Alterations in neurotransmitter amino acids in hippocampal kindled seizures

A Transient Upregulation of Glutamine Synthetase in the Dentate Gyrus Is Involved in Epileptogenesis Induced by Amygdala Kindling in the Rat

Reduction of glutamine synthetase (GS) function is closely related to established epilepsy, but little is known regarding its role in epileptogenesis. The present study aimed to elucidate the functional changes of GS in the brain and its involvement in epileptogenesis using the amygdala kindling model of epilepsy induced by daily electrical stimulation of basolateral amygdala in rats. Both expression and activity of GS in the ipsilateral dentate gyrus (DG) were upregulated when kindled seizures progressed to stage 4. A single dose of L-methionine sulfoximine (MSO, in 2 µl), a selective GS inhibitor, was administered into the ipsilateral DG on the third day following the first stage 3 seizure (just before GS was upregulated). It was found that low doses of MSO (5 or 10 µg) significantly and dose-dependently reduced the severity of and susceptibility to evoked seizures, whereas MSO at a high dose (20 µg) aggravated kindled seizures. In animals that seizure acquisition had been successfully suppressed with 10 µg MSO, GS upregulation reoccurred when seizures re-progressed to stage 4 and re-administration of 10 µg MSO consistently reduced the seizures. GLN at a dose of 1.5 µg abolished the alleviative effect of 10 µg MSO and deleterious effect of 20 µg MSO on kindled seizures. Moreover, appropriate artificial microRNA interference (1 and 1.5×10(6) TU/2 µl) of GS expression in the ipsilateral DG also inhibited seizure progression. In addition, a transient increase of GS expression and activity in the cortex was also observed during epileptogenesis evoked by pentylenetetrazole kindling. These results strongly suggest that a transient and region-specific upregulation of GS function occurs when epilepsy develops into a certain stage and eventually promotes the process of epileptogenesis. Inhibition of GS to an adequate degree and at an appropriate timing may be a potential therapeutic approach to interrupting epileptogenesis.

Blood-brain barrier dysfunction, TGFβ signaling, and astrocyte dysfunction in epilepsy

Brain insults, including traumatic and ischemic injuries, are frequently followed by acute seizures and delayed development of epilepsy. Dysfunction of the blood-brain barrier (BBB) is a hallmark of brain insults and is usually surrounding the core lesion. Recent studies from several laboratories confirmed that vascular pathology is involved in the development of epilepsy and demonstrate a key role for astroglia in this process. In this review, we focus on glia-related mechanisms linking vascular pathology, and specifically BBB dysfunction, to seizures and epilepsy. We summarize molecular and physiological experimental data demonstrating that the function of astrocytes is altered due to direct exposure to serum albumin, mediated by transforming growth factor beta signaling. We discuss the reported changes and their potential role in the observed hyperexcitability as well as potential implications of these findings for the future development of new diagnostic modalities and treatments to allow a full implementation of the gained knowledge for the benefit of patients with epilepsy.

Posttraumatic epilepsy: hemorrhage, free radicals and the molecular regulation of glutamate

Traumatic brain injury causes development of posttraumatic epilepsy. Bleeding within neuropil is followed by hemolysis and deposition of hemoglobin in neocortex. Iron from hemoglobin and transferring is deposited in brains of patients with posttraumatic epilepsy. Iron compounds form reactive free radical oxidants. Microinjection of ferric ions into rodent brain results in chronic recurrent seizures and liberation of glutamate into the neuropil, as is observed in humans with epilepsy. Termination of synaptic effects of glutamate is by removal via transporter proteins. EAAC-1 is within neurons while GLT-1 and GLAST are confined to glia. Persistent down regulation of GLAST production is present in hippocampal regions in chronic seizure models. Down regulation of GLAST may be fundamental to a sequence of free radical reactions initiated by brain injury with hemorrhage. Administration of antioxidants to animals causes interruption of the sequence of brain injury responses induced by hemorrhage, suggesting that such a strategy needs to be evaluated in patients with traumatic brain injury.

Decreased hippocampal volume on MRI is associated with increased extracellular glutamate in epilepsy patients

PURPOSE

Temporal lobe epilepsy (TLE) is associated with smaller hippocampal volume and with elevated extracellular (EC) glutamate levels. We investigated the relationship between the hippocampal volume and glutamate in refractory TLE patients.

