A

EseRo1ir©

K

mmvtsbirtrwntrfc

0d

Brain Research Bulletin 71 (2006) 160–166

Opposite effects of a GABAB antagonist in two modelsof epileptic seizures in developing rats

Pavel Mares a,∗, Romana Slamberova a,b

a Institute of Physiology, Academy of Sciences of the Czech Republic, Videnska 1083, 142 20 Prague 4, Czech Republicb Charles University, 3rd Medical School, Prague, Czech Republic

Received 9 March 2006; received in revised form 21 August 2006; accepted 24 August 2006Available online 20 September 2006

bstract

The action of a GABAB antagonist CGP 35348 and a GABAB agonist baclofen was studied in two models of epileptic seizures characterized byEG spike-and-wave rhythm in freely moving immature rats. Rhythmic metrazol activity (RMA, model of human absences) was induced by lowystemic dose of pentylenetetrazol (PTZ) in 18- and 25-day-old rats, epileptic after discharges (ADs, model of human myoclonic seizures) werelicited by electrical stimulation of sensorimotor cortex in rat pups 12, 18, and 25 days old. CGP 35348 (50, 100 and 200 mg/kg i.p.) suppressedMA in both age groups in a dose-dependent manner. Simultaneously it increased the incidence of clonic seizures, potentiating thus an effectf PTZ. Baclofen (1, 3 and 6 mg/kg i.p.) augmented markedly RMA in 25-day-old rats. On the contrary, baclofen suppressed RMA in a part of

8-day-old animals. Incidence of seizures was not changed by baclofen in either age group. As ADs are concerned CGP 35348 (100 and 200 mg/kg.p.) exhibited a proconvulsant action, baclofen (3, 6 or 12 mg/kg i.p.) was anticonvulsant, but again an irregularity of action was found in 18-day-oldats. The role of GABAB-mediated inhibition in epileptogenesis depends on the type of seizures and also on the stage of maturation.

2006 Published by Elsevier Inc.

kcaa

taweThtbp

eywords: GABAB; Seizures; Spike-and-wave rhythm; Ontogeny; Rat

Potentiation of GABAergic inhibition is one of the basicechanisms of anticonvulsant action. It represents the mainechanism of some classical antiepileptic drugs (phenobarbital,

alproate, benzodiazepines) as well as in some new antiepilep-ics developed intentionally as drugs influencing GABAergicystem (progabide, vigabatrin, tiagabine). Phenobarbital andenzodiazepines are acting through GABAA receptors, increas-ng the frequency or duration of opening of chloride channel,espectively. Other above mentioned antiepileptics may realizeheir anticonvulsant action through both GABAA and GABABeceptors. Progabide (which was withdrawn from clinical use)as demonstrated to be both GABAA and GABAB receptor ago-ist, vigabatrin and tiagabine increase the amount of GABA inhe synaptic cleft without a specificity to either type of GABA

eceptors (for review [23]). For a long time the attention wasocused on GABAA system because of insufficient data con-erning GABAB system, where only baclofen as an agonist was

∗ Corresponding author. Tel.: +420 24106 2549; fax: +420 24106 2488.E-mail address: maresp@biomed.cas.cz (P. Mares).

strt[

t

361-9230/$ – see front matter © 2006 Published by Elsevier Inc.oi:10.1016/j.brainresbull.2006.08.013

nown and specific antagonists were missing. This situation washanged since 1990 when the first systemically active GABABntagonist was synthesized and a number of chemically relateds well as unrelated drugs followed [5].

First data on GABAB receptor antagonists in experimen-al epilepsy models demonstrated their possible proconvulsantction in vitro as well as in vivo [17]. This was in agreementith older data on the action of GABAB agonist baclofen which

xhibited predominantly anticonvulsant effects (for review [18].hen a specific action of GABAB antagonists against models ofuman absences was discovered [15,31,35] and their possibleherapeutic use was discussed [4]. Absences represent the onlyasic type of seizures where inhibition represents an importantathogenetic mechanism [38] and both GABAA and GABABystems are involved [31]. Human absence seizures as well asheir animal models are characterized by spike-and-wave EEGhythm. This rhythm is generated in thalamocortical system [40];

he GABAB mechanism is involved in oscillations in this system10].

