Maturation of kainic acid seizure-brain damage syndrome in the rat. III. Postnatal development of kainic acid binding sites in the limbic system

Pathophysiological mechanisms of brain damage from status epilepticus

Human status epilepticus (SE) is consistently associated with cognitive problems, and with widespread neuronal necrosis in hippocampus and other brain regions. In animal models, convulsive SE causes extensive neuronal necrosis. Nonconvulsive SE in adult animals also leads to widespread neuronal necrosis in vulnerable regions, although lesions develop more slowly than they would in the presence of convulsions or anoxia. In very young rats, nonconvulsive normoxic SE spares hippocampal pyramidal cells, but other types of neurons may not show the same resistance, and inhibition of brain growth, DNA and protein synthesis, and of myelin formation and of synaptogenesis may lead to altered brain development. Lesions induced by SE may be epileptogenic by leading to misdirected regeneration. In SE, glutamate, aspartate, and acetylcholine play major roles as excitatory neurotransmitters, and GABA is the dominant inhibitory neurotransmitter. GABA metabolism in substantia nigra (SN) plays a key role in seizure arrest. When seizures stop, a major increase in GABA synthesis is seen in SN postictally. GABA synthesis in SN may fail in SE. Extrasynaptic factors may also play an important role in seizure spread and in maintaining SE. Glial immaturity, increased electronic coupling, and SN immaturity facilitate SE development in the immature brain. Major increases in cerebral blood flow (CBF) protect the brain in early SE, but CBF falls in late SE as blood pressure falters. At the same time, large increases in cerebral metabolic rate for glucose and oxygen continue throughout SE. Adenosine triphosphate (ATP) depletion and lactate accumulation are associated with hypermetabolic neuronal necrosis. Excitotoxic mechanisms mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors open ionic channels permeable to calcium and play a major role in neuronal injury from SE. Hypoxia, systemic lactic acidosis, CO2 narcosis, hyperkalemia, hypoglycemia, shock, cardiac arrhythmias, pulmonary edema, acute renal tubular necrosis, high output failure, aspiration pneumonia, hyperpyrexia, blood leukocytosis and CSF pleocytosis are common and potentially serious complications of SE. Our improved understanding of the pathophysiology of brain damage in SE should lead to further improvement in treatment and outcome.

Epileptogenic effects of status epilepticus

Determining whether and under what conditions status epilepticus (SE) leads to undesirable long-term sequelae has major clinical ramifications. In addition to structural brain damage and enduring neurological deficits following SE, it has been suggested that SE can establish a chronic condition of active epilepsy. These three residua (epileptic brain damage, neurological deficits, and epilepsy) have been especially linked to protracted SE. The older clinical literature indicates that these sequelae are especially likely if SE occurs in an immature brain, but this point has been challenged in recent studies. Clinical and animal model work that examines the issue of chronic nervous system deficits arising as a consequence of SE is reviewed, with particular attention to the question of the epileptogenic effect of SE. Because of the inherent problem of not being able to exclude occult neurological disease antecedent to SE in brain, animal model work promises to be especially relevant to the issues at hand. Work done on adult rats has shown that a previously normal brain can be "converted" after a bout of SE to an epileptic brain, as manifest both by epileptic brain damage resembling that found in the hippocampus of patients with intractable temporal lobe epilepsy and by spontaneous recurrent seizures registered in the hippocampus. A two-step model is proposed: morphological brain injury takes place first and this change, in turn, promotes seizures. This model is offered as one way in which chronic active epilepsy can be established by a transient episode of SE. Although some findings from work with animal models have been interpreted as not supporting the idea that the immature brain is sensitive to a chronic epileptogenic influence initiated by SE, the majority of such work is consistent with this idea. On the other hand, a considerable amount of animal work indicates that the brains of immature animals are quite resistant to SE-induced brain damage, in contrast to those of adults. Thus, under these circumstances, a different process of epileptogenesis than the two-step model may be operational. It is concluded that, under appropriate conditions, SE does exert an epileptogenic effect that persists.

