Epileptogenesis in organotypic hippocampal cultures has limited dependence on culture medium composition

Prolonged Hyperactivity Elicits Massive and Persistent Chloride Ion Redistribution in Subsets of Cultured Hippocampal Dentate Granule Cells

Chloride ions play a critical role in neuronal inhibition through the activity of chloride-permeable GABAA receptor channels. Ion transporters, chloride channels, and immobile ion species tightly regulate intracellular chloride concentrations. Several studies related to epilepsy suggest that chloride extrusion function may decrease in an activity-dependent manner. Consequently, it is crucial to investigate whether intense neuronal activity, as observed during status epilepticus, could lead to sustained increases in intracellular chloride levels in neurons, which in turn could contribute to epilepsy-associated hyperexcitability. This study utilized the chloride sensitive indicator (6-Methoxyquinolinio) acetic acid ethyl ester bromide (MQAE) combined with fluorescence lifetime imaging (FLIM) to examine whether application of the convulsant, pilocarpine, a muscarinic acetylcholine receptor agonist, could induce synchronous epileptiform activity and elevate intracellular chloride concentrations in hippocampal slice cultures. Using a Gaussian mixture model, we identified a multimodal distribution of intracellular chloride levels among neurons, with a significant subset of these cells exhibiting massive and prolonged (days) chloride accumulation. The combination of multicellular imaging and statistical analysis served as a powerful tool for studying the emergence of multiple, distinct populations of neurons in pathological conditions, in contrast to homogeneous populations evident under control conditions.

The NLRP3 Inflammasome in Neurodegenerative Disorders: Insights from Epileptic Models

Neuroinflammation represents a dynamic process of defense and protection against the harmful action of infectious agents or other detrimental stimuli in the central nervous system (CNS). However, the uncontrolled regulation of this physiological process is strongly associated with serious dysfunctional neuronal issues linked to the progression of CNS disorders. Moreover, it has been widely demonstrated that neuroinflammation is linked to epilepsy, one of the most prevalent and serious brain disorders worldwide. Indeed, NLRP3, one of the most well-studied inflammasomes, is involved in the generation of epileptic seizures, events that characterize this pathological condition. In this context, several pieces of evidence have shown that the NLRP3 inflammasome plays a central role in the pathophysiology of mesial temporal lobe epilepsy (mTLE). Based on an extensive review of the literature on the role of NLRP3-dependent inflammation in epilepsy, in this review we discuss our current understanding of the connection between NLRP3 inflammasome activation and progressive neurodegeneration in epilepsy. The goal of the review is to cover as many of the various known epilepsy models as possible, providing a broad overview of the current literature. Lastly, we also propose some of the present therapeutic strategies targeting NLRP3, aiming to provide potential insights for future studies.

A Dynamic Balance between Neuronal Death and Clearance in an in Vitro Model of Acute Brain Injury

In in vitro models of acute brain injury, neuronal death may overwhelm the capacity for microglial phagocytosis, creating a queue of dying neurons awaiting clearance. Neurons undergoing programmed cell death are in this queue, and are the most visible and frequently quantified measure of neuronal death after injury. However, the size of this queue should be equally sensitive to changes in neuronal death and the rate of phagocytosis. Using rodent organotypic hippocampal slice cultures as a model of acute perinatal brain injury, serial imaging demonstrated that the capacity for microglial phagocytosis of dying neurons was overwhelmed for 2 weeks. Altering phagocytosis rates (e.g., by changing the number of microglia) dramatically changed the number of visibly dying neurons. Similar effects were generated when the visibility of dying neurons was altered by changing the membrane permeability for stains that label dying neurons. Canonically neuroprotective interventions, such as seizure blockade, and neurotoxic maneuvers, such as perinatal ethanol exposure, were mediated by effects on microglial activity and the membrane permeability of neurons undergoing programmed cell death. These canonically neuroprotective and neurotoxic interventions had either no or opposing effects on healthy surviving neurons identified by the ongoing expression of transgenic fluorescent proteins.SIGNIFICANCE STATEMENT In in vitro models of acute brain injury, microglial phagocytosis is overwhelmed by the number of dying cells. Under these conditions, the assumptions on which assays for neuroprotective and neurotoxic effects are based are no longer valid. Thus, longitudinal assays of healthy cells, such as serial assessment of the fluorescence emission of transgenically expressed proteins, provide more accurate estimates of cell death than do single-time point anatomic or biochemical assays of the number of dying neurons. More accurate estimates of death rates in vitro will increase the translatability of preclinical studies of neuroprotection and neurotoxicity.

