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Title: Ghrelin stimulates growth hormone release from the pituitary via hypothalamic growth hormone-releasing hormone neurons in the cichlid, Oreochromis niloticus

item OGAWA, SATOSHI - Monash University
item SELVARAJAH, GEETA - Inti International University College
item XIAOCHUN, LIU - Guangzhou University
item Shepherd, Brian
item PARHAR, ISHWAR - Monash University

Submitted to: Cell and Tissue Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2018
Publication Date: 6/23/2018
Citation: Ogawa, S., Selvarajah, G., Xiaochun, L., Shepherd, B.S., Parhar, I.S. 2018. Ghrelin stimulates growth hormone release from the pituitary via hypothalamic growth hormone-releasing hormone neurons in the cichlid, Oreochromis niloticus. Cell and Tissue Research.

Interpretive Summary: In fish, growth is generally related to feed intake and the greater the feed intake, the greater the growth. Small proteins called neuropeptides, which are located in the brain and elsewhere, have been shown to affect feed intake in fish, but little is known of their interactions and regulation. Ghrelin is a peptide which is secreted by the stomach during fasting and signals the brain to increase feed intake. However, very little is known about how ghrelin interacts with neuropeptides in the brain to act as a signal for increased feed intake. The aim of this study was to map the location of specific neurons in the brain of tilapia and determine the effects of ghrelin in this commercially important finfish. Ghrelin treatment affected levels of growth hormone in the pituitary, insulin-like growth factor-I in blood, and growth hormone releasing hormone in the hypothalamus. This study increases our understanding of how ghrelin controls neuropeptides and other hormones related to growth and feed intake. This knowledge could help to improve feeding practices and diet formulations in commercially important finfish species.

Technical Abstract: Ghrelin, a gastric peptide, is implicated in a multiplicity of biological functions, including energy homeostasis and reproduction. Neuronal systems that are involved in energy homeostasis as well as reproduction traverse the hypothalamus, however, the mechanism by which they control energy homeostasis is not fully understood. In this study, we first examined neural associations among three hypothalamic neuropeptides controlling reproduction and homeostasis including gonadotropin-releasing hormone (GnRH), neuropeptide Y (NPY) and growth hormone-releasing hormone (GHRH) neurons by double-immunofluorescence. Further we examined the effect of centrally administered ghrelin on these neuropeptides containing neurons. In addition, we also examined plasma levels of growth hormone in the fish treated with ghrelin. Double-labeling revealed possible neural associations among GnRH, NPY and GHRH neurons in the brain and pituitary.Intracerebroventricular injection of ghrelin significantly decreased the number of GHRH expressing cells, which may indicate promotion of peptide release from the GHRH cell soma. However there was no effect of ghrelin on GnRH and NPY neurons. In addition, plasma GH levels were significantly increased in the animals injected with ghrelin. These results suggest that central administration of ghrelin primarily act on hypothalamic GHRH neurons to stimulate GH release from the pituitary in the tilapia.