N-ACETYL-LACTOSAMINE IN THE RAT OLFACTORY SYSTEM - EXPRESSION AND POTENTIAL ROLE IN NEURITE GROWTH

Primary sensory olfactory neurons exhibit a mosaic topographical proje ction from the olfactory neuroepithelium in the nasal cavity to the ol factory bulb formation of the telencephalon. Axons from primary neuron s that are widely scattered in the epithelium terminate in discrete re gions of the olfactory bulb. It has been hypothesised that carbohydrat es present on the surface of primary olfactory axons mediate selective fasciculation and the formation of the topographical pathway. We exam ined the expression of the disaccharide N-acetyl-lactosamine in both t he developing and the adult rat olfactory system. N-acetyl-lactosamine was expressed by all primary sensory olfactory neurons and by their t erminations in the olfactory bulb throughout embryonic development and early postnatal life. In adults, N-acetyl-lactosamine was restricted to a subpopulation of primary sensory olfactory neurons that were disp ersed throughout the neuroepithelium but that projected predominantly to the ventrolateral and ventromedial surfaces of the olfactory bulb. The axons of these neurons sort out in the outer layer of the bulb and preferentially self-fasciculate to form distinct axon bundles that te rminate within select glomeruli. The role of N-acetyl-lactosamine in a xon growth was tested by culturing primary sensory olfactory neurons o n substrate-bound carbohydrates. Olfactory neuroepithelium cultures fr om both embryonic and postnatal rats revealed that substrate-bound N-a cetyl-lactosamine was a strong and specific neurite growth-promoting a gent. These data suggest that, during development of the olfactory pro jection, N-acetyl-lactosamine, which is present on all olfactory axons , acts as a nonselective permissive substrate for axon growth. In adul ts, however, the restricted distribution of N-acetyl-lactosamine on a subpopulation of axons may facilitate sorting out and self-fasciculati on, which is necessary for preserving the mosaic nature of the olfacto ry pathway in this highly plastic region of the nervous system. These results support the hypothesis that cell surface carbohydrates are inv olved in axon growth in the olfactory system. (C) 1996 Wiley-Liss, Inc .