A chronic histopathological and electrophysiological analysis of a rodent hypoxic-ischemic brain injury model and its use as a model of epilepsy

Disrupting Epileptiform Activity by Preventing Parvalbumin Interneuron Depolarization Block

Inhibitory synaptic mechanisms oppose epileptic network activity in the brain. The breakdown in this inhibitory restraint and propagation of seizure activity has been linked to the overwhelming of feedforward inhibition, which is provided in large part by parvalbumin-expressing (PV) interneurons in the cortex. The underlying cellular processes therefore represent potential targets for understanding and preventing the propagation of seizure activity. Here we use an optogenetic strategy to test the hypothesis that depolarization block in PV interneurons is a significant factor during the loss of inhibitory restraint. Depolarization block results from the inactivation of voltage-gated sodium channels and leads to impaired action potential firing. We used focal NMDA stimulation to elicit reproducible epileptiform discharges in hippocampal organotypic brain slices from male and female mice and combined this with targeted recordings from defined neuronal populations. Simultaneous patch-clamp recordings from PV interneurons and pyramidal neurons revealed epileptiform activity that was associated with an overwhelming of inhibitory synaptic mechanisms and the emergence of a partial, and then complete, depolarization block in PV interneurons. To counteract this depolarization block, we developed protocols for eliciting pulsed membrane hyperpolarization via the inhibitory opsin, archaerhodopsin. This optical approach was effective in counteracting cumulative inactivation of voltage-gated channels, maintaining PV interneuron action potential firing properties during the inhibitory restraint period, and reducing the probability of initiating epileptiform activity. These experiments support the idea that depolarization block is a point of weakness in feedforward inhibitory synaptic mechanisms and represents a target for preventing the initiation and spread of seizure activity.SIGNIFICANCE STATEMENT GABA_A receptor-mediated synaptic transmission opposes seizure activity by establishing an inhibitory restraint against spreading excitation. Parvalbumin-expressing (PV) interneurons contribute significantly to this inhibitory restraint, but it has been suggested that these cells are overwhelmed as they enter a state of "depolarization block." Here we test the importance of this process by devising an optogenetic strategy to selectively relieve depolarization block in PV interneurons. By inducing brief membrane hyperpolarization, we show that it is possible to reduce depolarization block in PV interneurons, maintain their action potential firing in the face of strong excitation, and disrupt epileptiform activity in an in vitro model. This represents a proof of principle that targeting rate-limiting processes can strengthen the inhibitory restraint of epileptiform activity.

Immune Challenges and Seizures: How Do Early Life Insults Influence Epileptogenesis?

The development of epilepsy, a process known as epileptogenesis, often occurs later in life following a prenatal or early postnatal insult such as cerebral ischemia, stroke, brain trauma, or infection. These insults share common pathophysiological pathways involving innate immune activation including neuroinflammation, which is proposed to play a critical role in epileptogenesis. This review provides a comprehensive overview of the latest preclinical evidence demonstrating that early life immune challenges influence neuronal hyperexcitability and predispose an individual to later life epilepsy. Here, we consider the range of brain insults that may promote the onset of chronic recurrent spontaneous seizures at adulthood, spanning intrauterine insults (e.g. maternal immune activation), perinatal injuries (e.g. hypoxic–ischemic injury, perinatal stroke), and insults sustained during early postnatal life—such as fever-induced febrile seizures, traumatic brain injuries, infections, and environmental stressors. Importantly, all of these insults represent, to some extent, an immune challenge, triggering innate immune activation and implicating both central and systemic inflammation as drivers of epileptogenesis. Increasing evidence suggests that pro-inflammatory cytokines such as interleukin-1 and subsequent signaling pathways are important mediators of seizure onset and recurrence, as well as neuronal network plasticity changes in this context. Our current understanding of how early life immune challenges prime microglia and astrocytes will be explored, as well as how developmental age is a critical determinant of seizure susceptibility. Finally, we will consider the paradoxical phenomenon of preconditioning, whereby these same insults may conversely provide neuroprotection. Together, an improved appreciation of the neuroinflammatory mechanisms underlying the long-term epilepsy risk following early life insults may provide insight into opportunities to develop novel immunological anti-epileptogenic therapeutic strategies.

