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

Early life status epilepticus and stress have distinct and sex-specific effects on learning, subsequent seizure outcomes, including anticonvulsant response to phenobarbital

AIMS

Neonatal status epilepticus (SE) is often associated with adverse cognitive and epilepsy outcomes. We investigate the effects of three episodes of kainic acid-induced SE (3KA-SE) and maternal separation in immature rats on subsequent learning, seizure susceptibility, and consequences, and the anticonvulsant effects of phenobarbital, according to sex, type, and age at early life (EL) event.

METHODS

3KA-SE or maternal separation was induced on postnatal days (PN) 4-6 or 14-16. Rats were tested on Barnes maze (PN16-19), or lithium-pilocarpine SE (PN19) or flurothyl seizures (PN32). The anticonvulsant effects of phenobarbital (20 or 40 mg/kg/rat, intraperitoneally) pretreatment were tested on flurothyl seizures. FluoroJadeB staining assessed hippocampal injury.

RESULTS

3KA-SE or separation on PN4-6 caused more transient learning delays in males and did not alter lithium-pilocarpine SE latencies, but aggravated its outcomes in females. Anticonvulsant effects of phenobarbital were preserved and potentiated in specific groups depending on sex, type, and age at EL event.

CONCLUSIONS

Early life 3KA-SE and maternal separation cause more but transient cognitive deficits in males but aggravate the consequences of subsequent lithium-pilocarpine SE in females. In contrast, on flurothyl seizures, EL events showed either beneficial or no effect, depending on gender, type, and age at EL events.

Issues related to development of new antiseizure treatments

This report represents a summary of the discussions led by the antiseizure treatment working group of the International League Against Epilepsy (ILAE)/American Epilepsy Society (AES) Working Groups joint meeting in London (London Meeting). We review here what is currently known about the pharmacologic characteristics of current models of refractory seizures, both for adult and pediatric epilepsy. In addition, we address how the National Institute of Neurological Disorders and Stroke (NINDS)-funded Anticonvulsant Screening Program (ASP) is evolving to incorporate appropriate animal models in the search for molecules that might be sufficiently novel to warrant further pharmacologic development. We also briefly address what we believe is necessary, going forward, to achieve the goal of stopping seizures in all patients, with a call to arms for funding agencies, the pharmaceutical industry, and basic researchers.

Excitatory actions of GABA in the intact neonatal rodent hippocampus in vitro

The excitatory action of gamma-aminobutyric acid (GABA) is considered to be a hallmark of the developing nervous system. However, in immature brain slices, excitatory GABA actions may be secondary to neuronal injury during slice preparation. Here, we explored GABA actions in the rodent intact hippocampal preparations and at different depths of hippocampal slices during the early post-natal period [post-natal days (P) 1–7]. We found that in the intact hippocampus at P1–3: (i) GABA exerts depolarizing action as seen in cell-attached single GABA(A) channel recordings; (ii) GABA(A) receptor (GABA(A)-R) agonist isoguvacine and synaptic activation of the GABA(A)-Rs increase the frequency of multiple unit activity and the frequency of the network-driven giant depolarizing potentials (GDPs); and that (iii) Na⁺–K⁺–2Cl^- cotransporter (NKCC1) antagonist bumetanide suppresses GDPs and the excitatory actions of isoguvacine. In the hippocampal slices at P2–5, isoguvacine and synaptic activation of GABA(A)-Rs-evoked excitatory responses at all slice depths, including surface and core. Thus, GABA exerts excitatory actions in the intact hippocampus (P1–3) and at all depths of hippocampal slices (P2–5). Therefore, the excitatory actions of GABA in hippocampal slices during the first post-natal days are not due to neuronal injury during slice preparation, and the trauma-related excitatory GABA actions at the slice surface are a fundamentally different phenomenon observed during the second post-natal week.

