HYPOTHALAMIC TARGETS FOR GROWTH-HORMONE SECRETAGOGUES

Various novel growth hormone (GH) secretagogues have been developed. G H secretagogues release GH directly from the pituitary via a pathway d istinct from that involving GH-releasing hormone (GHRH). However, they also act centrally to activate hypothalamic neurones, and require an intact GHRH system for potent in vivo activity. Both normal and transg enic growth-retarded (Tgr) rats release GH in response to GH secretago gues, and their responses are sensitive to the pattern of secretagogue administration. GH secretagogues are not completely specific for GH r elease, but also activate the adrenocorticotrophin-adrenal axis, imply ing that they have additional central actions. The recent cloning of a n endogenous receptor for GH secretagogues now makes it possible to id entify central targets for their action. An endogenous receptor implie s the existence of an endogenous ligand, but its site of production, r elationship to the xenobiotic pharmacological agents and its underlyin g physiological relevance remain unclear.