Apelin and apj in common carp (Cyprinus carpio L.): Molecular characterization, tissue expression and responses to periprandial changes and fasting–refeeding

Apelin and its receptor apj have a variety of biological functions and are involved in appetite and energy regulation. However, the comparative studies of apelin/apj system in fish are still absent. In this study, the open reading frames of apelin and apja and apjb were obtained. Apelin and apj mRNAs were widely distributed in all tissues detected, with the highest level in the brain, followed by the spleen, kidney and foregut. The mRNA expression of apelin and apjs increased rapidly at mealtime and significantly decreased after 30 min of feeding. In addition, there were obvious circadian rhythms (higher at nighttime and lower at daytime) in the expression of apelin and apja and apjb. Furthermore, fasting elevated the mRNA levels of apelin and apjs in the hypothalamus, foregut and hepatopancreas. When the 8‐day fasted fish were refed, apelin and apjs mRNA expression significantly decreased. However, there were differences in the response to fasting and refeeding between apja and apjb. The roles of apelin and its receptor apjs in fish are poorly understood. Herein, we employed molecular and physiological studies to characterize their structures, distribution and mRNA expression responses to periprandial changes and fasting–refeeding in Cyprinus carpio L. for the first time.