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

Long-term vigabatrin treatment modifies pentylenetetrazole-induced seizures in mice: focused on GABA brain concentration

BACKGROUND

The goal of our study was to examine the long-term effect of vigabatrin (VGB), a γ-aminobutyric acid aminotransferase (GABA-AT) inhibitor on clonazepam (CLO), ethosuximide (ETX) and valproate (VPA) anticonvulsive activity against pentylenetetrazole (PTZ)-induced seizures in mice.

METHODS

VGB was administered for 3 and 7 days. Convulsions were evoked by PTZ at its CD97 (99 mg/kg). The influence of CLO, ETX and VPA alone or in combination with VGB on motor performance and long-term memory was analyzed. γ-aminobutyric acid (GABA) concentration in mice brain and plasma as well as glutamate decarboxylase (GAD) activity was measured.

RESULTS

After 3 days of treatment, VGB in doses up to 500 mg/kg increased PTZ-induced seizure threshold, whereas after 7 days VGB (at the dose of 125 mg/kg) inhibited clonic seizures in experimental mice. 7 days of VGB administration did not change the protective effect of CLO, ETX and VPA against PTZ-induced seizures. 7 days of VGB treatment at a subthreshold dose of 75 mg/kg decreased TD50 of ETX and CLO in the chimney test, but did not affect TD50 value for VPA. 7 days of VGB administration in combination with AEDs did not affect long-term memory in mice. VGB after 3 days or 7 days of administration increased brain GABA concentration. GAD activity was decreased after 3 and 7 days of VGB administration.

CONCLUSIONS

The presented results confirm anticonvulsive activity of VGB through GABA metabolism alteration and suggest care when combining VGB with ETX or CLO in the therapy.

Effects of L-theanine on anxiety-like behavior, cerebrospinal fluid amino acid profile, and hippocampal activity in Wistar Kyoto rats

RATIONALE AND OBJECTIVES

The amino acid L-theanine (N-ethyl-L-glutamine) has historically been considered a relaxing agent. In the present study, we examined the effects of repeated L-theanine administration on behavior, levels of amino acids in the cerebrospinal fluid (CSF), and hippocampal activity in Wistar Kyoto (WKY) rats, an animal model of anxiety and depressive disorders.

METHODS

Behavioral tests were performed after 7-10 days of L-theanine (0.4 mg kg^-1 day^-1) or saline administration, followed by CSF sampling for high-performance liquid chromatography (HPLC) analysis. An independent set of animals was subjected to [¹⁸F]fluorodeoxyglucose positron emission tomography (PET) scanning after the same dose of L-theanine or saline administration for 7 days.

RESULTS

In the elevated plus maze test, the time spent in the open arms was significantly longer in the L-theanine group than in the saline group (P = 0.035). In addition, significantly lower CSF glutamate (P = 0.039) and higher methionine (P = 0.024) concentrations were observed in the L-theanine group than in the saline group. A significant increase in the standard uptake value ratio was observed in the hippocampus/cerebellum of the L-theanine group (P < 0.001).

CONCLUSIONS

These results suggest that L-theanine enhances hippocampal activity and exerts anxiolytic effects, which may be mediated by changes in glutamate and methionine levels in the brain. Further study is required to more fully elucidate the mechanisms underlying the effects of L-theanine.

Neuropharmacological and neurobiological relevance of in vivo ¹H-MRS of GABA and glutamate for preclinical drug discovery in mental disorders

Proton magnetic resonance spectroscopy ((1)H-magnetic resonance spectroscopy (MRS)) is a translational modality with great appeal for neuroscience since the two major excitatory and inhibitory neurotransmitters, glutamate, and GABA, can be noninvasively quantified in vivo and have served to explore disease state and effects of drug treatment. Yet, if (1)H-MRS shall serve for decision making in preclinical pharmaceutical drug discovery, it has to meet stringent requirements. In particular, (1)H-MRS needs to reliably report neurobiologically relevant but rather small changes in neurometabolite levels upon pharmacological interventions and to faithfully appraise target engagement in the associated molecular pathways at pharmacologically relevant doses. Here, we thoroughly addressed these matters with a three-pronged approach. Firstly, we determined the sensitivity and reproducibility of (1)H-MRS in rat at 9.4 Tesla for detecting changes in GABA and glutamate levels in the striatum and the prefrontal cortex, respectively. Secondly, we evaluated the neuropharmacological and neurobiological relevance of the MRS readouts by pharmacological interventions with five well-characterized drugs (vigabatrin, 3-mercaptopropionate, tiagabine, methionine sulfoximine, and riluzole), which target key nodes in GABAergic and glutamatergic neurotransmission. Finally, we corroborated the MRS findings with ex vivo biochemical analyses of drug exposure and neurometabolite concentrations. For all five interventions tested, (1)H-MRS provided distinct drug dose-effect relationships in GABA and glutamate over preclinically relevant dose ranges and changes as low as 6% in glutamate and 12% in GABA were reliably detected from 16 mm(3) volumes-of-interest. Taken together, these findings demonstrate the value and limitation of quantitative (1)H-MRS of glutamate and GABA for preclinical pharmaceutical research in mental disorders.

The protective effect of myo-inositol on hippocamal cell loss and structural alterations in neurons and synapses triggered by kainic acid-induced status epilepticus

It is known that myo-inositol pretreatment attenuates the seizure severity and several biochemical changes provoked by experimentally induced status epilepticus. However, it remains unidentified whether such properties of myo-inositol influence the structure of epileptic brain. In the present light and electron microscopic research we elucidate if pretreatment with myo-inositol has positive effect on hippocampal cell loss, and cell and synapses damage provoked by kainic acid-induced status epilepticus. Adult male Wistar rats were treated with (i) saline, (ii) saline + kainic acid, (iii) myo-inositol + kainic acid. Assessment of cell loss at 2, 14, and 30 days after treatment demonstrate cytoprotective effect of myo-inositol in CA1 and CA3 areas. It was strongly expressed in pyramidal layer of CA1, radial and oriental layers of CA3 and in less degree-in other layers of both fields. Ultrastructural alterations were described in CA1, 14 days after treatment. The structure of neurons, synapses, and porosomes are well preserved in the rats pretreated with myo-inositol in comparing with rats treated with only kainic acid.

Insulin increases central apolipoprotein E levels as revealed by an improved technique for collection of cerebrospinal fluid from rats

Cerebrospinal fluid (CSF) provides an invaluable analytical window to the central nervous system (CNS) because it reflects the dynamically changing complement of CNS constituents. We describe an improved method for sampling CSF in rats that is easy to perform. It has a 96% success rate of CSF collection and consistently yields large volumes (150-200 μl) of CSF. The blood contamination rate is also low (6%) as determined by both visual inspection and the lack of molecular detection of apolipoprotein B, a plasma-derived protein, which is absent in the CNS. This improved method of CSF sampling can have broad applicability in physiological and pharmacological evaluation for diverse CNS targets. We used this technique to provide proof of principle by examining the effect of intraperitoneal insulin on the level of apolipoprotein E (apoE) in the CSF. Insulin (0.5 and 1 U/kg) led to a significant increase of insulin in both plasma and CSF at 2 h after intraperitoneal administration and decreased blood glucose for at least 2h. ApoE concentrations in CSF, but not in plasma, were also significantly increased, and its time-course was inversely correlated with the alterations in blood glucose over 2 h. These results provide a pharmacological validation of the novel CSF sampling and validation procedure for sampling rat CSF.

