Regulation of GABA metabolism in discrete rabbit brain regions under methoxypyridoxine--regional differences in cofactor saturation and the preictal activation of glutamate decarboxylase activity

[Food Poisoning by Ginkgo Seeds through Vitamin B6 Depletion]

Overconsumption of Ginkgo biloba seeds induces food poisoning characterized by tonic-clonic convulsions and vomiting. The primary toxic component, 4'-O-methylpyridoxine (MPN), was purified from the seeds in 1985. This review includes the following aspects of ginkgo seed poisoning: 1) toxicity related to the content of MPN and MPN glucoside in G. biloba seeds; 2) the effect of MPN on vitamin B₆ analogs, including an increase in pyridoxal and pyridoxic acid and decrease in pyridoxal-5'-phosphate plasma concentrations; 3) case reports of ginkgo seed poisoning in Asia, North America, and Europe, and their effective treatment via vitamin B₆ administration. Considering the increase in the use of G. biloba seeds, it is essential to raise global awareness of their potential toxicity.

Properties and interaction of heterologously expressed glutamate decarboxylase isoenzymes GAD(65kDa) and GAD(67kDa) from human brain with ginkgotoxin and its 5'-phosphate

Two isoforms of glutamate decarboxylase (GAD(65kDa) and GAD(67kDa)) from human brain, which had previously been overexpressed in Escherichia coli as fusion proteins containing a glutathione-S-transferase domain, were purified by affinity chromatography on glutathione Sepharose 4B. Both isoforms were also expressed in Saccharomyces cerevisiae. After modification of a HPLC based assay, the enzymes were characterized with respect to their biochemical properties. Comparison of kinetic data, pH, and temperature optima as well as of the mode of interaction with pyridoxal phosphate as a cofactor revealed several significant differences between the two isoenzymes reflecting their somewhat different physiological and molecular features. Investigation of the influence of 4'-O-methylpyridoxine (ginkgotoxin) (1), a neurotoxin occurring in Ginkgo biloba L., on the different isoenzymes, indicates that the phosphorylated form of the toxin, 4'-O-methylpyridoxine-5'-phosphate (2), decreases GAD(65kDa) activity, although in unphysiologically high concentrations, whereas GAD(67kDa) activity seems to be hardly affected.

GABA release and GAD67 mRNA expression in rat hippocampus following entorhinal cortex activation

This study investigate the effect of stimulation of glutamatergic afferents originating in the entorhinal cortex on possible changes of GABAergic transmission in the CA1 subregion of the hippocampus. Microdialysis was used to monitor extracellular GABA and in situ hybridization to measure levels of glutamic acid decarboxylase67 (GAD67) mRNA. A dose-dependent increase in extracellular levels of GABA in the dorsal CA1 subregion was detected following injection of 2.4 and 9.6 micrograms quisqualate into the lateral entorhinal cortex whereas 0.24 microgram had no effect. The GABA increase was attenuated by local administration of tetrodotoxin (TTX), indicating neuronal origin. A 60% decrease and a 160% increase were seen in levels of GAD67 mRNA in the CA1 following injection of 0.24 and 9.6 micrograms quisqualate, respectively. This study provides evidence of an entorhinal cortex influenced stimulatory effect on GABAergic activity in the CA1. However, no direct relationship was found between stimulated GABA release and subsequently measured GAD67 mRNA levels. The increased GABA release and the apparent adaptive increase in GAD67 mRNA levels by the strongest stimulation may be due to an endogenous inhibitory neuroprotective response to an excitotoxic influence.