Action of antiepileptic drugs against kainic acid-induced seizures and automatisms during ontogenesis in rats

Kainic acid (KA 4-14 mg/kg) administered intraperitoneally (i.p.) produces automatisms (scratching until third postnatal week, "wet dog" shakes thereafter), and clonic and tonic-clonic seizures in rats aged 7, 12, 18, 25, and 90 days. Administration of carbamazepine (CBZ) i.p. (25 or 50 mg/kg), phenobarbital (PB 20-80 mg/kg), clonazepam (CZP 0.2 or 1 mg/kg), or valproate (VPA 200 mg/kg) influenced neither incidence nor latency of automatisms. Clonic seizures that are regularly observed after the third postnatal week in controls were either abolished or substantially suppressed by any of the aforementioned antiepileptic drugs (AEDs). Tonic-clonic seizures observed in the first 3 postnatal weeks were suppressed only by solvent [including propyleneglycol (PEG), ethanol, and water]; the effect of AEDs on tonic-clonic seizures was proconvulsant instead. The automatisms were most resistant to AED therapy. These results induce some doubts about the adequacy of the KA model for identifying AEDs effective against complex partial seizures, but forthcoming AEDs that suppress automatisms in the KA rat model might also be active against human complex partial seizures.

Seizure activity-induced changes in polyamine metabolism and neuronal pathology during the postnatal period in rat brain

Systemic injection of kainic acid (KA) does not cause neuronal pathology in limbic structures in rat brain prior to postnatal day (PND) 21. The present study tested if the development of the pathogenic response is associated with the maturation of a link between seizure activity and polyamine metabolism. Pathology was assessed with histological techniques and with the binding of [3H]Ro5-4864, a ligand for the peripheral type benzodiazepine binding sites (PTBBS), a marker of glial cell proliferation. In agreement with previous results, peripherally administered kainate at doses sufficient to induce intense behavioral seizures produced a loss of Nissl staining in hippocampus after PND 21 but not at earlier ages. The pattern of neuronal damage observed after PND 21 resembled that found in adult animals: extensive losses of Nissl staining in area CA3 of hippocampus and in piriform cortex, more modest effects in CA1 and sparing of the granule cells of the dentate gyrus. Similarly, no increase in [3H]Ro5-4864 binding as a result of KA administration was observed in hippocampus and piriform cortex until PND 21. Ornithine decarboxylase (ODC) activity and putrescine levels were high in the neonatal brain and decreased to reach adult values by PND 21. KA-induced seizure activity did not significantly alter both variables until PND 21. After PND 21, ODC activity and putrescine levels markedly increased 16 h after KA-induced seizure activity in hippocampus and piriform cortex. The magnitude of the effects increased between PND 21 and PND 30, at which point the changes in both parameters were comparable to those found in adults. Polyamines stimulate the activity of the calcium-dependent proteases calpain in brain fractions and may increase calpain-mediated proteolysis in situ. In accord with this, kainate-induced breakdown of spectrin, a preferred substrate of calpain, measured 16 h after KA injection followed a developmental curve parallel to that for kainate-induced increases in putrescine levels. These results indicate that the onset of vulnerability to seizure activity triggered by kainic acid is correlated with the development of an ODC/polyamine response to the seizures and further support a critical role for the ODC/polyamine pathway in neuronal pathology following a variety of insults.

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.

Absence of c-fos induction in neonatal rat brain after seizures

Induction of the proto-oncogene c-fos is often considered to be a marker of increased neuronal activity. We have used in situ hybridization to study the pattern of c-fos expression in limbic structures following kainic acid-induced seizures during the postnatal period in the rat. Prior to postnatal day 13 (P13), seizure activity did not result in c-fos induction in any limbic structure. Between P13 and P25, a gradual increase in c-fos expression was observed in hippocampus and cortical structures. These results were corroborated by nuclear run-off transcription assay. Thus, alterations in c-fos transcription that may facilitate stimulus-transcription coupling occur during postnatal development. The possible relationship between the postnatal maturation of c-fos expression and the increase in susceptibility of specific neuronal populations to seizure-induced cell damage is discussed.

