Changes of thyrotropin-releasing hormone concentration in the brain and levels of prolactin and thyroxin in the serum during spawning migration of the chum salmon Oncorhynchus keta.

Changes in concentration of thyrotropin-releasing hormone (TRH) in various regions of the brain of female and male chum salmon Oncorhynchus keta during their spawning migration toward the home stream were examined to determine the physiological role of TRH. In the same specimens, serum levels of prolactin (PRL) and thyroxin (T4) were also examined and compared with the change of TRH concentration. TRH concentration in the olfactory bulb of males increased significantly just before the upstream migration or entering the river and dropped after entering the home stream. TRH concentration in the olfactory lobe was also slightly higher in Class 2 salmons with nuptial coloration captured in coastal water than in the other groups. TRH concentrations in hypothalamus and pituitary remained relatively low during the spawning migration. The change of TRH concentration in females was similar to that in males, but was smaller and with a wider range. The serum level of PRL was extremely low in both males and females collected in coastal waters (Classes 1 and 2) and increased after the fish entered the river. The serum level of T4 in both males and females tended to be low, as did the TRH concentration in the hypothalamus and pituitary. Immunohistochemistry of the brain using anti-proTRH serum revealed many proTRH cell bodies in the olfactory bulb and olfactory lobe. These results indicate that TRH in the homing salmon is produced mainly in the olfactory bulb and lobe and that its production may be related to changes in olfactory function during migration.