Implementing a bioassay to screen molecules for antiepileptogenic activity: chronic pilocarpine versus subdudral haematoma models.

BACKGROUND: There is a need to discover novel chemical compounds that will inhibit the pathological process of epileptogenesis (i.e. agents that will prevent the long-term formation of an active seizure focus following a brain insult). The goal of this paper is to identify a bioassay of value in drug design when screening new chemical entities as putative antiepileptogenic agents. METHODS: We focused on two models: the pilocarpine chronic seizure model of spontaneous recurrent seizures (SRSs) and a chronic subdural haematoma model of SRSs. Both models were evaluated using more than 20 Sprague-Dawley rats for each model. RESULTS: In the pilocarpine-induced model of SRSs, 80% of animals went on to develop SRSs when the dose of pilocarpine was 380 mg/kg i.p. In 50 animals that developed SRSs, the average number of seizures per 15 days of observation was 3.8 seizures with a range of 2-23 seizures per 15-day period. The chronic subdural model was inefficient in producing SRSs. CONCLUSIONS: A pilocarpine-induced SRS model of epilepsy affords a reliable model of epileptogenesis suitable for evaluating new chemical entities as putative antiepileptogenics.     

The pilocarpine model of epilepsy

The systemic administration of a potent muscarinic agonist pilocarpine in rats promotes sequential behavioural and electrographic changes that can be divided in three distinct periods: (a) an acute period that built up progressively into a limbic status epilepticus and that lasts 24h, (b) a silent period with a progressive normalization of EEG and behaviour which varies from 4 to 44 days, and (c) a chronic period with spontaneous recurrent seizures (SRSs). The main features of the SRSs observed during the long-term period resemble those of human complex partial seizures and recurs 2–3 times per week per animal. Therefore, this novel and unique experimental approach may serve as a model of epilepsy mimicking the human condition.This research was supported by FAPESP, CNPq and FINEP from Brazil.     

Role of the anterodorsal and anteroventral nuclei of the thalamus in spatial memory in the rat

This study tests the hypothesis that the anterior thalamic nuclei play a significant role in spatial learning and memory. Adult, male Sprague-Dawley rats with bilateral ibotenic acid lesions of the anterior thalamus were tested for 5 days in a repeated acquisition water maze task. Compared with Controls, rats with nearly complete lesions of both anterodorsal (AD) and anteroventral (AV) thalamic nuclei (AD/AV) were only mildly impaired in their spatial learning and memory. Larger lesions that extended into the anteromedial (AM) thalamic nucleus (AD/AV+) caused a more severe impairment and complete lesions of all three anterior nuclei (AD/AV/AM) resulted in even greater impairment that extended to all aspects of the task. In probe trials, only the Control animals had a preference for the correct quadrant. Approximately one-half of the rats were tested for a second week to determine if the impaired groups would benefit from further training. AD/AV/AM rats showed little improvement, but the other groups all improved significantly in all aspects of the task except the probe trial. Together, these data indicate that the anterior thalamic nuclei contribute to spatial learning and memory, but neither AV nor AD independently plays a dominant role.     

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.     

The course of untreated seizures in the pilocarpine model of epilepsy

The course of untreated epilepsy is not well established. This study uses a model of chronic limbic epilepsy (pilocarpine model of epilepsy) to determine the pattern of occurrence of seizures in untreated animals. Following pilocarpine administration, 21 rats were monitored continuously with a video system for 135 days after the first spontaneous seizure. Animals showed a great variability in seizure numbers and were divided in two subgroups presenting either a low frequency of seizures (n = 9 animals presenting ten or less seizures in the first 15 days of observation) or a high frequency of seizures (n = 12 animals presenting more than ten seizures during this period). Animals with low number of seizures during the first 15 days of observation showed a significant increase in seizure frequency in the following period of analysis (until 105 days). On the other hand, those with initial high number of seizures showed significant changes in seizure frequency only in the first 2 months. The duration of each spontaneous seizure did not change significantly over time. These findings show that in untreated epilepsy there is a maturation process in the early stages and this accelerating process can be of predictive value for the treatment of epilepsy.     

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.     

