Epilepsy and the blood-brain barrier.

A concern for the possible role of the blood-brain barrier (BBB) in the epilepsies was based on ultrastructural studies that demonstrated increased micropinocytosis in cerebral capillaries during seizures. Continued interest in the structure of the BBB has led to the demonstration that, in human psychomotor epilepsy, there is a thickening of the capillary basement membrane. These studies also suggest that an increase in capillary mitochondria and interendothelial tight junctions may characterize seizure-traumatized brain regions. These studies forecast an increased interest and understanding of the ultrastructural events associated with capillaries in seizure states. Additional focus on the BBB comes from the clinical use of anticonvulsant drug levels in the control and treatment of seizures. Debate as to whether free drug levels are appropriate continues. The brain capillary is the interface between blood-borne drug and the target site, and thus an increased understanding of the events associated with brain-plasma exchange has been sought. The concept that only that fraction of drug that is freely dialyzable is available for equilibration across the BBB is not supported by recent studies, which demonstrate that protein-bound ligands are able to dissociate and gain access to the brain in the course of a single capillary transit. It has been established that albumin-bound fatty acids, steroids, and anticonvulsant drugs more readily distribute into tissues than previously believed. Thus, traditional free drug hypotheses need to be expanded to account for the fact that dissociation constants measured in vitro are not the same as those measured in vivo. The BBB also regulates nutrient availability to the brain, and under normal conditions excess substrate is made available to the brain for metabolism. Indirect evidence is available to suggest that during seizures, BBB transport may indeed be the rate-limiting step. Specifically, glucose availability to the seizing brain may be restricted to such a degree that brain glucose utilization rates are no longer independent of plasma glucose levels. If it can be proven that BBB transport is the rate-limiting step during seizures, then it would be possible to augment brain glucose utilization rates by increasing plasma glucose levels. In addition, a depression of brain glucose utilization could be achieved by inducing hypoglycemia. It is not fully understood whether BBB rate limitation would persist postically, nor is it known whether BBB alterations may be global or restricted to the seizure focus.(ABSTRACT TRUNCATED AT 400 WORDS)