Journal of Neuroscience, Vol 11, 3752-3762, Copyright © 1991 by Society for Neuroscience
Evaluation of projection patterns in the primary olfactory system of rainbow trout
DR Riddle and B Oakley
Neuroscience Program, University of Michigan, Ann Arbor 48109.
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.