The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Abstract

The amygdala processes and directs inputs and outputs that are key to fear behavior. However, whether it directly senses fear-evoking stimuli is unknown. Because the amygdala expresses acid-sensing ion channel-1a (ASIC1a), and ASIC1a is required for normal fear responses, we hypothesized that the amygdala might detect a reduced pH. We found that inhaled CO(2) reduced brain pH and evoked fear behavior in mice. Eliminating or inhibiting ASIC1a markedly impaired this activity, and localized ASIC1a expression in the amygdala rescued the CO(2)-induced fear deficit of ASIC1a null animals. Buffering pH attenuated fear behavior, whereas directly reducing pH with amygdala microinjections reproduced the effect of CO(2). These data identify the amygdala as an important chemosensor that detects hypercarbia and acidosis and initiates behavioral responses. They also give a molecular explanation for how rising CO(2) concentrations elicit intense fear and provide a foundation for dissecting the bases of anxiety and panic disorders.

Figure 1

ASIC1a disruption attenuates CO₂ effects in multiple behavioral models of fear. (A) CO₂ evoked prominent fear-like freezing behavior compared to air. ASIC1a−/− mice were significantly impaired relative to wild-type controls. ANOVA revealed a significant CO₂ by genotype interaction (F(2, 35) = 4.62, p < 0.05). Planned contrast testing revealed a significant attenuation of freezing in the ASIC1a−/− mice in response to 10% CO₂ (*p < 0.001), but not in air or 5% CO₂ (p > 0.05). From left to right, groups contained 5, 6, 7, 6, 7 and 10 mice. (B) ASIC1a antagonists PcTx1 and A-317567 reduced 10% CO₂-evoked freezing in wild-type mice but had no effect on CO₂-evoked freezing in the ASIC1a−/− mice. ANOVA revealed significant effect of group (F(5, 43) = 3.87, p < 0.01). Significant pairwise comparisons are indicated (*p < 0.05; n = 7–10 mice per group). (C) CO₂ increases anxiety-like behavior in the open field test. ANOVA revealed significant main effects of CO₂ (F(3, 64) = 8.57, p < 0.001) and genotype (F(1, 64) = 15.95, p < 0.001), but not a CO₂ by genotype interaction (F(3, 64) = 1.38, p > 0.05). Planned contrast comparisons revealed significant differences between ASIC1a+/+ and ASIC1a−/− mice at the higher CO₂ concentrations (* p < 0.01; n = 7–10 mice per group). This difference was not due to differences in general locomotor activity; we previously showed that ASIC1a disruption does not alter total locomotor activity at baseline (Coryell et al., 2007). Similarly, in these studies during CO₂ exposure we found no effect of ASIC1a disruption on total beam breaks (F < 1). (D) ASIC1a+/+ mice avoided the CO₂ chamber, whereas ASIC1a−/− CO₂ did not. Consequently, the ASIC1a+/+ spent significantly less time on the CO₂ side than the ASIC1a−/− mice (* p = 0.004, Wilcoxon rank sum test; n = 7 and 9 per group). (E) CO₂ enhanced freezing in ASIC1a+/+ mice during context fear conditioning. Analysis of total freezing during training revealed a significant difference between the groups (F(3, 62) = 13.94, p < 0.001, ANOVA). Planned contrast comparisons revealed a significant effect of CO₂ in ASIC1a+/+ mice (p < 0.001), but not in ASIC1a−/− mice (p > 0.05). (F) Previous CO₂ exposure potentiated context-evoked fear memory in ASIC1a+/+ mice, but not in ASIC1a−/− mice. Significant overall differences were detected between the groups (F(3, 62) = 7.99, p < 0.001, ANOVA). Planned contrast testing revealed a significant effect of CO₂ in ASIC1a+/+ mice (* p < 0.05), but not in ASIC1a−/− mice (p > 0.05).

Figure 2

CO₂ inhalation increases arterial CO₂ partial pressure (PaCO₂) and stimulates ventilation normally in the ASIC1a−/− mice. (A) PaCO₂ did not differ between ASIC1a+/+ and ASIC1a−/− mice. ANOVA revealed significant main effect of CO₂ (F(1,8) = 50.2, p < 0.001; n = 3 per group), but not genotype (F < 1) and no interaction (F < 1). (B) CO₂-evoked increases in minute ventilation relative to baseline did not differ between ASIC1a+/+ and ASIC1a−/− mice (F(1, 46) = 0.065, p = 0.80, ANOVA; from left to right groups comprised of 17, 17, 12, and 4 mice).

Figure 3

CO₂ inhalation reduces amygdala pH. (A) Examples of pH recordings in amygdala showing effects of CO₂ inhalation. (B) Mean pH response in basolateral amygdala to CO₂ inhalation was normal in ASIC1a−/− mice. ANOVA revealed a significant effect of CO₂ (F (2, 15) = 7.8, p < 0.01), but no effect of genotype (F(1, 15) = 0.2, p = 0.66) and no interaction (F< 1, n = 3–4 mice per group). (C) Lateral ventricle pH response to CO₂ inhalation. ANOVA revealed a significant effect of CO₂ (F (2, 30) = 12.0, p < 0.001), but no effect of genotype (F(1, 30) = 1.2, p = 0.3) and no interaction (F< 1, n = 6 mice per group).