Human absences represent an age-bound type of seizures;hey do not appear before a certain level of maturation is reached

Resea

asotib[tatGt

tvGbwmvidsa–iGas

cnGs

1

1oso1rehatc

1p

orfrwt

oSrEiBsecwaArw

1

eelbcasbbdigwbdbacss

1

FAiwtw

2

2p

sgesT

P. Mares, R. Slamberova / Brain

nd their maximal incidence is in the late pre-school and earlychool age [2]. Two pharmacologically induced animal modelsf absence seizures [39] exhibit a developmental profile similaro human absences. gamma-Hydroxybutyrate and pentetrazol-nduced episodes of spike-and-wave activity do not appearefore the third post-natal week in rats (�-hydroxybutyrate –38], pentetrazol – [37]). Therefore the pentetrazol model (rou-inely used in our laboratory—[27]) was chosen to test thentiabsence action of a GABAB antagonist CGP 35348. Simul-aneously we performed a parallel series of experiments withABAB agonist baclofen to know the development of its poten-

iating action in this model.Recently GABA-B receptor antagonists were demonstrated

o be proconvulsant in other models of epileptic seizures inivo as well as in vitro [8,20,43,48]. In addition, high doses ofABA-B receptor antagonists were found not only to potentiateut also to elicit hippocampal seizures in rats [20]. Thereforee decided to compare effects of a GABAB antagonist in aodel of absence seizures with its action in a model of con-

ulsant seizures characterized also by spike-and-wave rhythmn the EEG. A developmental model of cortical epileptic afterischarges characterized by spike-and-wave rhythm and cloniceizures was described in our laboratory [29] and data for thection of drugs influencing GABAA receptors (benzodiazepines[19], phenobarbital – [33], neurosteroids – [28]) as well as act-

ng at both GABAA and GABAB receptors (progabide – [33],ABA reuptake blocker NNC-711 – [3]) are at disposal. These

fter discharges probably represent a model of human myocloniceizures.

Two questions have to be answered: (1) Are the two modelsharacterized by spike-and-wave rhythm in the EEG (one ofonconvulsive, the other of convulsive seizures) influenced byABAB drugs in the same way? (2) Does maturation of GABAB

ystem influence its role in the model of absence seizures?

. Methods

All experiments were performed in immature albino rats of the Wistar strain2, 18, and 25 days old (the day of birth was taken as 0) bred in the animal housef our Institute. Implantation of electrodes was made under ether anesthesia; thisurgical preparation lasted less than 15 min. Details are given in the descriptionf individual experiments. The animals were allowed to recover for at leasth, then 5% solution of sucrose was offered to be sucked, righting and placing

eflexes were examined and only animals exhibiting normal behavior entered thexperiments. Body temperature of the animals was maintained by means of a padeated to 34 ◦C, i.e. to the normal temperature in the nest. The experiments werepproved by the Animal Care and Use Committee of the Institute of Physiologyo be in agreement with the Animal Protection Law of the Czech Republic (fullyompatible with European Community Council directives 86/609/EEC).

.1. Experiment 1—spike-and-wave activity induced byentetrazol (rhythmic metrazol activity, RMA)

Only 18- and 25-day-old rats were used in this experiment because episodesf spike-and-wave rhythm cannot be generated in younger animals [37]. Four

ecording flat silver electrodes were implanted epidurally (over right and leftrontal, i.e. sensorimotor, and occipital, i.e. visual, cortical areas) together with aeference electrode localized in the nasal bone. Animals were pre-treated eitherith baclofen (Sigma, St. Louis, MO) in doses of 1, 3, or 6 mg/kg intraperi-

oneally (i.p.) or with CGP 35348 (a gift of Novartis, Basel) in doses of 50, 100,

d

ds

rch Bulletin 71 (2006) 160–166 161

r 200 mg/kg i.p. Thirty minutes later fresh water solution of pentetrazol (PTZ,igma, St. Louis, MO) was injected i.p. in a dose of 40 mg/kg. Control animalseceived only PTZ in the same dose. Volume of injection was always 1 ml/kg.ach age and dose group consisted of 9–13 animals. EEG activity was recorded

n both reference and bipolar connections for 30 min after PTZ administration.ehavior of animals was marked directly into the EEG recording. Incidence of

pike-and-wave episodes (rhythmic metrazol activity, RMA), latency to the firstpisode of RMA, and to the first generalized RMA (i.e. recorded in all four corti-al areas) were measured. In addition the number and duration of RMA episodesas counted between the 10th and 15th minute after PTZ administration. Total

s well as average duration of RMA activity during this period was calculated.ll other abnormal EEG and behavioral phenomena (e.g. convulsions) were also

egistered. Animals exhibiting motor seizures after the 40-mg/kg dose of PTZere not included into the quantification of spike-and-wave episodes.

