1-BP inhibits NF-kappaB activity and Bcl-xL expression in astrocytes in vitro and reduces Bcl-xL expression in the brains of rats in vivo.

1-Bromopropane (1-BP) has been widely used as a substitute for chlorofluorocarbon that destroys the ozone layer. Although the central neurotoxicity of 1-BP has been recently reported, a molecular mechanism is not clear. In particular, the effects on cells in brain have not been fully analyzed. Here, we studied the effects of 1-BP on the activation of transcription factors involved in anti-apoptotic function or cell survival in astrocytes. Astrocytoma cell lines, U251, U373 and VM, or murine primary astrocytes were used for in vitro assay. DNA binding activities of NF-kappaB in these cells induced by interleukin (IL)-1 or LPS were inhibited by 1-BP. Consequently, the treatment of U251 cells with 1-BP resulted in suppression of NF-kappaB reporter activity. Furthermore, 1-BP blocked IkappaBalpha degradation, which is important for NF-kappaB activation. In addition, the level of Bcl-xL mRNA, which is known as an anti-apoptotic gene, were reduced in U251 treated with 1-BP or in the brain from rat exposed to 1-BP (400 ppm, 12 weeks). These results suggest that subchronic inhalation exposure to 1-BP vapor may affect the Bcl-xL expression in astrocytes.

Antiepileptic action induced by a combination of vigabatrin and tiagabine.

Vigabatrin, an inhibitor of GABA breakdown by GABA transaminase and of GABA transporter isoform 1 (GAT1), and tiagabine, a highly specific inhibitor of GAT1, have successfully been applied in the treatment of epilepsy. We investigated the effects of individual and combined application of these drugs on GAT1 expressed in Xenopus oocytes, and examined the effects on epileptiform discharges in the CA3 area of brain slices of genetically epileptic El and control ddY mice, and on the occurrence of seizures in El mice. Simultaneous application of vigabatrin and tiagabine inhibited epileptiform discharges induced by high-K+ solution in the brain slices in an antagonistic fashion. The degree of inhibition by tiagabine after pre-treatment with vigabatrin was additive in ddY mice and synergistic in El mice. In Mg2+-free solution, co-treatment by the two drugs produced additive inhibition in slices from both mouse strains, but pre-treatment with vigabatrin produced synergistic inhibition in slices only from ddY mice. In the slices from El mice, a combination of drugs resulted in additive effects in both co- and pre-treatment by the drugs. Although these drugs are also effective in vivo at suppressing seizure occurrence in El mice, the combined application does not show synergistic effects, but rather is antagonistic under the experimental conditions in this particular variant of epilepsy. The synergistic inhibition of epileptiform discharges in brain slices may, in part, have originated from the complex interaction with GAT1. In experiments on the GAT1 expressed in oocytes it could be demonstrated that synergistic inhibition occurs only at low concentration (0.1 nM) of vigabatrin. This illustrates that the oocytes may form a powerful test system for drug screening and investigation of complex drug interactions. These results present a novel interpretation of synergistic inhibition of certain epileptic discharges using vigabatrin and another drug, and that for successful synergistic treatment of epilepsies carefully designed timed dosage regimens are essential.

Glucocorticoid hypersecretion following intracerebroventricular injection of ethylcholine mustard aziridinium ion in rats.

To investigate whether cholinergic hypofunctions in the brain influence hypothalamic-pituitary-adrenal activity, we examined the effects of cholinergic neurotoxin ethylcholine mustard aziridinium ion on basal and stress-induced levels of corticosterone in rats. Blood sampling from rats following intracerebroventricular injection of saline (5 microl, as a control) or this neurotoxin (5 nmol/5 microl) was performed over a day in one series, and was taken before, during and after an immobilization stress exposure in another series. Plasma levels of corticosterone and adrenocorticotropin were determined by the radioimmunoassay. The basal levels of plasma corticosterone and adrenocorticotropin over a day were significantly higher in the neurotoxin-treated rats, compared with the control rats (corticosterone, P<0.001; adrenocorticotropin, P<0.05). Further, relative adrenal gland weight of the neurotoxin-treated rats was significantly greater than that of the control rats (P<0.05). However, responses in plasma corticosterone level caused by the immobilization stress in the neurotoxin-treated rats were not different from those in the control rats. The present study demonstrated that damage to the cholinergic neurons in the brain increased hypothalamic-pituitary-adrenal activity over a day, probably due to freedom from inhibitory influences of the hippocampal cholinergic system, but that this damage did not influence stress-induced changes in plasma glucocorticoid level.

