Transcriptome analysis of  longissimus dorsi muscle treated with melatonin  in weaned piglets
Melatonin has been reported to play crucial roles in regulating meat quality, improving reproductive properties and maintaining intestinal health in animal production, but whether it regulates skeletal muscle development in weaned piglet is rarely studied. This study was conducted to investigate the effects of melatonin on growth performance, skeletal muscle development and lipid metabolism in animals by intragastric administration of melatonin solution. Twelve 28-day-old DLY (Duroc × Landrace × Yorkshire) weaned piglets with similar body weight were randomly divided into two groups: control group and melatonin group. The results showed that melatonin supplementation for 23 days had no effect on growth performance, but significantly reduced serum glucose content (P<0.05). Remarkably, melatonin increased longissimus dorsi muscle (LDM) weight, eye muscle area and decreased the liver weight in weaned piglets (P<0.05). In addition, the cross-sectional area of muscle fibers was increased (P<0.05), while triglyceride (TG) levels were decreased in LDM and psoas major muscle (PMM) by melatonin treatment (P<0.05). Transcriptome sequencing showed melatonin induced the expression of genes related to skeletal muscle hypertrophy and fatty acid oxidation. Enrichment analysis indicated that melatonin regulated cholesterol metabolism, protein digestion and absorption and mitophagy signaling pathways in muscle. Gene set enrichment analysis (GSEA) also confirmed the effects of melatonin on skeletal muscle development and mitochondrial structure and function. Moreover, quantitative real-time polymerase chain reaction (qPCR) analysis revealed that melatonin supplementation elevated the gene expression of cell differentiation and muscle fiber development, including paired box 7 (PAX7), myogenin (MYOG), myosin heavy chain (MYHC) ?A and MYHC ?B (P<0.05), which was accompanied by increased insulin like growth factor 1 (IGF1) and insulin like growth factor binding protein 5 (IGFBP5) expression in LDM (P<0.05). Additionally, melatonin regulated lipid metabolism and activated mitochondrial function in muscle by increasing the mRNA abundance of cytochrome c oxidase subunit 6A (COX6A), COX5B and carnitine palmitoyltransferase 2 (CPT2) and decreasing the mRNA expression of peroxisome proliferator activated receptor gamma (PPARG), Acetyl-CoA carboxylase (ACC) and fatty acid binding protein 4 (FABP4) (P<0.05). Together, our results suggest that melatonin could promote skeletal muscle growth and muscle fiber hypertrophy, improve mitochondrial function and decrease fat deposition in muscle. Overall design: A total of 12 weaned piglets  at 28 d of age DLY (Duroc × Landrace × Yorkshire) were randomly divided into two groups (n = 6). All the piglets were fed the same basal diet and water throughout the experiment. Following 5 days for adaptive rearing, all the piglets were given fasting gavage at 9 a.m every morning. Piglets from melatonin-treated group were infused with 10 mL of the sterile saline solution dissolved with melatonin on basis of body weight (5 mg/kg) and the control group in which piglets received same volume of sterile saline solution daily for 23 days. Melatonin solution was prepared daily and kept with an aluminum foil cover to prevent light-induced degradation before use. We then performed gene expression  analysis using RNA from longissimus dorsi muscle after  melatonin supplementation