tstx toxin isolated from tityus serrulatus scorpion venom induces spontaneous recurrent seizures and...

Epilepsia, 44(7):904–911, 2003Blackwell Publishing, Inc.C© 2003 International League Against Epilepsy

TsTx Toxin Isolated from Tityus serrulatus Scorpion VenomInduces Spontaneous Recurrent Seizures and Mossy

Fiber Sprouting

∗Maria Regina Lopes Sandoval and †Ivo Lebrun

∗Laboratory of Pharmacology and †Laboratory of Biochemistry and Biophysics, Butantan Institute, Sao Paulo, Brazil

Summary: Purpose: To characterize the long-term behavioral,electroencephalographic (EEG) and histopathologic features af-ter a single TsTx microinjection into the hippocampus of rats.

Methods: TsTx, 2 µg, or 1 µl of 0.1 M phosphate bufferwas injected into the right dorsal hippocampus of the rat. EEGrecords and behavioral observations were made over a period of10 h after injection. For a period of 4 months, the animals wereobserved for the occurrence of convulsive seizures. At the endof the experiment, the brains were processed by the neo-Timmand Nissl methods.

Results: After intrahippocampal TsTx injection, three distinctphases were observed: (a) an immediate period that lasted 1day, during which the motor and electrographic seizures char-acteristic of status epilepticus (SE) were seen; (b) a silent pe-riod (31–49 days), characterized by normal EEG and behavior;

and (c) a period of spontaneous recurrent seizures (SRSs). Theseizure frequency was one to two per week. Four months af-ter TsTx injection, hippocampal neuronal loss and mossy fibersprouting in the supragranular layer of the dentate gyrus wereobserved.

Conclusions: The SRSs observed in this study may be asso-ciated with the TsTx-induced SE and brain damage. All animalsinjected with the toxin showed massive pyramidal neuronalloss in the dorsal hippocampus as well as intense gliosis andatrophy. Mossy fiber sprouting in the supragranular layer of thedentate gyrus was observed in those animals that had SRSs. Theeffects observed may be due, at least in part, to TsTx-enhancedrelease of glutamate in hippocampal pathways. Key Words:Scorpion toxin—Epilepsy—Hippocampal damage—Mossyfiber sprouting.

A toxin isolated and sequenced from Tityus serrulatusscorpion venom in our laboratory showed full homologyup to 26 amino acid residues with a toxin named IV-5 (orTs IV) toxin by Possani et al. (1) and TsTx by Sampaioet al. (2). We had first named this toxin TS-8F (3), but lateradopted TsTx to unify the nomenclature for this toxin.The effects of TsTx on guinea pig pancreatic secretion (1)and on glutamate release from brain structures (4,5), itsantigenic properties (6), and the structural organization ofscorpion toxin genes (7) have been studied.

Sodium channels are the molecular targets for severalgroups of neurotoxins, which alter channel function bybinding to specific receptor sites. Studies investigatingthe effect of different neurotoxins on voltage-dependentsodium channels have identified six different receptorsites. Receptor sites 3 and 4 of sodium channels are oc-cupied by α- and β-scorpion toxins, respectively. TsTxhas been shown to be of the α type (8). Scorpion α-toxins

Accepted February 22, 2003.Address correspondence and reprint requests to Dr. M.R.L. Sandoval

at Laboratory of Pharmacology, Butantan Institute, Av. Dr. Vital Brasil,1500, 05503-900 Sao Paulo SP, Brazil. E-mail: [email protected]

bind to site 3 of the Na+ channel in a voltage-dependentway, and slow or block the mechanism of inactivation ofthese channels (9,10). Conversely, β-type toxins bind toreceptor site 4 in a way that is independent of membranepotential and affect channel activation (8). These toxinsincrease channel depolarization time and consequentlyinduce excessive neurotransmitters release. Experimen-tally, scorpion toxins enhance the release of glutamateand γ -aminobutyric acid (GABA) from preloaded rat cor-tical synaptosomes (11,12), and they stimulate release ofacetylcholine in cortical slices from rat brain (13). Stud-ies performed with TsTx have demonstrated that this toxinevokes glutamate release from cortical synaptosomes (4,5)and of the rat hippocampus (14).

