The blood-brain barrier (BBB) protects the neural tissue from variations in blood composition and toxins. Elsewhere in the body the extracellular concentrations of hormones, amino acids and potassium undergo frequent fluctuations, especially after meals, exercise or stressful times. Since many of these molecules regulate neuronal excitability, a similar change in the composition of interstitial fluid in the CNS can lead to uncontrolled brain activity. The endothelial cells forming the blood-brain barrier are highly specialized to allow precise control over the substances that enter or leave the brain. All areas of the brain do not have a blood-brain barrier. The structures located at strategic positions in the midline of the ventricular
