Cyclicity of spontaneous recurrent seizures in pilocarpine model of temporal lobe epilepsy in rat

Pilocarpine administration to rats results in status epilepticus (SE) and after a latency period to the occurrence of spontaneous seizures. The model is commonly used to investigate mechanisms of epileptogenesis as well as the antiepileptic effects of novel compounds. Surprisingly, there have been no video-EEG studies determining the duration of latency period from SE to the appearance of the first spontaneous seizures or the type and frequency of spontaneous seizures at early phase of pilocarpine-induced epilepsy even though such information is critical for design of such studies. To address these questions, we induced SE with pilocarpine in 29 adult male Wistar rats with cortical electrodes. Rats were continuously video-EEG monitored during SE and up to 23 days thereafter. The first spontaneous seizures occurred 7.2+/-3.6 days after SE. During the follow-up, the mean daily seizure frequency was 2.6+/-1.9, the mean seizure duration 47+/-7 s, and the mean behavioral seizure score 3.2+/-0.9. Typically first seizures were partial (score 1-2). Interestingly, spontaneous seizures occurred in clusters with cyclicity, peaking every 5 to 8 days. These data show that in the pilocarpine model of temporal lobe epilepsy the latency period is short. Because many of the early seizures are partial and the seizures occur in clusters, the true phenotype of epilepsy triggered by pilocarpine-induced SE may be difficult to characterize without continuous long-term video-EEG monitoring. Finally, our data suggest that the model can be used for studies aiming at identifying the mechanisms of seizure clustering.     

Effect of hormonal replacement therapy in the hippocampus of ovariectomized epileptic female rats using the pilocarpine experimental model

Amado and Cavalheiro [Amado, D., Cavalheiro, E.A., 1998. Hormonal and gestational parameters in female rats submitted to the pilocarpine model of epilepsy. Epilepsy Res. 32, 266-274], studying the establishment of the pilocarpine epilepsy model in female rats observed that the estrous cycle was dramatically altered during the three periods of this experimental model. This work was delineated to study the function of sexual hormones in the development of the epilepsy model induced by pilocarpine in ovariectomized rats. Experimental groups were: (a) control animals during estrus phase of the estrous cycle (E) and ovariectomized female rats (OVX) treated with saline instead of pilocarpine in the same volume, (b) experimental animals, that developed status epilepticus (SE) and were studied during the chronic phase of this model: intact chronic rats (CHRON) and ovariectomized chronic rats (OVX+CHRON) and (c) ovariectomized chronic rats, that were submitted to hormonal replacement therapy treated with: medroxyprogesterone (OVX+CHRON+MPA); 17beta-estradiol (OVX+CHRON+E2), or both (OVX+CHRON+E2+MPA). All ovariectomized animals showed genital atrophy 4 days after the surgical procedure. Moreover, all animals that developed SE and survived showed spontaneous recurrent seizures during the chronic phase. Concerning to seizure frequency, animals receiving medroxyprogesterone associated with 17beta-estradiol showed decreased seizures' number. However, animals that received only medroxyprogesterone therapy also showed reduction in the number of seizures. In addition, hormonal treatment was also able to stabilize the mossy fibers sprouting process, showing the importance of these hormones in the development of the epilepsy in female rats.     

Strain differences in seizure-induced cell death following pilocarpine-induced status epilepticus

Mouse strains differ from one another in their susceptibility to seizure-induced excitotoxic cell death. Previously, we have demonstrated that mature inbred strains of mice show remarkable genetic differences in susceptibility to the neuropathological consequences of seizures in the kainate model of status epilepticus. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. However, it remains unclear whether strain differences in susceptibility to seizure-induced cell death observed following kainate administration are observed following systemic administration of other chemoconvulsants. In rodents, the cholinomimetic convulsant pilocarpine is widely used to induce status epilepticus (SE), followed by hippocampal damage and spontaneous recurrent seizures, resembling temporal lobe epilepsy. This model has initially been described in rats, but is increasingly used in mice. We characterized neuronal pathologies after pilocarpine-induced status epilepticus (SE) in eight inbred strains of mice focusing on the hippocampus. A ramping-up dose protocol for pilocarpine was used and behavior was monitored for 4-5 h. While we did not observe any significant differences in seizure latency or duration to pilocarpine among the inbred strains, we did observe a significant difference in susceptibility to the neuropathological consequences of pilocarpine-induced SE. Of the eight genetically diverse mouse strains screened for pilocarpine-induced status, BALB/cJ and BALB/cByJ were the only two strains that were resistant to the neuropathological consequences of seizure-induced cell death. Additional studies of these murine strains may be useful for investigating genetic influences on pilocarpine-induced status epilepticus.     