.2. Experiment 2—cortical epileptic after discharges (ADs)

All three age groups – 12-, 18-, and 25-day-old rat pups – were used in thisxperiment. Surgical preparation was the same as in Experiment 1 with the onlyxception that right frontal electrode was replaced by two stimulation electrodesocalized at coordinates AP = −1 and +1, L = 2 mm. Fifteen-second series ofiphasic rectangular pulses (duration 1 ms, frequency 8 Hz) were generated by aonstant current stimulator. Intensity of stimulation was set individually to elicitn after discharge lasting at least 3 s (it varied from 2.5 to 5 mA). Four stimulationeries were repeated with an interval of 20 min between the end of ADs and theeginning of the next stimulation. Ten minutes after the end of the first AD eitheraclofen or CGP 35348 was injected intraperitoneally. Baclofen was freshlyissolved in physiological saline and administered in doses of 3, 6, or 12 mg/kg.p. CGP 35348 was dissolved in water and either 100- or 200-mg/kg dose wasiven i.p. Volume of injection was again 1 ml/kg; control animals were injectedith this volume of physiological saline. Each age and dose group was formedy 8–10 animals. EEG was recorded approximately 30 s before the stimulation,uring the stimulation and ADs and at least 30 s after the end of ADs. Motorehavior of rats was again marked into the EEG recording. Incidence, patternnd duration of ADs was evaluated, movements accompanying stimulation andlonic seizures accompanying ADs were quantified according to the Racine’scale [36], where only point 1 was changed so that it included all activities notynchronous with stimulation or sharp EEG graphoelements [29].

.3. Statistical evaluation

The incidence of RMA and seizures in experiment 1 were evaluated usingisher’s exact test. Other data were statistically evaluated using one-wayNOVA (RM ANOVA in the case of ADs) with subsequent comparison accord-

ng to Dunnett’s test (SigmaStat® SPSS). Statistical evaluation of ADs durationas performed with absolute values, relative values (duration of the first AD

aken as 100%) were used only in figures. The level of statistical significanceas set at 5%.

. Results

.1. Experiment 1—spike-and-wave activity induced byentetrazol (rhythmic metrazol activity, RMA)

Under control conditions, the 40-mg/kg dose of PTZ inducedpike-and-wave episodes (Fig. 1) in all animals in both ageroups. In addition, two 18-day-old and one 25-day-old ratxhibited clonic motor seizures of head and forelimbs corre-ponding to the pattern of minimal metrazol seizures (Table 1).hese convulsive seizures can be regularly elicited by higher

oses of PTZ [37].

CGP 35348 decreased the incidence of RMA episodes in aose-dependent manner in both age groups (Table 1). At theame time, it increased significantly the incidence of clonic

162 P. Mares, R. Slamberova / Brain Research Bulletin 71 (2006) 160–166

F rationa ys ine

saaC

ain2wit6cdwggrmia

2(

w

TI

R

C

orciiEn1dttebr

CGP 35348 did not block the prolongation of ADs in anyage group. On the contrary, there was a significant increase induration of ADs after both doses of the drug in 12- and 18-day-

ig. 1. Rhythmic metrazol activity in a 25-day-old rat 12 min after the administnd left (LF) frontal and right (RO) and left (LO) occipital cortical areas alwalectrode. Time mark 2 s, amplitude calibration 0.5 mV.

eizures, again in a dependence on the dose (Table 1). Due tolow number of spike-and-wave episodes and an early appear-

nce of seizures it was impossible to quantify RMA episodes inGP 35348-pre-treated rat pups.

The incidence of RMA in baclofen-pre-treated 25-day-oldnimals was as high as in control rats (Table 1). An increas-ng number of rats exhibited long episodes of RMA formingearly continuous spike-and-wave rhythm, especially after the0th minute. An activating effect of baclofen in this age groupas demonstrated also by a decrease of latencies and an increase

n number and duration of RMA episodes (Figs. 2 and 3). Thehree doses of baclofen prevented the appearance of RMA in 2,and 6 out of 10 18-day-old rats, respectively (Table 1). Laten-

ies of the first RMA episodes were increased by the 3-mg/kgose only, the number, total and average duration of episodeere decreased by baclofen in rats exhibiting RMA in this ageroup (Figs. 2 and 3). A tendency to longer latencies of the firsteneralized RMA in comparison with the very first RMA nevereached the level of statistical significance. The incidence ofotor seizures remained unchanged after all doses of baclofen,

.e. clonic seizures were observed only exceptionally in eitherge group (Table 1).