A loss of modulation of sodium inactivation in the CNSwould produce a hyperexcitable population of neurons,which could lead to the genesis of an epileptogenic fo-cus (15). Changes in neuronal excitability affect the sus-ceptibility of the CNS to convulsions induced by variousagents; thus seizures result from an imbalance betweenexcitation and inhibition in the brain, favoring excitation(16). Seizures induced by toxins that alter sodium and

904

TsTx TOXIN–INDUCED RECURRENT SEIZURES 905

potassium channel function have been identified in scor-pion and snake venoms (3,17,18). Long-term studies withthese toxins could reveal substances that could lead to thedevelopment of a new animal model to study epilepsy.

In a previous study we characterized the short-term ef-fects of microinjection of 1 or 2 µg/µl TsTx into the hip-pocampus (3). The toxin induced orofacial automatisms,wet-dog shakes, myoclonus, and postural loss. Concomi-tantly, the electroencephalographic (EEG) record showedhigh-frequency and high-voltage spikes that evolved toseizure activity. Neuronal damage was observed in CA1and CA2 pyramidal cells and in granular cells of the den-tate gyrus. Moreover, glutamate antagonists administeredbefore the TsTx were able to block these effects (14).

We have been particularly interested in the long-termeffects of injections of TsTx into the hippocampus. Thequestion we posed was whether the short-term seizuresand hippocampal damage induced by TsTx could causelong-term neuronal alterations that would lead to spon-taneous recurrent seizures (SRSs). The literature has de-scribed several long-term models to study the process ofepileptogenesis triggered by substances that induce con-vulsions such as kainic acid (19–22), tetanus toxin (23),and pilocarpine (24). The present investigation was de-signed to characterize the long-term behavioral, EEG, andhistologic effects of intrahippocampal injection of TsTxin the rat.

METHODS

MaterialsFreeze-dried fresh Tityus serrulatus venom was ob-

tained from the Arthropod Laboratory of Butantan Insti-tute. Gel-filtration chromatography and high-performanceliquid chromatography (HPLC), as described by Carvalhoet al. (3), were used to isolate TsTx. The purity of the pep-tide was of sequencing grade. Only one peak in HPLCand only one N-terminus for 30 amino acid residues wasfound.

Male Wistar rats weighing 230–250 g were used. Ontheir arrival in the laboratory (7 days before the experi-ments), the rats were individually housed in wire-meshcages and maintained in a room with constant tempera-ture (22◦C ± 1◦C) on a 12-h light/dark cycle (lights on at7:00 a.m.). Food and water were provided ad libitum. Ani-mals were maintained in accordance with the guidelines ofthe Department of Pathology of the School of VeterinaryMedicine, Sao Paulo University, based on the guidelinesfor animal care prepared by the Committee on Care andUse of Laboratory Animal Resources, National ResearchCouncil, U.S.A.

Rats were anesthetized with a mixture of pentobarbi-tone and chloral hydrate (1 g pentobarbitone + 4 g chloralhydrate in 100 ml 0.9% NaCl, 3 ml/kg, i.p.) for stereo-taxic implantation of electrodes and a brain cannula. For

brain injections, a stainless steel guide cannula was im-planted into the right dorsal hippocampus [AP, –4.3; L,3.5; and V, 3.0 mm, according to the coordinates fromPaxinos and Watson (25)] and fixed with dental acrylate.In the contralateral hippocampus (AP, –4.3; L, 3.5; and V,3.5 mm), we implanted bipolar twisted electrodes to recordthe hippocampal EEG. The electrodes were anchored tothe skull with dental acrylate. For surface EEG record-ings, we inserted jeweler screws into the skull over theleft and right occipital cortex to function as electrodes.An additional screw placed in the frontal sinus served asa reference (indifferent) electrode. After surgery the ani-mals were housed individually and allowed to recover for3 to 4 days.

On the day of the experiment, the rats were transferredto glass cages (30 × 30 × 39 cm) and, after 15 min ofacclimatization, the EEG was recorded for 15 min. Theneither the vehicle (1.0 µl of 0.1 M phosphate buffer, pH7.4; n = 5) or toxin (1.0 µl containing 2.0 µg TsTx; n =9) was infused in 5 min into the right hippocampus. Forthese brain injections, we used a metal injector that pro-truded 0.5 mm beyond the tip of the guide cannula. Thehippocampal recording electrode was unilateral. EEG andbehavior were recorded continuously for a period of 10 hafter injection. Continuous EEG seizures persisting for aperiod of ≥30 min, and repetitive behavioral seizures weredefined as an effective response to the toxin characterizingstatus epilepticus (SE). Additional EEG recordings wereobtained 24 h after injection. On the day of toxin injectionand on the two subsequent days, the animals received oralsucrose and saline. The animals were observed by directvisual observation for 8 h/day, 5 days/week for the occur-rence of convulsive seizures. The EEG was recorded for2 h/day ≤16 days after toxin injection. No EEG record-ings could be made after this period because the implantedelectrodes were dislodged by animal movements. Obser-vations began on the same day the animals were injectedand were performed from 09:00 to 18:00 h.

