Effects of bicuculline and baclofen on paired-pulse depression in the dentate gyrus of epileptic patients

Acute Stress Facilitates LTD Induction at Glutamatergic Synapses in the Hippocampal CA1 Region by Activating μ-Opioid Receptors on GABAergic Neurons

Acute stress impairs recall memory through the facilitation of long-term depression (LTD) of hippocampal synaptic transmission. The endogenous opioid system (EOS) plays essential roles in stress-related emotional and physiological responses. Specifically, behavioral studies have shown that the impairment of memory retrieval induced by stressful events involves the activation of opioid receptors. However, it is unclear whether signaling mediated by μ-opioid receptors (μRs), one of the three major opioid receptors, participates in acute stress-related hippocampal LTD facilitation. Here, we examined the effects of a single elevated platform (EP) stress exposure on excitatory synaptic transmission and plasticity at the Schaffer collateral-commissural (SC) to CA1 synapses by recording electrically evoked field excitatory postsynaptic potentials and population spikes of hippocampal pyramidal neurons in anesthetized adult mice. EP stress exposure attenuated GABAergic feedforward and feedback inhibition of CA1 pyramidal neurons and facilitated low-frequency stimulation (LFS)-induced long-term depression (LTD) at SC-CA1 glutamatergic synapses. These effects were reproduced by exogenously activating μRs in unstressed mice. The specific deletion of μRs on GABAergic neurons (μR_GABA) not only prevented the EP stress-induced memory impairment but also reversed the EP stress-induced attenuation of GABAergic inhibition and facilitation of LFS-LTD. Our results suggest that acute stress endogenously activates μR_GABA to attenuate hippocampal GABAergic signaling, thereby facilitating LTD induction at excitatory synapses and eliciting memory impairments.

Abnormal neuronal patterning occurs during early postnatal brain development of Scn1b-null mice and precedes hyperexcitability

Voltage-gated Na(+) channel (VGSC) β1 subunits, encoded by SCN1B, are multifunctional channel modulators and cell adhesion molecules (CAMs). Mutations in SCN1B are associated with the genetic epilepsy with febrile seizures plus (GEFS+) spectrum disorders in humans, and Scn1b-null mice display severe spontaneous seizures and ataxia from postnatal day (P)10. The goal of this study was to determine changes in neuronal pathfinding during early postnatal brain development of Scn1b-null mice to test the hypothesis that these CAM-mediated roles of Scn1b may contribute to the development of hyperexcitability. c-Fos, a protein induced in response to seizure activity, was up-regulated in the Scn1b-null brain at P16 but not at P5. Consistent with this, epileptiform activity was observed in hippocampal and cortical slices prepared from the P16 but not from the P5-P7 Scn1b-null brain. On the basis of these results, we investigated neuronal pathfinding at P5. We observed disrupted fasciculation of parallel fibers in the P5 null cerebellum. Further, P5 null mice showed reduced neuron density in the dentate gyrus granule cell layer, increased proliferation of granule cell precursors in the hilus, and defective axonal extension and misorientation of somata and processes of inhibitory neurons in the dentate gyrus and CA1. Thus, Scn1b is critical for neuronal proliferation, migration, and pathfinding during the critical postnatal period of brain development. We propose that defective neuronal proliferation, migration, and pathfinding in response to Scn1b deletion may contribute to the development of hyperexcitability.

Changes of paired-pulse evoked responses during the development of epileptic activity in the hippocampus

Dysfunction of inhibitory synaptic transmission can destroy the balance between excitatory and inhibitory synaptic inputs in neurons, thereby inducing epileptic activity. The aim of the paper is to investigate the effects of successive excitatory inputs on the epileptic activity induced in the absence of inhibitions. Paired-pulse orthodromic and antidromic stimulations were used to test the changes in the evoked responses in the hippocampus. Picrotoxin (PTX), γ-aminobutyric acid (GABA) type A (GABA(A)) receptor antagonist, was added to block the inhibitory synaptic transmission and to establish the epileptic model. Extracellular evoked population spike (PS) was recorded in the CA1 region of the hippocampus. The results showed that the application of PTX induced a biphasic change in the paired-pulse ratio of PS amplitude. A short latency increase of the second PS (PS2) was later followed by a reappearance of PS2 depression. This type of depression was observed in both orthodromic and antidromic paired-pulse responses, whereas the GABAergic PS2 depression [called paired-pulse depression (PPD)] during baseline recordings only appeared in orthodromic-evoked responses. In addition, the depression duration at approximately 100 ms was consistent with a relative silent period observed within spontaneous burst discharges induced by prolonged application of PTX. In conclusion, the neurons may ignore the excitatory inputs and intrinsically generate bursts during epileptic activity. The depolarization block could be the mechanisms underlying the PPD in the absence of GABA(A) inhibitions. The distinct neuronal responses to stimulations during different epileptic stages may implicate the different antiepileptic effects of electrical stimulation.