Comparative distribution of GAD65 and GAD67 mRNAs and proteins in the rat spinal cord supports a differential regulation of these two glutamate decarboxylases in vivo

Gamma-aminobutyric acid (GABA) synthesis can result from the action of at least two glutamic acid decarboxylase (GAD) isoforms, GAD65 and GAD67, possibly involved in distinct mechanisms. We have made the hypothesis that GAD65 may respond to short-term changes and is present in neurons with a phasic activity, while GAD67 may rather provide GABA for the metabolic pool and for supporting tonic levels of synaptic transmission (Erlander et al.: Neuron 7:91-100, 1991; Feldblum et al.: J Neurosci Res 34:689-706, 1993). In the present work we have tested this hypothesis in the rat spinal cord where both types of activities have been identified. The correlation of GABA immunodetection with the distribution of GAD65 and GAD67 mRNAs and proteins has evinced in the dorsal horn a differential regulation of the two isoforms. In situ hybridization has revealed, in the dorsal horn, relatively higher levels of GAD67 mRNA than of GAD65, while immunodetection of the proteins demonstrated numerous punctate profiles with both GAD antisera. Reverse transcription-polymerase chain reaction (RT-PCR) data confirmed the abundance of the GAD67 transcripts compared to GAD65 in the rat spinal cord. In contrast, within the ventral horn, there was a greter number of GAD67-immunoreactive (IR) profiles mostly located around motoneurons. The paucity of GAD65 immunoreactivity in the ventral horn cannot be related to a different accessibility of the antigens to the epitopes since on the same section a dense GAD65 staining was detected in the dorsal horn. Hence, a number of biochemical and electrophysiological data support the concept of the involvement of glycine as the major inhibitory system within the ventral horn which may explain the low levels of GAD transcription in this region. The paucity of GAD65 in the ventral horn may also reflect a functional difference, suggesting a predominance of GAD67 in neurons under tonic activity. In the dorsal horn, where neurons with phasic and tonic firing patterns have been disclosed, GAD65 may, in addition, provide GABA for responses to short-term changes.

Different distributions of GAD65 and GAD67 mRNAs suggest that the two glutamate decarboxylases play distinctive functional roles

Two genes encode two forms of glutamate decarboxylase, GAD65 and GAD67. Because the two GADs differ in subcellular distribution and interactions with the cofactor pyridoxal phosphate, the two enzymes may play different roles in gamma-aminobutyric acid (GABA) production. In this study we have used in situ hybridization to compare the regional and cellular distributions of the two GAD mRNAs in rat brain. Both GAD mRNAs are abundant in olfactory bulb, olfactory tubercle, zona incerta, reticular nucleus of the thalamus, oculomotor nuclei, and pontine tegmental area. GAD65 mRNA is more abundant in several structures of the visual system, including the lateral geniculate nuclei, superior colliculi, and olivary pretectal nucleus, as well as in several hypothalamic and pontine nuclei. In contrast, GAD67 mRNA is more abundant in neocortex, the granular layer of olfactory bulb, lateral and medial septum, globus pallidus, inferior colliculi, and cerebellar cortex. Both GAD mRNAs are present in interneurons as well as in projection neurons, and both are present in neurons with different types of synapses, including dendrodendiritic, axosomatic, and axodendritic synapses. GAD65 mRNA predominates in the visual and the neuroendocrine systems, which are more subject to phasic changes, while GAD67 is present at relatively higher concentrations in many tonically active neurons. GAD65 and GAD67 together may provide more flexibility in the regulation of GABA synthesis than either could alone.

Transient increase in expression of a glutamate decarboxylase (GAD) mRNA during the postnatal development of the rat striatum

We recently reported that the mammalian brain has two forms of the GABA synthetic enzyme glutamate decarboxylase (GAD, E.C. 4.1.1.15), which are the products of two genes. The two forms, which we call GAD65 and GAD67, differ from each other in sequence, molecular size, subcellular distribution, and interactions with the cofactor pyridoxal phosphate (PLP), with GAD65 activity more dependent than that of GAD67 on the continued presence of exogenous PLP. The existence of two GAD genes suggests that individual GABA neurons may be subject to differential regulation of GABA production. We have examined the expression of these two forms of GAD during postnatal development of the rat striatum to determine whether different classes of GABA neurons selectively express different amounts of the two GAD mRNAs. Here we present evidence for a dramatic developmental difference in the expression of the two mRNAs during postnatal development of the rat striatum. Using in situ hybridization to the two GAD mRNAs, we observed a selective increase in GAD65 mRNA during the second postnatal week, at the time when striatal matrix neurons innervate the substantia nigra (SN). PLP-dependent enzyme activity in the midbrain increases in parallel with increased expression of GAD65 mRNA in the striatum. We hypothesize that the innervation of the SN by striatal neurons triggers an increase in GAD65. The changing ratios of GAD65 and GAD67 in the striatum may contribute to the well-documented changes in seizure susceptibility that occur in early life.

