ELECTROPHYSIOLOGICAL AND IMMUNOCYTOCHEMICAL CHARACTERIZATION OF GABA AND DOPAMINE NEURONS IN THE SUBSTANTIA-NIGRA OF THE RAT

Neurons in the substantia nigra pars reticulata and pars compacta of t he rat were studied using a combination of intracellular electrophysio logical recording in in vitro and subsequent immunocytochemical double and triple labelling techniques. The neurons recorded in the pars ret iculata were identified as either GABA or dopamine neurons: neurons we re considered to be GABA neurons if they were immunopositive for gluta mate decarboxylase, whereas those neurons which were immunopositive fo r tyrosine hydroxylase were considered to be dopaminergic. The GABA ne urons had short duration action potentials (0.45+/-0.03 ms halfwidth), no appal ent rectifying currents, no low threshold calcium spikes, we re spontaneously active (7.4+/-3.7 Hz), and could maintain high firing rates. The dopamine neurons had long duration action potentials (1.49 +/-0.10 ms), displayed both anomalous inward and transient outward rec tifying currents, and more than half (12/17 neurons) displayed a low t hreshold calcium spike. Their spontaneous firing rate was lower than t hat of the GABA neurons (2.3+/-1.0 Hz), and they displayed strong freq uency adaptation. Morphological reconstruction of neurobiotin-filled n eurons revealed that the pars reticulata GABA neurons had more extensi ve local dendritic arborization than the dopamine neurons from either the pars reticulata or the pars compacta. All of the neurons recorded from the pars compacta were dopamine neurons; they were found not to b e different either electrophysiologically or morphologically from pars reticulata dopamine neurons. The electrophysiology of the GABA neuron s suggests that input activity is translated linearly to spike frequen cy. These GABA neurons probably represent the projection neurons of th e pars reticulata, and it is thus likely that this basal ganglia outpu t is frequency coded. The close similarity between the dopamine neuron s in the pars compacta, which give rise to the nigrostriatal pathway, and those in the pars reticulata supports the notion that the dopamine neurons in these two regions are part of the same neuronal population . (C) 1997 IBRO. Published by Elsevier Science Ltd.