Brain amino acid concentration changes during status epilepticus induced by lithium and pilocarpine

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Anticonvulsant effects after grafting of rat, porcine, and human mesencephalic neural progenitor cells into the rat subthalamic nucleus

Cell transplantation based therapy is a promising strategy for treating intractable epilepsies. Inhibition of the subthalamic nucleus (STN) or substantia nigra pars reticulata (SNr) is a powerful experimental approach for remote control of different partial seizure types, when targeting the seizure focus is not amenable. Here, we tested the hypothesis that grafting of embryonic/fetal neural precursor cells (NPCs) from various species (rat, human, pig) into STN or SNr of adult rats induces anticonvulsant effects. To rationally refine this approach, we included NPCs derived from the medial ganglionic eminence (MGE) and ventral mesencephalon (VM), both of which are able to develop a GABAergic phenotype. All VM- and MGE-derived cells showed intense migration behavior after grafting into adult rats, developed characteristics of inhibitory interneurons, and survived at least up to 4 months after transplantation. By using the intravenous pentylenetetrazole (PTZ) seizure threshold test in adult rats, transient anticonvulsant effects were observed after bilateral grafting of NPCs derived from human and porcine VM into STN, but not after SNr injection (site-specificity). In contrast, MGE-derived NPCs did not cause anticonvulsant effects after grafting into STN or SNr (cell-specificity). Neither induction of status epilepticus by lithium-pilocarpine to induce neuronal damage prior to the PTZ test nor pretreatment of MGE cells with retinoic acid and potassium chloride to increase differentiation into GABAergic neurons could enhance anticonvulsant effectiveness of MGE cells. This is the first proof-of-principle study showing anticonvulsant effects by bilateral xenotransplantation of NPCs into the STN. Our study highlights the value of VM-derived NPCs for interneuron-based cell grafting targeting the STN.

Post-seizure drug treatment in young rats determines clear incremental losses of frontal cortical and hippocampal neurons: the resultant damage is similar to very old brains

Loss of neurons occurs with aging and following lithium/pilocarpine-induced epileptic seizures. In the present study, the numbers of neurons within the layers from sample areas of the four lobes of the neocortices and the hippocampus were counted by light microscopy in brains of rats that had been administered lithium or pilocarpine and then injected immediately or shortly after seizure onset with either acepromazine, ketamine, or prazosin. The mean numbers of neocortical and hippocampal neurons were lowest in rats treated with acepromazine or prazosin 1h after seizure onset, while those of rats immediately treated with ketamine displayed the least decrements and were most similar to normal rats. The largest loss of neurons occurred within the CA1 field and layers 5 and 6 of the frontal cortices. The mean numbers of neurons within the cortices in rats whose treatments had been delayed for 1h were similar to those of normal rats over 700 days of age. These results support the hypothesis that neuronal loss from cumulative effects of seizure-induced brain damage simulates aging.

Consequences of pilocarpine-induced status epilepticus in immunodeficient mice

Systemic injection of pilocarpine in rodents induces status epilepticus (SE) and reproduces the main characteristics of temporal lobe epilepsy (TLE). Different mechanisms are activated by SE contributing to cell death and immune system activation. We used BALB/c nude mice, a mutant that is severely immunocompromised, to characterize seizure pattern, neurochemical changes, cell death and c-Fos activation secondarily to pilocarpine-induced SE. The behavioral seizures were less severe in BALB/c nude than in BALB/c wild type mice. However, nude mice presented more tonic-clonic episodes and higher mortality rate during SE. The c-Fos expression was most prominent in the caudate-putamen, CA3 (p<0.05), dentate gyrus, entorhinal cortex (p<0.001), basolateral nucleus of amygdala (p<0.01) and piriform cortex (p<0.05) of BALB/c nude mice than of BALB/c. Besides, nude mice subjected to SE presented high number of Fluorojade-B (FJB) stained cells in the piriform cortex, amygdala (p<0.05) and hilus (p<0.001) in comparison with BALB/c mice. A significant increase in the level of glutamate and GABA was found in the hippocampus and cortex of BALB/c mice presenting SE in comparison to controls. However, the level of glutamate was higher in the brains of BALB nude mice than in the brains of BALB/c wild type mice, while the levels of GABA were unchanged. These results indicate that the brains of immunodeficient nude mice are more vulnerable to the deleterious effects of pilocarpine-induced SE as they present intense activation, increased glutamate levels and more cell death.

