SODIUM CURRENT KINETICS IN FRESHLY ISOLATED NEOSTRIATAL NEURONS OF THE ADULT GUINEA-PIG

Neurones of the neostriatum were freshly dispersed from the adult guinea pig brain. A fast, transient inward Na+ current (INa) was analysed using the whole-cell patch-clamp technique. Upon depolarizations, INa developed with a sigmoidal time course, which was described by m3 kinetics. INa showed an activation threshold of about -60 mV, a peak current at -30 to -20 mV, and a reversal of polarity at +60 mV. The steady-state activation (m∞) curve for INa had a slope factor of about 9 mV with a mid-point potential of about -26 mV. The voltage dependence of the activation time constant, τm, had a bell-shaped configuration with a maximum value at -60 mV. The forward rate constant for INa activation (αm) increased as the membrane was depolarized (about 9 mV for a change in the rate constant by a factor of e) in the range between -50 mV and -20 mV. Conversely, the backward rate constant (βm) decreased as the membrane was depolarized (about 31 mV for an e-fold change). The steady-state inactivation (h∞) curve was well expressed by the Boltzmann's equation with a half-inactivation potential of -62 mV and a slope parameter of 6 mV. The time course of INa decay followed a second-order process, whereas the recovery from inactivation was described as a first-order process. The τh curve showed a bell-shaped configuration with a maximum value at -60 mV. The forward rate constants (αh) decreased as the membrane was depolarized (about 17 mV for an e-fold change) in the range between -50 mV and -20 mV. The backward rate constants (βh) increased as the membrane was depolarized (about 10 mV for an e-fold change). There was a significant overlap between m∞ and h∞ curves, suggesting a steady influx of Na+ (window current). © 1990 Springer-Verlag.