The role of the ventromedial nucleus of the hypothalamus in epileptogenesis

Ultrasound-induced seizures in a mouse model of KCNQ2-NEO-DEE

PURPOSE

KCNQ2 neonatal developmental and epileptic encephalopathy (NEO-DEE) is characterized by intractable seizures accompanied by an abnormal neurodevelopment. In a mouse model of NEO-DEE carrying the p.(Thr274Met) variant of Kcnq2, spontaneous generalized seizures occur unexpectedly preventing controlled studies and highlighting the necessity for a customized setup to trigger seizures on demand. We aimed to obtain a stable and objective read-out to control the efficacy of new antiepileptic drugs or to test seizure susceptibility. We developed a protocol to trigger ultrasound-induced seizures (UIS) on demand in this model.

METHODS

We tested the ability of our protocol to induce seizures at four developmental stages in the Kcnq2^{p.(Thr274Met/+)} mouse model. We mapped the activated brain regions using c-fos protein labeling 2 h after seizure induction.

RESULTS

We show that the UIS have the same phenotypic expression and the same severity as spontaneous generalized seizures (SGS) in the Kcnq2-NEO-DEE mouse model. The developmental period during which mice exhibit SGS corresponds to the period during which Kcnq2^{p.(Thr274Met/+)} mice are the most susceptible to US. C-fos labeling reveals a subset of 6 brain regions activated 2 h after the induction of the seizure. The same regions were identified in the context of seizure induction in other rodent models.

CONCLUSION

This study provides a non-invasive and easy to use method to induce seizures in a Kcnq2-NEO-DEE mouse model and documents early neuronal activation in specific brain regions. This method can be used to test the efficacy of new antiepileptic approaches for this intractable form of genetic epilepsy.

Parsing the Network Mechanisms of Electroconvulsive Therapy

Electroconvulsive therapy (ECT) is one of the oldest and most effective forms of neurostimulation, wherein electrical current is used to elicit brief, generalized seizures under general anesthesia. When electrodes are positioned to target frontotemporal cortex, ECT is arguably the most effective treatment for severe major depression, with response rates and times superior to other available antidepressant therapies. Neuroimaging research has been pivotal in improving the field's mechanistic understanding of ECT, with a growing number of magnetic resonance imaging studies demonstrating hippocampal plasticity after ECT, in line with evidence of upregulated neurotrophic processes in the hippocampus in animal models. However, the precise roles of the hippocampus and other brain regions in antidepressant response to ECT remain unclear. Seizure physiology may also play a role in antidepressant response to ECT, as indicated by early positron emission tomography, single-photon emission computed tomography, and electroencephalography research and corroborated by recent magnetic resonance imaging studies. In this review, we discuss the evidence supporting neuroplasticity in the hippocampus and other brain regions during and after ECT, and their associations with antidepressant response. We also offer a mechanistic, circuit-level model that proposes that core mechanisms of antidepressant response to ECT involve thalamocortical and cerebellar networks that are active during seizure generalization and termination over repeated ECT sessions, and their interactions with corticolimbic circuits that are dysfunctional prior to treatment and targeted with the electrical stimulus.

The O2-sensitive brain stem, hyperoxic hyperventilation, and CNS oxygen toxicity

Central nervous system oxygen toxicity (CNS-OT) is a complex disorder that presents, initially, as a sequence of cardio-respiratory abnormalities and nonconvulsive signs and symptoms (S/Sx) of brain stem origin that culminate in generalized seizures, loss of consciousness, and postictal cardiogenic pulmonary edema. The risk of CNS-OT and its antecedent “early toxic indications” are what limits the use of hyperbaric oxygen (HBO2) in hyperbaric and undersea medicine. The purpose of this review is to illustrate, based on animal research, how the temporal pattern of abnormal brain stem responses that precedes an “oxtox hit” provides researchers a window into the early neurological events underlying seizure genesis. Specifically, we focus on the phenomenon of hyperoxic hyperventilation, and the medullary neurons presumed to contribute in large part to this paradoxical respiratory response; neurons in the caudal Solitary complex (cSC) of the dorsomedial medulla, including putative CO2 chemoreceptor neurons. The electrophysiological and redox properties of O2-/CO2-sensitive cSC neurons identified in rat brain slice experiments are summarized. Additionally, evidence is summarized that supports the working hypothesis that seizure genesis originates in subcortical areas and involves cardio-respiratory centers and cranial nerve nuclei in the hind brain (brainstem and cerebellum) based on, respectively, the complex temporal pattern of abnormal cardio-respiratory responses and various nonconvulsive S/Sx that precede seizures during exposure to HBO2.