The effect of antihistamines on seizures induced by increasing-current electroshocks: ketotifen, but not olopatadine, promotes the seizures in infant rats

Clinical reports have shown that some antihistamines, such as ketotifen, occasionally produced seizures, especially in pre-school age children or young patients with epilepsy. The purpose of this study was to investigate whether olopatadine, one of the most efficacious antihistamines, promotes seizures induced by electroshocks in young rats. We investigated the seizures induced by electroshock using increasing-current delivery in 3- or 4-week-old rats, and found that the threshold-current of tonic extensor seizures was elevated with age in weeks in the vehicle-treatment groups. While caffeine decreased the threshold-current in every age group of rats, pentylenetetrazole, a γ-aminobutyric acid (GABA)(A) receptor antagonist, significantly decreased them only in 4-week-old rats. On the other hand, ketotifen decreased them only in 3-weeks-old rats. In the 3-week-old rats, neither olopatadine nor fexofenadine had any effect on the threshold-currents of tonic extensor seizures. These results showed that histaminergic neuro-transmission in the brain plays a crucial role in inhibiting seizures in rats soon after weaning, but is no longer effective in rats as they approach sexual maturation. In addition, unlike ketotifen, olopatadine, as well as fexofenadine, do not promote the occurrence of seizures in infant rats.

Decreased GABABR expression and increased neuronal cell death in developing rat brain after PTZ-induced seizure

The objective of this study was to evaluate the PTZ-induced seizures effects on GABAB receptor (R) expression and to observe its neurodegenerative effect in hippocampal part of developing rat brain. In the present study, high dose of pentylenetetrazol (PTZ 40 mg/kg) was injected in developing rats of age 5 weeks having average weight of 60-65 g for 4 days. Further, baclofen (B 3 mg/kg i.p) agonist and phaclofen (P 30 μg/rat) antagonist of GABABR were injected along with PTZ. Western blot analysis was used to elucidate expression of GABABR protein upon PTZ, baclofen and phaclofen exposure in the developing rat brain. Furthermore, PTZ-induced apoptotic neurodegeneration was also observed through the release of caspase-3 antibody and propidium iodide (PI) staining using confocal microscopy. Seizure was confirmed using electroencephalography (EEG) data obtained from the Laxtha EEG-monitoring device in the EEG recording room and EEG was monitored 5-15 min after PTZ injection. The results of the present study showed that PTZ-induced seizure significantly decreased GABABR expression and induced neuronal apoptosis in cortical and hippocampal part of brain. While, baclofen reverse the effect of PTZ by increasing the expression of GABABR as compared to the PTZ- , PTZ plus B- and PTZ plus P-treated groups. Our findings indicated that PTZ-induced seizure showed not only decrease in GABABR expression but also cause neuronal apoptosis in the developing rat brain.

Validation of the rat model of cryptogenic infantile spasms

PURPOSE

To determine whether a new model of cryptogenic infantile spasms consisting of prenatal priming with betamethasone and postnatal trigger of spasms by N-methyl-D-aspartate (NMDA) responds to chronic adrenocorticotropic hormone (ACTH) treatment, and has electroencephalography (EEG) signature, efficacy of treatments, and behavioral impairments similar to those in human infantile spasms.

METHODS

Rats prenatally primed with betamethasone on gestational day 15 were used. Spasms were triggered with NMDA between postnatal days (P) 10 and 15 in a single session or in multiple sessions in one subject. The expression of spasms was compared to prenatally saline-injected controls. Effects of relevant treatments (ACTH, vigabatrin, methylprednisolone, rapamycin) were determined in betamethasone-primed rats. In the rats after spasms, behavioral evaluation was performed in the open field and elevated plus maze on P20-22.

KEY FINDINGS

NMDA at P10-15 (the rat "infant" period) triggers the spasms significantly earlier and in greater numbers in the prenatal betamethasone-exposed brain compared to controls. Similar to human condition, the spasms occur in clusters. Repeated trigger of spasms is associated with ictal EEG electrodecrements and interictal large-amplitude waves, a possible rat variant of hypsarrhythmia. Chronic ACTH treatment in a randomized experiment, and chronic pretreatment with methylprednisolone significantly suppress the number of spasms similar to the human condition. Pretreatment with vigabatrin, but not rapamycin, suppressed the spasms. Significant behavioral changes occurred following multiple bouts of spasms.

SIGNIFICANCE

The model of infantile spasms has remarkable similarities with the human condition in semiology, EEG, pharmacologic response, and long-term outcome. Therefore, the model can be used to search for novel and more effective treatments for infantile spasms.

