Cycloheximide prevents kainate-induced neuronal death and c-fos expression in adult rat brain

The present study was directed at evaluating the possible involvement of protein synthesis in excitotoxin-induced neuronal damage and prolonged expression of the proto-oncogene, c-fos. Kainic acid-induced seizure activity elicited varying degrees of neuronal damage and cell loss in selectively vulnerable regions of the adult rat limbic system. Pretreatment with cycloheximide, a protein synthesis inhibitor, did not alter behavioral seizure characteristics, but markedly attenuated damage to susceptible neuronal populations. A prolonged increase in c-fos mRNA was observed byin situ hybridization up to 16 h after the onset of seizures in regions exhibiting neuronal death. Pretreatment with cycloheximide did not affect the transient induction of c-fos observed in numerous structures, but significantly reduced the prolonged expression of c-fos mRNA in kainatevulnerable regions. Despite producing massive seizure activity, systemic kainic acid administration during the early postnatal period did not induce any neuronal death, and did not result in prolonged c-fos expression in any brain structures. The developmental onset of selective neuronal vulnerability coincided with that of prolonged c-fos expression in susceptible neuronal populations. In adult rats, seizure activity induced by pentylenetetrazole did not produce neuronal damage nor did it produce prolonged c-fos expression. These results not only demonstrate that kainate-induced neurotoxicity and the prolonged expression of c-fos are both prevented by cycloheximide, but also strengthen the idea that prolonged c-fos expression is a marker of neuronal death.     

Effects of hypoxia-ischemia and seizures on neuronal and glial-like c-fos protein levels in the infant rat

Unilateral carotid ligation in immature rats, followed by 2 h of hypoxia led to ischemic cell change from 2 h after the insult, on the ligated side of the brain. There was a time-dependent induction of immunoreactive c-fos protein in neurones but not glia or ependyma on the non-ligated side of the brain. Induction only occurred in rats that had seizures post hypoxia-ischemia. In the ligated hemisphere c-fos protein was induced in glial-like cells in the corpus callosum, fornix/fimbria and internal capsule and in ependymal cells lining the lateral ventricle starting from 2 h after hypoxia but subsiding by 3 days. No neuronal c-fos induction was seen in areas showing neuronal damage. MK-801 or carbamazepine, which prevented hypoxia-ischemia-induced seizures, also prevented c-fos induction in the non-ligated hemisphere while MK-801 was associated with increased c-fos induction in hippocampal neurones from the ligated side, as well as in glial-like and ependymal cells.These results suggest several processes are involved following the hypoxic-ischemic insult. Firstly, severe hypoxia-ischemia is associated with a reduction in neuronal c-fos protein levels, probably as a result of neuronal failure and death. Secondly, post hypoxic seizures cause c-fos induction in surviving neurones. Thirdly, glial-like from regions in which there is neural loss also exhibit induction of c-fos, which may be important for their subsequent proliferation or for the production of growth factors.     

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.     

Flurothyl status epilepticus in developing rats: behavioral, electrographic histological and electrophysiological studies.

Status epilepticus and repeated seizures have age-dependent morphological and neurophysiological alterations in the hippocampus. In the present study, effects of flurothyl-induced status epilepticus were examined in awake and free moving immature (2 weeks old) and adult rats. Without exception, adult rats died of respiratory arrest before the onset of status epilepticus. We were unable to find a concentration of flurothyl that produced status epilepticus and a low mortality in adult rats. In contrast, immature rats survived flurothyl status epilepticus for up to 60 min with a very low mortality. In rat pups, behavioral manifestations correlated with electrographic seizures in both the cortex and hippocampus. Neuropathological damage (cell loss, pyknotic cells or gliosis) was not observed in the immature hippocampus, thalamus, amygdala, substantia nigra or cortex at 24 h, 2 days or 2 weeks after status epilepticus. In addition, no aberrant mossy fiber reorganization or decrease in cells counts were observed in the hippocampus. Young rats did not show alterations in paired-pulse perforant path inhibition following flurothyl status epilepticus. The present findings are consistent with studies in other seizure models, indicating that immature rats are highly resistant to seizure-induced changes.     

