Examination of the Blood-to-Brain Transfer of α-Aminoisobutyric Acid and Horseradish Peroxidase: Regional Alterations in Blood—Brain Barrier Function following Acute Hypertension

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Abstract
Blood–brain barrier (BBB) alterations following acute hypertension were studied in rats, employing as tracers in each animal both horseradish peroxidase (HRP) (MW 40,000) and [14C]α-aminoisobutyric acid ([14C]AIB) (MW 104). Eighteen animals were subjected to acute hypertension induced by the intravenous infusion of norepinephrine bitartrate (NE) (Levophed). Five animals injected with both tracers but not infused with NE served as controls. The brain of each animal was serially sectioned with adjacent sections processed either for macroautoradiography or for light microscopic visualization of HRP reaction product via histochemical reaction with tetramethylbenzidine. Quantitative blood-to-brain transfer constants for AIB were determined in each of 14 brain regions. Qualitative comparisons were also made between the AIB and HRP blood-to-brain extravasation patterns in each group. Acute hypertension increased cerebrovascular permeability to both AIB and HRP in most animals. Topographically, the sites of the most highly elevated AIB transfer corresponded with sites of HRP extravasation. Conversely, all sites of protein passage corresponded spatially to sites of elevated AIB transfer. Brain regions commonly showing increased permeability to both tracers included the cerebral cortices, corpus callosum, and thalamus. Importantly, some brain regions showed elevated AIB transfer constants where protein extravasation was absent. These regions included the caudate–putamen, hippocampus, basal forebrain, and cerebellum. These observations suggest that following acute hypertension, alterations in BBB permeability are not limited to vascular segments allowing protein extravasation.