Ruminal metabolism of serotonin metabolites in cattle.

Metabolism of serotonin (5-HT) by gut microorganisms has been demonstrated in several species. However, information regarding 5-HT metabolism by rumen microbiota remains limited. The objective of this study was to evaluate ruminal metabolism of 5-HT-related metabolites and subsequent changes in circulating concentrations in bovines following oral administration of 5-Hydroxytryptophan (5-HTP). In vitro incubation of rumen fluid with 5-HTP (100 µM), 5-HT (100 µM), or 5-Hydroxyindoleacetic acid (5-HIAA; 100 µM) were assessed to evaluate the degradation and metabolization of these compounds. In an in vivo experiment, cows received oral administration of 5-HTP at doses of 0 or 10 mg/kg of body weight. Ruminal fluid and blood samples were collected at multiple time points following 5-HTP administration. In vitro incubation of 5-HTP rapidly disappeared resulting in a significant reduction after 60 min and complete disappearance by 480 min, without a corresponding increase in 5-HT concentrations; however, 5-HIAA concentrations increased. The incubation of 5-HT in rumen fluid caused a slight and transient increase in 5-HIAA concentrations. Oral administration of 5-HTP increased the area under the curve (AUC) for ruminal concentrations of tryptophan, 5-HTP, 5-HT, and 5-HIAA. The intake of 5-HTP increased ruminal concentration of 5-HT and 5-HIAA. Additionally, the AUC of serum 5-HT and 5-HIAA concentrations increased. Rumen microorganisms rapidly metabolize free 5-HTP, resulting in low efficiency of 5-HT production; however, this process still makes a significant contribution to circulating 5-HT concentrations in the host.