Hippocampal afterdischarges in rats. I. Effects of antiepileptics

Hippocampal afterdischarges (ADs) are considered to be a model of complex partial seizures. To study the pharmacology of these ADs, stimulation electrodes were implanted into the dorsal hippocampus of 33 male Wistar rats. Stimulation (15-s series of monophasic rectangular pulses with a duration of 1 ms and frequency of 8 Hz) was applied four times with interstimulation intervals of 15 min. Drugs (carbamazepine 50 and 100 mg/kg; clonazepam 0.2 and 0.5 mg/kg; ethosuximide 125 and 250 mg/kg; phenobarbital 40 and 80 mg/kg) as well as solvent and isotonic saline were injected intraperitoneally 2 min after the cessation of the first AD. Duration of AD, of the latent period between AD and recurrent AD and duration of recurrent AD and the number of wet dog shakes were measured. ADs were markedly shortened by both doses of clonazepam and phenobarbital and by the higher dose of carbamazepine. The action of ethosuximide was negligible. Wet dog shakes were influenced in the same way as AD duration. Recurrent ADs were more sensitive to antiepileptics than ADs and wet dog shakes.

Temporal Lobe Epileptogenesis and Epilepsy in the Developing Brain: Bridging the Gap Between the Laboratory and the Clinic. Progression, But in What Direction?

The origins of human mesial temporal lobe epilepsy and hippocampal sclerosis are still not well understood. Hippocampal sclerosis and temporal lobe epileptogenesis involve a series of pathologies including hippocampal neuronal loss and gliosis, axonal reorganization, and maybe hippocampal neoneurogenesis. However, the causality of these events is unclear as well as their relation to the factors that may precipitate epileptogenesis. Significant differences between temporal lobe epileptogenesis in the adult and immature brain may require differential approaches. Hereditary factors also may participate in some cases of hippocampal sclerosis. The key point is to identify the significance of these age-dependent changes and to design preventive treatments. Novel strategies for the prevention and treatment of mesial temporal lobe epilepsy and hippocampal sclerosis may include rational use of neuroprotective agents, hormonotherapy, immunizations, and immunotherapy.

Plasticity of excitatory amino acid transporters in experimental epilepsy

PURPOSE

To examine the relationship between seizures and excitatory amino acid transporter (EAAT) activity and whether up-regulation of EAAT activity alters epileptogenicity.

METHODS

In this study, we exposed rat hippocampal slices to different convulsants before measuring EAAT activity. Rats were exposed to the EAAT inhibitor pyrrolidine-2,4-dicarboxylic acid (PDC) before entorhinal cortex/hippocampal slices were obtained. These slices were exposed to low-Mg2+ buffer while electrophysiological recordings were obtained from the entorhinal cortex. mGluR III acting agents were used to study whether activation of mGluR III could regulate EAAT activity and if this regulation could overcome the effects on EAAT activity induced by the convulsants.

RESULTS

Veratridine, kainic acid (KA), and pilocarpine reduced EAAT activity in rat hippocampal slices. L-2-Amino-4-phosphonobutyric acid (an mGluR III agonist) restored EAAT activity and reduced epileptiform activity to near control levels. The saturation curve for glutamate uptake in slices from KA-seized rats killed 2 hours after the first forelimb clonus was displaced to the left, suggesting a compensatory change for the enhanced excitation. On the other hand, rats injected with the EAAT inhibitor PDC (by intracerebroventricular injection) had more severe KA-induced seizures and N-methyl-D-aspartate epileptiform activity than control rats. Furthermore, hippocampal slices from KA- or KA+PDC-treated rats exposed to low Mg2+ reduced their firing rate to nearly zero once they returned to normal solution, whereas their control counterparts continued to fire, although at a lower rate.

CONCLUSIONS

These results suggest a significant contribution of EAATs in some experimental epilepsy models and point to their short-term regulation by mGluR III as a possible source of their plasticity.

Neuroprotective effects of estrogens on hippocampal cells in adult female rats after status epilepticus

PURPOSE

Estrogens have neuroprotective effects in ischemia, stroke, and other conditions leading to neuronal cell death (e.g., Alzheimer's disease). The present study examined whether estrogens may have neuroprotective effects after seizures.

METHODS

The kainic acid model was used to determine if estrogens protect hippocampal cells after status epilepticus in adult female rats. Rats were ovariectomized 1 week before hormone replacement. beta-Estradiol benzoate (EB; 2 microg in 0.1 mL of oil) was injected subcutaneously 48 and 24 hours before seizure testing. We administered kainic acid (16 mg/kg intraperitoneally) and behaviorally monitored the rats for 5 hours. After this time, all rats were injected with pentobarbital (50 mg/kg intraperitoneally) irrespective of seizure severity. Some rats received two additional doses of EB, one immediately and one 24 hours after the seizures. Another group of rats received only these two doses of EB after the seizures, and yet another group of rats received pretreatment with the intracellular EB receptor antagonist tamoxifen before each of four EB injections. Control rats received oil instead of EB. Rats were killed 48 hours after seizures. Neuronal damage was evaluated in silver-impregnated and Nissl-stained sections.

RESULTS

Estrogen treatment before kainic acid administration significantly delayed the onset of kainic acid-induced clonic seizures, whereas it did not change the onset of status epilepticus compared with oil-treated controls. Furthermore, estrogen treatment significantly protected against kainic acid-induced seizure-related mortality. In control rats, examination of Nissl-stained and silver-impregnated slides revealed severe neuronal damage in the vulnerable pyramidal neurons of the hippocampal CA3 subfield and in the hilus of the dentate gyrus. Estrogen pretreatment, as well as the combination of pretreatment and posttreatment, significantly reduced the number of argyrophilic neurons in both the CA3 and the dentate gyrus. Posttreatment only had no protective effects. The data indicate that intracellular EB receptors mediate this type of neuroprotective effect, because the tamoxifen pretreatment abolished EB neuroprotection.

CONCLUSIONS

Our results suggest that estrogens can be beneficial in protecting against status epilepticus-induced hippocampal damage. Hormonal conditions may have differential effects on underlying epileptic state in some patients. Therefore, more studies are necessary to determine the prospective therapeutic advantage of hormonal treatment in seizure-related damage.

Prenatal methotrexate exposure delays onset of low Mg(2+)-induced epileptiform discharges in the entorhinal cortex

We determined the effects of prenatal exposure to DNA synthesis inhibitor methotrexate (MTX) on: (a) the susceptibility to low Mg(2+)-induced epileptiform activity in deep layers (IV-V) of medial entorhinal cortex in vitro; and (b) neuronal counts in this area. Low Mg(2+)-induced discharges developed significantly later in slices from prenatally MTX-exposed rats than in control slices. Neuronal counts were increased in the layer V of medial entorhinal cortex of prenatally MTX-exposed rats. Results indicate that: (a) MTX-induced prenatal brain DNA impairment is antiepileptogenic; and (b) simple increases in neuronal numbers may not be associated with epileptogenic effects.

Effect of ganaxolone on flurothyl seizures in developing rats

PURPOSE

To determine the effects of a newly synthesized epalon, ganaxolone (GNX), on primarily generalized seizures in rats of various ages during development. Epalons are classified as neuroactive steroids that interact at unique site of the GABAA receptor-Cl- channel complex in the central nervous system.

