Cholecystokinin (CCK)
In mammals, CCK inhibits food intake and induces the release of digestive enzymes from intestine/pancreas and gallbladder (Boguszewski et al., 2010; Dockray, 2012).
In fish, CCK was first shown to have a role in digestion, as, for example, it stimulated contraction of the gallbadder in coho (Vigna and Gorbman, 1977) and Atlantic (Aldman and Holmgren, 1987) salmon, as well as bluegill (Lepomis macrochirus), killifish (Fundulus heteroclitus), and the holostean bowfin (Amia calva) (Rajjo et al., 1988), stimulated lipase secretion in the stomachless killifish (Honkanen et al., 1988) and inhibited gastric secretion in Atlantic cod (Holstein, 1982). The first direct evidence of the actions of CCK on feeding was provided by injections in goldfish (Himick and Peter, 1994), followed by cloning of goldfish CCK cDNA (Peyon et al., 1998) and the demonstration of periprandial variations in CCK mRNA expression levels (Peyon et al., 1999). Subsequently, a number of studies have characterized CCK in several fish, including other Cypriniformes (e.g., common carp Zhong et al., 2013; zebrafish Koven and Schulte, 2012; Tian et al., 2015; grass carp; blunt snout bream Ping et al., 2013; Ji et al., 2015), Characiformes (e.g., cavefish Wall and Volkoff, 2013, dourado Pereira et al., 2015; Volkoff et al., 2016, thin dogfish Oligosarcus hepsetus Vieira-Lopes et al., 2013, pirapitinga Volkoff, 2015a, red-bellied piranha Volkoff, 2014a, pacu Volkoff et al., 2017), Salmoniformes (e.g., Atlantic salmon Valen et al., 2011), Gadiformes (Atlantic cod Tillner et al., 2013), Perciformes [e.g., yellowtail (Furutani et al., 2013; Hosomi et al., 2014); Astatotilapia burtoni (Grone et al., 2012); cunner (Babichuk and Volkoff, 2013; Hayes and Volkoff, 2014); sea bass (Tillner et al., 2014); yellow croaker (Larimichthys crocea) (Cai et al., 2015); white sea bream, Diplodus sargus (Micale et al., 2012, 2014)], Pleuronectiformes (e.g., winter flounder (MacDonald and Volkoff, 2009a), Atlantic halibut Kamisaka et al., 2001, olive flounder Kurokawa et al., 2000) and Siluriformes (channel catfish Peterson et al., 2012).
Overall, in all fish species studied to date, CCK appears to have similar roles in feeding and digestive processes to its role in mammals, i.e., it acts as a satiety/appetite-inhibiting factor and induces the release of digestive enzymes from the GIT.