Abstract
Globally, rates of obesity are increasing. Given that body weight homeostasis is regulated by arcuate nucleus (ARC) melanocortin circuitry, and that offspring born to obese mothers have impairments in this circuitry, resulting in obesity later in life, recent focus has shifted to how this important homeostatic neural circuit develops. In rodents, the majority of ARC melanocortin circuitry development occurs during the early postnatal period, a time when the anorexigenic hormone leptin is endogenously elevated in the circulation, suggesting a link between leptin signalling and ARC melanocortin circuitry development. Supporting this, ARC melanocortin projections are impaired in leptin-deficient ob/ob mice, although exogenous leptin treatment during the early postnatal period can rescue these projection deficits. Leptin receptor mRNA (Leprb) is upregulated in the developing ARC during the same developmental period when melanocortin projections are growing toward their postsynaptic targets, suggesting an increased leptin sensitivity during this important developmental window. However, as Leprb mRNA is expressed in hypothalamic regions in which ARC melanocortin neurons project to in the early postnatal period, exactly where leptin signals in the developing hypothalamus to exert its developmental effect remains unclear. Therefore, I hypothesized that leptin signalling in the postsynaptic target regions of ARC melanocortin projections was necessary for complete ARC melanocortin circuitry maturation.
To test this hypothesis, I first validated that the mice used in this thesis exhibited normal postnatal leptin surge and body weight profiles, compared to previously published data. I found a significant increase in plasma leptin during the early postnatal period, and observed similar increases in body weight when compared to previous reports, suggesting that the mice used in this thesis develop normally. Secondly, I used RT-qPCR to determine the temporal pattern of Leprb expression during the early postnatal period. I found that Leprb was upregulated in the ARC, the dorsomedial nucleus of the hypothalamus (DMH) and the paraventricular nucleus (PVN) during the early postnatal period in both male and female mice, suggesting that these postsynaptic target nuclei become more sensitive to leptin during the early postnatal period. I then used single-plex RNAscope® to determine the spatial pattern of Leprb during the early postnatal period. By comparing the spatial pattern of Leprb expression to the distribution to other genes and proteins, I was able to narrow down the phenotype of Leprb-expressing cells in each nucleus. Finally, as the expression of Leprb mRNA in the PVN had not been previously described during the early postnatal period, I used duplex RNAscope® to reveal the phenotype of PVN Leprb-expressing cells. I found that a subpopulation of melanocortin 4 receptor- expressing cells co-expressed Leprb mRNA, suggesting that leptin signalling at the postsynaptic targets of ARC melanocortin projections may be required for complete ARC melanocortin circuitry maturation.
Overall, the data presented in this thesis suggest that, in addition to leptin signalling in the ARC, leptin signals in the postsynaptic target nuclei of ARC melanocortin projections to promote the development of ARC melanocortin circuitry. This work contributes to an improved understanding of how this circuitry develops and it is hoped that these data and any subsequent research leads to improved outcomes in human health and disease.