Potentiometric study of resting potential, contributing K+ channels and the onset of Na+ channel excitability in embryonic rat cortical cells

Resting membrane potential (RMP), K+ channel contribution to RMP and the development of excitability were investigated in the entire population of acutely dissociated embryonic (E) rat cortical cells over E11–22 using a voltage‐sensitive fluorescent indicator dye and flow cytometry. During the period of intense proliferation (E11–13), two cell subpopulations with distinct estimated RMPs were recorded: one polarized at ∼–70 mV and the other relatively less‐polarized at ∼–40 mV. Ca2+o was critical in sustaining the RMP of the majority of less‐polarized cells, while the well‐polarized cells were characterized by membrane potentials exhibiting a ∼Nernstian relationship between RMP and [K+]o. Analysis of these two subpopulations revealed that > 80% of less‐polarized cells were proliferative, while > 90% of well‐polarized cells were postmitotic. Throughout embryonic development, the disappearance of Ca2+o‐sensitive, less‐polarized cells correlated with the disappearance of the proliferating population, while the appearance of the K+o‐sensitive, well‐polarized population correlated with the appearance of terminally postmitotic neurons, immuno‐identified as BrdU–, tetanus toxin+ cells. Differentiating neurons were estimated to contain increased K+i relative to less‐polarized cells, coinciding with the developmental expression of Cs+/Ba2+‐sensitive and Ca2+‐dependent K+ channels. Both K+ channels contributed to the RMP of well‐polarized cells, which became more negative toward the end of neurogenesis. Depolarizing effects of veratridine, first observed at E11, progressively changed from Ca2+o‐dependent and tetrodotoxin‐insensitive to Na+o‐dependent and tetrodotoxin‐sensitive response by E18. The results reveal a dynamic development of RMP, contributing K+ channels and voltage‐dependent Na+ channels in the developing cortex as it transforms from proliferative to primarily differentiating tissue.