Identification and electrophysiology of isolated pars compacta neurons from guinea-pig substantia nigra

Single neurons from the substantia nigra of the adult guinea-pig were dissociated. Two morphological types of neurons were identified: (i) large multipolar neurons (20-40 microns greatest diameter) and (ii) small fusiform neurons (less than 20 microns diameter). Neurons of both types showed catecholamine fluorescence and were retrogradely labelled by rhodamine-conjugated microspheres injected into the striatum. Two types of neuron (Type A and Type B) were also identified electrophysiologically in perforated patch recordings of membrane potential and whole-cell current. Type A neurons had a low spontaneous firing frequency (5 Hz) broad action potentials (4 ms) and a threshold of approximately -45 mV. They were inhibited by the dopamine D2 receptor agonist, quinpirole. Type B neurons were characterized by a faster firing frequency (25 Hz), shorter action potential duration (1.5 ms) and a threshold of approximately -50 mV. These cells were unaffected by quinpirole. Large multipolar neurons were usually found to be of the A type and small fusiform, neurons of the B type. The properties of the large multipolar, Type A neurons suggest that they form part of the nigrostriatal, dopaminergic population of the pars compacta.