|Budget Amount *help
¥2,600,000 (Direct Cost : ¥2,600,000)
Fiscal Year 1999 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1998 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1997 : ¥1,300,000 (Direct Cost : ¥1,300,000)
Gonadotropin-releasing hormone (GnPH) is a pivotal hormone in the control of reproduction in all vertebrates. During evolution, GnRH peptide underwent gene duplication and structural changes to give rise to multiple molecular forms of GnRHs. Vertebrate species, including primates and humans have more than one GnRH variant in individual brains. Several species of bonyfish have three molecular variants of GnRH; in the terminal nerve (salmon GnRH), preoptic area (seabream GnRH) and midbrain tegmentum (chicken GnRH II). In fish, tilapia Oreochromis niloticus, we have characterized the primary structure and demonstrated that neurons containing the three GnRH molecular forms have different embryonic origins. Further, we speculate that the three GnRH molecules might have different functions and different regulatory mechanisms.
We have constructed specific GnRH messenger RNA probes from known GnRH cDNA sequences, and have used them successfully to localize GnRHs in the brain of tilapia. In the present study, using quantitative radioactive in situ hybridization, we examined the effects of castration and steroid replacement therapy on the terminal nerve, preoptic and midbrain GnRH mRNA synthesis. The terminal nerve GnRH neurons are positively regulated by testosterone and negatively by triiodothyronine ; and preoptic GnRH neurons by estrogen and testosterone but not ketotestosterone (nonaromatizable androgen). The present study emphasizes that the midbrain GnRH neurons are insensitive to sex steroids, indicating an as yet unknown regulatory mechanism may operate these cells. The terminal nerve and preoptic GnRH neurons are under the influence of an estrogenic pathway.