The effects of kindling on GABA-mediated inhibition in the dentate gyrus of the rat. II. Receptor binding

Pentylenetetrazole kindling induces dynamic changes in GAD65 expression in hippocampal somatostatin interneurons

INTRODUCTION

One of the mechanisms of epileptgenesis is impairment of inhibitory neural circuits. Several studies have compared neural changes among subtypes of gamma-aminobutyric acid-related (GABAergic) neurons after acquired epileptic seizure. However, it is unclear that GABAergic neural modifications that occur during acquisition process of epileptic seizure.

METHODS

Male rats were injected with pentylenetetrazole (PTZ kindling: n = 30) or saline (control: n = 15) every other day to observe the development of epileptic seizure stages. Two time points were identified: the point at which seizures were most difficult to induce, and the point at which seizures were most easy to induce. The expression of GABAergic neuron-related proteins in the hippocampus was immunohistochemically compared among GABAergic subtypes at each of these time points.

RESULTS

Bimodal changes in seizure stages were observed in response to PTZ kindling. The increase of seizure stage was transiently suppressed after 8 or 10 injections, and then progressed again by the 16th injection. Based on these results, we defined 10 injections as a short-term injection period during which seizures are less likely to occur, and 20 injections as a long-term injection period during which continuous seizures are likely to occur. The immunohistochemical analysis showed that hippocampal glutamic acid decarboxylase 65 (GAD65) expression was increased after short-term kindling but unchanged after long-term kindling. Increased GAD65 expression was limited to somatostatin-positive (SOM+) cells among several GABAergic subtypes. By contrast, GAD, GABA, GABAAR α1, GABABR1, and VGAT cells showed no change following short- or long-term PTZ kindling.

CONCLUSION

PTZ kindling induces bimodal changes in the epileptic seizure stage. Seizure stage is transiently suppressed after short-term PTZ injection with GAD65 upregulation in SOM+ cells. The seizure stage is progressed again after long-term PTZ injection with GAD65 reduction to baseline level.

Spatial exploration induced expression of immediate early genes Fos and Zif268 in adult-born neurons Is reduced after pentylenetetrazole kindling

Seizure activity stimulates adult neurogenesis, the birth of new neurons, in the hippocampus. Many new neurons that develop in the presence of repeatedly induced seizures acquire abnormal morphological and functional characteristics that can promote network hyperexcitability and hippocampal dysfunction. However, the impact of seizure induced neurogenesis on behaviour remains poorly understood. In this study, we investigated whether adult-born neurons generated immediately before and during chronic seizures were capable of integration into behaviorally relevant hippocampal networks. Adult rats underwent pentylenetetrazole (PTZ) kindling for either 1 or 2 weeks. Proliferating cells were labelled with BrdU immediately before kindling commenced. Twenty-four hours after receiving their last kindling treatment, rats were placed in a novel environment and allowed to freely explore for 30 min. The rats were euthanized 90 min later to examine for behaviourally-induced immediate early gene expression (c-fos, Zif268). Using this approach, we found that PTZ kindled rats did not differ from control rats in regards to exploratory behaviour, but there was a marked attenuation in behaviour-induced expression of Fos and Zif268 for rats that received 2 weeks of PTZ kindling. Further examination revealed that PTZ kindled rats showed reduced colocalization of Fos and Zif268 in 2.5 week old BrdU + cells. The proportion of immature granule cells (doublecortin-positive) expressing behaviorally induced Zif268 was also significantly lower for PTZ kindled rats than control rats. These results suggest that chronic seizures can potentially disrupt the ability of adult-born cells to functionally integrate into hippocampal circuits important for the processing of spatial information.

Effects of Low Frequency Stimulation on Spontaneous Inhibitory and Excitatory Post-Synaptic Currents in Hippocampal CA1 Pyramidal Cells of Kindled Rats

OBJECTIVE

Low-frequency stimulation (LFS) exerts suppressive effects in kindled animals. It is believed that overstimulated glutamatergic and decreased GABAergic transmission have long been associated with seizure activity. In this study, we investigated the effect of electrical LFS on different parameters of spontaneous excitatory and inhibitory post-synaptic currents (sEPSCs and sIPSCs) in hippocampal CA1 pyramidal cells in kindled animals.

MATERIALS AND METHODS

In this experimental study, rats were kindled by electrical stimulation of the hippocampal CA1 area in a semi-rapid manner (12 stimulations/day). The animals were considered fully kindled when they showed stage 5 seizures on three consecutive days. One group of animals received LFS 4 times at 30 seconds, 6 hours, 18 and 24 hours following the last kindling stimulation. Each LFS consisted of 4 packages at 5 minutes intervals. Each package of LFS consisted of 200 pulses at 1 Hz and each monophasic square wave pulse duration was 0.1 millisecond. At 2-3 hours post-LFS, acute hippocampal slices were prepared and a whole cell patch clamp recording was performed in all animals to measure the different parameters of sEPSCs and sIPSCs.

RESULTS

In kindled animals, the inter-event interval (as an index of occurrence) of sEPSCs decreased, whereas sIPSC increased. In addition, the decay time constant of sIPSCs as an index of the duration of its activity decreased compared to the control group. There was no significant difference in other parameters between the kindled and control groups. Application of LFS in kindled animals prevented the observed changes. There was no significant difference between the measured parameters in kindled+LFS and control groups.

CONCLUSION

LFS application may prevent seizure-induced increase in the occurrence of sEPSCs and seizure-induced decrease in occurrence and activity duration of sIPSCs.

Increased inhibitory synaptic activity in the hippocampus (CA1) of genetic absence epilepsy rats: Relevance of kindling resistance

PURPOSE

Genetic absence epilepsy rats from Strasbourg (GAERS), a well-validated genetic rat model for typical absence epilepsy, are known to manifest a resistance to secondary generalization of abnormal focal electrical activity evoked by kindling. The mechanism of this resistance is still unclear. In order to understand the possible mechanism of kindling resistance, we investigated for the first time, the differences of short-term synaptic plasticity by using a paired-pulse paradigm as an indicator of GABAergic activity in CA1 region of hippocampus in GAERS and non-epileptic Wistar rats in-vivo.

METHODS

Rats were subjected to kindling process, basolateral amygdala was stimulated twice a day, with a supra-threshold current, until they displayed limbic or convulsive seizures. One hour after the last kindling stimulation, evoked field potentials from CA1 pyramidal layer of hippocampus were recorded in-vivo under urethane anesthesia.

RESULTS

In response to supra-threshold kindling stimulations GAERS showed a significantly delayed kindling progression and displayed a significant increase in hippocampal excitability at early stages of kindling that is the critical for the development of convulsive seizures. In control rats that were not received kindling stimulation, paired-pulse depression (PPD) was significantly pronounced in GAERS with respect to the Wistar group. During the kindling course, PPD was gradually reduced in the Wistar rats as kindling progression was advanced. However in GAERS, PPD ratios were not significantly changed at early stages of kindling. When GAERS reached convulsive stage, their PPD ratios became similar to that of Wistar rats.

DISCUSSION

The increased inhibition in paired-pulse responses at early stages of kindling in GAERS suggests the role of augmented GABAergic activity as one of the underlying mechanisms of kindling resistance observed in genetic rat models of absence epilepsy.

