Effects of gonadotropin-releasing hormone on pituitary-ovarian axis of one-year old pre-pubertal red seabream.

To identify the pubertal development of the brain-pituitary-gonad (BPG) axis in female red seabream (Pagrus major), we investigated the effects of gonadotropin-releasing hormone agonist (GnRHa) on seabream (sb) GnRH mRNA levels in the brain, gonadotropin subunit mRNA levels in the pituitary, and serum concentrations of luteinizing hormone (LH), testosterone (T) and estradiol-17beta (E2) in pre-pubertal fish. Sexually immature 12-month-old fish were implanted with a cholesterol pellet containing GnRHa and maintained for 10-20 days. In the brain, GnRHa had no effect on sbGnRH mRNA levels. In the pituitary, although no marked changes were observed in follicle-stimulating hormone (FSH) beta subunit mRNA levels, the expression of glycoprotein (GP) alpha, and LHbeta subunit genes in the pituitary was drastically up-regulated (approximately 4- and 5-fold, respectively) and serum LH levels were also increased (approximately 3-fold) by GnRHa implantation. However, ovaries of GnRHa treated fish contained only oocytes at the peri-nucleolus stage, and oocyte development such as vitellogenesis and oocyte maturation did not occur throughout the experimental period. In these fish, even though LH was released, only slight increases in serum concentrations of T and E2 were observed. These results indicate that the pituitary gonadotropin cells of pre-pubertal 12-month-old fish were already receptive to GnRH stimulus, and acquired the ability to synthesize and release of LH as in the case of adult fish. Deficient factors for the onset of puberty by GnRHa treatment will be discussed.