1. Depolarization-activated, calcium independent potassium (K+) currents were studied with the use of whole cell voltage clamp recording from neostriatal neurons acutely isolated from adult (≤4 wk old) rats. The whole cell K+ current was composed of transient and persistent components. The aims of the experiments were to isolate the persistent component and then to characterize its voltage dependence and kinetics. 2. Application of 10 mM 4- aminopyridine (4-AP) completely blocked the transient currents while reducing the persistent current by ~40% [50% inhibitory concentration (IC50), of blockable current = 125 μM]. The persistent K+ current also was reduced by tetraethylammonium (TEA). Two components to the TEA block were present, having IC50s of 125 μM (23% of the blockable current) and 5.9 mM (77% of the blockable current). Collectively, these results suggested that the persistent component of the total K+ current was pharmacologically heterogeneous. The properties of the 4-AP-resistant, persistent K+ current (I(Krp)) were subsequently studied. 3. The kinetics of activation and deactivation of I(Krp) were voltage dependent. Examination of the entire activation/deactivation time constant profile showed that it was bell shaped, with time constants being moderately rapid (τ ~ 50 ms) at membrane potentials corresponding to the resting potential of neostriatal cells (approximately -80 mV), becoming considerably longer (τ ~ 100 ms) at potentials near the cells' spike thresholds (approximately -45 mV), and decreasing to a minimum (τ ~ 5 ms) at potentials associated with the peak of the cells' action potentials (approximately +20 mV). The inactivation kinetics of I(Krp) also were voltage dependent. The time constants of inactivation varied between 1 and 8 s at potentials between -10 and +35 mV. 4. Unlike persistent K+ currents in many other cell types. I(Krp) activated at relatively hyperpolarized membrane potentials (approximately -70 mV). The Boltzmann function describing activation had a half-activation voltage of - 13 mV and a slope factor of 12 mV. In addition, the Boltzmann function describing the voltage dependence of inactivation of I(Krp) had a relatively depolarized half-inactivation voltage of -55 mV and a large slope factor of 19 mV, indicating that this current was available over a broad range of membrane potentials (between -100 and -10 mV). 5. Neostriatal neurons recorded in vivo exhibit subthreshold shifts in membrane potential of variable duration (tens of ms to s) from a hyperpolarized resting state to a depolarized state that is limited in amplitude just below spike threshold. The voltage dependence of activation and inactivation of I(Krp) indicates that it will be available on depolarization from the hyperpolarized state. However, the slow activation rate of this current suggests that it will contribute little either to limiting the amplitude of the initial depolarization associated with entry into the depolarized state or to depolarizing episodes of short duration (e.g., <50 ms). However, I(Krp) should limit the amplitude of membrane depolarizations associated with prolonged excursions into the depolarized state.
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