Comparative distribution of messenger RNAs encoding glutamic acid decarboxylases (Mr 65,000 and Mr 67,000) in the basal ganglia of the rat

Glutamic acid decarboxylase, the enzyme required for GABA synthesis, exists as distinct isoforms, which have recently been found to be encoded by different genes. The relative expression of messenger RNAs encoding two isoforms of glutamic acid decarboxylase (Mr 67,000 and Mr 65,000) was measured at the single-cell level in neurons of the rat basal ganglia with in situ hybridization histochemistry. Both messenger RNAs were expressed in neurons of the striatum, pallidum, and substantia nigra pars reticulata, but marked differences in the relative level of labelling were observed with the two probes. In striatum, efferent neurons were more densely labelled for the messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) than for the messenger RNA encoding glutamic acid decarboxylase (Mr 67,000), whereas the reverse was observed for GABA-ergic interneurons. Neurons of the entopeduncular nucleus were much more densely labelled for messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) than for messenger RNA encoding glutamic acid decarboxylase (Mr 67,000). In addition, labelling for messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) was higher in the entopeduncular nucleus (internal pallidum) than in the globus pallidus (external pallidum), a structure which expressed similar levels of both mRNAs. In contrast to neurons of the internal pallidum, efferent neurons of the substantia nigra pars reticulata expressed slightly more messenger RNA encoding glutamic acid decarboxylase (Mr 67,000) than that encoding the other isoform of the enzyme. The results suggest a differential expression of the messenger RNAs encoding the two isoforms of glutamic acid decarboxylase in subpopulations of basal ganglia neurons in rats.

Seizure activity in pyridoxine-deficient adult rats

This investigation tested the hypothesis that the degree of pyridoxine depletion rather than the status of neuronal maturity determines seizure proneness in the pyridoxine-deficient rat. Dietary pyridoxine deficiency was induced in neuronally mature rats. Seizure activity was monitored using computerized EEG analysis. Dietary pyridoxine deficiency of 10 weeks' duration induced in neuronally mature rats led to spontaneous convulsive seizure activity. Even moderately pyridoxine-deficient adult rats (on the deficient diet for less than 8 weeks) exhibited seizure-like diffuse spike and wave activity and electrocortical inhibition. Picrotoxin-, pentylenetetrazol-, or domoic acid-induced seizure thresholds were significantly reduced in pyridoxine-deficient rats when compared with normal controls. Pyridoxine-deficient rats exhibited increased dominance of delta and theta activities and increased hemispherical asymmetries in response to convulsant treatment.

Distribution of glutamic acid decarboxylase (Mr 67,000) in the basal ganglia of the rat: an immunohistochemical study with a selective cDNA-generated polyclonal antibody

