Ultrastructural Contributions to Extrasynaptic Glutamatergic Signaling in Olfactory Bulb Glomeruli

Olfactory bulb glomeruli have a complex organization that includes axodendritic synapses between olfactory sensory neurons (OSNs) and excitatory mitral cells (MCs) and tufted cells (TCs), as well as dendrodendritic synapses between MCs/TCs and GABAergic periglomerular cells (PGCs). MCs also receive excitatory signals from one subclass of TCs, the external tufted cells (eTCs). While these signals are driven by glutamate released from eTC dendrites, they appear not to reflect direct eTC > MC synaptic connections but rather "spill-over" of glutamate released at eTC > PGC synapses acting on nearby MC dendrites. Here, we used serial section electron microscopy images of rat olfactory bulb glomeruli with biocytin-labeled MC and eTC dendrites to evaluate potential ultrastructural underpinnings of "extrasynaptic" signaling. We compared the environment around eTC > PGC synapses with that of MC > PGC synapses using several quantitative measures and, as a further point of comparison, also evaluated axodendritic OSN > MC and OSN > eTC synapses. Across the four synapse types, one unique feature of eTC > PGC synapses was their much closer distance to the nearest dendrites of excitatory cells (including MCs), averaging ∼160 nm. In contrast, astroglial processes were positioned quite far away from eTC > PGC synapses, with a mean distance of ∼500 nm. These distance values would suggest that glutamate released from eTC > PGC synapses may access nearby excitatory dendrites without interference from glial glutamate transporters, thus providing an ultrastructural basis for extrasynaptic signaling. Our battery of ultrastructural measures, which included brick analyses, also supported a long-standing model for the organization of glomeruli in which OSN and dendrodendritic synapses are segregated into discrete compartments.