Human Brain γ-Aminobutyric Acid Levels and Seizure Control Following Initiation of Vigabatrin Therapy

Abstract: Vigabatrin is a novel antiepileptic drug designed to control seizures by raising brain γ-aminobutyric acid (GABA) concentrations. Seizure control is not improved significantly when the daily dose is increased beyond 50 mg/kg. Serial, in vivo measurements of GABA levels in human occipital lobe were made using ¹H NMR spectroscopy before and after the start of vigabatrin treatment. We used a 2.1-T magnetic resonance imagerspectrometer and an 8-cm surface coil to examine serially a 14-cm³ volume in the occipital lobe of 26 patients with complex partial seizures. Brain GABA content increased following the start of vigabatrin treatment up to a daily dose of 60 mg/kg. Additional increases in dose failed to increase brain GABA content further. GABA synthesis may decrease with sustained elevations of human brain GABA levels. Starting vigabatrin treatment reduced seizure frequency by >50%, from six to seven per month to three. Improved seizure control was not associated with further increases of vigabatrin dose. Increased brain GABA concentration was associated with improved seizure control. Starting vigabatrin treatment improved seizure control twofold when GABA levels increased above 1.8 mmol/kg. Further increases in brain GABA content above 2.5 mmol/kg provided less protection. Measuring occipital lobe GABA concentrations may predict improved seizure control when using antiepileptic drugs designed to increase brain GABA levels.     

Neuroactive Amino Acids in Focally Epileptic Human Brain: A Review

Studies of neuroactive amino acids and their regulatory enzymes in surgically excised focally epileptic human brain are reviewed. Concentrations of glutamate, aspartate and glycine are significantly increased in epileptogenic cerebral cortex. The activities of the enzymes, glutamate dehydrogenase and aspartate aminotransferase, involved in glutamate and aspartate metabolism are also increased. Polyamine synthesis is enhanced in epileptogenic cortex and may contribute to the activation of N-methyl-D-aspartate (NMDA) receptors. Nuclear magnetic resonance spectroscopy (NMRS) reveals that patients with poorly controlled complex partial seizures have a significant diminution in occipital lobe gamma aminobutyric acid (GABA) concentration. The activity of the enzyme GABA-aminotransaminase (GABA-T) which catalyzes GABA degredation is not altered in epileptogenic cortex. NMRS studies show that vigabatrin, a GABA-T inhibitor and effective antiepileptic, significantly increases brain GABA. Glutamate decarboxylase (GAD), responsible for GABA synthesis, is diminished in interneurons in discrete regions of epileptogenic cortex and hippocampus. In vivo microdialysis performed in epilepsy surgery patients provides measurements of extracellular amino acid levels during spontaneous seizures. Glutamate concentrations are higher in epileptic hippocampi and increase before seizure onset reaching potentially excitotoxic levels. Frontal or temporal cortical epileptogenic foci also release aspartate, glutamate and serine particularly during intense seizures or status epilepticus. GABA in contrast, exhibits a delayed and feeble rise in the epileptic hippocampus possibly due to a reduction in the number and/or efficiency of GABA transporters.     

Amino Acid Levels in the Cerebrospinal Fluid of Newly Diagnosed Epileptic Patients: Effect of Vigabatrin and Carbamazepine Monotherapies

Abstract: We studied the CSF amino acid levels of 42 patients with newly diagnosed epilepsy before treatment with antiepileptic medication and during monotherapy with either vigabatrin or carabamzepine. The present study shows that patients with newly diagnosed epilepsy have elevated levels of the excitatory amino acid glutamate in CSF. Vigabatrin monotherapy effectively prevents the appearance of seizures in patients with high baseline CSF glutamate levels. In these patients, vigabatrin not only elevates the levels of γ-aminobutyric acid, but also decreases the elevated levels of glutamate in CSF, which may also be important to the antiepileptic efficacy of vigabatrin. Patients with low CSF glutamate levels did not benefit from vigabatrin-induced changes in amino acid levels and successful monotherapy with carbamazepine did not affect CSF amino acid levels. The elevation of γ-aminobutyric acid is thus not the only way to achieve seizure control and there are several factors underlying the generation and control of seizures. Follow-up of the patients with high baseline glutamate CSF levels will show if the observed abnormalities are related to the severity of epilepsy in individual patients and if early treatment with vigabatrin of these patients could prevent the development of intractable epilepsy.     

