Electrophysiology of dentate granule cells after kainate-induced synaptic reorganization of the mossy fibers

Excitatory synaptic input to granule cells increases with time after kainate treatment

Temporal lobe epilepsy is usually associated with a latent period and an increased seizure frequency following a precipitating insult. After kainate treatment, the mossy fibers of the dentate gyrus are hypothesized to form recurrent excitatory circuits between granule cells, thus leading to a progressive increase in the excitatory input to granule cells. Three groups of animals were studied as a function of time after kainate treatment: 1-2 wk, 2-4 wk, and 10-51 wk. All the animals studied 10-51 wk after kainate treatment were observed to have repetitive spontaneous seizures. Whole cell patch-clamp recordings in hippocampal slices showed that the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) in granule cells increased with time after kainate treatment. This increased excitatory synaptic input was correlated with the intensity of the Timm stain in the inner molecular layer (IML). Flash photolysis of caged glutamate applied in the granule cell layer evoked repetitive EPSCs in 10, 32, and 66% of the granule cells at the different times after kainate treatment. When inhibition was reduced with bicuculline, photostimulation of the granule cell layer evoked epileptiform bursts of action potentials only in granule cells from rats 10-51 wk after kainate treatment. These data support the hypothesis that kainate-induced mossy fiber sprouting in the IML results in the progressive formation of aberrant excitatory connections between granule cells. They also suggest that the probability of occurrence of electrographic seizures in the dentate gyrus increases with time after kainate treatment.

Age-dependent consequences of seizures: relationship to seizure frequency, brain damage, and circuitry reorganization

Seizures in the developing brain pose a challenge to the clinician. In addition to the acute effects of the seizure, there are questions regarding the impact of severe or recurrent seizures on the developing brain. Whether provoked seizures cause brain damage, synaptic reorganization, or epilepsy is of paramount importance to patients and physicians. Such questions are especially relevant in the decision to treat or not treat febrile seizures, a common occurrence in childhood. These clinical questions have been addressed using clinical and animal research. The largest prospective studies do not find a causal connection between febrile seizures and later temporal lobe epilepsy. The immature brain seems relatively resistant to the seizure-induced neuronal loss and new synapse formation seen in the mature brain. Laboratory investigations using a developmental rat model corresponding to human febrile seizures find that even though structural changes do not result from hyperthermic seizures, synaptic function may be chronically altered. The increased understanding of the cellular and synaptic mechanisms of seizure-induced damage may benefit patients and clinicians in the form of improved therapies to attenuate damage and changes induced by seizures and to prevent the development of epilepsy.

Kainic acid-induced mossy fiber sprouting and synapse formation in the dentate gyrus of rats

In the kainic acid (KA) model of temporal lobe epilepsy, mossy fibers (MFs) are thought to establish recurrent excitatory synaptic contacts onto granule cells. This hypothesis was tested by intracellular labeling of granule cells with biocytin and identifying their synaptic contacts in the dentate molecular layer with electron microscopic (EM) techniques. Twenty-three granule cells from KA-treated animals and 14 granule cells from control rats were examined 2 to 4 months following KA at the light microscopic (LM) level; four cells showing MF sprouting were further characterized at the EM level. Timm staining revealed a time-dependent growth of aberrant MFs into the dentate inner molecular layer. The degree of sprouting was generally (but not invariably) correlated with the severity and frequency of seizures. LM examination of individual biocytin-labeled MF axon collaterals revealed enhanced collateralization and significantly increased numbers of synaptic MF boutons in the hilus compared to controls, as well as aberrant MF growth into the granule cell and molecular layers. EM examination of serially reconstructed, biocytin-labeled MF collaterals in the molecular layer revealed MF boutons that form asymmetrical synapses with dendritic shafts and spines of granule cells, including likely autaptic contacts on parent dendrites of the biocytin-labeled granule cell. These results constitute ultrastructural evidence for newly formed excitatory recurrent circuits, which might provide a structural basis for enhanced excitation and epileptogenesis in the hippocampus of KA-treated rats.

Brain-derived neurotrophic factor delays hippocampal kindling in the rat

Epileptic seizures increase the expression of brain-derived neurotrophic factor in the hippocampus. Since this neurotrophin exerts modulatory effects on neuronal excitability in this structure, it may play an important role in hippocampal epileptogenesis. This question was addressed by studying the effects of chronic infusions of recombinant brain-derived neurotrophic factor and brain-derived neurotrophic factor antisense in the hippocampus during the first seven days of hippocampal kindling. Infusion with brain-derived neurotrophic factor (6-24 microg/day) significantly delayed the progression of standard hippocampal kindling and strongly suppressed seizures induced by rapid hippocampal kindling. These suppressive effects were dose dependent, long lasting, not secondary to neuronal toxicity and specific to this neurotrophin, as nerve growth factor accelerated hippocampal kindling progression. They also appeared to be specific to the hippocampus, as infusion of brain-derived neurotrophic factor (48 microg/day) in the amygdala only resulted in a slight and transient delay of amygdala kindling. Conversely to the protective effects of exogenous brain-derived neurotrophic factor, chronic hippocampal infusion of antisense oligodeoxynucleotides (12 nmol/day), resulting in reduced expression of endogenous brain-derived neurotrophic factor in the hippocampus, aggravated seizures during hippocampal kindling. Taken together, our results lead us to suggest that the seizure-induced increase in brain-derived neurotrophic factor expression in the hippocampus may constitute an endogenous regulatory mechanism able to restrain hippocampal epileptogenesis.

Ultrastructural features and synaptic connections of hilar ectopic granule cells in the rat dentate gyrus are different from those of granule cells in the granule cell layer

Several investigators have shown the existence of dentate granule cells in ectopic locations within the hilus and molecular layer using both Golgi and retrograde tracing studies but the ultrastructural features and synaptic connections of ectopic granule cells were not previously examined. In the present study, the biocytin retrograde tracing technique was used to label ectopic granule cells following injections into stratum lucidum of CA3b of hippocampal slices obtained from epileptic rats. Electron microscopy was used to study hilar ectopic granule cells that were located 20-40 microm from the granule cell layer (GCL). They had ultrastructural features similar to those of granule cells in the GCL but showed differences, including nuclei that often displayed infoldings and thicker apical dendrites. At their origin, these dendrites were 6 microm in diameter and they tapered down to 2 microm at the border with the GCL. Both biocytin-labeled and unlabeled axon terminals formed exclusively asymmetric synapses with the somata and proximal dendrites of hilar ectopic granule cells. The mean number of axosomatic synapses for these cells was three times that for granule cells in the GCL. Together, these data indicate that hilar ectopic granule cells are postsynaptic to mossy fibers and have less inhibitory input on their somata and proximal dendrites than granule cells in the GCL. This finding is consistent with recent physiological results showing that hilar ectopic granule cells from epileptic rats are more hyperexcitable than granule cells in the GCL.

Short- and long-term changes in CA1 network excitability after kainate treatment in rats

Neuron loss, axon sprouting, and the formation of new synaptic circuits have been hypothesized to contribute to seizures in temporal lobe epilepsy (TLE). Using the kainate-treated rat, we examined how alterations in the density of CA1 pyramidal cells and interneurons, and subsequent sprouting of CA1 pyramidal cell axons, were temporally associated with functional changes in the network properties of the CA1 area. Control rats were compared with animals during the first week after kainate treatment versus several weeks after treatment. The density of CA1 pyramidal cells and putative inhibitory neurons in stratum oriens was reduced within 8 days after kainate treatment. Axon branching of CA1 pyramidal cells was similar between controls and animals examined in the first week after kainate treatment but was increased several weeks after kainate treatment. Stimulation of afferent fibers in brain slices containing the isolated CA1 region produced graded responses in slices from controls and kainate-treated rats tested <8 days after treatment. In contrast, synchronous all-or-none bursts of spikes at low stimulus intensity (i.e., "network bursts") were only observed in the CA1 several weeks after kainate treatment. In the presence of bicuculline, the duration of evoked bursts was significantly longer in CA1 pyramidal cells weeks after kainate treatment than from controls or those examined in the first week posttreatment. Spontaneous network bursts were also observed in the isolated CA1 several weeks after kainate treatment in bicuculline-treated slices. The data suggest that the early loss of neurons directly associated with kainate-induced status epilepticus is followed by increased axon sprouting and new recurrent excitatory circuits in CA1 pyramidal cells. These changes characterize the transition from the initial acute effects of the kainate-induced insult to the eventual development of all-or-none epileptiform discharges in the CA1 area.

