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

Abstract

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. Serotonin is a neurotransmitter that helps regulate many body functions, including mood and appetite, metabolism, and blood flow. Serotonin is produced from the amino acid tryptophan, but this pathway is limited, only a small portion of tryptophan normally enters. One way to increase serotonin production is by providing 5-hydroxytryptophan (5-HTP), the direct precursor to serotonin. However, it is not known whether 5-HTP can be effectively supplied through the rumen of adult cattle or whether it can influence blood flow. This study evaluated how delivering 5-HTP into the rumen or the abomasum affected serotonin levels and blood flow in growing beef steers. Five ruminally-cannulated steers received water, abomasal 5-HTP, or increasing doses of ruminal 5-HTP. Blood samples were collected to measure serotonin and related compounds, and blood flow in the tail artery was measured using ultrasound. Feed intake and indicators of nutrient metabolism were not affected by 5-HTP. Abomasal delivery increased circulating serotonin, as expected. Importantly, ruminal delivery of 5-HTP increased serotonin in a dose-dependent manner, suggesting that some 5-HTP escaped ruminal breakdown. The highest ruminal dose increased artery size relative to baseline, indicating improved blood vessel dilation. These findings demonstrate that ruminal 5-HTP supplementation can increase serotonin and enhance peripheral blood flow, providing new insight into how serotonin metabolism can be manipulated in cattle.

Neurotransmitters

• Serotonin (5-HT)

Related

• 5-HTP
• Tryptophan