Basolateral Amygdala Projections To The Prelimbic Subdivision Of VmPFC Involved In Cued Fear Memory Erasure

BackgroundMemory extinction is a normal physiological function of human.If memory extinction failed,especially fear memory,it would lead to mental disorders,such as post-traumatic stress disorder(PTSD)and depression.Clinically,fear extinction known as an exposure therapy is a major approach to treat these mental disorders.Under some conditions,however,fear memory is observed to recur after the exposure therapy,indicating that extinction does not erase the original fear memory.Some reports show that fear memory can be erased via drug or behavioral intervention in a labile period after memory retrieval,for example,presenting a retrieval trial 10 min?6 h before extinction session(Retrieval + Extinction training)is able to induce memory erasure,but the mechanism remains unclear.It is well known that the high-level functions of brain require specific neural circuit's involvement.Both Retrieval + Extinction pattern and Extinction alone contain an extinction session.Converging evidences from animal studies indicate that the Basolateral amygdala(BLA),ventromedial prefrontal cortex(vmPFC)and other brain regions as well as the circuits among them play an important role in fear extinction.There is an extinction session in both trainings,in order to research whether the neural circuits changed during the Retrieval + Extinction session,using retrograde neuroanatomical tracer and DREADD in a cue dependent fear conditioning paradigm,we found that there were BLA projection neurons directly targeting the infralimbic subdivision of vmPFC(IL)as well as prelimbic subdivision(PL).Moreover,the circuit of BLA-IL was involved in both fear memory extinction and erasure,whereas,BLA inputs to PL were excessively activated after Retrieval +Extinction than regular extinction training.Furthermore,specifically inhibited or activated the BLA projections to the PL could promote or block the erasure of cued fear memory correspondingly.Finally,during Retrieval + Extinction treatment,the projections from BLA might exert inhibitory effects mediated by local interneurons in the PL to balance the circuit locally,which led to the erasure of fear memory.The study will not only perfect relevant theories of fear erasure,but also provide new approaches to treat mental disorders such as PTSD.ObjectiveDiscover the differential neuronal circuits participated in cued fear memory erasureResults:1.Retrieval + Extinction but not Extinction resulted in fear memory erasureMice were divided into two groups to undergo Extinction and Retrieval +Extinction training respectively after fear conditioning.Spontaneous recovery,Renewal and Reinstatement were used to test whether fear memory was erased after Extinction or Retrieval + Extinction training.We found the Freezing level in mice underwent Retrieval + Extinction training was obviously lower than single Extinction group,which suggested the training model was successful.2.BLA-IL neurons were activated during fear extinction as well as fear erasureWe utilized retrograde neuroanatomical trace analysis in vivo and found that there were BLA projection neurons targeting the IL.Moreover,compared with control group,subpopulation of BLA projection neurons targeting the IL in the mice exposed to both Extinction and Retrieval + Extinction were obviously activated(P<0.01).However,there were no significant difference between Extinction group and Retrieval+ Extinction group(P>0.05).3.BLA-PL neurons were participated in fear erasure but not fear extinctionUsing the same methods,we found there were BLA projection neurons targeting the PL.Moreover,we analyzed the projection neurons targeting the PL,and found that there were more BLA-PL neurons were activated in the Retrieval + Extinction group(P<0.01)than control and Extinction group.However,the BLA-PL neurons in the Extinction group were not activated obviously(P>0.05).4.Mice exposed to extinction training in conjunction with inhibition of BLA-PL circuit exhibited fear memory erasureWe used the DREADD system to selectively silence the BLA-PL pathway during extinction training,and mice were then exposed to Renewal or Spontaneous recovery test.The results showed,compared with control group,mice selectively silenced of the BLA-PL pathway exhibited weaker freezing level(P<0.001).5.Fear memory was not erased in mice that were exposed to Retrieval + Extinction training in conjunction with activation of BLA-PL circuitWe used the DREADD system to selectively activate the BLA-PL pathway during Retrieval + Extinction training,and mice were then exposed to Renewal or Spontaneous recovery test.The results showed,compared with control group,mice selectively activated of the BLA-PL pathway exhibited higher freezing level(P<0.001).6.BLA-PL projections involved in fear memory erasure probably by activating the inhibitory neurons in the PLWe analyzed the excitatory neurons in PL region after Extinction or Retrieval +Extinction.We found that excitatory neurons were not activated significantly during Retrieval + Extinction(P>0.05),however,more inhibitory neurons were activated in Retrieval + Extinction group but not Extinction group(P<0.01).On the other hand,we found that the inhibitory neurons in PL and the BLA projections were co-marked abundantly.Conclusions:We found for the first time that BLA-vmPFC circuits' plastic changes occurred in fear memory erasure during Retrieval + Extinction training.There were not only BLA-IL but also BLA-PL circuit participated in fear memory erasure.Moreover,inhibition or activation of this circuit by chemical genetics was able to promote or inhibit fear erasure,which might be achieved by regulating the inhibitory neurons in the PL.