The EEG obtained during the first hour after TsTx in-jection (acute period) was analyzed quantitatively. Wecounted all isolated and cluster spikes and all epilepticdischarges (seizures) during this period. The duration ofeach cluster of spikes and seizures was measured, and theresults were expressed as the sum of these episodes. Afterthis 1-h period, the rats started to have prolonged epilep-tic discharges. We quantified the number and duration ofseizures between 1 and 8 h after injection, and the episodeswere rated on a 4-point scale (0–3 crosses). In the imme-diate and SRS period, we described the behavioral signsfor each rat (Table 1).

At the end of the experiment (120 days after TsTx in-jection), the animals were deeply anesthetized with etherand fixed by transaortic perfusion of 25 ml of Millonig’sbuffer, 50 ml of 0.1% sodium sulfide fixative inMillonig’s buffer, 100 ml of 3% glutaraldehyde, and

Epilepsia, Vol. 44, No. 7, 2003

906 M. R. L. SANDOVAL AND I. LEBRUN

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200 ml of 0.1% sodium sulfide fixative in Millonig’sbuffer. After fixation, the brains were removed from theskull and immersed overnight in 15% sucrose solution.Cryostat-frozen coronal sections of 40 µm were then pro-cessed for modified Timm staining to enhance the vis-ualization of zinc-containing granule cell mossy fibers(26). For cell counts, 20-µm sections adjacent to thoseused for neo-Timm staining were processed for Nisslstaining. Cells were counted in the hippocampal CA1,CA3, and CA4 pyramidal cell layers, and in the granulatecell layer of the dentate gyrus. Seven sections per ani-mal were counted. The septotemporal levels correspond-ing to bregma, −3.30 to −4.30 mm, were evaluated forcell counts and mossy fiber sprouting. To complement thisstudy, a qualitative analysis of the septotemporal levels ofthe hippocampus was performed from bregma −2.0 to−4.5 (25). The cells were counted by a blind observer byusing the method of Bageta et al. (17). Only cells withclearly visible nuclei were counted. We magnified theslices 400 times and counted all cells within a 100-µm2

grid (17).Cell counts in experimental groups and the control were

compared by analysis of variance (ANOVA) followed bythe Dunnett test. Data are reported as mean ± standarddeviation (SD). Values were considered significant whenp ≤ 0.05.

RESULTS

Immediate period

Behavioral and EEG effectsShort-term behavioral alterations began 10–15 min after

TsTx injection (2 µg/µl) into the CA1 hippocampal area.Data from individual rats are shown in Table 1. At firstthe animals showed alternating periods of wet-dog shakesand staring. After a few minutes, behavior consisted offacial automatisms, rearing, masticatory jaw movements,sniffing, and bilateral forelimb clonus, rearing and falling

FIG. 1. Electroencephalographicrecords of an individual rat illustrat-ing the alterations observed afterinjection of TsTx (2 µg) into thedorsal hippocampus. A: Controlrecord obtained from an animalinjected into the hippocampus with1.0 µl of 0.1 M phosphate buffer;(B) cluster of spikes and wet-dogshakes (•); (C) moderate epilepticdischarges; (D) seizures observedrepeatedly 3 h after TsTx injection.The records are from cortex (Cx)and hippocampus (Hpc).

[score 5 of Pinel and Rovner (27)]. In addition to thesesigns, salivation (rat 5), circling behavior (rat 4), vigor-ous jumps, and periods of complete immobility were ob-served. About 90 min after the injections, the behavioralalterations evolved to limbic convulsions characterized bybilateral forelimb clonus with rearing and falling, followedby generalized clonic convulsions with jumping, wild run-ning, and falling [score 7 of Pinel and Rovner (27)]. Theconvulsive seizures recurred at 20- to 30-min intervalsthat alternated with periods of immobility, but electricalseizures were uninterrupted. These behaviors were ob-served ≤8 h after toxin injection.