A dynamic balance between neuronal death and clearance after acute brain injury

After acute brain injury, neuronal apoptosis may overwhelm the capacity for microglial phagocytosis, creating a queue of dying neurons awaiting clearance. The size of this queue should be equally sensitive to changes in neuronal death and the rate of phagocytosis. Using rodent organotypic hippocampal slice cultures as a model of acute perinatal brain injury, serial imaging demonstrated that the capacity for microglial phagocytosis of dying neurons was overwhelmed for two weeks. Altering phagocytosis rates, e.g. by changing the number of microglia, dramatically changed the number of visibly dying neurons. Similar effects were generated when the visibility of dying neurons was altered by changing the membrane permeability for vital stains. Canonically neuroprotective interventions such as seizure blockade and neurotoxic maneuvers such as perinatal ethanol exposure were mediated by effects on microglial activity and the membrane permeability of apoptotic neurons, and had either no or opposing effects on healthy surviving neurons.

Significance

After acute brain injury, microglial phagocytosis is overwhelmed by the number of dying cells. Under these conditions, the assumptions on which assays for neuroprotective and neurotoxic effects are based are no longer valid. Thus longitudinal assays of healthy cells, such as assessment of the fluorescence emission of transgenically-expressed proteins, provide more accurate estimates of cell death than do single-time-point anatomical or biochemical assays. More accurate estimates of death rates will increase the translatability of preclinical studies of neuroprotection and neurotoxicity.

Lab-on-Chip Microsystems for Ex Vivo Network of Neurons Studies: A Review

Increasing population is suffering from neurological disorders nowadays, with no effective therapy available to treat them. Explicit knowledge of network of neurons (NoN) in the human brain is key to understanding the pathology of neurological diseases. Research in NoN developed slower than expected due to the complexity of the human brain and the ethical considerations for in vivo studies. However, advances in nanomaterials and micro-/nano-microfabrication have opened up the chances for a deeper understanding of NoN ex vivo, one step closer to in vivo studies. This review therefore summarizes the latest advances in lab-on-chip microsystems for ex vivo NoN studies by focusing on the advanced materials, techniques, and models for ex vivo NoN studies. The essential methods for constructing lab-on-chip models are microfluidics and microelectrode arrays. Through combination with functional biomaterials and biocompatible materials, the microfluidics and microelectrode arrays enable the development of various models for ex vivo NoN studies. This review also includes the state-of-the-art brain slide and organoid-on-chip models. The end of this review discusses the previous issues and future perspectives for NoN studies.

Microvascular changes associated with epilepsy: A narrative review

The blood-brain barrier (BBB) is dysfunctional in temporal lobe epilepsy (TLE). In this regard, microvascular changes are likely present. The aim of this review is to provide an overview of the current knowledge on microvascular changes in epilepsy, and includes clinical and preclinical evidence of seizure induced angiogenesis, barriergenesis and microcirculatory alterations. Anatomical studies show increased microvascular density in the hippocampus, amygdala, and neocortex accompanied by BBB leakage in various rodent epilepsy models. In human TLE, a decrease in afferent vessels, morphologically abnormal vessels, and an increase in endothelial basement membranes have been observed. Both clinical and experimental evidence suggests that basement membrane changes, such as string vessels and protrusions, indicate and visualize a misbalance between endothelial cell proliferation and barriergenesis. Vascular endothelial growth factor (VEGF) appears to play a crucial role. Following an altered vascular anatomy, its physiological functioning is affected as expressed by neurovascular decoupling that subsequently leads to hypoperfusion, disrupted parenchymal homeostasis and potentially to seizures". Thus, epilepsy might be a condition characterized by disturbed cerebral microvasculature in which VEGF plays a pivotal role. Additional physiological data from patients is however required to validate findings from models and histological studies on patient biopsies.

Variability of seizure-like activity in an in vitro model of epilepsy depends on the electrical recording method

Background

Hippocampal and cortical slice-based models are widely used to study seizures and epilepsy. Seizure detection and quantification are essential components for studying mechanisms of epilepsy and assessing therapeutic interventions. To obtain meaningful signals and maximize experimental throughput, variability should be minimized. Some electrical recording methods require insertion of an electrode into neuronal tissue, change in slice chemical microenvironment, and transients in temperature and pH. These perturbations can cause acute and long-term alterations of the neuronal network which may be reflected in the variability of the recorded signal.

New method

In this study we investigated the effect of experimental perturbations in three local field potential (LFP) recording methods including substrate micro-wires (s-MWs), multiple electrode arrays (MEAs), and inserted micro wire electrodes (i-MW). These methods enabled us to isolate effects of different perturbations. We used organotypic hippocampal slices (OHCs) as an in-vitro model of posttraumatic epilepsy. To investigate the effect of the disturbances caused by the recording method on the paroxysmal events, we introduced jitter analysis, which is sensitive to small differences in the seizure spike timing.

