Differences in Potassium Channel Composition Underlie Distinct Action Potential Kinetics in Transcriptomically Identified Neocortical Mouse Cell Types

Abstract

Neocortical neuronal cell types differ in their intrinsic electrical properties, leading to input - output characteristics tuned to their function. To understand how gene expression in a given cell type gives rise to these electrical characteristics, we established a data generation pipeline using the ‘Patch-seq’ technique - a powerful approach to characterize the electrophysiological, morphological and transcriptomic features from single neurons. We first capture the neuron's passive and active electrical properties in current clamp using a standard set of square pulse, ramp, and noise stimuli. We then extract the cellular contents for downstream RNA-seq processing - ultimately mapping the cell to its transcriptomic type. To narrow the gap between gene expression and electrical behavior, we also characterize macroscopic ion channel properties using the outside-out ‘nucleated’ voltage clamp configuration. Here we use voltage clamp protocols designed to isolate and characterize the fast and slow inactivating potassium ion channel currents. In the mouse primary visual cortex, we find differences in potassium channel current density amplitude and kinetics in cells representing the major inhibitory transcriptomic cell subclasses - Pvalb, Sst, VIP, and Lamp5. Within a major subclass, we find that some specific transcriptomic types can be distinguished by their electrophysiological properties, while others overlap. Interestingly, properties of the macroscopic potassium current correlate with action potential kinetics and the expression of certain ion channels. This richer dataset drives both perturbation experiments to establish a causal link between ion channel currents and active properties as well as a refined taxonomy that incorporates active ion channel currents to further discriminate between cell types.