Distinct isoforms of glutamic acid decarboxylase, the synthetic enzyme for GABA, exist in brain. Their distribution at the cellular level is not known, because previous studies have been confounded by the lack of monospecificity of available antibodies. We have examined the distribution of glutamic acid decarboxylase (Mr 67,000; GAD67) in the basal ganglia of the rat with a polyclonal antibody generated against the protein expressed in bacteria transformed with the corresponding cDNA. This antibody, which is directed against a portion of GAD67 non homologous to other known glutamic acid decarboxylase isoforms, selectively recognizes GAD67 on western blots. We show that GAD67 is present to various degree in all types of GABAergic neurons previously described in these regions. In contrast with results obtained with non-selective antibodies for glutamic acid decarboxylase, GAD67-positive neuronal cell bodies were readily detected in sections of the striatum, pallidum and substantia nigra in the absence of colchicine treatment. Modifications in the immunohistochemical procedure favoured staining of glutamic acid decarboxylase-positive fibres with the same antibody, indicating that GAD67 is also present in axon terminals of GABAergic neurons. The results suggest that GAD67 may be involved in GABA synthesis in both cell bodies and axon terminals of all GABAergic neurons of the basal ganglia, but is particularly abundant or accessible in their cell bodies.

Two genes encode distinct glutamate decarboxylases

gamma-Aminobutyric acid (GABA) is the most widely distributed known inhibitory neurotransmitter in the vertebrate brain. GABA also serves regulatory and trophic roles in several other organs, including the pancreas. The brain contains two forms of the GABA synthetic enzyme glutamate decarboxylase (GAD), which differ in molecular size, amino acid sequence, antigenicity, cellular and subcellular location, and interaction with the GAD cofactor pyridoxal phosphate. These forms, GAD65 and GAD67, derive from two genes. The distinctive properties of the two GADs provide a substrate for understanding not only the multiple roles of GABA in the nervous system, but also the autoimmune response to GAD in insulin-dependent diabetes mellitus.

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.

Postnatal expression of glutamate decarboxylases in developing rat cerebellum

The recent identification of two genes encoding distinct forms of the GABA synthetic enzyme, glutamate decarboxylase (GAD), raises the possibility that varying expression of the two genes may contribute to the regulation of GABA production in individual neurons. We investigated the postnatal development the two forms of GAD in the rat cerebellum. The mRNA for GAD67, the form which is less dependent on the presence of the cofactor, pyridoxal phosphate (PLP), is present at birth in presumptive Purkinje cells and increases during postnatal development. GAD67 mRNA predominates in the cerebellum. The mRNA for GAD65, which displays marked PLP-dependence for enzyme activity, cannot be detected in cerebellar cortex by in situ hybridization until P7 in Purkinje cells, and later in other GABA neurons. In deep cerebellar nuclei, which mature prenatally, both forms of GAD mRNA can be detected at birth. The amounts of immunoreactice GAD and GAD enzyme activity parallel changes in mRNA levels. We suggest that the delayed appearance of GAD65 is coincident with synapse formation between GABA neurons and their targets during the second postnatal week. GAD67 mRNA may be present prior to synaptogenesis to produce GABA for trophic and metabolic functions.

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.

Effects of acute barbiturate administration, tolerance and dependence on brain GABA system: comparison to alcohol and benzodiazepines

Central nervous system depressants, e.g., barbiturates, alcohol and benzodiazepines, have a wide spectrum of activity in humans and animals. Evidence accumulated suggests that some of the pharmacological actions exerted by these agents may be mediated through GABA system by mimicking GABAergic transmission. This review attempts to summarize the evidence available as to how the GABA system plays a part in the barbiturate actions and the development of tolerance to and physical dependence on barbiturates. The comparisons of the effects of alcohol, barbiturates and benzodiazepines at different steps of GABA synapse are also presented. Furthermore, the results which have been reported in the literature are inconsistent. This may be due to differences in: (a) animal models used; (b) brain regions used; (c) protocols (dose, duration, form and route of administration, etc.) used in treating animals and/or (d) techniques (pharmacological, biochemical, physiological, etc.) used.

