Circulating concentrations of nocturnal leptin, growth hormone, and insulin-like growth factor-I increase before the onset of puberty in agonadal male monkeys: Potential signals for the initiation of puberty

The factor(s) responsible for initiating the developmental increase in noct urnal gonadotropin-releasing hormone secretion, defining the onset of puber ty, are not known. Although signals regulating prepubertal growth seem to b e obvious candidates to control such a process, it is unclear whether prepu bertal alterations occur in these growth-related factors such that they mig ht provide the brain information on changing body size. Using samples analy zed previously describing the initiation of nocturnal pulsatile LH secretio n in agonadal male monkeys (Endocrinology 139: 2774-2783, 1998), developmen tal changes in plasma concentrations of leptin, GH, and insulin-like growth factor I(IGF-I) were determined to test the hypothesis that an increase in circulating levels of one or all of these growth-derived signals precedes the onset of puberty. Hormone concentrations were determined in live juveni le males at 10-day intervals from approximately 60 days before and 50 days after the initiation of pulsatile nocturnal LH secretion. Leptin concentrat ions were determined in samples obtained at 1000and 2200 h, 36 and 48 h bef ore the nocturnal assessment of pulsatile LH. Mean nocturnal GH concentrati ons were determined from the sequential samples collected at night. IGF-I w as determined in the 1000- or 2200-h presequential samples. Although daytim e leptin concentrations did not increase developmentally, nocturnal leptin levels increased significantly during the 30 days before the onset of puber ty. Furthermore, both nocturnal GH and IGF-I concentrations showed a signif icant sustained increase from the early prepubertal period to the 30 days p receding the onset of puberty. These data are the first to demonstrate an i ncrease in nocturnal leptin and GH-induced IGF-I secretion prior to the ons et of puberty in the agonadal male monkey and that these developmental chan ges occur independent of the gonadal influences. These findings protide jus tification for empirical investigation of the role of leptin and the GH axi s, in particular IGF-I, in regulating developmental increases in pulsatile nocturnal gonadotropin-releasing hormone secretion initiating puberty in pr imates.