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 nocturnal gonadotropin-releasing hormone secretion, defining the onset of puberty, are not known. Although signals regulating prepubertal growth seem to be obvious candidates to control such a process, it is unclear whether prepubertal alterations occur in these growth-related factors such that they might provide the brain information on changing body size. Using samples analyzed previously describing the initiation of nocturnal pulsatile LH secretion in agonadal male monkeys (Endocrinology 139: 2774-2783, 1998), developmental 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 juvenile males at 10-day intervals from approximately 60 days before and 50 days after the initiation of pulsatile nocturnal LH secretion. Leptin concentrations were determined in samples obtained at 1000and 2200 h, 36 and 48 h before the nocturnal assessment of pulsatile LH. Mean nocturnal GH concentrations were determined from the sequential samples collected at night. IGF-I was determined in the 1000- or 2200-h presequential samples. Although daytime leptin concentrations did not increase developmentally, nocturnal leptin levels increased significantly during the 30 days before the onset of puberty. Furthermore, both nocturnal GH and IGF-I concentrations showed a significant sustained increase from the early prepubertal period to the 30 days preceding the onset of puberty. These data are the first to demonstrate an increase in nocturnal leptin and GH-induced IGF-I secretion prior to the onset of puberty in the agonadal male monkey and that these developmental changes occur independent of the gonadal influences. These findings protide justification for empirical investigation of the role of leptin and the GH axis, in particular IGF-I, in regulating developmental increases in pulsatile nocturnal gonadotropin-releasing hormone secretion initiating puberty in primates.