Glutamate decarboxylase activity in striatal slices: characterization of the increase following depolarization

Reduced Glx and GABA Inductions in the Anterior Cingulate Cortex and Caudate Nucleus Are Related to Impaired Control of Attention in Attention-Deficit/Hyperactivity Disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that impairs the control of attention and behavioral inhibition in affected individuals. Recent genome-wide association findings have revealed an association between glutamate and GABA gene sets and ADHD symptoms. Consistently, people with ADHD show altered glutamate and GABA content in the brain circuitry that is important for attention control function. Yet, it remains unknown how glutamate and GABA content in the attention control circuitry change when people are controlling their attention, and whether these changes can predict impaired attention control in people with ADHD. To study these questions, we recruited 18 adults with ADHD (31-51 years) and 16 adults without ADHD (28-54 years). We studied glutamate + glutamine (Glx) and GABA content in the fronto-striatal circuitry while participants performed attention control tasks. We found that Glx and GABA concentrations at rest did not differ between participants with ADHD or without ADHD. However, while participants were performing the attention control tasks, participants with ADHD showed smaller Glx and GABA increases than participants without ADHD. Notably, smaller GABA increases in participants with ADHD significantly predicted their poor task performance. Together, these findings provide the first demonstration showing that attention control deficits in people with ADHD may be related to insufficient responses of the GABAergic system in the fronto-striatal circuitry.

Comparison of the expression of two forms of glutamic acid decarboxylase (GAD67 and GAD65) in the visual cortex of normal and dark-reared cats

In normal development, there are dramatic changes in both the level and the laminar pattern of expression of the two forms of glutamate decarboxylase (GAD67, GAD65), the synthetic enzyme for gamma-aminobutyric acid (GABA). We have used antibodies to determine whether these normal postnatal changes in the expression of the two GADs depend on visual input by comparing normal and dark-reared cat visual cortex. Western blot analysis showed no significant differences in the levels of expression of the two enzymes between rearing conditions at either 5 or 20 weeks. Immunohistochemistry was used to compare the laminar distribution of the GADs in the two rearing conditions. At 1 week of age, both GAD67 and GAD65 immunoreactivity is concentrated in deep layers of visual cortex. At 5 and 20 weeks in both rearing conditions, GAD67-stained cells bodies were distributed rather uniformly across all cortical layers. GAD65 primarily labeled puncta (synaptic terminals) and these were also distributed rather uniformly across all visual cortical layers in both rearing conditions. Counts of GAD67-positive cell bodies and GAD65-positive puncta also revealed no differences between the rearing conditions. Thus, both GAD67, which produces the basal pool of GABA, and GAD65, which is specialized to respond to short-term increases in demand in synaptic terminals, developed normal levels of expression and normal intracellular and laminar distributions in the absence of visual input. Physiological studies suggest immaturity in the GABA system of dark-reared visual cortex. The present results indicate that such abnormalities are not due to presynaptic alterations in GABA synthetic enzymes.

Role of glutamatergic and GABAergic systems in alcoholism

The pharmacological effects of ethanol are complex and widespread without a well-defined target. Since glutamatergic and GABAergic innervation are both dense and diffuse and account for more than 80% of the neuronal circuitry in the human brain, alterations in glutamatergic and GABAergic function could affect the function of all neurotransmitter systems. Here, we review recent progress in glutamatergic and GABAergic systems with a special focus on their roles in alcohol dependence and alcohol withdrawal-induced seizures. In particular, NMDA-receptors appear to play a central role in alcohol dependence and alcohol-induced neurological disorders. Hence, NMDA receptor antagonists may have multiple functions in treating alcoholism and other addictions and they may become important therapeutics for numerous disorders including epilepsy, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's chorea, anxiety, neurotoxicity, ischemic stroke, and chronic pain. One of the new family of NMDA receptor antagonists, such as DETC-MESO, which regulate the redox site of NMDA receptors, may prove to be the drug of choice for treating alcoholism as well as many neurological diseases.

