Voltage-activated membrane currents in rat cerebellar granule neurones

1. Voltage-activated currents have been recorded from cerebellar granule neurones in explant cultures from young rats (1-9 days old). Cells were examined with whole-cell patch-clamp methods. Depolarizing pulses from a pre-pulse potential of -100 mV evoked a rapidly activated transient inward current, and an outward current which decayed in two phases. The ionic dependence, kinetics and pharmacological properties of these currents have been studied. 2. Peak inward Na+ currents in cells from 7-day-old rats were in the range 350-450 pA. No evidence was found for the presence of calcium currents. Thus, inward current was unchanged in zero Ca2+, 1 mM-EGTA solution. No inward current was obtained in medium containing 10 mM-Ba2+ and tetrodotoxin (TTX). Supplementing the pipette (i.e. intracellular) solution with Mg-ATP did not reveal any Ca2+ current. 3. Depolarizing steps (from -100 mV) in TTX-containing solution gave an early transient outward current and a late outward current. The transient current resembled IA described in other cells, and reversed close to EK in both normal and elevated potassium concentrations, indicating that K+ is the predominant charge carrier. Depolarizing steps from -50 mV failed to give a transient outward current, and gave only a slowly rising current which resembled the late potassium current, IK. 4. Inactivation of the transient current was examined by applying test depolarizations from increasingly negative pre-pulse potentials (-50 to -120 mV): half-inactivation occurred at -72 mV. Transient outward currents decayed exponentially with time constants, tau, of 7.3-25.3 ms at 0 mV. The time course of removal of inactivation in cells held at -50 mV, and given increasingly long pre-pulses to -100 mV, was exponential with tau = 35 ms. 5. Both transient and late outward currents were reversibly abolished by addition to the bathing medium of 10 mM-Ba2+ or 1 mM-quinine. Outward K+ current was not dependent on external calcium. Tetraethylammonium (20 mM) selectively reduced the late outward current; the peak transient current was reduced by less than 20%. 4-Aminopyridine (2 mM) showed little selectivity between transient and late outward currents. 6. It is concluded that cerebellar granule cells from young rats possess voltage-activated inward Na+ current as well as two types of K+ current, IA and IK. In terms of neuronal functioning, the properties of the transient outward current may confer a role in regulating excitability and in repolarization, but a definitive statement will require knowledge of the cellular location and relative densities of channels in granule cells in vivo.