Downregulation of kainate receptors in the hippocampus following repeated seizures in immature rats

The Reactive Plasticity of Hippocampal Ionotropic Glutamate Receptors in Animal Epilepsies

Ionotropic glutamate receptors (iGluRs) mediate the synaptic and metabolic actions of glutamate. These iGluRs are classified within the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type, kainate-type, and N-methyl-d-aspartate (NMDA)-type functional receptor families. The iGluR assemblies are regulated by transcription, alternative splicing, and cytoplasmic post-translational modifications. The iGluR subunit proteins are transported from the endoplasmic reticulum, inserted into the synaptic membranes, and anchored at their action site by different scaffolding and interacting proteins. The functional properties of iGluRs depend on their subunit composition, the amino acid sequence of the protein domains, and the scaffolding proteins in the synaptic membranes. The iGluRs are removed from the membranes by enzymatic action and endocytosis. Hippocampal iGluRs are rearranged through the upregulation and downregulation of the subunits following deafferentation and epileptic seizures. The rearrangement of iGluRs and the alteration of their subunit composition transform neurons into “pathological” cells, determining the further plasticity or pathology of the hippocampal formation. In the present review, we summarize the expression of AMPA, kainate, and NMDA receptor subunits following deafferentation, repeated mild seizures, and status epilepticus. We compare our results to literature descriptions, and draw conclusions as to the reactive plasticity of iGluRs in the hippocampus.

Neonatal seizures induced by pentylenetetrazol or kainic acid disrupt primary cilia growth on developing mouse cortical neurons

Neonatal or early-life seizures (ELS) are often associated with life-long neurophysiological, cognitive and behavioral deficits, but the underlying mechanisms contributing to these deficits remain poorly understood. Newborn, post-migratory cortical neurons sprout ciliary buds (procilia) that mature into primary cilia. Disruption of the growth or signaling capabilities of these cilia has been linked to atypical neurite outgrowth from neurons and abnormalities in neuronal circuitry. Here, we tested the hypothesis that generalized seizures induced by pentylenetetrazol (PTZ) or kainic acid (KA) during early postnatal development impair neuronal and/or glial ciliogenesis. Mice received PTZ (50 or 100mg/kg), KA (2mg/kg), or saline either once at birth (P0), or once daily from P0 to P4. Using immunohistochemistry and electron microscopy, the cilia of neurons and glia were examined at P7, P14, and P42. A total of 83 regions were analyzed, representing 13 unique neocortical and hippocampal regions. Neuronal cilia were identified by co-expression of NeuN and type 3 adenylyl cyclase (ACIII) or somatostatin receptor 3 (SSTR3), while glial cilia were identified by co-expression of GFAP, Arl13b, and gamma-tubulin. We found that PTZ exposure at either P0 or from P0 to P4 induced convulsive behavior, followed by acute and lasting effects on neuronal cilia lengths that varied depending on the cortical region, PTZ dose, injection frequency, and time post-PTZ. Both increases and decreases in neuronal cilia length were observed. No changes in the length of glial cilia were observed under any of the test conditions. Lastly, we found that a single KA seizure at P0 led to similar abnormalities in neuronal cilia lengths. Our results suggest that seizure(s) occurring during early stages of cortical development induce persistent and widespread changes in neuronal cilia length. Given the impact neuronal cilia have on neuronal differentiation, ELS-induced changes in ciliogenesis may contribute to long-term pathology and abnormal cortical function.

Tolerance to seizure induced by kainic acid is produced in a specific period of zebrafish development

During brain development, the electrical disturbance promoted by a seizure can have several consequences, because it can disturb a set of steps extremely regulated needed to the correct brain maturation. Animal modeling of seizure is invaluable to contribute to the mechanistic understanding of punctual seizure event, and those that triggered in an immature neural network could alter the mature brain physiology. In the present study we observed that the exposure to kainic acid diluted directly in water of zebrafish decreased the locomotor activity at 7 days post-fertilization (dpf) animals and increased at 15 dpf, despite the absence of more specific seizure features. Pre-exposure to kainic acid (500 μM) diluted in water at 7 dpf animals reduced the susceptibility to a second exposure 2 months later by intraperitoneal injection. The current data suggest that these different responses are associated with neuronal maturation process and open a question about the window of development that are crucial to long lasting effects related to seizure in this animal model.