Influence of medial septal cholinoceptive cells on c-Fos-like proteins induced by soman

The effects of intraseptal application of atropine on c-fos proto-oncogene expression related to soman treatment were studied by immunohistochemistry for c-Fos-likke proteins. In control rats, 2 h after the onset of convulsion, c-Fos-like immunoreactivity was intense in the piriform and entorhinal cortices, but also in the cingulate, frontoparietal and retrosplenial cortices. In addition, the staining was moderate in the hypothalamus, amygdala and fascia dentata. The intraseptal application of atropine, which prevented soman-induced convulsions, reduced or even blocked c-Fos-like protein production related to soman treatment. This inhibition of Fos induction was significant in most of the limbic structures but also in non-limbic areas. The data in this study strongly suggest that the cholinergic cells of the medial septal area play a key role in soman-induced seizures, and confirm that c-Fos-like protein induction is closely related to neuronal hyperactivity.     

Mapping of neuronal networks underlying generalized seizures induced by increasing doses of pentylenetetrazol in the immature and adult rat: a c‐Fos immunohistochemical study

Previous studies from our group have shown that pentylenetetrazol (PTZ)-induced status epilepticus (SE) leads to age-dependent acute and long-term metabolic and circulatory changes in immature rats. In order to define the neural substrates involved in PTZ seizures according to age, the purpose of the present study was to map the areas of cellular activation during seizures of increasing severity in 10-day-old (P10), 21-day-old (P21) and adult rats. Seizures were induced by repetitive injections of subconvulsive doses of PTZ. The total dose received by the animals ranged from 4 to 125 mg/kg. These doses induced a variety of seizure profiles including absence-like, clonic seizures and SE. The cellular activation was measured as the density of c-Fos immunoreactive cells in animals at 2 h after the onset of the seizures. In P10 rats receiving a behaviourally non-active dose of PTZ, c-Fos immunoreactivity appeared only in the amygdala. The dose of 40 mg/kg that induced absence-like seizures led to a weak c-Fos expression in the medial thalamus, some cortical areas and globus pallidus. Clonic seizures reinforced labelling in the previous areas and induced a spread of c-Fos immunoreactivity to other cortical areas, thalamus, hypothalamus and some brainstem nuclei. At that age, only SE led to a widespread and stronger expression of c-Fos which was, however, totally lacking in the midbrain, and remained incomplete in the brainstem and forebrain limbic system, including the hippocampus. In P21 and adult rats, the inactive dose of PTZ induced c-Fos immunoreactivity in thalamus and hypothalamus. With absence-like seizures, c-Fos labelling spread to the cerebral cortex, amygdala, septum and some brainstem regions. With clonic seizures, immunoreactivity was reinforced in all areas already activated by absence-like seizures, and appeared in the striatum, accumbens, brainstem and hippocampus, except in CA1. After SE, c-Fos was strongly expressed in all brain areas. The intensity of c-Fos labelling was higher in most regions of P21 compared to adult rats. These data are in agreement with the immaturity of cellular and synaptic connectivity in P10 rats, the known greater sensitivity of rats to various kinds of seizures during the third week of life and the nature of the neural substrates involved in PTZ seizures.     

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.     

Sexual stimulation activates c-fos within estrogen-concentrating regions of the female rat forebrain

Regions of the brain that concentrate estrogen and progesterone are thought to regulate female sexual behavior by altering gene expression and neural sensitivity to afferent stimulation. We used immunocytochemistry and in situ hybridization to examine c-fos gene expression within estrogen-concentrating regions of the forebrain following various types of sexual stimulation with or without hormone treatment. Ovariectomized rats received injections of estradiol benzoate 48 h and progesterone 4 h before testing. Control rats that had been ovariectomized at least 5 months before testing did not receive hormone treatment. Rats were then either placed into bilevel testing chambers with sexually vigorous males, received manual stimulation of the flanks, received vaginocervical stimulation with a glass rod, or were left in their home cages. Copulation with intromission and ejaculation in hormone-treated rats, or stimulation of the vaginal cervix in both hormone-treated and control rats, produced a dramatic induction of c-fos mRNA and Fos-like immunoreactivity in estrogen-concentrating regions, such as the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, ventromedial hypothalamus, lateral habenula, and medial amygdala, in addition to regions that do not readily concentrate estrogen, such as the neocortex, thalamus, and striatum. Mechanical stimulation of the flanks produced a smaller induction of Fos in these rats, whereas hormone treatment alone had no effect. These data demonstrate that afferent sensory stimulation, but not estrogen or progesterone, regulates c-fos gene expression within different estrogen-concentrating and non-concentrating regions of the female rat forebrain.     