Neuroprotectants FK-506 and cyclosporin A ameliorate the course of pilocarpine-induced seizures

The present study was designed to examine whether neuroprotective agents, FK506 or cyclosporin A (CsA), applied to rats undergoing pilocarpine-induced seizures can minimize further development of the status epilepticus. In order to solve this problem, pilocarpine was injected in 60-day-old Wistar rats to evoke status epilepticus. When epileptic seizures reached a defined, moderate level of intensity, the rats received a single FK506 or CsA injections. During a 6-h period following pilocarpine injection, the animals were observed continuously and motor symptoms were recorded and rated. In epileptic rats injected with FK-506 or CsA, signs of significant amelioration of the course of epilepsy accompanied by longer survival periods were observed. Moreover, some differences between effects of the two agents were seen. The obtained results appear to show that, in addition to neuroprotective action, FK506 and CsA can exert also antiepileptic influences.     

Persistence and atrophy of septal/diagonal band neurons expressing the p75 neurotrophin receptor in pilocarpine-induced chronic epilepsy in the rat

Systemic administration of pilocarpine, which results in status epilepticus followed by recurrent seizures in rats, is a widely used experimental model of chronic limbic epilepsy. Marked structural alterations have been documented in pilocarpine-induced epilepsy, and these include cell loss in the hippocampus and other brain areas, and sprouting of mossy and cholinergic fibers in the hippocampus. Evidence is accumulating that neurotrophins and neurotrophin receptors are involved in the cascade of these events. Two and 4 months after pilocarpine-induced epilepsy, neurons containing the 75-kDa low affinity neurotrophin receptor (p75^NTR) were investigated with immunohistochemistry in the medial septal and diagonal band nuclei. No significant differences in the distribution and number of immunoreactive neurons were found in the epileptic rats compared to control saline-treated animals. However, in the epileptic animals, a significant decrease in the perikaryal size of p75^NTR-immunoreactive neurons of the septal/diagonal band region was found by 60 days, and such atrophic changes were more marked in the diagonal band nuclei by 120 days. These findings indicate that the p75^NTR-containing cell bodies, which include the neurons projecting to the hippocampal formation and are cholinergic in the normal brain, survive after months of spontaneous recurrent seizures, during which, therefore, a supply of p75^NTR to target regions is maintained in the chronic epileptic brain. However, the present data point out that these p75^NTR-containing neurons undergo a significant shrinkage in pilocarpine-induced chronic epilepsy, thus indicating that they are involved in the brain pathology of temporal lobe epilepsy.     

Serotonin depletion effects on the pilocarpine model of epilepsy

The monoamine content in cerebral structures has been related to neuronal excitability and several approaches have been used to study this phenomenon during seizure vulnerability. In the present work, we have described the effects of serotonin (5-HT) depletion after the administration of 5,7-dihydroxytryptamine (5,7-DHT) into the median raphe nucleus in rats submitted to the pilocarpine model of epilepsy. Susceptibility to pilocarpine-induced status epilepticus as well as the spontaneous seizure frequency during the chronic period of the model was determined. Since the hippocampus is one of the main structures in the development of this epilepsy model, the 5-HT levels in this region were also determined after drug administration. Sixty-three percent of 5,7-DHT pre-treated rats (15/24) and only 33.4% of those receiving the control solution (9/24) progressed to motor limbic seizures evolving to status epilepticus, following the administration of pilocarpine. The frequency of seizures during the chronic period, in epileptic rats that received 5,7-DHT, showed a significant (58%) increase after the treatment, when compared with control group. Our data showed that serotonin may play an important role on seizure activity which seems to be exerted by its inhibitory action on the expression of overt behavior seizures departing from an established focus in the limbic system.     

