Elsevier

Neurochemistry International

Volume 33, Issue 3, 1 September 1998, Pages 251-254
Neurochemistry International

Enhancement of serotonin transporter function by tumor necrosis factor alpha but not by interleukin-6

https://doi.org/10.1016/S0197-0186(98)00026-6Get rights and content

Abstract

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-α) 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-α 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-α modulates 5-HTT function. Furthermore, we propose a molecular mechanism for this effect. Since both 5-HT and TNF-α are elevated at sites of inflammation, TNF-α 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-α effect shown here can aid in preventing disturbances of serotonergic neurotransmission.

Introduction

Interactions between the nervous system and the immune system are of great interest. A key player in this regard may be serotonin (5-HT), which acts both as a neurotransmitter and is released at high concentrations at sites of inflammation (Sternberg et al., 1987).Neurotransmission mediated by 5-HT influences many physiologic functions including motor activity, food intake, reproductive activity, sleep, and neuroendocrine rhythms, as well as cognition and emotional states, including mood and anxiety (Lesch et al., 1996). The serotonin transporter (5-HTT) is central to the fine-tuning of serotonergic neurotransmission and regulates the magnitude and duration of serotonergic responses (Lesch et al., 1996). Modulation of 5-HTT function is therefore an important mechanism to influence serotonergic neurotransmission (Heils et al., 1998).

Tumor necrosis factor alpha (TNF-α) is an inflammatory mediator that is now recognized as a critical component of the immune system. It initiates many different aspects of host inflammatory defenses including cellular proliferation, differentiation, induction of cell surface molecules, antiviral activity, and the induction of cell death (Beutler, 1990; Wong et al., 1996). It is rapidly released by macrophages and lymphocytes upon activation (Beutler, 1992). Interleukin-6 is another inflammatory mediator that is released early during immune responses and plays an important part in host defense (Akira et al., 1993). We therefore investigated whether TNF-α and interleukin-6 as key mediators of the immune system influence the function of the 5-HTT, using as cellular model system the JAR choriocarcinoma cell line, which constitutively expresses the 5-HTT.

Section snippets

Cell culture

The human choriocarcinoma cell line JAR was obtained from American Type Culture Collection (Rockville, Md.) and grown in RPMI 1640 medium supplemented with 10% fetal calf serum (FCS), glutamine, and gentamicin at 37°C in a humidified atmosphere at 5% CO2. For treatment with recombinant human TNF-α or interleukin-6 (R+D Systems, Wiesbaden, Germany), JAR cells were washed once before plating at a density of 2×105 cells⧹well into 24-well plates (Falcon, Becton-Dickinson, Heidelberg, Germany), 1 ml

Effect of TNF-α on the 5-HTT

JAR cells incubated with TNF-α (50 ng⧹ml) for two days showed markedly enhanced 5-HT uptake, compared to control cells (Fig. 1). This effect is characterized by an increase in the maximal velocity (Vmax) of 5-HT uptake, with Vmax approx. 22 pmol for TNF-treated and Vmax approx. 11 pmol for untreated cells. The affinity of the transporter (Km) showed no significant change, with a Km of between 0.5 and 0.6 μM. To assess the time course of the effect of TNF-α , JAR cells were incubated with the

Discussion

We have shown that exposure of the JAR cell line to the cytokine TNF-α enhances 5-HT transporter-mediated uptake of 5-HT. This effect is characterized by an increase in the maximal velocity of 5-HT uptake, with only a slight change in the affinity of the 5-HTT. The effect is dose- and time-dependent, with a maximum observed after exposure to TNF-α for 48 h. In contrast to TNF-α, interleukin-6 had no effect on 5-HTT function.

We have excluded the possibility that the increased 5-HT uptake after

Acknowledgements

This work was supported by the Bundesministerium für Bildung und Forschung (BMBF, Nr. 512-4001-01KS9603).

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