METHODS

Sprague-Dawley male rats were used at 9, 15, 30, and 60 postnatal days (PN). GNX dissolved in 2-hydroxypropylbeta-cyclodextrine was administered intraperitoneally in different doses at various time points before flurothyl testing. The incidence and threshold of clonic and tonic-clonic flurothyl seizures were evaluated. Behavioral changes were also assessed.

RESULTS

In all age groups, the effects of GNX were dose dependent and more prominent 10 min after its administration. In PN 60 and PN 30 rats, GNX had dose-dependent anticonvulsant effects; tonic-clonic seizures were more sensitive to GNX treatment than clonic seizures. In PN 15 and PN 9 rats, GNX demonstrated dose-and time-dependent anticonvulsant effects against both types of flurothyl-induced seizures. GNX was more effective in PN 15 rats than in other age groups, but at doses that altered motor behavior.

CONCLUSIONS

GNX has anticonvulsant effects against flurothylinduced seizures in all age groups tested. Its effects are more prominent in the two younger age groups, especially in PN 15 rats, but are associated with motor side effects.

Prenatal morphine exposure enhances seizure susceptibility but suppresses long-term potentiation in the limbic system of adult male rats

The present study examined the effects of prenatal morphine exposure on NMDA-dependent seizure susceptibility in the entorhinal cortex (EC), and on activity-dependent synaptic plasticity at Schaffer collateral and perforant path synapses in the hippocampus. During perfusion with Mg(2+)-free ACSF, an enhancement of epileptiform discharges was found in the EC of slices from prenatally morphine-exposed male rats. A submaximal tetanic stimulation (2x50 Hz/1 s) in control slices elicited LTP at the Schaffer collateral-CA1 synapses, but neither LTP nor LTD was evoked at the perforant path-DG synapses. In slices from prenatally morphine-exposed adult male rats, long-term potentiation of synaptic transmission was not observed at Schaffer collateral-CA1 synapses, while the submaximal tetanus now elicited frank LTD of synaptic EPSPs at perforant path synapses. These data suggest that prenatal morphine exposure enhances the susceptibility of entorhinal cortex to the induction of epileptiform activity, but shifts long-term plasticity of hippocampal synapses in favor of LTD.

Prenatal morphine exposure alters N-methyl-D-aspartate- and kainate-induced seizures in adult male rats

The purpose of the present study was to investigate whether prenatal exposure to morphine has effects on excitatory amino acid-induced seizures. Adult male rats, exposed on embryonic days 11-18 to saline or morphine, were injected with N-methyl-D-aspartate (NMDA) (150, 175, 200, 225, and 250 mg/kg) or kainic acid (KA) (15 or 20 mg/kg) in adulthood to assess the occurrence and latency to onset of stereotypy and seizures. The latency to onset of stereotypy was significantly increased after 175 mg/kg, and decreased after 200 mg/kg of NMDA in morphine-exposed animals. The lowest dose of NMDA (150 mg/kg) induced seizures in prenatally saline-treated control male rats but not in the morphine-exposed male rats. In the KA-injected group, prenatally morphine-exposed males had shorter latency to onset of wet-dog shakes, but there were no effects on the latency to onset of clonic seizures. The data suggest that prenatal morphine exposure has long-term effects on seizure susceptibility and the onset of stereotypy in the excitatory amino acid-induced seizure models.

Prenatal methotrexate exposure decreases seizure susceptibility in young rats of two strains

Effects of prenatal exposure to methotrexate (MTX) administered in Sprague-Dawley (one 5 mg/kg dose of MTX on gestational day 15; E15) or Wistar (one 5 mg/kg dose of MTX on E14 or E15 or two such doses on E15) pregnant rat dams were studied in developing offspring. Young Sprague-Dawley rats were subjected to rapid kindling on postnatal days (PN) 15 and 16, and to flurothyl seizures on PN 15 and PN 30. Offspring of the Wistar strain were tested in flurothyl on PN 30. In Sprague-Dawley rats, prenatal exposure to MTX decreased susceptibility to kindling-induced seizures on PN 15 and to flurothyl-induced clonic seizures on PN 30. In Wistar rats, a single dose of MTX on E15 was ineffective, but two doses significantly decreased susceptibility to flurothyl-induced seizures. Additionally, due to a shorter duration of pregnancy in Wistar rats, exposure to a single dose of MTX on E14 also decreased susceptibility to flurothyl seizures. MTX, as folic acid antagonist, interferes with DNA synthesis. However, unlike other treatments that suppress DNA synthesis (such as methylazoxymethanol exposure or X-ray radiation), MTX exposure results in anticonvulsant effects in surviving offspring. The data suggest that not all prenatal impairments of DNA have proconvulsant features postnatally.

Age-specific N-methyl-D-aspartate-induced seizures: perspectives for the West syndrome model

PURPOSE

With intraperitoneal N-methyl-D-aspartate (NMDA; 15-200 mg/kg) administration, we attempted to develop an animal model of age-specific West syndrome to serve for testing of putative anticonvulsant drugs and to determine the mechanisms of this disorder.

METHODS

Experiments were performed in 12-, 18-, and 60-day-old (adult) rats. The effects of systemic pretreatment with hydrocortisone (5-25 mg/kg), pyridoxine (20-250 mg/kg), and sodium valproate (VPA; 200 and 400 mg/kg) against the NMDA-induced automatisms, emprosthotonic (hyperflexion), and clonic-tonic seizures were determined. NMDA-induced EEG changes and alterations of the performance in horizontal bar, rotorod, open field, and elevated plus-maze tests were recorded.

RESULTS

In young rats, hydrocortisone had proconvulsant effects. High doses of pyridoxine induced epileptiform activity independent of and distinct from that induced by NMDA. Only VPA had moderate effects against the NMDA-induced syndrome. EEG consisted of periods of suppression mixed with ictal activity of serrated waves and high-voltage chaotic EEG activity. In adult rats, EEG alterations involved spike and spike-and-wave activity. NMDA also deteriorated performance of young rats in the open field, rotorod, and elevated plus maze tests.

CONCLUSIONS

NMDA syndrome in rats fulfills some, but not all, criteria of the West syndrome model, such as occurrence of flexion seizures, nonspecific diffuse EEG changes, refractoriness to antiepileptic therapy (but a response to VPA), as well as long-term alteration of behavioral tasks. However, NMDA-induced seizures represent an acute model without the occurrence of spontaneous seizures, whereas in the clinical situation, both the seizures and neurologic deterioration are chronic. Further, in the West syndrome and the NMDA seizure model, there is an incongruent response to therapy with antiepileptic drugs.

