GONADOTROPIN-RELEASING-HORMONE (GNRH) - EXPRESSION DURING SALMON MIGRATION

In the seaward migrating chum salmon, immunocytochemical and in situ h ybridization techniques revealed isolated GnRH neurons at the base of the nasal epithelium, along the nervus terminalis and as ganglia at th e rostroventral (gROB) and caudalmost (GT) olfactory bulb. A novel GnR H ganglion was seen at the cribriform bone (gCB). GnRH immunoreactivit y but not the hybridization signal was detected in the midbrain neuron s. 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 expressio n and the migratory behavior of chum salmon.