1. ¹Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN, USA
2. ²Department of Psychiatry, Vanderbilt University School of Medicine, Nashville, TN, USA
3. ³Center for Molecular Neuroscience, Vanderbilt University School of Medicine, Nashville, TN, USA

Correspondence: Dr WA Hewlett, Department of Psychiatry/Pharmacology, Vanderbilt School of Medicine, Nashville, TN 37232-8645, USA. Tel: +1 615 343 0795; Fax: +1 615 322 5298; E-mail: william.a.hewlett@vanderbilt.edu

Received 2 September 2005; Revised 16 November 2005; Accepted 28 November 2005; Published online 1 February 2006.

Abstract

Proinflammatory cytokines and serotonergic homeostasis have both been implicated in the pathophysiology of major psychiatric disorders. We have demonstrated that activation of p38 mitogen-activated protein kinase (MAPK) induces a catalytic activation of the serotonin transporter (SERT) arising from a reduction in the SERT K_m for 5-hydroxytryptamine (5-HT). As inflammatory cytokines can activate p38 MAPK, we hypothesized that they might also activate neuronal SERT. Indeed, Interleukin-1beta (IL-1) and tumor necrosis factor alpha (TNF-) stimulated serotonin uptake in both the rat embryonic raphe cell line, RN46A, and in mouse midbrain and striatal synaptosomes. In RN46A cells, IL-1 stimulated 5-HT uptake in a dose- and time-dependent manner, peaking in 20 min at 100 ng/ml. This was abolished by IL-1ra (20 ng/ml), an antagonist of the IL-1 receptor, and by SB203580 (5 M), a p38 MAPK inhibitor. TNF- also dose- and time-dependently stimulated 5-HT uptake that was only partially blocked by SB203580. Western blots showed that IL-1 and TNF- activated p38 MAPK, in an SB203580-sensitive manner. IL-1 induced an SB203580-sensitive decrease in 5-HT K_m with no significant change in V_max. In contrast, TNF- stimulation decreased 5-HT K_m and increased SERT V_max. SB203580 selectively blocked the TNF--induced change in SERT K_m. In mouse midbrain and striatal synaptosomes, maximal stimulatory effects on 5-HT uptake occurred at lower concentrations (IL-1, 10 ng/ml; TNF-, 20 ng/ml), and over shorter incubation times (10 min). As with RN46A cells, the effects of IL-1 and TNF- were completely (IL-1) or partially (TNF-) blocked by SB203580. These results provide the first evidence that proinflammatory cytokines can acutely regulate neuronal SERT activity via p38 MAPK-linked pathways.