Effect of long-term administration of Hexarelin on the somatotrophic axis in aged rats.

The growth hormone-releasing peptide Hexarelin (Hexa; 80 micrograms/kg-1, s.c.) was administered for 30 and 60 days to old rats. The GH-releasing effect of Hexa was maintained during chronic treatment. At the end of the treatment, old rats were administered once with Hexa which elicited a greater GH response in rats chronically treated with the peptide than in those receiving a placebo. Pituitary GHmRNA concentrations were significantly lower in the older rats than in the younger animals, irrespective of Hexa treatment, while the GH protein content was similar in all the groups studied. The same was true for hypothalamic GHRH, whose synthesis was reduced in all the older animals but not in the young, in the presence of maintained concentrations of the peptide. Somatostatin mRNA concentrations were significantly higher in the hypothalami of older rats and administration of Hexa for 30 or 60 days brought the concentrations of somatostatin mRNA of aged rats to 'young' levels. Treatments with Hexa failed to alter the circulating levels of IGF-1. The data reported in this article indicate that long-term treatment with Hexa normalized some biological indices of somatotrophic function in aged rats.