Behavioral features and effects of transport procedures on endocrine variables of horses.

This review focuses on the behavioral features of horses and the effects of transport procedures on their endocrine variables. It highlights the contribution of diverse hormones in equine responses to transport stress. Data obtained on the effects of the length of transport, influence of confinement, temperament, and previous experience of horses, also in relation to their utilization (competition and breeding) on hormonal changes are reported. The data highlight the particular role of β-endorphin (β-END), hypothalamus-pituitary-adrenal (HPA) and hypothalamus-pituitary-thyroid (HPT) axes hormones and iodothyronines, and notably of triiodothyronine (T3), in the general adaptive response to different stress conditions, but also provide some evidence about their specific temporal and individual action in these responses. These data confirmed that sympatho-adrenal hormones and β-endorphin in equines are released during the early stages of stress to respond to stressors and to facilitate the hormonal cascade of the HPA axis. The HPA hormones allow modulation of the homeostatic response to specific transport stress conditions so as to maintain functional and behavioral homeostasis. These data also suggest that iodothyronines could help facilitate the specific requirements of different types of stressors. The results show a specific T3 (tri-iodothyronine) decrease as an effect of isolation, but a significant T3 increase after long-distance transport. As a result, a modulation of homeostatic responses to different energy demands during transport in equines could be hypothesized, with T3 playing a central role in physiological stress response. Stressors could modulate, through T3 downregulation, paraventricular nucleus activity and HPA secretion, thus controlling the ability to maintain a physiological stress homeostatic response. This review could offer further data to support the proposed regulation of hypophysiotropic TRHergic neurons as metabolic integrators during stress.