CNS function and dysfunction during exposure to hyperbaric oxygen in operational and clinical settings

Hyperbaric oxygen (HBO2) is breathed during hyperbaric oxygen therapy and during certain undersea pursuits in diving and submarine operations. What limits exposure to HBO2 in these situations is the acute onset of central nervous system oxygen toxicity (CNS-OT) following a latent period of safe oxygen breathing. CNS-OT presents as various non-convulsive signs and symptoms, many of which appear to be of brainstem origin involving cranial nerve nuclei and autonomic and cardiorespiratory centers, which ultimately spread to higher cortical centers and terminate as generalized tonic-clonic seizures. The initial safe latent period makes the use of HBO2 practical in hyperbaric and undersea medicine; however, the latent period is highly variable between individuals and within the same individual on different days, making it difficult to predict onset of toxic indications. Consequently, currently accepted guidelines for safe HBO2 exposure are highly conservative. This review examines the disorder of CNS-OT and summarizes current ideas on its underlying pathophysiology, including specific areas of the CNS and fundamental neural and redox signaling mechanisms that are thought to be involved in seizure genesis and propagation. In addition, conditions that accelerate the onset of seizures are discussed, as are current mitigation strategies under investigation for neuroprotection against redox stress while breathing HBO2 that extend the latent period, thus enabling safer and longer exposures for diving and medical therapies.

The Repeated Flurothyl Seizure Model in Mice

Development of spontaneous seizures is the hallmark of human epilepsy. There is a critical need for new epilepsy models in order to elucidate mechanisms responsible for leading to the development of spontaneous seizures and for testing new anti-epileptic compounds. Moreover, rodent models of epilepsy have clearly demonstrated that there are two independent seizure systems in the brain: 1) the forebrain seizure network required for the expression of clonic seizures mediated by forebrain neurocircuitry, and 2) the brainstem seizure network necessary for the expression of brainstem or tonic seizures mediated by brainstem neurocircuitry. In seizure naïve animals, these two systems are separate, but developing models that can explore the intersection of the forebrain and brainstem seizure systems or for elucidating mechanisms responsible for bringing these two seizure systems together may aid in our understanding of: 1) how seizures can become more complex overtime, and 2) sudden unexpected death in epilepsy (SUDEP) since propagation of seizure discharge from the forebrain seizure system to the brainstem seizure system may have an important role in SUDEP because many cardiorespiratory systems are localized in the brainstem. The repeated flurothyl seizure model of epileptogenesis, as described here, may aid in providing insight into these important epilepsy issues in addition to understanding how spontaneous seizures develop.

Dissociation of spontaneous seizures and brainstem seizure thresholds in mice exposed to eight flurothyl-induced generalized seizures

Objective

C57BL/6J mice exposed to eight flurothyl‐induced generalized clonic seizures exhibit a change in seizure phenotype following a 28‐day incubation period and subsequent flurothyl rechallenge. Mice now develop a complex seizure semiology originating in the forebrain and propagating into the brainstem seizure network (a forebrain→brainstem seizure). In contrast, this phenotype change does not occur in seizure‐sensitive DBA/2J mice. The underlying mechanism was the focus of this study.

Methods

DBA/2J mice were exposed to eight flurothyl‐induced seizures (1/day) followed by 24‐h video‐electroencephalographic recordings for 28 days. Forebrain and brainstem seizure thresholds were determined in C57BL/6J and DBA/2J mice following one or eight flurothyl‐induced seizures, or after eight flurothyl‐induced seizures, a 28‐day incubation period, and final flurothyl rechallenge.