Neonatal Seizure Models to Study Epileptogenesis

Current therapeutic strategies for epilepsy include anti-epileptic drugs and surgical treatments that are mainly focused on the suppression of existing seizures rather than the occurrence of the first spontaneous seizure. These symptomatic treatments help a certain proportion of patients, but these strategies are not intended to clarify the cellular and molecular mechanisms underlying the primary process of epilepsy development, i.e., epileptogenesis. Epileptogenic changes include reorganization of neural and glial circuits, resulting in the formation of an epileptogenic focus. To achieve the goal of developing “anti-epileptogenic” drugs, we need to clarify the step-by-step mechanisms underlying epileptogenesis for patients whose seizures are not controllable with existing “anti-epileptic” drugs. Epileptogenesis has been studied using animal models of neonatal seizures because such models are useful for studying the latent period before the occurrence of spontaneous seizures and the lowering of the seizure threshold. Further, neonatal seizure models are generally easy to handle and can be applied for in vitro studies because cells in the neonatal brain are suitable for culture. Here, we review two animal models of neonatal seizures for studying epileptogenesis and discuss their features, specifically focusing on hypoxia-ischemia (HI)-induced seizures and febrile seizures (FSs). Studying these models will contribute to identifying the potential therapeutic targets and biomarkers of epileptogenesis.

Models of hypoxia and ischemia-induced seizures

Despite greater understanding and improved management, seizures continue to be a major problem in childhood. Neonatal seizures are often refractory to conventional antiepileptic drugs, and can result in later life epilepsy and cognitive deficits, conditions for which there are no specific treatments. Hypoxic and/or ischemic encephalopathy (HIE) is the most common cause for neonatal seizures, and accounts for more than two-thirds of neonatal seizure cases. A better understanding of the cellular and molecular mechanisms is essential for identifying new therapeutic strategies that control the neonatal seizures and its cognitive consequences. This heavily relies on animal models that play a critical role in discovering novel mechanisms underlying both epileptogenesis and associated cognitive impairments. To date, a number of animal models have provided a tremendous amount of information regarding the pathophysiology of HIE-induced neonatal seizures. This review provides an overview on the most important features of the main animal models of HIE-induced seizures. In particular, we focus on the methodology of seizure induction and the characterizations of post-HIE injury consequences. These aspects of HIE-induced seizure models are discussed in the light of the suitability of these models in studying human HIE-induced seizures.

Posttraumatic seizures and epilepsy in adult rats after controlled cortical impact

Posttraumatic epilepsy (PTE) has been modeled with different techniques of experimental traumatic brain injury (TBI) using mice and rats at various ages. We hypothesized that the technique of controlled cortical impact (CCI) could be used to establish a model of PTE in young adult rats. A total of 156 male Sprague-Dawley rats of 2-3 months of age (128 CCI-injured and 28 controls) was used for monitoring and/or anatomical studies. Provoked class 3-5 seizures were recorded by video monitoring in 7/57 (12.3%) animals in the week immediately following CCI of the right parietal cortex; none of the 7 animals demonstrated subsequent spontaneous convulsive seizures. Monitoring with video and/or video-EEG was performed on 128 animals at various time points 8-619 days beyond one week following CCI during which 26 (20.3%) demonstrated nonconvulsive or convulsive epileptic seizures. Nonconvulsive epileptic seizures of >10s were demonstrated in 7/40 (17.5%) animals implanted with 2 or 3 depth electrodes and usually characterized by an initial change in behavior (head raising or animal alerting) followed by motor arrest during an ictal discharge that consisted of high-amplitude spikes or spike-waves with frequencies ranging between 1 and 2Hz class 3-5 epileptic seizures were recorded by video monitoring in 17/88 (19%) and by video-EEG in 2/40 (5%) CCI-injured animals. Ninety of 156 (58%) animals (79 CCI-injured, 13 controls) underwent transcardial perfusion for gross and microscopic studies. CCI caused severe brain tissue loss and cavitation of the ipsilateral cerebral hemisphere associated with cell loss in the hippocampal CA1 and CA3 regions, hilus, and dentate granule cells, and thalamus. All Timm-stained CCI-injured brains demonstrated ipsilateral hippocampal mossy fiber sprouting in the inner molecular layer. These results indicate that the CCI model of TBI in adult rats can be used to study the structure-function relationships that underlie epileptogenesis and PTE.

