Acute effects of sodium valproate and gamma-vinyl GABA on regional amino acid metabolism in the rat brain: incorporation of 2-[14C]glucose into amino acids

Assessment of behavioral, morphological and electrophysiological changes in prenatal and postnatal valproate induced rat models of autism spectrum disorder

Autism spectrum disorders (ASD) are neurodevelopmental disorders, that are characterized by core symptoms, such as alterations of social communication and restrictive or repetitive behavior. The etiology and pathophysiology of disease is still unknown, however, there is a strong interaction between genetic and environmental factors. An intriguing point in autism research is identification the vulnerable time periods of brain development that lack compensatory homeostatic corrections. Valproic acid (VPA) is an antiepileptic drug with a pronounced teratogenic effect associated with a high risk of ASD, and its administration to rats during the gestation is used for autism modeling. It has been hypothesized that valproate induced damage and functional alterations of autism target structures may occur and evolve during early postnatal life. Here, we used prenatal and postnatal administrations of VPA to investigate the main behavioral features which are associated with autism spectrum disorders core symptoms were tested in early juvenile and adult rats. Neuroanatomical lesion of autism target structures and electrophysiological studies in specific neural circuits. Our results showed that prenatal and early postnatal administration of valproate led to the behavioral alterations that were similar to ASD. Postnatally treated group showed tendency to normalize in adulthood. We found pronounced structural changes in the brain target regions of prenatally VPA-treated groups, and an absence of abnormalities in postnatally VPA-treated groups, which confirmed the different severity of VPA across different stages of brain development. The results of this study clearly show time dependent effect of VPA on neurodevelopment, which might be explained by temporal differences of brain regions' development process. Presumably, postnatal administration of valproate leads to the dysfunction of synaptic networks that is recovered during the lifespan, due to the brain plasticity and compensatory ability of circuit refinement. Therefore, investigations of compensatory homeostatic mechanisms activated after VPA administration and directed to eliminate the defects in postnatal brain, may elucidate strategies to improve the course of disease.

Propionate enters GABAergic neurons, inhibits GABA transaminase, causes GABA accumulation and lethargy in a model of propionic acidemia

Propionic acidemia is the accumulation of propionate in blood due to dysfunction of propionyl-CoA carboxylase. The condition causes lethargy and striatal degeneration with motor impairment in humans. How propionate exerts its toxic effect is unclear. Here, we show that intravenous administration of propionate causes dose-dependent propionate accumulation in the brain and transient lethargy in mice. Propionate, an inhibitor of histone deacetylase, entered GABAergic neurons, as could be seen from increased neuronal histone H4 acetylation in the striatum and neocortex. Propionate caused an increase in GABA (γ-amino butyric acid) levels in the brain, suggesting inhibition of GABA breakdown. In vitro propionate inhibited GABA transaminase with a Ki of ∼1 mmol/l. In isolated nerve endings, propionate caused increased release of GABA to the extracellular fluid. In vivo, propionate reduced cerebral glucose metabolism in both striatum and neocortex. We conclude that propionate-induced inhibition of GABA transaminase causes accumulation of GABA in the brain, leading to increased extracellular GABA concentration, which inhibits neuronal activity and causes lethargy. Propionate-mediated inhibition of neuronal GABA transaminase, an enzyme of the inner mitochondrial membrane, indicates entry of propionate into neuronal mitochondria. However, previous work has shown that neurons are unable to metabolize propionate oxidatively, leading us to conclude that propionyl-CoA synthetase is probably absent from neuronal mitochondria. Propionate-induced inhibition of energy metabolism in GABAergic neurons may render the striatum, in which >90% of the neurons are GABAergic, particularly vulnerable to degeneration in propionic acidemia.

Polymorphisms of ABAT, SCN2A and ALDH5A1 may affect valproic acid responses in the treatment of epilepsy in Chinese

Aim: The clinical efficacy of valproic acid (VPA) varies greatly among epileptic patients. To find the potential genetic factors related to VPA responses, the pharmacogenetics study was conducted. Methods: Two hundred and one Chinese Han epileptic patients who were treated by VPA for at least 1 year were recruited. Up to 24 SNPs in 11 candidate genes that correlate with the metabolism, transport or target of VPA were genotyped. Results: Three SNPs, rs1731017 (ABAT), rs2304016 (SCN2A) and rs1054899 (ALDH5A1) were found associated with VPA responses with the p-values of 0.003, 0.007 and 0.048, respectively. Further interaction analysis showed that the interaction between rs17183814 (ABAT) and rs1641022 (SCN2A) was also correlated with the response of VPA (p = 0.006). Conclusion: This study found three SNPs and one interaction among ABAT, SCN2A and ALDH5A1 were significantly associated with VPA response, which indicated that these genes may play important roles in the pharmacological mechanism of VPA.

Long-term valproate treatment increases brain neuropeptide Y expression and decreases seizure expression in a genetic rat model of absence epilepsy

The mechanisms by which valproate, one of the most widely prescribed anti-epileptic drugs, suppresses seizures have not been fully elucidated but may involve up-regulation of neuropeptide Y (NPY). We investigated the effects of valproate treatment in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) on brain NPY mRNA expression and seizure control. GAERS were administered either valproate (42 mg.kg⁻¹ hr⁻¹) or saline continuously for 5 days. Electroencephalograms were recorded for 24 hrs on treatment days 1, 3 and 5 and the percentage of time spent in seizure activity was analysed. NPY mRNA expression was measured in different brain regions using qPCR. Valproate treatment suppressed seizures by 80% in GAERS (p<0.05) and increased NPY mRNA expression in the thalamus (p<0.05) compared to saline treatment. These results demonstrate that long-term valproate treatment results in an upregulation of thalamic expression of NPY implicating this as a potential contributor to the mechanism by which valproate suppresses absence seizures.

Differential levels of brain amino acids in rat models presenting learned helplessness or non-learned helplessness

RATIONALE

Glutamatergic and γ-aminobutyric acid (GABA)ergic abnormalities have recently been proposed to contribute to depression. The learned helplessness (LH) paradigm produces a reliable animal model of depression that expresses a deficit in escape behavior (LH model); an alternative phenotype that does not exhibit LH is a model of resilience to depression (non-LH model).

OBJECTIVES

We measured the contents of amino acids in the brain to investigate the mechanisms involved in the pathology of depression.

METHODS

LH and non-LH models were subjected to inescapable electric footshocks at random intervals following a conditioned avoidance test to determine acquirement of predicted escape deficits. Tissue amino acid contents in eight brain regions were measured via high-performance liquid chromatography.

RESULTS

The non-LH model showed increased GABA levels in the dentate gyrus and nucleus accumbens and increased glutamine levels in the dentate gyrus and the orbitofrontal cortex. The LH model had reduced glutamine levels in the medial prefrontal cortex. Changes in the ratios of GABA, glutamine, and glutamate were detected in the non-LH model, but not in the LH model. Reductions in threonine levels occurred in the medial prefrontal cortex in both models, whereas elevated alanine levels were detected in the medial prefrontal cortex in non-LH animals.

