Evaluation of a Bovine Temperament Model for Endophenotypes Associated with Hypothalamic-Pituitary-Adrenal Axis Dysfunction
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Dynamic interactions of behavior-related traits and the physiological stress response bear upon the beef industry by impacting animal welfare, health, and productivity. The specific mechanisms of hypothalamic-pituitary-adrenal (HPA) axis dysfunction as related to cattle temperament remain unclear. To further characterize endophenotypes associated with the complex interaction of environment and genotype, the following experiments focused on stimulation and regulation of the pituitary gland in cattle of differing genetic background and temperament. Using serial blood sampling, via jugular cannula, the pituitary and subsequent adrenal response to exogenous vasopressin (VP) was characterized for steers of an excitable or calm temperament. Exit velocity (EV) measured at weaning was used to determine steer temperament. Endocrine parameters were measured for 6 h before and 6 h after the VP administration to quantify the stress response to both the handling associated with the experimental procedures and pharmacological challenge. Elevated concentrations of cortisol in excitable steers during the pre-challenge period reflected an increased initial adrenal reactivity to interactions with humans. Subsequent acclimation to the experimental surroundings yielded greater baseline cortisol concentrations in the cattle with an excitable temperament. Pituitary stimulation with VP resulted in a greater adrenocorticotropic hormone (ACTH) output from the excitable compared to the calm animals. A separate experiment employed the same 12-h blood sampling protocol with a different pituitary secretagogue, corticotrophin-releasing hormone (CRH), in order to evaluate pituitary-adrenal responsiveness in cattle with differing temperaments and genetic backgrounds. Measures of EV at weaning identified the calmest and most excitable steers from two separate calf crops; one Angus and the other Brahman. Within breed, adrenal medullary response to initial handling was influenced by temperament as concentrations of epinephrine and norepinephrine were higher in the excitable steers of both breedtypes. Additionally, concentrations of cortisol also differed by temperament in the Angus steers at this time point. An effect of temperament on pituitary responsiveness to exogenous CRH was observed in the Angus but not the Brahman steers. Unlike what was observed with the previously described VP challenge, the pituitary responsiveness to CRH was blunted in the excitable steers. The specific endophenotypes which have been identified or reinforced through these experiments suggest that there are aspects of HPA dysfunction associated with bovine temperament.
Curley, Kevin (2012). Evaluation of a Bovine Temperament Model for Endophenotypes Associated with Hypothalamic-Pituitary-Adrenal Axis Dysfunction. Doctoral dissertation, Texas A&M University. Available electronically from