Short-term synaptic plasticity in the dentate gyrus of monkeys

The hippocampus plays an important role in learning and memory. Synaptic plasticity in the hippocampus, short-term and long-term, is postulated to be a neural substrate of memory trace. Paired-pulse stimulation is a standard technique for evaluating a form of short-term synaptic plasticity in rodents. However, evidence is lacking for paired-pulse responses in the primate hippocampus. In the present study, we recorded paired-pulse responses in the dentate gyrus of monkeys while stimulating to the medial part of the perforant path at several inter-pulse intervals (IPIs) using low and high stimulus intensities. When the stimulus intensity was low, the first pulse produced early strong depression (at IPIs of 10–30 ms) and late slight depression (at IPIs of 100–1000 ms) of field excitatory postsynaptic potentials (fEPSPs) generated by the second pulse, interposing no depression IPIs (50–70 ms). When the stimulus intensity was high, fEPSPs generated by the second pulse were depressed by the first pulse at all IPIs except for the longest one (2000 ms). Population spikes (PSs) generated by the second pulse were completely blocked or strongly depressed at shorter IPIs (10–100 or 200 ms, respectively), while no depression or slight facilitation occurred at longer IPIs (500–2000 ms). Administration of diazepam slightly increased fEPSPs, while it decreased PSs produced by the first pulse. It also enhanced the facilitation of PSs produced by the second stimulation at longer IPIs. The present results, in comparison with previous studies using rodents, indicate that paired-pulse responses of fEPSPs in the monkey are basically similar to those of rodents, although paired-pulse responses of PSs in the monkey are more delayed than those in rodents and have a different sensitivity to diazepam.

Low level postnatal methylmercury exposure in vivo alters developmental forms of short-term synaptic plasticity in the visual cortex of rat

Methylmercury (MeHg) has been previously shown to affect neurotransmitter release. Short-term synaptic plasticity (STP) is primarily related to changes in the probability of neurotransmitter release. To determine if MeHg affects STP development, we examined STP forms in the visual cortex of rat following in vivo MeHg exposure. Neonatal rats received 0 (0.9% NaCl), 0.75 or 1.5 mg/kg/day MeHg subcutaneously for 15 or 30 days beginning on postnatal day 5, after which visual cortical slices were prepared for field potential recordings. In slices prepared from rats treated with vehicle, field excitatory postsynaptic potentials (fEPSPs) evoked by paired-pulse stimulation at 20-200 ms inter-stimulus intervals showed a depression (PPD) of the second fEPSP (fEPSP2). PPD was also seen in slices prepared from rats after 15 day treatment with 0.75 or 1.5 mg/kg/day MeHg. However, longer duration treatment (30 days) with either dose of MeHg resulted in paired-pulse facilitation (PPF) of fEPSP2 in the majority of slices examined. PPF remained observable in slices prepared from animals in which MeHg exposure had been terminated for 30 days after completion of the initial 30 day MeHg treatment, whereas slices from control animals still showed PPD. MeHg did not cause any frequency- or region-preferential effect on STP. Manipulations of [Ca2+](e) or application of the GABA(A) receptor antagonist bicuculline could alter the strength and polarity of MeHg-induced changes in STP. Thus, these data suggest that low level postnatal MeHg exposure interferes with the developmental transformation of STP in the visual cortex, which is a long-lasting effect.