Relationship Between GABA Concentrations in Cerebrospinal Fluid and Seizure Excitability

Abstract: Various studies suggest that alterations in GABAergic function may be connected to epileptic seizures. Low CSF GABA levels have been reported in epilepsy and also febrile convulsions of children. In this study the pentet-razole seizure threshold of dogs was compared with the concentration of GABA in the CSF and blood plasma. A highly significant positive correlation was found between seizure excitability and CSF GABA level, but not between CSF and plasma GABA concentrations.     

Clinical relevance of measuring GABA concentrations in cerebrospinal fluid

Determination of GABA concentrations in human cerebrospinal fluid can be used to assess GABA-ergic activity in the central nervous system. As CSF free GABA concentrations may vary with age, sex, CSF fraction, and collection and storage conditions, careful attention to these factors are necessary to allow interpretation of results. Longitudinal studies to investigate the influence of pharmacological agents on CSF GABA have proven especially useful to define clinical biochemical activity and have been utilized to attribute the anti-epileptic action of vigabatrin, a selective inhibitor of GABA-transaminase, to its effects on brain GABA metabolism.Special issue dedicated to Dr. Sidney Udenfriend.     

Ubiquitin Carboxy-Terminal Hydrolase L1 (UCH-L1) is increased in cerebrospinal fluid and plasma of patients after epileptic seizure

ABSTRACT: BACKGROUND: Clinical and experimental studies have demonstrated that seizures can cause molecular and cellular responses resulting in neuronal damage. At present, there are no valid tests for assessing organic damage to the brain associated with seizure. The aim of this study was to investigate cerebrospinal fluid (CSF) and plasma concentrations of Ubiquitin carboxy-terminal hydrolase L1 (UCH-L1), a sensitive indicator of acute injury to brain neurons, in patients with tonic--clonic or partial secondarily generalized seizures due to various etiologies. METHODS: CSF and plasma concentrations of UCH-L1 were assessed in 52 patients within 48 hours after epileptic seizure and in 19 controls using ELISA assays. RESULTS: CSF obtained within 48 hours after seizure or status epilepticus (SE) presented significantly higher levels of UCH-L1 compared to controls (p = 0.008). Plasma UCH-L1 concentrations were negatively correlated with time to sample withdrawal. An analysis conducted using only the first 12 hours post-seizure revealed significant differences between concentrations of UCH-L1 in plasma and controls (p = 0.025). CSF and plasma concentrations were strongly correlated with age in patients with seizure, but not in control patients. Plasma UCH-L1 levels were also significantly higher in patients after recurrent seizures (n = 4) than in those after one or two seizures (p = 0.013 and p = 0.024, respectively). CONCLUSION: Our results suggest that determining levels of neuronal proteins may provide valuable information on the assessment of brain damage following seizure. These data might allow clinicians to make more accurate therapeutic decisions, to identify patients at risk of progression and, ultimately, to provide new opportunities for monitoring therapy and targeted therapeutic interventions.     

Susceptibility to kainate-induced seizures under dietary zinc deficiency

Zinc homeostasis in the brain is altered by dietary zinc deficiency, and its alteration may be associated with the etiology and manifestation of epileptic seizures. In the present study, susceptibility to kainate-induced seizures was enhanced in mice fed a zinc-deficient diet for 4 weeks. When Timm's stain was performed to estimate zinc concentrations in synaptic vesicles, Timm's stain in the brain was attenuated in the zinc-deficient mice. In rats fed the zinc-deficient diet for 4 weeks, susceptibility to kainate-induced seizures was also enhanced. When the release of zinc and neurotransmitters in the hippocampal extracellular fluid of the zinc-deficient rats was studied using in vivo microdialysis, the zinc concentration in the perfusate was less than 50% of that of the control rats and the increased levels of zinc by treatment with kainate were lower than the basal level in control rats, suggesting that vesicular zinc is responsive to dietary zinc deficiency. The levels of glutamate in the perfusate of the zinc-deficient rats were more increased than in the control rats, whereas the levels of GABA in the perfusate were not at all increased in the zinc-deficient rats, unlike in the control rats. The present results demonstrate an enhanced release of glutamate associated with a decrease in GABA concentrations as a possible mechanism for the increased seizure susceptibility under zinc deficiency.     