Status epilepticus-induced hilar basal dendrites on rodent granule cells contribute to recurrent excitatory circuitry

Mossy fiber sprouting into the inner molecular layer of the dentate gyrus is an important neuroplastic change found in animal models of temporal lobe epilepsy and in humans with this type of epilepsy. Recently, we reported in the perforant path stimulation model another neuroplastic change for dentate granule cells following seizures: hilar basal dendrites (HBDs). The present study determined whether status epilepticus-induced HBDs on dentate granule cells occur in the pilocarpine model of temporal lobe epilepsy and whether these dendrites are targeted by mossy fibers. Retrograde transport of biocytin following its ejection into stratum lucidum of CA3 was used to label granule cells for both light and electron microscopy. Granule cells with a heterogeneous morphology, including recurrent basal dendrites, and locations outside the granule cell layer were observed in control preparations. Preparations from both pilocarpine and kainate models of temporal lobe epilepsy also showed granule cells with HBDs. These dendrites branched and extended into the hilus of the dentate gyrus and were shown to be present on 5% of the granule cells in pilocarpine-treated rats with status epilepticus, whereas control rats had virtually none. Electron microscopy was used to determine whether HBDs were postsynaptic to axon terminals in the hilus, a site where mossy fiber collaterals are prevalent. Labeled granule cell axon terminals were found to form asymmetric synapses with labeled HBDs. Also, unlabeled, large mossy fiber boutons were presynaptic to HBDs of granule cells. These results indicate that HBDs are present in the pilocarpine model of temporal lobe epilepsy, confirm the presence of HBDs in the kainate model, and show that HBDs are postsynaptic to mossy fibers. These new mossy fiber synapses with HBDs may contribute to additional recurrent excitatory circuitry for granule cells.

Synaptic connections from multiple subfields contribute to granule cell hyperexcitability in hippocampal slice cultures

Limbic status epilepticus and preparation of hippocampal slice cultures both produce cell loss and denervation. This commonality led us to hypothesize that morphological and physiological alterations in hippocampal slice cultures may be similar to those observed in human limbic epilepsy and animal models. To test this hypothesis, we performed electrophysiological and morphological analyses in long-term (postnatal day 11; 40-60 days in vitro) organotypic hippocampal slice cultures. Electrophysiological analyses of dentate granule cell excitability revealed that granule cells in slice cultures were hyperexcitable compared with acute slices from normal rats. In physiological buffer, spontaneous electrographic granule cell seizures were seen in 22% of cultures; in the presence of a GABA(A) receptor antagonist, seizures were documented in 75% of cultures. Hilar stimulation evoked postsynaptic potentials (PSPs) and multiple population spikes in the granule cell layer, which were eliminated by glutamate receptor antagonists, demonstrating the requirement for excitatory synaptic transmission. By contrast, under identical recording conditions, acute hippocampal slices isolated from normal rats exhibited a lack of seizures, and hilar stimulation evoked an isolated population spike without PSPs. To examine the possibility that newly formed excitatory synaptic connections to the dentate gyrus contribute to granule cell hyperexcitability in slice cultures, anatomical labeling and electrophysiological recordings following knife cuts were performed. Anatomical labeling of individual dentate granule, CA3 and CA1 pyramidal cells with neurobiotin illustrated the presence of axonal projections that may provide reciprocal excitatory synaptic connections among these regions and contribute to granule cell hyperexcitability. Knife cuts severing connections between CA1 and the dentate gyrus/CA3c region reduced but did not abolish hilar-evoked excitatory PSPs, suggesting the presence of newly formed, functional synaptic connections to the granule cells from CA1 and CA3 as well as from neurons intrinsic to the dentate gyrus. Many of the electrophysiological and morphological abnormalities reported here for long-term hippocampal slice cultures bear striking similarities to both human and in vivo models, making this in vitro model a simple, powerful system to begin to elucidate the molecular and cellular mechanisms underlying synaptic rearrangements and epileptogenesis.

Modest increase in extracellular potassium unmasks effect of recurrent mossy fiber growth

The recurrent mossy fiber pathway of the dentate gyrus expands dramatically in many persons with temporal lobe epilepsy. The new connections among granule cells provide a novel mechanism of synchronization that could enhance the participation of these cells in seizures. Despite the presence of robust recurrent mossy fiber growth, orthodromic or antidromic activation of granule cells usually does not evoke repetitive discharge. This study tested the ability of modestly elevated [K(+)](o), reduced GABA(A) receptor-mediated inhibition and frequency facilitation to unmask the effect of recurrent excitation. Transverse slices of the caudal hippocampal formation were prepared from pilocarpine-treated rats that either had or had not developed status epilepticus with subsequent recurrent mossy fiber growth. During superfusion with standard medium (3.5 mM K(+)), antidromic stimulation of the mossy fibers evoked epileptiform activity in 14% of slices with recurrent mossy fiber growth. This value increased to approximately 50% when [K(+)](o) was raised to either 4.75 or 6 mM. Addition of bicuculline (3 or 30 microM) to the superfusion medium did not enhance the probability of evoking epileptiform activity but did increase the magnitude of epileptiform discharge if such activity was already present. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (1 microM), which selectively activates type II metabotropic glutamate receptors present on mossy fiber terminals, strongly depressed epileptiform responses. This result implies a critical role for the recurrent mossy fiber pathway. No enhancement of the epileptiform discharge occurred during repetitive antidromic stimulation at frequencies of 0.2, 1, or 10 Hz. In fact, antidromically evoked epileptiform activity became progressively attenuated during a 10-Hz train. Antidromic stimulation of the mossy fibers never evoked epileptiform activity in slices from control rats under any condition tested. These results indicate that even modest changes in [K(+)](o) dramatically affect granule cell epileptiform activity supported by the recurrent mossy fiber pathway. A small increase in [K(+)](o) reduces the amount of recurrent mossy fiber growth required to synchronize granule cell discharge. Block of GABA(A) receptor-mediated inhibition is less efficacious and frequency facilitation may not be a significant factor.

Endogenous control of hippocampal epileptogenesis: a molecular cascade involving brain-derived neurotrophic factor and neuropeptide Y

PURPOSE

Seizures increase the expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. Because this neurotrophin exerts modulatory effects on hippocampal neuronal excitability, it may play an important role in epileptogenesis initiated in this structure. Moreover BDNF is known to regulate the expression of neuropeptide Y (NPY), which displays modulatory properties on seizure activity. This suggests that the effects of BDNF on epileptogenesis may be mediated by NPY.

METHODS

Adult male rats received a 7-day chronic intrahippocampal infusion of BDNF, BDNF antisense oligodeoxynucleotides, NPY, or anti-NPY immunoglobulin G during kindling of the hippocampus. The long-term regulation of NPY expression by BDNF was also studied by immunohistochemistry and radioimmunoassay.

RESULTS

BDNF applied during the first week of hippocampal stimulation significantly delayed the progression of kindling, an effect that outlasted the end of the infusion by at least 7 days. Conversely, infusion of BDNF antisense oligodeoxynucleotides to reduce the expression of endogenous BDNF in the hippocampus aggravated the electroencephalographic expression of seizures. Chronic infusion of BDNF increased the expression of NPY in the hippocampus, with a time course similar to that of the protective effect of the neurotrophin on kindling. Finally, chronic infusion of NPY in the hippocampus delayed the progression of hippocampal kindling, whereas anti-NPY antibodies had an aggravating effect.

CONCLUSIONS

Our results suggest that the seizure-induced increase in BDNF expression in the hippocampus may constitute an endogenous protective mechanism able to counteract hippocampal epileptogenesis. This protective effect appears to be mediated at least in part through the regulation of NPY expression.

What is GABAergic inhibition? How is it modified in epilepsy?

A deficit of gamma-aminobutyric acid-ergic (GABAergic) inhibition is hypothesized to underlie most forms of epilepsy. Although apparently a straightforward and logical hypothesis to test, the search for a deficit of GABAergic inhibition in epileptic tissue has revealed itself to be as difficult as the quest for the Holy Grail. The investigator faces many obstacles, including the multiplicity of GABAergic inhibitory pathways and the multiplicity of variables that characterize the potency of inhibition within each inhibitory pathway. Perhaps more importantly, there seems to be no consensual definition of GABAergic inhibition. The first goal of this review is to try to clarify the notion of GABAergic inhibition. The second goal is to summarize our current knowledge of the various alterations that occur in the GABAergic pathways in temporal lobe epilepsy. Two important features will emerge: (a) according to the variable used to measure GABAergic inhibition, it may appear increased, decreased, or unchanged; and (b) these modifications are brain area- and inhibitory pathway-specific. The possible functional consequences of these alterations are discussed.

Association between the density of mossy fiber sprouting and seizure frequency in experimental and human temporal lobe epilepsy

PURPOSE

If the sprouting of granule cell axons or mossy fibers in the dentate gyrus is critical for the generation of spontaneous seizures in temporal lobe epilepsy (TLE), one could hypothesize that epileptic animals or humans with increased sprouting would have more frequent seizures. This hypothesis was tested by analyzing the data gathered from experimental and human epilepsy.