雌激素调控瞬时外向钾通道对大鼠慢性颞叶癫发作的影响

目的探讨雌激素对大鼠慢性颞叶癫发作的影响及其与瞬时外向钾通道(kv4.2)之间的关系。方法 SD雌性大鼠随机分为正常对照组、去势对照组、正常致组、去势致组。应用毛果芸香碱诱导大鼠癫持续状态(SE),观察大鼠行为学变化。经4周后成为慢性颞叶癫大鼠。Western bolt方法检测大鼠海马瞬时外向钾通道Kv4.2蛋白表达水平。结果与正常致组比较,去势致组达到SE的潜伏期延长、死亡率降低,但差异无统计学意义(P0.05);去势致组的癫发作强度较正常致组轻,差异有统计学意义(P0.05)。去势致组和正常致组在SE 4周后海马Kv4.2均较正常对照组显著升高,均差异有统计学意义(P0.05和P0.01)。去势对照组和去势致组比较,均未见对Kv4.2蛋白表达产生影响,说明Kv4.2蛋白表达升高为癫发作所致。结论大鼠癫发作后Kv4.2表达增加、瞬时外向钾电流增强、神经冲动的发放频率减慢,可能是产生代偿性自我保护反应的结果;虽然去势雌性大鼠未对Kv4.2表达产生影响,但出现潜伏期延长和死亡率降低、发作强度降低的行为学趋势,提示低剂量雌激素可能存在对癫发作的保护作用。     

Malnutrition in infancy as a susceptibility factor for temporal lobe epilepsy in adulthood induced by the pilocarpine experimental model

Malnutrition during the earliest stages of life may result in innumerable brain problems. Moreover, this condition could increase the chances of developing neurological diseases, such as epilepsy. We analyzed the effects of early-life malnutrition on susceptibility to epileptic seizures induced by the pilocarpine model of epilepsy. Wistar rat pups were kept on a starvation regimen from day 1 to day 21 after birth. At day 60, 16 animals (8 = well-nourished; 8 = malnourished) were exposed to the pilocarpine experimental model of epilepsy. Age-matched well-nourished (n = 8) and malnourished (n = 8) rats were used as controls. Animals were video-monitored over 9 weeks. The following behavioral parameters were evaluated: first seizure threshold (acute period of the pilocarpine model); status epilepticus (SE) latency; first spontaneous seizure latency (silent period), and spontaneous seizure frequency during the chronic phase. The cell and mossy fiber sprouting (MFS) density were evaluated in the hippocampal formation. Our results showed that the malnourished animals required a lower pilocarpine dose in order to develop SE (200 mg/kg), lower latency to reach SE, less time for the first spontaneous seizure and higher seizure frequency, when compared to well-nourished pilocarpine rats. Histopathological findings revealed a significant cell density reduction in the CA1 region and intense MFS among the malnourished animals. Our data indicate that early malnutrition greatly influences susceptibility to seizures and behavioral manifestations in adult life. These findings suggest that malnutrition in infancy reduces the threshold for epilepsy and promotes alterations in the brain that persist into adult life.     

Increased GABA(A)-receptor alpha1-subunit expression in hippocampal dentate gyrus after early-life status epilepticus

