Blood-brain barrier permeability and the brain extracellular space in acute cerebral inflammation

The diffusion properties of the brain cortical extracellular space have never been examined in models of inflammation, even though inflammation can cause increased blood-brain barrier permeability. Uptake of intravascular 125I-labelled albumin and the diffusion of the tetramethylammonium ion within the brain extracellular space was measured in an experimental brain abscess to determine the effect of acute inflammation upon blood-brain barrier permeability and diffusion properties of the cortical extracellular space. The blood-brain transfer constant for albumin was increased in the abscess region, indicating that an increase in blood-brain barrier permeability occurred in animals inoculated with a weakly pathogenic strain of Staphylococcus aureus. The volume fraction of the extracellular space, as measured by the diffusion of tetramethylammonium ion, ranged from 0.19 to 0.23 in bacteria inoculated subjects and from 0.21 to 0.22 in controls. The tortuosity of the extracellular space ranged from 1.40 to 1.42 in bacteria inoculated subjects and was 1.39 in controls. These results showed that the volume fraction and tortuosity of the cortical extracellular space were not affected by inflammation even though vascular permeability was increased. This result was supported by the finding that brain water content, measured in the same animals, was increased to a non-significant extent in the bacteria inoculated subjects. These findings lead to the conclusion that acute inflammation induced by a weak pathogen can cause increased blood-brain barrier permeability without a significant change in the diffusion properties of the brain cortical space.