BLOOD TO BRAIN AND BRAIN TO BLOOD PASSAGE OF NATIVE HORSERADISH-PEROXIDASE, WHEAT-GERM-AGGLUTININ, AND ALBUMIN - PHARMACOKINETIC AND MORPHOLOGICAL ASSESSMENTS

Native horseradish peroxidase (HRP) and the lectin wheat germ agglutin in (WGA) conjugated to HRP are protein probes represented in the blood -brain barrier (BBB) literature for elucidating morphological routes o f passage between blood and brain. We report the application of establ ished pharmacokinetic methods, e.g., multiple-time regression analysis and capillary depletion technique, to measure and compare bidirection al rates of passage between blood and brain for radioactive iodine-lab eled HRP (I-HRP), WGA-HRP (I-WGA-HRP), and the serum protein albumin ( I-ALB) following administration of the probes intravenously (i.v.) or by intracerebroventricular (i.c.v.) injection in mice. The pharmacokin etic data are supplemented with light and electron microscopic analyse s of HRP and WGA-HRP delivered i.v. or by i.c.v. injection. The rates of bidirectional movement between blood and brain are the same for coi njected I-HRP and I-ALB. Blood-borne HRP, unlike WGA-HRP, has unimpede d access to the CNS extracellularly through sites deficient in a BBB, such as the circumventricular organs and subarachnoid space/ pial surf ace. Nevertheless, blood-borne I-WGA-HRP enters the brain similar to 1 0 times more rapidly than I-HRP and I-ALB. Separation of blood vessels from the neocortical parenchyma confirms the entry of blood-borne I-W GA-HRP to the brain and sequestration of I-WGA-HRP by cerebral endothe lial cells. Nearly half the I-WGA-HRP radioactivity associated with co rtical vessels is judged to be subcellular. Light microscopic results suggest the extracellular pathways into the brain available to blood-b orne native HRP do not represent predominant routes of entry for blood -borne WGA-HRP. Ultrastructural analysis further suggests WGA-HRP is l ikely to undergo adsorptive transcytosis through cerebral endothelia f rom blood to brain via specific subcellular compartments within the en dothelium. Entry of blood-borne I-WGA-HRP, but not of I-ALB, is stimul ated with coinjected unlabeled WGA-HRP, suggesting the latter may enha nce the adsorptive endocytosis of blood-borne I-WGA-HRP. With i.c.v. c oinjection of I-WGA-HRP and I-ALB, I-WGA-HRP exits the brain more slow ly than I-ALB. The brain to blood passage of I-WGA-HRP is nil with inc lusion of unlabeled WGA-HRP, which does not alter the exit of I-ALB. A dsorptive endocytosis of i.c.v. injected WGA-HRP appears restricted la rgely to cells lining the ventricular cavities, e.g., ependymal and ch oroid plexus epithelia. In summary, the data suggest that the bidirect ional rates of passage between brain and blood for native HRP are comp arable to those for albumin. Blood-borne WGA-HRP is assessed to enter the brain more rapidly than native HRP and albumin, perhaps by the pro cess of adsorptive transcytosis through BBB endothelia, but has diffic ulty leaving the CNS; the latter result may be due to avid binding and adsorptive endocytosis of WGA-HRP by exposed CNS cells. Neither nativ e HRP nor WGA-HRP alters the integrity of the BBB to albumin. For this reason, both native HRP and WGA-HRP are suitable probes for investiga ting the permeability of the BBB to macromolecules in vivo.