Figure 7.

Identification of principal dopaminergic and GABAergic neurons in substantia nigra from current-clamp recordings. A1, A2, Dopaminergic neurons exhibit slow pacemaker-like firing with broad action potentials exceeding 2.5 msec in duration and a large-amplitude, long-duration spike afterhyperpolarization. A3, Dopaminergic neurons exhibit a prominent time-dependent sag in the voltage deflection in response to hyperpolarizing current injection because of a slowly activating I_h. A4, I-V plots both before (filled circles) and during (open circles) activation of I_h, calculated at the times indicated by the filled and open arrows in A3 above. The I-V relationship was best fit by linear regression before activation of I_h but showed a pronounced inward rectification after its activation, which was best fit by a fourth-power polynomial. B1, B2, In contrast to dopaminergic neurons, pars reticulata principal GABAergic neurons fire spontaneously at higher rates, have a shorter-duration action potential of ∼1 msec in duration, and do not exhibit a prominent large-amplitude spike afterhyperpolarization. B3, B4, Principal GABAergic neurons lack I_h, and the I-V plot, calculated at the time indicated by the arrow, is best fit by linear regression (B4). In both cases, the hyperpolarizing current was applied from rest. Traces in A3 and B3 are the digital averages of four consecutive single sweeps.