Acid-Sensing Ion Channels Contribute to Type III Adenylyl Cyclase-Independent Acid Sensing of Mouse Olfactory Sensory Neurons

Acids can disturb the ecosystem of wild animals through altering their olfaction and olfaction-related survival behaviors. It is known that the main olfactory epithelia (MOE) of mammals rely on odorant receptors and type III adenylyl cyclase (AC3) to detect general odorants. However, it is unknown how the olfactory system sense protons or acidic odorants. Here, we show that while the MOE of AC3 knockout (KO) mice failed to respond to an odor mix in electro-olfactogram (EOG) recordings, it retained a small fraction of acid-evoked EOG responses. The acetic acid-induced EOG responses in wild-type (WT) MOE can be dissected into two components: the big component dependent on the AC3-mediated cAMP pathway and the much smaller component not. The small acid-evoked EOG response of the AC3 KOs was blocked by diminazene, an inhibitor of acid-sensing ion channels (ASICs), but not by forskolin/IBMX that desensitize the cAMP pathway. AC3 KO mice lost their sensitivity to detect pungent odorants but maintained sniffing behavior to acetic acid. Immunofluorescence staining demonstrated that ASIC1 proteins were highly expressed in olfactory sensory neurons (OSNs), mostly enriched in the knobs, dendrites, and somata, but not in olfactory cilia. Real-time polymerase chain reaction further detected the mRNA expression of ASIC1a, ASIC2b, and ASIC3 in the MOE. Additionally, mice exhibited reduced preference to attractive objects when placed in an environment with acidic volatiles. Together, we conclude that the mouse olfactory system has a non-conventional, likely ASIC-mediated ionotropic mechanism for acid sensing.

Microglial Acid Sensing Regulates Carbon Dioxide-Evoked Fear

BACKGROUND

Carbon dioxide (CO2) inhalation, a biological challenge and pathologic marker in panic disorder, evokes intense fear and panic attacks in susceptible individuals. The molecular identity and anatomic location of CO2-sensing systems that translate CO2-evoked fear remain unclear. We investigated contributions of microglial acid sensor T cell death-associated gene-8 (TDAG8) and microglial proinflammatory responses in CO2-evoked behavioral and physiological responses.

METHODS

CO2-evoked freezing, autonomic, and respiratory responses were assessed in TDAG8-deficient ((-/-)) and wild-type ((+/+)) mice. Involvement of TDAG8-dependent microglial activation and proinflammatory cytokine interleukin (IL)-1β with CO2-evoked responses was investigated using microglial blocker, minocycline, and IL-1β antagonist IL-1RA. CO2-chemosensitive firing responses using single-cell patch clamping were measured in TDAG8(-/-) and TDAG8(+/+) mice to gain functional insights.

RESULTS

TDAG8 expression was localized in microglia enriched within the sensory circumventricular organs. TDAG8(-/-) mice displayed attenuated CO2-evoked freezing and sympathetic responses. TDAG8 deficiency was associated with reduced microglial activation and proinflammatory cytokine IL-1β within the subfornical organ. Central infusion of microglial activation blocker minocycline and IL-1β antagonist IL-1RA attenuated CO2-evoked freezing. Finally, CO2-evoked neuronal firing in patch-clamped subfornical organ neurons was dependent on acid sensor TDAG8 and IL-1β.

CONCLUSIONS

Our data identify TDAG8-dependent microglial acid sensing as a unique chemosensor for detecting and translating hypercapnia to fear-associated behavioral and physiological responses, providing a novel mechanism for homeostatic threat detection of relevance to psychiatric conditions such as panic disorder.