Leptin is primarily an adipose-derived hormone that acts in the hypothalamus to regulate body fat levels by suppressing appetite and increasing metabolic rate. Pregnancy is characterised by increased food intake and fat mass to meet the metabolic demands of this physiological state. Leptin concentrations also increase during pregnancy, but this does not prevent the pregnancy-induced hyperphagia, suggesting a state of leptin resistance. The aims of this thesis were to measure hypothalamic leptin responsiveness during pregnancy and to investigate the potential mechanisms underlying pregnancy-induced leptin resistance. The satiety response to intracerebroventricular (i.c.v) leptin was measured in fasted non-pregnant (diestrous), early pregnant (day 7), and mid-pregnant (day 14) rats. Serial blood samples collected from another group of rats demonstrated that despite initial elevated plasma leptin concentrations in pregnant rats, fasting significantly decreased leptin concentrations so that pregnant and non-pregnant groups had similar, low leptin concentrations. Leptin treatment significantly reduced food intake in non-pregnant and early pregnant rats but not in mid-pregnant rats. In addition, there was no post-fasting hyperphagic response in the pregnant rats. These results indicate that pregnant rats become resistant to the satiety action of leptin. To investigate the mechanisms underlying pregnancy-induced leptin resistance, leptin-induced activation of hypothalamic leptin-target neurons was examined. Signal transducer and activator of transcription 3 (STAT3) phosphorylation was measured in non- pregnant and mid-pregnant rats following i.c.v. administration of leptin. Western blot and immunohistochemistry analysis indicated that leptin-induced STAT3 phosphorylation was significantly reduced in the ventromedial nucleus of the hypothalamus (VMH) during pregnancy. A suppression in the amount of leptin-induced STAT3 activation was observed in the arcuate nucleus during pregnancy, yet there was no overall change in the number of leptin responsive neurons compared to non-pregnant rats. This raises the possibility of a decrease in the degree of responsiveness of arcuate nucleus neurons to leptin during pregnancy. Using double-labelled immuno-histochemistry for alpha-melanocyte stimulating hormone (α-MSH) and leptin-induced pSTAT3 it was demonstrated that pro-opiomelanocortin (POMC) neurons remain responsive to leptin during pregnancy. In the VMH, consistent with the reduced pSTAT3, pregnancy also induced a 2-fold reduction in mRNA for the long form of the leptin receptor (Ob-Rb), the only isoform with full signal transduction capabilities. Expression of mRNA for one of the short forms of the leptin receptor (Ob-Ra) in the choroid plexus was decreased in early and late pregnancy, suggesting that reduced leptin transport into the brain may contribute to pregnancy-induced leptin resistance. CSF/plasma leptin concentration ratios did not differ between pregnant and non-pregnant rats however, suggesting unimpaired leptin transport during pregnancy. These results indicate that pregnancy is a state of hypothalamic leptin resistance and is associated with impaired activation of the leptin-induced JAK/STAT3 signalling pathway in the VMH and arcuate nucleus, and reduced expression of Ob-Rb mRNA in the VMH. This state of leptin resistance represents an important adaptation of the maternal brain allowing increased food intake and fat mass so that the maternal body can meet the metabolic demands of pregnancy and prepare for the subsequent demands of lactation.

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.