METHODS

In experiment I (rats with "newly diagnosed" TLE), self-sustained status epilepticus was induced in rats by electrically stimulating the amygdala. Thereafter, the appearance of spontaneous seizures was monitored by continuous video-electroencephalography (EEG) until the animal developed two spontaneous seizures and for 11 d thereafter. Rats were perfused for histology, and mossy fibers were stained using the Timm method. In experiment II (rats with "recently diagnosed" TLE), status epilepticus was induced in rats and the development of seizures was monitored by video-EEG for 24 h/d every other day for 60 days. All animals were then perfused for histology. In experiment III (rats with "chronic" TLE), animals were monitored by video-EEG for 24 h/d every other day for 6 months before histologic analysis. To assess mossy fiber sprouting in human TLE, hippocampal sections from 31 patients who had undergone surgery for drug-refractory TLE were stained with an antibody raised against dynorphin.

RESULTS AND CONCLUSIONS

Our data indicate that the density of mossy fiber sprouting is not associated with the total number of lifetime seizures or the seizure frequency in experimental or human TLE.

Mossy fiber zinc and temporal lobe epilepsy: pathological association with altered "epileptic" gamma-aminobutyric acid A receptors in dentate granule cells

Temporal lobe epilepsy is associated with circuit rearrangements within the hippocampus. Mossy fibers sprout and pathologically innervate the inner molecular layer of the dentate gyrus, providing a recurrent excitatory pathway not present in the control brain. In addition to releasing glutamate, these recurrent collaterals also release zinc, which can accumulate in high concentrations in the extracellular space. Accompanying these dentate gyrus circuit rearrangements are alterations in the subunit expression patterns and pharmacology of gamma-aminobutyric acid A (GABAA) receptors in dentate granule cells. In normal, control granule cells, GABAA receptors are zinc insensitive as a result of high levels of expression of the alpha1 subunit in these cells. In epileptic brain, expression of alpha1 subunits decreases and expression of alpha4 and delta subunits increases, leading to the assembly of GABAA receptors that are exquisitely zinc sensitive. This temporal and spatial association of the expression of zinc-sensitive GABAA receptors and the emergence of a zinc-delivery system unique to the epileptic hippocampus has led to the formulation of an hypothesis that suggests that zinc release during repetitive activation of the dentate gyrus may lead to a catastrophic failure of inhibition under conditions mediating seizure initiation. This could contribute to the limbic hyperexcitability characteristic of temporal lobe epilepsy.

L-type Ca(2+) channel blocker inhibits mossy fiber sprouting and cognitive deficits following pilocarpine seizures in immature mice

Behavioral and cognitive deficits are one of the most frequent sequelae of childhood epilepsy. Accumulating evidence indicates that epilepsy induces aberrant development of the mossy fibers in the hippocampus, the region that is commonly accepted to play a key role in learning and memory. We have therefore proposed that such abnormal maturation of the central nervous system may cause the adverse prognoses following epilepsy. Based on this hypothesis, using primary cultures of the dentate granule cells, we showed that the L-type Ca(2+) channel blocker nicardipine was neuroprotective against excessive mossy fiber synaptogenesis induced by prolonged depolarization that was assumed to mimic epileptiform conditions. Therefore, we evaluated the in vivo effect of nicardipine on aversive sequelae following epileptiform seizures. We found aberrant sprouting of the mossy fibers and poor performance of spatial and contextual tasks in the mice that had received treatment with pilocarpine at their early postnatal age. Repetitive administration of nicardipine prevented the mossy fiber sprouting and ameliorated the cognitive deterioration, although it did not show anticonvulsant actions against pilocarpine seizures. In the present study, we proposed two in vitro and in vivo models for evaluating epilepsy sequelae and noticed that L-type Ca(2+) channel blocker nicardipine was effective in both models. L-type Ca(2+) channel blocker may be a good candidate for a preventive for childhood epilepsy sequelae. Likewise, these useful systems will disclose additional candidates in future.

Lipid peroxidation in hippocampus early and late after status epilepticus induced by pilocarpine or kainic acid in Wistar rats

Oxidative stress has been implicated in a variety of acute and chronic neurologic conditions, including epilepsy. Both the kainic acid and pilocarpine are useful models of temporal lobe epilepsy in rodents. As an index of lipid peroxidation the level thiobarbituric acid reactive substances (TBARS) was measured after the status epileticus induced by pilocarpine or kainic acid. In hippocampus there was a slight enhancement in the TBARS levels measured 12-14 h after the end of status epileticus induced by pilocarpine and kainic acid. The TBARS levels in pilocarpine treated animals was significantly decreased late after status epileticus and in kainic acid model the TBARS returned to basal levels. These results indicating a putative role of reactive oxygen species in kainic acid and pilocarpine induced epilepsy.

Opioid modulation of recurrent excitation in the hippocampal dentate gyrus

kappa opioid receptor activation inhibits granule cell-mediated excitatory neurotransmission in the hippocampal formation via a decrease in glutamate release from both perforant path and mossy fiber terminals. We now report a third, anatomically and pharmacologically distinct site of such kappa opioid inhibition within the hippocampus. Granule cell population responses to selective stimulation of an excitatory hilar pathway were decreased by the kappa(1) opioid receptor agonist U69,593, an effect blocked by the kappa(1) antagonist norbinaltorphimine. U69,593 also inhibited hilar path induced long-term potentiation (LTP) of granule cell responses. LTP in this pathway was also blocked by the NMDA receptor antagonist d-2-amino-5-phosphonovalerate, unlike granule cell mossy fiber LTP in CA3. The kappa opioid peptide dynorphin is present in hilar mossy fiber collaterals. Ultrastructural analysis of these collaterals demonstrated dynorphin-containing vesicles in asymmetric synapses formed between axon terminals and granule cell dendrites, suggesting direct granule cell-granule cell connections. Evoked release of endogenous dynorphin within the hilus was effective in reducing hilar excitation of granule cells, although this release, in contrast to the release of dynorphin in the dentate molecular layer, was not dependent on L-type calcium channels. No hilar path excitation was observed in the absence of bicuculline, suggesting a strong GABA(A)-mediated inhibition of this pathway. However, hilar path activity could be seen after LTP, with or without bicuculline. Thus, kappa opioids can inhibit granule cell recurrent excitation, likely via effects on excitatory mossy fiber collaterals. Such collaterals are thought to be important in mediating temporal lobe epilepsy.

Changes in synaptosomal ectonucleotidase activities in two rat models of temporal lobe epilepsy

Adenosine has been proposed as an endogenous anticonvulsant which can play an important role in seizure initiation, propagation and arrest. Besides the release of adenosine per se, the ectonucleotidase pathway is an important metabolic source of extracellular adenosine. Here we evaluated ATP diphosphohydrolase and 5'-nucleotidase activities in synaptosomes from hippocampus and cerebral cortex at different periods after induction of status epilepticus (SE) by intraperitoneal administration of pilocarpine or kainate. Ectonucleotidase activities from synaptosomes of hippocampus and cerebral cortex of rats were significantly increased at 48-52 h, 7-9 days and 45-50 days after induction of SE by pilocarpine. In relation to kainate model, both hippocampal enzymes were enhanced at 7-9 days and 45-50 days, but only 5'-nucleotidase remained elevated at 100-110 days after the treatment. In cerebral cortex, an increase in ATP diphosphohydrolase was observed at 48-52 h, 7-9 days and 45-50 days after induction of SE by kainate. However, 5'-nucleotidase activity only presented significant changes at 45-50 and 100-110 days. Our results suggest that SE can induce late and prolonged changes in ectonucleotidases activities. The regulation of the ectonucleotidase pathway may play a modulatory role during the evolution of behavioral and pathophysiological changes related to temporal lobe epilepsy.

Optical recording study of granule cell activities in the hippocampal dentate gyrus of kainate-treated rats

In the epileptic hippocampus, newly sprouted mossy fibers are considered to form recurrent excitatory connections to granule cells in the dentate gyrus and thereby increase seizure susceptibility. To study the effects of mossy fiber sprouting on neural activity in individual lamellae of the dentate gyrus, we used high-speed optical recording to record signals from voltage-sensitive dye in hippocampal slices prepared from kainate-treated epileptic rats (KA rats). In 14 of 24 slices from KA rats, hilar stimulation evoked a large depolarization in almost the entire molecular layer in which granule cell apical dendrites are located. The signals were identified as postsynaptic responses because of their dependence on extracellular Ca(2+). The depolarization amplitude was largest in the inner molecular layer (the target area of sprouted mossy fibers) and declined with increasing distance from the granule cell layer. In the inner molecular layer, a good correlation was obtained between depolarization size and the density of mossy fiber terminals detected by Timm staining methods. Blockade of GABAergic inhibition by bicuculline enlarged the depolarization in granule cell dendrites. Our data indicate that mossy fiber sprouting results in a large and prolonged synaptic depolarization in an extensive dendritic area and that the enhanced GABAergic inhibition partly masks the synaptic depolarization. However, despite the large dendritic excitation induced by the sprouted mossy fibers, seizure-like activity of granule cells was never observed, even when GABAergic inhibition was blocked. Therefore, mossy fiber sprouting may not play a critical role in epileptogenesis.

