In vivo effects of insulin-like growth factor-I on the development of sensory pathways: analysis of the primary somatic sensory cortex (S1) of transgenic mice.

In the rodent brain, insulin-like growth factor I (IGF-I) messenger RNA is transiently expressed in sensory projection neurons during periods of synaptogenesis and neuronal growth. Transgenic (Tg) mice with brain IGF-I overexpression and ectopic brain expression of IGF-binding protein-1 (IGFBP-1), an inhibitor of IGF-I actions, show changes in brain size and myelination. We used these mouse models to evaluate in vivo IGF-I effects on sensory pathway development by conducting anatomical studies in the S1 barrel field. Brain size, cortical area, and barrel field dimensions were increased in IGF-I and reduced in IGFBP-1 Tg mice compared with those in wild-type (wt) mice. The brain and cerebral cortex of Tg mice with the highest transgene expression were the most altered in size. Cortex and barrel field size changes were not precisely proportional, because in some Tg mice barrels were relatively more affected than the cortex, whereas in others the opposite was observed. Brain IGF-I overexpression increased the average number of neurons per barrel, neuronal cell body cross-sectional area, and barrel neuropil volume, whereas brain expression of IGFBP-1 reduced each. Neuronal density was greatly reduced in IGF-I Tg mice and increased in IGFBP-1 Tg mice. No differences in body weight, whisker pad and follicle areas, and whisker pad innervation density were found among Tg and wt mice. These observations indicate that IGF-I enhances neuronal growth in developing sensory pathways and support the concept that modified availability of local trophic factors, such as IGF-I, changes brain, neocortical, and S1 relative dimensions by altering neuronal survival and neuropil elaboration. Study of the S1 cortex provides an excellent model to probe the in vivo mechanisms of IGF actions.