Characterization of Ca2+ channels in rat subthalamic nucleus neurons

The subthalamic nucleus (STN) plays a key role in motor control. Although previous studies have suggested that Ca2+ conductances may be involved in regulating the activity of STN neurons, Ca2+ channels in this region have not yet been characterized. We have therefore investigated the subtypes and functional characteristics of Ca2+ conductances in STN neurons, in both acutely isolated and slice preparations. Acutely isolated STN cells were identified by retrograde filling with the fluorescent dye, Fluoro-Gold. In acutely isolated STN neurons, Cd2+-sensitive, depolarization-activated Ba2+ currents were observed in all cells studied. The current-voltage relationship and current kinetics were characteristic of high-voltage-activated Ca2+ channels. The steady-state voltage-dependent activation curves and inactivation curves could both be fitted with a single Boltzmann function. Currents evoked with a prolonged pulse, however, inactivated with multiple time constants, suggesting either the presence of more than one Ca2+ channel subtype or multiple inactivation processes with a single channel type in STN neurons. Experiments using organic Ca2+ channel blockers revealed that on average, 21% of the current was nifedipine sensitive, 52% was sensitive to omega -conotoxin GVIA, 16% was blocked by a high concentration of omega -agatoxin IVA (200 nM), and the remainder of the current (9%) was resistant to the co-application of all blockers. These currents had similar voltage dependencies, but the nifedipine-sensitive current and the resistant current activated at slightly lower voltages. omega -Agatoxin IVA at 20 nM was ineffective in blocking the current. Together, the above results suggest that acutely isolated STN neurons have all subtypes of high-voltage-activated Ca2+ channels except for P-type, but have no low-voltage-activated channels. Although acutely isolated neurons provide a good preparation for whole cell voltage-clamp study, dendritic processes are lost during dissociation. To gain information on Ca2+ channels in dendrites, we thus studied Ca2+ channels of STN neurons in a slice preparation, focusing on low-voltage-activated channels. In current-clamp recordings, a slow spike was always observed following termination of an injected hyperpolarizing current. The slow spike occurred at resting membrane potentials and was sensitive to micromolar concentrations of Ni2+, suggesting that it is a low-threshold Ca2+ spike. Together, our results suggest that STN neurons express low-voltage-activated Ca2+ channels and several high-voltage-activated subtypes. Our results also suggest the possibility that the low-voltage-activated channels have a preferential distribution to the dendritic processes.