Effect of short-and long-term exposure to low environmental temperature on brain regional GABA metabolism

Single exposure of adult male rats to low environmental temperature (LET, 12 ± 0.5°C) for 2 h significantly increased the hypothalamic and striatal GABA levels without affecting those in other regions of brain. The activity of glutamate decarboxylase (GAD) was elevated in hypothalamus (H) and corpus striatum (CS) under these conditions. GABA accumulation rate (measured with ethanolamine-O-sulfate, an inhibitor of GABA-transaminase) was also increased in both H and CS of rats exposed to LET for 2 h. Unlike after a single exposure, the repeated exposure (2 h/day) for 7, 15, and 30 onsecutive days did not change the hypothalamic GABA metabolism. No change in GABA metabolism was observed in CS when rats were repeatedly exposed to LET for 7 consecutive days. Prolongation of repeated exposure to LET (2 h/day) for 15 and 30 consecutive days decreased the striatal GABA level and increased the activity of GABA-transaminase, although GAD activity was not altered under these conditions. These results suggest that single exposure to LET accelerates GABA synthesis and may reduce the GABAergic activity in both H and CS; whereas repeated exposure to LET for 15 or 30 consecutive days enhances GABAergic activity with the stimulation of GABA utilization only in CS without affecting its synthesizing process. Thus, it may be suggested that the hypothalamic and striatal GABA system may play a characteristic role in response to short-and long-term exposure to LET.     

The effect of vibration on brain GABA metabolism in rats with hypo- and hypercalcemia]

The horizontal vibration (30 min, 20 Hz, A = 0.4 mm) has been studied for its influence on the level of GABA components and dicarbonic acids in brain tissues of adult male rats with hypo- and hypercalcemia. It is assumed, that hypercalcemia vibration situation prevents excessive stress agent, thus leading to a decrease in the level of GABA and activity of glutamate decarboxylase in various structures of the brain.     

Sodium valproate exerts anti-conflict activity in rats without any concomitant rise in forebrain GABA level

The role of GABA in the mediation of anti-conflict activity by drugs remains controversial. Amino-oxyacetic acid (AOAA), 30 mg/kg, i.p., 2 h, and γ-vinyl GABA (GVG) 900 mg/kg, i.p., 4 h, elevated rat forebrain GABA, but failed to exert any anti-conflict activity in a waterlick paradigm in rats. The GABA analogue, THIP, 0.1–10.0 mg/kg, i.p., 30 min, was also ineffective. Sodium valproate (VPA), 400 mg/kg, i.p., showed no increase in forebrain GABA at 5 min and 4 h, and a very weak elevation, to 106% of control, at 30 min. However, VPA elicited anti-conflict activity at 5 as well as at 30 min. The VPA mediated anti-conflict behavior at 5 min unrelated to increased forebrain GABA level and the lack of anti-conflict activity of AOAA and GVG in spite of significantly elevated GABA suggest an anti-conflict mechanism independent of increased brain GABA concentration. A GABA receptor involvement in the anti-conflict mechanism of VPA was nevertheless indicated by the ability of bicuculline, 0.3 mg/kg, s.c., 15 min, to completely suppress VPA elicited anti-conflic response at 5 min.     

Effect of vibration on gamma-aminobutyric acid metabolism in the brain in various functional conditions of the adrenal cortex]

The low-frequency vibration during 30 min (20 Hz, A = 0.4 mm) has been studied for its influence on the level of components of the GABA system and dicarbonic ++amino acids in male rats at hypo- and hyperfunction of the adrenal cortex. It is shown that under these conditions of the experiment the GABA level and glutamate-decarboxylase activity increase. Hyperfunction of the adrenal cortex against the background of vibration causes a relatively less pronounced increase in the GABA content, than the vibration alone or against the background of inhibition of adrenocortical function in the organism.     

