Changes in transcription of pituitary glycoprotein hormone α and gonadotropin IIβ subunits during ovarian development induced by repeated injections of salmon pituitary homogenate in the Japanese eel, Anguilla japonica

Abstract

To investigate transcription of pituitary glycoprotein hormone a and gonadotropin IIβ (GTH IIβ) subunits in female Japanese eel, gene quantitation systems using reverse transcription-polymerase chain reaction (RT-PCR) were developed, using β-actin as an internal control gene. These systems were not only sensitive, but also precise, with low coefficients of variation within and between assays (below 20 and 30%, respectively). Using these quantitation systems, we investigated how transcription of both subunits changed in Japanese eel during ovarian development induced by repeated injections of salmon pituitary homogenate (SPH). Transcriptional level of pituitary glycoprotein hormone α was similar to that of s-actin before SPH injection, and increased slowly during ovarian development, reaching a maximum (3-fold increase over initial levels) at the migratory nucleus stage. In contrast, the level of GTH IIβ mRNA was very low (about one-tenth that of β-actin) before SPH injection, but expression increased markedly during ovarian development with an approximate 120-fold increase over initial levels (12 times that of β-actin) at the migratory nucleus stage. These results suggest that GTH II synthesis in female eel was activated during ovarian development induced by repeated SPH injections. The increase in mRNA levels of both subunits mRNA is probably due to sex steroids produced in eel ovarian follicles. Unexpectedly, transcriptional level of GTH IIβ was abnormally high compared to that of pituitary glycoprotein hormone α at the migratory nucleus stage. These results suggest that repeated SPH injections induced aberrant endocrine conditions in female Japanese eels.