GAD levels and muscimol binding in rat inferior colliculus following acoustic trauma

Pharmacological studies of the inferior colliculus (IC) suggest that the inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in shaping responses to simple and complex acoustic stimuli. Several models of auditory dysfunction, including age-related hearing loss, tinnitus, and peripheral deafferentation, suggest an alteration of normal GABA neurotransmission in central auditory pathways. The present study attempts to further characterize noise-induced changes in GABA markers in the IC. Four groups (unexposed control, 0 h post-exposure, 42 h post-exposure, and 30 days post-exposure) of 3-month-old male Fischer 344 rats were exposed to a high intensity sound (12 kHz, 106 dB) for 10 h. Observed hair cell damage was primarily confined to the basal half of the cochlea. There was a significant decrease in glutamic acid decarboxylase (GAD(65)) immunoreactivity in the IC membrane fraction compared to controls (P<0.05) at 0 h (-41%) and 42 h (-28%) post-exposure, with complete recovery by 30 days post-exposure (P>0.98). Observed decreases in cytosolic levels of GAD(65) were not significant. Quantitative muscimol receptor binding revealed a significant increase (+20%) in IC 30 days after sound exposure (P<0.05). These data suggest that changes in GABA neurotransmission occur in the IC of animals exposed to intense sound. Additional studies are needed to determine whether these changes are a result of protective/compensatory mechanisms or merely peripheral differentiation, as well as whether these changes preserve or diminish central auditory system function.

Detection of glutamate decarboxylase isoforms in rat inferior colliculus following acoustic exposure

The inferior colliculus is a central auditory structure which serves as a site for the integration of ascending and descending auditory information. Changes in central auditory structures may occur with acoustic exposure, which cannot be explained by alterations in cochlear function alone. Rats were exposed to a 10-kHz tone at 100 dB SPL for 9 h. Auditory brainstem response measures showed an initial 25-30-dB threshold shift across all tested frequencies. By 30 days post-exposure, thresholds for clicks and most frequencies returned to near control levels; however, thresholds remained elevated at 10 and 20 kHz. Inner hair cell loss was confined to apical and basal ends of the cochlea, and did not exceed 20%. Inferior colliculus levels of the two isoforms of the GABA synthetic enzyme glutamate decarboxylase (65,000 and 67,000 mol. wt forms) were measured immediately post-exposure (0 h) and at two and 30 days post-exposure using quantitative immunocytochemical and western blotting techniques. Zero-hour measures revealed a significant increase in the level of glutamate decarboxylase (mol. wt 67,000) protein (118%), as well as in the optical density (35%) of immunolabeled cells. By 30 days post-exposure, inferior colliculus protein levels of both glutamate decarboxylase isoforms were significantly below unexposed controls (39% and 21% for the 65,000 and 67,000 mol. wt forms, respectively). These studies describe increased markers for GABA immediately following acoustic exposure, followed by a decline to below control levels from two to 30 days post-exposure. It remains to be determined whether noise trauma-induced changes in glutamate decarboxylase levels in the inferior colliculus reflect protective up-regulation in response to intense stimulation, followed by the establishment of new neurotransmitter equilibrium levels.

Expression of two forms of glutamic acid decarboxylase (GAD67 and GAD65) during postnatal development of the cat visual cortex

The postnatal development of GAD67 and GAD65 protein expression and of GAD67 positive neurons and GAD65 containing axon terminals in cat visual cortex was studied. Western blot analysis showed that the expression of both GAD67 and GAD65 increased to approximately two-thirds of the adult level during the first 5 postnatal weeks and gradually increased thereafter. In adult cats, immunohistochemistry showed that GABA and GAD67 containing neurons were found in all cortical layers. Faint cell body staining was seen with the antibody to GAD65, but it densely labeled puncta. In neonates, GABA and GAD67 immunoreactivity was most intense in two distinct bands, one superficial (Layer 1/Marginal zone), another deep (Layer VI/Subplate). Unlike in adults, GAD65 positive cell bodies were clearly evident in neonates and distributed similarly to, but less frequently than, GABA and GAD67. These GAD65 positive cells frequently had morphologies suggestive of embryonic cells and largely disappeared in older animals. During postnatal development, the neurochemical differentiation of GAD67 positive neurons and GAD65 positive axon terminals across visual cortical laminae followed an inside-outside developmental pattern, which reached adult levels after 10 weeks of age. These results suggest that postnatal development of the visual cortical GABA system involves three distinct processes: (A) a dying off of embryonic GABA cells which could play a role in formation of the cortical plate; (B) a period of relative quiescence of the VC GABA system in the first 5 postnatal weeks which could maximize excitatory NMDA effects during the rising phase of the critical period; (C) the prolonged postnatal maturation of the adult GABA system which could be involved in the crystallization of adult physiological properties and the disappearance of neural plasticity.