Age-dependent long-term structural and functional effects of early-life seizures: evidence for a hippocampal critical period influencing plasticity in adulthood

Neural activity promotes circuit formation in developing systems and during critical periods permanently modifies circuit organization and functional properties. These observations suggest that excessive neural activity, as occurs during seizures, might influence developing neural circuitry with long-term outcomes that depend on age at the time of seizures. We systematically examined long-term structural and functional consequences of seizures induced in rats by kainic acid, pentylenetetrazol, and hyperthermia across postnatal ages from birth through postnatal day 90 in adulthood (P90). Magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), and electrophysiological methods at ⩾P95 following seizures induced from P1 to P90 demonstrated consistent patterns of gross atrophy, microstructural abnormalities in the corpus callosum (CC) and hippocampus, and functional alterations in hippocampal circuitry at ⩾P95 that were independent of the method of seizure induction and varied systematically as a function of age at the time of seizures. Three distinct epochs were observed in which seizures resulted in distinct long-term structural and functional outcomes at ⩾P95. Seizures prior to P20 resulted in DTI abnormalities in CC and hippocampus in the absence of gross cerebral atrophy, and increased paired-pulse inhibition (PPI) in the dentate gyrus (DG) at ⩾P95. Seizures after P30 induced a different pattern of DTI abnormalities in the fimbria and hippocampus accompanied by gross cerebral atrophy with increases in lateral ventricular volume, as well as increased PPI in the DG at ⩾P95. In contrast, seizures between P20 and P30 did not result in cerebral atrophy or significant imaging abnormalities in the hippocampus or white matter, but irreversibly decreased PPI in the DG compared to normal adult controls. These age-specific long-term structural and functional outcomes identify P20-30 as a potential critical period in hippocampal development defined by distinctive long-term structural and functional properties in adult hippocampal circuitry, including loss of capacity for seizure-induced plasticity in adulthood that could influence epileptogenesis and other hippocampal-dependent behaviors and functional properties.

Temporal lobe epileptiform activity following systemic administration of 4-aminopyridine in rats

PURPOSE

  The K(+) channel blocker 4-aminopyridine (4AP) induces epileptiform synchronization in brain slices maintained in vitro without interfering with γ-aminobutyric acid (GABA)A receptor-mediated inhibition and, actually, even enhancing it. The hypothesis that similar electrographic epileptiform patterns occur in vivo following systemic 4AP injection was tested here.

METHODS

  Sprague-Dawley rats (n = 13) were implanted with bipolar electrodes aimed at the hippocampal CA3 region, entorhinal cortex, subiculum, dentate gyrus, and amygdala. They were then injected with a single dose of 4AP (4-5 mg/kg, i.p.), and video-monitoring/electroencephalography (EEG) recordings were performed.

KEY FINDINGS

  4AP induced convulsive or nonconvulsive seizures in 12 of 13 rats, along with generalized fascicular twitching, wet-dog shakes, and myoclonic jerks. On EEG, we observed in 7 (58.3%) of 12 animals long-lasting interictal spikes from the subiculum before the occurrence of the first seizure. Once seizures had started, interictal spikes occurred in all areas with no fixed site of origin. Most seizures (41/60, 68.3%) were characterized by a low-voltage fast-activity onset pattern and were convulsive (48/60, 80%). 4AP also induced highly rhythmic theta (6-11 Hz) oscillations in CA3 and entorhinal cortex before seizure occurrence.

SIGNIFICANCE

  Our study shows that systemic 4AP administration in vivo can enhance theta oscillations and induce slow interictal spikes and low-voltage fast-onset seizures similar to those reported in brain slices. We propose that these effects may reflect, at least in part, enhanced GABAA receptor-mediated inhibition as reported in in vitro studies.

Regional changes in gene expression after limbic kindling

Repeated electrical stimulation results in development of seizures and a permanent increase in seizure susceptibility (kindling). The permanence of kindling suggests that chronic changes in gene expression are involved. Kindling at different sites produces specific effects on interictal behaviors such as spatial cognition and anxiety, suggesting that causal changes in gene expression might be restricted to the stimulated site. We employed focused microarray analysis to characterize changes in gene expression associated with amygdaloid and hippocampal kindling. Male Long-Evans rats received 1 s trains of electrical stimulation to either the amygdala or hippocampus once daily until five generalized seizures had been kindled. Yoked control rats carried electrodes but were not stimulated. Rats were euthanized 14 days after the last seizures, both amygdala and hippocampus dissected, and transcriptome profiles compared. Of the 1,200 rat brain-associated genes evaluated, 39 genes exhibited statistically significant expression differences between the kindled and non-kindled amygdala and 106 genes exhibited statistically significant differences between the kindled and non-kindled hippocampus. In the amygdala, subsequent ontological analyses using the GOMiner algorithm demonstrated significant enrichment in categories related to cytoskeletal reorganization and cation transport, as well as in gene families related to synaptic transmission and neurogenesis. In the hippocampus, significant enrichment in gene expression within categories related to cytoskeletal reorganization and cation transport was similarly observed. Furthermore, unique to the hippocampus, enrichment in transcription factor activity and GTPase-mediated signal transduction was identified. Overall, these data identify specific and unique neurochemical pathways chronically altered following kindling in the two sites, and provide a platform for defining the molecular basis for the differential behaviors observed in the interictal period.

The role of collybistin in gephyrin clustering at inhibitory synapses: facts and open questions

Collybistin (Cb) is a brain-specific GDP/GTP-exchange factor, which interacts with the inhibitory receptor anchoring protein gephyrin. Data from mice carrying an inactivated Cb gene indicate that Cb is required for the formation and maintenance of gephyrin and gephyrin-dependent GABA(A) receptor (GABA(A)R) clusters at inhibitory postsynapses in selected regions of the mammalian forebrain. However, important aspects of how Cb's GDP/GTP-exchange activity, structure, and regulation contribute to gephyrin and GABA(A)R clustering, as well as its role in synaptic plasticity, remain poorly understood. Here we review the current state of knowledge about Cb's function and address open questions concerning its contribution to synapse formation, maintenance, plasticity, and adaptive changes in response to altered network activity.

Cortical hyperexcitability and epileptogenesis: Understanding the mechanisms of epilepsy - part 2

Epilepsy encompasses a diverse group of seizure disorders caused by a variety of structural, cellular and molecular alterations of the brain primarily affecting the cerebral cortex, leading to recurrent unprovoked epileptic seizures. In this two-part review we examine the mechanisms underlying normal neuronal function and those predisposing to recurrent epileptic seizures starting at the most basic cellular derangements (Part 1, Volume 16, Issue 3) and working up to the highly complex epileptic networks and factors that modulate the predisposition to seizures (Part 2). We attempt to show that multiple factors can modify the epileptic process and that different mechanisms underlie different types of epilepsy, and in most situations there is an interplay between multiple genetic and environmental factors.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Three brief epileptic seizures reduce inhibitory synaptic currents, GABA(A) currents, and GABA(A)-receptor subunits

PURPOSE

Cellular mechanisms activated during seizures may exacerbate epilepsy. gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in brain, and we hypothesized that brief epileptic seizures may reduce GABA function.

METHODS

We used audiogenic seizures (AGSs) in genetically epilepsy-prone rats (GEPRs) to investigate effects of seizures on GABA-mediated inhibition in the presence of epilepsy. GEPRs are uniformly susceptible to AGSs beginning at 21 postnatal days. AGSs are brief convulsions lasting approximately 20 s, and they begin in inferior colliculus (IC). We evoked three seizures in GEPRs and compared the results with those in seizure-naive GEPRs and nonepileptic Sprague-Dawley (SD) rats, the GEPR parent strain.

RESULTS

Whole-cell recording in IC slices showed that GABA-mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were reduced 55% by three brief epileptic seizures. Whole-cell recording in IC neuronal cultures showed that currents elicited by GABA were reduced 67% by three seizures. Western blotting for the alpha1 and alpha4 subunits of the GABA(A) receptor showed no statistically significant effects. In contrast, three brief epileptic seizures reduced gamma2 subunit levels by 80%.

