Circulating ACTH and cortisol investigations in standardbred racehorses under training and racing sessions

The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine system involved in the coping response to stressful challenges during exercise stimuli. Exercise represents a significant disruptor of homeostasis, inducing an ACTH-cortisol co-secretion, based on different characteristics of exercise in sport horses. Based on this statement, the aim of this study is to evaluate the circulating adrenocorticotropin and cortisol changes in Standardbred trotters, after training and racing sessions, considering the different age and sex. In particular, the aim is to determine to what extent the level of ACTH and cortisol increases during maximum effort in competition conditions (racing), and to compare two exercise conditions of different intensity, training and racing sessions, and effects on ACTH and cortisol responses. Ten Standardbreds, three females and seven males, clinically healthy, were enrolled and subjected to two exercise conditions: a non-competitive session (training) and then a competitive event (racing). Four of them were 2-year-olds and a further six were 3-year-olds. Training and racing effects on both ACTH (p < 0.01) and cortisol (p < 0.01) values were obtained. Compared to the training session, horses showed greater ACTH concentrations at rest (p < 0.001), at 5 (p < 0.01) and 30 min (p < 0.001), and lower cortisol concentrations only at rest (p < 0.01) after racing; 2- and 3-year-old horses showed the greater ACTH concentrations at 5 and 30 min (p < 0.01) post-racing; males showed the greater ACTH concentrations at 5 min and 30 min (p < 0.01) post-racing. The different stimuli of the two contexts, and differences in exercise intensity, such as training and competitive event, may have affected the direction of hypothalamic-pituitary-adrenal (HPA) axis response, both as an ability to adapt to physical stress of different intensity and as a preparatory activity for coping with stimuli. In conclusion, training and racing events induced a different HPA axis response in which both emotional experience and physical maturity could induce a significant adaptive response. As ACTH and cortisol concentrations in adult equids are extremely heterogeneous, further investigation is required to explore how different variables can influence the hormonal dynamics and their role as expressions of adaptive strategies to stress in horses.