Optimized methods for epilepsy therapy development using an etiologically realistic model of focal epilepsy in the rat

Conventionally developed antiseizure drugs fail to control epileptic seizures in about 30% of patients, and no treatment prevents epilepsy. New etiologically realistic, syndrome-specific epilepsy models are expected to identify better treatments by capturing currently unknown ictogenic and epileptogenic mechanisms that operate in the corresponding patient populations. Additionally, the use of electrocorticography permits better monitoring of epileptogenesis and the full spectrum of acquired seizures, including focal nonconvulsive seizures that are typically difficult to treat in humans. Thus, the combined use of etiologically realistic models and electrocorticography may improve our understanding of the genesis and progression of epilepsy, and facilitate discovery and translation of novel treatments. However, this approach is labor intensive and must be optimized. To this end, we used an etiologically realistic rat model of posttraumatic epilepsy, in which the initiating fluid percussion injury closely replicates contusive closed-head injury in humans, and has been adapted to maximize epileptogenesis and focal non-convulsive seizures. We obtained week-long 5-electrode electrocorticography 1 month post-injury, and used a Monte-Carlo-based non-parametric bootstrap strategy to test the impact of electrode montage design, duration-based seizure definitions, group size and duration of recordings on the assessment of posttraumatic epilepsy, and on statistical power to detect antiseizure and antiepileptogenic treatment effects. We found that use of seizure definition based on clinical criteria rather than event duration, and of recording montages closely sampling the activity of epileptic foci, maximize the power to detect treatment effects. Detection of treatment effects was marginally improved by prolonged recording, and 24h recording epochs were sufficient to provide 80% power to detect clinically interesting seizure control or prevention of seizures with small groups of animals. We conclude that appropriate electrode montage and clinically relevant seizure definition permit convenient deployment of fluid percussion injury and electrocorticography for epilepsy therapy development.

Novel frontiers in epilepsy treatments: preventing epileptogenesis by targeting inflammation

Currently available epilepsy drugs only affect the symptoms (seizures), and there is a need for innovative treatments that target the underlying disease. Increasing evidence points to inflammation as a potentially important mechanism in epileptogenesis. In the last decade, a new generation of etiologically realistic syndrome-specific experimental models have been developed, which are expected to capture the epileptogenic mechanisms operating in corresponding patient populations, and to exhibit similar treatment responsiveness. Recently, an intervention known to have broad-ranging anti-inflammatory effects (selective brain cooling) has been found to prevent the development of spontaneously occurring seizures in an etiologically realistic rat model of post-traumatic epilepsy. Several drugs used clinically for other indications also have the potential for inhibiting inflammation, and should be investigated for antiepileptogenic activity in these models. If results of such studies are positive, these compounds could rapidly enter Phase III trials in patients at high risk of developing epilepsy.

Low-dose GYKI-52466: prophylactic preconditioning confers long-term neuroprotection and functional recovery following hypoxic-ischaemic brain injury

Experimental preconditioning provides beneficial outcomes in conditions such as cardiac surgery, brain surgery and stroke. Here we evaluated the protective effects of low-dose subcutaneous GYKI-52466 preconditioning in a rat model of hypoxic-ischaemic (HI) brain injury. Male Sprague-Dawley rats (postnatal day 26) were administered saline or GYKI-52466 (GYKI; 3-mg/kg, 90 min; 1-mg/kg, twice in 120 min; or 0.5-mg/kg, thrice in 180 min) prior to left common carotid artery occlusion. Animals were allowed to recover for 2h, and then placed in a hypoxia chamber (8% O₂/92% N₂; 33 ± 1°C) for 1h. A sham surgery group received saline without HI. Seizure activity was scored during hypoxia and sensorimotor tests performed before surgery and at 1, 7, 14 and 90 days post-HI. On days 14 and 90 brains were fixed and sectioned for the assessment of infarct size and ventricular enlargement. Low-dose GYKI-52466 preconditioning significantly reduced infarct volume and ventricular enlargement relative to saline-treated controls at day 14 after HI. On day 90, tissue loss was significantly reduced by GYKI 3-mg/kg compared to saline. Foot-faults, paw use asymmetry, and postural reflex scores were significantly improved in all GYKI treatment groups. Our results show that GYKI-52466 is effective at doses well-below, and at pre-administration intervals well-beyond previous studies, and suggest that a classical blockade of ionotropic AMPA receptors does not underlie its neuroprotective effects. Low-dose GYKI-52466 preconditioning represents a novel, prophylactic strategy for neuroprotection in a field almost devoid of effective pharmaceuticals.

Primary and secondary mechanisms of epileptogenesis in the temporal lobe: there is a before and an after

Extensive data involving several animal models of temporal lobe epilepsy highlight synaptic alterations that likely act synergistically during acquired epileptogenesis. Most of this research has utilized experimental models in which intense electrical activity in adult animals, primarily involving status epilepticus, causes variable neuronal death in the hippocampus and other temporal lobe structures. Neuronal death, including principal cells and specific interneurons, likely has several roles in epileptogenesis after brain injury. Both reduction of GABA-mediated inhibition from selective interneuron loss and the progressive formation of new recurrent excitatory circuits after death of principal neurons enhance excitability and promote seizures during the development of epilepsy. These epileptogenic circuits hypothetically continue to undergo secondary epileptogenesis, which involves further modifications that contribute to a progressive, albeit variable, increase in the frequency and severity of spontaneous recurrent seizures.

