327. Effects of 5-hydroxytryptophan Supplementation and Ergot Alkaloids on Hypothalamic Transcriptomics, Serotonin Bioavailability, and Feed Intake in Beef Cattle.

Abstract Previous research has demonstrated that ergot alkaloids (EA) decrease circulating serotonin, and 5-hydroxytryptophan (5-HTP) supplementation increases circulating serotonin. It remains unclear if changes in serotonin bioavailability and/or feed intake are associated with alterations in hypothalamic function. The objective was to evaluate the effects of EA and 5-HTP supplementation on serotonin bioavailability, feed intake, and the transcriptomic profile of the hypothalamus in beef cattle. AnguśHolstein steers [n = 24; body weight (BW)=427±101 kg] were ruminally-dosed with non-toxic or toxic endophyte-infected tall fescue seed to provide 0 or 15 mg ergovaline/kg of BW. Additionally, steers were ruminally dosed with 0 or 3 mg 5-HTP/kg of BW. Therefore, the four treatments were: control, 5-HTP, EA, EA + 5-HTP. Steers were housed indoors in individual pens, exposed to cyclical temperatures mimicking summer conditions (21 °C–32 °C), and limit-fed alfalfa cubes to supply 1.5 times the net energy required for maintenance. Feed intake and feeding behavior were measured daily. Blood samples were collected in 7-d intervals and analyzed for serotonin concentration using high-performance liquid chromatography. On day 16, the hypothalamus was collected for RNA sequencing after slaughter. There were four unique serotonergic phenotypes (P = 0.05): 5-HTP increased serum serotonin relative to control; EA decreased serum serotonin relative to control, and the combination (EA + 5-HTP) decreased serum serotonin compared to all other treatments. Ergot alkaloid consumption decreased dry matter intake (P = 0.04), meal duration (P = 0.03), and meal size (P < 0.01). 5-hydroxytryptophan supplementation did not influence (P > 0.05) meal duration, meal size, or dry matter intake versus no 5-HTP supplementation. 5-hydroxytryptophan, EA, or their combination (EA + 5-HTP) downregulated (P < 0.01) genes associated with the serotonergic synapse. 5-hydroxytryptophan, EA, or their combination (EA + 5HTP) downregulated (3.5-fold, 5.5-fold, 3.9-fold, respectively; P < 0.01) mRNA expression of tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme catalyzing the conversion of 5-HTP to serotonin in the hypothalamus. 5-hydroxytryptophan, EA, and their combination (EA + 5-HTP) downregulated (3.3-fold, 3.8-fold, and 4.9-fold, respectively; P < 0.01) mRNA expression of the serotonin reuptake transporter (SLC6A4). 5-hydroxytryptophan, EA, or their combination (EA + 5-HTP) downregulated (3.2-fold, 3.3-fold, 5-fold, respectively; P < 0.01) mRNA expression of FEV, a transcription factor essential for differentiation, growth, and development of hypothalamic serotonergic neurons. Ergot alkaloids and 5-HTP downregulated (P < 0.01) mRNA expression of serotonergic receptors, including HTR1A and HTR1B. Ergot alkaloids downregulated (P < 0.01) several transcripts (NPY, CCK, GLP1R, and GHSR) associated with neuroactive ligand-receptor interactions. 5-hydroxytryptophan upregulated (P < 0.01) several transcripts (VIP, NMS, and POMC) associated with neuroactive ligand-receptor interactions. Transcripts associated with hypothalamic serotonin synthesis, transport, signaling, and reuptake were downregulated by EA and 5-HTP supplementation. Transcripts associated with hypothalamic neuroactive ligand-receptor interactions were differentially regulated by EA and 5-HTP. These findings demonstrate that EA and 5-HTP modulate hypothalamic serotonergic signaling and appetite-related pathways, suggesting associations with the regulation of feed intake and systemic serotonin bioavailability in cattle.