Fyn tyrosine kinase is highly expressed in the limbic system and mice lacking Fyn tyrosine kinase showed increased fearfulness in a variety of tests for anxiety-related behaviors. To investigate the possible role of Fyn tyrosine kinase in aggression, we assessed the aggressive behaviors of the mice lacking the Fyn tyrosine kinase using the resident-intruder and restraint-induced target biting paradigms. The percentage of Fyn-deficient mice that attacked an inanimate target in a restraint tube was higher than that of the control mice. On the contrary, in the resident-intruder paradigm, the percentage of Fyn-deficient mice that attacked the intruder was lower and the Fyn-deficient mice showed a longer latency to attack an intruder. These results suggest a distinct role of Fyn tyrosine kinase in enhancing the offensive aggression and decreasing the defensive aggression. A possible influence of anxiety-phenotype of the Fyn-deficient mice on their abnormal aggressive behavior was discussed.
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