The role of NPY in reproductive and metabolic signalling in a mouthbrooding female tilapia, Oreochromis niloticus
2017-05-24T05:30:08Z (GMT) by
Appetite is regulated by a balance between excitatory (drive) and inhibitory (satiety) processes. Dysregulation of this balance leads to eating disorders. However, the neuronal mechanism of control of appetite has not been clearly understood. A cichlid fish, the Nile tilapia(Oreochromis niloticus) is a maternal mouthbrooder in which the female churns the eggs in her mouth for 10-14 days. During mouthbrooding, female fish stops feeding and minimizes energy expenditure to maintain energy homeostasis. Thus, the mouthbrooding tilapia is an ideal model to study the mechanism of appetite regulation and energy homeostasis in the reproductive state. Neuropeptide Y (NPY) is a potent orexigenic neuropeptide which is highly conserved in vertebrates. Although central NPY is known to essentially promote feeding and reduce energy expenditure, recent research has revealed that NPY expression is also influenced by different physiological states. In this thesis, we hypothesize that NPY plays an integral role in linking reproductive and metabolic functions. In Objective I (Chapter 2), we examined the distribution of NPY neurons in the brain to understand its possible role regulation of reproduction in the mouthbrooding tilapia. The 5’and 3’ rapid amplification of cDNA ends method was used to obtain the full length sequence of NPY cDNA. In situ hybridization technique was used to localize NPY cells in three major central brain areas which includes the telencephalon, preoptic area and hypothalamus. In Objective II (Chapter 3), to examine potential roles of three NPY cell populations in the appetite control, npy mRNA expression levels were examined in three brain regions containing NPY cells in mouthbrooding and induced fasting female fish. In the mouthbrooding female, npy mRNA levels were significantly decreased in the hypothalamus of 4 and 12 day mouthbrooding females in comparison to non-mouthbrooding females. In induced fasting female, npy mRNA levels were significantly increased only in the telencephalon. These results suggest different roles of the NPY neuronal populations in the regulation of appetite and energy homeostasis. A significant increase in leptin receptor (Lep-R) and ghrelin receptors (GHS-R) subtypes namely GHS-R1a and GHS-R1b gene expression was measured in mouthbrooding fish. We also found a significant increase in kisspeptin (Kiss2) gene expression in the hypothalamus. In Objective III (Chapter 4), to identify potential association between metabolism and reproduction, effect of kisspeptin on NPY gene was examined in female tilapia under different feeding regimes. Intraperitoneal injection of tilapia kisspeptin (Kiss2) decreased NPY mRNA expression in all feeding groups. Furthermore, in situ hybridization confirmed the distribution of kisspeptin receptor in NPY cell containing regions, suggesting its potential action on NPY systems. The colocalization of the kiss2R mRNA in some NPY mRNA containing cells using the double in situ hybridization in the telencephalon, preoptic area and the hypothalamus, confirmed the role of NPY neurons an important candidate gene in the regulation of feeding and energy metabolism during reproduction. Collectively, the outcome of the three main objectives revealed the neural population specific function of NPY under different feeding conditions. In addition, the different NPY systems are possibly differentially regulated by other neuroendocrine factors including kisspeptin as well as peripheral cues from leptin and ghrelin under different physiological conditions. This study led to the identification of an alternative NPY signaling by Kiss2 to influence feeding behavior and energy homeostasis under specific physiological conditions.