CONCLUSIONS

The effects of the very first seizures, in animals known to be epileptic, in an area of brain known to be critical to the seizure network, were studied. The results indicate that even brief epileptic seizures can markedly reduce IPSCs and GABA currents and alter GABA(A)-receptor subunit protein levels. The cause of the reductions in IPSCs and GABA currents is likely to be altered receptor subunit composition, with reduced gamma2 levels causing reduced GABA(A)-receptor sensitivity to GABA. Seizure-induced reductions in GABA-mediated inhibition could exacerbate epilepsy.

Kindling-induced alterations in GABAAreceptor-mediated inhibition and neurosteroid activity in the rat piriform cortex

The piriform cortex makes strong interconnections with limbic structures (amygdala, entorhinal cortex and hippocampus) that are involved in memory processing. These connections have also been implicated in the development of temporal lobe epilepsy. However, little is known about how neurones in this region may change during seizure genesis. Here we tested the hypothesis that in the kindling model of temporal lobe epilepsy GABAA receptor-mediated inhibition is altered in the piriform cortex. To do this we performed whole-cell patch-clamp recordings in piriform cortex brain slices obtained from non-kindled and amygdala-kindled adult rats. We found that kindling coincided with an increase in the amplitude and duration of miniature inhibitory post-synaptic currents (mIPSCs) recorded from non-pyramidal neurones, whereas the mIPSCs occurring on pyramidal (excitatory) cells did not change. Non-stationary noise analysis of mIPSCs occurring on the non-pyramidal neurones showed that inferred unitary conductance of synaptic channels were the same before and after kindling, implying that the channel number increased significantly. Immunocytochemical analysis of the inhibitory innervation showed that it was also unaltered by seizure induction. We also found that the effect of the positive modulator tetrahydrodeoxycorticosterone was reduced on the pyramidal neurones after kindling. In contrast, the potentiating effects of tetrahydrodeoxycorticosterone on non-pyramidal cells were about the same after kindling as in control (sham) rats. These data indicate that amygdala kindling causes a shift in the inhibition 'balance' between the pyramidal and non-pyramidal cells, perhaps leading to the disinhibition of pyramidal cells.

Vigabatrin in Low Doses Selectively Suppresses the Clonic Component of Audiogenically Kindled Seizures in Rats

PURPOSE

The effect of systemic administration of the gamma-aminobutyric acid (GABA)-transaminase inhibitor vigabatrin (VGB) on different components of convulsions was tested in the model of audiogenically kindled seizures, which consist of brainstem (running, tonus) and forebrain (clonus) elements.

METHODS

Audiogenically susceptible rats of Krushinsky-Molodkina (KM), Wistar, and WAG/Rij strains received repeated sound stimulation (60 dB, 10-80 kHz) until kindled audiogenic seizures were reliably elicited. Kindled audiogenic seizures consisted of running, tonic, and generalized clonic phases in KM rats (severe audiogenic seizures) and of running and Racine stage 5 facial/forelimb clonus in Wistar and WAG/Rij rats (moderate seizures). Vehicle, 100, or 200 mg/kg of VGB was intraperitoneally injected 2, 4 and 24 h before the induction of kindled audiogenic seizures.

RESULTS

At both doses, VGB did not change the seizure latency and the duration of running and tonic convulsions, but suppressed clonic ones in all rat strains. In KM rats, the mean duration of posttonic clonus was significantly reduced at 24 h after 100 mg/kg and from 4 h after 200 mg/kg. In Wistar and WAG/Rij rats, the mean duration of facial/forelimb clonus was reduced from 4 and 2 h after 100- and 200-mg/kg administration, respectively; 24 h after the high-dose injection, clonus was completely blocked in all rats of both strains. No difference in efficacy of VGB between Wistar and WAG/Rij rats was observed.

CONCLUSIONS

VGB more effectively suppresses clonic convulsions than running and tonic ones in audiogenically kindled rats. It is supposed that this selective anticonvulsive effect of VGB results from different sensitivities of forebrain and brainstem epileptic networks to the presumed GABA enhancement.

Kindling Limits the Interictal Neuronal Temporal Response Properties in Cat Primary Auditory Cortex

PURPOSE

The present study examined the effect of electrical kindling on the interictal temporal response properties of single units recorded from primary auditory cortex (AI) of the adult cat.

METHODS

Cats were permanently implanted with electrodes in AI, kindled twice daily for 40 sessions, and the contralateral AI was subsequently mapped. Kindling stimulation consisted of 1-s trains of biphasic square-wave pulses applied at a frequency of 60 Hz, 100 microA above the afterdischarge (AD) threshold. The EEG activity was recorded during each kindling session, and the behavioral manifestation was scored. Subsequent to kindling, multiple single-unit responses were recorded under ketamine anesthesia in response to 1-s-long periodic click trains, with click rates between 2 and 64 Hz. Neuronal responses were characterized according to their ability to respond in time-locked fashion to the clicks.

RESULTS

Kindling stimulation resulted in progression of the AD characteristics and seizure behavior, with six of 10 kindled cats reaching a fully generalized state. In the fully kindled cats, the best modulation frequencies and limiting following rates for the single-unit responses were significantly lower compared with those of naive and sham controls.

CONCLUSIONS

Repeated epileptiform activity interferes with temporal processing in cat auditory cortex in the interictal state. This may have implications for people with epileptic foci in auditory-related areas.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Effects of some synthetic kynurenines on brain amino acids and nitric oxide after pentylenetetrazole administration to rats

We have previously proven that some synthetic kynurenines behave as antagonists of the N-methyl-d-aspartate receptor inhibiting neuronal subtype of nitric oxide synthase activity. We now investigate the anticonvulsant activity of four of these kynurenines in pentylenetetrazole (PTZ)-treated rats. The rats were treated with each kynurenine (10-160 mg/kg, s.c.) 30 min before PTZ administration (100 mg/kg, s.c.). Then, latency, duration and intensity of the first seizure and the percent animal survival were noted. PTZ-induced death was counteracted by high doses of kynurenines. Latency of the first seizure was significantly increased and its intensity reduced at the same doses, whereas the duration of the first seizure significantly decreased with doses of 20 mg/kg in most of the kynurenines tested. Three hours after PTZ administration, the surviving animals were sacrificed and the levels of brain amino acids and nitrite were measured. PTZ administration increased glutamate, glutamine, serine and taurine levels in different brain areas. High doses of kynurenines generally counteracted the effects of PTZ on excitatory amino acids, but they also reduced inhibitory aminoacids. However, the most consistent effect of kynurenines was the dose-dependent reduction of brain nitrite levels induced by PTZ. These results reveal a new family of anticonvulsant drugs that affect mainly to nitric oxide production in the brain.