.2. Experiment 2—cortical epileptic after discharges

ADs)

First stimulation elicited ADs in all animals. These ADsere formed by spike-and-wave rhythm in 18- and 25-day-

able 1ncidence of rhythmic episodes and seizures

CGP 35348 Baclofen

18 days 25 days 18 days 25 days

MAControl 10/10 8/8 C 10/10 8/850 mg/kg 4/10 4/9 1 mg/kg 8/10 9/11100 mg/kg 1/10 3/10 3 mg/kg 4/10 10/10200 mg/kg 0/13 1/13 6 mg/kg 4/10 10/10

onvulsionsControl 2/10 1/8 C 2/10 1/850 mg/kg 3/10 3/8 1 mg/kg 3/10 1/11100 mg/kg 8/10 5/10 3 mg/kg 2/10 0/10200 mg/kg 11/13 11/13 6 mg/kg 1/10 2/10

Faagp

of pentetrazol (40 mg/kg i.p.). Individual leads from top to bottom: right (RF)a reference connection. An upward deflection means negativity at the cortical

ld rats (Fig. 4), whereas 12-day-old rat pups exhibited onlyhythmic sharp delta waves. Stimulation was accompanied bylonic movements of head and/or forelimbs synchronous withndividual stimuli, the same behavioral correlate was seen dur-ng ADs (jerks were synchronous with sharp elements in theEG). Duration of the first AD in 12-day-old rat pups was sig-ificantly longer than in the two older groups (11.8 ± 1.7 s in2-day-old rats versus 6.8 ± 0.6 and 7.1 ± 0.5 s in 18- and 25-ay-old rats, respectively; p < 0.001). Repeated stimulations ledo a progressive increase in duration of ADs in all age groups buthis prolongation reached the level of statistical significance onlyxceptionally (Fig. 5). Intensity of movements elicited directlyy cortical stimulation as well as intensity of clonic seizureselated to ADs did not change (data not shown).

ig. 2. Latencies (mean + S.E.M.) to the first RMA (A) and to the first gener-lized RMA (B) under the influence of baclofen in 18-day-old rats (left parts)nd 25-day-old rats (right parts). Abscissae: doses of baclofen, C means controlroup; ordinates: latency in seconds. Statistical significant difference in com-arison with control group is marked by an asterisks.

P. Mares, R. Slamberova / Brain Research Bulletin 71 (2006) 160–166 163

FbA

obsnesd

Fta

Fig. 5. Influence of CGP 35348 on relative duration of after discharges (ADs,mean + S.E.M.). From top to bottom: animals 12, 18, and 25 days old. Abscissae:control and two drug groups, always the first to fourth AD (see inset in theright upper corner); drug or physiological saline was injected after the firstAta

g

ig. 3. Number of RMA episodes (A), their total (B) and average duration (C)etween the 10th and 15th minute after pentetrazol administration. Ordinates:, number of episodes; B and C, duration in seconds. Other details as in Fig. 2.

ld rats (Fig. 5). The results in 25-day-old animals stayed justelow the level of statistical significance (p = 0.0573). The inten-ity of movements accompanying both stimulation and ADs did

ot change under the influence of CGP 35348 with only onexception—an increased intensity of movements accompanyingtimulation during the fourth stimulation series after the higherose of CGP 35348 in 12-day-old rats (data not shown).

ig. 4. Cortical after discharge in an 18-day-old rat. Individual leads from topo bottom: left frontal (LF), right occipital (RO), and left occipital (LO) corticalrea in a reference connections. Time mark 1 s, amplitude calibration 1 mV.

6edAdpwshcf

3

sab

D. Ordinates: relative duration of ADs, the average of the first AD is alwaysaken as 100%. Asterisks denote significant differences in comparison with theppropriate first AD.

All three doses of baclofen blocked the progressive prolon-ation of ADs in all age groups with the exception of the 3- and-mg/kg doses in 18-day-old rats. In addition, there was a short-ning of the duration of all post-drug ADs after the two higheroses of baclofen in 25-day-old rats and of the third and fourthDs after the highest dose in 12-day-old animals (Fig. 6). Alloses of baclofen suppressed the intensity of movements accom-anying stimulation in 18- and 25-day-old groups; no changesere observed in 12-day-old rats (Fig. 7). Intensity of clonic

eizures related to ADs decreased significantly after the twoigher doses of baclofen in 25-day-old animals. No significanthanges in the intensity of seizures accompanying ADs wereound in any other age and dose group (data not shown).

