Inhibitory and excitatory amino acids in CSF of patients suffering from complex partial seizures during chronic treatment with gamma-vinyl GABA (vigabatrin)

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

Plasma taurine levels are not affected by vigabatrin in pediatric patients

Vigabatrin is a highly effective antiseizure medication, but its use is limited due to concerns about retinal toxicity. One proposed mechanism for this toxicity is vigabatrin-mediated reduction of taurine. Herein we assess plasma taurine levels in a retrospective cohort of children with epilepsy, including a subset receiving vigabatrin. All children who underwent a plasma amino acid analysis as part of their clinical evaluation between 2006 and 2015 at Stanford Children's Health were included in the analysis. There were no significant differences in plasma taurine levels between children taking vigabatrin (n = 16), children taking other anti-seizure medications, and children not taking any anti-seizure medication (n = 556) (analysis of variance [ANOVA] p = 0.841). There were, however, age-dependent decreases in plasma taurine levels. Multiple linear regression revealed no significant association between vigabatrin use and plasma taurine level (p = 0.87) when controlling for age. These results suggest that children taking vigabatrin maintain normal plasma taurine levels, although they leave unanswered whether taurine supplementation is necessary or sufficient to prevent vigabatrin-associated visual field loss. They also indicate that age should be taken into consideration when evaluating taurine levels in young children.

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Monitoring vigabatrin in head injury patients by cerebral microdialysis: obtaining pharmacokinetic measurements in a neurocritical care setting

Aims

The aims were to determine blood–brain barrier penetration and brain extracellular pharmacokinetics for the anticonvulsant vigabatrin (VGB; γ-vinyl-γ-aminobutyric acid) in brain extracellular fluid and plasma from severe traumatic brain injury (TBI) patients, and to measure the response of γ-aminobutyric acid (GABA) concentration in brain extracellular fluid.

Methods

Severe TBI patients (n = 10) received VGB (0.5 g enterally, every 12 h). Each patient had a cerebral microdialysis catheter; two patients had a second catheter in a different region of the brain. Plasma samples were collected 0.5 h before and 2, 4 and 11.5 h after the first VGB dose. Cerebral microdialysis commenced before the first VGB dose and continued through at least three doses of VGB. Controls were seven severe TBI patients with microdialysis, without VGB.

Results

After the first VGB dose, the maximum concentration of VGB (C_max) was 31.7 (26.9–42.6) μmol l⁻¹ (median and interquartile range for eight patients) in plasma and 2.41 (2.03–5.94) μmol l⁻¹ in brain microdialysates (nine patients, 11 catheters), without significant plasma–brain correlation. After three doses, median C_max in microdialysates increased to 5.22 (4.24–7.14) μmol l⁻¹ (eight patients, 10 catheters). Microdialysate VGB concentrations were higher close to focal lesions than in distant sites. Microdialysate GABA concentrations increased modestly in some of the patients after VGB administration.

Conclusions

Vigabatrin, given enterally to severe TBI patients, crosses the blood–brain barrier into the brain extracellular fluid, where it accumulates with multiple dosing. Pharmacokinetics suggest delayed uptake from the blood.

Taurine deficiency is a cause of vigabatrin-induced retinal phototoxicity

OBJECTIVE

Although vigabatrin irreversibly constricts the visual field, it remains a potent therapy for infantile spasms and a third-line drug for refractory epilepsies. In albino animals, this drug induces a reduction in retinal cell function, retinal disorganization, and cone photoreceptor damage. The objective of this study was to investigate the light dependence of the vigabatrin-elicited retinal toxicity and to screen for molecules preventing this secondary effect of vigabatrin.

METHODS

Rats and mice were treated daily with 40 and 3mg vigabatrin, respectively. Retinal cell lesions were demonstrated by assessing cell function with electroretinogram measurements, and quantifying retinal disorganization, gliosis, and cone cell densities.

RESULTS

Vigabatrin-elicited retinal lesions were prevented by maintaining animals in darkness during treatment. Different mechanisms including taurine deficiency were reported to produce such phototoxicity; we therefore measured amino acid plasma levels in vigabatrin-treated animals. Taurine levels were 67% lower in vigabatrin-treated animals than in control animals. Taurine supplementation reduced all components of retinal lesions in both rats and mice. Among six vigabatrin-treated infants, the taurine plasma level was found to be below normal in three patients and undetectable in two patients.

