Thyroxine (T4) transfer from CSF to choroid plexus and ventricular brain regions in rabbit: Contributory role of P-glycoprotein and organic anion transporting polypeptides

This study investigated the transfer of T4 from cerebrospinal fluid (CSF) into the choroid plexuses (CP) and ventricular brain regions, and the role of P-glycoprotein (P-gp), multidrug resistance protein 1 (mrp1) and organic anion transporting polypeptides (oatps). During in vivo ventriculo-cisternal (V-C) perfusion in the anesthetized rabbit (meditomidine hydrochloride 0.5 mg kg− 1, pentobarbitone 10 mg kg− 1 i.v.), 125I-T4 was perfused continuously into ventricular CSF with reference molecules 14C-mannitol and blue dextran. Over 2 h, 36.9 ± 4.6% 125I-T4 was recovered in cisternal CSF. Addition of P-gp substrate verapamil increased CSF 125I-T4 recovery to 51.4 ± 2.8%, although mrp1 and oatp substrates had no significant effect. In brain, 125I-T4 showed greatest accumulation in the CP (1.52 ± 0.31 ml g− 1), followed by ventricular regions (caudate putamen, ependyma, hippocampus, 0.05–0.14 ml g− 1). At the CP, verapamil and probenecid (but not indomethacin) significantly increased 125I-T4 accumulation, implicating a role for P-gp and oatps. Of other brain regions, all three drugs increased accumulation in caudate putamen 3–5 times, and indomethacin and probenecid increased accumulation in ependyma 4–5 times. The role of P-gp was investigated further in isolated incubated CPs using 5 μg/ml C219 anti-P-gp antibody. Both 125I-T4 and 3H-cyclosporin accumulation increased by 80%, suggesting that P-gp is functional in the CP and has a role in removal of T4. Combined with the in vivo results, these studies suggest that P-gp provides a local homeostatic mechanism, maintaining CSF T4 levels. We conclude that P-gp and oatps contribute to the transfer of 125I-T4 between the CSF, CP and brain, hence regulating 125I-T4 availability in CSF.