. Discussion

Our results are in good agreement with data on anticonvul-ant action of GABAB antagonists against absence models indult rats and mice [15,31] as well as on proconvulsant action ofaclofen in these models [49]. Proconvulsant action of GABAB

164 P. Mares, R. Slamberova / Brain Research Bulletin 71 (2006) 160–166

Fi

adsrwAogaomttpcspddissioi

Fig. 7. Intensity of movements directly induced by cortical stimulation undertt

atsr

ccbcads[ctAtcm

ig. 6. Influence of baclofen on relative duration of after discharges. Details asn Fig.5.

ntagonists in “in vivo” models of other seizure types wasescribed at first for motor seizures [17,48], confirmation in EEGtudies was published only recently [8,20]. Spike-and-wavehythm is generally accepted as a thalamo-cortical phenomenonhere GABAB transmission plays an important role [1,21,38].ll the experiments in inbred strains of rodents were focusednly on spike-and-wave rhythm as a part of models of primaryeneralized absence seizures. In addition to these models, spike-nd-wave rhythm can be also recorded as an EEG counterpartf other types of seizures in experimental animals—minimaletrazol seizures and cortical epileptic after discharges charac-

erized by marked clonic convulsions. These two models can beaken as an analogue of human myoclonic seizures [22,30]. Ourresent finding that GABAB antagonist CGP 35348 aggravatesortical epileptic after discharges is in agreement with conclu-ions made by Marescaux’ group that GABAB antagonists areroconvulsant in “convulsive” seizures [48]. In addition, theose of pentetrazol used in our experiments was rather high asemonstrated by a sporadic appearance of minimal seizures evenn control groups. Due to such nearly threshold dose for cloniceizures the animals were prone to generation of convulsive

eizures and even moderate proconvulsant effect can result in anncreased seizure incidence. A moderate anticonvulsant effectf baclofen in the model of cortical epileptic after discharges isn agreement with our data on a (more marked) anticonvulsant

atph

he influence of baclofen. Details as in Fig. 5, only ordinates: score accordingo the five-point scale (see Section 1).

ction of drugs acting at GABAA receptor [19,28,33]. A quan-itative difference speaks in favor of primary effect of GABAAystem in the action of drugs acting at both types of GABAeceptors [3,33].

On the other hand, our “mirror” effects in the two modelsharacterized by spike-and-wave rhythm in the EEG (and espe-ially prolongation of spike-and-wave after discharges inducedy cortical stimulation after GABAB antagonist administration)ast doubts on the single mechanism of generation of spike-nd-wave rhythm. We have found differences in effects of otherrugs on EEG in these two models in adult rats—e.g. etho-uximide suppressed RMA leaving unchanged cortical ADs30]. Possible explanation of the difference of pharmacologi-al sensitivity could be in a possibility that the common finalhalamocortical path can be triggered from different structures.nticonvulsant drugs might influence these triggering struc-

ures rather than thalamocortical generator. Triggering structuresould either activate basal forebrain structures as a generator ofinimal, clonic seizures [7] or block the cortical input from

scending reticular formation. The spike-and-wave rhythm in

he two models exhibits some differences: frequency of com-lexes in the episodes of rhythmic metrazol activity is stable andigher (4–5 Hz) than in cortical after discharges (around 3 Hz at

Resea

t[Ri1ttncctpmr[[araaS[Ptaoi

wSadtsasddpdsmGidtecrras[wspt

d[wa

pocupioet[p

A

G5

R

[

P. Mares, R. Slamberova / Brain

he beginning and 2 Hz at the end of AD in adult Wistar rats45] as well as in 18- and 25-day-old ones in the present study);MA is with the exception of a few very first episodes general-

zed but ADs can exhibit regional differences especially in the2-day-old animals where activity restricted to one or two cor-ical regions is rather a rule. An alternative explanation must beaken into account: the origin of spike-and-wave rhythm mustot be identical in the two cases. It is possible to quote Mar-us [26] who succeeded to induce spike-and-wave rhythm inats and monkeys by formation of symmetrical neocortical foci;his rhythm was elicited even if only callosal connections werereserved in undercut cortex, i.e. without a participation of thala-us. Detailed analysis of spike-and-wave episodes in WAG/Rij

ats demonstrated that these episodes start in the cerebral cortex32]. It is in agreement with results of Timofeev and Steriade44] in cats. Studies on the suppression of genetically determinedbsence seizures by ethosuximide also indicated an importantole of primary somatosensory cortex [25]. It must be taken intoccount that the same thalamocortical system is able to gener-te oscillations at different frequencies as was demonstrated byteriade for sleep spindles and epileptic spike-and-wave rhythm41,42]. To make the explanation even more difficult Staak andape [40] concluded that GABA-A and GABA-B inhibitory sys-

ems play a different role in thalamocortical rhythmic activitiesccording to the frequencies. Analysis of possible different rolef the two GABAergic systems will be the next step in our stud-es.

