Cholecystokinin: Molecular cloning and immunohistochemical localization in the gastrointestinal tract of larval red drum, Sciaenops ocellatus (L.)

The current study sought to clarify the role of cholecystokinin (CCK) in the digestion of larval red drum (Sciaenops ocellatus) in order to better characterize the processes limiting the utilization of microparticulate diets at first feeding. The red drum CCK cDNA, isolated from adult anterior intestine and pyloric caeca, contains a 414 base pair (bp) open reading frame encoding a deduced amino acid sequence of 138 residues which is highly similar to preprocholecystokinin from other vertebrates. The mature CCK octapeptide has the same amino acid sequence as that found in mammals and in Atlantic herring (Clupea harengus). Tissue distribution analysis of adult and juvenile red drum using primers specific for red drum CCK mRNA revealed bright bands in samples from the brain, pyloric caeca, anterior intestine, and gonad with fainter bands seen in all other tissues. Immunohistochemical analysis of larval red drum showed that CCK-immunoreactive (CCK-IR) cells were present as early as 3 days post hatch (DPH) in some fish and were present in all fish by 6 DPH. CCK-IR cells were found in the anterior midgut in early larvae and had spread to the first bend of the gut by day 6. In older larvae (18+ DPH), CCK-IR cells were found in large numbers in the anterior intestine and in the developing pyloric caeca. The sequence and distribution of CCK mRNA along with the presence of CCK-IR cells in early red drum larvae suggest that CCK is present and may be capable of regulating pancreatic secretion in early red drum larvae.