Low frequency stimulation modifies receptor binding in rat brain

Experiments were designed to reproduce the antiepileptic effects of low frequency stimulation (LFS) during the amygdala kindling process and to examine LFS-induced changes in receptor binding levels of different neurotransmitters in normal brain. Male Wistar rats were stereotactically implanted in the right amygdala with a bipolar electrode. Rats (n = 14) received twice daily LFS (15 min train of 1Hz, 0.1 ms at an intensity of 100 to 400 microA) immediately after amygdala kindling stimulation (1s train of 60 Hz biphasic square waves, each 1 ms at amplitude of 200-500 microA) during 20 days. The LFS suppressed epileptogenesis (full attainment of stage V kindling) but not the presence of partial seizures (lower stages of kindling) in 85.7% of the rats. Thereafter, normal rats (n = 7) received amygdala LFS twice daily for 40 trials. Animals were sacrificed 24 h after last stimulation and their brain used for labeling mu opioid, benzodiazepine (BZD), alpha(1)-adrenergic, and adenylyl cyclase binding. Autoradiography experiments revealed increased BZD receptor binding in basolateral amygdala (20.5%) and thalamus (29.3%) ipsilateral to the place of stimulation and in contralateral temporal cortex (18%) as well as decreased values in ipsilateral frontal cortex (24.2%). Concerning mu receptors, LFS decreased binding values in ipsilateral sensorimotor (7.2%) and temporal (5.6%) cortices, dentate gyrus (5.8% ipsi and 6.8% contralateral, respectively), and contralateral CA1 area of dorsal hippocampus (5.5%). LFS did not modify alpha(1) receptor and adenylyl cyclase binding values. These findings suggest that the antiepileptic effects of LFS may involve activation of GABA-BZD and endogenous opioid systems.

Perinatal exposure to polychlorinated biphenyls alters excitatory synaptic transmission and short-term plasticity in the hippocampus of the adult rat

Developmental exposure to polychlorinated biphenyls (PCBs) has been associated with cognitive deficits in humans and laboratory animals. Previous work has demonstrated a reduced capacity to support long-term potentiation (LTP) in animals exposed to a PCB mixture, Aroclor 1254 (A1254) via the dam in utero and throughout the preweaning period [Brain Res. 850;1999:87-95; Toxicol. Sci. 57;2000:102-11]. Assessment of normalized input/output (I/O) functions collected prior to LTP induction failed to reveal consistent differences in baseline synaptic transmission between control and PCB-exposed groups. The present study was designed to systematically evaluate excitatory and inhibitory synaptic transmission using a more extensive I/O analysis and paired pulse functions to assess short-term plasticity. Pregnant Long-Evans rats were administered either corn oil (control) or 6 mg/kg per day of A1254 by gavage from gestational day (GD) 6 until pups were weaned on postnatal day (PND) 21. In adult male offspring (5-11 months of age), field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. Detailed I/O functions were assessed by averaging the responses evoked in the dentate gyrus to stimulus pulses delivered to the perforant path in an extensive ascending intensity series. Population spike (PS) and postsynaptic potential (PSP) amplitudes recorded in the dentate gyrus were significantly enhanced in PCB-exposed animals relative to controls at midrange intensities. No group differences were observed in EPSP slope amplitudes. Short-term plasticity was assessed by delivering pairs of stimulus pulses at interpulse intervals (IPIs) ranging from 10 to 70 ms. In the dentate gyrus this range of intervals activates both inhibitory and excitatory mechanisms leading to a pattern of depression at brief intervals (<30 ms) followed by facilitation as the interval between pulses is extended. Paired pulse depression was decreased at an intermediate IPI (30 ms) with submaximal stimulus intensities. These data augment previous work demonstrating persistent changes in hippocampal plasticity as a result of exposure to PCBs during development. Furthermore, as increases in field potential amplitudes were observed, these findings support previous conclusions that A1254-induced LTP deficits are not readily attributable to reductions in synaptic excitability. Thus, in addition to impairment in use-dependent synaptic plasticity reported previously, the present report reveals that basic components of information processing within the hippocampus are permanently altered as a result of perinatal exposure to PCBs.

GABA and its receptors in epilepsy

Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian brain. It acts through 2 classes of receptors, GABAA receptors that are ligand-operated ion channels and the G-protein-coupled metabotropic GABAB receptors. Impairment of GABAergic transmission by genetic mutations or application of GABA receptor antagonists induces epileptic seizures, whereas drugs augmenting GABAergic transmission are used for antiepileptic therapy. In animal epilepsy models and in tissue from patients with temporal lobe epilepsy, loss in subsets of hippocampal GABA neurons is observed. On the other hand, electrophysiological and neurochemical studies indicate a compensatory increase in GABAergic transmission at certain synapses. Also, at the level of the GABAA receptor, neurodegeneration-induced loss in receptors is accompanied by markedly altered expression of receptor subunits in the dentate gyrus and other parts of the hippocampal formation, indicating altered physiology and pharmacology of GABAA receptors. Such mechanisms may be highly relevant for seizure induction, augmentation of endogenous protective mechanisms, and resistance to antiepileptic drug therapy. Other studies suggest a role of GABAB receptors in absence seizures. Presynaptic GABAB receptors suppress neurotransmitter release. Depending on whether this action is exerted in GABAergic or glutamatergic neurons, there may be anticonvulsant or proconvulsant actions.

Partial hippocampal kindling increases GABAB receptor-mediated postsynaptic currents in CA1 pyramidal cells

In previous studies, we showed that partial hippocampal kindling decreased the efficacy of the presynaptic GABAB receptors on both GABAergic and glutamatergic terminals of CA1 neurons in hippocampal slices in vitro. In this study, GABAB receptor-mediated inhibitory postsynaptic currents (GABAB-IPSCs) were assessed by whole-cell recordings in CA1 pyramidal neurons in hippocampal slices of male Long-Evans rats. The peak GABAB-IPSC evoked by a brief train of supramaximal stratum radiatum stimuli (20 pulses of 300 Hz) in the presence of picrotoxin (0.1 mM) and kynurenic acid (1 mM) was larger in neurons of kindled (65.9 +/- 5.2 pA, N=42 cells) than control (45.8 +/- 4.8 pA, N=32 cells) rats (P<0.01). Adding GABA uptake blocker nipecotic acid (1 mM) or GABAB receptor agonist baclofen (0.01 mM) in the perfusate induced outward currents that were blocked by GABAB receptor antagonist CGP 55845A (1 microM). The peak outward current induced by nipecotic acid was larger in neurons of the kindled (55.4 +/- 5.7 pA, N=30) than the control group (39.8 +/- 4.5 pA, N=28) (P<0.05). However, the magnitude of the baclofen-induced current was not different between kindled (90.8 +/- 6.9 pA, N=29) and control (87.2 +/- 5.9 pA, N=21) groups (P>0.05). We concluded that partial hippocampal kindling increased GABAB-IPSCs in hippocampal CA1 pyramidal cells via multiple presynaptic mechanisms.

Spontaneous seizures and loss of axo-axonic and axo-somatic inhibition induced by repeated brief seizures in kindled rats

Repeated brief seizures evoked by kindling progressively increase seizure susceptibility and eventually induce spontaneous seizures. Previous studies have demonstrated that the initial seizures evoked by kindling increase paired-pulse inhibition at 15-25 msec interpulse intervals in the dentate gyrus and also induce apoptosis, progressive neuronal loss, mossy fiber sprouting, and neurogenesis, which could potentially alter the balance of excitation and/or inhibition and modify functional properties of hippocampal circuits. In these experiments, paired-pulse inhibition in the dentate gyrus was reduced or lost after approximately 90-100 evoked seizures in association with emergence of spontaneous seizures. Evoked IPSCs examined by single electrode voltage-clamp methods in granule cells from kindled rats experiencing spontaneous seizures demonstrated altered kinetics (reductions of approximately 48% in 10-90% decay time, approximately 40% in tau, and approximately 65% in charge transfer) and confirmed that decreased inhibition contributed to the reduced paired-pulse inhibition. The loss of inhibition was accompanied by loss of subclasses of inhibitory interneurons labeled by cholecystokinin and the neuronal GABA transporter GAT-1, which project axo-somatic and axo-axonic GABAergic inhibitory terminals to granule cells and axon initial segments. Seizure-induced loss of interneurons providing axo-somatic and axo-axonic inhibition may regulate spike output to pyramidal neurons in CA3 and could play an important role in generation of spontaneous seizures. The sequence of progressive cellular alterations induced by repeated seizures, particularly loss of GABAergic interneurons providing axo-somatic and axo-axonic inhibition, may be important in the development of intractable epilepsy.

