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

Reduction of vesicle-associated membrane protein 2 expression leads to a kindling-resistant phenotype in a murine model of epilepsy

Our previous work has correlated permanent alterations in the rat neurosecretory machinery with epileptogenesis. Such findings highlighted the need for a greater understanding of the molecular mechanisms underlying epilepsy so that novel therapeutic regimens can be designed. To this end, we examined kindling in transgenic mice with a defined reduction of a key element of the neurosecretory machinery: the v-SNARE (vesicle-bound SNAP [soluble NSF attachment protein] receptor), synaptobrevin/vesicle-associated membrane protein 2 (VAMP2). Initial analysis of biochemical markers, which previously displayed kindling-dependent alterations in rat hippocampal synaptosomes, showed similar trends in both wild-type and VAMP2(+/-) mice, demonstrating that kindled rat and mouse models are comparable. This report focuses on the effects that a ~50% reduction of synaptosomal VAMP2 has on the progression of electrical kindling and on glutamate release in hippocampal subregions. Our studies show that epileptogenesis is dramatically attenuated in VAMP2(+/-) mice, requiring both higher current and more stimulations to reach a fully kindled state (two successive Racine stage 5 seizures). Progression through the five identifiable Racine stages was slower and more variable in the VAMP2(+/-) animals compared with the almost linear progression seen in wild-type littermates. Consistent with the expected effects of reducing a major neuronal v-SNARE, glutamate-selective, microelectrode array (MEA) measurements in specific hippocampal subregions of VAMP2(+/-) mice showed significant reductions in potassium-evoked glutamate release. Taken together these studies demonstrate that manipulating the levels of the neurosecretory machinery not only affects neurotransmitter release but also mitigates kindling-induced epileptogenesis.

Kindling-induced asymmetric accumulation of hippocampal 7S SNARE complexes correlates with enhanced glutamate release

PURPOSE

To correlate kindling-associated alterations of the neurotransmitter secretory machinery, glutamate release in the trisynaptic hippocampal excitatory pathway, and the behavioral evolution of kindling-induced epileptogenesis.

METHOD

Neurotransmitter release requires the fusion of vesicle and plasma membranes; it is initiated by formation of a stable, ternary complex (7SC) of SNARE [soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptor] proteins. Quantitative Western blotting was used to monitor levels of 7SC and SNARE regulators [NSF, SV2 (synaptic vesicle protein 2)] in hippocampal synaptosomes from amygdala-kindled animals. Hippocampal synaptic glutamate release was measured in vivo with a unique microelectrode array (MEA) that uses glutamate oxidase to catalyze the breakdown of glutamate into a reporter molecule.

KEY FINDINGS

Ipsilateral hippocampal accumulation of 7SC developed with onset of amygdalar kindling, but became permanent only in animals stimulated to at least Racine stage 3; the ratio peaked and did not increase with more than two consecutive stage 5 seizures. Chronic 7SC asymmetry was seen in entorhinal cortex and the hippocampal formation, particularly in dentate gyrus (DG) and CA1, but not in the other brain areas examined. There was a strong correlation between asymmetric 7SC accumulation and increased total hippocampal SV2. Following a 30-day latent period, amplitudes of spontaneous synaptic glutamate release were enhanced in ipsilateral DG and reduced in ipsilateral CA3 of kindled animals; increased volleys of synaptic glutamate activity were seen in ipsilateral CA1.

SIGNIFICANCE

Amygdalar kindling is associated with chronic changes in the flow of glutamate signaling in the excitatory trisynaptic pathway and with early but permanent changes in the mechanics of vesicular release in ipsilateral hippocampal formation.

Functional role for redox in the epileptogenesis: molecular regulation of glutamate in the hippocampus of FeCl3-induced limbic epilepsy model

We used western blotting to measure the quantity of glutamate and gamma-aminobutyric acid (GABA) transporters proteins within hippocampal tissue obtained from rats who had undergone epileptogenesis. Chronic seizures were induced by amygdalar injection of FeCl(3). We found that the glial glutamate transporters GLAST and GLT-1 were down-regulated at 60 days after initiation of chronic and recurrent seizures. However, the neuronal glutamate transporter EAAC-1 and the GABA transporter GAT-3 were increased. We performed in vivo microdialysis in freely moving animals to estimate in vivo redox state. We found that the hippocampal tissues were oxidized, resulting in even further impairment of glutamate transport. Our data show that epileptogenesis in rats resulting in chronic and recurrent seizures is associated with collapse of glutamate regulation caused by both the molecular down-regulation of glial glutamate transporters combined with the functional failure due to oxidation.

Asymmetric accumulation of hippocampal 7S SNARE complexes occurs regardless of kindling paradigm

Modifications of neurotransmission may contribute to the synchronization of neuronal networks that are a hallmark of epileptic seizures. In this study we examine the synaptosomal proteins involved in neurotransmitter release to determine if alterations in their interactions correlate with the chronic epileptic state. Using quantitative western blotting, we measured the levels of 7S SNARE complexes and SNARE effectors in the effected hippocampi from animals that were electrically kindled through stimulation from one of three different foci. All three kindling paradigms, amygdalar, entorhinal, and septal, were associated with an accumulation of 7S SNARE complexes in the ipsilateral hippocampus, measured 1 month after completion of kindling. Of the eight SNARE effectors examined (alpha-SNAP, NSF, SV2A/B, Munc18a/nSec1, Munc13-1, Complexins 1 and 2, and synaptotagmin I), there was a statistically significant bihemispheric increase of hippocampal SV2 and decrease of NSF upon kindling; neither by itself would be expected to account for the asymmetry of SNARE complex distribution. These data suggest that an ipsilateral hippocampal accumulation of SNARE complexes is a permanent alteration of kindling-induced epilepsy, regardless of stimulation pathway. The significance of these findings toward a molecular understanding of epilepsy will be discussed.

