A breath of fresh air into the neurobiology of anxiety and panic

1. A breath of fresh air into the neurobiology of anxiety and panic The Amygdala Is a Chemosensor that Detects Carbon Dioxide and Acidosis to Elicit Fear Behavior , Ziemann et al. Cell 2009

2. The Amygdala 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. 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…..

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

4. Paper results - Loss or Inhibition of (acid-sensing ion channel-1a ) ASIC1a Impairs CO2-Induced Fear Behavior • 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 CO2 evoked prominent fear-like freezing behavior compared to air. ASIC1a−/− mice were significantly impaired relative to wild-type controls

5. Paper results - Loss or Inhibition of ASIC1a Impairs CO2-Induced Fear Behavior • 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 CO2 increases anxiety-like behavior in the open field test

6. Paper results - Loss or Inhibition of ASIC1a Impairs CO2-Induced Fear Behavior • Previous CO2 exposure potentiated context-evoked fear memory in ASIC1a+/+ mice but not in ASIC1a−/− mice 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 basolateralamygdala Ziemann et al. Cell 2009 CO2 enhanced freezing in ASIC1a+/+ mice during context fear conditioning

7. Breathing CO2 Reduces Amygdala pH Ziemann et al. Cell 2009 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.

8. Are those changes sufficient to activate ASICs??? YES Ziemann et al. Cell 2009

9. Does minimizing the ph reduction attenuate the CO2 response? YES Ziemann et al. Cell 2009 • 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

10. Is it Amygdala specific? • 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 amygdalaversus those that missed. • (D) local acidic and vehicle injections hits vs. miss – freezing response Ziemann et al. Cell 2009 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?

11. Is it Amygdala specific? 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

12. The paper in a nutshell Ziemann et al. Cell 2009 • 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.