Microglial M1 proinflammatory gene mRNA levels after intraperitoneal LPS injection. Young adult and middle-aged mice were injected with PBS or LPS (5 mg/kg ip) at times 0 and 24 h (arrows). Microglia were immunomagnetically isolated at 0, 3, and 24 h after the 1st LPS injection and 3 h after the 2nd PBS or LPS injection (24+3 h). qRT-PCR was used to evaluate the expression of iNos (A), Tnfα (B), Il-6 (C), and Il-1β (D). 1 symbol, P < 0.05; 2 symbols, P < 0.01; 3 symbols, P < 0.001; *time point vs. baseline (0 h) in young adult mice; +time point vs. baseline (0 h) in middle-aged mice; #young adult vs. middle-aged mice at the indicated time point.