Pattern of rat olfactory bulb mitral and tufted cell connections to the anterior olfactory nucleus pars externa

Abstract

The pattern of output of mitral and tufted cells of the rat olfactory bulb (OB) to layer Ia overlying the pars externa (pE) of the anterior olfactory nucleus (AON) has been studied in the rat by iontophoresis of horseradish peroxidase and Phaseolus vulgaris-leucoagglutinin. These agents labeled mitral and tufted cells and at least the proximal portion of their axons. In most cases we observed small branches from axons of the lateral olfactory tract that appear to terminate in the region of the pE AON, while the main axon could often be traced for considerable distances past these branches. These branches are assumed to terminate in the pE AON because they could not be traced to other terminal regions, because they ramify in layer Ia, and because they usually show small swellings characteristic of axons in terminal regions. Although each ramification could be extensive, we found that the positions of these small branches were related to the positions of the injections within the OB. Dorsal medial injections labeled dorsal branches. Ventral medial injections labeled ventral branches. Injections on the lateral face of the OB labeled intermediate branches. The centers of the regions within which branches were labeled were strongly correlated with the positions of the injection around the circumference. Comparison of the anterior-posterior axis of the OB produced no such strong correlation. Reconstructions of axons showed that terminal branches arise from both mitral and tufted cells, although at least some mitral cells are shown not to have such branches in the pE AON. Studies of the patterns of dendrites and terminals in the pE AON indicate that this region has the same pattern of layer Ia and Ib terminals seen in other olfactory cortical regions. The pE AON cell layer is intercalated just below the boundary between layers Ia and Ib. Since dendrites of the underlying pars lateralis of the AON (pL AON) penetrate into layer Ia over much of the pE AON, it is necessary to remember that at least part of the pL AON may also receive topographically organized inputs.

Neurons in the anterior olfactory nucleus pars externa detect right or left localization of odor sources

“…In accordance with this hypothesis, Johnson et al demonstrated that sulfide-category odorants activate glomeruli at the ventro-caudal region in the ventral zone of the OB (19), and mitral/tufted cells in these regions project axons to the ventrocaudal region of the AONpE (11,18). sponses of ipsilateral AONpE neurons to contranostril inputs.…”

“…AONpE receives topographical axonal projections from the ipsilateral OB (11,(16)(17)(18), raising the possibility that the odorant category tuning of ipsinostril input to individual AONpE neurons is a result of the topography of afferent projection from the ipsilateral OB. In accordance with this hypothesis, Johnson et al demonstrated that sulfide-category odorants activate glomeruli at the ventro-caudal region in the ventral zone of the OB (19), and mitral/tufted cells in these regions project axons to the ventrocaudal region of the AONpE (11,18).…”

“…The anterior olfactory nucleus (AON) is the most rostral region of the olfactory cortex and receives excitatory axonal inputs from the ipsilateral OB (8)(9)(10)(11). In addition, AON neurons receive inputs from the contralateral olfactory cortex via the anterior commissure (12,13), suggesting that individual AON neurons receive odor information originating from both ipsilateral and contralateral OEs.…”

Functional imaging of cortical feedback projections to the olfactory bulb

“…The AON receives sensory input from the OB and sends “ascending” outputs to other olfactory and non-olfactory areas including anterior piriform cortex, olfactory tubercle, entorhinal cortex and periamygdaloid cortex (for review, see Brunjes et al, 2005). Features of the connections between the AON and the OB include a coarse topography in the centripetal projections from the OB to the AON (Schoenfeld et al, 1985; Scott et al, 1985; Yan et al, 2008; Miyamichi et al, 2011) as well as descending projections that innervate not only the ipsilateral but also the contralateral OB (Schoenfeld and Macrides, 1984; Shipley and Adamek, 1984; Kay and Brunjes, 2014). In addition, laminar differences in the distribution of AON projections to OB have been observed for both zones (Davis and Macrides, 1981; Luskin and Price, 1983).…”

Membrane and synaptic properties of pyramidal neurons in the anterior olfactory nucleus

“…Tufted cells in the olfactory bulb project selectively to the AON as well as the neighboring ventrorostral anterior piriform cortex (APC V-R ) (Matsutani et al 1989). Tufted cells show enhanced excitation relative to mitral cells, which project to the entire olfactory cortex (Schneider and Scott 1983;Orona et al 1984;Scott et al 1985;Christie et al 2001;Nagayama et al 2004). AON projections to the APC terminate directly adjacent to the cell bodies of pyramidal neurons (Haberly and Price 1978;Luskin and Price 1983a).…”