BRAIN CAPILLARY PERFUSION DURING SLEEP

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, Evan s blue (EB); it remained unchanged across the three behavioral conditi ons, QW, QS, and AS. The anatomical network was assessed by alkaline p hosphatase (AP) endothelial staining, which is known to underestimate the number of existing capillaries. The resulting number of AP profile s 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 indi cate that no capillary recruitment accompanies the wake-sleep cycle. C apillary surface al ea is a relevant factor in determining exchanges a cross the blood-brain barrier. In the absence of capillary recruitment (relative constancy of the surface area), the CBF changes during slee p should preferentially affect flow-limited with respect to diffusion- limited transport.