Microvessels isolated from 6, 12, 18 and 24 month old rats were resuspended in serum-free media for 8 h, equal volumes of conditioned medium (50 μl) was added to cerebral cortical cultures and incubated for 24 h at 37°C. Cell viability was determined by MTT assay. Viable cells not exposed to conditioned medium were defined as 100%. Results are mean ± SD from 2 separate experiments performed in triplicate. **p < 0.01 vs. 6 months; ***p < 0.001 vs. 6 months.