Contribution of Gamma amino butyric acid (GABA) to salt stress responses of Nicotiana sylvestris CMSII mutant and wild type plants

Plants accumulate high levels of Gamma amino butyric acid (GABA) in response to different environmental stresses and GABA metabolism has different functions such as osmotic and pH regulation, bypass of tricarboxylic acid cycle, and C:N balance. The cytoplasmic male sterile (CMS) II mutant of Nicotiana sylvestris has a deletion in the mitochondrial gene nad7 which encodes the NAD7 subunit of complex I which causes increased leaf respiration, impaired photosynthesis, slower growth and increased amino acid levels. In this study we aimed to elucidate the role of GABA and GABA metabolism in different genotypes of the same plant system under salt stress (100mM NaCl) in short (24h) and long (7, 14 and 21 days) terms. We have investigated the differences in leaf fresh and dry weights, relative water content, photosynthetic efficiency (F(v)/F(m)), glutamate dehydrogenase (GDH, EC 1.4.1.4) and glutamate decarboxylase (GAD, EC 4.1.1.15) enzyme activities, GABA content and GAD gene expression profiles. GDH activity showed variations in CMSII and wild type (WT) plants in the first 24h. GAD gene expression profiles were in good agreement with the GAD enzyme activity levels in CMSII and WT plants after 24h. In long-term salinity, GAD activities increased in WT but, decreased in CMSII. GABA accumulation in WT and CMSII plants in short and long term was induced by salt stress. Variations in GDH and GAD activities in relation to GABA levels were discussed and GABA metabolism has been proposed to be involved in better performance of CMSII plants under long term salinity.     

Glutamic acid decarboxylase of embryonic avian retina cells in culture: Regulation byγ-aminobutyric acid (GABA)

1. Retina-cell aggregate cultures expressed glutamate decarboxylase activity (L-glutamate 1-carboxylase; EC 4.1.1.15) as a function of culture differentiation. 2. Glutamic acid decarboxylase (GAD) activity was low in the initial phases of culture and increased eight-fold until culture day 7, remaining high up to day 13 (last stage studied). 3. The addition of GABA to the culture medium 24 h after cell seeding almost totally prevented the expression of GAD activity. 4. In association with decreased enzyme activity, aggregates exposed to GABA did not display immunoreactivity for GAD, suggesting that GAD molecules were either lost from GABAergic neurons or significantly altered with GABA treatment. 5. Control, untreated aggregates showed intense GAD immunoreactivity in neurons. Positive cell bodies were characterized by a thin rim of labeled cytoplasm with thickest labeling at the emergence of the main neurite. 6. Heavily labeled patches were also observed throughout the aggregates, possibly reflecting regions enriched in neurites. 7. The GABA-mediated reduction of GAD immunoreactivity was a reversible phenomenon and could be prevented by picrotoxin.     

Use of Inhibitors of γ-Aminobutyric Acid (GABA) Transaminase for the Estimation of GABA Turnover in Various Brain Regions of Rats: A Reevaluation of Aminooxyacetic Acid

The technique of estimating gamma-aminobutyric acid (GABA) turnover by inhibiting its major degrading enzyme GABA-T (4-aminobutyrate:2-oxoglutarate aminotransferase; EC 2.6.1.19) and measuring GABA accumulation has been used repeatedly, but, at least in rats, its usefulness has been limited by several difficulties, including marked differences in the degree of GABA-T inhibition in different brain regions after systemic injection of GABA-T inhibitors. In an attempt to improve this type of approach for measuring GABA turnover, the time course of GABA-T inhibition and accumulation of GABA in 12 regions of rat brain has been studied after systemic administration of aminooxyacetic acid (AOAA), injected at various doses and with different routes of administration. A total and rapidly occurring inhibition of GABA-T in all regions was obtained with intraperitoneal injection of 100 mg/kg AOAA, whereas after lower doses, marked regional differences in the degree of GABA-T inhibition were found, thus leading to underestimation of GABA synthesis rates, e.g., in substantia nigra. The activity of the GABA-synthesizing enzyme GAD (L-glutamate-1-decarboxylase; EC 4.1.1.15) was not reduced significantly at any time after intraperitoneal injection of AOAA, except for a small decrease in olfactory bulbs. Even the highest dose of AOAA tested (100 mg/kg) was not associated with toxicity in rats, but induced motor impairment, which was obviously related to the marked GABA accumulation found with this dose. The increase in GABA concentrations induced with intraperitoneal injection of 100 mg/kg AOAA was rapid in onset, allowing one to estimate GABA turnover rates from the initial rate of GABA accumulation, i.e., during the first 30 min after AOAA injection. GABA turnover rates thus determined were correlated in a highly significant fashion with the GAD activities determined in brain regions, with highest turnover rates measured in substantia nigra, hypothalamus, olfactory bulb, and tectum. Pretreatment of rats with diazepam, 5 mg/kg i.p., 5-30 min prior to AOAA, reduced the AOAA-induced GABA accumulation in all 12 regions examined, most probably as a result of potentiation of postsynaptic GABA function. The data indicate that AOAA is a valuable tool for regional GABA turnover studies in rats, provided the GABA-T inhibitor is administered in sufficiently high doses to obtain complete inhibition of GABA degradation.     

