Topographic organization of connections between the main olfactory bulb and pars externa of the anterior olfactory nucleus in the hamster

Abstract

The organization of connections between the main olfactory bulb (MOB) and pars externa (pE) of the anterior olfactory nucleus was studied in hamsters by using wheat germ agglutinin-horseradish peroxidase as both an anterograde and a retrograde neuronal tracer. Bulbar efferents of pE project exclusively to the contralateral MOB. A topographic organization is evident in these efferents, such that distinct sectors of pE project predominantly to certain sectors of the contralateral MOB and lightly to other sectors. The predominant projection to any bulbar sector in the coronal plane is repeated at nearly all rostral-caudal levels, i.e., the pE efferents to the contralateral MOB terminate within long strips or wedges that show a sector-to-sector topographic organization with respect to the medial-lateral and dorsal-ventral axes but not the rostral-caudal axis of the MOB. Afferents to pE arising in the ipsilateral MOB also show a sector-to-sector topographic organization. Injections into restricted sectors along the circumference of pE label all classes of output neurons (mitral cells and internal, middle, and external tufted cells) in restricted sectors of the ipsilateral MOB, and the sectors that have retrograde neuronal labeling are homotopic to those in the contralateral MOB that have dense anterograde terminal labeling. External tufted cells are not labeled and the other classes of MOB output neurons do not have prominent topographic patterns of labeling in cases with injections caudal to pE. The somata of external tufted cell that project to pE are predominantly in the deep part of the glomerular layer; most of the external tufted cells that lie more superficially in the glomerular layer do not appear to have projections extrinsic to the MOB. These results indicate that both the afferent and efferent connections of pE with the MOB are topographically organized and provide a short synaptic pathway between homotopic sectors of the two main olfactory bulbs.

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).…”

“…In one candidate pathway (Fig. 5A), contralateral AONpE neurons send topographically organized commissural projections to the ipsilateral OB (17,18,20,21) and form excitatory synaptic inputs on granule cells ("G" in Fig. 5A) that inhibit mitral and tufted cells (22).…”

Neonatal and Adult Neurogenesis Provide Two Distinct Populations of Newborn Neurons to the Mouse Olfactory Bulb

“…In contrast, superficial GCs might provide the means for lateral inhibition of tufted cells. Furthermore, because tufted and mitral cells show distinct patterns of axonal projections to the olfactory cortex (Skeen and Hall, 1977;Scott et al, 1980;Schoenfeld and Macrides, 1984), it is possible that GC subtypes that contact either mitral or tufted cells handle different aspects of odorant information. Given that tufted cells are adapted to detect low concentrations of odorants (Nagayama et al, 2004), we propose that early born GCs might preferentially regulate this function, whereas late born interneurons might rather control odorant identity by participating in the odor-specific temporal shaping of mitral cell spike responses.…”

Compensatory Rapid Switching of Binasal Inputs in the Olfactory Cortex

“…AON neurons project their axons as association fibers to the ipsi-OC, and to the contralateral (contra)-AON and contra-OC via the anterior commissure (AC) ( Fig. 1) (Schoenfeld and Macrides, 1984;Brunjes et al, 2005). It appears that the AON receives axonal inputs via the AC from the contra-AON or contra-OC (Haberly and Price, 1978), and thus receives sensory signals from both the ipsi-and contra-OEs (Wilson, 1997;Lei et al, 2006).…”