Striking differences in individual anticonvulsant response to phenobarbital in rats with spontaneous seizures after status epilepticus

PURPOSE: More than one third of patients with epilepsy have inadequate control of seizures with drug therapy, but mechanisms of intractability are largely unknown. Because of this large number of pharmacoresistant patients with epilepsy, the existing process of antiepileptic drug (AED) discovery and development must be reevaluated with a focus on preclinical models of therapy-resistant epilepsy syndromes such as mesial temporal lobe epilepsy (TLE). However, although various rodent models of TLE are available, the pharmacoresponsiveness of most models is not well known. In the present study, we used a post-status epilepticus model of TLE to examine whether rats with spontaneous recurrent seizures (SRSs) differ in their individual responses to phenobarbital (PB). METHODS: Status epilepticus was induced in Sprague-Dawley rats by prolonged electrical stimulation of the basolateral amygdala. Once the rats had developed SRSs, seizure frequency and severity were determined by continuous EEG/video recording over a 6-week period (i.e., a predrug control period of 2 weeks, followed by PB treatment for 2 weeks, and a postdrug control period of 2 weeks). PB was administered twice daily at maximal tolerated doses. RESULTS: Analysis of plasma drug concentrations showed that drug concentrations within the therapeutic range (10-40 microg/ml) were maintained in all rats throughout the period of treatment. In six (55%) of 11 rats, complete control of seizures was achieved, and another rat exhibited a >90% reduction of seizure frequency. These seven rats were considered responders. The remaining four (36%) rats showed either no response at all (n=3) or only moderate reduction in seizure frequency and were therefore considered nonresponders. Plasma drug concentrations did not differ between these two groups of rats. CONCLUSIONS: These data demonstrate that, similar to patients with epilepsy, rats with SRSs markedly differ in their individual responses to AED treatment. Pharmacoresistant rats selected by prolonged drug treatment from groups of rats with SRSs may provide a unique model to study mechanisms of pharmacoresistance and to identify novel AEDs for treating seizures of patients currently not controlled with existing therapies.     

Effects of the novel antiepileptic drug levetiracetam on spontaneous recurrent seizures in the rat pilocarpine model of temporal lobe epilepsy

PURPOSE: Animal models in which seizures are elicited by chemical or electrical means are commonly used for identification and preclinical testing of novel antiepileptic drugs (AEDs). Such models have been successful in discovering all the new AEDs. However, despite the high efficacy of AEDs against elicited seizures in rodent models, a significant proportion of epilepsy patients with spontaneous recurrent seizures is resistant to these drugs. It is not known whether drug testing in rodent models with spontaneous recurrent seizures would yield a more predictive result with respect to AED efficacy in the clinic. This led us to test one of the novel AEDs, levetiracetam (LEV), in a rat model of temporal lobe epilepsy (TLE) with spontaneous recurrent seizures. METHODS: Wistar rats were subjected to pilocarpine-induced status epilepticus and recorded for spontaneous recurrent seizures in the months after pilocarpine treatment. A group of rats with frequent spontaneous seizures was used for the drug trial with LEV. The experimental protocol for drug testing in these rats was as follows. For 2 weeks, rats received subcutaneous implantation of osmotic minipumps filled with saline (predrug control period), followed by a 2-week period with implantation of LEV-filled minipumps (drug period), after which pumps were replaced by drug-free pumps for 2 weeks (postdrug control period). The LEV concentration in the pumps during the drug period was adjusted to give daily doses resulting in the maximal plasma concentration range determined previously in patients with TLE during prolonged treatment with LEV. During the 6 weeks of the experiment in epileptic rats, seizures were recorded by video monitoring. RESULTS: Average seizure frequency during the pre- and postdrug control period in a group of eight epileptic rats was 21 and 25 seizures. This was reduced to an average seizure frequency of 8 seizures during the 2 weeks of treatment with LEV. However, the individual response of rats to LEV varied markedly from complete seizure control to no effect at all, although plasma drug levels were within the therapeutic range in all rats. When seizure frequency was separately calculated for the first and second week of treatment, the significant anticonvulsant effect determined in the first week was partially diminished in the second week, suggesting that tolerance may have developed in some of the rats. CONCLUSIONS: The data demonstrate that interesting results can be obtained by drug testing in epileptic rats, giving a more realistic prediction of clinical efficacy than results from drug testing in animal models with elicited seizures. Thus, although drug trials in rats with spontaneous recurrent seizures are laborious and time-consuming, such trials should be added to the preclinical characterization of novel AEDs.     