Administration of Vigabatrin (γ-Vinyl-γ-Aminobutyric Acid) Affects the Levels of Both Inhibitory and Excitatory Amino Acids in Rat Cerebrospinal Fluid

The effect of vigabatrin (gamma-vinyl-gamma-aminobutyric acid), a new anticonvulsant drug, on the transmitter amino acids in rat cisternal CSF was studied. CSF was collected through a permanently implanted polyethylene cannula from freely moving rats at 5, 24, 48, and 96 h after administration of 1,000 mg/kg of vigabatrin. The free gamma-aminobutyric acid (GABA) level was elevated maximally (13.5-fold; p less than 0.01) at 24 h after injection. The homocarnosine (GABA-histidine) level also was increased (123%; p less than 0.01) at 24 h after injection, and its concentration remained at the same level for the next 3 days. Glycine and taurine concentrations had increased [31% (p less than 0.05) and 63% (p less than 0.01), respectively] at 5 h after injection. It is interesting that the levels of glutamate and aspartate increased [330% (p less than 0.05) and 421% (p less than 0.01), respectively] at 96 h after injection, the time when the free GABA level had returned to the baseline concentration and the vigabatrin level was 3% of the maximal concentration. The present study indicates that a single dose of vigabatrin in rats elevates levels of both the inhibitory and excitatory amino acids in CSF. However, the temporal profile of observed changes in relation to vigabatrin injection shows that neither the long-lasting elevation of GABA content nor the increase in glutamate and aspartate levels correlates with the level of vigabatrin in CSF. These findings suggest that the excitatory mechanisms are also augmented following acute administration of vigabatrin, especially when the content of GABA had decreased to the baseline level and the level of vigabatrin was low.     

Regional changes in the concentrations of glutamate,glycine, taurine,and GABA in the vitamin B-6 deficient developing rat brain: Association with neonatal seizures

It is well known that a dietary restriction of vitamin B-6 during gestation and lactation produces spontaneous seizures in neonatal animals. Since pyridoxal phosphate, one of the biologically active forms of vitamin B-6, is the cofactor for GAD the neonatal seizures have been attributed to low levels of brain GABA as a result of cofactor depletion. Although GABA levels are significantly lower in B-6 restricted neonatal rats with spontaneous seizures, seizure activity is not present in B-6 deficient adult rats or 28 day old rats in the present study, despite significantly low levels of brain GABA. These facts suggest that depletion of GABA is not the only biochemical alteration essential for the emergence of seizures. In the present study, the effect of vitamin B-6 undernutrition on the concentrations of the neuroactive amino acids, Glu, Gly, Tau, and GABA was determined in selected regions of the developing rat brain. The results show that the concentrations of Glu, Tau, and GABA were significantly lower and GLY significantly higher in selected brain regions of the B-6 restricted 14 day old rat compared to control tissue. Most of these changes were unique to 14 days of age, the time when spontaneous seizures are observed, and not present at 28 or 56 days of age when seizures are absent. This pattern of amino acid changes in the brain and the magnitude of the changes was consistent with those measured in a variety of chemically-induced animal models of epilepsy and in human epileptic foci. The regional distribution of amino acid changes was associated with brain regions which have been suggested to be responsible for the initiation and propagation of seizure activity. Two unique findings were also made in this study. First, there was a regional brain heterogeneity in the age-associated loss of brain Tau concentrations with the pons/medulla and substantia nigra appearing to be highly vulnerable and the hippocampus quite resistant to the loss of Tau. A second finding was the normalization of the neonatal GABA deficit in most brain regions by 56 days of age. The normalization of brain GABA was present in the face of continued dietary vitamin B-6 restriction. In summary, this study shows that the neuroactive amino acids Glu, Gly, Tau, and GABA are markedly altered in the seizure-prone vitamin B-6 restricted neonatal rat brain. The alterations in the brain concentration of Glu, Gly, and Tau may play an equally important role as GABA in the underlying mechanism of seizures associated with this condition.Abbreviations GAD Glutamic acid decarboxylase - GABA gamma-aminobutyric acid - Glu glutamate - Gly glycine - Tau taurine - CNS central nervous system - CTX cortex - HIPP hippocampus - C/P caudate/putamen - SN substantia nigra - Cb cerebellum - P/M pons/medulla     

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.     

