The Amygdala Is a Chemosensor that Detects Carbon Dioxide and Acidosis to Elicit Fear Behavior , Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 2

The brain's defensive system needs to be highly adaptive to generate rapid autonomic and behavioral responses to threatening stimuli, such as a predator in the forest, a bully at the office, or an aversive stimulus in the laboratory…..

The Amygdala, a collection of nuclei buried in the temporal lobe of the brain, is essential for both innate and learned fear in rodents and humans.

04/21/23 Johannes Passecker 3

Ziemann et al. Cell 2009

Neuroanatomy proved the amaygdala as a convergence point of multimodel sensory information – linked and derived closely from Pavlovian fear conditioning.

Coincident activity among sensory afferents in the BLA -> potentiation of glutamatergic synapses in the conditioned stimulus pathway -> learned fear response.

mediated by direct connections between the BLA and the central nucleus of the amygdala (CEA), which in turn projects to hypothalamic, midbrain, and medullary centers that regulate heart rate, freezing behavior, and respiration

BLA = the lateral, basolateral, and basomedial nuclei)

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CO2 evoked prominent fear-like freezing behavior compared to air. ASIC1a−/− mice were significantly impaired relative to wild-type controls

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.

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 5

CO2 increases anxiety-like behavior in the open field test

ASIC1a+/+ mice avoided the CO2 chamber, whereas ASIC1a−/− did not. Consequently, the ASIC1a+/+ spent significantly less time on the CO2 side than the ASIC1a−/− mice

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 6

Previous CO2 exposure potentiated context-evoked fear memory in ASIC1a+/+ mice but not in ASIC1a−/− mice

CO2 enhanced freezing in ASIC1a+/+ mice during context fear conditioning

acid-sensing ion channel-1a as a mediator of CO2 induced fear

Why the Amygdala?

Amygdala as the central brain structure involved in panic and

fear response

robust ASIC1a expression in the basolateral amygdala

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 7

They measured pH in the basolateral amygdala of anesthetized mice and found that breathing CO2 reduced pH of both genotypes similarly . They recorded similar reductions in the lateral ventricle. Baseline pH in the mice breathing air was less than the expected range likely due to respiratory suppression during anesthesia.

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 8Ziemann et al. Cell 2009

YES

04/21/23 Johannes Passecker 9

Systemic HCO3−

administration in order to raise Amygdala ph

In ASIC1a+/+ mice:

Attenuates CO2 freezing Inhibition of contectually

conditioned fear response

Attenuates freezing response to predator odor

Ziemann et al. Cell 2009

YES

04/21/23 Johannes Passecker 10

breathing CO2 reduces pH throughout the brain

Although ASIC1a expression is abundant in the amygdala, it is also widely distributed in the CNS.

Would pH reductions limited to the amygdala produce fear behavior?

Microinjections of acidic ACSF into the amygdala + a ph sensor

(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 amygdala versus those that missed.

(D) local acidic and vehicle injections hits vs. miss – freezing response

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 11

Injection of adeno-associated virus encoding ASIC1a into the amygdala of ASIC1a-/- mice

expressing ASIC1a in the basolateral amygdala restored CO2-evoked freezing

injections that missed the amygdala or expression of GFP had minimal effects on CO2-evoked freezing.

Ziemann et al. Cell 2009

04/21/23 Johannes Passecker 12

Ziemann et al. 2009 suggests

that changes in extracellular pH in the amygdala trigger cationic currents mediated by ASIC1a channels.

that inhalation of carbon dioxide (CO2) decreases the pH in the amygdala and yields freezing behavior in mice.

Genetic deletion or pharmacological disruption of ASIC1a channels reduces fear associated with CO2 inhalation

and viral-mediated expression of ASIC1a in the BLA of ASIC1a-deficient mice restores CO2-induced fear. Ziemann et al. Cell 2009