Somatostatin: an endogenous antiepileptic

The neuropeptide somatostatin (SST) is highly expressed in brain regions associated with seizures. In hippocampus, SST expression and release is regulated by seizures, and SST-containing neurons within the hilus of the dentate gyrus are sensitive to seizure-induced death. In vivo and in vitro studies suggest that the loss of SST function in the dentate could contribute to epileptogenesis and seizure susceptibility. SST also has inhibitory actions in the CA1 and CA3 hippocampus indicating this peptide is an important homeostatic regulator throughout the hippocampus. In vivo studies show SST has robust antiepileptic properties with the major site of action being hippocampus. In rodents, somatostatin receptor subtype 2 (SST(2)) and SST(4) appear to mediate the majority of the antiepileptic actions of SST, with SST(2) predominate in rat and SST(4) in mouse. Thus SST receptors may be appropriate targets for new antiepileptic drugs (AEDs), although validation in human tissue is lacking.

Computational models of neuronal biophysics and the characterization of potential neuropharmacological targets

The identification and characterization of potential pharmacological targets in neurology and psychiatry is a fundamental problem at the intersection between medicinal chemistry and the neurosciences. Exciting new techniques in proteomics and genomics have fostered rapid progress, opening numerous questions as to the functional consequences of ligand binding at the systems level. Psycho- and neuro-active drugs typically work in nerve cells by affecting one or more aspects of electrophysiological activity. Thus, an integrated understanding of neuropharmacological agents requires bridging the gap between their molecular mechanisms and the biophysical determinants of neuronal function. Computational neuroscience and bioinformatics can play a major role in this functional connection. Robust quantitative models exist describing all major active membrane properties under endogenous and exogenous chemical control. These include voltage-dependent ionic channels (sodium, potassium, calcium, etc.), synaptic receptor channels (e.g. glutamatergic, GABAergic, cholinergic), and G protein coupled signaling pathways (protein kinases, phosphatases, and other enzymatic cascades). This brief review of neuromolecular medicine from the computational perspective provides compelling examples of how simulations can elucidate, explain, and predict the effect of chemical agonists, antagonists, and modulators in the nervous system.

Anticonvulsive Effect of a Selective mGluR8 Agonist (S)-3,4-Dicarboxyphenylglycine (S-3,4-DCPG) in the Mouse Pilocarpine Model of Status Epilepticus

PURPOSE

We sought to investigate the anticonvulsive and neuroprotective effect of a selective metabotropic glutamate receptor 8 (mGluR8) agonist (S)-3,4-dicarboxyphenylglycines (S-3,4-DCPG) on pilocarpine-induced status epilepticus (PISE) and subsequent loss of hilar neurons in the dentate gyrus after systemic (intravenous) or local (intracerebroventricular) administration. We compared the difference in granular cell responses after paired-pulse stimulation of the perforant pathway and the sensitivity to local injection of S-3,4-DCPG into the stratum granulosum in the control and mice at 2 months after PISE.

METHODS

We used intravenous, intracerebroventricular, or intrahippocampal administration of S-3,4-DCPG to mice with status epilepticus or temporal lobe epilepsy and neurophysiologic recording of somatic field excitatory postsynaptic potential (sfEPSP) and population spike (PS) of granular cells in response to perforant-pathway stimulation or S-3,4-DCPG treatment.

RESULTS

Intracerebroventricular (1.91 micromol) but not systemic administration of S-3,4-DCPG (at doses of 12.5, 50, 100, 200, 400, 800, and 1,200 mg/kg) could control PISE with no neuroprotective effect. In epileptic mice, mGluR8-mediated inhibition of fEPSPs was reduced significantly in granular cell bodies.

CONCLUSIONS

At doses ranging from 12.5 to 1,200 mg/kg, intravenous administration of S-3,4-DCPG may not be effective in controlling status epilepticus. Down-regulation of mGluR8 may be related to reduced S-3,4-DCPG-mediated inhibition and the subsequent occurrence of spontaneously recurrent seizures.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Reduced inhibition of dentate granule cells in a model of temporal lobe epilepsy

