When housed together, juvenile rainbow trout (Oncorhynchus mykiss) form social hierarchies in which subordinate animals show stress-related changes in behaviour, endocrine function and neurochemistry. Similar changes are observed in the subordinate animals when social hierarchies are studied in mammalian species. These animals also show a marked reduction in hippocampal cell proliferation. To determine whether this effect of stressful social interaction on cell proliferation exists in rainbow trout as well, the bromodeoxyuridine (BrdU) method of investigating proliferation was adapted for utilization in rainbow trout.
Pairs of fish were allowed to fight for dominance, and were then left to interact for 4 days. 24h before the end of the experiment, the deoxythymidine analogue BrdU was administred intraperitonally to all fish. BrdU is incorporated into the DNA of cells going through S-phase and is thus a marker of proliferation. Proliferating cells were visualized immunohistochemically and quantified in transverse sections of the telencephalon, which is suggested to contain structures embryologically and functionally homologous to the hippocampus.
Even though the method appeared to be stressful to the animals, a reduction in telencephalic cell proliferation of almost 40% was found in the subordinates compared to isolated controls. The proliferation in dominant animals did not differ significantly from any of the other groups, but there was a strong tendency of reduced proliferation in these animals as well.
These results confirm that stressful social interaction reduces brain cell proliferation in rainbow trout in a similar manner as in mammals. This change is suggested to be caused by the increased plasma cortisol levels that were found in the subordinate animals, and may be a component of the adaptive stress response.