Evidence for a role of glutamate decarboxylase activity as a regulatory mechanism of cerebral excitability

Epilepsy and hippocampal neurodegeneration induced by glutamate decarboxylase inhibitors in awake rats

Glutamic acid decarboxylase (GAD), the enzyme responsible for GABA synthesis, requires pyridoxal phosphate (PLP) as a cofactor. Thiosemicarbazide (TSC) and γ-glutamyl-hydrazone (PLPGH) inhibit the free PLP-dependent isoform (GAD65) activity after systemic administration, leading to epilepsy in mice and in young, but not in adult rats. However, the competitive GAD inhibitor 3-mercaptopropionic acid (MPA) induces convulsions in both immature and adult rats. In the present study we tested comparatively the epileptogenic and neurotoxic effects of PLPGH, TSC and MPA, administered by microdialysis in the hippocampus of adult awake rats. Cortical EEG and motor behavior were analyzed during the next 2h, and aspartate, glutamate and GABA were measured by HPLC in the microdialysis-collected fractions. Twenty-four hours after drug administration rats were fixed for histological analysis of the hippocampus. PLPGH or TSC did not affect the motor behavior, EEG or cellular morphology, although the extracellular concentration of GABA was decreased. In contrast, MPA produced intense wet-dog shakes, EEG epileptiform discharges, a >75% reduction of extracellular GABA levels and remarkable neurodegeneration of the CA1 region, with >80% neuronal loss. The systemic administration of the NMDA glutamate receptor antagonist MK-801 30 min before MPA did not prevent the MPA-induced epilepsy but significantly protected against its neurotoxic effect, reducing neuronal loss to <30%. We conclude that in adult awake rats, drugs acting on PLP availability have only a weak effect on GABA neurotransmission, whereas direct GAD inhibition produced by MPA induces hyperexcitation leading to epilepsy and hippocampal neurodegeneration. Because this degeneration was prevented by the blockade of NMDA receptors, we conclude that it is due to glutamate-mediated excitotoxicity consequent to disinhibition of the hippocampal excitatory circuits.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

Sex steroids effects on the content of GAD, TH, GABA(A), and glutamate receptors in the olfactory bulb of the male rat

Sex steroids exert multiple functions in the central nervous system. They modulate responses to olfactory information in mammals but their participation in the regulation of neurotransmission in the olfactory bulb is unknown. We studied by Western blot the effects of estradiol (E2), progesterone (P4), and allopregnanolone (Allo) on the content of glutamic acid decarboxylase (GAD), gamma-aminobutyric acid A receptor alpha-2 subunit (GABA(A)R alpha-2), glutamate receptor 2/3 (GlutR 2/3), and tyrosine hydroxylase (TH) in the olfactory bulb of gonadectomized male rats. GAD content was increased by all steroids administered alone. Interestingly, progestins reduced E2 effects on GAD content. Steroids increased the content of TH and GABA(A)R alpha-2. In contrast, GlutR 2/3 content was decreased by E2 and P4, whereas Allo did not modify it. These results suggest that estrogens and progestins regulate olfactory bulb functions in the male rat by modulating the expression of key proteins involved in several neurotransmission systems.

Molecular Identification of the 62 kd Form of Glutamic Acid Decarboxylase from the Mouse

A series of overlapping clones coding for L-glutamic acid decarboxylase was purified from a mouse brain cDNA library, the longest of which contains a 1869 bp open reading frame and 913 bp of non-coding sequence. By comparison with the corresponding sequences from the mouse genome, it was determined that the first methionine in the longest cDNA represents the initiation codon. Expression of this cDNA in eukaryotic cells produces a 62 kd protein that is recognized by antiserum against rat GAD and which displays GAD activity commensurate with the amount of protein produced. Antibodies raised against the purified product of this cDNA recognize a 62 kd protein from mouse brain on immunoblots, specifically stain GABA-ergic neurons in brain sections, and are capable of immunoprecipitating most GAD activity from mouse brain extracts. These results provide the first definitive identification of a cDNA coding for the larger of two forms of GAD in mouse brain, and suggest that the two forms are closely related.