Transcranial magnetic stimulation and epilepsy

Epileptic conditions are characterized by an altered balance between excitatory and inhibitory influences at the cortical level. Transcranial magnetic stimulation (TMS) provides a noninvasive evaluation of separate excitatory and inhibitory functions of the cerebral cortex. In addition, repetitive TMS (rTMS) can modulate the excitability of cortical networks. We review the different ways that TMS has been used to investigate pathophysiological mechanisms and effects of antiepileptic drugs in patients with epilepsy and epileptic myoclonus. The safety of different TMS techniques is discussed too. Finally, we discuss the therapeutic prospects of rTMS in this field.

Functional and morphological changes in the hippocampal neuronal circuits associated with epileptic seizures

PURPOSE

We review what is currently known about functional and morphologic plasticity of hippocampal neuronal circuits in animal epilepsy models.

RESULTS

Perforant-path kindling has been shown to enhance excitatory synaptic transmission in the dentate gyrus significantly, and this enhancement has lasted for > or =1 month after kindling. However, in our rapid kindling experiment with an interstimulus interval of 5 min, perforant-path kindling resulted in behavioral convulsions and prolonged the afterdischarge duration, but dentate synaptic transmission remained depressed during kindling.

CONCLUSIONS

These results indicate that kindling-induced synaptic potentiation is not necessary for kindling development. Conversely, epileptic seizures have led to the anomalous sprouting of mossy fibers, a process thought to induce hyperexcitation of granule cells. We conclude that the sprouting of mossy fibers did not play a critical role in kindling, because we observed no significant correlation between the mossy fiber sprouting and kindling development. Epileptic seizures also have been shown to increase gamma-aminobutyric acid (GABA)A receptor-mediated inhibition during interictal periods. Although this strengthening of GABAergic inhibition is likely to be a defensive measure against seizure initiation, some studies have suggested that another type of GABAergic response facilitates seizure activity. The possibility of GABAA receptor-mediated excitation during ictal periods is theoretically examined.

Repeated generalized audiogenic seizures induce plastic changes on acoustically evoked neuronal firing in the amygdala

Repetition of audiogenic seizures (AGS) (AGS kindling) results in increases in the duration of convulsive behavior and the emergence of cortical epileptiform EEG activity. These changes involve expansion of the neuronal network subserving these seizures. The amygdala (AMG) is postulated to become involved in this expanded network, but the neurophysiological basis of this process is unknown. The present study examined changes in chronically-recorded extracellular neuronal firing patterns in the lateral nucleus of AMG (LAMG) induced by AGS kindling in behaving genetically epilepsy-prone rats (GEPR-9s). Before AGS kindling, onset-only (36.1%), onset-delayed (50%) and delayed-only (13.9%) patterns of response to acoustic stimuli were observed. Neuronal firing was greatly suppressed following systemically administered uncompetitive NMDA antagonist (ketamine, 30 mg/kg, i.p.) with complete recovery by 4 h. After AGS kindling, LAMG neurons displayed a significantly increased incidence of onset-only patterns (93.3%, at 0.5 Hz), and mean acoustic responsiveness was also significantly increased (516.2% of control). LAMG neurons fired tonically during tonic convulsions and exhibited burst firing during post-tonic clonus. Greater acoustically-induced synchronization of LAMG firing, as indicated by elevated responsiveness and increased concentration of firing near the stimulus onset, may be critical for mediating the behavioral and EEG changes induced by AGS kindling. LAMG neuronal firing increases induced by AGS kindling may initiate these pathophysiological alterations, in part, by enhanced glutamate receptor-mediated excitation. This possibility is supported by the previously observed ability of an NMDA antagonist to reverse AGS kindling when focally microinjected into AMG, and the blockade of LAMG firing by administration of an uncompetitive NMDA antagonist observed in the present study.

Do fits really beget fits? The effect of previous epileptic activity on the subsequent induction of the tetanus toxin model of limbic epilepsy in the rat

The effect of pretreatment with either tetanus toxin (in ventral hippocampus) or kainic acid (into dorsal hippocampus, with or without suppression of seizures by phenobarbital) on the subsequent development of epilepsy in rats injected with tetanus toxin (into ventral hippocampus) has been studied. Both treatments advanced the timing of the development of the subsequent epilepsy by a few days but did not affect the severity of the syndrome. The fits stopped after 3 weeks in all the rats but recurred in 6 of 20 of those given kainic acid, with or without phenobarbital, but not in those given only tetanus toxin. It is concluded that while fits make the brain more sensitive to a further epileptogenic stimulus they do not themselves increase their severity or persistence. It is the destruction of the CA3/4 area of the hippocampus which results in this advance and in the predisposition to permanent epilepsy.

Cortical excitability in cryptogenic localization-related epilepsy: interictal transcranial magnetic stimulation studies

PURPOSE

To assess whether single-and paired-pulse transcranial magnetic stimulation (TMS) can measure the interictal brain excitability of medicated patients with cryptogenic localization related epilepsy (CLE). Changes in the balance between excitation and inhibition are the core phenomena in focal epileptogenesis. TMS can assess this balance in the primary motor cortex.

METHODS

We selected 18 patients with CLE and similar clinical features in whom we located the epileptogenic area reliably, with 11 age-and sex-matched healthy controls. For both motor cortices, we determined the threshold to TMS, the duration of the cortical silent period, and the corticocortical inhibition and facilitation curve.

RESULTS

TMS was safe. The more antiepileptic drugs (AEDs) taken by the patients, the higher their threshold to TMS. The silent period duration failed to show significant changes. On paired TMS, a cluster analysis identified a homogeneous subgroup of patients (n = 7) who showed a significantly defective corticocortical inhibition and excess facilitation. With respect to the epileptogenic area, the phenomenon was bilateral in four of these patients, ipsilateral in two, and contralateral in one. The phenomenon was independent of AEDs and many other clinical variables. However, this patient group had a higher seizure frequency and a higher proportion of electroencephalograms (EEGs) showing interictal generalized epileptic discharges than the rest of the patients.

CONCLUSION

Paired TMS provided a valuable pathophysiologic insight into the interictal excitatory state of the cortex in CLE. This method can potentially supply useful prognostic clinical information.

Kindling induced by pentylenetetrazole in rats is not directly associated with changes in the expression of NMDA or benzodiazepine receptors

Repeated injections of a subconvulsant dose of pentylenetetrazole (PTZ, 30 mg/kg IP three times weekly for 13 injections) in Wistar and hooded Lister rats resulted in kindled seizures, the extent of which varied between strains. Wistar rats achieved stage 4 of clonic-tonic seizures, whereas hooded Lister rats only reached stage 2 of convulsive waves axially through the body. Rats were killed 10 days after their final injection, and radioligand binding was used to measure the expression of NMDA receptors in cortex and hippocampus using [3H]MK-801 and [3H]L-689,560, the latter binding specifically to the NR1 subunit. [3H]Ro 15-1788 measured expression of GABA(A)-benzodiazepine binding sites containing alpha1, alpha2, alpha3, or alpha5 subunits. Specific analysis of GABA(A) receptors containing the alpha5 subunit, which are preferentially localized in the hippocampus, was assessed with [3H]L-655,708. In the cortex, there was no effect of strain or treatment on the K(D) or B(max) of any of the ligands. Similarly, there was no effect of strain or treatment on hippocampal [3H]L-689,560 or [3H]Ro 15-1788 binding. However, in the hippocampus there was a significant, albeit modest, effect of treatment on the B(max) of [3H]MK-801 binding and the B(max) and K(D) of [3H]L-655,708 binding, i.e., PTZ-treated rats had fewer [3H]MK-801 and [3H]L-655,708 binding sites (NMDA and alpha5-containing GABA(A) receptors, respectively), but, these reductions were significant only in the relatively seizure-insensitive hooded Lister strain. This suggests that the increased susceptibility to kindling in Wistar rats is not directly related to alterations in the expression of NMDA or GABA(A) receptors.