Gamma-vinyl GABA increases nonvesicular release of GABA and glutamate in the nucleus accumbens in rats via action on anion channels and GABA transporters

RATIONALE

gamma-Amino butyric acid (GABA) is a well-characterized inhibitory neurotransmitter in the central nervous system, which may also stimulate nonvesicular release of other neurotransmitters under certain conditions. We have recently reported that gamma-vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, elevates extracellular GABA but fails to alter dopamine release in the nucleus accumbens (NAc).

OBJECTIVES

Here, we investigated the mechanism(s) by which GVG elevates extracellular GABA levels and whether GVG also alters glutamate release in the NAc.

MATERIALS AND METHODS

In vivo microdialysis was used to simultaneously measure extracellular NAc GABA and glutamate before and after GVG administration in freely moving rats.

RESULTS

Systemic administration of GVG or intra-NAc local perfusion of GVG significantly increased extracellular NAc GABA and glutamate. GVG-enhanced GABA was completely blocked by intra-NAc local perfusion of 5-nitro-2, 3-(phenylpropylamino)-benzoic acid (NPPB), a selective anion channel blocker and partially blocked by SKF89976A, a type 1 GABA transporter inhibitor. GVG-enhanced glutamate was completely blocked by NPPB or SKF89976A. Tetrodotoxin, a voltage-dependent Na(+)-channel blocker, failed to alter GVG-enhanced GABA and glutamate.

CONCLUSIONS

These data suggest that GVG-enhanced extracellular GABA and glutamate are mediated predominantly by the opening of anion channels and partially by the reversal of GABA transporters. Enhanced extracellular glutamate may functionally attenuate the pharmacological action of GABA and prevent enhanced GABA-induced excess inhibition.

Treatment of epilepsy: the GABA-transaminase inhibitor, vigabatrin, induces neuronal plasticity in the mouse retina

Vigabatrin was a major drug in the treatment of epilepsy until the discovery that it was associated with an irreversible constriction of the visual field. Nevertheless, the drug is still prescribed for infantile spasms and refractory epilepsy. Disorganization of the photoreceptor nuclear layer and cone photoreceptor damage have been described in albino rats. To investigate the vigabatrin-elicited retinal toxicity further, we examined the retinal tissue of albino mice treated with two vigabatrin doses. The higher dose did not always cause the photoreceptor layer disorganization after 1 month of treatment. However, it triggered a massive synaptic plasticity in retinal areas showing a normal layering of the retina. This plasticity was shown by the withdrawal of rod but not cone photoreceptor terminals from the outer plexiform layers towards their cell bodies. Furthermore, both rod bipolar cells and horizontal cells exhibited dendritic sprouting into the photoreceptor nuclear layer. Withdrawing rod photoreceptors appeared to form ectopic contacts with growing postsynaptic dendrites. Indeed, contacts between rods and bipolar cells, and between bipolar cells and horizontal cells were observed deep inside the outer nuclear layer. This neuronal plasticity is highly suggestive of an impaired glutamate release by photoreceptors because similar observations have been reported in different genetically modified mice with deficient synaptic transmission. Such a synaptic deficit is consistent with the decrease in glutamate concentration induced by vigabatrin. This description of the neuronal plasticity associated with vigabatrin provides new insights into its retinal toxicity in epileptic patients.

Subchronic treatment with antiepileptic drugs modifies pentylenetetrazol-induced seizures in mice: Its correlation with benzodiazepine receptor binding

Experiments using male CD1 mice were carried out to investigate the effects of subchronic (daily administration for 8 days) pretreatments with drugs enhancing GABAergic transmission (diazepam, 10 mg/kg, ip; gabapentin, 100 mg/kg, po; or vigabatrin, 500 mg/kg, po) on pentylenetetrazol (PTZ)-induced seizures, 24 h after the last injection. Subchronic administration of diazepam reduced latencies to clonus, tonic extension and death induced by PTZ. Subchronic vigabatrin produced enhanced latency to the first clonus but faster occurrence of tonic extension and death induced by PTZ. Subchronic gabapentin did not modify PTZ-induced seizures. Autoradiography experiments revealed reduced benzodiazepine receptor binding in several brain areas after subchronic treatment with diazepam or gabapentin, whereas subchronic vigabatrin did not induce significant receptor changes. The present results indicate differential effects induced by the subchronic administration of diazepam, vigabatrin, and gabapentin on the susceptibility to PTZ-induced seizures, benzodiazepine receptor binding, or both.

The pharmacokinetics of vigabatrin in rat blood and cerebrospinal fluid

PURPOSE

Data on the blood pharmacokinetics of vigabatrin, an antiepileptic drug with a unique and novel mechanism of action, in the rat are sparse. Additionally, little is known of the kinetics of vigabatrin in the central cerebrospinal fluid (CSF) compartment. We therefore investigated the rate of penetration into and the inter-relationship between serum and CSF compartments following systemic administration of vigabatrin in the rat.

METHODS

Sprague-Dawley rats were implanted with a jugular vein catheter and a cisterna magna catheter for blood and CSF sampling, respectively. Vigabatrin was administered by intraperitonial injection at three different doses (250, 500 and 1000mg/kg) and blood and CSF collected at timed intervals up to 8h. Vigabatrin concentrations in sera and CSF were determined by high performance liquid chromatography.

RESULTS

Vigabatrin concentrations in blood and CSF rose linearly and dose-dependently and the time to maximum concentration (Tmax) was 0.4 and 1.0h, respectively. Vigabatrin is not protein bound in serum and its elimination from serum (mean t1/2 values, 1.1-1.4 h) is rapid and dose-independent. The efflux of vigabatrin from CSF was significantly slower than that seen for serum (mean t1/2 values, 2.2-3.3h).

CONCLUSIONS

The kinetics of vigabatrin are linear with rapid entry into CSF. However, although vigabatrin CSF kinetics parallel that seen in serum, CSF vigabatrin concentrations represent only 2% of concentrations seen in serum and do not reflect free drug concentrations in serum.

Tiagabine and vigabatrin reduce the severity of NMDA-induced excitotoxicity in chick retina

The possible neuroprotective effects of two GABAergic drugs, tiagabine (TGB) and vigabatrin (VGB), against N-methyl-D-aspartate (NMDA)-induced excitotoxicity have been investigated in the isolated chick embryo retina model. Retina segments were incubated either with NMDA alone (100 microM) or with NMDA and TGB or VGB (10-1,000 microM, added 5 min before NMDA). Retina damage was assessed after 24 h by measuring lactate dehydrogenase (LDH) activity present in the medium and by histological analysis. Both drugs reduced LDH release in a dose-dependent manner with comparable mean maximal values of 56.6-63.7% achieved at concentration of 1 mM. Histological analysis of retina slices was in line with the biochemical assays and showed partial preservation of drug exposed retina structure with reduced edema especially in the inner plexiform layer. The present data provide pharmacological evidence that both TGB and VGB reduce the severity of NMDA-induced excitotoxic damage. Although an increase in GABAergic transmission might play a role, this in itself is insufficient to explain the neuroprotective effect of the two drugs and the exact mechanism remains to be elucidated.

