Effects of 5-hydroxytryptophan on Glucose Tolerance and Lactation in Dairy Cows.

Peripheral serotonin is an important regulator of many aspects of lactation. Administration of the serotonin precursor, 5-hydroxytryptophan (5-HTP), has been shown to alter some aspects of energy metabolism and endocrine signals in dairy cattle. However, neither the effect of 5-HTP on insulin sensitivity nor the long-term effect on milk production have been studied in ruminants. The objectives of this study were to determine the effects of intravenous (IV) infusion of 5-HTP on response to an IV glucose tolerance test (GTT) and on lactation performance in dairy cows. We hypothesized that 5-HTP would alter insulin sensitivity and improve lactation performance. Twenty-four Holstein cows were assigned to IV 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 GTT was performed immediately after IV infusion on day 3. For the GTT, differential effects were observed for glucose, insulin, and free fatty acids (FFA). Neither maximal glucose nor area under the curve (AUC) were affected by 5-HTP (P≥0.15). On the other hand, basal insulin (difference of 0.116 µg/L, P=0.004) and AUC (P=0.04) were both decreased for 5-HTP, with maximal insulin tending to be decreased by 0.35 µg/L (P=0.06). Basal FFA concentration was unaffected by treatment (P =0.55), but both minimal (difference of 82 µmol/L, P=0.015) and AUC (P=0.034) FFA were increased by 5-HTP treatment. During the final 7 days of the period, feed intake was decreased by 4.0 kg/day during infusion days for 5-HTP cows (P<0.01), but not on rest days (P=0.34). Milk yield was decreased for 5-HTP cows only during infusion days (4.4 kg/day, P<0.01), returning to control levels on rest days (P=0.69). Similarly, milk protein yield was decreased (80 g/day, P<0.01) and milk fat yield tended to be decreased (140 g/day, P=0.06) by 5-HTP on infusion days, recovering on rest days. Over the 21-day period, 5-HTP cows lost 0.87 kg/day of body weight, while control cows gained 0.45 kg/day (P<0.01). Given the glucose and insulin responses during the GTT, it is evident that less insulin was required to clear the same glucose load for cows receiving 5-HTP. However, it is unclear if the lack of responsiveness in FFA to glucose challenge is due to effects of serotonin on the adipose tissue, or if there is a relationship between these FFA and decreased synthetic capacity of the mammary glands. Overall, this level of 5-HTP altered insulin sensitivity, and acutely but transiently depressed production. More work is needed to delineate the effects of 5-HTP on energy metabolism and production.