Nonconvulsive Kainic Acid-Induced Seizures Elicit Age-Dependent Impairment of Memory for the Elevated Plus-Maze

The goal of this study was to evaluate changes in spatial learning in adult and immature rats during and after nonconvulsive seizures. An elevated plus-maze was used in 18- and 25-day-old and adult rats. Kainic acid (KA 6 mg/kg) was administered 60 minutes before the first exposure (Experiment 1) or after a 3-day pretraining (Experiment 2, only adult rats). Animals were retested three times with 24-hour intervals. EEG activity was monitored in 18-day-old rats. KA prolonged the transfer latency (TL) in all age groups. In the youngest group the TL was prolonged 24 hours after KA when epileptic EEG graphoelements were still registered. In both older groups, prolonged TL was measured only 60 minutes after KA. In the pretrained adults, significantly prolonged TLs persisted for 24 hours after KA. KA changed the performance of adult and immature rats in the elevated plus maze not only during nonconvulsive seizures but also 24 hours later.     

Seizures Induced by Homocysteine in Rats During Ontogenesis

Summary: We studied the convulsant action of homocysteine in 211 immature and adult Wistar albino rats. Homocysteine elicited minimal, predominantly clonic, and major generalized tonic-clonic seizures at six different developmental stages, from 7 days to adulthood. Nevertheless, some age-dependent differences in the seizure pattern were apparent. Minimal seizures in immature rats lasted ≤20 min, thus representing an epileptic status, whereas in adult animals these seizures were much shorter, lasting only ≥40 s. In addition, flexion seizures were observed in 7-and 12-day-old rats, only rarely in 15-and 18-day-old animals, and never in the 25-day-old and adult rats. ECoG recordings demonstrated a nearly iso-electric pattern during homocysteine-induced seizures in 7-and 12-day-old rat pups. In older rats, spikes or sharp waves were recorded, but precise electroclinical correlations were poor. The greater sensitivity of younger animals to kainic acid (KA) and N-methyl-D-aspartate (NMDA), as reported previously, was not evident in the case of homocysteine-induced seizures. This observation, together with a different behavioral pattern, suggests that homocysteine cannot be considered a simple agonist of the kainate or NMDA type of excitatory amino acid receptors. The exact mechanism of the convulsant action of homocysteine, both during development and in adulthood, remains to be clarified.     

Kainic acid seizures in the developing brain: status epilepticus and spontaneous recurrent seizures.

Acute and chronic effects of seizures induced by intraperitoneal (i.p.) injection of kainic acid (KA) were studied in developing rats (postnatal days (P) 5, 10, 20, 30, and adult 60). For 3 months following KA-induced status epilepticus, spontaneous recurrent seizure (SRS) occurrence was quantified using intermittent video monitoring. Latency to generalized seizures was then tested using flurothyl, and brains were histologically analyzed for CA3 lesions. In P5-10 rats, KA caused generalized tonic-clonic ('swimming') seizures. SRS did not develop, and there was no significant difference between control and KA-treated rats in latency to flurothyl-induced seizures. In contrast, rats P20 and older exhibited limbic automatisms followed by limbic motor seizures which secondarily generalized. Incidence and frequency of SRS increased with age. P20-30 rats with SRS had shorter latencies to flurothyl seizures than did KA-treated P20-30 rats without SRS or controls. KA-treated P60 rats (with or without SRS) had shorter latencies than controls to flurothyl seizure onset. SRS in P60 rats occurred sooner after KA than in P20-30 rats. CA3 lesions were seen in P20-60 rats with and without SRS, but not in P5-10 rats. These data suggest that there are developmental differences in both acute and chronic responses to KA, with immature animals relatively protected from the long-term deleterious effects of this convulsant.     

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling.

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.     