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

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

L-phosphoserine, a metabolite elevated in Alzheimer's disease, interacts with specific L-glutamate receptor subtypes

L-Phosphoserine is one of the phosphomonoesters elevated in Alzheimer's disease brain and has close structural similarity to L-glutamate. This study attempts to define precisely the actions of L-phosphoserine at L-glutamate receptor subtypes. L-Phosphoserine is shown to bind to N-methyl-D-aspartate and kainic acid receptor subtypes, but not to the quisqualic acid subtype. Studies of [3H]MK-801 binding in the presence and absence of L-glutamate and glycine show L-phosphoserine to be a competitive N-methyl-D-aspartate antagonist. The IC50 of L-phosphoserine in these studies varies from 373 to 721 microM. This may indicate a physiologically relevant action of L-phosphoserine in Alzheimer's disease brain because whole brain concentrations may reach over 1 mM.

Depression of a sustained calcium current by kainate in rat hippocampal neurones in vitro

1. High-threshold, slow inactivating inward Ca2+ currents were studied in CA1 pyramidal neurones from rat hippocampal slices using the single-electrode voltage clamp technique. 2. Kainate (50-400 nM) induced a dose-dependent depression of the amplitude of the slow Ca2+ current. At a dose of 200 nM the current amplitude was reduced from -0.63 +/- -0.06 to -0.32 +/- 0.06 nA. Such an effect of kainate was associated with the development of a small inward current (-0.11 +/- 0.03 nA). Kynurenic acid (1 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microM) fully prevented these actions of kainate. 3. The structurally related kainate analogue alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA; 200 nM) depressed the slow Ca2+ current by 30 +/- 7%, an effect also blocked by CNQX. 4. In low-Na+ medium slow Ca2+ currents were followed by sustained inward tail currents. Kainate reduced both the steady-state Ca2+ current (from -0.98 +/- 0.14 to -0.63 +/- 0.15 nA) and the tail current (from -0.40 +/- 0.04 to -0.14 +/- 0.03 nA). 5. The inactivation process of the slow Ca2+ current was tested by a double-pulse protocol and was found to be enhanced by kainate. 6. Equimolar replacement of Ca2+ by Ba2+ produced larger inward currents followed by prolonged tails. Kainate reduced the Ba2+ steady-state current from -1.77 +/- 0.18 to -1.44 +/- 0.24 nA and the tail current from -0.47 +/- 0.15 to -0.17 +/- 0.05 nA. 7. In current clamp experiments Ca2+ action potentials were recorded from cells loaded with the Ca2+ chelator BAPTA. In these conditions kainate failed to reduce the Ca2+ action potential, while in the absence of BAPTA kainate shortened the Ca2+ action potentials by 30%. 8. It is suggested that low concentrations of kainate reduced the slow Ca2+ current by promoting its inactivation perhaps through a rise in free intracellular Ca2+.

Neuronal degeneration in hippocampus and cerebellum of mutant spastic Han-Wistar rats

The neuropathology of the brain of mutant spastic Han-Wistar rats (Han-Wist SPA/SPA) was investigated using histological techniques. A surprising result was the detection of neuronal degeneration in the hippocampus and cerebellum of mutant spastic rat brains, whereas other regions, e.g. neocortex, isocortex, basal ganglia and thalamus, were overall normal. The CA3 sector in the septal third of the hippocampus including the cell band reaching into the hilus ('CA3c') showed a severe neuronal degeneration, whereas the granule cells of the dentate gyrus, several hilar neurons ('CA4') and the pyramidal cells in CA1 were found normal. In the cerebellum, a variable patchy degeneration of Purkinje cells was detected while the general layering was normal and granule cells and Golgi cells appeared preserved.

Excitatory amino acids in the developing brain: ontogeny, plasticity, and excitotoxicity

Besides their role as neurotransmitters, excitatory amino acids (EAAs) in the developing brain are crucially involved in plasticity and excitotoxicity which are modified by their distinct ontogeny. Along with incomplete neuritogenesis and synaptogenesis, presynaptic markers of the EAA system are immature in the developing brain; however, postsynaptic EAA system activities, particularly of the N-methyl-D-aspartate and quisqualate receptors, are transiently enhanced early in life. This transient enhancement is presumably beneficial to the immature brain because physiologic activation of the EAA system plays a critical role in plasticity of early learning and morphogenesis. At the same time, this transient hypersensitivity renders the immature brain vulnerable to pathologic excitation of the EAA system (excitotoxicity) as observed during neonatal hypoxia-ischemia.

