Molecular mechanisms of onset of puberty in salmonids : regulation of GnRH receptor genes

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The onset of puberty is controlled by hypothalamus-pituitary system in vertebrates. However, molecular mechanisms of neuroendocrine regulation of puberty are unknown, particularly in lower vertebrates like fish. Salmonids are an interesting model for investigating the neuroendocrine mechanisms of puberty, because their sexual maturation progress slowly with seasons. We focused on regulation of expression of genes encoding gonadotropin-releasing hormone receptor(GnRH-R), which is a key molecule of responsiveness to GnRH. In salmonids, a single type I GnRH-R was originally identified in the brain of rainbow trout. We demonstrated in masu salmon that five different GnRH-R genes, termed msGnRH-R1,R2,R3,R4,and R5,are expressed in the brain, pituitary and other peripheral tissues with different pattern. A splicing variant of msGnRH-R1 is also expressed in these tissues. All these receptors are type I receptors, but are divided into two subtypes, one subtype (type Ia) includes R1,R2, and R3 a … More nd the other (type Ib) includes R4 and R5. The identity of nucleotide sequences among R1,R2 and R3 is 96-99%, while that of R4 and R5 is 81%. The identity between the two subtypes decreases to 59-71%. The effect of GnRH on gonadotropin (GTH) secretion may be attributed in part to changes in expression of GnRH-Rs. We therefore examined seasonal variations in expression of the five msGnRH-R genes in the pituitary during the reproductive cycle by real-time PCR. All five subtypes of msGnRH-R genes were expressed in the pituitary, although R4 mRNA was dominant. Interestingly, the expression patterns of five msGnRH-R genes differed among different subtypes. Among them, R4 mRNA increased only in the pre-spawning period, when GnRH stimulates expression of luteinizing hormone(LH) genes. These results suggest that R4 subtype is involved in the GnRH-induced LH synthesis. Other subtype mRNAs increased in different periods and may be related to other GnRH action in the pituitary such as GnRH-induced prolactin gene expression. To obtain more accurate information on the function of the multiple GnRH-R subtypes, cell type specific distribution of the GnRH-Rs in the pituitary needs to be determined. Less