On the Blood‐Brain Barrier to Peptides: [3H]βCasomorphin‐5 Uptake by Eighteen Brain Regions In Vivo

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Abstract
After intracarotid injection of [3H]β-casomorphin-5 (βCM5) in rats, the accumulation of radioactivity was determined in 18 brain regions and the anterior pituitary. The relative accumulation in all regions significantly exceeded that of [3H]inulin by a factor of 2.5, indicating a low but measurable brain uptake of the peptide. In blood-brain barrier-free areas, the accumulation of radioactivity was 15-fold higher than in blood-brain barrierprotected areas. The relative accumulation was not dependent on the total βCM5 concentration in the range of 0.3–1.1μM, and was not depressed by 400μM L-tyrosine. We conclude that βCM5, like other peptides, is accumulated in the blood-brain barrier-free areas to a relatively high but differing degree, whereas in the areas with a tight endothelium the accumulation is relatively low and nearly uniform. A binding to endothelial cells may contribute to the low accumulation of βCM5, especially in blood-brain barrier-protected areas.