Functional Expression of Carnitine/Organic Cation Transporter 1 in Murine Choroid Plexus

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Membrane transporters expressed in the choroid plexus (CP) are involved in the transport of substances between the blood and cerebrospinal fluid (CSF). Carnitine/organic cation transporter 1 (OCTN1, also known as SLC22A4) is expressed in rodent CP; however, its specific roles in blood–CSF transport remain unclear. Therefore, in this study, we aimed to evaluate the potential role of OCTN1 in the elimination of substances from CSF. Tritium-labeled ergothioneine ([³H]ERGO), a typical in vivo substrate of OCTN1, was injected into the lateral ventricles of wild-type and octn1 gene knockout (octn1^−/−) mice. Clearance of [³H]ERGO from CSF was higher than that of the bulk flow marker, [¹⁴C]mannitol, in wild-type mice. However, [³H]ERGO clearance was significantly lower in octn1^−/− mice than in wild-type mice. Furthermore, OCTN1 expression in CP was determined via immunohistochemical analysis. CP/CSF ratio of [³H]ERGO was significantly lower in octn1^−/− mice than in wild-type mice. These results suggest that OCTN1 is functionally expressed in CP and involved in the elimination of ERGO from CSF in mice.