Homeostasis of the neural microenvironment of the central nervous system (CNS) is essential for the normal function of neuronal networks and is protected by the blood-brain barrier (BBB). The BBB is formed by highly specialized capillary endothelial cells, which inhibit transendothelial passage of molecules from blood to brain by an extremely low pinocytotic activity and the lack of fenestrae, and the BBB restricts the paracellular diffusion of hydrophilic molecules due to an elaborate network of complex tight junctions between the endothelial cells. On the other hand, in order to meet the high metabolic requirements of the CNS tissue, specific transport systems are selectively expressed in the capillary brain endothelial cell membranes, which mediate the directed transport of nutrients into the CNS (in particular the glucose transporter) or of toxic metabolites out of the CNS (the multidrug resistance system) (Greenwood et al. 1995).