Effect of maternal administration of vigabatrin during late gestation on fetoplacental amino acid profile in the mouse

The objectives were to determine if vigabatrin in late gestation affects fetal growth and causes alterations in amino acid concentrations in the mouse. A single dose of 450 mg/kg VGB in saline or a proportionate volume of saline was administered intraperitoneally (IP) to Theiler outbred (TO) mice on gestation day 15 and maternal plasma, placentae and fetuses were collected at different time intervals after treatment. VGB attained peak concentration in the maternal plasma and the fetus at 2 h after treatment and in the placenta at 4 h. At 12 h significantly lower concentrations of several amino acid including methionine were found in the placentae and fetuses in the treated group. After 24 h, no difference was seen in the plasma amino acid concentrations but in the placentae and fetuses a significant decrease occurred in some amino acids in the treated group. At 48 and 72 h, a generalized increase in most amino acid levels occurred in the fetus and placenta but not in maternal plasma of the treated group although the fetal and placental weights were significantly reduced. VGB during late gestation causes fetal growth retardation accompanied by an initial disruption of amino acid supply followed by an increase in amino acid concentrations for the next 2 days. This increase did not help restore growth suggesting that early fetal period is particularly vulnerable to VGB-induced intrauterine growth retardation in the mouse.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

Biphasic action of vigabatrin on cortical epileptic after-discharges in rats

The time course of the anticonvulsant effect of vigabatrin against cortically induced epileptic after-discharges (ADs) was studied in freely moving rats with implanted electrodes. Adult rats (n=30) were exposed to five stimulation sessions each consisting of six stimulation series at 20-min intervals. The first session was a control one, then two groups of animals (n=10 each) were given vigabatrin (600 or 1,200 mg/kg i.p.), the control animals received physiological saline. Stimulation sessions were repeated 1, 24, 48, and 96 hours after the injection. Control animals exhibited an increased transition from the spike-and-wave type of AD to the second, "limbic" type and an increased intensity of movements accompanying stimulation. ADs in the second and subsequent sessions were, however, shorter than in the first session. Vigabatrin facilitated the transition to the second type of AD 1 h after administration but suppressed this transition as well as decreased the number of stimulations eliciting ADs 48 h later. AD duration and the severity of clonic seizures accompanying spike-and-wave ADs were influenced similarly. The effects of the lower dose of vigabatrin were more marked than those of the higher dose. The biphasic action of vigabatrin in our model might be due either to uneven changes of GABA concentration in different brain structures or to an additional mechanism of action. Our results in a cortical model of seizure demonstrate that the sequence of pro- and anticonvulsant actions of vigabatrin is not restricted to seizures of limbic origin and might represent a general phenomenon.

Opiate tolerance by heroin self-administration: An fMRI study in rat

Functional MRI (fMRI) was employed to determine whether repeated heroin self-administration (SA) produces tolerance or sensitization in the brain of heroin-SA rats. Twelve rats were evenly divided into saline and heroin (0.06 mg/kg, 4 hr/day) SA groups. There was a progressive increase in drug-SA behavior and daily heroin intake during the 8-9 days of heroin-SA training. Within 24 hr after the last session of daily SA, acute heroin (0.1 mg/kg) administration induced regional blood oxygen level-dependent (BOLD) signals in both groups of rats. The positive BOLD signals appeared mainly in the cortical regions, including the prefrontal cortex, cingulate, and olfactory cortex, while the negative BOLD signals were predominantly located in subcortical regions such as caudate and putamen, nucleus accumbens, thalamus, and hypothalamus. However, the number of activated voxels or BOLD-signal intensity was significantly less in heroin-SA rat in regions of prefrontal cortex, nucleus accumbens, and thalamus, etc., compared to the changes in the saline control rats. Application of gamma-vinyl GABA (100 mg/kg), an irreversible GABA-transaminase inhibitor, failed to block opiate actions in the heroin-SA rats. Together, these data suggest that repeated heroin-SA produces tolerance or desensitization of opiate actions in the rat brain, which may in turn potentiate drug SA behavior and drug intake.

Vigabatrin, but not gabapentin or topiramate, produces concentration-related effects on enzymes and intermediates of the GABA shunt in rat brain and retina

PURPOSE

The antiepileptic drug (AED) vigabatrin (VGB), which exerts its pharmacologic effects on the gamma-aminobutyric acid (GABA) system, causes concentric visual field constriction in >40% of exposed adults. This may be a class effect of all agents with GABA-related mechanisms of action. We compared the concentration-related effects of VGB in rat brain and eye with those of gabapentin (GBP) and topiramate (TPM), both of which have been reported to elevate brain GABA concentrations in humans.

METHODS

Adult male rats (n = 10) were administered 0.9% saline (control), VGB (250, 500, 1,000 mg/kg), GBP (50, 100, 200 mg/kg), or TPM (12.5, 25, 50, 100 mg/kg). At 2 h after dosing, animals were killed, a blood sample obtained, the brain dissected into eight distinct regions, and the retina and vitreous humor isolated from each eye. Samples were analyzed for several GABA-related neurochemical parameters, and serum and tissue drug concentrations determined.

RESULTS

VGB treatment produced a significant (p < 0.05) dose-related increase in GABA concentrations and decrease in GABA-transaminase activity in all tissues investigated. This effect was most pronounced in the retina, where VGB concentrations were 18.5-fold higher than those in brain. In contrast, GBP and TPM were without effect on any of the neurochemical parameters investigated and did not accumulate appreciably in the retina.

CONCLUSIONS

These findings corroborate a previously reported accumulation of VGB in the retina, which may be responsible for the visual field constriction observed clinically. This phenomenon does not appear to extend to other GABAergic drugs, suggesting that these agents might not cause visual field defects.

Effect of vigabatrin on glutamate dehydrogenase in the hippocampus of seizure prone gerbils

Vigabatrin (VGB, gamma-vinyl-gamma-aminobutyric acid (GABA)), an irreversible inhibitor of GABA transaminase, increases regional inhibitory effects by elevating GABA concentration and reducing glutamate synthesis. In the present study, changes in glutamate dehydrogenase (GDH) activity and its immunoreactivity in the seizure prone gerbil hippocampus after treating VGB were investigated to identify the effect of VGB on energy and/or glutamate metabolism via GDH. In the VGB treated group, GDH immunoreactivity and its activity in the hippocampus were significantly decreased, as compared with those of controls. These findings suggest that VGB administration may suppress the development and spread of seizures not only by elevating the level of GABA, but also by affecting the glutamate signaling and energy metabolism in neurons.

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.

Spatial and temporal alterations in the GABA shunt in the gerbil hippocampus following transient ischemia

In the present study, we have identified the alteration in the expressions of GABA shunt-associated enzymes and the GABA transporter in order to determine the relationship between the neuronal damage and GABA metabolism following ischemia. At 30 min post-ischemia, the immunoreactivities of the glutamic acid decarboxylase (GAD) isoforms were markedly elevated in the CA1 region, as compared with the sham operated group. At 3-12 h post-ischemia, their immunoreactivities recovered at the sham level. These patterns were similarly observed up to 12 h following ischemia insult. However, the intensity of GAD67 was markedly increased at 24 h post-ischemic insult. The temporal changes in GABA transporter 1 (GAT-1) expressions were similar to that of GAD67, but not GAD65, expression, at least prior to 12 h after ischemic insults. GAT-1 immunoreactivity was significantly elevated in the CA1 region posterior to 12 h post-ischemia. Both succinic semialdehyde dehydrogenase (SSADH) and succinic semialdehyde reductase (SSAR) immunoreactivities were not altered in GABAergic neurons following ischemia. In contrast, in pyramidal cells, both SSADH and SSAR immunoreactivities showed chronological alterations in the CA1 region. Thus, our findings suggest that the differential alterations of GABA metabolism may be one of the important factors in neuronal damages induced by ischemia.