Progressive NKCC1-dependent neuronal chloride accumulation during neonatal seizures

Seizures induce excitatory shifts in the reversal potential for GABA(A)-receptor-mediated responses, which may contribute to the intractability of electro-encephalographic seizures and preclude the efficacy of widely used GABAergic anticonvulsants such as phenobarbital. We now report that, in intact hippocampi prepared from neonatal rats and transgenic mice expressing Clomeleon, recurrent seizures progressively increase the intracellular chloride concentration ([Cl(-)](i)) assayed by Clomeleon imaging and invert the net effect of GABA(A) receptor activation from inhibition to excitation assayed by the frequency of action potentials and intracellular Ca(2+) transients. These changes correlate with increasing frequency of seizure-like events and reduction in phenobarbital efficacy. The Na(+)-K(+)-2Cl(-) (NKCC1) cotransporter blocker bumetanide inhibited seizure-induced neuronal Cl(-) accumulation and the consequent facilitation of recurrent seizures. Our results demonstrate a novel mechanism by which seizure activity leads to [Cl(-)](i) accumulation, thereby increasing the probability of subsequent seizures. This provides a potential mechanism for the early crescendo phase of neonatal seizures.

Effect of cross-fostering on seizures in adult male offspring of methamphetamine-treated rat mothers

Stimulant drugs are often associated with increased seizure susceptibility. Inhibitory gamma-aminobutyric acid (GABA) and excitatory N-methyl-D-aspartate (NMDA) systems play a role in the effect of stimulants in the genesis of epileptic seizures. Our previous studies showed that prenatal methamphetamine (MA) exposure induced long-term changes in seizure susceptibility. The aim of the present study was to investigate the effect of cross-fostering on the prenatal and postnatal MA-exposed rats, respectively, on their seizures in adulthood. Bicuculline (GABA(A) receptor antagonist), NMDA (NMDA receptor agonist) and flurothyl (a convulsant gas) were used to induce seizures in adult male offsprings. Female dams were injected with MA (5 mg/kg daily) or physiological saline (S) for approx. 9 week [about 3 week prior to impregnation, for the entire gestation period (22 days) and in preweaning period (21 days)]. Absolute controls (C) did not receive any injections. On postnatal day 1, pups were cross-fostered so that each mother received pups from all three treatments. Thus, nine groups (based on the prenatal and postnatal drug exposure) of adult male rats were tested in each seizure test: C/C; C/S; C/MA; S/C; S/S; S/MA; MA/C; MA/S; MA/MA. The present study demonstrates that the effect of prenatal and/or postnatal MA exposure is seizure model specific. In addition, our data show that there is an effect of cross-fostering on seizures; particularly, the effect of prenatal MA exposure shown in animals fostered by control mothers is no longer apparent in animals fostered postnatally by MA-treated mothers. Such effect of postnatal treatment is not manifested in prenatal controls. In summary, it seems that: (1) prenatal MA exposure alters seizure susceptibility more than postnatal MA exposure; (2) especially in seizures induced by chemicals that affect GABAergic system (bicuculline, flurothyl) notable effect of adoption (cross-fostering) is apparent; (3) in seizure models that are associated with NMDA system (NMDA, flurothyl), effect of prenatal stress seems to play a role.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

Timing of the developmental switch in GABA(A) mediated signaling from excitation to inhibition in CA3 rat hippocampus using gramicidin perforated patch and extracellular recordings

The timing of the developmental switch in the GABA(A) mediated responses from excitatory to inhibitory was studied in Wistar rat CA3 hippocampal pyramidal cells using gramicidin perforated patch-clamp and extracellular recordings. Gramicidin perforated patch recordings revealed a gradual developmental shift in the reversal potential of synaptic and isoguvacine-induced GABA(A) mediated responses from -55 +/- 4 mV at postnatal days P0-2 to -74 +/- 3 mV at P13-15 with a midpoint of disappearance of the excitatory effects of GABA at around P8. Extracellular recordings in CA3 pyramidal cell layer revealed that the effect of isoguvacine on multiple unit activity (MUA) switched from an increase to a decrease at around P10. The effect of synaptic GABA(A) mediated responses on MUA switched from an increase to a decrease at around P8. It is concluded that the developmental switch in the action of GABA via GABA(A) receptors from excitatory to inhibitory occurs in Wistar rat CA3 pyramidal cells at around P8-10, an age that coincides with the transition from immature to mature hippocampal rhythms. We propose that excitatory GABA contributes to enhanced excitability and ictogenesis in the neonatal rat hippocampus.