Electrically evoked GABA release in rat hippocampus CA1 region and its changes during kindling epileptogenesis

Previous findings on changes in K+-induced GABA release from hippocampal slices during kindling epileptogenesis were reinvestigated using physiological electrical stimulation. For that purpose, a procedure was developed enabling neurochemical monitoring of GABA release locally in the CA1 region of rat hippocampal slices upon tetanic stimulation of Schaffer-collateral fibers. In the presence of a GABA reuptake blocker, subsequent application of short (3 s) pulses of 50-Hz stimuli induced a local transient increase in GABA release. In slices from fully kindled animals, 24 h after the last generalized seizure, tetanically stimulated GABA release was increased in comparison to control slices. In slices from long-term kindled animals, 4-5 weeks after the last seizure, tetanically stimulated GABA release had returned to control levels. Application of the broad low-affinity GABAB receptor antagonist saclofen increased the tetanically stimulated GABA release in control slices, but had no effect in fully kindled slices. In slices from long-term kindled animals, however, saclofen enhanced GABA release similarly as in control slices. We conclude that the transient increase in tetanus-induced GABA release during kindling epileptogenesis is seizure-related, and probably caused by temporarily impaired presynaptic GABAB receptors. The possible relevance of this finding for GABA transmission in epilepsy is discussed.

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.

Differential effects of pentylenetetrazol-kindling on long-term potentiation of population excitatory postsynaptic potentials and population spikes in the CA1 region of rat hippocampus

The effects of pentylenetetrazol-kindling on synaptic transmission and the effectiveness of &theta; pattern primed-bursts (PBs) for the induction of long-term potentiation (LTP) of population excitatory postsynaptic potentials and population spikes were investigated in hippocampal CA1 of pentylenetetrazol-kindled rats. Experiments were carried out in the control and kindled animals at two post-kindling periods, i.e., 48-144 h (early phase) and 30-33 days (long lasting phase). Field potentials (population excitatory postsynaptic potentials, pEPSPs; and population spikes, PSs) were recorded at the stratum radiatum and the stratum pyramidale following stimulation of the stratum fibers, respectively. PBs were delivered to stratum fibers and PB potentiation was assessed. The results showed that 48-144 h after kindling there was no significant difference for pEPSP slope and PS amplitude between two groups. But at 30-33 days after kindling, the pEPSP slope in the stratum radiatum of kindled animals decreased, whereas the amplitude of PSs increased compared to those of controls. Shortly after kindling, control animals had normal LTP of pEPSP slope and PS amplitude in response to PBs, but kindled rats lack LTP of pEPSP slope and PBs induced LTP of PS amplitude in most of kindled animals. In 30-33 days after kindling, PB potentiation was not observed in the stratum radiatum of kindled animals but PBs induced LTP of PS amplitude, which was significantly greater than that of control animals. The effect is compatible with the hypothesis, which postulates kindling-associated functional deficit in hippocampus, especially CA1, as an explanation for the behavioral deficits seen with the kindling model of epilepsy.

Central nervous system-initiated inflammation and neurotrophism in trauma: IL-1 beta is required for the production of ciliary neurotrophic factor

Injury to the CNS results in the production and accumulation of inflammatory cytokines within this tissue. The origin and role of inflammation within the CNS remains controversial. In this paper we demonstrate that an acute trauma to the mouse brain results in the rapid elevation of IL-1beta. This increase is detectable by 15 min after injury and significantly precedes the influx of leukocytes that occurs hours after. To confirm that IL-1beta up-regulation is initiated by cells within the CNS, in situ hybridization for cytokine transcript was combined with cell type immunohistochemistry. The results reveal parenchymal microglia to be the sole source of IL-1beta at 3 h postinjury. A role for CNS-initiated inflammation was addressed by examining the expression of the neurotrophic factor, ciliary neurotrophic factor (CNTF). Analysis of their temporal relationship suggests the up-regulation of CNTF by IL-1beta, which was confirmed through three lines of evidence. First, the application of IL-1 receptor antagonist into the lesion site attenuated the up-regulation of CNTF. Second, the examination of corticectomized animals genetically deficient for IL-1beta found no CNTF up-regulation. Third, the lack of CNTF elevation in IL-1beta null mice was rescued through exogenous application of IL-1beta into the lesion site. These findings provide the first evidence of the requirement for IL-1beta in the production of CNTF following CNS trauma, and suggest that inflammation can have a beneficial impact on the regenerative capacity of the CNS.

Phenobarbital administration directed against kindled seizures delays functional recovery following brain insult

Anti-convulsant drug administration or recurrent seizures can impact functional recovery following brain insult. The nature of that impact depends on a variety of factors, including timing of drug administration and drug mechanism of action, as well as seizure number, timing, and severity. The objective of this study was to determine the functional consequences of anti-convulsant administration directed against seizure activity in brain-damaged animals. To this end, phenobarbital was coupled with daily electrical kindling of the amygdala beginning 48 h after a unilateral anteromedial cortex lesion. Recovery from somatosensory deficits was assessed, as was regional atrophy and basic fibroblast growth factor (bFGF) expression. Animals receiving phenobarbital prior to daily kindling failed to recover within 2 months of testing. In contrast, animals receiving saline prior to kindling as well as phenobarbital-treated non-kindled animals recovered within 2 months after the lesion. Though the exact mechanisms underlying these behavioral phenomena remain uncertain, patterns of bFGF expression among the groups provide some insight. Taken together, results from the present study suggest that anti-convulsant drug administration directed against subclinical seizure activity can be more detrimental to functional recovery than seizures alone or anti-convulsant drug treatment after seizure activity has occurred.