Regulation of gamma-aminobutyric acid synthesis in the brain

gamma-Aminobutyric acid (GABA) is synthesized in brain in at least two compartments, commonly called the transmitter and metabolic compartments, and because regulatory processes must serve the physiologic function of each compartment, the regulation of GABA synthesis presents a complex problem. Brain contains at least two molecular forms of glutamate decarboxylase (GAD), the principal synthetic enzyme for GABA. Two forms, termed GAD65 and GAD67, are the products of two genes and differ in sequence, molecular weight, interaction with the cofactor, pyridoxal 5'-phosphate (pyridoxal-P), and level of expression among brain regions. GAD65 appears to be localized in nerve terminals to a greater degree than GAD67, which appears to be more uniformly distributed throughout the cell. The interaction of GAD with pyridoxal-P is a major factor in the short-term regulation of GAD activity. At least 50% of GAD is present in brain as apoenzyme (GAD without bound cofactor; apoGAD), which serves as a reservoir of inactive GAD that can be drawn on when additional GABA synthesis is needed. A substantial majority of apoGAD in brain is accounted for by GAD65, but GAD67 also contributes to the pool of apoGAD. The apparent localization of GAD65 in nerve terminals and the large reserve of apoGAD65 suggest that GAD65 is specialized to respond to short-term changes in demand for transmitter GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo release of newly synthesized [3H]GABA in the substantia nigra of the rat: relative contribution of GABA striato-pallido-nigral afferents and nigral GABA neurons

The release of [3H]gamma-aminobutyric acid ([3H]GABA) continuously formed from [3H]glutamine has been measured with a push-pull cannula implanted in the substantia nigra of the rat anesthetized with ketamine. Consistent with the high density of GABA terminals coming from both the striato-pallido-nigral afferents, and from GABA nigrofugal neurons, our results showed that a large amount of [3H]GABA was spontaneously released in the reticulata, about 4 times higher than in the compacta. In the absence of calcium the spontaneous [3H]GABA release was reduced (-30%), as well as the K(+)-induced release of [3H]GABA (-66%). Bicuculline (10(-4) M) did not affect the K(+)-evoked release of [3H]GABA, suggesting that autoreceptors on GABA afferent fibers are distinct from the GABAA subtype. Partial lesions of striato- and pallido-nigral GABA neurons with kainic acid (1.2 micrograms) decrease by 40% the glutamic acid decarboxylase (GAD) activity in the ipsilateral SN without decreasing the spontaneous release of [3H]GABA; even following extensive lesions with kainic acid (2.5 micrograms), GAD activity (-72%) and spontaneous [3H]GABA release (-83%) were not completely abolished. These results suggest that a non-negligible contribution of GABA nigral neurons accounts for the spontaneous GABA release measured in the substantia nigra. This is further supported by the decrease (-20%), and the increase (+40%) of [3H]GABA release produced by the local application of glycine (10(-6) M), and bicuculline (10(-4) M), which respectively, inhibits and activates the nigral neuron activity. The contribution of nigral GABA neurons to the amount of [3H]GABA release from the substantia nigra, is likely linked to their high spontaneous firing rate.

