Characterization of pallidonigral neurons in the rat by a combination of Golgi impregnation and retrograde transport of horseradish peroxidase: their monosynaptic input from the neostriatum.

After injection of horseradish peroxidase, or its conjugate with wheatgerm agglutinin, into the substantia nigra of rats, retrogradely labelled cells were found in the globus pallidus. Forty-six of these neurons were also impregnated by the Golgi procedure and then gold-toned: their somata ranged from 15 to 30 microns in diameter and these pallidonigral neurons had from two to five primary dendrites that were long and smooth, that branched infrequently and that bore occasional spines on their distal regions. Most of the neurons studied came from the lateral part of the globus pallidus. At the ultrastructural level, the identified pallidonigral neurons were found to have deeply infolded nuclei and an abundant cytoplasm; their perikarya were richly innervated by two distinct types of bouton, both of which formed symmetrical synaptic contacts. The dendrites of pallidonigral neurons were ensheathed in boutons, the majority forming symmetrical synaptic contacts. After placement of electrolytic lesions in the rostro-dorsal neostriatum, degenerating boutons were found in symmetrical synaptic contact with the cell bodies and dendrites of six identified pallidonigral neurons. It is concluded that pallidonigral neurons belong to the Golgi category of large pallidal neurons with smooth dendrites and that they receive monosynaptic input from the neostriatum. Thus, in addition to the direct striatonigral pathway, the neostriatum can influence the substantia nigra by a monosynaptic relay through the globus pallidus, which might allow other pallidal afferents to influence the transfer of information from neostriatum to substantia nigra.