Compensatory change in EAAC1 glutamate transporter in rat hippocampus CA1 region during kindling epileptogenesis

Functional and molecular changes in glutamate transporters during kindling epileptogenesis were investigated in hippocampus CA1-region of rats. In control animals total glutamate transporter activity was indicated by the stimulatory effect of the high-affinity transporter blocker L-trans-pyrrolidine-2,4-dicarboxylate on extracellular glutamate and aspartate concentrations, as measured by in vivo microdialysis. This blocker-induced elevation was absent already early during epileptogenesis. CA1 levels of the glutamate transporter subtypes GLAST and GLT-1, analyzed by quantitative immunoblotting, did not change during kindling epileptogenesis. However, the 60% decrease in EAAC-1 level observed in age-matched controls was fully compensated for in kindled animals 4-5 weeks after the last generalized seizure. These results indicate a compensatory change of the neuronal EAAC-1 glutamate transporter in CA1 region during kindling epileptogenesis, which may be the consequence of a decrease in total transporter activity.

Pentylenetetrazol-induced kindling stimulates the polyamine interconversion pathway in rat brain

The levels of polyamines, N-acetylpolyamines, and GABA in the cerebral cortex and brainstem of rat brain after completion of pentylenetetrazol (PTZ)-induced kindling were investigated. Pretreatment with the polyamine oxidase inhibitor MDL72527 caused an accumulation of N1-acetylspermidine and N1-acetylspermine in normal rats. After a kindling seizure, the levels of N-acetylpolyamines were elevated, particularly in the cerebral cortex, indicating activation of polyamine interconversion. The levels of putrescine and GABA were lower in kindled rats pretreated with MDL72527. In addition, pretreatment with MDL72527 enhanced the seizure susceptibility to PTZ in normal rats. These results suggest that the polyamine interconversion pathway is involved in brain excitability, probably through the regulation of putrescine and GABA levels.

Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

The effect of partial hippocampal kindling, a model of temporal lobe seizure, on monosynaptic inhibition mediated by GABA was studied. Kindled rats were given 15 nonconvulsive hippocampal afterdischarges, and control rats were given low frequency or no stimulations. At 1-2 d after kindling, paired-pulse depression (PPD) of the IPSCs recorded in CA1 neurons in vitro was significantly smaller in kindled as compared with control rats. The difference in PPD persisted for at least 21 d after kindling. The decrease in PPD of the IPSCs after partial hippocampal kindling was likely caused by a reduced GABA autoinhibition after downregulation of presynaptic GABAB receptors. The GABAB antagonist CGP35348 (1 mM) suppressed PPD of the IPSCs more strongly in control than in kindled rats. Direct activation of the presynaptic GABAB receptors by baclofen suppressed the monosynaptic IPSCs significantly more in control than in kindled rats. The decay rate of a single-pulse IPSC was faster in kindled than in control rats on day 1 or day 21 after partial kindling. The difference in IPSC decay between kindled and control rats was found with or without a GABAB receptor antagonist. The low efficacy of the presynaptic GABAB receptors in kindled rats may provide compensatory stabilization of the postsynaptic membrane against further seizures or plasticity.

Changes in voltage-dependent calcium channel alpha1-subunit mRNA levels in the kindling model of epileptogenesis

The establishment of a focus of epileptiform activity in the hippocampus of the rat, using the kindling paradigm, leads to enhanced voltage-dependent calcium conductance of CA1 pyramidal neurones (G.C. Faas, M. Vreugdenhil, W.J. Wadman, Calcium currents in pyramidal CA1 neurones in vitro after kindling epileptogenesis in the hippocampus of the rat, Neuroscience 75 (1996) 57-67; M. Vreugdenhil, W.J. Wadman, Kindling-induced long-lasting enhancement of calcium in hippocampal CA1 area of the rat: relation to calcium-dependent inactivation, Neuroscience 59 (1994) 105-114). Using semi-quantitative in situ hybridization techniques, we investigated whether these changes were associated with an altered expression of the genes that encode for the alpha1A-E-subunits of the voltage-dependent calcium channels (VDCC). Kindling epileptogenesis was induced in rats that received an electrical tetanic stimulation of the Schaffer collateral/commissural fibre pathway in the hippocampus twice daily. Two groups of rats were studied before the appearance of generalized seizures, one group after at least 5 generalized seizures (fully kindled) and one group was investigated at long-term (28 days) after the last seizure. During the initial stages of epileptogenesis, the alpha1A-, alpha1D- and alpha1E-subunit mRNA levels were significantly increased in the different hippocampal subareas in comparison to the levels in control animals. In contrast, alpha1B-subunit gene expression decreased in the CA area and dentate gyrus. No significant change was observed in the alpha1C-I and alpha1C-II expression. At the fully kindled stage, the only significant change was an up-regulation of the alpha1B-subunit mRNA levels in the CA3 area, 24 h after the last seizure. No change in VDCC alpha1-subunit gene expression was found in animals investigated long-term after the establishment of the fully kindled state. Thus, the VDCC alpha1-subunit gene expression is altered in a subclass-specific manner during the early stages of kindling and may play a role in the establishment of a kindled focus, possibly caused by an alteration of the population of VDCCs involved in neurotransmitter release. The absence of long-lasting changes suggests that the maintenance of a kindled focus is not due to persisting alterations in VDCC alpha1 mRNA levels.

Tiagabine exerts an anti-epileptogenic effect in amygdala kindling epileptogenesis in the rat

Tiagabine (TGB) is a novel antiepileptic drug whose anticonvulsant effects are due to inhibition of gamma-aminobutyric acid (GABA) transport mediated by the GABA transporter-1. We have previously shown that TGB is effective in acute amygdala kindled seizures, and consequently we wanted to test the hypothesis that TGB also could suppress the development of kindling epileptogenesis. Rats had stereotaxically implanted stimulating/recording electrodes in the basolateral amygdala and recording electrode in the contralateral occipital cortex. Rats were divided in three groups (n = 8 for each group) intraperitoneally (i.p.) administered vehicle, TGB 7.3 micromol/kg and TGB 24.3 micromol/kg, respectively, 30 min before stimulation. TGB dose-dependently suppressed the development of the behavioral seizure score and afterdischarge (AD) duration recorded from the amygdala and cortex. Vehicle treated animals displayed at the 16th stimulation an average behavioral score of 4.7 +/- 0.2 (mean +/- SEM) compared to 3.9 +/- 0.2 in the 7.3 micromol/kg TGB treated group and 1.4 +/- 0.3 in the 24.3 micromol/kg TGB treated group. Amygdaloid AD in controls on the 16th stimulation was 92 +/- 10 s compared to 56 +/- 12 s in group 2 and 25 +/- 3 s in group 3. Cortical AD was at the same time 92 +/- 10, 55 +/- 13 and 20 +/- 5 s, respectively. Groups 2 and 3 required four and seven further stimulations, respectively, without TGB administration to reach the AD level in the control group. At the 17th stimulation, rats in group 1 were administered TGB 24.3 micromol/kg and displayed an average behavioral score of 0.5 +/- 0.2. Amygdaloid and cortical AD were both 6 +/- 1 s. Tiagabine 24.3 micromol/kg suppresses both the kindling process and the expression of the fully kindled seizure.