Rapid inactivation of brain glutamate decarboxylase by aspartate

In the absence of its cofactor, pyridoxal 5'-phosphate (pyridoxal-P), glutamate decarboxylase is rapidly inactivated by aspartate. Inactivation is a first-order process and the apparent rate constant is a simple saturation function of the concentration of aspartate. For the beta-form of the enzyme, the concentration of aspartate giving the half-maximal rate of inactivation is 6.1 +/- 1.3 mM and the maximal apparent rate constant is 1.02 +/- 0.09 min-1, which corresponds to a half-time of inactivation of 41 s. The rate of inactivation by aspartate is about 25 times faster than inactivation by glutamate or gamma-aminobutyric acid (GABA). Inactivation is accompanied by a rapid conversion of holoenzyme to apoenzyme and is opposed by pyridoxal-P, suggesting that inactivation results from an alternative transamination of aspartate catalyzed by the enzyme, as previously observed with glutamate and GABA. Consistent with this mechanism pyridoxamine 5'-phosphate, an expected transamination product, was formed when the enzyme was incubated with aspartate and pyridoxal-P. The rate of transamination relative to the rate of decarboxylation was much greater for aspartate than for glutamate. Apoenzyme formed by transamination of aspartate was reactivated with pyridoxal-P. In view of the high rate of inactivation, aspartate may affect the level of apoenzyme in brain.

Non-steady-state kinetics of brain glutamate decarboxylase resulting from interconversion of the apo- and holoenzyme

In addition to its primary reaction, brain glutamate decarboxylase (L-glutamate 1-carboxy-lyase, EC 4.1.1.15) catalyses an alternative transamination reaction that leads to the production of apoenzyme. Apoenzyme can be converted to holoenzyme by reaction with pyridoxal 5'-phosphate, thereby completing a cyclic interconversion of the apo- and holoenzyme. The effect of the cycle on the kinetic behavior of the enzyme was investigated with the aid of a kinetic model that combines a steady-state description of the primary reaction and a non-steady-state description of the cycle. In the presence of saturating levels of the cofactor, pyridoxal 5'-phosphate, the cycle had little effect on the kinetics of slowly transaminated substrates such as glutamate. However, the kinetic behavior of aspartate, a rapidly transaminated substrate, was strongly affected by the cycle. With aspartate, a large proportion of apoenzyme was produced, resulting in non-linear decarboxylation time courses. Estimates of the steady-state kinetic parameters for aspartate (Km, Ki, Vmax) and the apparent type of inhibition were found to depend strongly on the assay time and procedure. Similar dependencies were found for the aspartate analogues, methyl alpha-DL-aspartate, cysteine sulfinate and beta-alanine, suggesting that they also undergo rapid transamination. The kinetic model accurately predicted holoenzyme levels and accurately described the decarboxylation time courses for glutamate, aspartate and mixtures of these substrates.

Dietary pyridoxine and the susceptibility to limbic motor seizures in rats

Dietary pyridoxine (PN) deficiency in adult female rats produced a 32% decrease in hippocampal gamma-aminobutyric acid content, measured by a radioreceptor assay. No spontaneous seizures were observed in pyridoxine-deficient animals, but the seizure latency after a systemic kainic acid challenge decreased by 35%. The results suggest that latency is a useful measure of limbic seizure susceptibility; this susceptibility can be manipulated by diet; and in adults pyridoxine-dependent mechanisms normally participate in preventing, rather than initiating, limbic seizures.

Pathophysiological aspects of blood-brain barrier permeability in epileptic seizures

Blood-brain barrier (BBB) permeability to macromolecules was assessed during seizures induced by pentylenetetrazole, bicuculline, methoxypyridoxine, methionine sulfoximine, and kainic acid. It was observed that each convulsant induced a specific pattern of regional BBB opening. This was, however, only the case when systemic blood pressure (BP) rose with seizure onset. The analysis of regional cerebral blood flow revealed that a high increase in flow in rabbits with BP rise is related to the normal flow at rest in the single brain region, but not to BBB permeability. In rabbits without BP increase, regional flow increase was low but well modulated and is possibly a better indicator for neuronal activity. The ultrastructural analysis showed that macromolecular transport over the cerebrovascular endothelium is by pinocytosis, an neurotransmitter controlled process. It is suggested that seizure-induced regional BBB opening is determined by two factors: release of neurotransmitters due to the process of auto-regulation during peripheral pressure increase, and change in local neurotransmitter milieu due to the action of the convulsant and/or the seizure activity.

