The role of hypothalamic neuropeptides in the development of obesity related hypertension

Field: Neuroscience (Life Sciences)

The relationship between body fat and blood pressure is such that relatively minor weight gains are associated with increases in mean arterial pressure (MAP). Whilst the relative risk of cardiovascular disease is greater once systolic and diastolic pressures reach 140/90 mmHg, respectively, it is important to note that there is no specific level of MAP at which disease develops and the correlation between MAP and risk of developing cardiovascular disease is strong, positive and continuous in nature (Carretero & Oparil, 2000). It is in this context that the increase in the prevalence of obesity must be examined. Obesity is now considered a global epidemic placing a considerable economic and health burden on society. Of major concern is the association between obesity and hypertension with some reports suggesting up to 78 % of newly diagnosed hypertension is attributable to obesity. The current thesis examines the effect of consuming a high fat diet (HFD) on metabolic profiles, haemodynamic rhythms and hypothalamic neurons pertinent to energy homeostasis and cardiovascular regulation. Experiments were conducted on New Zealand White rabbits given free access to either a normal fat diet (NFD) or a HFD for three weeks. The main advantage of short-term exposure to an obesogenic stimulus is removal of confounding variables associated with obesity. Therefore, early changes that are key to the development of hypertension can be detected more easily. An additional benefit of the model is that diet-induced obesity in rabbits manifests in a manner analogous to human obesity, namely increased adiposity, hypertension and augmented renal sympathetic nerve activity. Moreover, hypothalamic neuropeptides regulating energy and cardiovascular homeostasis as well as sympathetic tone in humans are well conserved evolutionarily and are thus at play in the rabbit. Recent evidence suggests specific lipid groups such as ceramides and cholesterol esters play a direct role in obesity-related pathologies. The haemodynamic and sympathetic response to the HFD develops within the firstfew days of consumption and it is possible that rapid changes in plasma lipid profiles accompany early weight gain and drive, at least initially, the observed hypertension. In addition to dyslipidaemia, hyperinsulinemia and hyperleptinaemia are known to exert a deleterious effect on cardiovascular health. Consequently, the first investigation described in Chapter 3 examined the effect of three week fat-feeding on metabolic profiles in the rabbit. Circulating insulin and glucose levels were increased within the first three days of the diet in HFD rabbits alone, remaining elevated at week 1 and returning to baseline thereafter. By contrast, plasma leptin levels remained unchanged in both dietary groups, increasing by week 3 in HFD rabbits alone. There was no dyslipidaemia in either NFD or HFD-fed rabbits. The main finding was that plasma insulin and leptin, but not dyslipidaemia, are likely involved in the genesis of hypertension in obesity albeit at different time frames. Rabbits fed a single meal a day exhibit meal-associated haemodynamic rhythms that are characterised by pre-prandial troughs and post-prandial peaks. Consumption of a HFD ad libitum increases pre-prandial values resulting in ‘diurnal non-dipping’, the failure of blood pressure and heart rate to decrease during the day. This shift in pattern means arterial pressure remains elevated for the duration of the 24-hour cycle subsequently increasing the risk of developing cardiovascular disease. It remains unclear whether the cardiovascular changes are due to the fat content of the diet or increased caloric intake from hyperphagia. Thus the novel aspect introduced to the rabbit model in Chapter 4 was the granting of ad libitum access to both NFD and HFD-fed rabbits. Whilst subtle, this change to dietary habits enabled the effect of increased total caloric intake to be distinguished from the effect of increased caloric intake from fat. Rabbits fed a HFD had greater MAP and heart rate (HR) and exhibited marked increases in pre-prandial HR. By contrast, rabbits fed a NFD showed no change in cardiovascular parameters despite consuming more calories. Average daily locomotor activity was decreased in both dietary groups. Thus, increased calories, specifically from fat, affects cardiovascular variables whilst decreased locomotor activity appears to result from greater caloric load irrespective of source. Obesity is associated with augmented sympathetic nerve activity that is suggested to play a key role in the development of hypertension. The central actions of the hormone leptin, an anorectic adipokine, are thought to link excess adiposity and enhanced sympathetic tone. The actions of leptin are dependent on secondary neurons in the hypothalamus which synthesise and release neuropeptide Y (NPY) and alpha-melanocortin stimulating hormone (α-MSH). Thus, in Chapter 5 the contribution of both neuronal populations to the development of obesity related hypertension was assessed. HFD-fed rabbits, but not NFD rabbits, exhibited marked renal sympathetic nerve activity (RSNA) and HR responses to intracerebroventricular (ICV) α-MSH which were concomitant with decreased neuronal activity (c-Fos positive cells) in the dorsomedial, ventromedial and paraventricular nuclei of the hypothalamus. ICV NPY reduced MAP in HFD but not control animals. Taken together the results suggest three-week fat-feeding produces marked hypersensitivity to both α-MSH and NPY, enhancing RSNA and resulting in hypertension. Of the several hypothalamic nuclei involved in the transduction of leptin signals, the ventromedial hypothalamus (VMH) receives input from both NPY and α-MSH containing neurons in the arcuate (ARC). Moreover, high levels of the leptin receptor are expressed in the VMH whilst projections to hindbrain nuclei regulating sympathetic tone are known to originate in the VMH. In addition, the VMH was the only nucleus in which targeted injections of exogenous leptin increased both cardiovascular parameters and RSNA. Consequently, the study described in Chapter 6 examined the contribution of the VMH to obesity related neurogenic hypertension. Bilateral injections of α-MSH into the VMH of HFD rabbits produced marked renal sympathoexcitation, tachycardia and a small increase in blood pressure. Similarly, microinjections of NPY produce sympathoexcitation but failed to elicit a depressor response observed in NFD-fed animals. Microinjections of the leptin receptor antagonist decreased MAP, HR and RSNA in HFD but not NFD-fed rabbits. Combined, the results obtained in Chapter 6 suggest VMH neurons amplify both leptin and α-MSH signalling, contributing to enhanced sympathetic tone in HFD rabbits. Moreover, the loss of NPY-mediated depressor response concomitant with the fact NPY enhanced RSNA implies subpopulations of NPY neurons are affected in discrete ways by consumption of a HFD. Collectively, the work presented in this thesis suggests that relatively short-term exposure to a high-fat diet enhances central sensitivity to α-MSH and NPY. These rapid changes occur prior to considerable changes in bodyweight and are preceded by increased circulating insulin and leptin levels. Both hormones affect cardiovascular regulation and are likely to contribute to the functional changes of hypothalamic neurons which lead to obesity related hypertension. The findings presented in this thesis form a new approach to the development of obesity related neurogenic hypertension.