Interneuron-Driven Ictogenesis in the 4-Aminopyridine Model: Depolarization Block and Potassium Accumulation Initiate Seizure-like Activity

TL;DR

This study reveals how seizures start in a mouse model, showing that overactive inhibitory brain cells trigger a chain of events leading to seizure-like activity.

Contribution

The paper identifies a precise sequence involving interneuron depolarization block and potassium accumulation as a trigger for seizure initiation.

Findings

Interneurons fire rapidly before entering depolarization block, which coincides with potassium accumulation.

Pyramidal neurons start firing 1.1 seconds after depolarization block, marking seizure onset.

Hippocampal neurons show distinct activity, suggesting region-specific seizure mechanisms.

Abstract

The mechanisms of ictal discharge initiation remain incompletely understood, particularly the paradoxical role of inhibitory fast-spiking interneurons in seizure generation. Using simultaneous whole-cell recordings of interneurons and pyramidal neurons combined with extracellular [K+]o monitoring in mouse entorhinal cortex-hippocampal slices (4-aminopyridine model of epileptiform activity), we identified a critical transition sequence: interneurons displayed high-frequency firing during the preictal phase before entering depolarization block (DB). DB onset coincided with the peak of rate of extracellular [K+] accumulation. Pyramidal cells remained largely silent during interneuronal hyperactivity but started firing within 1.1 ± 0.3 s after DB onset, marking the transition to ictal discharges. This consistent sequence (interneuron DB → [K+]o rate peak → pyramidal cell firing) was observed in 100% of entorhinal cortex recordings. Importantly, while neurons across all entorhinal cortical layers synchronously fired during the first ictal discharge, hippocampal CA1 neurons showed fundamentally different activity: they generated high-frequency interictal bursts but did not participate in ictal events, indicating region-specific seizure initiation mechanisms. Our results demonstrate that interneuron depolarization block acts as a precise temporal switch for ictogenesis and suggest that the combined effect of disinhibition and K+-mediated depolarization triggers synchronous pyramidal neuron recruitment. These findings provide a mechanistic framework for seizure initiation in focal epilepsy, highlighting fast-spiking interneurons dysfunction as a potential therapeutic target.

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