Shunting and hyperpolarizing GABAergic inhibition in the high-potassium model of ictogenesis in the developing rat hippocampus

Ontogenesis of GABAergic signaling may play an important role in developmental changes in seizure susceptibility in the high-potassium model of ictogenesis in vitro. The age-dependent effects of [K(+)](o) on the reversal potential of the GABA(A)-mediated responses and membrane potential in hippocampal slices in vitro were compared with the effect of GABA(A)-receptors antagonists and GABA(A) modulators on high-potassium induced seizures in the CA3 pyramidal layer of rat hippocampus in vivo. GABA(A) responses were depolarizing at P8-12 and hyperpolarizing at P17-21. In P8-12 rats, GABA(A) responses switch their polarity from depolarizing to hyperpolarizing upon elevation of extracellular potassium. At approximately 10 mM [K(+)](o), activation of GABA(A) receptors produced an isoelectric, purely shunting response characterized by no changes in the membrane potential but an increase in the membrane conductance. In P17-21 rats, the hyperpolarizing GABA(A) driving force progressively increased with elevation of [K(+)](o). In P8-12 rats in vivo, GABA(A)-receptor antagonists did not affect the occurrence of ictal discharges induced by intrahippocampal injection of 10 mM [K(+)](o), but significantly increased seizure duration. Diazepam and isoguvacine completely prevented seizures induced by 10 mM [K(+)](o). In P17-21 rats, GABA(A)-receptor antagonists strongly increased the occurrence of ictal activity induced both by 10 mM [K(+)](o). Taken together, these results suggest that anticonvulsive effects of GABA are because of the combination of shunting and hyperpolarizing actions of GABA. Although shunting GABA is already efficient in the young age group, a developmental increase in the hyperpolarizing GABA(A) driving force likely contributes to the increase in the GABAergic control of seizures upon maturation.

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

The action of a GABA(B) antagonist CGP 35348 and a GABA(B) agonist baclofen was studied in two models of epileptic seizures characterized by EEG spike-and-wave rhythm in freely moving immature rats. Rhythmic metrazol activity (RMA, model of human absences) was induced by low systemic dose of pentylenetetrazol (PTZ) in 18- and 25-day-old rats, epileptic after discharges (ADs, model of human myoclonic seizures) were elicited by electrical stimulation of sensorimotor cortex in rat pups 12, 18, and 25 days old. CGP 35348 (50, 100 and 200 mg/kg i.p.) suppressed RMA in both age groups in a dose-dependent manner. Simultaneously it increased the incidence of clonic seizures, potentiating thus an effect of PTZ. Baclofen (1, 3 and 6 mg/kg i.p.) augmented markedly RMA in 25-day-old rats. On the contrary, baclofen suppressed RMA in a part of 18-day-old animals. Incidence of seizures was not changed by baclofen in either age group. As ADs are concerned CGP 35348 (100 and 200 mg/kg i.p.) exhibited a proconvulsant action, baclofen (3, 6 or 12 mg/kg i.p.) was anticonvulsant, but again an irregularity of action was found in 18-day-old rats. The role of GABA(B)-mediated inhibition in epileptogenesis depends on the type of seizures and also on the stage of maturation.

Anticonvulsant action of GABA in the high potassium-low magnesium model of ictogenesis in the neonatal rat hippocampus in vivo and in vitro

Previous developmental studies in vitro suggested that the inhibitory neurotransmitter GABA exerts depolarizing and excitatory actions on the immature neurons and that depolarizing GABA is causally linked to ictal activity during the first weeks of postnatal life. However, remarkably little is known on the role of GABA in the generation of neonatal seizures in vivo. Here, using extracellular recordings from CA3 hippocampus, we studied the effects of GABA(A)-acting drugs on electrographic seizures induced by local intrahippocampal injection of the epileptogenic agents (high K(+)/low Mg(2+)) in the nonanesthetized rats in vivo and in the hippocampal slices in vitro during the second postnatal week (postnatal days P8-12). We found that in vivo, the induction of ictal-like events was facilitated by co-infusion of high-K(+)/low Mg(2+) together with the GABA(A) antagonist bicuculline or gabazine. Moreover, the infusion of bicuculline alone caused ictal-like activity in approximately 30% of cases. Co-infusion of the GABA(A) receptor agonist isoguvacine or the GABA(A)-positive allosteric modulator diazepam completely prevented high-K(+)/low Mg(2+)-induced seizures. In in vitro studies using hippocampal slices, we also found that high-K(+)/low Mg(2+) produced ictal activity that was exacerbated by bicuculline and gabazine and reduced by isoguvacine. Thus in the model of high-K(+)/low Mg(2+)-induced seizures both in in vivo and in vitro conditions, GABA, acting via GABA(A) receptors, has an anticonvulsant effect during the critical developmental period of enhanced excitability.