Seizures induced by intracerebral administration of pyridoxal-5'-phosphate: effect of GABAergic drugs and glutamate receptor antagonists

Pyridoxal-5'-phosphate (PLP), the cofactor of glutamate decarboxylase, paradoxically induces convulsions when injected intracranially in adult mammals. We have tested the effect of some GABAergic and antiglutamatergic drugs on the behavioral and electroencephalographic (EEG) seizures produced by intracerebroventricular (i.c.v.) microinjection of 1 micromol PLP in the rat. PLP induced barrel turning, running fits and tonic-clonic convulsions, which started 5-10 min after recovery from the anesthesia (halothane), peaked at 20 min and disappeared at about 50 min. These symptoms were accompanied by frequent high amplitude EEG spike burst discharges. Pyridoxal, pyridoxamine-5'-phosphate or deoxypyridoxine were ineffective. The i.c.v. microinjection of the GABAergic compounds muscimol, isoguvacine, aminooxyacetic acid or GABA itself, significantly protected against PLP effects. In contrast, the NMDA receptor antagonists MK-801 and the non-NMDA receptor antagonist NBQX, failed to protect and induced motor alterations and mortality. We conclude that a temporary decrease of the GABA(A) receptor function is involved in the convulsant effect of PLP. This decrease might be due to the formation of a Schiff base between the carbonyl group of PLP and the epsilon-amino group of a functionally crucial lysine residue located in one extracellular loop of the GABA(A) receptor.

Is hypothalamic GABA involved in immune function in relation to dietary protein during aging?

Hypothalamic GABAergic activity and immune response in spleen were not significantly changed with the increase of age from 3 to 6 months in adult male albino rats. Further increase of age from 6 to 9 months increase the GABAergic activity and decreased the cell viability in spleen without any change in its T-lymphocyte cytotoxicity. Consumption of low protein diet (LPD) for a short-term period (STP; 7 consecutive days) increased the hypothalamic GABAergic activity without changing the immune response in 3 months old rats. When supplemented for a long-term period (LTP; 30 consecutive days) to 3 months old rats, a reduction of hypothalamic GABAergic activity and the immune response was observed. Intake of high protein diet (HPD) for both STP and LTP increased the GABAergic activity and immune response, but the increase of GABAergic activity in hypothalamus under STP was greater than that observed under LTP. In 6 months old rats consumption of LPD for STP reduced the GABAergic activity without any alteration of its immune response. Long-term supplementation of this LPD to the same age group increased GABAergic activity and the mitotic activity of spleen cells without any alteration of the functional activity of the T-cells in spleen. Consumption of HPD for STP failed to produce any change in hypothalamic GABAergic activity and the immune response of 6 months old rats. Supplementation of HPD for LTP reduced the hypothalamic GABAergic activity and the immune response of the same age group. The reduction in hypothalamic GABAergic activity without any change in the immune response was observed following the supplementation of low protein diet to 9 months old rat for STP. Intake of the LPD for LTP also reduced the hypothalamic GABAergic activity and the mitotic activity of the spleen cells without any alteration of the functional activity of the T-cells in spleen of 9 months old rats. Supplementation of HPD for STP to this aged rat, on the other hand, failed to produced any change in hypothalamic GABAergic activity and the immune response. Intake of HPD for LTP by this aged rats increased the hypothalamic GABAergic activity along with the immune response. The results of this study, thus, suggest that hypothalamic GABAergic activity during aging is an index of immune response and it is modulated following the short- and long-term consumption of protein poor and protein rich diet.

Determination of glutamate decarboxylase by high-performance liquid chromatography

An improved method for the determination of glutamate decarboxylase (GAD) activity is described. The enzyme was evaluated by incubation with glutamic acid (L-Glu) in the presence of pyridoxal 5'-phosphate (PLP): the gamma-aminobutyric acid (GABA) formed was derivatized to PTC-GABA; the latter was subsequently separated and assayed by isocratic HPLC (LiChrospher RP-18 column; isocratic elution with pH 5.8 acetate buffer in acetonitrile-water) with UV absorbance detection at 254 nm. The method described is a sensitive, reproducible and specific assay useful for following variations of GAD activity in vitro; this assay was subsequently used for the evaluation of GAD activity variations after irradiation with low doses of He-Ne laser radiation.

