Submitted to: International Symposium on Fish Endocrinology
Publication Type: Abstract Only
Publication Acceptance Date: July 15, 2004
Publication Date: September 15, 2004
Citation: Shepherd, B.S., Johnson, J., Silverstein, J., Weber, G.M. 2004. Endocrine and orexigenic effects of growth hormone secretogogues in rainbow trout (oncorhynchus mykiss). International Symposium on Fish Endocrinology. Technical Abstract: The pituitary hormone, growth hormone (GH), and its intermediary, insulin-like growth factor I (IGF-I), have fundamental roles in the regulation of growth in teleosts. Growth hormone secretion is principally controlled by the two neuroendocrine factors, growth hormone-releasing hormone (GHRH) and somatostatin (SS). Recently, a newly discovered hormone, termed "Ghrelin", and growth hormone-releasing peptides (GHRPs: synthetic hexapeptides), which bind to the same receptor (GHRP-receptor) on the pituitary & hypothalamus, have been shown to stimulate GH secretion in a variety of teleosts. However, a recent study reported no effects of the synthetic GHRPs to stimulate GH release in juvenile grass carp, suggesting species differences. Indeed, a major difference between the GHRPs, and Ghrelin(s), appears to reside in their specificity to the somatotrope. In this regard, studies in tilapia, eel and goldfish show that the release of other pituitary hormones follows in vitro exposure to Ghrelin(s), whereas this was not seen in tilapia treated with GHRP (KP-102) or bGHRH in vivo. By contrast, a recent study in trout (Kaiya, H. et al., 2003, Endocrinology: 144: 5215-5226) showed a lack of this non-specific action (in vivo or in vitro) for either of the trout Ghrelins identified. Despite these findings, there have been no studies that have examined the full effects of either Ghrelin or GHRPs on the somatotropic axis in a teleost. Against this background we have undertaken work to examine the effects of these GH secretogogues on the somatotropic axis, in the rainbow trout (Oncorhynchus mykiss), by measuring plasma levels of GH, IGF-I and the IGFBPs following secretogogue treatment. We have also evaluated the physiological response of rainbow trout to secretogogue treatment by determining their effects (orexigenic) on feeding. We will present findings that show dose- and time-dependent responses of the somatropic axis, and feeding, in rainbow trout treated with Ghrelin, hGHRH and KP-102.