Rapamycin has age‐, treatment paradigm‐, and model‐specific anticonvulsant effects and modulates neuropeptide Y expression in rats

Purpose: Rapamycin (RAP) has certain antiepileptogenic features. However, it is unclear whether these effects can be explained by the anticonvulsant action of RAP, which has not been studied. To address this question, we tested potential anticonvulsant effects of RAP in immature and adult rats using different seizure models and treatment paradigms. In addition, we studied changes in the expression of neuropeptide Y (NPY) induced by RAP, which may serve as an indirect target of the RAP action. Methods: A complex approach was adopted to evaluate the anticonvulsant potential of RAP: We used flurothyl‐, pentylenetetrazole (PTZ)–, N‐methyl‐d ‐aspartate (NMDA)–, and kainic acid (KA)–induced seizures to test the effects of RAP using different pretreatment protocols in immature and adult rats. We also evaluated expression of NPY within the primary motor cortex, hippocampal CA1, and dentate gyrus (DG) after different pretreatments with RAP in immature rats. Key Findings: We found the following: (1) RAP administered with short‐term pretreatment paradigms has a weak anticonvulsant potential in the seizure models with compromised inhibition. (2) Lack of RAP efficacy correlates with decreased NPY expression in the cortex, CA1, and DG. Specifically in immature rats, a single dose of RAP (3 mg/kg) 4 or 24 h before seizure testing had anticonvulsant effects against PTZ‐induced seizures. In the flurothyl seizure model only the 4‐h pretreatment with RAP was anticonvulsant in the both age groups. Short‐term pretreatments with RAP had no effects against NMDA‐ and KA‐induced seizures tested in immature rats. Long‐term pretreatments with RAP over 8 days did not show beneficial effect in all tested seizure models in developing rats. Moreover, the long‐term pretreatment with RAP had a slight proconvulsant effect on KA‐induced seizures. In immature rats, any lack of anticonvulsant effect (including proconvulsant effect of multiple doses of RAP) was associated with downregulation of NPY expression in the cortex and DG. In immature animals, after a single dose of RAP with 24 h delay, we found a decrease of NPY expression in DG, and CA1 as well. Significance: Our data show weak age‐, treatment paradigm‐, and model‐specific anticonvulsant effects of RAP as well as loss of those effects after long‐term RAP pretreatment associated with downregulation of NPY expression. These findings suggest that RAP is a poor anticonvulsant and may have beneficial effects only against epileptogenesis. In addition, our data present new insights into mechanisms of RAP action on seizures indicating a possible connection between mammalian target of rapamycin (mTOR) signaling and NPY system.     

The expression of Fos following kainic acid-induced seizures is age-dependent

The expression of limbic seizures following kainic acid (KA) administration starts at approximately postnatal day (P) 19 in rats. In this study we investigated whether the expression of Fos-like immunoreactivity (Fos-IR) in limbic regions occurs concomitantly with the behavioural expression of limbic seizures. Immunohistochemistry for c-Fos protein was examined 1, 2, 4, 12 and 24 h following seizure onset (KA-treated rats) or saline injections (controls) in immature and adult rats at P7, P13, P20 and P60. The expression of Fos-IR in limbic structures following KA-induced seizures is age-dependent. There is a strong and selective induction of Fos-IR in the CA3 region of the hippocampus following KA-induced seizures in rats at P7. However, the expression of Fos-IR in KA-treated rats at P13, P20 and P60 involved other hippocampal structures in addition to CA3. Abundant induction of Fos-IR was found in the CA1, CA3 and dentate gyrus (DG) in KA-treated rats at P13, P20 and P60. While immature rats at P7 and P13 showed very few or no Fos-IR neurons in most amygdala nuclei, rat pups at P20 showed strong induction of Fos-IR in the amygdala. Our results demonstrated that the induction of Fos-IR in most amygdala nuclei and the full expression of behavioural limbic seizures occur at the same developmental age, which is consistent with the idea that the amygdala may play a role in the modulation of limbic seizures.     

