Administration of STZ downregulates the phosphorylation levels of IR. (A) Western blots showing the expression levels of phosphorylated insulin receptor (p-IR) and total IR proteins in APP/PS1 transgenic mouse brain 20 weeks after STZ administration. β-actin was used as a loading control. (B) There were no significant differences in total IR levels between vehicle controls and STZ-treated mice. (C) Insulin deficiency induced by STZ administration led to a marked reduction in the p-IR level in the transgenic mouse brain. (D) The ratio of p-IR/IR was significantly reduced in STZ-treated mouse brain, compared with the vehicle control. **p < 0.01 (n = 7).