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.

Pyridoxal phosphate-unrelated inhibition of hippocampal glutamic acid decarboxylase by convulsant pyridoxal sulphate

Previous studies from this laboratory have shown that pyridoxal-5'-sulphate, the synthetic analogue of pyridoxal phosphate, causes epileptic seizures including tonic-clonic convulsions. These seizure activities are prevented or reversed by GABA or muscimol. In an attempt to delineate the biochemical basis of these seizure processes further, we have studied and shown that pyridoxal sulphate is a competitive inhibitor of glutamic acid decarboxylase. In addition, the chronic administration of pyridoxal sulphate was shown to reduce the concentration of pyridoxal phosphate in the cerebellum, the cerebrum, and basal ganglion, but not in the hippocampus. The activity of hippocampal glutamic acid decarboxylase was reduced after 1, 3, and 5 days of chronic application of pyridoxal sulphate. The inhibition was demonstrated, whether glutamic acid decarboxylase was assayed in the presence or absence of its coenzyme pyridoxal phosphate. Unlike findings in the hippocampus, the activity of glutamic acid decarboxylase in other brain regions was unaffected following chronic application of pyridoxal sulphate. The selective toxic effects of pyridoxal sulfate to the hippocampus, a brain area well known for its high susceptibility to seizure discharges, deserve additional indepth investigation.

Comparative characterization of glutamate decarboxylase in crude homogenates of oviduct, ovary, and hypothalamus

Some biochemical characteristics of L-glutamate decarboxylase (GAD) were compared using crude homogenates of the rat oviduct, ovary, and hypothalamus. As estimated by the measurement of CO2 production, the specific activities of oviductal and ovarian GAD were about 10 and 3% of the hypothalamic value, respectively. Stoichiometric formation of gamma-aminobutyric acid (GABA) and CO2 from L-glutamate could be observed in oviduct and hypothalamus, while in ovarian homogenates the production of CO2 was more than nine times that of GABA. Hypothalamic and tubal GAD showed similar time course, temperature dependence, and pH dependence. The dependence on protein concentration and on exogenous cofactor supply was also similar in these two tissues. The kinetic constant for L-glutamate as a substrate was nearly the same in oviduct (1.30 mM) and hypothalamus (1.64 mM). The responsiveness of tubal and hypothalamic GAD to various inhibitors was also similar. The present findings indicate that the oviductal and the hypothalamic GAD may be identical, and they suggest a possible GABAergic innervation of the Fallopian tube.

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.

Alterations in the content of amino acid neurotransmitters before the onset and during the course of methoxypyridoxine-induced seizures in individual rabbit brain regions

In rabbits, generalized seizures were induced by methoxypyridoxine, and changes in amino acid concentrations of 15 brain regions were investigated before seizure onset and during the course of sustained epileptiform activity. As previously reported, gamma-aminobutyric acid (GABA) concentration decreased preictally in most regions. At the same time, taurine level was elevated in the hypothalamus, thalamus, hippocampus, caudatum, and frontal cortex. After 90 min of seizures, it was significantly decreased in the hypothalamus, periaqueductal grey, substantia nigra, frontal cortex, and cerebellum. Glycine content was reduced preictally only in the substantia nigra; after seizure onset its concentration rose in all brain areas. Glutamate content in the frontal cortex decreased before seizure onset; after 1.5 h of seizures, its concentration in cerebellum, caudatum, and hippocampus was reduced. Aspartate level was decreased in most areas after sustained seizures; in putamen, however, it was elevated. In contrast, glutamine content increased preictally in the superior colliculus and in all brain areas by approximately 200% after 90 min of seizures. Alanine and valine content also rose markedly in most brain areas after prolonged seizures, and threonine showed the same tendency. The single brain regions were observed to respond to methoxypyridoxine in highly individualistic ways. For example, the glycine content of the substantia nigra, which is believed to utilize this amino acid as a neurotransmitter, decreased preictally. The potential importance of the superior colliculus in seizure induction is considered in view of the early rise in glutamine level. The antagonistic preictal behavior of taurine and GABA is discussed with respect to synthesis, uptake from the blood, and antiepileptic properties.