EFFECTS OF CHRONIC TREATMENT WITH AMINO-OXYACETIC ACID OR SODIUM n-DIPROPYLACETATE ON BRAIN GABA LEVELS AND THE DEVELOPMENT AND REGRESSION OF COBALT EPILEPTIC FOCI IN RATS

Abstract– Acute treatment of cobalt-induced epilepsy in rats with amino-oxyacetic acid (20-60 mg/kg intraperitoneally) resulted in a short period (30-90 min) of epileptic spike suppression. In contrast sodium n -dipropylacetate (100-400 mg/kg intraperitoneally) had no effect on spike frequencies. Chronic treatment of cobalt epileptic rats with amino-oxyacetic acid (2.5-10 mg/kg intraperitoneally daily) or sodium n -dipropylacetate (200-400 mg/kg intraperitoneally daily) elevated brain GABA concentrations significantly and reduced brain glutamate decarboxylase activity relative to control saline-injected cobalt epileptic rats. Brain γ-aminobutyrate aminotransferase activity was significantly reduced by chronic treatment with amino-oxyacetic acid, whereas chronic sodium n -dipropylacetate had no effect on brain γ-aminobutyrate aminotransferase activity although elevating brain GABA. Amino-oxyacetic acid (2.5-10 mg/kg intraperitoneally per day) reduced the frequency of epileptic spikes in the secondary foci of cobalt epileptic rats. The anticonvulsant action of amino-oxyacetic acid was most marked at 5 mg/kg intraperitoneally where a secondary focus failed to develop in treated cobalt epileptic rats. However, there was no simple relationship between the elevation of brain GABA and the anticonvulsant action of amino-oxyacetic acid. Thus focal GABA was higher in rats given intraperitoneal amino-oxyacetic acid (10 mg/kg) but the anticonvulsant action of amino-oxyacetic acid was less marked at this dose. Sodium n -dipropylacetate (200-400 mg/kg intraperitoneally per day) had no long-term anticonvulsant action in this model of epilepsy. It is concluded that the anticonvulsant action of sodium n -dipropylacetate, and probably that of amino-oxyacetic acid, is not likely to be mediated through a mechanism involving elevation of brain GABA.     

Regional Variation in γ-Aminobutyric Acid Turnover: Effect of Castration on γ-Aminobutyric Acid Turnover in Microdissected Brain Regions of the Male Rat

Abstract: This study compared the turnover of GABA neurons in different brain areas of the male rat and examined the effect of castration on GABA turnover in regions of the brain associated with the control of gonadotropin secretion. To estimate GABA turnover, GABA was quantified by HPLC in microdissected brain regions 0,30,60,90, and 120 min after inhibition of GABA degradation by aminooxyacetic acid (100 mg/kg, i.p.). GABA accumulation was linear in all areas for 90 min ( p < 0.01), and GABA turnover was estimated as the slope of the line formed by increased GABA concentration versus time, determined by linear regression. There was considerable regional variation both in the initial steady-state concentrations of GABA and in the rates of GABA turnover. Of 10 discrete brain structures, GABA turnover was highest in the medial preoptic nucleus and lowest in the caudate nucleus. Turnover times in the terminal fields of known GABAergic projection neurons ranged sevenfold, from 2.6 h in the substantia nigra to 0.4 h in the lateral vestibular nucleus. The effect of castration on GABA turnover in 13 microdissected brain regions was investigated by measuring regional GABA concentrations before and 30 min after injection of aminooxyacetic acid in intact rats or 2 or 6 days postcastration. Following castration, steady-state GABA concentrations were increased, and GABA turnover decreased in the diagonal band of Broca, the medial preoptic area, and the median eminence. GABA turnover increased in the medial septal nucleus and was unaffected in the cortex, striatum, and hindbrain. These results are consistent with the hypothesis that testosterone negative-feedback control of luteinizing hormone-releasing hormone involves steroid-sensitive GABAergic neurons in the rostral and medial basal hypothalamus.     