Results

Similar to C57BL/6J mice, DBA/2J mice expressed spontaneous seizures. However, unlike C57BL/6J mice, DBA/2J mice continued to have spontaneous seizures without remission. Because DBA/2J mice did not express forebrain→brainstem seizures following flurothyl rechallenge after a 28‐day incubation period, this indicated that spontaneous seizures were not sufficient for the evolution of forebrain→brainstem seizures. Therefore, we determined whether brainstem seizure thresholds were changing during this repeated‐flurothyl model and whether this could account for the expression of forebrain→brainstem seizures. Brainstem seizure thresholds were not different between C57BL/6J and DBA/2J mice on day 1 or on the last induction seizure trial (day 8). However, brainstem seizure thresholds did differ significantly on flurothyl rechallenge (day 28), with DBA/2J mice showing no lowering of their brainstem seizure thresholds.

Significance

These results demonstrate that DBA/2J mice exposed to the repeated‐flurothyl model develop spontaneous seizures without evidence of seizure remission and provide a new model of epileptogenesis. Moreover, these findings indicated that the transition of forebrain ictal discharge into the brainstem seizure network occurs as a result of changes in brainstem seizure thresholds that are independent of spontaneous seizure expression.

Spatiotemporal differences in the c-fos pathway between C57BL/6J and DBA/2J mice following flurothyl-induced seizures: A dissociation of hippocampal Fos from seizure activity

Significant differences in seizure characteristics between inbred mouse strains highlight the importance of genetic predisposition to epilepsy. Here, we examined the genetic differences between the seizure-resistant C57BL/6J (B6) mouse strain and the seizure-susceptible DBA/2J (D2) strain in the phospho-Erk and Fos pathways to examine seizure-induced neuronal activity to uncover potential mechanistic correlates to these disparate seizure responsivities. Expression of neural activity markers was examined following 1, 5, or 8 seizures, or after 8 seizures, a 28 day rest period, and a final flurothyl rechallenge. Two brain regions, the hippocampus and ventromedial nucleus of the hypothalamus (VMH), had significantly different Fos expression profiles following seizures. Fos expression was highly robust in B6 hippocampus following one seizure and remained elevated following multiple seizures. Conversely, there was an absence of Fos (and phospho-Erk) expression in D2 hippocampus following one generalized seizure that increased with multiple seizures. This lack of Fos expression occurred despite intracranial electroencephalographic recordings indicating that the D2 hippocampus propagated ictal discharge during the first flurothyl seizure suggesting a dissociation of seizure discharge from Fos and phospho-Erk expression. Global transcriptional analysis confirmed a dysregulation of the c-fos pathway in D2 mice following 1 seizure. Moreover, global analysis of RNA expression differences between B6 and D2 hippocampus revealed a unique pattern of transcripts that were co-regulated with Fos in D2 hippocampus following 1 seizure. These expression differences could, in part, account for D2's seizure susceptibility phenotype. Following 8 seizures, a 28 day rest period, and a final flurothyl rechallenge, ∼85% of B6 mice develop a more complex seizure phenotype consisting of a clonic-forebrain seizure that uninterruptedly progresses into a brainstem seizure. This seizure phenotype in B6 mice is highly correlated with bilateral Fos expression in the VMH and was not observed in D2 mice, which always express clonic-forebrain seizures upon flurothyl retest. Overall, these results illustrate specific differences in protein and RNA expression in different inbred strains following seizures that precede the reorganizational events that affect seizure susceptibility and changes in seizure semiology over time.

Segregation of seizure traits in C57 black mouse substrains using the repeated-flurothyl model