Examination of persistent effects of repeated administration of pentylenetetrazol on rat hippocampal CA1: evidence from in vitro study on hippocampal slices

The early and long-lasting effects of pentylenetetrazol-kindling on hippocampal CA1 synaptic transmission were investigated. Experiments were carried out in the hippocampal slices from control and kindled rats at two post-kindling periods, i.e. 48-144 h (early phase) and 30-33 days (long-lasting phase). Field potentials, i.e. population excitatory postsynaptic potential (pEPSP) and population spike (PS) were recorded at the stratum pyramidale following stimulation of the stratum radiatum. Kindling-induced changes in synaptic transmission were assessed by stimulus-response functions and paired-pulse responses. The results showed that 48-144 h after kindling, the PS amplitude in the CA1 of kindled slices enhanced, and a second PS appeared compared to control slices. But at 30-33 days after kindling, the pEPSP slope in the CA1 of kindled slices enhanced without any change in the PS compared with those in the control slices. Evaluation of paired-pulse responses showed a significant reduction in paired-pulse inhibition for PS 48-144 h after kindling and a significant increase in paired-pulse inhibition for pEPSP 30-33 days after kindling. Our results suggest that pentylenetetrazol-kindling is accompanied by enhanced excitability and a reduction of paired-pulse inhibition in hippocampal CA1. The increased paired-pulse inhibition one month after kindling, may be interpreted as an adaptive process to cope with subsequent seizures.

Expression of glial glutamate transporters GLT-1 and GLAST is unchanged in the hippocampus in fully kindled rats

In situ hybridization techniques and quantitative western blotting were used to study the expression of the glial glutamate transporter GLT-1 and GLAST in the brains of normal (implanted, non-kindled) and fully kindled rats. Wistar rats were implanted with stimulating electrodes in the basolateral amygdala, and killed 28 days after the stimulated group had shown stage 5 seizures on five occasions. The brains were processed for in situ hybridization of messenger RNA for GLT-1 using 35S-labelled oligonucleotide probes or digoxigenin-labelled riboprobes. Paired (kindled and non-kindled) sections were used for qualitative and quantitative analyses. Image analysis of autoradiograms showed no change in expression of GLT-1 messenger RNA in any region of the hippocampus or in the cortex. An increase in expression of GLT-1 messenger RNA (expressed as percentage difference of control) was observed bilaterally in the striatum in kindled animals (16-21%, P<0.05). Nuclear emulsion-dipped sections showed predominant glial cell labelling in the hippocampus. Particle density analysis revealed reduced cell labelling in some kindled vs control pairs but overall there was no significant reduction in labelling in CA1. Equivalent results were found in CA1 using digoxigenin-labelled riboprobes. Quantitative immunoblotting also revealed no change in GLT-1 or GLAST transporter protein in the hippocampus of kindled animals. From these data we conclude that the enduring seizure susceptibility associated with the fully kindled state is unlikely to involve alterations in hippocampal GLT-1 messenger RNA or GLT-1 and GLAST transporter protein expression.

Loss of Calbindin-D28K immunoreactivity from dentate granule cells in human temporal lobe epilepsy

The loss of the calcium binding protein, Calbindin-D28k, from dentate granule cells has been observed in different animal models of epilepsy and in ischaemia. This decrease is accompanied by alterations of calcium and N-methyl-D-aspartate currents, which may explain the hyperexcitability of the dentate gyrus. In the present study, we found a loss of calbindin immunoreactivity from over 90% of the dentate granule cells in lobectomy samples from four of 10 temporal lobe epilepsy patients. In another four patients, over 50%, of dentate granule cells were devoid of calbindin immunoreactivity, whereas the remaining two cases showed a 20-30% decrease. Electron microscopy revealed a normal ultrastructure both in calbindin-containing and calbindin-negative granule cells. Both calbindin-positive and -negative mossy fibre collaterals participated in supragranular sprouting. As inferred from data in animal models, the lack of calbindin in dentate granule cells of human epileptic subjects is likely to result in hyperexcitability of the dentate gyrus, which may then function as a "motor" for seizures.

Persistent reduction of GABA(A) receptor-mediated inhibition in rat hippocampus after chronic intermittent ethanol treatment

GABA(A) receptor-mediated function was studied in rats treated with chronic intermittent ethanol (CIE). Rats were given 60 doses of 6g/kg ethanol every 24 h by gastric intubation, with repeated intoxicating and withdrawal episodes leading to a kindling-like increase in seizure susceptibility (Kokka et al., Alcohol: Clin. Exp. Res., 17 (1993) 525-531). Efflux of 36Cl-, evoked by application of muscimol, a measure of GABA(A) receptor function, was examined in 300 mu m slices obtained from frontal, parietal, and temporal cortex, hippocampus, and inferior colliculus, one day after the last administration of ethanol. Compared to controls, the 36Cl- efflux in hippocampal slices of CIE rats was significantly reduced by 29%, while there were no changes in the other brain regions studied. In hippocampal slices, paired-pulse inhibition in CA1 pyramidal neurons, measured extracellularly using homosynaptic orthodromic stimulation at an interval of 10 ms, was significantly reduced in CIE rats. A significant decrease by 40% both at 2 and 40 days after 60 doses of ethanol was found, implying a persistent decrease in GABA(A) receptor-mediated inhibition in CIE rats. These reductions in paired-pulse inhibition are consistent with the decrease in the pentylenetetrazol (PTZ) seizure threshold which was previously observed in CIE rats. Therefore, we suggest that this reduction of GABA(A) receptor-mediated inhibition contributes to the persistent increase in seizure susceptibility of CIE rats.

