Widespread patterns of neuronal damage following systemic or intracerebral injections of kainic acid: a histological study

Distribution and kainate-mediated induction of the DNA mismatch repair protein MSH2 in rat brain

DNA repair is one of the most essential systems for maintaining the inherited nucleotide sequence of genomic DNA over time. Repair of DNA damage would be particularly important in neurons, because these cells are among the longest-living cells in the body. MSH2 is one of the proteins which are involved in the recognition and repair of a specific type of DNA damage that is characterized by pair mismatches. We studied the distribution of MSH2 in rat brain by immunohistochemical analysis. We found the level of MSH2 expression in rat brain to be clearly heterogeneous. The highest intensity of staining was found in the pyramidal neurons of the hippocampus and in the entorhinal and frontoparietal cortices. Positive cells were observed in the substantia nigra pars compacta, in cerebellar granular and Purkinje cells, and in the motor neurons of the spinal cord. We investigated the possible modulation of MSH2 expression after injection of kainate. Systemic administration of kainate induces various behavioural alterations and a typical pattern of neuropathology, with cell death in the hippocampal pyramidal neurons of the CA3/CA4 fields. Kainate injection also resulted in a marked, dose-dependent increase of MSH2 immunoreactivity in the hippocampal neurons of the CA3/CA4 fields. The effect was specific, since no changes in immunoreactivity were detected in the dentate gyrus nor in other brain areas. In summary, our data suggest that a mismatch DNA repair system, of which MSH2 protein is a representative component, is heterogeneously expressed in the rat brain and specifically induced by an experimental paradigm of excitotoxicity.

Changes of hippocampal Cu/Zn-superoxide dismutase after kainate treatment in the rat

In order to evaluate the putative role of Cu,Zn-superoxide dismutase (SOD-1) in the antioxidant defense mechanism during the neurodegenerative process, we examined the level of mRNA, the specific activity and immunocytochemical distribution for SOD-1 in the rat hippocampus after systemic injection of kainic acid (KA). Hippocampal SOD-1 mRNA levels were significantly increased by the seizure intensity 3 and 7 days after KA. These enhanced mRNA levels for SOD-1 were consistent with the increased specific activities for SOD-1, suggesting that the superoxide radical generated in neurotoxic lesion, induced SOD-1 mRNA. The CA1 and CA3 neurons lost their SOD-1-like immunoreactivity, whereas SOD-1-positive glia-like cells mainly proliferated throughout the CA1 sector and had an intense immunoreactivity at 3 and 7 days after KA. This immunocytochemical distribution for SOD-1-positive non-neuronal elements was similar to that for glial fibrillary acidic protein (GFAP)-positive cells. Each immunoreactivity for SOD-1-positive non-neuronal cell or GFAP in the layers of CA1 and CA3 disappeared 3 and 7 days after a maximal stage 5 seizure. On the other hand, activated microglial cells as selectively marked with the lectin occurred in the areas affected by KA-induced lesion. Double-labeling immunocytochemical analysis demonstrated the co-localization of SOD-1-positive glia-like cells and reactive astrocytes as labeled by GFAP or S-100 protein immunoreactivity. This finding suggested that the mobilization of astroglial cells for the synthesis of SOD-1 protein is a response to the KA insult designed to decrease the neurotoxicity induced by oxygen-derived free radicals. Therefore, these alterations might reflect the regulatory role of SOD-1 against oxygen-derived free radical-induced neuronal degeneration after systemic KA administration.

Norepinephrine-deficient mice have increased susceptibility to seizure-inducing stimuli

Several lines of evidence suggest that norepinephrine (NE) can modulate seizure activity. However, the experimental methods used in the past cannot exclude the possible role of other neurotransmitters coreleased with NE from noradrenergic terminals. We have assessed the seizure susceptibility of genetically engineered mice that lack NE. Seizure susceptibility was determined in the dopamine beta-hydroxylase null mutant (Dbh -/-) mouse using four different convulsant stimuli: 2,2,2-trifluroethyl ether (flurothyl), pentylenetetrazol (PTZ), kainic acid, and high-decibel sound. Dbh -/- mice demonstrated enhanced susceptibility (i.e., lower threshold) compared with littermate heterozygous (Dbh +/-) controls to flurothyl, PTZ, kainic acid, and audiogenic seizures and enhanced sensitivity (i.e., seizure severity and mortality) to flurothyl, PTZ, and kainic acid. c-Fos mRNA expression in the cortex, hippocampus (CA1 and CA3), and amygdala was increased in Dbh -/- mice in association with flurothyl-induced seizures. Enhanced seizure susceptibility to flurothyl and increased seizure-induced c-fos mRNA expression were reversed by pretreatment with L-threo-3, 4-dihydroxyphenylserine, which partially restores the NE content in Dbh -/- mice. These genetically engineered mice confirm unambiguously the potent effects of the noradrenergic system in modulating epileptogenicity and illustrate the unique opportunity offered by Dbh -/- mice for elucidating the pathways through which NE can regulate seizure activity.

Evidence for enhanced neuro-inflammatory processes in neurodegenerative diseases and the action of nitrones as potential therapeutics

A brief review is presented on observations leading to the current notions regarding neuro-inflammatory processes. The greatest focus is on Alzheimer's disease (AD) since this is where the most convincing data has been obtained. A brief summary of observations on the neuroprotective action of alpha-phenyl-tert-butyl-nitrone (PBN) as well as results of research designed to understand its mechanism of action is presented. We hypothesize that the mechanism of action of PBN involves inhibition of signal transduction processes, which are involved in the upregulation of genes mediated by pro-inflammatory cytokines and H2O2 that cause formation of toxic gene products. Results from recent experiments on Kainic acid (KA) mediated brain damage are provided to suggest the validity of the in vivo action of PBN to inhibit neuro-inflammatory processes. The accumulating scientific facts are helping to provide concepts that may become the basis for novel therapeutic approaches to the treatment of several neurodegenerative diseases.

Nuclear factor kappa B-mediated kainate neurotoxicity in the rat and hamster hippocampus

Administration of the excitotoxin kainate produces seizure activity and selective neuronal death in various brain areas. We examined the degeneration pattern of hippocampal neurons following systemic injections of kainate in the hamster and the rat. As reported, treatment with kainate resulted in severe neuronal loss in the hilus and CA3 in the rat. While the hilar neurons were also highly vulnerable to kainate in the hamster, neurons in the CA1 area, but not CA3, were highly sensitive to kainate. In both animals, immunoreactivity to anti-p50 nuclear factor kappa B antibody was increased in nuclei of the hilar neurons within 4 h following administration of kainate. Kainate treatment also increased the nuclear factor kappa B immunoreactivity in hamster CA1 neurons and rat CA3 neurons 24 h later. Neurons showing intense nuclear factor kappa B signal were stained with acid fuchsin. Kainate also increased DNA binding activity of p50 and p65 nuclear factor kappa B in the nuclear extract of the hippocampal formation as analysed by electrophoretic mobility shift assay in the hamster, suggesting that activation of nuclear factor kappa B may contribute to kainate-induced hippocampal degeneration. Administration of 100 nmol dizocilpine maleate 3 h prior to kainate attenuated kainate-induced activation of nuclear factor kappa B and neuronal death in CA1 in the hamster. The present study provides evidence that the differential vulnerability of neurons in the rat and the hamster hippocampus to kainate is partly mediated by mechanisms involving N-methyl-D-aspartate-dependent activation of nuclear factor kappa B.

The hamster circadian rhythm system includes nuclei of the subcortical visual shell

The clock regulating mammalian circadian rhythmicity resides in the suprachiasmatic nucleus. The intergeniculate leaflet, a major component of the subcortical visual system, has been shown to be essential for certain aspects of circadian rhythm regulation. We now report that midbrain visual nuclei afferent to the intergeniculate leaflet are also components of the hamster circadian rhythm system. Loss of connections between the intergeniculate leaflet and visual midbrain or neurotoxic lesions of pretectum or deep superior colliculus (but not of the superficial superior colliculus) blocked phase shifts of the circadian activity rhythm in response to a benzodiazepine injection during the subjective day. Such damage did not disturb phase response to a novel wheel stimulus. The amount of wheel running or open field locomotion were equivalent in lesioned and control groups after benzodiazepine treatment. Electrical stimulation of the deep superior colliculus, without its own effect on circadian rhythm phase, greatly attenuated light-induced phase shifts. Such stimulation was associated with increased FOS protein immunoreactivity in the suprachiasmatic nucleus. The results show that the circadian rhythm system includes the visual midbrain and distinguishes between mechanisms necessary for phase response to benzodiazepine and those for phase response to locomotion in a novel wheel. The results also refute the idea that benzodiazepine-induced phase shifts are the consequence of induced locomotion. Finally, the data provide the first indication that the visual midbrain can modulate circadian rhythm response to light. A variety of environmental stimuli may gain access to the circadian clock mechanism through subcortical nuclei projecting to the intergeniculate leaflet and, via the final common path of the geniculohypothalamic tract, from the leaflet to the suprachiasmatic nucleus.

