Fig. 4.

IGF-I and insulin cooperate to produce enhanced numbers of neurons. EGF-generated precursors were cultured with 1 μm BrdU in the absence or presence of 10 nminsulin, 10 nm IGF-I, or the two combined, for 7 DIV.A, With the numbers of β-tubulin-immunoreactive neurons produced by 7 DIV of 10 nm IGF-I serving as 100% (see legend to Fig. 3 and Materials and Methods for details), the combined actions of 10 nm IGF-I plus 10 nminsulin are found to be more than additive (*p < 0.05; n = 4 independent cultures).B, In each condition, 100 β-tubulin-immunoreactive neurons were examined for incorporation of BrdU, and the percentage of double-labeled cells was calculated. The combined actions of 10 nm IGF-I plus 10 nm insulin result in a twofold increase in newly generated neurons (**p < 0.01 relative to control or insulin; p < 0.05 relative to IGF-I; n = 4 independent cultures). Taken together, these data suggest that increased neuronal numbers caused by combined insulin plus IGF-I is attributable to actions on mitotically active neuronal precursors.