Region-specific modulation of limbic seizure susceptibility by ovarian steroids

Gonadal steroid hormones can markedly affect seizure susceptibility. Ovariohysterectomized female rats given ovarian steroid hormone supplements were used to evaluate the effects of ovarian steroids on epileptiform activity in hippocampal slices in vitro and on flurothyl-induced seizures in vivo. Seizure susceptibility was compared in the entorhinal cortex (EC) and CA1 regions of the hippocampus perfused with Mg(2+)-free medium, which leads to epileptiform discharges caused by a relief of voltage-dependent NMDA receptor block. After in vivo treatment with 500 microg of progesterone for 2 h prior to slice preparation, the latency to onset of low Mg(2+)-induced epileptiform activity of slices was significantly prolonged compared to slices from controls. In contrast, progesterone replacement accelerated the development of epileptiform activity in the CA1 region. Neither estrogen alone (2 x 2 microg of estradiol benzoate, 48 and 24 h prior to the experiment), nor a combined treatment with estrogen plus progesterone, significantly affected seizure susceptibility in either CA1 or the EC. There were no consistent effects of estrogen or progesterone, alone or in combination, on flurothyl-induced seizures in vivo. The data suggest that in vitro, progesterone alters seizure susceptibility in a site- and seizure model-specific fashion. The differential effects of progesterone may be due to differential expression of progesterone receptor isoforms or metabolites in specific brain areas suggesting that selective modulation of NMDA receptor-dependent epileptiform activity may play a role in hormonal effects on epileptogenesis.

Bicuculline-induced rhythmic EEG episodes: gender differences and the effects of ethosuximide and baclofen treatment

PURPOSE

There are gender differences in the expression of seizures. We tested rhythmic EEG episodes induced by low doses of bicuculline in rats for gender differences. To verify the validity of these discharges as a model of absence seizures in both male and female rats, we tested the antiabsence drug ethosuximide (ESM) and a gamma-aminobutyric acid (GABA(B))-receptor agonist, baclofen, which may exacerbate absence seizures.

METHODS

Adult rats of both sexes were used. Under general anesthesia, EEG electrodes were implanted over frontal and occipital cortex, and some females were ovariectomized. After recovery, male, intact female rats, and female rats ovariectomized and ovariectomized rats with estradiol replacement were compared for occurrence of rhythmic EEG episodes (approximately 6 cycles/ s) induced by 2.5 mg/kg of bicuculline, s.c. Because of gender differences in sensitivity to bicuculline, further pharmacologic effects of ESM (125 and 250 mg/kg, i.p.) and baclofen (2 mg/kg, i.p.) were tested separately in male (3.0 mg/kg of bicuculline), and female (2.5 mg/kg of bicuculline) rats.

RESULTS

After the identical dose of bicuculline, s.c., male and female rats differed in the incidence of rhythmic episodes and in the latency to onset of the first as well as the generalized episode. Female rats with natural or exogenous estrogens (but not ovariectomized rats) developed EEG episodes more often than did males, and this effect could be attributed to the presence of estrogens. ESM pretreatment suppressed the episodes, whereas baclofen enhanced their occurrence, as well as the total duration of episodes without gender-specific differences.

CONCLUSIONS

The study demonstrates gender differences (related probably to the presence of circulating estrogens) in the susceptibility of rats to develop rhythmic EEG episodes induced by threshold doses of bicuculline. This activity has some features of an acute absence seizure model.

Prenatal morphine exposure alters ovarian steroid hormonal regulation of seizure susceptibility

The present study examined the ovarian hormonal regulation of seizure susceptibility in prenatally morphine- and saline-exposed adult female rats in the flurothyl seizure model in vivo, and in low-magnesium-induced epileptiform activity in brain slices, in vitro. All females were ovariohysterectomized (OVX); some received either estrogen (E) or progesterone (P) replacement, while others were injected with E + P sequentially. In prenatally saline-treated control females, there was an increase in the flurothyl-induced clonic seizure threshold (anticonvulsant effect) in the presence of both hormones (E + P) compared to OVX controls. In morphine-exposed females, there was an increase in the flurothyl-induced clonic seizure threshold after an E injection alone while there was a reduced tonic--clonic seizure threshold in the presence of both hormones (E + P) compared to the hormone treatment-matched group of saline-exposed females. In control females, in low magnesium medium in vitro, the development of two types of epileptiform activity (seizure-like events and status of short discharges) was not affected by the different hormonal conditions. However, prenatal morphine exposure suppressed the development of both types of epileptiform activity in the E-injected females compared to the E-injected, control females. The present data demonstrate that the anticonvulsant effects of P on seizure susceptibility requires the presence of E. Furthermore, prenatal morphine exposure alters ovarian steroid hormone-regulated seizure susceptibility.

West syndrome model: seek and you will find

West syndrome is an age-specific epileptic syndrome with onset in infancy. It comprises infantile spasms (usually flexion convulsions), EEG pattern of hypsarrhythmia and mental retardation. Current therapy involves ACTH, corticosteroids, valproic acid, pyridoxine and vigabatrine. The treatment is difficult and more effective antiepileptic drugs are required. Unfortunately, there is no animal model of West syndrome that would accurately depict the situation found in humans. N-methyl-D-asparate (NMDA)-induced seizures in infant rats have certain features of the West syndrome model. These seizures are age-specific (they occur before 25 days of age), include hyperflection (emprosthotonus), their EEG is not specific and they somewhat respond to treatment with the benzodiazepine clonazepam. In 12 and 18 day old rats, we tested the effects of hydrocortisone, pyridoxine and sodium valproate against the seizures induced by 15 and 45 mg/kg of NMDA i.p., respectively. There were weak effects of sodium valproate against the NMDA-induced emprosthotonus. In contrast, high doses of pyridoxine were proconvulsant and hydrocortisone worsened the damage of nerve cells induced by NMDA. The data show that NMDA-induced seizures although similar to West syndrome are extremely resistant to therapy and may not be a good model of the West syndrome. However, the search for an adequate model that would allow for determination of possible mechanisms and testing of putative antiepileptic drugs will continue.

Extracellular acidosis and high levels of carbon dioxide suppress synaptic transmission and prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices

Long-term potentiation (LTP) is a long-lasting increase in synaptic strength induced by high frequency stimulation. LTP may participate in learning and memory formation. In many synaptic systems, LTP is dependent on intact function of N-methyl-D-aspartate (NMDA) receptors. NMDA receptors may be inhibited in different conditions involving also extracellular acidosis. A decrease in the extracellular pH accompanies many pathological states such as ischemia, hypoxia, and the CNS injury. The study was designed to determine whether comparable extracellular acid-base imbalances are able to interfere with the LTP induction. Hippocampal slices from adult rats were stimulated with high frequency stimulation (1 x 100 Hz/1 s) at Schaffer collateral-commissural synaptic system in the environment with different pH (6.7-7.8) and the field responses were recorded in CA1. Acidosis was achieved by supplying excessive CO2 or by HCO3-decrease in standard bicarbonate-containing buffer or by a direct acidification of the buffer containing Na-HEPES. Invariably, all forms of acidification suppressed the efficacy of normal, low frequency synaptic transmission and prevented the induction of LTP in a reversible manner; i.e., after reperfusion of the slices at pH 7.3 and restimulation, there was a return of synaptic transmission back to baseline, and a significant amount of LTP occurred. In contrast, alkalization to pH 7.8, although enhancing synaptic transmission efficacy, did not further increase the LTP magnitude compared to control environment with pH 7.3. The results suggest that extracellular acidosis associated with several pathological conditions in the CNS may significantly diminish the LTP induction, and thus negatively affect all physiological processes that utilize LTP.

