The irregular firing of cortical interneurons in vitro arises from stochastic processes.

Pharmacologically isolated GABAergic interneurons in the mouse visual cortex display highly irregular spike times (coefficient of variation ? 1) in response to DC de-polarization. This is in marked contrast to cortical pyramidal cells. We used non-linear time series analysis methods to distinguish between the presence of non-linear deterministic processes or the amplification of sub threshold noise giving rise to the observed dynamics. No evidence for non-linear deterministic processes was found. This leaves a high sensibility of the interneuronal spike initiation process for membrane potential noise as the most likely explanation for the high CV. We propose that this intrinsically irregular spiking of an important subpopulation of cortical neurons contributes to the overall irregularity of cortical activity.