Anticonvulsive and free radical scavenging actions of two herbs, Uncaria rhynchophylla (MIQ) Jack and Gastrodia elata Bl., in kainic acid-treated rats

Uncaria rhynchophylla (Miq.) Jack (UR) and Gastrodia elata BI. (GE) are traditional Chinese herbs that are usually used in combination to treat convulsive disorders, such as epilepsy, in China. The aim of this study was to compare the anticonvulsive and free radical scavenging activities of UR alone and UR in combination with GE in rats. For the in vitro studies, brain tissues from 6 male Sprague-Dawley (SD) rats were treated with 120 microg/ml kainic acid (KA), with or without varied concentrations of UR or UR plus GE. For the in vivo studies, male SD rats (6 per group) received intraperitoneal (i.p.) injection of KA 12 mg/kg to induce epileptic seizures and generation of free radicals, with or without oral administration of UR 1 g/kg alone or UR 1 g/kg plus GE 1 g/kg. Epileptic seizures were verified by behavioral observations, and electroencephalography (EEG) and electromyography (EMG) recordings. These results showed that UR alone decreased KA-induced lipid peroxide levels in vitro, whereas UR plus GE did not produce a greater effect than UR alone. UR significantly reduced counts of wet dog shakes (WDS), paw tremor (PT) and facial myoclonia (FM) in KA-treated rats and significantly delayed the onset time of WDS, from 27 min in the control group to 40 min in the UR group. UR plus GE did not inhibit seizures more effectively than UR alone, but did further prolong the onset time of WDS to 63 min (P < 0.05 vs. UR alone). UR alone reduced the levels of free radicals in vivo, as measured by lipid peroxidation in the brain and luminol-chemiluminescence (CL) counts and lucigenin-CL counts in the peripheral whole blood, but the combination of GE and UR did not reduce free radical levels more markedly than UR alone. In conclusion, our results indicate that UR has anticonvulsive and free radical scavenging activities, and UR combined with GE exhibit greater inhibition on the onset time of WDS than UR alone. These findings suggest that the anticonvulsive effects of UR and GE may be synergistic. However, the mechanism of interaction between UR and GE remains unknown.

In vivo regulation of glial cell line-derived neurotrophic factor-inducible transcription factor by kainic acid

A putative transcription factor induced in vitro by glial cell line-derived neurotrophic factor (GDNF) and transforming growth factor-beta was recently cloned and characterized [Yajima S. et al. (1997) J. Neurosci. 17, 8657-8666]. The messenger RNA of this protein, termed murine GDNF-inducible transcription factor (mGIF, hereafter referred to as GIF), is localized within cortical and hippocampal regions of brain, suggesting that GIF might be regulated by perturbations of these brain regions. In an effort to learn more about the role of GIF in vivo, we examined GIF messenger RNA in the brains of rats treated with the glutamatergic agonist kainic acid. This treatment is known to induce seizures and alter the messenger RNA expression of several growth factors, including GDNF, in several brain regions. Rats were given intraperitoneal saline (1 ml/kg) or kainic acid (15 mg/kg) and were killed at various time-points for in situ hybridization of brain sections with a GIF messenger RNA riboprobe. In saline-treated rats, GIF messenger RNA was present at low levels in cerebral cortex, hippocampus and hippocampal remnants such as the taenia tecta. Kainic acid treatment induced robust increases in GIF messenger RNA in several brain regions, including cerebral cortex, hippocampus, caudate-putamen, nucleus accumbens, and several nuclei of the amygdala and hypothalamus. Most brain regions showed the greatest increase in GIF messenger RNA 4-6 h after kainic acid administration and a return towards normal levels at 48 h. The CA3 region of hippocampus, however, showed a more rapid increase in GIF messenger RNA that was also evident 48 h after kainic acid administration. These results demonstrate that GIF messenger RNA can be regulated in vivo, and that this novel factor warrants further study as a central mediator of GDNF and perhaps other neurotrophic factors.

Parenterally administered kainic acid induces a persistent hyperalgesia in the mouse and rat

Nociceptive primary afferent C-fibers express a subset of glutamate receptors that are sensitive to kainic acid. Thus, we tested the possibility that activation of these receptors alters nociception. Intraperitoneal (i.p.) injection of kainic acid induced a persistent thermal hyperalgesia, when tested using the hot plate (mice) and tail flick (mice and rats) assays, and mechanical hyperalgesia when tested using von Frey monofilaments (rats), but had no effect on acetic acid-induced chemical nociception (mice). When administered i. p., 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid HBr/kainate (AMPA/KA) antagonist, completely blocked hyperalgesia. When injected intrathecally (i.t.), kainic acid itself failed to induce hyperalgesia and AMPA/KA antagonists given i.t. also failed to attenuate the hyperalgesic effect of kainic acid administered i.p. , indicating that the spinal cord is not the primary site of action. Kainic acid injected subcutaneously in the back of mice decreased response latencies in the hot plate and tail flick assays, indicating that hyperalgesia is achieved by a variety of parenteral routes of injection. Histological evaluation of rat spinal cord and dorsal root ganglia revealed no neurodegenerative changes 24 h after kainic acid. Together these data suggest that a persistent hyperalgesia results from the transient activation of AMPA/KA receptors that are located outside the spinal cord, perhaps on the distal projections of primary afferent fibers.

Differential expression of brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the adult rat spinal cord: regulation by the glutamate receptor agonist kainic acid

Previous in vitro studies indicate that select members of the neurotrophin gene family, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), contribute to survival and differentiation of spinal cord motoneurons. To investigate the potential roles of these factors in the adult spinal cord, we examined their cellular localization and regulation after systemic exposure to an excitotoxic stimulus, kainic acid (KA). Of the neurotrophins examined, NT-4/5 mRNA was most robustly expressed in the lumbosacral spinal cord of the normal adult rat, including expression by neurons throughout the gray matter, and in a subpopulation of white and gray matter glia. Both BDNF and NT-3 mRNAs were also densely expressed by alpha motoneurons of lamina IX, but were detected at lower levels elsewhere in the gray matter. NT-3 mRNA was additionally expressed by spinal cord glia, but was less widespread compared to NT-4/5. In response to systemic administration of KA, NT-4/5 and BDNF mRNAs were dramatically upregulated in a spatially and temporally restricted fashion, whereas levels of NT-3 mRNA were unchanged. These results provide strong in vivo evidence to support the idea that BDNF, NT-3, and in particular, NT-4/5, play a role in the normal function of the adult spinal cord. Furthermore, our results indicate that the actions of BDNF and NT-4/5 participate in the response of the cord to excitotoxic stimuli, and that those of NT-4/5 and NT-3 include both neurons and glia.

Brain-derived neurotrophic factor, nerve growth and neurotrophin-3 selected regions of the rat brain following kainic acid-induced seizure activity

Changes in levels of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) in various regions of the rat brain following kainic acid-induced seizure activity were investigated. BDNF protein, as measured by a two-site enzyme immunoassay, increased transiently 12-24 h after the intraperitoneal administration of kainic acid to 61.6 ng/g wet weight in the hippocampus (approximately 10-fold increase), 19.5 ng/g in the piriform plus entorhinal cortex (approximately 10-fold) and 8.2 ng/g in the olfactory bulb (approximately 16-fold), and then rapidly decreased. Increases of 2- to 4-fold in levels of BDNF were also detected in the septum, cerebral cortex, striatum and hypothalamus, but not in the cerebellum. In contrast, levels of NGF and NT-3 decreased 24 h after the administration of kainic acid. Western and Northern blotting analyses of hippocampal tissues, respectively, revealed increase in levels of a 14-kDa protein corresponding to BDNF and its mRNA at both 4.2 and 1.4 kb. Hippocampal mRNAs for NGF and NT-3 increased and decreased, respectively, in kainic acid-treated rats. Immunohistological investigations showed that, in the hippocampus, the administration of kainic acid enhanced a homogeneous immunoreactivity of BDNF in the polymorph inner layer (the stratum radiatum of the CA3/CA4 regions and the hilar region) and in granule cells of the dentate gyrus. BDNF protein was found in neurons, but not at all in glial cells or in blood vessels, and was localized in the cytoplasm, the nucleoplasm and the primary dendrites of neurons as well as in perisynaptic extracellular spaces, but hardly in their axons. Our results show that kainic acid treatment increases levels of BDNF, but not NGF or NT-3, in various regions of the rat brain, other than the cerebellum. Also, the majority of BDNF newly synthesized by hippocampal granule neurons is secreted into the perisynaptic extracellular space in the polymorph inner layer of the dentate gyrus, supporting an autocrine-like role for the factor in synaptic functions.

Differential regulation of cytokine expression following pilocarpine-induced seizure

While the pathological changes that occur in the brain following seizure have been well characterized, the molecular mechanisms underlying these events are poorly understood. Cell death, reactive gliosis, and axonal sprouting are among the best studied alterations in the epileptic brain. Previous work in both the peripheral and the central nervous systems suggests that cytokines are capable of affecting each of these processes. To better understand the role of cytokines in seizures and their sequelae, we have characterized cytokine expression in an animal model of epilepsy. Using pilocarpine to chemically induce seizures, and RNase protection assays to assess mRNA levels, we have quantified changes in expression of several members of the neuropoietic cytokine family following a single, prolonged seizure. Levels of oncostatin M (OSM), leukemia inhibitory factor (LIF), cardiotrophin-1, and ciliary neurotrophic factor were all increased in the hippocampus after seizure, though to differing extents and with markedly different time courses. Cells expressing the most dramatically up-regulated cytokines, LIF and OSM, were identified by combined in situ hybridization and immunohistochemistry. The majority of LIF(+) cells in the hippocampus were glial fibrillary acidic protein(+) astrocytes, while the majority of OSM(+) cells had the morphology of interneurons and were occasionally colabeled with neurofilament markers. Both the time course and the localization of cytokine up-regulation following seizure suggest possible roles for these intercellular signaling molecules in epilepsy.