Effect of pentylenetetrazol on the expression of tyrosine hydroxylase mRNA and norepinephrine and dopamine transporter mRNA

Seizure activity has been shown to have differential effects on the terminal content of the monoamines, norepinephrine (NE) and dopamine (DA). Induction of seizure activity reduces the terminal content of NE, while DA levels remain unchanged or slightly elevated. This study examined the effect of the chemoconvulsant pentylenetetrazol (PTZ) on the mRNA expression of regulatory proteins which maintain the terminal content of NE and DA (i.e., synthesis and re-uptake). The areas examined were the noradrenergic neurons of the locus coeruleus (LC) and dopaminergic neurons of the substantia nigra pars compacta/ventral tegmentum area (SNpc/VTA) in the rat. In the LC, PTZ increased mRNA expression of the immediate early gene, c-fos, and mRNA expression of the synthesizing enzyme, tyrosine hydroxylase (TH), and the re-uptake protein, norepinephrine transporter (NET). This effect on TH and NET was observed only 1 day after the administration of PTZ. In contrast, PTZ did not alter the expression of c-fos mRNA in the SNpc/VTA, but reduced the expression of the dopamine transporter (DAT) mRNA. This effect was observed only 1 day after the administration of PTZ. TH mRNA expression in dopaminergic neurons was elevated initially in a manner similar to that observed in the LC. However, the effect of PTZ on TH mRNA expression in dopaminergic neurons was more prolonged (still elevated 3 days later). These results indicate that the chemoconvulsant PTZ has differential effects on the mRNA expression of regulatory systems (TH and neurotransporter proteins) in noradrenergic and dopaminergic neurons.     

l-Dopa stimulates c-fos expression in dopamine denervated striatum by combined activation of D-1 and D-2 receptors

Administration of l-dopa to unilaterally 6-hydroxydopamine-lesioned rats, activates the early gene c-fos in the lesioned caudate-putamen. D-1 receptor blockade by SCH 23390, preventedl-dopa-induced Fos-like immunoreactivity in the whole caudate-putamen, while D-2 receptor blockade by raclopride reduced Fos-like immunoreactivity only in the dorso-lateral portion. The results suggest thatl-dopa induces c-fos primarily through an activation of D-1 receptors, while D-2 receptor stimulation plays a facilitatory influence on D-1 mediated c-fos expression.     

Decreased expression of brain cAMP response element-binding protein gene following pentylenetetrazol seizure

The present study investigated whether the expression of the cAMP response element-binding protein (CREB) in the rat brain is altered following an acute self-limited seizure induced by pentylenetetrazol (PTZ). Male rats were injected intraperitoneally with a single convulsive dose (45 mg/kg) of PTZ, and the matched controls were given saline. For immunohistochemistry, animals were perfused with 4% parafomaldehyde at 24 h following PTZ seizures, and CREB immunoreactivity was examined in rat brain. For real-time RT-PCR, animals were sacrificed at 2 and 24 h and 1 week following PTZ seizures. Tissues from different rat brain regions were micropunched and subjected to real-time RT-PCR using Taqman probe. The CREB immunoreactive profiles were significantly decreased in CA3 and dentate gyrus of hippocampal formation, sensory cerebral cortex and thalamus at 24 h after PTZ seizures. Consistent with changes in CREB immunoreactivity, levels of CREB mRNA were significantly decreased in the hippocampus, cerebral cortex, amygdala and thalamus at 24 h after PTZ seizures. No significant change was found for CREB mRNA expression in these regions at 2 h or 1 week following PTZ seizures. These results show that a brief seizure caused a decline in CREB expression up to 24 h later.     

Recurrent neonatal seizures result in long‐term increases in neuronal network excitability in the rat neocortex

Neonatal seizures are associated with a high likelihood of adverse neurological outcomes, including mental retardation, behavioral disorders, and epilepsy. Early seizures typically involve the neocortex, and post‐neonatal epilepsy is often of neocortical origin. However, our understanding of the consequences of neonatal seizures for neocortical function is limited. In the present study, we show that neonatal seizures induced by flurothyl result in markedly enhanced susceptibility of the neocortex to seizure‐like activity. This change occurs in young rats studied weeks after the last induced seizure and in adult rats studied months after the initial seizures. Neonatal seizures resulted in reductions in the amplitude of spontaneous inhibitory postsynaptic currents and the frequency of miniature inhibitory postsynaptic currents, and significant increases in the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in pyramidal cells of layer 2/3 of the somatosensory cortex. The selective N‐methyl‐d ‐aspartate (NMDA) receptor antagonist d ‐2‐amino‐5‐phosphonovalerate eliminated the differences in amplitude and frequency of sEPSCs and mEPSCs in the control and flurothyl groups, suggesting that NMDA receptors contribute significantly to the enhanced excitability seen in slices from rats that experienced recurrent neonatal seizures. Taken together, our results suggest that recurrent seizures in infancy result in a persistent enhancement of neocortical excitability.     