Regional patterns of blood-brain barrier breakdown during epileptiform seizures induced by various convulsive agents

In unrestrained rabbits with generalized epileptic seizures induced by systemic application of convulsant drugs, regional changes in blood-brain barrier (BBB) permeability to macromolecules were investigated using Evans Blue (EB) as indicator. BBB leakage due to seizures was present only in animals in which the mean arterial blood pressure rose about 50 mm Hg with the onset of convulsive motor activity. However, a blood pressure increase was not necessarily associated with the occurrence of BBB opening. Pentylenetetrazole-induced seizures resulted in bilateral EB leakage mainly in the hypothalamus, with exception of the mammillary bodies, and the preoptic area, and they were associated, in most cases, with an intensive staining of the cerebellum and also of the midbrain tegmentum. In contrast, seizures due to the GABA receptor blocker bicuculline brought about a penetration of the dye in the region of the pallidum, whereas the GABA synthesis inhibitor methoxypyridoxine produced BBB breakdown in the hippocampus. Methionine-sulfoximine convulsions resulted in a selective stain of the corpora mammillaria, and kainic acid induced a diffuse leakage in neocortical brain areas. As a rule, BBB breakdown was bilateral and confined to anatomically limited brain areas, suggesting that BBB integrity was not only disturbed by abrupt increases in the intraluminal pressure, but was also influenced from the brain tissue. The fluorescence microscopic observations revealed that the tracer penetrated into the neuropil through larger vessels. It had the tendency to accumulate in neurons. In case of the hippocampus, CA2 pyramidal cells revealed more intense uptake of EB than those of the adjacent fields.

GABA, glutamic acid decarboxylase, and GABA transaminase levels in the myenteric plexus in the intestine of humans and other mammals

Regional distribution of endogenous gamma-aminobutyric acid (GABA), its synthesizing enzyme, glutamic acid decarboxylase (GAD), and metabolic enzyme, GABA transaminase (GABA-T), were determined in the intestinal tract of guinea pigs and cats and the findings compared with the number of ganglion cells in Auerbach's plexus. There were positive correlations among the GABA contents and the numbers of neural cells of the plexus. The precise localization of GABA and GAD in individual layers (mucosa, circular and longitudinal muscles, and Auerbach's plexus) in the human and cat colon was also determined. The endogenous GABA contents and GAD activity were the highest in Auerbach's plexus in tissues of both species. These results indicate that GABA is synthesized and localized in Auerbach's plexus and probably plays a significant role in the enteric nervous system.

Dissociation between epileptic seizures induced by convulsant drugs and alteration in the concentrations of pyridoxal phosphate in rat brain regions

Allylglycine increased the concentration of pyridoxal phosphate in cerebral cortex from 1011.4 +/- 25.0 to 1318.0 +/- 66.3 and decreased it in cerebellum from 1289.0 +/- 49 to 1147.7 +/- 119.4 ng/g wet tissue during the preictal period. Mercaptopropionic acid increased the concentration of pyridoxal phosphate in cerebellum from 1525 +/- 91 to 1985.7 +/- 275 ng/g wet tissue. Similar effects were noted in hippocampus and cerebral cortex. Picrotoxin increased the concentration of pyridoxal phosphate in hippocampus from 938.7 +/- 44 to 1043 +/- 118 but decreased it in cerebral cortex from 1124.52 +/- 124 to 979.4 +/- 15 ng/g wet brain. The effects of strychnine were identical to those of allylglycine. Bicuculline reduced the concentration of pyridoxal phosphate in cerebral cortex from 1184 +/- 61 to 1075.14 +/- 78 ng/g wet brain.