Excitotoxic effects of non-NMDA receptor agonists in organotypic corticostriatal slice cultures

The excitotoxic effects of the glutamate receptor agonists kainic acid (KA) and 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and the corresponding neuroprotective effects of the AMPA/KA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) were examined in corticostriatal slice cultures. The purpose was to examine the feasibility of these cultures for excitotoxic studies, and to demonstrate possible differential excitotoxic effects of KA and AMPA on striatal and cortical neurons. Slices of dorsolateral striatum with overlying neocortex were obtained from neonatal rats and grown on semiporous membranes in serum-free medium for 3-4 weeks before exposure to KA or AMPA for 48 h. The uptake by injured cells of the fluorescent dye propidium iodide (PI) added to the culture medium was used as a quantifiable measure for neuronal degeneration and compared with efflux of the cytosolic enzyme lactate dehydrogenase (LDH) into the culture medium and loss of glutamic acid decarboxylase (GAD) activity in the tissue. Histological sections were also stained by the fluorescent dye Fluoro-Jade (FJ), for degenerating neurons and by immunocytochemical staining for gamma-aminobutyric acid (GABA). Digitized images showed a dose (0-24 microM KA, 0-6 microM AMPA) and time (0-48 h) dependent increase in PI uptake in both striatum and cortex. In other cultures exposed to KA (24 microM) or AMPA (6 microM) together with NBQX (0.1-9 microM), NBQX was found to exert a differential neuroprotective effect on striatum and cortex at low doses. NBQX was thus more protective against KA in the cortex than in the striatum, while the opposite was seen in relation to AMPA. Regarding neurodegenerative markers, PI uptake was significantly correlated with (1) LDH release into the culture medium, (2) optical density of Fluoro-Jade staining, (3) loss of GAD-activity in tissue homogenates, and (4) loss of GABA-immunostained neurons. We conclude that both differences between compounds (AMPA vs. KA) and brain areas (striatum vs. cortex) can be demonstrated in corticostriatal slice cultures, which in conjunction with an established set of markers for neuronal cell damage appears to be a feasible model for studies of the neurotoxic and neuroprotective effects of glutamate receptor agonists and antagonists.

Immunohistochemical localization of interleukin-1beta, interleukin-1 receptor antagonist and interleukin-1beta converting enzyme/caspase-1 in the rat brain after peripheral administration of kainic acid

The temporal and anatomical distribution of members of the interleukin-1 system in the rat brain following intraperitoneal kainic acid administration was studied in relation to neurodegeneration as detected with in situ end labelling. Kainic acid administration (10 mg/kg, i.p.) resulted in the induced expression of interleukin-1beta, interleukin- receptor antagonist and caspase-1p10 immunoreactivity in areas known to display neuronal and tissue damage upon excitotoxic lesions. The induction of these proteins was transient. Interleukin-1 immunoreactivity appeared at 5 h, and the interleukin-1 receptor antagonist-immunoreactive cells were first detected at 12 h, whereas the induction of caspase- 1p10 expression was first detected 24 h after kainic acid injection. Double labelling with the microglial marker Ox42 confirmed that both interleukin-1beta and interleukin-1 receptor antagonist were mainly localized in microglial cells. The regional distribution of in situ end-labelled neurons was similar to the distribution of cells expressing interleukin-1beta and interleukin-1 receptor antagonist, whereas the distribution of caspase-1 was more limited. The in situ end-labelled neurons, were, similarly to the interleukin-1beta-positive cells, first detected at 5 h, which is earlier than the induction of caspase-1. Our results show that the induction of IL-1beta and IL-1 receptor antagonist proteins after kainic acid are closely associated with the temporal as well as the anatomical distribution of in situ end-labelled neurons, whereas the induction of caspase-1 protein exhibited a delayed temporal profile and limited distribution. Since cytokine production occurs in activated microglial cells, the inflammatory component seems to be a strong mediator of this type of excitotoxic damage. The late onset of the caspase-1 expression would seem to indicate that this enzyme has no fundamental role in directly causing neuronal cell death induced by systemic kainic acid.

Evidence for altered trisynaptic circuitry in schizophrenic hippocampus

Recent postmortem studies have demonstrated subtle alterations in the hippocampal formation (HIPP) of patients with schizophrenia (SZ). These changes include a decreased density of nonpyramidal neurons (NPs), an increase of the GABAA, but not benzodiazepine receptors and a neuroleptic-dose-related increase of GAD65-IR terminals, particularly in sectors CA3 and CA2. High resolution studies of the GABAA receptor have further suggested that a decrease of disinhibitory GABAergic activity (i.e., GABA-to-GABA) in stratum pyramidale of CA3 may coexist with reduced inhibitory modulation (i.e., GABA-to-excitatory pyramidal neuron) in the stratum oriens of this same sector. These changes could potentially involve excitotoxic damage to interneurons in CA2; but, the precise time frame for the induction of such an injury during pre- versus postnatal life cannot as yet be inferred from the available data. These findings are consistent with reports of abnormal oscillatory rhythms and increased basal metabolic activity in the HIPP of patients with SZ. The fact that patients with manic depression also show a decrease of NPs in CA2 suggests that changes in the GABA system may not be related to a susceptibility gene for SZ. Rather, these alterations could be associated with a nonspecific factor, such as stress, experienced either early in life or much later during adolescence or adulthood. Presumably, there are also changes associated in other transmitter systems that may play a more specific role in establishing the SZ phenotype.

Anticonvulsant effect of Uncaria rhynchophylla (Miq) Jack. in rats with kainic acid-induced epileptic seizure

This study investigated the anticonvulsant effect of Uncaria rhynchophylla (UR) and the physiological mechanisms of its action in rats. A total of 70 male Sprague-Dawley (SD) rats were selected for study. Thirty four of these rats were divided into 5 groups as follows: 1) CONTROL GROUP (n = 6): received intraperitoneal injection (i.p.) of kainic acid (KA, 12 mg/kg); 2) UR1000 group (n = 10), 3) UR500 group (n = 6) 4) UR250 group, received UR 1000, 500, 250 mg/kg i.p. 30 min prior to KA administration, respectively; 5) Contrast group: received carbamazepine 20 mg/kg i.p. 30 min prior to KA administration. Behavior and EEG were monitored from 15 min prior to drug administration to 3 hours after KA administration. The number of wet dog shakes were counted at 10 min intervals throughout the experimental course. The remaining 36 rats were used to measure the lipid peroxide level in the cerebral cortex one hour after KA administration. These rats were divided into 6 groups of 6 rats as follows: 1) Normal group: no treatment was given; 2) CONTROL GROUP: received KA (12 mg/kg) i.p.; 3) UR1000 group, 4) UR500 group, 5) UR250 group, received UR 1000, 500, 250 mg/kg i.p. 30 min prior to KA administration, respectively; 6) Contrast group: received carbamazepine 20 mg/kg i.p. 30 min prior to KA administration. Our results indicated that both UR 1000 and 500 mg/kg decreased the incidence of KA-induced wet dog shakes, no similar effect was observed in the UR 250 mg/kg and carbamazepine 20 mg/kg group. Treatment with UR 1000 mg/kg, 500 mg/kg, or 250 mg/kg and carbamazepine 20 mg/kg decreased KA-induced lipid peroxide level in the cerebral cortex and was dose-dependent. These findings suggest that the anticonvulsant effect of UR possibly results from its suppressive effect on lipid peroxidation in the brain.

Platelet-activating factor (PAF) acetylhydrolase activity, LIS1 expression, and seizures

Lissencephaly patients are born with severe brain malformations and suffer from recurrent seizures. LIS1, the gene mutated in isolated lissencephaly patients, is a subunit of the heterotrimeric cytosolic enzyme platelet-activating factor acetylhydrolase (PAF-AH), interacts with tubulin, and affects microtubule dynamics. In order to gain molecular insights into the possible involvement of LIS1 in seizures in lissencephaly patients, we induced seizures in rats by injection of kainate. PAF-AH activity was markedly reduced as early as 30 min following initiation of seizures, making this parameter a sensitive indicator of seizure events. PAF-AH activity returned to and surpassed control values 1 week following initiation of seizures. Expression of LIS1 in the dentate gyrus changed significantly in a manner similar to that of PAF-AH enzymatic activity. This is the first correlation found between LIS1 expression and PAF-AH activity. Furthermore, the expression of the alpha2 catalytic subunit, which is the major PAF-AH catalytic subunit in rat adult brain, changed in a dramatic fashion. An additional higher-mobility LIS1 cross-reactive band was detected in samples isolated a week following seizure occurrence. This LIS1 isoform was enriched in the microtubule-associated fraction. We propose that LIS1 expression is an important factor in regulation of PAF-AH activity. We postulate that reductions in LIS1 protein levels found in lissencephaly patients may render them more susceptible to seizures.

