Interneurons spark seizure-like activity in the entorhinal cortex

Abstract

Excessive neuronal synchronization is presumably involved in epileptiform synchronization. However, the respective roles played by interneurons (GABAergic) and principal (glutamatergic) cells during interictal and ictal discharges remain unclear. Here, we employed tetrode wire recordings to establish the involvement of these two cell types in 4-aminopyridine-induced interictal- and low-voltage fast (LVF) onset ictal-like discharges in the rat entorhinal cortex in an in vitro slice preparation. We recorded a total of 90 single units (69 putative interneurons, 17 putative principal and 4 unclassified cells) from 36 slices, and found that: (i) interneurons (66.7%) were more likely to fire during interictal discharges than principal cells (35.3%); (ii) interneuron activity increased shortly before LVF ictal onset, whereas principal cell activity did not change; (iii) interneurons and principal cells fired at high rates throughout the tonic phase of the ictal discharge; however, (iv) only interneurons showed phase-locked relationship with LVF activity at 5-15Hz during the tonic phase. Finally, the association of interneuron firing with interictal discharges was maintained during blockade of ionotropic glutamatergic transmission. Our findings demonstrate the prominent involvement of interneurons in interictal discharge generation and in the transition to LVF ictal activity in this in vitro model of epileptiform synchronization.

Low‐voltage fast seizures in humans begin with increased interneuron firing

“…8,9 Findings obtained in vitro from entorhinal cortex and hippocampal tissue slices and from isolated guinea pig preparation show that 4-aminopyridine or bicuculline application induces LVF seizurelike discharges that coincide with sustained interneuron discharges and robust inhibitory postsynaptic events in principal (glutamatergic) neurons. 4,[10][11][12][13][14] In line with these findings, optogenetic stimulation of parvalbumin-or somatostatin-positive interneurons can initiate ictallike LVF onset events similar to those occurring spontaneously. [15][16][17] Experimentally, LVF onset seizures are also recorded in status epilepticusinduced models of mesial-temporal lobe epilepsy.…”

“…4,10–14 In line with these findings, optogenetic stimulation of parvalbumin- or somatostatin-positive interneurons can initiate ictallike LVF onset events similar to those occurring spontaneously. 15–17 Experimentally, LVF onset seizures are also recorded in status epilepticus-induced models of mesial–temporal lobe epilepsy.…”

“…14 The action potentials generated by human putative excitatory and inhibitory neurons both exhibited phase locking during spontaneous LVF onset seizures (see Fig 1D), but the action potentials generated by putative inhibitory neurons (n = 94) exhibited stronger phase locking to the LVF activity (20–30Hz) compared to the action potentials recorded from putative excitatory neurons (n = 48; see Fig 1E, Wilcoxon rank sum test, p < 0.001), confirming the accuracy of neuron classification using waveform features. 14 To further confirm the accuracy of our assignments, in a subsample of the data we characterized the mean ISI of all units during a baseline (5–40 minute) epoch, a 5-minute preictal-LVF onset epoch, and a 5- to 40-minute postictal epoch. We found that action potentials generated by putative inhibitory neurons (n = 29) exhibited relatively briefer ISIs, as compared to excitatory neurons (n = 91), during the baseline and preictal epochs (Wilcoxon rank sum, p < 0.016, Bonferroni corrected; see Fig 1F).…”

“…14,33,38,39 Other approaches to discriminating principal neurons from interneurons have addressed the temporal autocorrelation of the spike train. 21,29 This approach, however, may be inappropriate for ictal epochs due to the atypical neuronal firing patterns.…”

“…The cluster containing spike waveforms with a shorter half width, shorter trough to peak duration, and larger peak amplitude asymmetry were classified as putative inhibitory interneurons (see Fig 1B, C), whereas the second cluster of spike waveforms was classified as putative excitatory principal cells. 14 Overall, we identified 100 excitatory neurons and 40 inhibitory neurons in the mesial-temporal SOZ using waveform morphology criteria. We also identified 49 excitatory neurons, and 13 inhibitory mesialtemporal neurons contralateral to the SOZ.…”