Interictal inhibitory mechanisms in patients with cryptogenic motor cortex epilepsy: a study of the silent period following transcranial magnetic stimulation

The silent period (SP) following transcranial magnetic stimulation (TMS) of the motor cortex is mainly due to cortical inhibitory mechanisms. The aim of the present study was to investigate these inhibitory phenomena in primary motor cortex epilepsy. We studied the TMS-induced SP in both the first dorsal interosseous (FDI) muscles in 8 patients who suffered from cryptogenic partial epilepsy with seizures starting with clonic movements of the right upper limb. All patients were on chronic medication with antiepileptic drugs. Therefore, besides contrasting the results with 16 age-matched normal controls, we also studied 10 patients receiving similar antiepileptic treatments who suffered from cryptogenic partial epilepsy with seizures characterised by the absence of clonic manifestations. The duration of the SP was bilaterally increased in the patients with clonic seizures when compared with the two other groups of subjects. The SP was longer in the left FDI muscle (contralateral to the side of the clonic manifestation in all the patients). Our findings likely indicate enhanced interictal inhibitory mechanisms in patients with partial epilepsy involving the primary motor cortex. The resulting inhibitory effect could be greater in the intact hemisphere rather than in the affected one, in which the hyperexcitability of the epileptic focus had to be counterbalanced.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.

Time-dependent and regional expression of GABA transporter mRNAs following amygdala-kindled seizures in rats

To investigate the role played by GABA transporters in epileptic seizures, we examined time-dependent and regional changes in expression of GAT-1 and GAT-3 GABA transporter mRNA in amygdala-kindled rat brain using an in situ hybridization method. GAT-1 mRNA was significantly increased bilaterally in the hippocampal dentate gyrus (111-116%) at 1 h after kindled generalized seizures. GAT-1 mRNA was also significantly increased bilaterally in the hippocampal subfields (CA1-4 and dentate gyrus [110-117%]) at 4 h after kindled seizures. There were no significant changes in GAT-1 mRNA level in the amygdalar nuclei, pyriform cortex or cerebral cortex either ipsilaterally or contralaterally at any time after kindled seizures. In contrast, GAT-3 mRNA was significantly increased bilaterally in the amygdalar nuclei and in the contralateral pyriform cortex and cerebral cortex 1 h after seizures. Since all these changes returned to control levels by 8 or 24 h after kindled seizures, the increases in GABA transporter mRNA appeared to be transient responses to seizure activity. These findings indicate that GAT-1 subtype transporter is specifically involved in seizure activity in the hippocampus, while GAT-3 subtype transporter is mainly involved in seizure activity in the amygdalar nuclei and pyriform cortex following amygdala-kindled generalized seizures.

The effects of traumatic brain injury on inhibition in the hippocampus and dentate gyrus

Changes in inhibitory neuronal functioning may contribute to morbidity following traumatic brain injury (TBI). Evoked responses to orthodromic paired-pulse stimulation were examined in the hippocampus and dentate gyrus at 2 and 15 days following lateral fluid percussion TBI in adult rats. The relative strength of inhibition was estimated by measuring evoked paired pulses in three afferent systems: the CA3 commissural input to the CA1 region of the hippocampus; the entorhinal cortical input to the ipsilateral CA1 area (temporoammonic system); and the entorhinal input to the ipsilateral dentate gyrus (perforant path). In addition to quantitative electrophysiological estimates of inhibitory efficacy, levels of gamma-aminobutyric acid (GABA) were qualitatively examined with immunohistochemical techniques. Effects of TBI on paired-pulse responses were pathway-specific, and dependent on time postinjury. At 2 days following TBI, inhibition of population spikes was significantly reduced in the CA3 commissural input to CA1, which contrasted with injury-induced increases in inhibition in the dentate gyrus seen at both 2 and 15 days postinjury. Low-level stimulation, subthreshold for population spikes, also revealed changes in paired-pulse facilitation of field extracellular postsynaptic potentials (fEPSPs), which depended on fiber pathway and time postinjury. Significant injury-induced electrophysiological changes were almost entirely confined to the hemisphere ipsilateral to injury. Intensity of GABA immunobinding exhibited a regional association with electrophysiological indices of inhibition, with the most pronounced increases in GABA levels and inhibition found in the dentate gyrus. TBI-induced effects showed a regional pattern within the hippocampus which corresponds closely to inhibitory changes reported to follow ischemia and kindling. This degree of similarity in outcome following dissimilar injuries may indicate common mechanisms in the nervous system response to injury.

Regional excitatory and inhibitory amino acid concentrations in pentylenetetrazol kindling and kindled rat brain

We determined regional concentrations of excitatory and inhibitory amino acids in pentylenetetrazol (PTZ) kindling and kindled rat brains in order to investigate the mechanisms responsible for the PTZ kindling process and kindled state. Compared with control rats, PTZ kindling rats had significantly higher concentrations of aspartate in the striatum and the temporal cortex 24 h after the 14th injection of PTZ. Glutamate and GABA concentrations were also significantly higher in the brainstem of PTZ kindling rats 24 h after the 14th injection of PTZ. These findings suggest that the striatum and temporal cortex may participate in a pathway for propagation of the PTZ kindling process, and that the brainstem may be a primary site of PTZ kindling epileptogenesis or a part of the pathways for propagation. On the other hand, the concentrations of amino acids in PTZ kindled rats and controls did not differ 4 weeks after the 14th injection. This suggests that the alterations we detected in amino acid metabolism are not related to retention of the PTZ kindled state, and that some other mechanism for it must exist.

Changes in hippocampal circuitry after pilocarpine-induced seizures as revealed by opioid receptor distribution and activation

The pilocarpine model of temporal lobe epilepsy was used to study the time-dependent changes in dentate gyrus circuitry after seizures. Seizures caused a decrease in mu- and delta-opioid receptor immunoreactive (MOR-IR and DOR-IR, respectively) neurons in the hilus and MOR-IR neurons in the granule cell layer. Additionally, diffuse DOR-IR, MOR-IR, and GABA immunoreactivities (GABA-IR) were increased in the inner molecular layer. Using the in vitro hippocampal slice preparation to study the physiological consequences of the anatomical changes, we found that the disinhibitory effects of the mu-opioid receptor agonist [D-Ala2, MePhe4,Gly-(ol)5]-enkephalin (DAMGO) and the GABAA receptor antagonist bicuculline were greatly depressed 5-13 d after pilocarpine injection but returned to control levels within 6 weeks. The amplitudes of monosynaptic evoked IPSCs and the effects of DAMGO on this parameter were also slightly decreased 5-13 d after pilocarpine injection but significantly increased at 6 weeks. DAMGO significantly decreased the mean amplitude of spontaneous IPSCs (sIPSCs) at 6 weeks after pilocarpine injection but not in controls. The delta-opioid receptor agonist [D-Pen2,5]-enkephalin (DPDPE) principally inhibited excitatory transmission in saline-treated animals without affecting either sIPSCs or evoked IPSCs. The DPDPE-induced inhibition of excitatory transmission became more pronounced at 6 weeks after pilocarpine injection. These results illustrate the anatomical reorganization and functional changes in dentate gyrus circuitry evident in an animal model of temporal lobe epilepsy and provide evidence of compensatory changes after trauma to the hippocampal formation.

