Role of paroxysmal depolarization in focal seizure activity

Abstract

We analyze the role of inhibition in sustaining focal epileptic seizure activity. We review ongoing seizure activity at the mesoscopic scale that can be observed with microelectrode arrays as well as at the macroscale of standard clinical EEG. We provide clinical, experimental, and modeling data to support the hypothesis that paroxysmal depolarization (PD) is a critical component of the ictal machinery. We present dual-patch recordings in cortical cultures showing reduced synaptic transmission associated with presynaptic occurrence of PD, and we find that the PD threshold is cell size related. We further find evidence that optically evoked PD activity in parvalbumin neurons can promote propagation of neuronal excitation in neocortical networks in vitro. Spike sorting results from microelectrode array measurements around ictal wave propagation in human focal seizures demonstrate a strong increase in putative inhibitory firing with an approaching excitatory wave, followed by a sudden reduction of firing at passage. At the macroscopic level, we summarize evidence that this excitatory ictal wave activity is strongly correlated with oscillatory activity across a centimeter-sized cortical network. We summarize Wilson-Cowan-type modeling showing how inhibitory function is crucial for this behavior. Our findings motivated us to develop a network motif of neurons in silico, governed by a reduced version of the Hodgkin-Huxley formalism, to show how feedforward, feedback, PD, and local failure of inhibition contribute to observed dynamics across network scales. The presented multidisciplinary evidence suggests that the PD not only is a cellular marker or epiphenomenon but actively contributes to seizure activity.NEW & NOTEWORTHY We present mechanisms of ongoing focal seizures across meso- and macroscales of microelectrode array and standard clinical recordings, respectively. We find modeling, experimental, and clinical evidence for a dual role of inhibition across these scales: local failure of inhibition allows propagation of a mesoscopic ictal wave, whereas inhibition elsewhere remains intact and sustains macroscopic oscillatory activity. We present evidence for paroxysmal depolarization as a mechanism behind this dual role of inhibition in shaping ictal activity.

Keywords: ictal perturbator; multiscale analysis; neural inhibition; oscillator network.

Fig. 1.

Simplified representation of 2 (seemingly contradictory) roles of excitatory (E) and inhibitory (I) activity ascribed to clinical seizure activity (e.g., Fisher et al. 2005). A: hyperexcitation is a disturbed balance in favor of excitation. B: hypersynchronous oscillation that is often characteristic for clinical recording of ongoing seizures requires back and forth switches between dominant excitation and inhibition.

Fig. 2.

Candidate mechanisms leading to hyperexcitation, failure of inhibition, and hypersynchrony. A: a diagram of an inhibitory cell (red) connected to an excitatory cell (black), both stimulated by external input. Reduced inhibitory efficacy can be caused by saturation of the inhibitory cell leading to paroxysmal depolarization shift (PDS) and increased Cl⁻ concentration in the excitatory neuron. The overall activity of both cells can be increased by increasing K⁺ levels in the extracellular space. The components in the box at bottom right represent that ephaptic effects may increase when synaptic transmission fails because of a low Ca²⁺ concentration. B: a Wilson-Cowan type macroscopic population model that is capable of generating sustained oscillations when inhibitory function is intact. E, excitatory; I, inhibitory.

Fig. 3.

Size effect of the neuronal response to synaptic input simulated with the Hodgkin–Huxley formalism. Two model neurons of different sizes with identical ion channel dynamics show different susceptibility for saturation. The larger neuron (top) starts responding (generating action potentials, rest > firing) at higher input levels than the smaller neuron representing an interneuron (middle). Also, saturation (firing > saturation) occurs at lower levels in the smaller neuron. Note that at the input levels used her, the larger neuron does not reach saturation levels. The synaptic input current for each neuron is plotted at bottom.

Fig. 4.