Increase in 5'-nucleotidase and decrease in guanylate kinase activity in the brain and liver of irradiated rats

The activity of 5-nucleotidase (EC 3.1.3.5) and guanylate kinase (EC 2.7.4.8) in the mitochondria and supernatant of the brain and liver was determined in experiments on Wistar male rats 1, 3, 6, 24 and 48 h after the single total irradiation by gamma-quanta in a dose of 30 Gy. It is established that the activity of 5-nucleotidase in the liver endures phase changes with the predominance of the enzyme activation; in the brain it is higher during the whole period of investigation. The guanylate kinase activity lowers in the both fractions of the organs under study during the whole period of the experiment.     

RELATIONSHIP BETWEEN THE 4-AMINOBUTYRATE BYPATH AND THE OXIDATION OF 2-OXOGLUTARATE IN RAT BRAIN MITOCHONDRIA

Abstract— A method is described to evaluate simultaneously the contributions of 2-oxoglutarate oxidation and the GABA bypath to succinate production in isolated rat brain mitochondria.
2-Oxoglutarate oxidation is under respiratory control whereas the activity of the GABA shunt is but slightly affected by the mitochondrial energy state.
The oxidation of GABA is half-maximal with 5m m -GABA. GABA does not affect 2-oxoglutarate oxidation. 1 m m -2-oxoglutarate is optimal for GABA oxidation, whereas higher concentrations inhibit the shunt activity.
The rate of GABA oxidation observed in vitro (5 nmol/min.mg mitochondrial protein) is comparable to the activity of the shunt under in viuo conditions.
The control and the compartmentation of GABA oxidation are discussed.     

High-affinity GABA uptake and GABA-metabolizing enzymes in pig forebrain white matter: a quantitative study

GABA receptor activation in central nervous white matter may be protective during white matter hypoxia in the adult, and it may modify axonal conduction, especially in the developing brain. GABA uptake is important for the shaping of the GABA signal, but quantitative data are lacking for GABA uptake and GABA-metabolizing enzymes in central nervous white matter. We report that high-affinity uptake of GABA in adult pig corpus callosum, fimbria, subcortical pyramidal tracts, and occipital white matter is approximately 20% of that in temporal cortex gray matter. Tiagabine (0.1 microM), an antiepileptic drug that specifically inhibits the GAT-1 GABA transporter inhibited GABA uptake 50% in temporal cortex and 60-68% in white structures. This finding indicates that GAT-1 is an important GABA transporter in white matter and suggests that white matter GABA uptake is inhibited during tiagabine therapy. GABA transaminase activity in white structures was approximately 20% of neocortical values. Glutamate decarboxylase (GAD) activity in white structures was only 4% of that in neocortex (7-12 pmol/mg tissue x min(-1) versus approximately 200 pmol/mg tissue x min(-1)). Since white matter activity of citrate synthase of the tricarboxylic acid cycle was approximately 25% of neocortical values ( approximately 0.4 nmol/mg tissue x min(-1) versus approximately 1.5 nmol/mg tissue x min(-1)), the low GAD activity suggests a slower metabolic turnover of GABA in white than in gray matter.     

THE DISTRIBUTION OF GLUTAMATE DECARBOXYLASE IN RAT TISSUES; ISOTOPIC VS FLUORIMETRIC ASSAYS

—The activity of glutamate decarboxylase (GAD, EC 4.1.1.15) in normal and neoplastic rat tissues was determined by two assay methods, one based on the production of ¹⁴CO₂ from [¹⁴C]glutamic acid and the other on the fluorimetric measurement of γ-aminobutyric acid (GABA) formation. Activities obtained with the isotopic assay were high in every tissue (ranging from over 800 in liver and brain to 107nmol CO₂/min/g in lung). They were drastically diminished by Triton X-100, by an oxygen-free atmosphere or by the mitochondrial electron transport inhibitors, rotenone and antimycin A. Activities measured fluorimetrically were significant in only a few tissues and were stimulated by Triton (e.g. from 299 to 569 nmol GABA/min/g brain) but were unaffected by rotenone. For several tissues after Triton treatment the fluorimetric and isotopic assays (in air) gave the same results (i.e. the two end products, CO₂ and GABA were in stoichiometric agreement); however, the fluorimetric assay remains the more reliable measure of GAD activity since Triton may not inhibit completely the non-GAD dependent decarboxylation of glutamate in all types of tissue preparations. The hepatic, renal and mammary tumours tested were devoid of GAD; among non-neural normal tissues, kidney, liver and, possibly, adrenal gland contained significant GAD activity. In kidney and liver the activity was 15 and 10 per cent of that in brain.     