Diurnal rhythms of spontaneous recurrent seizures and behavioral alterations of Wistar and spontaneously hypertensive rats in the kainate model of epilepsy

Attention deficit hyperactivity disorder (ADHD) can coexist with epilepsy. Spontaneously hypertensive rats (SHRs) are considered to model ADHD with overactivity, impulsiveness, deficient sustained attention, and alterations in circadian autonomic profiles. The present study explored spontaneous recurrent seizures (SRSs) and behavioral diurnal activity rhythms in normotensive Wistar rats and SHRs in the kainate model of epilepsy. Rats were video monitored (24 h/3 months) to detect SRSs. SHRs manifested a lower seizure frequency during the light phase in the 8th and 10th weeks and a lower frequency of SRSs during the night phase accompanied by attenuated responses in hyperexcitability tests. Both epileptic strains were hyperactive, with lower anxiety levels, and their diurnal rhythms were abolished. Epileptic Wistar rats and SHRs exhibited less exploration during the dark phase. This study suggests that SHRs may be useful in modeling some aspects (particularly hypertension-related diurnal rhythm disturbance) of behavior associated with epilepsy.     

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.     

Effects of A1 receptor agonist/antagonist on spontaneous seizures in pilocarpine-induced epileptic rats

Adenosine is an endogenous anticonvulsant that activates pre- and postsynaptic adenosine A₁ receptors. A₁ receptor agonists increase the latency for the development of seizures and status epilepticus following pilocarpine administration. Although hippocampal adenosine is increased in the chronic phase of the pilocarpine model, it is not known whether the modulation of A₁ receptors may influence the frequency of spontaneous recurrent seizures (SRS). Here, we tested the hypothesis that the A1 receptor agonist RPia ([R]-N-phenylisopropyladenosine) and the A1 antagonist DPCPX (8-Cyclopentyl-1,3-dipropylxanthine) administered to chronic pilocarpine epileptic rats would respectively decrease and increase the frequency of SRS and hippocampal excitability. Four months after Pilo-induced SE, chronic epileptic rats were video-monitored for the recording of SRS before (basal) and after a 2-week treatment with RPia (25 μg/kg) or DPCPX (50 μg/kg). Following sacrifice, brain slices were studied with electrophysiology. We found that rats given RPia had a 93% nonsignificant reduction in the frequency of seizures compared with their own pretreatment baseline. In contrast, the administration of DPCPX resulted in an 87% significant increase in seizure rate. Nontreated epileptic rats had a similar frequency of seizures along the study. Corroborating our behavioral data, in vitro recordings showed that slices from animals previously given DPCPX had a shorter latency to develop epileptiform activity, longer and higher DC shifts, and higher spike amplitude compared with slices from nontreated Pilo controls. In contrast, smaller spike amplitude was recorded in slices from animals given RPia. In summary, the administration of A1 agonists reduced hippocampal excitability but not the frequency of spontaneous recurrent seizures in chronic epileptic rats, whereas A1 receptor antagonists increased both.     

Effects of fluoxetine and TFMPP on spontaneous seizures in rats with pilocarpine-induced epilepsy

PURPOSE: Fluoxetine is a selective serotonin [5-hydroxytryptamine (5-HT)] reuptake inhibitor (SSRI) commonly used to treat depression. Some uncontrolled clinical studies have reported that SSRIs increase seizures, but animal experiments with evoked-seizure models have suggested that SSRIs at therapeutic doses decrease seizure susceptibility. We tested the hypothesis that fluoxetine and trifluoromethylphenylpiperazine (TFMPP, a nonselective 5-HT-receptor agonist) reduce the frequency of spontaneous motor seizures in pilocarpine-treated rats. METHODS: Fluoxetine (20 mg/kg) and TFMPP (5 mg/kg) were administered to rats with pilocarpine-induced epilepsy. Phenobarbital (PB; 10 mg/kg) was a positive control, and saline (i.e., 0.5 ml) controlled for the injection protocol. Each rat received each treatment (intraperitoneally) once per day for 5 consecutive days with 1 week between treatments. Rats were continuously video-monitored for the last 72 h of each treatment. RESULTS: When compared with saline over the entire 72-h observation period, PB and fluoxetine treatment, but not TFMPP, reduced the spontaneous-seizure rate. Plots of magnitude of the drug effect as a function of seizure frequency after saline treatment revealed larger drug effects for fluoxetine and PB in the rats with the highest control seizure rate. When the data from the five rats with the highest seizure frequency in saline were analyzed for the first 6 h after treatment, TFMPP also significantly reduced seizure frequency. CONCLUSIONS: Animal models with spontaneous seizures can be used to screen potential antiepileptic drugs, and fluoxetine and TFMPP reduce spontaneous seizures in the pilocarpine model of temporal lobe epilepsy.     