Proconvulsant effects induced by pyridoxine in young rats

High doses of pyridoxine (vitamin B6) can be neurotoxic in adults. Effects of intraperitoneally administered B6 (100, 250 and 400 mg/kg) were studied in 7, 12, 18 and 60 day old rats. B6 elicited epileptiform EEG discharges without any motor correlate in all age groups. In contrast, motor seizures were rare, seen only in 18 day old rats (250 mg/kg of B6). Data indicate that in young rats, B6 may have proconvulsant effects in doses relevant to those sometimes used in pediatric neurology.

Site-specific effects of local pH changes in the substantia nigra pars reticulata on flurothyl-induced seizures

Local cerebral changes of acid-base balance may interfere with neuronal communication. Acidosis enhances and alkalosis suppresses GABAA receptor neurotransmission while there are opposite effects on NMDA receptor transmission. In this study, we determined site-specific effects of acidified solutions of Na-HEPES-artificial cerebrospinal fluid infused into the anterior or posterior area of the substantia nigra pars reticulata (SNR) in rats. Two levels of pH were compared: 6.7 and 7.4. Rats were challenged with flurothyl and the threshold for clonic and tonic-clonic seizures was determined. In the anterior SNR, there were no differences between the effects of the solution with pH 6.7 and 7.4 on flurothyl seizures. In contrast in the posterior SNR, microinfusions with pH 6.7 had proconvulsant effects. The results suggest that local pH changes may have site-specific effects on seizure susceptibility in vivo.

An indirect method for absorption rate estimation: flurothyl-induced seizures

This paper develops a method to estimate a minimal amount of flurothyl necessary to induce the seizures (the seizure threshold). A simple mathematical model is proposed which permits one to determine the drug absorption rate from the amount which has been administered and from the measured latency to onset of seizure. Experimental animal (rats) were exposed to a continuous intake of flurothyl in two different situations: either being alone in the airtight chamber or sharing it in a pair. In the latter case, we assume that the two rats uniformly share the infused drug. Our calculations estimate that approximately 20 microliters of flurothyl is necessary to induce twitches, whereas 25 microliters of flurothyl is the dose required for the induction of clonic seizures. The model can be used to estimate the threshold amounts of any drug producing obvious behavioral changes irrespective of the route of administration.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. IV. Effects of CGP 39551

We determined anticonvulsant effects of CGP 39551 [(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid 1-ethylester] against pentylenetetrazol-induced seizures in developing, 7-90 day old, rats. The rats received CGP 39551 in doses of 10, 20 or 40 mg/kg IP 30 min prior to the pentylenetetrazol administration (100 mg/kg s.c.). In addition, the 20 mg/kg dose of CGP 39551 was injected 120 min prior to pentylenetetrazol. In adult rats, all doses of CGP 39551 blocked generalized tonic-clonic pentylenetetrazol-induced seizures. In younger rats, higher doses of CGP 39551 and/or a longer delay between the CGP 39551 pretreatment and pentylenetetrazol administration was necessary for similar anticonvulsant effects against tonic-clonic seizures. In contrast, there was no effect of CGP on pentylenetetrazol-induced clonic seizures. The results indicate that CGP 39551 has anticonvulsant features similar to other competitive NMDA receptor antagonists. High doses of CGP 39551 and long pretreatment latency which are necessary in young rats for anticonvulsant effects may reflect the overexpression of NMDA transmission during the second and third postnatal week of the rat. Alternatively in adult rats, we can speculate an anticonvulsant role of a CGP 39551 metabolite or maturation of brain uptake mechanism for CGP 39551.

Developmental regulation of regional functionality of substantial nigra GABAA receptors involved in seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata is critical for seizure control. We tested the hypothesis that there is a differential regional distribution and functionality of nigral GABAA receptor sites that is developmentally regulated. In adult rats, we determined the effects on flurothyl seizures of (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid (ZAPA, a presumed agonist of the low-affinity GABAA receptor site), bicuculline (an antagonist of the low-affinity GABAA receptor site) and gamma-vinyl-GABA (a GABA-transaminase inhibitor), infused bilaterally in anterior or posterior substantia nigra pars reticulata. ZAPA infusions (8 micrograms) were anticonvulsant in anterior substantia nigra but proconvulsant in posterior substantia nigra. Bicuculline infusions (100 ng) were proconvulsant in anterior substantia nigra but ineffective in posterior substantia nigra. An anticonvulsant dose of gamma-vinyl-GABA, when infused in anterior substantia nigra, was proconvulsant when infused in posterior substantia nigra. In 15 day old rats, the effects of ZAPA, were biphasic: 2 micrograms was anticonvulsant while 8 micrograms was proconvulsant. There was no regional specificity. The data suggest that with maturation there is functional segregation of specific GABAA receptor subtypes involved in substantia nigra-mediated seizure control.

Age-specific effects of baclofen on pentylenetetrazol-induced seizures in developing rats

PURPOSE

To determine whether seizures have age-specific features, we studied the role of gamma-aminobutyric acidB (GABAB) transmission in rats of various ages (9, 15, 30, and 60 postnatal days).

METHODS

We used a GABAB receptor agonist baclofen (2 or 5 mg/kg intraperitoneally, i.p.) and a GABAB receptor antagonist CGP 35348 (100 or 600 mg/kg i.p.) in the pentylenetetrazol (PTZ)-induced model of clonic and tonic-clonic seizures (100 mg/kg subcutaneously, s.c.).

RESULTS

Whereas baclofen was anticonvulsant and CGP 35348 proconvulsant in most animals, there were distinct age-related differences in the effectiveness of these drugs and the antagonist had some anticonvulsant activity in adults. Furthermore, the two drugs acting at GABAB receptors had a different profile of action in clonic seizures as compared with tonic-clonic seizures.

CONCLUSIONS

The differences in the age-specific action of the GABAB agonist and antagonist suggest the different GABAB receptor subsets may mediate the drug effects. The results indicate that putative antiepileptic drugs (AEDs) must be tested during development because it may not be possible to extrapolate age-specific anticonvulsant effects from studies in adult animals.

Increased susceptibility of brain slices from carbonic anhydrase II-deficient mice to low [Mg2+]O-induced seizures

Brain pH is thought to be an influential factor in determining susceptibility to seizures. We compared the susceptibility of brain slices from carbonic anhydrase II (CA II)-deficient mice to epileptiform activity induced by low extracellular [Mg2+], with slices from normal littermates, both bathed in artificial cerebrospinal fluid at pH 7.3. In both entorhinal cortex and hippocampal field CA1, epileptiform activity started earlier in CA II-deficient slices. Raising extracellular [CO2] (20%; extracellular pH, 6.7) reversibly blocked the epileptiform activity in normal, but not in CA II-deficient, slices. The data, combined with previous in vivo findings showing an increased resistance of mutants to seizures, suggest the presence of in vivo anticonvulsant acidosis with long-term compensatory changes that lead to in vitro 'proconvulsant' behavior in CA II-deficient slices clamped at pH 7.3.

