POTENTIOMETRIC STUDY OF RESTING POTENTIAL, CONTRIBUTING K-CELLS( CHANNELS AND THE ONSET OF NA+ CHANNEL EXCITABILITY IN EMBRYONIC RAT CORTICAL)

Resting membrane potential (RMP), K+ channel contribution to RMP and t he development of excitability were investigated in the entire populat ion of acutely dissociated embryonic (E) rat cortical cells over E11-2 2 using a voltage-sensitive fluorescent indicator dye and flow cytomet ry. During the period of intense proliferation (E11-13), two cell subp opulations with distinct estimated RMPs were recorded: one polarized a t similar to -70 mV and the other relatively less-polarized at similar to -40 mV. Ca-o(2+) was critical in sustaining the RMP of the majorit y of less-polarized cells, while the well-polarized cells were charact erized by membrane potentials exhibiting a similar to Nernstian relati onship between RMP and [K+](o). Analysis of these two subpopulations r evealed that > 80% of less-polarized cells were proliferative, while > 90% of well-polarized cells were postmitotic. Throughout embryonic de velopment, the disappearance of Ca-o(2+)-sensitive, less-polarized cel ls correlated with the disappearance of the proliferating population, while the appearance of the K-o(+)-sensitive, well-polarized populatio n correlated with the appearance of terminally postmitotic neurons, im mune-identified as BrdU(-), tetanus toxin(+) cells. Differentiating ne urons were estimated to contain increased K-i(+) relative to less-pola rized 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 o bserved at E11, progressively changed from Ca-o(2+)-dependent and tetr odotoxin-insensitive to Na-o(+)-dependent and tetrodotoxin-sensitive r esponse by E18. The results reveal a dynamic development of RMP, contr ibuting K+ channels and voltage-dependent Na+ channels in the developi ng cortex as it transforms from proliferative to primarily differentia ting tissue.