Elsevier

Experimental Neurology

Volume 283, Part A, September 2016, Pages 341-352
Experimental Neurology

Research Paper
Role of CA3 theta-modulated interneurons during the transition to spontaneous seizures

https://doi.org/10.1016/j.expneurol.2016.06.027Get rights and content

Highlights

  • Theta oscillations are different in epileptic rats and precede seizure onset.

  • Subtypes of interneurons are differentially affected in epileptic rats, even during interictal periods.

  • Most neurons, except theta-on interneurons, display decreased firing and increased synchrony during theta epochs.

  • Theta-on interneurons are functionally preserved and maintain similar firing patterns as in controls.

  • These interneurons selectively exhibit aberrantly high firing during the transition to seizures.

Abstract

Multiple studies have observed heterogeneous neuronal firing patterns as a local network transitions to spontaneous seizures. We demonstrated that separately examining interneurons and pyramidal cells during this transition in a rat model of temporal lobe epilepsy elucidates some of this heterogeneity. Recently, it was demonstrated that classifying cells into specific theta-related subtypes further clarified the heterogeneity. Moreover, changes in neuronal synchrony with the local field potential were identified and determined to be specific to interneurons during the transition to seizures. To extend our understanding of the chronic changes in epileptic networks, we examined field potentials and single neuron activity in the CA3 hippocampus of pilocarpine-treated rats during interictal periods and compared these to neuronal activity in healthy controls and during preictal periods. Neurons were classified into theta-subtypes based on changes in firing patterns during theta periods. As previously reported, we find a high probability of theta oscillations before seizure onset and a selective increase in theta-on interneuron firing rate immediately preceding seizure onset. However, we also find overall slower theta rhythm and a general decrease in subtype-specific firing during interictal periods compared to that in control animals. The decrease in subtype specific interneuron activity is accompanied by increases in synchrony. Exceptionally, theta-on interneurons, that selectively increase their firing rate at seizure onset, maintain similar firing rates and synchrony as controls during interictal period. These data suggest that increased synchrony during interictal periods may compensate for low firing rates creating instability during theta that is prone to seizure initiation via a transition to hyper-synchronous activation of theta-on interneurons.

Introduction

Understanding changes in neuronal firing patterns during the transition to seizures is essential for understanding epilepsy and for developing therapeutic interventions. Multiple studies show that during the transition to seizures, some neurons increase their firing rate while others decrease, (Babb et al., 1987, Bower and Buckmaster, 2008, Bower et al., 2012, Truccolo et al., 2011) suggesting heterogeneous changes in firing rate. CA3 subfield is a particularly interesting structure within the hippocampus because while it is reported to be a common site of seizure initiation in pilocarpine-treated rats (Lévesque et al., 2012), we recently showed that prior to seizure onset, pyramidal cells are at best firing more slowly and only increase their firing rates after the onset of rhythmic LFP spiking (Grasse et al., 2013). Moreover, approximately 40% of CA3 interneurons increase their firing rate at onset (Grasse et al., 2013) and Toyoda et al. (2015) demonstrated that these cells are predominately theta-on interneurons. These results suggest that identifying the neuronal subtypes could be critical to understanding the physiological mechanisms underlying the transition to seizures.

Neuronal subtypes, identified by their firing patterns in relation to theta rhythms, might be particularly important because there is mounting evidence that theta oscillations are dominant in the hippocampus and medial septum prior to chemically induced seizures (Butuzova and Kitchigina, 2008, Kitchigina and Butuzova, 2009) and spontaneous seizures (Grasse et al., 2013). In fact, a recent study reported that theta oscillations preceded a majority (81%) of spontaneous seizures in a rat model of epilepsy (Sedigh-Sarvestani et al., 2014). These studies emphasize the potential role of theta oscillations during the transition to seizures. Unlike in epileptic animals, neuronal firing during theta oscillations in healthy animals is widely studied (Buzsáki, 2002, Csicsvari et al., 1999). Within temporal lobe structures, both interneurons and pyramidal cells can be further classified by changes in their firing patterns during theta periods. Studies in anesthetized animals identified a subtype of neurons whose firing rate increased during theta oscillations (theta-on) compared to non-theta periods and a subtype whose firing rate decreased (theta-off cells) (Bland et al., 1999, Colom and Bland, 1987, Smythe et al., 1991) and these subtypes are differentially modulated at the transition to seizure (Toyoda et al., 2015).

