In several species, including humans, circulating insulin-like growth factor I (IGF-I) levels increase during the onset of puberty, suggesting that this peptide contributes to attaining sexual maturity. Because IGF-I elicits LHRH release from the median eminence (ME) of immature female rats in vitro, we hypothesized that it may represent one of the peripheral signals suspected to link somatic development to the LHRH-releasing system at puberty. We now present evidence in support of this concept. Quantitation of IGF-I messenger RNA (mRNA) levels by ribonuclease protection assay revealed that expression of the IGF-I gene did not change in the medial basal hypothalamus or preoptic area of female rats during peripubertal development. In contrast, the contents of both IGF-Ia and IGF-Ib mRNA, the two alternatively spliced forms of the IGF-I gene, increased significantly in the liver during the early proestrous phase of puberty. This change was followed by an elevation in serum IGF-I levels during the late proestrous phase of puberty along with a concomitant increase in serum gonadotropin levels. The proestrous change in serum IGF-I levels was accompanied by a selective increase in IGF-I receptor (IGF-IR) mRNA in the ME. Small doses of IGF-I (2-200 ng), administered intraventricularly, effectively induced LH release in both juvenile and peripubertal female rats, an increase prevented by prior immunoneutralization of LHRH actions. Importantly, intraventricular injections of IGF-I (20 ng), administered twice dally in the afternoon to immature animals, significantly advanced puberty. Thus, these results suggest that IGF-I of peripheral origin contributes to the initiation of female puberty by stimulating LHRH release from the hypothalamus, an effect that appears to be amplified by the increased synthesis of IGF-I receptors in the ME during first proestrus.
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