Early life seizures: evidence for chronic deficits linked to autism and intellectual disability across species and models

Recent work in Exp Neurol by Lugo et al. (2014b) demonstrated chronic alterations in sociability, learning and memory following multiple early life seizures (ELS) in a mouse model. This work adds to the growing body of evidence supporting the detrimental nature of ELS on the developing brain to contribute to aspects of an autistic phenotype with intellectual disability. Review of the face validity of behavioral testing and the construct validity of the models used informs the predictive ability and thus the utility of these models to translate underlying molecular and cellular mechanisms into future human studies.

Pharmacological Preconditioning with GYKI 52466: A Prophylactic Approach to Neuroprotection

Some toxins and drugs can trigger lasting neuroprotective mechanisms that enable neurons to resist a subsequent severe insult. This “pharmacological preconditioning” has far-reaching implications for conditions in which blood flow to the brain is interrupted. We have previously shown that in vitro preconditioning with the AMPA receptor antagonist GYKI 52466 induces tolerance to kainic acid (KA) toxicity in hippocampus. This effect persists well after washout of the drug and may be mediated via inverse agonism of G-protein coupled receptors (GPCRs). Given the amplifying nature of metabotropic modulation, we hypothesized that GYKI 52466 may be effective in reducing seizure severity at doses well below those normally associated with adverse side effects. Here we report that pharmacological preconditioning with low-dose GYKI imparts a significant protection against KA-induced seizures in vivo. GYKI (3 mg/kg, s.c.), 90–180 min prior to high-dose KA, markedly reduced seizure scores, virtually abolished all level 3 and level 4 seizures, and completely suppressed KA-induced hippocampal c-FOS expression. In addition, preconditioned animals exhibited significant reductions in high frequency/high amplitude spiking and ECoG power in the delta, theta, alpha, and beta bands during KA. Adverse behaviors often associated with higher doses of GYKI were not evident during preconditioning. The fact that GYKI is effective at doses well-below, and at pre-administration intervals well-beyond previous studies, suggests that a classical blockade of ionotropic AMPA receptors does not underlie anticonvulsant effects. Low-dose GYKI preconditioning may represent a novel, prophylactic strategy for neuroprotection in a field almost completely devoid of effective pharmaceuticals.

Kindling in the early postnatal period: Effects on the dynamics of age-related changes in electrophysiological characteristics of hippocampal neurons

The effects of chronic administration of pentylenetetrazole (PTZ) during early ontogenesis (from postnatal day 14) on the dynamics of age-related changes in electrophysiological characteristics of rat hippocampal slices were studied. Unlike the situation in adult animals, convulsive activity did not develop in rat pups in response to repeated injections. Comparison of the amplitude characteristics of total monosynaptic responses of neurons in hippocampal field CA1 to application of single and paired (separated by 70 msec) stimulation of Schäffer collaterals during the period of maximally intense hippocampal synaptogenesis (at weeks 2-3 of postnatal development) revealed no significant differences between the control group (administration of isotonic saline) and the group given PTZ. The level of suppression of facilitation in paired-pulse stimulation with a short interstimulus interval (15 msec) was significantly less in hippocampal slices from rat pups from the PTZ group. However, as compared with the passive control, the direction of rearrangements in the two experimental group was essentially the same. Nonetheless, regular administration of PTZ during the period of maximally intense hippocampal maturation affected the development of its characteristics. This was not only apparent as a deficiency of inhibitory processes. Increases in the intensity of test stimuli applied to hippocampal slices from PTZ-treated rat pups at 27-48 days of age led to relatively lower response amplitudes as compared with those seen in hippocampal slices from control (given isotonic saline) rats of the same age. The level of facilitation in paired-pulse stimulation with an interstimulus interval of 70 msec showed no difference, decreasing to similar extents in both groups as compared with the passive control group. In addition, hippocampal slices from the PTZ group showed significant decreases in the magnitude of long-term potentiation. Changes occurring in the hippocampus after regular administration of PTZ did not correlate with the development of convulsive activity. The only significant relationship involving the intensity of convulsions was with the increase (compared with the normal for age) in the amplitudes of responses to minimal-intensity test stimuli.