2-amino-4-phosphonobutyric acid exerts a light-dependent effect on post-gabaculine levels of retinal gamma-aminobutyric acid (GABA): evidence that ON synaptic pathways regulate retinal GABAergic transmission

The effects of light, 2-amino-4-phosphonobutyric acid (APB), and kainic acid on rat retinal gamma-aminobutyric acid (GABA)-ergic transmission were studied by measuring levels of retinal GABA following subcutaneous injection of gabaculine, an irreversible inhibitor of GABA-transaminase. Post-gabaculine levels of retinal GABA in light-exposed rats were significantly greater than those in rats held in darkness. The synaptic mechanism of this effect of light was examined by measuring post-gabaculine levels of retinal GABA in rats placed into either lighted or darkened conditions after receiving unilateral intravitreal injections of APB, a glutamate analogue that selectively decreases the activity of ON synaptic pathways in the retina. APB attenuated the post-gabaculine accumulation of GABA in rats held in the light, but not in those placed into darkness. Furthermore, the light-dependent increment in post-gabaculine accumulation of retinal GABA was entirely APB sensitive, and the effect of APB was entirely light dependent. In contrast to APB, kainic acid stimulated the post-gabaculine accumulation of retinal GABA in vivo. Our findings suggest that APB and kainic acid influence GABAergic transmission at different sites in the retina and that some retinal GABAergic neurons are either ON or ON-OFF amacrine cells.

Development of changes in endogenous GABA release during kindling epileptogenesis in rat hippocampus

The calcium-dependent gamma-aminobutyric acid (GABA) and glutamate release from rat hippocampal CA1 slices, evoked by a 1-min depolarization with 50 mM K+, was investigated in different stages of kindling epileptogenesis. Kindling was induced by tetanic stimulation of the Schaffer collateral/commissural pathway. In agreement with our previous results, we found a significantly increased calcium-dependent GABA release compared to that of implanted controls, in a group of fully kindled animals 1 day after the last seizure and also 25-36 days after the last seizure. In addition, we found that the increase in GABA release was associated with late phases of kindling epileptogenesis since no significant alterations were found in partly kindled animals that had received only 6 kindling stimulations while a significant increase was apparent in animals that had received 14 tetanic stimuli. When the release protocol was carried out in the presence of SK&F 89776-A, a blocker of the GABA uptake carrier, an additional amount of GABA was found after depolarization. This additional amount of GABA, reflecting the amount of GABA taken up under conditions without blocker, was in kindled animals not different from controls which demonstrates that a reduced GABA uptake does not account for the observed enhanced release in kindled animals. The calcium-dependent release of glutamate evoked by 1 min of high potassium depolarization was not significantly changed in the kindled groups. Only after prolonged depolarization during 4 subsequent minutes a significant increase in animals of the fully kindled group and at long-term after kindling was observed. The threshold K+ concentration for eliciting a calcium-dependent release of GABA and glutamate, was not changed in the kindled animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The structural and functional heterogeneity of glutamic acid decarboxylase: a review

Studies of the GABA-synthetic enzyme glutamate decarboxylase (glutamic acid decarboxylase; GAD; E.C.4.1.1.15) began in 1951 with the work of Roberts and his colleagues. Since then, many investigators have demonstrated the structural and functional heterogeneity of brain GAD. At least part of this heterogeneity derives from the existence of two GAD genes.

Two forms of the gamma-aminobutyric acid synthetic enzyme glutamate decarboxylase have distinct intraneuronal distributions and cofactor interactions

Glutamate decarboxylase (GAD) catalyzes the production of gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter. The mammalian brain contains two forms of GAD, with Mrs of 67,000 and 65,000 (GAD67 and GAD65). Using a new antiserum specific for GAD67 and a monoclonal antibody specific for GAD65, we show that the two forms of GAD differ in their intraneuronal distributions: GAD67 is widely distributed throughout the neuron, whereas GAD65 lies primarily in axon terminals. In brain extracts, almost all GAD67 is in an active holoenzyme form, saturated with its cofactor, pyridoxal phosphate. In contrast, only about half of GAD65 (which is found in synaptic terminals) exists as active holoenzyme. We suggest that the relative levels of apo-GAD65 and holo-GAD65 in synaptic terminals may couple GABA production to neuronal activity.

