Immunocytochemical localization of the GABAB2 receptor subunit in the glomeruli of the mouse main olfactory bulb.

The olfactory input to the brain is carried out by olfactory nerve axons that terminate in the olfactory bulb glomeruli and make synapses onto dendrites of glutamatergic projection neurons, mitral and tufted cells, and GABAergic interneurons, periglomerular cells. The dendrites are reciprocally connected through asymmetric synapses of mitral/tufted cells with periglomerular cells and symmetric synapses of the opposite direction. Transmission at the first synapse in the olfactory pathway is regulated presynaptically, and this regulation is mediated, in part, by metabotropic GABA B receptors that, when activated, inhibit transmitter release from the olfactory nerve. Functional GABA B receptors are heterodimers composed of the GABA B1 and GABA B2 subunits. Studies using double immunofluorescence have shown colocalization of both subunits in the glomerular neuropil, and ultrastructural studies have localized GABA B1 to extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of olfactory nerve terminals. We studied the subcellular localization of GABA B2 in the mouse olfactory glomeruli using a subunit-specific antibody and preembedding immunogold labeling. Immunoreactivity for GABA B2 was associated with symmetric dendrodendritic synapses of periglomerular cells with mitral/tufted cells and was localized to the extrasynaptic plasma membrane of presynaptic dendrites, and extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of postsynaptic dendrites. The results suggest that postsynaptic, and perhaps presynaptic, GABA B receptors may be expressed at GABAergic synapses between dendrites of periglomerular interneurons and projection neurons. Immunolabeling was observed at junctions of the olfactory nerve with mitral/tufted cell dendrites, providing ultrastructural evidence for the expression of the GABA B2 subunit at the primary olfactory synapse.