PURPOSE: Previous studies in neonatal (postnatal day 10) and adult rats suggest that status epilepticus (SE) induces changes in the alpha1 subunit of the GABA(A) receptor (GABRA1) in dentate granule neurons (DGNs) that are age dependent and vary inversely with the likelihood of epilepsy development. In the present study, we examined GABRA1 expression after SE at postnatal day 20 (P20), an intermediate age when only a subset of SE-exposed animals develop epilepsy. METHODS: SE was induced with lithium-pilocarpine or kainate at P20. Animals were video-EEG monitored after SE to determine the presence or absence of spontaneous seizures. GABRA1 mRNA and protein levels were determined 7 days or 3 months later in SE-exposed and control animals by using a combination of aRNA amplification, Western blotting, and immunohistochemistry techniques. RESULTS: GABRA1 mRNA levels in DGNs of SE-exposed rats that did not become epileptic were higher than those in control rats, but were not different from DGNs in epileptic SE-exposed rats. GABRA1 protein levels in dentate gyrus were significantly increased in both epileptic and nonepileptic SE-exposed rats compared with controls. GABRA1 mRNA changes were region specific and did not occur in CA1 or CA3 areas of hippocampus. GABRA1 alterations were present by 1 week after P20 SE and were similar whether pilocarpine or kainate was used to induced SE. CONCLUSIONS: P20 SE results in persistent increases in GABRA1 levels selectively in dentate gyrus. These changes preceded the onset of epilepsy, were not model specific, and occurred in both epileptic and nonepileptic animals.     

The sleep–wake cycle in adult rats following pilocarpine-induced temporal lobe epilepsy

The relationship between sleep and epilepsy is both complex and clinically significant. Temporal lobe epilepsy (TLE) influences sleep architecture, while sleep plays an important role in facilitating and/or inhibiting possible epileptic seizures. The pilocarpine experimental model reproduces several features of human temporal lobe epilepsy and is one of the most widely used models in basic research. The aim of the present study was to characterize, behaviorally and electrophysiologically, the phases of sleep–wake cycles (SWC) in male rats with pilocarpine-induced epilepsy. Epileptic rats presented spikes in all phases of the SWC as well as atypical cortical synchronization during attentive wakefulness and paradoxical sleep. The architecture of the sleep–wake phases was altered in epileptic rats, as was the integrity of the SWC. Because our findings reproduce many relevant features observed in patients with epilepsy, this model is suitable to study sleep dysfunction in epilepsy.     

Long-term decrease in calbindin-D28K expression in the hippocampus of epileptic rats following pilocarpine-induced status epilepticus

Acquired epilepsy (AE) is characterized by spontaneous recurrent seizures and long-term changes that occur in surviving neurons following an injury such as status epilepticus (SE). Long-lasting alterations in hippocampal Ca²⁺ homeostasis have been observed in both in vivo and in vitro models of AE. One major regulator of Ca²⁺ homeostasis is the neuronal calcium binding protein, calbindin-D28k that serves to buffer and transport Ca²⁺ ions. This study evaluated the expression of hippocampal calbindin levels in the rat pilocarpine model of AE. Calbindin protein expression was reduced over 50% in the hippocampus in epileptic animals. This decrease was observed in the pyramidal layer of CA1, stratum lucidum of CA3, hilus, and stratum granulosum and stratum moleculare of the dentate gyrus when corrected for cell loss. Furthermore, calbindin levels in individual neurons were also significantly reduced. In addition, the expression of calbindin mRNA was decreased in epileptic animals. Time course studies demonstrated that decreased calbindin expression was initially present 1 month following pilocarpine-induced SE and lasted for up to 2 years after the initial episode of SE. The results indicate that calbindin is essentially permanently decreased in the hippocampus in AE. This decrease in hippocampal calbindin may be a major contributing factor underlying some of the plasticity changes that occur in epileptogenesis and contribute to the alterations in Ca²⁺ homeostasis associated with AE.     

Expression of vitamin D receptor mRNA in the hippocampal formation of rats submitted to a model of temporal lobe epilepsy induced by pilocarpine

