Febrile Seizures Cause a Rapid Depletion of Calcium-Permeable AMPA Receptors at the Synapses of Principal Neurons in the Entorhinal Cortex and Hippocampus of the Rat

Febrile seizures (FSs) are a relatively common early-life condition that can cause CNS developmental disorders, but the specific mechanisms of action of FS are poorly understood. In this work, we used hyperthermia-induced FS in 10-day-old rats. We demonstrated that the efficiency of glutamatergic synaptic transmission decreased rapidly after FS by recording local field potentials. This effect was transient, and after two days there were no differences between control and post-FS groups. During early ontogeny, the proportion of calcium-permeable (CP)-AMPA receptors in the synapses of the principal cortical and hippocampal neurons is high. Therefore, rapid internalization of CP-AMPA receptors may be one of the mechanisms underlying this phenomenon. Using the whole-cell patch-clamp method and the selective CP-AMPA receptor blocker IEM-1460, we tested whether the proportion of CP-AMPA receptors changed. We have demonstrated that FS rapidly reduces synaptic CP-AMPA receptors in both the hippocampus and the entorhinal cortex. This process was accompanied by a sharp decrease in the calcium permeability of the membrane of principal neurons, which we revealed in experiments with kainate-induced cobalt uptake. Our experiments show that FSs cause rapid changes in the function of the glutamatergic system, which may have compensatory effects that prevent excessive excitotoxicity and neuronal death.

Modulation of seizure-like events by the small conductance and ATP-sensitive potassium ion channels

Potassium ion channels are extensively involved in the regulation of epileptic seizures. The small conductance calcium-sensitive potassium channels (SK channels) and ATP-sensitive potassium (K_ATP) channels are activated by calcium ion entry and decrease ATP levels, respectively. These channels can underlie the post-burst afterhyperpolarization and be upregulated during seizures, providing negative feedback during epileptic activity. Using the whole-cell patch-clamp method in rat brain slices, we investigated the effect of SK- and K_ATP-affecting drugs on seizure-like events (SLEs) in the 4-aminopyridine model of epileptic seizures in vitro. We demonstrate that SK and K_ATP channels contribute to sustaining the high-frequency firing of the principal neurons in the deep layers of the entorhinal cortex during injections of depolarizing current and epileptiform discharges. Neither the pharmacological blockade nor the activation of these channels was able to prevent the epileptiform activity in brain slices. However, the blockade of K_ATP channels increases the SLE duration, suggesting that these channels may contribute to the termination of SLEs. Thus, K_ATP channels can be considered a promising target for pharmacological interventions for the treatment of epilepsy.

Maternal Hypoxia Increases the Excitability of Neurons in the Entorhinal Cortex and Dorsal Hippocampus of Rat Offspring

Prenatal hypoxia is a widespread condition that causes various disturbances in later life, including aberrant central nervous system development, abnormalities in EEG rhythms, and susceptibility to seizures. Hypoxia in rats on the 14th day of embryogenesis (E14) disrupts cortical neuroblast radial migration, mainly affecting the progenitors of cortical glutamatergic neurons but not GABAergic interneurons or hippocampal neurons. Thus, hypoxia at this time point might affect the development of the neocortex to a greater extent than the hippocampus. In the present study, we investigated the long-term effects of hypoxia on the properties of the pyramidal neurons in the hippocampus and entorhinal cortex (EC) in 3-week-old rats subjected to hypoxia on E14. We observed a reduction in the total number of NeuN-positive neurons in EC but not in the CA1 field of the hippocampus, indicating an increased cell loss in EC. However, the principal neuron electrophysiological characteristics were altered in the EC and hippocampus of animals exposed to hypoxia. The whole-cell patch-clamp recordings revealed a similar increase in input resistance in neurons from the hippocampus and EC. However, the resting membrane potential was increased in the EC neurons only. The recordings of field postsynaptic potentials (fPSPs) in the CA1 hippocampal area showed that both the threshold currents inducing fPSPs and population spikes were lower in hypoxic animals compared to age-matched controls. Using the dosed electroshock paradigm, we found that seizure thresholds were lower in the hypoxic group. Thus, the obtained results suggest that maternal hypoxia during the generation of the pyramidal cortical neurons leads to the increased excitability of neuronal circuitries in the brain of young rats. The increased excitability can be attributed to the changes in intrinsic neuronal properties.

