The interaction between rumen microbiota and neurotransmitters plays an important role in the adaptation of phenological changes in Tibetan sheep.

The gut-brain axis is regarded as the "second brain" of the host. Gut microbiota and their metabolites affect intestinal homeostasis, function, and phenotype by regulating bidirectional communication between the gut and brain. This serves as a vital strategy for understanding how gut microbiota regulate nutrient metabolism and adaptability in animals. This study explored the metabolic mechanisms through which Tibetan sheep adapt to high-altitude environments via the rumen microbiota-gut-brain axis across different phenological periods (returning-green period, fresh grass period and withered grass period). By analyzing metabolic indicators, neurotransmitters, and gene and protein expression in serum, rumen, adipose, and hypothalamic tissues, we discovered that energy metabolism markers (creatine kinase, lactate dehydrogenase, glucose) and immunoglobulins (IgG, IgM) in the serum were significantly elevated during the fresh grass period (P < 0.05). In contrast, thyroid hormones T3 and T4 were at higher levels during the returning-green period (P < 0.05). The density of rumen fiber-degrading bacteria was higher during the returning-green period (P < 0.05). Meanwhile, the densities of Butyrivibrio fibrisolvens, Selenomonas ruminantium, and Treponema bryantii microbiota significantly during the fresh grass period and were positively correlated with isovaleric acid concentration (P < 0.05). Neurotransmitters (5-HT, DOPAC, 5-HIAA, and NE) were significantly elevated in both the rumen epithelium and hypothalamus during the fresh grass period (P < 0.05). The analysis of the cAMP-PKA-pCREB pathway showed that the genes and proteins of UCP1, PKA, and CREB1 were highly expressed in adipose tissue during the fresh grass and withered grass periods, and there significant negative correlations to specific microbiota (P < 0.05). In summary, Tibetan sheep adapt to high-altitude environments through the rumen microbiota-gut-brain axis, regulating metabolic and neurotransmitter changes to establish a unique metabolic adaptation mechanism.