AIDA,a class I metabotropic glutamate-receptor antagonist limits kainate-induced hippocampal dysfunction

PURPOSE: In the developing animal, intraperitoneal injections of kainic acid (KA) lead to a prolonged initial seizure followed by chronic recurrent seizures and long-term hippocampal dysfunction. We investigated whether the class I metabotropic glutamate receptor (mGluR) antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA) is neuroprotective in the KA model of epilepsy. METHODS: Immature rats aged postnatal day 20 (P20) and P30 were injected with fixed volumes of KA, KA + AIDA, AIDA, or saline. We monitored recurrent seizures. Thirty days later, we tested hippocampal function with the Morris water-maze test or prepared hippocampal slices to record extracellularly evoked and spontaneous potentials from the CA1 area. In a third group, we performed neuronal counts. RESULTS: In both age groups, acute seizures were similar in KA and KA + AIDA groups. Rare spontaneous recurrent seizures occurred only in KA-injected rats. The KA P20 group performed significantly worse than controls in the water-maze test. The KA + AIDA group showed impaired performance on day 1, but learning improved substantially, reaching control values in the remaining 3 days. The P30 KA rats performed worse than controls on all trial days, whereas the KA + AIDA rats improved by day 3, but did not reach control values. Electrophysiologic recordings showed small but consistent differences between KA and control animals, suggestive of an adaptive modification in the gamma-aminobutyric acid (GABA)ergic system, reversed by AIDA. On histology, we observed a loss of CA1 interneurons in both ages. Cell loss was reversed by the use of AIDA. CONCLUSIONS: Blockade of the class I mGluR during KA-induced seizures in the developing brain limits seizure-induced hippocampal dysfunction.     

大鼠出生前后胆碱补充对癫痫发作后认知功能的影响

目的 探讨出生前后胆碱补充或缺乏对癫痫发作后认知功能的影响。方法 3组大鼠自受孕后第11天至子鼠出生后第7天分别给予胆碱丰富、胆碱缺乏和正常胆碱含量的饮食。子鼠出生后第42天，海人酸诱导癫痫发作。癫痫发作10d后，用Morris水迷宫对大鼠空间学习记忆能力进行测试并测量海马中胆碱乙酰转移酶和乙酰胆碱酯酶的活性。结果 出生前后用胆碱丰富食物饲养的大鼠较胆碱缺乏饮食和服碱正常饮食饲养的大鼠在水迷宫测试中有较好表现。经胆碱缺乏饮食饲养的大鼠海马中胆碱乙酰转移酶的活性较对照组低18．8％，较胆碱补充组低21．3％。结论 出生前后饮食中补充胆碱可以减轻癫痫发作引起的认知功能损害，这一作用可能与海马内胆碱乙酰转移酶的水平有关。     

Effects of Blocking GABA Degradation on Corticotropin-Releasing Hormone Gene Expression in Selected Brain Regions

PURPOSE: The gamma-aminobutyric acid (GABA) degradation blocker gamma-vinyl-GABA (VGB) is used clinically to treat seizures in both adult and immature individuals. The mechanism by which VGB controls developmental seizures is not fully understood. Specifically, whether the anticonvulsant properties of VGB arise only from its elevation of brain GABA levels and the resulting activation of GABA receptors, or also from associated mechanisms, remains unresolved. Corticotropin-releasing hormone (CRH), a neuropeptide present in many brain regions involved in developmental seizures, is a known convulsant in the immature brain and has been implicated in some developmental seizures. In certain brain regions, it has been suggested that CRH synthesis and release may be regulated by GABA. Therefore we tested the hypothesis that VGB decreases CRH gene expression in the immature rat brain, consistent with the notion that VGB may decrease seizures also by reducing the levels of the convulsant molecule, CRH. METHODS: VGB was administered to immature, 9-day-old rats in clinically relevant doses, whereas littermate controls received vehicle. RESULTS: In situ hybridization histochemistry demonstrated a downregulation of CRH mRNA levels in the hypothalamic paraventricular nucleus but not in other limbic regions of VGB-treated pups compared with controls. In addition, VGB-treated pups had increased CRH peptide levels in the anterior hypothalamus, as shown by radioimmunoassay. CONCLUSIONS: These findings are consistent with a reduction of both CRH gene expression and secretion in the hypothalamus, but do not support an indirect anticonvulsant mechanism of VGB via downregulation of CRH levels in limbic structures. However, the data support a region-specific regulation of CRH gene expression by GABA.     

