The expression of the growth associated protein B50/GAP43 in the olfactory system of neonatal and adult rats.

B50/GAP43 is a neuron-specific phosphoprotein whose expression is associated with neural development and synaptic plasticity. Its postnatal ontogeny was investigated in the primary olfactory pathway of the rat using immunohistochemical methods. The unique ability of the olfactory neuroepithelium to generate new neurons from a population of precursor cells present in the basal cell layer of this tissue makes it a valuable model in the study of neural development. In newborn rats B50/GAP43 is present throughout the entire population of olfactory receptor neurons. These cells are stained throughout, from the ciliated dendritic knob to their axon terminals in the bulb. This appears to be the first example of unambiguous B50/GAP43 expression in dendritic processes. With increasing age the distribution of this protein becomes progressively restricted to a subpopulation of olfactory neurons. Comparison of the expression of B50/GAP43 and the olfactory marker protein (OMP), a polypeptide only present in mature olfactory neurons, revealed that during postnatal development of the olfactory system these 2 proteins are expressed in a nearly reciprocal fashion. In adult animals (3.5 months-6 months of age), B50/GAP43-positive cells are exclusively present adjacent to the basal cell layer of the neuroepithelium. Basal cells appear to be unstained. The region of the epithelium containing the B50/GAP43-positive cells is virtually devoid of OMP-positive neurons. A significant fraction of these B50/GAP43-containing cells bear dendritic and neuritic processes. However, these cells do not express olfactory cilia. It is probable that the olfactory neurons expressing the growth-associated B50/GAP43 protein may correspond to a particular subset of olfactory neurons at an intermediate state of maturation.