Patients and models of temporal lobe epilepsy have fewer inhibitory interneurons in the dentate gyrus than controls, but it is unclear whether granule cell inhibition is reduced. We report the loss of GABAergic inhibition of granule cells in the temporal dentate gyrus of pilocarpine-induced epileptic rats. In situ hybridization for GAD65 mRNA and immunocytochemistry for parvalbumin and somatostatin confirmed the loss of inhibitory interneurons. In epileptic rats, granule cells had prolonged EPSPs, and they discharged more action potentials than controls. Although the conductances of evoked IPSPs recorded in normal ACSF were not significantly reduced and paired-pulse responses showed enhanced inhibition of granule cells from epileptic rats, more direct measures of granule cell inhibition revealed significant deficiencies. In granule cells from epileptic rats, evoked monosynaptic IPSP conductances were <40% of controls, and the frequency of GABA(A) receptor-mediated spontaneous and miniature IPSCs (mIPSCs) was <50% of controls. Within 3-7 d after pilocarpine-induced status epilepticus, miniature IPSC frequency had decreased, and it remained low, without functional evidence of compensatory synaptogenesis by GABAergic axons in chronically epileptic rats. Both parvalbumin- and somatostatin-immunoreactive interneuron numbers and the frequency of both fast- and slow-rising GABA(A) receptor-mediated mIPSCs were reduced, suggesting that loss of inhibitory synaptic input to granule cells occurred at both proximal/somatic and distal/dendritic sites. Reduced granule cell inhibition in the temporal dentate gyrus preceded the onset of spontaneous recurrent seizures by days to weeks, so it may contribute, but is insufficient, to cause epilepsy.

Neural growth, neural damage and neurotrophins in the kindling model of epilepsy

Do seizures change the brain? Studies on the kindling model--a widely used animal model of epilepsy--suggest that they do. Dr. Racine, one of the pioneers in the kindling field, describes the basic phenomena of kindling, and discusses the possible roles of cell growth and cell death in this model.

GABABR1 receptor protein expression in human mesial temporal cortex: changes in temporal lobe epilepsy

Immunocytochemistry was used to examine gamma-aminobutyric acid beta (GABA)(B)R1a-b protein expression in the human hippocampal formation (including dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex) and perirhinal cortex. Overall, GABA(B)R1a-b immunostaining was intense and widespread but showed differential areal and laminar distributions of labeled cells. GABA(B)R1a-b-immunoreactive (-ir) neurons were found in the three main layers of the dentate gyrus, the most intense labeling being present in the polymorphic layer, whereas the granule cells were moderately immunoreactive. Except for slight variations, similar distribution patterns of GABA(B)R1a-b immunostaining were found along the different subfields of the Ammon's horn (CA1-CA4). The highest density of GABA(B)R1a-b-ir neurons was localized in the stratum pyramidale, where virtually every pyramidal cell was intensely immunoreactive, including the proximal part of the apical dendrites. Within the subicular complex, a more intense GABA(B)R1a-b immunostaining was found in the subiculum than in the presubiculum or parasubiculum, especially in the pyramidal and polymorphic cell layers. In the entorhinal cortex, distribution of GABA(B)R1a-b immunoreactivity was localized mainly in both pyramidal and nonpyramidal cells of layers II, III, and VI and in the superficial part of layer V, with layers I, IV, and deep layer V being less intensely stained. In the perirhinal cortex, the most intense GABA(B)R1a-b immunoreactivity was located in the deep part of layer III and in layer V and was mainly confined to medium-sized and large pyramidal cells. Thus, the differential expression, but widespread distribution, of GABA(B)R1a-b protein found in the present study suggests the involvement of GABA(B) receptors in many circuits of the human hippocampal formation and adjacent cortical structures. Interestingly, the hippocampal formation of epileptic patients (n = 8) with hippocampal sclerosis showed similar intensity of GABA(B)R1a-b immunostaining in the surviving neurons located within or adjacent to those regions presenting neuronal loss than in the controls. However, surviving neurons in the granule cell layer of the dentate gyrus displayed a significant reduction in immunostaining in 7 of 8 patients. Therefore, alterations in inhibitory synaptic transmission through GABA(B) receptors appears to affect differentially certain hippocampal circuits in a population of epileptic patients. This reduction in GABA(B)R1a-b expression could contribute to the pathophysiology of temporal lobe epilepsy.

