RESUMEN
The blood-arachnoid barrier (BAB), which is formed by arachnoid epithelial cells linked by tight junctions, has generally been considered impermeable to water-soluble substances. However, we recently demonstrated that organic anion transporters 1 and 3 (Oat1 and Oat3) play roles in drug clearance at the BAB. Here, we examined whether an organic anion-transporting polypeptide (Oatp) also plays a role, using the fluorescent organic anion sulforhodamine-101 (SR-101) as a model substrate. SR-101 was injected into the cisterna magna of rats in order to minimize the contribution of choroid plexus transport. The in vivo cerebrospinal fluid (CSF) elimination clearance of SR-101 after intracisternal administration was ninefold greater than that of fluorescein-labeled inulin, a bulk flow marker. In the case of pre-administration of taurocholate, a broad-spectrum inhibitor of Oatps, or digoxin, a strong substrate/inhibitor for Oatp1a4 but not for Oatp1a1, Oat1, and Oat3, the CSF elimination of SR-101 was significantly reduced, becoming similar to that of inulin, and thus indicating complete inhibition of SR-101 clearance from the CSF. The distribution of SR-101 fluorescence was restricted to the arachnoid mater in the absence of inhibitor, whereas the fluorescence was increased in the parenchyma of the spinal cord after co-injection of taurocholate or digoxin. Immunostaining confirmed the localization of Oatp1a4 in the arachnoid mater. These results indicate that Oatp1a4 at the BAB acts as an avid clearance pathway of SR-101 in the CSF to the blood. Thus, Oatp1a4 appears to play a major role in CSF detoxification by limiting the distribution of organic anions to the brain and spinal cord.