Electrophysiology and immunohistochemistry in the hippocampal ca1 and the dentate gyrus of rats chronically exposed to 1-bromopropane, a substitute for specific chlorofluorocarbons.

1-Bromopropane is a newly introduced substitute for specific chlorofluorocarbons whose production was prohibited because of depletion of ozone layers. In this study, we analyzed disinhibitory effects induced by repetitive inhalation of 1-bromopropane for 12 weeks in the hippocampal CA1 and the dentate gyrus. In addition, reversal of the disinhibitory effects was examined 4 weeks after 1-bromopropane inhalation ceased. Exposure rats were placed in a stainless steel inhalation chamber at a concentration of 700 ppm, while the control group was provided only room air in the same type of chamber. Paired-pulse inhibition of population spike was considerably decreased (P<0.05) at 5 ms interpulse intervals in the CA1, and at 10 and 20 ms (P<0.05) interpulse intervals in the dentate gyrus in slices obtained from exposed rats following 4-, 8- and 12-week inhalation periods. The paired-pulse inhibition was decreased at 5 ms interpulse intervals in the dentate gyrus after 12 weeks of inhalation. These changes were not associated with the paired-pulse ratio of field excitatory postsynaptic potentials, suggesting a reduction of recurrent inhibition. The disinhibition was counteracted with the N-methyl-d-aspartate receptor antagonist dl-2-amino-5-phosphonopentameric acid in the dentate gyrus, whereas it was unchanged in the CA1. Tiagabine, a selective inhibitor of GABA transporter GAT1, increased the paired-pulse inhibition in the dentate gyrus, and the increase was less in the exposed rats compared with control rats (P<0.0003). The changes in both areas recovered to control levels 4 weeks after cessation of inhalation. Our electrophysiological studies suggest differential and reversible disinhibitory effects in the dentate gyrus and the CA1. 1-Bromopropane-induced disinhibition was further analyzed by immunohistochemical methods. There were no apparent morphological defects in either excitatory or inhibitory neuronal components, supporting the reversibility of physiological changes. In conclusion, chronic inhalation of 1-bromopropane induces a disinhibition in the CA1 and dentate gyrus that is reversible following cessation of exposure.

Down-regulation of GABA-transporter function by hippocampal translation products: its possible role in epilepsy.

The genetically epileptic mouse strain (El) is used as a model for human temporal lobe epilepsy. To address the question of whether altered function of the neuronal GABA transporter GAT1 is involved in the pathology of epilepsy of El mice, we expressed in Xenopus oocytes cloned GAT1 of mouse brain by injection of complementary ribonucleic acid (cRNA) and co-injected messenger ribonucleic acid (mRNA) isolated from the hippocampus of non-epileptic control mother strain (ddY) mice and from El mice. GABA transporter activity was investigated by measurements of [(3)H]-GABA uptake as well as by steady-state and transient current measurements under voltage clamp.Co-injection of hippocampal mRNA into oocytes reduced GAT1-mediated transport. This effect was more pronounced for mRNA from ddY mice than for that from El mice that never experienced seizures, El(-), and being absent for mRNA from El mice that have had high seizure experience, El(+). The pronounced inhibition of GABA transport after injection of mRNA from the ddY strain results from reduced expression of functional GAT1, but to about one third also from a reduced GABA translocation rate. The reduced translocation can be attributed to a reduced forward rate of a step associated with extracellular Na(+) binding. If the results can be applied to the mouse brain, we may hypothesise that in ddY mice some GAT down-regulating factor translated from hippocampal mRNA may be involved to keep GAT1 activity low, and hence GABA concentration in synaptic cleft high. In El(-) mice such regulatory mechanism may be reduced or counteracted by another unknown factor present in El(-) brain. The repeated seizure experience in El(+) mice enhances this compensatory effect.

Influences of cholinergic neurotoxin ethylcholine aziridinium ion on circadian rhythms in rats.