Just after toxin injection, the EEG record showed high-voltage fast activity with isolated spikes initially restrictedto the hippocampus. Electrical paroxysmal activity alsooccurred during periods of complete immobility and star-ing. EEG changes included ictal and interictal epilepti-form activity. After ∼10 min, the EEG showed clusters ofspikes and short epileptic discharges that started in the hip-pocampus and evolved to the cortex. The quantification ofthese electrical alterations is shown in Table 1. One hourafter TsTx injection, isolated seizures and short-lastingepileptic discharges were observed. The discharges lasted1–2 min and recurred repeatedly. After 2 h, these episodeswere >30 min, characterizing SE (Table 1 and Fig. 1),and during the last few hours of the 8-h observation pe-riod, they were continuous. SE was observed in sevenof nine rats injected with TsTx (Table 1). The remainingtwo animals had wet-dog shake, facial automatisms, rear-ing, masticatory jaw movements, forelimb clonus, clonicconvulsion (rat 5 had two episodes, and rat 6 had fourepisodes) and short-lasting epileptic discharges (Table1).At the end of the 10-h observation period, the animals wereakinetic, and they remained so the next day. Generally thebehavioral and EEG alterations were simultaneous. Noneof the animals died. Seizures were no longer observed 24h after the injection. Only isolated spikes were recordedat that time.



Silent period

Behavioral and EEG effectsNo behavioral abnormalities were observed from the

day after toxin injection until day 31, and the silent pe-riod extended to 49 days after injection of TsTx. The ani-mals were manipulated 3 times per week for cage cleaningand daily for EEG recording, but no aggressive behaviorwas observed during these manipulations. The EEG wasrecorded for 2 h/day until the animal dislodged the record-ing apparatus from its head (9–16 days after TsTx injec-tion). The EEG pattern during this period seemed normal.

Spontaneous recurrent seizure period

Behavioral effectsAfter the silent period, the rats began to show SRSs at a

frequency of one to two per week. The SRS period lasteduntil the time of death (5–7 weeks after the start of thisperiod). All seven rats that showed SE during the immedi-ate period developed SRSs, whereas the animals that didnot show SE in the immediate period (numbers 5 and 6)did not develop SRS (Table 1). Animals 7 and 8 had thehighest severity of seizures, with forelimb clonus, rear-ing, postural loss, and falling. In the remaining four rats,convulsions were characterized by eye blinking, mastica-tory jaw movements, chewing, head nodding, and forelimbclonus (Table 1). Our data do not provide information onthe preferred convulsion period, because we did not ob-serve the rats during the dark cycle.

In contrast to the kainic acid model, the convulsionswere not evoked by manipulation of the animals (28). Theduration of SRSs was ∼30–50 s and was observed onlyonce on a given day, in contrast to the pilocarpine model,in which the animals have clusters of many seizures onthe same day (29). No EEG recording was made duringthis period because the animals dislodged the recordingapparatus from their heads.

Histopathologic analysisAnalysis of hippocampal sections by light microscopy

showed lesions characterized by massive neuronal loss

FIG. 2. Histograms showing thenumber of cells/100 µm2 in thepyramidal (subfields CA1, CA3, andCA4) and dentate granular layersquantified 120 days after intrahip-pocampal injection of vehicle or2 µg TsTx. Injected side, side ofthe hippocampus where the toxinor phosphate buffer was injected;contralateral side, the side oppositethe side of injection. Data are repre-sented as means ± SD; p < 0.05compared with the control group(analysis of variance followed by theDunnett test).

and hippocampal gliosis. All rats injected with TsTx hadsignificantly fewer cells in the CA1, CA3, and CA4 sub-fields of the hippocampal formation on the ipsilateral andcontralateral side to the injection site compared with con-trol animals (Fig. 2). Rats 7 and 8 showed the most ex-tensive neuronal loss, and rats 5 and 6 showed the leastneuronal loss (no SRSs were observed in these animals).Rats that had SE displayed a marked presence of neo-Timm–positive granules on the ipsilateral side of the den-tate gyrus (Fig. 3). The side contralateral to the injectionshowed a weak presence of staining mossy fiber. A quali-tative analysis of septotemporal levels of the hippocampusperformed from bregma −2.0 to −4.5 (25) showed neu-ronal death at all these levels and mossy fiber sprouting.Neuronal loss was not observed in the ventral hippocam-pus [plates −4.30 and −4.50 (25)]. One hundred twentydays after toxin injection, a clear atrophy of the hippocam-pus was noted, creating a cavity between the cortex andthe hippocampus, and the cannula scar had disappeared(Fig. 3).