INTERPRETATION

These results indicate that vigabatrin generates a taurine deficiency responsible for its retinal phototoxicity. Future studies will investigate whether cotreatment with taurine and vigabatrin can limit epileptic seizures without inducing the constriction of the visual field. Patients taking vigabatrin could gain immediate benefit from reduced light exposures and dietetic advice on taurine-rich foods.

Brain Homocarnosine and Seizure Control of Patients Taking Gabapentin or Topiramate

PURPOSE

To assess the relation between seizure control and brain homocarnosine and gamma-aminobutyric acid (GABA) levels of patients with complex partial seizures taking gabapentin (GBP) or topiramate (TPM) as adjunctive therapy.

METHODS

In vivo measurements of GABA and homocarnosine were made of a 14-cc volume in the occipital cortex by using (1)H spectroscopy with a 2.1-Tesla magnetic resonance spectrometer and an 8-cm surface coil. Poor seizure control was defined as more recent seizures than the median for the two groups of patients studied.

RESULTS

Homocarnosine levels were higher in patients with better seizure control than in those with poor control. No differences were found in the intracellular GABA levels between the patients who responded to GBP or TPM compared with those who did not.

CONCLUSIONS

In the visual neocortex, which is remote from the presumed seizure-onset zone, higher homocarnosine levels were associated with better seizure control in the patients taking GBP or TPM as adjunctive therapy; elevated intracellular GABA levels appeared to offer no additional protection.

The effects of vigabatrin on electrophysiology and visual fields in epileptics: a controlled study with a discussion of possible mechanisms

PURPOSE

To compare the visual electrophysiology and visual fields of patients taking vigabatrin to those of a control group of epileptics on other anti-epileptic drugs (AEDs).

METHODS

Fourteen epileptics treated with vigabatrin and 10 control patients treated with other AEDs underwent ERG and EOG. Goldmann visual fields were performed and analysed using standard software to measure areas contained within I4e isopters.

RESULTS

The cone and rod b-waves of the ERG, the oscillatory potential amplitudes and Arden indices were reduced in vigabatrin-treated subjects and the oscillatory potentials delayed. The Arden indices were reduced due to an increased dark trough. The areas contained within the I4e isopter of vigabatrin treated subjects were reduced compared to the control group and these areas correlated well with oscillatory potential amplitudes and b-wave amplitudes in the vigabatrin group only.

CONCLUSIONS

The use of vigabatrin is associated with a reduction of the ERG cone b-wave amplitude and oscillatory potentials which correlates with visual field loss. The Arden ratio is reduced in subjects taking vigabatrin but may recover after cessation. However, visual loss may persist in the presence of a recovered EOG. These findings suggest further effects of the drug than those mediated by GABA receptors, and support the contention that the cause of the field loss may be at least in part due to retinal effects. Possible mechanisms are discussed.

Effects of vigabatrin on brain GABA+/CR signals in patients with epilepsy monitored by 1H-NMR-spectroscopy: responder characteristics

PURPOSE

Vigabatrin (VGB) is a new antiepileptic drug that increases the human brain gamma-aminobutyric acid (GABA) level by irreversibly inhibiting GABA transaminase. Although some patients respond to VGB with a significant seizure reduction, others do not. The aim of this study was to identify possible responders before or in an early phase of VGB treatment by measuring the GABA and homocarnosine contaminated with macromolecules/creatine and phosphocreatine ratio (GABA+/Cr) signal by means of proton-nuclear magnetic resonance (1H NMR) spectroscopy.

METHODS

Measurements were performed immediately before and after a titration period of 1 month (2 g/day during the past 2 weeks). A third measurement followed a maintenance period of 3 months (2 or 3 g/day). In 14 patients with drug-resistant temporal lobe epilepsy and 3 patients with occipital lobe epilepsy, GABA+/Cr was measured in the ipsilateral (i.e., epileptogenic) hemisphere and contralateral (i.e., nonepileptogenic) hemisphere in a volume of 8 cm3.

RESULTS

Depending on the therapeutic efficacy of VGB, we defined three groups: (a) full responders (n = 7), (b) nonresponders (n = 7), and (c) partial responders (n = 3). The nonresponders had no significant change in the GABA+/Cr signal during the treatment compared with baseline. The full responders had a significant increase of the GABA+/Cr signal during the whole treatment phase and a lower ipsilateral level at baseline. The partial responders had also a lowered ipsilateral GABA+/Cr signal at baseline and an increase during treatment but a decrease when the seizures started again.

