Evaluation of brain angiotensin and plasticity related genes following fear memory retrieval

Abstract

Background Fear learning is an adaptive response to traumatic events and gradual decline of fear response on repeated exposure to stimuli can lead to fear extinction. However, a single presentation of a cue threat can reactive the fear memory through an alternative process called reconsolidation. Here we sought to investigate the role of the renin angiotensin systems (RAS) in reconsolidation to determine whether recent findings from RAS extinction studies in rodents and humans might also be explained by interference with reconsolidation. Methods Using Pavlovian auditory fear conditioning, a single retrieval tone was presented to initiate memory recall. It has been previously shown that fear memories, after retrieval, are in a labile state for a time‐limited window of six hours and require protein synthesis for consolidation. Without any pharmacological intervention, mRNA levels at early (40mins) and late (4hr) phases of reconsolidation were compared to examine gene expression pattern changes in common RAS and plasticity genes in different brain regions during memory reconsolidation. Results To validate our behavior model of single retrieval presentation, we compared the expression of immediate early genes (IEGs) ‐ cfos, Egr‐1 and Arc in the basolateral amygdala (BLA) – critical for fear learning and memory. These IEG’s were induced rapidly at 40mins with 2–3‐fold (p<0.001; n=12) increases in mRNA expression and returned to the baseline within 4hrs comparable to animals which did not receive the retrieval tone. The RAS distribution in the brain follows a distinct pattern which we confirmed in our lab by measuring the expression level of angiotensinogen (Agt) in telencephalon (forebrain) and diencephalon (mid and hindbrain) relative to liver. There is a 3‐fold increase in Agt (p=0.025, n=6) mRNA levels at 40mins. Angiotensin converting enzyme (Ace) mRNA levels decreased by half (p = 0.0169, n=6) and consequently Ace2 levels increased 2‐fold at early reconsolidation phase in BLA. The angiotensin type1 receptor (Agtr1) showed increased expression (p=0.0213, n=10) at 4hrs whereas angiotensin type 2 receptor (Agtr2) had steady rise at both 40mins and 4hrs (p=0.0016, n=10). The BLA mRNA levels for tachykinin receptor 3 (Tac3r), previously shown to modulate fear memory and angiotensin II, was elevated by 3‐fold at both 40mins and 4hrs. We also examined the CA1 hippocampal region, but unlike the BLA, there was no significant change in the expression levels of these. Conclusion Here we identified two distinct pathways for brain RAS in the BLA during a time course of memory reconsolidation, including early phase activation of Ace2 and a later phase increase in angiotensin receptors (Agtr1 and Agtr2) mRNA levels. These data suggest that, similar to transcriptional regulation of IEGs in the BLA, angiotensin‐related genes undergo dynamic regulation during reconsolidation following fear memory retrieval. Support or Funding Information NIH1R01HL137103‐01, AHA15CSA24340001 Evaluation of brain angiotensin and plasticity related ge...