Convulsant action of systemically administered glutamate and bicuculline methiodide in immature rats

Developmental changes of transport of drugs into the brain play an important role in ontogenetic neuropharmacology. Two convulsant drugs with different mechanisms of action (glutamate and bicuculline methiodide) were chosen to demonstrate these changes in developing rats. High dose of glutamate (4 g/kg i.p.) induced both minimal (predominantly clonic) and generalized tonic-clonic seizures in rat pups 7, 12, and 18 days old. In contrast, seizures were only exceptionally observed in 25 and 90 days old animals. Bicuculline methiodide was administered in a dose of 2 or 20 mg/kg i.p. The first sign of bicuculline methiodide action in all age groups was represented by automatisms, a symptomatology never seen after bicuculline hydrochloride administration. Minimal seizures were induced in 12-day-old and in a few 18-day-old and adult rats. Generalized seizures were common after the higher dose of bicuculline methiodide in 7- and 12-day-old rat pups, seldom in 18-day-old ones and never seen in 25-day-old and adult animals. Both glutamate and bicuculline methiodide enter the brain in immature rats but the mechanisms are probably different - glutamate is transported actively through the blood-brain barrier whereas no similar system is known for bicuculline methiodide.     

NMDA-induced seizures in developing rats cause long-term learning impairment and increased seizure susceptibility

N-methyl-D-aspartate (NMDA) receptors play a prominent role in the pathogenesis of epilepsy, yet few studies have used NMDA as a convulsant in whole animals. In developing rats, systemic NMDA induces seizures with a unique seizure phenotype ("emprosthotonic" or hyperflexion seizures) and electrographic pattern (electrodecrement). These features are not seen in kainic acid-induced seizures, suggesting that seizures activated by NMDA might cause different long-term consequences. Therefore, we investigated the effects of NMDA seizures during development on cognitive function and susceptibility to seizures in adulthood. Rat pups (P12-20) were injected with saline (n=36) or NMDA (n=64) at convulsant doses (15-30mg/kg, i.p.). After NMDA injection, a characteristic sequence of seizure activity was seen: initial behavioral arrest, followed by hyperactivity, agitation, and then emprosthotonus and generalized tonic-clonic seizures. Seizures were terminated 30min later by ketamine (50mg/kg, i.p.). On P85, rats underwent behavioral testing in the water maze. Rats that had experienced NMDA seizures as pups took significantly longer to learn the platform location over 5 days of testing, compared to controls. On P90, rats were injected with pentylenetetrazol (PTZ, 50mg/kg, i.p.) to assess their susceptibility to generalized seizures. NMDA-treated rats had decreased latency and increased duration of class V PTZ seizures. Cresyl violet-stained sections of cortex and hippocampus had no obvious cell loss or gliosis. In summary, NMDA causes a unique seizure phenotype in the developing brain, with subsequent deficits in spatial learning and an increased susceptibility to PTZ seizures in adulthood. This study provides additional evidence for long-term alterations of neuronal excitability and cognitive capacity associated with seizures during development.     