The influence of estrogen deficiency in physically active women with functional hypothalamic amenorrhea (ExFHA) on cardiovascular regulation is unknown. Three mechanistic studies compared cardiovascular responses to exercise and orthostatic stress in ExFHA women with responses in physically active (ExOv) and sedentary (SedOv) eumenorrheic ovulatory women. Measures included calf blood flow (BF), brachial artery (BA) endothelial dependent and independent function, shear rate (SR), vascular resistance (VR), blood pressure (BP), heart rate (HR), HR variability (HRV), muscle sympathetic nervous activity (MSNA), and serum renin-angiotensin-aldosterone system (RAAS) components. Study one examined the effects of a single bout of dynamic exercise on vascular function in ExFHA (n=12), ExOv (n=14), and SedOv (n=15) women. Pre-exercise, calf BF and BA endothelium-dependent flow-mediated vasodilation (FMD%) were lower (p<0.05) in ExFHA versus ovulatory women in association with higher (p<0.05) calf VR and lower (p<0.05) SR, respectively. Endothelium-independent vasodilation, assessed at baseline only, was also lower (p<0.05) in ExFHA. Post-exercise, calf BF was increased and VR decreased (p<0.05) in ExFHA women, similar (p>0.05) to that observed in ovulatory women. FMD% and SR were augmented (p<0.05) post-exercise, but both remained lower (p<0.05) in ExFHA versus ovulatory women (p<0.05). Study two investigated neurohumoral (MSNA and RAAS) BP regulation during orthostatic stress in ExFHA (n=12) and ExOv (n=17) women. Baseline systolic BP was lower (p<0.05) in ExFHA versus ExOv. Neurohumoral measures did not differ (p>0.05) between the groups at baseline. However, during hypotensive stimuli, MSNA increased to a greater extent (p<0.05), yet angiotensin II and renin were not activated in ExFHA women. Study three examined autonomic control of HR during orthostatic stress in ExFHA (n=11), ExOv (n=17), and SedOv (n=17) women. Lower HR (p<0.05) at rest and during orthostatic stress in ExFHA was associated with markedly elevated (p<0.05) HRV due to higher (p<0.05) parasympathetic modulation. Sympathetic modulation did not differ (p>0.05) between the groups. These studies indicate altered cardiovascular regulation in otherwise healthy ExFHA women. The influence of estrogen deficiency per se in these alterations are not clear, but in light of the etiology of amenorrhea, it is likely that complex interactions between estrogen and energy deficiency and exercise training are involved.

The influence of estrogen deficiency in physically active women with functional hypothalamic amenorrhea (ExFHA) on cardiovascular regulation is unknown. Three mechanistic studies compared cardiovascular responses to exercise and orthostatic stress in ExFHA women with responses in physically active (ExOv) and sedentary (SedOv) eumenorrheic ovulatory women. Measures included calf blood flow (BF), brachial artery (BA) endothelial dependent and independent function, shear rate (SR), vascular resistance (VR), blood pressure (BP), heart rate (HR), HR variability (HRV), muscle sympathetic nervous activity (MSNA), and serum renin-angiotensin-aldosterone system (RAAS) components. Study one examined the effects of a single bout of dynamic exercise on vascular function in ExFHA (n=12), ExOv (n=14), and SedOv (n=15) women. Pre-exercise, calf BF and BA endothelium-dependent flow-mediated vasodilation (FMD%) were lower (p<0.05) in ExFHA versus ovulatory women in association with higher (p<0.05) calf VR and lower (p<0.05) SR, respectively. Endothelium-independent vasodilation, assessed at baseline only, was also lower (p<0.05) in ExFHA. Post-exercise, calf BF was increased and VR decreased (p<0.05) in ExFHA women, similar (p>0.05) to that observed in ovulatory women. FMD% and SR were augmented (p<0.05) post-exercise, but both remained lower (p<0.05) in ExFHA versus ovulatory women (p<0.05). Study two investigated neurohumoral (MSNA and RAAS) BP regulation during orthostatic stress in ExFHA (n=12) and ExOv (n=17) women. Baseline systolic BP was lower (p<0.05) in ExFHA versus ExOv. Neurohumoral measures did not differ (p>0.05) between the groups at baseline. However, during hypotensive stimuli, MSNA increased to a greater extent (p<0.05), yet angiotensin II and renin were not activated in ExFHA women. Study three examined autonomic control of HR during orthostatic stress in ExFHA (n=11), ExOv (n=17), and SedOv (n=17) women. Lower HR (p<0.05) at rest and during orthostatic stress in ExFHA was associated with markedly elevated (p<0.05) HRV due to higher (p<0.05) parasympathetic modulation. Sympathetic modulation did not differ (p>0.05) between the groups. These studies indicate altered cardiovascular regulation in otherwise healthy ExFHA women. The influence of estrogen deficiency per se in these alterations are not clear, but in light of the etiology of amenorrhea, it is likely that complex interactions between estrogen and energy deficiency and exercise training are involved.

