) on endogenous amino acid release from rat hippocampus slices was studied. LTP was induced in vivo by application of a tetanus (200 Hz, 200 ms) to the Schaffer collateral fibers in unanesthetized rats. Endogenous release of glutamate and gamma-aminobutyric acid (GABA) was investigated 60 min after tetanization in CA1 subslices of potentiated and control rats. No significant effects of LTP were observed in basal and K(+)-induced Ca(2+)-independent release components of these amino acids. In contrast, K(+)-induced Ca(2+)-dependent release of both glutamate and GABA increased approximately 100% in slices from potentiated rats. No differences were observed in total content of glutamate and GABA between the subslices from control and LTP animals. These results suggest a persistent increase in the recruitment of the presynaptic vesicular pool of glutamate and GABA during LTP.     

Reduction of brain taurine: Effects on neurotoxic and metabolic actions of kainate

The effects of chronic administration of 2-guanidinoethane sulfonic acid on the levels of intra- and extracellular amino acids in the rat hippocampus were studied. The tissue content of taurine was selectively reduced by almost one third after 9 days of peroral administration of 1% 2-guanidinoethane sulfonate. Extracellular levels of amino acids were monitored with the brain microdialysis method. The taurine concentration in the extracellular fluid was depressed in relation to the decrease in intracellular taurine. Unexpectedly, extracellular (but not intracellular) glutamate was doubled in 2-guanidinoethane sulfonate treated animals. The kainic acid evoked release of taurine was suppressed in the 2-guanidinoethane sulfonate group, whereas the kainate stimulated efflux of glutamate was elevated after 2-guanidinoethane sulfonate administration. The acute metabolic effects of kainate were studied by measuring the efflux of the adenosine triphosphate breakdown products hypoxanthine, xanthine, inosine and adenosine. No differences were found between control and 2-guanidinoethane sulfonate treated rats with respect to basal or kainic acid evoked release of purine catabolites. Also, the neuronal loss caused by kainate injection into the hippocampus was not modified by 2-guanidinoethane sulfonate treatment, suggesting that endogenous taurine does not affect these responses. We conclude that chronic administration of 2-guanidinoethane sulfonate does not sensitize central neurons to the metabolic and toxic actions of kainate.     

Taurine and GABA release from mouse cerebral cortex slices: Effects of structural analogues and drugs

The effects of structural analogues, excitatory amino acids and certain drugs on spontaneous and potassium-stimulated exogenous taurine and GABA release were investigated in mouse cerebral cortex slices using a superfusion system. Spontaneous efflux of both amino acids was rather slow but could be enhanced by their uptake inhibitors. Taurine efflux was facilitated by exogenous taurine, hypotaurine, -alanine and GABA, whereas GABA, nipecotic acid and homotaurine effectively enhanced GABA release. The stimulatory potency of the analogues closely corresponded to their ability to inhibit taurine and GABA uptake, respectively, indicating that these efflux processes could be mediated by the carriers operating outwards. Glutamate induced GABA release, whereas taurine efflux was potentiated by aspartate, glutamate, cysteate, homocysteate and kainate. The centrally acting drugs, including GABA agonists and antagonists, as well as the proposed taurine antagonist TAG (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide), had no marked effects on spontaneous taurine and GABA release. Potassium ions stimulated dosedependently both taurine and GABA release from the slices, the responses of taurine being strikingly slow but sustained. Exogenous GABA and nipecotic acid accelerated the potassium-stimulated GABA release, whereas picrotoxin and bicuculline were ineffective. The potassium-stimulated taurine release was unaltered or suppressed by exogenous taurine and analogues, differing in this respect from GABA release. The apparent magnitude of the depolarization-induced GABA release is thus influenced by the function of membrane transport sites, but the same conclusion cannot be drawn with regard to taurine. Haloperidol and imipramine were able to affect the evoked release of both taurine and GABA.     

Extracellular Levels of Amino Acids and Choline in Human High Grade Gliomas: An Intraoperative Microdialysis Study

The concentrations of endogenous amino acids and choline in the extracellular fluid of human cerebral gliomas have been measured, for the first time, by in vivo microdialysis. Glioblastoma growth was associated with increased concentrations of choline, GABA, isoleucine, leucine, lysine, phenylalanine, taurine, tyrosine, and valine. There was no difference between grade III and grade IV tumors in the concentrations of phenylalanine, isoleucine, tyrosine, valine, and lysine, whereas the concentrations of choline, aspartate, taurine, GABA, leucine, and glutamate were significantly different in the two tumor-grade subgroups. In contrast to the other compounds, the concentration of glutamate was decreased in glioma. The parenchyma adjacent to the tumor showed significant changes only in the extracellular concentration of glutamate, isoleucine, and valine. The concentrations of choline and the amino acids, glutamate, leucine, taurine, and tyrosine showed significant positive correlations with the degree of cell proliferation. Epilepsy, which is relatively common in subjects with gliomas, was shown to be a significant confounding variable when the extracellular concentrations of aspartate, glutamate and GABA were considered.     