Effects of GABA-transaminase inhibitor Vigabatrin on thermoregulation in rats

Vigabatrin is a GABA derivative (gamma-vinyl GABA) which inhibits irreversibly the enzyme activity of GABA transaminase and thus increased indirectly brain GABA concentrations. We have used body temperature assay to examine the effects of Vigabatrin on thermoregulation in intact rats. In order to understand the mechanism of thermoregulatory action of Vigabatrin at cellular level, we have investigated its effect on individual warm-sensitive preoptic area/anterior hypothalamus (PO/AH) neurons in rat brain slice preparations. The results of the present study suggest that Vigabatrin produced dose-dependent hypothermia in rats and also increased temperature sensitivity of warm-sensitive PO/AH neurons.     

Epoxy Fatty Acids and Inhibition of the Soluble Epoxide Hydrolase Selectively Modulate GABA Mediated Neurotransmission to Delay Onset of Seizures

In the brain, seizures lead to release of large amounts of polyunsaturated fatty acids including arachidonic acid (ARA). ARA is a substrate for three major enzymatic routes of metabolism by cyclooxygenase, lipoxygenase and cytochrome P450 enzymes. These enzymes convert ARA to potent lipid mediators including prostanoids, leukotrienes and epoxyeicosatrienoic acids (EETs). The prostanoids and leukotrienes are largely pro-inflammatory molecules that sensitize neurons whereas EETs are anti-inflammatory and reduce the excitability of neurons. Recent evidence suggests a GABA-related mode of action potentially mediated by neurosteroids. Here we tested this hypothesis using models of chemically induced seizures. The level of EETs in the brain was modulated by inhibiting the soluble epoxide hydrolase (sEH), the major enzyme that metabolizes EETs to inactive molecules, by genetic deletion of sEH and by direct administration of EETs into the brain. All three approaches delayed onset of seizures instigated by GABA antagonists but not seizures through other mechanisms. Inhibition of neurosteroid synthesis by finasteride partially blocked the anticonvulsant effects of sEH inhibitors while the efficacy of an inactive dose of neurosteroid allopregnanolone was enhanced by sEH inhibition. Consistent with earlier findings, levels of prostanoids in the brain were elevated. In contrast, levels of bioactive EpFAs were decreased following seizures. Overall these results demonstrate that EETs are natural molecules which suppress the tonic component of seizure related excitability through modulating the GABA activity and that exploration of the EET mediated signaling in the brain could yield alternative approaches to treat convulsive disorders.     

Anticonvulsant properties of the cerebrospinal fluid during activation of the antiepileptic system of the brain

Cerebrospinal fluid (CSF) was obtained after 30-40 sessions of daily electrical stimulation of the cat cerebellum vermis. The intraventricular injection of CSF (10 microliters) to Wistar rats increased the latent period of initial seizure manifestations, significantly reduced the number of animals with seizures and reduced the severity of seizures induced by korazol injection (40 mg/kg). Analogous seizure changes were observed in rats after intraventricular injection of CSF (10 microliters) from cats subject to 3-10 electroshock seizure fits. Intraventricular injection of CSF (250 microliters) obtained from cats after electroshock to cats with strychnine-induced epileptic foci in the brain cortex led to the suppression of the epileptic activity. The conclusion was made that different ways of antiepileptic system activation cause the accumulation of endogenous antiepileptic substances in CSF.     

Increased gamma-aminobutyric acid (GABA), homocarnosine and beta-alanine in cerebrospinal fluid of patients treated with gamma-vinyl GABA (4-amino-hex-5-enoic acid)

γ-Vinyl GABA, an enzyme-activated irreversible inhibitor of GABA transaminase (GABA-T), was administered orally to 15 patients with various neurological conditions at daily doses of 0.5, 1, 2 or 6 g/day for 3 days. CSF samples were obtained by lumbar puncture before treatment and within 24 hours after the last dose and the CSF concentrations of free GABA, total GABA, homocarnosine, β-alanine and γ-vinyl GABA determined by ion-exchange chromatography with fluorometric detection. γ-Vinyl GABA treatment produced dose-dependent increases in free GABA, conjugated GABA (defined as total minus free GABA), homocarnosine and β-alanine. The concentrations of CSF γ-vinyl GABA also depended on the dose administered. These results indicate that γ-vinyl GABA enters the CNS after oral administration and alters GABA metabolism by inhibition of GABA-T and suggest that such treatment may achieve therapeutic benefit in conditions where such neurochemical alterations are desirable.     

