To define the role of GH during central nervous system development, we performed studies in cultured rat cerebral cortical cells from 14- (E14) and 17-d-old embryos (E17). The expression of GH receptor, IGF-I receptor, and IGF-I mRNAs was confirmed. In E17, GH increased total cell number (3.9-fold), [(3)H]-thymidine incorporation (3.5-fold), proliferating cell nuclear antigen levels (2.5-fold), and bromodeoxyuridine (BrdU)-positive cells (2.5-fold). GH action on nestin/BrdU-positive cells was increased in E14 cells at 3 d in vitro (80-fold) but not at 7 d in vitro. In E14 cells, GH increased (9.5-fold) beta-tubulin/BrdU cells. In E17 cells, GH induced neuronal differentiation, as indicated by the absence of beta-tubulin/BrdU-positive cells and the 5.9-fold increment of beta-tubulin protein, and increased glial fibrillary acidic protein/BrdU-positive cells (2.5-fold) and glial fibrillary acidic protein expression (4.5-fold). GH-induced proliferation and differentiation was blocked by IGF-I antiserum. GH increased IGF-binding protein-3 (IGFBP-3), IGF-I receptor protein and its phosphorylation. This study shows that GH promotes proliferation of neural precursors, neurogenesis, and gliogenesis during brain development. These responses are mediated by locally produced IGF-I. GH-induced IGFBP-3 may also have a role in these responses. Therefore, GH is able to activate the IGF-I/IGFBP-3 system in these cerebral cells and induce a physiological action of IGF-I.

Diminished GH secretion is a well known association of obesity. As in obese humans, Zucker fatty rats develop a progressive GH deficiency, present at 6 weeks of age and maximal at 10 to 12 weeks. The aim of this study was to investigate the GH dependence of IGF-I gene expression in liver and extrahepatic tissues of the obese Zucker rat as a model of progressive GH reduction during adult life. Six-and 11-week-old obese Zucker rats and their lean littermates were used to compare body weight, glycemia, insulinemia, serum GH and IGF-I levels and IGF-I mRNA expression in liver, heart, aorta, kidney and skeletal muscle. In comparison with lean controls, obese Zucker rats showed at both ages comparable glycemia, severe hyperinsulinemia (mU/ml, mean ...; 6 weeks 138 10 vs 45 6 P<0·001; 11 weeks 147 14 vs 46 3, P<0·001) and lower GH (ng/ml; 6 weeks 1·7 0·9 vs 2·7 1·1; 11 weeks 1·5 0·9 vs 4·2 1·2) in the presence of similar circulating IGF-I levels (ng/ml; 6 weeks 774 26 vs 694 28; 11 weeks 1439 182 vs 1516 121). Hepatic IGF-I mRNA expression was already reduced at 6 weeks of age due to a significant decrease in the IGF-Ib transcript compared with lean controls (relative units; IGF-Ia: 99 2% vs 100 5%; IGF-Ib: 69 10% vs 100 2%, P<0·05) and this reduction was more marked in 11-week-old animals when both IGF-I transcripts were significantly diminished (relative units; IGF-Ia: 80 6% vs 100 1%, P<0·05; IGF-Ib: 65 5% vs 100 2%, P<0·01). Extrahepatic tissues expressed almost exclusively the IGF-Ia transcript, the amount of which relative to controls was: (1) similar at 6 weeks and decreased at 11 weeks in kidney and skeletal muscle extracts (relative units; kidney: 6 weeks 88 10% vs 100 2%; 11 weeks 76 3% vs 100 4%, P<0·05; vastus lateralis: 6 weeks 95 7% vs 100 10%; 11 weeks 59 4% vs 100 2%, P<0·001); (2) similar at both ages in thoracic aorta (relative units; 6 weeks 121 6% vs 105 5%; 11 weeks: 91 14% vs 100 4%); and (3) increased at both ages in left ventricle extracts (relative units; 6 weeks 114 2% vs 99 9%, P<0·05; 11 weeks 119 7% vs 95 3%, P<0·05).These data support the existence of tissue-specific dependence of IGF-I mRNA on GH levels during adulthood, reflected by the different behavior of IGF-I expression for each tissue in conditions of progressive decrease of GH levels.