Pilot study of β-endorphin concentrations in horses with pituitary pars intermedia dysfunction using a newly validated enzyme-linked immunosorbent assay

β-endorphin, a proopiomelanocortin (POMC)-derived peptide secreted by pars intermedia melanotropes, may play a significant but underexplored role in pituitary pars intermedia dysfunction (PPID) pathophysiology and diagnosis. This study aimed to (1) validate a commercially available human β-endorphin enzyme-linked immunosorbent assay (ELISA) kit for equine samples, and (2) compare β-endorphin concentrations between horses with PPID and healthy controls. Assay validation included the generation of standard curves using purified synthetic equine β-endorphin and human β-endorphin standards, with both curves showing full parallelism. Intra- and inter-assay coefficients of variation (CV) were determined by measuring 37 equine serum samples in duplicate on a single plate and five serum samples across seven different plates. The intra-assay CV was 11.3 % for standards and 5.3 % for equine samples, whereas the inter-assay CV was 6.9 % for standards and 15.6 % for equine samples. Plasma β-endorphin concentrations remained stable over 24 hours regardless of centrifugation timing, storage temperature, or duration. β-endorphin concentrations were determined in five horses with PPID and 20 healthy aged controls. Horses in the PPID group had significantly higher β-endorphin concentrations (median, 506 pg/mL; IQR, 213-762) compared to horses in the control group (median, 35 pg/mL; IQR, 16-55) (P < 0.001). This study may serve as a basis for further research on the role of β-endorphin in horses, particularly in horses with PPID.