Two pharmacologically and kinetically distinct transient potassium currents in cultured embryonic mouse hippocampal neurons

Abstract

Transient potassium currents in mammalian central neurons influence both the repolarization of single action potentials and the timing of repetitive action potential generation. How these currents are integrated into neuronal function will depend on their specific properties: channel availability at the resting potential, activation threshold, inactivation rate, and current density. We here report on the voltage-gated transient potassium currents in embryonic mouse hippocampal neurons dissected at embryonic days 15-16 and grown in dissociated cell culture for up to 3 d. Two transient potassium currents, A-current and D-current, were isolated based on steady state inactivation and sensitivity to 4-aminopyridine (4-AP) and dendrotoxin (DTx). A-current had an activation threshold of approximately -50 mV and was half-inactivated at approximately -81 mV. A-current relaxations at voltages between -40 and +40 mV could be fit by single exponential functions with time constants of 20-25 msec; these time constants showed little sensitivity to voltage. In contrast, D-current had an activation threshold of between -40 and -30 mV and was half-inactivated at approximately -22 mV. D-current inactivation was voltage dependent; time constants of fitted exponential functions ranged from approximately 7 sec at -40 mV to 200 msec at +40 mV. A slower component of inactivation was also evident. D-current was preferentially blocked by 4-AP (100 microM) and DTx (1 microM). Operationally, A- and D-currents could be cleanly separated based on conditioning pulse potential and 4-AP sensitivity. Total transient potassium current amplitude increased during the time that neurons were in culture (recordings were made between 2 hr after dissociation and 3 d in culture). When normalized for cell capacitance (an index of membrane area), A-current density (pA/pF) decreased and D-current density increased, even during a period between days 1 and 3 when total transient current density remained constant. This observation suggests that A- and D-currents may be reciprocally modulated. Since blockade of D-current (with 100 microM 4-AP) increased both action potential duration and repetitive firing in response to constant current stimulation, long-term modulation of the A-current:D-current ratio may affect the excitability of hippocampal neurons.

Three Kinetically Distinct Ca2+-Independent Depolarization-Activated K+ Currents in Callosal-Projecting Rat Visual Cortical Neurons

“…The 50-μM-sensitive currents ( Fig. 3D) inactivate slowly and resemble currents referred to as I D , or slow transient currents, in other preparations Storm 1988;Wu and Barish 1992). The currents blocked by 5 mM (but not by 50 μM) 4-AP, in contrast, inactivate rapidly ( Fig.…”

“…Rapidly activating and inactivating currents, similar to I A in CP neurons, have been characterized in neurons from rat hippocampus Segal and Barker 1984;Storm 1988), dentate gyrus (Beck et al 1992), neostriatum (Surmeier et al 1989), lateral geniculate nucleus (Budde et al 1992), thalamus , sensorimotor cortex (Foehring and Surmeier 1993;Hamill et al 1991), and visual cortex (Albert and Nerbonne 1995;Foehring and Surmeier 1993); the guinea pig (Numann et al 1987) and embryonic mouse (Wu and Barish 1992) hippocampus; and cat sensorimotor cortex (Spain et al 1991). The time-and voltage-dependent properties and the pharmacological sensitivities of the currents termed I A in these different preparations are similar.…”

“…Slow transient currents, which are distinct from I A and I K and often are referred to as I D , have been identified in neurons from rat prefrontal cortex (Hammond and Crepel 1992), neostriatum (Surmeier et al 1991), hippocampus , visual cortex (Albert and Nerbonne 1995;Foehring and Surmeier 1993), globus pal-lidus (Stefani et al 1995) and dorsal root ganglia (DRG) (Stansfeld et al 1986) and from rat sensorimotor cortex (Foehring and Surmeier 1993), guinea pig lateral geniculate nucleus (McCormick 1991) and DRG (Penner et al 1986), and mouse hippocampus (Wu and Barish 1992). In general, slow transient currents activate fast on membrane depolarization, inactivate slowly and are blocked by micromolar concentrations of 4-AP and often by DTX (Foehring and Surmeier 1993;Penner et al 1986;Stansfeld et al 1986;Surmeier et al 1991).…”

“…Two types of K + currents that are likely to subserve these functions have been described in a number of mammalian CNS neurons: a rapidly activating and inactivating current that is blocked by 4-aminopyridine and usually is referred to as I A (Beck et al 1992;Budde et al 1992;Ficker and Heinemann 1992;Numann et al 1987;Segal and Barker 1984;Storm 1988;Surmeier et al 1989;Wu and Barish 1992); and a 4-aminopyridine-insensitive, delayed, outwardly rectifying K + current, usually referred to as I K (Beck et al 1992;Budde et al 1992;Numann et al 1987;Rudy 1988;Segal and Barker 1984;Storm 1990). These are broad classifications, however, and the detailed time-and voltagedependent properties of the various currents termed I A and I K do vary among cells (Rudy 1988;Storm 1990).…”

“…These are broad classifications, however, and the detailed time-and voltagedependent properties of the various currents termed I A and I K do vary among cells (Rudy 1988;Storm 1990). In addition, in some CNS neurons, an additional, rapidly activating, slowly inactivating Ca 2+ -independent K + current has been identified based on sensitivity to micromolar concentrations of 4-aminopyridine and, in some cases, also to the dendrotoxins Hammond and Crepel 1992;McCormick 1991;Stefani et al 1995;Surmeier et al 1991;Wu and Barish 1992). These currents have been referred to variably as I D or "slow transient" currents to distinguish them from I A and I K .…”