Structural and functional age-related changes in brain vasculature might af
fect the cerebral microcirculation. The present study evaluated the density
of perfused brain capillaries and the perfusion fraction (perfused/existin
g capillaries) in aged rats (24 months) during the states of the wake-sleep
cycle (quiet wakefulness, quiet sleep, active sleep) characterized by diff
erent levels of brain activation. The number of perfused capillaries was de
termined by intravascular injection of the fluorescent marker Evans Blue; h
istochemical staining of the capillary endothelium identified the alkaline-
phosphatase (AP) reactive quota of the anatomical population. No sleep-rela
ted changes in perfused capillary density were found, and the perfusion fra
ction in the AP-stained subpopulation was high and stable (95%) across the
sleep-wake cycle: changing levels of brain activation during sleep do not a
ffect functional capillary density in aged rats.