Potassium currents in precursor cells isolated from the anterior subventricular zone of the neonatal rat forebrain.

Abstract

The progenitor cells from the anterior part of the neonatal subventricular zone, the SVZa, are unusual in that, although they undergo division, they have a neuronal phenotype. To characterize the electrophysiological properties of the SVZa precursor cells, recordings were made of potassium and sodium currents from SVZa cells that were removed from postnatal day 0-1 rats and cultured for 1 day. The properties of the delayed rectifier and A-type potassium currents were described by classical Hodgkin and Huxley analyses of activation and inactivation. In addition, cells were assessed under current clamp for their ability to generate action potentials. The A-type potassium current (IK(A)) was completely inactivated at a holding potential of -50 mV. The remaining potassium current resembled the delayed rectifier current (IK(DR)) in that it was blocked by tetraethylammonium (TEA; IC50 4.1 mM) and activated and inactivated slowly compared with IK(A). The conductance-voltage (G-V) curve revealed that G increased continuously from 0.2 nS at -40 mV to a peak of 2.6 nS at +10 or +20 mV, and then decreased for voltages above +30 mV. Activation time constants were largest at -40 mV ( approximately 11 ms) and smallest at 100 mV ( approximately 1.5 ms). The properties of IK(A) were studied in the presence of 20 mM TEA, to block IK(DR), and from a holding potential of -15 mV, to inactivate both IK(DR) and IK(A). IK(A) was then allowed to recover from inactivation to negative potentials during 200- to 800-ms pulses. Recovery from inactivation was fastest at -130 mV ( approximately 21 ms) and slowest at -90 mV ( approximately 135 ms). Inactivation was voltage independent from -60 to +60 mV with a time constant of approximately 15 ms. At steady state, IK(A) was half inactivated at -90 mV. GK(A) increased from 0.2 nS at -60 mV to a peak of 2.4 nS at +40 mV. Finally, the activation time constants ranged from approximately 1.9 ms at -50 mV to 0.7 ms at +60 mV. The properties of IK(A) resembled those of IK(A) found in differentiating cerebellar granule neurons. Most SVZa cells had sodium currents (28/32 cells). However, in current clamp 11 of 12 cells were incapable of generating action potentials from voltages of -30 to -100 mV, suggesting that the available current densities were too low to support excitability.