The role of circulating β-endorphin in different stress models in Equines: a review.

To evaluate whether β-endorphin (β-END) is a reliable marker of stress in equines, the reference values and changes in different models of stress (exercise, transportation) have been considered. Baseline circulating β-END concentrations of horses showed seasonal and circadian fluctuations. Increased β-END concentrations were associated with the severity and type of exercise stress and were positively correlated with both exercise speed and intensity. Positive correlations between β-END or adrenocorticotropic hormone (ACTH) and cortisol were recorded after competitive and experimental jumping over different fence height, related to the emotional content of trials. In Standardbreds, an exercise effect on β-END changes during both training and competition emerged. In trained horses, after exercise with psychomotor disables, no β-END and ACTH changes were detected. After short road transport (100 km), concomitant variations in β-END and ACTH concentrations suggested the effect of loading and confinement. In experienced stallions, after simulated transport, higher β-END concentrations than basal, while lower concentrations than after conventional transport, were recorded. Positive correlations among β-END, ACTH, and cortisol changes in both conventional and simulated transport were observed. After a very short road transportation (50 km), no significant differences in β-END concentrations were recorded in experienced equines. Horses facing forward had significantly higher β-END concentrations than horses facing backward. It can be supposed that, in equines, β-END is released during the early stages of stress, so improving the hormonal cascade of hypothalamic–pituitary–adrenal axis and β-END could facilitate the active coping strategies under stressful conditions and can be regarded as a precocious marker of stress.