Spatial and temporal mRNA expression of key gastrointestinal peptide hormones in Atlantic salmon during processing of a single meal- towards an understanding of their roles in regulation of digestion and appetite
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Ghrelin (GRLN), Cholecystokinin (CCK) and the Neuropeptide Y (NPY) family member; PYY are gastrointestinal (GI) tract peptide hormones involved in the optimization of the digestive process and regulation of appetite in mammals. Although the functions of these hormones have been extensively studied in higher vertebrates, less is known for fish. The high diversity between fish species, suggest a species specific approach when studying the role of GI tract peptide hormones. Two isoformes of GRLN (GRLN-1, GRLN-2) and CCK (CCK-L, CCK-N), in addition to PYY were recently cloned and characterized in Atlantic salmon. However, no information is available on the postprandial involvement of these peptide hormones in Atlantic salmon GI tract. Therefore an experiment was initiated where the aim was to increase the knowledge of regulation of the digestive process and appetite by these key GI tract peptide hormones in Atlantic salmon. This study describes the spatial distribution and postprandial changes in GRLN, CCK and PYY mRNA expression in the GI tract during the complete processing of a single meal in Atlantic salmon (average body weight: 44.7 ± 2.1 g). The mRNA expression values were obtained by quantitative PCR from four GI tract segments covering stomach, pyloric cecae, midgut and hindgut. In order to assess the GI tract transit, the content of feed and chyme in each GI tract segment, and gallbladder content were assessed. Unfed fish were used as control, and these fish were also used to assess potential diurnal patterns of GRLN-1, GRLN-2, CCK-L, CCK-N and PYY mRNA expression in Atlantic salmon. With the exception of CCK-N which was not expressed in midgut and hindgut, the genes for all peptides were expressed in all GI tract segments analyzed in Atlantic salmon, although at different levels. Both CCK-L and CCK-N mRNA expression increased within 1.5 hours past feeding (hpf) when the stomach was still ca 80 % full, the gallbladder was nearly empty and chyme had started to be transferred into the midgut. The CCK isoforms had different spatial and temporal mRNA expression patterns during the 24 h processing of the meal. PYY mRNA expression increased at 1.5 hpf and then decreased after 3 hpf. No differences were detected between GRLN isoforms and both forms increased after processing of a single meal, indicating minor effect of GRLN on regulation of digestion. Taken together this suggests that CCK and PYY have in part similar roles in regulating digestive processes to that observed in higher vertebrates. The two isoforms of CCK appear to have different functions. The results did not identify a role for GRLN in regulating digestion and appetite comparable to mammals.