CONCLUSIONS

The present study demonstrates region-specific compensatory elevations in GABA levels in the dentate gyrus and nucleus accumbens of non-LH animals, supporting the implication of the GABAergic system in the recovery of depression.

Elevated endogenous GABA concentration attenuates glutamate-glutamine cycling between neurons and astroglia

In this study, the relationship between endogenous brain GABA concentration and glutamate-glutamine cycling flux (V (cyc)) was investigated using in vivo (1)H and (1)H{(13)C} magnetic resonance spectroscopy techniques. Graded elevations of brain GABA levels were induced in rat brain after administration of the highly specific GABA-transaminase inhibitor vigabatrin (gamma-vinyl-GABA). The glial-specific substrate [2-(13)C]acetate and (1)H{(13)C} magnetic resonance spectroscopy were used to measure V (cyc) at different GABA levels. Significantly reduced V (cyc) was found in rats pretreated with vigabatrin. The reduction in group mean V (cyc) over the range of GABA concentrations investigated in this study (1.0 +/- 0.3-5.1 +/- 0.5 micromol/g) was found to be nonlinear: Delta V (cyc)/V (cyc) = [GABA (micromol/g)](-0.35 )- 1.0 (r (2) = 0.98). The results demonstrate that V (cyc) is modulated by endogenous GABA levels, and that glutamatergic and GABAergic interactions can be studied in vivo using noninvasive magnetic resonance spectroscopy techniques.

Mechanisms of action of the mood stabilizer valproate: a focus on GSK-3 inhibition

Valproate is the most widely prescribed antiepileptic drug worldwide, and it is also used in the treatment of bipolar affective disorder, migraine headache and cancer. However, the therapeutic mechanism of action of valproate in these illness states is not understood. This article reviews the pharmacological effects of valproate that may explain its therapeutic efficacy. It focuses on the hypothesis that inhibition of glycogen synthase kinase-3 by valproate is a crucial therapeutic mechanism of this drug in the treatment of bipolar affective disorder. Other cellular pathways and signaling molecules that are targets of valproate (such as inositol de novo biosynthesis, histone deacetylase, protein kinase C, γ-aminobutyric acid, the extracellular signal-regulated kinase pathway and others) are also discussed.

CBF changes in drug naive juvenile myoclonic epilepsy patients

PURPOSE

The role of thalamus and brainstem in generalized epilepsy has been suggested in previous studies. The aim of the present study was to assess regional cerebral blood flow (rCBF) abnormality in juvenile myoclonic epilepsy (JME) patients.

METHODS

(99m)Tc-ethylcysteinate dimer brain single photon emission computed tomography (SPECT) was performed in 19 drug naive JME patients and 25 normal controls with the similar age and gender distribution. Differences of rCBF between a JME group and a normal control group were examined by the statistical parametric mapping of brain SPECT images using independent t test. The regression analyses in SPM were also performed between rCBF and the age of seizure onset or the disease duration in JME group.

RESULTS

Compared to normal controls, the JME group showed a significant rCBF reduction in bilateral thalami, red nucleus, midbrain, pons, left hippocampus, and in the cerebelli (FDR corrected p < 0.01) whereas rCBF increase in the left superior frontal gyrus (uncorrected p < 0.001 but FDR corrected p > 0.05). Disease duration was negatively correlated with rCBF in bilateral frontal cortices, caudate nuclei, brainstem and cerebellar tonsils.

CONCLUSIONS

Our results suggest that abnormal neural networks in the thalamus, hippocampus, brainstem and cerebellum are associated with JME.

Structural brain abnormalities in juvenile myoclonic epilepsy patients: volumetry and voxel-based morphometry

Objective

We aimed to find structural brain abnormalities in juvenile myoclonic epilepsy (JME) patients.

Materials and Methods

The volumes of the cerebrum, hippocampus and frontal lobe and the area of the corpus callosum's subdivisions were all semiautomatically measured, and then optimized voxel-based morphometry (VBM) was performed in 19 JME patients and 19 age/gender matched normal controls.

Results

The rostrum and rostral body of the corpus callosum and the left hippocampus were significantly smaller than those of the normal controls, whereas the volume of the JME's left frontal lobe was significantly larger than that of the controls. The area of the rostral body had a significant positive correlation with the age of seizure onset (r = 0.56, p = 0.012), and the volume of the right frontal lobe had a significant negative correlation with the duration of disease (r = -0.51, p = 0.025). On the VBM, the gray matter concentration of the prefrontal lobe (bilateral gyri rectus, anterior orbital gyri, left anterior middle frontal gyrus and right anterior superior frontal gyrus) was decreased in the JME group (corrected p < 0.05).

Conclusion

The JME patients showed complex structural abnormalities in the corpus callosum, frontal lobe and hippocampus, and also a decreased gray matter concentration of the prefrontal region, which all suggests there is an abnormal neural network in the JME brain.

Progress in pharmaceutical development presentation with improved pharmacokinetics: a new formulation for valproate

Successful long-term treatment of patients with epilepsy requires selection of an appropriate antiepileptic regimen, optimal dosing and patient compliance. Recent advances in our understanding of the biological basis of epilepsy and in the choice of treatment options are transforming the global management of these patients. If the achievement of seizure freedom remains the primary goal of any antiepileptic treatment, issues associated with drug acceptability and tolerability, and with quality of life of patients, have gained increasing attention as major determinants of ultimate therapeutic success. Sustained-release formulations of antiepileptic drugs can be very helpful in achieving treatment objectives. Stable serum levels without marked peak-to-trough fluctuations, reduced frequency of dosing and the possibility of dosing flexibility may all improve compliance, patient satisfaction and ultimately quality of life. The efficacy of sodium valproate for the treatment of most types of epilepsy has been demonstrated extensively and this drug remains the mainstay of treatment for many clinical situations. Among the various valproate formulations, extended-release tablets have shown improved patient compliance and satisfaction. However, the tablet size and the limited dosing flexibility could be unsuitable for individualized treatment in special populations such as children, the elderly and patients with swallowing difficulties. A new sustained-release formulation of sodium valproate consisting of tasteless microspheres that can be sprinkled on semi-solid food such as yoghurt or jam has been developed. A stick pack presentation allows individualized dosing and greater convenience.