METHODS

We used quantitative MRI volumetrics to measure the hippocampal volume and zero-flow microdialysis to measure the interictal glutamate, glutamine, and GABA levels in the epileptogenic hippocampus of 17 patients with medication-resistant epilepsy undergoing intracranial EEG evaluation. The relationships between hippocampal volume, neurochemical levels, and relevant clinical factors were examined.

RESULTS

Increased EC glutamate in the epileptogenic hippocampus was significantly related to smaller ipsilateral (R(2)= 0.75, p < 0.0001), but not contralateral hippocampal volume when controlled for glutamine and GABA levels, and for clinical factors known to influence hippocampal volume. Glutamate in the atrophic hippocampus was significantly higher (p = 0.008, n = 9), with the threshold for hippocampal atrophy estimated as 5 microM. GABA and glutamine levels in the atrophic and nonatrophic hippocampus were comparable. Decreased hippocampal volume was related to higher seizure frequency (p = 0.008), but not to disease duration or febrile seizure history. None of these clinical factors were related to the neurochemical levels.

CONCLUSIONS

We provide evidence for a significant association between increased EC glutamate and decreased ipsilateral epileptogenic hippocampal volume in TLE. Future work will be needed to determine whether the increase in glutamate has a causal relationship with hippocampal atrophy, or whether another, yet unknown factor results in both. This work has implications for the understanding and treatment of epilepsy as well as other neurodegenerative disorders associated with hippocampal atrophy.

Seizure activity and changes in hippocampal extracellular glutamate, GABA, dopamine and serotonin

Increases in hippocampal extracellular neurotransmitter levels have consistently been observed during temporal lobe seizures in humans, but animal studies on this subject have yielded conflicting results. Our aim was to better characterise the relationship between seizure activity and changes in hippocampal glutamate, GABA, dopamine and serotonin by comparing three limbic seizure models which differ only in the pharmacological mechanism used to induce seizures. Seizures were evoked in freely moving rats by intrahippocampal microperfusion, via a microdialysis probe, of the muscarinic receptor agonist pilocarpine (10mM), GABA(A) receptor antagonist picrotoxin (100microM) or group I metabotropic glutamate receptor agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) (1mM). Seizure-related behavioural changes were scored and hippocampal extracellular glutamate, GABA, dopamine and serotonin concentrations were monitored. Seizures were of comparable severity in all groups. During seizures, hippocampal glutamate, GABA and dopamine concentrations increased in all groups. Glutamate increases were significantly higher in the picrotoxin group. Hippocampal serotonin concentration increased following pilocarpine and picrotoxin, but not DHPG. Our results suggest a direct relationship between seizure activity and increased hippocampal extracellular concentrations of glutamate, GABA and dopamine, but not serotonin. The fact that picrotoxin induces seizures by disinhibition, rather than direct excitation, may account for the larger glutamate increases in this group.

Effect of zonisamide on molecular regulation of glutamate and GABA transporter proteins during epileptogenesis in rats with hippocampal seizures

Epileptiform discharges and behavioral seizures may be the consequences of excess excitation associated with the neurotransmitter glutamate, or from inadequate inhibitory effects associated with gamma-aminobutyric acid (GABA). Synaptic effects of these neurotransmitters are terminated by the action of transporter proteins that remove amino acids from the synaptic cleft. Excitation initiated by the synaptic release of glutamate is attenuated by the action of glial transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1), and the neuronal transporter excitatory amino-acid carrier-1 (EAAC-1). GABA is removed from synaptic regions by the action of the transporters proteins GABA transporter-1 (GAT-1) and GABA transporter-3 (GAT-3). In this experiment, albino rats with chronic, spontaneous recurrent seizures induced by the amygdalar injection of FeCl3 were treated for 14 days with zonisamide (ZNS) (40 mg/kg, i.p.). Control animals underwent saline injection into the same amygdalar regions. Treatment control for both groups of intracerebrally injected animals was i.p. injection of equal volumes of saline. Western blotting was used to measure the quantity of glutamate and GABA transporters in hippocampus and frontal cortex. ZNS caused increase in the quantity of EAAC-1 protein in hippocampus and cortex and down regulation of the GABA transporter GAT-1. These changes occurred in both experimental and ZNS treated control animals. These data show that the molecular effect of ZNS, with up-regulation of EAAC-1 and decreased production of GABA transporters, should result in increased tissue and synaptic concentrations of GABA. Although many antiepileptic drugs have effects on ion channels when measured in vitro our study suggests that additional mechanisms of action may be operant. Molecular effects on regulation of transporter proteins may aid in understanding epileptogenesis and inform investigators about future design and development of drugs to treat epilepsy.