To investigate whether damages of cholinergic neurons in the brain produce aging-like changes in circadian rhythms, we examined the influences of intracerebroventricular injection of cholinergic neurotoxin ethylcholine aziridinium ion (AF64A, 5 nmol/5 microl) on circadian rhythms in rats, by measuring locomotor activity and body temperature with the automatic behavioral measurement system combined with the telemetry. Daily rhythms in locomotor activity and body temperature were observed in AF64A-treated rats under a 12:12 h light:dark (LD) cycle, however, in AF64A-treated rats, the amplitude of activity and temperature rhythms was significantly decreased, the phase of the both rhythms was advanced and the amount of activity was decreased, compared with control rats. Locomotor activity and body temperature also showed a circadian rhythm in AF64A-treated rats under the constant dark condition with the period similar to that in the control rats. The present findings are in accordance with the observation in aged animals in which cholinergic hypofunction are often observed, suggesting that hypofunctions of the cholinergic systems in the brain might be involved in aging-like changes in the circadian rhythms.

Large frequency potentiation induced by 2 Hz stimulation in the hippocampus of epileptic El mice.

El mouse has been found to be characteristics with hippocampal disinhibition, and has been suggested decrease in GABAergic synaptic transmission [Ono et al., Brain Res. 745 (1997) 165-172; Fueta et al. , Brain Res. 779 (1998) 324-328]. The efficacy of GABAergic synapses can be modulated in response to trains of low frequency stimulation. The frequency potentiation of a population spike (PS) and the field excitatory postsynaptic potential (fEPSP) induced by a low frequency stimulation (2 Hz for 15 s) were recorded for the CA3 subfield, and PS alone for the CA1 subfield and dentate gyrus. PS frequency potentiation was greater in El mice than in non-epileptic control ddY mice. Especially the CA3 subfield exhibited a high PS frequency potentiation (300+/-73%) compared to age-matched ddY mice (64+/-24%). However, EPSP frequency potentiation was similar in El and ddY mice. The degree of PS frequency potentiation in CA3 was decreased by the reduction of extracellular Ca2+ from 2 to 1 mM in both strains, suggesting presynaptic involvement. The potentiation in El mice was suppressed by AMPA/kainate type receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dion (CNQX), but more than half of the control value remained at 5 microM, whereas the potentiation in ddY mice was abolished at this concentration. N-methyl-d-aspartate (NMDA) type receptor antagonist 3-3 (2-carboxypiperazine-4-yl) propyl-1-phosphonate (10 microM; CPP) did not affect the potentiation. Bicuculline (5 microM), GABAA receptor antagonist, did not increase the amplitude of PS during stimulation but induced epileptic (multiple PSs) potentials. High PS frequency potentiation of El mice was mimicked to the degree of that in ddY mice by a low dose of GABAB receptor agonist baclofen (3 microM). The suppression by baclofen was partially reversed by the antagonist saclofen (500 microM). The large frequency potentiation in young El mice, which do not have seizure-susceptibility, indicates an intrinsic property in El mice. It is suggested that the high synchronization of CA3 neurons in El mice is due to a little activation of GABAB receptor activation and also to enhancement of non-NMDA type synaptic transmission.

Granule cell disinhibition in dentate gyrus of genetically seizure susceptible El mice.

Paired-pulse inhibition was investigated electrophysiologically in the dentate gyrus using hippocampal slices from epileptic El mice. At short interpulse intervals (IPIs), the inhibition was 30% in the El, and 90% in the control ddY mice at the ages of 10 and 15 weeks. No difference in inhibition was observed at the age of 5 weeks. Bicuculline, a GABAA receptor antagonists, attenuated the inhibition during short IPIs n the ddY mice, while in the El mice, phenobarbital and flunitrazepam, which enhance GABAA receptor function, restored the inhibitory activity comparable to that of the ddY. The disinhibition progressed with growth, closely correlating with seizure development in El mice. These results suggest that decrease in the GABAergic inhibition occurs in the dentate gyrus of the El mice with growth. GABA concentration in the hippocampus was also quantified using HPLC. In El mice, GABA level was significantly lower than that in ddY mice at the ages of 5 and 15 weeks. Thus, the disinhibition observed in the El dentate gyrus at 15 weeks of age does not appear to be directly related to the content of GABA. GABAergic disinhibition suggests possible loss of unknown inhibition control factor(s) in the El dentate gyrus as growth progresses. The growth-dependent disinhibition in the granule cells may be prerequisite for epileptogenesis in El mice.