Convulsions and inhibition of glutamate decarboxylase by pyridoxal phosphate-gamma-glutamyl hydrazone in the developing rat

We have previously shown that in the adult rat the inhibition of brain glutamate decarboxylase (GAD) activity by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH) administration does not result in convulsions, whereas in the adult mouse intense convulsions invariably occur. In the present study we report that, surprisingly, immature rats from 2 to 20 days of age treated with PLPGH (80 mg/kg) showed generalized tonic-clonic convulsions, whereas no convulsions at all were present in 30 days-old or older rats. GAD activity, measured by enzymic determination of GABA formed in forebrain homogenates, was inhibited by about 60% at the time of convulsions in 15 days-old and younger rats, whereas the inhibition was between 40 and 50% in older animals. The addition of the coenzyme pyridoxal 5'-phosphate to the incubation medium completely reversed this inhibition. In all treated animals GABA levels were lower compared to controls. The results indicate that the susceptibility of GAD in vivo to a diminished cofactor concentration decreases with age. It seems possible that changes in the expression of enzyme forms are reflected in developmental variations in the susceptibility to seizures induced by vitamin B6 depletion, but alterations of other B6-dependent biochemical pathways cannot be discarded.

Efficacy of a putative GABA analog on synaptic transmission in the cat spinal cord

4-Hydroxy-4-phenylcaproamide (HPhCA), at high doses or rates of IV injection depressed the ventral root reflexes elicited by nerve or dorsal root stimulation. The D (direct) and I (synaptic) ventral root waves and the antidromic (A) dorsal root wave evoked by intraspinal stimulation were also depressed. Similar effects were produced when HPhCA was applied topically on the cord dorsum. At 80 mg/kg and 8 mg/kg/min, the spinal reflexes and the I wave were facilitated for 4 to 6 h, but the D and A waves were depressed. Intracellular recordings from motoneurons showed that HPhCA injection produced: hyperpolarization that lasted several hours, short lasting (< 20 min) facilitation of both EPSPs and IPSPs as well as spike-like potentials (SLPs) that were triggered by EPSPs even though the neuron was hyperpolarized. SLPs may reach the threshold for full spikes. Our results suggest that the spinal depression results from hyperpolarization of motoneurons and the initial facilitation appears to be presynaptic. The late facilitation may be produced by SLPs. HPhCA does not appear to mimic the actions of GABA in primary afferents fibers and motoneurons.

Glutamate decarboxylase in developing rat neocortex: does it correlate with the differentiation of GABAergic neurons and synapses?

Postnatal development of glutamate decarboxylase was studied in the rat cerebral cortex. Two methods were used: estimation of the enzymatic activity of glutamate decarboxylase in homogenates of developing cortical tissue and visualization of structures containing glutamate decarboxylase-like immunoreactivity. Glutamate decarboxylase-like immunoreactivity appeared first in perikarya and dendrites and only later in axons and axon varicosities. The most rapid increase in the glutamate decarboxylase activity took place during the second postnatal week and this coincided with a rapid increase in the density of axon varicosities containing glutamate decarboxylase-like immunoreactivity but preceded the most rapid phase in the formation of GABAergic synapses by several days. However, there was a change in the characteristics of glutamate decarboxylase which correlated with GABA synaptogenesis: two fractions of glutamate decarboxylase with different sensitivities to the activating effects of Triton X-100 could be distinguished as from about the time when most of the GABAergic synapses are formed.

Inhibition of brain glutamate decarboxylase activity is related to febrile seizures in rat pups

Because previous work showed that in the newborn brain, but not in the adult brain, glutamate decarboxylase (GAD) is notably susceptible to heat, we have studied the possible involvement of GAD inhibition in febrile convulsions and the related changes in gamma-aminobutyric acid (GABA) content. Rats of different ages were subjected to hyperthermia, and GAD activity was determined in brain homogenates by measuring the release of 14CO2 from labeled glutamate and by measuring the formation of GABA. The latter method gave considerably lower values than the former in the youngest rats, and was considered more reliable. With this method, we found a 37-48% inhibition of GAD activity in rat pups 2-5 days old, which showed febrile seizures at progressively higher body temperatures, whereas in 10- and 15-day-old animals, which did not show convulsions, GAD activity was not affected by hyperthermia. Whole-brain GABA levels, however, did not change at any age. In contrast to GAD, choline acetyltransferase and lactic dehydrogenase activities were not altered by hyperthermia at any of the ages studied. These results suggest that a decreased efficiency of the inhibitory neurotransmission mediated by GABA, consequent to the inhibition of GAD activity, may be a factor related to febrile convulsions.

