5-hydroxytryptophan strongly stimulates serotonin synthesis in the Holstein steers

Abstract Although serotonin has been extensively studied in many species, there is a lack of information in ruminants and no research has evaluated if its precursor, 5-hydroxytryptophan (5-HTP), administered into the abomasum may be used as a means to manipulate serotonin metabolism. Thus, the objective of this study was to evaluate if intra-abomasal infusion of 5-HTP increases circulating serotonin in the steer. Eight Holstein steers (471 ± 8.9 kg) were used in a replicated 4 x 4 Latin Square design experiment. The treatments were intra-abomasal infusion of 5-HTP at 0.5, 1, 2.5 and 5 mg/kg BW. Blood was collected at 0, 2, 4, 6, 8 and 24 h after infusion. The serum concentration of 5-HTP increased quadratically (P = 0.005) with a peak at 2 h after administration. The 5-HTP administration increased (P 0.05) between the doses of 5-HTP. When 5-HTP was dosed at 2.5 mg/kg BW or higher, intake decreased and there was an altered manure consistency. The serum 5-hydroxyindole acetic acid concentrations followed the same pattern as 5-HTP. Plasma glucose content was not affected (P > 0.05) by 5-HTP dosing. However, free fatty acid concentration in the plasma was lower (P > 0.05) than baseline for the infusion levels of 0.5 and 1 mg/kg BW. Intra-abomasal infusion of 5-HTP efficiently increases serum serotonin cattle.