Evidence for coexistence of GABA and dopamine in neurons of the rat olfactory bulb

Immunoreactivities for gamma-aminobutyric acid (GABA) and the dopamine-synthesizing enzyme tyrosine hydroxylase (TH) were localized ultrastructurally and colocalized at the light microscopic level in neurons of the rat main olfactory bulb. By means of a simultaneous indirect immunofluorescence technique, GABA and TH immunoreactivities were found to coexist in a large number of neurons in the glomerular and external plexiform layers. Virtually all the TH-immunoreactive periglomerular neurons also contained GABA immunoreactivity (GABA-I) while there was an additional number of GABA-immunoreactive periglomerular cells (27%) which did not contain TH immunoreactivity (TH-I). In contrast, the numerous tufted-type neurons in the glomerular and superficial external plexiform layers which contained TH-I did not contain GABA-I. In the external plexiform layer (EPL), 41% of the immunoreactive neurons contained GABA-I alone, 24% contained TH-I alone, and 35% contained both. EPL neurons containing GABA-I only or both GABA-I and TH-I never exhibited tufted cell morphological characteristics and were generally of the short-axon type. Electron microscopic examination of GABA-I and TH-I elements in the glomerular layer detected morphologically similar periglomerular perikarya and intraglomerular processes immunoreactive for each substance and other neurons and processes of the same type containing neither GABA-I or TH-I. These data indicate that the classical neurotransmitters GABA and dopamine coexist in large numbers of neurons in the rat main olfactory bulb including characteristic periglomerular cells and certain other local-circuit neuronal types.