Sodium current kinetics in freshly isolated neostriatal neurones of the adult guinea pig

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Abstract
Neurones of the neostriatum were freshly dispersed from the adult guinea pig brain. A fast, transient inward Na+ current (I Na) was analysed using the whole-cell patch-clamp technique. Upon depolarizations, I Na developed with a sigmoidal time course, which was described by m 3 kinetics. I Na 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 I Na 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, τ m , had a bell-shaped configuration with a maximum value at −60 mV. The forward rate constant for I Na activation (α m ) 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 (β m) 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 I Na decay followed a second-order process, whereas the recovery from inactivation was described as a first-order process. The τ h curve showed a bell-shaped configuration with a maximum value at −60 mV. The forward rate constants (α h ) 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 (β h) 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).