Effects of vigabatrin on brain GABA+/Cr signals in focus-distant and focus-near brain regions monitored by 1H-NMR spectroscopy

The new antiepileptic drug vigabatrin (VGB) increases gamma-aminobutyric acid (GABA) in the brain. We compared GABA+/Cr signals measured focus-near and focus-distant and correlated it with the degree of response to VGB. Brain GABA+/Cr signals were measured in 17 epileptic patients in structurally normal appearing tissue by nuclear proton magnetic resonance (1H-NMR) spectroscopy using a special editing sequence for GABA. In 11 patients the measurements were done in brain areas distant to focus and in six near to focus. Full-responders (seizure reduction of >or=50% at the end of the treatment phase) and partial-responders (seizure reduction of >or=50% at the end of the first month of treatment but <or=50% at end of treatment) had lower GABA+/Cr signals in the hemisphere with the epileptogenic focus and increases of the GABA+/Cr signals with VGB. Non-responders (seizure reduction of <or=50%) had no side difference in the GABA+/Cr signals before treatment and no increase during treatment. These observations were made in structurally normal appearing tissue near to the focus and distant to the focus. A side difference in brain GABA+/Cr signal between the epileptogenic and non-epileptogenic hemisphere before VGB treatment correlates with an improved seizure control under VGB treatment regardless whether the measurement is done focus-near or focus-distant.

Expression of glutamate transporters and ionotropic glutamate receptors in GLAST knockout mice

In order to investigate the molecular mechanism underlying high seizure susceptibility of GLAST knockout mice, we carried out Western blotting for the expression of GLT-1, EAAC-1, and several kinds of glutamate receptors in the hippocampus and the cortex. Although no significant difference was observed between GLAST (+/+) and (-/-) mice in terms of expression of GLT-1 and EAAC-1 in the hippocampus, these proteins were over-expressed in the frontal cortex in GLAST (-/-) mice (GLT-1, about 210% increase; EAAC-1, about 180% increase). Expression of hippocampal Glu-R1 and Glu-R2 in GLAST (-/-) mice was remarkably increased (Glu-R1, about 140% increase; Glu-R2, about 160% increase), while Glu-R3 and NMDA receptors levels (NMDA-R1, 2A and 2B) were equal to those in control. Cortical levels of Glu-R1, -R2 and -R3 receptors in GLAST (-/-) mice were remarkably decreased (Glu-R1, about 60% decrease; Glu-R2, about 60% decrease; Glu-R3, about 70% decrease), while NMDA receptors were remarkably increased in comparison to those in GLAST (+/+) mice (N-R1, about 150% increase; N-R2A, about 150% increase; N-R2B, about 140% increase). These data suggest that the increased susceptibility to seizures in GLAST (-/-) mice might be derived from increased expression of Glu-R1 in the hippocampus coupled with decreased cortical expression of Glu-R2 and increased NMDA-R1 and -2A, -2B expression.

Changes in extracellular levels of amygdala amino acids in genetically fast and slow kindling rat strains

A neurochemical basis for many of the epilepsies has long been suspected to result from an imbalance between excitatory and inhibitory neurotransmitter mechanisms. Data supporting changes in extrasynaptic amino acid levels during epileptogenesis, however, remain controversial. In the present study, we used in vivo microdialysis to measure the levels of extracellular GABA (gamma-aminobutyric acid) and glutamate during seizure development in rats with a genetic predisposition for (Fast), or against (Slow), amygdala kindling. Dialysates were collected from both amygdalae before, during, and up to 12 min after a threshold-triggered amygdala afterdischarge (AD). One hour later, samples were again collected from both amygdalae in response to a hippocampal threshold AD. Daily amygdala kindling commenced the next day but without dialysis. After the rats were fully kindled, the same protocol was again employed. Amino acid levels were not consistently increased above baseline with triggered seizures in either strain. Instead, before kindling, a focal seizure in the Slow rats was associated with a large decrease in GABA in the non-stimulated amygdala, while amino acid levels in the Fast rats remained near baseline in both amygdalae. Similar results were seen after kindling. By contrast, before and after kindling, hippocampal stimulation caused large decreases in all amino acid levels in both amygdalae in both strains. These data suggest that, in response to direct stimulation, extracellular amino acid concentrations remain stable in tissues associated with either greater natural (Fast) or induced (kindled Fast/Slow) excitability, but are lowered with indirect stimulation (hippocampus) and/or low excitability.

