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Physiological Studies of Transgenic Mice Overexpressing Growth Hormone (GH) Secretagogue Receptor 1A in GH-Releasing Hormone Neurons

Division of Molecular Neuroendocrinology, National Institute for Medical Research, London NW7 1AA, United Kingdom; Beth Israel Deaconess Medical Center, Division of Endocrinology, Harvard Medical School (N.B.), Boston, Massachusetts 02215; Department of Neurosurgery, Barts, and The London School of Medicine and Dentistry, Queen Mary, University of London (C.M.), London E1 4NS, United Kingdom; and Imperial College London, Department of Neuroendocrinology, Hammersmith Hospital (P.H.), London W12 ONN, United Kingdom

Address all correspondence and requests for reprints to: Prof. Iain C. A. F. Robinson, Division of Molecular Neuroendocrinology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom. E-mail: irobins{at}nimr.mrc.ac.uk.

The type 1A GH secretagogue (GHS) receptor (GHSR) has been proposed to mediate the effects of ghrelin on GH release, food intake, and body composition. We have overexpressed GHSR in GH-producing GC cells and GHRH neurons in an attempt to enhance signaling via this pathway selectively, in the GH axis. Constitutive overexpression of human GHSR in rat GC cell lines resulted in increased basal phosphoinositol turnover and rendered them responsive to GHS ligands. We then generated transgenic mice overexpressing human GHSR in GHRH neurons using a 38-kb rat GHRH cosmid promoter. GHRH-GHSR transgenic mice showed increased hypothalamic GHRH expression, pituitary GH contents, and postweaning growth rates. Body weights of the transgenic mice became similar in adulthood, whereas adipose mass was reduced, particularly so in female GHRH-GHSR mice. Organ and muscle weights of transgenic mice were increased despite chronic exposure to a high fat diet. These results suggest that constitutive overexpression of GHSR in GHRH neurons up-regulates basal activity in the GHRH-GH axis. However, GHRH-GHSR mice showed no evidence of increased sensitivity to acute or chronic treatment with exogenous GHS ligands. Food intake and adipose tissue responses to chronic high fat feeding and treatment with GHS ligands were unaffected, as were locomotor and anxiety behaviors, although GHRH-GHSR mice remained significantly leaner than wild-type littermates. Thus, constitutive overexpression of GHSR can up-regulate basal signaling activity in the GHRH/GH axis and reduce adiposity without affecting other GHSR-mediated signals.

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