The amygdala differentially regulates defensive behaviors evoked by CO2.

ABSTRACT

CO2 inhalation can provoke panic attacks in humans, and the likelihood is increased in patients with panic disorder. Identifying brain sites involved could provide important mechanistic insight into the illness. In mice, the amygdala has been suggested to promote CO2 responses; however, recent studies in humans with amygdala damage indicate the amygdala is not required for CO2-induced fear and panic and might actually oppose these responses. To clarify the role of the amygdala, we produced lesions in mice paralleling the human lesions, and characterized behavioral responses to CO2. Compared to sham controls, we found that amygdala-lesioned mice froze less to 10% CO2, and unlike shams they also began to jump frenetically. At 20% CO2, controls also exhibited jumping, suggesting it is a normal response to more extreme CO2 concentrations. The effect of amygdala lesions was specific to CO2 as amygdala-lesioned mice did not jump in response to a predator odor or to an auditory conditioned stimulus. In amygdala-lesioned mice, jumping evoked by 10% CO2 was eliminated by co-lesioning the dorsal periaqueductal gray, a structure implicated in panic and escape-related behaviors. Together, these observations suggest a dual role for the amygdala in the CO2 response promoting CO2-induced freezing, and opposing CO2-induced jumping, which may help explain the exaggerated CO2 responses in humans with amygdala lesions.