Postnatal development of the excitatory amino acid system in visual cortex of the rat. Changes in ligand binding to NMDA, quisqualate and kainate receptors

The postnatal development of the ligand binding to N-methyl-D-aspartate (NMDA), quisqualate and kainate receptor sites was examined in whole homogenates of the visual cortex of rats, aged 2-360 days. As selective ligands, [3H]CPP (3-(2-carboxypyperazine-4-yl)-propyl-1-phosphonic acid, [3H]AMPA (RS-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid) and [3H]KA (kainic acid) were used, respectively. The binding of CPP was low in newborns, rapidly increased from the second postnatal week, reached its maximum between weeks 2 and 3, then slowly declined up to the age of 1 year. In contrast, the binding of AMPA and kainate was high perinatally, increased rapidly up to day 6 after birth to reach an early maximum value, then gradually decreased to adult values which were attained at an age of 3-4 weeks. These age-related changes were derived from alterations in the density of binding sites, which, in the case of AMPA, was accompanied by an increase in binding affinity. The results, compared with the developmental time-course of excitatory synapses, indicate that, in the immature cerebral cortex, NMDA receptors may be primarily involved in synaptic transmission, whereas quisqualate and kainate receptors may play some other (e.g. trophic) roles.

The ontogeny of excitatory amino acid receptors in the rat forebrain--II. Kainic acid receptors

The ontogeny of [3H]kainic acid binding in rat forebrain was studied quantitatively using in vitro receptor autoradiography. Specific binding was detectable in ventral thalamus, hippocampus, striatum and olfactory bulb by postnatal day 1. In regions with high densities of receptors in adulthood, such as CA3, dentate gyrus and striatum, binding increased progressively across development peaking at postnatal day 21. In ventral thalamus and the inner lamina of the neocortex, [3H]kainic acid binding was high in the first three postnatal weeks and relatively low thereafter. Saturation studies performed on adults and 14-day-old animals suggest differences in both the affinity and the maximal binding capacity contributed to the observed developmental changes in binding of [3H]kainic acid.

Transient increase in ligand binding to quisqualate and kainate sites in cerebral cortex of immature rats

The postnatal changes in the specific binding of [3H]kainate and [3H]AMPA (RS-alpha-amino-3-hydroxy-5-methyl-4-isoxasolopropionic acid, an agonist of quisqualate receptors) were studied in cerebral cortex of rats, aged 2-360 days. The binding of the two ligands was assayed in whole-tissue homogenates. Similar developmental time courses were found for kainate and AMPA binding, characterized by high perinatal levels, a further increase during the first few days after birth, an early maximum value around the age of one week, and a gradual decrease to adult values which were attained at an age of 3-4 weeks. As revealed by Scatchard analysis, the transient elevation of ligand binding was derived from an increased density of binding sites, which, in the case of AMPA, was accompanied also by an increase in binding affinity. The results indicate that, in the immature cerebral cortex, kainate and quisqualate receptors may play a role other than in synaptic transmission.

Effects of ketamine on metrazol-induced seizures during ontogenesis in rats

An anticonvulsant action of ketamine, a noncompetitive NMDA receptor antagonist (1-40 mg/kg IP), on the metrazol-induced seizures was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused ataxia even in the lowest dose used. As concerens its interaction with metrazol it exerted a clearcut anticonvulsant effect against generalized tonic-clonic seizures at all developmental stages. On the contrary, the effects on clonic (i.e., minimal) seizures were only moderate or absent. Higher efficacy of ketamine was observed in young animals. Our results suggest a role of excitatory amino acids in the generation of generalized tonic-clonic metrazol seizures, but their share on the induction of clonic (minimal) seizures seems to be very small.

