The adult rat olfactory system expresses microtubule-associated proteins found in the developing brain.

We have compared the expression and localization of neuronal microtubule-associated proteins (MAPs) MAP5, MAP2, and tau in the adult rat olfactory system and cerebral cortex. Each of these MAPS is known to exist as distinct "early" and "late" forms in the developing and adult brain, respectively. Because axonal growth and dendritic reinnervation continue in the adult olfactory system, it can serve as a test of whether expression of early MAP forms is necessary for neuronal growth and plasticity. We found that for all three MAPs, the early forms continue to be expressed in the adult olfactory system, whereas the cerebral cortex switches to the late forms during neuronal maturation (between 10 and 20 days of age in the rat). For MAP2, the 6 kb mRNA and low-molecular weight MAP2c, both of which are typically found in embryonic tissue, persist in the adult olfactory bulb. For MAP5, an early highly phosphorylated form, MAP5a, is present throughout the brain and disappears during maturation simultaneously with a several-fold drop in the overall level of MAP5. However, in the olfactory bulb, MAP5 levels do not fall, and MAP5a persists in the adult. The early form of tau is also prominent in the adult olfactory bulb and, by immunohistochemistry, is mainly confined to a subset of olfactory axons, the vomeronasal nerve. Thus, in the adult olfactory bulb, both MAP protein synthesis and phosphorylation conform to a pattern associated with the developing brain. Immunohistochemistry also showed that MAP5 is concentrated in the olfactory nerve axons and the mitral cell dendrites of the olfactory bulb, i.e., exactly those elements that are involved in the olfactory nerve innervation that takes place in the adult. These results suggest that the expression of the early MAP forms is closely associated with neurite outgrowth and plasticity.