Figure 4

Acid-evoked currents in amygdala neurons are mediated by ASIC1a. (A) Representative examples of responses to pH 7.2 and 7.0 in cultured amygdala neurons from ASIC1a+/+ and ASIC1a−/− mice recorded by whole-cell voltage clamp. (B) Mean peak current evoked by increasingly acidic pH levels in cultured amygdala neurons from ASIC1a+/+ and ASIC1a−/− mice. From left to right, ASIC1a+/+ groups included 14, 17, 19, and 9 neurons. ASIC1a−/− groups each included 7 neurons. (C) As pH was lowered, a greater percentage of amygdala neurons from ASIC1a+/+ mice exhibited acid-evoked current. None of the ASIC1a−/− neurons showed acid-evoked current. (D) Examples of acid-evoked currents in basolateral amygdala neurons in acute slices from ASIC1a+/+ and ASIC1a−/− mice. The number of cells exhibiting acid-evoked currents versus the total number tested at each pH value were as follows: (ASIC1a+/+: pH 5 = 16/21, pH 6.9 = 6/9, pH 7.2 = 2/5), (ASIC1−/−: pH 5 = 0/9, pH 6.9 = 0/7, pH 7.2 = 0/4).

Figure 5

Raising brain pH with HCO₃⁻ blocks CO₂–evoked freezing, fear conditioning, and predator odor-evoked fear. (A) Representative tracing of amygdala pH response to HCO₃⁻ injection (2 mM/kg, ip). (B) Effect of HCO₃⁻ injection on pH response to CO₂ inhalation (black trace) versus no HCO₃⁻ injection (gray trace). (C) HCO₃⁻ reduced CO₂-evoked freezing in ASIC1a+/+ mice, but not in ASIC1a−/− mice. ANOVA revealed significant differences between groups (F(3, 30) = 3.92, p < 0.05). Planned contrast testing revealed a significant effect of HCO₃⁻ in ASIC1a+/+ mice (*p < 0.05), but not in ASIC1a−/− mice (p = 0.8). From left to right, group sizes were 8, 7, 10, and 9 mice. (D) HCO₃⁻ injection prior to fear conditioning inhibited the contextually conditioned fear response in ASIC1a+/+ mice but not in ASIC1a−/− mice. ANOVA revealed significant differences between groups (F(3, 30) = 5.91, p < 0.01). Planned contrast testing found a significant effect of HCO₃⁻ in ASIC1a+/+ mice (*p < 0.05), but not in ASIC1a−/− mice (p = 0.9). From left to right, group sizes were 14, 6, 7, and 7 mice. (E) HCO₃⁻ injection prior to predator odor exposure reduced freezing in ASIC1a+/+ mice but not in ASIC1a−/− mice. ANOVA revealed significant differences between groups (F(3, 26) = 3.46, p < 0.05). Planned contrast testing found a significant effect of HCO₃⁻ in ASIC1a+/+ mice (*p < 0.01), but not in ASIC1a−/− mice (p = 0.9). From left to right, group sizes were 5, 7, 10, and 8 mice.

Figure 6

Amygdala acidosis evokes ASIC1a-dependent freezing behavior. (A) Injecting acidic ACSF into the amygdala of an anaesthetized mouse lowered amygdala pH by several tenths of a pH unit. (B) Acid injection lowered pH to 6.8 or below. (C) Acidic injections that hit the basolateral amygdala (gray boundary) versus those that missed. (D) Acidic injections that hit the amygdala in awake and freely moving mice triggered robust freezing in ASIC1a+/+ mice and very little freezing in ASIC1a−/− mice. ANOVA revealed a significant pH by genotype interaction (F(1, 28) = 12.8, p < 0.001). In the low-pH-injected mice, planned contrast testing revealed significantly greater freezing in ASIC1a+/+ hit mice compared to ASIC1a−/− hit mice (**p < 0.001) and compared to ASIC1a+/+ miss mice (*p < 0.05) (group sizes from left to right were 11, 10, 4, 7, 2, and 4 mice). (E) Electrically stimulating the left amygdala with a bipolar electrode evokes similar rate of freezing responses in ASIC1a+/+ and ASIC1a−/− mice. Shown are percentages of trials per 5 attempts that produced freezing at each voltage (n = 5 mice per group; ASIC1a+/+ ED₅₀ = 1.99 V, 95% CI = 1.64–2.27; ASIC1a−/− ED₅₀ = 1.81 V, 95% CI = 1.44 – 2.09).

Figure 7

Restoring ASIC1a expression bilaterally in the basolateral amygdala of ASIC1a−/− mice rescues CO₂-evoked freezing. (A) Example of AAV-mediated ASIC1a expression and eGFP expression in bilateral amygdala of ASIC1a−/− mouse. (B) AAV-ASIC1a injections that hit the basolateral amygdala (BLA) bilaterally (ASIC1a-hit) increased CO₂-evoked freezing. One-way ANOVA revealed significant differences between the groups (F(4, 42) = 4.85, p < 0.01), and planned contrast tests revealed a significant increase in CO₂-evoked freezing in the ASIC1a-hit group relative to other ASIC1a−/− groups (*p < 0.001). From left to right groups sizes were 14, 8, 5, 12, and 8.