Protective effects of mossy fiber lesions against kainic acid-induced seizures and neuronal degeneration

The effects of a hippocampal mossy fiber lesion have been determined on neuronal degeneration and limbic seizures provoked by the subsequent intracerebroventricular administration of kainic acid to unanesthetized rats. Mossy fiber lesions were made either by transecting this pathway unilaterally or by destroying the dentate granule cells unilaterally or bilaterally with colchicine. All control rats eventually developed status epilepticus and each temporally discrete seizure that preceded status epilepticus was recorded from the hippocampus ipsilateral to the kainic acid infusion before the contralateral hippocampus. A mossy fiber lesion of the ipsilateral hippocampus prevented the development of status epilepticus in 26% of subjects and in 52% of subjects seizures were recorded from the contralateral hippocampus before the ipsilateral hippocampus. Unlike electrographic records from other treatment groups, those from rats which had received a bilateral colchicine lesion exhibited no consistent pattern indicative of seizure propagation from one limbic region to another. A bilateral, but not a unilateral, mossy fiber lesion also dramatically attenuated the behavioral expression of the seizures. Regardless of its effects on kainic acid-induced electrographic and behavioral seizures, a mossy fiber lesion always substantially reduced or completely prevented the degeneration of ipsilateral hippocampal CA3-CA4 neurons. This protective effect was specific for those hippocampal neurons deprived of mossy fiber innervation. Neurons in other regions of the brain were protected from degeneration only when the mossy fiber lesion also prevented the development of electrographic status epilepticus. These results suggest that the hippocampal mossy fibers constitute an important, though probably not an obligatory, link in the circuit responsible for the spread of kainic acid seizures. Degeneration of CA3-CA4 neurons appears to depend upon (1) the duration of hippocampal seizure activity and (2) an as yet undefined influence of or interaction with the mossy fiber projection which enhances the neurodegenerative effect of the seizures.

Effects of kainic acid on seizure susceptibility in the developing brain

The short and long-term effects of systemic administration of kainic acid to immature animals were studied in rats. Kainic acid was administered systemically to rats of 1-30 days of age. The rats were monitored for both behavioral and EEG changes. To study the effects of kainic acid on seizure susceptibility, status epilepticus was induced in 12-, 18-, and 27-day-old rats by systemic administration of kainic acid. Seizure susceptibility was assessed 3 days later using the kindling technique. In addition, another group of 27-day-old rats that developed status epilepticus following systemic administration of kainic acid were kindled as adults. Young rats (1 day of age) developed behavioral status epilepticus after kainic acid and ictal electroencephalographic changes were seen beginning at age 6 days. The 15- and 21-day-old rats kindled 3 days after kainic acid administration kindled at the same rate as controls. However, 30-day-old rats that received kainic acid at age 27 days kindled more quickly to stage-5 seizures than controls. Rats that received kainic acid at age 27 days and maintained until adulthood developed spontaneous recurrent seizures and kindled faster as adults than controls. These results demonstrate that the effect of kainic acid on seizure susceptibility is an age-dependent phenomenon.

Effects of serial administration of kainic acid on the developing brain

The purpose of this study was to determine whether the repeated administration of kainic acid at 24-hr intervals results in pharmacological kindling. Rather than developing an increased intensity of seizures, this method of serial administration of kainic acid resulted in the development of tolerance to its effects.

Kainate binding sites in the hippocampal mossy fibers: localization and plasticity

The regional distribution of high affinity binding sites for kainic acid has been determined in rat hippocampi by quantitative autoradiography. Selective lesions were made in order to determine the exact localization of these sites in the mossy fiber system, and to evaluate whether the sprouting and synaptic reorganization of the mossy fibers are associated with alterations in the distribution of these binding sites. The results show that kainate binding sites in the stratum lucidum are more vulnerable to destruction of the granules and their mossy fibers by intrahippocampal colchicine injections than to destruction of the CA3/CA4 pyramidal cells by injection of kainate into the amygdala. This suggests that a substantial proportion of the kainate binding sites is associated with the mossy fiber terminals (i.e. the presynaptic elements). Furthermore, in keeping with an earlier study, destruction of the pyramidal neurons of CA3 by intracerebral kainate produced a dark Timm positive band in the supragranular zone which is due to the sprouting of mossy fibers. This was associated with an increase in the density of kainate binding sites, which further stresses the parallelism between the distribution of these sites and mossy fiber terminals.