Increasing treatment options for cannabis dependence: a review of potential pharmacotherapies

Despite the fact that rates of cannabis dependence have increased substantially over the past several years, there are no medications approved for the treatment of cannabis dependence. This paper reviews data from recent research on cannabinoids that may be relevant for the development of pharmacotherapies for cannabis dependence. Included in the discussion are findings from studies that have assessed the ability of medications to ameliorate cannabis-related abstinence symptoms in laboratory animals and human research participants. Data from studies that have investigated the effects of pharmacological agents on cannabis self-administration are also reviewed because these data may provide information critical for informing relapse prevention medication development efforts. The majority of published studies evaluating cannabis pharmacotherapies have focused on decreasing withdrawal symptoms: a growing number of medications reduce symptoms in laboratory animals, but the majority of these medications have not been tested in humans. Fewer studies have assessed the effects of potential cannabis treatment medications on cannabinoid-related reinforcing effects. In laboratory animals, only the CB1 cannabinoid antagonist rimonabant has shown promise. In humans, this medication has not been tested on cannabis reinforcing effects. To date, no medication has been shown to alter cannabis self-administration by humans.

In vivo detection of cortical GABA turnover from intravenously infused [1-13C]D-glucose

In this study [2-(13)C] gamma-aminobutyric acid (GABA) was spectrally resolved in vivo and detected simultaneously with [4-(13)C]glutamate (Glu) and [4-(13)C]glutamine (Gln) in the proton spectra obtained from a localized 40 microL voxel in rat neocortex with the use of an adiabatic (1)H-observed, (13)C-edited (POCE) spectroscopy method and an 89-mm-bore vertical 11.7 Tesla microimager. The time-resolved kinetics of (13)C label incorporation from intravenously infused [1-(13)C]glucose into [4-(13)C]Glu, [4-(13)C]Gln, and [2-(13)C]GABA were measured after acute administration of gabaculine, a potent and specific inhibitor of GABA-transaminase. In contrast to previous observations of a rapid turnover of [2-(13)C]GABA from [1-(13)C]glucose in intact rat brain, the rate of (13)C incorporation from [1-(13)C]glucose into [2-(13)C]GABA in the gabaculine-treated rats was found to be significantly reduced as a result of the blockade of the GABA shunt.

Valproate suppresses status epilepticus induced by 4-aminopyridine in CA1 hippocampus region

PURPOSE

We investigated the effects of valproate (VPA) on an in vivo model of status epilepticus (SE) induced by intrahippocampal application of 4-aminopyridine (4-AP).

METHODS

To induce continuous epileptiform activity without a clinical component, 4-AP (100 mM) was slowly injected in the hippocampus of adult rats. Extracellular field potential from the CA1 region of the rat hippocampus was recorded to assess abnormal epileptiform activity. Once the SE seizures were induced by 4-AP, the test drug was injected. In some experiments to test the ability of a drug to prevent the induction of SE, the drug was administered before 4-AP injection.

RESULTS

Intrahippocampal injection of 4-AP induced continuous epileptic activity without a clinical component that lasted >60 min. The intravenous injection of 400-600 mg/kg VPA rapidly (approximately 100 s) abolished the SE, and this effect persisted for >/=4 h in our experimental model. The intravenous injection of 100-300 mg/kg VPA did not abolish previously induced SE, but prevented the appearance of SE when applied before the induction of SE. The intravenous injection of 80 mg/kg phenytoin or carbamazepine did not abolish or prevent SE.

CONCLUSIONS

We conclude that 4-AP-induced SE was suppressed by VPA at 400-600 mg/kg, whereas minor doses (100-300 mg/kg) only prevent the 4-AP-induced SE. Present results suggest the revisiting of VPA as a useful drug for the treatment of SE.

Valproate: past, present, and future

Preclinical studies have been carried out during the past four decades to investigate the different mechanisms of action of valproate (VPA). The mechanisms of VPA which seem to be of clinical importance include increased GABAergic activity, reduction in excitatory neurotransmission, and modification of monoamines. These mechanisms are discussed in relation to the various clinical uses of the drug. VPA is widely used as an antiepileptic drug with a broad spectrum of activity. In patients, VPA possesses efficacy in the treatment of various epileptic seizures such as absence, myoclonic, and generalized tonic-clonic seizures. It is also effective in the treatment of partial seizures with or without secondary generalization and acutely in status epilepticus. The pharmacokinetic aspects of VPA and the frequent drug interactions between VPA and other drugs are discussed. The available methods for the determination of VPA in body fluids are briefly evaluated. At present, investigations and clinical trials are carried out and evaluated to explore the new indications for VPA in other conditions such as in psychiatric disorders, migraine and neuropathic pain. Furthermore, the toxicity of VPA, both regarding commonly occurring side effects and potential idiosyncratic reactions are described. Derivatives of VPA with improved efficacy and tolerability are in development.

Effect of lamotrigine on plasma GABA levels in healthy humans

Lamotrigine, a new anticonvulsant, has been reported to be useful in treating bipolar depression, rapid cycling, and other phases of bipolar disorder. However, the mechanism of action underlying its efficacy in mood disorders is still not known. Since there is evidence for gamma-aminobutyric acid (GABA) involvement in the pathophysiology and treatment response of patients with bipolar disorder, this study was designed to examine the effect of lamotrigine on plasma GABA levels in healthy humans. Eleven healthy volunteers with no lifetime history of psychiatric illness or family history in first-degree relatives were recruited. Each subject received lamotrigine 100 mg/day for 1 week. Blood samples for assay of plasma levels of GABA were taken from each subject before and after administration of lamotrigine. Plasma GABA levels were analyzed using high-pressure liquid chromatography (HPLC) with fluorescence detection after derivatization with o-phthaldialdehyde (OPA). We found no significant difference in the plasma GABA levels of the study subjects before and after treatment with lamotrigine. The finding of this study suggests that lamotrigine in the dose used in this study does not appear to enhance GABA levels in humans.

Immunohistochemistry as a tool for topographical semi-quantification of neurotransmitters in the brain

Immunohistochemistry is a powerful tool to detect neurotransmitter (NT) presence in different brain structures with a high spatial resolution. However, it is only scarcely used in quantitative approach due to lack of reproducibility and sensitivity. We developed a protocol of NT detection based on immunohistochemistry and image analysis to show that this approach could also be useful to evaluate NT content variations. We focused our study on the GABAergic system in the cerebellum and measured different accurate parameters, namely the optical density (O.D.), the stained area and the number of immunoreactive cells in each cerebellar cell layer. In order to modify the GABA content, we used gamma-vinyl-GABA (GVG), an inhibitor of GABA-transaminase, known to dramatically increase GABA concentration in the central nervous system (CNS) and especially in the cerebellum. We observed a significant increase in the three parameters measured in the molecular and the granular layers of the cerebellum after treatment with GVG, reflecting the well-established increase in GABA content after such a treatment. Therefore, our technical approach allows not only a precise determination of the effects in particular cell layers but also a semi-quantification of GABA content variations. This technique could be suitable for monitoring NT variations following any treatment.