Delayed apoptotic pyramidal cell death in CA4 and CA1 hippocampal subfields after a single intraseptal injection of kainate

We have performed a detailed time-course analysis of cell death in the hippocampal formation, basal forebrain and amygdala following a single intraseptal injection of kainate in adult rats. Acetylcholinesterase histochemistry revealed a profound loss of staining in the medial septum but not in the diagonal band, and cholinergic fiber density was highly reduced in the hippocampus and amygdala at 10 days postinjection. Terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphatebiotin nick end labeling (TUNEL) histochemistry was performed for precise location of apoptotic cells. Both the medial septum and amygdala exhibited numerous TUNEL-positive nuclei after the intraseptal injection of kainate, while the lateral septum exhibited a lower but significant incidence in terms of apoptotic cells. In the medial septum, the presence of apoptotic cells was at a location displaying acetylcholinesterase staining. TUNEL histochemistry revealed a time-dependent sequential apoptotic cell death in hippocampal pyramidal cells. During the first two days postinjection, apoptosis in the hippocampus was only evident in the CA3 region. At five days postinjection, the entire CA4 region became apoptotic. At 10 days postinjection, the whole extent of the CA1 pyramidal cell layer exhibited numerous TUNEL-positive nuclei. The time-course of kainate-induced apoptosis in Ammons's horn correlated with the disappearance of hippocampal pyramidal neurons as detected by Nissl staining, which is suggestive of a prominent apoptotic death for these cells. The temporal delayed distant damage to CA4 and CA1 hippocampal subfields after a single intraseptal kainate injection is not seen in other models employing kainate and may be a valuable tool for exploring the cellular mechanisms leading to cell death in conditions of status epilepticus.

Recurrent excitatory connectivity in the dentate gyrus of kindled and kainic acid-treated rats

Repeated seizures induce mossy fiber axon sprouting, which reorganizes synaptic connectivity in the dentate gyrus. To examine the possibility that sprouted mossy fiber axons may form recurrent excitatory circuits, connectivity between granule cells in the dentate gyrus was examined in transverse hippocampal slices from normal rats and epileptic rats that experienced seizures induced by kindling and kainic acid. The experiments were designed to functionally assess seizure-induced development of recurrent circuitry by exploiting information available about the time course of seizure-induced synaptic reorganization in the kindling model and detailed anatomic characterization of sprouted fibers in the kainic acid model. When recurrent inhibitory circuits were blocked by the GABA(A) receptor antagonist bicuculline, focal application of glutamate microdrops at locations in the granule cell layer remote from the recorded granule cell evoked trains of excitatory postsynaptic potentials (EPSPs) and population burst discharges in epileptic rats, which were never observed in slices from normal rats. The EPSPs and burst discharges were blocked by bath application of 1 microM tetrodotoxin and were therefore dependent on network-driven synaptic events. Excitatory connections were detected between blades of the dentate gyrus in hippocampal slices from rats that experienced kainic acid-induced status epilepticus. Trains of EPSPs and burst discharges were also evoked in granule cells from kindled rats obtained after > or = 1 wk of kindled seizures, but were not evoked in slices examined 24 h after a single afterdischarge, before the development of sprouting. Excitatory connectivity between blades of the dentate gyrus was also assessed in slices deafferented by transection of the perforant path, and bathed in artificial cerebrospinal fluid (ACSF) containing bicuculline to block GABA(A) receptor-dependent recurrent inhibitory circuits and 10 mM [Ca(2+)](o) to suppress polysynaptic activity. Low-intensity electrical stimulation of the infrapyramidal blade under these conditions failed to evoke a response in suprapyramidal granule cells from normal rats (n = 15), but in slices from epileptic rats evoked an EPSP at a short latency (2.59 +/- 0.36 ms) in 5 of 18 suprapyramidal granule cells. The results are consistent with formation of monosynaptic excitatory connections between blades of the dentate gyrus. Recurrent excitatory circuits developed in the dentate gyrus of epileptic rats in a time course that corresponded to the development of mossy fiber sprouting and demonstrated patterns of functional connectivity corresponding to anatomic features of the sprouted mossy fiber pathway.

Genetic dissection of the signals that induce synaptic reorganization

Synaptic reorganization of mossy fibers following kainic acid (KA) administration has been reported to contribute to the formation of recurrent excitatory circuits, resulting in an epileptogenic state. It is unclear, however, whether KA-induced mossy fiber sprouting results from neuronal cell loss or the seizure activity that KA induces. We have recently demonstrated that certain strains of mice are resistant to excitotoxic cell death, yet exhibit seizure activity similar to what has been observed in rodents susceptible to KA. The present study takes advantage of these strain differences to explore the roles of seizure activity vs cell loss in triggering mossy fiber sprouting. In order to understand the relationships between gene induction, cell death, and the sprouting response, we assessed the regulation of two molecules associated with the sprouting response, c-fos and GAP-43, in mice resistant (C57BL/6) and susceptible (FVB/N) to KA-induced cell death. Following administration of KA, increases in c-fos immunoreactivity were observed in both strains, although prolonged induction of c-fos was present only in the hippocampal neurons of FVB/N mice. Mossy fiber sprouting following KA administration was also only observed in FVB/N mice, while induction of GAP-43, a marker associated with mossy fiber sprouting, was not observed in either strain. These results indicate that: (i) KA-induced seizure activity alone is insufficient to induce mossy fiber sprouting; (ii) mossy fiber sprouting may be due to the loss of hilar neurons following kainate administration; and (iii) induction of GAP-43 is not a necessary component of the sprouting response that occurs following KA in mice.

Evidence for enhanced neuro-inflammatory processes in neurodegenerative diseases and the action of nitrones as potential therapeutics

A brief review is presented on observations leading to the current notions regarding neuro-inflammatory processes. The greatest focus is on Alzheimer's disease (AD) since this is where the most convincing data has been obtained. A brief summary of observations on the neuroprotective action of alpha-phenyl-tert-butyl-nitrone (PBN) as well as results of research designed to understand its mechanism of action is presented. We hypothesize that the mechanism of action of PBN involves inhibition of signal transduction processes, which are involved in the upregulation of genes mediated by pro-inflammatory cytokines and H2O2 that cause formation of toxic gene products. Results from recent experiments on Kainic acid (KA) mediated brain damage are provided to suggest the validity of the in vivo action of PBN to inhibit neuro-inflammatory processes. The accumulating scientific facts are helping to provide concepts that may become the basis for novel therapeutic approaches to the treatment of several neurodegenerative diseases.

In vitro status epilepticus causes sustained elevation of intracellular calcium levels in hippocampal neurons

Calcium ions and calcium-dependent systems have been implicated in the pathophysiology of status epilepticus (SE). However, the dynamics of intracellular calcium ([Ca2+]i) levels during SE has not yet been studied. We have employed the hippocampal neuronal culture (HNC) model of in vitro SE that produces continuous epileptiform discharges to study spatial and dynamic changes in [Ca2+]i levels utilizing confocal laser scanning microscopy and the calcium binding dye, indo-1. During SE, the average [Ca2+]i levels increased from control levels of 150-200 nM to levels of 450-600 nM. This increased [Ca2+]i was maintained for the duration of SE. Following SE, [Ca2+]i levels gradually returned to basal values. The duration of SE was shown to affect the ability of the neuron to restore resting [Ca2+]i levels. Both N-methyl-D-aspartate (NMDA) receptor-gated and voltage-gated Ca2+ channels (VGCCs) contributed to the increased calcium entry during SE. Moreover, this elevation in [Ca2+]i occurred in both the nucleus and cytosol. These results provide the first dynamic measurement of [Ca2+]i during prolonged electrographic seizure discharges in an in vitro SE model and suggest that prolonged epileptiform discharges give rise to abnormal sustained increases in [Ca2+]i levels that may play a role in the neuronal cell damage and long-term plasticity changes associated with SE.