Kainic acid-induced seizure upregulates Na(+)/myo-inositol cotransporter mRNA in rat brain

A major organic osmolyte, myo-inositol protects cells from perturbing effects of high intracellular concentrations of electrolytes. Myo-inositol is accumulated into cells through Na(+)/myo-inositol cotransporter (SMIT). In order to investigate the regulation of SMIT in generalized seizure, we employed Northern blot analysis and in situ hybridization to study the changes in SMIT mRNA expression in kainic acid-injected rats. Northern blot analysis demonstrated that SMIT mRNA began to increase in the brain 2 h after onset of seizure, and peaked at 12 h. In situ hybridization revealed rapid increase of SMIT mRNA (2 h of seizure) in the CA3 hippocampal pyramidal cells and in the dentate granular cells. Then, at 4-6 h SMIT mRNA expression was observed in the other limbic structure such as amygdala and piriform cortex. Finally, in neocortex and in CA1 pyramidal cells, SMIT mRNA was slowly increased and peaked at 12 h. Microautoradiogram demonstrated that cells expressed SMIT mRNA were mainly neurons. These results suggest that SMIT mRNA is upregulated by kainic acid-induced seizure primarily in structures involved in seizure activity.

Bcl-2, Bax and Bcl-x expression following kainic acid administration at convulsant doses in the rat

Neuronal death was produced in the CA1 and CA3 areas of the hippocampus, amygdala, and piriform and entorhinal cortices after intraperitioneal administration of kainic acid at convulsant doses to adult rats. To assess the involvement of members of the Bcl-2 family in cell death or survival, immunohistochemistry, western and northern blotting to Bcl-2, Bcl-x and Bax, and in situ hybridization to Bax were examined at different time-points after kainic acid treatment. Members of the Bcl-2 family were expressed in the cytoplasm of pyramidal neurons in the hippocampus, and in a subset of neurons of the piriform and the entorhinal cortices, amygdala and neocortex in the normal adult brain. Dying neurons in the pyramidal cell layer of CA1 and CA3 areas, entorhinal and piriform cortices, and amygdala also expressed Bcl-2, Bax and Bcl-x following excitotoxicity, although many dying cells did not. In addition, a number of cells in the affected areas showed Bax immunoreactivity in their nuclei at 24-48 h following kainic acid administration, thus indicating Bax nuclear translocation in a subset of dying cells. Western blots disclosed no modifications in the intensity of the bands corresponding to Bcl-2, Bcl-x and Bax, between control and kainic acid-treated rats. No modifications in the intensity of the bcl-2 messenger RNA band on northern blots was observed in kainic acid-treated rats. However, a progressive increase in the intensity of the bax messenger RNA band was found in kainic acid-treated rats at 6 h, 12 h and 24 h following kainic acid administration. Interestingly, a slight increase in Bax immunoreactivity was observed in the cytoplasm of neurons of the dentate gyrus at 24-48 h, a feature which matches the increase of bax messenger RNA in the same area, as shown by in situ hybridization at 12-24 h following kainic acid injection. The present results suggest that cell death or survival does not correlate with modifications of Bcl-2, Bax and Bcl-x protein, and messenger RNA expression, but rather that kainic acid excitotoxicity is associated with Bax translocation to the nucleus in a subset of dying cells.

Expression of the thymosin beta10 gene in normal and kainic acid-treated rat forebrain

Thymosin beta10 (Tbeta10) is a small actin-sequestering peptide widely distributed in mammalian tissues including nervous system. Here, we analyze the expression of Tbeta10 gene in normal and kainic acid (KA)-treated rat forebrain by in situ hybridization. Our results showed a defined regional pattern of the mRNA encoding for Tbeta10 in both normal and KA-treated rat forebrain. The presence of this transcript in different regions of the rat forebrain, including hippocampus, neocortex and several brain nuclei, provides evidence for the participation of Tbeta10 in the control of the actin dynamics that takes place in neurons. Furthermore, the analysis of the forebrain in KA-treated rats revealed an activation of the Tbeta10 gene linked to gliosis.

Selective early induction of synaptosomal-associated protein (molecular weight 25,000) following systemic administration of kainate at convulsant doses in the rat

SNAP-25 (synaptosomal-associated protein of mol. wt 25,000) is an essential component for neurotransmitter release, and its expression has been related to the plastic responses that follow CNS injury. In the present study, transient induction of SNAP-25 in selected brain areas is shown by immunohistochemistry at short times after a single intraperitoneal injection of kainate at convulsant doses. Six hours after kainate injection, SNAP-25 immunoreactivity was noticed in the perikarya of certain neurons of the perirhinal and lateral cortices, polymorphic layer of the dentate gyrus, CA3 pyramidal area of the hippocampus, and thalamus. In the same areas, a strong increase in SNAP-25 immunorectivity was detected at 12 and 24 h after kainate injection in cell bodies and fibers. Four days after kainate administration, the immunostaining pattern was similar to that observed in control animals. Intraperitoneal injection of cycloheximide blocked the expression of SNAP-25, thus suggesting de novo SNAP-25 protein synthesis following kainate administration. Kainate-dependent induction of SNAP-25a messenger RNA synthesis was observed by in situ hybridization in the mentioned brain areas. Heat shock protein of mol. wt 72,000 (HSP70/72) is a chaperone whose expression is induced early under stress conditions. Its expression and distribution were compared to that of SNAP-25 after the excitotoxic insult. Brain areas overexpressing SNAP-25 and HSP70/72 overlapped. In addition, partial co-localization of both antigens was observed by double-labeling immunohistochemistry. These results provide evidence of an involvement of SNAP-25 in the reactive response that follows kainate administration, and support the role of this protein in the plastic events that take place after kainate excitotoxicity.

Expression of thymosin beta4 messenger RNA in normal and kainate-treated rat forebrain

Thymosin beta4 is a major actin-sequestering peptide widely distributed in mammalian tissues, including the nervous system. In the present study, we analyse the expression of thymosin beta4 in normal and kainate-treated rat forebrain. In untreated animals, thymosin beta4 messenger RNA is mainly expressed in neurons of the hippocampal formation, neocortex and amygdaloid complex, as well as in oligodendrocytes. Other high-expressing areas are the tanycytic ependyma of the infundibulum, the substantia nigra pars compacta, and the supraoptic and premammillary nuclei. In rats treated with kainate, an excitotoxin that induces synaptic activation in the CA1-CA3 pyramidal neurons of the hippocampus, the levels of thymosin beta4 were clearly increased in the hippocampus and neocortex during the first 2-3 h after injection. In the long term, kainate causes neuronal degeneration in the CA1-CA3 regions of the hippocampus and functionally related structures, provoking a depletion of thymosin beta4 messenger RNA in these areas; however, the levels of this transcript are restored two weeks after kainate injection. Moreover, we have found that, in these degenerating zones, gliosis is accompanied by an elevation of the levels of thymosin beta4 messenger RNA, particularly in the CA1-CA3 region of the hippocampus, the lateral geniculate nucleus and the mammillothalamic tract. The present results demonstrate the existence of relatively high levels of thymosin beta4 messenger RNA in several areas of the rat forebrain, indicating that this peptide plays an important role in the regulation of actin polymerization in these regions of the brain. Moreover, the elevation of this messenger RNA after kainate treatment suggests a function of thymosin beta4 in the production and remodelling of neuronal processes. Finally, our findings provide evidence for a participation of this actin-sequestering molecule in the reactivity of certain types of glial cell that follows kainate lesions.

Lithium-pilocarpine-induced status epilepticus produces necrotic neurons with internucleosomal DNA fragmentation in adult rats

Prolonged and continuous epileptic seizures [status epilepticus (SE)] produce a widespread pattern of neuronal death, primarily in limbic brain regions. Because it has been suggested that seizure-induced neuronal death may be apoptotic in nature, we tested the hypothesis that lithium-pilocarpine-induced status epilepticus (LPCSE) produces apoptotic neurons. LPCSE lasting 3 h was induced in male Wistar rats which were allowed to recover for 24 or 72 h before perfusion-fixation. Neuronal death was assessed by light microscopy with the haematoxylin-and-eosin stain (H&E), with in situ DNA nick-end labelling (TUNEL stain), by electron microscopy, and by agarose gel electrophoresis of DNA extracted from vulnerable brain regions. Ultrastructurally, acidophilic neurons identified with H&E were dark, shrunken and necrotic in appearance, exhibiting pyknotic nuclei, irregular, dispersed chromatin clumps and cytoplasmic vacuolization. No cells with apoptotic features were seen. Acidophilic neurons were found in 21 out of 23 brain regions examined, and comprised 26-45% of the total number of neurons examined. A subset of these neurons (< 10% of the total number of neurons) were TUNEL-positive at 72 h, but not 24 h, after SE. Internucleosomal DNA cleavage (DNA 'laddering') was found in the six brain regions examined ultrastructurally 24 and 72 h after SE. These results indicate that, in adult rats, LPCSE produces neuronal injury with the appearance of necrosis rather than apoptosis. The necrotic neurons show nuclear pyknosis, chromatin condensation and internucleosomal DNA fragmentation, confirming the nonspecificity of these nuclear changes. Internucleosomal DNA cleavage and other programmed cell death mechanisms can be activated by SE in neurons which become necrotic.