Sequential changes in AMPA and NMDA protein levels during Fe(3+)-induced epileptogenesis

Seizure susceptibility is related to enhanced glutamatergic excitatory synaptic transmission with alterations in the expressions of ionotropic glutamate receptors. We wondered if levels of AMPA and NMDA receptor subunits changed following epileptogenesis induced by amygdalar FeCl(3) injection. We used Western blots to measure levels of subunits in the ipsilateral and contralateral hippocampus at various times after FeCl(3) injection into the amygdaloid body. With acute seizures, at +5 days after the injection, levels of GluR1, NMDAR1, and NMDAR2 were markedly increased in both hippocampi, with quantities at least 2-4 times baseline. By +15 and +30 days after injection, when chronic spontaneous seizures were occurring, the levels of GluR2 were increased, while GluR1 and NMDAR1&2A/B were decreased. Increased NMDAR1&2A/B levels at +5 days are consistent with the occurrence of upregulation of NMDA receptor production in the early stages of epileptogenesis. Since GluR2 suppresses glutamate receptor-mediated Ca(2+)-influx, increased expression of GluR2 with development of chronic, recurrent seizures may be a compensatory effect during epileptogenesis from neural responses to propagated seizures.

Kindling phenomena induced by the repeated short-term high potassium stimuli in the ventral hippocampus of rats: on-line monitoring of extracellular glutamate overflow

We observed in this study that transient periodic stimuli in response to high potassium (40 mM, 5 min at 40-min intervals, 13-15 stimuli) perfusion in the ventral hippocampus of rats led to the appearance of a kindling-like phenomenon. In this kindling-like phenomenon, we confirmed the augmentation of glutamate release and the prolongation of spike discharge. Changes in the extracellular glutamate levels before and after the stimuli were monitored by the application of in vivo microdialysis combined with on-line enzyme fluorometric detection of glutamate. This kindling-like phenomenon was not observed when microdialysis was carried out using a Ca++-free medium. The augmentation of glutamate release and the prolongation of spike discharge with epileptic convulsions are completely Ca++ dependent. These data show that repeated short-term increases in extracellular glutamate levels results in the enhancement of excitatory neuronal systems, causing an excessive propagation of seizure activity and culminating in secondary generalized seizures.

Sequential changes in glutamate transporter protein levels during Fe(3+)-induced epileptogenesis

Severe head injury in humans causes recurrent seizures; this form of epilepsy appears to correlate with occurrence of parenchymal hemorrhage. Injection of ferric cations, one component of hemoglobin, into rat amygdala, causes lipid peroxidation, and recurrent spontaneous seizures. We wondered whether regulation of extracellular glutamate might be perturbed as a mechanism of chronic epileptogenesis, therefore levels of glutamate transporter proteins GLT-1, GLAST and EAAC-1 were measured in ipsilateral and contralateral hippocampi removed from rats having spontaneous iron-induced limbic seizures. The neuronal transporter EAAC-1 was elevated bilaterally up to 30 days following the microinjection that initiated seizures. The neuronal transporter EAAC-1 was elevated bilaterally up to 30 days following the microinjection that initiated seizures. The glial transporter GLT-1 increased 5 and 15 days after iron injection on the side contralateral to the injection then returned to basal levels 30 days after the lesion. GLAST also showed an initial increase but at 15 and 30 days after injection, when experimental animals were experiencing spontaneous limbic behavioral seizures, this protein was down-regulated. The results suggest that iron-induced epileptogenesis involves alteration in glial glutamate transport that may lead to enhanced excitation within the hippocampus.