Seizures in the Developing Brain

PURPOSE

Development and sex hormones are important determinants of seizure susceptibility. Seizures develop in the immature brain more readily than in the mature brain. Male children experience a higher incidence of epilepsy or unprovoked seizures than do female children. Sex-specific differences in the development of seizure-suppressing neuronal networks may account, at least in part, for this increased age- and sex-related susceptibility to seizures. The control of seizures can be influenced by the substantia nigra pars reticulata (SNR) in an age- and sex-specific manner. In the adult male rat SNR, two topographically discrete regions (SNRanterior and SNRposterior) mediate distinct effects on seizures, by using divergent output networks in response to localized infusions of gamma-aminobutyric acid (GABA)A agents, such as muscimol. The GABAA-sensitive "anticonvulsant" region is located in the SNRanterior, whereas the GABAA-sensitive "proconvulsant region is in the SNRposterior. In immature postnatal day (PN)15-21 male rats, the SNR is not topographically segregated, and GABAAergic drug infusions produce similar effects when applied in the SNRanterior or SNRposterior. Only a GABAA-sensitive proconvulsant network is evident. By contrast, female SNR does not contain any region that mediates muscimol-related proconvulsant effects. As with the adult, immature female rats do not develop a proconvulsant SNR region at any age.

METHODS

We measured the effects of SNR muscimol infusions on seizures in male rats castrated at birth to better understand the effects of testosterone on the formation of age- and sex-specific features of the SNR.

RESULTS

Neonatal castration permanently alters the maturation of the muscimol-sensitive SNR effect on seizures. The SNR of neonatally castrated rats develops functionally like the "female" SNR. The "proconvulsant" SNR region does not develop in the absence of testosterone in the immediate postnatal period. The "male" type of SNR effects can be induced in neonatally castrated rats by restoration of testosterone levels or in female rats by artificially increasing testosterone levels. Dihydrotestosterone and estrogen, produced by the reduction and aromatization of testosterone, respectively, are the direct mediators of testosterone actions. At PN0, only beta estrogen receptors are equally expressed in the SNRs of males and females and may be responsible for testosterone-mediated effects in both sexes.

CONCLUSIONS

The phenotype of SNR GABAergic neurons, as characterized by GABAA-receptor subunit composition, by muscimol-induced electrophysiologic responses, and by connectivity of output networks each may be altered by the presence of testosterone. Higher KCC2 messenger RNA (mRNA) expression in female PN15 SNR neurons compared with males may be responsible for sex-related differences in muscimol-induced electrophysiologic responses. In summary, a growing body of compelling evidence identifying sex-related differences in the SNR implicates postnatal testosterone as a critical factor in the development of pro- or anticonvulsant circuits. The recognition of sex- and age-related features in the SNR holds the promise that these findings can be translated into the development of specific and effective treatments for seizure disorders.

Theophylline impairs memory/learning in developing mice

We studied the relationship between theophylline and memory/learning using an elevated plus-maze test and measuring spontaneous locomotor activity in developing mice. There were no significant differences in transfer latency (TL) in 21-, 30- and 42-day-old mice in the acquisition trial, but theophylline significantly prolonged TL in comparison with the control group in 21- and 30-day-old mice in the retention trial (n = 20; 21-day-old mice, P = 0.0009; 30-day-old mice, P = 0.03). The dose-dependent stimulant effects of theophylline were shown in spontaneous locomotor activity (n = 16; 21-day-old mice, P = 0.0005; 30-day-old mice, P = 0.0007; 42-day-old mice, P = 0.0003) in all groups and in rearing in 21-day-old mice (P = 0.02) 1 h after drug administration, but there were no significant effects 24 h after drug administration in all groups. Our study showed that even a therapeutic dose of theophylline impaired memory/learning only in developing mice, and theophylline increased locomotor activity both in developing and adolescent mice. These observations are consistent with several clinical studies, which reported the relationships between theophylline use and memory/learning and learning-related behaviors in children, such as difficulty in studying and hyperactivity. Theophylline possibly induces memory/learning disabilities and hyperactivity in infants and young children.

