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.

Keywords: Amygdala; Carbon dioxide; Panic; Periacqueductal gray.

Figure 1:. Amygdala lesions alter CO₂-evoked behaviors.

A) Diagram [58] showing size and location of the largest (red) and smallest (blue) amygdala lesions. B) Amygdala lesions reduce CO₂-evoked freezing. ANOVA revealed an interaction between CO₂ concentration and lesion status (F(2,57) = 7.382, p = 0.0014, n = 15, 7, 18, 10, 6, 6). Planned contrast testing revealed that amygdala lesions reduce freezing to both 10% (***p = 0.0004) and 20% CO₂ (*p = 0.0481). C) Amygdala lesions elicit 10% CO₂-evoked jumping. ANOVA revealed an interaction between CO₂ concentration and lesion status (F(2,57) = 5.248, p = 0.0081. Planned contrast testing revealed that mice with amygdala lesions exhibit increased jumping in response to 10% (***p < 0.0001) but not 20% (p = 0.9709) CO₂, and that sham controls jumped more in response to 20% CO₂ than 10% CO₂ (p = 0.0066).

Figure 2:. Amygdala-lesioned mice do not jump to an auditory conditioned stimulus or a predator odor.

A) Amygdala lesions greatly attenuate conditioned freezing to a tone that had been previously paired with footshocks (t(34) = 5.676, ***p < 0.0001, n = 18/group). However, exposure to this tone does not elicit jumping (p > 0.9999). B) Amygdala lesions do not alter freezing in response to the predator odor TMT (t(11) = 0.3492, p = 0.7336, n = 6, 7), nor do they elicit jumping.

Figure 3:. BNST and hippocampus lesions do not elicit 10% CO₂-evoked jumping.

A) As previously reported [14], BNST lesions reduce 10% CO₂-evoked freezing. However, BNST lesions do not elicit 10% CO₂-evoked jumping. ANOVA revealed an interaction between CO₂ concentration and lesion status (F(1,49) = 7.56, p = 0.0083, n = 15, 9, 18, 11). Planned contrast testing revealed that BNST-lesioned mice freeze less than sham-lesioned mice in 10% CO₂ (**p = 0.0032). However, ANOVA revealed no effect of CO₂ concentration on jumping (F(1,49) = 1.4, p = 0.2425), no effect of lesion (F(1,49) = 2.797, p = 0.1008), and no interaction (F(1,49) = 1.4, p = 0.2425). B) Hippocampus lesions do not alter 10% CO₂-evoked freezing behavior or elicit 10% CO₂-evoked jumping. ANOVA revealed an effect of CO₂ concentration on freezing (F(1,43) = 253.2, p < 0.0001, n = 15, 7, 18, 7), but no effect of lesion (F(1,43) = 0.0439, p = 0.8350), and no interaction (F(1,43) = 0.5889, p = 0.5889). ANOVA revealed no effect of CO₂ on jumping (F(1,43) = 3.14, p = 0.0835), no effect of lesion (F(1,43) = 0.004056, p = 9495), and no interaction (F(1,43) = 0.06299, p = 0.8030).

Figure 4:. dPAG lesions reduce jumping behavior in amygdala-lesioned mice.

10% CO₂-evoked behavior of mice with both amygdala and dPAG lesions was compared to that of sham controls and mice with only amygdala lesions (from Figure 1). A) ANOVA revealed a difference in freezing among groups (F(2,35) = 11.35, p = 0.0002, n = 18, 10, 10). Planned contrast testing revealed that freezing in mice with both amygdala and dPAG lesions was less than in sham controls (***p = 0.0005) and similar to mice with only amygdala lesions (p = 0.3445). B) ANOVA revealed a difference in jumping among groups (F(2,35) = 30.00, p < 0.0001). dPAG lesions greatly attenuated the CO₂-evoked jumping seen in amygdala-lesioned mice (***p = 0.0005) and mice with both amygdala and dPAG lesions exhibited similar jumping as sham controls (p = 0.6950).