Repeated Administration of γ-VinylGABA Reduces Rat Brain Glutamine Synthetase Activity

Abstract: The glutamine cycle has been proposed as a pathway in which glutamine synthesized in glia provides substrate for synthesis of the neurotransmitters glutamate and GABA as they are lost from neurons. To test whether GABA may regulate this pathway, the effect of elevated GABA on the glial enzyme glutamine synthetase was examined in rat brain. Repeated subcutaneous injections of the antiepileptic GABA transaminase inhibitor γ-vinylGABA at a dose of 150 mg/kg per day for 21 days reduced glutamine synthetase activity by 36% in the cortex and 22% in the cerebellum. At 30 mg/kg per day, glutamine synthetase activity was reduced by 9.5% in the cortex but unchanged in the cerebellum. The reductions were brain specific because the skeletal muscle and liver enzymes were unaffected by γ-vinylGABA administration. Amino acid analysis of the cortex from γ-vinylGABA-treated rats demonstrated a 270% increase in GABA levels after 150 mg/kg but no change after 30 mg/kg. GABA levels and glutamine synthetase activity were inversely correlated. The 150 mg/kg dose significantly lowered cortical glutamine and glutamate levels. The decline in brain glutamine synthetase activity with chronic γ-vinylGABA administration developed gradually over time and may be due to the slow turnover of this enzyme in vivo.     

EFFECT OF DIAZEPAM AND PENTOBARBITAL ON AMINOOXYACETIC ACID-INDUCED ACCUMULATION OF GABA

Abstract— The effect of diazepam and pentobarbital on γ-aminobutyric acid (GABA) levels, the aminooxyacetic acid (AOAA)-induced accumulation of GABA, and the in vitro activity of l -glutamate 1-carboxyl-lyase (EC 4.1.1.15) [GAD] were studied in various regions of rat brain. Diazepam increased GABA levels in the substantia nigra, diminished the AOAA-induced accumulation of GABA in the caudate nucleus, cingulate, parietal and entorhinal cortex and had no effect on GABA accumulation in the pyriform and cerebellar cortex. After pentobarbital, GABA levels were elevated in the caudate nucleus but decreased in the parietal and pyriform cortex; the AOAA-induced accumulation of GABA also diminished in all cortical regions studied. No correlation was found between the apparent changes in GABA synthesis, as estimated by accumulation after inhibition of 4-aminobutyrate-2-oxoglu-tarate (EC 2.6.1.19) [GABA-T] with AOAA, and the changes in GABA levels induced by these drugs. The reduction in AOAA-induced GABA accumulation after diazepam and pentobarbital treatment was most pronounced in regions which showed the greatest accumulation of GABA after AOAA administration. Neither diazepam nor pentobarbital administration affected the activity of GAD in homogenates of cingulate cortex. Chlorpromazine, at a dose which decreased spontaneous activity, enhanced the AOAA-induced GABA accumulation in the cingulate cortex, suggesting that drug-induced sedation is not necessarily associated with decreased GABA synthesis. While regional differences were observed in the effects of diazepam and pentobarbital on GABA synthesis, both agents appear to inhibit GABA synthesis in vivo and both do so, in at least some brain areas, at subsedative doses.     

Increased Intracellular γ-Aminobutyric Acid Selectively Lowers the Level of the Larger of Two Glutamate Decarboxylase Proteins in Cultured GABAergic Neurons from Rat Cerebral Cortex

The regulation of glutamate decarboxylase (GAD; EC 4.1.1.15) was studied by using cultures of cerebral cortical neurons from rat brain grown in serum-free medium. About 50% of the neurons in the cultures were gamma-aminobutyric acid (GABA)ergic as determined by two double-staining procedures. Immunoblotting experiments with four anti-GAD sera that recognize the two forms to varying degrees, demonstrated that the cultures contained the two forms of GAD that are present in rat brain (apparent molecular masses = 63 and 66 kDa). GAD activity was reduced by 60-70% when intracellular GABA levels were increased by incubating the cultures with the GABA-transaminase inhibitor gamma-vinyl-GABA for greater than 5-10 h or with 1 mM GABA itself. Neither baclofen nor muscimol (100 microM) affected GAD activity. Immunoblotting experiments showed that only the larger of the two forms of GAD (66 kDa) was decreased by elevated GABA levels. These results, together with previous results indicating that the smaller form of GAD is more strongly regulated by pyridoxal 5'-phosphate (the cofactor for GAD), suggest that the two forms of GAD are regulated by different mechanisms.     