Developmental changes in GABAergic actions and seizure susceptibility in the rat hippocampus

The immature brain is prone to seizures but the underlying mechanisms are poorly understood. We explored the hypothesis that increased seizure susceptibility during early development is due to the excitatory action of GABA. Using noninvasive extracellular field potential and cell-attached recordings in CA3 of Sprague-Dawley rat hippocampal slices, we compared the developmental alterations in three parameters: excitatory actions of GABA, presence of the immature pattern of giant depolarizing potentials (GDPs) and severity of epileptiform activity generated by high potassium. The GABA(A) receptor agonist isoguvacine increased firing of CA3 pyramidal cells in neonatal slices while inhibiting activity in adults. A switch in the GABA(A) signalling from excitation to inhibition occurred at postnatal day (P) 13.5 +/- 0.4. Field GDPs were present in the form of spontaneous population bursts until P12.7 +/- 0.3. High potassium (8.5 mm) induced seizure-like events (SLEs) in 35% of slices at P7-16 (peak at P11.3 +/- 0.4), but only interictal activity before and after that age. The GABA(A) receptor antagonist bicuculline reduced the frequency or completely blocked SLEs and induced interictal clonic-like activity accompanied by a reduction in the frequency but an increase in the amplitude of the population spikes. In slices with interictal activity, bicuculline typically caused a large amplitude interictal clonic-like activity at all ages; in slices from P5-16 rats it was often preceded by one SLE at the beginning of bicuculline application. These results suggest that, in the immature hippocampus, GABA exerts dual (both excitatory and inhibitory) actions and that the excitatory component in the action of GABA may contribute to increased excitability during early development.

The effects of seizures on the connectivity and circuitry of the developing brain

Recurring seizures in infants and children are often associated with cognitive deficits, but the reason for the learning difficulties is unclear. Recent studies in several animal models suggest that seizures themselves may contribute in important ways to these deficits. Other studies in animals have shown that recurring seizures result in dendritic spine loss. This change, coupled with a down-regulation in NMDA receptor subunit expression, suggests that repetitive seizures may interrupt the normal development of glutamatergic synaptic transmission. We hypothesize that homeostatic, neuroprotective processes are induced by recurring early-life seizures. These processes, by diminishing glutamatergic synaptic transmission, are aimed at preventing the continuation of seizures. However, by preventing the normal development of glutamatergic synapses, and particularly NMDA receptor-mediated synaptic transmission, such homeostatic processes also reduce synaptic plasticity and diminish the ability of neuronal circuits to learn and store memories.

Oral baclofen in cerebral palsy: possible seizure potentiation?

Baclofen, a gamma-aminobutyric acid agonist, is widely used to treat spasticity of cerebral and spinal origin. Patients with both acute baclofen overdose and withdrawal have developed seizures. After several reports of new-onset seizures in children treated with oral baclofen at our institution, we reviewed our experience regarding possible effects of baclofen on seizure induction in a childhood movement disorders program over a 2-year period. Of 54 children (ages 1-10) treated with oral baclofen, 19 (35%) had a prior history of seizures. Five children (14%) developed new-onset seizures after starting baclofen. Although epilepsy is very common in children with cerebral palsy, these findings raise the possibility that baclofen may potentiate seizures in certain young children with cerebral palsy. Further study of the effects of baclofen on seizures is warranted.

Role of subcortical structures in the pathogenesis of infantile spasms: what are possible subcortical mediators?

Infantile spasms present a constellation of symptoms and laboratory findings that suggest a role of subcortical circuits in the pathogenesis of this illness. The clinical features of spasms and the influence of subcortical circuits in the regulation of the electroencephologram, along with frequent abnormalities in subcortical structure and functional anatomy, brain stem electrophysiology, sleep regulation, and subcortical neurotransmitter levels, point to the importance of subcortical circuits in the generation of spasms. Furthermore, laboratory evidence shows that modulation of subcortical nuclei may attenuate and ameliorate seizures. We review clinical evidence indicating abnormal function in subcortical circuits and present a hypothesis that the development of infantile spasms requires dysfunction in both cortical and subcortical circuits. The confluence of evidence suggesting a role of subcortical structures in the origin of spasms and laboratory data indicating an anticonvulsant role on some subcortical nuclei raise the possibility of novel approaches to the treatment of infantile spasms.

