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Regulation of adult olfactory neurogenesis by insulin‐like growth factor‐I

Insulin‐like growth factor‐I (IGF‐I) has multiple effects within the developing nervous system but its role in neurogenesis in the adult nervous system is less clear. The adult olfactory mucosa is a site of continuing neurogenesis that expresses IGF‐I, its receptor and its binding proteins. The aim of the present study was to investigate the roles of IGF‐I in regulating proliferation and differentiation in the olfactory mucosa. The action of IGF‐I was assayed in serum‐free culture combined with bromodeoxyuridine‐labelling of proliferating cells and immunochemistry for specific cell types. IGF‐I and its receptor were expressed by globose basal cells (the neuronal precursor) and by olfactory neurons. IGF‐I reduced the numbers of proliferating neuronal precursors, induced their differentiation into neurons and promoted morphological differentiation of neurons. The evidence suggests that IGF‐I is an autocrine and/or paracrine signal that induces neuronal precursors to differentiate into olfactory sensory neurons. These effects appear to be similar to the cellular effects of IGF‐I in the developing nervous system.