Sequential changes in glutamate transporter mRNA levels during Fe(3+)-induced epileptogenesis

Severe head injury in humans can cause recurrent seizures; this form of epilepsy appears to correlate with the occurrence of parenchymal hemorrhage. The injection of ferric cations, one component of hemoglobin, into rat amygdala, causes lipid peroxidation, and recurrent spontaneous seizures. We wondered whether the regulation of glutamate might be perturbed as a result of severe head injury, which might then act as a mechanism of chronic epileptogenesis. Levels of glutamate transporter glutamate-aspartate transporter (GLAST), glutamate transporter-1 (GLT-1), and excitatory amino-acid carrier (EAAC-1) mRNA were measured in ipsilateral and contralateral hippocampi and cerebral cortex removed from rats at 60 min, 24 h, and 5, 15 and 30 days after FeCl(3) injection into the amygdaloid body. While the neuronal transporter EAAC-1 mRNA was elevated bilaterally for up to 30 days following the microinjection that initiated seizures, GLT-1 mRNA, derived from glial cells, returned to basal levels. At 15 and 30 days after injection, however, when the experimental animals were experiencing spontaneous limbic behavioral seizures, GLAST mRNA was down-regulated. Epileptogenesis may correlate with the impairment of glial glutamate transport, leading to an excitation and imbalance of transmitter influences within the hippocampi and cerebral cortex.

Effect of depth electrode implantation with or without subsequent kindling on GABA turnover in various rat brain regions

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied electrical stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that impairment of GABAergic inhibition may be involved. In the present experiments, GABA turnover was determined in vivo by the GABA aminotransferase (GABA-T) inhibition method in 13 brain regions in three groups of rats: (1) a group which was kindled via electrical stimulation of intra-amygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (2) a group of implanted but non-stimulated rats (sham control group) in which neurochemical measurements were done at the same time after electrode implantation as in the kindled group; and (3) a group of non-implanted, naive control rats. Regional GABA levels were determined after vehicle injection as well as 30 and 90 min after administration of aminooxyacetic acid (AOAA) at a dose which completely inhibits GABA-T. Compared to naive controls, prolonged electrode implantation in the amygdala induced a significant reduction of AOAA-induced GABA accumulation in amygdala, hippocampus, piriform cortex, olfactory bulb, frontal cortex, striatum, hypothalamus, tectum, and cerebellar cortex. In view of the GABA hypothesis of kindling, reduced GABA turnover in response to electrode implantation would suggest that the implantation per se exerts a pro-kindling effect, which was recently demonstrated in rats with intraamygdala electrodes. However, amygdala kindling itself appeared to antagonize the effect of electrode implantation in most regions. Thus, although, compared to naive controls, the predominant change in kindled rats was a decrease in GABA turnover, this decrease was less marked than in sham controls. In thalamus and brainstem kindling markedly increased GABA turnover above the levels determined in both naive and sham controls, possibly in response to impaired postsynaptic GABAergic function. The data indicate that both electrode implantation and kindling significantly alter regional GABA turnover, which might contribute to the pathophysiology of the kindling phenomenon. Furthermore, the data substantiate that the choice of adequate controls is critical in neurochemical and functional studies on the kindling phenomenon.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Decrease in inhibition in dentate granule cells from patients with medial temporal lobe epilepsy

Alterations in synaptic inhibition are associated with epileptiform activity in several acute animal models; however, it is not clear if there are changes in inhibition in chronically epileptic tissue. We have used intracellular recordings from granule cells of patients with temporal lobe epilepsy to determine whether synaptic inhibition is compromised. Two groups of patients with medial temporal lobe epilepsy were used, those with medial temporal lobe sclerosis (MTLE), and those with extrahippocampal masses (MaTLE) where the cell loss and synaptic reorganization that characterize MTLE are not seen. Although the level of tonic inhibition at the somata was not significantly different in the two patient groups, there was a reduction in the conductance of polysynaptic perforant path-evoked fast and slow inhibitory postsynaptic potentials (IPSPs) (53% and 66%, respectively). We found that there was a comparable decrease in the monosynaptic IPSP conductances examined in the presence of glutamatergic antagonists as that seen for the polysynaptically evoked IPSPs. These data suggest that the decrease in inhibition seen in normal artificial cerebrospinal fluid in MTLE granule cells cannot be solely explained by a decrease in excitatory input onto inhibitory interneurons and may reflect changes at the interneuron-granule cells synapse or in the number of specific inhibitory interneurons.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

Decreased GABA release following tonic-clonic seizures is associated with an increase in extracellular glutamate in rat hippocampus in vivo