Seizures in the developing brain: cellular and molecular mechanisms of neuronal damage, neurogenesis and cellular reorganization

Epilepsy is a common neurological disorder that occurs more frequently in children than in adults. The extent that prolonged seizure activity, i.e. status epilepticus (SE), and repeated, brief seizures affect neuronal structure and function in both the immature and mature brain has been the subject of increasing clinical and experimental research. Earlier studies suggest that seizure-induced effects in the immature brain compared with the adult brain are different. This is manifested as differences in neuronal vulnerability, cellular and synaptic reorganization and regenerative processes. The focus of this review is first to give a short overview of currently used experimental models of epilepsy in immature rats, and then discuss more thoroughly seizure-induced acute and sub-acute cellular and molecular alterations, highlight the contribution of inflammatory-like reactions and intracellular cytoskeleton to the insult, and reveal changes in the structure and function of inhibitory GABA(A) and excitatory glutamate receptors. The role of seizure-activated reparative, plastic processes, synaptic remodelling, neurogenesis as well as the long-term consequences of seizures are briefly outlined. The main emphasis is put on studies carried out in experimental animals, and the focus of interest is the hippocampus, the brain area of great vulnerability in epilepsy. In vitro studies are discussed only to limited extent. Collectively, recent studies suggest that the deleterious effects of seizures may not solely be a consequence of neuronal damage and loss per se, but could be due to the fact that seizures interfere with the highly regulated developmental processes in the immature brain.

Domoic acid preconditioning and seizure induction in young and aged rats

Clinical reports suggest that the elderly are hypersensitive to the neurological effects of domoic acid (DOM). In the present study we assessed DOM-induced seizures in young and aged rats, and seizure attenuation following low-dose DOM pretreatment (i.e. preconditioning). Seizure behaviours following saline or DOM administration (0.5-2mg/kg i.p.) were continuously monitored for 2.5h in naïve and DOM preconditioned rats. Competitive ELISA was used to determine serum and brain DOM concentrations. Dose- and age-dependent increases in seizure activity were evident in response to DOM. Lower doses of DOM in young and aged rats promoted low level seizure behaviours. Animals administered high doses (2mg/kg in young; 1mg/kg in aged) progressed through various stages of stereotypical behaviour (e.g., head tics, scratching, wet dog shakes) before ultimately exhibiting tonic-clonic convulsions. Serum and brain DOM analysis indicated impaired renal clearance as contributory to increased DOM sensitivity in aged animals, and this was supported by seizure analysis following direct intrahippocampal administration of DOM. Preconditioning young and aged animals with low-dose DOM 45-90 min before high-dose DOM significantly reduced seizure intensity. We conclude that age-related supersensitivity to DOM is related to reduced clearance rather than increased neuronal sensitivity, and that preconditioning mechanisms underlying an inducible tolerance to excitotoxins are robustly expressed in both young and aged CNS.

Neonatal seizures: multicenter variability in current treatment practices

Standardized approaches to the treatment of neonatal seizures remain undeveloped. We assessed the type and number of anticonvulsants selected, blood levels attained, and postdischarge anticonvulsant treatment of neonatal seizures among five neonatal intensive care units in the United States between 2000-2003. Almost all of the 480 neonates (94%) with seizures were treated, initially with phenobarbital (82%), lorazepam (9%), phenytoin (2%), other anticonvulsants (1%), or a combination of the first two drugs (6%). While the majority of neonates were treated with one drug (59%), the number of anticonvulsants varied (P<0.0001), as did the peak serum phenobarbital levels (P<0.0001). The majority (75%) of survivors received anticonvulsant treatment after discharge. These neonates were more likely to have had abnormal electroencephalography or brain imaging, or to have needed a second anticonvulsant, compared with neonates whose drug therapy was discontinued. Anticonvulsant therapy is used in the majority of neonates with seizures, mostly with phenobarbital, and treatment is continued beyond discharge. The observed wide therapeutic variability may reflect a lack of standardized diagnostic and treatment approaches, particularly for seizures refractory to initial phenobarbital therapy. Trials of anticonvulsants with long-term neurodevelopmental follow-up are needed to develop evidence-based treatment guidelines.

