Epileptiform synchronization and high-frequency oscillations in brain slices comprising piriform and entorhinal cortices

Principal neurons in the olfactory cortex mediate bidirectional modulation of seizures

Although the piriform cortex (PC) has been previously implicated as a critical node for seizure generation and propagation, the underlying neural mechanism has remained unclear. Here, we found increased excitability in PC neurons during amygdala kindling acquisition. Optogenetic or chemogenetic activation of PC pyramidal neurons promoted kindling progression, whereas inhibition of these neurons retarded seizure activities induced by electrical kindling in the amygdala. Furthermore, chemogenetic inhibition of PC pyramidal neurons alleviated the severity of kainic acid-induced acute seizures. These results demonstrate that PC pyramidal neurons bidirectionally modulate seizures in temporal lobe epilepsy, providing evidence for the efficacy of PC pyramidal neurons as a potential therapeutic target for epileptogenesis. KEY POINTS: While the piriform cortex (PC) is an important olfactory centre critically involved in olfactory processing and plays a crucial role in epilepsy due to its close connection with the limbic system, how the PC regulates epileptogenesis is largely unknown. In this study, we evaluated the neuronal activity and the role of pyramidal neurons in the PC in the mouse amygdala kindling model of epilepsy. PC pyramidal neurons are hyperexcited during epileptogenesis. Optogenetic and chemogenetic activation of PC pyramidal neurons significantly promoted seizures in the amygdala kindling model, whereas selective inhibition of these neurons produced an anti-epileptic effect for both electrical kindling and kainic acid-induced acute seizures. The results of the present study indicate that PC pyramidal neurons bidirectionally modulate seizure activity.

Limbic and olfactory cortical circuits in focal seizures

Epilepsies affecting the limbic regions are common and generate seizures often resistant to pharmacological treatment. Clinical evidence demonstrates that diverse regions of the mesial portion of the temporal lobe participate in limbic seizures; these include the hippocampus, the entorhinal, perirhinal and parahippocampal regions and the piriform cortex. The network mechanisms involved in the generation of olfactory-limbic epileptiform patterns will be here examined, with particular emphasis on acute interictal and ictal epileptiform discharges obtained by treatment with pro-convulsive drugs and by high-frequency stimulations on in vitro preparations, such as brain slices and the isolated guinea pig brain. The interactions within olfactory-limbic circuits can be summarized as follows: independent, region-specific seizure-like events (SLE) are generated in the olfactory and in the limbic cortex; SLEs generated in the hippocampal-parahippocampal regions tend to remain within these areas; the perirhinal region controls the neocortical propagation and the generalization of limbic seizures; interictal spiking in the olfactory regions prevents the invasion by SLEs generated in limbic regions. The potential relevance of these observations for human focal epilepsy is discussed.

Processing of cell assemblies in the lateral entorhinal cortex

There is evidence that olfactory cortex responds to its afferent input with the generation of cell assemblies: collections of principal neurons that fire together over a time scale of tens of ms. If such assemblies form an odor representation, then a fundamental question is how each assembly then induces neuronal activity in downstream structures. We have addressed this question in a detailed model of superficial layers of lateral entorhinal cortex, a recipient of input from olfactory cortex and olfactory bulb. Our results predict that the response of the fan cell subpopulation can be approximated by a relatively simple Boolean process, somewhat along the lines of the McCulloch/Pitts scheme; this is the case because of the sparsity of recurrent excitation amongst fan cells. However, because of recurrent excitatory connections between layer 2 and layer 3 pyramidal cells, synaptic and probably also gap junctional, the response of pyramidal cell subnetworks cannot be so approximated. Because of the highly structured anatomy of entorhinal output projections, our model suggests that downstream targets of entorhinal cortex (dentate gyrus, hippocampal CA3, CA1, piriform cortex, olfactory bulb) receive differentially processed information.