The opposite effects of the GABAB agonist and antagonistere observed in both seizure models only in 25-day-old rats.urprisingly, baclofen exhibited an anticonvulsant action againstn absence model in 18-day-old animals (a decrease in the inci-ence, number and duration of RMA episodes). In contrast tohis finding baclofen did not increase the incidence of motoreizures like CGP 35348. Developmental changes of baclofenction were described in other models (e.g. flurothyl-inducedeizures—[47]; failure of baclofen action against cortical afterischarges in 18-day-old rats in the present experiment). Thisevelopmental irregularity cannot yet be explained, only someossibilities might be outlined. Turgeon and Albin [46] in theirevelopmental study on GABAB receptors are discussing pos-ible changes of baclofen binding during post-natal develop-ent of rats. An uneven development of pre- and post-synapticABAB receptors [13] and at least quantitative differences

n pharmacological sensitivity of these receptors [11] wereescribed in the cerebral cortex. Pre-synaptic GABAB recep-ors suppress release of both glutamate and GABA [16] and theffects on excitatory and inhibitory pre-synaptic terminals couldhange during ontogeny. The representation of two GABAB-eceptor splice variants exhbit opposite developmental trends inat cerebral cortex—GB1a decreases and GB1b increases withge [12]. Laminar distribution of GABAB receptor protein in theomatosensory cortex also changes during early development34], affinity of both GB1a and GB1b for baclofen increases

ith age [24]. Developmental study of baclofen binding demon-

trated that it is highest at post-natal day 12 in thalamus, later (atost-natal day 25) in cerebral cortex; on the other hand, nega-ive modulation of adenylyl cyclase activity by baclofen could be

[

[

rch Bulletin 71 (2006) 160–166 165

emonstrated since post-natal day 12 in both cortex and thalamus14]. Developmental changes of coupling of GABAB receptorsith G proteins cannot be excluded but no systematic study is

t disposal.Our finding that the effects of baclofen on movements accom-

anying stimulation (present study) as well as on the tonic phasef generalized tonic–clonic seizures [18] did not qualitativelyhange during the post-natal development in contrast to fail-re of this drug in 18-day-old rats in both models studied in theresent experiments suggests different sites of action of baclofenn these situations. The action of baclofen on direct activationf the motor system might be exerted at another level than anffect against spike-and-wave episodes because GABAB recep-ors are present in many brain areas and also in the spinal cord6,9]. Only further studies may solve the question which of theseossibilities (if any) is the right one.

cknowledgements

This study was supported by grant no. 305/05/2581 of therant Agency of the Czech Republic and by a project AV0Z no.011922.

eferences

[1] G. Avanzini, M. de Curtis, C. Marescaux, F. Panzica, R. Spreafico, M.Vergnes, Role of the thalamic reticular nucleus in the generation of rhythmicthalamo-cortical activities subserving spike and waves, J. Neural Transm.Suppl. 35 (1992) 85–95.

[2] S.F. Berkovic, Childhood absence epilepsy and juvenile absence epilepsy,in: E. Wyllie (Ed.), The Treatment of Epilepsy: Principles and Practice,Lea & Fibiger, Philadelphia, 1993, pp. 547–551.

[3] K. Bernaskova, R. Slamberova, P. Mares, GABA uptake blocker NNC 711exhibits marked anticonvulsant action in two cortical epileptic models inimmature rats, Epilepsia 40 (1999) 1184–1189.

[4] H. Bittiger, W. Froestl, S.J. Mickel, H.-R. Olpe, GABAB receptor antago-nists: from synthesis to therapeutic application, Trends Pharmacol. Sci. 14(1993) 391–394.

[5] N.G. Bowery, B. Bettler, W. Froestl, J.P. Gallagher, F. Marshall, M. Rai-teri, T.I. Bonner, S.J. Enna, International Union of Pharmacology XXXIII.Mammalian �-aminobutyric acidB receptors: structure and function, Phar-macol. Rev. 54 (2002) 217–264.

[6] N.G. Bowery, A.L. Hudson, G.W. Price, GABAA and GABAB receptorsite distribution in the rat central nervous system, Neuroscience 20 (1987)365–383.

[7] R.A. Browning, D.K. Nelson, Modification of electroshock andpentylenetetrazol seizure patterns in rats after precollicular transections,Exp. Neurol. 93 (1986) 546–556.

[8] M.A.M. Carai, G. Brunetti, C. Lobina, S. Serra, G. Vacca, G. Minardi,G. Colombo, G.L. Gessa, Proconvulsive effect of the GABAB receptorantagonist SCH 50911 in rats undergoing ethanol withdrawal syndrome,Eur. J. Pharmacol. 445 (2002) 195–199.

[9] D.C.M. Chu, R.L. Albin, A.B. Young, J.B. Penney, Distribution and kinet-ics of GABAB binding sites in rat central nervous system: a quantitativeautoradiographic study, Neuroscience 34 (1990) 341–357.