Aromatase expression by astrocytes after brain injury: implications for local estrogen formation in brain repair

Recent evidence indicates that 17beta-estradiol may have neuroprotective and neuroregenerative properties. Estradiol is formed locally in neural tissue from precursor androgens. The expression of aromatase, the enzyme that catalyses the conversion of androgens to estrogens, is restricted, under normal circumstances, to specific neuronal populations. These neurons are located in brain areas in which local estrogen formation may be involved in neuroendocrine control and in the modulation of reproductive or sex dimorphic behaviours. In this study the distribution of aromatase immunoreactivity has been assessed in the brain of mice and rats after a neurotoxic lesion induced by the systemic administration of kainic acid. This treatment resulted in the induction of aromatase expression by reactive glia in the hippocampus and in other brain areas that are affected by kainic acid. The reactive glia were identified as astrocytes by co-localization of aromatase with glial fibrillary acidic protein and by ultrastructural analysis. No immunoreactive astrocytes were detected in control animals. The same result, the de novo induction of aromatase expression in reactive astrocytes on the hippocampus, was observed after a penetrating brain injury. Furthermore, using a 3H2O assay, aromatase activity was found to increase significantly in the injured hippocampus. These findings indicate that although astrocytes do not normally express aromatase, the enzyme expression is induced in these glial cells by different forms of brain injury. The results suggest a role for local astroglial estrogen formation in brain repair.

Recurrent seizures and hippocampal sclerosis following intrahippocampal kainate injection in adult mice: electroencephalography, histopathology and synaptic reorganization similar to mesial temporal lobe epilepsy

Human mesial temporal lobe epilepsy is characterized by hippocampal seizures associated with pyramidal cell loss in the hippocampus and dispersion of dentate gyrus granule cells. A similar histological pattern was recently described in a model of extensive neuroplasticity in adult mice after injection of kainate into the dorsal hippocampus [Suzuki et al. (1995) Neuroscience 64, 665-674]. The aim of the present study was to determine whether (i) recurrent seizures develop in mice after intrahippocampal injection of kainate, and (ii) the electroencephalographic, histopathological and behavioural changes in such mice are similar to those in human mesial temporal lobe epilepsy. Adult mice receiving a unilateral injection of kainate (0.2 microg; 50 nl) or saline into the dorsal hippocampus displayed recurrent paroxysmal discharges on the electroencephalographic recordings associated with immobility, staring and, occasionally, clonic components. These seizures started immediately after kainate injection and recurrid for up to eight months. Epileptiform activities occurred most often during sleep but occasionally while awake. The pattern of seizures did not change over time nor did they secondarily generalize. Glucose metabolic changes assessed by [14C]2-deoxyglucose autoradiography were restricted to the ipsilateral hippocampus for 30 days, but had spread to the thalamus by 120 days after kainate. Ipsilateral cell loss was prominent in hippocampal pyramidal cells and hilar neurons. An unusual pattern of progressive enlargement of the dentate gyrus was observed with a marked radial dispersion of the granule cells associated with reactive astrocytes. Mossy fibre sprouting occurred both in the supragranular molecular layer and infrapyramidal stratum oriens layer of CA3. The expression of the embryonic form of the neural cell adhesion molecule coincided over time with granule cell dispersion. Our data describe the first histological, electrophysiological and behavioural evidence suggesting that discrete excitotoxic lesions of the hippocampus in mice can be used as an isomorphic model of mesial temporal lobe epilepsy.

Hippocampal mossy fiber sprouting induced by chronic electroconvulsive seizures

Stress, which can precipitate and exacerbate depression, causes atrophy and in severe cases death of hippocampal neurons. Atrophy of the hippocampus has also been observed in patients suffering from recurrent major depression. The present study examines the influence of electroconvulsive seizures, one of the most effective treatments for depression, on the morphology and survival of hippocampal neurons. The results demonstrate that chronic administration of electroconvulsive seizures induces sprouting of the granule cell mossy fiber pathway in the hippocampus. This sprouting is dependent on repeated administration of electroconvulsive seizures, reaches a maximum 12 days after the last treatment and is long lasting (i.e. up to six months). Electroconvulsive seizure-induced sprouting occurs in the absence of neuronal loss, indicating that sprouting is not a compensatory response to cell death. This is different from the sprouting induced by kindling or excitotoxin treatment, which induce cell death along with recurrent seizures. Electroconvulsive seizure-induced sprouting is significantly diminished in brain-derived neurotrophic factor heterozygote knockout mice, indicating that this neurotrophic factor contributes to mossy fiber sprouting. However, infusion of brain-derived neurotrophic factor into the hippocampus does not induce sprouting of the mossy fiber pathway. The results demonstrate that chronic administration of electroconvulsive seizures induces mossy fiber sprouting and suggest that increased expression of brain-derived neurotrophic factor is necessary, but not sufficient for the induction of this sprouting. Although the functional consequences remain unclear, sprouting of the mossy fiber pathway would appear to oppose the actions of stress and could thereby contribute to the therapeutic actions of electroconvulsive seizure therapy.

Changes in expression of neuronal and glial glutamate transporters in rat hippocampus following kainate-induced seizure activity

The expression of excitatory amino acid transporters (EAATs) in rat hippocampus was studied following kainic acid-induced seizure activity in vivo and in hippocampal slice cultures. Protein and mRNA levels of the glial (EAAT2) and neuronal (EAAT3) transporters were determined with affinity-purified antibodies and oligonucleotide probes, respectively. Kainate treatment decreased EAAT3 immunoreactivity in stratum lacunosum moleculare within 4 h of seizure onset. Upon pyramidal cell death (5 days after kainate treatment), EAAT3 immunoreactivity in stratum pyramidale of CA1 and in stratum lacunosum moleculare was almost completely eliminated. The rapid effect of kainate on EAAT3 expression was confirmed by in situ hybridization; EAAT3 mRNA levels were decreased in CA1 and CA3 regions within 4-8 h of seizure onset. Kainate treatment had an opposite effect on levels and expression of EAAT2. Developmental studies indicated that the rapid regulation of transporter expression was not observed in rats younger than 21 days, an observation congruent with previous reports regarding the resistance of young rats to kainate. In hippocampal organotypic cultures, which lack a major excitatory input from the entorhinal cortex, kainate produced a slow decrease in [3H]d-aspartate uptake. This study indicates that an early effect of kainate treatment consists of down-regulation of the neuronal transporter EAAT3 in restricted hippocampal regions, together with a modest increase in the expression of the glial transporter EAAT2. Differential regulation of neuronal and glial glutamate transporters may thus play a role in kainate-induced seizure, neurotoxicity and neuronal plasticity.

Kainic and domoic acids differentially affect NADPH-diaphorase neurons in the mouse hippocampal formation

We investigated changes in numbers of nitric-oxide-producing cells in the hippocampal formation, striatum, and temporal cortex of mice 24 h after intraperitoneal administration of kainic acid (5, 10, 15, and 20 mg/kg) or domoic acid (1, 2, and 4 mg/kg). Nitric-oxide-producing cells were demonstrated histochemically by staining for nicotinamide adenine dinucleotide phosphate diaphorase. Nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the dentate gyrus and the subiculum did not change in number following administration of kainic acid or domoic acid at any dose. Positive neurons in the CA3 region of mice treated with kainic acid or domoic acid at any dose were significantly fewer than in controls. Although the numbers of positive neurons in the CA1/CA2 regions did not differ from those of controls at any of the four doses of kainic acid, positive cells in the CA1/CA2 were significantly more numerous than in controls at any dose of domoic acid. Although no significant differences in the numbers of positive neurons in the striatum were apparent between controls and any of the four doses of kainic acid, domoic acid significantly decreased the numbers of such cells. These results suggest that systemically administered kainic acid and domoic acid affect differentially nitric-oxide-producing cells in the hippocampal formation.

Muscarinic receptor loss and preservation of presynaptic cholinergic terminals in hippocampal sclerosis

PURPOSE

Prior single-photon emission tomography studies showed losses of muscarinic acetylcholine receptor (MAChR) binding in patients with refractory mesial temporal lobe epilepsy. Experimental animal studies demonstrated transient losses of MAChR due to electrically induced seizures originating in the amygdala. However, the relations between cholinergic synaptic markers, seizures, and underlying neuropathology in human temporal lobe epilepsy are unknown. We tested the hypotheses that human brain MAChR changes are attributable to hippocampal sclerosis (HS), and that HS resembles axon-sparing lesions in experimental animal models.

METHODS

We measured MAChR binding-site density, an intrinsic neuronal marker, within the hippocampal formation (HF) in anterior temporal lobectomy specimens from 10 patients with HS and in 10 autopsy controls. Binding-site density of the presynaptic vesicular acetylcholine transporter (VAChT) was measured as a marker of extrinsic cholinergic afferent integrity. MAChR and VAChT results were compared with neuronal cell counts to assess their relations to local neuronal losses.

RESULTS

Reduced MAChR binding-site density was demonstrated throughout the HF in the epilepsy specimens compared with autopsy controls and correlated in severity with reductions in cell counts in several HF regions. In contrast to MAChR, VAChT binding-site density was unchanged in the epilepsy specimens compared with autopsy controls.

CONCLUSIONS

Reduction in MAChR binding in HS is attributable to intrinsic neuronal losses. Sparing of afferent septal cholinergic terminals is consistent with the hypothesis that an excitotoxic mechanism may contribute to the development of HS and refractory partial epilepsy in humans.