DISCUSSION

Our results provide a new experimental model to in-duce SRSs in rats. The sequential progression of the ma-jor events after intrahippocampal injection of TsTx can beclassified as follows.

1. An immediate period characterized by staring, im-mobility, facial automatism, rearing, masticatoryjaw movements, sniffing, and limbic convulsion.These symptoms classify the seizures during theimmediate period at level 7 on the 8-point scaledescribed by Pinel and Rovner (27). The epilepticbehavioral signs are quite similar to those inducedby pilocarpine (24). Electrical paroxysmal activ-ity occurred during periods of complete immobil-ity and staring. Similar effects were observed afterintrahippocampal injection of kainic acid (21). Inthe EEG record, epileptic discharges recurred re-peatedly, characterizing SE. Similar electrical signsare observed after injection of kainic acid into the



FIG. 3. Supragranular neo-Timmstaining of the dentate gyrus ob-served 120 days after injection of2 µg TsTx into the dorsal hip-pocampus in rats (B, D). A, C:Control animal. The arrows showmossy fiber sprouting. The pho-tomicrograph was made 0.2 mmfrom the injection site. Scale bars,100 µm.

amygdaloid complex (21) or the hippocampus (19)of the rat or after systemic administration of pilo-carpine to rats (24). Because our animal preparationhad hippocampal electrodes implanted only on theside contralateral to the TsTx injection, it was notpossible to determine the source of the epilepticdischarges.

2. A silent period ranging from 31 to 49 days, duringwhich no behavioral signs of an epileptic naturewere observed. The EEG during this period wasrecorded for ≤16 days after toxin injection and wasnormal. It is clear that a major difference is foundbetween the onset of the SRSs elicited by TsTx,pilocarpine [mean of 14.8 days (29)], and kainate[5–21 days (28)]. However, all three models have aperiod during which the EEG and behavior returnto normal.

3. A period of SRSs. These seizures involved limbicconvulsions characterized by eye blinking, fore-limb clonus, rearing, postural loss with falling, andone to two per week were seen until the animalswere killed (120 days after TsTx injection).

The characteristics of TsTx-induced SRSs resemblethose observed after pilocarpine. In pilocarpine-treatedrats, the SRSs occur at constant frequency and per-sist throughout the observation period (29). In contrast,in kainic acid–treated rats, SRSs are observed for a periodof 22–46 days, after which EEG and behavioral seizuresremit (28). However, multiple intraperitoneal injectionsof low doses of kainate induced a chronic epileptic state,with continued SRSs until the rats died or were killed(30). In the present experiments performed with TsTx,

the SRSs occurred at regular frequency over a period of120 days. Nevertheless, the motor signs were similar tothose observed after microinjection of kainic acid into thehippocampus (28) and systemic pilocarpine injection (29).In our model, convulsions are elicited spontaneously. Inthe kainate model, most seizures, and particularly the firstones, were elicited by handling the animal (28). In con-trast with the pilocarpine model, TsTx-treated rats did nothave clusters of seizures in a given day or an evolution ofthese convulsions resembling kindling (27).

These results suggest that the SRSs observed in thisstudy may be the consequence of the TsTx-induced SE,because animals that showed SE during the immediate pe-riod developed SRSs. All animals injected with the toxinshowed massive pyramidal neuronal loss in the dorsal hip-pocampal CA1, CA3, and CA4 subfields ipsi- and con-tralateral to the site of toxin injection as well as intensegliosis and atrophy. Similar results were obtained in stud-ies on humans with temporal lobe epilepsy, showing cellloss in the pyramidal layer of the hippocampal formationand a proliferation of astrocytes, a pattern referred to ashippocampal sclerosis (31,32).

We previously showed pyramidal cell loss 7 days afterTsTx injection (3). In the present study, we observed notonly a massive neuronal loss 120 days after toxin injec-tion but also an evident gliosis and atrophy of the dorsalhippocampus, creating a cavity between the cortex andthe hippocampus. Thus the process of neurodegenerationbegins soon after injection of the toxin and seems to beintensified by the SRSs. We noted a positive associationbetween frequency and severity of SRSs and hippocam-pal cell loss. Rats 7 and 8 showed the most intense cellloss and the highest number and severity of SRSs [score 5,



Pinel and Rovner (27)]. The pattern of supragranular neo-Timm staining was dramatically altered in the supragran-ular layer of the dentate gyrus only in those animals thathad SRSs. Therefore TsTx-treated rats have indications ofsynaptic reorganization that also are observed in humanepileptogenic tissue (26,33) and animal models of limbicseizures (34,35).

