Responses to ramp current stimulation of the neurons in substantia nigra pars compacta in vitro

The response to ramp current stimulation was studied in the pars compacta neurons in guinea pig substantia nigra slices. Although accommodation was not seen with current duration up to 1000 ms, a threshold peak emerged on the threshold-latency curve at 30-400 ms when the cell was hyperpolarized. The appearance of the peak was closely correlated with an inflection in the potential trajectory before the spike. A voltage-dependent, fast inactivating outward current may underlie these responses. They persisted in Ca2+-free solution, but disappeared when Cd2+ (0.4 mM) or Co2+ (5 mM) ions were applied extracellularly.