Unique properties of R-type calcium currents in neocortical and neostriatal neurons

Whole cell recordings from acutely dissociated neocortical pyramidal neuron s and striatal medium spiny neurons exhibited a calcium-channel current res istant to known blockers of L-, N-, and P/Q-type Ca2+ channels. These R-typ e currents were characterized as high-voltage-activated (HVA) by their rapi d deactivation kinetics, half-activation and half-inactivation voltages, an d sensitivity to depolarized holding potentials. In both cell types, the R- type current activated at potentials relatively negative to other HVA curre nts in the same cell type and inactivated rapidly compared with the other H VA currents. The main difference between cell types was that R-type current s in neocortical pyramidal neurons inactivated at more negative potentials than R-type currents in medium spiny neurons. Ni2+ sensitivity was not diag nostic for R-type currents in either cell type. Single-cell RT-PCR revealed that both cell types expressed the alpha 1E mRNA, consistent with this sub unit being associated with the R-type current.