Short-term effects of kainic acid on CA1 hippocampal interneurons differentially vulnerable to excitotoxicity

PURPOSE

We sought to identify the inhibitory interneurons of the rat hippocampal CA1 region selectively vulnerable in the kainic acid (KA) model of temporal lobe epilepsy and to determine whether their selective vulnerability could be due to differential short-term KA effects.

METHODS

We quantified vulnerable interneurons in stratum oriens-alveus (O/A) by using immunohistochemistry for glutamic acid decarboxylase (GAD), parvalbumin (PV), and somatostatin (SS) after KA injections in rats, and then compared in normal slices the effects of KA on interneurons either in O/A (vulnerable to KA) or in strata radiatum and lacunosum-moleculare (R/LM) (resistant to KA) by using whole-cell recording and calcium imaging.

RESULTS

GAD-, PV- and SS-positive cells in O/A were decreased after KA treatment in P20 and P30 rats. Both short (1-min) and long (10-min) applications of KA produced similar tetrodotoxin (TTX)-insensitive membrane depolarization and decrease in input resistance in O/A and R/LM interneurons. KA responses were antagonized by CNQX and GYKI52466, suggesting AMPA receptor activation. KA also generated a similar increase in intracellular Ca2+ in O/A and R/LM interneurons, which was antagonized by CNQX and GYKI52466.

CONCLUSIONS

The selective vulnerability of GAD-, PV-, and SS-immunopositive O/A interneurons in the KA model may not arise from cell-specific short-term membrane effects or calcium responses induced by KA, but from other glutamate receptor-mediated excitotoxic processes.

Advances in the pathophysiology of developmental epilepsies

Pediatric epilepsies display unique characteristics that differ significantly from epilepsy in adults. The immature brain exhibits a decreased seizure threshold and an age-specific response to seizure-induced brain injury. Many idiopathic epilepsy syndromes and symptomatic epilepsies commonly present during childhood. This review highlights recent advances in the pathophysiology of developmental epilepsies. Cortical development involves maturational regulation of multiple cellular and molecular processes, such as neurogenesis, neuronal migration, synaptogenesis, and expression of neurotransmitter receptors and ion channels. These normal developmental changes of the immature brain also contribute to the increased risk for seizures and unique responses to seizure-induced brain injury in pediatric epilepsies. Recent technological advances, especially in genetics and imaging, have yielded exciting discoveries about the pathophysiology of specific pediatric epilepsy syndromes, such as the emergence of channelopathies as the cause of many idiopathic epilepsies and identification of malformations of cortical development as a major source of symptomatic epilepsies in children.

Kainic acid induces rapid cell death followed by transiently reduced cell proliferation in the immature granule cell layer of rat organotypic hippocampal slice cultures

Brain injury due to seizures results in transiently increased cell proliferation and neurogenesis in the subgranular zone of the adult dentate gyrus. In contrast, the immature postnatal brain appears to be more resistant to cell death after seizure-induced brain injury and paradoxically reacts to seizures by reducing SGZ proliferation. Organotypic hippocampal slice cultures are a useful paradigm for modelling the early postnatal hippocampus. We have investigated the temporal relationship between cell death and cell proliferation after kainate in the granule cell layer of rat organotypic hippocampal slice cultures equivalent to post natal day 11 animals. We found stable numbers and densities of mature thionine stained cells in the granule cell layer over 72 h in control cultures grown in defined medium. We also found a slowly declining cell proliferation rate over the same time period under control conditions. We report evidence of early cell death in the granule cell layer after just 2 h exposure to 5 microM kainate, followed by a significant decrease in cell proliferation in the granule cell layer at 24 h. In contrast to control conditions, cell proliferation rose significantly in the kainate exposed cultures by 72 h back to levels seen at 2 h. There were no significant changes in cell labelling with antibody to activated caspase-3 between kainate treated and control cultures at any time point examined. Our results suggest that kainate-induced injury in the early postnatal hippocampus damages precursor cells contributing to a reduction in granule layer cell proliferation.