The effects of maximal electroshock, used as a model of generalized seizures, were studied on extracellular GABA and glutamate levels in the ventral hippocampus of the freely-moving rat, using in vivo microdialysis. Following a maximal electroshock there was a rapid decline in GABA levels (46 +/- 5%) in the 20 min immediately after the seizure and levels remained depressed for a further 60 min. However, although there was a transient small decrease (11 +/- 2%) in glutamate levels in the first 20 min post-ictally, there followed a more prolonged, larger increase in the next 40 min. Maximal electroshock, administered in the absence of extracellular calcium, did not change GABA levels, while glutamate levels were again increased (42 +/- 8%) in the 40-80 min after the shock. Local perfusion with nickel (1 mM) to block T-type calcium channels had no effect on basal GABA or glutamate levels but prevented maximal electroshock-induced changes in both amino acids. Experiments were carried out to test the hypothesis that the post-ictal increased glutamate release was due to the decrease in GABA release. Perfusion with the potent GABA re-uptake inhibitor NNC-711, for 60 min prior to administration of maximal electroshock, increased GABA levels (436 +/- 58%) and abolished the seizure-induced decrease. Basal glutamate levels were not affected by perfusion with NNC-711 but subsequent maximal electroshock also failed to affect levels. Local perfusion with the GABAA receptor antagonist bicuculline (1, 10 and 100 microM) had no effect on basal GABA levels but glutamate levels were increased (46 +/- 5%) after perfusion with 100 microM bicuculline.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of in vivo amino acid neurotransmitters by high-performance liquid chromatography with o-phthalaldehyde-sulphite derivatisation

The measurement of amino acid neurotransmitters by high-performance liquid chromatography (HPLC) has emerged as a reliable and sensitive method. This paper describes a method which employs electrochemical (EC) detection of amino acid derivatives formed by a reaction with o-phthalaldehyde (OPA) in the presence of sulphite ions. This is discussed in relation to the problems of previously reported methods based on OPA derivatisation. Precise separation of the following 7 amino acid standards is achieved using isocratic elution: serine, glycine, taurine, glutamate, arginine, alanine and GABA, in order of increasing retention time. Total elution time is 25 min. Derivatisation proceeds at room temperature and the derivatives are stable for up to 5 h. This technique has the sensitivity to determine the concentrations of amino acid neurotransmitters in cerebrospinal fluid (CSF) and an in vivo microdialysis method is discussed for the detection of basal and potassium-stimulated levels of gamma-aminobutyric acid and glutamate from rat hippocampus.

Simultaneous monitoring of the seizure-related changes in extracellular glutamate and gamma-aminobutyric acid concentration in bilateral hippocampi following development of amygdaloid kindling

We simultaneously monitored the seizure-related changes in extracellular hippocampal glutamate (Glu) and gamma-aminobutyric acid (GABA) concentration in brain dialysates in order to clarify the role of Glu and GABA in the development of kindling. Brain dialysates were collected every 5 min from 10 min prior to 80 min after stimulus in the three developing conditions consisting of pre-kindling state, stage 3 (C-3), and five consecutive stage 5 (5*C-5) following kindling in the same rat. Extracellular Glu level increased rapidly, lasting for only 5 min after stimulus. The post-stimulus ratio of Glu increase in partially kindled rats (C-3) was 2.5-3.5 times of the baseline, and in fully kindled rats it was about 5 times of the baseline. Extracellular GABA concentration enhanced gradually, reaching a plateau level at 15-20 min and lasting for several hours after stimulus at each stage. The enhancement of GABA level was about 1.5 times of the baseline in partially kindled stage, and was about 2.5 times of the baseline in fully kindled stage. There was no significant difference between the two hemispheres with respect to either the time-course or the magnitude of Glu and GABA increase respectively. These data show that progressive, transient and stimulus-induced enhancement of extracellular Glu levels combined with long-lasting elevation of extracellular GABA levels in the bilateral ventral hippocampi results in imbalance between the excitatory and inhibitory neuronal systems, causing excessive propagation of seizure activity, culminating in the secondary generalized seizure of amygdaloid kindling.

Bilateral seizure-related changes of extracellular glutamate concentration in hippocampi during development of amygdaloid kindling

To monitor the seizure-related changes of extracellular hippocampal glutamate (Glu) concentration during the development of amygdaloid kindling, we used brain dialysates and an enzymatic cycling technique for Glu determination with a highly sensitive assay and high time resolution (1 min). The extracellular Glu level was transiently (for 3 min) enhanced after stimulus and returned rapidly to baseline. In partially kindled rats (stage 3), the extracellular Glu level during the first minute post stimulus was 2.5-3.5-fold that of baseline, while fully kindled rats exhibited about a 5-fold increase in Glu level. Amygdaloid kindling is accompanied by a progressive, transient, stimulus-induced enhancement of extracellular Glu levels during the first minute post stimulus in both hippocampi.

