The role of satiety hormones in developmental programming of obesity, stress, and neuroimmune function

Abstract

Ghrelin and leptin, two important metabolic hormones involved in regulation of energy balance in the adult, have recently been shown to have a neurotrophic role during early life development in that they regulate neuronal connectivity within regions of the hypothalamus responsible for control of energy balance. Disturbances to the early life nutritional environment including overnutrition have been highly correlated with long-life metabolic problems, such as childhood obesity and its complications. (Throughout this thesis the term overnutrition refers to the neonatally overfed animal model due to litter manipulation and not as a form of malnutrition, which may elicit similar offspring phenotypes.) In addition to their metabolic roles, leptin and ghrelin appear to be important for early brain development, with leptin stimulating and ghrelin inhibiting the establishment of appetite regulatory circuitry. Recent research suggests that the neurodevelopmental actions of leptin and ghrelin are restricted to a crucial developmental period that occurs during the first two weeks after birth in rodents, which in humans this critical period encompasses the third trimester of pregnancy and approximately 1000 days after birth. However, the effects of overnutrition on the developing brain during this period and the mechanisms that underlie neurodevelopmental actions of leptin and ghrelin are currently unknown. It is also unclear whether overnutrition affects brain development in males and females to the same extent. The early developmental period is associated with enhanced neuronal plasticity, and disturbances during this critical time have the potential to program later life health outcomes. It is therefore important to shed further insight into the roles of leptin and ghrelin, to develop interventions that alleviate the incidence of childhood obesity and associated comorbidities. An animal model of childhood obesity in rats was used to investigate the short- and long-term effects of neonatal overfeeding on the leptin and ghrelin system. The animal model was achieved by manipulating litter sizes into control litters of 12 pups and small litters of 4 pups, the later representing the neonatal overfeeding environment. The reduction of litter size immediately after birth, led to a significant increase in dietary intake. Consequently, neonatally overfed pups experienced accelerated weight gain and disrupted circulating leptin and ghrelin levels in comparison to control counterparts. Using this model we examined the short- and long-term effects of neonatal overnutrition on the leptin and ghrelin systems in males and females. We investigated here the effects of neonatal overnutrition on hypothalamic orexigenic and anorexigenic neuronal fibre immunoreactivity and if these changes were mediated by leptin in male rats. We also investigated whether these effects could be normalised by a leptin antagonist. We found that neonatal overnutrition in males was associated with short-term central leptin resistance, altered orexigenic neuronal fibre immunoreactivity and no changes in anorexigenic immunoreactivity. We also found that short-term neonatal leptin antagonism did not reverse excess body weight or hyperleptinemia. Our findings suggest that factors other than leptin contribute to the obese phenotype. Most importantly, we found that in males the early life effects of neonatal overfeeding are partly resolved in adulthood, emphasising the importance of brain plasticity. Interestingly, neonatally overfed females, despite an obese phenotype characterised by hyperleptinemia and increased body weight compared to controls, did not show the same changes in the central feeding circuitry as observed in males. These findings are suggestive of sex differences in the effects of neonatal overfeeding and of differences in the ability of the male and female central systems to respond to challenges in the early life nutritional environment. Similarly to leptin, ghrelin along with its appetite-stimulating role in adults is involved in the establishment of hypothalamic feeding pathways during neonatal development in rodents. Here was investigated long-term effect of neonatal overfeeding on the ghrelin system including the ability of both forms of ghrelin (acyl ghrelin and des-acyl ghrelin) to access the hypothalamus in male rats. We demonstrated that neonatal overfeeding affects the ghrelin system during early life by suppressing circulating ghrelin levels and increasing hypothalamic responsiveness to exogenous acyl but not des-acyl ghrelin. Neonatal overnutrition also affects the ability of acyl ghrelin to reach the hypothalamus. Importantly, the influences of neonatal overnutrition observed during early life were resolved in adulthood. In contrast to males, where neonatal overnutrition resulted in compromised hypothalamic ghrelin signalling, in females, the central ghrelin system and circulating ghrelin remained intact. However, neonatal overnutrition in females was associated with long-term alterations in the capacity for the pituitary gland to respond to ghrelin. Ghrelin has recently been shown to have additional properties to those involved in satiety signalling, including regulation of hypothalamic-pituitary-adrenal (HPA) axis responses to psychological stress, as well as being a potent anti-inflammatory agent. Here were investigated both forms of ghrelin (acyl and des-acyl ghrelin) and their anti-inflammatory activity hypothesising that this activity is mediated via the HPA axis. In experiments where male rats were concomitantly injected with acyl or des-acyl ghrelin and lipopolysaccharide (LPS) acyl, but not des-acyl ghrelin, suppressed the inflammatory cytokine response to LPS. Des-acyl ghrelin also had no effects on components of the HPA axis. Acyl ghrelin, despite stimulating neuronal activation in the paraventricular nucleus of the hypothalamus in vivo and stimulating adrenocorticotropic hormone release from the pituitary in vitro, did not affect the HPA axis response to LPS. These findings suggest acyl ghrelin¿s anti-inflammatory effects are independent of its actions on the HPA axis and have implications for the potential use of this peptide for treatment of inflammatory conditions without compromising HPA axis activity. In conclusion, findings from this thesis indicate that metabolic hormones leptin and ghrelin have immense importance in the development of central centres of energy control and metabolism. Altered nutritional environment during early development affects both leptin and ghrelin systems in the rat, however it resolves to a certain extent in adulthood. Importantly, these central developmental changes mediated by leptin and ghrelin have a strong sex-specific factor. In this thesis we also added to the understanding of the mechanisms by which ghrelin exerts its anti-inflammatory properties, which is of great importance in development of therapeutic strategies for treatment of inflammatory conditions