Endocrinological and behavioural adaptations to experimentally induced physical stress in horses

Abstract

The major objective of this thesis is to find parameters to diagnose early overtraining (a stress-related disorder) in horses. Diagnosing overtraining remains still a major challenge. In man, a decrease in performance despite normal training, is indicative for the syndrome. The diagnosis is made by use of the Profile of the Mood (POMS) questionnaire, a valuable but subjective tool. Hormonal disbalance due to prolonged severe stress is considered as a possible cause of the syndrome. In this thesis, the results of an overtraining study in Standardbred horses will be presented. The horses underwent training on a high speed treadmill. The study consisted of four periods, namely an acclimatisation, training, intensified training and recovery period with at the end of each period a test week. After the training period the horses were randomly divided into a control group and an intensified trained (IT) group. Standardized exercise tests were used to monitor performance. Behavioural studies were performed to monitor mood changes. The growth hormone axis as well as glucose metabolism were monitored to detect hormonal disturbances. The performance of the horses increased during the training period, however during the intensified training period the IT group showed a decrease in performance despite the fact that the horses were more intensively trained than the control horses. The horses changed their gait from trot to galop or even stopped during the training. Symptoms are indicative of overtraining. The behavioural study consisted of three different assessments exclusively developed for the current experiment and aimed to distinguish between the mood states as described in the POMS. During the intensified training period the IT horses showed obvious changes in mood state. The horses were less interested in the environment and in other unknown horses as was shown in different behavioural changes. Growth hormone is secreted in a pulsatile fashion which is apparently random in nature and therefore difficult to quantify: concentration time series are needed instead of single blood samples. Deconvolution algorithm is used in this thesis to quantify equine GH secretory and elimination dynamics for the first time. Approximate Entropy was used to quantify orderliness of the GH time series. The growth hormone profile changed significantly for the intensified trained horse during the intensified training period. The profile changed to a more irregular pattern with smaller but more frequent pulses with a longer GH half-life. It was hypothesized that the increase in GH pulsatility might be beneficial to restore homeostatic balance. Glucose metabolism and peripheral insulin sensitivity were measured by use of the euglycaemic hyperinsulinaemic clamp technique. The intensified trained horses showed dissociation of correlation between glucose metabolism and the mean glucose metabolism rate to plasma insulin concentration ratio during the intensified training period indicative of loss of coordinated control of glucose metabolism. In conclusion: after the detraining period the intensified horses were not fully recovered which indicates that an early form of overtraining was induced. This thesis clearly shows that overtraining and possibly other stress related syndromes induce mental problems before physical problems become apparent.