Obesity is a worldwide epidemic and a major risk factor for numerous diseases. The regulation of feeding behavior and body weight depends on a wide range of neuronal pathways influencing satiety and hunger. Serotonin (5-HT) is one of those players identified to have a profound effect on energy homeostasis. The effect of obesity on 5-HT metabolism in the gastrointestinal (GI) tract and its underlying mechanisms still needs to be further elaborated. The aim of the present study was to investigate the effect of diet-induced obesity (DIO) on 5-HT in the enteric nervous system, the expression of different enzymes and receptors of the 5-HT pathway in the brain and GI tract, GI transit and behavior. Zebrafish were fed either a high caloric diet during 4 weeks or a normal diet (CNTL). The proportion of serotonergic neurons in the GI tract was analyzed using immunofluorescent double staining. Quantitative PCR (qPCR) was performed on brain and GI tissue to analyze the expression of 5-HT receptors, the 5-HT precursor, tryptophan hydroxylase (tph), 5-HT transporter (SERTa/b) and monoamine oxidase (MAO). GI transit was measured by gavaging glass beads or providing fluorescently labeled food and calculating the geometric centre (GC). Swim behavior was calculated as preferential swim area, swim speed and distance. Results showed an increase in body mass index after 4 weeks. Overfeeding increased the proportion of serotonergic neurons in the proximal GI tract. qPCR revealed significantly elevated levels for tph2, but not for tph1a/b, in the brain and the intestine of DIO fish. Furthermore, a significant increase in the expression of the 5-HT4 receptor and SERTa were observed in the brain, but not in the GI tract, while 5-HT2b receptor showed to be upregulated in the GI tract, but not the brain. GC was increased after feeding with fluorescently labeled food. Also, the intestinal length in DIO fish was significantly larger, indicating higher transit rates compared to CNTL fish. No differences in behavior were observed between the two groups. This study, revealed an increase in 5-HT expression in enteric neurons probably due to an increased tph2 expression in the intestine, resulting in increased GI transit. Furthermore, DIO exhibited increased expression of the 5-HT4 receptor and SERTa in the brain, and 5-HT2b receptor in the GI tract, respectively. The present data obtained from zebrafish are in line with earlier findings in mammalian models and further validate the zebrafish as a model for GI research.
Diet induced obesity, Serotonin, Gastrointestinal transit, Zebrafish, Enteric nervous system