Firing patterns in substantia nigra compacta identified neurons in vitro

Dopamine-containing neurons of the midbrain are required to control voluntary movements, behavior and motivation. Damage of these neurons in the substantia nigra zona compacta (SNc) is considered as the cause of Parkinson's disease. In this work, the firing characteristics of SNc neurons were studied in vitro. Neurons were identified by intracellular labeling tyrosine hydroxylase (TH) immunocytochemistry, and their electrophysiological characteristics. TH-positive neurons displayed three different firing modes. At around -55 mV neurons exhibited a regular single spike firing mode (pacemaking). At around -65 mV, neurons discharged in an irregular single spike firing mode (idling). At around -75 mV, many neurons exhibited a mode of discharge characterized by low threshold spikes (LTS), oscillations and, on some occasions, bursting. When present, bursts were of short duration and commonly made up of incomplete somatodendritic action potentials. TH-negative neurons also exhibited single spikes and bursting modes of firing. Both types of SNc neurons generated LTS oscillations following a hyperpolarization. However, LTS in TH-negative neurons could be evoked from less negative membrane potentials and produced more prolonged bursts. LTS from both types of neurons could be differentiated. LTS were blocked by 100 microM Ni2+ and became prolonged in the presence of TEA.