Leukocyte rolling on unstimulated endothelium is decreased and faster in the absence of platelet serotonin. (A) Rhodamine-stained leukocyte rolling in resting mesenteric veins of WT and Tph1^−/− mice after laparotomy without further stimulation (n = 8). (B) Resting leukocyte-rolling velocity in these mice. (C) In vitro uptake of labeled serotonin after incubation of isolated platelets with fluoxetine (n = 3). (D) Serum serotonin in Tph1^−/− and WT mice after a 3-week treatment with fluoxetine or vehicle in the drinking water (n = 11). Leukocyte rolling (E) and velocity (F) in WT mice after treatment with fluoxetine for 3 weeks (n = 9). Only negligible leukocyte adhesion was observed in these experiments. Bar represents 50 μm.