Mossy fiber-granule cell synapses in the normal and epileptic rat dentate gyrus studied with minimal laser photostimulation

Dentate granule cells become synaptically interconnected in the hippocampus of persons with temporal lobe epilepsy, forming a recurrent mossy fiber pathway. This pathway may contribute to the development and propagation of seizures. The physiology of mossy fiber-granule cell synapses is difficult to characterize unambiguously, because electrical stimulation may activate other pathways and because there is a low probability of granule cell interconnection. These problems were addressed by the use of scanning laser photostimulation in slices of the caudal hippocampal formation. Glutamate was released from a caged precursor with highly focused ultraviolet light to evoke action potentials in a small population of granule cells. Excitatory synaptic currents were recorded in the presence of bicuculline. Minimal laser photostimulation evoked an apparently unitary excitatory postsynaptic current (EPSC) in 61% of granule cells from rats that had experienced pilocarpine-induced status epilepticus followed by recurrent mossy fiber growth. An EPSC was also evoked in 13-16% of granule cells from the control groups. EPSCs from status epilepticus and control groups had similar peak amplitudes ( approximately 30 pA), 20-80% rise times (approximately 1.2 ms), decay time constants ( approximately 10 ms), and half-widths (approximately 8 ms). The mean failure rate was high (approximately 70%) in both groups, and in both groups activation of N-methyl-D-aspartate receptors contributed a small component to the EPSC. The strong similarity between responses from the status epilepticus and control groups suggests that they resulted from activation of a similar synaptic population. No EPSC was recorded when the laser beam was focused in the dentate hilus, suggesting that indirect activation of hilar mossy cells contributed little, if at all, to these results. Recurrent mossy fiber growth increases the density of mossy fiber-granule cell synapses in the caudal dentate gyrus by perhaps sixfold, but the new synapses appear to operate very similarly to preexisting mossy fiber-granule cell synapses.

5'-Nucleotidase activity of mossy fibers in the dentate gyrus of normal and epileptic rats

Sprouting of mossy fibers in the hippocampus of rats that underwent limbic epileptogenesis by amygdala kindling or kainate injection was studied at the light microscopic and ultrastructural levels by cytochemical demonstration of the enzyme 5'-nucleotidase. This adenosine-producing ectoenzyme has previously been shown to characterize malleable terminals during brain development and lesion-induced synaptogenesis, but to be otherwise associated with glial membranes. At the light microscopic level, kainate-treated but not control or kindled rats showed 5'-nucleotidase activity in the CA3 region and in the inner molecular layer of the dentate gyrus. At the ultrastructural level, in control animals, the synapses of the molecular and granular layers were enzyme negative. Only some mossy fiber boutons of the dentate hilus exhibited 5'-nucleotidase activity. In epileptic rats, synaptic labeling within the hilus appeared more intense. Moreover, 5'-nucleotidase-containing terminals within the inner molecular layer, presumably ectopic mossy fiber boutons, were found in both kindled and kainate-treated rats. It is concluded that, in both the normal and epileptic hippocampus, 5'-nucleotidase is associated with axons capable of a plastic sprouting response. The synaptic enzyme may attenuate the glutamatergic transmission of mossy fibers, in particular of the aberrant mossy fibers in epileptic rats, by producing the inhibitory neuromodulator adenosine. Alternatively, 5'-nucleotidase may influence synapse formation by its putative non-enzymatic, adhesive functions.

Flurothyl status epilepticus in developing rats: behavioral, electrographic histological and electrophysiological studies

Status epilepticus and repeated seizures have age-dependent morphological and neurophysiological alterations in the hippocampus. In the present study, effects of flurothyl-induced status epilepticus were examined in awake and free moving immature (2 weeks old) and adult rats. Without exception, adult rats died of respiratory arrest before the onset of status epilepticus. We were unable to find a concentration of flurothyl that produced status epilepticus and a low mortality in adult rats. In contrast, immature rats survived flurothyl status epilepticus for up to 60 min with a very low mortality. In rat pups, behavioral manifestations correlated with electrographic seizures in both the cortex and hippocampus. Neuropathological damage (cell loss, pyknotic cells or gliosis) was not observed in the immature hippocampus, thalamus, amygdala, substantia nigra or cortex at 24 h, 2 days or 2 weeks after status epilepticus. In addition, no aberrant mossy fiber reorganization or decrease in cells counts were observed in the hippocampus. Young rats did not show alterations in paired-pulse perforant path inhibition following flurothyl status epilepticus. The present findings are consistent with studies in other seizure models, indicating that immature rats are highly resistant to seizure-induced changes.

Abnormal responses to perforant path stimulation in the dentate gyrus of slices from rats with kainate-induced epilepsy and mossy fiber reorganization

Previous electrophysiological studies have demonstrated that in a subset of hippocampal slices from tissue resected from patients with mesial temporal lobe epilepsy, perforant path stimulation can elicit prolonged negative field-potential shifts in the dentate granule cell layer (Masukawa et al., 1989. Brain Res. 493, 168-174; Isokawa and Fried, 1996. Neuroscience 72, 31-37). In this investigation, hippocampal slices were prepared from rats: (1) 2-4 days following kainate treatment, when little or no reorganization of the mossy fibers would be present and (2) 3-13 months after kainate treatment, when mossy fiber reorganization would have occurred. In saline-treated controls, perforant path stimulation typically evoked a single population spike. In contrast, perforant path stimulation could evoke 3-12 population spikes in nearly all slices from kainate-injected rats 2-4 days and 3-13 months after treatment. The majority of slices from kainate-injected rats 3-13 months after treatment had qualitatively similar responses to perforant path stimulation as that observed in slices from kainate-injected rats 2-4 days after treatment. However, in 17% of the slices from kainate-treated rats 3-13 months after treatment (29% of rats), the multiple population spikes were followed by a prolonged negative field-potential shift (duration: 140 ms-1.5 s) with variable superimposed population spike activity. This type of epileptiform activity was only observed in slices with robust Timm's staining in the inner molecular layer and similar responses could also be evoked in these slices with hilar stimulation. Furthermore, pharmacological depression of inhibition by adding the GABA(A) receptor antagonist bicuculline unmasked hilar-evoked prolonged negative field-potential shifts in most slices from kainate-treated rats 3-13 months following treatment, and these slices had robust Timm's staining in the inner molecular layer. Such events were not observed in slices from saline-treated controls or kainate-injected rats 2-4 days after treatment. In conclusion, the prolonged negative field-potential shifts evoked to perforant path stimulation in normal ACSF were associated with mossy fiber reorganization, but the relative contribution of altered inhibition, increased synaptic excitation, or even non-synaptic mechanisms is unknown.

Chronic epileptogenic cellular alterations in the limbic system after status epilepticus

Status epilepticus (SE) is associated with both acute and permanent pathological sequellae. One common long term consequence of SE is the subsequent development of a chronic epileptic condition, with seizures frequently originating from and involving the limbic system. Following SE, many studies have demonstrated selective loss of neurons within the hilar region of the dentate gyrus, CA1 and CA3 pyramidal neurons. Selective loss of distinct subpopulations of interneurons throughout the hippocampus is also frequently evident, although interneurons as a whole are selectively spared relative to principal cells. Accompanying this loss of neurons are circuit rearrangements, the most widely studied being the sprouting of dentate granule cell (DGC) axons back onto the inner molecular layer of the dentate gyrus, termed mossy fiber sprouting. Less studied are the receptor properties of the surviving neurons within the epileptic hippocampus following SE. DGCs in epileptic animals exhibit marked alterations in the functional and pharmacological properties of gamma-aminobutyric acid (GABA) receptors. DGCs have a significantly elevated density of GABA(A) receptors in chronically epileptic animals. In addition, the pharmacological properties of GABA(A) receptors in post-SE epileptic animals are quite different compared to controls. In particular, GABA(A) receptors in DGCs from epileptic animals show an enhanced sensitivity to blockade by zinc, and a markedly altered sensitivity to modulation by benzodiazepines. These pharmacological differences may be due to a decreased expression of alpha1 subunits of the GABA(A) receptor relative to other alpha subunits in DGCs of post-SE epileptic animals. These GABA(A) receptor alterations precede the onset of spontaneous seizures in post-SE DGCs, and so are temporally positioned to contribute to the process of epileptogenesis in the limbic system. The presence of zinc sensitive GABA receptors combined with the presence of zinc-containing "sprouted" mossy fiber terminals innervating the proximal dendrites of DGCs in the post-SE epileptic hippocampus prompted the development of the hypothesis that repetitive activation of the DG in the epileptic brain could result in the release of zine. This zinc in turn may diffuse to and block "epileptic" zinc-sensitive GABA(A) receptors in DGCs, leading to a catastrophic failure of inhibition and concomitant enhanced seizure propensity in the post-SE epileptic limbic system.