Effect of glucoprivation on glutamate decarboxylase activity in the ventromedial nucleus

Activity of the GABA-synthesizing enzyme, glutamate decarboxylase (GAD), was measured in brain areas involved in glucoregulation 60 min after a glucoprivie challenge. The rate of GAD activity in the ventromedial nucleus of the hypothalamus (VMN) increased in a dose-dependent manner in response to intraperitoneal injection of 2-deoxy-D-glucose (2-DG). The increase in VMN GAD activity was significantly correlated with an increase in food intake (r2 = .77, p less than 0.01). The increase in VMN GAD activity was not due to the higher food intake since rats receiving 2-DG and denied access to food also had elevated rates of VMN GAD activity. VMN GAD activity was increased 28% and 32% after intracerebroventricular injection of 2-DG or 5-thioglucose, respectively. The rates of GAD activity in the lateral hypothalamus and area postrema were not affected by either peripherally or centrally administered 2-DG. The increase in VMN GAD activity after glucoprivation may be involved in the regulation of blood glucose by influencing food intake.

Kindling increases the K(+)-evoked Ca2(+)-dependent release of endogenous GABA in area CA1 of rat hippocampus

The release of endogenous amino acids from hippocampal CA1 subslices under basal conditions and the release evoked by high potassium (50 mM K+) depolarization was studied during kindling epileptogenesis. Emphasis was put on the release of the amino acid neurotransmitters gamma-aminobutyric acid (GABA) and glutamate. Kindling was induced by tetanic stimulation of the Schaffer-collaterals/commissural fibers of the dorsal hippocampus of the rat. The calcium-dependent GABA release in the presence of high K+ was significantly increased (40-46%) in fully kindled animals, 24 h after the last seizure, in comparison to controls. At long-term, 28 days after the last seizure, the calcium-dependent GABA release was still significantly increased (45-49%). An increased release of GABA in kindled animals was still found when GABA uptake was blocked by nipecotic acid. In contrast, no significant alterations were encountered in the basal or high potassium induced release of the excitatory amino acids aspartate and glutamate. These results suggest that kindling epileptogenesis is accompanied by a specific and long-lasting enhancement of GABA exocytosis which may lead to a desensitization of the GABA receptor, and thus determine the increase of seizure sensitivity.

Stability and activation of glutamate apodecarboxylase from pig brain

The stability and activation of glutamate apodecarboxylase was studied with three forms of the enzyme from pig brain (referred to as the alpha, beta, and gamma forms). Apoenzyme was prepared by incubating the holoenzyme with aspartate followed by chromatography on Sephadex G-25. Apoenzyme was much less stable than holoenzyme to inactivation by heat (for beta-glutamate decarboxylase (beta-GAD) at 30 degrees C, t1/2 values of apo- and holoenzyme were 17 and greater than 100 min). ATP protected holoenzyme and apoenzyme against heat inactivation. The kinetics of reactivation of apoenzyme by pyridoxal-P was consistent with a two-step mechanism comprised of a rapid, reversible association of the cofactor with apoenzyme followed by a slow conversion of the complex to active holoenzyme. The reactivation rate constant (kr) and apparent dissociation constant (KD) for the binding of pyridoxal-P to apoenzyme differed substantially among the forms (for alpha-, beta-, and gamma-GAD, kr = 0.032, 0.17, and 0.27 min-1, and KD = 0.014, 0.018, and 0.04 microM). ATP was a strong competitive inhibitor of activation (Ki = 0.45, 0.18, and 0.39 microM for alpha-, beta-, and gamma-GAD). In contrast, Pi stimulated activation at 1-5 mM but inhibited at much higher concentrations. The results suggest that ATP is important in stabilizing the apoenzyme in brain and that ATP, Pi, and other compounds regulate its activation.

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.

Kinetically different, multiple forms of glutamate decarboxylase in rat brain

Four molecular forms of rat-brain glutamate decarboxylase were resolved by hydrophobic interaction chromatography on phenyl-Sepharose and affinity chromatography on ATP-agarose. SDS-polyacrylamide gel electrophoresis of purified enzyme and immunoblots of SDS gels indicated a subunit molecular weight of approximately 60,000 for each form of the enzyme, and cross-linking with dimethyl suberimidate prior to electrophoresis indicated that each form has dimeric subunit structure. Immunoblots of non-denaturing gels showed differing electrophoretic mobilities among the forms. The kinetic properties of the 4 enzyme forms were found to be significantly different. The Km for glutamate ranged from 0.17 +/- 0.05 to 1.18 +/- 0.08 mM, and there was a greater than two-fold range in their rates of inactivation by glutamate and GABA in the absence of pyridoxal 5'-phosphate. In subcellular fractionation experiments the forms with greater electrophoretic mobility were recovered in the synaptosomal fraction, and the form with the lowest electrophoretic mobility was the most abundant in the postmicrosomal supernatant. Calcium-dependent binding of glutamate decarboxylase in crude enzyme preparations to phospholipid vesicles was observed, but none of the purified enzyme forms showed an appreciable degree of binding to the vesicles.

