Long‐lasting modulation of synaptic plasticity in rat hippocampus after early‐life complex febrile seizures

A small fraction of children with febrile seizures appears to develop cognitive impairments. Recent studies in a rat model of hyperthermia‐induced febrile seizures indicate that prolonged febrile seizures early in life have long‐lasting effects on the hippocampus and induce cognitive deficits. However, data on network plasticity and the nature of cognitive deficits are conflicting. We examined three specific measures of hippocampal plasticity in adult rats with a prior history of experimental febrile seizures: (i) activity‐dependent synaptic plasticity (long‐term potentiation and depression) by electrophysiological recordings of Schaffer collateral/commissural‐evoked field excitatory synaptic potentials in CA1 of acute hippocampal slices; (ii) Morris water maze spatial learning and memory; and (iii) hippocampal mossy fiber plasticity by Timm histochemistry and quantification of terminal sprouting in CA3 and the dentate gyrus. We found enhanced hippocampal CA1 long‐term potentiation and reduced long‐term depression but normal spatial learning and memory in adult rats that were subjected to experimental febrile seizures on postnatal day 10. Furthermore, rats with experimental febrile seizures showed modest but significant sprouting of mossy fiber collaterals into the inner molecular layer of the dentate gyrus in adulthood. We conclude that enhanced CA1 long‐term potentiation and mild mossy fiber sprouting occur after experimental febrile seizures, without affecting spatial learning and memory in the Morris water maze. These long‐term functional and structural alterations in hippocampal plasticity are likely to play a role in the enhanced seizure susceptibility in this model of prolonged human febrile seizures but do not correlate with overt cognitive deficits.     

Long-term effects of status epilepticus in the immature brain are specific for age and model

PURPOSE: Status epilepticus (SE) is more common in children than adults and has a high mortality and morbidity rate. SE in adult rats results in long-term disturbances in learning and memory, as well as an enhanced seizure susceptibility to further seizures. In contrast, a number of studies suggest that the immature brain is less vulnerable to the morphologic and physiologic alterations after SE. The goal of this study was to determine whether the long-term consequences of SE during development on hippocampal plasticity and cognitive function are age and model specific. METHODS: We used lithium-pilocarpine (Li-PC) to induce SE at different age points during development (P12, P16, P20) and evaluated the effects of this abnormal neural activity on spatial memory performance and seizure susceptibility in the animals beginning at P55, corresponding to young adulthood. RESULTS: We demonstrated that SE at P12 did not result in any structural or functional changes detectable in adulthood, whereas SE at both P16 and P20 induced cell loss and mossy fiber sprouting within the hippocampus and cognitive impairment when the animals were tested as adults. CONCLUSIONS: Whereas the seizure threshold to generalized seizures was not altered, animals with SE at P20 showed an increased susceptibility to kindling in adulthood.     

Long-term effects of neonatal seizures: a behavioral, electrophysiological, and histological study

Previous studies have demonstrated that recurrent seizures during the neonatal period lead to permanent changes in seizure threshold and learning and memory. The pathophysiological mechanisms for these changes are not clear. To determine if neonatal seizures cause changes in hippocampal excitability or inhibition, we subjected rats to 50 flurothyl-induced seizures during the first 10 days of life (five seizures per day). When the rats were adults, we examined seizure threshold using flurothyl inhalation, and learning and memory in the water maze. In separate groups of animals, we evaluated in vivo paired-pulse facilitation and inhibition in either CA1 with stimulation of the Schaffer collaterals or dentate gyrus with stimulation of the perforant path. Following these studies, the animals were sacrificed and the brains evaluated for mossy fiber sprouting with the Timm stain. Compared to control animals, rats with 50 flurothyl seizures had a reduced seizure threshold, impaired learning and memory in the water maze, and sprouting of mossy fibers in the CA3 pyramidal cell layer and molecular layer of the dentate gyrus. No significant differences in impaired paired-pulse inhibition was noted between the flurothyl-treated and control rats. This study demonstrates that recurrent neonatal seizures result in changes of neuronal connectivity and alterations in seizure susceptibility, learning and memory. However, the degree of impairment following 50 seizures was modest, demonstrating that the immature brain is remarkably resilient to seizure-induced damage.     

