Functional characterization of rat plasma membrane monoamine transporter in the blood-brain and blood-cerebrospinal fluid barriers

This study investigated the expression and functional roles of rat plasma membrane monoamine transporter (rPMAT) in the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier by using in vitro brain barrier model cells (TR-BBB13 and TR-CSFB3 cells) and multiple in vivo experimental techniques. Quantitative reverse transcription-polymerase chain reaction analysis showed relatively high expression of rPMAT mRNA in TR-BBB13 and TR-CSFB3 cells. 1-Methyl-4-phenylpyridinium (MPP⁺) was transported into rPMAT-expressing cells in a sodium-independent manner. [³H]MPP⁺ was taken up concentration dependently by TR-BBB13 and TR-CSFB3 cells with K_m values similar to that of rPMAT-expressing cells. [³H]MPP⁺ transports into these cells were markedly inhibited by serotonin, dopamine, and cationic drugs. rPMAT small interfering RNA (siRNA) significantly suppressed the [³H]MPP⁺ uptake by TR-BBB13 cells. Intracerebrally injected [³H]MPP⁺ was eliminated from the brain parenchymal region, whereas brain [³H]MPP⁺ uptake did not increase with time during in situ brain perfusion, suggesting that the brain-to-blood transport across the BBB predominates over the blood-to-brain transport. Brain microdialysis studies revealed that the elimination across the BBB was significantly decreased by coperfusion of unlabelled MPP⁺, serotonin, or dopamine. [³H]MPP⁺ was also eliminated from the CSF. These findings suggest that PMAT in brain barriers functions as the brain-to-blood transporter to regulate brain concentrations of organic cations including monoamines and cationic neurotoxins.