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Serotonin (5-HT) is a prime candidate for studies of the interaction between the nervous and immune systems, since it is both an important neurotransmitter and released at high concentrations at sites of inflammation. Serotonergic neurotransmission is regulated by the 5-HT transporter (5-HTT), which determines the magnitude and duration of serotonergic responses. Since tumor necrosis factor alpha (TNF-alpha) and interleukin-6 are two inflammatory mediators that are central to the initiation of inflammation, we studied the impact of these cytokines on the 5-HTT. As model system we used a cell line which constitutively expresses the 5-HTT, namely the choriocarcinoma cell line JAR. We found that TNF-alpha enhances 5-HT uptake, with a doubling of the maximal velocity of uptake. Interleukin-6, on the other hand, had no effect. We thus show for the first time that the cytokine TNF-alpha modulates 5-HTT function. Furthermore, we propose a molecular mechanism for this effect. Since both 5-HT and TNF-alpha are elevated at sites of inflammation, TNF-alpha may act to renormalize 5-HT levels by way of its effect on the 5-HTT. This is especially important for the central nervous system, where the TNF-alpha effect shown here can aid in preventing disturbances of serotonergic neurotransmission.