Is the concentration of gamma-aminobutyric acid in the nerve terminal regulated via product inhibition of glutamic acid decarboxylase?

Fractions of synaptosomes were used to study the regulation of gamma-aminobutyric acid (GABA) synthesis. The isolated synaptosomes were superfused in media of various compositions. [3H]GABA and GABA released into the medium or remaining in the synaptosomes were analyzed by liquid scintillation and HPLC techniques. Different conditions, designed to increase the GABA efflux rate were used: the rate of superfusion was varied and the concentrations of K+ and Ca2+ were altered. Stimulation of GABA efflux was paralleled with an increased synthesis of GABA, since, in spite of the increased GABA efflux, a relatively constant intraterminal level was found. The findings suggest that the intraterminal concentration of GABA and thus also its synthesis is regulated via product inhibition. In addition, [3H]GABA, exogenous, and GABA, endogenous, responded to external stimulae (Ca2+, veretradine, various GABA concentrations and the glutaminase inhibitor diazo-nor-leucine) in a way which was compatible with them being localized in and/or released from different compartments.

Characterization of three kinetically distinct forms of glutamate decarboxylase from pig brain

Pig brain contains three forms of glutamate decarboxylase with pI values of 5.3, 5.5 and 5.8, referred to as the alpha-, beta- and gamma-forms respectively. These forms were purified and kinetically characterized. The major synaptic form of glutamate decarboxylase (the beta-form) migrated as a single band on electrophoresis in sodium dodecyl sulphate/polyacrylamide gels with an apparent Mr of 60 000. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis followed by immunoblotting with an affinity-purified antibody to the enzyme indicated a subunit Mr of 60 000 for the alpha- and gamma-forms as well. An extensive kinetic analysis, aided by an integrated equation that describes the inactivation and re-activation cycle of the enzyme, revealed that the three forms of the enzyme differ markedly in kinetic properties. The Km values for L-glutamate were 0.17, 0.45 and 1.24 mM respectively for the alpha-, beta- and gamma-forms. The Ki for 4-aminobutyrate, the first-order rate constants for inactivation by L-glutamate and 4-aminobutyrate, the rate constant for re-activation of the apoenzyme by pyridoxal 5'-phosphate and the dissociation constant for pyridoxal 5'-phosphate also differed in a similar way among the three forms; the values were in the order alpha-form less than beta-form less than gamma-form.

beta-N-Oxalylamino-L-alanine: action on high-affinity transport of neurotransmitters in rat brain and spinal cord synaptosomes

beta-N-Oxalylamino-L-alanine (BOAA) is a dicarboxylic diamino acid present in Lathyrus sativus (chickling pea). Excessive oral intake of this legume in remote areas of the world causes humans and animals to develop a type of spastic paraparesis known as lathyrism. BOAA is one of several neuroactive glutamate analogs reported to stimulate excitatory receptors and, in high concentrations, cause neuronal vacuolation and necrosis. The present study investigates the action of BOAA in vitro on CNS high-affinity transport systems for glutamate, gamma-aminobutyric acid (GABA), aspartate, glycine, and choline and in the activity of glutamate decarboxylase (GAD), the rate-limiting enzyme in the decarboxylation of glutamate to GABA. Crude synaptosomal fractions (P2) from rat brain and spinal cord were used for all studies. [3H]Aspartate transport in brain and spinal cord synaptosomes was reduced as a function of BOAA concentration, with reductions to 40 and 30% of control values, respectively, after 15-min preincubation with 1 mM BOAA. Under similar conditions, transport of [3H]glutamate was reduced to 74% (brain) and 60% (spinal cord) of control values. High-affinity transport of [3H]GABA, [3H]glycine, and [3H]choline, and the enzyme activity of GAD, were unaffected by 1 mM BOAA. While these data are consistent with the excitotoxic (convulsant) activity of BOAA, their relationship to the pathogenesis of lathyrism is unknown.

