Mechanistic Study Of Acid-sensing Ion Channel In Fear Extinction

Learning and memory represent a basic function of the brain,among which extinction learning is necessary for adaptation of the organism to the constantly changing environment.Many brain diseases,such as anxiety disorders,are closely related to deficiencies in extinction learning.One of the hotspots in neuroscience research has been elucidating the mechanisms underlying extinction of learned fear(i.e.fear extinction),a process that relies on the dynamic changes in neuronal connectivity of particular neural circuits,mainly composed of hippocampus,medial prefrontal cortex(mPFC),and basolateral amygdala(BLA).However,it remains unclear how the plasticity of these circuits,as well as the corresponding molecules,underpins fear extinction.Acid-sensing ion channels(ASICs)are a group of extracellular proton-gated cation channels.In the central nervous system,postsynaptic ASIC1 a channel is believed to be activated by protons released from presynaptic vesicles during synaptic transmission,and this activation contributes to synaptic plasticity,and in turn learning and memory.However,whether ASIC1 a is involved in fear extinction remains unclear.Here,we focused on whether and how ASIC1 a plays a role in fear extinction,aiming to validate if ASIC1 a represents a promising target for clinical treatment of aberrant fear-related disorders.First,we reconfirmed the crucial role of ASIC1 a in fear acquisition.Global knockout or conditional knockout(cKO)in BLA of ASIC1 a gene severely inhibited fear learning.On the other hand,ASIC1 a cKO in ventral hippocampus(vHPC)did not affect the acquisition of fear memory,but severely impaired fear extinction.Local delivery of ASIC1 a inhibitor into vHPC suppressed fear extinction,while inhibiting ASIC1 a activity in BLA had no such effect.Therefore,vHPC ASIC1 a plays a specific and key role in fear extinction.In order to explore the mechanism by which ASIC1 a regulates fear extinction,we performed RNA sequencing for whole-transcriptome analysis of vHPC tissues after extinction learning.The results suggest that brain-derived neurotrophic factor(BDNF)is a potential key molecule for ASIC1 a regulation of fear extinction.Using fluorescence quantitative PCR and enzyme-linked immunosorbent assay(ELISA),we confirmed that both mRNA and protein contents of BDNF in vHPC tissues increased significantly after extinction learning,but these effects were eliminated in ASIC1 a vHPC cKO mice.Overexpression of BDNF gene in vHPC restored the impaired extinction learning of ASIC1 a cKO mice to the control level,indicating that vHPC ASIC1 a regulates fear extinction by affecting BDNF expression.Next,by combining optogenetics with in vitro electrophysiological recording,we explored the plasticity of neuronal circuit underlying the contribution of vHPC ASIC1 a to fear extinction,as well as the molecular mechanism.Indeed,extinction learning caused a significant enhancement of both presynaptic neurotransmitter release and postsynaptic N-methyl-D-aspartate receptor(NMDAR)function in the vHPC-infralimbic cortex(IL)synapse,but decreased these indexes in the vHPCprelimbic cortex(PL)synapse.However,ASIC1 a cKO in vHPC abolished extinction learning-associated synaptic adaptations described above in the vHPC-PL and vHPC-PL synapses.Notably,in vitro treatment of the mPFC slice from ASIC1 a vHPC cKO mice with recombinant BDNF completely restored extinction learningassociated potentiation of NMDAR function in the vHPC-IL synapse,but didn’t affect the presynaptic neurotransmitter release in the vHPC-IL and vHPC-PL synapses.Accordingly,in vivo delivery of BDNF to mPFC reversed the impairment of fear extinction in ASIC1 a vHPC cKO mice.Interestingly,intervening ASIC1 a in mPFC had no impact on fear extinction,indicating that ASIC1 a in presynaptic neurons of vHPC-mPFC circuit regulates fear extinction through BDNF signaling.Finally,gene overexpression and pharmacological activation of ASIC1 a in vHPC promoted fear extinction.Therefore,we propose that ASIC1 a may be a potential therapeutic target for clinical treatment of fear disorders.We conclude that ASIC1 a regulates extinction learning-associated plasticity of vHPC-mPFC circuit via BDNF signaling,especially enhancement of postsynaptic NMDAR function of the vHPC-IL synapse,and thereby contributes to fear extinction.