Compartmental organization of the olfactory bulb glomerulus

Olfactory receptor cell (ORC) axons terminate in the olfactory bulb glomeru lar neuropil, where they synapse with dendrites of mitral, tufted, and peri glomerular neurons. We investigated the organization of the glomerular neur opil 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. Adu lt Sprague-Dawley rats were processed for immunocytochemistry with olfactor y 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 r evealed two distinctive subcompartments within glomeruli: an axonal compart ment containing predominately primary afferent axons with individual dendri tic inserts and a complementary dendritic compartment that excluded primary afferent axons. Areas not occupied by OMP or MAP2 immunoreactivity were ei ther immunoreactive for GFAP, indicating a glial process, or were blood ves sels. Synaptophysin and synapsin 1 also showed differential labeling within the glomerulus. Synaptophysin strongly colocalized with OMP, whereas synap sin 1 was associated most strongly with MAP2. Reconstructions of glomeruli from EM montages revealed interdigitating axonal and dendritic subcompartme nts. The axonal subcompartments were composed primarily of ORC processes wi th individual or small groups of dendrites interspersed. Dendritic subcompa rtments were composed predominately of dendritic processes. Primary afferen t 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 withi n olfactory bulb glomeruli. J. Comp. Neurol. 407:261-274, 1999. (C) 1999 Wi ley-Liss, Inc.