Figure 1. A schematic diagram illustrating the constituents of the CNS barriers. The endothelial cells are connected by tight junctions (TJs), preventing molecules permeating from the blood into the CNS parenchyma via the paracellular route. Influx transporters such as L-type amino acid transporter 1 (LAT1) and glucose transporter 1 (GLUT1) facilitate entry of nutrients into the parenchyma, whereas efflux transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) actively exclude potential toxins from entering the brain.