Vigabatrin directed against kindled seizures following cortical insult: impact on epileptogenesis and somatosensory recovery

The anticonvulsant drug vigabatrin has not been found to be detrimental to the recovery process when administered following focal cortical insult. This finding is in contrast to the negative postinjury consequences of other anticonvulsant drugs (e.g., phenobarbital and diazepam) with more direct activation of the GABA/benzodiazepine receptor complex. Moreover, phenobarbital directed against kindled seizures affects functional recovery more adversely than either the drug or subconvulsive seizures alone. The purpose of the present study was to determine whether vigabatrin (150, 200, and 250 mg/kg) directed against kindled seizures would impact recovery from lesion-induced somatosensory deficits. Vigabatrin was coupled with daily electrical kindling of the amygdala during the first week after a unilateral anteromedial cortex (AMC) lesion. Somatosensory recovery was assessed using bilateral tactile stimulation tests. Animals receiving the highest dose of vigabatrin prior to electrical kindling (250 mg/kg vigabatrin/kindled) remained significantly impaired even after two months of testing relative to vehicle/kindled, kindled/250 mg/kg vigabatrin, which received vigabatrin after electrical kindling, and the 150, 200, and 250 mg/kg vigabatrin/nonkindled groups (p < 0.0001). In contrast, neither vigabatrin (at any of the doses tested) nor subconvulsive kindled seizures impacted the recovery process (p > 0.05) when administered alone (i.e., without the drug + seizure interaction). These data add to the accumulating experimental and clinical evidence suggesting that the neurobehavioral consequences of the interaction between anticonvulsant drugs and subclinical seizures after brain insult are detrimental to functional recovery.

Vigabatrin protects against hippocampal damage but is not antiepileptogenic in the lithium-pilocarpine model of temporal lobe epilepsy

In temporal lobe epilepsy (TLE), the nature of the structures involved in the development of the epileptogenic circuit is still not clearly identified. In the lithium-pilocarpine model, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as well as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). In order to determine whether protection of some brain areas could prevent the epileptogenesis induced by status epilepticus (SE) and to identify the cerebral structures involved in the genesis of TLE, we studied the effects of the chronic exposure to Vigabatrin (gamma-vinyl-GABA, GVG) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE. The animals were subjected to SE and GVG treatment (250 mg/kg) was initiated at 10 min after pilocarpine injection and maintained daily for 45 days. These pilo-GVG rats were compared with rats subjected to SE followed by a daily saline treatment (pilo-saline) and to control rats not subjected to SE (saline-saline). GVG treatment induced a marked, almost total neuroprotection in CA3, an efficient protection in CA1 and a moderate one in the hilus of the dentate gyrus while damage in the entorhinal cortex was slightly worsened by the treatment. All pilo-GVG and pilo-saline rats became epileptic after the same latency. Glutamic acid decarboxylase (GAD67) immunoreactivity was restored in pilo-GVG rats compared with pilo-saline rats in all areas of the hippocampus, while it was increased over control levels in the optical layer of the superior colliculus and the substantia nigra pars reticulata. Thus, the present data indicate that neuroprotection of principal cells in the Ammon's horn of the hippocampus is not sufficient to prevent epileptogenesis, suggesting that the hilus and extra-hippocampal structures, that were not protected in this study, may play a role in the genesis of spontaneous recurrent seizures in this model. Furthermore, the study performed in non-epileptic rats indicates that chronic treatment with a GABAmimetic drug upregulates the expression of the protein GAD67 in specific areas of the brain, independently from the seizures.

Chronic elevation of brain GABA levels beginning two days after status epilepticus does not prevent epileptogenesis in rats

Vigabatrin (VGB) treatment is neuroprotective in various models of status epilepticus (SE) and delays the development of kindling via mechanisms that are assumed to relate to the elevation of GABA levels in the brain. Here, we tested the hypothesis that a chronic elevation of brain GABA levels obtained by VGB treatment prevents the development of spontaneous seizures (i.e. epilepsy) following SE in rats. Self-sustained SE (SSSE) was induced by stimulating the lateral nucleus of the amygdala. Two days later, chronic VGB (75 mg/kg/day) or saline treatment was started via subcutaneous osmotic minipumps. The development of spontaneous seizures was monitored once a week (24 h at a time) using video-EEG recording. Rats were perfused for histology either at the end of the 10-week drug treatment, or later at the end of an 8-week drug-free follow-up period. Before perfusion for histology, spatial learning and memory perform was tested in the Morris water-maze. Spontaneous seizures were observed in 55% (6/11) of the saline-treated and 73% (8/11) of the VGB-treated rats during the 10-week treatment period. Seizure frequency, severity, and duration were similar in VGB-treated rats and controls during and after the drug-treatment period. VGB treatment did not decrease neuronal damage in various temporal lobe regions or mossy fiber sprouting. VGB treatment also did not attenuate spatial learning or memory impairments. These findings indicate that the augmentation of GABAergic neurotransmission by VGB does not prevent the development of epilepsy when treatment is started 2 days after SE.

Increased vigabatrin entry into the brain by polysorbate 80 and sodium caprate

The effects of a non-ionic surfactant, polysorbate 80, and the sodium salt of the saturated fatty acid, sodium caprate (C10), as potential brain absorption enhancers for vigabatrin were studied. Vigabatrin is an enzyme-activated irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase that increases brain and cerebrospinal GABA concentrations in animals and man. Before intravenous administration, a range of concentrations of the surfactants were tested using erythrocyte lysis or the red blood cell lysis test to establish the non-toxic concentration range. Vigabatrin was dissolved in 0.1% polysorbate 80 and 0.1% sodium caprate and administered intravenously in doses of 4 mL kg(-1) to male Wistar rats (230-250 g; n = 3). Rats were killed 2 h after drug and surfactant administration and the brains were immediately removed and homogenized in 0.4 M perchloric acid. Selected ion monitoring electrospray mass spectrometry was used to determine the concentration of vigabatrin and GABA directly from the perchloric acid extract of the rat brain. This method was developed to increase the speed and efficiency of the analysis by removing the need for complex extraction and derivatization procedures while retaining the specificity of the mass spectrometer as a detector. The stability of both vigabatrin and GABA in perchloric acid was established by monitoring their pseudo molecular ions in standard solutions at timed intervals over 24 h. Although the detection level for vigabatrin and GABA was at least 50 pg, only GABA was detected in rat brain. Vigabatrin caused a small increase in whole brain GABA. However, GABA levels were higher in the samples with vigabatrin + enhancer than in the samples where vigabatrin alone was administered. One-way analysis of variance indicated a significant effect of the surfactants on GABA levels (F (5,17) = 11.86, P < 0.01) and vigabatrin absorption was presumed. The rectal temperature of the rats is lowered by the presence of vigabatrin in the brain. Vigabatrin alone decreased rectal temperature by 6%. When given with either polysorbate 80 or sodium caprate, the extent of temperature lowering was significantly greater (P < 0.001). There was no significant difference after 2 h between polysorbate 80 + vigabatrin, and sodium caprate + vigabatrin.

Elevation of the gamma-aminobutyric acid transaminase expression in the gerbil CA1 area after ischemia-reperfusion damage

gamma-Aminobutyric acid-transaminase (GABA-T) plays an important role in the metabolism of GABA, particularly in the neurons or glial cells. The present study was undertaken to determine the alteration of GABA-T expression in the gerbil hippocampus after ischemia-reperfusion. In the sham, GABA-T(+) neurons were scattered in the hippocampus proper and dentate gyrus. The intensity of the GABA-T immunoreactivity had nearly disappeared in the interneurons at 12 h after ischemia. In contrast, 24 h post-ischemia the dramatic augmentation of GABA-T immunoreactivity in the pyramidal cells was observed in the CA1 area but not in the CA2 or CA3 areas. Forty-eight hours after ischemia-reperfusion, its immunoreactivity was preserved in the CA1 neurons. These results suggest that the over-expression of GABA-T in the CA1 area may be related to delayed neuronal death after ischemia-reperfusion insult.