In vivo intracellular analysis of granule cell axon reorganization in epileptic rats

In vivo intracellular recording and labeling in kainate-induced epileptic rats was used to address questions about granule cell axon reorganization in temporal lobe epilepsy. Individually labeled granule cells were reconstructed three dimensionally and in their entirety. Compared with controls, granule cells in epileptic rats had longer average axon length per cell; the difference was significant in all strata of the dentate gyrus including the hilus. In epileptic rats, at least one-third of the granule cells extended an aberrant axon collateral into the molecular layer. Axon projections into the molecular layer had an average summed length of 1 mm per cell and spanned 600 microm of the septotemporal axis of the hippocampus-a distance within the normal span of granule cell axon collaterals. These findings in vivo confirm results from previous in vitro studies. Surprisingly, 12% of the granule cells in epileptic rats, and none in controls, extended a basal dendrite into the hilus, providing another route for recurrent excitation. Consistent with recurrent excitation, many granule cells (56%) in epileptic rats displayed a long-latency depolarization superimposed on a normal inhibitory postsynaptic potential. These findings demonstrate changes, occurring at the single-cell level after an epileptogenic hippocampal injury, that could result in novel, local, recurrent circuits.

Characterization of neuronal migration disorders in neocortical structures: quantitative receptor autoradiography of ionotropic glutamate, GABA(A) and GABA(B) receptors

Epileptiform activity was previously described [Luhmann et al. (1998) Eur. J. Neurosci., 10, 3085-3094] in the neocortex of the adult rat following freeze lesioning of the newborn neocortex. After a survival time of 3 months, a small area of dysplastic cortex surrounded by histologically normal (exofocal) neocortex was observed. The dysplastic cortex is characterized by the formation of a small sulcus and a three- to four-layered architecture. Two questions are addressed here: (i) is the hyperexcitability associated with changes in binding to major excitatory and inhibitory transmitter receptors in the dysplastic cortex?; and (ii) do such changes also occur in the exofocal cortex? Alterations in binding to glutamatergic N-methyl-D-aspartate (NMDA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate and GABA(A) and GABA(B) (gamma-aminobutyric acid) receptors are demonstrated with quantitative in vitro receptor autoradiography by using the ligands [3H]MK-801, [3H]AMPA, [3H]kainate, [3H]muscimol and [3H]baclofen, respectively. In the dysplastic cortex, the binding to NMDA, AMPA and kainate receptors is significantly increased, whereas the binding to GABA(A) and GABA(B) receptors is reduced. Exofocal areas of the lesioned hemisphere show an imbalance between excitatory and inhibitory receptor binding with an up-regulation of the binding to AMPA and kainate, and a down-regulation to GABA(A) receptors. The binding to GABA(B) and NMDA receptors is not significantly changed in the exofocal areas. The imbalance between excitatory and inhibitory receptors may cause the hyperexcitability, as previously found in the identical experimental model, and may also induce epileptiform activity in the human cortex with migration disorders.

Paired pulse suppression and facilitation in human epileptogenic hippocampal formation

Paired pulse stimulation has commonly been employed to investigate changes in excitability in epileptic hippocampal tissue employing the in vitro slice preparation. We used paired pulse stimulation in the intact temporal lobe of patients with temporal lobe seizures to compare the excitability of pathways in the epileptogenic hippocampus (located in the temporal lobe in which seizures arise) with those in the non-epileptogenic hippocampus of the contralateral temporal lobe (in the hemisphere to which seizures spread). A total of 20 patients with temporal lobe seizure onsets were studied during chronic depth electrode monitoring for seizure localization. Intracranial in vivo stimulation and recording sites included the hippocampus, entorhinal cortex, subicular cortex and parahippocampal gyrus. A comparison of all hippocampal pathways located in the temporal lobe where seizures typically started (n = 37) with those in temporal lobes contralateral to seizure onset (n = 53) showed significantly greater paired pulse suppression of population post-synaptic potentials on the epileptogenic side (F(1,87) = 6.1, P < 0.01). Similarly, mean paired pulse suppression was significantly greater for epileptogenic perforant path responses than for contralateral perforant path responses (F(1,13) = 7.5, P < 0.01). In contrast, local stimulation activating intrinsic associational pathways of the epileptogenic hippocampus showed decreased paired pulse suppression in comparison to the epileptogenic perforant path. These results may be a functional consequence of the formation of abnormal recurrent inhibitory and recurrent excitatory pathways in the sclerotic hippocampus. Enhanced inhibition may be adaptive in suppressing seizures during interictal periods, while abnormal recurrent excitatory circuits in the presence of enhanced inhibition may drive the hypersynchronization of principal neurons necessary for seizure genesis.

