Density of perfused brain capillaries in the aged rat during the wake-sleep cycle.

Structural and functional age-related changes in brain vasculature might affect the cerebral microcirculation. The present study evaluated the density of perfused brain capillaries and the perfusion fraction (perfused/existing capillaries) in aged rats (24 months) during the states of the wake-sleep cycle (quiet wakefulness, quiet sleep, active sleep) characterized by different levels of brain activation. The number of perfused capillaries was determined by intravascular injection of the fluorescent marker Evans Blue; histochemical staining of the capillary endothelium identified the alkaline-phosphatase (AP) reactive quota of the anatomical population. No sleep-related changes in perfused capillary density were found, and the perfusion fraction in the AP-stained sub-population was high and stable (95%) across the sleep-wake cycle: changing levels of brain activation during sleep do not affect functional capillary density in aged rats.