Identifying the genetic basis of epilepsy in humans is difficult due to its complexity, thereby underlying the need for preclinical models with specific aspects of seizure susceptibility that are tractable to genetic analyses. In the repeated-flurothyl model, mice are given 8 flurothyl-induced seizures, once per day (the induction phase), followed by a 28-day rest period (incubation phase) and final flurothyl challenge. This paradigm allows for the tracking of multiple phenotypes including: initial generalized seizure threshold, decreases in generalized seizure threshold with repeated flurothyl exposures, and changes in the complexity of seizures over time. Given the responses we previously reported in C57BL/6J mice, we analyzed substrains of the C57BL lineage to determine if any of these phenotypes segregated in these substrains. We found that the generalized seizure thresholds of C57BL/10SNJ and C57BL/10J mice were similar to C57BL/6J mice, whereas C57BL/6NJ and C57BLKS/J mice showed lower generalized seizure thresholds. In addition, C57BL/6J mice had the largest decreases in generalized seizure thresholds over the induction phase, while the other substrains were less pronounced. Notably, we observed only clonic seizures during the induction phase in all substrains, but when rechallenged with flurothyl after a 28-day incubation phase, ∼80% of C57BL/6J and 25% of C57BL/10SNJ and C57BL/10J mice expressed more complex seizures with tonic manifestations with none of the C57BL/6NJ and C57BLKS/J mice having complex seizures with tonic manifestations. These data indicate that while closely related, the C57BL lineage has significant diversity in aspects of epilepsy that are genetically controlled. Such differences further highlight the importance of genetic background in assessing the effects of targeted deletions of genes in preclinical epilepsy models.

Dissociation of seizure traits in inbred strains of mice using the flurothyl kindling model of epileptogenesis

Previous seizure models have demonstrated genetic differences in generalized seizure threshold (GST) in inbred mice, but the genetic control of epileptogenesis is relatively unexplored. The present study examined, through analysis of inbred strains of mice, whether the seizure characteristics observed in the flurothyl kindling model are under genetic control. Eight consecutive, daily generalized seizures were induced by flurothyl in mice from five inbred strains. Following a 28-day rest period, mice were retested with flurothyl. The five strains of mice demonstrated inter-strain differences in GST, decreases in GST across seizure trials, and differences in the behavioral seizure phenotypes expressed. Since many of the seizure characteristics that we examined in the flurothyl kindling model were dissociable between C57BL/6J and DBA/2J mice, we analyzed these strains in detail. Unlike C57BL/6J mice, DBA/2J mice had a lower GST on trial 1, did not demonstrate a decrease in GST across trials, nor did they show an alteration in seizure phenotype upon flurothyl retest. Surprisingly, [C57BL/6JxDBA/2J] F1-hybrids had initial GST on trial 1 and GST decreases across trials similar to what was found for C57BL/6J, but they did not undergo the alteration in behavioral seizure phenotype that had been observed for C57BL/6J mice. Our data establish the significance of the genetic background in flurothyl-induced epileptogenesis. The [C57BL/6JxDBA/2J] F1-hybrid data demonstrate that initial GST, the decrease in GST across trials, and the change in seizure phenotype differ from the characteristics of the parental strains, suggesting that these phenotypes are controlled by independent genetic loci.

Seizure responses and induction of Fos by the NMDA agonist (tetrazol-5-yl)glycine in a genetic model of NMDA receptor hypofunction

Effects of the direct NMDA agonist (tetrazol-5-yl)glycine (TZG) were examined in a genetic mouse model of reduced NMDA receptor function. In this model, expression of the NR1 subunit is reduced but not eliminated and the mice are therefore designated as NR1 hypomorphic. Previous work suggested that the reduced NR1 subunit expression produced a functional subsensitivity as judged by a blunted Fos induction response to a sub-seizure dose of TZG. In the present study seizure threshold doses of TZG were tested in the wild type and mutant mice. Surprisingly, there was no difference in the seizure sensitivity between the wild type mice and mice presumed to express very low levels of the NR1 subunit. An extensive neuroanatomical analysis of Fos induction was conducted after the threshold seizure doses of TZG. The results demonstrate that some brain regions of the NR1 -/- mice exhibit much lower Fos induction in comparison to the NR1 +/+ mice. These regions include hippocampus, amygdala, and cerebral cortical regions. However, in other regions, similar induction of Fos was observed in both genotypes in response to the NMDA agonist. Regions showing similar Fos induction in the NR1 +/+ and NR1 -/- mice include the lateral septum, nucleus of the solitary tract, and medial hypothalamic regions. The results suggest that the NMDA receptor hypofunction in the NR1 -/- mice is not global but regionally specific and that subcortical structures are responsible for the seizure-inducing effects of TZG.