Effect of theophylline and trimethobenzamide when given during kainate-induced status epilepticus: an improved histopathologic rat model of human hippocampal sclerosis

PURPOSE: The most common pathology in temporal lobe epilepsy (TLE) is hippocampal sclerosis. It is controversial whether status epilepticus (SE) or prolonged seizures plus secondary cerebral injuries are pathogenic mechanisms of hippocampal sclerosis. This study addressed this question in rat models of TLE. METHODS: Hippocampal neuron densities and supragranular mossy fiber sprouting were determined in adult rats subjected to systemic kainate-induced SE (KA-only) and KA-induced SE followed 75 minutes later by theophylline (KA/Theo) or trimethobenzamide (KA/Tri). These drugs probably decrease seizure-induced cerebral hyperemia or hypertension. RESULTS: Compared with controls and KA-only rats, KA/Tri and KA/Theo rats showed decreased CA3b and CA1 neuron densities (i.e., greater Sommer's sector injury). In addition, KA/Tri rats showed that increased trimethobenzamide dosages were associated with decreased hilar, CA3c, CA3b, CA1, and subiculum neuron densities. There were no significant differences in supragranular mossy fiber sprouting between KA-only, KA/Tri, and KA/Theo rats. CONCLUSIONS: Pharmacologic manipulations during KA-induced SE are associated with differences in hippocampal pathology, especially in Sommer's sector, and the final pattern of damage and axon sprouting shows histopathologic similarities to that in patients with hippocampal sclerosis. Our findings support the hypothesis that secondary physiologic insults during SE that are likely to decrease seizure-induced cerebral hyperemia and hypertension may generate greater hippocampal neuronal injury compared with SE alone, and this may be a pathogenic mechanism of human hippocampal sclerosis in patients with TLE.     

One hour of pilocarpine-induced status epilepticus is sufficient to develop chronic epilepsy in mice, and is associated with mossy fiber sprouting but not neuronal death

Determining the minimal duration of status epilepticus (SE) that leads to the development of subsequent spontaneous seizures (i.e., epilepsy) is important, because it provides a critical time-window for seizure intervention and epilepsy prevention. In the present study, male ICR (Imprinting Control Region) mice were injected with pilocarpine to induce acute seizures. SE was terminated by diazepam at 10 min, 30 min, 1 h, 2 h and 4 h after seizure onset. Spontaneous seizures occurred in the 1, 2 and 4 h SE groups, and the seizure frequency increased with the prolongation of SE. Similarly, the Morris water maze revealed that the escape latency was significantly increased and the number of target quadrant crossings was markedly decreased in the 1, 2 and 4 h SE groups. Robust mossy fiber sprouting was observed in these groups, but not in the 10 or 30 min group. In contrast, Fluoro-Jade B staining revealed significant cell death only in the 4 h SE group. The incidence and frequency of spontaneous seizures were correlated with Timm score (P = 0.004) and escape latency (P = 0.004). These data suggest that SE longer than one hour results in spontaneous motor seizures and memory deficits, and spontaneous seizures are likely associated with robust mossy fiber sprouting but not neuronal death.     

Timing of ketogenic diet initiation in an experimental epilepsy model

Following kainic acid (KA)-induced status epilepticus (SE), the ketogenic diet (KD) retards the development of epileptogenesis, with fewer spontaneous recurrent seizures (SRS) and less mossy fiber sprouting than rats on a normal diet. In this study, we investigated whether there is a critical period for initiation of the KD, in terms of the diet's effectiveness in reducing SRS. In addition, we investigated whether early treatment with the KD prevents the deficits in spatial learning and memory that ordinarily follow KA-induced SE. Young rats (P30) underwent KA-induced SE, followed by assignment to one of three treatment groups: control diet ('KA'), KD begun 2 days after SE ('KD2'), and KD begun fourteen days after SE ('KD14'). For 12 weeks following SE, rats were monitored by closed circuit video recording (12 h/wk) to detect SRS. KD2 rats had significantly fewer SRS than rats in the control or KD14 groups. On water maze testing to assess spatial learning and memory, KD2 rats had significantly poorer acquisition of place learning than control (KA alone) or KD14 rats. KD2 rats also failed to gain weight well. There was no difference between groups on routine histologic examination of the hippocampus. In summary, P30 rats placed on the KD 2 days after SE were relatively protected from recurrent seizures, but showed behavioral and physical impairment. Rats placed on the KD 14 days after KA-induced SE did not differ from controls with regard to spontaneous seizure rate.     

