Effects of elevated serotonin on energy metabolism in lactating dairy cows

Authors: Virginia L. Pszczolkowski, Meghan K. Connelly, Adam D. Beard, Jimena Laporta, Laura L.
Hernandez, Sebastian I. Arriola Apelo

Title: Effects of elevated serotonin on energy metabolism in lactating dairy cows

Introduction: Peripheral serotonin is an important regulator of many aspects of lactation. Intravenous administration of the serotonin precursor, 5-hydroxytryptophan (5-HTP), has been shown to
alter some aspects of energy metabolism and endocrine signaling in dairy cattle. However, neither
the effect of 5-HTP on insulin sensitivity nor long term effects on metabolism have been studied.

Objective: The objectives of this study were to determine the effects of intermittent intravenous
infusion of 5-HTP on response to an intravenous glucose tolerance test (IVGTT) and on energy
metabolism after 3 weeks.

Methods: Twenty Holstein cows were assigned to infusion of either 5-HTP (1 mg/kg/day) or saline
in a 2-period crossover design. Treatments were infused via jugular catheters for 1 h/day, with 3
days of infusion followed by 4 days of rest, repeated through the 21-day period. The IVGTT was
performed immediately after infusion on day 3.

Results: By day 3, whole blood serotonin in cows receiving 5-HTP was elevated 35% above control
cows (p=0.002), and remained elevated until day 21. Insulin sensitivity indices showed improvement in 5-HTP cows, with HOMA-IR decreased by 71% (p<0.001) and RQUICKI increased by 19%
(p=0.05). During the IVGTT, glucose area under the curve (AUC) was not different between treatments (p>0.05), but insulin AUC was lower (p=0.02) and FFA AUC was greater (p=0.008) for 5-HTP
cows. During the final three days of infusion (days 15-17), 5-HTP cows produced on average 4.8
kg/day less milk (p<0.05), but recovered to control yield on final rest (days 18-21). Circulating FFA
and mammary FFA extraction were both elevated in 5-HTP cows up to 4 h after final infusion
(p<0.05), but neither milk fat yield nor percent were improved (p>0.05). Over the 21-day period,
5-HTP cows lost 0.87 kg/day of bodyweight, while control cows gained 0.45 kg/day (p<0.001).

Conclusions: Overall, elevated serotonin transiently improved insulin sensitivity and FFA availability and mammary extraction, but at the expense of lactation performance.