The precise microenvironment required within the brain to conserve optimal function is maintained by the presence of specialised barriers situated between the neural tissue and the circulating blood . These barriers protect the brain from the vast majority of toxins and pathogens as well as regulate the exchange of nutrients, metabolites, molecules and ions between the brain parenchyma and the blood by means of specific transporters and ion channels . The most extensive barrier type is found between the blood and the brain parenchyma and is formed through a complex functional interplay between brain microvascular endothelial cells, which are bound together by sophisticated ‘tight junctions’, pericytes, astrocytes, neurons and microglial cells. Together these cells constitute the neurovascular units that comprise the ‘classical’ parenchymal BBB . There are additional barriers in the choroid plexus separating the blood from the ventricular CSF, and between the blood and subarachnoid CSF. These barriers have endothelial cells joined by tight junctions but lack the other cellular components of the parenchymal BBB .