Lipoic acid effects on glutamate and taurine concentrations in rat hippocampus after pilocarpine-induced seizures

Pilocarpine-induced seizures can be mediated by increases in oxidative stress and by cerebral amino acid changes. The present research suggests that antioxidant compounds may afford some level of neuroprotection against the neurotoxicity of seizures in cellular level. The objective of the present study was to evaluate the lipoic acid (LA) effects in glutamate and taurine contents in rat hippocampus after pilocarpine-induced seizures. Wistar rats were treated intraperitoneally (i.p.) with 0.9% saline (Control), pilocarpine (400 mg/kg, Pilocarpine), LA (10 mg/kg, LA), and the association of LA (10 mg/kg) plus pilocarpine (400 mg/kg), that was injected 30 min before of administration of LA (LA plus pilocarpine). Animals were observed during 24 h. The amino acid concentrations were measured using high-performance liquid chromatograph (HPLC). In pilocarpine group, it was observed a significant increase in glutamate content (37%) and a decrease in taurine level (18%) in rat hippocampus, when compared to control group. Antioxidant pretreatment significantly reduced the glutamate level (28%) and augmented taurine content (32%) in rat hippocampus, when compared to pilocarpine group. Our findings strongly support amino acid changes in hippocampus during seizures induced by pilocarpine, and suggest that glutamate-induced brain damage plays a crucial role in pathogenic consequences of seizures, and imply that strong protective effect could be achieved using lipoic acid through the release or decrease in metabolization rate of taurine amino acid during seizures.

Subunit-specific trafficking of GABA(A) receptors during status epilepticus

It is proposed that a reduced surface expression of GABA(A) receptors (GABARs) contributes to the pathogenesis of status epilepticus (SE), a condition characterized by prolonged seizures. This hypothesis was based on the finding that prolonged epileptiform bursting (repetitive bursts of prolonged depolarizations with superimposed action potentials) in cultures of dissociated hippocampal pyramidal neurons (dissociated cultures) results in the increased intracellular accumulation of GABARs. However, it is not known whether this rapid modification in the surface-expressed GABAR pool results from selective, subunit-dependent or nonselective, subunit-independent internalization of GABARs. In hippocampal slices obtained from animals undergoing prolonged SE (SE-treated slices), we found that the surface expression of the GABAR beta2/3 and gamma2 subunits was reduced, whereas that of the delta subunit was not. Complementary electrophysiological recordings from dentate granule cells in SE-treated slices demonstrated a reduction in GABAR-mediated synaptic inhibition, but not tonic inhibition. A reduction in the surface expression of the gamma2 subunit, but not the delta subunit was also observed in dissociated cultures and organotypic hippocampal slice cultures when incubated in an elevated KCl external medium or an elevated KCl external medium supplemented with NMDA, respectively. Additional studies demonstrated that the reduction in the surface expression of the gamma2 subunit was independent of direct ligand binding of the GABAR. These findings demonstrate that the regulation of surface-expressed GABAR pool during SE is subunit-specific and occurs independent of ligand binding. The differential modulation of the surface expression of GABARs during SE has potential implications for the treatment of this neurological emergency.

Trafficking of GABA(A) receptors, loss of inhibition, and a mechanism for pharmacoresistance in status epilepticus

During status epilepticus (SE), GABAergic mechanisms fail and seizures become self-sustaining and pharmacoresistant. During lithiumpilocarpine-induced SE, our studies of postsynaptic GABA(A) receptors in dentate gyrus granule cells show a reduction in the amplitude of miniature IPSCs (mIPSCs). Anatomical studies show a reduction in the colocalization of the beta2/beta3 and gamma2 subunits of GABA(A) receptors with the presynaptic marker synaptophysin and an increase in the proportion of those subunits in the interior of dentate granule cells and other hippocampal neurons with SE. Unlike synaptic mIPSCs, the amplitude of extrasynaptic GABA(A) tonic currents is augmented during SE. Mathematical modeling suggests that the change of mIPSCs with SE reflects a decrease in the number of functional postsynaptic GABA(A) receptors. It also suggests that increases in extracellular [GABA] during SE can account for the tonic current changes and can affect postsynaptic receptor kinetics with a loss of paired-pulse inhibition. GABA exposure mimics the effects of SE on mIPSC and tonic GABA(A) current amplitudes in granule cells, consistent with the model predictions. These results provide a potential mechanism for the inhibitory loss that characterizes initiation of SE and for the pharmacoresistance to benzodiazepines, as a reduction of available functional GABA(A) postsynaptic receptors. Novel therapies for SE might be directed toward prevention or reversal of these losses.