A study on gamma-aminobutyric acid (GABA) and its analogues by using molecular orbital methods: on epileptogenicity of new quinolones

Using the molecular orbital methods, we examined molecular structure, electron density distribution, electrostatic potential field and receptor structure of gamma-aminobutyric acid (GABA), and its analogues. The following findings were obtained: a comparison of the biological activity and the morphology electrostatic potentials of GABA analogues disclosed that the active site is the amino group, and the biological activity correlates closely with the electrostatic potential structure around the amino group. The active sites were compared between the receptor-binding molecules and the GABA uptake inhibitory molecules, and the results suggested that the receptor structure differed between the two groups of molecules and that the GABA A receptors had two subtypes. On these results, the epileptogenicity of new quinolones was studied using this method. These results suggested that the new quinolones blockaded the GABA receptor-binding system and that the important active site of the new quinolones for GABA receptor-binding was the the piperazyl amino group. These results suggested that the concentration of zwitterion type of the new quinolones was very important clinically.

Transmitter amino acid levels in rat brain regions after amygdala-kindling or chronic electrode implantation without kindling: evidence for a pro-kindling effect of prolonged electrode implantation

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that alterations in amino acidergic neurotransmission may be involved. In the present experiments, levels of glutamate, aspartate, GABA, glycine, and taurine were determined in 12 brain regions by HPLC in 3 groups of animals: (a) a group which was kindled via electrical stimulation of intraamygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (b) a group of implanted but nonstimulated rats (surgical control group) in which neurochemical measurements were done at the same time after electrode implantation as the kindled group, and (c) a group of non-implanted, naive control rats. Compared to surgical controls, kindling induced a significant reduction of glutamate, GABA, and taurine in the brain stem (pons/medulla), whereas no differences between both groups were found in any of the other regions. However, both electrode-implanted groups differed significantly from non-implanted naive rats in several regions, indicating that electrode-implantation per se induced long-lasting alterations in transmitter amino acids. The most striking difference to naive controls was an increase of glycine levels in several regions in which this amino acid is known to potentiate glutamatergic transmission. In order to examine the functional consequences of prolonged electrode implantation, seizure thresholds were determined in groups of rats with short and prolonged electrode implantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Kindling produces long-lasting and selective changes in gene expression of hippocampal neurons

To test the hypothesis that repeated subconvulsive stimulations required to induce kindling can permanently alter gene expression of hippocampal neurons, we used Northern and in situ hybridization analyses to measure steady-state mRNA levels encoding several phenotypic proteins. mRNA encoding a membrane-bound protein, ligatin, was significantly reduced in kindled brains relative to naive and sham control animals. This change in gene expression persisted for over 4 months after kindling, was associated with a decrease in ligatin protein expression, was not produced by single or multiple seizures that did not induce the kindling phenomena, and was blocked by MK801. These results provide direct evidence that kindling can cause persistent changes in the expression of specific genes in hippocampal neurons and suggest that N-methyl-D-aspartate receptor-activated changes in gene expression may be a basic molecular mechanism underlying some of the long-lasting plasticity changes seen in kindling or models of long-term memory.

Loss of dendritic synapses in the medial amygdala associated with kindling

Kindling stimulation was given in the basolateral amygdala (BLA), the septal area or the corpus callosum in the right hemisphere of adult rats. The density of dendritic synapses was electron microscopically studied in the medial amygdaloid nucleus (MAN) ipsi- and contralateral to the stimulation side. The number of dendritic synapses was markedly decreased in both sides of the MAN of 3 groups of kindled rats. Such reduction occurred in both dendritic shaft and spine synapses. The most remarkable decrease was obtained in the BLA kindling. These results suggest that a decrease of synapses may provide a morphological basis for kindling.