Adenovirus vector-mediated gene transfer into human epileptogenic brain slices: prospects for gene therapy in epilepsy

As a first step in the development of a gene therapy approach to epilepsy, we evaluated the ability of adenovirus vectors to direct the transfer into and expression of a marker gene in human brain slices obtained from patients undergoing surgery for medically intractable epilepsy. Following injection of adenovirus vectors containing the Escherichia coli lacZ gene into hippocampal and cortical brain slices, lacZ mRNA, beta-galactosidase protein, and enzymatic activity were detected, confirming successful gene transfer, transcription, and translation into a functional protein. Transfected cells were predominantly glial, with some neurons expressing beta-galactosidase as well. These results support the potential of adenovirus vectors to transfer genetic information into human epileptogenic brain, resulting in expression of the gene into a functional protein. These findings also have implications for the development of gene therapy approaches to certain seizure disorders. A number of potential therapeutic approaches are discussed, including the elevation of inhibitory neurotransmitter or neuropeptide levels, expression or modulation of postsynaptic receptors, and manipulation of signal transduction systems.

Direct gene transfer into human epileptogenic hippocampal tissue with an adeno-associated virus vector: implications for a gene therapy approach to epilepsy

PURPOSE

Virus vectors capable of transferring genetic information into human cells provide hope for improved therapy in several neurological diseases, including epilepsy. We evaluated the ability of an adeno-associated virus (AAV) vector to transfer and cause expression of a lacZ marker gene in brain slices obtained from patients undergoing temporal lobectomy for control of medically intractable seizures.

METHODS

Human brain slices were injected with an AAV vector (AAVlacZ) encoding Escherichia coli beta-galactosidase and incubated for as long as 24 h. The presence of lacZ mRNA. beta-galactosidase protein and enzymatic activity were assayed by reverse transcriptase polymerase chain reaction (rtPCR), immunocytochemistry, and the X-Gal technique, respectively.

RESULTS

AAVlacZ directed the expression in human epileptogenic brain of E. coli beta-galactosidase that had functional activity. Expression was observed in < or =5 h and was sustained for as long as the slices were viable. Morphological analysis indicated that neurons were preferentially transfected, and there was no evidence of cytotoxicity.

CONCLUSIONS

Our results confirm the feasibility of using AAV vectors to transfer genes into the human CNS and in particular, into neurons. Replacement of the lacZ gene with a functional gene modulating hippocampal neuronal physiology, might allow a localized genetic intervention for focal seizures based on the stereotaxic or endovascular delivery of such a vector system into the appropriate brain region.

In contrast to kindled seizures, the frequency of spontaneous epilepsy in the limbic status model correlates with greater aberrant fascia dentata excitatory and inhibitory axon sprouting, and increased staining for N-methyl-D-aspartate, AMPA and GABA(A) receptors

