Evaluation of projection patterns in the primary olfactory system of rainbow trout.

Neuroscience Program, University of Michigan, Ann Arbor 48109.
Journal of Neuroscience (Impact Factor: 6.75). 01/1992; 11(12):3752-62.
Source: PubMed

ABSTRACT Topographic projections are important for coding sensory information in the visual, auditory, and somatosensory systems but are of uncertain importance in the coding of olfactory information. We searched for topographic projections between olfactory receptor cells and the olfactory bulb of the rainbow trout Oncorhynchus mykiss. Anterograde axonal tracing with HRP revealed that the olfactory axons arising from discrete regions of the olfactory epithelium travel together within the olfactory nerve. The abrupt resorting and redistribution of these axons at the interface between the olfactory nerve and olfactory bulb imply that local cues control and organize axonal projections. The sites of termination of HRP-labeled axons in the glomerular layer could not be predicted from the location of their cell bodies in the periphery. Retrograde tracing with fluorescently labeled latex beads, injected into glomerular subregions as small as 1% of the total glomerular volume, labeled receptor cells dispersed throughout the olfactory epithelium. The distributions of labeled receptor cells were uncorrelated with the bulbar injection sites. Double-labeling experiments revealed that even widely separated sites in the glomerular layer receive axons from comingled populations of receptor cells. Hence, the evidence indicates that the spatial arrangement of olfactory receptor cells in the epithelium is not preserved in the termination of their axons in the olfactory bulb. We conclude that the primary olfactory in trout lacks point-to-point or regionally topographic organization and that the entire extent of the olfactory epithelium contributes axons to each region of the glomerular layer.

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    ABSTRACT: We have used a combination of techniques to analyze the primary olfactory projection in trout: anterograde tract tracing with horseradish peroxidase (HRP) and immunocytochemistry with antisera to olfactory marker protein (OMP) and to keyhole limpet hemocyanin (KLH). HRP labeling and the OMP antiserum revealed a subset of ciliated receptor neurons with a wide dendrite that lacked the protruding knob found on other receptor neurons. The organization of the primary olfactory axons was clearly revealed by antisera to KLH, which reacted with no other neurons. When visualized with anti-KLH, fascicles of olfactory axons penetrated the basal lamina of the olfactory rosette at scattered sites and converged to form the olfactory nerve. Fascicles within the olfactory nerve traveled parallel to the long axis of the nerve until resorted by extensive intermixing as they entered the olfactory bulb. Within the olfactory bulb, most axons terminated in nine discrete terminal fields in the glomerular layer; however, a few olfactory nerve axons projected into the ventral medial telencephalon. Fascicles supplying each terminal field in the glomerular layer followed distinctive trajectories within the olfactory nerve layer. Axons ending in two terminal fields made brush-like terminations rather than the glomerular terminations characteristic of the remaining seven fields. After unilateral olfactory nerve transection, returning olfactory axons reestablished the normal pattern of terminal fields within 14 weeks. It is likely that the organization of afferents in the trout olfactory bulb is similarly well regulated during normal receptor cell replacement.
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