Age-dependent anticonvulsant action of clonazepam in the N-methyl-D-aspartate model of seizures

Seizures may result from an impaired balance between excitation and inhibition. We tested whether clonazepam [a benzodiazepine that enhances GABAA inhibitory transmission (0.2 or 1.0 mg/kg, intraperitoneally [i.p.])] suppresses an age-dependent pattern of N-methyl-D-aspartate (NMDA)-induced phenomena in 7-, 12-, 18-, 25-, and 60-day-old rats (10, 40, 100, 100, and 200 mg/kg of NMDA, i.p., respectively). There were no effects of clonazepam against the NMDA-induced automatisms and emprosthotonus. In 7-day-old rats, clonazepam was proconvulsant in clonic-tonic seizures (it decreased the latency to onset of seizures, whereas it was anticonvulsant in 25-day-old rats. There was no difference between anticonvulsant effects of clonazepam and its solvent in 12- and 60-day-old rats. Both cortical and hippocampal EEG seizures was extremely poor in this model. There was no improvement of EEG recording after clonazepam. The results demonstrate that impaired excitation cannot be simply balanced by an enhanced inhibition and that the drug effects in young animals cannot be predicted from the effects in adults.

Age-dependent effects of gamma-aminobutyric acid agents on flurothyl seizures

Behavioral characteristics of seizures have age-dependent features, which suggests that effective treatment of seizures may be age-specific as well. In experiments that used the flurothyl seizure model, we examined the effects of several drugs that affect GABAergic neurotransmission in rats of various ages. Systemic administration of phenobarbital (PB, a drug that enhances GABAA receptor-mediated inhibition) was anticonvulsant in most age groups. In contrast, gamma-vinyl GABA (VGB, a drug that increases endogenous GABA levels and enhances both GABAA and GABAB receptor transmission) did not have anticonvulsant effects. Baclofen (a GABAB receptor agonist) was proconvulsant in 9-day-old rat pups, and anticonvulsant in 15-30-day-old rats and lost its anticonvulsant activity in 60-day-old rats. CGP 35348 (a GABAB receptor antagonist) was proconvulsant in developing rats but not in 60-day-old rats. A novel GABAB receptor antagonist, CGP 36742, was proconvulsant in 9- and 15-day-old rats but had no effects in 30- and 60-day-old rats. These results indicate that the effects of presumed GABAergic agents are not uniform across the age span. The differences may reflect age-dependent maturational changes of GABA receptor subtypes, differential action of the drugs on pre- and postsynaptic sites and possible non-GABAergic effects.

Kainate/AMPA receptor antagonists are anticonvulsant against the tonic hindlimb component of pentylenetetrazol-induced seizures in developing rats

Non-NMDA receptor antagonists CNQX, DNQX, and NBQX (10-40 mg/kg IP) were tested against pentylenetetrazol-induced (100 mg/kg SC) seizures in 7 to 90-day-old rats. All three drugs significantly decreased the incidence of tonic hindlimb component of tonic-clonic pentylenetetrazol seizures, often in favor of increased incidence of forelimb tonus throughout development. In addition, in 7 to 25-day-old rats, DNQX and NBQX decreased the severity of seizures due to a decrease in total incidence of the tonic component of tonic-clonic seizures compared to age-matched controls. However, neither drug was able to consistently suppress the incidence or increase latency to onset of clonic and tonic-clonic pentylenetetrazol seizures. The data suggest that, during development, non-NMDA receptor transmission may play a role in the generation of the tonic component, but not in the generation of other components of pentylenetetrazol-induced seizures.

Resistance of hippocampal synaptic transmission to hypoxia in carbonic anhydrase II-deficient mice

Mutant Car2n/Car2n mice deficient in carbonic anhydrase II (CA II; a major brain CA isozyme) suffer from systemic acidosis and are more resistant to experimental seizures than their normal littermates (+/+ or +/Car2n). The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor has been shown to contribute to long-term potentiation (LTP) of synaptic transmission, hypoxic/ischemic neuronal injury and to be blocked by extracellular protons (acidosis). We compared the effects of hypoxia on synaptic transmission and LTP in field CA1 of hippocampal slices from CA II-deficient mice to their normal littermates. Slices were subjected to successive 5, 10 and 15 min-periods of hypoxia with 30 min-recovery periods in between. Hippocampal slices from mutant, CA II-deficient mice, were more resistant to all periods of hypoxia tested than slices from normal littermates. In a separate set of mutant and normal slices, there were no differences in LTP of population spike amplitude. The relative resistance of CA II-deficient mice to hypoxia-induced damage may be a consequence of severe interstitial acidosis. The sustained influence of increased extracellular proton concentrations may change the characteristics of NMDA receptors resulting in an increased resistance of synaptic transmission in CA II-deficient mice to hypoxia compared to controls.

Developmental seizure models

Seizures frequently occur in children. There are significant differences in the generation, expression and modification of seizures as a function of age. Animal models of epilepsy should depict these age-related differences. In this paper, we summarize the main features of generalized flurothyl, pentylenetetrazol, bicuculline and picrotoxin-induced seizures in adult and immature rats as well as the characteristics of focal seizures induced by amygdala kindling, systemic NMDA and kainic acid. Some of the models may be more advantageous for screening antiepileptic drug effects whereas other models may be more helpful in studying the basic mechanisms of epilepsy.

Effects of MK-801 and phenytoin on flurothyl-induced seizures during development

We determined the effects of the N-methyl-D-aspartate (NMDA) receptor blocker MK-801 (0.05, 0.1, and 0.5 mg/kg intraperitoneally, i.p.) and phenytoin (PHT, 5, 10, and 20 mg/kg i.p.) on flurothyl-induced clonic and tonic-clonic seizures in 9-, 15-, 30-, and 60-day-old male rats. Both agents had seizure-, age-, and dose-specific effects. The highest dose of MK-801 was anticonvulsant against clonic flurothyl-induced seizures only in 9- and 60-day-old rats, but suppressed tonic-clonic seizures in all ages. The lowest dose of MK-801 (0.05 mg/kg) produced significant anticonvulsant effects only in 15 day old rats. PHT did not have any effect on clonic seizures throughout development. Both doses of PHT (10 and 20 mg/kg) were anticonvulsant against tonic-clonic seizures in adult rats but not in any other age group. The results indicate that NMDA receptors play an important role in tonic-clonic flurothyl-induced seizures throughout development (especially in 15-day-old rats) and that the anticonvulsant effects of PHT may vary at different stages of brain development.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis: III. The action of kynurenic acid and glutamic acid diethylester

N-methyl-D-aspartate (NMDA) receptor antagonists are anticonvulsant drugs with specific activity against tonic-clonic pentylenetetrazol-induced seizures. However, they do not affect clonic seizures with preserved righting reflexes. In these experiments, we tested the anticonvulsant activity of strychnine-insensitive glycine receptor (at the NMDA site) antagonist kynurenic acid and nonspecific excitatory amino acid receptor antagonist glutamic acid diethylester (GDEE) in the pentylenetetrazol-induced seizure model in developing rats 7, 12, 18, 25, and 90 days old. Control rats were injected with pentylenetetrazol (100 mg/kg subcutaneously). Other rats were pretreated either with kynurenic acid (40, 80, or 240 mg/kg IP) or with GDEE (0.48-480 mg/kg IP), followed by pentylenetetrazol (100 mg/kg). In very young rats (7 and 12 days), both kynurenic acid and GDEE increased the incidence of clonic seizures, whereas the occurrence of tonic-clonic seizures was suppressed or delayed compared to controls. This effect is very similar to the anticonvulsant action of the competitive and noncompetitive NMDA receptor antagonists. In adult rats, the pretreatment with rather higher doses of kynurenic acid or GDEE suppressed or delayed clonic seizures as well as tonic-clonic seizures. Both drugs also induced behavioral side effects: repetitive orientation, wet dog shakes, and frequent jumping. Our data show that there are only weak and nonconsistent age-specific anticonvulsant effects resulting from the blockade of strychnine-insensitive glycine receptor often associated with serious side effects, thus decreasing chances to develop effective antiepileptic treatment in this drug class.