A microassay for the determination of soluble and membrane-bound glutamate decarboxylase activity--influences of cations, lipid composition, and pyridoxal 5'-phosphate on the glutamate decarboxylase binding to liposomes

A radiochemical microassay for soluble and membrane-bound glutamate decarboxylase (GAD) is described. Up to 180 samples can be determined per day with a variation coefficient of 2%. The method detects newly synthesized gamma-amino-n-butyric acid in the picomole range and can easily be applied to other enzymes whose substrate and product differ by charge. In an aqueous homogenate of brain (1 + 10; w/v) about 15% of the total GAD activity are spun down by centrifugation (1 h, 100,000g) increasing to 35% of the total GAD activity in solutions with 8 mM calcium chloride or 100 mM potassium acetate. There is similar dependence on the cation concentration when GAD binds to phospholipid vesicles (liposomes) as well as dependence on lipid concentration and lipid composition. The coenzyme pyridoxal 5'-phosphate has no influence on GAD binding to liposomes.

Neurochemical evidence for afferent GABAergic and efferent cholinergic neurotransmission in the frog vestibule

Glutamate decarboxylase and choline acetyltransferase activities with magnitudes similar to those of their homologous enzymes in frog nervous tissue were found in homogenates of the frog labyrinth. Transection of the vestibular nerve resulted in a gradual diminution of choline acetyltransferase activity until it reached an 88% decrease 6 weeks after surgery. In contrast, glutamate decarboxylase activity did not suffer any alteration at any time after nerve excision. The presence of their enzymes of synthesis is evidence of the neurotransmitter participation of GABA and acetylcholine in the frog vestibule; the observed decrease of choline acetyltransferase following vestibule nerve excision supports the efferent synaptic bouton localization of choline acetyltransferase. The suggestion that glutamate decarboxylase is located in a cell type (or compartment) that may well be the hair cell is supported by the fact that this enzyme does not suffer any modification after surgery. These results are in accordance with an efferent cholinergic neurotransmission and a putative afferent role of GABA in the frog vestibule.

Two forms of rat brain glutamic acid decarboxylase differ in their dependence on free pyridoxal phosphate

There are two forms of glutamate decarboxylase (GAD) found in the rat brain. One form (form A) does not require exogenous pyridoxal-5'-phosphate (PLP) for activity whereas another form (form B) requires exogenous PLP for activity. These two forms differ greatly in temperature sensitivity, inactivation, and reactivation by the removal and readdition of PLP, electrophoretic mobility, and regional distribution. For instance, forms A and B are inactivated to an extent of 91% and 10%, respectively, by the treatment at 45 degrees C for 30 min; form A is greatly inactivated (77%) by the removal of PLP by aminooxyacetic acid and the readdition of PLP, whereas form B is only slightly inactivated (7%). Forms A and B can be clearly separated by 5% polyacrylamide gel electrophoresis in which form A migrates faster than form B. In all 10 brain regions studied, form A is present in smaller amounts than form B. This difference is greatest in the superior colliculus (the ratio of B to A is about 5), while in the locus coeruleus and cerebellum, forms A and B are present in nearly equal proportion. Forms A and B are similar with respect to relative abundance in hypotonic, isotonic, and hypertonic preparations, inhibition of catalytic activity by a carbonyl-trapping agent, immunochemical properties, and chromatographic patterns in a variety of systems. The significance of forms A and B and PLP in the regulation of gamma-amino-butyric acid (GABA) level is also discussed.