Pharmacological investigation of convulsant gamma-aminobutyric acid (GABA) antagonists in amygdala-kindled rats

The convulsant potencies of several drugs that antagonize GABAergic neurotransmission were assessed in amygdala-kindled and control rats. The potencies of picrotoxin, pentylenetetrazol, and a convulsant barbiturate were increased by amygdala kindling. No significant potentiation, however, was observed with isoniazid, 3-mercaptopropionic acid, bicuculline, or the glycine antagonist strychnine. Destruction, by electrolytic lesion, of the amygdala focus did not reverse the kindling-induced potentiation of picrotoxin convulsions. The potentiation of picrotoxin convulsions was correlated with the pattern of kindling development but not with the number of stimulations or seizures. The present results are consistent with the hypotheses that (1) kindling potentiates only some convulsant agents, (2) kindling-induced convulsant potentiation takes place outside of the kindled focus, and (3) kindling is not produced by a failure of the GABA system.

Large dense-core vesicle exocytosis and membrane recycling in the mossy fibre synapses of the rabbit hippocampus during epileptiform seizures

The ultrastructure of the hippocampal mossy fibre layer was studied in ultrathin sections and freeze-fracture preparations of rabbits under deep Nembutal anaesthesia, after recovery from ether anaesthesia, and 40 min after a single injection of methoxypyridoxine, that is, during the second generalized seizure discharge. The giant mossy fibre boutons contain two types of vesicles: evenly distributed, small round clear vesicles (50 nm) and a few scattered large dense-core vesicles (100 nm). In rare instances fusion of dense-core vesicles with the presynaptic membrane was observed. No differences in the morphology of the mossy fibre synapses were found between anaesthetized and unanaesthetized animals. During epileptiform seizures, however, the size and shape of clear and dense-core vesicles varied greatly. The active synaptic zones were covered with large, core-containing omega profiles or bumps and indentations. Only dense-core vesicles seem to undergo exocytosis. A fusion of clear vesicles with presynaptic membrane was not observed. Various explanations for the fact that only dense-core vesicles seem to undergo exocytosis are discussed. The hypothesis is put forward that in the mossy fibre bouton two morphologically and functionally distinct populations of synaptic vesicles exist and that only one of them undergoes visible irreversible exocytosis, whereas the majority, that is, the small vesicles discharge their transmitter by reversible fusion. After MP injection features of membrane retrieval were also prominent. Frequently, at the borders of the active synaptic zones coated membrane convolutes of both pre- and postsynaptic membranes had invaded the terminals as well as the postsynaptic spine. Thus, in contrast to electrical stimulation, the self-sustained seizures allows energy-expensive processes such as extensive membrane internalization to take place during the interictal pauses.

Selected ion monitoring determination of acetylcholine during methoxypyridoxine seizures

The neurotransmitter gamma-aminobutyric acid is known to decrease preictally after administration of the potent convulsant methyoxypyridoxine, a competitive inhibitor of glutamate decarboxylase. An attempt was made to determine the effect of this gamma-aminobutyric acid decrease on the cholinergic system. Rabbits were immobilized and artificially ventilated in order to avoid hypoxidosis. Seizures were induced by intravenous injection of 100 mg kg-1 methoxypyridoxine; 40 minutes later the animals were decapitated and discrete brain areas removed. Tissue contents of acetylcholine and choline were estimated by gas chromatographic mass spectrometric analysis of the beta-dimethylaminoethyl acetate and propionate derivatives using deuterated internal standards. Gas chromatographic column optimization resulted in a considerable sensitivity gain. Computerized selected ion monitoring was carried out on the dimethylmethyleneimmonium ions using voltage switching. The use of a computer controlled solvent dump valve was implemented to increase precision. No significant difference was observed in the concentration of acetylcholine in the frontal cortex, cerebellar cortex, septum, hippocampus, or caudate of seizure versus control animals; septal choline increased, however. This suggests that the acetylcholine turnover could be increased during seizure.