[Exchanges through the blood-brain barrier]

The blood-brain barrier (BBB) is the main interface controlling the exchange of nutrients and drugs between the blood and brain. Its specificity is given by some specific properties of the endothelium of the brain capillaries. They include the presence of tight junctions sealing adjacent endothelial cells and the absence of fenestrations preventing paracellular transport pathway across the BBB. The BBB is also a metabolic and pharmacological barrier because of the activity of many cytosolic enzymes and transporters expressed both or either at the luminal or abluminal faces of the brain microvessel endothelial cells. Macromolecules like insulin, leptin and transferrin may cross the BBB via receptor mediated transcytosis. More recently the discovery of P-glycoprotein, an ABC protein, at the luminal membrane of the brain endothelial cells has shown that several lipophilic antimitotic and psychotropic drugs are pumped out of the brain by this transporter. All these properties illustrate how complex the exchanges of nutrients and drugs across the BBB are.