Evidence for feedback regulation of glutamate decarboxylase by gamma-aminobutyric acid

Although feedback control mechanisms for regulating the synthesis of various neurotransmitters have been demonstrated no such mechanism has been described for gamma-aminobutyric acid (GABA) in mammalian brain. Physiological concentrations of GABA inactivated glutamate decarboxylase, the enzyme responsible for GABA synthesis, by converting it to apoenzyme. This inactivation was opposed by the cofactor, pyridoxal 5'-phosphate (pyridoxal-P), and was promoted by ATP. GABA also competitively inhibited the enzyme, and the Ki for inhibition was essentially the same as the concentration of GABA giving the half-maximal rate of inactivation (16 mM). These results provide a mechanism for direct feedback control of presynaptic GABA synthesis and provide further support for the regulation of glutamate decarboxylase in vivo by a cycle of inactivation and reactivation.

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.

Storage and release of endogenous and labelled GABA formed from [3H]glutamine and [14C]glucose in hippocampal slices: effect of depolarization

To study the effect of depolarization on the synthesis, storage and release of GABA, hippocampal slices were incubated in 0.25 mM [3H]glutamine and 2.5 mM [14C]glucose in the presence of 3 or 50 mM K+. Total and labelled glutamine, glutamate and GABA contents were measured by high-performance liquid chromatography. Depolarization in the presence of Ca2+ led to a two-fold increase of labelled glutamate and a 3-fold increase of labelled GABA content originating from both labelled precursors. In the absence of Ca2+ and in the presence of 10 mM Mg2+, depolarization failed to increase labelled glutamate content and labelled GABA formation was increased by only 30%. Following superfusion with unlabelled 0.25 mM glutamine and 2.5 mM glucose a second depolarization with 50 mM K+ released twice as much labelled GABA from slices that had been incubated in the presence of 50 mM K+, than from those incubated in 3 mM K+. This difference remained unchanged in slices that were superfused with 1 mM aminooxyacetic acid, an inhibitor of GABA synthesis. The contribution of labelled GABA, especially of GABA derived from [3H]glutamine, to released GABA was significantly higher than to GABA stored in the slices. Results suggest that depolarization in the presence of Ca2+ results in increased glutamate and GABA synthesis from both glutamine and glucose and that part of GABA released by high K+ originates from preformed GABA stores.

The release of [3H]GABA formed from [3H]glutamate in rat hippocampal slices: comparison with endogenous and exogenous labeled GABA

to compare the storage and release of endogenous GABA, of [3H]GABA formed endogenously from glutamate, and of exogenous [14C]GABA, hippocampal slices were incubated with 5 microCi/ml [3,4-3H]1-glutamate and 0.5 microCi/ml [U-14C]GABA and then were superfused in the presence or absence of Ca+ with either 50 mM K+ or 50 microM veratridine. Endogenous GABA was determined by high performance liquid chromatography which separated labeled GABA from its precursors and metabolites. Exogenous [14C]GABA content of the slices declined spontaneously while endogenous GABA and endogenously formed [3H]GABA stayed constant over a 48 min period. In the presence of Ca+ 50 mM K+ and in the presence or absence of Ca2+ veratridine released exogenous [14C]GABA more rapidly than endogenous or endogenously formed [3H]GABA, the release of the latter two occurring always in parallel. The initial specific activity of released exogenous [14C]GABA was three times, while that of endogenously formed [3H]GABA was only 50% higher than that in the slices. There was an excess of endogenous GABA content following superfusion with 50 mM K+ and Ca2+, which did not occur in the absence of Ca2+ or after veratridine. The observation that endogenous GABA and [3H]GABA formed endogenously from glutamate are stored and released in parallel but differently from exogenous labelled GABA, suggests that exogenous [3H] glutamate can enter a glutamate pool that normally serves as precursor of GABA.

