Compartmental organization of the olfactory bulb glomerulus

Olfactory receptor cell (ORC) axons terminate in the olfactory bulb glomerular neuropil, where they synapse with dendrites of mitral, tufted, and periglomerular neurons. We investigated the organization of the glomerular neuropil by using antibodies to both single- and double-label constituents for analyses with confocal microscopy. Electron microscopy (EM) was employed to assess the distribution of synaptic appositions within the glomerulus. Adult Sprague-Dawley rats were processed for immunocytochemistry with olfactory marker protein (OMP), synaptophysin, synapsin 1, glial fibrillary acidic protein (GFAP), and/or microtubule-associated protein 2 (MAP2). Equivalent rats were processed for transmission EM. Double labeling for OMP and MAP2 revealed two distinctive subcompartments within glomeruli: an axonal compartment containing predominately primary afferent axons with individual dendritic inserts and a complementary dendritic compartment that excluded primary afferent axons. Areas not occupied by OMP or MAP2 immunoreactivity were either immunoreactive for GFAP, indicating a glial process, or were blood vessels. Synaptophysin and synapsin 1 also showed differential labeling within the glomerulus. Synaptophysin strongly colocalized with OMP, whereas synapsin 1 was associated most strongly with MAP2. Reconstructions of glomeruli from EM montages revealed interdigitating axonal and dendritic subcompartments. The axonal subcompartments were composed primarily of ORC processes with individual or small groups of dendrites interspersed. Dendritic subcompartments were composed predominately of dendritic processes. Primary afferent axodendritic and local-circuit dendrodendritic synapses segregated within the glomerulus into the axonal and dendritic subcompartments, respectively. The results support the hypothesis of subcompartmental organization within olfactory bulb glomeruli.