Locus Coeruleus and Neuronal Plasticity in a Model of Focal Limbic Epilepsy

Summary:  Purpose: A lesion of the noradrenergic nucleus Locus Coeruleus (LC) converts sporadic seizures evoked by microinfusion of bicuculline into the anterior piriform cortex (APC) of rats into limbic status epilepticus (SE). The purpose of this study was to evaluate the chronic effects of this new model of SE on the onset of secondary epileptogenesis. We further related the loss of noradrenaline (NE) with hippocampal mossy fiber sprouting.
Methods: Male Sprague Dawley rats were treated with systemic saline or DSP-4 (a neurotoxin selective for noradrenergic terminals originating from the LC), microinfused with bicuculline into the APC three days later, and sacrificed after 45 days. Naïve and DSP-4 pretreated sham-operated rats served as respective controls. The following evaluations were performed: (a) monitoring of acute seizures and delayed occurrence of spontaneous recurrent seizures (SRS); (b) NE levels in the hippocampus, frontal and olfactory cortex; (c) occurrence of mossy fiber sprouting into the inner molecular layer of the dentate gyrus of the dorsal hippocampus.
Results: In 30% of rats lacking noradrenergic terminals, SE evoked from the APC was followed by SRS. Conversely, seizures evoked in intact rats did not result in chronic epileptogenesis. Seizures/SE did not modify NE levels as compared with baseline levels both in naïve and DSP-4-pretreated rats. Rats undergoing SE following DSP-4 + bicuculline developed SRS which were accompanied by hippocampal mossy fiber sprouting.
Conclusions: Noradrenergic loss converts focally induced sporadic seizures into an epileptogenic SE, which is accompanied by mossy fiber sprouting within the dentate gyrus.     

Pentylenetetrazol-induced recurrent seizures in rat pups: time course on spatial learning and long-term effects

PURPOSE: Recurrent seizures in infants are associated with a high incidence of neurocognitive deficits. Animal models have suggested that the immature brain is less vulnerable to seizure-induced injury than is that in adult animals. We studied the effects of recurrent neonatal seizures on cognitive tasks performed when the animals were in adolescence and adulthood. METHODS: Seizures were induced by intraperitoneal injection of pentylenetetrazol (PTZ) for 5 consecutive days, starting from postnatal day 10 (P10). At P35 and P60, rats were tested for spatial memory by using the Morris water maze task. In adulthood, motor performance was examined by the Rotarod test, and activity level was assessed by the open field test. Seizure threshold was examined by inhalant flurothyl. To assess presence or absence of spontaneous seizures, rats were video recorded for 4 h/day for 10 consecutive days for the detection of spontaneous seizures. Finally, brains were examined for histologic evidence of injury with cresyl violet stain and Timm staining in the supragranular zone and CA3 pyramidal cell layers of the hippocampus. RESULTS: PTZ-treated rats showed significant spatial deficits in the Morris water maze at both P35 and P60. There were no differences in seizure threshold, motor balance, or activity level during the open field test. Spontaneous seizures were not recorded in any rat. The cresyl violet stain showed no cell loss in either the control or experimental rats. PTZ-treated rats exhibited more Timm staining in the CA3 subfield. However, the control and experimental rats showed similar Timm staining within the supragranular zone. CONCLUSIONS: Our findings indicate that recurrent PTZ-induced seizures result in long-term cognitive deficits and morphologic changes in the developing brain. Furthermore, these cognitive deficits could be detected during pubescence.     

Strain differences affect the induction of status epilepticus and seizure-induced morphological changes

Genetic deficits have been discovered in human epilepsy, which lead to alteration of the balance between excitation and inhibition, and ultimately result in seizures. Rodents show similar genetic determinants of seizure induction. To test whether seizure‐prone phenotypes exhibit increased seizure‐related morphological changes, we compared two standard rat strains (Long–Evans hooded and Wistar) and two specially bred strains following status epilepticus. The special strains, namely the kindling‐prone (FAST) and kindling‐resistant (SLOW) strains, were selectively bred based on their amygdala kindling rate. Although the Wistar and Long–Evans hooded strains experienced similar amounts of seizure activity, Wistar rats showed greater mossy fiber sprouting and hilar neuronal loss than Long–Evans hooded rats. The mossy fiber system was affected differently in FAST and SLOW rats. FAST animals showed more mossy fiber granules in the naïve state, but were more resistant to seizure‐induced mossy fiber sprouting than SLOW rats. These properties of the FAST strain are consistent with those observed in juvenile animals, further supporting the hypothesis that the FAST strain shares circuit properties similar to those seen in immature animals. Furthermore, the extent of mossy fiber sprouting was not well correlated with sensitivity to status epilepticus, but was positively correlated with the frequency of spontaneous recurrent seizures in the FAST rats only, suggesting a possible role for axonal sprouting in the development of spontaneous seizures in these animals. We conclude that genetic factors clearly affect seizure development and related morphological changes in both standard laboratory strains and the selectively bred seizure‐prone and seizure‐resistant strains.     

