Light and electron microscopic localization of glutamic acid decarboxylase and substance P in the dorsal column nuclei of the cat.

Using immunocytochemical methods, glutamic acid decarboxylase (GAD)-immunoreactive boutons were demonstrated throughout the dorsal column nuclei of the cat brain. They were small and were generally presynaptic to larger unlabelled axon terminals, the latter probably originating from primary afferent fibres. The middle-ventral 'reticular' region of the cuneate nucleus also contained substance P-positive terminals, which were large and synapsed on dendritic profiles. The findings strongly indicate that gamma-aminobutyric acid (GABA) is the neurotransmitter that is responsible for the presynaptic inhibition in the dorsal column nuclei. The substance P-positive terminals probably originate from extrinsic fibers.