Comparative fos immunoreactivity in the brain after forebrain, brainstem, or combined seizures induced by electroshock, pentylenetetrazol, focally induced and audiogenic seizures in rats

To help discern sites of focal activation during seizures of different phenotype, the numbers of Fos immunoreactive (FI) neurons in specific brain regions were analyzed following "brainstem-evoked," "forebrain-evoked" and forebrain/brainstem combination seizures induced by a variety of methods. First, pentylenetetrazol (PTZ, 50 mg/kg) induced forebrain-type seizures in some rats, or forebrain seizures that progressed to tonic/clonic brainstem-type seizures in other rats. Second, minimal electroshock induced forebrain seizures whereas maximal electroshock (MES) induced tonic brainstem-type seizures in rats. Third, forebrain seizures were induced in genetically epilepsy-prone rats (GEPRs) by microinfusion of bicuculline into the area tempestas (AT), while brainstem seizures in GEPRs were induced by audiogenic stimulation. A final set was included in which AT bicuculline-induced forebrain seizures in GEPRs were transiently interrupted by audiogenic seizures (AGS) in the same animals. These animals exhibited a sequence combination of forebrain clonic seizure, brainstem tonic seizure and back to forebrain clonic seizures. Irrespective of the methods of induction, clonic forebrain- and tonic/clonic brainstem-type seizures were associated with considerable Fos immunoreactivity in several forebrain structures. Tonic/clonic brainstem seizures, irrespective of the methods of induction, were also associated with FI in consistent brainstem regions. Thus, based on Fos numerical densities (FND, numbers of Fos-stained profiles), forebrain structures appear to be highly activated during both forebrain and brainstem seizures; however, facial and forelimb clonus characteristic of forebrain seizures are not observable during a brainstem seizure. This observation suggests that forebrain-seizure behaviors may be behaviorally masked during the more severe tonic brainstem seizures induced either by MES, PTZ or AGS in GEPRs. This suggestion was corroborated using the sequential seizure paradigm. Similar to findings using MES and PTZ, forebrain regions activated by AT bicuculline were similar to those activated by AGS in the GEPR. However, in the combination seizure group, those areas that showed increased FND in the forebrain showed even greater FND in the combination trial. Likewise, those areas of the brainstem showing FI in the AGS model, showed an even greater effect in the combination paradigm. Finally, the medial amygdala, ventral hypothalamus and cortices of the inferior colliculi showed markedly increased FND that appeared dependent upon activation of both forebrain and brainstem seizure activity in the same animal. These findings suggest these latter areas may be transitional areas between forebrain and brainstem seizure interactions. Collectively, these data illustrate a generally consistent pattern of forebrain Fos staining associated with forebrain-type seizures and a consistent pattern of brainstem Fos staining associated with brainstem-type seizures. Additionally, these data are consistent with a notion that separate seizure circuitries in the forebrain and brainstem mutually interact to facilitate one another, possibly through involvement of specific "transition mediating" nuclei.

Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.

Increased mitotic activity in the dentate gyrus of the hippocampus of adult C57BL/6J mice exposed to the flurothyl kindling model of epileptogenesis