Diabetic Hyperglycemia Aggravates Seizures and Status Epilepticus-induced Hippocampal Damage

Epileptic seizures in diabetic hyperglycemia (DH) are not uncommon. This study aimed to determine the acute behavioral, pathological, and electrophysiological effects of status epilepticus (SE) on diabetic animals. Adult male Sprague-Dawley rats were first divided into groups with and without streptozotocin (STZ)-induced diabetes, and then into treatment groups given a normal saline (NS) (STZ-only and NS-only) or a lithium-pilocarpine injection to induce status epilepticus (STZ + SE and NS + SE). Seizure susceptibility, severity, and mortality were evaluated. Serial Morris water maze test and hippocampal histopathology results were examined before and 24 h after SE. Tetanic stimulation-induced long-term potentiation (LTP) in a hippocampal slice was recorded in a multi-electrode dish system. We also used a simulation model to evaluate intracellular adenosine triphosphate (ATP) and neuroexcitability. The STZ + SE group had a significantly higher percentage of severe seizures and SE-related death and worse learning and memory performances than the other three groups 24 h after SE. The STZ + SE group, and then the NS + SE group, showed the most severe neuronal loss and mossy fiber sprouting in the hippocampal CA3 area. In addition, LTP was markedly attenuated in the STZ + SE group, and then the NS + SE group. In the simulation, increased intracellular ATP concentration promoted action potential firing. This finding that rats with DH had more brain damage after SE than rats without diabetes suggests the importance of intensively treating hyperglycemia and seizures in diabetic patients with epilepsy.     

Progression of spontaneous seizures after status epilepticus is associated with mossy fibre sprouting and extensive bilateral loss of hilar parvalbumin and somatostatin-immunoreactive neurons

The development of spontaneous limbic seizures was investigated in a rat model in which electrical tetanic stimulation of the angular bundle was applied for up to 90 min. This stimulation produced behavioural and electrographic seizures that led to a status epilepticus (SE) in most rats (71%). Long-term EEG monitoring showed that the majority of the rats (67%) that underwent SE, displayed a progressive increase of seizure activity once the first seizure was recorded after a latent period of about 1 week. The other SE rats (33%) did not show this progression of seizure activity. We investigated whether these different patterns of evolution of spontaneous seizures could be related to differences in cellular or structural changes in the hippocampus. This was the case regarding the following changes. (i) Cell loss in the hilar region: in progressive SE rats this was extensive and bilateral whereas in nonprogressive SE rats it was mainly unilateral. (ii) Parvalbumin and somatostatin-immunoreactive neurons: in the hilar region these were almost completely eliminated in progressive SE rats but were still largely present unilaterally in nonprogressive SE rats. (iii) Mossy fibre sprouting: in progressive SE rats, extensive mossy fibre sprouting was prominent in the inner molecular layer. In nonprogressive SE rats, mossy fibre sprouting was also present but less prominent than in progressive SE rats. Although mossy fibre sprouting has been proposed to be a prerequisite for chronic seizure activity in experimental temporal lobe epilepsy, the extent of hilar cell death also appears to be an important factor that differentiates between whether or not seizure progression will occur.     

Effects of early-life LiCl-pilocarpine-induced status epilepticus on memory and anxiety in adult rats are associated with mossy fiber sprouting and elevated CSF S100B protein