Isozyme specific changes in the expression of protein kinase C isozyme (alpha-zeta) genes in the hippocampus of rats induced by kindling epileptogenesis

The transcript levels of the protein kinase C (PKC) isoform genes during the development of a kindled epileptogenic focus, elicited by stimulation of Schaffer collateral/commissural fibres in the CA1 area of the rat hippocampus, were compared with the expression levels in control animals using a semi-quantitative in situ hybridization approach. In the hippocampus of control animals, the levels of PKC-alpha-zeta transcripts showed a gene-specific expression pattern and significant differences in expression level were observed between the neurons of CA1, CA3 and fascia dentata. In the early stages of kindling epileptogenesis, i.e. following 6 and 14 afterdischarges, specific changes in the expression levels of PKC-beta, -epsilon, and -zeta but not of PKC-alpha, -gamma, and -delta were found. PKC-beta expression was decreased in CA1, while the PKC-epsilon and -zeta specific hybridization signals were increased in CA1, CA3 and fascia dentata. In fully kindled animals, that had experienced 10 generalized seizures, most expression levels tended to return to control values. One month after the last seizure no significant alterations were encountered. These results indicate an involvement of specific PKC-isoform gene expression in the induction of an epileptogenic focus, but not in the maintenance of the long-lasting kindled state.

Long-term and regional specific changes in [3H]flunitrazepam binding in kindled rat hippocampus

The binding of the GABAA/benzodiazepine receptor agonist [3H]flunitrazepam was studied in the hippocampus of rats kindled by daily stimulation of the Schaffer collaterals, using semi-quantitative autoradiography. Two kindled stages were investigated: (i) 24 h after the last generalized tonic-clonic seizure (fully kindled) and (ii) 28 days after the last generalized seizure (long-term). The binding of [3H]flunitrazepam was determined at two concentrations, 3 and 16 nM. In the CA1 area, we found a small but significant decrease (ca. 10%), both in the 3 and 16 nM [3H]flunitrazepam binding at the fully kindled stage. In contrast, there was a significant increase in the 3 nM binding (c. 15%) at the long-term stage. The 16 nM binding was not significantly different from control binding at this stage. In the granular and molecular layers of the fascia dentata, we found at both kindled stages a significantly increased 3 nM (ca. 9 and 19%, respectively) and 16 nM (ca. 19 and 14%, respectively) binding. Furthermore, we found that muscimol was still able to enhance the [3H]flunitrazepam binding in kindled animals, indicating that the GABAA receptor agonist binding site and benzodiazepine agonist binding site are still functionally coupled. The changes in [3H]flunitrazepam binding at the fully kindled stage are in agreement with the recently observed kindling-induced changes in [3H]muscimol binding in the hippocampal formation of the same animals [Titulaer M. N. G. et al. (1994) Neuroscience 59, 817-826] and extend these observations to the benzodiazepine modulatory site of the GABAA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of GABAA receptor subunit mRNAs in hippocampal pyramidal and granular neurons in the kindling model of epileptogenesis: an in situ hybridization study

To investigate the molecular changes underlying kindling epileptogenesis in the rat hippocampus, the expression levels of the genes encoding for 13 different gamma-aminobutyric acid type-A receptor (GABAAR) subunits were measured in hippocampal principal neurons using in situ hybridization techniques and semi-quantitative analysis of the autoradiograms. Schaffer collateral-commissural pathway kindled rats were investigated at three different stages of kindling acquisition, at 24 h after the last seizure and at long-term (28 days) after termination of kindling stimulations. Changes were distinct for the different subunits in the three analyzed regions (CA1, CA3, fascia dentata) and also different for the various kindling stages. In all hippocampal areas at the early phases of kindling epileptogenesis, before the appearance of generalized seizures, an increase was found of those transcripts that constituted the majority of the expressed variants in control animals (alpha 1, alpha 2, alpha 4, beta 1, beta 2, beta 3, gamma 2/gamma 2L mRNA). In these stages, the increased levels of different variants in the granular neurons of the fascia dentata were more pronounced when compared to the pattern of changes in pyramidal cells of CA1 and CA3. In fully kindled animals, the expression levels of several subunits returned to control levels, whereas beta 3 and gamma 2/gamma 2L mRNA expression was still significantly enhanced in all areas. At long-term, few changes were encountered. The long-splice variant of gamma 2 was decreased within pyramidal and granular neurons while the total level of gamma 2 mRNA was not different from controls. The increased GABAAR subunit expression in the fascia dentata may underly the reported increased GABAAR ligand binding and the increased GABA mediated inhibition. However, the decreased GABAAR binding and the attenuation of GABAergic inhibition in CA1, could not be explained by a decrement of receptor subunit expression.

Autoradiographic analysis of [35S]t-butylbicyclophosphorothionate binding in kindled rat hippocampus shows different changes in CA1 area and fascia dentata

We studied the binding of [35S]t-butylbicyclophosphorothionate to the GABAA receptor-mediated chloride channel in the CA1 area and fascia dentata of control and Schaffer collateral kindled rats, by means of semi-quantitative autoradiography. The [35S]t-butylbicyclophosphorothionate binding was determined at three stages during kindling acquisition: (i) after six afterdischarges, (ii) after 14 afterdischarges and (iii) after the induction of fully kindled seizures. Furthermore, the binding was studied at the long-term stage, 28 days after the last generalized tonic-clonic seizure [Racine R. J. (1972) Electroenceph. clin. Neurophysiol. 32, 281-294]. The binding was investigated at three [35S]t-butylbicyclophosphorothionate concentrations, 4, 47.5 (KD value) and 180 nM (Bmax value). A significant decrease in [35S]t-butylbicyclophosphorothionate binding in the CA1 area (-6 to -20%) and hilar formation (-17 to -37%), in one or more of the three [35S]t-butylbicyclophosphorothionate concentrations tested at the six and 14 afterdischarges and fully kindled stages was observed, but no significant changes at the long-term kindling stage were found. In contrast, the granular and molecular layers of the fascia dentata presented a significant increase in [35S]t-butylbicyclophosphorothionate binding (+15 to +38%) at the 14 afterdischarges, fully kindled and long-term kindled stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatiotemporal alterations of central alpha 1-adrenergic receptor binding sites following amygdaloid kindling seizures in the rat: autoradiographic studies using [3H]prazosin