Actions of brain-derived neurotrophic factor in slices from rats with spontaneous seizures and mossy fiber sprouting in the dentate gyrus

This study examined the acute actions of brain-derived neurotrophic factor (BDNF) in the rat dentate gyrus after seizures, because previous studies have shown that BDNF has acute effects on dentate granule cell synaptic transmission, and other studies have demonstrated that BDNF expression increases in granule cells after seizures. Pilocarpine-treated rats were studied because they not only have seizures and increased BDNF expression in granule cells, but they also have reorganization of granule cell "mossy fiber" axons. This reorganization, referred to as "sprouting," involves collaterals that grow into novel areas, i.e., the inner molecular layer, where granule cell and interneuron dendrites are located. Thus, this animal model allowed us to address the effects of BDNF in the dentate gyrus after seizures, as well as the actions of BDNF on mossy fiber transmission after reorganization. In slices with sprouting, BDNF bath application enhanced responses recorded in the inner molecular layer to mossy fiber stimulation. Spontaneous bursts of granule cells occurred, and these were apparently generated at the site of the sprouted axon plexus. These effects were not accompanied by major changes in perforant path-evoked responses or paired-pulse inhibition, occurred only after prolonged (30-60 min) exposure to BDNF, and were blocked by K252a. The results suggest a preferential action of BDNF at mossy fiber synapses, even after substantial changes in the dentate gyrus network. Moreover, the results suggest that activation of trkB receptors could contribute to the hyperexcitability observed in animals with sprouting. Because human granule cells also express increased BDNF mRNA after seizures, and sprouting can occur in temporal lobe epileptics, the results may have implications for understanding temporal lobe epilepsy.

Inhibition of dentate granule cell neurogenesis with brain irradiation does not prevent seizure-induced mossy fiber synaptic reorganization in the rat

Aberrant reorganization of dentate granule cell axons, the mossy fibers, occurs in human temporal lobe epilepsy and rodent epilepsy models. Whether this plasticity results from the remodeling of preexisting mossy fibers or instead reflects an abnormality of developing dentate granule cells is unknown. Because these neurons continue to be generated in the adult rodent and their production increases after seizures, mossy fibers that arise from either developing or mature granule cells are potential substrates for this network plasticity. Therefore, to determine whether seizure-induced, mossy fiber synaptic reorganization arises from either developing or mature granule cell populations, we used low-dose, whole-brain x-irradiation to eliminate proliferating dentate granule cell progenitors in adult rats. A single dose of 5 Gy irradiation blocked cell proliferation and eliminated putative progenitor cells in the dentate subgranular proliferative zone. Irradiation 1 d before pilocarpine-induced status epilepticus significantly attenuated dentate granule cell neurogenesis after seizures. Two irradiations, 1 d before and 4 d after status epilepticus, essentially abolished dentate granule cell neurogenesis but failed to prevent mossy fiber reorganization in the dentate molecular layer. These results indicate that dentate granule cell neurogenesis in the mature hippocampal formation is vulnerable to the effects of low-dose ionizing irradiation. Furthermore, the development of aberrant mossy fiber remodeling in the absence of neurogenesis suggests that mature dentate granule cells contribute substantially to seizure-induced network reorganization.

Newly formed excitatory pathways provide a substrate for hyperexcitability in experimental temporal lobe epilepsy

Temporal lobe epilepsy (TLE) in humans and animals is associated with axonal sprouting of glutamatergic neurons and neosynaptogenesis in the hippocampal formation. We examined whether this plasticity of excitatory pathways contributes to an increased level of glutamatergic excitation in the CA1 region of rats experiencing chronic spontaneous limbic seizures following kainic acid or pilocarpine treatment. In chronic cases, we report an extensive axonal sprouting of CA1 pyramidal neurons, with many axonal branches entering the pyramidal cell layer and stratum radiatum, regions that are not innervated by axonal collaterals of CA1 pyramidal neurons in control animals. Concurrently with this anatomical reorganization, a large increase of the spontaneous glutamatergic drive is observed in the dendrites and somata of CA1 pyramidal cells. Furthermore, electrical activation of the reorganized CA1 associational pathway evokes epileptiform bursts in CA1 pyramidal cells. These findings suggest that reactive plasticity could contribute to the hyperexcitability of CA1 pyramidal neurons and to the propagation of seizures in these two models of TLE.

Recurrent mossy fiber pathway in rat dentate gyrus: synaptic currents evoked in presence and absence of seizure-induced growth

A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant.

Activity and injury-dependent expression of inducible transcription factors, growth factors and apoptosis-related genes within the central nervous system

This review primarily discusses work that has been performed in our laboratories and that of our direct collaborators and therefore does not represent an exhaustive review of the current literature. Our aim is to further discuss the role that gene expression plays in neuronal plasticity and pathology. In the first part of this review we examine activity-dependent changes in the expression of inducible transcription factors (ITFs) and neurotrophins with long-term potentiation (LTP) and kindling. This work has identified particular ITFs (Krox-20 and Krox-24) and neurotrophin systems (particularly the brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase-B, Trk-B system) that may be involved in stabilizing long-lasting LTP (i.e. LTP3). We also show that changes in the expression of other ITFs (Fos, Jun-D and Krox-20) and the BDNF/trkB neurotrophin system may play a central role in the development of hippocampal kindling, an animal model of human temporal lobe epilepsy. In the next part of this review we examine changes in gene expression after neuronal injuries (ischemia, prolonged seizure activity and focal brain injury) and after nerve transection (axotomy). We identify apoptosis-related genes (p53, c-Jun, Bax) whose delayed expression selectively increases in degenerating neurons, further suggesting that some forms of neuronal death may involve apoptosis. Moreover, since overexpression of the tumour-suppressor gene p53 induces apoptosis in a wide variety of dividing cell types we speculate that it may perform the same function in post-mitotic neurons following brain injuries. Additionally, we show that neuronal injury is associated with rapid, transient, activity-dependent expression of neurotrophins (BDNF and activinA) in neurons, contrasting with a delayed and more persistent injury-induced expression of certain growth factors (IGF-1 and TGFbeta) in glia. In this section we also describe results linking ITFs and neurotrophic factor expression. Firstly, we show that while BDNF and trkB are induced as immediate-early genes following injury, the injury-induced expression of activinA and trkC may be regulated by ITFs. We also discuss whether loss of retrograde transport of neurotrophic factors such as nerve growth factor following nerve transection triggers the selective and prolonged expression of c-Jun in axotomized neurons and whether c-Jun is responsible for regeneration or degeneration of these axotomized neurons. In the last section we further examine the role that gene expression may play in memory formation, epileptogenesis and neuronal degeneration, lastly speculating whether the expression of various growth factors after brain injury represents an endogenous neuroprotective response of the brain to injury. Here we discuss our results which show that pharmacological enhancement of this response with exogenous application of IGF-1 or TGF-beta reduces neuronal loss after brain injury.

Dissociation of mossy fiber sprouting and electrically-induced seizure sensitivity: rapid kindling versus adaptation

It has been shown that massed stimulation (MS) of the amygdala or hippocampus does not result in seizure progression but in the 'phenomenon of adaptation', whereas alternate day rapid kindling (ADRK) produces reliable kindling (Lothman, E.W., Williamson, J.M., 1994. Brain Res. 649, 71-84). The goal of the present experiment was to determine if the two different effects are due to differences in mossy fiber sprouting and/or different seizure and postictal spike propagation patterns. Nine rats underwent MS (66-70 stimulations separated by 5-min interstimulus interval), six were exposed to ADRK (12 stimulations/day, every 30 min, with 4 stimulus days, each separated by 1 stimulus-free day), five rats served as control. All rats had electrodes implanted bilaterally in dorsal and ventral hippocampi (VH) and 14 of them had additional electrodes in the piriform cortex. Animals were stimulated in the left VH at afterdischarge threshold. There was no potentiation in seizure response 4-7 weeks after MS. In contrast, ADRK produced not only kindling but also ongoing epileptogenesis resulting 4-7 weeks later in spontaneous seizures and development of a prolonged convulsive state in response to the initially subconvulsive stimulus. Epileptiform activity during MS was mostly restricted to VH, whereas during ADRK it spread widely among studied structures including piriform cortex. Afterdischarges during MS were elicited frequently but seizures did not progress beyond stage 2-3. During ADRK, afterdischarges were evoked less frequently but seizures reached stage 4-7 by the end of the 3rd and 4th stimulus days. The fully kindled state was not reached at this time, but epileptogenic changes continued to progress. Seven weeks after the initial stimulation, both groups demonstrated mossy fiber sprouting of similar intensity in VH. We suggest, (1) frequent but predominantly local hippocampal afterdischarges induce mossy fiber sprouting, but this is not sufficient to produce significant enhancement in seizure susceptibility, and (2) the involvement of extra-hippocampal structures, possibly piriform cortex, and formation of an aberrant hippocampal-para-hippocampal circuit is required to result in a condition of progressive epileptogenesis.