The pilocarpine model of temporal lobe epilepsy

Understanding the pathophysiogenesis of temporal lobe epilepsy (TLE) largely rests on the use of models of status epilepticus (SE), as in the case of the pilocarpine model. The main features of TLE are: (i) epileptic foci in the limbic system; (ii) an "initial precipitating injury"; (iii) the so-called "latent period"; and (iv) the presence of hippocampal sclerosis leading to reorganization of neuronal networks. Many of these characteristics can be reproduced in rodents by systemic injection of pilocarpine; in this animal model, SE is followed by a latent period and later by the appearance of spontaneous recurrent seizures (SRSs). These processes are, however, influenced by experimental conditions such as rodent species, strain, gender, age, doses and routes of pilocarpine administration, as well as combinations with other drugs administered before and/or after SE. In the attempt to limit these sources of variability, we evaluated the methodological procedures used by several investigators in the pilocarpine model; in particular, we have focused on the behavioural, electrophysiological and histopathological findings obtained with different protocols. We addressed the various experimental approaches published to date, by comparing mortality rates, onset of SRSs, neuronal damage, and network reorganization. Based on the evidence reviewed here, we propose that the pilocarpine model can be a valuable tool to investigate the mechanisms involved in TLE, and even more so when standardized to reduce mortality at the time of pilocarpine injection, differences in latent period duration, variability in the lesion extent, and SRS frequency.     

Repeated low-dose treatment of rats with pilocarpine: low mortality but high proportion of rats developing epilepsy.

Systemic administration of pilocarpine in rats can result in a chronic behavioral state that is similar to human temporal lobe epilepsy. The pilocarpine model of epilepsy is widely used for studying the factors that contribute to the development of epilepsy as a consequence of status epilepticus (SE). For this purpose, pilocarpine is either administered alone at a high systemic dose or in combination with lithium, which markedly potentiates the convulsant effect of pilocarpine. Both experimental protocols, however, are associated with high mortality rates. In the present study, we evaluated whether mortality rate in rats can be decreased by repeated administration of low doses of pilocarpine. The time the rats spent in SE was limited by diazepam. Preliminary experiments in lithium-free rats indicated that repeated low-dose administration of pilocarpine is too time-consuming to produce SE compared to single high-dose administration. All subsequent experiments were performed in lithium-pretreated rats. Single-dose injection of 30 mg/kg pilocarpine produced SE in approximately 70% of the animals, but 45% of the rats died although SE was interrupted by diazepam after 90 min. Repeated i.p. administration of 10 mg/kg pilocarpine at 30-min intervals resulted in SE after 2-4 injections; the mean dose of pilocarpine needed to induce SE was 26 mg/kg. When SE was interrupted after 90 min, mortality rate was below 10%, which was significantly lower compared to the protocol with one single administration of 30 mg/kg pilocarpine. In contrast to mortality rate, the development of spontaneous recurrent seizures did not differ between experimental protocols. Almost all rats which had experienced a SE of at least 60 min developed chronic epilepsy. Average latency to the first spontaneous seizure was approximately 40 days. The frequency and severity of spontaneous seizures was not significantly different between protocols, although animal groups with repeated low-dose treatment tended to have higher frequencies of spontaneous seizures compared to single-dose administration. The present study demonstrates that systemic treatment of lithium-pretreated rats with several low doses of pilocarpine efficiently produces SE and chronic epilepsy with much lower mortality rates than single-dose pilocarpine.     