Electrical stimulation of substantia nigra pars reticulata is anticonvulsant in adult and young male rats

Electrical stimulation of deep brain structures has been used for pain relief and treatment of refractory Parkinson's disease. Recently, stimulation of the subthalamic nucleus or anterior nuclei of the thalamus was introduced for the treatment of refractory epilepsy when other treatments failed. The substantia nigra pars reticulata (SNR) is another crucial site involved in the control of seizures. We studied the effects of continuous electrical stimulation of the SNR as a function of age in male rats. Adult [postnatal day (PN) 60] and young (PN 15) rats with electrodes symmetrically implanted in the SNR were used. The rats were stimulated with continuous constant current pulses (130 Hz) and simultaneously challenged with flurothyl to induce seizures. Control rats had the electrodes implanted but were not stimulated. High-frequency electrical stimulation of the SNR had anticonvulsant effects in both age groups. However, we identified age-specific features: In PN 60 rats, both unilateral and bilateral stimulation of the anterior region of the SNR produced anticonvulsant effects against clonic seizures, while stimulation of the posterior region of the SNR was ineffective. Stimulation of either SNR region had no effects on tonic-clonic seizures. In PN 15 rats, irrespective of the stimulation site within the SNR, bilateral stimulations of the SNR produced anticonvulsant effects against both clonic and tonic-clonic flurothyl-induced seizures, while unilateral stimulation was without effect. The data suggest that the SNR may be a candidate site for deep brain stimulation for the treatment of epilepsy.

Prenatal morphine exposure induces age- and sex-dependent changes in seizure susceptibility

1. Prenatal exposure to morphine induces long-term alterations in seizure susceptibility, which are age-, sex-, and seizure model-specific. 2. Adult male and female rats exposed prenatally to morphine show decreased susceptibility to GABA-regulated seizures. 3. Prenatally morphine-exposed, adult male rats are more sensitive to excitatory amino acid receptor-mediated seizures than control males, control females, or morphine-exposed females.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

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.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.

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.

Prenatal morphine exposure differentially alters seizure susceptibility in developing female rats

We examined the effect of prenatal morphine exposure (5-10 mg/kg on days 11-18 of gestation) on seizure susceptibility in female rats during development. The effect of morphine exposure on flurothyl-induced seizures was age-dependent. At postnatal day (PN) 15, morphine exposure decreased both clonic and tonic-clonic seizure thresholds compare to saline controls. At PN 25, morphine exposure did not alter the clonic seizure threshold but increased the threshold to tonic-clonic seizure. At PN 38, morphine exposure did not influence either threshold. The data suggest that the effects of prenatal exposure to opioids on seizures are age-related and transient.

GluR2 hippocampal knockdown reveals developmental regulation of epileptogenicity and neurodegeneration

In adult rats, kainic acid-induced status epilepticus reduces GluR2 subunit expression prior to neurodegeneration of hippocampal CA3 neurons. Increased formation of Ca2+ permeable AMPA receptors may contribute to the delayed neurodegenerative process. In rat pups, highly prone to seizures but resistant to seizure-induced hippocampal damage, GluR2 mRNA and protein expression remain constant in CA3 neurons possibly contributing to their survival. To investigate whether reduced GluR2 expression in hippocampus may lead to enhanced hippocampal vulnerability in an age-dependent manner and whether changes correspond to altered electroencephalography (EEG) patterns, unilateral microinfusion of GluR2 antisense oligodeoxynucleotides (AS-ODNs) into hippocampus was performed at three ages (postnatal (P8), P13, and adult). At P13, GluR2 knockdown resulted in spontaneous seizure-like behavioral manifestations and neurodegeneration of CA3 neurons lateral and distal from the cannula infusion site. EEG recordings revealed high rhythmic activity associated with seizure-like behavior. In P8 pups and adult rats, there were no behavioral manifestations; distant hippocampal damage of the CA3 was not observed. Results indicate that unilateral knockdown of hippocampal GluR2 subunit expression induces age-dependent seizure-like behavioral manifestations, altered EEG recording patterns, and reduces the survival of CA3 neurons in the hippocampus of young rats during a specific postnatal period (3rd week), when GluR2 expression peaks in development and glutamatergic inputs are maturing.

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.