Further studies on the role of calcium in the regulation of glutamate decarboxylase activity in brain slices

[3H]GABA synthesis in brain slices was used as a model to study the role of Ca2+ in the regulation of GAD activity. Experimental conditions were chosen to increase and decrease the flux of Ca2+ and to promote the increase in free intracellular Ca2+. The blockade of electron transport and the inhibition of oxidative phosphorylation in the slices inhibited [3H]GABA synthesis. High K+ depolarization stimulated [3H]GABA synthesis and this effect was not blocked by lidocaine, trifluoperazine, or verapamil, but the stimulation was blocked by the intracellular Ca2+ antagonist TMB-8. The data do not differentiate between the relative contributions of extra- and intracellular Ca2+ but reflect that GAD activity is modulated by a dynamic balance between these two compartments as well as between stored and free Ca2+ within the cells.

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.

The release of gamma-aminobutyric acid, glutamate, and acetylcholine from striatal slices: a mass fragmentographic study

The release processes of endogenous Acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate (Glu) and glutamine (GLN) were studied in superfused guinea-pig caudatal slices. Basal ACh release remained constant for up to 2 h, while the basal release of GABA, Glu and GLN declined to half or less of its initial values after 1 h of superfusion. Electrical stimulation increased the ACh release by 700-800% and that of GABA by 80% whereas it decreased the output of Glu by 50% and failed to modify the GLN efflux. KCl (25 nM) increased the output of ACh by 400%, that of GABA by approximately 500% and decreased that of Glu by 40%. Substituting of CaCl(2) by MgCl(2) in the superfusion medium reduced the basal efflux of GABA, Glu and GLN. Under these conditions, no evoked release of ACh or of GABA was detected, following electrical or KCl stimulation. Tetrodotoxin 5 x 10(-7) decreased the basal ACh release by 60% and increased the GABA efflux by 40%. The toxin abolished the stimulus-evoked ACh efflux but scarcely affected that of GABA. These results are consistent with a possible neurotransmitter role of ACh and GABA in the striatum and show some differences in the ionic mechanisms underlying GABA and ACh release.

Alterations in the compartmentalized metabolism of glutamic acid with changed cerebral conditions

Data obtained from combined determinations of the nervous tissue content of glutamic acid, taurine and glutamine were examined in terms of the well established concept of a compartmentalized metabolism for glutamic acid. Three different situations associated with altered cortical conditions were studied: cortical hyperexcitability induced by cobalt epilepsy (mouse); chronic stimulation of the optic tectum by light adaptation (fish); and anatomic alteration of the optic tectum following unilateral enucleation (fish). All 3 situations appear to cause a reduction in the ability of glial elements to capture free glutamic acid released from neuronal structures. However, the underlying causes for such an insufficiency seem to differ in each instance. In epilepsy the release of glutamic acid and taurine exceeds the glial capture rate; during chronic stimulation of a normal cortex a diminished glial uptake rate for both amino acids seems apparent; anatomical degenerative changes seem to diminish especially the glutamine retention capacity of the cortex, possibly in combination with a reduced glial taurine uptake.

In vivo release of endogenously synthesized [3H]GABA from the cat substantia nigra and the pallido-entopeduncular nuclei

Halothane anesthetized cats were implanted with push--pull cannulae to study the release of [3H]GABA continuously formed from [3H]glutamine in the substantia nigra (SN) and in the pallido-entopeduncular nuclei (PEP). A spontaneous release of [3H]GABA was observed from both structures and it reached a steady state level 1 h after the beginning of the superfusion with [3H]glutamine. In cats implanted with two push--pull cannulae, the local application of potassium (47 mM) in the PEP stimulated the release of [3H]GABA from the ipsilateral SN. In cats implanted with 4 push--pull cannulae, the unilateral 10 min electrical stimulation of the caudate nucleus evoked the release of [3H]GABA not only from the ipsilateral SN, but also in most cases from the contralateral structure. This stimulus also enhanced the release of [3H]GABA from PEP but the effects were mainly observed in the medio-caudal part of the ipsilateral PEP and in the latero-rostral part of the contralateral structure. In all cases, the changes in [3H]GABA release were observed during and after the electrical stimulation. The ipsilateral effects can be attributed to the direct activation of the caudato-PEP or caudato-SN GABAergic neurons. A polysynaptic neuronal loop must be involved in the symmetric contralateral effects.