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.

A speculative model of affective illness cyclicity based on patterns of drug tolerance observed in amygdala-kindled seizures

In this article, we discuss molecular mechanisms involved in the evolution of amygdala kindling and the episodic loss of response to pharmacological treatments during tolerance development. These phenomena allow us to consider how similar principles (in different neurochemical systems) could account for illness progression, cyclicity, and drug tolerance in affective disorders. We describe the phenomenon of amygdala-kindled seizures episodically breaking through effective daily pharmacotherapy with carbamazepine and valproate, suggesting that these observations could reflect the balance of pathological vs compensatory illness-induced changes in gene expression. Under certain circumstances, amygdala-kindled animals that were initially drug responsive can develop highly individualized patterns of seizure breakthroughs progressing toward a complete loss of drug efficacy. This initial drug efficacy may reflect the combination of drug-related exogenous neurochemical mechanisms and illness-induced endogenous compensatory mechanisms. However, we postulate that when seizures are inhibited, the endogenous illness-induced adaptations dissipate (the "time-off seizure" effect), leading to the re-emergence of seizures, a re-induction of a new, but diminished, set of endogenous compensatory mechanisms, and a temporary period of renewed drug efficacy. As this pattern repeats, an intermittent or cyclic response to the anticonvulsant treatment emerges, leading toward complete drug tolerance. We also postulate that the cyclic pattern accelerates over time because of both the failure of robust illness-induced endogenous adaptations to emerge and the progression in pathophysiological mechanisms (mediated by long-lasting changes in gene expression and their downstream consequences) as a result of repeated occurrences of seizures. In this seizure model, this pattern can be inhibited and drug responsivity can be temporarily reinstated by several manipulations, including lowering illness drive (decreasing the stimulation current), increasing drug dosage, switching to a new drug that does not show crosstolerance to the original medication, or temporarily discontinuing treatment, allowing the illness to re-emerge in an unmedicated animal. Each of these variables is discussed in relation to the potential relevance to the emergence, progression, and suppression of individual patterns of episodic cyclicity in the recurrent affective disorders. A variety of clinical studies are outlined that specifically test the hypotheses derived from this formulation. Data from animal studies suggest that illness cyclicity can develop from the relative ratio between primary pathological processes and secondary endogenous adaptations (assisted by exogenous medications). If this proposition is verified, it further suggests that illness cyclicity is inherent to the neurobiological processes of episode emergence and amelioration, and one does not need to postulate a separate defect in the biological clock. The formulation predicts that early and aggressive long-term interventions may be optimal in order to prevent illness emergence and progression and its associated accumulating neurobiological vulnerability factors.

Effects of chronic morphine pretreatment on amygdaloid kindling development, postictal seizure and suppression and benzodiazepine receptor binding in rats

Effects of chronic morphine pretreatment on the development of amygdaloid kindling, seizure suppression and benzodiazepine (BDZ) receptor binding in rats were evaluated. The morphine-pretreated animals showed faster acquisition of seizure activity. Further evaluation of the postictal seizure suppression immediately after a fully kindled seizure demonstrated that morphine-pretreated rats had a decreased sensitivity to subsequent kindling stimulations. Twenty-four hours after the last electrical stimulation, saline-pretreated fully kindled rats showed enhanced BDZ receptor binding in dentate gyrus, and decreased binding in cingulate cortex ipsilateral to the stimulation site, compared to saline controls. Morphine-pretreated amygdala-kindled rats had significantly higher BDZ binding in piriform, entorhinal and sensorimotor cortices, basolateral and cortical amygdaloid nuclei, dentate gyrus, CAI-3 areas, substantia nigra pars reticulata and periaqueductal gray. The present study indicates that the previous experience with chronic morphine modifies the kindling process and that the enhanced BDZ receptor binding detected in our experiments may be involved in the enhanced postictal seizure suppression observed in these animals.

[125I]iomazenil binding in the brains of spontaneously epileptic rats: an ex vivo quantitative autoradiographic study

Central benzodiazepine receptor binding was studied in spontaneously epileptic rats (SER) and in their parent strain. Kyoto-Wistar (KW) rats, by ex vivo quantitative autoradiography with [125I]iomazenil. Thirteen-week-old SER developed tonic and absence-like seizures following mild stimulation by tapping at 5 min before injection of the radioligand. When compared with age-matched KW rats, a significant 76% elevation of radioactivity was observed in the SER hippocampus, while there was no difference in hippocampal blood flow obtained using [125I]N-isopropyl-p-iodoamphetamine. Since benzodiazepine receptors act to enhance inhibitory GABA transmission, this finding suggests a possible mechanism for seizure-induced enhancement of inhibition.

Lasting potentiation of inhibition is associated with an increased number of gamma-aminobutyric acid type A receptors activated during miniature inhibitory postsynaptic currents

Whole-cell patch-clamp recordings unveiled a substantial increase in the amplitude, but no change in the frequency, of miniature inhibitory postsynaptic currents (mIPSCs) in dentate gyrus granule cells following chronic epilepsy induced by kindling. This novel and persistent enhancement of gamma-aminobutyric acid type A (GABAA) receptor-mediated inhibition lasted for at least 48 hr following its induction. Nearly a doubling of the number of activated functional postsynaptic GABAA receptor channels during mIPSCs without any change in single-channel conductance or kinetics could be demonstrated using nonstationary fluctuation analysis. As postsynaptic GABAA receptors are likely to be pharmacologically saturated by the transmitter concentration in the cleft, incrementing the number of functional receptor channels may be the most effective means to augment inhibition in the mammalian brain.

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.

CL 218,872 a triazolopyridazine with a selective affinity for the benzodiazepine BZ1 receptor subtype, retards the development and expression of amygdaloid-kindled seizures: effects of flumazenil

To clarify the role of benzodiazepine receptors in kindling, the present experiment assessed the effects of CL 218,872 (1, 5, 10, and 20 mg/kg), a triazolopyridazine with a selective affinity for the putative benzodiazepine BZ1 receptor subtype, on the development and expression of amygdaloid-kindled seizures. Additionally, we assessed the effects of flumazenil (10 mg/kg), a non-specific benzodiazepine receptor antagonist, on kindling and the expression of kindled seizures alone or concomitantly with CL 218,872 (20 mg/kg). CL 218,872 retarded the development of kindled seizures in a linear dose-dependent manner; rats treated with 5, 10, and 20 mg/kg, but not 1 mg/kg, of CL 218,872 required a greater number of afterdischarges (ADs) to develop generalized seizures than controls. Flumazenil also retarded kindling and failed to attenuate the prophylactic effect of CL 218,872. In a cross-over procedure rats that did not develop generalized seizures after 30 ADs while under drug were rekindled under vehicle and rats kindled under vehicle were subsequently tested under drug. Rats crossed over to vehicle rekindled at a faster rate than did controls during initial kindling, suggesting that some kindling had occurred under the drug. CL 218,872 also dose-dependently depressed kindled seizures and this was attenuated by flumazenil, which had little effect on kindled seizures by itself. Together, these data suggest that CL 218,872 is a potent anticonvulsant, implicating the BZ1 receptor subtype in seizure development and in the expression of kindled seizures.