Response of the cockroach brain gamma-aminobutyric acid system to isonicotinic acid hydrazide and mercaptopropionic acid

The presence of gamma-aminobutyric acid (GABA) as well as glutamic acid decarboxylase (GAD) and GABA-transaminase (GABA-T) enzymes was demonstrated in the cockroach (Periplaneta americana) brain. Isonicotinic acid hydrazide (INH) in vivo (2.19 mumol/g) inhibited brain GAD activity, the inhibition lasted for about 2 hours and the normal activity levels reappeared at 4 h after INH administration. Brain GABA levels increased initially but then declined and were restored to normal levels at 4 h after INH administration. GABA-T activity was strongly inhibited by INH and a total 100% inhibition was observed at 2-3 h following INH treatment. The GABA-T activity, however, began to recover after 3 h but only 37% of the total enzyme activity was released from inhibition. Mercaptopropionic acid (MPA) in vivo (32 micrograms/g) inhibited brain GAD activity and depleted GABA level also. Results indicate that INH response of the cockroach brain GABA system is similar to that reported for the chick brain but differs from that of the mammalian brain.     

Studies on the Pyruvate Dehydrogenase Complex in Brain with the Arylamine Acetyltransferase-Coupled Assay

Abstract: A spectrophotometric assay for the brain pyruvate dehydrogenase complex (PDHC) with arylamine acetyltransferase (ArAT; EC 2.3.1.5) to follow the production of acetyl-CoA has been standardized. Activity was proportional to time and protein. It depended completely on added pyruvate, CoA, NAD, and MgCI₂, and partially on thiamine pyrophosphate, Triton X-100, and a sulfhydryl compound. The activities are the highest in the literature for brain PDHC (50 nmol/min/mg protein) and equal the maximum recorded rates of pyruvate flux for brain in vivo . Activities as low as 0.6 nmol/min could be measured. Use of ArAT of different purities (1–2-fold and 11–%-fold) allowed convenient measurement of total PDHC (ArAT-I) and of the active form of PDHC (ArAT-II). The proportion of PDHC in the active form was 50% in mouse brain, 30% in rat brain, and 10% in mouse liver. Total PDHC activity was unchanged postmortem during storage of mouse brain in situ at +4°C or at -20°C for 3 days or at +20°C for 24 h. The relative specific activity of PDHC in cytoplasmic or synaptoplasmic fractions was less than that of two other mitochondrial enzymes, fumarase (EC 4.2.1.2) and monoamine oxidase (EC 1.4.3.4), which argues strongly against the hypothesis of a cytoplasmic PDHC in cholinergic nerve endings.     

Action of microwaves with different modulation frequencies and exposure times on GABA receptor concentration in the cerebral cortex of rats]

The effect of 800 mHz microwaves of 0, 3, 5, 7, 16, and 30 Hz modulation on GABA receptor concentration in rat brain cortex has been investigated. Irradiation of the whole body at a modulation frequency of 16 Hz readily decreases the GABA receptor concentration. Irradiation at other modulation frequencies is ineffective. Irradiation of the whole body modulated at 16 Hz with various exposure times (5, 15 and 60 min) has revealed the highest effect at 5 min, while at a longer exposure the effect decreases.     

A correlation between changes in gamma-aminobutyric acid metabolism and seizures induced by antivitamin B6.

The effects of DL-penicillamine (DL-PeA), hydrazine and toxopyrimidine (TXP, 2-methyl-6-amino-5-hydroxymethylpyrimidine) on gamma-aminobutyric acid (GABA) metabolism in mouse brain were studied. All these compounds inhibited the activity of glutamate decarboxylase [EC 4.1.1.15] (GAD) and slightly inhibited that of 4-aminobutyrate: 2-oxoglutarate aminotransferase [EC 2.6.1.19] (GABA-T). In contrast, very different effects were observed on GABA levels; hydrazine caused a marked increase, DL-PeA had no effect, and TXP caused a slight decrease in the content of the amino acid. These results could be described by an equation which related the excitable state to changes in the flux of the GABA bypass. Since the values obtained from the equation clearly reflect the seizure activity, it is suggested that the decreased GABA flux might be a cause of convulsions induced by these drugs.