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Fabulous fat – assessment of plasma ghrelin levels will help elucidate adiposity regulation in salmonid fish

By: Elisabeth Jönsson-Bergman and Björn Thrandur Björnsson, Fish Endocrinology Laboratory, Göteborg University

 

 

A SEAFOODplus research group has successfully developed a method for assessing the peptide ghrelin in fish plasma. This will help optimize lipid levels in tailor made aquaculture products. 

 

Marine lipids are considered to be of major health benefit to humans, and fattiness is thus an important quality trait of seafood. For optimizing lipid levels of tailormade aquacultured products, a basic biological understanding is needed for how energy balance and adiposity of fish is regulated. This is one of the goals of SEAFOODplus project 5.1 BIOQUAL.

 

In 1999, a peptide termed ghrelin (Ghr) was discovered in the rat stomach (Kojima et al 1999), and was subsequently found in fish (Unniappan et al 2002, Kaiya et al 2003). What made this discovery particularly exciting was that Ghr stimulates a receptor in the mammalian pituitary and brain, causing the release of growth hormone (GH). Further, Ghr levels rise during fasting and decrease following a meal, indicating a role in the shortterm control of premeal hunger. Ghr simulates adiposity and is the only known humeral orexigen (appetite stimulant) in mammals (Tschop et al 2000, Nakazato et al 2001). Not surprisingly, Ghr is being intensely researched in humans, e.g. in the FP6 Food Quality and Safety IP  DIABESITY, which runs in parallel with SEAFOODplus.

 

Ghrelin has recently been isolated and cloned in a few fish spe¬cies, including the rainbow trout. Similar to mammals, ghrelin is produced in the gastrointestinal tract and stimulates GH release (Kaiya et al 2003). Ghrelin also stimulates appetite in goldfish (Unniappan et al 2002), but further data on the physiological function of ghrelin in fish are scarce.

 

A key step in further research is the establishment of a method for assessing plasma Ghr levels in salmonids. This has now been accomplished by the UGOT team within SEAFOODplus, with the validation of a specific radioimmunoassay (RIA) for rain¬bow trout and Atlantic salmon. The RIA is being applied to samples from quantitative and qualitative feeding trials carried out by BIOMAR and NIFA, where UGOT studies the role of Ghr in regulating adiposity, energy status and growth.

 

Standard curves validating how specific binding of ghrelin occurs in rainbow trout and Atlantic salmon by the parallelity test procedure.
A graphical sketch of a ghrelin molecule showing the positions of the amino acids in the peptide chain.

References:

 

Kojima M, Hosada H, Date Y, Nakazato M, Matsuo H, Kangawa K 1999. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 402:656-660.

 

Tschop M, Smiley DL, Heiman ML 2000. Ghrelin induces adiposity in rodents. Nature 407: 908-913.

 

Nakazato M, Murakami N, Date Y, Kojima M, Matsuo H, Kangawa K 2001. A role for ghrelin in the central regulation of feeding. Nature 409:194-198.

 

Kaiya, H, Kojima M, Hosoda H, Moriyama S, Takahashi A, Kawauchi H, Kangawa K. 2003. Peptide purification, complementary deoxyribonucleic acid (DNA) and genomic DNA clon¬ing, and functional characterization of ghrelin in rainbow trout. Endocrinology 144:5215-5226.

 

Unniappan S, Lin X, Cervini L, Rivier J, Kaiya H, Kangawa K, Peter RE. 2002. Goldfish ghrelin: molecular characterization of the complementary deoxyribonucleic acid, partial gene structure and evidence for its stimulatory role in food intake. Endocrinology 143:4143-4146.