Insulin-like growth factor I stimulates dendritic growth in primary somatosensory cortex

The temporal and spatial distributions of several growth factors suggest roles in the regulation of neuronal differentiation in the neocortex. Among such growth factors, the insulin-like growth factors (IGF-I and -II) are of particular interest because they are available to neurons from multiple sources under independent control. IGF-I is produced by many neurons throughout the brain and also by cells in the cerebral vasculature. IGF-II is found at high levels in the CSF, and both IGF-I and IGF-II cross the blood-brain barrier. Thus, the IGFs may act as both paracrine and endocrine regulators of neuronal development. As an initial step toward understanding the influence of IGFs in the developing cerebral cortex, the present study examined the effects of IGF-I and of the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) on the dendritic complexity of layer 2 pyramidal neurons. The results demonstrate that IGF-I increased the branching and total extent of both apical and basal dendrites of pyramidal cells in organotypic slices of rat primary somatosensory cortex. BDNF and NT-3 also enhanced dendritic development, but the two neurotrophins increased the extent of only basal, not apical, dendrites and promoted greater elongation than was seen after IGF-I treatment. These results provide direct evidence that IGF-I can regulate the dendritic elaboration of cortical neurons and indicate that endogenous IGFs may influence dendritic differentiation and the formation of cortical connections. In addition, IGF-dependent regulation of dendritic structure may represent a link between age-related declines in IGFs and cognitive deficits seen in senescence.