Effects of vigabatrin treatment on status epilepticus-induced neuronal damage and mossy fiber sprouting in the rat hippocampus

Selective neuronal damage and mossy fiber sprouting may underlie epileptogenesis and spontaneous seizure generation in the epileptic hippocampus. It may be beneficial to prevent their development after cerebral insults that are known to be associated with a high risk of epilepsy later in life in humans. In the present study, we investigated whether chronic treatment with an anticonvulsant, vigabatrin (gamma-vinyl GABA), would prevent the damage to hilar neurons and the development of mossy fiber sprouting. Vigabatrin treatment was started either 1 h, or 2 or 7 days after the beginning of kainic acid-induced (9 mg/kg, i.p.) status epilepticus and continued via subcutaneous osmotic minipumps for 2 months (75 mg/kg per day). Thereafter, rats were perfused for histological analyses. One series of horizontal sections was stained with thionine to estimate the total number of hilar neurons by unbiased stereology. One series was prepared for somatostatin immunohistochemistry and another for Timm histochemistry to detect mossy fiber sprouting. Our data show that vigabatrin treatment did not prevent the decrease in the total number of hilar cells, nor the decrease in hilar somatostatin-immunoreactive (SOM-ir) neurons when SOM-ir neuronal numbers were averaged from all septotemporal levels. However, when vigabatrin was administered 2 days after the onset of status epilepticus, we found a mild neuroprotective effect on SOM-ir neurons in the septal end of the hippocampus (92% SOM-ir neurons remaining; P < 0.05 compared to the vehicle group). Vigabatrin did not prevent mossy fiber sprouting regardless of when treatment was started. Rather, sprouting actually increased in the septal end of the hippocampus when vigabatrin treatment began 1 h after the onset of status epilepticus (P < 0.05 compared to the vehicle group). Our data show that chronic elevation of brain GABA levels after status epilepticus does not have any substantial effects on neuronal loss or mossy fiber sprouting in the rat hippocampus.

Adverse psychological impact, glutamatergic dysfunction, and risk factors for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cell loss and pathological changes in neuronal transmission. In particular, malfunction in glutamatergic activity may be associated with the impairment of memory seen in Alzheimer patients. Both hypoactivation and hyperactivation of glutamatergic systems seem to cause impeded cognitive processing in animals. Rats subjected to rearing in isolation display reduced levels of glutamate in temporal regions accompanied by impaired learning and memory. Similar cognitive deficits are also seen in animals exposed to behavioral stress. Stress appears to have deleterious effects on cognition caused by glutamate neurotoxicity leading to attenuated synaptic activity. It is suggested that stress may represent a potential risk factor for AD. The known risk factors for AD (age, heredity, head trauma, low education, depression) may all be related to glutamatergic dysfunction. Some difficulties with pharmacological approaches based on glutamatergic agonists are discussed. It is suggested that optimal glutamate-mediated neurotransmission throughout life may prevent the occurrence of mental decline associated with AD.

The effect of acute kainic acid treatment on mu-opioid receptors in rat brain

There is evidence that acute exposure to kainic acid (KA) induces the release of endogenous ligands for opioid receptors and that mu-opioid agonists intensify KA-induced neurodegeneration. The aim of the present study was to investigate any acute toxic effects of KA upon mu-opioid receptors labelled with [3H]-DAMGO. 200-250 g rats were injected intraperitoneally with either saline or 16 mg/kg KA and brains were removed after 4 h. Membrane homogenates were prepared from the cerebellum, cortex, hippocampus, medulla and pons, midbrain and hypothalamus and striatum and in separate studies, from whole brain. In addition, frozen coronal sections were processed for comparative quantitative autoradiography. KA produced a two-fold increase in receptor affinity for [3H]-DAMGO in all regions and significant increases in receptor number in cortex, medulla and pons and striatum. Quantitative autoradiography showed similar significantly increased mu-labelling of structures comprising these gross anatomical regions. The findings demonstrate region specific changes in rat brain mu-opioid receptors after acute KA treatment which may be functionally related to the convulsant effects of this excitotoxin.

A gene expression approach to mapping the functional maturation of the hippocampus

Previous studies have shown an association among seizures, neuronal death and the expression of cellular immediate-early genes (cIEG). To understand further the relationship between these processes, we investigated the ability of kainic acid (KAI) to induce behavioral responses and gene expression in the hippocampus of developing fos-lacZ transgenic mice. Despite the fact that KAI elicited seizure-like activity from P2 onwards, Fos-lacZ was first detected at P5 in CA3 pyramidal neurons. Thus, intense behavioral responses were not invariably associated with fos-lacZ expression. Furthermore, while adult CA3 neurons are highly susceptible to KAI toxicity, they are resistant at P5. Therefore, the presence of Fos-lacZ in CA3 neurons is not necessarily predictive of their fate. By P10, Fos-lacZ was induced in CA3 neurons and in the most mature granule neurons of the dentate gyrus (DG). Between P15 and P20, KAI induced fos-lacZ in all CA1 and CA3 pyramidal neurons and most granule neurons of the DG. This stereotypical pattern of fos-lacZ expression mirrors the ontogeny of hippocampal circuitry and glutamate signalling. Thus the fos-lacZ mice can be used to map the functional maturation of the nervous system with single cell resolution. The scope of this approach was extended by administration of additional chemoconvulsants to fos-lacZ mice and by analysis of fos-lacZ transgenic mice with mutations in their FAP site. These additional studies revealed anatomical and mechanistic differences in glutamate receptor-mediated transcriptional responses in the nervous system.

Hippocampal damage after injection of kainic acid into the rat entorhinal cortex

Several experimental models of epilepsy have used kainic acid in animals to induce seizures and neuropathological changes which mimic those observed in human temporal lobe epilepsy. These models differ in the location and manner in which kainic acid is applied. In the present study, we characterized the seizure activity and neuropathological changes that occur in awake rats after kainic acid (25 ng/250 nl) is injected into the entorhinal cortex of freely moving rats. In 91% of the animals, this induced generalized motor seizures. Moreover, all of the animals survived status epilepticus. Animals were perfused two weeks after the injection for neuropathological examination. Silver-impregnation revealed that kainic acid caused pyramidal cell damage which was most severe in the CA1 subfield and to a lesser degree in the CA3c area. A loss of NADPH diaphorase-containing neurons in the hilus and the CA1 area was also consistently seen and, in most cases, a population of somatostatin-immunoreactive neurons was diminished. Our findings show that a minute amount of kainic acid delivered directly to the entorhinal cortex on unanesthetized animals reliably produces generalized seizures as well as a consistent pattern of cell damage in the hippocampus. Therefore, this model may be suitable for investigating the mechanisms underlying temporal lobe epilepsy, and may prove useful in assessing different treatment strategies for preventing seizure-induced structural damage.

Selective reduction of GluR2 protein in adult hippocampal CA3 neurons following status epilepticus but prior to cell loss

Kainic acid (KA) induces status epilepticus and delayed neurodegeneration of CA3 hippocampal neurons. Downregulation of glutamate receptor 2 (GluR2) subunit mRNA [the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) subunit that limits Ca2+ permeability] is thought to a play role in this neurodegeneration, possibly by increased formation of Ca2+ permeable AMPA receptors. The present study examined early hippocampal decreases in GluR2 mRNA and protein following kainate-induced status epilepticus and correlated expression changes with the appearance of dead or dying cells by several histological procedures. At 12 h, in situ hybridization followed by emulsion dipping showed nonuniform decreases in GluR2 mRNA hybridization grains overlying morphologically healthy-appearing CA3 neurons. GluR1 and N-methyl-D-aspartate receptor mRNAs were unchanged. At 12-16 h, when little argyrophilia or cells with some features of apoptosis were detected by silver impregnation or electron microscopy, single immunohistochemistry with GluR2 and GluR2/3 subunit-specific antibodies demonstrated a pattern of decreased GluR2 receptor protein within CA3 neurons that appeared to predict a pattern of damage, similar to the mRNA observations. Double immunolabeling showed that GluR2 immunofluorescence was depleted and that GluR1 immunofluorescence was sustained in clusters of the same CA3 neurons. Quantitation of Western blots showed increased GluR1:GluR2 ratios in CA3 but not in CA1 or dentate gyrus subfields. Findings indicate that the GluR1:GluR2 protein ratio is increased in a population of CA3 neurons prior to significant cell loss. Data are consistent with the "GluR2 hypothesis" that reduced expression of GluR2 subunits will increase formation of AMPA receptors permeable to Ca2+ and predict vulnerability to a particular subset of pyramidal neurons following status epilepticus.

A short episode of seizure activity protects from status epilepticus-induced neuronal damage in rat brain

Kainic acid (KA)-induced status epilepticus (SE) in adult rats results in extensive neuronal damage throughout the limbic system and the loss of selectively vulnerable neuronal populations, particularly CA3 neurons. We investigated the effects of a short episode of seizure activity on neuronal death elicited by a subsequent prolonged SE episode. A short episode of seizure activity was produced by sub-cutaneous (s.c.) injection of KA followed after 1 h by pentobarbital administration. Twenty-four hours later, KA was administered again, and animals were sacrificed 3 days later. Neuronal damage was estimated by visual analysis of neuronal density. Our results show that a short episode of seizure activity did not produce neuronal damage but almost completely protected vulnerable neurons from KA-induced neuronal damage. These results extend to epileptic tolerance the notion of tolerance previously described in the case of ischemia.

