support-information-section).

Somatostatin (SST) expressing interneurons are the second most abundant group of inhibitory neurons in the neocortex. They mainly target the apical dendrites of excitatory pyramidal cells and are implicated in feedforward and feedback inhibition. In the present study, we employ a conditional knockout mouse, in which the transcription factor Satb1 is selectively deleted in SST‐expressing interneurons resulting to the reduction of their number across the somatosensory barrel field. Our goal was to investigate the effect of the reduced number of Satb1 mutant SST‐interneurons on (i) the endogenous cortical network activity (spontaneously recurring Up/Down states), and (ii) the transition to epileptiform activity. By conducting LFP recordings in acute brain slices from young male and female mice, we demonstrate that mutant animals exhibit significant changes in network excitability, reflected in increased Up state occurrence, decreased Up state duration and higher levels of extracellular spiking activity. Epileptiform activity was induced through two distinct and widely used in vitro protocols: the low magnesium and the 4‐Aminopyridine (4‐AP) model. In the former, slices from mutant animals manifested shorter latency for the expression of stable seizure‐like events. In contrast, when epilepsy was induced by 4‐AP, no significant differences were reported. We conclude that normal SST‐interneuron function has a significant role both in the regulation of the endogenous network activity, and in the transition to seizure‐like discharges in a context‐dependent manner.

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