Repeated flurothyl-induced generalized forebrain seizures result in a progressive and permanent lowering of the generalized seizure threshold in mice and an increase in the percentage of animals expressing forebrain-brainstem seizures, when rechallenged with flurothyl, after a stimulation-free period. Since this seizure paradigm serves as an excellent model for examining changes in seizure threshold and seizure propagation, we were interested in examining mitotic activity in hippocampal progenitors following flurothyl-induced epileptogenesis. In the present studies, we investigated (1). the effect of one or eight flurothyl-induced seizures on mitotic activity in the hippocampal dentate gyrus of adult mice measured by 5-bromo-2'-deoxyuridine incorporation, (2). the time course of change in hippocampal mitotic activity, (3). the cellular phenotype of these mitotically active cells, and (4). the relationship of changes in mitotic activity to changes in seizure threshold and phenotype. Significant increases in hippocampal mitotic activity were observed in mice exposed to either one or eight flurothyl-induced seizures. Increases were observed at 1 and 3 days following one seizure, and at 0, 1, 3, and 7 days following eight seizures. Confocal analyses, using neuronal and glial markers, suggest that the majority of these mitotic cells are neurons. In addition, no correlation was observed between hippocampal mitotic activity and the final seizure type that animals expressed following incubation and flurothyl retest. A significant correlation was observed between hippocampal mitotic activity and seizure threshold in flurothyl-kindled mice. Overall, these results indicate that both one and eight flurothyl-induced seizures are potent inducers of hippocampal neurogenesis in adult mice. Results further suggest that the increases in hippocampal neurogenesis are not directly related to the processes that underlie the shift in behavioral seizure phenotype, but may be involved in either the establishment or the maintenance of seizure threshold in this flurothyl model of epileptogenesis.

The effects of brain-irradiation-induced decreases in hippocampal mitotic activity on flurothyl-induced epileptogenesis in adult C57BL/6J mice

Previous studies have demonstrated that seizures are potent inducers of mitotic activity in the rodent hippocampus. The role of this mitotic activity in epileptogenesis currently remains unknown. In the present study, we investigated the effect of alterations in hippocampal mitotic activity on changes in seizure threshold and phenotype using flurothyl kindling. In flurothyl kindling, eight repeated flurothyl-induced generalized forebrain (clonic) seizures result in a rapid, progressive, and permanent lowering of the generalized seizure threshold in mice and in a slowly evolving increase in the percentage of animals expressing forebrain-brain stem (clonic-tonic) seizures when reexposed to flurothyl following a 2- to 4-week stimulation-free period. Therefore, flurothyl kindling serves as an excellent model for evaluating mechanisms of generalized seizure threshold and seizure propagation. To investigate this relationship between hippocampal mitotic activity and epileptogenesis, mice were given brain irradiation, focused mainly on the hippocampus, bilaterally, and were exposed to the flurothyl kindling model of epileptogenesis. Brain irradiation virtually eliminated all basal and seizure-induced mitotic activity in the hippocampal dentate gyrus of mice. In addition, animals that underwent irradiation and flurothyl kindling did not differ from control mice on measures of seizure threshold (threshold induction and maintenance) and seizure phenotype. Overall, these results suggest that seizure-induced increases in mitotic activity in the hippocampal dentate gyrus are not directly related to the processes that underlie the shift in behavioral seizure phenotype or in either the induction or the maintenance of lowered seizure threshold that is observed in this flurothyl model of epileptogenesis.

Behavioral and metabolic features of repetitive seizures in immature and mature rats

Seizure incidence varies significantly with age, with seizure susceptibility particularly high during the first few years of life. Of significant concern is what effects do brief, repetitive seizures have on the developing brain. We approached this issue by examining the change in seizure threshold, and related markers of neuronal activity and metabolic activity (c-fos mRNA and 2-deoxyglucose [2DG]), as a function of repetitive seizure episodes in immature and mature rats. Starting on postnatal day 15 (P15) (immature) or P60 (adult) rats were given two flurothyl seizures a day for 5 days (nine or ten seizures). The seizure latency profile, our measure of threshold, in immature versus adult rats across the 5-day testing period was different. In immature rats, threshold for the second seizure on each day was significantly lower than for the first seizure, suggesting that there was little refractoriness after the first seizure of the day. In contrast, the mature animal had a significantly longer threshold latency to the second seizure for the first 3 days of testing. The immature animal was also more likely than the adult to exhibit tonic extension as a feature of the first seizure of the day. Following repetitive seizures, more regions of the CNS showed c-fos mRNA expression in the immature animal than adults, suggesting that repetitive seizures in the immature animal activated a greater percentage of the brain. Compared with the effects of a single seizure, repetitive seizures resulted in less 2DG labeling in most regions of the brain (except the hippocampus); in the immature brain this difference was more distinct than in adults. The consequences of repetitive seizures in the immature animal results in distinctly different seizure behavior and neuronal activity pattern (c-fos expression) than that observed in the mature animal.