Effects of GH and IGF-I administration on GHRH and somatostatin mRNA levels: II. A study in the infant rat

It is generally accepted that growth hormone influences its own secretion by modulating the activity of GHRH and SRIF neurons. To investigate if GH feedback mechanisms are already operating in the early postnatal life of the rat, we have studied in 10-day-old pups the effects of rhGH and rhIGF-I administration on GHRH and somatostatin mRNA levels. The same experiment was also performed in pups passively immunized with an anti-GHRH antiserum from the day of birth. The latter animal model had been previously characterized for presenting reduced levels of circulating GH and IGF-I. In control pups, neither rhGH (250 μg/kg, b.i.d., sc) nor rhIGF-I (150 μg/kg, b.i.d., sc) administration induced significant changes of GHRH and SRIF gene expression. The passive immunization against GHRH induced per se a trend toward an increase and a reduction of GHRH and SRIF mRNA levels, respectively. Also in these rats the treatment for 3 days with rhGH and rhIGF-I did not further modify the GHRH and SRIF mRNA levels. Based on these results, we conclude that in the 10-day-old rat GH feedback mechanisms are poorly operative, though a direct ultra-short loop mechanism involving the GHRH and SRIF systems seems already operating.