Zif268 and Fos-like immunoreactivity in tetanus toxin-induced epilepsy: reciprocal changes in the epileptic focus and the surround

Altered gene expression for a number of molecules has been suggested as one of the underlying mechanisms of epileptogenesis. Changes in expression of the immediate early genes, zif268 and c-fos, were investigated in chronic focal epilepsy induced by tetanus toxin (TT, 20–35 ng) injected in the rat motor cortex. Most rats injected with TT and perfused on postoperative day 5, 7 or 14 had recurrent focal seizures after a latent period of 4–13 days, and showed enhanced Zif268 immunoreactivity in a cluster of neurons at the injection site, as well as reduced Zif268 immunoreactivity in a distinct cortical zone around this cluster. C-fos or Fos-related immunoreactivity was decreased over widespread areas of frontoparietal and piriform cortex in epileptic rats, except for a focus at the injection site which, in most cases, showed increases in Fos-like immunoreactivity. Some epileptic rats showed increased Zif268 immunoreactivity in neurons of the ipsilateral ventral lateral and central lateral thalamic nuclei and increased Zif268 and Fos-like immunoreactivity in the pontine nuclei. Rats perfused before onset of seizures, showed no overt changes other than a slight decrease in Zif268 and Fos-like immunoreactivity at the injection site. The reciprocal changes in Zif268 immunoreactive neurons in the epileptic focus and the immediate surround parallel changes in gene expression for a number of molecules important in epileptogenesis and suggest a state of functional disconnection of the epileptic focus from other cortical areas that may contribute to the development and maintenance of focal epilepsy.     

Interical spiking increases 2-deoxy[14C]glucose uptake and c-fos—like reacitivity

Although interical spikes are thought to share pathophysiological mechanisms with partial-onset seizure discharges, positron emission tomographic studies of the interictal stare have paradoxically shown focal hypometabolism whereas seizures produce hypermetabolism. To address this question, we performed functional mapping studies in an interictal spiking model in the rat. Recording screw electrodes were inserted through the skull bone so as to depress underlying cortex. Interictal spiking was subsequently induced by systemic administration of bicuculline methiodide. 2-deoxy[¹⁴C]glucose studies revealed increased glucose utilization in superficial and middle cortical layers at spiking screw sites. Nonspiking screw sites in the same animals and in controls did not show increased uptake. Convulsive seizures caused additional 2-deoxy[¹⁴C]glucose uptake at screw sites and in widespread forebrain areas. c-fos immunoreactivity occurred in superficial cortex at interictal spiking, but not nonspiking, sites. Convulsive seizures induced widespread forebrain c-fos immunoreactivity. These data suggest interictal epileptiform activity occurs in cells adjacent to cortical injury; these activate deeper layers via local connections. Interictal and ictal epileptiform states share common mechanisms, as both induce glucose hypermetabolism and immediate-early gene product activation. Possible reasons for failure to detect hypermetabolism in interictal human subjects are discussed.     

Pentylenetetrazol-induced recurrent seizures in rat pups: time course on spatial learning and long-term effects

PURPOSE: Recurrent seizures in infants are associated with a high incidence of neurocognitive deficits. Animal models have suggested that the immature brain is less vulnerable to seizure-induced injury than is that in adult animals. We studied the effects of recurrent neonatal seizures on cognitive tasks performed when the animals were in adolescence and adulthood. METHODS: Seizures were induced by intraperitoneal injection of pentylenetetrazol (PTZ) for 5 consecutive days, starting from postnatal day 10 (P10). At P35 and P60, rats were tested for spatial memory by using the Morris water maze task. In adulthood, motor performance was examined by the Rotarod test, and activity level was assessed by the open field test. Seizure threshold was examined by inhalant flurothyl. To assess presence or absence of spontaneous seizures, rats were video recorded for 4 h/day for 10 consecutive days for the detection of spontaneous seizures. Finally, brains were examined for histologic evidence of injury with cresyl violet stain and Timm staining in the supragranular zone and CA3 pyramidal cell layers of the hippocampus. RESULTS: PTZ-treated rats showed significant spatial deficits in the Morris water maze at both P35 and P60. There were no differences in seizure threshold, motor balance, or activity level during the open field test. Spontaneous seizures were not recorded in any rat. The cresyl violet stain showed no cell loss in either the control or experimental rats. PTZ-treated rats exhibited more Timm staining in the CA3 subfield. However, the control and experimental rats showed similar Timm staining within the supragranular zone. CONCLUSIONS: Our findings indicate that recurrent PTZ-induced seizures result in long-term cognitive deficits and morphologic changes in the developing brain. Furthermore, these cognitive deficits could be detected during pubescence.     