Vitamin D (VD), is a steroid hormone with multiple functions in the central nervous system (CNS), producing numerous physiological effects mediated by its receptor (VDR). Clinical and experimental studies have shown a link between VD dysfunction and epilepsy. Along these lines, the purpose of our work was to analyze the relative expression of VDR mRNA in the hippocampal formation of rats during the three periods of pilocarpine-induced epilepsy. Male Wistar rats were divided into five groups: (1) control group; rats that received saline 0.9%, i.p. and were killed 7 days after its administration (CTRL, n = 8), (2) SE group; rats that received pilocarpine and were killed 4 h after SE (SE, n = 8), (3) Silent group—7 days; rats that received pilocarpine and were killed 7 days after SE (SIL 7d, n = 8), (4) Silent group—14 days; rats that received pilocarpine and were killed 14 days after SE (SIL 14d, n = 8), (5) Chronic group; rats that received pilocarpine and were killed 60 days after the first spontaneous seizure, (chronic, n = 8). The relative expression of VDR mRNA was determined by real-time PCR. Our results showed an increase of the relative expression of VDR mRNA in the SIL 7 days, SIL 14 days and Chronic groups, respectively (0.060 ± 0.024; 0.052 ± 0.035; 0.085 ± 0.055) when compared with the CTRL and SE groups (0.019 ± 0.017; 0.019 ± 0.025). These data suggest the VDR as a possible candidate participating in the epileptogenesis process of the pilocarpine model of epilepsy.     

Pilocarpine-induced status epilepticus results in mossy fiber sprouting and spontaneous seizures in C57BL/6 and CD-1 mice

Several rodent models are available to study the cellular mechanisms associated with the development of temporal lobe epilepsy (TLE), but few have been successfully transferred to inbred mouse strains commonly used in genetic mutation studies. We examined spontaneous seizure development and correlative axon sprouting in the dentate gyrus of CD-1 and C57BL/6 mice after systemic injection of pilocarpine. Pilocarpine induced seizures and status epilepticus (SE) after systemic injection in both strains, although SE onset latency was greater for C57BL/6 mice. There were also animals of both strains which did not experience SE after pilocarpine treatment. After a period of normal behavior for several days after the pilocarpine treatment, spontaneous tonic-clonic seizures were observed in most CD-1 mice and all C57BL/6 that survived pilocarpine-induced SE. Robust mossy fiber sprouting into the inner molecular layer was observed after 4-8 weeks in mice from both strains which had experienced SE, and cell loss was apparent in the hippocampus. Mossy fiber sprouting and spontaneous seizures were not observed in mice that did not experience a period of SE. These results indicate that pilocarpine induces spontaneous seizures and mossy fiber sprouting in both CD-1 and C57BL/6 mouse strains. Unlike systemic kainic acid treatment, the pilocarpine model offers a potentially useful tool for studying TLE development in genetically modified mice raised on the C57BL/6 background.     

Transplantation of bone marrow mononuclear cells decreases seizure incidence, mitigates neuronal loss and modulates pro-inflammatory cytokine production in epileptic rats

Approximately 30% of patients with mesial temporal lobe epilepsy do not respond to treatment with antiepileptic drugs. We have previously shown that transplantation of mononuclear bone marrow cells (BMC) has an anticonvulsant effect in acute epilepsy. Here, we used pilocarpine to induce epilepsy in rats and studied the effects of BMC injected intravenously either at the onset of seizures or after 10 months of recurrent seizures. BMC effectively decreased seizure frequency and duration. In addition, decreased levels of proinflammatory cytokines (TNF-α, IL-1β and IL-6) and increased levels of anti-inflammatory cytokine (IL-10) were observed in the brain and serum of BMC-treated rats. Transplants performed at seizure-onset protected against pilocarpine-induced neuronal loss and gliosis and stimulated the proliferation of new neurons in epileptic rats. Our data demonstrate that BMC transplantation has potent therapeutic effects and could be a potential therapy for clinically intractable epilepsies.     

Superoxide dismutase, glutathione peroxidase activities and the hydroperoxide concentration are modified in the hippocampus of epileptic rats.

The relationship between free radical and scavenger enzymes has been found in the epileptic phenomena and reactive oxygen species have been implicated in seizure-induced neurodegeneration. Using the epilepsy model obtained by systemic administration of pilocarpine (PILO) in rats, we investigated the superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities as well as the hydroperoxide (HPx) concentration in the hippocampus of rats during status epilepticus (SE), silent and chronic periods. The enzyme activities as well as the HPx concentration were measured using spectrophotometric methods and the results compared to values obtained from saline-treated animals. The SOD activity decreased after long-lasting SE period and during the chronic phase. In addition, HPx levels increased in same periods whereas the GPx activity increased only in the hippocampus of animals submitted to 1 h of SE. Animals presenting partial seizures, those submitted to 5 h of SE and animals from the silent period (seizure free) showed normal levels of SOD, GPx and HPx. These results show a direct evidence of lipid peroxidation during seizure activity that could be responsible for neuronal damage in the hippocampus of rats, during the establishment of PILO model of epilepsy.     