Variations of dopamine, serotonin, and amino acid concentrations in Noda epileptic rat (NER) retina

Noda epileptic rats (NER) exhibit frequent spontaneous tonic-clonic convulsions which represent a valuable model of human epilepsy. If implication of brain neurotransmitters was largely reported, little is known about retina. However, it has been reported that human epilepsy syndrome varies not only with the location of seizure foci but also according to rhythmic patterns, for which retina has a major role in the transmission of external light-dark cycle information. The purpose of this work was to evaluate dopamine (DA), DA metabolites, serotonin (5-HT), and amino acid [glutamate, aspartate, glycine, gamma aminobutyric acid (GABA), and taurine] level variations in retina from NER, at two different nycthemeral periods (11 a.m. and 11 p.m.) and at different ages (2, 6, and 12 months). In NER, retinal dopaminergic function was decreased as soon as 2 months, whereas GABA levels were increased, even if no differences among the different ages could be distinguished. These variations were associated to a slight increase in 5-HT. Other amino acids tested were not affected by epilepsy, whereas taurine decreased with aging in NER as well as in control rats. Retinal 5-HT occurs principally as a precursor of melatonin (MEL). A triangular interaction may be hypothesized: MEL could decrease DA synthesis or release by enhancing GABA activity. Taken together, these results suggest that the retinal physiology is affected by the epileptic status and that information transmitted from retina to the brain should be affected by epilepsy in NER.

GABA synapses and the rapid loss of inhibition to dentate gyrus granule cells after brief perforant-path stimulation

PURPOSE

To study the pharmacologic and synaptic basis for the early loss of paired-pulse inhibition that occurs in the perforant-path stimulation model of status epilepticus.

METHODS

Hippocampal slices were prepared from male Wistar rats. Test paired pulses (20- to 50-ms interstimulus interval) of the perforant path were used before and after an abbreviated period of perforant-path stimulation (1-5 min; 2-Hz continuous with 20 Hz of 10 s/min pulses) while either recording field potentials from the dentate gyrus granule cell layer or directly measuring whole-cell patch-clamp currents from granule cells. Paired-pulse field recordings also were obtained during perfusion of the gamma-aminobutyric acid (GABA)(A) antagonist bicuculline.

RESULTS

Prolonged loss of paired-pulse inhibition occurs after brief (< 5 min) perforant-path stimulation in vitro (similar to results in vivo) with the paired-pulse population spike amplitude ratio (P2/P1) increasing from a baseline of 0.53 +/- 0.29 to 1.17 +/- 0.09 after perforant-path stimulation (p < 0.05). After perfusion with the GABA(A) antagonist, bicuculline, the P2/P1 ratio also increased from a baseline of 0.52 +/- 0.16 to 1.15 +/- 0.26 (p < 0.05). After 1-2 min of perforant-path stimulation, a 22 +/- 6% (p < 0.05) decrease occurred in the P2/P1 amplitude ratio of paired-pulse evoked inhibitory postsynaptic currents.

CONCLUSIONS

Similar to in vivo, loss of paired-pulse inhibition occurs with brief perforant-path stimulation in vitro. GABA(A) antagonism causes a similar loss of paired-pulse inhibition, and the effects of perforant-path stimulation on postsynaptic inhibitory currents also are consistent with the involvement of GABA(A) synaptic receptors. The findings suggest that loss of inhibition at GABA synapses may be an important early event in the initiation of status epilepticus.

Structural and functional alterations in mitochondrial membrane in picrotoxin-induced epileptic rat brain

