'Intact' dopaminergic midbrain neurons of the rat display unclamped dendritic Ca2+ currents

Calcium channels play an important role in generating the complex electrophysiology of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNc). To directly study these calcium currents in dendritically intact neurons, two issues needed to be addressed: the identity of the neuron (DAergic versus non-DAergic) and control of putative dendritic calcium conductances. A measure of the anomalous rectifier (Ih) was used to identify DAergic neurons. Both groups of neurons produced large (approximately 1 nA), sustained inward currents that persisted as 'plateau currents' for hundreds of milliseconds after the termination of the depolarizing voltage clamp step. We conclude that intact DAergic neurons can be identified under conditions optimal for recording calcium currents (i.e. Cs+ internal solution), but powerful dendritic currents limit rigorous biophysical analysis.