CONCLUSIONS

Responders to VGB could be identified by a lower ipsilateral baseline GABA+/Cr signal and a steeper increase during VGB treatment. However, it was not possible to predict the duration of the response (full versus partial responder) with these criteria.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Vigabatrin: a novel therapy for seizure disorders

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and adverse effects of vigabatrin and its role in the management of seizure disorders.

METHODS

A MEDLINE search of English-language literature from January 1993 through January 1999 was conducted using vigabatrin as a search term to identify pertinent studies and review articles. Additional studies were identified from the bibliographies of reviewed literature. The manufacturer provided postmarketing surveillance data. Priority was given to randomized, double-blind, placebo-controlled studies.

FINDINGS

Vigabatrin is a selective and irreversible inhibitor of gamma-aminobutyric acid transaminase. In controlled clinical trials of vigabatrin add-on therapy in patients with uncontrolled partial seizures, 24-67% of patients achieved a < or =50% reduction in seizure frequency. Data from two comparative trials with carbamazepine monotherapy indicate that vigabatrin monotherapy reduces the frequency of partial seizures in patients with newly diagnosed epilepsy. Vigabatrin also controls infantile spasms, particularly those associated with tuberous sclerosis. Vigabatrin is more effective in patients with partial seizures than in those with generalized seizures. The drug is generally well tolerated. Headache and drowsiness were the most common adverse effects observed in controlled clinical trials; visual field defects, psychiatric reactions, and hyperactivity also have been reported. There are no known clinically significant drug interactions.

CONCLUSIONS

Vigabatrin improves seizure control as add-on therapy for refractory partial seizures and may produce therapeutic benefits in the treatment of infantile spasms. Vigabatrin is generally well tolerated, with a convenient administration schedule, a lack of known significant drug interactions, and no need for routine monitoring of plasma concentrations.

Analysis of CSF amino acids in young patients with generalised refractory epilepsy during an add-on study with lamotrigine

The effect of add-on administration of lamotrigine (1-12 mg/kg per day, 2-12 months) on the levels of neurotransmission related amino acids including gamma-aminobutyric acid (GABA), glutamate, aspartate, glycine and antiepileptic drugs (AEDs) in lumbar cerebrospinal fluid (CSF) was studied in 22 children and young adults with generalised therapy resistant epilepsy. Two lumbar punctures were performed, one prior to, and one following a mean of 5 months (2-12 months) of lamotrigine treatment. Lamotrigine decreased seizure incidence and severity in 12 of the 22 patients without influencing CSF GABA, glutamate, aspartate or glycine levels. Lamotrigine did not alter the concentrations of AEDs in CSF or plasma. However, CSF GABA levels were 86% higher in those patients also treated with gamma-vinyl-GABA (vigabatrin, GVG) compared with patients treated with other combinations and this was not altered by co-medication with lamotrigine. The proposed mechanism of action of lamotrigine, namely that it may inhibit glutamate release in the CNS, is not reflected by changes in CSF glutamate levels. The present findings indicate that CSF GABA, glutamate, aspartate and glycine levels may not be useful as in vivo neurochemical markers in young patients responding to the therapeutic dose of lamotrigine used in this study.

Vigabatrin increases human brain homocarnosine and improves seizure control

Homocarnosine, a dipeptide of gamma-aminobutyric acid (GABA) and histidine, is thought to be an inhibitory neuromodulator synthesized in subclasses of GABAergic neurons. Homocarnosine is present in human brain in greater amounts (0.4-1.0 micromol/g) than in other animals. The antiepileptic drug vigabatrin increases human cerebrospinal fluid homocarnosine linearly with daily dose. By using 1H nuclear magnetic resonance spectroscopy, serial occipital lobe GABA and homocarnosine concentrations were measured in 11 patients started on vigabatrin. Daily low-dose (2 g) vigabatrin increased both homocarnosine and GABA. Larger doses of vigabatrin (4 g) further increased homocarnosine but changed GABA levels minimally. Seizure control improved with increasing homocarnosine and GABA concentrations. Patients whose seizure control improved with the addition of vigabatrin had higher mean homocarnosine, but the same mean GABA concentrations, than those whose seizure control did not improve. Increased homocarnosine may contribute to improved seizure control.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

Cerebrospinal fluid gamma-aminobutyric acid in patients with panic disorder

Cerebrospinal fluid (CSF) gamma-aminobutyric acid (GABA) levels were measured in 11 patients with panic disorder (PD) prior to and following 7 months of treatment with alprazolam or imipramine and in six neurological control patients. Although a clear treatment response was observed in patients with PD, neither alprazolam nor imipramine significantly changed CSF GABA during the treatment period. A negative correlation was demonstrated between baseline CSF GABA and posttreatment overt psychopathology. Low pretreatment level of CSF GABA correlated significantly with poor therapeutic outcome, judged by the amount of anxiety and depression as well as by the frequency of panic attacks at the end of follow-up.