In vivo monitoring of rat brain metabolites during vigabatrin treatment using localized 2D-COSY

A two-dimensional COSY-based localization sequence was designed to allow the in vivo monitoring of proton metabolites in rat brain [particularly gamma-aminobutyric acid (GABA), glutamine, taurine and myo-inositol]. The sequence incorporated OSIRIS signal localization, B1-insensitive water suppression and phase-sensitive COSY acquisition. The method was used to study the effects of the GABA-transaminase inhibitor vigabatrin on rat brain metabolite concentrations. Wistar rats were treated daily for 3 days with an oral dose of vigabatrin (200 mg/kg, n = 4). Localized COSY spectra were obtained during a 120 min acquisition from a 270 microl central brain voxel and compared with nine untreated control animals. Significant elevations were observed in GABA (267% of control, p < 0.005, Mann-Witney test), glutamine (130% of control, p < 0.005) and taurine (113% of control, p < 0.05). Changes in GABA and taurine were consistent with previous data on the action of Vigabatrin, and support a previously hypothesized link between these compounds. The increase in glutamine was more surprising and may reflect the balance between the level and/or site of GABA-transaminase inhibition and downregulation of GABA synthesis.

Effects of radiofrequency exposure on the GABAergic system in the rat cerebellum: clues from semi-quantitative immunohistochemistry

The widespread use of cellular phones raises the problem of interaction of electromagnetic fields with the central nervous system (CNS). In order to measure these effects on neurotransmitter content in the CNS, we developed a protocol of neurotransmitter detection based on immunohistochemistry and image analysis. Gamma-vinyl-GABA (GVG), an inhibitor of the GABA-transaminase was injected in rats to increase GABA concentration in the CNS. The cellular GABA contents were then revealed by immunohistochemistry and semi-quantified by image analysis thanks to three parameters: optical density (O.D.), staining area, and number of positive cells. The increase in cerebellar GABA content induced by GVG 1200 mg/kg was reflected in these three parameters in the molecular and the granular layers. Therefore, control of immunohistochemistry parameters, together with appropriate image analysis, allowed both the location and the detection of variations in cellular neurotransmitter content. This protocol was used to investigate the effects of exposure to 900 MHz radiofrequencies on cerebellar GABA content. Both pulsed emission with a specific absorption rate (SAR) of 4 W/kg and continuous emission with high SAR (32 W/kg) were tested. We observed a selective diminution of the stained processes area in the Purkinje cell layer after exposure to pulsed radiofrequency and, in addition, a decrease in O.D. in the three cell layers after exposure to continuous waves. Whether this effect is, at least partly, due to a local heating of the tissues is not known. Overall, it appears that high energetic radiofrequency exposure induces a diminution in cellular GABA content in the cerebellum.

Competitive and noncompetitive NMDA antagonist effects in rats trained to discriminate lever-press counts

The glutamate activated N-methyl-D-aspartate (NMDA) receptor may play a role in short-term memory processing. Among the evidence for this is that NMDA antagonists can impair accuracy in fixed consecutive number (FCN) tasks. This study was designed to further characterize this effect by examining NMDA antagonists differing in their cellular mechanisms of action. Rats were trained to respond under an FCN operant schedule, which required eight presses on one lever (counting lever) before one press at an alternate lever (reinforcement lever) would produce food reinforcement. The effects of three noncompetitive [MK-801 (0.01-0.56 mg/kg); phencyclidine (0.3-3.0 mg/kg); memantine (1-10 mg/kg)] and two competitive [SDZ EAA 494 (0.3-3.0 mg/kg) and NPC 17742 (2.0-16 mg/kg)] NMDA antagonists were analyzed. MK-801 and phencyclidine decreased accuracy at doses not reducing response rates. Memantine, and both of the competitive antagonists, also reduced accuracy, but did so only at doses that markedly reduced response rates. These results suggest that both the affinity and the site bound on the NMDA glutamate receptor by antagonists can determine their effects on FCN performance. Subsequent studies investigated whether SCH 23390, a dopamine D1 receptor antagonist, and NMDA could modulate the effects by phencyclidine and SDZ EAA 494, respectively, on FCN performance.

Up-regulation of hippocampal glutamate transport during chronic treatment with sodium valproate

Excessive glutamatergic neurotransmission has been implicated in some neurodegenerative disorders. It would be of value to know whether glutamate transport, which terminates the glutamate signal, can be up-regulated pharmacologically. Here we show that chronic treatment of rats with the anti-epileptic drug sodium valproate (200 mg or 400 mg/kg bodyweight, twice per day for 90 days) leads to a dose-dependent increase in hippocampal glutamate uptake capacity as measured by uptake of [(3)H]glutamate into proteoliposomes. The level of glutamate transporters EAAT1 and EAAT2 in hippocampus also increased dose-dependently. No effect of sodium valproate on glutamate transport was seen in frontal or parietal cortices or in cerebellum. The hippocampal levels of glial fibrillary acidic protein and glutamine synthetase were unaffected by valproate treatment, whereas the levels of synapsin I and phosphate-activated glutaminase were reduced by valproate treatment, suggesting that the increase in glutamate transporters was not caused by astrocytosis or increased synaptogenesis. A direct effect of sodium valproate on the glutamate transporters could be excluded. The results show that hippocampal glutamate transport is an accessible target for pharmacological intervention and that sodium valproate may have a role in the treatment of excitotoxic states in the hippocampus.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.

Effects of valproate on amino acid and monoamine concentrations in striatum of audiogenic seizure-prone Balb/c mice

The effects of valproate on CNS concentrations of gamma-aminobutyric acid (GABA), glulamate (GLU), glutamine (GLN); dopamine (DA), serotonin (5-HT), and metabolites were examined in tissue extracts of caudate nucleus of genetic substrains of Balb/c mice susceptible (EP) or resistant (ER) to audiogenic seizures. Generalized tonic-clonic seizures observed in EP mice were inhibited by valproate, administered 1 h prior to testing, in a dose-response fashion. Concentrations of GABA, GLU, and GLN, which were lower in EP mice than in ER mice, were significantly increased by valproate at doses of 180 and 360 mg/kg. Concentrations of homovanillic acid (HVA) and hydroxyindoleacetic acid (5-HIAA), metabolites of DA and 5-HT, were substantially increased by valproate at these doses. The in situ activity of tyrosine hydroxylase (TH) was not significantly influenced by valproate, whereas a valproate-induced increase in tryptophan hydroxylase (TPH) activity was observed in both striatum and in midbrain tegmentum. The data are consistent with the interpretation that anti-convulsive doses of valproate influences the intraneuronal metabolism of monoamines, GABA, and glutamate concurrently. Valproate's influence on the metabolism of both major inhibitory (GABA) and excitatory (GLY amino acids in striatum could contribute to its anti-convulsive effects in genetically seizure prone mice, as well as to the accumulation of DA and 5-HT metabolites.

