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Title: ENDOCRINE AND OREXIGENIC EFFECTS OF GROWTH HORMONE SECRETOGOGUES IN RAINBOW TROUT (ONCORHYNCHUS MYKISS)

Author
item Shepherd, Brian
item JOHNSON, JAIME - UNIVERSITY OF KENTUCKY
item Silverstein, Jeffrey
item PARHAR, ISHWAR - NIPPON MED SCHOOL, JAPAN
item VIJAYAN, MATHILIKATH - UNIV OF WATERLOO, ONT, CA
item MCGUIRE, ALISON - UNI OF WATERLOO, ONT CANA
item Weber, Gregory - Greg

Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2006
Publication Date: 11/25/2006
Citation: Shepherd, B.S., Johnson, J.K., Silverstein, J., Parhar, I., Vijayan, M.M., McGuire, A., Weber, G.M. 2006. Endocrine and orexigenic effects of growth hormone secretogogues in rainbow trout (Oncorhynchus mykiss). Comparative Biochemistry and Physiology. Part A 146(2007):390-399.

Interpretive Summary: Using synthetic peptides, we recently demonstrated that a new regulatory pathway, that controls pituitary growth hormone (GH) secretion, is present in fish. This work demonstrates the existence of a new endocrine axis that controls growth hormone (GH) secretion in teleosts. Recently, other researchers identified a new hormone, called "ghrelin", that is produced by the stomach, and which regulates pituitary GH secretion via the same mechanism. Our work in rainbow trout has shown that ghrelin, and other GH-releasing peptides (the synthetic GH-releasing peptide, GHRP-2 and human GH-releasing hormone, GHRH), stimulate important components (GH, insulin-like growth factor-I and the insulin-like growth factor-binding proteins) of the endocrine growth axis, and feed intake, in this species. This is the first demonstration of an effect of ghrelin treatment on these components of the endocrine growth axis. It is expected that these findings will result in novel ways to promote growth in commercially-important teleosts.

Technical Abstract: In teleosts, the effects of growth hormone secretagogues (GHSs) on the somatotropic axis are poorly understood, particularly with respect to Insulin-like growth factor-I and the IGF-binding proteins (IGFBPs). To assess the endocrine and orexigenic responses of rainbow trout (Oncorhynchus mykiss) to GHS treatment, animals were injected with human GHRH1-29-amide, KP-102 or rat ghrelin (0, 1 or 10 picomole/gram body weight) and feed intake (2 and 5 h post-injection) and plasma levels of growth hormone (GH), IGF-I and the IGFBPs were determined at 3, 6 and 12 h post-injection. Feed intake was significantly elevated by all of the GHSs tested. All GHSs elevated plasma GH levels in a time-dependent manner. Plasma IGF-I levels were elevated by all GHSs at 3 h post-injection, whereas those animals treated with KP-102 and ghrelin exhibited depressions at 6 h. Four IGFBPs were identified in the plasma by western blotting. Levels of the 20 kDa IGFBP were only altered by sampling time. Levels of the 32 kDa IGFBP were significantly depressed by all GHSs tested. Levels of the 42 kDa IGFBP were significantly elevated by all GHSs tested. Plasma levels of the 50 kDa IGFBP was decreased in some treatment groups at 3 h, but elevated by 6 h in the ghrelin-treated groups and elevated in all treatment groups by 12 h post-injection. These endocrine and orexigenic responses demonstrate that GHSs influence the teleost neuroendocrine system well after short-term actions (< 3 hr post-injection) on GH release, and the responses of the IGFBPs to GHS treatment support this notion and clarify their identification as functional homologues to mammalian IGFBPs.