Mitochondrial function is a key determinant of both excitability and viability of neurons. Present studies were carried out to decipher cerebral mitochondrial oxidative energy metabolism and membrane function in the chronic condition of generalized seizures induced by picrotoxin (PTX) in rats. PTX-induced convulsions resulted in decreased respiration rates (14-41%) with glutamate, pyruvate + malate, and succinate as substrate. The ADP phosphorylation rates were drastically reduced by 44-65%. An opposite trend was observed with ascorbate + N,N,N',N'-tetramethyl-p-phenylenediamine [corrected] (TMPD) as substrate. In general, uncoupling of the mitochondrial electron transport was observed after PTX treatment. Malate dehydrogenase (MDH) and succinate dehydrogenase (SDH) activities were decreased by 20-80%; also, there was significant reduction in cytochrome b content after PTX treatment, while the F(o)F(1) ATPase (complex V) activity increased in basal and 2,4-dinitrophenol (DNP)-stimulated condition, indicating increased membrane fragility. The substrate kinetics analysis had shown that K(m) and V(max) of the higher affinity kinetic component of ATPase increased significantly by 1.2- to 1.4-fold in epileptic condition. Temperature kinetic analysis revealed 1.2-fold increase in energies of activation with decreased transition temperature. The total phospholipid (TPL) and cholesterol (CHL) contents decreased significantly with lowering of diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS), while lysophospholipid (lyso), sphingomyelin (SPM), and phosphatidylcholine components were found to be elevated. Brain mitochondrial membrane was somewhat more fluidized in epileptic animals. Possible consequences of mitochondrial respiratory chain (MRC) dysfunction are discussed. In conclusion, impairment of MRC function along with structural alterations suggests novel pathophysiological mechanisms important for chronic epileptic condition.

Pilocarpine-induced status epilepticus increases glutamate release in rat hippocampal synaptosomes

A pronounced glutamate release has been related to neuronal death in several structures due to status epilepticus (SE). We investigated the glutamate uptake and release by both cortical and hippocampal synaptosome in pilocarpine model of epilepsy. Animals were submitted to long-lasting SE (12 h) induced by pilocarpine and compared with non-treated animals. Animals presenting SE did not modify the glutamate uptake by synaptosomes. An increase in the glutamate efflux in the absence (1.43-fold) and in the presence of KCl (1.25-fold) was found in hippocampal synaptosomes. Pilocarpine added to the medium did not modify the glutamate release profile, showing that SE is necessary to modify the glutamate release. As the glutamate uptake is not modified, the hippocampal excitotoxicity may be related to impairment only in the mechanism of the glutamate release.

Electrophysiological, neurochemical and regional effects of levetiracetam in the rat pilocarpine model of temporal lobe epilepsy

This study compared levetiracetam (Keppra) with reference antiepileptic drugs (AEDs) in the rat pilocarpine model of temporal lobe epilepsy. Electroencephalogram (EEG) recordings showed that i.p. administration of valproate (300 mg/kg), phenobarbital (5 mg/kg) and clonazepam (0.5 mg/kg) all significantly delayed the appearance of the first epileptic spike discharge in hippocampus as well as synchronous epileptiform activity in hippocampus and cortex. In contrast, i.p. administration of levetiracetam (17 mg/kg) only significantly delayed the appearance of the latter. This was corroborated by findings showing that i.p. administration of levetiracetam (17 mg/kg) significantly opposed pilocarpine-induced increases in the amplitude of the orthodromic population spike in the hippocampal CA3 area of urethane-anaesthetised rats, while valproate (200 mg/kg), phenobarbital (10 mg/kg) and clonazepam (1 mg/kg) had no effect. Pre-treatment i.p. with phenobarbital (10 mg/kg) and clonazepam (0.5 mg/kg) significantly reversed seizure-induced changes in aspartate and GABA concentrations while valproate (300 mg/kg) significantly reduced aspartate concentrations further. In contrast, levetiracetam (34 mg/kg) significantly counteracted all seizure-induced alterations in amino acid concentrations. Midazolam induced significant seizure protection after microinjection into substantia nigra pars reticulata (SNR, 50 nmol), nucleus accumbens (NA, 25 nmol) and caudate putamen (CP, 25 nmol), whereas phenytoin (50 nmol) only showed significant seizure protection after injection into the latter area. Levetiracetam differed by significant seizure protection after injection into SNR (1,000 nmol) and NA (3,000 nmol). These results suggest that levetiracetam is distinct from other AEDs by its ability to selectively suppress synchronisation of neuronal spike and burst firing in hippocampus.

Seizure onset times for rats receiving systemic lithium and pilocarpine: sources of variability

Injection of 30 mg/kg of pilocarpine 24 h after systemic injection of lithium (3 mEq/kg) results in overt limbic motor seizures within about 30 min. Results of several experiments indicated that whereas food deprivation or repeated nociceptive stimulation during the previous 24 h decreased seizure onset times (SOTs) by about 11 to 12 min, food restriction, continuous lighting, or, handling during the previous 7 to 14 days increased SOTs by comparable durations. Early handling before weaning but not injections of clomimpramine also decreased SOTs. A difference of 18 min in the means of SOTs was produced by injecting either 1.0 (increased SOT) or 1.5 mg/kg (decreased SOT) of dexamethasome during the previous 24 h. A strong (multiple r=.87) association between SOTs and the amount of damage within five specific thalamic-limbic nuclei was observed. These results, in conjunction with blood corticosterone levels taken before and after induction of the seizures, suggest the neurochemical mechanisms affecting the range in SOTs could involve the adrenocorticotropic hormone (ACTH)-corticosterone system and influence the amount of post-seizure-induced damage.

