Seasonal changes in expression of genes encoding five types of gonadotropin-releasing hormone receptors and responses to GnRH analog in the pituitary of masu salmon.

Five types of gonadotropin-releasing hormone receptor (GnRH-R) genes, designated as msGnRH-R1, R2, R3, R4, and R5, are expressed in the brain and pituitary of masu salmon (Oncorhynchus masou). In the present study, seasonal changes in the expression of these five genes were examined in the pituitary to elucidate their roles in GnRH action during growth and sexual maturation. In addition, the seasonal variation of these genes in response to GnRH was examined in a GnRH analog (GnRHa) implantation experiment. Pituitary samples were collected 1 week after the implantation every month from immaturity through spawning. The absolute amount of GnRH-R mRNA in single pituitaries was determined by real-time PCR assays. Among the five genes, R4 was predominantly expressed in the pituitaries. In the immature fish, the amount of GnRH-R mRNA varied with seasons and subtypes. In the pre-spawning period, R1 and R4 mRNAs in both sexes and R2 and R3 mRNAs in the females increased 4- to 20-fold and then decreased in the spawning season. The effects of GnRHa treatment were significantly different in both sexes. In the females, GnRHa tended to elevate the expression of all the subtypes of GnRH-R genes in various stages during the experimental period, whereas it had almost no apparent effects in the males. These results indicate that the expression of the five GnRH-R genes is seasonally variable and may be related to the responses of the pituitary hormone genes to GnRH, and the regulation of GnRH-R genes by GnRH is different in both sexes.