GABA changes with vigabatrin in the developing human brain

PURPOSE

Changes in gamma-aminobutyric acid (GABA) physiology are important in determining seizure susceptibility in the developing nervous system. Noninvasive measurements of brain GABA in adults with epilepsy have demonstrated important relations among seizure control, brain GABA levels, and changes in brain GABA with drugs designed to alter GABA metabolism. The purpose of this study was to demonstrate the changes in GABA in the occipital lobes of children with epilepsy after treatment with vigabatrin (VGB).

METHODS

Ten proton nuclear magnetic resonance spectroscopic (NMRS) studies were obtained on four subjects with epilepsy. The subjects were between ages 1 and 5 years. Occipital lobe GABA levels were measured before and after treatment with VGB.

RESULTS

Brain GABA levels increased significantly in these subjects after VGB treatment (p < 0.05, paired Student's t test). In one subject, brain GABA was decreased in the region of the epileptic focus compared with the homologous region of the opposite hemisphere. A nearly fivefold increase in GABA occurred in the epileptic region after VGB treatment in this subject.

CONCLUSIONS

VGB increases brain GABA levels in children with epilepsy. NMRS can be used to monitor the response of brain GABA levels to drugs known to alter GABA physiology and serve as an important tool to understand the role of GABA-mediated inhibition in pediatric epilepsies.

Neurochemical actions of gabapentin in mouse brain

Gabapentin (GBP) is a recently licensed antiepileptic, drug whose mode of action remains to be fully elucidated. The following studies were designed to investigate the effects of GBP on several gamma-aminobutyric acid (GABA) related neurochemical parameters in mouse brain. GBP (0-75 mg/kg) was administered by intraperitoneal injection either as a single dose or twice daily for 8 days. Animals were sacrificed 4 h after the final administration and their brains removed and analysed for concentrations of GABA, glutamate and glutamine and the activities of GABA-transaminase (GABA-T) and glutamic acid decarboxylase (GAD). Single dose GBP increased brain GABA-T activity and glutamine concentration but was without effect on GAD activity or the concentrations of GABA and glutamate. Following repeated treatment with GBP, brain GABA-T activity was consistently decreased and there was also a decrease in brain glutamate concentration. Repeated drug treatment was without effect on the activity of GAD or on the concentrations of GABA and glutamine. These results suggest that GBP has effects on the GABAergic system which may contribute to its antiepileptic and/or neuroprotective actions.

Vigabatrin and carbamazepine have different efficacies in the prevention of status epilepticus induced neuronal damage in the hippocampus and amygdala

The present study compares the efficacy of carbamazepine (20 mg/kg/day) and vigabatrin (250 mg/kg/day) in preventing hippocampal and amygdaloid damage in the perforant pathway stimulation model of status epilepticus in the rat. One group of rats received a combination of the drugs. Drug treatments were started one week before the stimulation and continued for two weeks thereafter. Gallyas silver impregnation and somatostatin immunohistochemistry were used to detect neuronal damage. All drug treatments were equally effective in decreasing the number and severity of seizures during electrical stimulation. In the vigabatrin group, the damage to the hilar somatostatin-immunoreactive (SOM-ir) neurons and hippocampal CA3c pyramidal cells was less severe than in the vehicle (SOM-ir, P < 0.01; CA3c, P < 0.05) and carbamazepine (SOM-ir, P < 0.01; CA3c, P < 0.05) groups. In the carbamazepine and combination groups, the severity of neuronal damage in the hippocampus did not differ from that in vehicle-treated animals. The amygdaloid neurons were not protected by any of the treatments. Our results show that even though vigabatrin and carbamazepine treatments had similar anticonvulsant efficacy during the perforant pathway stimulation, only vigabatrin but not carbamazepine decreased seizure-induced neuronal damage. Vigabatrin decreased neuronal damage in the hippocampus but not in the amygdala. These results demonstrate that different brain regions and neuronal networks may be protected unequally by different anticonvulsants.

Tiagabine prevents seizures, neuronal damage and memory impairment in experimental status epilepticus

A novel antiepileptic drug, tiagabine ((R)-N-[4,4-di-(3-methylthien-2-yl) but-3-enyl] nipecotic acid hydrochloride), was studied in rats in order to determine its efficacy in preventing seizures, seizure-induced neuronal damage and impairment of spatial memory in the perforant pathway stimulation model of status epilepticus. In pilot experiments, administration of tiagabine (50, 100 or 200 mg/kg/day) with subcutaneously implanted Alzet osmotic pumps led to a dose-dependent increase in tiagabine concentrations in the serum and brain. Two days of tiagabine treatment at a dose range of 50-200 mg/kg/day did not change the levels of gamma-aminobutyric acid (GABA), glutamate or aspartate in cisternal cerebrospinal fluid (CSF) compared to the controls. In the pentylenetetrazol test, the maximal anticonvulsive effect of tiagabine administered via osmotic pumps was achieved already with a dose of 50 mg/kg/day. In the perforant pathway model of status epilepticus, subchronic treatment with tiagabine (Alzet pumps, 50 mg/kg/day) completely prevented the appearance of generalized clonic seizures during stimulation (P < 0.001). In the same rats, tiagabine treatment reduced the loss of pyramidal cells in the CA3c and CA1 fields of the hippocampus (P < 0.05) but not the loss of somatostatin immunoreactive neurons in the hilus. Two weeks after perforant pathway stimulation, the tiagabine-treated rats performed better in the Morris water-maze test than the vehicle-treated rats did (P < 0.001). Our results show that tiagabine treatment reduces the severity of seizures in the perforant pathway stimulation model of status epilepticus. Possibly associated with the reduction in seizure number and severity, tiagabine treatment also reduced seizure-induced damage to pyramidal cells in the hippocampus as well as the impairment of the spatial memory associated with hippocampal damage.

The neuroprotective effects of gamma-vinyl GABA in transient global ischemia: a morphological study with early and delayed evaluations

Enhancing inhibitory mechanisms has been shown to improve neuronal survival after transient focal or global ischemia. In most studies, histological evaluations have been confined to the CA1 region of the hippocampus up to 7 days after an ischemic insult. We have previously shown that continuous intra-ventricular infusion of gamma-vinyl GABA (GVG) results in significant protection after cerebral ischemia. This present study was designed to assess histological and behavioral function at 7 and 28 days after a single 5 min ischemic episode in gerbils. One set of animals received the medication 30 min before the insult and the other set at 1 h after the insult. Evaluation at 7 days showed significant protection in most regions of the brain in both the pre- and post-ischemic treated animals in comparison to the controls. Delayed evaluation at 28 days showed significant protection only in the pre-ischemic treated animals. Behavioral testing with Morris water maze showed no differences in either pre- or post-ischemic treated animals when compared to saline-treated ischemic controls. Our study clearly demonstrates the usefulness of delayed evaluation in the assessment of 'true' neuronal protection. Pre-ischemic treated animals showed persistent and true neuronal protection, in contrast to a temporary protection as seen at 7 days in the post-ischemic treated animals. The lack of behavioral improvement in the pre- and post-ischemic treated animals suggests that morphological protection alone cannot be considered as the sole criterion for successful outcome.