Effects of lesions of basal forebrain cholinergic neurons in newborn rats on susceptibility to seizures

The cholinergic system modulates cerebral excitability. We recently reported that immunolesions of the basal forebrain (BF) cholinergic neurons in adult rats increase the susceptibility to generalized seizures. In this study we investigated the effects of lesions of the BF cholinergic neurons in neonatal rats on seizure susceptibility and cognitive function. Neonatal rats at postnatal day (P) 7 received intracerebroventricular (i.c.v.) injections of 192 IgG-saporin (SAP) or phosphate-buffered saline. Following 3 weeks after the injection the first group of rats was implanted with hippocampal electrodes for electroencephalogram (EEG) recordings while the second group of rats was tested for visual spatial memory using the hidden platform version of the water maze test. The first group of rats was then tested for seizure susceptibility using flurothyl 1 week after the electrode implantation. Rats that received immunolesions of the BF cholinergic neurons at P7 had significantly shorter latencies to onset of myoclonic jerks and tonic-clonic seizures than controls. However, no significant differences were found in the duration of seizures, or EEG ictal duration. No significant deficits in spatial learning were found between rats that received i.c.v. injections of SAP at P7 and controls. As in adult rats, lesions of the BF cholinergic system in rat pups result in subsequent increase in seizure susceptibility.     

Suppression of hippocampal neurogenesis is associated with developmental stage, number of perinatal seizure episodes, and glucocorticosteroid level

Seizures increase dentate granule cell proliferation in adult rats but decrease proliferation in young pups. The particular period and number of perinatal seizures required to cause newborn granule cell suppression in development are unknown. Therefore, we examined cell proliferation with bromodeoxyuridine (BrdU) immunohistochemistry during the peak of neurogenesis (e.g., P6 and P9) and at later postnatal ages (e.g., P13, P20, or P30) following single and multiple episodes of perinatal status epilepticus induced by kainate (KA). Because an inverse relationship exists between glucocorticosteroids (CORT) levels and granule cell proliferation, plasma CORT levels and electroencephalographic (EEG) activity were simultaneously monitored to elucidate underlying mechanisms that inhibit cell proliferation. In control animals, the number of BrdU-labeled cells increased then declined with maturation. After 1x KA or 2x KA administered on P6 and P9, the numbers of BrdU-labeled cells were not different from age-matched controls. However, rat pups with 3x KA (on P6, P9, and P13) had marked suppression of BrdU-labeled cells 48-72 h after the last seizure (43 +/- 6.5% of control). Cell proliferation was also significantly inhibited on P20 after 2x KA (to 56 +/- 6.9%) or 3x KA (to 54 +/- 7.9%) and on P30 with 3x KA (to 74.5 +/- 8.2% of age-matched controls). Cell death was not apparent as chromatin stains showed increased basophilia of only inner cells lining the granule cell layers, in the absence of eosinophilia, argyrophilia, or terminal deoxynucleotidyl dUTP nick endlabeling (TUNEL) labeling at times examined. In P13 pups with 3x KA, electron microscopy revealed an increased number of immature granule cells and putative stem cells with irregular shape, condensed cytoplasm, and electron dense nuclei, and they were also BrdU positive. The EEG showed no relationship between neurogenesis and duration of high-synchronous ictal activity. However, endocrine studies showed a correlation with BrdU number and age, sustained increases in circulating CORT levels following 1x KA on P6 (0.7 +/- 0.1 to 2.40 +/- 0.86 microg/dl), and cumulative increases that exceeded 10 microg/dl at 4-8 h after 3x KA on P13 or P20. In conclusion, a history of only one or two perinatal seizure(s) can suppress neurogenesis if a second or third seizure recurs after a critical developmental period associated with a marked surge in CORT. During the first 2 weeks of postnatal life sustained increases in postictal circulating CORT levels but not duration or intensity of ictal activity has long-term consequences on neurogenesis. The occurrence of an increased proportion of immature granule cells and putative stem cells with irregular morphology in the absence of neurodegeneration suggests that progenitors may not differentiate properly and remain in an immature state.     

The long-term effects of seizures on the developing brain: clinical and laboratory issues.

