Expression of growth hormone secretagogue-receptors by growth hormone-releasing hormone neurons in the mediobasal hypothalamus.

A novel class of synthetic compounds, termed GH-secretagogues (GHSs), have been shown to be potent stimulators of GH release, although their mechanism of action and functional significance remains obscure. The recent cloning of the rat GHS receptor (GHS-R) permitted the identification of numerous sites of expression of GHS-R in brain, but nothing is yet known about the cell types that express this receptor. We performed dual chromogenic and autoradiographic in situ hybridization to test the hypothesis that GHRH neurons in the hypothalamus coexpress GHS-R mRNA. GHS-R-hybridizing cells showed extensive overlap with GHRH-expressing neurons in both the arcuate (Arc) and ventromedial (VMN) hypothalamic nuclei. Quantification of the double-labeled cells revealed that approximately 27% of GHRH-hybridizing neurons in the Arc, and 22% of those in the VMN, expressed the GHS-R gene. These studies are the first to colocalize the GHS-R to any neurochemical cell type in rat brain. The results provide evidence that the GHSs may directly modulate GHRH release, and thereby stimulate GH secretion, through interaction with the GHS-R on hypothalamic GHRH mRNA-containing neurons.