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

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 cells that project to pE are predominantly in the deep part of the glomerular layer; most of the external tufted cells that He 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.