The development of epilepsy in the paediatric brain

The immature central nervous system (CNS) is more susceptible to the development of seizures than its adult counterpart. Developmental studies of experimental seizures have suggested that young animals have unique behavioural seizure patterns, including the presence of bilateral, though asymmetric, convulsions. There are differences in the mechanisms responsible for the generation of seizures, propagation patterns and seizure arrest and recurrences. These differences are due to local factors as well as factors that affect neural systems consisting of long neuronal circuits. The substantia nigra, a site involved in the control of seizures, will be used as an example to demonstrate how evolving neurobiological processes modulates the suppression or exacerbation of seizures with age. Evidence will also be presented indicating that early in life, seizures may not produce hippocampal damage. An understanding of the age-related differences is important for the development of rational approaches to treating seizures and their consequences.

Lowering of extracellular pH suppresses low-Mg(2+)-induces seizures in combined entorhinal cortex-hippocampal slices

Lowering [Mg2+]o induces epileptiform bursting in hippocampus and entorhinal cortex (EC), presumably by activation of N-methyl-D-aspartate (NMDA) receptors. Since increasing [H+]o has been shown to reduce NMDA receptor activation, we hypothesized that this could contribute to anticonvulsant actions of acidic pH. To test this, we studied the effects of raising extracellular PCO2 (20.6%, pH = 6.7) or lowering extracellular pH (6.7 or 6.2) on low-Mg(2+)-induced epileptiform discharges. Lowering the pH to 6.7 by either means increased the interval between seizure-like events (SLEs), decreased the maximal amplitude of SLEs, and if the site of seizure generation was at a distance from the recording site, acidification slowed the rate of seizure propagation. In contrast, the duration of SLEs was unaffected by acidic pH or high PCO2. Raising PCO2 or lowering pH to 6.7 also blocked early (8-10 min) but not late (> 20 min) phases of status-like discharges. All effects of the extracellular pH changes were fully reversible. Further lowering of extracellular pH to 6.2 completely and reversibly blocked both SLEs and status-like discharges. Our data show that the effects of high PCO2 and low pH on seizures in the EC in vitro may be dose-dependent and consistent with induction by proton blockade of NMDA receptors. Thus, blockade of NMDA currents by protons may be an important component of the anticonvulsant action of extracellular acidosis. The results also suggest that acidosis may be a desirable property for new antiepileptic treatments.

Developmental changes of ketamine action against epileptic afterdischarges induced by hippocampal stimulation in rats

Action of ketamine (5-40 mg/kg) was tested against electrically induced hippocampal afterdischarges (four stimulations in one session; 8 Hz, 15 s) in rats 7, 12, 18, 25 and 90 days old. In control sessions, there was either stable afterdischarge (AD) duration and wet dog shakes (WDS) number or there was an increase in ADs' duration with repeated stimulations. Ketamine had dose-dependent and age-dependent effects. In 7-18-day-old rats, ketamine suppressed better WDS number than AD duration, with nearly absent action on AD duration in 18-day-old animals. Ketamine was equipotent for both phenomena in 25-day-old rats and, in contrast, it decreased more AD duration than WDS number in 90-day-old rats. The data suggest a differentiation induced by ketamine in the expression of motor and electrographic phenomena of the experimental seizures.

Clonidine, but not ritanserin, suppresses kainic acid-induced automatisms in developing rats

In young rats, systemic or local administration of kainic acid (KA) elicits scratching as the prevailing automatism whereas in adult rats, wet dog shakes (WDS) are usually recorded. We tested the effects of the alpha 2-adrenergic agonist clonidine (0.25 mg/kg, IP; also acting, however, on imidazoline receptors), which has been reported to block KA-induced WDS in adult rats and the 5-HT2 antagonist ritanserin (20 mg/kg, IP) in rats aged 7, 12, 18, 25, and 90 days treated IP with doses of KA that induce maximum number of automatisms with minimal early lethal effects (i.e., 4, 6, 8, 10, and 14 mg/kg, respectively). Both WDS and scratching were frequently recorded together in one animal. Neither ritanserin nor its solvent had significant effects on the total number of automatisms or on their distribution between WDS and scratching. In contrast, clonidine suppressed automatisms throughout the development studied. In 90-day-old (adult) rats clonidine decreased the incidence of both WDS and scratching, whereas it usually attenuated scratching at younger ages. Kainic acid-induced seizures were also recorded because of reported incompatibility between tonic-clonic seizures and WDS in adult rats. In 18-90-day-old rats, tonic-clonic seizures and WDS were found incompatible. In 7-18-day-old pups, scratching and KA-induced tonic-clonic seizures occurred together. Moreover, in 7-day-old rats, clonidine was anticonvulsant. We have demonstrated that KA-induced automatisms develop from scratching in pups to prevailing WDS in adult rats, whereas the incidence of scratching rather decreases during development.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a benzodiazepine, bretazenil (Ro 16-6028), on rhythmic metrazol EEG activity: comparison with standard anticonvulsants

A novel anticonvulsant benzodiazepine bretazenil (Ro 16-6028) was studied electrophysiologically in a model of human absence seizures: rhythmic metrazol activity (RMA) in rats. The effects of Ro 16-6028 pretreatment (0.01, 0.05, or 0.1 mg/kg intraperitoneally, i.p.) were compared with those of clonazepam (CZP, 0.02 or 0.1 mg/kg i.p.), valproate (VPA, 200, 300, or 400 mg/kg) and ethosuximide (ESM, 31.25, 62.5, or 125 mg/kg i.p.) in 45 rats with implanted electrocorticographic electrodes. RMA was elicited by an injection of pentylenetetrazol (metrazol, PTZ) in a dose of 40 or 35 mg/kg i.p. The effects of Ro 16-6028 were similar to those of CZP and VPA, i.e., suppression of RMA episodes, an increase in latency and a decrease in number, and total as well as mean duration. On the other hand, ESM differed from these antiepileptic drugs (AEDs) in inability to shorten the duration of RMA episodes. Based on these results, Ro 16-6028 might be predicted to be efficient against human absence seizures.

