Two small and intimately related regions of the brains of zebrafish have been found to control the outcome of a fight before the fight begins. Fighting is a necessary evil in any animal society that maintains social position and dominance in mating. The discovery was reported in the March 31, 2016, edition of the journal Science by Hitoshi Okamoto and colleagues at the RIKEN Brain Science Institute in Japan.
Two neural circuits that originate in the habenula and pass through the interpeduncular nucleus and terminate in the dorsal tegmental area of zebrafish brains determine if the fish will win or lose a fight with another fish. An equivalent set of circuits in the human brain is located in the periaqueductal gray area. The researchers measured electrical activity in the two circuits before, during, and after a zebrafish fight. The lateral and medial parts of the dorsal habenula were found to be associated with surrender or continuing aggression, respectively.
The researchers tested their findings with zebrafish that had been modified to have no electrical activity in either the aggressive circuit or the submissive circuit by a nerve toxin. Silencing activity in the submissive circuit caused fish to win fights more frequently. Silencing the aggressive circuit caused fish to lose fights more and also caused the fish to come back for another fight and lose again.
The discovery may have applications in sports and mental aberrations that produce violent behavior. The trick for sports would be to find an inhibitor of the circuit that allows the possibility of a loss to be silenced. The treatment of aggression or mental defect that leads to violent behavior would require a drug that turns the aggressive circuit off. Sports applications might be deemed illegal with drug screens.