Regional differences in hippocampal excitability manifested by paired-pulse stimulation of genetically epileptic El mice.

Hippocampal excitability in El mice was studied by analyzing paired-pulse responses of population excitatory postsynaptic potentials (EPSPs) and population spikes (PSs). In vitro slice preparations from seizure-susceptible adult (15 weeks old) and non-seizure susceptible young (5 weeks old) El mice were compared with age-matched mother strain ddY mice. In CA1 area, paired-pulse inhibition of PSs was reduced by about 50% at 10 ms interpulse interval (IPI) in both 5 and 15 weeks old El mice when compared to ddY mice. Phenobarbital (200 microM) decreased paired-pulse ratio (PPR) by 30% in El mice, and bicuculline (1 microM) increased PPR by 80% in ddY mice at 10 ms IPI. These results suggest an intrinsic existence of decreased GABA(A) receptor-mediated inhibition in CA1 of El mice. In dentate gyrus (DG), an increase in paired-pulse facilitation of PSs was observed at intermediate IPIs (50-200 ms) in El mice at both ages, especially at 15 weeks of age, when 52%-increased PPR was recorded. The facilitation was not due to GABA(A) receptor-mediated inhibition and was not age-dependent. In CA3 area, increased paired-pulse facilitation of PSs and EPSPs over the range of 10-1000 ms IPIs was observed only in the 15-week-old El mice. The age-dependent appearance of seizure susceptibility was associated with the increase in excitatory synaptic transmission in CA3. Our results show that El mice possess excitatory/inhibitory synaptic transmission abnormalities in the hippocampus that could contribute to seizure predisposition.

Tetraethylammonium-induced epileptiform activity in young and adult rat hippocampus.

Extracellular field potential recordings were used to study the epileptiform activity evoked by tetraethylammonium (TEA) in the CA3 subfield of hippocampal slices obtained from young (12-18 day-old) and adult (> 60-day-old) rats. During TEA application (5-10 mM), young slices generated both ictal-like (duration: up to 28 s, rate of occurrence 1-3 x 10(-2) s-1) and interictal-like (duration: 1.5-2 s; rate of occurrence: 1-3 x 10(-1) s-1) activity. In adult slices only interictal-like activity was induced by TEA (3-10 mM). Depending on the concentrations of TEA, these events lasted 80-600 ms and occurred at 5-60 x 10(-2) s-1. Both the N-methyl-D-aspartate (NMDA) receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (5-10 microM; CPP) and the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5-10 microM; CNQX) were necessary to suppress ictal-likeand interictal-like discharges in young slices. By contrast, interictal-like activity in adult slices was reduced and eventually blocked by CNQX (0.5-3 microM) alone. Furthermore the pattern of epileptiform discharges seen in young slices was modified by CPP (i.e. decrease in the rate of occurrence of ictal events and reduction in the duration of interictal discharges), while the activity recorded in adult slices was resistant to this NMDA antagonist. Bicuculline methiodide (5 microM; BMI) enhanced the duration of epileptiform activities in both young and adult slices. Our data demonstrate that the epileptiform discharges induced by TEA in the CA3 subfield of the rat hippocampus display age-dependent patterns of activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Unusual biochemical development of genetically seizure-susceptible El mice.

Looking for the 'epilepsy gene', we used ddY derived, genetically seizure-susceptible El mice. To find biochemical abnormalities, we examined the amino acid metabolism and gene activity, including poly(A)+ RNA and sodium channel mRNA expressions, in the developmental growth of El mice. At the early postnatal stage, abnormalities in amino acid metabolism were aberrant free amino acid fluctuations. Almost all free amino acids in the liver of newborn El mice showed considerably lower levels than did ddY mice. Among those amino acids, Asp, Glu and Tyr were extremely low, but rapidly recovered to the ddY level within a week. During the successive growth period, we observed no significant difference in hepatic amino acid levels between El and ddY mice. No such drastic changes were noted in the amino acid levels in the brains of ddY and El mice; only the Gly level was greater in El mice than in ddY mice on the day of birth. Rotatory stimulation which evokes convulsions in El mice but not in ddY mice was applied to adult mice and changes in the amino acid level were assessed. The level of Glu and Tyr in seizure-induced El mice was approximately twice that noted in the liver and brain of El mice, which did not experience seizures. It was also somewhat increased in ddY mice subjected to rotational stress which did not induce seizures in that strain. Gene activity that expresses poly(A)+ RNAs, including sodium channel mRNA, was determined by Northern blot analysis, which reveals unscheduled mRNA synthesis by the appearance of an extra band approximately 3 kb in size.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of vesl-1S/homer-1a, a gene associated with long-term potentiation, in the brain of the epileptic EI mouse.