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.

Synaptic reorganization in explanted cultures of rat hippocampus

Due to loss of afferent innervation, synaptic reorganization occurs in organotypic hippocampal slice cultures. With extra- and intracellular recordings, we confirm that the excitatory loop from the dentate gyrus (DG) to CA3 and further to CA1 is preserved. However, hilar stimulation evoked antidromic population spikes in the DG which were followed by a population postsynaptic potential (PPSP); intracellularly, an antidromic spike with a broad shoulder or EPSP/IPSP sequences were induced. Synaptic responses were blocked by glutamate receptor antagonists. Stimulation of CA1 induced a PPSP in DG. Dextranamine stained pyramidal cells of CA1 were shown to project to DG. After removal of area CA3, DG's and mossy fibers' (MF) stimulation still elicited PPSPs and EPSP/IPSP sequences in area CA1 which disappeared when a cut was made through the hippocampal fissure. During bicuculline perfusion, hilar stimulation caused EPSPs in granule cells and spontaneous and evoked repetitive firing appeared even after its isolation from areas CA3 and CA1. Collateral excitatory synaptic coupling between granule cells was confirmed by paired recordings. Besides the preservation of the trisynaptic pathway in this preparation, new functional synaptic contacts appear, presumably due to MF collateral sprouting and formation of pathways between areas CA1 and DG.

Brain-derived neurotrophic factor transgenic mice exhibit passive avoidance deficits, increased seizure severity and in vitro hyperexcitability in the hippocampus and entorhinal cortex

Transgenic mice overexpressing brain-derived neurotrophic factor from the beta-actin promoter were tested for behavioral, gross anatomical and physiological abnormalities. Brain-derived neurotrophic factor messenger RNA overexpression was widespread throughout brain. Overexpression declined with age, such that levels of overexpression decreased sharply by nine months. Brain-derived neurotrophic factor transgenic mice had no gross deformities or behavioral abnormalities. However, they showed a significant passive avoidance deficit. This deficit was dependent on continued overexpression, and resolved with age as brain-derived neurotrophic factor transcripts decreased. In addition, the brain-derived neurotrophic factor transgenic mice showed increased seizure severity in response to kainic acid. Hippocampal slices from brain-derived neurotrophic factor transgenic mice showed hyperexcitability in area CA3 and entorhinal cortex, but not in dentate gyrus. Finally, area CA1 long-term potentiation was disrupted, indicating abnormal plasticity. Our data suggest that overexpression of brain-derived neurotrophic factor in the brain can interfere with normal brain function by causing learning impairments and increased excitability. The results also support the hypothesis that excess brain-derived neurotrophic factor could be pro-convulsant in the limbic system.

Remodeling dendritic spines in the rat pilocarpine model of temporal lobe epilepsy

Dendritic degeneration is a common pathology in temporal lobe epilepsy and its animal models. However, little is known when and how the degeneration occurs. In the present study of the rat pilocarpine model, visualization of dendrites of the hippocampal dentate granule cells (DGCs) by biocytin revealed a generalized spine loss immediately after the acute seizure induced by pilocarpine. However, this generalized damage was followed by recovery and plastic changes in spine shape and density, which occurred 15-35 days after the initial acute seizure, i.e., during the period of establishing a chronic phase of this model with the induction of spontaneous seizures. The present finding suggests that initial acute seizures do not cause permanent damages in dendrites and spines of DGCs; instead, dendritic spines are dynamically maintained in the course of the establishment and maintenance of spontaneous seizures. Local dendritic spine degeneration, detected later in the chronic phase of epilepsy, is likely to have a separate cause from initial acute insults.

Nonobligate role of early or sustained expression of immediate-early gene proteins c-fos, c-jun, and Zif/268 in hippocampal mossy fiber sprouting

Axon sprouting in dentate granule cells is an important model of structural plasticity in the hippocampus. Although the process can be triggered by deafferentation, intense activation of glutamate receptors, and other convulsant stimuli, the specific molecular steps required to initiate and sustain mossy fiber (MF) reorganization are unknown. The cellular immediate early genes (IEGs) c-fos, c-jun, and zif/268 are major candidates for the initial steps of this plasticity, because they encode transcription factors that may trigger cascades of activity-dependent neuronal gene expression and are strongly induced in all experimental models of MF sprouting. The mutant mouse stargazer offers an important opportunity to test the specific role of IEGs, because it displays generalized nonconvulsive epilepsy and intense MF sprouting in the absence of regional cell injury. Here we report that stargazer mice show no detectable elevations in c-Fos, c-Jun, or Zif/268 immediate early gene proteins (IEGPs) before or during MF growth. Experimental results in stargazer, including (1) a strong IEGP response to kainate-induced convulsive seizures, (2) no IEGP response after prolongation of spike-wave synchronization, (3) no IEGP increase at the developmental onset of seizures or after prolonged seizure suppression, and (4) unaltered levels of the intracellular Ca2+-buffering proteins calbindin-D28k or parvalbumin, exclude the possibility that absence of an IEGP response in stargazer is either gene-linked or suppressed by known refractory mechanisms. These data demonstrate that increased levels of these IEGPs are not an obligatory step in MF-reactive sprouting and differentiate the early downstream molecular cascades of two major seizure types.

Status epilepticus causes long-term NMDA receptor-dependent behavioral changes and cognitive deficits

PURPOSE

The role of N-methyl-D-aspartate (NMDA)-receptor activation on behavioral and cognitive changes after status epilepticus (SE) is unknown. In this study, behavioral and cognitive changes after SE were evaluated in the short and long term and in rats in which the NMDA receptor was inactivated during SE.

METHODS

Pilocarpine (350 mg/kg) was injected to induce SE. Inhibition of the NMDA receptor during SE was achieved with MK-801 (4 mg/kg). Seizure intensity during SE was monitored by electroencephalography (EEG). After SE, behavioral studies were performed to identify abnormal behavior by using behavioral tests adapted from Moser's functional observational battery. Cognitive changes were assessed by using the Morris Water Maze (MWM).

RESULTS

Pilocarpine-treated animals scored significantly higher on two of the behavioral tests: the Touch test and the Pick-Up test. These behavioral changes occurred very soon after SE, with the earliest changes observed 2 days after SE and persisting for the life of the animal. Inhibition of the NMDA receptor with MK-801 completely inhibited these behavioral changes under conditions that did not alter the duration of SE. In addition, pilocarpine-treated animals exhibited cognitive deficits as determined by using the MWM. Six weeks after SE, the animals displayed significantly longer latencies to locate the hidden platform on this test. The impaired performance on the MWM also occurred as early as 5 days after SE. These cognitive deficits were prevented in animals treated with MK-801 during SE.

CONCLUSIONS

The results indicate that behavioral and cognitive changes occur soon after SE, are permanent, and are dependent on NMDA-receptor activation during SE. NMDA-receptor activation may play an important role in causing cognitive and behavioral morbidity after recovery from SE.

Hilar somatostatin-mRNA containing neurons are preserved after perforant path kindling in the rat

The loss of neurons in the dentate gyrus have been evaluated in rats in which kindling had been induced through stimulation of the hippocampal perforant path. The total number of granule cells, hilar neurons and the total number of neurons expressing the message for somatostatin (SS-mRNA) were estimated with stereological methods in five rats sacrificed 24 h after the last seizure and compared with estimates made in five control animals. The mean total number of granule cells was 981,000 in control rats and 943,000 in kindled rats, a non-significant difference at the 5% level of significance (P = 0.78, Student's t-test). The mean total number of hilar neurons was 52,100 compared with a mean of 58,520 in the control group, the difference being statistically significant (P = 0.04). The mean total number of SSergic hilar neurons was 16,630 compared with a mean of 15,430 in the control rats (non-significant, P = 0.12). These findings indicate that hilar neurons are lost after kindling but that these neurons are not somatostatinergic.

Supragranular mossy fiber sprouting is not necessary for spontaneous seizures in the intrahippocampal kainate model of epilepsy in the rat

In a previous study, we suggested a dissociation between spontaneous recurrent epileptic seizures (SRS) and hippocampal supragranular mossy fiber sprouting (MFS) in the pilocarpine model of epilepsy (PILO). One possible explanation, would be that SRS in the PILO model do not originate in the hippocampus and thus would not depend on MFS. In the present study, we investigated whether MFS is necessary for the SRS that develop after a small intrahippocampal dose of kainic acid (KA), a model where seizures are more likely to start in the hippocampus. Intrahippocampal injections of KA were performed in rats, with and without the concomitant administration of cycloheximide (CHX) (0.5 microg of KA and 6 microg of CHX). After injection, recording electrodes were positioned in the same stereotaxic location. Here again, CHX was able to completely block (5/8 animals) MFS, visualized by neo-Timm staining, without altering the frequency and intensity of spontaneous ictal and interictal EEG events. From these data, we can conclude that, in the intra-hippocampal KA model, MFS is not necessary for the occurrence of ictal events. We suggest that CHX can be used together with classic epileptogenic agents, as a means to study temporal lobe epilepsy (TLE) without the contributing effect of MFS--as seen in TLE patients with mass lesions in the lateral temporal lobe.

