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

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.