Neuronal stress and injury in C57/BL mice after systemic kainic acid administration

Kainate-induced seizures are widely studied as a model of human temporal lobe epilepsy due to behavioral and pathological similarities. While kainate-induced neuronal injury is well characterized in rats, relatively little data is available on the use of kainate and its consequences in mice. The growing availability of genetically altered mice has focused attention on the need for well characterized mouse seizure models in which the effects of specific genetic manipulations can be examined. We therefore examined the kainate dose-response relationship and the time-course of specific histopathological changes in C57/BL mice, a commonly used founder strain for transgenic technology. Seizures were induced in male C57/BL mice (kainate 10-40 mg/kg i.p.) and animals were sacrificed at various time-points after injection. Seizures were graded using a behavioral scale developed in our laboratory. Neuronal injury was assayed by examining DNA fragmentation using in situ nick translation histochemistry. In parallel experiments, we examined the expression an inducible member of the heat shock protein family, HSP-72, another putative marker of neuronal injury, using a monoclonal antibody. Seizure severity paralleled kainate dosage. At higher doses DNA fragmentation is seen mainly in hippocampus in area CA3, and variably in CA1, thalamus and amygdala within 24 h, is maximal within 72 h, and is largely gone by 7 days after administration of kainate. HSP-72 expression is also highly selective, occurring in limbic structures, and it evolves over a characteristic time-course. HSP-72 is expressed mainly in structures that also manifest DNA fragmentation. Using double-labeling techniques, however, we find essentially no overlap between neurons expressing HSP-72 and DNA fragmentation. These findings indicate that DNA fragmentation and HSP-72 expression are complementary markers of seizure-induced stress and injury, and support the notion that HSP-72 expression is neuroprotective following kainate-induced seizures.

Micromolar L-glutamate induces extensive apoptosis in an apoptotic-necrotic continuum of insult-dependent, excitotoxic injury in cultured cortical neurones

Excitotoxicity induced by L-glutamate (Glu), when examined in a pure neuronal cortical culture, involved widespread apoptosis at concentrations of 1-10 microM as part of a continuum of injury, which at its most servere was purely necrotic. Cells, maintained in chemically defined neurobasal/B27 medium, were exposed at d7 for 2 h to Glu (1-500 microM), and cellular injury was analysed 2 and 24 h after insult using morphology (phase-contrast microscopy), a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) viability assay, nuclear staining with 4,6-diamidino-2-phenylindole (DAPI), terminal transferase-mediated dUTP nick end-labelling (TUNEL) and DNA fragmentation by gel electrophoresis. Glu-mediated neurotoxicity was prevented by MK-801 (5 microM), whilst CNQX (20 microM) attenuated injury by 20%. Exposure to intensive insults (100 and 500 microM Glu) induced necrosis characterized by rapid cell swelling (< 2 h) and lack of chromatin condensation, confirmed by DAPI nuclear staining. In contrast, mild insults (< 20 microM Glu) failed to produce acute neuronal swelling at < 2 h, but 24 h after injury resulted in a large number of apoptotic nuclei as confirmed by TUNEL and electrophoretic evidence of DNA fragmentation, which was attenuated by cycloheximide (0.1 microg/ml). Our findings indicate for the first time that physiological concentrations of Glu produce neuronal injury across a continuum involving apoptosis (< 20 microM) and increasingly necrosis(> 20 microM), dependent on the severity of the initial insult.

An immunosuppressant, FK506, protects against neuronal dysfunction and death but has no effect on electrographic and behavioral activities induced by systemic kainate

Kainate is a potent agonist of an excitatory amino acid receptor subtype in the central nervous system, and causes neuronal death in several regions of the brain. Neurons are preferentially killed in the hippocampus, especially in the CA1 region, by systemic administration of kainate. It is speculated that functional alterations occur in the neurons preceding death. We examined the effect of FK506 on kainate-induced neuronal death and functional alterations in the rat hippocampal CA1 region. FK506 had no effect on electrographic and behavioral seizure activities induced by kainate; however, it prevented neuronal death measured seven days after administration. Although neither death nor morphological alterations of neurons were observed in the CA1 region 24 h after administration, the neurons exhibited decreased excitatory postsynaptic potentials and enhanced long-term potentiation. This functional alteration was not detected in the rats administered FK506 prior to kainate. Taken together, these observations indicate that functional alteration precedes neuronal death in rats systemically administered kainate and that FK506 prevents both. It is suggested that FK506 exerts its neuroprotective effect not by attenuating electrographic and behavioral seizure activities, but by protecting neurons from kainate-induced functional disorders.

Amygdala damage in experimental and human temporal lobe epilepsy

The amygdala complex is one component of the temporal lobe that may be damaged unilaterally or bilaterally in children and adults with temporal lobe epilepsy (TLE) or following status epilepticus. Most MR (magnetic resonance) imaging studies of epileptic patients have shown that volume reduction of the amygdala ranges from 10-30%. In the human amygdala, neuronal loss and gliosis have been reported in the lateral and basal nuclei. Studies in rats have more specifically identified the amygdaloid regions that are sensitive to status epilepticus-induced neuronal damage. These areas include the medial division of the lateral nucleus, the parvicellular division of the basal nucleus, the accessory basal nucleus, the posterior cortical nucleus, and portions of the anterior cortical and medial nuclei. Otherwise, other amygdala nuclei, such as the magnocellular and intermediate divisions of the basal nucleus and the central nucleus, remain relatively well preserved. Amygdala kindling studies in rats have shown that the density of a subpopulation of GABAergic inhibitory neurons that also contain somatostatin may be reduced even after a low number of generalized seizures. While analyses of histological sections and MR images indicate that in approximately 10% of TLE patients, seizure-induced damage is isolated to the amygdala, more often amygdala damage is combined with damage to the hippocampus and/or other brain areas. Moreover, recent data from rodents and nonhuman primates suggest that structural and functional alterations caused by seizure activity originating in the amygdala are not limited to the amygdala itself, but may also affect other temporal lobe structures. The information gathered so far on damage to the amygdala in epilepsy or after status epilepticus suggests that local alterations in inhibitory circuitries may contribute to a lowered seizure threshold and greater excitability within the amygdala. Furthermore, damage to select nuclei in the amygdala may predict impairment of performance in behavioral tasks that depend on the integrity of the amygdaloid circuits.

The effects of kainic acid in rats with spontaneous recurrent seizures

1. Rats with spontaneous recurrent seizures (SRS) were obtained by injection of kainic acid (KA; 10 mg/kg SC) to drug-naive rats that regularly developed wet-dog shakes followed by complex partial seizures and status epilepticus. Three to five weeks later, the rats with manifest SRS were selected. 2. The SRS rats were challenged with KA (10 mg/kg SC). The seizures induced in SRS rats by KA were similar to SRS regarding their clinical stage and duration (mean duration of seizures: 44 sec and 43 sec, respectively). The frequency of seizures was, however, increased compared with the frequency of SRS in control, vehicle-treated SRS rats (mean frequency of seizures: 12.9 and 0.4 per 3 hr, respectively). The KA-induced seizures in SRS rats differ behaviorally from KA-induced seizures in naive rats-namely, neither wet-dog shakes nor the status epilepticus could be induced. 3. Repeated injection of an equal dose of KA, applied to the SRS rats 1 day after the previous KA challenge, did not induce seizures. The loss of seizure susceptibility to KA was only temporary, as shown after a 7-day drug-free period, when the repeated injection of KA regained its seizure-triggering capacity. 4. The results indicate that reactivity to the seizure-inducing agent kainic acid changes in rats with spontaneous recurrent seizures.

Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid-induced seizure activity

Nurr1 is an immediate early gene encoding a member of the steroid-thyroid hormone receptor family. In PC12 cells, Nurr1 is readily induced by membrane depolarization, but not by growth factors. Nurr1 is predominantly expressed in the brain, and is essential to the differentiation of midbrain dopaminergic neurons. However, Nurr1 is also expressed in brain regions unrelated to dopaminergic neurons, e.g., hippocampus and cerebral cortex, and its immediate induction following seizure activity suggests a potential involvement of this transcription factor in modulating gene expression in the nervous system. To investigate the response of Nurr1 to neuronal activation, we analyzed Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid (KA)-induced seizure. In P7 animals, systemic injection of KA increased Nurr1 mRNA levels in a few hilar cells of the dentate gyrus and some pyramidal cells of the CA3 region of the hippocampus. In older animals, Nurr1 induction progressively expanded to all hippocampal regions (P14, P21) and eventually to cortical regions (adult). The increase was rapid and transient in the dentate gyrus, a structure resistant to the neurotoxic effect of KA, and was more prolonged in other regions more susceptible to KA toxicity. Induction of Nurr1 at early postnatal stages and rapid increase in the dentate gyrus following KA-induced seizure, suggest that Nurr1 expression is modulated by neuronal activity. On the other hand, prolonged Nurr1 induction in regions sensitive to KA toxicity indicates a possible involvement of Nurr1 in selective neuronal vulnerability.

