Low-Threshold Ca2+ Currents in Dendritic Recordings from Purkinje Cells in Rat Cerebellar Slice Cultures

Voltage-dependent Ca2+ conductances were investigated in Purkinje cells in rat cerebellar slice cultures using the whole-cell and cell-attached configurations of the patch-clamp technique. In the presence of 0.5 mm Ca2+ in the extracellular solution, the inward current activated with a threshold of −55 ± 1.5 mV and reached a maximal amplitude of 2.3 ± 0.4 nA at −31 ± 2 mV. Decay kinetics revealed three distinct components: a fast (24.6 ± 2 msec time constant), a slow (304 ± 46 msec time constant), and a nondecaying component. Rundown of the slow and sustained components of the current, or application of antagonists for the P/Q-type Ca2+ channels, allowed isolation of the fast-inactivating Ca2+ current, which had a threshold for activation of −60 mV and reached a maximal amplitude of 0.7 nA at a membrane potential of −33 mV. Both activation and steady-state inactivation of this fast-inactivating Ca2+ current were described with Boltzmann equations, with half-activation and inactivation at −51 mV and −86 mV, respectively. This Ca2+ current was nifedipine-insensitive, but its amplitude was reduced reversibly by bath-application of NiCl2 and amiloride, thus allowing its identification as a T-type Ca2+ current. Channels with a conductance of 7 pS giving rise to a fast T-type ensemble current (insensitive to ω-Aga-IVA) were localized with a high density on the dendritic membrane. Channel activity responsible for the ensemble current sensitive to ω-Aga-IVA was detected with 10 mm Ba2+ as the charge carrier. These channels were distributed with a high density on dendritic membranes and in rare cases were also seen in somatic membrane patches.