Functional expression of the serotonin transporter in immortalized rat brain microvessel endothelial cells

There is evidence from recent studies that the brain endothelium (of capillaries and/or larger vessels) may serve as a specific target for serotonin (5-HT). This neurotransmitter is expected to be involved in the regulation of the BBB permeability and/or of the cerebral blood flow via receptor-mediated mechanisms. Effective control of these processes depends on a speedy uptake and metabolism of released 5-HT molecules. To realise this, a similar mechanism of 5-HT uptake as in brain may exist at the BBB. In this study, we have demonstrated using reverse transcriptase-polymerase chain reaction (RT-PCR) that 5-HT transporter mRNA is present in the brain endothelium and that a saturable transport system for 5-HT is functionally expressed at immortalized rat brain endothelial cells (RBE4 cells). These cells take up [³H]5-HT by an active saturable process with a K _m-value of 397± 64 nmol/L and a transport capacity of 51.7 ± 3.5 pmol•g^-1•min^-1. The 5-HT uptake depends on Na⁺, as indicated by the replacement of NaCl by LiCl. The 5-HT uptake was sensitive to specific 5-HT transport inhibitors such as paroxetine, clomipramine, fluoxetine and citalopram, but not to inhibitors of the vesicular amine transporter such as reserpine or tetrabenazine. Our results demonstrate that cerebral endothelial cells are able to participate actively in the removal and metabolism of the released 5-HT, which supports the concept of direct serotonergic regulation of the BBB function.