In vitro Oscillation Patterns Throughout the Hippocampal Formation in a Rodent Model of Epilepsy

Specific oscillatory patterns are considered biomarkers of pathological neuronal network in brain diseases, such as epilepsy. However, the dynamics of underlying oscillations during the epileptogenesis throughout the hippocampal formation in the temporal lobe epilepsy is not clear. Here, we characterized in vitro oscillatory patterns within the hippocampal formation of epileptic rats, under 4-aminopyridine (4-AP)-induced hyperexcitability and during the spontaneous network activity, at two periods of epileptogenesis. First, at the beginning of epileptic chronic phase, 30 days post-pilocarpine-induced Status Epilepticus (SE). Second, at the established epilepsy, 60 days post-SE. The 4-AP-bathed slices from epileptic rats had increased susceptibility to ictogenesis in CA1 at 30 days post-SE, and in entorhinal cortex and dentate gyrus at 60 days post-SE. Higher power and phase coherence were detected mainly for gamma and/or high frequency oscillations (HFOs), in a region- and stage-specific manner. Interestingly, under spontaneous network activity, even without 4-AP-induced hyperexcitability, slices from epileptic animals already exhibited higher power of gamma and HFOs in different areas of hippocampal formation at both periods of epileptogenesis, and higher phase coherence in fast ripples at 60 days post-SE. These findings reinforce the critical role of gamma and HFOs in each one of the hippocampal formation areas during ongoing neuropathological processes, tuning the neuronal network to epilepsy.

Astrocyte Role in Temporal Lobe Epilepsy and Development of Mossy Fiber Sprouting

Epilepsy affects approximately 50 million people worldwide, with 60% of adult epilepsies presenting an onset of focal origin. The most common focal epilepsy is temporal lobe epilepsy (TLE). The role of astrocytes in the presentation and development of TLE has been increasingly studied and discussed within the literature. The most common histopathological diagnosis of TLE is hippocampal sclerosis. Hippocampal sclerosis is characterized by neuronal cell loss within the Cornu ammonis and reactive astrogliosis. In some cases, mossy fiber sprouting may be observed. Mossy fiber sprouting has been controversial in its contribution to epileptogenesis in TLE patients, and the mechanisms surrounding the phenomenon have yet to be elucidated. Several studies have reported that mossy fiber sprouting has an almost certain co-existence with reactive astrogliosis within the hippocampus under epileptic conditions. Astrocytes are known to play an important role in the survival and axonal outgrowth of central and peripheral nervous system neurons, pointing to a potential role of astrocytes in TLE and associated cellular alterations. Herein, we review the recent developments surrounding the role of astrocytes in the pathogenic process of TLE and mossy fiber sprouting, with a focus on proposed signaling pathways and cellular mechanisms, histological observations, and clinical correlations in human patients.

Cell assembly formation and structure in a piriform cortex model

The piriform cortex is rich in recurrent excitatory synaptic connections between pyramidal neurons. We asked how such connections could shape cortical responses to olfactory lateral olfactory tract (LOT) inputs. For this, we constructed a computational network model of anterior piriform cortex with 2000 multicompartment, multiconductance neurons (500 semilunar, 1000 layer 2 and 500 layer 3 pyramids; 200 superficial interneurons of two types; 500 deep interneurons of three types; 500 LOT afferents), incorporating published and unpublished data. With a given distribution of LOT firing patterns, and increasing the strength of recurrent excitation, a small number of firing patterns were observed in pyramidal cell networks: first, sparse firings; then temporally and spatially concentrated epochs of action potentials, wherein each neuron fires one or two spikes; then more synchronized events, associated with bursts of action potentials in some pyramidal neurons. We suggest that one function of anterior piriform cortex is to transform ongoing streams of input spikes into temporally focused spike patterns, called here "cell assemblies", that are salient for downstream projection areas.

The potential antiepileptogenic effect of neuronal Cx36 gap junction channel blockage

Epilepsy is one of the most prevalent neurological disorders and can result in neuronal injury and degeneration. Consequently, research into new antiepileptic drugs capable of providing protection against neuronal injury and degeneration is extremely important. Neuronal Cx36 gap junction channels have been found to play an important role in epilepsy; thus, pharmacological interference using Cx36 gap junction channel blockers may be a promising strategy for disrupting the synchronization of neurons during seizure activity and protecting neurons. Based on these promising findings, several in vivo and in vitro studies are ongoing and the first encouraging results have been published. The results bring hope that neurons can be protected from injury and degeneration in patients with epilepsy, which is currently impossible.