Impairments of Long-Term Synaptic Plasticity in the Hippocampus of Young Rats during the Latent Phase of the Lithium-Pilocarpine Model of Temporal Lobe Epilepsy

Status epilepticus (SE) causes persistent abnormalities in the functioning of neuronal networks, often resulting in worsening epileptic seizures. Many details of cellular and molecular mechanisms of seizure-induced changes are still unknown. The lithium–pilocarpine model of epilepsy in rats reproduces many features of human temporal lobe epilepsy. In this work, using the lithium–pilocarpine model in three-week-old rats, we examined the morphological and electrophysiological changes in the hippocampus within a week following pilocarpine-induced seizures. We found that almost a third of the neurons in the hippocampus and dentate gyrus died on the first day, but this was not accompanied by impaired synaptic plasticity at that time. A diminished long-term potentiation (LTP) was observed following three days, and the negative effect of SE on plasticity increased one week later, being accompanied by astrogliosis. The attenuation of LTP was caused by the weakening of N-methyl-D-aspartate receptor (NMDAR)-dependent signaling. NMDAR-current was more than two-fold weaker during high-frequency stimulation in the post-SE rats than in the control group. Application of glial transmitter D-serine, a coagonist of NMDARs, allows the enhancement of the NMDAR-dependent current and the restoration of LTP. These results suggest that the disorder of neuron–astrocyte interactions plays a critical role in the impairment of synaptic plasticity.

Insertion of Calcium-Permeable AMPA Receptors during Epileptiform Activity In Vitro Modulates Excitability of Principal Neurons in the Rat Entorhinal Cortex

Epileptic activity leads to rapid insertion of calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) into the synapses of cortical and hippocampal glutamatergic neurons, which generally do not express them. The physiological significance of this process is not yet fully understood; however, it is usually assumed to be a pathological process that augments epileptic activity. Using whole-cell patch-clamp recordings in rat entorhinal cortex slices, we demonstrate that the timing of epileptiform discharges, induced by 4-aminopyridine and gabazine, is determined by the shunting effect of Ca2+-dependent slow conductance, mediated predominantly by K+-channels. The blockade of CP-AMPARs by IEM-1460 eliminates this extra conductance and consequently increases the rate of discharge generation. The blockade of NMDARs reduced the additional conductance to a lesser extent than the blockade of CP-AMPARs, indicating that CP-AMPARs are a more significant source of intracellular Ca2+. The study's main findings were implemented in a mathematical model, which reproduces the shunting effect of activity-dependent conductance on the generation of discharges. The obtained results suggest that the expression of CP-AMPARs in principal neurons reduces the discharge generation rate and may be considered as a protective mechanism.

Short-Term Epileptiform Activity Potentiates Excitatory Synapses but Does Not Affect Intrinsic Membrane Properties of Pyramidal Neurons in the Rat Hippocampus In Vitro

Even brief epileptic seizures can lead to activity-dependent structural remodeling of neural circuitry. Animal models show that the functional plasticity of synapses and changes in the intrinsic excitability of neurons can be crucial for epileptogenesis. However, the exact mechanisms underlying epileptogenesis remain unclear. We induced epileptiform activity in rat hippocampal slices for 15 min using a 4-aminopyridine (4-AP) in vitro model and observed hippocampal hyperexcitability for at least 1 h. We tested several possible mechanisms of this hyperexcitability, including changes in intrinsic membrane properties of neurons and presynaptic and postsynaptic alterations. Neither input resistance nor other essential biophysical properties of hippocampal CA1 pyramidal neurons were affected by epileptiform activity. The glutamate release probability also remained unchanged, as the frequency of miniature EPSCs and the paired amplitude ratio of evoked responses did not change after epileptiform activity. However, we found an increase in the AMPA/NMDA ratio, suggesting alterations in the properties of postsynaptic glutamatergic receptors. Thus, the increase in excitability of hippocampal neural networks is realized through postsynaptic mechanisms. In contrast, the intrinsic membrane properties of neurons and the probability of glutamate release from presynaptic terminals are not affected in a 4-AP model.

Early Life Febrile Seizures Impair Hippocampal Synaptic Plasticity in Young Rats

Febrile seizures (FSs) in early life are significant risk factors of neurological disorders and cognitive impairment in later life. However, existing data about the impact of FSs on the developing brain are conflicting. We aimed to investigate morphological and functional changes in the hippocampus of young rats exposed to hyperthermia-induced seizures at postnatal day 10. We found that FSs led to a slight morphological disturbance. The cell numbers decreased by 10% in the CA1 and hilus but did not reduce in the CA3 or dentate gyrus areas. In contrast, functional impairments were robust. Long-term potentiation (LTP) in CA3-CA1 synapses was strongly reduced, which we attribute to the insufficient activity of N-methyl-D-aspartate receptors (NMDARs). Using whole-cell recordings, we found higher desensitization of NMDAR currents in the FS group. Since the desensitization of NMDARs depends on subunit composition, we analyzed NMDAR current decays and gene expression of subunits, which revealed no differences between control and FS rats. We suggest that an increased desensitization is due to insufficient activation of the glycine site of NMDARs, as the application of D-serine, the glycine site agonist, allows the restoration of LTP to a control value. Our results reveal a new molecular mechanism of FS impact on the developing brain.

