Exploring the role of ASIC1a in mouse models of anxiety

Carbon dioxide (CO2) inhalation lowers brain pH and induces anxiety, fear, and panic responses in humans. In mice, CO2 produces freezing and avoidance behavior that has been suggested to depend on the amygdala. However, a recent study in humans with bilateral amygdala lesions revealed that CO2 can trigger fear and panic even in the absence of amygdalae, suggesting and important role for extra-amygdalar brain structures. Because the bed nucleus of the stria terminalis (BNST) contributes to fear- and anxiety-related behaviors and expresses acid sensing ion channel-1A (ASIC1A), we hypothesized that the BNST plays an important role in CO2-evoked fear-related behaviors in mice. We found that BNST lesions decreased both CO2-evoked freezing and CO2-conditioned place avoidance. In addition, we found that CO2 inhalation caused BNST acidosis, and that acidosis was sufficient to depolarize BNST neurons and induce freezing behavior; both responses depended on ASIC1A. Finally, disrupting Asic1a specifically in the BNST reduced CO2-evoked freezing whereas viral vector mediated expression of ASIC1A in the BNST of Asic1a-/- and Asic1a+/+ mice increased CO2-evoked freezing. Together, these findings identify the BNST as an extra-amygdalar fear circuit structure important in CO2-evoked fear-related behavior.

Genetic disruption of the acid-sensing ion channel-1A (ASIC1A) in mice results in deficits in several fear- and anxiety-related behaviors. These deficits have been largely attributed to the loss of ASIC1A in neurons. However, recent studies have identified ASIC1A in several types of non-neuronal cells, including glia. To test the hypothesis that it is the loss of ASIC1A in neurons that results in the behavioral deficits seen in Asic1a-/- mice, we generated SynCre+Asic1aloxP/loxP mice, in which ASIC1A is disrupted specifically in neurons. To validate these mice, we confirmed by PCR that the Asic1a floxed allele was disrupted in brain, but not tail DNA. We further detected a reduction in ASIC1A protein in the SynCre+Asic1aloxP/loxP mice by western blotting and ASIC1A immunohistochemsitry. Further characterization of cre expression with a Rosa26 cre reporter mouse revealed that cre expression did not occur in all neurons, but verified that cre expression was neuron-specific. This neuron-specific knockout of ASIC1A led to behavioral deficits in several models of fear and anxiety, including cued and context fear conditioning, predator odor-evoked freezing and CO2-evoked freezing. Together, these findings suggest that it is ASIC1A in neurons that mediates these fear- and anxiety-related behaviors.

Trimethylthiazoline (TMT), a predator odor isolated from fox feces, elicits freezing and avoidance responses in rodents. This TMT-evoked freezing behavior depends on the bed nucleus of the stria terminalis (BNST), a brain region thought to contribute to anxiety in both humans and mice. Because the acid-sensing ion channel-1A (ASIC1A) is robustly expressed in the BNST and has been previously implicated in TMT-evoked freezing, we hypothesized that the BNST might be a site of ASIC1A action in the TMT-evoked freezing response. Consistent with previous studies, we found that TMT-evoked freezing depended both on the olfactory bulb and on ASIC1A. Viral-mediated disruption of ASIC1A in the BNST reduced TMT-evoked freezing, whereas, viral mediated expression of ASIC1A in the BNST of Asic1a-/- mice increased TMT-evoked freezing. We further observed that TMT exposure induces a modest acidosis, likely due to TMT-induced respiratory suppression. However, this respiratory suppression was not unique to odors that evoke freezing, suggesting that it does not drive the TMT-evoked freezing response. Together, these findings suggest that the BNST is a key site of ASIC1A action in TMT-evoked freezing.

Regulation of cerebral blood flow (CBF) is critical to insure that the brain has adequate resources to maintain normal function. One of the strongest regulators of CBF is carbon dioxide (CO2). CO2 and acidosis are thought to induce vasodilation and increase CBF by initiating nitric oxide (NO) synthesis, though the mechanism by which this occurs is unknown. Recent unpublished studies have suggested that the acid-sensing ion channel-1A (ASIC1A) plays a role in hypercapnia-induced vasodilation. Therefore, we hypothesized that CO2-induced NO production would depend on ASIC1A. We found that CO2 induced robust NO production in Asic1a+/+ but not Asic1a-/- mice. To test the role of neuronal ASIC1A in CO2-induced NO production, we generated SynCre+Asic1aloxP/loxP mice, in which ASIC1A is disrupted specifically in neurons. We found that CO2 did not induce significant NO production in the SynCre+Asic1aloxP/loxP mice, suggesting that it is ASIC1A in neurons that mediates this response. Together, these studies suggest that ASIC1A may mediate neurovascular coupling and regulate CBF.