Subcellular distribution of glutamic acid decarboxylase in rat brain regions following electroconvulsive stimulation

Electroconvulsive stimulation of rats evoked significant increases of glutamic acid decarboxylase (GAD) activity in the synaptosomal fractions of neocortex (including white matter) and hippocampal formation. The elevation of synaptosomal-bound GAD activity was not significant in cingulate cortex, striatum, caudal brainstem and thalamus. The electroconvulsive shocks had no effect on the GAD activity of the cytoplasmic fractions of any brain regions investigated. The highest physiological level of synaptosomal GAD activity was found in thalamus, followed (in decreasing order) by striatum, hippocampus, cingulate cortex, caudal brainstem and neocortex.

Some characteristics of glutamic acid decarboxylase of chick ampullary cristae

In a previous study, it was demonstrated that enzyme-mediated gamma-aminobutyric acid (GABA) synthesis occurs in the vestibule of the chick inner ear. As deeper knowledge of the properties of its synthesizing enzyme might contribute to the understanding of the role of GABA in inner ear function, some characteristics of glutamate decarboxylase (GAD) were studied in chick isolated ampullary cristae under conditions in which 14CO2 release from [1-14C]glutamate and [14C]GABA formation from [U-14C]glutamate for estimating GAD activity were equal. It was found that Km for glutamate is 5 mM and that the enzyme pH optimum is 7.3. These values fall within the range described for the corresponding enzyme in nervous tissue of other species. Pyridoxal phosphate (PLP) activates the enzyme and aminooxyacetic acid inhibits it, the same as these agents activate or inhibit GAD from several nervous tissue sources. 2-Mercaptoethanol shows some protection from inactivation of the PLP-dependent enzyme and Triton X-100 exerts some inhibition of vestibular GAD activity, as previously shown in other nervous tissue preparations. Although its cellular localization is at present uncertain, these results indicate that GAD of chick vestibular tissue possesses properties resembling those of the brain enzyme and might be controlled in a manner similar to that of GAD in brain, thus possibly participating in the regulation of inner ear function.

Effect of haloperidol on glutamate decarboxylase activity in discrete brain areas of the rat

Using freeze-dried samples of rat brain, the effect of haloperidol on glutamate decarboxylase (GAD) activity without exogenously added pyridoxal-5'-phosphate (PLP) was studied in discrete brain nuclei and areas. Repeated injections of haloperidol produced significant changes in GAD activity in the dorsal part of the caudate nucleus, entopeduncular nucleus, pars reticulata of the substantia nigra, lateral hypothalamic area, and dorsomedial hypothalamic nucleus. A reduction of GAD activity after haloperidol was observed in the entopeduncular nucleus and pars reticulata of the substantia nigra. This finding demonstrates biochemically that haloperidol-induced extrapyramidal behavior may be involved in the reduction of GABAergic transmission in the entopeduncular nucleus and pars reticulata of the substantia nigra. A decrease in GAD activity in the lateral hypothalamic area indicates that interaction between GABAergic neurons as well as dopaminergic neurons may be involved in the haloperidol-induced behavioral changes. In addition, close interaction between GABAergic and dopaminergic systems in the dorsomedial hypothalamic nucleus and dorsal part to the caudate nucleus was demonstrated.

An analysis of the cortical and striatal involvement in dyskinesia induced in rats by intracerebral injection of GABA-transaminase inhibitors and picrotoxin

Unilateral intrastriatal injection of various substances induces a characteristic dyskinetic syndrome in rats. These substances include picrotoxin as well as a series of irreversible GABA-transaminase inhibitors. Using the degree of enzyme inhibition in various brain areas as a measure of drug distribution following intrastriatal administration of gamma-acetylenic GABA and gamma-vinyl GABA, there was found considerable retrodiffusion via the needle tract to the overlying cortex. Topical application of gamma-acetylenic GABA and gamma-vinyl GABA to the cortical surface overlying the striatum produced a high incidence of identical dyskinesias without any evidence of diffusion of drugs to the striatum. The cortically induced movements could be duplicated by picrotoxin application to a defined cortical area. These findings suggest that interference with gabaergic function in the striatum is not necessary for the production of the dyskinetic syndrome and that this syndrome may be a cortically induced phenomenon.

