Lack of Contribution of P-Glycoprotein (P-gp) to Transport via the Mouse Blood-Cerebrospinal Fluid Barrier

Abstract

The blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) work effectively to restrict the free exchange of compounds between blood and brain. It is well known that P-glycoprotein (P-gp) that is expressed in brain microvessel endothelial cells contributes to the BBB as an efflux transporter. Western blot analysis has shown that P-gp is also expressed in the choroid plexus forming the blood-CSF barrier. The purpose of this study was to examine the role of P-gp in the efflux transport of its substrates across the bloodCSF barrier. The CSF concentration of etoposide and digoxin after intracerebroventricular administration, and the CSF concentration of 99m Tc-sestamibi after intravenous administration were compared between wild-type and P-gp knockout mice. After intracerebroventricular administration, there was no significant difference in the remaining concentration of etoposide and digoxin in CSF between wild-type and P-gp knockout mice, respectively. After intravenous administration of 99m Tc-sestamibi, its CSF-to-serum concentration ratio in P-gp knockout mice was not significantly different from that in wild-type, whereas its brain-to-serum concentration ratio was significantly increased in P-gp knockout mice compared with wildtype mice (1.62-fold, p < 0.05). In addition, immunohistochemical analysis showed a low membrane expression of P-gp in mouse choroid plexus, compared with brain microvessels. These results suggest that P-gp does not play an important role in the transport of its substrates across the blood-CSF barrier.