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Stress affects virtually all organisms and can result in both physiological and behavioral changes. Conditioned defeat in Syrian hamsters is a model of stress-induced behavioral plasticity that occurs in a social context. In this model, hamsters are defeated by a larger, more aggressive counterpart. Defeated hamsters subsequently fail to defend their own territory and show striking and long-lasting increases in submissive behavior even when paired with a non-threatening counterpart. The present series of experiments seeks to identify the brain regions and molecular mediators that contribute to this behavioral plasticity. One brain region that has been overlooked by our laboratory is the hippocampus. The results of the first study suggested that the ventral, but not dorsal, hippocampus is important for the acquisition of conditioned defeat as temporary inactivation of the ventral hippocampus prior to defeat training significantly reduced submissive and defensive behaviors when hamsters were tested with a non-aggressive intruder. Next, we sought to identify a potential molecular mediator of social stress-induced behavioral plasticity in hamsters identified as winners or losers after a fight. Using in situ hybridization for brain-derived neurotrophic factor (BDNF) mRNA, we showed that winning and losing hamsters exhibited differences in BDNF mRNA in several regions including the basolateral and medial amygdala as well as the dentate gyrus of the dorsal hippocampus and CA1 of the ventral hippocampus. We next showed that neurotrophic activity in the basolateral amygdala is important for the acquisition of conditioned defeat because K252a infused into the basolateral amygdala prior to defeat training by an aggressive counterpart, significantly decreased submissive and defensive behavior during subsequent testing. Finally, existing data suggest that the amygdala and hippocampus interact to modulate the formation of emotional memories. To test the hypothesis that the basolateral amygdala and ventral hippocampus interact to mediate the behavioral plasticity observed in conditioned defeat, we simultaneously inactivated these regions either contralaterally or ipsilaterally prior to social defeat. Our results suggest that BLA and VHPC interact to mediate the acquisition of conditioned defeat, however, the nature of this interaction remains to be determined. Index words. Social stress, Behavioral plasticity, Hamster, Amygdala, Hippocampus, Brain-derived neurotrophic factor, Agonistic behavior, Learning and memory.
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