Experimental data gathered in cortical neurons on effects of paroxysmal depolarization on synaptic transmission and propagation. A: dual patch-clamp recording of coupled rat cortical neurons showing the effect of paroxysmal depolarization shift (PDS) on synaptic transmission between 2 excitatory neurons. B: dual patch-clamp recording of coupled rat cortical neurons showing effects between a presynaptic inhibitory cell (red traces) and a postsynaptic excitatory one (black traces). The recordings in A and B show that the postsynaptic output (bottom trace of each pair) and synaptic transmission is reduced when the presynaptic neuron (top trace of each pair) transitions from normal bursting (A1 and B1) to PDS activity (A2 and B2). A2 shows that the postsynaptic effect of PDS (3 bursts on left) is similar to that of a single presynaptic action potential (traces on right). Both the bursts in A1 and B1 were evoked with 20-pA injections; the injections to evoke the PDS in A2 and B2 were 80 pA and 45 pA, respectively. C: saturation effects shown with patch-clamp measurements in a red fluorescence identified parvalbumin-positive (PVn) inhibitory neuron upon optical stimulation in a brain slice from PVn-hChR2 mice ([postnatal day 30). C1: 2 superimposed responses at different light intensities: the lower intensity (150 μW delivered at slice surface) evoked rapid firing (light red) whereas the highest intensity (520 μW delivered at slice surface) resulted in a PDS (red). C2 depicts the same PDS response as in C1 and is obtained upon the same optical stimulation (520 μW) after the neuron was synaptically isolated from network excitation with 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). This demonstrates that the PDS behavior is an intrinsic property and not evoked via the network. D: image of fluorescent neocortical PVn neuron in brain slice from PVn-hChR2 mice (arrow) with micropipette. E: setup for the optical stimulation of the inhibitory PVn neurons. Electrical stimulation is indicated by the pulse and lightning flash on left; optical stimulation of the PVn neurons at the blue symbol at a wavelength (λ) of 480 nm. The extracellular measurement (Ctx) in the cortex is positioned right from the optical stimulus. F: the first 2 trials show how electrical stimuli alone do not result in propagation at the extracellular recording site: the responses in the integrated extracellular cortical signal (F1) and the associated spectrogram depicting frequencies 1–5 Hz (F2). The large deflections are stimulus artifacts. Interestingly, at the third stimulus given during strong optical activation of the inhibitory population, neural activity does propagate. The intensity of the optical stimulus in F is 520 μW, the same as used in C2. Color scale in F2: −35 to −20 dB/Hz.

Fig. 5.

Response characteristics of the regular-spiking (RS) and fast spiking (FS) models. Neuronal response profiles are frequently grouped into 3 classes, as first defined by Hodgkin (1948). The underlying nonlinear dynamics and bifurcations can be described by using different parameter sets in Eq. 1. The parameters used for this figure are summarized in Table 1. A: an excitatory pyramidal cell model of the RS type with Class I properties. B: an inhibitory cell model of the FS type with class II excitability. In addition, it can be seen that saturation at high input levels occurs at a much lower level for the inhibitory cells than for the larger pyramidal neurons (360 pA vs. 2,000 pA).

Fig. 6.

A motif with feedback and feedforward connections, modeling the activity, and recorded activity during human focal seizure activity: part of a network with excitatory neurons (E1 and E2) and inhibitory neurons (I1 and I2). Neuron E2 can be inhibited via neuron I1 in a feedforward loop (FF) and via neuron I2 in a feedback loop (FB). The circuit is excited with a strong external input (blue arrows). A depicts a detail within the perturbator with a slowly propagating ictal wave (1 mm/s) starting at the arrow at top left. As the ictal wave propagates, neuron E2 will eventually get excited so that it experiences both FF and FB inhibition. B depicts the situation of the same circuit within the oscillator. The network now receives input from the small and far-away perturbator. Via long-range excitatory connections, the perturbator excites the neurons in the oscillator, E1 and E2. Compared with the scenario in A, the input at the oscillator is weaker and broader, thereby creating similar relatively strong inputs to both neurons (blue arrows). As a result both excitatory cells will simultaneously exhibit similar activities. In this case, the inhibitory effect of both inhibitory cells on neuron E2 will be also similar, in both strength and delay. As a result, a clear distinction between the effect of inhibition via FF and FB on E2 vanishes, and FF now resembles FB. C and D depict the response of networks in A and B, respectively. C1 and D1 depict the input function (note the different vertical calibrations in C1 and D1). C2–C4 depict responses of E2 (black, C2) and I1 (red, C3) and the associated extracellular pseudo-EEG evoked by the neural activity (C4) for the scenario in A. Note that inhibitory activity precedes the excitatory activity. D2–D4 are the same as in C but for the network scenario shown in B. Note that once the oscillation is established the inhibitory activity persists after offset of excitation (vertical dashed line across D2 and D3). E depicts the spike sorting result of extracellular activity during a human focal seizure at the passage of an ictal wave at time 0 indicated by the vertical dashed line. Putative inhibitory neural firing (red) is reduced substantially before the increase in putative excitatory firing (black) as the wave front passes. F depicts the spike sorting result of the activity in an area away from the wave front during oscillatory activity of the local field potential (top). Spike sorting shows that putative excitation and inhibition are, on average (bottom), synchronized with the oscillation and that here putative inhibition is active after offset of the putative excitation. Note the similarities between the recordings in E and F and the simulations shown in C and D, respectively. In E, where FF inhibition dominates, the inhibitory activity peak precedes the excitatory one. In contrast, in F, where feedback inhibition rules to create oscillations, the inhibitory activity persists after the excitatory activity peaks (data from Merricks et al. 2018). AU, arbitrary units.

Fig. 7.

Macroscopic effect of the perturbator located at the gray square close to the temporal tip: lateral view (top) and basal view (bottom) showing the spike triggered averages (STAs) arranged spatially. At each location, the average (black) and its noise estimate (green) are depicted. It can be seen that the effect of the ictal wave activity ranges across a macroscopic centimeter-sized area (reproduced from Eissa et al. 2017).