Genetic mapping of ASIC 4 and contrasting phenotype to ASIC 1a in modulating innate fear and anxiety

Although ASIC 4 is a member of the acid‐sensing ion channel ( ASIC ) family, we have limited knowledge of its expression and physiological function in vivo . To trace the expression of this ion channel, we generated the ASIC 4‐knockout/Cre ERT 2 ‐knockin ( Asic4 CreERT2 ) mouse line. After tamoxifen induction in the Asic4 CreERT2 :: CAG ‐ STOP floxed ‐Td‐tomato double transgenic mice, we mapped the expression of ASIC 4 at the cellular level in the central nervous system ( CNS ). ASIC 4 was expressed in many brain regions, including the olfactory bulb, cerebral cortex, striatum, hippocampus, amygdala, thalamus, hypothalamus, brain stem, cerebellum, spinal cord and pituitary gland. Colocalisation studies further revealed that ASIC 4 was expressed mainly in three types of cells in the CNS : (i) calretinin ( CR )‐positive and/or vasoactive intestine peptide ( VIP )‐positive interneurons; (ii) neural/glial antigen 2 ( NG 2)‐positive glia, also known as oligodendrocyte precursor cells; and (iii) cerebellar granule cells. To probe the possible role of ASIC 4, we hypothesised that ASIC 4 could modulate the membrane expression of ASIC 1a and thus ASIC 1a signaling in vivo . We conducted behavioral phenotyping of Asic4 CreERT2 mice by screening many of the known behavioral phenotypes found in Asic1a knockouts and found ASIC 4 not involved in shock‐evoked fear learning and memory, seizure termination or psychostimulant‐induced locomotion/rewarding effects. In contrast, ASIC 4 might play an important role in modulating the innate fear response to predator odor and anxious state because ASIC 4‐mutant mice showed increased freezing response to 2,4,5‐trimethylthiazoline and elevated anxiety‐like behavior in both the open‐field and elevated‐plus maze. ASIC 4 may modulate fear and anxiety by counteracting ASIC 1a activity in the brain.