In the early 1960s, the location of the BBB was still not established with certainty; one view was that the apparent restriction on compound entry to the brain was due to a limited extracellular space.19 Brain slices were used to establish that the brain contained appreciable extracellular fluid, so that restriction on uptake in vivo must be due to a barrier at the capillary level.49 In the 1970s brain slices were used to make significant advances in understanding of brain biochemistry and neurochemistry, for example for glucose transport.50'51 However, they were not suitable for kinetic studies, since diffusion in the extracellular space rather than the transporter itself often proved the rate-limiting step, and cell swelling caused changes in the fluid compartments.51
More recently, improvements in techniques for isolation and maintenance of brain slices with well-preserved extracellular space,52 together with new imaging techniques (such as 2-photon confocal microscopy) allowing examination of structures within the slice, have allowed studies of cell-cell interaction within the neurovascular unit, including influences of astrocytes on blood vessels.6 However, as the vessels within the slice are unperfused, these preparations are not suitable for examining BBB permeability and transport. For studies of gene and protein expression, the powerful technique of laser capture microdissection (LCM) can be used not only to sample from individual blood vessels in defined regions of brain slices, but also to separate relatively cleanly the endothelial and perivascular cells.53 This tool has significant applications in determining changes in phenotype of blood vessels in pathological human brain material (resectioned, biopsy, or post-mortem), to guide drug delivery strategies.
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