Abstract
Three-dimensional (3D) reconstruction of synaptic arrangement on a particular dendrite provides essential information regarding neuronal properties and neural microcircuits. Unconventional synapses are particularly good candidates for such steric attribution. In main and accessory olfactory bulbs (MOBs and AOBs), there are dendrodendritic reciprocal synapses (RSs) between excitatory projection neurons and inhibitory interneurons. Although the fine structure and configuration of these synapses have been investigated in MOB, their characteristics in AOB were unknown. In this study, we performed 3D AOB reconstruction using serial section transmission electron microscopy. We found numerous RSs on primary dendrites from glomeruli to mitral/tufted (MT) cell somas. These synapses formed between dendritic shafts of MT cells and large dendritic spines, or so-called gemmules, of granule (Gr) cells. This indicates that chemical signals received by a glomerulus are regulated in the primary dendrite of an MT cell before reaching its soma. In MOB, RSs are located on secondary dendrites and act as lateral and self-inhibiting following mitral cell depolarization. Our results indicate that AOB intrabulbar microcircuitry is quite different from that in the MOB.
Highlights
Dendrodendritic reciprocal synapses (RSs) are unusual synaptic structures in the glomerular layer (GL) and external plexiform layer (EPL) that are constructed between excitatory projection neurons and inhibitory interneurons in olfactory bulb (Shepherd and Greer, 2004)
RSs in the EPL are primarily distributed on secondary dendrites of mitral cells, and glutamate released from the mitral cell dendritic shafts to Gr cell dendritic spines is followed by γ-aminobutyric acid (GABA) release back onto mitral cells or lateral inhibition of other mitral cells from Gr cells (Mori and Takagi, 1978; Shepherd and Greer, 2004)
MT cells have the largest somas in the accessory olfactory bulb (AOB), but these are smaller than the somas of typical main olfactory bulb (MOB) mitral cells
Summary
Dendrodendritic reciprocal synapses (RSs) are unusual synaptic structures in the glomerular layer (GL) and external plexiform layer (EPL) that are constructed between excitatory projection neurons and inhibitory interneurons in olfactory bulb (Shepherd and Greer, 2004). In the main olfactory bulb (MOB), mitral cells have a single thick, primary dendrite that receives input from olfactory sensory neurons and forms glomeruli, with some long secondary dendrites that extend laterally and terminate in the EPL (Shepherd and Greer, 2004). The accessory olfactory bulb (AOB) has a laminar structure that is similar to the MOB, but recent studies have indicated real differences in cytoarchitecture, glomerular formation, and physiological and morphological properties of projection neurons between the 2 bulbs (Urban and Castro, 2005; Dulac and Wagner, 2006; Larriva-Sahd, 2008; Yonekura and Yokoi, 2008; Yokosuka, 2012). Our result indicates that intrabulbar microcircuits are quite different between the MOB and AOB
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