Sub region-specific modulation of synchronous neuronal burst firing after a kainic acid insult in organotypic hippocampal cultures

Exploring Kainic Acid-Induced Alterations in Circular Tripartite Networks with Advanced Analysis Tools

Brain activity implies the orchestrated functioning of interconnected brain regions. Typical in vitro models aim to mimic the brain using single human pluripotent stem cell-derived neuronal networks. However, the field is constantly evolving to model brain functions more accurately through the use of new paradigms, e.g., brain-on-a-chip models with compartmentalized structures and integrated sensors. These methods create novel data requiring more complex analysis approaches. The previously introduced circular tripartite network concept models the connectivity between spatially diverse neuronal structures. The model consists of a microfluidic device allowing axonal connectivity between separated neuronal networks with an embedded microelectrode array to record both local and global electrophysiological activity patterns in the closed circuitry. The existing tools are suboptimal for the analysis of the data produced with this model. Here, we introduce advanced tools for synchronization and functional connectivity assessment. We used our custom-designed analysis to assess the interrelations between the kainic acid (KA)-exposed proximal compartment and its nonexposed distal neighbors before and after KA. Novel multilevel circuitry bursting patterns were detected and analyzed in parallel with the inter- and intracompartmental functional connectivity. The effect of KA on the proximal compartment was captured, and the spread of this effect to the nonexposed distal compartments was revealed. KA induced divergent changes in bursting behaviors, which may be explained by distinct baseline activity and varied intra- and intercompartmental connectivity strengths. The circular tripartite network concept combined with our developed analysis advances importantly both face and construct validity in modeling human epilepsy in vitro.

Human neural organoids: Models for developmental neurobiology and disease

Human organoids stand at the forefront of basic and translational research, providing experimentally tractable systems to study human development and disease. These stem cell-derived, in vitro cultures can generate a multitude of tissue and organ types, including distinct brain regions and sensory systems. Neural organoid systems have provided fundamental insights into molecular mechanisms governing cell fate specification and neural circuit assembly and serve as promising tools for drug discovery and understanding disease pathogenesis. In this review, we discuss several human neural organoid systems, how they are generated, advances in 3D imaging and bioengineering, and the impact of organoid studies on our understanding of the human nervous system.

The Contribution of Astrocyte and Neuronal Panx1 to Seizures Is Model and Brain Region Dependent

Pannexin1 (Panx1) is an ATP release channel expressed in neurons and astrocytes that plays important roles in CNS physiology and pathology. Evidence for the involvement of Panx1 in seizures includes the reduction of epileptiform activity and ictal discharges following Panx1 channel blockade or deletion. However, very little is known about the relative contribution of astrocyte and neuronal Panx1 channels to hyperexcitability. To this end, mice with global and cell type specific deletion of Panx1 were used in one in vivo and two in vitro seizure models. In the low-Mg²⁺ in vitro model, global deletion but not cell-type specific deletion of Panx1 reduced the frequency of epileptiform discharges. This reduced frequency of discharges did not impact the overall power spectra obtained from local field potentials. In the in vitro KA model, in contrast, global or cell type specific deletion of Panx1 did not affect the frequency of discharges, but reduced the overall power spectra. EEG recordings following KA-injection in vivo revealed that although global deletion of Panx1 did not affect the onset of status epilepticus (SE), SE onset was delayed in mice lacking neuronal Panx1 and accelerated in mice lacking astrocyte Panx1. EEG power spectral analysis disclosed a Panx1-dependent cortical region effect; while in the occipital region, overall spectral power was reduced in all three Panx1 genotypes; in the frontal cortex, the overall power was not affected by deletion of Panx1. Together, our results show that the contribution of Panx1 to ictal activity is model, cell-type and brain region dependent.

Prenatal betamethasone exposure increases corticotropin-releasing hormone expression along with increased hippocampal slice excitability in the developing hippocampus

BACKGROUND

The objective of this study was to determine whether prenatal exposure to betamethasone alters hippocampal expression of corticotropin-releasing hormone (CRH) and resultant hippocampal circuit excitability.

METHODS

Real time (RT)-PCR and western blots were used to determine CRH mRNA and protein expression levels, respectively, in hippocampal extracts of two-week old rat pups prenatally primed with betamethasone or saline on gestational day 15. The data were compared to changes in epileptiform activity induced by kainic acid (KA) or depletion of [Mg²⁺]₀ in combined hippocampus-entorhinal cortex slices.

RESULTS

RT-PCR analysis showed 3-fold increased levels of CRH mRNA in hippocampal extracts from prenatally betamethasone-primed pups compared to saline controls (p < 0.05), but no changes in mRNA expression of CRH receptors (1 and 2). Changes in CRH protein isoform ratio in hippocampal extracts suggest 30 % increase in mature CRH levels in betamethasone-primed hippocampi (p < 0.05). No changes in mRNA expression in CRH feedback loop associated genes, GR and FKBP51, were found. Compared to saline-exposed pups, slices from betamethasone-primed pups had faster onset of epileptiform-like activity (inter-ictal discharges and seizure-like-events) after bath application of 4 μM KA (p < 0.05) suggesting a "more hyperexcitable" state. The epileptiform-like activity after KA application was significantly reduced following bath application of a CRH R2 antagonist (p < 0.05) but CRH R1 antagonist had no effect (p > 0.05). Also in the low-Mg²⁺-induced epileptiform activity, there was increased excitability, in the form of enhanced inter-ictal discharges, in slices from betamethasone primed compared to saline exposed rat pups (p < 0.05).

