Behavioral, circuit, and molecular mechanisms underlying UCS deflation

SUMMARY/ABSTRACT While extinction learning can reduce excessive fear characteristic of anxiety-based disorders like post-traumatic stress disorder and generalized anxiety disorder, behavioral inhibition achieved through extinction is highly susceptible to relapse. This is because extinction functions to create a new inhibitory memory while leaving the original fear memory largely unaltered and intact. We have recently identified and characterized behaviorally a novel approach to behavioral reduction that seems to circumvent the behavioral relapse common to extinction. In this procedure, weak versions of the fear-provoking unconditional stimulus are presented to alter the representation of that fearful stimulus to ultimately weaken fear responding to cues previously associated with it. The present proposal aims to expand on our understanding of this phenomenon and apply it to novel situations to reduce behavioral relapse following fear-reducing procedures. Our overarching hypothesis is that unconditional stimulus deflation will be distinct from extinction at the behavioral, circuit, cellular, and molecular levels and will be tested in three complementary but independent aims. In Aim 1, we will first directly compare UCS deflation to extinction in the three most commonly-studied relapse paradigms: renewal, spontaneous recovery, and reinstatement with the prediction that UCS deflation, but not extinction, will reduce all three types of relapse. Then, we will test the requirement for neural activity in the IL BLA pathway during UCS deflation in creating these long-lasting reductions in fear responding using an optogenetic approach. In Aim 2, we will examine the degree to which extinction and deflation activate the original memory ensemble using a robust activity marker (RAM) approach. Here, our prediction is that UCS deflation will preferentially activate the memory ensemble tagged during acquisition, with less activation of this ensemble in extinction learning. We will then quantify the degree to which the original training ensemble is activated following renewal testing. Here, we hypothesize more overlap between the neural ensemble in renewal following extinction than following UCS deflation. Finally, we will test the prediction that the ubiquitin proteasome system will have a unique role in UCS deflation but not extinction. First, we will examine changes in nuclear and synaptic K48 polyubiquitin tagging following UCS deflation or extinction with the prediction that UCS deflation will produce changes in the synapse indicative of memory retrieval and updating whereas extinction will produce changes in the nucleus indicative of new memory formation. We will then test the hypothesis that blocking proteolytic activity will impair UCS deflation but not extinction using an intracranial approach. These studies will be the first to examine how UCS deflation functions to reduce fear across multiple levels of analysis (behavioral, circuit, cellular, and molecular). Further, they will provide detailed mechanistic insight and identify specific targets through which UCS deflation can weaken the original learning to persistently reduce fear and render this reduction less susceptible to relapse.