Noradrenergic neurons are thought to be involved in the process of seizure development and long-term central nervous system plasticity associated with kindling and epilepsy. These processes involve actions of noradrenaline at alpha 1-, alpha 2- and beta 1-adrenergic receptors. In this study, quantitative in vitro autoradiography was used to investigate possible changes in the density of brain alpha 1-adrenergic receptors in a kindling model of epilepsy in the rat. Kindling was produced by daily unilateral stimulation of the amygdala. The alpha 1A+alpha 1B subtypes of adrenergic receptors were labelled with the alpha 1-selective antagonist, [3H]prazosin and alpha 1B receptors, detected in the presence of 10 nM WB4101 to selectively occupy alpha 1A receptors, accounted for 50% of total alpha 1 receptors in cerebral cortex. Autoradiographic studies identified significant and long-lasting, ipsilateral increases in specific [3H]prazosin binding throughout layers I-III of the cortex in sham-operated, unstimulated rats, presumably caused by the surgical implantation of the stimulating electrode within the basolateral amygdaloid nucleus. Binding to alpha 1A + alpha 1B receptors and alpha 1B receptors was increased by an average of 35 and 60%, respectively under these conditions. Stimulation-evoked seizures produced dramatic bilateral increases in specific [3H]prazosin binding to alpha 1A + alpha 1B receptors and particularly to alpha 1B receptors in layers I-III of all cortical areas examined. These changes were rapidly induced and the largest increases (range alpha 1A + alpha 1B 80-340%; alpha 1B 165-380%) occurred at 0.5-2 h after the last stage 5 kindled seizure. At 1 and 3 days after the last seizure, increases were measured for both alpha 1A + alpha 1B and alpha 1B receptors in layers I-III of particular cortical regions, but not overall (e.g. 60-210% increase in perirhinal cortex at both times, with increases also in retrosplenial, hindlimb, occipital, parietal and temporal cortices). Between 2-8 wk post-stimulation specific receptor binding levels were equivalent to those in sham-operated, unstimulated rats. In contrast to the large and widespread increases in outer cortical [3H]prazosin binding, smaller increases were detected in the inner cortex (layer V-VI) at individual times (65-75% increase at 30 min), while no significant changes occurred in several other brain regions examined, including thalamus, which contained a high density of alpha 1A and alpha 1B receptors, or hippocampus which has a low density of both alpha 1 receptor subtypes.(ABSTRACT TRUNCATED AT 400 WORDS)

An experimental model of progressive epilepsy: the development of kindling of the hippocampus of the rat

Kindling epileptogenesis was induced by periodic electrical stimulation of the Schaffer collateral/commissural pathway in the CA1 area of the rat hippocampus. The progressive nature of hippocampal kindling is demonstrated by a detailed description of the behavioral signs and the progressive increase of the after-discharge duration in the course of kindling acquisition. Furthermore, the evolution of the alterations in the paired-pulse local evoked field potentials and the modifications of the GABAA receptor binding and of the expression of mRNAs encoding for the subunits of the GABAA and glutamate receptors are considered. Evidence is presented that during kindling opposite changes occur in the CA1 and the fascia dentata in terms of the balance between excitation and inhibition due to contrasting changes in GABA-mediated inhibitory function.

Rat hippocampal kindling induces changes in the glutamate receptor mRNA expression patterns in dentate granule neurons

The expression level of the mRNAs encoding the Flip and Flop versions of the AMPA-selective glutamate receptor subunits A, B, C and D was studied using in situ hybridization in the hippocampus of rats kindled by Schaffer collateral/commissural fibre stimulation. The expression levels of the Flip variant of GluR-A, B and C mRNAs were bilaterally enhanced in the dentate granule neurons of fully kindled animals 24 h after the last seizure. These changes were already observed after the sixth kindling stimulation (preconvulsive-stage), but not after a single afterdischarge. Four weeks after the last seizure, when the animals were still hypersensitive to kindling stimulations, only GluR-A Flip expression was enhanced. These results suggest that kindling epileptogenesis is accompanied by an increased number and enhanced sensitivity of the expressed AMPA type glutamate receptors in the fascia dentata, leading to an enhanced excitatory synaptic transmission which may contribute to the process of kindling epileptogenesis.

Biphasic differential changes of GABAA receptor subunit mRNA levels in dentate gyrus granule cells following recurrent kindling-induced seizures

GABAA receptor alpha 1, beta 3 and gamma 2 subunit mRNA levels have been measured in hippocampus using in situ hybridization, following 1, 10 and 40 seizures produced by rapid kindling stimulations. Major alterations of gene expression were largely confined to the dentate gyrus. One stimulus-induced seizure reduced gamma 2 mRNA levels in the dentate gyrus by 30%. In contrast, mRNA expression increased for alpha 1 in CA1 and CA3 and for beta 3 in CA1 to around 30% above control values. Ten stimulations reduced beta 3 (by 19%) and gamma 2 (by 37%) mRNA expression in the dentate gyrus. No changes were observed in other hippocampal subregions. Forty kindling-induced seizures led to biphasic alterations of subunit mRNA levels in dentate gyrus with only minor changes in CA1-CA3. Up to 4 h after the last seizure mRNA expression for alpha 1 was slightly decreased in dentate gyrus, whereas marked reductions were observed for beta 3 and gamma 2 (by 41% and 48%, respectively). Between 12 and 48 h there were major increases of alpha 1 (by 59%) and gamma 2 (by 35%) mRNA levels but no significant changes of beta 3 mRNA expression. Subunit mRNA levels had returned to control values after 5 days, which argues against a direct involvement of GABAA receptor in kindling-evoked hyperexcitability. The rapid and transient, biphasic changes of GABAA receptor subunits following recurrent seizures could play an important role in stabilizing granule cell excitability, thereby reducing seizure susceptibility. The differential regulation of subunit mRNA levels following seizures suggests a novel mechanism for changing the physiological properties of dentate granule cells through possible GABAA receptor complexes with different subunit composition.