The long-term effects of seizures on the developing brain is a difficult clinical problem to study since cognitive impairment and behavioral abnormalities may be related to the etiological agent responsible for the seizures, age at time of onset of seizures, the type, frequency, or duration of the seizures, or the antiepileptic drugs used to treat the seizures. Many of these variables can be eliminated by using animal models of epilepsy. Work in our laboratory using the kainic acid (KA) model has demonstrated that status epilepticus in prepubescent and mature rats leads to significant deficits in memory, learning and behavior as adults when compared to control littermates without seizures. These rats also had a high incidence of spontaneous recurrent seizures (SRS) and an increased susceptibility to seizures using kindling and flurothyl. However, younger animals (less than or equal to 20 day old) with KA-induced seizures of similar severity were not associated with later neurological deficits. The immature animals also had a low rate of SRS and did not differ from controls in susceptibility to kindling or flurothyl. Studies using the continuous hippocampal stimulation model of epilepsy have also demonstrated that prolonged seizures in the developing brain are less severe than those in the mature animal. The pathophysiological mechanisms that "protect" the young brain from long-term detrimental effects of prolonged seizures are unknown.     

Synaptic reorganization in subiculum and CA3 after early-life status epilepticus in the kainic acid rat model

PURPOSE: The immature rat brain is highly susceptible to seizures, but has a resistance to pathological changes induced by seizures as compared to adult rats. However, prolonged seizures during early-life enhance cellular injury and hyperexcitability induced by convulsive insults later in adulthood. The mechanisms underlying these phenomena are not understood. In adult models, the CA1 axons reorganize their projections to subiculum. Seizure induced plasticity in this pathway has not been investigated in immature seizure models, and may contribute to the vulnerability to later seizures. METHODS: On postnatal day 15, rats experienced convulsive status epilepticus with kainic acid (KA). Seizure induced plasticity was examined with Timm histochemistry and iontophoretic injections of sodium selenite, a retrograde tracer. Cellular injury was evaluated with Fluoro-Jade B histochemistry. RESULTS: Retrograde tracing experiments determined a 67% larger dorsoventral extent of retrograde labeling in the CA1 pyramidal region after tracer injections in subiculum. The synaptic reorganization of the CA1 projection to subiculum was noted in the absence of overt neuronal injury in subiculum or CA1. In contrast, mossy fiber sprouting was detected into the stratum oriens of CA3 with limited neuronal injury to CA3 pyramidal neurons. No mossy fiber sprouting into the inner molecular layer of the dentate gyrus, or CA1 sprouting into the stratum moleculare of CA1 were noted. CONCLUSIONS: The results indicate that the developing brain has distinct mechanisms of seizure induced reorganization as compared to the adult brain. Our experiments show that the concept of "resistance of the immature brain to excitotoxicity" is considerably more complicated than generally believed. Morphological plasticity in the immature brain appears more extensive in distal, but not proximal, projections of hippocampal pathways, and across hippocampal lamellae. The abnormal connectivity between hippocampal lamellae might play a role in the increased susceptibility to injury and hyperexcitability associated with later convulsive insults.     

Arginine vasopressin in the pathogenesis of febrile convulsion and temporal lobe epilepsy

We aimed to investigate the possible convulsant action of arginine vasopressin (AVP) in both a febrile convulsion model in rat pups and a temporal lobe epilepsy model in adult rats and to define the receptor type which mediates this effect. In rat pups, 125 ng V2 receptor antagonist significantly prevented hyperthermic seizures, but did not affect seizure latency. In adult rats, the only effective dose and agent was 125 ng V2 receptor antagonist, which prevented pilocarpine-induced status epilepticus, extended the status epilepticus latency and improved the 24 h survival rate. These data suggest that AVP has a convulsant activity in febrile convulsions and also in seizures independent of fever, and this effect is mediated by V2 receptors.     

