GROWTH-HORMONE INDUCES EXPRESSION OF THE C-FOS GENE ON HYPOTHALAMIC NEUROPEPTIDE-Y AND SOMATOSTATIN NEURONS IN HYPOPHYSECTOMIZED RATS

The neuronal expression of the protooncogene c-fos may serve as a mark er of neural activity. We previously examined brain sites upon which G H exerts an immediate early influence in rats and determined that the c-fos gene was transiently expressed in the hypothalamic periventricul ar nucleus (PeV) and arcuate nucleus (ARC) after recombinant human GH (rhGH) administration. As the distribution of c-fos messenger RNA (mRN A)-containing cells appeared to overlap with that of somatostatin (SS) neurons in both the PeV and ARC, we hypothesized that GH exerts a fee dback effect on hypothalamic SS neurons. To extend this hypothesis, we characterized the neurons expressing the c-fos gene in response to rh GH administration in hypophysectomized rats. Adult male Wistar rats we re hypophysectomized 10 days before use. After hypophysectomy, rats re ceived daily sc injections of cortisone acetate (0.5 mg/kg BW) and L-T -4 (20 mu g/kg BW). Four international units (1.33 mg) of rhGH were gi ven iv through an indwelling right atrial cannula. The vehicle was giv en to the control animals. Coronal sections of the hypothalamus were p rocessed for in situ hybridization after rhGH or vehicle administratio n. To estimate the localization of neurons expressing the c-fos gene, the adjacent hypothalamic sections, 30 mu m in thickness, were process ed for hybridization histochemistry for SS, neuropeptide-Y (NPY), or G RF mRNA. In the ARC, the distribution of c-fos mRNA-containing cells a ppeared to overlap with that of NPY and partially with that of SS mRNA -containing cells, but it clearly differed from the distribution of GR F mRNA-containing cells. In the PeV, distribution of the cells express ing the c-fos gene was comparable to that of SS mRNA-containing cells. To further ascertain the distribution, hypothalamic sections, 6 mu m in thickness, were processed by double label in situ hybridization usi ng a S-35-labeled c-fos cRNA probe and a digoxigenin-labeled NPY or SS cRNA probe. In the ARC, 65% of the c-fos gene-expressing cells were N PY neurons. In the PeV, 60% of the c-fos gene-expressing cells were SS neurons. NPY is known to act within the hypothalamus and inhibit GH s ecretion via SS in rats, and the NPY neurons in the ARC have been show n to project to SS neurons in the PeV. Our findings suggest that the f eedback effect of GH on the hypothalamus is mediated not only by SS ne urons in the PeV, but also by NPY neurons in the ARC.