Chronic pretreatment with naloxone modifies benzodiazepine receptor binding in amygdaloid kindled rats

Male Sprague-Dawley rats received either naloxone (75 micrograms/h) or saline (0.5 microliter/h) s.c. for 14 days delivered with osmotic minipumps. Two days after termination of either treatment, daily amygdala kindling stimulation was applied until the animals experienced stage V kindled seizures. Benzodiazepine (BDZ) binding sites were labeled with [3H]flunitrazepam (2 nM), and changes in specific brain areas were determined by in vitro quantitative autoradiography. Twenty-four hours after the last electrical stimulation, the saline pretreated fully kindled rats showed enhanced BDZ receptor binding in dentate gyrus, and decreased binding in cingulate cortex ipsilateral to the stimulation compared to saline controls. Twenty-eight days after the last stage V kindled seizure, the significant alterations were no longer evident. In agreement with a previous study, we found that naloxone pretreated amygdala kindled rats showed stage V kindled seizures followed by intervals of 3-5 days in which the same electrical stimulation failed to induce any behavioral and EEG alterations. In comparison with the saline pretreated kindled and saline control groups, the naloxone pretreated kindled rats had significantly higher BDZ binding in different cortical areas, amygdala complex, hippocampus, substantia nigra and periaqueductal gray, 24 h after the last electrical stimulation. The present study indicates that previous chronic exposure to naloxone increases BDZ receptor binding in kindled rats, and suggests that this effect may be associated with the enhanced seizure suppression observed in these animals.

The effect of kindling beyond the 'stage 5' criterion on paired-pulse depression and hilar cell counts in the dentate gyrus

Recent experiments have indicated that recurrent inhibition in the dentate gyrus, as measured with paired-pulse tests, is reduced following the induction of status epilepticus. Also, a loss of cells in the hilus has been reported, and it has been suggested that the two effect might be related. In this experiment, we have monitored paired-pulse depression and counted cells in the hilus in animals that have been kindled well beyond the typical stage 5 criterion. Responses evoked in the dentate gyrus by paired-pulse stimulation of the perforant path were monitored before and after kindling of the perforant path. One group of animals served as controls and received no kindling stimulations. Another group was kindled to 4 stage 5 seizures and then allowed to recover for 2 months. A third group was kindled to 44 stage 5 seizures and then allowed to recover for at least 5 weeks. Paired-pulse tests were taken at 1 week intervals during the kindling and recovery phases. Paired-pulse inhibition increased during kindling, peaked after 4 stage 5 seizures, remained enhanced throughout the additional 40 stage 5 seizures, and recovered towards baseline over a period of about 5 weeks. Upon completion of this phase of the experiment, cell counts were taken in the hilar regions of the Nissl stained brain sections. There was a significant reduction in number of cells in the tissue from kindled animals, compared to controls, but there was no significant difference between the 2 kindled groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid dephosphorylation of microtubule-associated protein 2 in the rat brain hippocampus after pentylenetetrazole-induced seizures

We have studied the effect of Pentylenetetrazole (PTZ)-induced seizures on the state of phosphorylation of microtubule-associated protein 2 (MAP-2) from rat hippocampus. A method for the in vivo 32P-labeling of hippocampal proteins has been established, consisting of intracerebro-ventricular injection of 32PO4 of high specific activity. The results obtained indicate that PTZ induces a rapid and transient dephosphorylation of high-molecular-mass MAP-2, which is prevented when the N-methyl-D-aspartate receptor antagonist MK-801 is previously administered. Phosphopeptide mapping of 32P-labeled MAP-2 obtained from hippocampi of PTZ-treated rats reveals a pattern of phosphorylation distinct from that obtained from control saline-treated rats or MK-801 plus PTZ treated rats. We discuss the possible implications of N-methyl-D-aspartate-receptor activation and MAP-2 dephosphorylation on the plastic changes induced in rat brain hippocampus after induced epileptiform activity.

Synaptic inhibition in primary and secondary chronic epileptic foci induced by intrahippocampal tetanus toxin in the rat

1. Injecting twelve mouse minimum lethal doses of tetanus toxin into one hippocampus of a rat leads to the development of chronic epileptic foci in both hippocampi. These generate intermittent epileptic discharges for 6-8 weeks. Here we compare GABAergic inhibition, 10-18 days after injection, in slices prepared from the injected and contralateral hippocampi (respectively the primary and the secondary or 'mirror' foci), using both neurochemical and electrophysiological methods. 2. Epileptic activity was recorded from slices of both hippocampi from all tetanus toxin-injected rats. Evoked epileptic discharges were similar on the two sides, but spontaneous epileptic discharges were more common contralaterally. 3. Ca(2+)-dependent, K(+)-stimulated (synaptic) release of radiolabelled GABA was depressed in slices from the injected hippocampus, compared with vehicle-injected controls. In contrast, slices from the contralateral hippocampus had normal levels of Ca(2+)-dependent, K(+)-stimulated GABA release, even though adjacent slices were epileptogenic. 4. Intracellular recordings revealed that both fast and slow stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were abolished in CA3 pyramidal cells in the primary focus. In the secondary focus, however, fast IPSPs were seen in seven of twenty-five cells, and slow IPSPs were seen in all cells if the stimulus was strong enough. 5. Monosynaptic IPSPs were isolated pharmacologically by blocking glutamatergic excitatory postsynaptic potentials (EPSPs) with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphopentanoic acid (AP-5). No monosynaptic IPSPs were uncovered in cells from the primary focus at any stimulus strength. Monosynaptic IPSPs were evoked in all cells from both the secondary focus and control slices. The estimated conductances of monosynaptic fast IPSPs were similar in cells from the secondary focus and from the controls, although the former required twice the stimulus strength. 6. Slow IPSPs were found in the secondary focus and in controls, but not in the primary focus. They were sensitive to 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH saclofen). The estimated conductances of slow IPSPs evoked by weak stimuli in the secondary focus were much smaller than in the controls. However, stimuli that could trigger epileptic discharges in the secondary focus, evoked 2-OH saclofen-sensitive slow IPSPs with estimated conductances approaching the controls. This marked increase in the slow IPSP did not occur when EPSPs, and epileptic bursts, were blocked with CNQX and AP-5, suggesting that a strong barrage of excitation is needed to generate full-sized slow IPSPs in the secondary focus.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of hypoxia on excitation and GABAergic inhibition in mature and developing rat neocortex