Prenatal malnutrition and development of the brain

In this review, we have summarized various aspects as to how prenatal protein malnutrition affects development of the brain and have attempted to integrate several broad principles, concepts, and trends in this field in relation to our findings and other studies of malnutrition insults. Nutrition is probably the single greatest environmental influence both on the fetus and neonate, and plays a necessary role in the maturation and functional development of the central nervous system. Prenatal protein malnutrition adversely affects the developing brain in numerous ways, depending largely on its timing in relation to various developmental events in the brain and, to a lesser extent, on the type and severity of the deprivation. Many of the effects of prenatal malnutrition are permanent, though some degree of amelioration may be produced by exposure to stimulating and enriched environments. Malnutrition exerts its effects during development, not only during the so-called brain growth spurt period, but also during early organizational processes such as neurogenesis, cell migration, and differentiation. Malnutrition results in a variety of minimal brain dysfunction-type syndromes and ultimately affects attentional processes and interactions of the organism with the environment, in particular producing functional isolation from the environment, often leading to various types of learning disabilities. In malnutrition insult, we are dealing with a distributed, not focal, brain pathology and various developmental failures. Quantitative assessments show distorted relations between neurons and glia, poor formation of neuronal circuits and alterations of normal regressive events, including cell death and axonal and dendritic pruning, resulting in modified patterns of brain organization. Malnutrition insult results in deviations in normal age-related sequences of brain maturation, particularly affecting coordinated development of various cell types and, ultimately, affecting the formation of neuronal circuits and the commencing of activity of neurotransmitter cell types and, ultimately, affecting the formation of neuronal circuits and the commencing of activity of neurotransmitter systems. It is obvious that such diffuse type "lesions" can be adequately assessed only by interdisciplinary studies across a broad range of approaches, including morphological, biochemical, neurophysiological, and behavioral analyses.

Characterization of mu opioid receptor binding during amygdala kindling in rats and effects of chronic naloxone pretreatment: an autoradiographic study

Using in vitro autoradiography, mu receptor binding in rat brain was characterized at different amygdala kindling stages and in amygdaloid kindled animals pretreated chronically with naloxone. Male Sprague-Dawley rats implanted with bipolar electrodes in the right amygdala received one of the following pretreatments s.c. for 14 days via osmotic minipumps: normal saline solution, 0.5 microliters/h, or naloxone HCl, 75 micrograms/h. Two days after treatments were accomplished animals were stimulated daily. Our data showed different patterns of mu receptor binding during the normal kindling process: during stage II-III, pronounced binding increase was detected in cingulate, temporal and entorhinal cortices, anterior amygdala, caudate putamen, thalamic nuclei, ventrolateral and dorsolateral portions of central gray, substantia nigra pars compacta and pars reticulata. Twenty-four hours after the last stage V kindled seizure, enhanced binding was observed in cingulate and frontoparietal cortices, anterior amygdala, caudate putamen and ventromedial thalamic nucleus. Twenty-eight days after the last stage V kindled seizure, binding augmentation was noticed in cingulate and frontoparietal cortices, whereas decreased binding was detected in amygdala complex, substantia nigra pars reticulata, piriform, perirhinal, parietal, temporal and entorhinal cortices. Mu receptor binding in kindled rats chronically pretreated with naloxone was significantly higher in several structures when compared with control and normal kindled groups. Our data indicate different regional selective patterns of mu receptor binding during amygdala kindling which may depend on epileptogenesis and long-term changes induced by this process. In addition, even higher mu receptor binding results from chronic naloxone administration prior to kindling.

Anatomic correlates of interhippocampal seizure propagation time

The relation between interhippocampal seizure propagation time (IHSPT) and anatomic alterations in the human epileptic hippocampus may provide insight into the pathophysiology of temporal lobe epilepsy (TLE). Using depth electrode recordings, we measured the time required for spontaneous seizures with onset in one hippocampus to become manifest in the contralateral hippocampus in 50 patients who underwent resection of the temporal lobe of seizure origin. Cell densities in individual hippocampal subfields were determined and correlated with mean IHSPT for each patient. Mean IHSPT was significantly and inversely correlated with cell counts in CA4 only (r = -0.38, p less than 0.01, Pearson's product correlation; r = -0.52, p less than 0.001, Spearman's rank order correlation). In 5 patients with bilateral independent hippocampal seizure onset who had temporal lobectomy and a diagnosis of mesial temporal sclerosis, mean IHSPT was consistently longer from the sclerotic temporal lobe than to it. These observations suggest that anatomic changes associated with chronic epilepsy alter propagation patterns. Because CA4 is believed to modulate the output of dentate granule cells and also has commissural connections to the contralateral homotopic area, the association of decreased CA4 cells with prolongation of IHSPT suggests that the observed anatomic alterations may actively (through increased inhibition) or passively (through decreased recruitment) interfere with various routes of seizure propagation.

Kindling and electrode effects on the benzodiazepine receptors density of olfactory bulb and hippocampus after olfactory bulb kindling

The olfactory bulb (OB) kindling is a model of limbic secondary generalized epilepsy. Ten days after the completion of OB kindling, we have studied the long term effects of both electrode insertion and kindling on the binding of [3H]diazepam to crude mitochondrial fractions. On the one hand, we have shown that electrode implantation in sham-operated controls induced an obvious increase in benzodiazepine (BZD) receptor density (Bmax) only at the site of the electrode in comparison to sham-unoperated rats. These results might indicate an additional mechanism extending earlier observations reported by others, who have shown that prolonged electrode implantation induced changes in sham-operated and kindled rats. On the other hand, the long lasting effect of OB kindling on the binding parameters of [3H]diazepam was examined in the focus and in the hippocampus. The results indicate a bilateral increase of BZD receptors in the OB and an ipsilateral increase in the hippocampus. These changes might be a regulation phenomenon in response to a hyperexcitability state and to focal stimulations.

Important roles of N-methyl-D-aspartate receptors in expression of amygdaloid-kindled seizure demonstrated by intraperitoneal administration of L-aspartate in dimethyl sulfoxide

Intraperitoneally (i.p.) administered L-aspartate (Asp) (20 mmol/kg) produced no behavioral or EEG change in nonkindled rats. Nonkindled rats that received 18, 19, or 20 mmol/kg Asp, dissolved in 10 or 15% dimethylsulfoxide (DMSO), i.p. developed masticatory movement, head nodding, and myoclonic jerks of the limbs, followed by wild running and subsequent tonic extension of the whole body. In contrast to the effects in nonkindled rats, i.p. injection of Asp 20 mmol/kg in 15% DMSO in amygdala-kindled rats precipitated electroclinical generalized seizures identical to kindled ones. When the kindled amygdala was pretreated with 2-amino-7-phosphonoheptanoic acid (2-APH), a potent and specific antagonist of N-methyl-D-aspartate (NMDA) receptors, the Asp/DMSO-induced generalized convulsion identical to kindled amygdala seizure was suppressed. 2-APH treatment of the contralateral amygdala was without such suppression. The results suggest that (a) Asp is ineffective when given alone (when given with DMSO, however, Asp evokes generalized seizures identical to kindled ones in amygdala-kindled rats, while it induces a qualitatively different generalized seizure in nonkindled rats; (b) NMDA receptors of the kindled amygdala play an important role in activation of the transsynaptic neurocircuit underlying the expression of kindled amygdala seizure; and (c) DMSO is useful in assessing potential central effects of compounds that do not readily penetrate the blood-brain barrier (BBB).