Ruminal 5-hydroxytryptophan increases serum serotonin and peripheral vasodilation in growing beef cattle

Serotonin is synthesized from tryptophan through hydroxylation and decarboxylation reactions, with 5-hydroxytryptophan (5-HTP) serving as the immediate precursor. Because 5-HTP bypasses the rate-limiting step of serotonin synthesis, it has been used to experimentally increase circulating serotonin in ruminants via enteral or intravenous administration; however, the capability of ruminally-supplied 5-HTP to escape microbial degradation and influence peripheral serotonin metabolism and vascular function has not been fully characterized. The objective of this study was to evaluate abomasal and ruminal 5-HTP administration on peripheral serotonin metabolism and caudal artery hemodynamics in growing beef steers. Five ruminally cannulated Angus × Holstein steers [body weight (BW) = 284 ± 10.6 kg] were randomly assigned to a 5 × 5 Latin square design. Treatments consisted of water (negative control), abomasal infusion of 0.5 mg 5-HTP/kg BW (positive control), or ruminal infusions of 1, 2, or 3 mg 5-HTP/kg BW administered as daily pulse doses for 5 d, with 5-d washout periods. Blood samples were collected serially over 24 h on days 1 and 5 of each period for analysis of tryptophan, kynurenine, 5-HTP, serotonin, and 5-hydroxyindolacetic acid. Caudal artery hemodynamics were assessed using color Doppler ultrasonography. Dry matter intake and plasma biomarkers of energy and protein metabolism were not affected (P ≥ 0.15) by enteral 5-HTP treatments. Abomasal 5-HTP infusion rapidly increased (P < 0.05) serum 5-HTP and increased serum serotonin concentration on day 5. Increasing ruminal 5-HTP infusion increased serum serotonin concentration in a dose-dependent manner, indicating partial escape from ruminal degradation and effective post-ruminal utilization. Relative to baseline, ruminal infusion of 3 mg 5-HTP/kg BW increased caudal artery cross-sectional area, demonstrating peripheral vasodilation (P < 0.05), whereas lower ruminal 5-HTP doses and abomasal 5-HTP infusion did not elicit measurable vascular responses. These results demonstrate that ruminal supplementation of 5-HTP can increase circulating serotonin and, at sufficient doses, promote peripheral vasodilation in growing beef steers. This study provides the first in vivo evidence of arterial vasodilation associated with 5-HTP supplementation in cattle and establishes ruminal delivery as a viable route for modulating serotonergic and vascular function.