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: valproic acid, clonazepam, and alpha-methyl-alpha-phenylsuccinimide

Spontaneous thalamocortical epileptiform activity was elicited in rodent thalamocortical slices by a medium containing no added Mg2+. Multiple varieties of activity were generated in these slices, including simple thalamocortical burst complex (sTBC) activity that resembled the spike-wave discharges of generalized absence epilepsy, and complex thalamocortical burst complex (cTBC) activity that resembled generalized tonic-clonic seizure discharges. In a further pharmacological characterization of this activity, the effects of the broad-spectrum anticonvulsants valproic acid, alpha-methyl-alpha-phenylsuccinimide (the active metabolite of methsuximide) and clonazepam were studied. All three drugs were found to be effective in controlling both sTBC and cTBC activity when applied in clinically relevant concentration ranges. The effectiveness of valproic acid against spontaneous rhythms in vitro was not due to augmentation of GABAergic inhibition. No effect of valproic acid on GABA-activated chloride currents was evident in patch-clamp recordings of acutely isolated thalamic or cortical neurons. The equivalent general clinical and experimental spectrum of action of broadly effective anticonvulsants provided an additional correlation between the clinical efficacy of anticonvulsant drugs and their effects against epileptiform discharges in rodent thalamocortical slices. This further validates spontaneous generalized low-Mg2+ thalamocortical activity as a potentially valuable in vitro model of the primary generalized epilepsies, in which the cellular mechanisms underlying generation and control of these seizure discharges can be studied.

Antiepileptic drug mechanisms of action

Established antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium channels, gamma-aminobutyric acid type A (GABAA) receptors, or calcium channels. Benzodiazepines and barbiturates enhance GABAA receptor-mediated inhibition. Phenytoin (PHT), carbamazepine (CBZ), and possibly valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing sodium-channel inactivation. Ethosuximide (ESM) and VPA reduce a low threshold (T-type) calcium-channel current. The mechanisms of action of the new AEDs are not fully established. Gabapentin (GBP) binds to a high-affinity site on neuronal membranes in a restricted regional distribution of the central nervous system. This binding site may be related to a possible active transport process of GBP into neurons; however, this has not been proven, and the mechanism of action of GBP remains uncertain. Lamotrigine (LTG) decreases sustained high-frequency repetitive firing of voltage-dependent sodium action potentials that may result in a preferential decreased release of presynaptic glutamate. The mechanism of action of oxcarbazepine (OCBZ) is not known; however, its similarity in structure and clinical efficacy to CBZ suggests that its mechanism of action may involve inhibition of sustained high-frequency repetitive firing of voltage-dependent sodium action potentials. Vigabatrin (VGB) irreversibly inhibits GABA transaminase, the enzyme that degrades GABA, thereby producing greater available pools of presynaptic GABA for release in central synapses. Increased activity of GABA at postsynaptic receptors may underline the clinical efficacy of VGB.

Measurement of GABA following GABA-transaminase inhibition by gabaculine: a 1H and 31P NMR spectroscopic study of rat brain in vivo

A selective 1H NMR spin-echo editing method was used to detect the 4-CH2 of GABA in rat brain in vivo before and after intravenous administration of the highly selective GABA transaminase inhibitor, gabaculine (3-amino-2,3-dihydrobenzoic acid-HCl; 100 mg/kg, intravenously). The effects of the inhibitor on high energy phosphates and pHi were determined by 31P NMR. GABA levels increased approximately linearly (r = 0.81 to 0.94; P < 0.0005) from 1.9 +/- 0.4 mumol/g (pre-gabaculine; mean +/- SD) to between 6 and 8 mumol/g after 4 hr at rates of accumulation of 1.1 to 2.9 mumol/hr/g. 1H NMR spectroscopic measurements of cerebral GABA and its rate of turnover offers a new approach in the study of GABA-mediated processes in vivo.

Excitatory and inhibitory amino acids in the cerebral cortex of nucleus basalis magnocellularis lesioned rats

It is well known that pathways arising from the nucleus basalis magnocellularis in the basal forebrain which terminate in the cerebral cortex are involved in cognitive function. The cholinergic system is generally thought to play a large part in these processes from lesion, pharmacological and transplantation studies. With increasing evidence suggesting the involvement of amino acid transmitters in learning and memory processes, it is of interest to also evaluate possible changes in the levels of amino acid transmitters in the cortex of nucleus basalis magnocellularis-lesioned rats. In the present study, 9 cortical amino acids were measured in rats with bilateral lesions of the nucleus basalis magnocellularis. We measured significant reductions in aspartate, alanine and gamma-aminobutyric acid; these were 80%, 75%, and 81%, respectively, of control brain values. These results suggest that changes in the amino acid content of the cerebral cortex following lesion of the nucleus basalis magnocellularis-lesioned rat should perhaps also be considered when evaluating behavioral effects in this model.

Valproate increases glutaminase and decreases glutamine synthetase activities in primary cultures of rat brain astrocytes

It has been proposed that hyperammonemia may be associated with valproate therapy. As astrocytes are the primary site of ammonia detoxification in brain, the effects of valproate on glutamate and glutamine metabolism in astrocytes were studied. It is well established that, because of compartmentation of glutamine synthetase, astrocytes are the site of synthesis of glutamine from glutamate and ammonia. The reverse reaction is catalyzed by the ubiquitous enzyme glutaminase, which is present in both neurons and astrocytes. In astrocytes exposed to 1.2 mM valproate, glutaminase activity increased 80% by day 2 and remained elevated at day 4; glutamine synthetase activity was decreased 30%. Direct addition of valproate to assay tubes with enzyme extracts from untreated astrocytes had significant effects only at concentrations of 10 and 20 mM. When astrocytes were exposed for 4 days to 0.3, 0.6, or 1.2 mM valproate and subsequently incubated with L-[U-14C]glutamate, label incorporation into [14C]glutamine was decreased by 11, 25, and 48%, respectively, and is consistent with a reduction in glutamine synthetase activity. Label incorporation from L-[U-14C]glutamate into [14C]aspartate also decreased with increasing concentrations of valproate. Following a 4-day exposure to 0.6 mM valproate, the glutamine levels increased 40% and the glutamate levels 100%. These effects were not directly proportional to valproate concentration, because exposure to 1.2 mM valproate resulted in a 15% decrease in glutamine levels and a 25% increase in glutamate levels compared with control cultures. Intracellular aspartate was inversely proportional to all concentrations of extracellular valproate, decreasing 60% with exposure to 1.2 mM valproate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of long-term vigabatrin therapy on GABA and other amino acid concentrations in the central nervous system--a case study

A 32 year old patient with refractory complex partial seizures was treated with vigabatrin for 3.5 years. Before starting treatment and at 42 months, lumbar punctures were done and the CSF analyzed for amino acids including GABA. Although the patient experienced a 50% seizure reduction, he underwent a left sided temporal lobectomy, and the tissue sample was also analyzed for amino acid content. It was found that vigabatrin caused a three-fold increase in total and free GABA in both the tissue sample and CSF. There were no other significant changes in the other amino acids analyzed. Seizure reduction seen initially was maintained over the long-term observation period. The finding of a specific increase of GABA in brain tissue and CSF of this patient treated with vigabatrin provides additional support to the concept that the primary effect of vigabatrin is as a selective enzyme activated irreversible inhibitor of GABA transaminase.