Age dependence of homosynaptic non-NMDA mediated long-term depression in field CA1 of rat hippocampal slices

It has been hypothesized that high levels of presynaptic activity that fail to activate postsynaptic N-methyl-D-aspartate (NMDA) receptors may lead to long-term depression (LTD). Therefore, we tested the ability of high-frequency (50 Hz) synaptic stimulation in the presence of a blocker of NMDA receptors to elicit homosynaptic LTD at Schaffer collateral-CA1 synapses in hippocampal slices from 15-, 30- and 60-day-old rats. In control slices, there were no developmental differences in the incidence of long-term potentiation (LTP) of either EPSP slope or population spike amplitude. However, while NMDA receptor blockade with the specific antagonist D-2-amino-5-phosphonopentanoic acid (AP5; 25 microM) completely eliminated LTP in 30 and 60-day-olds, a significant number of slices from 15-day-old rats displayed some non-NMDA LTP of synaptic transmission. Moreover, under NMDA receptor blockade, the same high-frequency stimulation now induced homosynaptic LTD of population spike amplitude in a significant number of slices from 15- and 60-day-old rats (47% and 42%, respectively) but not in 30-day-olds (7%). LTD of population spike amplitude was most pronounced in 15-day-old slices (27 +/- 6% of baseline), whereas, in 60-day-old slices, LTD was 81 +/- 3% of baseline. LTD of EPSP slopes occurred in 44% of 15-day-olds, 13% of 30-day-olds, and 33% of slices from 60-day-old rats; the magnitude of EPSP was similar in 15 and 60-day-old slices (70 +/- 9% versus 81 +/- 1% of baseline).(ABSTRACT TRUNCATED AT 250 WORDS)

Models of simple partial and absence seizures in freely moving rats: action of ketamine

The action of ketamine was studied in two models of seizures: a) bilateral neocortical discharges produced by topical application of pentylenetetrazol (model of simple partial seizures); and b) rhythmic spike-and-wave activity induced by systemic administration of pentylenetetrazol (model of absence seizures). Ketamine exerted biphasic effects. In the first model, the dose of 20 mg/kg ketamine significantly suppressed the ictal neocortical discharges (i.e., continuous spiking or ictal activity) accompanied by clonic motor seizures. However, at the dose of 40 mg/kg ketamine significantly accentuated the onset and increased the number of individual discharges (interictal spikes) in bilateral neocortical foci. In the model of rhythmic spike-and-wave activity, the lower dose of ketamine (20 mg/kg) decreased the number of rhythmic spike-and-wave episodes when compared to the higher dose (40 mg/kg) of ketamine, which increased the number of episodes. However, neither result differed significantly from control values. The present results suggest a dose-dependent action of ketamine: Lower doses (10 and 20 mg/kg in the rat) are able to suppress seizure activity, whereas a higher dose (40 mg/kg) potentiates the seizures. Moreover, the action of ketamine may be dependent upon the seizure model used. The study presents a new model of acute epileptic focus in freely moving rats.

Ethosuximide suppresses seizures and lethality induced by picrotoxin in developing rats

The action of ethosuximide (125 or 250 mg/kg, IP) against picrotoxin-induced seizures (3-6 mg/kg, IP) was assessed in rats 12, 18, 25, and 90 days old. In 18-day-old and older controls, picrotoxin regularly elicited clonic seizures; tonic-clonic seizures were induced in all age categories with high consequent mortality. Only the higher dose of ethosuximide (250 mg/kg) increased the latency of clonic seizures in 18- and 25-day-old pups. Tonic-clonic seizures were delayed by ethosuximide in 12-, 18-, and 90-day-old rats. Picrotoxin-induced lethality was suppressed only in 18- and 90-day-old rats by the 250-mg/kg dose of ethosuximide. In contrast, ethosuximide pretreatment increased the incidence of clonic seizures in 12-day-old rats. The results suggest that only high doses of ethosuximide can suppress clonic seizures, and this action is not consistent. Tonic-clonic seizures probably have model-specific sensitivity to ethosuximide because in previous studies ethosuximide completely suppressed pentylenetetrazol-induced tonic-clonic seizures but had no effect on kainic acid-induced tonic-clonic seizures. The suppression of mortality rates is probably due to nonspecific effects of high doses of ethosuximide.

Reduced susceptibility to seizures in carbonic anhydrase II deficient mutant mice

The seizure susceptibility of carbonic anhydrase II (CA) deficient mice and their normal littermates was determined and compared. In flurothyl-induced seizures, CA deficient mice displayed longer latencies to the onset of both clonic and tonic-clonic seizures. In pentylenetetrazole-induced seizures mutant mice exhibited a lower incidence of clonic seizures than did their normal littermates. Acetazolamide (a CA blocker) was used for the pretreatment of normal mice to compare them to CA deficient littermates. The pretreated mice displayed a lower incidence of flurothyl-induced tonic-clonic seizures and of both types of pentylenetetrazole seizures. The attempts to elicit audiogenic seizure did not reveal any difference between normal and mutant littermates. However, when the mice were primed by a loud sound during the critical period and retested for audiogenic seizures again at age 1.5 months, the CA deficient mice displayed a significantly lower incidence of seizures. The similarity between the anticonvulsant action of CA deficiency and the anticonvulsant action of acetazolamide suggests an important role of CA in seizures. The exact mechanism of anticonvulsant action by CA inhibition, however, remains to be elucidated.

Action of antiepileptic drugs against kainic acid-induced seizures and automatisms during ontogenesis in rats

Kainic acid (KA 4-14 mg/kg) administered intraperitoneally (i.p.) produces automatisms (scratching until third postnatal week, "wet dog" shakes thereafter), and clonic and tonic-clonic seizures in rats aged 7, 12, 18, 25, and 90 days. Administration of carbamazepine (CBZ) i.p. (25 or 50 mg/kg), phenobarbital (PB 20-80 mg/kg), clonazepam (CZP 0.2 or 1 mg/kg), or valproate (VPA 200 mg/kg) influenced neither incidence nor latency of automatisms. Clonic seizures that are regularly observed after the third postnatal week in controls were either abolished or substantially suppressed by any of the aforementioned antiepileptic drugs (AEDs). Tonic-clonic seizures observed in the first 3 postnatal weeks were suppressed only by solvent [including propyleneglycol (PEG), ethanol, and water]; the effect of AEDs on tonic-clonic seizures was proconvulsant instead. The automatisms were most resistant to AED therapy. These results induce some doubts about the adequacy of the KA model for identifying AEDs effective against complex partial seizures, but forthcoming AEDs that suppress automatisms in the KA rat model might also be active against human complex partial seizures.

Differential effects of naloxone on postictal depression

The influence of naloxone on the duration of the postictal depression was determined in 2 seizure models in the adult rats: hippocampal afterdischarges and maximal electroshock. For testing the intensity of postictal depression 2 subsequent stimulations were used. The interstimulation intervals were 3, 5, 10 and 60 min. Using interstimulation intervals 3, 5 and 10 min there was marked depression of the afterdischarge duration. Wet dog shakes accompanying hippocampal afterdischarges were suppressed only in 3- and 5-min intervals. Naioxone (1 mg/kg i.p.) abolished the suppression of afterdischarges when 10-min interstimulation interval was used. In maximal electroshock seizures where the duration of tonic flexion and extension was determined, no postictal depression was registered nor were any effects of naloxone present. The results suggest a limited role of the mu opioid receptor system in the late phases of postictal depression following hippocampal stimulation and different effects of the mu opioid system in tonic flexion/extension and behavioral depression induced by maximal electroshock.