Despite recent evidence of changes in firing patterns of theta-related cell types during theta epochs transitioning to seizure onset, it is unclear whether neuronal firing patterns during interictal theta oscillations in epilepsy are fundamentally different from those in healthy controls. Moreover, it is unknown whether the theta oscillations that precede seizures are different from those during interictal theta oscillations. To address these questions, theta oscillations and the underlying neuronal firing patterns were compared between control rats and epileptic (pilocarpine model of temporal lobe epilepsy) rats during interictal and preictal periods. Results show that theta and neuronal firing patterns in epileptic animals are distinctly different from those in healthy control animals, suggesting long-term changes that impact the CA3 network during interictal periods as well as preictal periods.

Section snippets

Overview

Animals were injected with pilocarpine to induce status epilepticus for 2 h and monitored until they exhibited chronic spontaneous seizures. Pilocarpine-treated rats (N = 6) with spontaneous seizures and naive age-matched control rats (N = 4) were implanted with drivable tetrodes in CA3 hippocampus. Wide band neural signals and video recordings were obtained for multiple 48 h recording sessions in each animal. Seizure frequency was variable and seizures sometimes occurred in clusters, but overall

Theta oscillations precede majority of seizures

A total of 25 spontaneous seizures were recorded from 6 Long-Evans rats. In the 2 min prior to rhythmic ictal spiking onset, most seizures were preceded by theta oscillations (84%) that occurred in small bursts (duration 4.3 ± 1.9 s, Fig. 2A, B). To determine if this theta was associated with REM sleep as previously described (Sedigh-Sarvestani et al., 2014), we classified the behavior of the animals and determined that 14 seizures were preceded by sleep, 8 were preceded by awake stationary

Discussion

While our data support earlier studies demonstrating that theta oscillations are likely to be dominant during the transition to seizures (Butuzova and Kitchigina, 2008, Fujita et al., 2014, Kitchigina and Butuzova, 2009, Kitchigina et al., 2013, Sedigh-Sarvestani et al., 2014) and that theta-on interneurons selectively increase their firing rate, our comparisons of neuronal activity from healthy controls and epileptic animals identified important differences describing the impact of epilepsy on

Acknowledgements

We would like to thank Dr. Liset Menendez de la Prida for feedback on earlier versions of the paper and Dr. Beatriz Gal for performing histological staining of sections used in the paper. This work was supported by a grant from Coulter Foundation.

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      For both kainic acid (Arabadzisz et al., 2005a; Dugladze et al., 2007; Riban et al., 2002) and pilocarpine (Chauvière et al., 2009; Colom, 2006; Karunakaran et al., 2016; Lee et al., 2017b; Marcelin et al., 2009) models of TLE in rodents, a significant reduction of theta power follows both acutely induced status-epilepticus as well as chronic spontaneous and recurring seizures (Fig. 5). Changes in oscillatory power occur bi-laterally, in the dorsal (Arabadzisz et al., 2005a; Colom, 2006; Dugladze et al., 2007; Karunakaran et al., 2016; Lee et al., 2017b; Marcelin et al., 2009) and ventral hippocampus (Arabadzisz et al., 2005a; Dugladze et al., 2007) as well as the medial septum (Arabadzisz et al., 2005a; Colom, 2006). Theta oscillations during interictal periods within hippocampal CA3 are also slower in epileptic animals compared to controls, consistent with observations in CA1 hippocampus of kainate-treated (Inostroza et al., 2013) or pilocarpine-treated (Marcelin et al., 2009) animals.

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