Purpose: This study investigated putative correlations among behavioral changes and: (1) neuronal loss, (2) hippocampal mossy fiber sprouting, and (3) reactive astrogliosis in adult rats submitted to early‐life LiCl‐pilocarpine‐induced status epilepticus (SE). Methods: Rats (P15) received LiCl (3 mEq/kg, i.p.) 12–18 h prior pilocarpine (60 mg/kg; s.c.). At adulthood, animals were submitted to behavioral tasks and after the completion of tasks biochemical and histological analysis were performed. Results: In SE group, it was observed an increased number of degenerating neurons in the CA1 subfield and in the hilus of animals 24 h after SE. At adulthood, SE group presented an aversive memory deficit in an inhibitory avoidance task and the animals that presented lower latency to the step down showed a higher score for mossy fiber sprouting. In the light‐dark exploration task, SE rats returned less and spent less time in the light compartment and present an increased number of risk assessment behavior (RA). There was a negative correlation between the time spent in the light compartment and the score for mossy fiber sprouting and a positive correlation between score for mossy fiber sprouting and number of RA. LiCl‐pilocarpine‐treated animals showed higher levels of S100B immunocontent in the CSF as well as a positive correlation between the score for sprouting and the GFAP immunocontent in the CA1 subfield, suggesting an astrocytic response to neuronal injury. Conclusions: We showed that LiCl‐pilocarpine‐induced SE during development produced long‐lasting behavioral abnormalities, which might be associated with mossy fiber sprouting and elevated CSF S100B levels at adulthood.     

A single episode of neonatal seizures permanently alters glutamatergic synapses

OBJECTIVE: The contribution of seizures to cognitive changes remains controversial. We tested the hypothesis that a single episode of neonatal seizures (sNS) on rat postnatal day (P) 7 permanently impairs hippocampal-dependent function in mature (P60) rats because of long-lasting changes at the synaptic level. METHODS: sNS was induced with subcutaneously injected kainate on P7. Learning, memory, mossy fiber sprouting, spine density, hippocampal synaptic plasticity, and glutamate receptor expression and subcellular distribution were measured at P60. RESULTS: sNS selectively impaired working memory in a hippocampal-dependent radial arm water-maze task without inducing mossy fiber sprouting or altering spine density. sNS impaired CA1 hippocampal long-term potentiation and enhanced long-term depression. Subcellular fractionation and cross-linking, used to determine whether glutamate receptor trafficking underlies the alterations of memory and synaptic plasticity, demonstrated that sNS induced a selective reduction in the membrane pool of glutamate receptor 1 subunits. sNS induced a decrease in the total amount of N-methyl-D-aspartate receptor 2A and an increase in the primary subsynaptic scaffold, PSD-95. INTERPRETATION: These molecular consequences are consistent with the alterations in plasticity and memory caused by sNS at the synaptic level. Our data demonstrate the cognitive impact of sNS and associate memory deficits with specific alterations in glutamatergic synaptic function.     

Timing of cognitive deficits following neonatal seizures: relationship to histological changes in the hippocampus.

Neonatal seizures are frequently associated with cognitive impairment and reduced seizure threshold. Previous studies in our laboratory have demonstrated that rats with recurrent neonatal seizures have impaired learning, lower seizure thresholds, and sprouting of mossy fibers in CA3 and the supragranular region of the dentate gyrus in the hippocampus when studied as adults. The goal of this study was to determine the age of onset of cognitive dysfunction and alterations in seizure susceptibility in rats subjected to recurrent neonatal seizures and the relation of this cognitive impairment to mossy fiber sprouting and expression of glutamate receptors. Starting at postnatal day (P) 0, rats were exposed to 45 flurothyl-induced seizures over a 9-day period of time. Visual-spatial learning in the water maze and seizure susceptibility were assessed in subsets of the rats at P20 or P35. Brains were evaluated for cell loss, mossy fiber distribution, and AMPA (GluR1) and NMDA (NMDAR1) subreceptor expression at these same time points. Rats with neonatal seizures showed significant impairment in the performance of the water maze and increased seizure susceptibility at both P20 and P35. Sprouting of mossy fibers into the CA3 and supragranular region of the dentate gyrus was seen at both P20 and P35. GluR1 expression was increased in CA3 at P20 and NMDAR1 was increased in expression in CA3 and the supragranular region of the dentate gyrus at P35. Our findings indicate that altered seizure susceptibility and cognitive impairment occurs prior to weaning following a series of neonatal seizures. Furthermore, these alterations in cognition and seizure susceptibility are paralleled by sprouting of mossy fibers and increased expression of glutamate receptors. To be effective, our results suggest that strategies to alter the adverse outcome following neonatal seizures will have to be initiated during, or shortly following, the seizures.     

