Voltage- and ligand-gated ion channels in floor plate neuroepithelia of the rat

Whole-cell patch-clamp recordings were used to characterize the membrane properties and ion channel complement of floor plate neuroepithelia in embryonic and neonatal rats. The average resting potential was close to -60 mV, the capacitance was similar to 7 pS and the membrane time constant averaged 31 ms, in both neonates and embryos. Two types of K+ current were identified (i) a slowly activating, slowly inactivating current that was present in all cells, and (ii) a rapidly inactivating current that was present in 39% of cells from neonates and 64% of cells from embryos. K+ currents were significantly larger in neonates than embryos. Nai currents were absent from all neuroepithelial cells examined. In contrast, the majority of floor plate cells exhibited a significant Ca2+ current. Biophysically this current activated at potentials positive to -60 mV and exhibited fast, voltage-dependent, inactivation. The Ca2+ current was equipermeant to Ca2+ and Ba2+, sensitive to 40-120 mu M Ni2+ and only slightly inhibited by 100 mu M Cd2+. These and other observations indicated this current is mediated by low-voltage-activated (i.e. T-type) Ca2+ channels. The majority of floor plate cells tested also exhibited responses to the neurotransmitter GABA which produced robust inward currents at negative membrane potentials, in chloride-loaded cells. Both the pharmacology and voltage-dependence of the GABA-activated currents indicated they arose from activation of GABA(A) receptors. (C) 1998 IBRO. Published by Elsevier Science Ltd.