Cholecystokinin (CCK) and its receptors (CCK1R and CCK2R) in chickens: functional analysis and tissue expression

Cholecystokinin (CCK) is widely distributed in the gastrointestinal tract and central nervous system, regulating a range of physiological functions by activating its receptors (CCK1R and CCK2R). Compared to those in mammals, the CCK gene and its receptors have already been cloned in various birds, such as chickens. However, knowledge regarding their functionality and tissue expression is limited. In this study, we examined the expression of CCK and its 2 receptors in chicken tissues. In addition, the functionality of the 2 receptors was investigated. Using 3 cell-based luciferase reporter systems and western blots, we demonstrated that chicken (c-) CCK1R could be potently activated by cCCK-8S but not cCCK-4, whereas cCCK2R could be activated by cCCK-8S and cCCK-4 with similar efficiency. Using RNA-sequencing, we revealed that cCCK is abundantly expressed in the testis, ileum, and several brain regions (cerebrum, midbrain, cerebellum, hindbrain, and hypothalamus). The abundant expression of CCK in the hypothalamus was further supported by immunofluorescence. In addition, cCCK1R is highly expressed in the pancreas and moderately expressed in various intestinal regions (ileum, cecum, and rectum) and the pituitary gland, whereas cCCK2R expression is primarily restricted to the brain. Our data reveal the differential specificities of CCK receptors for various CCK peptides. In combination with the differential tissue distribution of CCK and its receptors, the present study helps to understanding the physiological functions of CCK/CCKRs in birds.