Rodent amygdala under acute psychological stress: a review

Stress is a common experience in everyday life. Exposure of organisms to threat stimuli requires adaptive mechanisms to maintain homeostasis. Animals possess integrated circuits to generate defensive responses appropriate to the threat and approach responses proportional to potential gains. The amygdala is a brain structure critical for the stress response. It is subdivided into nuclei that, due to their connections and cytoarchitecture, specifically modulate stress-related behaviour. This review focuses on the role of the major amygdala nuclei in response to acute stressors that elicit emotional responses in rodents. Of the two nuclei, central (Ce) and medial (Me), which constitute the main amygdala output to hypothalamic and brainstem stress-related regions, it is Me that appears to be the more important in terms of generating anxiety-related responses to an acute psychological stressor. The Me neurons can influence the functioning of the hormonal and autonomic systems, thereby regulating defensive and reproductive behaviour under stress conditions. The Me not only plays a role in integrating and relaying sensory cues to downstream targets, but also controls behavioural output itself and represents the earliest stage at which specific behavioural responses are determined. The basolateral complex (BLC) appears to play less direct, more 'analytical', roles such as evaluating the valence of stimuli or regulating behaviour in situations involving uncertainty or unpredictability. Moreover, under stress, the BLC may influence regional stress-induced immune and metabolic changes in other limbic structures. There has been little investigation into the role of the remaining amygdala nuclei involved in stress response, and this awaits further research.