Electrical stimulation of the piriform cortex for the treatment of epilepsy: A review of the supporting evidence

In this review, we consider how the piriform cortex is engaged in both focal and generalized epilepsy networks and postulate the various neural pathways that can be effectively neuromodulated by stimulation at this site. This highlights the common involvement of the piriform cortex in epilepsy. We address both current and future preclinical studies of deep brain stimulation (DBS) of the piriform cortex, with attention to the critical features of these trials that will enable them to be of greatest utility in informing clinical translation. Although recent DBS trials have utilized thalamic targets, electrical stimulation of the piriform cortex may also be a useful intervention for people with epilepsy. However, more work is required to develop a solid foundation for this approach before considering human trials.

The effects of dynamical synapses on firing rate activity: a spiking neural network model

Accumulating evidence relates the fine-tuning of synaptic maturation and regulation of neural network activity to several key factors, including GABA_A signaling and a lateral spread length between neighboring neurons (i.e., local connectivity). Furthermore, a number of studies consider short-term synaptic plasticity (STP) as an essential element in the instant modification of synaptic efficacy in the neuronal network and in modulating responses to sustained ranges of external Poisson input frequency (IF). Nevertheless, evaluating the firing activity in response to the dynamical interaction between STP (triggered by ranges of IF) and these key parameters in vitro remains elusive. Therefore, we designed a spiking neural network (SNN) model in which we incorporated the following parameters: local density of arbor essences and a lateral spread length between neighboring neurons. We also created several network scenarios based on these key parameters. Then, we implemented two classes of STP: (1) short-term synaptic depression (STD) and (2) short-term synaptic facilitation (STF). Each class has two differential forms based on the parametric value of its synaptic time constant (either for depressing or facilitating synapses). Lastly, we compared the neural firing responses before and after the treatment with STP. We found that dynamical synapses (STP) have a critical differential role on evaluating and modulating the firing rate activity in each network scenario. Moreover, we investigated the impact of changing the balance between excitation (E) and inhibition (I) on stabilizing this firing activity.

High-frequency oscillations and seizure-like discharges in the entorhinal cortex of the in vitro isolated guinea pig brain

We analyzed the patterns of seizure-like activity and associated high-frequency oscillations (HFOs) induced by the K⁺ channel blocker 4-aminopyridine (4AP, 50μM) or the GABA_A receptor antagonist bicuculline methiodide (BMI, 50μM) in the in vitro isolated guinea pig brain preparation. Extracellular field recordings were obtained from the medial entorhinal cortex (EC) using glass pipettes or silicon probes; 4AP or BMI were applied through the basilar artery. Ripples (80-200Hz) or fast ripples (250-500Hz) occurred at higher rates shortly before ictal events induced by 4AP or BMI, respectively. In addition, during the ictal period, ripples were mostly associated with 4AP-induced ictal events whereas fast ripples predominated during ictal discharges induced by BMI. Finally, ripples occurred at higher rates during the clonic phase of 4AP-induced ictal events compared to the tonic phase, while higher rates of fast ripples characterized the clonic phase in both 4AP- and BMI-induced ictal discharges. These differences in HFO occurrence presumably reflect the diverse action of these two convulsants on GABA_A receptor signaling.

Synaptic Conductances during Interictal Discharges in Pyramidal Neurons of Rat Entorhinal Cortex

In epilepsy, the balance of excitation and inhibition underlying the basis of neural network activity shifts, resulting in neuronal network hyperexcitability and recurrent seizure-associated discharges. Mechanisms involved in ictal and interictal events are not fully understood, in particular, because of controversial data regarding the dynamics of excitatory and inhibitory synaptic conductances. In the present study, we estimated AMPAR-, NMDAR-, and GABA_A R-mediated conductances during two distinct types of interictal discharge (IID) in pyramidal neurons of rat entorhinal cortex in cortico-hippocampal slices. Repetitively emerging seizure-like events and IIDs were recorded in high extracellular potassium, 4-aminopyridine, and reduced magnesium-containing solution. An original procedure for estimating synaptic conductance during IIDs was based on the differences among the current-voltage characteristics of the synaptic components. The synaptic conductance dynamics obtained revealed that the first type of IID is determined by activity of GABA_A R channels with depolarized reversal potential. The second type of IID is determined by the interplay between excitation and inhibition, with early AMPAR and prolonged depolarized GABA_A R and NMDAR-mediated components. The study then validated the contribution of these components to IIDs by intracellular pharmacological isolation. These data provide new insights into the mechanisms of seizures generation, development, and cessation.