The El mouse is an animal model for hereditary epilepsy. It undergoes a convulsive seizure lasting 10-30 s after vestibular stimulation, such as "tossing-up". To better characterize the effects of seizure activity in the El mouse, we examined the expression profile of vesl-1S/homer-1a mRNA in the brain. It has been shown that this mRNA is expressed after long-term potentiation or seizures induced by electrical or pharmacological stimulations. The basal level of vesl-1S mRNA was very low even in seizure-susceptible El mice. The El mouse hippocampus, but not that of ddY mice, showed a remarkable increase in vesl-1S mRNA expression in the dentate granule cell layer after seizure induced by "tossing-up". The mRNA remained for about 3 h after the seizure and disappeared after 8 h.

Decreased susceptibility to pentylenetetrazol-induced seizures after low-frequency transcranial magnetic stimulation in rats.

We studied the effects of low-frequency repetitive transcranial magnetic stimulation (rTMS) on seizure susceptibility in rats. rTMS of 1000 pulses at 0.5 Hz led to a prolonged latency for seizure development after an intraperitoneal injection of pentylenetetrazol. The rTMS effectively prevented the development of status epilepticus of pentylenetetrazol-induced convulsions. These findings indicate that low-frequency rTMS affects the neural excitability, in the direction of anticonvulsive, and therefore, suggest the possibility of therapeutic use of rTMS in epilepsy.

Effects of antiepileptic drugs on 4-aminopyridine-induced epileptiform activity in young and adult rat hippocampus.

Extracellular field potential recordings were used to study the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PhB) and valproic acid (VPA) on the epileptiform activity evoked by 4-aminopyridine (4-AP, 50 microM) in the CA3 subfield of rat hippocampal slices obtained from young (8-23-day-old) and adult (> 60-day-old) male rats. Ictal (duration: 3-20 s; rate of occurrence: 3-12 x 10(-3) s-1) and interictal (duration: 0.2-0.8 s; rate of occurrence: 0.2-0.8 s-1) discharges were recorded in young slices, while only interictal activity (duration: 70-90 ms; rate of occurrence: 0.5-0.9 s-1) discharges were observed in adult slices. In addition, in both young and adult slices 4-AP disclosed a synchronous long-lasting potential (duration and rate of occurrence: 0.6-3 s, 7-70 x 10(-3) s-1 and 260-660 ms, 8-60 x 10(-3) s-1, respectively) that was caused by the activation of the gamma-aminobutyric acid type A (GABAA) receptor. In young slices, ictal discharges were blocked by CBZ (0.05 mM), PHT (0.1 mM), PhB (0.5 mM) and VPA (0.5 mM). With the exception of PhB, higher concentrations were necessary in these experiments for blocking the interictal activity (i.e., CBZ: 0.1 mM; PHT: > 0.2 mM; VPA: 2 mM). At these concentrations, none of the AEDs blocked the interictal activity in the adult hippocampus, but only reduced the rate of occurrence. PhB enhanced the rate of occurrence of the synchronous GABA-mediated long-lasting potentials both in young (increase: 190%) and in adult (increase: 145%) slices, while VPA increased their occurrence by 54% only in young slices. CBZ decreased the rate of occurrence of this long-lasting potential only in adult hippocampus. Our data indicate that the effects of the AEDs on 4-AP-induced epileptiform discharges are both pattern- and age-dependent. The rank order of potencies of the four AEDs was: (a) in young: CBZ > PHT > PhB > VPA; (b) in adult: CBZ > PhB > PHT > VPA.

Analysis of the kinetics of synaptic inhibition points to a reduction in GABA release in area CA1 of the genetically epileptic mouse, El.

