Neurogenesis decreases during brain maturation from adolescence to adulthood.

Adolescence is an important stage of brain development. Recent studies have indicated that neurogenesis in the brain occurs throughout life prompting comparisons of adolescent and adult neurogenesis. Since insulin-like growth factor 1 (IGF-1) has been implicated in promoting neurogenesis we investigated the levels of neurogenesis in adolescents (PND30) and adults (PND120) using IGF-1 over-expressing mice and IGFBP-1 (IGF binding protein-1) over-expressing mice. Proliferation and differentiation of neuroprogenitors were determined using bromodeoxyuridine (BrdU)- and doublecortin (DCX)-labeling. High levels of neurogenesis were found in both the hippocampal dentate gyrus (DG) and the forebrain subventricular zone (SVZ) of the adolescents as compared with the adults. Both adolescent IGF-1 and IGFBP-1 transgenic mice as well as their wildtype controls have significantly higher expression of BrdU and DCX in the hippocampus and SVZ when compared with their adult counterparts. However, no significant differences on BrdU-labeling were found when either of transgenic mice were compared with their wildtype littermates in both age groups. These studies indicate that adolescent mice have high levels of neurogenesis compared to adults suggesting a dramatic loss of neurogenesis during the transition from adolescence to adulthood. However, the role of IGF-1 during adolescent development is still unclear.