The effect of sodium valproate on extracellular GABA and other amino acids in the rat ventral hippocampus: an in vivo microdialysis study

We report the effects of i.p. administration of sodium valproate (VPA) on extracellular concentrations of various amino acids in the rat ventral hippocampus studied using in vivo microdialysis, followed by HPLC with fluorometric detection. At the doses used (100, 200 and 400 mg/kg), VPA had no effect on extracellular aspartate, glutamine and taurine, whilst inducing a small, but not statistically significant increase in glutamate at 200 and 400 mg/kg. In contrast, VPA administration produced a biphasic effect on extracellular GABA levels which was dependent on the dose used. At 100 mg/kg, VPA reduced GABA concentrations by 50% when compared to basal. 200 mg/kg VPA had virtually no effect, whilst 400 mg/kg VPA raised extracellular GABA levels to 200% of basal. The results are discussed in relation to the known pharmacological and anticonvulsant actions of VPA.

Valproate effects on pre-optic GABA release and pituitary LH secretion in the rat

The in vivo effects of the anticonvulsant drug sodium valproate (VPA) on pre-optic gamma-aminobutyric acid (GABA) release and pituitary luteinizing hormone (LH) secretion were studied by perfusing the pre-optic area of unanaesthetized, freely moving ovarectomized rats through push-pull cannulae at a flow-rate of 20 >μl/min with a fraction period of 15 min. Local treatment with 40, 100, 400 and 1600 μg VPA/ml differently affected pre-optic GABA release and pituitary LH secretion concerning mean level, mean pulse amplitude and mean pulse frequency. GABA levels in perfusate were suppressed by local treatment with 40 and 100 μg VPA/ml CSF, respectively, whereas no significant change could be observed at the highest concentration used (1600 μg VPA/ml CSF). Pituitary LH secretion was reduced by pre-optic perfusion with 100 μg VPA/ml CSF with regard to mean plasma level and pulse amplitude but no significant change in pulse frequency could be observed. By raising VPA concentration the effects became more pronounced, and at 1600 μ g VPA/ml CSF there was a marked reduction of LH secretion regarding mean plasma level, amplitude and frequency. In conclusion, the present data put forward our view that the mechanism of action of VPA generates an enhancement of GABAergic transmission different from that involving elevated extracellular GABA.

Effects of vigabatrin (gamma-vinyl GABA) on neurotransmission-related amino acids and on GABA and benzodiazepine receptor binding in rats

The effect of 12-day intraperitoneal i.p. administration of vigabatrin (GVG, gamma-vinyl GABA) to rats on the neurotransmission-related amino acids in various brain regions (cortex, hippocampus, cerebellum, and spinal cord), cisternal fluid (CSF) and blood was studied. Results showed that GVG administration increased the levels of GABA in cortical and subcortical regions of the brain and CSF without affecting GABA and benzodiazepine receptors in the cortex. In addition, a dose-dependent decrease was noted in the concentration of glutamate in the hippocampus and in the concentrations of aspartate and glutamine in the cortex, hippocampus, and cerebellum. The changes in the levels of amino acids in the brain, except for that of GABA, were not reflected in the CSF, however, and the levels of amino acids in discrete brain regions did not show any correlation with those in the serum or in the CSF. The results suggest that GVG administration might suppress development and spread of seizures not only by elevating the level of the inhibitory amino acid GABA, but also by decreasing the levels of excitatory amino acids in the brain.

Administration of vigabatrin (gamma-vinyl-gamma-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.

Reversal by apomorphine of the gabaculine-induced GABA accumulation in mouse cortex

To test the assumption that in the mice cortex the rate of accumulation of gamma-aminobutyric acid (GABA) after irreversible inhibition of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19; GABA-T) represents an index of GABA turnover, we examined whether the reversal of the gabaculine-induced accumulation of GABA elicited by apomorphine was due to a decrease in GABA turnover or to a modulation of the activity of the GABA-T inhibitor. Therefore, we simultaneously measured the action of apomorphine on gabaculine-induced accumulation of GABA and on GABA-T activity. In vitro, apomorphine (3 and 30 microM) did not alter the concentration-dependent inhibition of GABA-T by gabaculine. Ex vivo, apomorphine (2 x 0.5 mg/kg s.c.) markedly decreased (69%) gabaculine-induced (150 mg/kg i.p.) accumulation of GABA. This drug had no direct effect on GABA-T activity, but significantly reduced from 83 to 71% the inhibition of GABA-T by gabaculine. The linear correlation found between GABA levels and GABA-T activity allowed the quantification of the decrease in GABA turnover elicited by apomorphine. The results showed that apomorphine decreased significantly (P less than 0.001) the rate of GABA synthesis from 7.48 to 3.36 micromol GABA/g per h, if the partial reversal of gabaculine-induced inhibition of GABA-T is considered and 2.44 micromol/g per h if not. Apomorphine effect on GABA accumulation is mainly due to a decrease of the rate of GABA synthesis and to a lesser extent to a reversal of the inhibitory activity of gabaculine. Thus, inhibition of GABA-T by gabaculine is a sensitive and reliable method for the estimation of the rate of synthesis.

Neuronal activity, amino acid concentration and amino acid release in the substantia nigra of the rat after sodium valproate

The effects of sodium valproate on extracellularly recorded spontaneous neuronal activity and striatal-evoked inhibition in the substantia nigra zona reticulata of the rat were compared with its effects on the tissue concentration of endogenous amino acids and their spontaneous release into perfusates of this region obtained with a push-pull cannula. Valproate (200 mg/kg i.p.) produced a rapid and sustained reduction in the firing rate of all reticulata neurones tested and a concomitant increase in the duration of striatal-evoked inhibition. No change in the spontaneous release of any amino acid was observed. A significant elevation of nigral gamma-aminobutyric acid concentration was seen in both anaesthetized and non-anaesthetized animals, but this occurred only after 60 minutes. Valproate produced a rapid decline in nigral aspartate in non-anaesthetized but not in anaesthetized animals. The results of this study suggest that the acute depressant effect of valproate is unrelated to its ability to alter the concentration of GABA or aspartate in brain and is most likely due to a postsynaptic action.

Valproate enhances GABA turnover in the substantia nigra

The effects of the antiepileptic drug, valproate (VPA), on regional turnover of gamma-aminobutyric acid (GABA) in the rat brain were studied by determining the rate of GABA accumulation following complete inhibition of GABA degradation by aminooxyacetic acid (AOAA). VPA was administered at a dose of 200 mg/kg 5, 15 and 30 min prior to injection of AOAA, 100 mg/kg. In most of the 12 regions examined, VPA did not alter the AOAA-induced GABA accumulation. However, significant increases in GABA accumulation were found in corpus striatum and, more marked, in substantia nigra. Since the substantia nigra has been identified as a substrate for the anticonvulsant action of GABAergic drugs, the data may indicate that the effect of VPA on GABA synthesis rate in this region may be involved in its mechanism of anticonvulsant action.

