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A sensitive solution-hybridization assay was used to investigate the expression of genes encoding insulin-like growth factors I and II (IGF-I and -II) in the rat central nervous system (CNS). mRNAs for both IGFs are synthesized throughout the CNS of adult rats but exhibit distinct regional differences for each growth factor. IGF-I mRNA is 8-10 times more abundant in the cervical-thoracic spinal cord and in the olfactory bulb than in whole brain and is enriched 3-fold in the midbrain and cerebellum. IGF-II mRNA is minimally enriched in the medulla-pons and cerebellum but is 3-5 times less abundant in the midbrain and corpus striatum than in total brain. During CNS development the content of IGF-I and IGF-II mRNAs is highest at embryonic day 14 and declines by a factor of 3-4 at birth, to values found in adult brain. Embryonic neurons and glia synthesize IGF-I mRNA during short-term cell culture; only glia produce IGF-II mRNA. These observations show that IGF-I and IGF-II are differentially expressed in the developing and adult CNS and suggest that each growth factor may play a unique role in the mammalian nervous system.
Rotwein et al. (1988) studied this question.