Taurine release modified by GABAergic agents in hippocampal slices from adult and developing mice

Summary. In order to characterize the possible regulation of taurine release by GABAergic terminals, the effects of several agonists and antagonists of GABA receptors on the basal and K⁺-stimulated release of [³H]taurine were investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice using a superfusion system. Taurine release was concentration-dependently potentiated by GABA, which effect was reduced by phaclofen, saclofen and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) at both ages, suggesting regulation by both GABA_B and GABA_C receptors. The involvement of GABA_A receptors could not be excluded since the antagonist bicuculline was able to affect both basal and K⁺-evoked taurine release. Furthermore, several GABA_B receptor effectors were able to inhibit K⁺-stimulated taurine release in the adults, while the GABA_C receptor agonists trans-4-aminocrotonic acid (TACA) and cis-4-aminocrotonic acid (CACA) potentiated this release. The potentiation of taurine release by agents acting on the three types of GABA receptors in both adult and developing hippocampus further indicates the involvement of transporters operating in an outward direction. This inference is corroborated by the moderate but significant inhibition of taurine uptake by the same compounds. Received June 28, 1999, Accepted August 31, 1999     

Contrasting Effects of D- and L-(E)-4-(3-Phospiono-2- Propenyl) Piperazine-2-Carboxylic Acid as Anticonvulsants and as Inhibitors of Potassium-Evoked Increases in Hippocampal Extracellular Glutamate and Aspartate Levels in Freely Moving Rats I

Abstract: Microdialysis experiments performed in the dorsal hippocampus of freely moving rats showed that L-( E )- 4-(3-phosphono-2-propenyl) piperazine-2-carboxylic acid (L-CPPene) is 10 times as potent as D-CPPene in inhibiting potassium-induced increases in extracellular levels of aspartate and glutamate. In control experiments, two 100 m M KCI stimuli (S1 and S2) applied for 10 min each (separated by a 40-min recovery period) produced substantial (300–500%) increases in the extracellular levels of aspartate, glutamate, taurine, and GABA and a 50% decrease in the glutamine level. S2/S1 ratios in the control groups were 0.67 (aspartate), 0.78 (glutamate), 0.83 (GABA), and 0.85 (taurine). In the experimental groups, D- or L-CPPene was applied via the probe during the second potassium stimulus (S2). L-CPPene (25 or 250 μ M ) produced selective suppression of potassium-induced increases of extracellular glutamate (S2/S1 ratio: 0.25) and aspartate (S2/S1 ratio: 0.20) levels, whereas 250 μ M D-CPPene was required to inhibit the extracellular aspartate and glutamate increases. Neither enantiomer of CPPene affected the potassium-induced increases of GABA and taurine or the decrease in extracellular glutamine concentration. An addtional study comparing the anticonvulsant potencies of D- and L-CPPene was performed using audiogenic DBA/2 mice. The anticonvulsant potency of D-CPPene, as assessed against sound-induced seizures in DBA/2 mice, was an order of magnitude higher than that of L-CPPene [ED₅₀ clonic phase (intraperitoneal, 45 min): 1.64 μmol/kg and 16.8 μmol/kg, respectively]. We attribute the anticonvulsant action of D-CPPene to its antagonist action at the NMDA receptor. The selective inhibition by L-CPPene of potassium-induced increases in extracellular aspartate and glutamate levels is presumably due to an action on presynaptic glutamate receptors.     

Depolarization-Induced Release of Amino Acids From the Vestibular Nuclear Complex

There is evidence from immunohistochemistry, quantitative microchemistry, and pharmacology for several amino acids as neurotransmitters in the vestibular nuclear complex (VNC), including glutamate, γ-aminobutyrate (GABA), and glycine. However, evidence from measurements of release has been limited. The purpose of this study was to measure depolarization-stimulated calcium-dependent release of amino acids from the VNC in brain slices. Coronal slices containing predominantly the VNC were prepared from rats and perfused with artificial cerebrospinal fluid (ACSF) in an interface chamber. Fluid was collected from the chamber just downstream from the VNC using a microsiphon. Depolarization was induced by 50 mM potassium in either control calcium and magnesium concentrations or reduced calcium and elevated magnesium. Amino acid concentrations in effluent fluid were measured by high performance liquid chromatography. Glutamate release increased fivefold during depolarization in control calcium concentration and twofold in low calcium/high magnesium. These same ratios were 6 and 1.5 for GABA, 2 and 1.3 for glycine, and 2 and 1.5 for aspartate. Differences between release in control and low calcium/high magnesium ACSF were statistically significant for glutamate, GABA, and glycine. Glutamine release decreased during and after depolarization, and taurine release slowly increased. No evidence for calcium-dependent release was found for serine, glutamine, alanine, threonine, arginine, taurine, or tyrosine. Our results support glutamate and GABA as major neurotransmitters in the VNC. They also support glycine as a neurotransmitter and some function for taurine.     

Brain cortical amino acids measured by intracerebral dialysis in portacaval shunted rats

The extracellular amino acid content was measured in the parietal cortex in portacaval and sham operated rats, using the brain dialysis technique. The amino acid content of the perfusate was determined for 10 min before and during stimulation with potassium chloride. Basal levels of aspartate, glutamine, glycine, methionie, valine, phenylalanine and leucine were 2-to 6-fold higher in the PC-shunted as compared to the sham operated rats. For glutamate, taurine, and GABA no differences were observed between the two groups. After KCl stimulation the release of glutamate and GABA increased significantly in both groups. For GABA this rise was approximately twice as high in the PC-shunted rats (+300%,P<0.01) as in the sham operated rats (+150%,P<0.01 as compared to basal). In the sham operated, but not in the PC-shunted rats, methionine and valine levels rose significantly (+200%,P<0.05) and glutamine release decreased (–50%,P<0.05). These findings suggest that the brain metabolism of amino acids is altered after a portacaval shunt. This could in turn alter the neurotransmission and partly explain the low spontaneous motor activity seen in these animals.