GnRH analog stimulates gonadotropin II gene expression in maturing sockeye salmon

Our previous study suggested that, in the pituitaries of pre-spawning chum salmon, salmon gonadotropin-releasing hormone (sGnRH) stimulates expression of genes for gonadotropin (GTH) II beta but not for I beta, since the levels of mRNAs encoding sGnRH and GTH II but not I were increased during the final stages of spawning migration. In the present study, a capsule of GnRH analog (GnRHa) was implanted into the dorsal muscle of maturing sockeye salmon to clarify function of GnRH on expression of GTH subunit genes in pre-spawning homing salmonids. The amounts of GTH subunit mRNAs in the individual pituitaries were analyzed by a quantitative dot blot analysis using single-stranded sense DNA as the standard. The levels of GTH alpha and II beta mRNAs in the GnRHa-implanted fish were significantly higher than those in the control fish in both the males and females, whereas the levels of GTH I beta mRNA did not show any significant differences in both sexes. These results indicate that GnRH elevates expression of GTH subunit genes which encode the components of GTH II, alpha and II beta chains, in the pituitary of maturing sockeye salmon, and then accelerates final maturation.