Immunocytochemical localization of glutamate and gamma-aminobutyric acid in the accessory olfactory bulb of the rat

The synaptic organization of the accessory olfactory bulb (AOB) was studied in the rat with antibodies against the excitatory neurotransmitter glutama te (Glu) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) . To a large extent, the immunoreactivity patterns produced by the two anti bodies were complementary. Glu-like immunoreactivity (-LI) was observed in the glomerular neuropil, in the mitral cells, and in large neurons located in the periglomerular region. Immunogold electron microscopy revealed parti cularly high levels of Glu-LI in the axon terminals of vomeronasal neurons. GABA-LI was present in granule and periglomerular cells and in their proce sses. The dendritic spines of granule cells, which were presynaptic to mitr al cells, were strongly labelled by the antiserum against GABA. Labelling o f serial semithin sections showed that the GABA-positive and Glu-positive n eurons of the periglomerular region are generally distinct, and colocalizat ion of Glu and GABA occurred only in a few cells. These results are consist ent with electrophysiological studies indicating that the synaptic organiza tion of the AOB is similar to that of the main olfactory bulb. In both syst ems, Glu is the neurotransmitter used by primary afferents and output neuro ns, whereas GABA is involved in the circuits underlying lateral and feed-ba ck inhibition. (C) 1999 Wiley-Liss, Inc.