Dentate granule cells form novel basal dendrites in a rat model of temporal lobe epilepsy

Mossy fibre sprouting and re-organization in the inner molecular layer of the dentate gyrus is a characteristic of many models of temporal lobe epilepsy including that induced by perforant-path stimulation. However, neuroplastic changes on the dendrites of granule cells have been less-well studied. Basal dendrites are a transient morphological feature of rodent granule cells during development. The goal of the present study was to examine whether granule cell basal dendrites are generated in rats with epilepsy induced by perforant-path stimulation. Adult Wistar rats were stimulated for 24 h at 2 Hz and with intermittent (1/min) trains (10 s duration) of single stimuli at 20 Hz (20 V, 0.1 ms) delivered 1/min via an electrode placed in the angular bundle. The brains of these experimental rats and age- and litter-matched control animals were processed for the rapid Golgi method. All rats with perforant-path stimulation displayed basal dendrites on many Golgi-impregnated granule cells. These basal dendrites mainly originated from their somata at the hilar side and then extended into the hilus. Quantitative analysis of more than 800 granule cells in the experimental and matched control brains showed that 6-15% (mean=8.7%) of the impregnated granule cells have spiny basal dendrites on the stimulated side, as well as the contralateral side (mean=3.1%, range=2.9-3.9%) of experimental rats, whereas no basal dendrites were observed in the dentate gyrus from control animals. The formation of basal dendrites appears to be an adaptive morphological change for granule cells in addition to the previously described mossy fibre sprouting, as well as dendritic and somatic spine formation observed in the dentate gyrus of animal and human epileptic brains. The presence of these dendrites in the subgranular region of the hilus suggests that they may be postsynaptic targets of the mossy fibre collaterals.

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.

Responses of deep entorhinal cortex are epileptiform in an electrogenic rat model of chronic temporal lobe epilepsy

We investigated whether entorhinal cortex (EC) layer IV neurons are hyperexcitable in the post-selfsustaining limbic status epilepticus (post-SSLSE) animal model of temporal lobe epilepsy. We studied naive rats (n = 44), epileptic rats that had experienced SSLSE resulting in spontaneous seizures (n = 45), and electrode controls (n = 7). There were no differences between electrode control and naive groups, which were pooled into a single control group. Intracellular and extracellular recordings were made from deep layers of EC, targeting layer IV, which was activated by stimulation of the superficial layers of EC or the angular bundle. There were no differences between epileptic and control neurons in basic cellular characteristics, and all neurons were quiescent under resting conditions. In control tissue, 77% of evoked intracellular responses consisted of a short-duration [8.6 +/- 1.3 (SE) ms] excitatory postsynaptic potential and a single action potential followed by gamma-aminobutyric acid-A (GABAA) and GABAB inhibitory post synaptic potentials (IPSPs). Ten percent of controls did not contain IPSPs. In chronically epileptic tissue, evoked intracellular responses demonstrated prolonged depolarizing potentials (256 +/- 39 ms), multiple action potentials (13 +/- 4), and no IPSPs. Ten percent of epileptic responses were followed by rhythmic "clonic" depolarizations. Epileptic responses exhibited an all-or-none response to progressive increases in stimulus intensity and required less stimulation to elicit action potentials. In both epileptic and control animals, intracellular responses correlated precisely in morphology and duration with extracellular field potentials. Severing the hippocampus from the EC did not alter the responses. Duration of intracellular epileptic responses was reduced 22% by the N-methyl--aspartate (NMDA) antagonist (-)-2-amino-5-phosphonovaleric acid (APV), but they did not return to normal and IPSPs were not restored. Epileptic and control responses were abolished by the non-NMDA antagonist 6, 7-dinitroquinoxaline-2-3-dione (DNQX). A monosynaptic IPSP protocol was used to test connectivity of inhibitory interneurons to primary cells by direct activation of interneurons with a stimulating electrode placed near the recording electrode in the presence of APV and DNQX. Using this protocol, IPSPs similar to control (P > 0.05) were seen in epileptic cells. The findings demonstrate that deep layer EC cells are hyperexcitable or "epileptiform" in this model. Hyperexcitability is not due to interactions with the hippocampus. It is due partially to augmented NMDA-mediated excitation. The lack of IPSPs in epileptic neurons may suggest inhibition is impaired, but we found evidence that inhibitory interneurons are connected to their target cells and are capable of inducing IPSPs.

Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy

Human temporal lobe epilepsy is associated with complex partial seizures that can produce secondarily generalized seizures and motor convulsions. In some patients with temporal lobe epilepsy, the seizures and convulsions occur following a latent period after an initial injury and may progressively increase in frequency for much of the patient's life. Available animal models of temporal lobe epilepsy are produced by acute treatments that often have high mortality rates and/or are associated with a low proportion of animals developing spontaneous chronic motor seizures. In this study, rats were given multiple low-dose intraperitoneal (i.p.) injections of kainate in order to minimize the mortality rate usually associated with single high-dose injections. We tested the hypothesis that these kainate-treated rats consistently develop a chronic epileptic state (i.e. long-term occurrence of spontaneous, generalized seizures and motor convulsions) following a latent period after the initial treatment. Kainate (5 mg/kg per h, i.p.) was administered to rats every hour for several hours so that class III-V seizures were elicited for > or = 3 h, while control rats were treated similarly with saline. This treatment protocol had a relatively low mortality rate (15%). After acute treatment, rats were observed for the occurrence of motor seizures for 6-8 h/week. Nearly all of the kainate-treated rats (97%) had two or more spontaneous motor seizures months after treatment. With this observation protocol, the average latency for the first spontaneous motor seizure was 77+/-38 (+/-S.D.) days after treatment. Although variability was observed between rats, seizure frequency initially increased with time after treatment, and nearly all of the kainate-treated rats (91%) had spontaneous motor seizures until the time of euthanasia (i.e. 5-22 months after treatment). Therefore, multiple low-dose injections of kainate, which cause recurrent motor seizures for > or = 3 h, lead to the development of a chronic epileptic state that is characterized by (i) a latent period before the onset of chronic motor seizures, and (ii) a high but variable seizure frequency that initially increases with time after the first chronic seizure. This modification of the kainate-treatment protocol is efficient and relatively simple, and the properties of the chronic epileptic state appear similar to severe human temporal lobe epilepsy. Furthermore, the observation that seizure frequency initially increased as a function of time after kainate treatment supports the hypothesis that temporal lobe epilepsy can be a progressive syndrome.

Synaptic reorganization following kainic acid-induced seizures during development

Prolonged seizures in the adult brain causes neuronal loss in the hippocampus and aberrant growth (sprouting) of granule cell axons (mossy fibers) in the supragranular zone of the fascia dentata and stratum infrapyramidale of CA3. There is considerable evidence that these changes in neuronal growth following seizures are age related, with younger animals having fewer reactive changes following prolonged seizures than older animals. However, there is little information available regarding the age at which seizures in the developing brain result in alterations in axonal growth and synapse formation. In this study, we evaluated the effects of kainic acid (KA)-induced seizures during development on synaptic reorganization using the expression of growth-associated protein-43 (GAP-43), a marker for synaptogenesis and Timm stain which detects the presence of zinc in granule cell axons. Age specific doses of KA were used to induce seizures of similar intensity at various ages (postnatal days (P) 12, 21, 25, 35, 45, 60) in Sprague-Dawley rats. Up to the age of P25, there were no differences in either Timm or GAP-43 staining between animals with KA seizures and controls. In P25 and older KA-treated rats, Timm staining was found in the supragranular layer of the dentate gyrus. This staining increased with age at the time of KA injection. Seizures in adult (P60), but not younger rats also resulted in increased staining in the suprapyramidal layer of the CA3 subfields. Changes in GAP-43 were delayed compared to the Timm staining with no differences between KA-treated animals and controls until P35 when a band of GAP-43 immunostaining appeared in the supragranular inner molecular layer, progressively increasing in intensity and thickness with time. This study demonstrates that seizure-induced reactive synaptogenesis is age-related. Since both Timm and GAP-43 reflect different aspects of reactive synaptogenesis, used in combination these methods provide useful information about the structural changes following seizures during development.