EFFECT OF DRUGS ON RAT BRAIN, CEREBROSPINAL FLUID AND BLOOD GABA CONTENT

Acute administration of GABA transaminase inhibitors to rats results in a dose-dependent increase in both brain and blood GABA content and administration of isonicotinic acid hydrazide (INH), at a dose which decreases the amount of brain GABA, also lowers blood levels of this amino acid. Chronic treatment (10 days) with INH (20mg/kg), y-acetylenic-GABA (10 mg/kg) or aminooxyacetic acid (AOAA) (10 mg/kg) results in a significant elevation in both rat brain and blood GABA concentrations. At the doses studied, only AOAA caused a significant elevation in CSF GABA content. Co-administration of pyridoxal phosphate (2 mg/kg) blocks the chronic INH-induced rise in blood GABA but does not affect the increase in brain content of this amino acid. Chronic administration of di-n-propylacetate (20 mg/kg) did not significantly alter brain, blood or CSF GABA levels. The results suggest that, under the proper conditions, changes in blood GABA levels after administration of inhibitors of GABA synthesis or degradation may be an indirect indicator of changes in the brain content of this amino acid. Blood GABA determinations may be useful for studying the biochemical effectiveness of GABA transaminase inhibitors in man.     

Vigabatrin Inhibits Seizures and mTOR Pathway Activation in a Mouse Model of Tuberous Sclerosis Complex

Epilepsy is a common neurological disorder and cause of significant morbidity and mortality. Although antiseizure medication is the first-line treatment for epilepsy, currently available medications are ineffective in a significant percentage of patients and have not clearly been demonstrated to have disease-specific effects for epilepsy. While seizures are usually intractable to medication in tuberous sclerosis complex (TSC), a common genetic cause of epilepsy, vigabatrin appears to have unique efficacy for epilepsy in TSC. While vigabatrin increases gamma-aminobutyric acid (GABA) levels, the precise mechanism of action of vigabatrin in TSC is not known. In this study, we investigated the effects of vigabatrin on epilepsy in a knock-out mouse model of TSC and tested the novel hypothesis that vigabatrin inhibits the mammalian target of rapamycin (mTOR) pathway, a key signaling pathway that is dysregulated in TSC. We found that vigabatrin caused a modest increase in brain GABA levels and inhibited seizures in the mouse model of TSC. Furthermore, vigabatrin partially inhibited mTOR pathway activity and glial proliferation in the knock-out mice in vivo, as well as reduced mTOR pathway activation in cultured astrocytes from both knock-out and control mice. This study identifies a potential novel mechanism of action of an antiseizure medication involving the mTOR pathway, which may account for the unique efficacy of this drug for a genetic epilepsy.     

Detection of 14-3-3zeta in cerebrospinal fluid following experimentally evoked seizures

Surrogate and peripheral (bio)markers of neuronal injury may be of value in assessing effects of seizures on the brain or epilepsy development following trauma. The presence of 14-3-3 isoforms in cerebrospinal fluid (CSF) is a diagnostic indicator of Creutzfeldt-Jakob disease but these proteins may also be present following acute neurological insults. Here, we examined neuronal and 14-3-3 proteins in CSF from rats after seizures. Seizures induced by intra-amygdala microinjection of 0.1 microg kainic acid (KA) caused damage which was mainly restricted to the ipsilateral CA3 subfield of the hippocampus. 14-3-3zeta was detected at significant levels in CSF sampled 4 h after seizures compared with near absence in control CSF. Neuron-specific nuclear protein (NeuN) was also elevated in CSF in seizure rats. CSF 14-3-3zeta levels were significantly lower in rats treated with 0.01 microg KA. These data suggest the presence of 14-3-3zeta within CSF may be a biomarker of acute seizure damage.     

