Functional evidence for P-glycoprotein at the nose-brain barrier

Purpose. Experiments were performed to assess the brain distribution of [³H]-verapamil, including the influence of delivery route of inhibitor and substrate (nasal vs. systemic) on brain distribution. The anatomic location of P-glycoprotein (P-gp) at the nose-brain barrier also was investigated. Methods. Separate groups of mice were pretreated with rifampin or vehicle nasally or intravenously. [³H]-verapamil was administered either nasally or via in situ brain perfusion, and dose-response profiles were constructed for P-gp inhibition. Localization of P-gp in freshly obtained brain slices and olfactory tissue was evaluated by confocal microscopy. Results. Rifampin inhibited the P-gp-mediated efflux of [³H]-verapamil, regardless of delivery route (I_max = 62 ± 6%). The ED ₅₀ for enhancement of [³H]-verapamil uptake by nasal rifampin was ∼400-fold lower than for intravenous rifampin (0.16 vs. 65 mg/kg, respectively). Microscopy showed that P-gp was located in endothelial cells that line the olfactory bulb and within the olfactory epithelium. Conclusions. Nasal delivery of rifampin enhanced brain uptake of [ ³H]-verapamil. The magnitude of transport inhibition was dependent on the dose and route of the inhibitor, the time after administration of the inhibitor, and the specific brain region examined. P-gp is localized to both the olfactory epithelium and the endothelial cells that surround the olfactory bulb.