Regional and age specific effects of zolpidem microinfusions in the substantia nigra on seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata (SNR) is critical for seizure control. The SNR effects on seizures are site-specific within the SNR and developmentally regulated. These age- and site-specific effects may be due to differential regional distribution and functionality of SNR GABA(A) receptor sites. We investigated the role of GABA/benzodiazepine (BZD) receptors in the SNR in the control of seizures as a function of age. In adult rats, we determined the effects of bilateral zolpidem (an agonist of the BZD1 receptor site) microinfusions in the anterior or in the posterior SNR (SNRanterior or SNRposterior, respectively) on flurothyl-induced clonic and tonic-clonic seizures. In SNRanterior, zolpidem microinfusions were anticonvulsant but ineffective in SNRposterior against clonic seizures. Microinfusions of zolpidem in SNRposterior or above SNR, did not alter the threshold to clonic seizures. SNR microinfusions of zolpidem did not alter the threshold to tonic-clonic flurothyl-induced seizures. In 15 day old (PN 15) rats, the SNR microinfusions of zolpidem had anticonvulsant effects on clonic and tonic-clonic seizures. There was no regional specificity. Microinfusions of zolpidem above the SNR, did not alter the threshold to clonic or tonic-clonic seizures. Our data demonstrate that the BZD1 binding sites are involved in the SNR control of flurothyl seizures in adult and PN 15 male rats.

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.

Therapeutic doses of theophylline exert proconvulsant effects in developing mice

We studied the effect of therapeutic doses of theophylline on electrically-induced convulsions in developing mice. A theophylline dose as small as 3 mg/kg increased seizure susceptibility of 21-day-old mice, but not of 42-day-old mice. These findings were consistent with clinical reports that theophylline at the therapeutic blood concentrations occasionally induced convulsions in children. The age-dependent proconvulsant effect of theophylline was well inhibited by phenobarbital (PB), dose-dependently, but not by other well-established antiepileptic drugs (AEDs). PB may be a good choice of AED in patients with bronchial asthma and seizure disorders, if PB is indicative for their seizure types. The proconvulsant effect of theophylline in 21-day-old mice was counteracted by not only an adenosine A1 agonist, but also an NMDA antagonist and a histamine H3 antagonist. Several studies have established that the proconvulsant effect of theophylline intoxication is mainly due to the blockade of adenosine A1 receptors. The present findings suggested that the proconvulsant properties of therapeutic doses of theophylline in developing period were different from those of theophylline intoxication. Combination of therapeutic doses of theophylline and centrally-acting histamine H1 antagonists showed proconvulsant effects even in 42-day-old mice, suggesting that peripherally acting histamine H1 antagonists, such as astemizole, evastine and epinastine, were much safer than centrally acting histamine H1 antagonists for patients with both allergy and seizure history.

Vigabatrin

Vigabatrin is a structural analogue of gamma amino butyric acid (GABA), which binds irreversibly to GABA-transaminase causing increased brain levels of GABA. It is an important advance in the medical management of children with epilepsy. It appears to be particularly effective in the treatment of infantile spasms, especially when caused by tuberous sclerosis. It is also effective in the treatment of partial seizures and some generalized seizures including those of the Lennox-Gastaut syndrome. However, myoclonic seizures may be made worse by vigabatrin. It is not yet approved for use in the United States but it is approved throughout most of the rest of the world including Canada and Mexico. Release in the United States is expected in the near future.

Tiagabine

Tiagabine is a novel antiepileptic drug that was designed to block gamma-aminobutyric acid uptake by presynaptic neurons and glial cells. It has been shown to be effective against partial seizures in adults and adolescents. Preliminary pediatric data are also encouraging. This article reviews the available animal and human data on the pharmacokinetics, efficacy, and safety of tiagabine.

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.

Developmental aspects of epileptogenesis

Several factors may contribute to the propensity for the developing brain to have seizures and develop epilepsy. Hypersynchrony of neuronal circuits contributes to the seizure potential and several neurobiological features of the immature brain may support synchronized neuronal firing. The immature cerebral cortex and hippocampus have an increased density of synapses compared to adults and also a higher density of gap junctions and of excitatory amino acid receptors. Enhanced regenerative responses to injury in the developing brain may also contribute to the formation of abnormal hippocampal connections that support epilepsy. Molecular mechanisms that contribute to enhanced synaptic plasticity in the child's brain can also contribute to epileptogenesis in certain circumstances. The phenomenon of kindling, where repeated electrical stimulation of neuronal circuits leads to the development of epileptic seizures, is easily elicited in young animals. Long-term potentiation (LTP), where repeated synaptic stimulation leads to a reduced threshold for activation of that pathway and enhanced postsynaptic potentials, is much more robust in the immature cerebral cortex and may contribute to kindling and epileptogenesis. Age related enhancement of N-methyl-D-aspartate-type glutamate receptors, which are important for the activity dependent plasticity in the developing brain, appears to participate in LTP. This information suggests that normal developmental features of synaptic development make the immature brain more excitable than the adult brain and may contribute to epileptogenesis.