BDNF, and full length and truncated TrkB expression in the hippocampus of the rat following kainic acid excitotoxic damage. Evidence of complex time-dependent and cell-specific responses

Systemic administration of kainic acid (KA) at convulsant doses results in irreversible cell damage and neuron loss in the hilus of the dentate gyrus and in the CA1 area of the hippocampus. This is followed by reactive astrocytosis in these regions, and sprouting of mossy fibers into the molecular layer of the dentate gyrus. Since trophic factors are probably implicated in the cellular responses to the excitotoxic insult, and early induction of BDNF and TrkB mRNAs has been observed following KA injection, the present study examines BDNF, full-length and truncated TrkB protein expression in the hippocampus, as revealed by immunohistochemistry, up to 30 days following KA administration to adult rats. Reduction in BDNF and full-length TrkB immunoreactivity preceding neuron loss is observed in the damaged areas. However, transient increase in BDNF immunoreactivity is observed in surviving CA1 neurons and in granule cells of the dentate gyrus. In contrast, full-length TrkB immunoreactivity progressively increases in the molecular layer of the dentate gyrus up to day 30 following KA administration. A second peak in BDNF immunoreactivity is observed in reactive astrocytes, as revealed with double-labeling immunohistochemistry to BDNF and GFAP, in the plexiform layers of CA1 and, to a lesser degree, in the molecular layer of the dentate gyrus. In addition, strong truncated TrkB immunoreactivity is found in reactive astrocytes, as revealed with double-labeling immunohistochemistry to truncated TrkB and GFAP, in the same regions. These results, in concert with previous observations in the same model of hippocampal damage, suggest that BDNF participates in the early response to excitotoxic damage, and that expression of full-length TrkB at strategic sites in the molecular layer of the dentate gyrus has a role in the regenerative response linked to mossy fiber sprouting. Interestingly, delayed expression of BDNF and truncated TrkB in reactive astrocytes may act as negative regulators of neurite growth in devastated regions, such as the CA1 area, which are impoverished of putative postsynaptic sites.

Protection by an adenosine analogue against kainate-induced extrahippocampal neuropathology

1. The glutamate analogue kainic acid produces neuronal damage in the central nervous system. We have reported that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA) can, at doses as low as 10 microg/kg IP, prevent the hippocampal damage that follows the systemic administration of kainate. The present work was designed to examine purine protection against kainate in extrahippocampal regions by using histological methods. 2. The results show that R-PIA, at a dose of 25 microg/kg IP in rats, can protect against the neuronal damage caused by kainate in the basolateral amygdaloid nuclei, the pyriform cortex and around the rhinal fissure. This protection could be prevented by the simultaneous administration of the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine, confirming that the protection involved adenosine A1 receptors. No protection was observed in the posterior amygdaloid nuclei or the entorhinal cortex, suggesting the absence of relevant adenosine receptors or a different mechanism of excitotoxicity.

Changes in extracellular glutamate and GABA levels in the hippocampal CA3 and CA1 areas and the induction of glutamic acid decarboxylase-67 in dentate granule cells of rats treated with kainic acid

For the evaluation of glutamatergic and GABAergic transmission during seizures, rat hippocampal CA1 and CA3 areas were separately assessed by brain microdialysis, and extracelluar glutamate and GABA were measured through the course of the seizures after a systemic administration of kainic acid (KA). The generalized convulsion started at about 1.5 h and was suppressed by diazepam at 2 h after the KA treatment. In the CA3 area, extracellular glutamate started to increase soon after the KA injection and returned to the control level at about 1.5 h. A decrease and then slight increase of the extracellular glutamate level in CA3 followed the diazepam injection. In the CA1 area, in contrast, a long-lasting decrease of extracellular glutamate was observed. The extracellular GABA concentration in the CA3 area increased immediately after the systemic administration of KA and returned to the normal level at about 3.5 h. A second increase in the extracellular GABA in this area began at about 4.5 h after the KA treatment. In the CA1 area, an increase of extracellular GABA began at about 3.5 h after KA administration (much later than that observed in the CA3 area) and was maintained throughout the observation. In situ hybridization showed a transient expression of glutamic acid decarboxylase (GAD)-67 mRNA in the granule cell layer of the dentate gyrus at 4 and 6 h, whereas GAD65 mRNA was unaffected. GABA immunoreactivity in the same area and mossy fibers in the CA3 were increased most significantly at 8 h after administration of KA. The possible relation of GABA induction in mossy fibers with the delayed increase in extracellular GABA in CA3 was discussed.

Kainic acid induced expression of interleukin-1 receptor antagonist mRNA in the rat brain

The endogenous interleukin-1 receptor antagonist (IL-1ra), a protein with partial homology with the proinflammatory cytokine interleukin-1beta (IL-1beta), prevents binding of IL-1beta to the signalling receptor. Exogenous IL-1ra has been shown to reduce the neuronal damage occurring after excitotoxic amino acid administration and ischemia. In the present study, in situ hybridization histochemistry was employed to investigate the regulation of endogenous IL-1ra mRNA expression in the rat brain after peripheral administration of kainic acid (10 mg/kg). IL-1ra mRNA expression was markedly induced in the hippocampus, thalamus, amygdala, piriform cortex, perirhinal cortex, entorhinal cortex, and to a lesser extent in the hypothalamus, and parietal and temporal cortex. The expression was first detected at 5 h after the kainic acid administration and it was markedly increased at 24 h. No signal was detected at 4 days after the injection. The majority of the cells expressing IL-1ra mRNA displayed the morphological characteristics of microglia. Expression of IL-1ra mRNA in neurons occurred mainly in the piriform and perirhinal cortex. The distribution pattern of IL-1ra mRNA expressing microglia-like cells was similar to that of cells labelled with ED1, a marker for activated microglia. The induction of IL-1ra mRNA expression may represent an endogenous response to balance IL-1 receptor mediated activity in the brain following kainic acid administration, conceivably to elicit neuroprotective and/or antiinflammatory effects.

Effect of a subconvulsant dose of kainic acid on thresholds for phenomena elicited by electrical stimulation of sensorimotor cortex in rats

Electrical stimulation of sensorimotor cortex was used to study early and late effects of administration of kainic acid in a dose (6 mg/kg i.p.) eliciting only nonconvulsive seizures in rats. Thresholds for elicitation of four phenomena--movements directly related to stimulation; epileptic afterdischarges (ADs) of the spike-and-wave type; clonic seizures accompanying these ADs; and mixed type of ADs where spike-and-wave activity transgresses into limbic type of epileptic phenomena--were measured. Acute administration of kainic acid resulted in a decrease of the threshold for elicitation of mixed type of ADs. In contrast, 1 week after kainic acid administration, the thresholds for stimulation-bound movements, spike-and-wave ADs and concomitant clonic seizures were increased, but the threshold for mixed type of ADs remained unchanged. The changes in thresholds tended to decrease 2 weeks after kainic acid but statistical significance was reached only for stimulus-bound movements. In addition, repetition of stimulation series after 1 as well as 2 weeks markedly influenced the thresholds.

Effects of chronic nimodipine on working memory of old rats in relation to defects in synaptosomal calcium homeostasis

The present study was designed to investigate whether chronic (from 12 to 23 months of age) dietary treatment with the L-type Ca2+ channel blocker nimodipine (30 mg/kg body weight) enhances the cognitive behavior of aged animals and whether such a treatment would have long-term effects on the mechanisms of Ca2+ regulation in synaptic terminals from the aged rat brain. Cognitive behavior was evaluated in an 8-arm radial maze in 6 test series comprising a total of 105 test sessions, with intervals of no training between series. Nimodipine-treated rats performed better than vehicle-treated, aged-matched controls in all the test series, making more correct choices every time a new series was initiated. However, differences between nimodipine- and vehicle-treated rats were most remarkable in the last three test series, when the rats were 19 to 22 months. In these series 74% of the nimodipine-treated rats were able to perform the task in 4 to 9 test sessions whereas only 12%, 14% or none of the control rats learned the task. To study Ca2+ regulation in synaptosomes derived from cerebral cortex and hippocampus, we analyzed 45Ca2+ accumulation as well as the levels of the Ca2+-binding proteins calbindin-D28K and calreticulin by Western blotting. Nimodipine administration had no effect on hippocampal synaptosomes but increased the levels of calbindin-D28K and calreticulin in cerebral cortex preparations. These results indicate that chronic nimodipine treatment from 12 to 23 months of age prevents age-induced learning deficits without showing any signs of toxicity, and that these effects are associated with a small increase in the levels of synaptosomal Ca2+-binding proteins from cerebral cortex. The up-regulation of these proteins might provide a link between the long-term effects of nimodipine on gene expression and learning ability in old rats.

Seizure-induced neuronal injury: vulnerability to febrile seizures in an immature rat model

Febrile seizures are the most common seizure type in young children. Whether they induce death of hippocampal and amygdala neurons and consequent limbic (temporal lobe) epilepsy has remained controversial, with conflicting data from prospective and retrospective studies. Using an appropriate-age rat model of febrile seizures, we investigated the acute and chronic effects of hyperthermic seizures on neuronal integrity and survival in the hippocampus and amygdala via molecular and neuroanatomical methods. Hyperthermic seizures-but not hyperthermia alone-resulted in numerous argyrophilic neurons in discrete regions of the limbic system; within 24 hr of seizures, a significant proportion of neurons in the central nucleus of the amygdala and in the hippocampal CA3 and CA1 pyramidal cell layer were affected. These physicochemical alterations of hippocampal and amygdala neurons persisted for at least 2 weeks but were not accompanied by significant DNA fragmentation, as determined by in situ end labeling. By 4 weeks after the seizures, no significant neuronal dropout in these regions was evident. In conclusion, in the immature rat model, hyperthermic seizures lead to profound, yet primarily transient alterations in neuronal structure.