Temporal changes in neuronal dropout following inductions of lithium/pilocarpine seizures in the rat

Estimates of neuronal dropout for approximately 100 structures as defined by Paxinos-Watson were completed for brains of male Wistar albino rats between 1 and 50 days after status epilepticus was evoked by a single systemic injection of lithium and pilocarpine. Sample estimates of neuronal loss were strongly correlated with direct measures of cell density. The most extensive immediate damage occurred within the substantia nigra reticulata, CA1 field of the hippocampus, the piriform cortex and the reuniens and paratenial nuclei of the thalamus. Neuronal dropout continued in many other structures over a 50-day period. Structures that showed the greatest 2-deoxyglucose (2-DG) uptake during discrete seizures and waxing and waning seizures within the early stages of status epilepticus but the least 2-DG uptake at the time of late continuous spiking and fast spiking with pauses [Neuroscience 64 (1995) 1057, 1075] exhibited the most neuronal dropout. Relationships between the delay of injection of acepromazine (which facilitated survival) and the amount of damage suggested that the source of the process that results in permanent brain damage may originate within the region of the piriform cortices and its subcortices.

Lithium plus pilocarpine induced status epilepticus--biochemical changes

The aim of the study was to investigate the changes in biochemical mechanisms facilitating cellular damages in the lithium plus pilocarpine treatment and the resulting status epilepticus. The whole brain free fatty acid (FFA) level as well as the activities of superoxide dismutase (SOD), glutathione peroxidase (GPX), glutamate dehydrogenase, aspartate-aminotransferase (AST), alanine-aminotransferase, gamma-glutamoyl transferase, alkaline phosphatase (ALP), lactate dehydrogenase (LDH) and creatine kinase in the frontal cortex, cerebellum, hippocampus and pons-medulla region of Hannover-Wistar rats were determined. The control group was intact with no previous experimental history. LiCl (125 mg/kg i.p.) was injected 20 h prior to pilocarpine (30 mg/kg i.p.) and the treated rats were sacrificed 1 or 2 1/2 h after pilocarpine administration. The results show that lithium plus pilocarpine administration and the resulting status epilepticus produced the significant increase of the brain FFA content. Decreased GPX activities were detected in the frontal cortex, cerebellum and hippocampus of the treated rats without the accompanying decrease of SOD activity. Increased AST and LDH activities were observed in the frontal cortex, increased soluble ALP activities in the frontal cortex and pons-medulla region whereas the increased activity of membrane bound ALP was detected in the hippocampus of the rats with status epilepticus. Activities of the other analysed enzymes did not change in the examined brain regions. The presented data indicate clear regional differences of biochemical changes caused by lithium plus pilocarpine treatment and the resulting status epilepticus, frontal cortex being the most affected site.

Altered residual ATP content in rat brain cortex subcellular fractions following status epilepticus induced by lithium and pilocarpine

Changes in residual ATP concentrations were investigated following subcellular fractionation of rat brain cortex after a prolonged period of status epilepticus induced by sequential administration of lithium and pilocarpine. After 2 h of continuous high-amplitude rapid spiking on EEG, we found significantly decreased levels of residual ATP in the homogenate and mitochondria fractions from status epilepticus rat brains compared to matched controls. No difference in residual ATP level was observed in the synaptosomal preparations of status epilepticus animals compared to controls. Inorganic phosphate concentration in the status animals was higher than controls in the cytosolic fraction only. F1-ATPase activity, an enzymatic indicator of mitochondrial ATP synthesis rate, was significantly higher in the status brains, whereas other mitochondrial enzymes were not different in the status and control rat groups. These findings, together with our earlier report of reduced synaptosomal ecto-ATPase activity, suggest that either the corresponding in vivo ATP concentrations were reduced as a result of status epilepticus or other biochemical changes had occurred that facilitated the hydrolysis of ATP following decapitation. Controls for and measurement of such other changes failed to provide an explanation for the observed changes in residual ATP.