(4S)-4-amino-5,6-heptadienoic acid (MDL 72483): A potent anticonvulsant GABA-T inhibitor

(4S)-4-Amino-5,6-heptadienoic acid ((S)--allenyl-GABA; MDL 72483) is a potent inactivator of brain GABA-T in mice; (ED₅₀ (i.p.)=60 mg·kg^–1; ED₅₀ (oral)=70 mg·kg^–1). Its anticonvulsant effects against 3-mercaptopropionic acid (MPA)-induced seizures in mice is related to the elevation of whole brain GABA concentrations: The mentioned doses of MDL 72483 which cause a decrease of GABA-T activity by 50%, produce within 5 h after dosing an increase of GABA concentration by about 3 mol·g^–1, and protect 50% of the mice against seizures in this model of presynaptic GABA deficit. When given orally MDL 72483 is about five times more potent than vigabatrin ((4R/S)-4-amino-5-hexenoic acid) a known antiepileptic GABA-T inhibitor. Complete protection was achieved with a dose of 150 mg·kg^–1. Similar to vigabatrin, MDL 72483 does not protect significantly against metrazol-induced convulsions. However, at a dose of 300 mg·kg^–1, the time elapsing between metrazol administration and onset of convulsions was prolonged by a factor of 3.4. Oral administration of MDL 72483 for up to 19 days at a daily dose of 91–96 mg·kg^–1 did not produce any obvious behavioral changes in mice, nor was the ED₅₀ of the drug in MPA-seizure tests significantly altered by the pretreatment. These observations indicate that MDL 72483 is a promising drug for the treatment of certain epilepsies.Special issue dedicated to Dr. Eugene Roberts.     

Correlation between alterations in brain GABA metabolism and seizure excitability following administration of GABA aminotransferase inhibitors and valproic acid—a re-evaluation

The time course of the effects of aminooxyacetic acid, γ-vinyl GABA, γ-acetylenic GABA, gabaculine, ethanolamine-O-sulphate (EOS) and valproic acid (VPA) on brain GABA content and the activities of glutamic acid decarboxylase (GAD) and GABA aminotransferase (GABA-T), the enzymes involved in biosynthesis and degradation of GABA, was re-determined and compared with the action on the electroconvulsive threshold in mice. All drugs caused significant increases in the seizure threshold, and the temporal pattern of this effect correlated rather well with the induced elevation of brain GABA. However, no clear relationship was found between the extent of GABA increase and the relative increase of seizure threshold. Except for VPA, the time course of the increment in brain GABA followed closely the inhibition of GABA-T. The activity of GAD was gradually decreased by γ-acetylenic GABA and a slow decline of GAD activity was also observed after γ-vinyl GABA. EOS and gabaculine suggesting a feedback repression of GAD synthesis by highly elevated GABA concentrations. Concomitant with significant reduction of GAD activity, a decrease in seizure threshold occurred though brain GABA levels remained markedly elevated. On the other hand, following administration of VPA the effect of GABA levels was paralleled by an increase in GAD activity indicating that the GABA-elevating action of this drug can be attributed at least in part to an activation of GABA synthesis. The data suggest that reduction of GAD activity may be an inevitable consequence of increasing brain GABA concentrations over a certain extent and this effect seems to limit the anticonvulsant efficacy of GABA-T inhibitors.     

Placental transfer of vigabatrin (gamma-vinyl GABA) and its effect on concentration of amino acids in the embryo of TO mice

BACKGROUND: The mechanism of the teratogenicity of vigabatrin (VGB) is unknown. The objectives of this study were to determine the placental transfer of VGB and to evaluate the effect of VGB on maternal, placental, and fetal concentrations of amino acids. METHODS: A single dose of 400 mg/kg VGB in physiological saline was administered intraperitoneally to a group of Theiler outbred (TO) mice on gestational day (GD) 10. The controls received a proportionate volume of saline. Maternal blood samples, embryos, and placentas were collected at 3.5, 6.0, and 9.0 hr after treatment and their total amino acid concentrations determined in an ion-exchange amino acid analyzer. RESULTS: At 3.5 hr, there was a decrease in concentrations of some amino acids in the blood, placenta, and embryos of VGB-treated mice, but the decrease in methionine was most marked. gamma-aminobutyric acid (GABA) was significantly higher in the VGB group in both the embryos and the placentas at 3.5 hr but at 6.0 and 9.0 hr the differences were not significant. Vigabatrin levels were higher in the placenta than in the embryo at 3.5 hr, but at 6.0 hr there was an overlap of the VGB peak with that of tryptophan with very much lower levels than at 3.5 hr. At 9.0 hr, there was no vigabatrin peak in either the placenta or the embryo. CONCLUSIONS: Maternal exposure to VGB results in peak levels of the drug after 3.5 hr in the placenta and embryo. Methionine concentration is most severely affected in VGB-treated mothers, placentas, and fetuses. We speculate that this deficiency could be a possible mechanism for the teratogenic effects of vigabatrin.