CONCLUSIONS

Our study suggests a possible mechanistic link to prenatal betamethasone priming-induced increase in postnatal hippocampal excitability that involves enhanced expression of CRH acting at CRH R2. This is important in regards to the links between prenatal stress/corticosteroid-exposure and syndromes, such as epilepsy, autism spectrum disorders and other psychiatric disorders associated with neuronal hyperexcitability.

Seizure-like thalamocortical rhythms initiate in the deep layers of the cortex in a co-culture model

The oscillatory rhythms underlying many physiological and pathological states, including absence seizures, require both the thalamus and cortices for full expression. A co-culture preparation combining cortical and thalamic explants provides a unique model for investigating how such oscillations initiate and spread. Here we investigated the dynamics of synchronized thalamocortical activity by simultaneous measurement of field-potential recordings and rapid imaging of Ca(2+) transients by fluorescence methods. Spontaneous sustained hypersynchronized "seizure-like" oscillations required reciprocal cortico-thalamocortical connections. Isolated cortical explants can independently develop brief discharges, while thalamic explants alone were unable to do so. Rapid imaging of Ca(2+) transients demonstrated deep-layer cortical initiation of oscillatory network activity in both connected and isolated explants. Further, cortical explants derived from a rat model of genetic absence epilepsy showed increased bursting duration consistent with an excitable cortex. We propose that thalamocortical oscillatory network activity initiates in deep layers of the cortex with reciprocal thalamic interconnections enabling sustained hyper-synchronization.

Maternal immune activation by polyriboinosinic-polyribocytidilic acid injection produces synaptic dysfunction but not neuronal loss in the hippocampus of juvenile rat offspring

It has been suggested that maternal immune activation increases the risk of psychiatric disorders such as schizophrenia in offspring. There are many reports about hippocampal structural pathology in schizophrenia. Antipsychotic drug administration in adolescence prevented postpubertal hippocampal structural pathology in the maternal immune activation animal model. These findings suggest the possibility that maternal immune activation induces hippocampal dysfunction in juvenile offspring. To test this hypothesis, we investigated hippocampal function in juvenile offspring of maternal immune activation model rat. A synthetic double-stranded RNA polyriboinosinic-polyribocytidilic acid (Poly I:C; 4 mg/kg/day, I.P.) was injected to pregnant rats on gestation days 15 and 17, in order to cause immune activation by stimulating Toll-like receptor 3. Hippocampal synaptic function and morphology in their juvenile offspring (postnatal days 28-31) were compared to those in vehicle-injected control offspring. Field responses were recorded in the hippocampal CA1 region by stimulating commissural/Schaffer collaterals. Pre-synaptic fiber volley amplitudes (mV) and field excitatory post-synaptic potential slopes (mV/ms) were significantly lower in treated offspring. In addition, short-term synaptic plasticity, namely, the paired-pulse facilitation ratio, was significantly higher and long-term synaptic plasticity (long-term potentiation) was significantly impaired in treated offspring. Furthermore, major pre-synaptic protein (synaptophysin) expressions were decreased, but not major post-synaptic proteins (GluR1, GluR2/3, and NR1), in hippocampal CA1 of treated offspring, whereas neuronal loss was not detected in the hippocampal CA1-CA3 regions. These results indicate that maternal immune activation leads to synaptic dysfunction without neuronal loss in the hippocampus of juvenile offspring, and this may be one of the early stages of schizophrenia pathologies.

A simple method for multiday imaging of slice cultures

The organotypic slice culture (Stoppini et al. A simple method for organotypic cultures of nervous tissue. 1991;37:173-182) has become the method of choice to answer a variety of questions in neuroscience. For many experiments, however, it would be beneficial to image or manipulate a slice culture repeatedly, for example, over the course of many days. We prepared organotypic slice cultures of the auditory brainstem of P3 and P4 mice and kept them in vitro for up to 4 weeks. Single cells in the auditory brainstem were transfected with plasmids expressing fluorescent proteins by way of electroporation (Haas et al. Single-cell electroporation for gene transfer in vivo. 2001;29:583-591). The culture was then placed in a chamber perfused with oxygenated ACSF and the labeled cell imaged with an inverted wide-field microscope repeatedly for multiple days, recording several time-points per day, before returning the slice to the incubator. We describe a simple method to image a slice culture preparation during the course of multiple days and over many continuous hours, without noticeable damage to the tissue or photobleaching. Our method uses a simple, inexpensive custom-built insulator constructed around the microscope to maintain controlled temperature and uses a perfusion chamber as used for in vitro slice recordings.

Excitotoxic-mediated transcriptional decreases in HCN2 channel function increase network excitability in CA1

Changes in the conductance of the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel that mediates Ih are proposed to contribute to increased network excitability. Synchronous neuronal burst activity is a good reflection of network excitability and can be generated in isolated hippocampal slice cultures by removing Mg2+ from the extracellular fluid. We demonstrate that Ih contributes to this activity by increasing both the frequency and duration of bursting events. Changes in HCN channel function are also implicated in altered seizure susceptibility. Short-term application of kainic acid (KA) is known to initiate long lasting changes in neuronal networks that result in seizures, and in slice cultures was found to alter HCN mRNA levels in an isoform and hippocampal sub-region specific manner. These changes correlate with the ability of each sub-region to develop synchronous burst activity following KA that we have previously reported. Specifically, a loss of synchronous activity in the CA3 correlated with an increase in HCN2 mRNA levels that normalized concomitantly with the restoration of CA3 burst activity 7 days post insult. In contrast, in CA1 an increase in synchronous burst duration correlated with a reduction in HCN2 mRNA levels and both changes were still evident for 7 days post insult. Lamotrigine, known to increase Ih, reversed the impact of KA on burst duration in CA1 at both time-points linking a transcriptional reduction in HCN2 function to increased burst duration.