Physiological analysis of Rasmussen's encephalitis: patch clamp recordings of altered inhibitory neurotransmitter function in resected frontal cortical tissue

Rasmussen's encephalitis (RE) is a progressive, rare childhood disease characterized by severe epilepsy, hemiplegia, dementia, and inflammation of the brain. While one mechanism underlying the pathogenesis of RE has been hypothesized to be mediated by production of excitotoxic GluR3 autoantibodies to the AMPA receptor, other neuropathological etiologies have also been indicated. Whole-cell patch clamp recordings of GABA(A) receptor mediated responses were conducted in neurons acutely isolated from an RE patient, and compared to properties of non-focal human temporal cortical neurons. RE neurons appeared similar anatomically to control cortical neurons. Significant differences in GABAergic responses were evident between RE and control neurons. GABA was significantly more potent in RE than in control cortical neurons (EC50 of 13 microM vs 23 microM, respectively). In addition, the overall efficacy of GABA was significantly decreased in RE neurons, associated with a decrease in postsynaptic GABA current density in RE neurons (5.1 pA/microm2) in comparison to controls (9.2 pA/microm2). Augmentation of GABA responses by the benzodiazepine, clonazepam (CNZ), was significantly reduced in RE in comparison to control neurons (34% vs 99% augmentation at 100 nM). The RE-associated reduced functional efficacy and altered pharmacology of neuronal GABA(A) receptors is consistent with overall disinhibition in RE neurons, and could contribute to the generation of the severe epileptic activity evident in this disorder.

Physiological and pharmacological alterations in postsynaptic GABA(A) receptor function in a hippocampal culture model of chronic spontaneous seizures

Cultured rat hippocampal neurons previously exposed to a media containing no added Mg2+ for 3 h begin to spontaneously trigger recurrent epileptiform discharges following return to normal medium, and this altered population epileptiform activity persisted for the life of the neurons in culture (> 2 wk). Neurons in "epileptic" cultures appeared similar in somatic and dendritic morphology and cellular density to control, untreated cultures. In patch-clamp recordings from hippocampal pyramidal cells from "epileptic," low Mg2+ pretreated hippocampal cultures, a rapid (within 2 h of treatment), permanent (lasting > or = 8 days) and statistically significant 50-65% reduction in the current density of functional gamma-aminobutyric acid-A (GABA(A)) receptors was evident when the GABA responses of these cells were compared with control neurons. Functional GABA receptor current density was calculated by determining the maximal response of a cell to GABA 1 mM application and normalizing this response to cellular capacitance. Despite the marked GABA efficacy differences noted above, the potency of GABA in activating chloride currents was not significantly different when the responses to control and "epileptic" pyramidal cells to multiple concentrations of GABA were compared. The EC50 for GABA was 4.5 +/- 0.2 (mean +/- SE) for control neurons and 3.5 +/- 0.4 microM, 5.2 +/- 0.5 microM, 3.7 +/- 0.3 microM, and 4.6 +/- 0.3 microM for epileptic neurons 2 h, 2 days, 3 days, and 8 days after low Mg2+ pretreatment, respectively. Modulation of GABA responses by the benzodiazepine, clonazepam, was significantly reduced in epileptic neurons compared with controls. The kinetically determined clonazepam 100 nM GABA augmentation efficacy decreased from 44.1% in control neurons to 9.3% augmentation in neurons recorded from cultures 10 days posttreatment. The kinetics of GABA current block by the noncompetitive antagonist picrotoxin were determined in hippocampal cultured neurons, and an IC50 of 14 microM determined. Bath application of picrotoxin at half of the IC50 concentration (7 microM) induced epileptiform activity in control cultures and this activity appeared very similar to the epileptiform activity induced by prior low Mg2+ treatment. This concentration of picrotoxin was determined experimentally to block 30% of the GABA(A)-mediated receptor responses in these cultures, and this level of block was sufficient to trigger spontaneous epileptiform activity. The 50% reduction of GABA responses induced as a permanent consequence of low Mg2+ treatment therefore was determined to be sufficient in and of itself to induce the spontaneous epileptiform activity, which was also a consequence of this treatment.

