Stimulatory effects of insulin-like growth factor 1 on expression of gonadotropin subunit genes and release of follicle-stimulating hormone and luteinizing hormone in masu salmon pituitary cells early in gametogenesis

摘要

Insulin-like growth factor-I (IGF-I) has been shown to be involved in pubertal activation of gonadotropin (GTH) secretion. The aim of this study was to determine if IGF-I directly stimulates the synthesis and release of GTH at an early stage of gametogenesis. The effects of IGF-I on expression of genes encoding glycoprotein alpha (GP alpha ), follicle-stimulating hormone (FSH) beta and luteinizing hormone (LH) beta subunits and release of FSH and LH were examined using primary pituitary cells of masu salmon Oncorhynchus masou at three reproductive stages: early gametogenesis, maturing stage and spawning. IGF-I alone or IGF-I+salmon GnRH (sGnRH) were added to the primary pituitary cell cultures. The amounts of GP alpha , FSH beta and LH beta mRNAs were determined by real-time PCR. Plasma and medium levels of FSH and LH were determined by RIA. In males, IGF-I increased the amounts of all three subunit mRNAs early in gametogenesis in a dose-dependent manner, but not in the later stages. In females, IGF-I stimulated the release of FSH and LH early in gametogenesis, whereas no stimulatory effects on the sub-unit mRNA levels were observed at any stage. IGF-I+sGnRH stimulated the release of FSH and LH at all stages in both sexes, but had different effects on the subunit mRNA levels depending on the subunit and stage. The present results suggest that IGF-I itself directly stimulates the synthesis and release of GTH early in the gametogenesis of masu salmon, possibly acting as a metabolic signal that triggers the onset of puberty.