Carbonic anhydrase inhibition by acetazolamide reduces in vitro epileptiform synchronization

Depolarizing GABAA receptor-mediated currents are contributed by HCO3(-) efflux, and play a role in initiating ictal-like epileptiform events in several cortical structures supporting the view that GABAA receptor signaling actively participates to epileptiform synchronization. We employed here field potential recordings to analyze the effects of the carbonic anhydrase inhibitor acetazolamide (10 μM) on the epileptiform activity generated in vitro by piriform and entorhinal cortices (PC and EC, respectively) during application of the K(+) channel blocker 4-aminopyridine (4AP, 50 μM). Under these experimental conditions ictal- and interictal-like discharges along with high-frequency oscillations (ripples: 80-200 Hz, fast ripples: 250-500 Hz) occurred in these two regions. In both PC and EC, acetazolamide: (i) reduced the duration and the interval of occurrence of ictal discharges along with the associated ripples and fast ripples; (ii) decreased the interval of occurrence of interictal discharges and the rates of associated fast ripples; and (iii) diminished the duration and amplitude of pharmacologically isolated GABAergic events while increasing their interval of occurrence. Our results indicate that acetazolamide effectively controls 4AP-induced epileptiform synchronization in PC and EC. We propose that this action may rest on decreased GABAA receptor-mediated HCO3(-) efflux leading to diminished depolarization of principal cells and, perhaps, of interneurons.

KCC2 function modulates in vitro ictogenesis

GABAA receptor-mediated inhibition is active and may contribute to epileptiform synchronization. The efficacy of inhibition relies on low levels of intracellular Cl(-), which are controlled by KCC2 activity. This evidence has led us to analyze with field potential recordings the effects induced by the KCC2 blockers VU0240551 (10 μM) or bumetanide (50 μM) and by the KCC2 enhancer CLP257 (100 μM) on the epileptiform discharges generated by piriform and entorhinal cortices (PC and EC, respectively) in an in vitro brain slice preparation. Ictal- and interictal-like discharges along with high-frequency oscillations (HFOs, ripples: 80-200 Hz, fast ripples: 250-500 Hz) were recorded from these two regions during application of 4-aminopyridine (4AP, 50 μM). Blocking KCC2 activity with either VU024055 or high doses of bumetanide abolished ictal discharge in both PC and EC; in addition, these experimental procedures decreased the interval of occurrence and duration of interictal discharges. In contrast, enhancing KCC2 activity with CLP257 increased ictal discharge duration in both regions. Finally, blocking KCC2 activity decreased the duration and amplitude of pharmacologically isolated synchronous GABAergic events whereas enhancing KCC2 activity led to an increase in their duration. Our data demonstrate that in vitro ictogenesis is abolished or facilitated by inhibiting or enhancing KCC2 activity, respectively. We propose that these effects may result from the reduction of GABAA receptor-dependent increases in extracellular K(+) that are known to rest on KCC2 function.

Neurosteroidal modulation of in vitro epileptiform activity is enhanced in pilocarpine-treated epileptic rats

We employed field potential recordings in brain slices obtained from pilocarpine-treated epileptic (4-5weeks following a pilocarpine-induced status epilepticus) and age-matched, non-epileptic control (NEC) rats to establish the effects of the neurosteroid allotetrahydrodeoxycorticosterone (THDOC) on the epileptiform activity - including high frequency oscillations (HFOs; ripples: 80-200Hz, fast ripples: 250-500Hz) - induced by 4-aminopyridine (4AP) in piriform (PC) and entorhinal (EC) cortices. Both structures are highly susceptible to generate seizures and may also be involved in epileptogenesis. We found that THDOC application to pilocarpine-treated slices: (i) decreased interictal discharge frequency in PC while increasing it in EC; (ii) abolished ictal discharges in both areas in approx. one third of the experiments and reduced them in frequency and duration in the remaining experiments; and (iii) increased the occurrence of ripples and fast ripples associated to interictal events, and modified their pattern of occurrence during ictal discharges in both PC and EC. These effects were either weaker or absent in NEC tissue. Our results demonstrate that THDOC plays a structure-dependent modulatory role in epileptiform synchronization in the pilocarpine-treated epileptic rat brain where its actions are more pronounced than in NEC tissue. This evidence supports the application of neurosteroids as potential antiepileptic tools.