This study determined whether there were differences in hippocampal neuron loss and synaptic plasticity by comparing rats with spontaneous epilepsy after limbic status epilepticus and animals with a similar frequency of kindled seizures. At the University of Virginia, Sprague-Dawley rats were implanted with bilateral ventral hippocampal electrodes and treated as follows; no stimulation (electrode controls; n=5): hippocampal stimulation without status (stimulation controls; n=5); and limbic status from continuous hippocampal stimulation (n=12). The limbic status group were electrographically monitored for a minimum of four weeks. Four rats had no recorded chronic seizures (status controls), and all three control groups showed no differences in hippocampal pathology and were therefore incorporated into a single group (controls). Eight limbic status animals eventually developed chronic epilepsy (spontaneous seizures) and an additional eight rats were kindled to a similar number and frequency of stage 5 seizures (kindled) as the spontaneous seizures group. At the University of California (UCLA) the hippocampi were processed for: (i) Niss1 stain for densitometric neuron counts; (ii) neo-Timm's histochemistry for mossy fiber sprouting; and (iii) immunocytochemical staining for glutamate decarboxylase, N-methyl-D-aspartate receptor subunit 2, AMPA receptor subunit 1 and the GABA(A) receptor. In the fascia dentata inner and outer molecular layers the neo-Timm's stain and immunoreactivity was quantified as gray values using computer image analysis techniques. Statistically significant results (P<0.05) showed the following. Compared to controls and kindled animals, rats with spontaneous seizures had: (i) lower neuron counts for the fascia dentata hilus, CA3 and CA1 stratum pyramidale; (ii) greater supragranular inner molecular layer mossy fiber staining; and (iii) greater glutamate decarboxylase immunoreactivity in both molecular layers. Greater supragranular excitatory mossy fiber and GABAergic axon sprouting correlated with: (i) increases in N-methyl-D-aspartate receptor subunit 2 inner molecular layer staining; (ii) more AMPA receptor subunit 1 immunoreactivity in both molecular layers; and (iii) greater outer than inner molecular layer GABA(A) immunoreactivity. Furthermore, in contrast to kindled animals, rats with spontaneous seizures showed that increasing seizure frequency per week and the total number of natural seizures positively correlated with greater Timm's and GABAergic axon sprouting, and with increases in N-methyl-D-aspartate receptor subunit 2 and AMPA receptor subunit 1 receptor staining. In this rat limbic status model these findings indicate that chronic seizures are associated with hippocampal neuron loss, reactive axon sprouting and increases in excitatory receptor plasticity that differ from rats with an equal frequency of kindled seizures and controls. The hippocampal pathological findings in the limbic status model are similar to those in humans with hippocampal sclerosis and mesial temporal lobe epilepsy, and support the hypothesis that synaptic reorganization of both excitatory and inhibitory systems in the fascia dentata is an important pathophysiological mechanism that probably contributes to or generates chronic limbic seizures.

Granule cell disinhibition in dentate gyrus of genetically seizure susceptible El mice

Paired-pulse inhibition was investigated electrophysiologically in the dentate gyrus using hippocampal slices from epileptic El mice. At short interpulse intervals (IPIs), the inhibition was 30% in the El, and 90% in the control ddY mice at the ages of 10 and 15 weeks. No difference in inhibition was observed at the age of 5 weeks. Bicuculline, a GABAA receptor antagonists, attenuated the inhibition during short IPIs n the ddY mice, while in the El mice, phenobarbital and flunitrazepam, which enhance GABAA receptor function, restored the inhibitory activity comparable to that of the ddY. The disinhibition progressed with growth, closely correlating with seizure development in El mice. These results suggest that decrease in the GABAergic inhibition occurs in the dentate gyrus of the El mice with growth. GABA concentration in the hippocampus was also quantified using HPLC. In El mice, GABA level was significantly lower than that in ddY mice at the ages of 5 and 15 weeks. Thus, the disinhibition observed in the El dentate gyrus at 15 weeks of age does not appear to be directly related to the content of GABA. GABAergic disinhibition suggests possible loss of unknown inhibition control factor(s) in the El dentate gyrus as growth progresses. The growth-dependent disinhibition in the granule cells may be prerequisite for epileptogenesis in El mice.

Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyrus of temporal lobe epileptic human brain slices

An abnormal electrophysiological response in brain slices of the dentate gyrus from biopsy material from patients surgically treated for intractable epilepsy (46/57), exhibited characteristics similar to the physiological hallmark of epilepsy, the paroxysmal discharge, a prolonged (30-600 ms) and often large amplitude field potential. The most striking feature of the prolonged response to a single perforant path stimulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of substantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurred during both phases of the response. During a 1 Hz stimulus train applied to the perforant path, the magnitude and duration of the negative component of the field response was significantly increased. Approximately half of the cases (Group 1; 30/57) exhibited potentiation of the biphasic response, while the remaining cases (Group 2; 27/57) exhibited no negative field component during 1 Hz stimulation trains. This repetitive stimulation, in general, increased the area of the field response in a large majority of cases (44/57) regardless of the sign of the field potential. The number of population spikes following 1 Hz stimulation increased significantly for cases in both groups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Group 1) in comparison to cases that exhibited no negative field potentials (Group 2). Paired pulse depression at a 200 ms ISI was not significantly different between the groups. During a single stimulus, bicuculline disinhibition (20 microM) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to single perforant path stimuli also exhibited prolonged and large depolarizations that were comparable in time course to the duration of field potentials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculline was reduced by APV (20 microM). We suggest that the prolonged field response, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be due directly or indirectly to an increase in membrane depolarization mediated by activation of the NMDA receptor.