Growth Hormone-Releasing Activity of Hypothalamic Extracts at Different Ages

An investigation of the growth hormone-releasing activity of hypothalamic extracts from young growing animals and from animals which had reached growth stasis was undertaken to determine if a different growth hormone-releasing activity might be present in hypothalamic extracts with advancing age. For this purpose, hypothalamic extracts from 30-day-old rats and 2-year-old rats (5 mg/100 g body wt) of rat stalk-median eminence region were tested in vivo using intracarotid injections into 30-day-old recipient female rats. The pituitaries of the recipient animals were assayed for growth hormone content by the “tibia test” method. Hypothalamic extracts from 30-day-old rats induced an almost complete depletion of pituitary growth hormone content. This depletion effect of pituitary growth hormone content was not duplicated by hypothalamic extracts taken from 2-year-old animals and injected in equivalent amounts. These results strongly suggest the presence, in young animals, of a growth hormone-releasing factor (s) of hypothalamic origin: the concentration of this factor (s) seems to be markedly reduced in the hypothalamus of 2-year-old animals. (Endocrinology77: 241, 1965)