Social stress reduces forebrain cell proliferation in rainbow trout (Oncorhynchus mykiss).

Compared to mammals, teleost fish have a very high rate of adult brain cell proliferation. Still little is known about how this process is regulated in comparative models, and what its functional implications are. We investigated the effect of stressful social interaction on brain cell proliferation in size matched rainbow trout pairs after the formation of stable social hierarchies. After 4 days of interaction, socially subordinate fish displayed common signs of chronic stress including reduced feeding behaviour, elevated plasma cortisol levels, and up-regulated brain stem 5-HT activity. The number of newborn cells in the telencephalon was quantified using immunohistochemistry for the exogenously administered S-phase marker BrdU. Subordinate fish had 40% fewer BrdU-positive telencephalic cells compared to isolated controls, while dominant individuals showed a non-significant tendency towards reduced cell proliferation. Cell proliferation in subordinate animals correlated negatively with aggression received immediately after hierarchy formation, indicating that the level of cell division suppression is related to the severity of the social stressor. These findings are comparable to findings in mammalian models of psychosocial stress, indicating that the suppressive effect of social stress on brain cell proliferation is conserved, and thus likely confers adaptive benefits throughout the vertebrate subphylum.