A new approach for multiple sampling of cisternal cerebrospinal fluid in rodents with minimal trauma and inflammation

A new approach was developed to minimize inevitable damage to nervous and meningeal tissue due to implantation of a sampling tube allowing multiple withdrawal of cerebrospinal fluid (CSF) from the cisterna magna in adult rats. A tube was secured on the atlanto-occipital membrane. Thereafter, a hole was cut through the membrane, allowing flow of CSF from the cisterna magna to the tube. CSF could be sampled repeatedly for at least 1 week. There was no blood-brain barrier damage. The pressure in the cisterna magna remained normal as did the estimated rate of CSF formation. Very few blood cells contaminated the CSF. There was very little evidence of inflammation. The nervous tissue was undamaged as shown by exclusion of a dye-protein complex. The CSF concentrations of the cytosolic neuronal protein neuron-specific enolase (NSE), and of the astrocyte protein S-100 were very low. The pattern of amino acids remained within normal limits. Scanning electron microscopy revealed that clot and reactive changes were restricted to the vicinity of the connecting hole. We conclude that our approach to positioning a tube on the atlanto-occipital membrane and then connecting it to the cisterna magna reproducibly and reliably enables 'atraumatic' multiple sampling of CSF.

Mechanisms of action of new antiepileptic drugs: rational design and serendipitous findings

After years without any major breakthroughs in the treatment of epilepsy disorders, a new wave of antiepileptic drugs have become available to clinicians. Felbamate, gabapentin, lamotrigine and vigabatrin are among the most promising of this new generation of drugs and, when used as add-on therapy, provide some improvement in a significant number of patients suffering from previously refractory epilepsy whilst exhibiting a lower risk of unwanted side-effects than traditional antiepileptic drugs. In this article, Neil Upton reviews the recent discoveries that suggest these four new agents exert their antiepileptic properties by acting through diverse and often novel mechanisms, some of which are by design, and some of which are by chance. Also highlighted are examples of the most innovative mechanistic approaches currently being adopted to produce the next generation of antiepileptic drugs.

Inhibition of GABA transaminase enhances light-induced circadian phase delays but not advances

The CNS neurotransmitter GABA is distributed extensively throughout the suprachiasmatic nucleus, the site of circadian pacemaker cells in mammals. Pharmacological agents that act at GABAA receptors alter specific circadian responses to light and may induce phase shifts of circadian rhythms. In the present study, the role of endogenously released GABA in rhythm regulation was investigated using vigabatrin (gamma-vinyl GABA), an agent that has been shown to increase chronically or acutely the CNS levels of this neurotransmitter by inhibiting GABA transaminase. In Experiment 1, hamsters in constant darkness (DD) received a saline or a vigabatrin injection 1 hr before a 15-min, 700-lux light pulse. Vigabatrin increased photic phase delays but did not affect advances. In Experiment 2, vigabatrin delivered chronically via osmotic minipump treatment did not affect locomotor activity period in DD. However, after 14 days of infusion, photic phase delays (but not advances) were greatly increased in the vigabatrin group. In Experiment 3, in constant light (LL), chronic vigabatrin-treated animals showed an increased period that returned to pretreatment values after the 14-day drug infusion. The results are consistent with the phase-dependent effects of other agents that alter GABA neurotransmission. The data support the general hypothesis that GABA modulates the circadian responses to light in a phase-dependent manner, and may participate in entrainment to light-dark cycles by influencing the relative responsiveness to light in the early and late subjective night.

Gamma-vinyl GABA (vigabatrin) and mood disturbances

We explored factors that may predispose patients to adverse mood effects during treatment with vigabatrin (gamma-vinyl GABA; VGB): mood disorders before VGB treatment, type of epilepsy, seizure type and seizure frequency, type and number of comedication, and VGB dose. The clinical relevance of such a study is that it may help identify circumstances in which VGB should be administered with caution. Seventy-three patients (40 males, 33 females), all with refractory epilepsies, who received VGB as add-on therapy, were assessed by the Amsterdamse Stemmingslyst (ASL), a mood-rating scale, before the start of treatment, and demographic and clinical data were recorded. The patients were followed for 6 months after the start of VGB treatment. Treatment with VGB had to be discontinued in 38 patients (52% of the total sample). Mood problems were the main reason for discontinuation in 9 (12.3% of the total sample). In 6 other patients, mood problems were mentioned as the reason for discontinuing treatment, in combination with lack of drug efficacy. Development of adverse mood effects could not be predicted by a specific mood profile on the ASL. Before treatment, the "mood problems discontinuation group" did not show extreme scores for any assessed areas of mood and no significant differences from other patients were noted on the mood scales. Neither did clinical or demographic data show statistically confirmed specific characteristics for the mood problems discontinuation group, though the patients tended to use more antiepileptic drugs (AEDs) as cotherapy, to have a slightly lower daily dose of VGB, to be slightly older, and were mostly female.(ABSTRACT TRUNCATED AT 250 WORDS)

Limitation by gabapentin of high frequency action potential firing by mouse central neurons in cell culture

The investigational anticonvulsant drug, gabapentin (GP; 1-(aminomethyl) cyclohexaneacetic acid) limited repetitive firing of sodium-dependent action potentials of mouse spinal cord and neocortical neurons in monolayer dissociated cell culture. The effect developed slowly over time with sustained exposure. The IC50 was 1.3 x 10(-4) M for exposure times < or = 60 s, 1.9 x 10(-5) M for 10-60 min, and 4.0 x 10(-6) M for 12-48 h. Hyperpolarization restored sustained firing in the continuing presence of GP. Blockade of action potential firing by GP was frequency (use)-dependent. After preincubation with 2.9 x 10(-5) M GP (5 micrograms/ml), trains of brief stimuli at > or = 50 Hz elicited fewer action potentials than in control solution. Also, at 150 Hz, maximal rate of rise of action potentials decreased progressively with repetitive firing in GP-containing, but not control, solution. After overnight incubation in 2.9 x 10(-5) M GP, the absolute refractory period was prolonged from 2.4 +/- 0.6 ms in control solution (n = 11) to 4.7 +/- 0.3 ms (n = 10; P = 0.02 vs. control), and complete recovery from inactivation was prolonged from 8.0 +/- 1.3 ms to 17.0 +/- 2.6 ms (P < 0.001 vs. control). These findings suggest that GP may alter function of voltage-activated sodium channels, but the mechanism is unproven and may be indirect. Limitation of firing was observed in > or = 50% of neurons at concentrations in the range of those found in plasma and cerebrospinal fluid of patients treated successfully with GP. These results suggest that limiting the rate of firing of sodium-dependent action potentials may contribute to the anticonvulsant efficacy of gabapentin.

Pharmacological characterization of phenytoin-resistant amygdala-kindled rats, a new model of drug-resistant partial epilepsy

By repeated treatment with phenytoin, subgroups of animals with different response to phenytoin were selected from a large group of amygdala-kindled rats. In one subgroup ('phenytoin responders') phenytoin induced reproducible increases in focal seizure threshold, while in another subgroup ('phenytoin nonresponders') animals were resistant to phenytoin. These phenytoin nonresponders, which comprised about 12% of the kindled animals tested, did not differ from phenytoin responders in drug levels, drug adverse effects or location of the stimulation electrode in the amygdala. Treatment of phenytoin responders and nonresponders with other primary antiepileptic drugs showed that valproate and phenobarbital induced much smaller increases in focal seizure threshold in phenytoin nonresponders than in responders, whereas carbamazepine induced about the same threshold increase in both groups. The novel antiepileptic drug vigabatrin, which acts by increasing GABA levels, exerted anticonvulsant effects in phenytoin responders but was inactive in nonresponders. Determination of plasma amino acids before and after vigabatrin treatment demonstrated marked differences in biochemical responses to vigabatrin although drug levels were about the same in both groups. The data demonstrate that amygdala-kindled rats with phenytoin resistance offer a basic approach to the investigation of mechanisms of drug resistance in epilepsy. Furthermore, these animals may be used in the evaluation of new anticonvulsant drugs for treatment of partial seizures which do not respond to the currently available therapies.

