5A,B), while the apical transporter Mrp4 was down-regulated (Fig. 5D). Immunoblots confirmed the up-regulation of Ostα and Ostβ in Tph1−/− relative to WT kidney (Fig. 6A,B). The nonsignificant down-regulation of Asbt in Tph1−/− kidneys (Fig. 5C) could not be confirmed on the protein level. Instead, Asbt protein was similarly expressed in WT and Tph1−/− mice (Fig. 6A) and was Gefitinib mouse reduced by BDL relative

to sham-operated animals (Fig. 6B). To examine whether the observed expression changes may be associated with an altered bile salt regulation by the kidney, we measured urinary levels of bile salts at 3 days of BDL (Fig. 6C). Urinary bile salts were significantly reduced in Tph1−/− relative to WT mice. We next examined whether serotonin reloading by injection of the serotonin precursor see more 5-HTP can reverse the renal changes observed in Tph1−/− mice. The increased expression of Osta and Ostb genes was reduced by serotonin reloading of Tph1−/− mice (Fig. 7A). Expression levels of other transporters (Mrp2, Mrp3, and

Mrp4) were not affected by serotonin reloading (data not shown). Immunoblots on Tph1−/− kidneys demonstrated a reduction in Ostα and Ostβ, but no change in Asbt protein by serotonin (Fig. 7B,C). Immunohistochemical staining confirmed basolateral Ostβ down-regulation in serotonin-reloaded Tph1−/− kidney (Supporting Fig. 7). Finally, the lowered level of urinary bile salts in Tph1−/− were increased following reloading (Fig. CYTH4 7D). Taken together, these results indicate a relationship between increased plasma

bile salts, elevated levels of the Ostα·Ostβ transporter in the kidney, and a reduced urinary excretion of bile salts in serotonin-depleted mice. We hence conclude that serotonin affects bile salt homeostasis in acute cholestasis by controlling renal transporter expression in mice. Serotonin is a monoamine neurotransmitter regulating mood, appetite, sleep, and cognitive activities in the central nervous system. A significant amount of serotonin is also stored in the gut and platelets.30 We have shown that serotonin is important in liver regeneration, nonalcoholic steatohepatitis, and repair after ischemic injury.10, 11, 31 Here, we demonstrate that endogenous serotonin ameliorates liver injury in cholestatic mice. Lack of serotonin increases liver injury and bile salt levels in liver and circulation. Unexpectedly, serotonin appears to affect bile salt pools through adaptive regulation of renal rather than hepatic bile transporters. Cholestasis induced by BDL in the mouse is characterized by a series of phases; it includes an acute phase, an immune reaction, and fibrotic changes.32 Here, we focus on the acute phase at 3 days of BDL, where liver injury is maximal as determined in an initial time course experiment. At this time point, hepatic necrosis was exacerbated in serotonin-deficient Tph1−/− mice relative to WT mice and correlated with higher bile salt levels.