Acute fluoxetine treatment induces slow rolling of leukocytes on endothelium in mice.

Activated platelets release serotonin at sites of inflammation where it acts as inflammatory mediator and enhances recruitment of neutrophils. Chronic treatment with selective serotonin reuptake inhibitors (SSRI) depletes the serotonin storage pool in platelets, leading to reduced leukocyte recruitment in murine experiments. Here, we examined the direct and acute effects of SSRI on leukocyte recruitment in murine peritonitis.C57Bl/6 and Tph1-/- (Tryptophan hydroxylase1) mice underwent acute treatment with the SSRI fluoxetine or vehicle. Serotonin concentrations were measured by ELISA. Leukocyte rolling and adhesion on endothelium was analyzed by intravital microscopy in mesentery venules with and without lipopolysaccharide challenge. Leukocyte extravasation in sterile peritonitis was measured by flow cytometry of abdominal lavage fluid.Plasma serotonin levels were elevated 2 hours after fluoxetine treatment (0.70 ± 0.1 µg/ml versus 0.27 ± 0.1, p = 0.03, n = 14), while serum serotonin did not change. Without further stimulation, acute fluoxetine treatment increased the number of rolling leukocytes (63 ± 8 versus 165 ± 17/0.04 mm(2) min(-1)) and decreased their velocity (61 ± 6 versus 28 ± 1 µm/s, both p<0.0001, n = 10). In Tph1-/- mice leukocyte rolling was not significantly influenced by acute fluoxetine treatment. Stimulation with lipopolysaccharide decreased rolling velocity and induced leukocyte adhesion, which was enhanced after fluoxetine pretreatment (27 ± 3 versus 36 ± 2/0.04 mm(2), p = 0.008, n = 10). Leukocyte extravasation in sterile peritonitis, however, was not affected by acute fluoxetine treatment.Acute fluoxetine treatment increased plasma serotonin concentrations and promoted leukocyte-endothelial interactions in-vivo, suggesting that serotonin is a promoter of acute inflammation. E-selectin was upregulated on endothelial cells in the presence of serotonin, possibly explaining the observed increase in leukocyte-endothelial interactions. However transmigration of neutrophils in sterile peritonitis was not affected by higher serotonin concentrations, indicating that the effect of fluoxetine was restricted to early steps in the leukocyte recruitment. Whether SSRI use in humans alters leukocyte recruitment remains to be investigated

E-selectin expression is upregulated in the presence of peripheral serotonin.

<p>Immunofluorescence for E-selectin (red) and nuclei (blue). Mesenteric venules at baseline conditions (A), after fluoxetine treatment (B), after serotonin challenge (C) and after lipopolysaccharide stimulation (D) of WT mice. Mesenteric venules of Tph1−/− mice at baseline (E), after fluoxetine treatment (F), after serotonin challenge (G) and after lipopolysaccharide stimulation (H). Semi quantification of fluorescence-levels for E-selectin (I). Scale bar = 50 µm; Flx = fluoxetine, LPS = lipopolysaccharide. ** = p<0.001.</p

Neutrophil extravasation is unaltered after acute fluoxetine treatment in vivo.

<p>Gating strategies for Gr-1 neutrophil count with flow cytometry (A). Number of Gr-1 positive neutrophils in abdominal lavages of vehicle- and fluoxetine-treated WT mice 4 hours after intraperitoneal injection of 4% thioglycollate or vehicle (B, n = 9–10). n.s. = not significant.</p

Serotonin levels in serum and plasma.

<p>Serotonin concentrations in serum (A), plasma (B) and supernatant of PRP after fluoxetine challenge (C) measured by ELISA (n = 14). n.s. = not significant, * = p<0.01.</p