Selective kainic acid lesions in cultured explants of rat hippocampus

The influence of the excitotoxin kainic acid (KA) on cultivated explants of rat hippocampus was investigated. Addition of 3 microM KA to the culture medium over 24-48 h induced a destruction of the pyramidal cells in the CA3 region, whereas the CA1 pyramidal cells and the granule cells were left undamaged. Higher concentrations (10-100 microM) of KA destroyed also the latter cell groups. The selectivity of the KA lesion at 3 microM was further indicated by the fact that the acetylcholinesterase-positive neurons in the hippocampus were not destroyed through KA administration and that the stereoisomer dihydrokainic acid was ineffective in inducing lesions. Application of tetrodotoxin did not protect the CA3 pyramidal cells from KA lesion, whereas gamma-glutamylaminomethylsulphonic acid (GAMS) only offered a very small, statistically not significant, protection. Baclofen protected the cultures slightly from KA lesions but not when added together with GAMS. Possible mechanisms responsible for the KA lesions in these cultures are discussed.

Development of high affinity kainate binding sites in human and rat hippocampi

Autoradiographic localization of kainate binding sites has been determined in developing human and rat hippocampi. The results suggest differences. In particular, a transient high density of sites occurs in the supragranular layer of the fascia dentata of the human hippocampus.

Effect of seizures induced by intra-amygdaloid kainic acid on kainic acid binding sites in rat hippocampus and amygdala

[3H]Kainic acid binding sites with a slow dissociation rate in the rat limbic system were investigated in detail. Extensively washed membranes prepared from the hippocampal formation and from the region comprising the amygdala and the piriform cortex yielded non-linear Scatchard plots. Microdissection showed that the high-affinity component (affinity constant around 1 nM) was present in the hippocampal CA3 region (4.2 fmol/mg wet tissue) and the amygdaloid complex (4.6 fmol/mg wet tissue), whereas the remaining part of the hippocampal formation and the piriform lobe contained the low-affinity component (affinity constant 5-20 nM; 11.6 and 11.3 fmol/mg wet tissue, respectively). In the lateral + medial septum we detected only the low-affinity component. Severe limbic seizures, induced by unilateral injection of 0.7 or 0.8 microgram kainic acid in 0.3 microliter of phosphate-buffered saline into the amygdala, reduced kainic acid binding sites in the ipsilateral amygdala and CA3 region. The decline of kainic acid binding sites in the injected amygdala was followed by a similar effect in the contralateral amygdala ("mirror focus") and later by a moderate loss also in the contralateral CA3 region. Kainic acid receptor autoradiography demonstrated that binding sites were lost from the stratum lucidum in hippocampus. Septal lesion had no effect on kainic acid binding sites in the hippocampus. Comparison with previous results on the histopathological changes after this lesion shows that high-affinity kainic acid binding sites are preferentially located on neurons that undergo selective degenerations after severe kainic acid-induced seizures.

The distribution of [3H]kainate binding sites in primate hippocampus is similar to the distribution of both Ca2+-sensitive and Ca2+-insensitive [3H]kainate binding sites in rat hippocampus

The distribution of [3H]kainate binding sites was determined by quantitative autoradiography in three vertebrate species: rat, monkey, and human. These animals displayed a similar pattern of binding site density in the hippocampus. Highest levels were found within the stratum lucidum and moderate levels in the inner portion of the dentate gyrus molecular layer. Although the distribution is similar, there is a lower density of binding sites in the stratum lucidum of primates than in rodents. Experiments using rat brain synaptic plasma membrane fractions indicated that inclusion of Ca2+ ions results in a selective reduction in binding at the high affinity sites. The Ca2+-inhibited and Ca2+-inhibited binding sites in the high affinity sites. The Ca2+ -inhibited and Ca2+ -insensitive binding sites in the rat hippocampus exhibited a similar distribution. Together, these results suggest that in a variety of mammalian species kainate receptors exhibit similar regional distributions, and that the high and low affinity kainate binding sites also exhibit similar regional distributions.

