Immunolocalization and quantification of the phoenixin and GPR173 in the gastrointestinal tract of Holstein–Friesian bulls

Phoenixin (PNX), well-conserved but newly discovered neuropeptide, is involved in various physiological processes, such as food intake, cardiovascular functions, reproductive functions, and stress regulation. PNX is the predicted ligand of GPR173 receptor, but due to its relatively recent discovery in 2013, there is a lack of studies describing the exact mechanism of action of the peptide. In addition, the protein was not been well-studied in specific organs, particularly in the gastrointestinal tract (GIT) of ruminants, including domestic cattle, which are among the world's main livestock animals. Therefore, this study aimed to investigate the immunolocalization and quantification of PNX and GPR173 in the GIT of domestic cattle. Study material, including GIT sections of two age groups, calves and adult bulls (n = 6 per group), was obtained from a slaughterhouse. Enzyme-linked immunosorbent assay (ELISA) and immunohistochemical (IHC) analyses were performed. Analyses revealed low levels of PNX in the GIT of both age groups, with localization restricted to epithelial cells across all examined GIT segments, with statistically significant differences between age groups and GIT segments, which may result from the delayed development of forestomachs in calves. On the other hand, GPR173 levels were shown to be higher than those of PNX and to have a wider distribution extending beyond the epithelium to the blood vessels wall and the intrinsic nervous system. This may suggests that PNX is not the only ligand for this receptor. Overall, the results may suggest that both PNX and GPR173 could possibly play protective roles related to the immune response, regulate digestive and absorptive functions, and due to receptor presence in nerve fibres, may play a role in regulating GIT secretion and motility. These findings could potentially facilitate further research into the therapeutic potential of targeting PNX and GPR173 in managing gastrointestinal disorders in domestic cattle and other species, and can also be further used for experimental, clinical or pharmacological research into the treatment of eating disorders not only in humans, but also in farm animals.