N-methyl-D-aspartate (NMDA)-induced seizures in developing rats

Intraperitoneal administration of N-methyl-D-aspartate (NMDA) elicited epileptic motor seizures in developing rats aged from 7 to 25 days as well as in young adults. The very first sign of NMDA action is locomotor hyperactivity which is followed by clonic and tonic seizures. In rat pups during the first 3 postnatal weeks flexion seizures (emprosthotonus) appeared as the first pattern of motor seizures; later they were replaced by generalized tonic clonic seizures. Only regular tonic-clonic seizures were observed in 25-day-old and adult rats. The youngest animals are the most sensitive to NMDA. CD50 for tonic-colonic seizures is 6.7 mg/kg in 7-day-old rat pups and it increases up to 86.6 mg/kg in 25-day-old animals. Similar changes could also be demonstrated for LD50, a lethal outcome being very frequent. EEG recordings demonstrated mainly suppressed activity, thus exhibiting a lack of correlation with motor phenomena.

Bicuculline-induced neocortical epileptiform foci and the effects of 6-hydroxydopamine in developing rats

Catecholamines (dopamine and norepinephrine) are considered to be predominantly inhibitory neurotransmitters in the brain and their depletion produced by 6-hydroxydopamine may result in proconvulsant effects. In our experiments on rats aged 5, 7, 9, 12, 15, 18, 25 and 90 days under urethane anesthesia we demonstrated the development of neocortical epileptic focus evoked by topical application of bicuculline methiodide. In experimental groups aged 7, 12, 18, 25 and 90 days a chronic depletion of catecholamines was induced using pretreatment with 6-hydroxydopamine early postnatally. An epileptogenic focus was induced in all age groups; duration of a single discharge decreased with age in both control and experimental animals. The spread of activity from the primary focus to contralateral frontal cortex via callosal connections was as rapid as in controls. However, the transfer of discharge to occipital regions was delayed and the number of discharges decreased in experimental rats. Our study demonstrated a substantial role of catecholamines for synchronization of focal discharges in neocortex and a promoting role of catecholamines in association pathways within neocortex.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. II. The effects of MK-801

MK-801 is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist with anticonvulsant and neuroprotective properties. The action of MK-801 (0.05-10 mg/kg IP) was assessed against pentylenetetrazol-induced seizures (PTZ; 100 mg/kg SC; 30 min after MK-801) in rats aged 7, 12, 18, 25, and 90 days (N = 263). We observed pronounced ataxia and hypermobility after MK-801 pretreatment during the whole ontogenesis, and the animals exhibited head-weaving and body-rolls. After the combination of MK-801 and PTZ "wet dog shakes" were detected in 18-, 25-, and 90-day-old rats (never seen in controls receiving PTZ only). MK-801 only insignificantly modified the latencies of minimal (clonic) seizures in 18-day-old and older rats where this seizure type is regularly elicited. In 12-day-old rats an increased incidence of minimal seizures was detected. MK-801 nearly completely blocked or strongly delayed major (generalized tonic-clonic) seizures and attenuated the seizure severity during ontogenesis in a dose-dependent manner. Present results suggest the important role of NMDA receptors in the genesis of generalized tonic-clonic seizures whilst the role of NMDA receptors in minimal seizures appears to be negligible during the whole ontogenetic development.

Ketamine suppresses both bicuculline- and picrotoxin-induced generalized tonic-clonic seizures during ontogenesis

An anticonvulsant action of ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist (5-40 mg/kg IP), on the bicuculline-induced (3-8 mg/kg IP) or picrotoxin-induced seizures (3-6 mg/kg IP) was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused moderate ataxia which was more pronounced in younger animals. In combination with both aforementioned convulsants, ketamine exerted anticonvulsant effects against generalized tonic-clonic seizures in all developmental stages studied. This effect was more pronounced in bicuculline-treated animals. Moreover, ketamine also suppressed the lethality induced by both drugs during all the development. On the contrary, the action of ketamine on minimal (clonic) seizures was moderate or absent. Our results suggest an important role of ketamine-affected transmission in the generation of the generalized tonic-clonic seizure pattern; moreover, an action of high doses of ketamine on GABA-A receptors might be present.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. I. The effects of 2-amino-7-phosphonoheptanoate

The anticonvulsant action of 2-amino-7-phosphonoheptanoate (AP7) was assessed during ontogenesis of the rat. Animals of five age groups (7, 12, 18, 25, and 90 days) were pretreated with AP7 i.p. in the doses from 15 to 60 mg/kg 30 min prior to pentamethylenetetrazol (PTZ; metrazol; 100 mg/kg s.c.). The incidence and latency of minimal seizures (pure clonic without the loss of righting ability) and of generalized tonic-clonic seizures (major) were evaluated and compared with the control groups. Minimal metrazol seizures were not regularly observed in controls between ages 7 and 12 days. An increased incidence was noticed in AP7-treated groups. In animals of 18 days of age and older the AP7-pretreatment did not influence incidence of minimal seizures; the latencies were significantly lengthened only in 18-day-old animals. Major seizures were significantly suppressed with the highest dose of AP7 (60 mg/kg) in all groups except 7-day-old rats. In 90-day-old rats all doses of AP7 were effective in the suppression of major seizures. The latencies of major seizures were increased in 7 and 18 days old rats. It appears that the blockade of NMDA receptor substantially influences the major seizures induced by PTZ, whereas minimal (clonic) seizures are affected weakly. This suggests an important role of NMDA receptor-mediated transmission in the genesis of generalized tonic-clonic seizure pattern.

Ethosuximide affects both pentylenetetrazole- and kainate-induced clonic seizures but differentiates between tonic-clonic seizures

Young (25-day-old) and adult (90-day-old) rats pretreated with ethosuximide (62.5 or 125 mg/kg i.p.) were injected with either s.c. pentylenetetrazole (100 mg/kg) or i.p. kainate (10 or 14 mg/kg). The incidences and latencies of minor (clonic) and major (tonic-clonic) seizures were registered. Ethosuximide (125 mg/kg) completely blocked clonic seizures induced by the lower dose of kainate, and slightly suppressed or delayed those induced by the higher dose of kainate or pentylenetetrazole in both age groups. The effect of ethosuximide on major kainate-induced seizures (elicited in young rats only) was insignificant (ethosuximide only partially decreased the incidence of major seizures), whereas ethosuximide abolished major pentylenetetrazole-induced seizures in both age groups. Ethosuximide also failed to affect the latencies of kainate-induced automatisms (e.g., scratching, wet dog shakes). Similarities between kainate- and pentylenetetrazole-induced clonic seizures, as well as a similar action of ethosuximide on both, suggest a common generator for the pattern of clonic seizures.

Effects of ketamine on metrazol-induced seizures during ontogenesis in rats

An anticonvulsant action of ketamine, a noncompetitive NMDA receptor antagonist (1-40 mg/kg IP), on the metrazol-induced seizures was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused ataxia even in the lowest dose used. As concerens its interaction with metrazol it exerted a clearcut anticonvulsant effect against generalized tonic-clonic seizures at all developmental stages. On the contrary, the effects on clonic (i.e., minimal) seizures were only moderate or absent. Higher efficacy of ketamine was observed in young animals. Our results suggest a role of excitatory amino acids in the generation of generalized tonic-clonic metrazol seizures, but their share on the induction of clonic (minimal) seizures seems to be very small.