Paradoxical Anticonvulsant Effect of Cefepime in the Pentylenetetrazole Model of Seizures in Rats

Many β-lactam antibiotics, including cephalosporins, may cause neurotoxic and proconvulsant effects. The main molecular mechanism of such effects is considered to be γ-aminobutyric acid type a (GABAa) receptor blockade, leading to the suppression of GABAergic inhibition and subsequent overexcitation. We found that cefepime (CFP), a cephalosporin, has a pronounced antiepileptic effect in the pentylenetetrazole (PTZ)-induced seizure model by decreasing the duration and severity of the seizure and animal mortality. This effect was specific to the PTZ model. In line with findings of previous studies, CFP exhibited a proconvulsant effect in other models, including the maximal electroshock model and 4-aminopyridine model of epileptiform activity, in vitro. To determine the antiepileptic mechanism of CFP in the PTZ model, we used whole-cell patch-clamp recordings. We demonstrated that CFP or PTZ decreased the amplitude of GABAa receptor-mediated postsynaptic currents. PTZ also decreased the current decay time constant and temporal summation of synaptic responses. In contrast, CFP slightly increased the decay time constant and did not affect summation. When applied together, CFP prevented alterations to the summation of responses by PTZ, strongly reducing the effects of PTZ on repetitive inhibitory synaptic transmission. The latter may explain the antiepileptic effect of CFP in the PTZ model.

Seizure-Induced Potentiation of AMPA Receptor-Mediated Synaptic Transmission in the Entorhinal Cortex

Excessive excitation is considered one of the key mechanisms underlying epileptic seizures. We investigated changes in the evoked postsynaptic responses of medial entorhinal cortex (ERC) pyramidal neurons by seizure-like events (SLEs), using the modified 4-aminopyridine (4-AP) model of epileptiform activity. Rat brain slices were perfused with pro-epileptic solution contained 4-AP and elevated potassium and reduced magnesium concentration. We demonstrated that 15-min robust epileptiform activity in slices leads to an increase in the amplitude of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated component of the evoked response, as well as an increase in the polysynaptic γ-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptor-mediated components. The increase in AMPA-mediated postsynaptic conductance depends on NMDA receptor activation. It persists for at least 15 min after the cessation of SLEs and is partly attributed to the inclusion of calcium-permeable AMPA receptors in the postsynaptic membrane. The mathematical modeling of the evoked responses using the conductance-based refractory density (CBRD) approach indicated that such augmentation of the AMPA receptor function and depolarization by GABA receptors results in prolonged firing that explains the increase in polysynaptic components, which contribute to overall network excitability. Taken together, our data suggest that AMPA receptor enhancement could be a critical determinant of sustained status epilepticus (SE).

Adiabatic heating and convection in a porous medium filled with a near-critical fluid

Dynamics and heat transfer in a porous medium filled with a fluid phase at parameters near the gas-liquid critical point are studied. A two-dimensional numerical solver based on the hydrodynamic model for a porous medium with a high compressible fluid phase including the van der Waals equation of state is used. In weightlessness, adiabatic heating of fluid phase under the step-temperature heat supply is investigated analytically and numerically. In terrestrial conditions, gravity-driven convection in vertical rectangular cells generated by lateral heating in unsteady and steady-state regimes is simulated. The effects of high compressibility of near-critical fluid phase on convection are studied. Convective motions and heat transfer in horizontal rectangular cells consisting of two porous layers at different porosity and permeability heated from below are simulated as well. Adiabatic heating subjected to hydrostatic compressibility effects, the onset and development of convection, and convective structures in a steady-state regime are analyzed.

Hydrostatic compressibility phenomena: new opportunities for near-critical research in microgravity

Peculiarities of the isothermal and isentropic equilibrium in highly compressible media with nonperfect state equations are discussed. Formulation of the adiabatic temperature gradient using Schwarzschild criterion and equilibrium equations for nonperfect gas are considered. Criterion for the onset of convection in the van der Waals (VDW) gas is examined using direct numerical modeling on the basis of equation for convection in compressible viscous nonperfect gas. Results of three-dimensional modeling near-critical convection in the vicinity of the convection onset in space flight with quasi-steady microaccelerations are given. A proposal for the microgravity research of the near-critical convection is discussed.

Unsteady near-critical flows in microgravity

This paper presents analysis of the different time scales associated with unsteady fluid flow phenomena near the thermodynamical critical point and that are typical for experiments carried out in microgravity. A focus of the paper is modeling the initial stage of convection under low and zero gravity on the basis of the two-dimensional Navier-Stokes equations for a compressible gas with the Van der Waals state equation. We also consider a thermoacoustic problem on the basis of three-dimensional linearized equations for an isentropic inviscid gas near the critical point in zero gravity. We compare the heat transfer due to unsteady convection and the piston effect in an enclosure with side heating in zero and low gravity with pure conductivity.

Adiabatic heating and convection caused by a fixed-heat-flux source in a near-critical fluid

Dynamics and heat transfer in a near-critical fluid in a square cavity with a finite heat source located at the bottom are studied numerically. A thermally insulated enclosure and a fixed-heat-flux source are considered. The two-dimensional simulation is based on the full Navier-Stokes equations with two-scale splitting of the pressure and the van der Waals equation of state. It is shown that the piston effect is independent of convection. Near the critical point, this effect becomes independent of criticality and convective motions are damped.