ELECTROPHYSIOLOGICAL LOCALIZATION OF DISTINCT CALCIUM POTENTIALS AT SELECTIVE SOMATODENDRITIC SITES IN THE SUBSTANTIA-NIGRA

The dendrites of dopaminergic neurons in the substantia nigra play a pivotal role in the neurochemical homeostasis of the nucleus.5,17 It is conceivable therefore that the cell body and dendrites of these nigral neurons possess distinct and independent electroresponsive features. By means of differential polarization through applied electric fields, the cell body and dendrites have been activated in effective isolation during intracellular recordings from pars compacta neurons in the substantia nigra in vitro. In one class of neurons, which discharge in a "phasic" fashion and are located in the rostral substantia nigra, the dendrites are shown to be the origin of classic low-threshold and high-threshold type calcium potentials:9,8 indeed the high-threshold conductance appears to be exclusively dendritic. By contrast, in a second, more caudally located cell type, which discharges rhythmically, a high-threshold calcium spike is located principally in the cell body. The differential localization of these calcium conductances in sub-populations of neurons is likely to determine the functions for the calcium responses in each type of neuron, and moreover highlight the dendrites as dynamic and selective components in the physiology of the substantia nigra. The presence, for example, of the high-threshold calcium conductance in the dendrites of only one class of neuron suggests that this sub-population plays a prominent role in non-classical phenomena of dendritic release of a variety of chemical mediators.