Growth hormone(GH) is required for normal somatic growth and it also plays important roles in metabolism, immune function, and overall well being. A hypothalamic GH-releasing hormone (GHRH) deficit is now recognized as the primary cause of childhood onset GH deficiency and is linked to the lower circulating GH concentrations observed in elderly individuals. A variety of biopotent GHRH-like compounds are being developed for therapeutic use. However, relatively little is known about the regulation of endogenous GHRH synthesis and signaling. To better understand regulation of the GHRH/GHRH-receptor (GHRH-R) system this proposal focuses on counterregulatory mechanisms within the GH-axis which temper the pituitary somatotrope sensitivity to the actions of GHRH (homologous desensitization) and which regulate the availability of GHRH (GH/IGF-I negative feedback). Primary rat pituitary cell cultures will be used to study the effects of GHRH on GHRH-R synthesis (RT-PCR of GHRH-R mRNA) and sensitivity to GHRH (ligand stimulated adenylate cylase activity and cAMP generation). Studies will examine: ICER (cAMP inducible repressor), G-protein kinase-mediated receptor phosphorylation or receptor internalization roles in GHRH-mediated downregulation of the GHRH-R, in vitro. Animal models of aberrant GH production will examine the effects of GHRH hyper-(MT-hGHRH transgenic mice) and hypo-(monosodium glutamate treated rats) secretion on the GHRH-R signaling system, in vivo. Also, animal models will be used to determine 1) if circulating IGF-I plays an important role in hypothalamic feedback inhibition (GH receptor knockout mouse), 2) if GH induction of hypothalamic neuropeptide Y expression is an essential component of GH-mediated inhibition of GHRH synthesis (spontaneous dwarf rat [SDR] and TH-hGH transgenic dwarf mouse) and 3) if GH hypothalamic negative feedback contributes to diminished circulating GH concentrations observed with age (SDR). Finally, studies will be conducted to understand the relative contribution of the newly identified GH secretagogue receptor (GHS-R) system to the coordinate regulation of the GH-axis. The proposed studies represent a systematic approach to determine how counterregulatory mechanisms of the GH-axis maintain normal GHRH synthesis and signaling. A better understanding of these mechanisms will aid in the design and rational application of GHRH analogues and mimics which could serve to alleviate the deleterious effect of GH hyposecretion.
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