Results

Medium replacement can introduce long-lasting perturbations. Electrode insertion increased variability on a shorter time scale. OHCs also underwent spontaneous state transitions characterized by transient increases in variability.

Comparison with existing methods

This new method of seizure waveform analysis allows for more sensitive assessment of variability of ictal events than simply measuring seizure frequency and duration.

Conclusion

We demonstrated that some of the variability in OHC recordings are due to experimental perturbations while some are spontaneous and independent of recording method.

Current ex Vivo and in Vitro Approaches to Uncovering Mechanisms of Neurological Dysfunction after Traumatic Brain Injury

Traumatic brain injury often leads to progressive alterations at the molecular to circuit levels resulting in epilepsy and memory impairments. Ex vivo and in vitro models have provided a powerful platform for investigating the multimodal alteration after trauma. Recent ex vivo analyses using voltage sensitive dye imaging, optogenetics, and glutamate uncaging have revealed circuit abnormalities following in vivo brain injury. In vitro injury models have enabled examination of early and progressive changes in activity while development of three-dimensional organoids derived from human induced pluripotent stem cells have opened novel avenues for injury research. Here, we highlight recent advances in ex vivo and in vitro systems, focusing on their potential for advancing mechanistic understandings, possible limitations, and implications for therapeutics.

Epilepsy-on-a-Chip System for Antiepileptic Drug Discovery

OBJECTIVE

Hippocampal slice cultures spontaneously develop chronic epilepsy several days after slicing and are used as an in vitro model of post-traumatic epilepsy. Here, we describe a hybrid microfluidic-microelectrode array (μflow-MEA) technology that incorporates a microfluidic perfusion network and electrodes into a miniaturized device for hippocampal slice culture based antiepileptic drug discovery.

METHODS

Field potential simulation was conducted to help optimize the electrode design to detect a seizure-like population activity. Epilepsy-on-a-chip model was validated by chronic electrical recording, neuronal survival quantification, and anticonvulsant test. To demonstrate the application of μflow-MEA in drug discovery, we utilized a two-stage screening platform to identify potential targets for antiepileptic drugs. In Stage I, lactate and lactate dehydrogenase biomarker assays were performed to identify potential drug candidates. In Stage II, candidate compounds were retested with μflow-MEA-based chronic electrical assay to provide electrophysiological confirmation of biomarker results.

RESULTS AND CONCLUSION

We screened 12 receptor tyrosine kinases inhibitors, and EGFR/ErbB-2 and cFMS inhibitors were identified as novel antiepileptic compounds.

SIGNIFICANCE

This epilepsy-on-a-chip system provides the means for rapid dissection of complex signaling pathways in epileptogenesis, paving the way for high-throughput antiepileptic drug discovery.

Trigonelline protects hippocampus against intracerebral Aβ(1-40) as a model of Alzheimer's disease in the rat: insights into underlying mechanisms

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most common phenotype of dementia. Trigonelline is an alkaloid found in medicinal plants such as fenugreek seeds and coffee beans with neuroprotective potential and according to existing evidences, a favorable agent for treatment of neurodegenerative disorders. In this study, the possible protective effect of trigonelline against intracerebral Aβ(1-40) as a model of AD in the rat was investigated. For induction of AD, aggregated A(1-40) (10 μg/2 휇l for each side) was bilaterally microinjected into the hippocampal CA1 area. Trigonelline was administered p.o. at a dose of 100 mg/kg. The results showed that trigonelline pretreatment of Aβ-microinjected rats significantly improves spatial recognition memory in Y maze and performance in novel object recognition (NOR) task, mitigates hippocampal malondialdehyde (MDA), protein carbonyl, lactate dehydrogenase (LDH), and improves mitochondrial membrane potential (MMP), glutathione (GSH), and superoxide dismutase (SOD) with no significant change of catalase activity, nitrite level, caspase 3 activity, and DNA fragmentation. Additionally, trigonelline ameliorated hippocampal levels of glial fibrillary acidic protein (GFAP), S100b, cyclooxygenase 2 (Cox2), tumor necrosis factor α (TNFα), and interleukin 6 (IL-6) with no significant alteration of inducible nitric oxide synthase (iNOS). In addition, trigonelline pretreatment prevented loss of hippocampal CA1 neurons in Aβ-microinjected group. Therefore, our results suggest that trigonelline pretreatment in Aβ model of AD could improve cognition and is capable to alleviate neuronal loss through suppressing oxidative stress, astrocyte activity, and inflammation and also through preservation of mitochondrial integrity.

