GABAergic Interneurons in Seizures: Investigating Causality With Optogenetics

Abstract

Seizures are complex pathological network events characterized by excessive and hypersynchronized activity of neurons, including a highly diverse population of GABAergic interneurons. Although the primary function of inhibitory interneurons under normal conditions is to restrain excitation in the brain, this system appears to fail intermittently, allowing runaway excitation. Recent developments in optogenetics, combined with genetic tools and advanced electrophysiological and imaging techniques, allow us for the first time to assess the causal roles of identified cell-types in network dynamics. While these methods have greatly increased our understanding of cortical microcircuits in epilepsy, the roles played by individual GABAergic cell-types in controlling ictogenesis remain incompletely resolved. Indeed, the ability of interneurons to suppress epileptic discharges varies across different subtypes, and an accumulating body of evidence paradoxically implicates some interneuron subtypes in the initiation and maintenance of epileptiform activity. Here, we bring together findings from this growing field and discuss what can be inferred regarding the causal role of different GABAergic cell-types in seizures.

Keywords: epilepsy; interictal spikes; interneurons; optogenetics; seizures.

Figure 1.

Interictal spikes. (A) Three main types of activity observed in a human electroencephalography (EEG) recording. (a, b) Intracranial EEG traces showing interictal discharges (IID, blue), pre-ictal discharges (PID, pink) and ictal activity (yellow). (c) Amplitude distribution of IID and PID. (d) Spatial localization of each recording electrode and associated activity, showing a core of ictal activity (yellow spots) surrounded by IIDs (blue spots). Modified from Huberfeld and others (2011). (B) In vivo and in vitro recordings of interictal spikes in human mesial temporal lobe. Modified from Cohen and others (2002). (C) In vivo electrocorticogram (ECoG) recording of interictal spikes in a pilocarpine neocortical focal epilepsy mouse model (unpublished data). (D) In vitro local field potential (LFP) recording of interictal spikes in hippocampal slices superfused with high K⁺ solutions. Modified from Dzhala and Staley (2003).

Figure 2.

Possible mechanism of seizure induction by photo-activation of interneurons: post-inhibitory rebound spikes. Optogenetic activation of many interneurons (green) hyperpolarizes a large population of pyramidal neurons (black). When the photo-stimulation ends, pyramidal neurons are simultaneously released from inhibition and fire rebound action potentials initiating an ictal discharge.

Figure 3.

Possible mechanisms of seizure facilitation by interneurons. (A) Intense activity of interneurons during interictal bursts leads to GABA_A receptor–mediated Cl⁻ flux into principal neurons. This leads to KCC2-mediated efflux of K⁺ and Cl⁻. A seizure could then be triggered when extracellular K⁺ accumulation depolarizes a sufficient number of excitatory neurons. (B) When the capacity of principal neurons to extrude Cl⁻ is overwhelmed, Cl⁻ accumulation gradually shifts E_GABA to more depolarized potentials, weakening or even reversing the effect of GABA, and precipitating seizures. (C) Excessive activation of interneurons during interictal discharges causes them to enter a state of depolarization block. A seizure is generated when a sufficient number of interneurons cease firing and thus fail to contain excitatory activity.

Figure 4.

Factors influencing the effect of optogenetic stimulation of interneurons during seizures. (A) Stimulation frequency: the effect of PV+ (parvalbumin) but not SOM+ (somatostatin) cell photo-stimulation is frequency-dependent. (B) Location of stimulation relative to the seizure focus: photo-activation of interneurons within the focus is pro-ictogenic, while outside is anti-ictogenic. (C) Timing of stimulation relative to the seizure onset: optogenetic intervention before or early in the seizure can potentiate interneuron-mediated inhibition, while photo-activation at later time points—when the seizure has spread—may lead to pro-epileptic effects. (D) Interneuronal subtype: the effect of activating PV+ interneurons changes over time, while SOM+ neuron stimulation is anti-epileptic throughout seizures.