The antidepressants citalopram and reboxetine reduce seizure frequency in rats with chronic epilepsy

Purpose: For a long time, antidepressants have been thought to possess proconvulsant properties. This assumption, however, remains controversial, since anticonvulsant effects have been attributed to certain antidepressants. To date, it remains unclear which antidepressants can be used for the treatment of depression in patients with epilepsy. In this respect, studies investigating the convulsant liability of antidepressants in a chronic epilepsy model can give valuable information. The present study was designed to determine the seizure liability of citalopram and reboxetine in the kainic acid–induced post–status epilepticus model for temporal lobe epilepsy. Methods: Two months after the induction of status epilepticus, chronic epileptic rats (n = 16) were video‐electroencephalography (EEG) monitored during seven consecutive weeks. Weeks 1, 3, 5, and 7 served as sham weeks during which the rats received intraperitoneal saline injections for four consecutive days, followed by a 3‐day sham washout period during which no injections were given. During weeks 2, 4, and 6, rats received intraperitoneal injections with either citalopram (5, 10, and 15 mg/kg, once daily, n = 8) or reboxetine (10, 20, and 30 mg/kg, twice daily, n = 8) for 4 days, again followed by a washout period of 3 days. Drugs were administered in a randomly assigned fixed‐dose regimen per week. Each rat served as its own control. The drug doses were selected based on the doses reported to have antidepressant effects in rats. Key Findings: Citalopram significantly decreased the spontaneous seizure frequency at the highest dose tested, that is, the mean number of seizures decreased from 12.8 seizures to 8.8 seizures per week (31%) after treatment with 15 mg/kg citalopram. This dose also significantly decreased the cumulative seizure duration. Administration of 5 and 10 mg/kg citalopram did not alter the seizure frequency. The two highest doses of reboxetine significantly decreased the spontaneous seizure frequency, that is, 20 mg/kg reboxetine decreased the seizure frequency from 14.1 to 7.9 (44%) and 30 mg/kg reboxetine decreased the seizure frequency from 11.8 to 7.2 (39%). In addition, both doses significantly decreased the cumulative seizure duration. Administration of 10 mg/kg reboxetine did not alter seizure frequency. Citalopram and reboxetine had no effect on seizure severity and seizure duration in any of the doses tested. Significance: In general we can conclude that antidepressant doses of citalopram and reboxetine have, depending on the dose, an anticonvulsant effect or no effect on spontaneous seizures in the kainic acid–induced post–status epilepticus rat model.     

Electrical stimulation of left anterior thalamic nucleus with high-frequency and low-intensity currents reduces the rate of pilocarpine-induced epilepsy in rats

PurposeBilateral electrical stimulation of anterior nuclei of thalamus (ANT) has shown promising effects on epileptic seizures. However, bilateral implantation increases the risk of surgical complications and side effects. This study was undertaken to access the effectiveness of a stimulation paradigm involving high frequency and low intensity currents to stimulate the left ANT in rats.MethodsMale Sprague-Dawley rats were implanted with electroencephalogram (EEG) electrodes, and an additional concentric bipolar stimulation electrode into either the left or right ANT. The stimulus was a train of pulses (90 μs duration each) delivered with a frequency of 200 Hz and a current intensity of 50 μA. Thalamic stimuli were started 1 h before the first intraperitoneal pilocarpine injection (i.p., 300 mg/kg), and were applied for 5 h.ResultsEEG documented seizure activity and status epilepticus (SE) developed in 87.5% of rats treated with no ANT stimulation after a single dose of pilocarpine. Left ANT stimulation significantly increased the tolerance threshold for pilocarpine-induced EEG seizure activity; 20% of rats developed their EEG documented seizure activity after receiving the first dose, whereas 50%, 10% and 20% of rats did not develop seizure activity until they had received the 2nd, 3rd and 4th pilocarpine injection at 1-h intervals. Moreover, left thalamic stimulation reduced the occurrences of both EEG documented seizure activity and SE induced by single-dose pilocarpine to 25%. However, our result demonstrated that little effect on the occurrence rate of seizures and SE was found when rats received right ANT stimulation.ConclusionsThese results suggest that continuously 5-h left ANT stimulation with high frequency and low intensity currents, beginning from 1 h before the pilocarpine administration, may successfully reduce the occurrence rate of EEG documented seizure activity and SE development in rats.     

