Elevation of gene expression for salmon gonadotropin-releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Abstract

Our previous studies suggested that salmon gonadotropin-releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH-I, and sGnRH-II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real-time polymerase chain reactions. The amounts of sGnRH-II mRNA were higher than those of sGnRH-I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIbeta and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground.

Expression of hormone genes and osmoregulation in homing chum salmon: A minireview

“…This finding supports our hypothesis that sGnRH neurons concordantly facilitate upstream migratory behavior and final gonadal maturation (Onuma et al, 2005b). and release of VT and PRL in pre-spawning salmon.…”

“…In salmonid species, salmon gonadotropin-releasing hormone (sGnRH) neurons are the typical neuroendocrine cells that integrate information, and regulate reproductive behavior and pituitary hormone secretion (Onuma et al, 2005b). In homing chum salmon, expression of sGnRH genes was elevated in almost all forebrain loci during upstream migration from the coast through the natal hatchery (Onuma et al, 2005b).…”

“…In homing chum salmon, expression of sGnRH genes was elevated in almost all forebrain loci during upstream migration from the coast through the natal hatchery (Onuma et al, 2005b). Administration of GnRH analogue stimulated preference to FW, and also shortened homing duration in pre-spawning chum salmon from the mouth of the Ishikari River to the hatchery (Kitahashi et al, 2001).…”

Changes in the plasma levels of insulin-like growth factor-I from the onset of spawning migration through upstream migration in chum salmon

“…The decrease in the plasma levels of IGF-I in the homing adults probably occurred together with the increase in activity of the HPG-axis, because the amounts of salmon gonadotropin-releasing hormone mRNAs in the forebrain and those of LH mRNA in the pituitary increased during upstream migration of pre-spawning chum salmon (Onuma et al, 2003b;Onuma et al, 2005b). We thus consider that a decline in the circulating IGF-I permit the increase in the activity of the HPG-axis toward the final maturation, or, secretion of IGF-I was repressed by the increase in the HPG-axis activity during upstream migration.…”

“…In homing chum salmon, activity of the hypothalamus-pituitary-gonadal axis (HPG-axis) increased during upstream migration from the coast through the natal hatchery (Onuma et al, 2003a;Onuma et al, 2005b). In masu salmon, IGF-I modulated the amounts of gonadotropin (GTH) subunit mRNAs and release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in pituitary cells by season-and reproductive stage-dependent manners (Ando et al, 2006, Furukuma et al, 2008.…”