Age-related substantia nigra-mediated seizure facilitation

We investigated the role of the GABAergic system of the substantia nigra in seizures of rat pups. Rat pups had bilateral cannulae implanted chronically either in the substantia nigra or 1.5 mm dorsal to it. At age 16 to 17 days, seizures were induced in the cannulated rats and naive (intact) controls by exposure to flurothyl following pretreatment with the GABA agonist, muscimol, or vehicle. Naive rats were the most resistant to flurothyl seizures. Bilateral nigral infusions of muscimol markedly facilitated the development of flurothyl seizures in a dose-response manner and differed significantly from the vehicle controls or rats infused with muscimol dorsally to the substantia nigra. In contrast, bilateral nigral muscimol infusions protected adult rats against the development of flurothyl seizures. Our data indicate that the effects of nigral GABA agonist infusions on seizures are age-dependent; the altered responsivity of the GABAergic nigral system in immature rats may be responsible in part for the increased seizure susceptibility of the developing central nervous system.     

Long-term effects of neonatal seizures: a behavioral, electrophysiological, and histological study

Previous studies have demonstrated that recurrent seizures during the neonatal period lead to permanent changes in seizure threshold and learning and memory. The pathophysiological mechanisms for these changes are not clear. To determine if neonatal seizures cause changes in hippocampal excitability or inhibition, we subjected rats to 50 flurothyl-induced seizures during the first 10 days of life (five seizures per day). When the rats were adults, we examined seizure threshold using flurothyl inhalation, and learning and memory in the water maze. In separate groups of animals, we evaluated in vivo paired-pulse facilitation and inhibition in either CA1 with stimulation of the Schaffer collaterals or dentate gyrus with stimulation of the perforant path. Following these studies, the animals were sacrificed and the brains evaluated for mossy fiber sprouting with the Timm stain. Compared to control animals, rats with 50 flurothyl seizures had a reduced seizure threshold, impaired learning and memory in the water maze, and sprouting of mossy fibers in the CA3 pyramidal cell layer and molecular layer of the dentate gyrus. No significant differences in impaired paired-pulse inhibition was noted between the flurothyl-treated and control rats. This study demonstrates that recurrent neonatal seizures result in changes of neuronal connectivity and alterations in seizure susceptibility, learning and memory. However, the degree of impairment following 50 seizures was modest, demonstrating that the immature brain is remarkably resilient to seizure-induced damage.     

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.     

Mutual interactions between repeated flurothyl convulsions and electrical kindling

The interactions between repeated flurothyl seizures and electrical kindling induced from the neocortex or the amygdala were investigated. Three consecutive flurothyl-induced convulsions enhanced the rate of development of subsequent electrical kindling from the neocortex. Similarly multiple (more than 15) generalized kindled seizures, induced from either the neocortex or the amygdala, reduced the latency of onset of flurothyl seizures with repeated exposures producing a 'kindling-like' effect for flurothyl. These results indicate that eventually seizures will beget seizures independently of the site of origination of the seizures.     

Age-Dependent Changes in Long-Term Seizure Susceptibility and Behavior After Hypoxia in Rats

We showed that hypoxia is acutely epileptogenic in immature but not in adult rats. In the present study, we evaluated whether hypoxia results in an increase in long-term seizure susceptibility to flurothyl and whether this is associated with impaired performance on behavioral tests. We also determined whether these long-term outcomes are dependent on age at time of O2 deprivation. Long Evans hooded rats were rendered hypoxic on either postnatal day (P)5, P10, or P60. Sixty to 75 days after hypoxia, rats were tested for performance in water maze, open field, and handling tests and for seizure susceptibility to flurothyl. Hypoxia at P10 significantly increased seizure susceptibility to flurothyl, whereas hypoxia at P5 and P60 induced no long-term changes in seizure threshold. At P10, greater seizure severity during hypoxia and more prolonged exposure to hypoxia significantly increased long-term seizure susceptibility. This long-term change in seizure susceptibility appeared to be dissociated from any long-term neurobehavioral consequences, because only animals rendered hypoxic as adults (P60) had impaired behavioral performance. The results suggest that hypoxia-induced seizures can alter long-term seizure susceptibility and that this long-term effect is dependent on age and on severity of seizure activity at the time of previous hypoxia.