Vigabatrin

The discovery of gamma-aminobutyric acid (GABA) as the first major inhibitory neurotransmitter and a program exploring the use of enzyme inhibition as a therapeutic tool provided the basis for the conception of vigabatrin (VGB, Sabril). This molecule, an analogue of GABA, has a highly specific activity as an enzyme-activated irreversible inhibitor of GABA-transaminase causing several-fold increases in the concentration of brain GABA. In animal models for epilepsy, it was found to have a rather different spectrum of activity than conventional antiepileptic drugs (AEDs). The clinical development of VGB was delayed by the finding of focal areas of reversible microvacuolation in the white matter of the brains of rodents and dogs. An extensive human safety program has confirmed that this finding is species specific and does not occur in humans. Clinically, VGB is well tolerated and has been shown to be specially effective in the management of partial seizures that have failed to respond to other AEDs. In most controlled studies, about 50% of patients with previously uncontrolled seizures have a 50% reduction in frequency and about 4-5% become seizure-free. In children, it also appears to be especially effective in the management of infantile spasms as well as in partial seizures. VGB offers a significant improvement in the management of epilepsy and is now under development as a first-line agent.

Vigabatrin: effects on human brain GABA levels by nuclear magnetic resonance spectroscopy

Vigabatrin (VGB, Sabril) is a new antiepileptic drug used for treatment of partial and secondarily generalized tonic-clonic seizures. Many controlled short- and long-term trials have established efficacy as add-on therapy. Side effects have been infrequent. VGB acts as an irreversible substrate for gamma-aminobutyric acid (GABA) transaminase that leads to elevated brain GABA levels. Although this mechanism has been confirmed in animals and in cerebrospinal fluid of humans, we report the first study of brain GABA levels using noninvasive nuclear magnetic resonance spectroscopy. GABA elevation in brain closely parallels VGB dosage and reaches concentrations 2-3 times control values at daily dosage of 3 g. This technique offers promising potential to monitor changes induced by VGB as a function of time, dose, and clinical effect.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Amino acid levels in the cerebrospinal fluid of newly diagnosed epileptic patients: effect of vigabatrin and carbamazepine monotherapies

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 carbamazepine. 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 gamma-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 gamma-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.

Milacemide effects on the temporal inter-relationship of amino acids and monoamine metabolites in rat cerebrospinal fluid

The temporal inter-relationship of various amino acids and monoamine metabolites in rat cerebrospinal fluid was examined after acute administration of milacemide (100, 200 or 400 mg/kg i.p.), a glycine prodrug. Glycine concentrations rose linearly and dose dependently (20-190%) but were only significantly elevated at the higher milacemide dose (200 and 400 mg/kg). In animals given 400 mg/kg, glycine values were still significantly elevated 8 h later. A concomitant increase (20-25%) in serine and taurine and a decrease in alanine cerebrospinal fluid values were observed at the highest milacemide dose. Other amino acids were unaffected. While cerebrospinal fluid 5-hydroxyindoleacetic acid concentrations were unaffected, the dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, exhibited a linear dose-dependent reduction. However, only homovanillic acid values were significantly decreased after 400 mg/kg milacemide. Cerebrospinal fluid analysis may be useful as a first screen in ascertaining putative neurochemical changes associated with drug administration.

Acute effects of gamma-vinyl GABA on the GABAergic system in rats as studied by microdialysis

The acute effects of the irreversible gamma-aminobutyric acid (GABA) transaminase inhibitor, gamma-vinyl GABA (Vigabatrin), were studied in the central nervous system of the rat. GABA concentrations were monitored in the hippocampus by implantation of microdialysis probes. Two doses of gamma-vinyl GABA (1.6 and 8.0 mM) were administered via the probes and were found to cause a transient increase in the basal GABA outflow (10-fold) during the period of drug administration. In addition, gamma-vinyl GABA pretreatment (1.6 mM) seemed to decrease K(+)-evoked GABA release (P < 0.05). The immediate increase of GABA outflow after gamma-vinyl GABA administration may be the result of direct blockade of GABA uptake sites, a finding which further indicates that the action of GABA transaminase inhibitors may be mediated partly through GABA uptake inhibition.