Behavioral alterations in the pilocarpine model of temporal lobe epilepsy in mice

Psychiatric disorders frequently occur in patients with epilepsy, but the relationship between epilepsy and psychopathology is poorly understood. Frequent comorbidities in epilepsy patients comprise major depression, anxiety disorders, psychosis and cognitive dysfunction. Animal models of epilepsy, such as the pilocarpine model of acquired epilepsy, are useful to study the relationship between epilepsy and behavioral dysfunctions. However, despite the advantages of mice in studying the genetic underpinning of behavioral alterations in epilepsy, mice have only rarely been used to characterize behavioral correlates of epilepsy. This prompted us to study the behavioral and cognitive alterations developing in NMRI mice in the pilocarpine model of epilepsy, using an anxiety test battery as well as tests for depression, drug-induced psychosis, spatial memory, and motor functions. In order to ensure the occurrence of status epilepticus (SE) and decrease mortality, individual dosing of pilocarpine was performed by ramping up the dose until onset of SE. This protocol was used for studying the consequences of SE, i.e. hippocampal damage, incidence of epilepsy with spontaneous recurrent seizures, and behavioral alterations. SE was terminated by diazepam after either 60, 90 or 120 min. All mice that survived SE developed epilepsy, but the severity of hippocampal damage varied depending on SE length. In all anxiety tests, except the elevated plus maze test, epileptic mice exhibited significant increases of anxiety-related behavior. Surprisingly, a decrease in depression-like behavior was observed in the forced swimming and tail suspension tests. Furthermore, epileptic mice were less sensitive than controls to most of the behavioral effects induced by MK-801 (dizocilpine). Learning and memory were impaired in epileptic mice irrespective of SE duration. Thus, the pilocarpine-treated mice seem to reflect several of the behavioral and cognitive disturbances that are associated with epilepsy in humans. This makes these animals an ideal model to study the neurobiological mechanisms underlying the association between epilepsy and psychopathology.     

Carbamazepine enhances discriminative memory in a rat model of epilepsy

PURPOSE: Seizures and antiepileptic drugs (AEDs) are the main causes for cognitive impairment in persons with epilepsy. It is still a matter of debate whether carbamazepine (CBZ) improves cognition because of its own psychotropic effects or because it is more effective to treat temporal epilepsy. Our objective was to analyze the performance of CBZ-treated or nontreated pilocarpine epileptic rats in an object-recognition test. METHODS: Twelve chronic pilocarpine-induced epileptic rats were treated with CBZ, 40 mg/kg, or saline, t.i.d. for 8 days. Twenty-one nonepileptic controls were treated with CBZ or saline. On day 8 of treatment, all rats were tested with an object-recognition paradigm. RESULTS: No locomotor impairment was detected in chronic epilepsy or CBZ treatment, as exploration during training was not affected. Exploratory behaviors during the choice session were not decreased in rats treated with CBZ; therefore CBZ does not compromise procedural memory. Epileptic rats showed a nonsignificant change in the discrimination performance, and prolonged treatment with CBZ in epileptic rats induced a significant increase in object discrimination during the choice session. CONCLUSIONS: Even though pilocarpine-induced epileptic animals do not show compromised performance in the spontaneous object-recognition test, prolonged CBZ treatment has a positive effect on a simple object-discrimination task. These results may be associated with the psychotropic effects of CBZ.     

