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The Human Growth Hormone-Releasing Hormone Transgenic Mouse as a Model of Modest Obesity: Differential Changes in Leptin Receptor (OBR) Gene Expression in the Anterior Pituitary and Hypothalamus after Fasting and OBR Localization in Somatotrophs¹

Department of Anatomy and Neurobiology, University of Kentucky College of Medicine, Lexington, Kentucky 40536

Address all correspondence and requests for reprints to: James F. Hyde, Ph.D., Department of Anatomy and Neurobiology, University of Kentucky College of Medicine, 800 Rose Street (MN224), Lexington, Kentucky 40536-0084. E-mail: jfhyde00{at}pop.uky.edu

We reported previously an increase in leptin receptor (OBR) gene expression in the anterior pituitary of human GH-releasing hormone (hGHRH) transgenic mice. The primary goal of this study was to investigate the possible mechanisms regulating OBR expression in these mice. Compared with normal sibling controls, hGHRH transgenic mice had significantly greater amounts of abdominal fat, higher levels of leptin messenger RNA (mRNA), and a 2-fold increase in plasma leptin concentrations. Despite normal plasma glucose levels, hGHRH transgenic mice had 4.5-fold elevated levels of plasma insulin. Using a ribonuclease protection assay, we measured the mRNA levels of the OBR long form (OBR_L) in the anterior pituitary and hypothalamus after 48 h of fasting. In the anterior pituitary, food deprivation induced dramatic increases in OBR_L mRNA levels in both normal and transgenic mice. In contrast, in the hypothalamus, fasting resulted in a significant decrease in OBR_L gene expression in normal mice, and no changes were detected in hGHRH transgenic mice. Using dual in situ hybridization, OBR_L mRNA was detected in somatotrophs. Moreover, the number of OBR_L-positive pituitary cells as well as the percentage of OBR_L-positive cells that express GH mRNA were increased in transgenic mice. In conclusion, 1) the modest obesity in hGHRH transgenic mice is associated with increases in leptin synthesis and secretion as well as insulin secretion; 2) GH and/or GHRH as well as leptin and insulin may differentially contribute to the changes in OBR_L gene expression in the anterior pituitary and the hypothalamus; 3) the response of OBR_L gene expression in the hypothalamus to fasting is absent in the modestly obese hGHRH transgenic mice; and 4) somatotrophs are target cells for leptin, and the increase in OBR_L gene expression in the pituitary of hGHRH transgenic mice is due at least in part to the increase in the number of cells expressing OBR_L.

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