Gamma-vinyl GABA (vigabatrin) in epilepsy: clinical, neurochemical, and neurophysiologic monitoring in epileptic patients

We report long-term clinical, neurochemical, and electrophysiologic data of gamma-vinyl GABA (GVG, vigabatrin) in three groups of patients. GVG was started as add-on therapy for 75 patients with refractory complex partial seizures (group A) and for 36 mentally handicapped patients with severe epilepsy (group B). The third group (C) consisted of 20 patients with carbamazepine (CBZ) monotherapy, in half of whom GVG monotherapy was substituted. After 3 months, 55% of patients in group A and 42% in group B were responders (reduction in seizure frequency greater than 50%). After 6 (group A) and 3 years (group B) of follow-up, 27 and 33% of the patients, respectively, still had good response to GVG. Neurochemical measurements showed a twofold increase in CSF GABA concentrations and minimal or no changes in other neurotransmitter-related parameters. In group C, substitution of GVG as medication tended to normalize the lengthened latencies in somatosensory evoked potentials (SEPs) observed during CBZ treatment.

Amino acid levels in cerebrospinal fluid of rats after administration of pentylenetetrazol

1. We studied the effect of pentylenetetrazol (PTZ)-induced myoclonic jerks and generalized clonic-tonic convulsions (GC) on the levels of neurotransmitter amino acids in the cisternal CSF of rats. 2. The levels of aspartate, glutamate, glycine, and taurine were elevated in the CSF during myoclonic jerks and more distinctly immediately after GC. 3. During the recovery period of postictal depression seen in EEG (5 min after GC), the CSF levels of transmitter amino acids were lower than in the control group. 4. PTZ-induced irritative activity in the EEG disappeared in 24 hr but the levels of amino acids remained abnormal. 5. Amino acid changes in the CSF following PTZ-induced convulsions might indicate that the release of amino acids into the extracellular space is increased before and during the propagation of PTZ-induced seizure and decreased during postictal depression.

Effect of vigabatrin on the electroencephalogram in rats

Vigabatrin (gamma-vinyl GABA; GVG) is a new antiepileptic drug (AED) that increases the level of the inhibitory transmitter, gamma-aminobutyric acid (GABA) in the brain. We evaluated the effect of GVG on the EEG of normal rats. GVG was administered intraperitoneally (i.p.) at a dose of 100 mg/kg once a day for 12 days. EEG was recorded at baseline, on the fourth day, at the end of the 12-day GVG period and 10 days after discontinuation of GVG. GVG increased the amplitude of delta (1-4 Hz) and theta (4-8 Hz) frequency bands and resulted in slowing of the peak frequency (Fp) and mean frequency (Fm) in both the frontal and occipital cortex, especially during waking-immobility. EEG changes normalized within 10 days after the last GVG injections. The results suggest that a relationship may exist between the EEG changes and increase in GABA levels with GVG.

Enhanced GABAergic inhibition preserves hippocampal structure and function in a model of epilepsy

Extensive electrical stimulation of the perforant pathway input to the hippocampus results in a characteristic pattern of neuronal death, which is accompanied by an impairment of cognitive functions similar to that seen in human temporal lobe epilepsy. The excitotoxic hypothesis of epileptic cell death [Olney, J. W. (1978) in Kainic Acid as a Tool in Neurobiology, eds. McGeer, E., Olney, J. W. & McGeer, P. (Raven, New York), pp. 95-121; Olney, J. W. (1983) in Excitotoxins, eds. Fuxe, K., Roberts, P. J. & Schwartch, R. (Wenner-Gren International Symposium Series, Macmillan, London), Vol. 39, pp. 82-96; and Rothman, S. M. & Olney, J. W. (1986) Ann. Neurol. 19, 105-111] predicts an imbalance between excitation and inhibition, which occurs probably as a result of hyperactivity in afferent pathways or impaired inhibition. In the present study, we investigated whether the enhancement of gamma-aminobutyric acid (GABA)-mediated (GABAergic) inhibition of neurotransmission by blocking the GABA-metabolizing enzyme, GABA transaminase, could influence the histopathological and/or the behavioral outcome in this epilepsy model. We demonstrate that the loss of pyramidal cells and hilar somatostatin-containing neurons can be abolished by enhancing the level of synaptically released GABA, and that the preservation of hippocampal structure is accompanied by a significant sparing of spatial memory as compared with placebo-treated controls. These results suggest that enhanced GABAergic inhibition can effectively block the pathophysiological processes that lead to excitotoxic cell death and, as a result, protect the brain from seizure-induced cognitive impairment.