10] V. Crunelli, N. Leresche, A role for GABAB receptors in excitationand inhibition of thalamocortical cells, Trends Neurosci. 14 (1991)16–21.

11] R.A. Deisz, J.M. Billard, W. Ziegelgansberger, Presynaptic and postsynap-tic GABA receptors of neocortical neurons of the rat in vitro: differencesin pharmacology and ionic mechanisms, Synapse 25 (1997) 62–72.

12] J.-M. Fritschy, V. Meskenaite, O. Weinmann, M. Honer, D. Benke, H.Mohler, GABAB-receptor splice variants GB1a and GB1b in rat brain:

1 Resea

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

66 P. Mares, R. Slamberova / Brain

developmental regulation, cellular distribution and extrasynaptic localiza-tion, Eur. J. Neurosci. 11 (1999) 761–768.

13] A. Fukuda, I. Mody, D.A. Prince, Differential ontogenesis of presynapticand postsynaptic GABAB inhibition in rat somatosensory cortex, J. Neu-rophysiol. 70 (1993) 448–452.

14] L. Hejnova, I. Ihnatovych, J. Novotny, H. Kubova, P. Mares, P. Svoboda,Modulation of adenylyl cyclase activity by baclofen in the developing ratbrain: difference between cortex, thalamus and hippocampus, Neurosci.Lett. 330 (2002) 9–12.

15] D.A. Hosford, Y. Wang, C.C. Liu, O.C. Snead, Characterization of theantiabsence effects of SCH-50911, a GABAB receptor antagonist, in thelethargic mouse, �-hydroxybutyrate, and pentylenetetrazole models, J.Pharmacol. Exp. Ther. 274 (1995) 1399–1403.

16] J.R. Howe, B. Sutor, W. Ziegelgansberger, Baclofen reduces postsynapticpotentials of rat cortical neurones by an action other than its hyperpolarizingaction, J. Physiol. (London) 384 (1987) 539–569.

17] G. Karlsson, C. Kolb, A. Hausdorf, C. Portet, M. Schmutz, H.-R. Olpe,GABAB receptors in various in vitro and in vivo models of epilepsy: astudy with the GABAB receptor blocker CGP 35348, Neuroscience 47(1992) 63–68.

18] H. Kubova, R. Haugvicova, P. Mares, Moderate anticonvulsant action ofbaclofen does not change during development, Biol. Neonate 69 (1996)405–412.

19] H. Kubova, P. Mares, J. Vorlıcek, Stable anticonvulsant action of benzo-diazepines during development in rats, J. Pharm. Pharmacol. 45 (1993)807–810.

20] L.S. Leung, K.J. Canning, B. Shen, Hippocampal afterdischarges afterGABAB–receptor blockade in the freely moving rat, Epilepsia 46 (2005)203–216.

21] Z. Liu, M. Vergnes, A. Depaulis, C. Marescaux, Evidence for a critical roleof GABAergic transmission within the thalamus in the genesis and controlof absence seizures in the rat., Brain Res. 545 (1991) 1–7.

22] W. Loscher, D. Schmidt, Which animal models should be used in the searchfor new antiepileptic drugs? A proposal based on experimental and clinicalconsiderations, Epilepsy Res. 2 (1988) 145–181.

23] R.L. Macdonald, Cellular actions of antiepileptic drugs, in: M.J. Eadie,F.J.E. Vajda (Eds.), Antiepileptic Drugs. Pharmacoloy and Terapeutics,Springer Verlag, Berlin, 1999, pp. 123–150.

24] B. Malitschek, D. Ruegg, J. Held, K. Kaupmann, H. Bittiger, W. Frostl, B.Bettler, R. Kuhn, Developmental changes of agonist affinity at GABA R1receptor variants in rat brain, Mol. Cell Neurosci. 12 (1998) 56–64.

25] J.P. Manning, D.A. Richards, N. Leresche, V. Crunelli, N.G. Bowery,Cortical-area specific block of genetically determined absence seizures byethosuximide, Neuroscience 123 (2004) 5–9.

26] E.M. Marcus, Experimental models of petit mal epilepsy, in: D.P. Purpura,J.K. Penry, D. Tower, D.M. Woodbury, R. Walter (Eds.), ExperimentalModels of Epilepsy, Raven Press, New York, 1972, pp. 113–146.

27] P. Mares, Ontogeny of ethosuximide action against two seizure models inrats is different, Life Sci. (Pharmacol. Lett.) 63 (1998), PL 51–57.

