Transgenic mice overexpressing the growth-hormone-releasing hormone gene have high concentrations of tachykinins in the anterior pituitary gland

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Evidence That Growth Hormone Exerts a Feedback Effect on Stomach Ghrelin Production and Secretion

Ghrelin is a recently discovered stomach hormone that stimulates pituitary growth hormone (GH) secretion potently. The purpose of these experiments was to test the hypothesis that a stomach-ghrelin-pituitary-GH axis exists in which either an elevation or reduction in systemic GH levels will exert a negative or positive feedback action, respectively, on stomach ghrelin homeostasis. In rats, GH administration decreased stomach ghrelin mRNA levels and plasma ghrelin levels significantly. In GH-releasing hormone (GHRH) transgenic mice, GHRH overexpression decreased stomach ghrelin peptide levels when compared with control mice. In aged rats (25 months) stomach ghrelin mRNA and peptide levels and plasma ghrelin levels were decreased when compared with young rats (5 months). Because GH secretion is reduced in aged rats, the elevated stomach ghrelin production and secretion may reflect a decreased GH feedback on stomach ghrelin, homeostasis, and secretion. Together, these findings suggest that endogenous pituitary GH exerts a feedback action on stomach ghrelin homeostasis and support the hypothesis that a stomach-ghrelinpituitary GH axis exists.

No effect of nutrient restriction from gestational days 28 to 78 on immunocytochemically detectable growth hormone-releasing hormone (GHRH) neurons and GHRH receptor colocalization in somatotropes of the ovine female fetus

The maternal environment affects fetal development and may permanently affect the physiology of the adult. Fetal growth hormone (GH) secretion is increased by maternal undernutrition but the physiological mechanisms responsible for this increase are unknown. We have recently found evidence suggesting that the GHRH component of the fetal neuroendocrine GH axis may be perturbed by undernutrition. This study sought to determine the effect of maternal undernutrition on immunocytochemically detectable GHRH neurons and the expression of GHRH receptors by somatotropes in the pituitary gland. Ewes were grouped (n=12 per group) randomly into control (fed 100% of requirements) or nutrient restricted (fed 50% of requirements) from days 28 to 78 of gestation, corresponding to the period from implantation to the end of placentation. At day 78, half the ewes were killed and the fetal brains were perfused. The remaining ewes were re-alimented to 100% of nutritional requirements and killed at day 135. There was no effect of nutrition restriction or age on the number of GHRH neurons. Similarly, the mean density and percentage of somatotropes expressing GHRH receptors was not significantly different between treatment groups at either age. This study found no effect, as determined by immunocytochemistry, of nutrient restriction on the GHRH component of the fetal neuroendocrine GH axis. It remains to be established if the release of GHRH and responsiveness of somatotropes to GHRH in the fetus are affected by undernutrition.

Effect of nutrient restriction on the somatotropes and substance P-immunoreactive cells in the pituitary of the female ovine fetus

The maternal environment affects fetal development and may influence the physiology of the adult. Fetal growth hormone (GH) is increased by maternal undernutrition but the mechanisms responsible are unknown. This study determined the effect of maternal undernutrition on the development of fetal pituitary somatotropes in the female. Ewes were grouped randomly into control (fed 100% of requirements) or nutrient restricted (fed 50%) from Days 28 to 78 of gestation. At Day 78, the ewes were killed and fetuses collected (Day 78 NR (nutrient restricted): n=6; Day 78C (control): n=6). Remaining ewes were realimented to 100% of nutritional requirements and were killed at Day 135 (Day 135 NR (nutrient restricted): n=6; Day 135 C (control): n=6). Somatotropes were visualized immunocytochemically and the size, mean density, total percentage and proportion colocalized with substance P were determined for each group. Nutrient restriction increased (p<0.01) the density of pituitary cells in Day 78 fetuses but this difference was no longer apparent by Day 135 after realimentation. The density and proportion of somatotropes were not different between treatment groups at Day 78 but were significantly (p<0.05) lower in the nutrient restricted Day 135 fetuses as compared to the Day 135 control animals. Somatotropes from restricted fetuses were significantly (p<0.001) larger at Day 78. Nutrient restriction increased the density (p<0.001) and percentage (p<0.05) of substance P-immunoreactive cells Day 135 fetuses. Similarly, the proportion of somatotropes that expressed substance P was significantly (p<0.05) increased by nutrient restriction in the Day 135 fetuses. Although nearly two thirds of substance P-immunoreactive cells co-expressed GH, there was no significant effect of treatment on this co-expression. Additional studies are required to determine if other components of the neuroendocrine GH axis are affected by this nutritional insult, if the alterations that we have observed, particularly in the tachykinin system, persist into adulthood and, importantly, what are the long-term consequences of an altered GH axis.

Effects of overexpression of growth hormone-releasing hormone on the hypothalamo-pituitary-gonadal function in the mouse

In this investigation, the neuroendocrine alterations induced by high, chronic circulating levels of endogenous growth hormone (GH) were studied in transgenic mice with ectopic overexpression of the human growth hormone-releasing hormone (h-GH-RH) gene. In comparison with their normal littermates, transgenic h-GH-RH mice had elevated plasma levels of GH, prolactin (PRL), and corticosterone. In addition, they had elevated body, liver, kidney, spleen, and pituitary weights compared with normal mice. Testis and seminal vesicle weights were also increased in transgenic mice. Although basal plasma luteinizing hormone (LH) levels, plasma estradiol levels in females, and plasma testosterone levels in males did not differ significantly between normal and transgenic animals, the LH response to castration was severely impaired in transgenic mice of both sexes. Among the biogenic amines studied in the hypothalamus, only dopamine concentrations were significantly lower in transgenic animals compared with their normal littermates. This decrease in hypothalamic dopamine may be related to the hyperprolactinemia in transgenic animals. In vitro, pituitaries from transgenic mice released significantly higher amounts of GH, and although the basal release of LH was not different in both normal and transgenic mice, the response to gonadotropin-releasing hormone was significantly smaller in transgenic mice. Cultured anterior pituitary cells from transgenic mice secreted high quantities of GH and PRL in vitro, but these quantities significantly decreased from 1 to 8 wk in culture. These results show that high, persistent levels of circulating endogenous GH induce alterations in neuroendocrine functions related to the hypothalamo-pituitary-gonadal and the hypothalamo-pituitary-adrenal axes.