Development of changes in endogenous GABA release during kindling epileptogenesis in rat hippocampus

The calcium-dependent gamma-aminobutyric acid (GABA) and glutamate release from rat hippocampal CA1 slices, evoked by a 1-min depolarization with 50 mM K+, was investigated in different stages of kindling epileptogenesis. Kindling was induced by tetanic stimulation of the Schaffer collateral/commissural pathway. In agreement with our previous results, we found a significantly increased calcium-dependent GABA release compared to that of implanted controls, in a group of fully kindled animals 1 day after the last seizure and also 25-36 days after the last seizure. In addition, we found that the increase in GABA release was associated with late phases of kindling epileptogenesis since no significant alterations were found in partly kindled animals that had received only 6 kindling stimulations while a significant increase was apparent in animals that had received 14 tetanic stimuli. When the release protocol was carried out in the presence of SK&F 89776-A, a blocker of the GABA uptake carrier, an additional amount of GABA was found after depolarization. This additional amount of GABA, reflecting the amount of GABA taken up under conditions without blocker, was in kindled animals not different from controls which demonstrates that a reduced GABA uptake does not account for the observed enhanced release in kindled animals. The calcium-dependent release of glutamate evoked by 1 min of high potassium depolarization was not significantly changed in the kindled groups. Only after prolonged depolarization during 4 subsequent minutes a significant increase in animals of the fully kindled group and at long-term after kindling was observed. The threshold K+ concentration for eliciting a calcium-dependent release of GABA and glutamate, was not changed in the kindled animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term increase of glutamate decarboxylase mRNA in a rat model of temporal lobe epilepsy

Behavioral changes following injury, neural degeneration, and aging partly reflect the synaptic plasticity of the nervous system. Such long-term plastic changes are likely to depend on alterations in the production of proteins involved in synaptic structures and neurotransmission. We have studied the regulation of the mRNA encoding one such protein, glutamate decarboxylase (GAD), the rate limiting enzyme of GABA synthesis, after a unilateral lesion in the hippocampus that leads to increased seizure susceptibility. Quantitative in situ hybridization reveals a long-term increase in GAD mRNA in several bilateral structures, as well as in specific neurons in the ipsilateral dentate gyrus. Our data do not support the often stated hypothesis that seizure susceptibility depends on the malfunction of GABA neurons.

NMDA receptor activation mediates the loss of GABAergic inhibition induced by recurrent seizures

Previously we have shown that delivery of rapidly recurring hippocampal seizures (RRHS) to awake rats causes a rapid kindling and that RRHS in urethane-anesthetized rats leads to a progressive lengthening of afterdischarges and diminution of paired pulse inhibition. The present experiments examined the relationship between the changes in afterdischarge durations and inhibition. Pre-treatment before RRHS with the non-competitive NMDA receptor antagonists MK-801 and ketamine blocked afterdischarge lengthening. MK-801 also prevented RRHS-induced changes in paired pulse inhibition. For pharmacodynamic and pharmacokinetic reasons the ability of ketamine to counteract RRHS-induced changes of paired pulse inhibition was not examined. MK-801 also blocked the rightward shift of stimulus intensity vs. population spike curves which RRHS caused. We suggest that RRHS leads to an enduring diminution of GABAergic inhibition and that this accounts, at least in part, for the lengthening of afterdischarges seen with recurrent hippocampal seizures. In addition, NMDA receptor activation appears to play a role in this decrease of the potency of GABAergic inhibition. However, mechanisms which are not dependent on NMDA receptor activation also play a critical role in hippocampal epileptogenesis.

Evidence for a chronic loss of inhibition in the hippocampus after kindling: biochemical studies

Brain tissue from kindled animals prepared 1 month after their last seizure was compared to tissue from matched surgical control animals. Quantitative film autoradiography was used to study muscimol binding in the CA1 region and 3 other brain areas (dentate gyrus, cerebral cortex, and thalamus). The Kd values so obtained were constant from region to region and comparable to those published by others. Bmax values varied; of the 4 regions studied CA1 had the lowest value of Bmax. There were no differences in either Kd or Bmax values in any region studied between kindled and surgical control rats. The release of GABA from nerve terminals was assessed with hippocampal tissue maintained in vitro and perfused with different solutions in which the concentrations of K+ and Ca2+ were varied. This allowed the examination of K+-induced depolarization release and the Ca2+ dependence of this process. K+-induced, Ca2+-dependent release of GABA from hippocampus derived from kindled animals was significantly less than that from hippocampus derived from controls. The biochemical studies reported here provide additional support for the hypothesis that there is a chronic decrease in GABA-mediated inhibition in the hippocampus associated with kindling. The data point to a dysfunction at the presynaptic level, within the GABAergic interneuron, but do not exclude changes at a level postsynaptic to the GABAergic interneuron not detected with the methods employed.