28] P. Mares, Anticonvulsant action of three neurosteroids against cortical

epileptic afterdischarges in immature rats, Brain Res. Bull. 68 (2005)179–184.

29] P. Mares, R. Haugvicova, H. Kubova, Unequal development of thresholdsfor various phenomena induced by cortical stimulation in rats, EpilepsyRes. 49 (2002) 35–43.

[

rch Bulletin 71 (2006) 160–166

30] P. Mares, A. Zouhar, Do we possess adequate models of childhood epilep-sies? Physiol. Bohemoslov. 37 (1988) 1–9.

31] C. Marescaux, M. Vergnes, R. Bernasconi, GABAB receptor antagonists:potential new anti-absence drugs, J. Neural Transm. Suppl. 35 (1992)179–188.

32] H.K. Meeren, J.P. Pijn, E.L. Van Luijtelaar, A.M. Coenen, F.H. Lopes daSilva, Cortical focus drives widespread corticothalamic networks duringspontaneous absence seizures in rats, J. Neurosci. 22 (2002) 1480–1495.

33] R. Polasek, H. Kubova, R. Slamberova, P. Mares, J. Vorlıcek, Suppressionof cortical epileptic afterdischarges in developing rats by anticonvulsantsincreasing GABAergic inhibition, Epilepsy Res. 25 (1996) 177–184.

34] A. Princivalle, M.C. Regondi, C. Frassoni, N.G. Bowery, R. Spreafico, Dis-tribution of GABAB receptor protein in somatosensory cortex and thalamusof adult rats and during postnatal development, Brain Res. Bull. 5 (2000)397–405.

35] A. Puigcerver, E.L.J.M. vanLuijtelaar, W.H.I.M. Drinkenburg, A.L.M.Coenen, Effects of the GABA(B) antagonist CGP 35348 on sleep–wakestates, behaviour, and spike-wave discharges in old rats, Brain Res. Bull.40 (1996) 157–162.

36] R.J. Racine, Modification of seizure activity by electrical stimulation. II.Motor seizures, Electroenceph. Clin. Neurophysiol. 32 (1972) 281–294.

37] R. Schickerova, P. Mares, S. Trojan, Correlation between electrocortico-graphic and motor phenomena induced by metrazol during ontogenesis inrats, Exp. Neurol. 84 (1984) 153–164.

38] O.C. Snead III, Evidence for GABAB mediated mechanisms in experimen-tal generalized absence seizures, Eur. J. Pharmacol. 213 (1992) 343–349.

39] O.C. Snead III, Pharmacological models of generalized absence seizuresin rodents, J. Neural Transm. Suppl. 35 (1992) 7–19.

40] R. Staak, H.-C. Pape, Contribution of GABAA and GABAB receptors tothalamic neuronal activity during spontaneous absence seizures in rats, J.Neurosci. 21 (2001) 1378–1384.

41] M. Steriade, Sleep, epilepsy and thalamic reticular inhibitory neurons,Trends Neurosci. 28 (2005) 317–324.

42] M. Steriade, M. Deschenes, The thalamus as a neuronal oscillator, BrainRes. Rev. 8 (1984) 1–62.

43] B. Sutor, H.J. Luhmann, Involvement of GABAB receptors in convulsant-induced epileptiform activity in rat neocortex in vitro, Eur. J. Neurosci. 10(1998) 3417–3427.

44] I. Timofeev, M. Steriade, Neocortical seizures: initiation, development andcessation, Neuroscience 123 (2004) 299–336.

45] E. Tolmacheva, G. van Luijtelaar, S.A. Chepurnov, Yu. Kaminskij, P. Mares,Cortical and limbic excitability in rats with absence epilepsy, Epilepsy Res.62 (2004) 189–198.

46] S.M. Turgeon, R.L. Albin, Postnatal ontogeny of GABAB binding in ratbrain, Neuroscience 62 (1994) 601–613.

47] L. Velısek, J. Velıskova, Z. Ptachewich, J. Ortız, S. Shinnar, S.L. Moshe,Age-dependent effects of GABA agents on flurothyl seizures, Epilepsia 36(1995) 636–643.

48] M. Vergnes, A. Boehrer, S. Simler, R. Bernasconi, C. Marescaux, Oppo-site effects of GABAB receptor antagonists on absences and convulsive

seizures, Eur. J. Pharmacol. 332 (1997) 245–255.

49] M. Vergnes, C. Marescaux, G. Micheletti, A. Depaulis, L. Rumbach, J.M.Warter, Enhancement of spike-and-wave discharges by GABA-mimeticdrugs in rats with spontaneous petit mal-like epilepsy, Neurosci. Lett. 44(1984) 91–94.