Hippocampal and cerebellar extracellular amino acids during pilocarpine-induced seizures in freely moving rats

Limbic seizures were provoked in freely moving rats by intrahippocampal administration of the muscarinic receptor agonist pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microliters/min). Changes in extracellular hippocampal and cerebellar glutamate, aspartate and gamma-aminobutyric acid (GABA) levels were monitored during and after pilocarpine administration. Effects of systemic or local administration of anticonvulsants on the seizures and concomitant changes in amino-acid concentrations, were investigated. Pilocarpine-induced seizures were completely abolished after intraperitoneal premedication for 7 days with phenobarbital (15 mg/kg per day) and after intrahippocampal administration of 10 mM phenobarbital and 1 mM carbamazepine (180 min at 2 microliters/min). Rats premedicated with carbamazepine (5 mg/kg per day) still developed seizures. The changes in extracellular hippocampal amino-acid levels suggest that glutamate, aspartate and GABA are not involved in seizure onset, but may play a role in seizure maintenance and/or spread in the pilocarpine animal model of epilepsy. The increases in extracellular amino acids in ipsi- and contralateral cerebellum following limbic seizures provoked in the hippocampus, probably play a role in the 'reversed' diaschisis phenomenon.

GABA metabolism during status epilepticus in the developing rat brain

The rate of synthesis of GABA, the major inhibitory neurotransmitter, was determined in parietal cortex and hippocampus during SE induced by systemic administration of lithium (3 mEq/kg) followed 20 h later by pilocarpine (100 mg/kg) in 1-4-week-old rats. Our results show that the immature hippocampus is better capable of maintaining GABA synthesis in the face of SE at the earliest stages of development studied (74.1% of basal in 1-week-old) and that development results in a progressive decline in the ability to maintain GABA synthesis in the face of SE (44.1% of basal by 4 weeks) that may parallel the ontogeny of self-sustaining seizures. Our data describe an aspect of developmental GABA neurochemistry which may in part explain the relative resistance of the immature hippocampus to seizure spread and of certain types of seizure-induced damage.

Anticonvulsant activity of new and potent inhibitors of nitric oxide synthase

The effects of new and potent NOS inhibitors, S-methyl-L-thiocitrulline (S-Me-TC), 3-bromo 7-nitro indazole (3-Br-7-NI), and 1-(2-trifluoromethylphenyl) imidazole (TRIM), were examined on the pilocarpine-induced seizures in mice. 3-Br-7-NI and TRIM decreased the frequency of status epilepticus and mortality, while TRIM. In addition, significantly reduced the incidence of seizures. The latencies to onsets of seizures, status epilepticus, and mortality were significantly prolonged by all three NOS inhibitors, while duration of seizures was reduced by 3-Br-7-NI and TRIM. These data suggest an excitatory effect of NO in the neuronal structure involved in the pilocarpine-induced seizures.

Experimental status epilepticus alters gamma-aminobutyric acid type A receptor function in CA1 pyramidal neurons

There is a reduction of gamma-aminobutyric acid (GABA)-mediated inhibition of the CA1 pyramidal region of the hippocampus during status epilepticus (SE). The cellular basis of this loss of GABA-mediated inhibition is not known. This study tested the possibility that GABA type A (GABAA) receptor function in CA1 pyramidal neurons was reduced or blocked during SE, at least in part by postsynaptic cellular mechanisms. GABAA receptor currents (IGABA) were studied by whole-cell patch-clamp techniques in CA1 pyramidal neurons acutely dissociated from rats undergoing lithium/pilocarpine-induced limbic status epilepticus (SE neurons) and from naive rats (naive neurons). SE neurons had more depolarized resting membrane potential (-17.3 mV) compared with naive neurons (-56 mV). IGABA was absent in 47% of SE neurons and reduced in 55% of the remainder, compared with naive neurons. The reduction in IGABA in SE neurons resulted from a combination of factors, including reduced potency and reduced efficacy of GABA in activating chloride channels, and diminished driving force for the GABA-induced chloride currents once activated. These postsynaptic cellular mechanisms resulted in a net reduction or loss in GABA-mediated inhibition and may explain previous in vivo findings reporting a loss of inhibition in hippocampus during limbic SE.

Cardiac hypertrophy secondary to status epilepticus in the rat

Status epilepticus was induced in rats by sequential injections of lithium and pilocarpine. Seizure activity was aborted by a combination of MK-801 and diazepam, with status duration ranging from 3 to 180 min. When the hearts were examined 8-12 days later, rats that had experienced an episode of status epilepticus had significantly heavier hearts than did controls. The nature of the cardiac tissue changes was not examined, and deserves further study.