Ontogenetic profile of ectonucleotidase activities from brain synaptosomes of pilocarpine-treated rats

Adenosine, a well-known neuromodulator, can act as an endogenous anticonvulsant via the activation of adenosine A₁ receptors. This adenine nucleoside can be produced in the synaptic cleft by the ectonucleotidase cascade, which includes the nucleoside triphosphate diphosphohydrolase (NTPDase) family and ecto-5′-nucleotidase. It has been previously reported that ectonucleotidase activities are increased in female adult rats submitted to the pilocarpine model of epilepsy. Several studies have suggested that the immature brain is less vulnerable to morphologic and physiologic alterations after status epilepticus (SE). Here, we evaluate the ectonucleotidase activities of synaptosomes from the hippocampus and cerebral cortex of male and female rats at different ages (7–9, 14–16 and 27–30-day old) submitted to the pilocarpine model of epilepsy. Our results show that ATP and ADP hydrolysis in the hippocampus and cerebral cortex were not altered by the pilocarpine treatment in female and male rats at 7–9, 14–16 and 27–30 days. There were no changes in AMP hydrolysis in female and male rats submitted to the model at different ages, but a significant increase in AMP hydrolysis (71%) was observed in synaptosomes from the cerebral cortex of male rats at 27–30 days. Pilocarpine-treated male rats (60–70-day old) presented an enhancement in ectonucleotidase activities in the synaptosomes of the cerebral cortex (33, 40 and 64% for ATP, ADP and AMP hydrolysis, respectively) and hippocampus (55, 98 and 101% for ATP, ADP and AMP hydrolysis, respectively). These findings highlight differences between the purinergic system of young and adult rats submitted to the pilocarpine model of epilepsy.     

Long-lasting modification of intrinsic discharge properties in subicular neurons following status epilepticus

A single episode of status epilepticus (SE) induces neuropathological changes in the brain that may lead to the development of a permanent epileptic condition. Most studies of this plasticity have focused on the hippocampus, where both synaptic function and intrinsic neuronal excitability have been shown to be persistently modified by SE. However, many other brain structures are activated during SE and may also be involved in the subsequent epileptogenic process. Here we have investigated whether SE, induced in rats with pilocarpine and terminated after 40 min with diazepam, persistently modifies the intrinsic excitability of pyramidal neurons in the subiculum. Subicular slices were prepared from control and SE-experienced rats (2-5 weeks after SE). In the control group, only 4% of the neurons fired bursts in response to intrasomatic, threshold-straddling depolarizing current pulses (low-threshold bursters). The remaining neurons either fired bursts in response to strong (3x threshold) depolarizations (35%; high-threshold bursters) or fired in a completely regular mode (61%; nonbursters). In the SE-experienced group, the fractions of low- and high-threshold bursters markedly increased to 29% and 53%, respectively. This change in firing behaviour was associated with a marked increase in the size of the spike after depolarization, particularly in low-threshold bursters. Experimental suppression of Ca2+ currents selectively blocked low-threshold bursting but did not affect high-threshold bursting, suggesting that a dual Ca2+- dependent and Ca2+- independent mechanism controls bursting in these neurons. The persistent up-regulation of intrinsic bursting in the subiculum, in concert with similar changes in the hippocampus, undoubtedly contributes to epileptogenesis following pilocarpine-induced SE.     

Epileptogenesis in immature rats following recurrent status epilepticus

Strong evidences link status epilepticus (SE) in childhood with the later development of epilepsy. Pilocarpine-induced SE in developing rats leads to late appearance of spontaneous epileptic seizures only when SE is induced after the 18th day of life. We examined the possibility that 3 consecutive episodes of pilocarpine-induced SE on postnatal days 7, 8 and 9 could induce behavioral, electrographic and histological epileptic changes in adult life. The animals also underwent behavioral tests (inhibitory step-down avoidance, skinner box, rota-rod, open field and elevated plus-maze). EEG recordings made at the age of 30, 60 and 90 days showed the occurrence of several episodes of spikes and/or polyspikes appearing simultaneously in hippocampus and cortex. Only three isolated spontaneous seizures were observed during the whole period of observation (120 days). The long-term effects of three consecutive episodes of SE include increased spontaneous exploratory activity, learning impairment, and reduced anxiety when tested on P60. Our findings provide evidence for EEG changes and cognitive deficits in adult life following recurrent SE on postnatal days 7–9.