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 somatotrophs

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 mecha nisms regulating OBR expression in these mice. Compared with normal sibling controls, hGHRH transgenic mice had significantly greater amounts of abdom inal fat, higher levels of leptin messenger RNA (mRNA), and a 2-fold increa se in plasma leptin concentrations. Despite normal plasma glucose levels, h GHRH transgenic mice had 4.5-fold elevated levels of plasma insulin. Using a ribonuclease protection assay, we measured the mRNA levels of the OBR lon g form (OBRL) in the anterior pituitary and hypothalamus after 48 h of fast ing. In the anterior pituitary, food deprivation induced dramatic increases in OBRL mRNA levels in both normal and transgenic mice. In contrast, in th e hypothalamus, fasting resulted in a significant decrease in OBRL gene exp ression in normal mice, and no changes were detected in hGHRH transgenic mi ce. Using dual in situ hybridization, OBRL mRNA was detected in somatotroph s. Moreover, the number of OBRL-positive pituitary cells as well as the per centage of OBRL-positive cells that express GH mRNA were increased in trans genic mice. In conclusion, 1) the modest obesity in hGHRH transgenic mice i s associated with increases in leptin synthesis and secretion as well as in sulin secretion; 2) GH and/or GHRH as well as leptin and insulin may differ entially contribute to the changes in OBRL gene expression in the anterior pituitary and the hypothalamus; 3) the response of OBRL gene expression in the hypothalamus to fasting is absent in the modestly obese hGHRH transgeni c mice; and 4) somatotrophs are target cells for leptin, and the increase i n OBRL gene expression in the pituitary of hGHRH transgenic mice is due at least in part to the increase in the number of cells expressing OBRL.