Effect of topiramate following recurrent and prolonged seizures during early development

Topiramate, an antiepileptic drug with a number of mechanisms of action including inhibition of glutamate activity at the AMPA and KA receptors, was assessed as a neuroprotective agent following seizures. We administered topiramate, 80 mg/kg, or saline for 4 weeks following a series of 25 neonatal seizures or status epilepticus (SE) induced by lithium-pilocarpine in postnatal day 20 rats. Age-matched control rats without a history of seizures were administered topiramate or saline. Following completion of the topiramate injections, animals were tested in the water maze for spatial learning and the brains examined for cell loss and sprouting of mossy fibers. While there was a trend for improved visual-spatial performance in the water maze following topiramate therapy in rats with neonatal seizures, no differences were found in the histological examination of the hippocampus. Neonatal rats exposed to 4 weeks of topiramate did not differ from non-treated controls in water maze performance or histological examination. In weanling rats subjected to SE, topiramate provided a moderate degree of neuroprotection, with topiramate-treated rats performing better in the water maze than rats receiving saline. However, no differences in cell loss or mossy fiber sprouting were found in the histological examination of the brains. These findings demonstrate that chronic treatment with topiramate following SE improves cognitive function. In addition, long-term administration of high-dose topiramate in the normal developing rat brain does not appear to impair cognitive performance.     

幼鼠早期癫痫持续状态对海马结构损伤的远期影响

目的　探讨幼鼠早期癫痫持续状态 (status epilepticus,SE)对成鼠后海马结构损伤的远期影响。方法　健康生后 1 5～ 2 0 d Wistar幼鼠 48只 ,随机分为生理盐水对照组和氯化锂 -匹罗卡品腹腔注射诱导的 SE组 ,并持续追踪至成鼠阶段 ,应用常规病理及电镜观察海马结构的形态学改变 ,同时应用 Timm组织化学染色方法进行苔藓纤维发芽研究。结果　约 2 /3 SE幼鼠发育至成鼠阶段后 ,海马结构的神经元仍可发现变性和坏死性改变 ,以 CA1区、CA3区和齿状回为重。 Timm染色见 CA3区有苔藓纤维发芽现象。约 1 /3 SE幼鼠发育至成鼠后未见海马结构损伤性改变。结论　幼鼠早期 SE造成的海马结构损伤性改变可持续至成鼠阶段 ,但存在个体差异 ,可能与个体耐受性有关。     

Sprouting of mossy fibers and the vacating of postsynaptic targets in the inner molecular layer of the dentate gyrus

Aberrant mossy fiber sprouting, which presumably results from hilar mossy cell death after status epilepticus (SE), is a frequently studied feature of temporal lobe epilepsy. Although mossy fiber sprouting can be suppressed by the protein synthesis inhibitor cycloheximide, spontaneous seizures remain unaltered. We have investigated the mechanisms underlying the ability of cycloheximide to block SE-induced mossy fiber sprouting in the inner molecular layer of dentate gyrus (IML). Pilocarpine-induced SE in the presence of cycloheximide resulted in a reduced number of injured hilar cells compared to rats not pretreated with cycloheximide. Presumed mossy cells, identified by calcitonin gene related peptide (CGRP) immunohistochemistry, were not significantly reduced in either group 60 days after SE. Whereas controls had a strong band of CGRP-positive fibers (putative mossy cell axons) and no neo-Timm stained fibers in the IML, pilocarpine-treated rats had no CGRP fibers and strong neo-Timm staining. Cycloheximide-pilocarpine-treated animals, in contrast, had CGRP and neo-Timm staining similar to controls. Cycloheximide might protect hilar CGRP-positive cells during SE and, by allowing those cells to retain their normal axonal projection, prevent mossy fiber sprouting. The recently suggested "irritable" mossy cell hypothesis relies on the survival of mossy cells for network hyperexcitability. We hypothesized that CGRP may be a marker for a subpopulation of relatively resistant mossy cells in rats, which, if they survive injury, may become irritable and contribute to hyperexcitability. We suggest that cycloheximide prevents SE-induced mossy fiber sprouting by preventing the loss of hilar CGRP-positive cells (putative mossy cells).     

