Brain capillary perfusion was evaluated in the different states of the wake-sleep cycle-quiet wakefulness (QW), quiet sleep (QS), and active sleep (AS)-in rats. The extent of the perfused capillary network was determined by intravascular distribution of a fluorescent marker. Evans blue (EB); it remained unchanged across the three behavioral conditions, QW, QS, and AS. The anatomical network was assessed by alkaline phosphatase (AP) endothelial staining, which is known to underestimate the number of existing capillaries. The resulting number of AP profiles were, therefore, significantly lower than the number of EB profiles, but the percentage of AP-stained capillaries that were perfused (96%) was also unchanged across the behavioral conditions. The results indicate that no capillary recruitment accompanies the wake-sleep cycle. Capillary surface area is a relevant factor in determining exchanges across the blood-brain barrier. In the absence of capillary recruitment (relative constancy of the surface area), the CBF changes during sleep should preferentially affect flow-limited with respect to diffusion-limited transport.

No abstract

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.