A Proposed Mechanism for Spontaneous Transitions between Interictal and Ictal Activity

Epileptic networks are characterized by two outputs: brief interictal spikes and rarer, more prolonged seizures. Although either output state is readily modeled in silico and induced experimentally, the transition mechanisms are unknown, in part because no models exhibit both output states spontaneously. In silico small-world neural networks were built using single-compartment neurons whose physiological parameters were derived from dual whole-cell recordings of pyramidal cells in organotypic hippocampal slice cultures that were generating spontaneous seizure-like activity. In silico, neurons were connected by abundant local synapses and rare long-distance synapses. Activity-dependent synaptic depression and gradual recovery delimited synchronous activity. Full synaptic recovery engendered interictal population spikes that spread via long-distance synapses. When synaptic recovery was incomplete, postsynaptic neurons required coincident activation of multiple presynaptic terminals to reach firing threshold. Only local connections were sufficiently dense to spread activity under these conditions. This coalesced network activity into traveling waves whose velocity varied with synaptic recovery. Seizures were comprised of sustained traveling waves that were similar to those recorded during experimental and human neocortical seizures. Sustained traveling waves occurred only when wave velocity, network dimensions, and the rate of synaptic recovery enabled wave reentry into previously depressed areas at precisely ictogenic levels of synaptic recovery. Wide-field, cellular-resolution GCamP7b calcium imaging demonstrated similar initial patterns of activation in the hippocampus, although the anatomical distribution of traveling waves of synaptic activation was altered by the pattern of synaptic connectivity in the organotypic hippocampal cultures.SIGNIFICANCE STATEMENT When computerized distributed neural network models are required to generate both features of epileptic networks (i.e., spontaneous interictal population spikes and seizures), the network structure is substantially constrained. These constraints provide important new hypotheses regarding the nature of epileptic networks and mechanisms of seizure onset.

How do we use in vitro models to understand epileptiform and ictal activity? A report of the TASK1-WG4 group of the ILAE/AES Joint Translational Task Force

In vitro brain tissue preparations allow the convenient and affordable study of brain networks and have allowed us to garner molecular, cellular, and electrophysiologic insights into brain function with a detail not achievable in vivo. Preparations from both rodent and human postsurgical tissue have been utilized to generate in vitro electrical activity similar to electrographic activity seen in patients with epilepsy. A great deal of knowledge about how brain networks generate various forms of epileptiform activity has been gained, but due to the multiple in vitro models and manipulations used, there is a need for a standardization across studies. Here, we describe epileptiform patterns generated using in vitro brain preparations, focusing on issues and best practices pertaining to recording, reporting, and interpretation of the electrophysiologic patterns observed. We also discuss criteria for defining in vitro seizure‐like patterns (i.e., ictal) and interictal discharges. Unifying terminologies and definitions are proposed. We suggest a set of best practices for reporting in vitro studies to favor both efficient across‐lab comparisons and translation to in vivo models and human studies.

In vitro Models for Seizure-Liability Testing Using Induced Pluripotent Stem Cells

The brain is the most complex organ in the body, controlling our highest functions, as well as regulating myriad processes which incorporate the entire physiological system. The effects of prospective therapeutic entities on the brain and central nervous system (CNS) may potentially cause significant injury, hence, CNS toxicity testing forms part of the “core battery” of safety pharmacology studies. Drug-induced seizure is a major reason for compound attrition during drug development. Currently, the rat ex vivo hippocampal slice assay is the standard option for seizure-liability studies, followed by primary rodent cultures. These models can respond to diverse agents and predict seizure outcome, yet controversy over the relevance, efficacy, and cost of these animal-based methods has led to interest in the development of human-derived models. Existing platforms often utilize rodents, and so lack human receptors and other drug targets, which may produce misleading data, with difficulties in inter-species extrapolation. Current electrophysiological approaches are typically used in a low-throughput capacity and network function may be overlooked. Human-derived induced pluripotent stem cells (iPSCs) are a promising avenue for neurotoxicity testing, increasingly utilized in drug screening and disease modeling. Furthermore, the combination of iPSC-derived models with functional techniques such as multi-electrode array (MEA) analysis can provide information on neuronal network function, with increased sensitivity to neurotoxic effects which disrupt different pathways. The use of an in vitro human iPSC-derived neural model for neurotoxicity studies, combined with high-throughput techniques such as MEA recordings, could be a suitable addition to existing pre-clinical seizure-liability testing strategies.