The neuronal loss in the CA3 and hilar (CA4) sub-fields of the hippocampus and the mossy fiber sproutingin the present study are in accordance with previous re-ports. It is known that the granular cells send their axons(mossy fibers) to the CA3 pyramidal cells and hilar in-terneurons. Several lines of evidence suggest that mossyfiber sprouting could be a consequence of the massiveneuronal loss in the CA3 and/or hilar subfields of the hip-pocampal formation (36–38). Although the precise mech-anism and the functional consequences of seizure-inducedmossy fiber reorganization remain to be defined (39–41),the neuronal reorganization responsible for epileptogene-sis presumably takes place during the silent interval; how-ever, the functional correlates of this process are poorlyunderstood (41).

Probably the hippocampal damage observed in the CA4subarea ipsilateral to the site of injection and in the CA1,CA3, and CA4 subareas contralateral to the site of injec-tion was the consequence of the spread of epileptic activityrather than of diffusion of the toxin. It is unlikely that in-jections in the CA1 subfield that are applied slowly invery small volumes (0.2 µl/min) can diffuse to the con-tralateral hippocampus (20,21). In addition, we observedthat the side contralateral to the site of toxin injection alsoshowed mossy fiber sprouting (of low intensity).

Concerning dose–response effects, we observed thatdoses >2 µg caused the death of the animals (data notshown), and doses <2 µg induced behavioral and EEGepileptic signs but not SE. These effects were describedpreviously (3). The current study reports more severeseizures induced by 2 µg of TsTx than our previous study(3). The difference may be related to the rat strains used.The rats were obtained from different suppliers, and thismay account for the difference in response to the samedose of toxin.

The mechanisms of TsTx-induced seizures and neu-ronal loss are poorly understood, but we know that TsTxincreases the open time of sodium channels (8). This actionincreases neurotransmitter release (11–13) with the con-sequent occurrence of an imbalance between inhibitionand excitation (42). It is known that the rat hippocampushas a high density of glutamate receptors. It is widely ac-cepted that different subtypes of glutamate receptors areinvolved in mechanisms of neuronal degeneration (43–45) and in epileptic activity (46,47). Thus we suggestthat the effects of TsTx observed in the immediate periodmay be owing, at least in part, to the enhanced releaseof glutamate in the hippocampal pathways. Supporting

this idea, it has been shown that TsTx induces release ofglutamate in brain synaptosomes (5) and that a microdial-ysis study revealed enhanced levels of extracellular gluta-mate in the hippocampal area 1 h after injection of TsTxinto the hippocampus (14). Moreover, pretreating rats withan intrahippocampal injection of MK-801 (dizocilpinemaleate) and AP-5 (2-amino-5-phosphonopentanoic acid)fully blocked the EEG alterations seen in the immedi-ate period after injection of TsTx into the hippocampus,and also blocked the cell loss in the CA1 area. CNQX(6-cyano-7-nitroquinoxaline-2, 3-dione), AP-3 (L9+)-2-amino-3-phosphonopropionic acid) and MCPG [(+)-α-methyl-4-carboxyphenylglycine] partially blocked theepileptiform discharges, and no hippocampal damage wasobserved (14). These data support the hypothesis thatTsTx increases excitatory amino acid release by inhibitingsodium channel inactivation (8) and also is in accordancewith the finding that Na+ channel disorders may be in-volved in some forms of epilepsy (15).

Concerning the characteristics of the TsTx model, weshowed that it has some similarities with the pilocarpineand the kainic acid models, but also has its own peculiarproperties. The present study has established parameterssuch as the onset, duration, and the behavioral character-istics of the SRSs.

Much of what is known about the mechanisms ofepilepsy and of anticonvulsant drug therapy has beenlearned from studies of animal models of the epilepsies.Epilepsy is not a single disease, and no one animal modelrepresents all the forms of epilepsy (48). These resultsindicate that TsTx may be a useful tool for studies on neu-ronal lesions and on the mechanisms underlying epilepsy,as well for screening antiepileptic drugs.

Acknowledgment: We are indebted to Geane A. Lourencoand Fatima Canhoto for valuable technical assistance. This workwas supported by a grant from Fundacao de Amparo a Pesquisado Estado de Sao Paulo (FAPESP).

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