EFFECTS OF GH AND IGF-I ADMINISTRATION ON GHRH AND SOMATOSTATIN MESSENGER-RNA LEVELS .1. A STUDY ON AD-LIBITUM FED AND STARVED ADULT MALE-RATS

The individual role played by GH and IGF-I in the regulation of hypoth alamic GHRH and SRIF gene expression is still object of debate. We hav e investigated the effect of exogenously administered recombinant hGH (rhGH) and recombinant hIGF-I (rhIGF-I) in ad libitum fed control and starved rats, the latter an animal model which is characterized by low circulating levels of endogenous GH and IGF-I. Adult male rats were f ed ad libitum (C) or food-deprived (S) for 72 hours; rats in either C or S groups were treated with systemic administration of rhGH and rhIG F-I for 3 days. GHRH, SRIF and GH mRNA levels were evaluated by Northe rn and slot blot hybridization. Administration of rhGH (250 mu g/kg/tw ice daily, sc) induced a significant inhibition of GHRH and a signific ant stimulation of SRIF mRNA levels in C rats; GH treatment was, howev er, ineffective on both neuropeptide mRNA levels in the S group. Conti nuous infusion of rhIGF-I (300 mu g/kg/day, sc) induced a significant increase of SRIF levels in both C and S rats but did not modify GHRH m RNA levels in either group. In the pituitary, GH mRNA levels followed a pattern very similar to that of GHRH. These results provide evidence for a direct role of GH in the inhibition of GHRH mRNA levels; IGF-I appears more involved in the direct stimulation of SRIF mRNA levels. ( C) 1997, Editrice Kurtis.