Blockade of FG 7142 kindling by anticonvulsants acting at sites distant from the benzodiazepine receptor

Repeated administration of the beta-carboline FG 7142 results in sensitisation to its convulsant effects (chemical kindling); acutely FG 7142 is not convulsant, but following several treatments full seizures develop. It has been suggested that the increased sensitivity results from changes in benzodiazepine (BZ)/GABA receptor function. The present experiments studied the ability of BZ receptor ligands and anticonvulsant drugs with diverse mechanisms of action to block the expression and development of kindling to once daily injection of FG 7142 (40 mg/kg, i.p.) in mice. In fully kindled mice, the BZ receptor agonists clonazepam, ZK 93,423 and CL 218,872, and the antagonists flumazenil and ZK 93,426 prevented FG 7142 convulsions, as did 2 anticonvulsants, sodium valproate, possibly acting by influencing GABAergic transmission, and ethosuximide. A further two substances, MK 801 and 2-chloradenosine which act respectively via glutamatergic and purinergic mechanisms were also effective. When administered concomitantly with repeated FG 7142, all of these substances prevented or strongly reduced the development of kindling. Phenytoin and carbamazepine were ineffective in protecting against FG 7142 convulsions in kindled mice, and in preventing the development of kindling when administered repeatedly together with FG 7142. Since MK 801 and 2-chloradenosine prevented kindling, these results suggest that an interaction of FG 7142 with BZ receptors is not sufficient to induce kindling, which may instead result from secondary changes in sites distant from BZ/GABA receptors.

Effects of pharmacological manipulations on basal and newly synthesized levels of GABA, glutamate, aspartate and glutamine in mouse brain cortex

Concentrations of basal and newly synthesized inhibitory (gamma-aminobutyric acid, GABA) and excitatory (glutamate and aspartate) neurotransmitter amino acids and glutamine were determined in mouse brain cortex. Isotopic enrichment following an intravenous infusion of a stable-labeled precursor, [13C6]D-glucose, was used to estimate the newly synthesized amino acid content. Effects of various pharmacological agents (valproate, aminooxyacetic acid, 3-mercaptopropionic acid, N-methyl-D-aspartate, and 2-amino-7-phosphonohepatanoic acid) were evaluated. The effects of 3-mercaptopropionic acid (an inhibitor of glutamate decarboxylase, a GABA-synthesizing enzyme) were restricted to the GABAergic system. On the other hand, N-methyl-D-aspartate (an agonist of a glutamate receptor subtype) was selective for the glutamate-glutamine system, and its effects were prevented by its selective antagonist, 2-amino-7-phosphonoheptanoic acid. In some cases, divergent effects were observed on basal and new amino acids. This suggested that basal and new amino acids may represent different compartments. The anticonvulsant drug valproate caused an increase in basal but a decrease in newly synthesized GABA. Aminooxyacetic acid caused a dramatic increase in basal GABA without affecting the newly synthesized GABA. This approach may be useful in studying compartmentation and fluxes of neurotransmitters.

Gamma-vinyl GABA: comparison of neurochemical and anticonvulsant effects in mice

Biochemical and pharmacological effects of gamma-vinyl GABA (Vigabatrin, GVG), and irreversible enzyme-activated inhibitor of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19; GABA-T), were measured in mice. This anticonvulsant produced a time- and dose-dependent elevation of the GABA, phenylalanine and lysine contents of cortical tissue and simultaneously decreased glutamate, aspartate and alanine levels. In addition, GVG caused a biphasic change in glutamine concentrations (a decline 1-4 hours after administration, followed 20 hours later by an increase). Moreover, we found a new, as yet unidentified amino acid in the brain eluting with the same retention time as alpha-aminoadipic acid from an HPLC cation-exchange column. The level of this novel chemical entity was greatly increased by GVG 20 hours after injection of the drug. At all tested intervals between 1 and 60 hours after injection, GVG was ineffective against maximal electroshock. The GABA-T inhibitor dose-dependently protected mice against isoniazid-induced seizures, simultaneously causing an increase in brain GABA concentrations. However, this apparent correlation applied only until 4 hours after treatment. To better define the anticonvulsant profile of GVG, groups of mice were treated, 1, 2, 4, and 24 hours prior to challenge with convulsant doses of strychnine, pentetrazole (PTZ), and picrotoxin, and brain amino acid levels, including brain concentrations of GVG, were measured. In all instances, the time dependency of the anticonvulsant effects of GVG and of increases in brain GABA levels differed. Amino acid concentrations in animals treated only with GVG were similar to those in animals given GVG and a chemical convulsant. GVG showed no selectivity for seizures produced by impairment of GABA-ergic neurotransmission. Although GVG is an effective GABA-T inhibitor, it apparently affects several other pyridoxal-phosphate-dependent cerebral enzymes and/or interacts with other neurotransmitter systems as well.

Anticonvulsant drug action and regional neurotransmitter amino acid changes

The role played by the inhibitory transmitters, GABA, glycine and taurine, and by excitatory (aspartate/glutamate) antagonists in mediating anticonvulsant action will be documented. This study provides examples of one anticonvulsant compound that affects glycine metabolism (milacemide), and another that affects aspartate metabolism (beta-methylene-aspartate). Beta-Methylene-aspartate, a selective inhibitor of glutamate-aspartate transaminase activity, protects against sound-induced seizures in audiogenic DBA/2 mice, with an ED50 value of 1.9 mumoles (icv; clonic phase). Forebrain and cerebellar aspartate, glutamate and GABA levels are reduced by 15-30% following the administration of beta-methylene-aspartate. Milacemide, a glycinamide derivative with experimental and clinical anticonvulsant activity, is ineffective against sound-induced seizures in DBA/2 mice. Following the ip administration of milacemide (100 mg/kg; 3 hours) there were significant increases in rat brain glycine levels in the cerebellum (+137%), cortex (+45%) and hippocampus (+59%).