To analyze the functional consequences of hypoxia on the efficacy of intracortical inhibitory mechanisms mediated by gamma-aminobutyric acid (GABA), extra- and intracellular recordings were obtained from rat primary somatosensory cortex in vitro. Hypoxia, induced by transient N2 aeration, caused a decrease in stimulus-evoked inhibitory postsynaptic potentials (IPSPs), followed by a pronounced anoxic depolarization. Upon reoxygenation, the fast (f-) and long-latency (l-) IPSP showed a positive shift in the reversal potential by 24.4 and 14.9 mV, respectively. The peak conductance of the f- and l-IPSP was reversibly reduced in the postanoxic period by 72% and 94%, respectively. Extracellular field potential recordings and application of a paired-pulse inhibition protocol confirmed the enhanced sensitivity of inhibitory synaptic transmission for transient oxygen deprivation. Intracellular recordings from morphologically or electrophysiologically identified interneurons did not reveal any enhanced susceptibility for hypoxia as compared to pyramidal cells, suggesting that inhibitory neurons are not selectively impaired in their functional properties. Intracellularly recorded spontaneous IPSPs were transiently augmented in the postanoxic period, indicating that presynaptic GABA release was not suppressed. Developmental studies in adult (older than postnatal day 28), juvenile (P14-18), and young (P5-8) neocortical slices revealed a prominent functional resistance of immature tissue for hypoxia. In comparison with adult cortex, the hypoxia-induced reduction in excitatory and inhibitory synaptic transmission was significantly smaller in immature cortex. Our data indicate a hypoxia-induced distinct reduction of postsynaptic GABAergic mechanisms, leading to the manifestation of intracortical hyperexcitability as a possible functional consequence.

Effect of amygdala kindling on the in vivo release of GABA and 5-HT in the dorsal raphe nucleus in freely moving rats

Our laboratory has previously reported a significant subsensitivity to iontophoretically applied GABA (gamma-aminobutyric acid) in dorsal raphe neurons of amygdala-kindled rats. This subsensitivity was selective for GABA and persisted at least 3 months after the last kindled seizure. In the present series of experiments, we explored mechanisms by which kindling could result in persistent GABA sensitivity changes, using in vivo microdialysis to quantitate neurotransmitter [including GABA and 5-hydroxytryptamine (5-HT)] release in the dorsal raphe nucleus of awake, unrestrained amygdala-kindled rats. Depolarization-induced release of GABA is markedly increased in the dorsal raphe nucleus in amygdala-kindled animals. This change in depolarization-induced GABA release appeared to be graded, dependent upon the stage to which the animal is kindled. Thus GABA release is increased in animals kindled to Stage 2 and even greater in animals kindled to Stage 5 seizures. The change in GABA release is also selective, since no consistent change in the release of other putative amino acid neurotransmitters or 5-HT was observed in these same animals. We hypothesize that this increase in depolarization-induced release of GABA in the amygdala-kindled animal underlies the development of subsensitivity to GABA in dorsal raphe neurons.

In vivo microdialysis of amino acid neurotransmitters in the hippocampus in amygdaloid kindled rat

Extracellular concentrations of gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) were determined by microdialysis in rat hippocampus during various amygdaloid kindled stages. The values of GABA and Glu were increased 3-4 times in C2-C3 stages in comparison with the values in control animals. After reaching the C5 stages, these values were increased 3-7 times. However, the concentration of Asp decreased depending on the kindling stage, reaching the lowest value of 33% in comparison with the normal value. The observed changes may be related to kindling induced seizures.

A long-lasting decrease in the inhibitory effect of GABA on glutamate responses of hippocampal pyramidal neurons induced by kindling epileptogenesis

Experiments were carried out to test whether changes in the sensitivity of hippocampal pyramidal neurons to the neurotransmitters glutamate, GABA and noradrenaline may be associated with the establishment of an epileptogenic focus induced by kindling. The effects of iontophoretically applied neurotransmitters on the firing rate of single units were quantified in the rat hippocampal CA1 area in kindled and control animals. Kindling was induced by electrical tetanic stimulation of the Schaffer collateral/commissural fibers. Firing was evoked by local glutamate iontophoresis while simultaneous GABA or noradrenaline application suppressed this response. A significant reduction of the GABAergic inhibitory action on the firing rate in kindled animals studied around four or around 42 days after the last convulsion was found. In the same neurons, the suppressive effect of noradrenaline was not different from controls. The neurons of kindled animals, investigated around four days after the last seizure, had a reduced sensitivity for glutamate; more glutamate ejection current was needed to evoke firing or to evoke the maximum firing rate. In contrast, the responsiveness for glutamate was significantly increased long-term after the last convulsion. These findings demonstrate that hippocampal Schaffer collateral kindling is associated with a long-lasting reduced effectiveness of the GABA-mediated response on glutamate-evoked firing in CA1.