Effects of single and repeated administration of vigabatrin on the performance of non-epileptic rats in a delayed non-matching to position task

The present experiments were performed to investigate the effects of single and repeated administration of vigabatrin (gamma-vinyl-GABA), a novel antiepileptic drug, on a working memory task (delayed non-matching to position task) in non-epileptic rats. At doses of 100, 300 and 500 mg/kg single administrations of vigabatrin and 50, 100 and 200 mg/kg repeated administrations, vigabatrin did not affect the choice accuracy in the delayed non-matching to position task employing delays of 0, 1, 2, 4, 8 and 16 s, whereas repeated administration with 300 mg/kg and a single dose of 1000 mg/kg decreased the behavioral activity as compared to saline treatment. Previous studies have shown that at doses of 50-200 mg/kg (daily administration) and 200-1000 mg/kg (single administration) vigabatrin has anticonvulsant activity. The present results suggest that vigabatrin does not markedly impair working memory in the low range of antiepileptic/anticonvulsive doses.

In situ measurements of enzyme activities in the brain

The present review focuses on enzymes involved in the metabolism of amino acid neurotransmitters and the microphotometric determinations of their activities in various layers of the rat hippocampus. The enzymes are NAD-linked isocitrate dehydrogenase (NAD-ICDH), glutamate dehydrogenase (GDH), and GABA transaminase (GABAT), all of which are localized in mitochondria. GDH seems to be restricted to astrocytes, whereas NAD-ICDH and GABAT are localized in neurons as well as in astrocytes. NAD-ICDH is an important enzyme of the tricarboxylic acid cycle and may deliver alpha-ketoglutarate for the formation of glutamate and GABA, which serve as neurotransmitters in the hippocampus. GDH catalyses the interconversion of alpha-ketoglutarate and glutamate, whereas GABAT is the important GABA-degrading enzyme and requires alpha-ketoglutarate for its activity. While differing in their cellular distribution and activity levels, NAD-ICDH, GDH and GABAT are significantly correlated in their hippocampal distribution. Furthermore, developmental and pharmacohistochemical studies suggest that the distribution and activity of astrocytic GDH is correlated with amino-acidergic neurotransmission in the hippocampus. The data reported give further evidence for a metabolic relationship between neurons and astrocytes in the turnover and metabolism of glutamate and GABA.

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.

Determination of GABA and vigabatrin in human plasma by a rapid and simple HPLC method: correlation between clinical response to vigabatrin and increase in plasma GABA

The novel antiepileptic drug vigabatrin (Sabril) acts by inhibiting degradation of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), increasing the GABA concentrations in the brain. Because the GABA degrading enzyme GABA aminotransferase (GABA-T) is also present in peripheral tissues, including blood platelets, measurement of plasma GABA levels might be a useful indication of the pharmacological response to vigabatrin during therapeutic monitoring. However, because of the very low concentrations of GABA in plasma, the few methods available for plasma GABA analysis are time-consuming, difficult to perform and/or not selective enough because of potential interference with other plasma constituents. In the present study, a rapid, selective and sensitive amino acid analysis HPLC method has been developed for plasma GABA determination with fluorescence detection, using o-phthaldialdehyde as a precolumn derivatizing agent. By employing a 3 microns particle size reversed-phase column and a multi-step gradient system of two solvents, the very low endogenous concentration of GABA in human plasma could be reproducibly quantitated without interference of other endogenous compounds. Incubation of human plasma samples with GABA degrading enzyme(s) resulted in an almost total loss of the GABA peak, thus demonstrating the specificity of the method for GABA analysis. In addition to GABA and other endogenous amino acids, the HPLC method could be used to quantitate plasma levels of vigabatrin. Thus, this improved HPLC amino acid assay might be used to examine whether concomitant monitoring of plasma GABA and vigabatrin is useful for clinical purposes. This was examined in 20 epileptic patients undergoing chronic treatment with vigabatrin. The average plasma GABA level of these 20 patients did not differ significantly from non-epileptic controls. However, when epileptic patients were subdivided according to their clinical response to vigabatrin, vigabatrin responders had significantly higher GABA levels than nonresponders or controls. In contrast to the difference in plasma GABA, vigabatrin responders and nonresponders did not differ in dose or plasma level of vigabatrin. These data may indicate that determination of plasma GABA is a valuable non-invasive method for therapeutic monitoring in patients on medication with vigabatrin.

Vigabatrin

OBJECTIVE

To introduce the reader to the use of a new agent, vigabatrin, in the treatment of refractory complex partial seizures. Clinical trials and pharmacokinetic data are reviewed, as well as neuropathology, adverse effects, drug interactions, and dosage guidelines.

DATA SOURCES

A MEDLINE search through March 1992 was used to identify pertinent English-language literature, including clinical trials, reviews, abstracts, and conference proceedings. Indexing terms included vigabatrin and anticonvulsants.

STUDY SELECTIONS

All clinical trials (total of 21) were reviewed, as were all pharmacokinetic studies (total of 8). Selected studies highlighting chemistry, pharmacology, neuropathology, and adverse effects were also reviewed.

DATA EXTRACTION

Performed subjectively by the author. Trials were assessed by design, sample size, types of seizures of the subjects, and clinical response.

DATA SYNTHESIS

Vigabatrin represents the first of a new class of antiepileptic drugs (AEDs)--the gamma-aminobutyric acid transaminase (GABA-T) inhibitors. Vigabatrin works by selective, irreversible inhibition of GABA-T, thus preventing the breakdown of GABA. It has been shown to produce dose-dependent increases in cerebrospinal fluid GABA concentrations, and decreases in GABA-T activity. Vigabatrin may also cause a decrease in excitation-related amino acids. It is well absorbed, is not protein bound, and is eliminated by glomerular filtration. However, even with a short half-life (5-7 h), vigabatrin may be given once or twice daily because of its mechanism of action. Few drug interactions have been reported with this agent, although decreases in phenytoin concentration may reach clinical significance. Concern over neuropathologic findings (microvacuolization of white matter) in animals caused trials of vigabatrin to be halted in 1983, but trials have now resumed as there is no evidence of toxicity in humans. Clinical efficacy of vigabatrin has been evaluated in controlled trials and appears to be most effective in complex partial seizures, producing a 50 percent or greater reduction in seizure frequency in approximately 50 percent of the adult patients studied. Efficacy in children with partial seizures also appears promising, and one uncontrolled study suggests that further study of vigabatrin in infantile spasms may be warranted.

CONCLUSIONS

Vigabatrin appears to be effective in treating refractory complex partial seizures in adults and refractory partial seizures in children. Its relatively benign adverse-effect profile and few known drug interactions may given this agent an advantage over existing anticonvulsants. However, definitive conclusions about the role of vigabatrin in epilepsy treatment should await the completion of ongoing Phase II and Phase III trials.