Experimental studies and controlled clinical testing of valproate and vigabatrin

Gamma-aminobutyric acid (GABA) is the most important inhibitory transmitter, quantitatively, in the CNS. Evidence exists that decreased GABAergic neurotransmission may play a role in some forms of epilepsy. Consequently, manipulating the GABA system may be a therapeutic possibility in the treatment of this disease. Inhibition of the major GABA degrading enzyme, GABA-transaminase (GABA-T), seems to be the most promising approach. Currently, 2 antiepileptic drugs, valproate (VPA) and vigabatrin, gamma-vinyl GABA (GVG), are available, which are supposed to inhibit the degradation of GABA. Both drugs cause an increase in the total concentration of GABA in the brain, but to a different extent. VPA produces a moderate elevation, which seems to be the result of a marked increase in the transmitter-related GABA pool, while the pronounced elevation in GABA concentration observed during treatment with GVG seems to be caused mainly by an increase in the non-transmitter-related (glial) GABA pool. In order to investigate this apparently differential influence of VPA and GVG on the GABA system, a number of studies were undertaken in selectively cultured astrocytes and neurons from mice. For both drugs neuronal GABA-T proved far more sensitive with regard to inhibition than glial GABA-T. In order to obtain a more direct measure of a potential GABAergic mechanism of action of VPA and GVG, synaptic release of endogenous GABA was determined after culturing neurons in the presence of clinically relevant concentrations of the drugs. GVG caused a significant increase in GABA release, even at concentrations as low as 25 microM. For VPA only the highest of the investigated concentrations (300 microM) augmented GABA release. It is concluded that the antiepileptic effect of GVG seems to be caused by a direct GABAergic mechanism of action. For VPA an influence on the GABA system may play a role in the antiepileptic effect of the drug. However, the lack of definite data on human brain levels of VPA after chronic treatment, combined with evidence that VPA exhibits a number of other effects that may be relevant for its antiepileptic properties, makes the interpretation of a GABAergic mechanism of action difficult. Controlled clinical trials have been increasingly applied within all areas of medicine. In 1982 a survey of the literature identified 29 studies of antiepileptic drugs, where the design involved randomization, the double-blind principle and a statistical analysis of the results.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of transmitter gamma-aminobutyric acid (GABA) synthesis and metabolism illustrated by the effect of gamma-vinyl GABA and hypoglycemia

The effect of different treatments on amino acid levels in neostriatum was studied to throw some light on the synthesis and metabolism of gamma-aminobutyric acid (GABA). Irreversible inhibition of GABA transaminase by microinjection of gamma-vinyl GABA (GVG) led to a decrease in aspartate, glutamate, and glutamine levels and an increase in the GABA level, such that the nitrogen pool remained constant. The results indicate that a large part of brain glutamine is derived from GABA. Hypoglycemia led to an increase in the aspartate level and a decrease in glutamate, glutamine, and GABA levels. The total amino acid pool was decreased compared with amino acid levels in normoglycemic rats. GVG treatment of hypoglycemic rats led to a decrease in the aspartate level and a further reduction in glutamate and glutamine levels. In this case, GABA accumulation continued, although the glutamine pool was almost depleted. The GABA level increased postmortem, but there were no detectable changes in levels of the other amino acids. Pretreatment of the rats with hypoglycemia reduced both glutamate and glutamine levels with a subsequent decreased postmortem GABA accumulation. The half-maximal GABA synthesis rate was obtained when the glutamate level was reduced by 50% and the glutamine level was reduced by 80%.

Common anticonvulsants inhibit Ca2+ uptake and amino acid neurotransmitter release in vitro

Phenytoin (PHT), phenobarbital (PB), and carbamazepine (CBZ) dose-dependently inhibited veratrine-stimulated calcium influx and evoked amino acid neurotransmitter release in rat cortical slices at relatively low concentrations. The action on Ca2+ influx was in the clinical effective dose range for these anticonvulsant drugs, whereas the action on amino acid release was mostly well above this range. Neither ethosuximide (ESM) nor sodium valproate (VPA) had any effect on the veratrine-stimulated Ca2+ influx. Stimulated amino acid release was not affected by ESM, whereas VPA specifically inhibited the release of aspartate in preference to glutamate and GABA at concentrations well within the clinically effective dose range. The actions of VPA and ESM on other parameters measured were detectable only at very high concentrations, which are likely to be irrelevant in defining their clinical mode of action.

Regional amino acid concentration in the brains of rats exposed to high pressures

Regional amino acid concentrations were measured in rat brain fixed by microwave irradiation at three levels of elevated atmospheric pressure corresponding to different phases of the high-pressure neurological syndrome [20 atmospheres absolute (ATA), no clinical signs; 60 ATA, tremor; 85 ATA, severe tremor and myoclonic jerks]. No changes in amino acid content occurred at 20 or 60 ATA. At 85 ATA glutamine content increased in hippocampus, striatum, cerebellum, and substantia nigra, and gamma-aminobutyric acid content increased in hippocampus. It is suggested that enhanced glutamate release in various subcortical structures contributes to the myoclonic activity observed at 85 ATA.

Gamma-aminobutyric acid and the high pressure neurological syndrome

Sodium valproate, nipecotic acid, diaminobutyric acid (DABA) and beta-alanine are drugs which enhance transmission mediated by gamma-aminobutyric acid (GABA) by a variety of mechanisms. They were used to study the role of GABA in the high pressure neurological syndrome (HPNS) in the rat. Sodium valproate, nipecotic acid and DABA reduced the increase in slow waves seen in the electroencephalogram (EEG) of control rats at pressures above 10-20 ATA; however, only sodium valproate had a beneficial effect on the behavioural signs of the high pressure neurological syndrome (tremor, myoclonus and convulsions). Sodium valproate is also thought to decrease neurotransmission produced by excitatory amino acids; thus, these results suggest that GABA is not one of the major neurotransmitters involved in all aspects of the high pressure neurological syndrome and that changes in excitatory neurotransmission may affect the behavioural signs.

Amino acids in rat striatal dialysates: methodological aspects and changes after electroconvulsive shock

Extracellular levels of amino acids were estimated in dialysates of the rat striatum that were collected 1, 2, and/or more than 5 days after surgery, before (resting release) and during exposure to high K concentrations (50 mM) or electroconvulsive shocks. The resting release of several amino acids (Glu, Asn, Thr, Tau, Tyr, Gly, and Ala) was higher 9 days as compared to 1 day after surgery. In the 1-day preparation the resting release correlated highly with that observed with push-pull cannulas. The correlation with the tissue content of the amino acids was high only when they were divided into two groups (putative transmitters and metabolic intermediates). High K exposure produced increased output of Ala, ethanolamine (Eam), Asp, Glu, Tau, and Gly and a decrease in the egress of Gln 1 or 2 days after surgery. The effects on Asp and Glu had disappeared, and that on Gln reversed after 4-9 days. Electrically induced convulsions produced increased output of Ala, Gln, and Eam 1 or 2 days and 2 weeks after implantation of the probe. Changes were seen not only during but also (and some cases even more prominent) after the seizure. This study shows the usefulness of dialysis to monitor extracellular transmitter amino acids in the striatum of conscious rats (also bilateral dialysis was possible) for only a limited time after implantation of the probe. The dialysis method is suitable for longer time, when metabolic changes in amino acids are to be followed. In addition to transmitter release, glycolysis can be monitored by the measurement of Ala in the dialysate.