Relationship between diurnal feeding rhythm and agrp gene expression level in Cyprinus carpio rubrofuscus

Feeding rhythms in fish show inter- and even intra-species variation. In order to detect the circadian feeding rhythm of Cyprinus carpio rubrofuscus and to provide a scientific basis for the establishment of standardized indoor aquaculture methods, we investigated the circadian feeding rhythm of C. carpio rubrofuscus and the relationship between the expression level of Agouti-related protein (AgRP) gene and circadian feeding rhythm under natural light. Besides, we measured the food intake of C. carpio rubrofuscus during the spring and autumn equinoxes in Guangzhou by segmented continuous feeding. We applied quantitative real-time PCR (qRT-PCR) to characterize the circadian expression profile of agrp genes in its brain, foregut, midgut and hindgut. The results show that the food intake of C. carpio rubrofuscus had similar circadian rhythm, lowest at 8:00 in both spring equinox and autumn equinox. Two feeding peaks appeared at 12:00 and 24:00 in the spring equinox, and appeared at 12:00 and 20:00 in the autumn equinox. The qRT-PCR results show that the expressions of agrp1 and agrp2 genes had circadian rhythms, highest at 8:00 and 20:00 but lowest at 24:00 and 4:00 in brain. In foregut, midgut and hindgut, the highest agrp1 expression levels were observed at 16:00, 20:00 and 24:00, respectively, and the lowest level was observed at 8:00. The highest agrp2 expression levels were observed at 16:00, 24:00 and 24:00 but the lowest levels were observed at 12:00, 4:00 and 20:00. In conclusion, when the feeding intake level of C. carpio rubrofuscus was the lowest, the agrp1 gene had the highest expression level in brain to promote subsequent feeding behavior. Between the two feeding peaks (12:00−20:00), the expression levels of agrp1 reached the maximum in foregut, midgut and hindgut in turn. This trend is consistent with the movement of food in intestines. Therefore, the results suggest that agrp1 might be involved in the regulation of circadian feeding rhythm in C. carpio rubrofuscus. However, there was no correlation between the expression level of agrp2 and feeding rhythm. It is indicated that agrp2 might have different functions in the evolutionary process.