Biochemical and morphological changes in the rat hippocampus following transection of the fimbria-fornix

According to electrophysiological studies, the subcortically denervated hippocampus has been suggested as a model for limbic epilepsy. We investigated a) whether fimbrial lesioning leads to any biochemical or morphological changes in the rat hippocampus, b) if these changes give any explanation to the previously indicated hyperexcitability, and c) if the changes are in line with the findings in other experimental models and human epilepsy. The fimbria-fornix transection was done by aspiration. Four months later, spontaneous EEG activities were recorded, and the hippocampal formation was processed for histology. In addition, a separate group of lesioned rats was used for hippocampal amino acid analysis. Hyperexcitable functioning of the hippocampus was seen as frequent and rhythmic spiking activity in 25% of the fimbria-fornix-lesioned rats, although the rest of them had spikes occasionally. The amino acids analysis revealed a notable decrease in the concentration of GABA but no significant changes in the amount of excitatory amino acids. This suggests impaired GABAergic functioning but does not exclude possible abnormalities in the release of both excitatory and inhibitory amino acids. The number of somatostatin-immunoreactive (SOM-IR) neurons, a subpopulation of GABAergic neurons, was decreased in all the areas of the hippocampus (CA3 > CA1 > hilus), but this was statistically significant only in the CA3 area. Interestingly, it is the region from which interictal spiking activity in the subcortically denervated rat presumable originates. Immunostaining for synaptophysin showed a dense band of granules in the inner molecular layer of the dentate gyrus, indicating probable synaptic reorganization of associational afferents.

Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by nicotinic blockade

1. The present study investigated whether stimulation of the GABA-ergic system affects spatial navigation (water-maze, WM) deficit induced by nicotinic blockade (mecamylamine). 2. The effects of various doses of gamma-vinyl-GABA (GVG: 50, 150 and 300 mg/kg) and mecamylamine (2.5 and 10 mg/kg) were examined alone and in combination. 3. GVG at the dose 150 mg/kg alone did not impair the performance of rats in the WM task. 4. Mecamylamine at the dose 2.5 and 10 mg/kg clearly impaired the performance of rats in WM task. 5. When the two drugs were co-administered, no interaction between mecamylamine and GVG was observed. 6. Combined nicotinic and muscarinic blockade did not interact as well with GVG administration. 7. Our results do not provide support for any interaction between cholinergic and GABA-ergic mechanisms.

Effects of single and repeated administration of vigabatrin on the performance of rats in a 5-choice serial reaction time task

The present study investigated whether pharmacological stimulation of the GABAergic system can affect attention. The effects of vigabatrin, a novel antiepileptic drug, on the performance of rats in a 5-choice serial reaction time task assessing selective attention were studied. The effects of acute (100, 300, 500 and 1000 mg/kg) and subchronic (50, 100, 200 and 300 mg/kg/day) administration of vigabatrin were investigated. Previous studies have shown that with acute administration of 300-500 mg/kg or subchronic administration of 50-200 mg/kg/day vigabatrin has anticonvulsant activity. At acute doses of 100 and 300 mg/kg or subchronic administration of 50-200 mg/kg/day vigabatrin had no effect on selective attention. At acute dosing of 500 mg/kg, vigabatrin slightly decreased behavioral activity of rats through decreasing the number of trials completed and percent of correct responses. The highest doses used caused an overall behavioral impairment with no marked depletion of any particular function. The results showed that administration of vigabatrin at antiepileptic doses produced no or slight impairment in attentional function. The deficits seen with higher doses were possibly due to a decrease in behavioral activity.

Vigabatrin has an anxiolytic effect in the elevated plus-maze test of anxiety

tau-Vinyl GABA (vigabatrin, GVG) is a novel antiepileptic drug that irreversibly inhibits GABA transaminase and elevates GABA levels in all parts of the brain. In the present study, we investigated the anxiolytic and behavioral effects of GVG in the elevated plus-maze and the hole board compared to diazepam. Doses of 500 and 1,000 mg/kg GVG were injected IP to different groups of male Wistar rats and animals were tested either 4 or 24 h after injection. Animals administered diazepam (1.5 mg/kg, IP) and saline (1 ml) were tested 20 min after injection. GVG and diazepam were found to decrease significantly the number of squares visited and rearing; both had a suppressant effect on locomotor activity. Neither drug had an effect on exploration (head dipping). GVG at a dose of 1,000 mg/kg was shown to have a similar anxiolytic activity either after 4 or 24 h as diazepam, while GVG at 500 mg/kg did not show any significant anxiolytic effect.

Gamma-vinyl GABA prevents hippocampal and substantia nigra reticulata damage in repetitive transient forebrain ischemia

GABAergic inhibitory mechanisms may offer protection to neurons after global ischemia. We tested the effects of gamma-vinyl GABA, a GABA-transaminase inhibitor, via continuous infusion in the third ventricle (Alza pumps) in a gerbil model of repetitive forebrain ischemia. We used two episodes of 3 min duration with a 'reperfusion' interval of 1 h between the insults. Histological analysis was done with silver staining 5 days after the insult. Our results show that there is significant protection of the hippocampus CA1 region and substantia nigra reticulata in treated animals compared to controls. An increase in GABA levels, decrease in glutamate, or mild hypothermia, may be potential mechanisms for this protection. GABAergic agents may prove useful agents in repetitive ischemia.

Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by muscarinic blockade

The present study investigated whether stimulation of the GABAergic system affects spatial navigation [water-maze (WM)] deficit induced by muscarinic blockade (scopolamine). The effects of various doses of gamma-vinyl-GABA (GVG) (50, 150, and 300 mg/kg) and scopolamine (0.4 and 0.1 mg/kg) were examined alone and in combination. GVG at 50 and 150 mg/kg alone did not impair the performance of rats in the WM yask. At 300 mg/kg, GVG caused slight impairment, increasing latency and total distance swim during training trials. Scopolamine at 0.4 mg/kg clearly impaired the performance of rats in the WM task. When the two drugs were coadministered, no interaction between scopolamine and GVG was observed. Our results do not provide support for any interaction between cholinergic muscarinic and GABAergic mechanisms.

Development of tolerance to the anticonvulsant effect of vigabatrin in amygdala-kindled rats

The anticonvulsant, adverse and biochemical effects of the novel antiepileptic drug vigabatrin (gamma-vinyl GABA), which increases GABA (gamma-aminobutyric acid) levels by inhibition of the GABA degrading enzyme GABA aminotransferase, were examined in amygdala-kindled rats after acute and chronic administration. Vigabatrin proved to be a potent anticonvulsant drug at acute doses (100-200 mg/kg), but during chronic administration, the anticonvulsant activity of the treatment was lost already in the second week of treatment. Tolerance also developed to the adverse effects, i.e. hypothermia, sedation and motor impairment. Determination of vigabatrin in plasma indicated that tolerance was not due to declining drug levels. Furthermore, determination of endogenous amino acids in plasma showed that GABA levels were highly elevated throughout the period of treatment, although the extent of GABA accumulation decreased in the second week. After cessation of chronic treatment with vigabatrin, there was no clear indication of withdrawal symptoms, except a prolonged seizure or afterdischarge duration in experiments with 100 mg/kg per day. The data suggest that chronic treatment with vigabatrin may be associated with a loss of anticonvulsant efficacy, at least when the drug is given as monotherapy.

Hippocampal mobility-related theta activity is not diminished by vigabatrin, a GABAmimetic antiepileptic drug, in normal rats

Lesions causing loss of hippocampal theta activity have been shown to result in spatial memory deficits in rats. On the other hand, hippocampal theta activity is thought to be associated only with motor activity, and its role in learning/memory is not clear. Vigabatrin, an inhibitor of GABA-tranasminase, causes elevation of brain GABA levels. Previously, we have found that subchronic administration of vigabatrin did not impair spatial learning/memory in a water maze task. This experiment was carried out to further examine the hippocampal effects of vigabatrin by studying whether vigabatrin at antiepileptic doses affects mobility-related hippocampal EEG. Administration of vigabatrin (100 mg/kg or 500 mg/kg, IP) to nonepileptic rats caused no significant changes in mobility-related rhythmic theta activity, and the relative spectral power of theta frequency had a slight increasing tendency. These results suggest that the vigabatrin-induced enhanced GABAergic inhibition does not disturb normal mobility-related hippocampal theta activity.

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.