Electrophysiological properties of identified output neurons of the rat substantia nigra (pars compacta and pars reticulata): evidences for the existence of branched neurons.

In Ketamine-anaesthetized rats the nigral efferents to the ipsilateral striatum, to both VL/VM thalamic nuclei and to both superior colliculi were studied. Nigral output neurons were antidromically activated from these target nuclei and characterized by their spontaneous activity and cellular localization within the substantia nigra. Neurons in the pars compacta, which give rise to the dopaminergic nigro-striatal pathway, exhibited a low spontaneous discharg rate (3 to 6/sec) and their axon had a slow conduction velocity (0.33 to 1 m/sec). They were antidromically activated from the striatum only. Neurons in the pars reticulata were characterized by a higher spontaneous activity (20 to 40/sec) anda faster conduction velocity (1.9 m/sec to 10 m/sec). They were antidromically activated from the thalamus, superior colliculus and striatum. Furthermore, some of these neurons were found to have branching axons projecting at least to two of the different target nuclei studied.