A possible pacemaker mechanism in pars compacta neurons of the guinea-pig substantia nigra revealed by various ion channel blocking agents.

The membrane properties of pars compacta neurons in the in vitro guinea-pig substantia nigra have been studied in the presence of sodium, calcium and potassium channel blockers. The following properties, which have already been described for dopamine-containing substantia nigra zona compacta neurons were observed: high and low threshold calcium spikes; a calcium-activated potassium-mediated transient; inward rectification. Inward rectification was sensitive to caesium ions. An additional property was seen reminiscent of an "A" current, although resistant to 4-aminopyridine. It is suggested that this outward transient is in fact a calcium activated potassium conductance. Under certain conditions calcium-mediated rhythmic depolarizations were observed. It is suggested that at least two of the properties seen (outward rectification and low threshold calcium spike) could interact to provide the basis for a pacemaker mechanism in pars compacta neurons.