Effects of insulin-like growth factor 1 on olfactory neurogenesis in vivo and in vitro

The rate of neurogenesis in the peripheral olfactory neuroepithelium is regulated by unknown mechanisms. The members of the insulin-like growth factor (IGF) family can influence neuronal generation, survival and/or differentiation. Several members of this family, in particular IGF-1, are expressed at high levels in the olfactory bulb and epithelium, where they could influence the generation and/or survival of olfactory receptor neurons (ORNs). To explore the role of IGF-1 in the olfactory epithelium (OE), we asked which cells expressed IGF-1 receptors (IGF-1Rs), using olfactory cell cultures and cryostat-cut tissue sections of neonatal (postnatal day four) and adult rat OE. An antibody specific for the alpha subunit of the IGF-1R densely labeled a subset of ORNs but not other cell types in sections and cultures. These ORNs were primarily immature, as determined by double labeling with neuronal markers. The number of IGF-1R-labeled cells as well as the levels of IGF-1R protein (determined by immunoprecipitation and Western blotting) decreased with age, which is consistent with normal developmental changes. To study IGF-1 effects in the intact animal, we infused IGF-1 and related growth factors into the noses of newborn Sprague-Dawley rats, i.e., when the epithelium is still developing. Growth factors or carrier solution (0.9% NaCl with 0.25% bovine serum albumin to prevent nonspecific binding) were applied (10 microliters) to the left nostril once per day starting shortly after birth on postnatal day 1 (P1), P2 and P3, and the animals were sacrificed on P4 by decapitation. After paraformaldehyde immersion fixation, cryostat sections of the olfactory area of the nose were immunostained for the proliferating cell nuclear antigen (PCNA). Sections were position-matched by turbinate structure and then epithelial height and area of PCNA staining at the base of the epithelium (which represents division of primarily neuronal precursors) were measured by image analysis. Both were significantly increased by rat IGF-1 (20 ng/ml, 2.6 nM), but not insulin (20 ng/ml, 2.6 nM) or an IGF-1 derivative, LongR3 IGF-1 (200 ng/ml, 22 nM), that does not bind to the IGF-1 binding proteins (IGFBPs). Thus IGF-1 appears to influence the rate of olfactory neurogenesis, and its actions are not modified by the IGFBPs. These data suggest an important role for IGF-1 in the OE.