Prevention of sudden cardiac death by the atypical neuroleptic acepromazine following status epilepticus in rats

Normal male rats in which status epilepticus has been induced by injecting 30 mg/kg of pilocarpine after a single systemic administration of lithium (sufficient to produce blood levels of 0.2 mEq/L) invariably die within 24 hr. Real-time monitoring indicated sudden cardiac death; it was preceded by progressive intensification of arrhythmia. A single systemic injection (25 mg/kg) of the atypical phenothiazine acepromazine prevented the mortality and virtually eliminated the cardiac instability.

Spontaneous recurrent seizures in rats: amino acid and monoamine determination in the hippocampus

Rats subjected to structural brain damage induced by sustained convulsions triggered by systemic administration of pilocarpine (PILO) are a useful model for investigation of the mechanisms essential for seizure generation and spread in rodents. After PILO administration, three distinct phases are observed: (a) an acute period of 1-2 days' duration corresponding to a pattern of repetitive limbic seizures and status epilepticus; (b) a seizure-free (silent) period characterized by a progressive return to normal EEG and behavior of 4-44 days' duration; and (c) a period of spontaneous recurrent seizures (SRS) starting 5-45 days after PILO administration and lasting throughout the animal's life. PILO (320-350 mg/kg intraperitoneally, i.p.) was administered to rats, and the content of hippocampal monoamines and amino acids was measured in the acute, silent, and SRS periods by liquid chromatography. Norepinephrine (NE) level was decreased during all periods whereas dopamine (DA) content was increased. Serotonin (5-hydroxytryptamine, 5-HT) was increased only in the acute period. Utilization rate measurement of monoamines showed increased NE consumption and decreased DA consumption during all phases. 5-HT utilization rate was increased only in the acute period. Amino acid content showed a decrease in aspartate (ASP) and glutamate (GLU) concentrations associated with increased gamma-aminobutyric acid (GABA) level during the acute period. The silent phase was characterized by a decrease in glycine (GLY) and GABA levels and an increase in GLU concentration. The SRS period showed an increase in all amino acid concentrations. These findings show important neurochemical changes in the course of establishment of an epileptic focus after brain damage induced by status epilepticus triggered by pilocarpine.

Paradoxical facilitation of pilocarpine-induced seizures in the mouse by MK-801 and the nitric oxide synthesis inhibitor L-NAME

The sensitivity of pilocarpine-induced seizures to NMDA receptor blockade with MK-801, or to inhibition of synthesis of the second messenger nitric oxide (NO) with N omega-nitro-L-arginine methyl ester (L-NAME), was studied in mice. The NO precursor L-arginine (100-500 mg/kg, IP) and L-NAME (1-125 mg/kg, IP) had no overt effects on animals' behaviour by themselves, while MK-801 (0.1-0.8 mg/kg, IP) caused motor excitability at low doses and sedation and paraplegia at high ones. Contrary to expectation, MK-801 and L-NAME failed to protect mice against limbic motor seizures induced by pilocarpine (400 mg/kg, IP), and L-arginine was not proconvulsant in mice challenged with a threshold convulsant dose of the cholinomimetic (100 mg/kg, IP). Surprisingly, both MK-801 and L-NAME were found to be proconvulsant when injected in conjunction with 100 mg/kg pilocarpine, and in both cases this convulsant action synergised with that produced by the dopamine D1 agonist SK&F38393 (10 mg/kg, IP). Concomitant administration of L-arginine (500 mg/kg) prevented the convulsant effect of 5 mg/kg L-NAME but was ineffective against 25 mg/kg L-NAME and MK-801. It is concluded that glutamate, acting through the NMDA receptor and NO production, normally suppresses epileptogenesis in the mouse pilocarpine model of limbic epilepsy.

Behaviors of rats with insidious, multifocal brain damage induced by seizures following single peripheral injections of lithium and pilocarpine

Several domains of behavior were measured in rats (n = 465) 10 days to 100 days after induction of limbic seizures by a single subcutaneous injection of lithium and pilocarpine. These rats displayed enhanced intragroup aggression but normal muricide; gustatory neophobia and conditioned taste aversion were virtually eliminated. Severe working and reference memory deficits were evident within the radial arm maze. Both state-dependent memory and possible situation-dependent precipitation of spontaneous seizures were suggested. The behavioral changes were considered commensurate with the multifocal pattern of thalamic, hippocampal/amygdaloid, and limbic cortical damage.