Synapses between GABA-immunoreactive axonal and dendritic elements in monkey substantia nigra.

gamma-Aminobutyric acid (GABA)-positive elements in the monkey substantia nigra pars reticulata (SNr) were identified using the peroxidase-antiperoxidase immunocytochemical technique with a direct GABA antibody. Light-microscopic examination revealed fine-caliber immunostained fibers piercing the cerebral peduncle and entering the SNr, where they formed a dense plexus. Ultrastructural observations included boutons with densely packed ovoid synaptic vesicles forming synapses with labeled and unlabeled dendritic elements. Most terminals contained small dark mitochondria, although some endings held larger, paler and fewer of these organelles. The synapses observed between GABA-immunoreactive profiles may represent the morphologic basis for disinhibition of pars compacta neurons reported to follow neostriatal or pallidal stimulation.