No description available.

The Siberian hamster (Phodopus sungorus) undergoes a suite of physiological changes in response to short day (SD) photoperiod which includes a marked reduction in body mass (up to 40%). This altered physiology can be reversed by a return to long day (LD) photoperiod and is driven by changes hypothalamic gene expression. Additionally, stimulation of weight regain occurs through spontaneous exercise when hamsters are provided with a running wheel (RW), despite intact photoperiod appropriate hypothalamic gene expression. The foundation hypothesis for this investigation was that the change in body weight in both paradigms is underpinned by an alteration of the growth hormone (GH) axis. Pasireotide, a somatostatin agonist, was utilised to inhibit GH secretion from the pituitary in both paradigms. Measurement of body mass, mass of internal organs, body composition by magnetic resonance imaging, hormonal analysis and in situ hybridization were used to determine the effect of a blockade of GH secretion by pasireotide. Pasireotide suppressed the GH axis in Siberian hamsters; with reduced circulating insulin-like growth factor-1 and altered hypothalamic gene expression of somatostatin (srif) and growth hormone – releasing hormone (ghrh) consistent with an inhibition of pituitary GH secretion. Pasireotide treatment inhibited RW and LD stimulated growth, and when administered to LD hamsters caused weight loss in a similar manner to that which occurs in SD and accompanied by testicular atrophy. In addition, pasireotide increased the incidence of torpor and increased bout length of this hypometabolic state in sedentary SD hamsters. In conclusion, evidence is provided for the hypothalamic – pituitary – growth hormone axis in the determination of photoperiod and RW induced body weight changes. Furthermore, the data show evidence for a novel muscle – brain pathway and evidence for a neuroendocrine pathway involved in torpor induction.

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.

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

Thesis (Ph. D.)--West Virginia University, 2010. / Title from document title page. Document formatted into pages; contains vi, 67 p. : ill. Includes abstract. Includes bibliographical references (p. 59-67).

Thesis (M.D.)--University of Hong Kong, 1990.

Thesis (M.Sc.)--York University, 2003. Graduate Programme in Kinesiology and Health Science. / Typescript. Includes bibliographical references (leaves 113-122). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pMQ99371

Thesis (M.D.)--University of Hong Kong, 1990. / Also available in print.

Thesis (M.A.)--State University of New York at Binghamton, Psychology Department, 2006. / Includes bibliographical references.

Thesis (Ph. D.)--University of Massachusetts Amherst, 2009. / Open access. Includes bibliographical references (p. 89-87). Print copy also available.