In vitro models to study the blood brain barrier.

The blood brain barrier regulates the transport of chemicals from entering and leaving the brain. Brain capillaries establish the barrier and restrict transport into the brain by providing a physical and chemical barrier. The physical barrier is due to tight membrane junctions separating the capillary endothelial cells resulting in limited paracellular transport. The chemical barrier is due to the expression of multidrug transporters that mediate the efflux of a broad range of hydrophobic chemicals. Because of the unusual nutrient demands of the brain, this limited permeability is compensated by the expression of a large number of transporters that are responsive to the metabolic demands of the brain. Consequently, the blood brain barrier indirectly regulates brain function by directly controlling the uptake of nutrients. Two widely used methods for studying the blood brain are a cell culture model using rat, pig, or cow brain endothelial cells and isolated microvessels. The cell culture model is more popular likely because it is easier to use and less costly compared to isolated microvessels. In some laboratories, brain endothelial cells are cocultured with astrocyte- or astroglial-conditioned media. The endothelial cells express many of the transporters displayed in vivo but not all. Although cell culture models vary, none express the tight barrier observed in vivo. Because microvessels are isolated directly from the brain, they express all of the transporters displayed in vivo. Their disadvantage is that the preparation is laborious, requires animals, and has a shorter lifespan in vitro. We present an approach in which transport is first verified in isolated microvessels, and then the mechanism is studied in cell culture.