# 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.

## Key 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.