In order to determine whether changes in synaptic inhibition are involved in chronic models of epilepsy, it is necessary to understand the factors which determine the kinetics of fast gamma-aminobutyric acid (GABA)ergic inhibition. For this purpose, we analyzed the decaying phase of isolated inhibitory postsynaptic currents (IPSC) in rats CA1 pyramidal cells. Reduction of GABA release (by reducing [Ca2+]o or paired-pulse stimulation) or blockade of GABA uptake (with tiagabine) led to the conclusion that small changes in the amount of GABA available for postsynaptic binding have little effect on the peak amplitude, but have marked effect on the duration of the IPSC. We then studied isolated GABAA receptor-mediated inhibition in area CA1 of the El mouse strain, which is genetically predisposed to epilepsy. Results were compared with the non-epileptogenic mother strain, ddY. Inhibitory postsynaptic potentials (IPSPs) in El mice (IPSPEl) were not significantly different in amplitude of those from ddY mice (IPSPddY). However, the rise-time and duration of IPSPEl were respectively about 25% and 50% shorter than those of IPSPddY. With appropriate pharmacological manipulation of GABA release or uptake, IPSPEl could be made to resemble the IPSPddY and vice versa. It is concluded that the synaptic release of GABA in area CA1 of the El mouse is decreased compared to that of the ddY mouse.

Susceptibility of chimera mice to epileptic convulsion.

The ddY mouse-derived E1 strain is a mutant susceptible to severe epileptic convulsion. Using a method of aggregation of embryos from the F1 progenies of ddY female X El male and ddY female X C57BL/6J male, seven chimera mice were produced. These seven chimera mice manifested distribution of white coat color derived from F1 progeny of ddY X El, in two 100%, one 50%, two 24%, and two less than 4% and the frequency of the seizure occurrence were 71.4%, 50%, 28.6%, 28.6%, 21.4%, 0.7% and 0%, respectively. It was found that the chimera mouse which had a larger percent of white coat color had higher susceptibility to epileptic convulsion, the susceptibility having been proportional to the degree of the tissue mosaicism.

Crossbreeding analysis of the mouse epilepsy.

Genetic control of the mouse epilepsy of E1 mouse isolated from ddY strain was investigated by crossbreeding with C57BL/6 mouse. A concept of the dominance deviation was applied for the explanation of the epileptic seizure inheritance in the generations of P1, F1, F2, B1 (F1 X E1 mouse) and B2 (F1 X C57BL/6 mouse), in which initial rate increase of the seizure frequency, seizure-inducing rotation numbers and seizure formative period as a function of the population growth were undertaken to be phenotypic measures. It was conclusive that the mouse epilepsy is apparently under genetic control by at least single gene with the seizure activity influencing factor(s) or by plural genes. Those are partially recessive for the allele assumed in cross-hybridized C57BL/6 mouse. No relevance of the coat color genotype and sex determination to the seizure segregation was observed obviously. Possible existence of epileptic substance(s) responsible for the enhancement of the seizure activity is also discussed.

Experimental animal epilepsy. A new device for the induction of epileptic seizures in the murine, El mouse.

A new apparatus was constructed for the induction of seizures in El mouse carrying inheritable epileptic features. They were rotated vertically in a drum with a 15 cm radius at the rate of two rotations per second. This procedure was repeated once a week. At the beginning of the experiment, the seizures occurred on an average of 20 rotations per mouse. This markedly decreased to only two rotations, six weeks thereafter. The acceleration which was effective for the induction of seizures in the El mouse, calculated in this apparatus, was 2369 +/- 980 cm/sec2. A rodent, Mongolian gerbil was also briefly described, comparing the seizure patterns with the El mouse.

Pattern- and age-dependency of the antiepileptic effects induced by valproic acid in the rat hippocampus.

The effects induced by the antiepileptic drug valproic acid were studied in the CA3 subfield of in vitro hippocampal slices obtained from young (16- to 27-day-old) and adult (over 60-day-old) rats. Spontaneous epileptiform discharges were induced by the addition of the convulsant 4-aminopyridine to the medium. Valproic acid (0.5 mM) selectively blocked the ictal epileptiform discharges in slices obtained from young rats. Interictal epileptiform discharges disappeared during perfusion with higher doses of valproic acid (2 mM). This blockade of interictal epileptiform activity was not observed when valproic acid (0.5-5 mM) was tested in hippocampal slices from adult rats. Thus, in the hippocampus of young rats, 4-aminopyridine-induced ictal activity is more sensitive to valproic acid than are interictal discharges. Moreover, valproic acid is effective in controlling interictal discharges in the young, but not in the adult rat hippocampus.