The annual fish Nothobranchius rachovii as a model for anorexia of ageing study

Background: One of the most common diseases that affect older people is anorexia of ageing. The latter is defined as a decrease in appetite and a predominant loss of body weight in late life. The basis for appetite loss in elderly people was described as an increase in CCK concentration in humans and rats. Cholecystokinin (CCK) is a peptide hormone that is responsible for regulating food intake and satiety in higher vertebrates. However, it is still unclear whether increased plasma CCK concentrations mediate the age-related decrease in appetite. Even though in vitro studies are excellent for investigating ageing, the use of a model organism that shares and emulates human physiological processes ensures a better comprehension of the in vivo mechanisms. Biogerontology and developmental biology have been developing a strong interest in African annual fishes from the genus Nothobranchius because of their short captive lifespan. The purpose of this study is to evaluate the possibility of using the genus Nothobranchius as a model for anorexia of ageing. Therefore, ensure a potential contribution to understanding the pathway through which CCK induces loss of appetite in older people by comparative/evolutionary localization of the current study model among canonical models of ageing, the morphology of the gastrointestinal tract, and the expression of CCK. Methods: The comparative/evolutionary investigation was conducted using the NCBI blastp (protein-protein BLAST) and NCBI Tree Viewer. The macroscopic morphology, histological features, ultrastructural organization of Nothobranchius rachovii gastrointestinal tract were investigated using stereomicroscope, Masson’s trichrome and alcian blue–PAS staining, and transmission electron microscopy, respectively. The CCK expression pattern was studied through immunofluorescence labeling, western blotting, and quantitative RT-PCR. Results: Several segments formed the intestine which was divided into different sections, with an anterior intestine consisting of a rostral intestinal bulb and an intestinal annex with lower diameter, mid, and posterior intestine. The epithelium during the transition from the rostral intestinal bulb to the posterior intestine sections showed a gradual decrease in the number of striated muscular bundles, villi height, and goblet mucous cell count. The lining epithelium of the intestinal villi was identified as having typical brush border enterocytes that were abundant in mitochondria. Additionally, CCK expression was observed in scattered intraepithelial cells located in the anterior tract of the intestine. Conclusions: This study introduces Nothobranchius rachovii as a model for anorexia of ageing giving the first bases on the gastrointestinal tract morphology and CCK expression pattern. Moreover, the comparative studies on young and elderly Notobranchius can help clarify the involvement of CCK in the mechanisms of anorexia associated with ageing.