Seizures in the developing brain cause adverse long-term effects on spatial learning and anxiety

PURPOSE: Seizures in the developing brain cause less macroscopic structural damage than do seizures in adulthood, but accumulating evidence shows that seizures early in life can be associated with persistent behavioral and cognitive impairments. We previously showed that long-term spatial memory in the eight-arm radial-arm maze was impaired in rats that experienced a single episode of kainic acid (KA)-induced status epilepticus during early development (postnatal days (P) 1-14). Here we extend those findings by using a set of behavioral paradigms that are sensitive to additional aspects of learning and behavior. METHODS: On P1, P7, P14, or P24, rats underwent status epilepticus induced by intraperitoneal injections of age-specific doses of KA. In adulthood (P90-P100), the behavioral performance of these rats was compared with that of control rats that did not receive KA. A modified version of the radial-arm maze was used to assess short-term spatial memory; the Morris water maze was used to evaluate long-term spatial memory and retrieval; and the elevated plus maze was used to determine anxiety. RESULTS: Compared with controls, rats with KA seizures at each tested age had impaired short-term spatial memory in the radial-arm maze (longer latency to criterion and more reference errors), deficient long-term spatial learning and retrieval in the water maze (longer escape latencies and memory for platform location), and a greater degree of anxiety in the elevated plus maze (greater time spent in open arms). CONCLUSIONS: These findings provide additional support for the concept that seizures early in life may be followed by life-long impairment of certain cognitive and behavioral functions. These results may have clinical implications, favoring early and aggressive control of seizures during development.     

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.     

Resistance of the immature hippocampus to seizure-induced synaptic reorganization.

Temporal lobe epilepsy is a common form of epilepsy in human adults and is associated with a unique pattern of damage in the hippocampus. The damage includes cell loss of the CA3 and CA4 areas and synaptic growth (sprouting) of mossy fibers in the supragranular layer of the dentate gyrus. Experimental evidence indicates that in adult rats the excitatory amino acid, kainic acid, induces a similar pattern of changes in hippocampal circuitry associated with alterations in perforant path excitation and inhibition. It has been suggested that, in humans, this type of damage may be a result of seizures early in life. In this study we examined the effects of kainic acid-induced status epilepticus on synaptic reorganization and paired-pulse electrophysiology in developing rats and adults. Kainic acid induced more severe seizures in 15-day-old rat pups than in adults. In contrast to adult rats, these seizures did not produce CA3/CA4 neuronal loss, mossy fiber sprouting or changes in paired-pulse excitation or inhibition in the hippocampus of rat pups tested 2-4 weeks after status epilepticus. Our results provide evidence that the immature hippocampus may be more resistant to seizure-induced changes than the mature hippocampus.     

Differences in c-fos immunoreactivity due to age and mode of seizure induction

The aim of this study was to determine whether the regional distribition and time course of immunoreactivity to the c-fos protein varies with maturation and method of seizure induction. The effect of the two chemical convulsants, pentylenetetrazol (PTZ) and flurothyl, on the spatial and temporal pattern of c-fos-like immunoreactivity in immature (postnatal day (P) 10) was compared to that in adult rats. Patterns of c-fos-like immunoreactivity following O₂ deprivation were also evaluated at the 2 ages because hypoxia is acutely epileptogenic in immature animals but not adults. C-fos-like immunoreactivity was examined at 2, 4, and 6 h after onset of chemically induced seizures or O₂ deprivation at both ages. After PTZ or flurothyl seizures, both ages exhibited similar patterns of IR in amygdala, pyriform cortex, and hypothalamus. Age-dependent regional differences were most prominent in cortex: superficial layers of retrosplenial, cingulate, and neocortex stained in adults; staining was confined to deep layers of neocortex in P10 rats. Intense staining of dentate gyrus and hippocampus occurred with more prolonged seizures, but not brief seizures. PTZ administration resulted in staining at 2 h after seizure onset and was reduced by 4 h in adults, but immunoreactivity was not seen until 4 and 6 h after seizure onset in immature rats, indicating an age effect on the time course of IR. In immature rats, immunoreactivity patterns after hypoxia were markedly different from PTZ or flurothyl; staining was confined to layer VI of neocortex in these animals, and rarely involved limbic structures. These differences in the pattern of c-fos immunoreactivity suggest that the neuronal populations involved in epileptogenesis are influenced by age as well as seizure phenotype and intensity.