Expression of insulin-like growth factor-I (IGF-I) and IGF-II in the avianbrain: relationship of in situ hybridization patterns with IGF type 1 receptor expression

Insulin-like growth factors (IGFs) are expressed in defined spatiotemporal patterns during the development of the mammalian central nervous system (CN S). Since IGF expression in avian species is less a ell documented, we stud ied here the expression of IGF-I and IGF-II during chicken CNS development, using in situ hybridization and reverse transcriptase-PCR, and compared th e results with the expression of the IGF type 1 receptor (IGF-1R). IGF-II e xpression started early in embryonic life, shortly after the onset of IGF-1 R expression. During organogenesis, IGF-II was strongly expressed in kidney , liver and gut primordia, in contrast with IGF-1R mRNA, which is highly en riched in proliferating neuroepithelia. During the second half of embryonic development. IGF-I and IGF-II had distinct expression patterns, suggesting specific roles for each ligand during brain maturation. IGF-II mRNA was fo und in numerous brainstem nuclei and in the optic tectum, whereas IGF-I mRN A was found predominantly in telencephalic regions. Both ligands were expre ssed in the cerebellum, but each by different cell layers. Some brain regio ns (olfactory bulb and olivo-cerebellar system) did nor exhibit the postnat al downregulation typical of extrahepatic IGF-I expression. but continued t o express IGF-I into adulthood. Purkinje cells expressed IGF-II in the embr yo, but switched to IGF-I expression in the adult. The conservation of embr yonic and postnatal IGF expression patterns in the CNS between avians and m ammals suggests that the involvement of the IGF system in neurogenesis and differentiation, and possibly in neural plasticity and learning, may have a risen early during tetrapode/vertebrate evolution. (C) 2000 ISDN. Published by Elsevier Science Ltd. All rights reserved.