Effects of amygdala lesions on behavioral and neuroendocrine responses to defeat in male syrian hamsters
Duncan, Judith Alicia Askew
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In hamsters, territorial aggression normally directed toward a non-aggressive intruder is replaced by submissive/defensive behaviors in hamsters that have previously experienced social defeat. This phenomenon has been labeled conditioned defeat. The present study investigated the role of the amygdala in the modulation of behavioral and neuroendocrine responses to defeat. In Experiments 1 and 2, male Syrian hamsters received either bilateral electrolytic or sham lesions of the central/basolateral (CE/BLA) nuclei of the amygdala. In Experiment 1, agonistic behavior was recorded during a 10- min social encounter with an aggressive male resident. Bilateral amygdala lesions had no effect on behavioral and neuroendocrine responses of the hamsters during an encounter with an aggressive resident. In Experiment 2, subjects were placed in an aggressive resident’s home cage for 10-min with either the resident present or absent. As in Experiment 1, agonistic behavior was recorded during social encounters with aggressive male residents. Amygdala lesions had no effect on behavioral responses to an aggressive resident during the initial defeat. Twenty-four hours later, defeated and non-defeated hamsters were exposed to a non-aggressive intruder for 10 min. Lesioned, non-defeated hamsters exposed to a non-aggressive opponent exhibited increases in non-social and decreases in social behavior but exhibited similar levels of aggression and submission. By contrast, lesioned hamsters that had previously experienced defeat exhibited significantly less submissive behavior during testing. Plasma levels of ACTH and cortisol were similar for all subjects. While the effects of CE/BLA lesions on behavior appear to depend upon the social history of the subject, these lesions do not block the ability of subjects to produce normal submissive responses to an aggressive opponent. In addition, it appears that the CE/BLA amygdala region is not critical in the modulation of neuroendocrine responses of defeat stress. Data suggest that the CE/BLA is involved in the behavioral plasticity observed following an acute social defeat in Syrian hamsters.