THE ROLE OF MATERNAL HIGH FAT-HIGH SUCROSE DIET ON THE FETAL PROGRAMMING OF HYPERTENSION THROUGH INCREASED CARDIOVASCULAR REACTIVITY TO STRESS

Abstract

Modern societies face an increasing prevalence of non-infectious, chronic diseases associated with modern lifestyle. In particular, cardiovascular (and cerebrovascular) disease remains the leading cause of morbidity and mortality, according to the World Health Organization, the American Heart Association and the Australian National Heart Foundation. Our diet, typically high in fat and sugar is one of the main causes, leading to what has been described as an obesity epidemic. Such diets are associated with diabetes, high cholesterol and high blood pressure: all major risk factors for cardiovascular disease. However, there appears to be an even more insidious threat to the cardiovascular health of the next generation. Recent evidence demonstrates that maternal obesity during pregnancy increases the risk of cardiovascular disease in the offspring in later life (McMillen and Robinson 2005). This is termed developmental origins of disease, and is now the subject of research that has expanded exponentially in the last 15 years. It is unclear whether maternal obesity increases cardiovascular risk in the offspring because of the obesity itself or because of mother’s diet, which is the likely cause of the obesity. Moreover, the pathophysiological changes that predispose the adult offspring to cardiovascular disease are unclear. This thesis exams 2 main questions: (1) Does a Western “fast food” diet, high in fat and sugar, during pregnancy, cause high blood pressure in the offspring, even when the mother is not obese? (2) Do the offspring exposed to a perinatal high fat/high sugar diet show altered cardiovascular responses to psychological stress? This may partly explain the pathophysiology of hypertension in the offspring, in the form of neurogenic hypertension. Sachini K. Jayaratne IX The contents of this thesis are as follows. In Chapter 1, the literature surrounding the hypothesis of the Developmental Origins of Health and Disease are reviewed, with a focus on programmed hypertension. Possible causative factors are discussed, including renal and vascular dysfunction, but with the primary focus on a neurogenic cause. The role of increased reactivity to stress is considered in the context of stress-induced causes of cardiovascular disease, and a brief overview of current methodologies used in research to assess autonomic function is given. Finally, the role of oxidative stress in hypertension is reviewed. Chapter 2 presents the results of a study that investigated in rats the impact of a non-obesogenic maternal high fat-high sucrose (HFS) diet on offspring blood pressure and cardiovascular responses to psychological stress. Dams were placed on a “W estern fast food diet” containing 21% (w/w) fat, 34% (w/w) sucrose and 19% protein for 4 weeks prior to conception, during gestation and lactation. The offspring were then weaned and placed on a normal chow diet. Control dams were kept on a normal chow diet for the entire period. At approximately 9-12 months age, the offspring were implanted with blood pressure telemetry probes and blood pressure, heart rate and derived indices of autonomic control (heart rate variability, blood pressure variability and spontaneous baroreflex gain) were measured at rest and in response to air jet (AJS) and restraint stress (RS). The results showed that adult HFS programmed offspring were hypertensive at rest and had increased blood pressure reactivity to AJS. Male, but not female offspring also showed increased reactivity to RS. The results demonstrated that a maternal HFS diet during the perinatal period can cause hypertension in the offspring in later Sachini K. Jayaratne X life, and that this can occur even if the dam is of normal body weight. The manuscript will be submitted to the Journal of Physiology for publication. Chapter 3 compared c-Fos expression in key cardiovascular control regions in the brainstem and hypothalamic following AJS (considered a mild psychological stressor) and RS (considered a more severe stress). Adult male Sprague Dawley rats were subjected to either AJS or RS. After 2 hours post-stress, rats were deeply anaesthetized with pentobarbital sodium (100 mg/kg) and perfused transcardially with physiological saline, followed by 4% paraformaldehyde (w/v) in 0.1M phosphate buffer. Brains were removed, sectioned and reacted immunohistochemically for Fos IgG. RS caused greater overall Fos expression than AJS with regional increases in the dorsomedial hypothalamus (DMH), hypothalamic perifornical area (PeF), lateral periaqueductal grey (lPAG), ventrolateral PAG (vlPAG), locus coeruleus, parabrachial complex and rostral ventrolateral medulla. A differential pattern of Fos expression was noted particularly in the PAG, with AJS predominately activating dorsolateral and lPAG, while RS predominately activating lPAG and vlPAG. The results confirm that RS is a more potent stressor than AJS. In addition, the results support the argument that RS includes a more physical, or interoceptive, component of stress than AJS. This manuscript has been submitted to Neuroscience Letters for publication. Chapter 4 examines the hypothesis that HFS programming of hypertension may have a neurogenic cause due to increased oxidative stress in key brainstem regions of cardiovascular control. HFS programmed offspring were produced as in Chapter 2. The rats were euthanized and the ventrolateral medulla, PAG and DMH/PeF were rapidly collected on ice. The tissue underwent assays to test levels of protein carbonylation, a common marker of oxidative stress, Sachini K. Jayaratne XI and glutathione, an important antioxidant that prevents oxidation of protein side chains. In a separate series of rats, cardiovascular function at rest and in response to AJS was examined following treatment with the antioxidant Tempol in the drinking water for 4 weeks. The results showed that there was increased protein carbonyls and decreased glutathione in the DMH/PeF of HFS offspring, but not in the PAG or RVLM. Tempol treatment abolished the hypertension and cardiovascular reactivity these offspring. These results are the first to demonstrated that oxidative stress plays an important role in the aetiology of high blood pressure and increased cardiovascular reactivity to stress in a non-obesogenic HFS model of developmental hypertension. This manuscript will be submitted to the Journal of Physiology for publication. In a preliminary study, Chapter 5 compares Fos expression, as a marker of neuronal activation, in cardiovascular control regions in HFS and control rats following AJS and RS. HFS and control offspring were produced as described above and at 9 months age male rats were subjected to either AJS or RS. At 2 hours post-stress test, the rats were deeply anaesthetized and perfused with 4% paraformaldehyde. Brains were removed, sectioned and processed for Fos immunohistochemistry. The results showed that in HFS programmed rats, RS produced increased Fos expression in the paraventricular nucleus and reduced Fos expression in the dorsomedial PAG, compared to control rats. Following AJS there was reduced Fos expression in the lateral PAG in HFS offspring than controls. These results provide preliminary data to elucidate the neuroanatomical substrate that underlies the difference in cardiovascular responses to stress in HFS programmed offspring.