The effect of subchronic administration of vigabatrin on learning and memory in nonepileptic rats

The present experiments investigated whether subchronic administration of vigabatrin, a GABA-mimetic drug, affects the performance of normal rats in the behavioural tasks assessing learning and memory. The effects of vigabatrin [50-200 mg/kg (IP)/day] administration on the acquisition and retention of water maze and passive avoidance task were studied. According to the results of three experiments, vigabatrin treatment did not markedly impair the acquisition or retention of water maze task. Furthermore, vigabatrin-treated rats were not inferior to saline-treated rats in reversal learning of water maze task. On the other hand, vigabatrin treatment slightly increased the speed of swimming in rats. The administration of vigabatrin did not affect the performance (training latency, number of training trials, testing latency) of rats in the passive avoidance task. According to these results, the effects of vigabatrin, a new antiepileptic drug, on the performance of nonepileptic rats were modest in behavioural tasks used to assess learning and memory.

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.

The effect of gamma-vinyl-GABA on the performance of nucleus basalis-lesioned rats in spatial navigation task

The present study investigates whether the stimulation of gamma-aminobutyric acid (GABA)ergic system affects spatial navigation deficits induced by the lesioning of the nucleus basalis (NB). Thus, the effect of gamma-vinyl-GABA treatment which elevates the GABA levels in brain was studied on water maze task both in unoperated and NB-lesioned (ibotenic acid) rats. The subchronic administration of gamma-vinyl-GABA aggravated dose-dependently NB lesion-induced deficits, although it did not impair the performance of unoperated rats in this task. The imbalance between GABAergic system and cholinergic or non-cholinergic systems of the NB may contribute to spatial navigation deficits in rats.

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.

Effect of antiepileptic drugs on somatostatin release in vitro

In the present study we investigated the effect of antiepileptic drugs on high potassium (50 mM) stimulated somatostatin release in rat cortical slices in a superfusion system. The somatostatin-like immunoreactivity (SLI) in superfusate was determined by radioimmunoassay. The antiepileptic drugs studied, vigabatrin, valproate, carbamazepine, phenobarbital, primidone, clonazepam and phenytoin were tested at a concentration range of 1-1000 microM). Of the drugs used vigabatrin had the most significant inhibitory effect on SLI release (IC50 = 240 microM). Vigabatrin also caused a concomitant, dose-dependent increase in superfusate gamma-amino butyric acid (GABA) level. A 30% decrease in the release of SLI followed incubation with valproate and carbamazepine, but only at high drug concentrations (1000 microM). Phenobarbital, primidone, clonazepam and phenytoin did not affect SLI release. Addition of GABA to superfusate caused a dose-dependent decrease in the amount of SLI release (IC50 = 56 microM). In conclusion, at low concentrations the antiepileptic drugs had only minor effects on SLI release. At higher concentrations, however, vigabatrin and valproate decreased the release of SLI, which may relate to their ability to elevate tissue levels of GABA.

Specificity of vigabatrin for the GABAergic system in human epilepsy

The therapeutic action of vigabatrin (gamma vinyl GABA, GVG) has been reported to be mediated by GABAergic neurotransmission. In the present study, we evaluated different neurotransmitter systems in the cerebrospinal fluid (CSF) of patients with complex partial epilepsy, before and during GVG treatment. The markers of the GABAergic system (free GABA, total GABA, homocarnosine) showed a two- to threefold elevation. There was also an increase in glycine during the 6 months of GVG treatment. In contrast, we did not find any constant CSF changes in either excitatory amino acids or in markers of the cholinergic (acetylcholinesterase), dopaminergic (homovanillic acid), serotonergic (5-hydroxyindoleacetic acid), or peptidergic (somatostatin, prolactin, beta-endorphin) systems. This finding (except an elevation in glycine) was in agreement with previous studies which suggest a specific action of GVG on the GABAergic system. The role of glycine in antiepileptic efficacy of GVG needs further evaluation.