The hypothalamus is a central regulator of energy homeostasis. Recently, proglucagon-derived peptides have emerged as potential appetite regulators. The proglucagon gene is expressed in the periphery and also in selective hypothalamic neurons. The regulation of hypothalamic proglucagon by two key regulators of energy balance, insulin and leptin, remains unstudied. Central glucagon-like peptide (GLP)-1 receptor (GLP-1R) activation by exendin-4, a long-acting GLP-1R agonist, induces anorexia; however, the specific hypothalamic neuronal populations activated by exendin-4 remain largely unknown. The role of GLP-2 as a central appetite regulator is poorly understood. In this thesis, using murine hypothalamic cell lines and mice as experimental models, mechanisms involved in the direct regulation of proglucagon gene by insulin and leptin were studied, and the actions of exendin-4 and GLP-2 on hypothalamic neuropeptides were determined. It was found that insulin and leptin regulate hypothalamic proglucagon mRNA by activating Akt and signal transducer and activator of transcription 3, respectively. Insulin and leptin did not regulate human proglucagon promoter regions, but affected proglucagon mRNA stability. In mice, intracerebroventricular exendin-4 and GLP-2 induced anorexia, activated proopiomelanocortin- and neuropeptide Y-expressing neurons in the arcuate nucleus and neurotensin- and ghrelin-expressing neurons in major hypothalamic appetite-regulating regions. In the hypothalamic neuronal models, exendin-4 and GLP-2 activated cAMP-response element-binding protein/activating transcription factor-1, and regulated neurotensin and ghrelin mRNA levels via a protein kinase A-dependent mechanism. Overall, the in vivo and in vitro findings suggest that these neuropeptides may serve as potential downstream mediators of exendin-4 and GLP-2 action. This research demonstrates direct regulation of hypothalamic proglucagon by insulin and leptin in vitro, and reports a previously unrecognized link between central GLP-1R and GLP-2R activation and regulation of hypothalamic neuropeptides. A better understanding of the regulation of hypothalamic proglucagon and central GLP-1R and GLP-2R activation is important to further expand our knowledge of feeding circuits.

The hypothalamus is a central regulator of energy homeostasis. Recently, proglucagon-derived peptides have emerged as potential appetite regulators. The proglucagon gene is expressed in the periphery and also in selective hypothalamic neurons. The regulation of hypothalamic proglucagon by two key regulators of energy balance, insulin and leptin, remains unstudied. Central glucagon-like peptide (GLP)-1 receptor (GLP-1R) activation by exendin-4, a long-acting GLP-1R agonist, induces anorexia; however, the specific hypothalamic neuronal populations activated by exendin-4 remain largely unknown. The role of GLP-2 as a central appetite regulator is poorly understood. In this thesis, using murine hypothalamic cell lines and mice as experimental models, mechanisms involved in the direct regulation of proglucagon gene by insulin and leptin were studied, and the actions of exendin-4 and GLP-2 on hypothalamic neuropeptides were determined. It was found that insulin and leptin regulate hypothalamic proglucagon mRNA by activating Akt and signal transducer and activator of transcription 3, respectively. Insulin and leptin did not regulate human proglucagon promoter regions, but affected proglucagon mRNA stability. In mice, intracerebroventricular exendin-4 and GLP-2 induced anorexia, activated proopiomelanocortin- and neuropeptide Y-expressing neurons in the arcuate nucleus and neurotensin- and ghrelin-expressing neurons in major hypothalamic appetite-regulating regions. In the hypothalamic neuronal models, exendin-4 and GLP-2 activated cAMP-response element-binding protein/activating transcription factor-1, and regulated neurotensin and ghrelin mRNA levels via a protein kinase A-dependent mechanism. Overall, the in vivo and in vitro findings suggest that these neuropeptides may serve as potential downstream mediators of exendin-4 and GLP-2 action. This research demonstrates direct regulation of hypothalamic proglucagon by insulin and leptin in vitro, and reports a previously unrecognized link between central GLP-1R and GLP-2R activation and regulation of hypothalamic neuropeptides. A better understanding of the regulation of hypothalamic proglucagon and central GLP-1R and GLP-2R activation is important to further expand our knowledge of feeding circuits.

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