Fig. 1.

Substantia nigra pars reticulata neurons may be discriminated from dopaminergic neurons by their basic electrophysiological properties. A, Records of membrane potential from an SNr neuron (top) and a putative dopamine neuron (bottom), recorded at resting potential. Both cells fire action potentials spontaneously (full amplitude not reproduced) at regular rates but at markedly different frequencies (SNr neuron, 25 Hz; dopamine neuron, 1.75 Hz). B, Voltage steps (200 msec) to potentials in the range −40 to −140 mV performed under voltage clamp (holding potential −60 mV) demonstrate pronounced time-dependent inward rectification on hyperpolarization (I_h) in the dopaminergic neuron (bottom records), which is virtually absent in the SNr neuron (top records). The top pair of records shows the series of voltage steps applied to the same SNr neuron as in A and the resultant membrane currents, whereas the bottom pair shows the voltage steps and currents produced in the dopamine neuron in A.