Gonadotropin-releasing hormone (GnRH): expression during salmon migration.

In the seaward migrating chum salmon, immunocytochemical and in situ hybridization techniques revealed isolated GnRH neurons at the base of the nasal epithelium, along the nervus terminalis and as ganglia at the rostroventral (gROB) and caudalmost (GT) olfactory bulb. A novel GnRH ganglion was seen at the cribriform bone (gCB). GnRH immunoreactivity but not the hybridization signal was detected in the midbrain neurons. During the migratory period, there were trends towards an increase in GnRH mRNA in the gCB and the gROB and a significant surge in plasma thyroid hormones was also evident. Therefore, we hypothesise thyroid hormones might be crucial for the increased tendency of GnRH expression and the migratory behavior of chum salmon.