Pregnancy decreases the frequency of spontaneous recurrent seizures in rats with kainic acid lesions of the hippocampus

Spontaneous recurrent seizures (SRSs) were observed in female rats following the injection of kainic acid into the dorsal hippocampus. Pregnancy and nursing decreased the frequency of SRSs in such animals. The finding of a protective effect of pregnancy and lactation in this animal model of temporal lobe epilepsy stresses the usefulness of this model.     

The susceptibility of rats to pilocarpine-induced seizures is age-dependent

Behavioral, electroencephalographic and morphological changes induced by systemic administration of pilocarpine hydrochloride were studied in 3-90-day-old rats. Pilocarpine, 100, 200 and 380 mg/kg, presented a characteristic array of behavioral patterns in developing rats. Hyper- or hypoactivity, tremor, loss of postural control, scratching, head bobbing and myoclonic movements of the limbs dominated the behavior in 3-9-day-old rats. No overt motor seizures were observed in this age group. More intense behavioral signs evolving in some animals to limbic seizures and status epilepticus occurred when pilocarpine was administered in 12-day-old-rats. The electrographic activity in these animals progressed from low voltage spiking registered concurrently in the hippocampus and cortex during the first week of life into localized epileptic activity in the hippocampus, which spread to cortical recordings during the second week of life. No morphological alterations were detected in the brains of 3-12-day-old rats subjected to the action of pilocarpine, 100-380 mg/kg. The adult pattern of behavioral and electroencephalographic sequelae after pilocarpine was encountered in 15-21-day-old rats. Akinesia, tremor and head bobbing progressed in 15-21-day-old rats given pilocarpine, 100-380 mg/kg, to motor limbic seizures and status epilepticus. The lethal toxicity of pilocarpine reached 50% during the third week of life. This increased susceptibility to the convulsant action of pilocarpine was characterized by a shortened latency for behavioral and electrographic signs, and an increased severity of seizures relative to older and younger rats. In 15-21-day-old rats subjected to pilocarpine-induced convulsions high voltage fast activity superposed over hippocampal theta-rhythm, progressed into high voltage spiking and spread to cortical records. The electrographic activity became well synchronized and then developed into seizures and status epilepticus. Morphological analysis of frontal forebrain sections in 15-21-day-old rats which underwent status epilepticus after pilocarpine revealed no damage or an attenuated pattern of damage. In 15-21-day-old rats which presented epilepsy-related brain damage, morphological breakdown was seen in the hippocampus, amygdala, olfactory cortex, neocortex and certain thalamic nuclei. No damage was detected in the substantia nigra and lateral thalamic nucleus. An adult pattern of the damage to the brain, in terms of extent and topography, was present in 4-5-week-old rats.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Effects of lesions in ventral anterior thalamus on experimental focal epilepsy

The effect of stereotaxic lesions of the ventral anterior thalamus, or the adjacent inferior thalamic peduncle, on experimental models of focal cortical epilepsy was studied. Acute epileptic foci in cat sensorimotor cortex were made by injection of tungstic acid gel. Following ipsilateral lesions of ventral anterior thalamus or the adjacent inferior thalamic peduncle in these animals there was a highly significant reduction in electrographic seizure frequency and duration compared to prelesion control periods. Interictal activity at the focus was not altered. The frequency and duration of spontaneous clinical seizures in five rhesus monkeys with chronic alumina cream foci in motor strip was continuously monitored in activity chairs. Both seizure frequency and duration decreased in all animals in the 4-week period after ipsilateral ventral anterior thalamic lesions as compared to the 4-week control period. Sham lesions did not have these effects. The thalamic lesions did not discernably alter behavior or neurologic function in these primates. Thus ventral anterior thalamic lesions decrease seizure frequency and duration in both acute and chronic experimental models of focal cortical epilepsy. These findings indicate that pathways originating in or passing through the ventral anterior thalamus play a role in the generalization of focal cortical seizures. These lesions in ventral anterior thalamus may be useful in the treatment of medically intractable seizures secondary to foci inaccessible to direct excision.