Abstract

This thesis deals with changes in gene expression during activation of the brain-pituitary-gonadal axis at puberty in early maturing male Atlantic salmon (Salmo salar) parr. To help elucidate the physiological roles of gonadotropins and their receptors in the regulation of puberty, cDNAs encoding FSH and LH receptors (FSHR and LHR, respectively) of Atlantic salmon were cloned and characterized. Gene expression of the receptors in the testes was analyzed in parallel with pituitary expression of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) β-subunit genes by RT-PCR. In addition, functional genes encoding proteins involved in the steroidogenic pathway and anti-Müllerian hormone (AMH) were studied in the testes, and plasma 11-ketostestosterone levels were measured. One-summer-old male Atlantic salmon parr were sampled from the prepubertal stage in December until spermiation in October. Sequence analysis of the Atlantic salmon FSHR and LHR showed the typical structure features of glycoprotein receptors including a large extracellular domain connected to a G protein-coupled transmembrane domain. Both of these gonadotropin receptors were expressed in immature testis, FSHR more abundantly than LHR. FSHR transcript levels increased in parallel with FSHβ levels from early spermatogenesis onwards while LHR mRNA levels started to increase prior to any major changes in LHβ expression. De novo transcription of genes encoding steroidogenic acute regulatory protein, 3β-hydroxysteroid dehydrogenase, cytochrome P450 17α-hydroxylase/17,20-lyase, and 11β-hydroxysteroid dehydrogenase was observed during the initiation of spermatogenesis in parallel with the changes in FSHβ levels. In contrast, AMH expression was downregulated and AMH levels were lowest during spermiogenesis. During spermatogenesis, large increases in the expression of LHR and all of the steroidogenic genes studied occurred concomitantly with the rise in LHβ transcripts. These findings suggest that FSH is involved in regulation of the expression of several testicular genes during the initiation of puberty and LH during the later stages of spermatogenesis. In addition, results of in vitro studies using serum-free primary cultures of pituitary cells indicate that IGF-I differentially modulates gonadotropin expression in the pituitary cells. IGF-I may stimulate FSHβ expression levels though interactions with gonadotropin-releasing hormone (GnRH) in immature males while it directly activates LHβ expression.