GABA-mediated behavioral inhibition during ontogeny in the mouse

Although immature rats and mice generally demonstrate poor behavioral inhibitory capacities, some recent evidence may indicate the presence of substantial inhibitory control. The present experiment investigated the possibility that gamma-aminobutyric acid (GABA) systems may mediate some behavioral inhibition during early development. Mice 9-100 days old were injected with the GABA-elevating agent amino-oxyacetic acid (AOAA) and tested for behavioral activity. High levels of locomotor activity characteristic of immature control mice were attenuated following AOAA injection, whereas AOAA had little effect on the activity of adult mice. Moreover, AOAA produced a period of rebound hyperactivity for young but not for adult mice. These findings suggest that although GABA systems may mediate early behavioral inhibition, coordination between excitatory and inhibitory capacities matures slowly. In a second experiment the dopamine-beta-hydroxylase inhibitor FLA-63 prevented rebound hyperactivity in young mice pretreated with AOAA, suggesting that the excitatory component may be mediated by noradrenergic systems.

Neurophysiological and neurochemical studies on the action of the anticonvulsant gamma-hydroxy, gamma-ethyl, gamma-phenyl-butyramide

The effect of gamma-hydroxy, gamma-ethyl, gamma-phenyl-butyramide (HEPB) on afterdischarges produced by hippocampal stimulation in cats was studied. HEPB notably diminished the duration of afterdischarges and in some cats blocked their propagation into the substantia nigra and the amygdala. HEPB treatment also antagonized the enhancement of afterdischarge duration produced by subconvulsive doses of bicuculline, whereas treatment with diphenylhydantoin strongly potentiated this effect of bicuculline. The intracisternal injection of HEPB or gamma-aminobutyric acid (GABA) in mice resulted in a potentiation of strychnine-induced convulsions. On the other hand, neurochemical experiments in mouse brain cortex slices and in synaptosomes demonstrated that HEPB did not affect the high affinity uptake of [3H] GABA, its spontaneous or Ca2+ dependent release stimulated by depolarizing K+ concentrations, and its Na+ independent binding to synaptic plasma membranes.

Glutamate decarboxylase activity in preoptic and hypothalamic nuclei of the rat

Freeze-dried samples were used for the determination of glutamate decarboxylase activity in discrete preoptic and hypothalamic nuclei. The activity showed marked regional variation in distribution.

The anticonvulsant activity of ketamine agains siezures induced by pentylenetetrazol and mercaptopropionic acid

The activity of the dissociative anaesthetics ketamine and gamma-hydroxybutyrate against seizures induced by mercaptopropionate and pentylenetetrazol have been determined. Ketamine (90 mg/kg) prevented the seizures induced by both convulsants, but gamma-hydroxybutyrate had negligible anticonvulsant activity. Mercaptopropionate (150 mg/kg) produced a rapid fall in whole brain glutamate decarboxylase activity which correlated with the onset of convulsions. Ketamine given prior to the mercaptopropionate prevented the convulsions, but had no effect on the reduction of enzyme activity. It was concluded that although ketamine was an anticonvulsant it did not act by preventing the inhibition of glutamate decarboxylase responsible for mercaptopropionate-induced convulsions.

Effect of vitamin B6 on gamma-aminobutyric acid level in rat brain

Vitamin B6 injected intraperitoneally into rats 400 mg/kg body weight, has produced a statistically significant decrease in GABA level concentrations in hippocampus and cerebellum. Cerebral cortex, caudate nucleus and thalamus have shown the decrease in GABA concentrations, but these changes were not statistically significant. No remarkable behavioural changes were noted under such circumstances. The possible functional meaning of these results is discussed in relation to the role of GABA distribution in different brain regions and development of convulsions.

GABA metabolism and cerebral protein synthesis

Glutamate decarboxylase (GAD) activity and [14C)leucine incorporation into brain protein in vivo were measured in mice injected with L-glutamic acid-gamma-hydrazide and 60 min later with pyridoxal-5'-phosphate; this combined treatment produces a decrease in GAD activity and an increase in GABA levels. Under such conditions, protein synthesis was inhibited to the same extent as GAD activity. A parellelism between the decrease in GAD activity and that in protein synthesis was also observed in brain cortex slices treated with GAD inhibitors. This treatment did not affect leucine incorporation into protein in liver slices. The results support the previously suggested hypothesis that a certain pool of GABA may have a role in the regulation of protein synthesis in brain, and further suggest that the synthesis of GABA may be more important in this respect than its total concentration.