Intrahippocampal injection of kainic acid produces significant pyramidal cell loss in neonatal rats

Previous reports have indicated that pyramidal cells in the developing rat hippocampal formation are not destroyed by intraventricular or intraperitoneal administration of kainic acid. We examined the neurotoxic properties of kainic acid and ibotenic acid following intrahippocampal injection in neonatal rats and found significant pyramidal cell death following injection of 1.0 microgram kainic acid in 6, 7 and 9-day-old pups. At doses 2.5 or five times this amount, significant pyramidal cell loss was obtained in 5-day-old rats as well. The susceptibility of pyramidal neurons to kainic acid increased as a function of age. The developing hippocampus was considerably more vulnerable to ibotenic acid compared with kainic acid, in contrast to the order of potency reported in adult rats. The increased sensitivity of CA3 pyramidal cells parallels the development of the mossy fiber innervation to the dendrites of these cells supporting the twofold mechanism suggested by Coyle for kainic acid neurotoxicity; that is, a direct cytotoxic action via postsynaptic receptors as well as increased sensitivity due to the presence of excitatory inputs.

Maturation of kainic acid seizure-brain damage syndrome in the rat. II. Histopathological sequelae

The histopathological sequelae of parenteral administration of kainic acid were investigated in immature rats (3-35 days of age). The brains were fixed 1-14 days after the administration of kainate and the damage evaluated by means of argyrophylic (Fink-Heimer, Gallyas or Nauta-Gygax) and Nissl stains. In animals of less than 18 days of age there was no sign of damage even after 1-2 h of severe tonico-clonic convulsions. Between 18 and 35 days after birth, there was a progressive increase in the severity of the damage, the adult pattern being reached at the latter age. As in adult animals, brain damage was most severe in structures which are part of the limbic system, i.e. the hippocampal formation, lateral septum, amygdaloid complex, claustrum, piriform cortex, etc. In addition to neuronal abnormalities, the following reactions were observed: hypertrophy and swelling of satellite oligodendroglia, proliferation of hypertrophic microglia, proliferation of astroglia and hypertrophy of endothelial cells in the capillary wall. The latter type of change, together with local coagulative necrosis, was almost exclusively restricted to the granular and molecular layers of the fascia dentata. In the hippocampal formation we found a temporal gradient of vulnerability. The earliest and most consistent neuronal alterations were largely restricted to interneurons of the hilar region and to a lesser extent to non-pyramidal neurons of strata oriens and radiatum. The severe necrotic destruction of the pyramidal layer of CA3 is conspicuous at a later age (postnatal day 30-35) and with longer survival times. Our results suggest that: (1) the neurotoxin only induces brain damage once it also causes limbic motor seizures and its associated metabolic activations, notably in the amygdala; (2) the earliest pathological sequelae occur in interneurons of the hilar region and (3) sclerosis of the vulnerable region of the Ammon's horn--the CA3 region--is only obtained once the dentate granules and their mossy fibres are fully operational, thereby reflecting the crucial role of this axonal connection in eliciting hippocampal damage.

Maturation of kainic acid seizure-brain damage syndrome in the rat. I. Clinical, electrographic and metabolic observations

The maturation of the seizure/brain damage syndrome produced by parenteral administration of kainate was studied in the rat. The motor, electrographic and metabolic alterations are described in the present report, the maturation of the pathological abnormalities and of the specific kainate binding sites are described in the two following companion papers. Parenteral kainate produces tonico-clonic seizures until the end of the third week of age when limbic motor signs (wet-dog shakes, facial myoclonia, paw tremor etc.) were first produced. Using the 2-deoxyglucose autoradiographic method, we found that in animals of 3 days of age and until the third week of age, kainate produced a rise in metabolism restricted to the hippocampus and lateral septum. This was paralleled by paroxysmal discharges which were recorded in the hippocampus. Starting from the end of the third week of age approximately--i.e. when the toxin produced limbic motor seizures--there was a rise of labelling in other structures which are part of or closely associated to the limbic system i.e. the amygdaloid complex, the mediodorsal and adjacent thalamic nuclei, piriform, entorhinal and rostral limbic cortices and areas of projection of the fornix. These metabolic maps are thus similar to those seen in adults. Two main conclusions can be drawn from these experiments: kainate activates the hippocampus from a very early age probably by means of specific receptors present in this structure and the limbic syndrome will only be produced by the toxin once the limbic circuitry--including in particular the amygdaloid complex--is activated by the procedure i.e. after the third week of age.