IDENTIFICATION OF TARGET-CELLS FOR GROWTH-HORMONES ACTION IN THE ARCUATE NUCLEUS

Growth hormone (GH) participates in the regulation of its own secretio n by acting through a short-loop feedback mechanism to regulate the sy nthesis and secretion of somatostatin (SS) and growth hormone-releasin g hormone (GHRH). The mechanism of GH's action in certain peripheral t argets involves the induction of c-fos. Similarly, we hypothesized tha t GH induces the expression of c-fos mRNA in SS and GHRH neurons in th e hypothalamus. Using in situ hybridization, we observed a significant induction of c-fos mRNA in the arcuate nucleus of human GH-treated co mpared with control animals. Contrary to our hypothesis, only 11% of G HRH mRNA-containing and 5% of SS mRNA-containing neurons colabeled for c-fos mRNA. These findings indicate that GH feedback on the hypothala mus includes the induction of c-fos mRNA primarily in neurons other th an GHRH and SS in the arcuate nucleus and suggest that these unidentif ied neurons located in the arcuate nucleus are directly involved in tr ansducing the effects of GH in the brain.