Circuit Mechanisms of Seizures in the Pilocarpine Model of Chronic Epilepsy: Cell Loss and Mossy Fiber Sprouting

We used the pilocarpine model of chronic spontaneous recurrent seizures to evaluate the time course of supragranular dentate sprouting and to assess the relation between several changes that occur in epilep tic tissue with different behavioral manifestations of this experimental model of temporal lobe epilepsy. Pilo carpine-induced status epilepticus (SE) invariably led to cell loss in the hilus of the dentate gyrus (DG) and to spontaneous recurrent seizures. Cell loss was often also noted in the DG and in hippocampal subfields CA1 and CA3. The seizures began to appear at a mean of 15 days after SE induction (silent period), recurred at variable frequencies for each animal, and lasted for as long as the animals were allowed to survive (325 days). The granule cell layer of the DG was dispersed in epileptic animals, and neo-Timm stains showed supra-and intragranular mossy fiber sprouting. Supragranular mossy fiber sprout ing and dentate granule cell dispersion began to appear early after SE (as early as 4 and 9 days, respectively) and reached a plateau by 100 days. Animals with a greater degree of cell loss in hippocampal field CAS showed later onset of chronic epilepsy (r= 0.83, p < 0.0005), suggest ing that CA3 represents one of the routes for seizure spread. These results demonstrate that the pilocarpine model of chronic seizures replicates several of the fea tures of human temporal lobe epilepsy (hippocampal cell loss, suprar and intragranular mossy fiber sprouting, den tate granule cell dispersion, spontaneous recurrent sei zures) and that it may be a useful model for studying this human condition. The results also suggest that even though a certain amount of cell loss in specific areas may be essential for chronic seizures to occur, excessive cell loss may hinder epileptogenesis.     

Effect of levetiracetam on visual-spatial memory following status epilepticus

Status epilepticus (SE) is often followed by severe cognitive impairment, including memory impairment. Previous studies have shown that SE is associated with impairment of single cells in the hippocampus that fire action potentials when the animal is in a specific location in space, the so-called place cells, and that place cell function correlates well with performance in tasks of visual-spatial memory. Place cell patterns therefore appear to be an excellent measure of spatial memory and may serve as a tool to assess seizure-induced impairment in memory. In this study we determined the relationship between visual-spatial memory and place cell function following SE. In addition, we determined if levetiracetam (LEV), an antiepileptic drug with a novel mechanism of action, can improve cognitive function and place cell firing patterns when administered following SE. SE was induced in adult male rats which were then randomized to post-SE treatment with LEV or normal saline (NS) treatment for 14 days. Non-SE control rats also were randomized to LEV or NS. Following discontinuation of LEV rats were tested for visual-spatial memory in the Morris water-maze and then underwent unit recording in the CA1 region of the hippocampus. Brains were then evaluated for cell loss and mossy fiber sprouting. SE was associated with severely impaired performance in the water-maze with SE rats demonstrating no learning over four days of testing. Paralleling this memory deficit was a marked disturbance in firing patterns of pyramidal neurons in CA1. Non-SE rats learned quickly over four days of water-maze testing and had normal pyramidal cell firing patterns. LEV had no major effects on water-maze performance or place cell function. Histopathological examination of the brains showed severe cell loss in CA1 in all of the SE rats with lesser degrees of injury in CA3 and the hilus. LEV treatment resulted in less histological damage in the hippocampus but had no effect on visual-spatial function or place cell physiology in either control or SE rats.     

Epilepsy and synaptic reorganization in a perinatal rat model of hypoxia-ischemia

PURPOSE: One of the potential consequences of perinatal hypoxia-ischemia (H-I) is the development of epilepsy, and synaptic reorganization in the hippocampus has been associated with epilepsy after an injury. We tested the hypothesis that perinatal H-I will induce spontaneous motor seizures, hippocampal lesions, and synaptic reorganization in the dentate gyrus. METHODS: The right common carotid artery of 7-day-old rats was permanently ligated, and the rats were placed for 120 min into a chamber filled with 8% oxygen (37 degrees C). Animals were directly observed for chronic motor seizures for 7 to 24 months after the H-I insult. RESULTS: Nearly half of the rats (i.e., eight of 20) were seen to have spontaneous motor seizures after the H-I injury. The ipsilateral hippocampi from both the rats with seizures and the rats not seen to have seizures had hippocampal lesions and increased amounts of Timm stain in the inner molecular layer (IML) compared with controls. The contralateral hippocampi from the rats with seizures, but not the hippocampi from the rats not seen to have seizures, had significantly increased amounts of Timm stain in the IML. CONCLUSIONS: These results suggest that perinatal H-I can induce epilepsy, ipsilateral hippocampal lesions, and mossy fiber sprouting in the lesioned and contralateral hippocampus.