Aging-related changes in in vivo release of growth hormone-releasing hormone and somatostatin from the stalk-median eminence in female rhesus monkeys (Macaca mulatta).

GH release decreases with aging in primates. However, it is unclear whether the age-related decrease in GH release is due to a decrease in stimulatory GHRH or an increase in inhibitory somatostatin (SS) from the hypothalamus. In the present study, we measured the release of GHRH and SS in the stalk-median eminence of conscious aged (n = 7, 27.0 +/- 0.7 yr old) and young adult female monkeys (n = 12, 5.0 +/- 0.3 yr old) using the push-pull perfusion method. Mean GHRH levels during morning (0600-1200 h) and evening (1800-2400 h) in aged monkeys were 3- to 4-fold lower than in young monkeys. Pulse analysis indicated that pulse frequency, pulse amplitude, and baseline GHRH release in aged monkeys were much lower than in young adults. In contrast, mean SS levels in aged monkeys during mornings and evenings were 2-fold higher than in young monkeys. Pulse analyses indicated that amplitude and baseline levels of SS were significantly higher in aged monkeys than in young adults. There were no significant changes in the pulse frequency of SS release. Therefore, the aging-related decrease in GH release is due to a substantial decrease in GHRH release and an increase in SS release from the hypothalamus.