ECoG studies of valproate, carbamazepine and halothane in frontal-lobe epilepsy induced by head injury in the rat

The use of electrocorticography (ECoG) with etiologically realistic epilepsy models promises to facilitate the discovery of better anti-epileptic drugs (AEDs). However, this novel approach is labor intensive, and must be optimized. To this end, we employed rostral parasagittal fluid percussion injury (rpFPI) in the adolescent rat, which closely replicates human contusive closed head injury and results in posttraumatic epilepsy (PTE). We systematically examined variables affecting the power to detect anti-epileptic effects by ECoG and used a non-parametric bootstrap strategy to test several different statistics, study designs, statistical tests, and impact of non-responders. We found that logarithmically transformed data acquired in repeated-measures experiments provided the greatest statistical power to detect decreases in seizure frequencies of preclinical interest with just 8 subjects and with up to approximately 40% non-responders. We then used this optimized design to study the anti-epileptic effects of acute exposure to halothane, and chronic (1 week) exposures to carbamazepine (CBZ) and valproate (VPA) 1 month post-injury. While CBZ was ineffective in all animals, VPA induced, during treatment, a progressive decrease in seizure frequency in animals primarily suffering from non-spreading neocortical seizures, but was ineffective in animals with a high frequency of spreading seizures. Halothane powerfully blocked all seizure activity. The data show that rpFPI and chronic ECoG can conveniently be employed for the evaluation of AEDs, suggest that VPA may be more effective than CBZ to treat some forms of PTE, and support the theory that pharmacoresistance may depend on the severity of epilepsy. The data also demonstrate the utility of chronic exposures to experimental drugs in preclinical studies and highlight the need for greater attention to etiology in clinical studies of AEDs.

Continuous electroencephalographic monitoring with radio-telemetry in a rat model of perinatal hypoxia-ischemia reveals progressive post-stroke epilepsy

The development of acquired epilepsy after a perinatal hypoxic-ischemic (HI) insult was investigated in rats. After unilateral carotid ligation with hypoxia on postnatal day 7, cortical electroencephalographic and behavioral seizures were recorded with continuous radio-telemetry and video. Chronic recordings were obtained between 2 and 12 months of age in freely behaving HI-treated and sham control rats. The hypotheses were that the acquired epilepsy is directly associated with an ischemic infarct (i.e., no lesion, no epilepsy), and the resultant epilepsy is temporally progressive. Every HI-treated rat with a cerebral infarct developed spontaneous epileptiform discharges and recurrent seizures (100%); in contrast, no spontaneous epileptiform discharges or seizures were detected with continuous monitoring in the HI-treated rats without infarcts. The initial seizures at 2 months generally showed focal onset and were nonconvulsive. Subsequent seizures had focal onsets that propagated to the homotopic contralateral cortex and were nonconvulsive or partial; later seizures often appeared to have bilateral onset and were convulsive. Spontaneous epileptiform discharges were initially lateralized to ipsilateral neocortex but became bilateral over time. The severity and frequency of the spontaneous behavioral and electrographic seizures progressively increased over time. In every epileptic rat, seizures occurred in distinct clusters with seizure-free periods as long as a few weeks. The progressive increase in seizure frequency over time was associated with increases in cluster frequency and seizures within each cluster. Thus, prolonged, continuous seizure monitoring directly demonstrated that the acquired epilepsy after perinatal HI was progressive with seizure clusters and was consistently associated with a cerebral infarct.

Dentate granule cells form hilar basal dendrites in a rat model of hypoxia-ischemia

Hilar basal dendrites form on dentate granule cells following seizures. To determine whether other brain insults cause the formation of hilar basal dendrites, a model of global cerebral hypoxia/ischemia was used. Rats underwent a transient induction of ischemia by occlusion of both common carotid arteries followed by reperfusion. Hippocampal slices were prepared from these animals 1 month after the ischemic insult, and granule cells were labeled with a retrograde tracing technique after biocytin injections into stratum lucidum of CA3b. Ischemic rats had numerous biocytin-labeled granule cells with hilar basal dendrites located at the hilar border of the granule cell layer. Quantitative analysis of ischemic rats compared to controls showed a significant increase in the percentage of biocytin-labeled granule cells with hilar basal dendrites. These data demonstrate that other brain insults in addition to epilepsy may result in the formation of hilar basal dendrites on granule cells.