Posttraumatic stress disorder (PTSD) is a significant psychiatric disorder affecting an estimated 3.9% of the global population. A key symptom differentiating PTSD from other anxiety disorders is reexperiencing, characterized by involuntary retrieval of traumatic memories (flashbacks, nightmares, intrusive thoughts). Understanding the mechanisms underlying reexperiencing is crucial for developing effective treatments. Fear memory, a learned response to a previously neutral stimulus, is believed to play a central role. Recent research has implicated genes associated with the cyclic adenosine monophosphate (cAMP) signaling pathway in PTSD. This study aims to explore the relationship between cAMP signaling and PTSD by investigating its impact on fear memory in mice and comparing the transcriptomes of fear memory-activated mice and PTSD patients experiencing reexperiencing symptoms. The hypothesis is that dysregulation of cAMP signaling, potentially through altered PDE4B expression, contributes significantly to PTSD reexperiencing.

Previous studies have linked PTSD to genes involved in the cAMP signaling pathway, including PACAP, PAC1 receptor (*ADCYAP1R1*), and corticotropin-releasing hormone receptor 1 gene (*CRHR1*). These genes' involvement suggests a potential role for cAMP signaling in PTSD etiology. Animal models of PTSD, using Pavlovian fear conditioning and inhibitory avoidance tasks, allow the investigation of fear memory mechanisms. These models show that fear memory retrieval is not passive, and the consolidated memory becomes labile upon retrieval and is subsequently maintained or enhanced through reconsolidation. The activation of the cAMP signaling pathway has been shown to facilitate memory retrieval in animal studies, highlighting its potential involvement in PTSD reexperiencing, which may reflect dysregulation of memory processes involved in retrieval, maintenance, and reconsolidation of traumatic memories.

The study employed both mouse and human components. The mouse study utilized pharmacological and optogenetic manipulations of cAMP signaling. In the pharmacological approach, mice received systemic injections of rolipram (ROL, a PDE4 inhibitor) or NB001 (an adenylyl cyclase 1 inhibitor) before fear memory retrieval (contextual fear conditioning and inhibitory avoidance tests). Optogenetic manipulation involved micro-infusion of AAV vectors expressing a photoactivatable adenylyl cyclase (bPAC) or a light-activated phosphodiesterase (LAPD) into the dorsal hippocampus, followed by blue light stimulation before memory retrieval. RNA extraction and quantitative reverse transcription PCR (qRT-PCR), RNA sequencing (RNA-seq) were performed on hippocampal and peripheral blood samples. The human study comprised 32 female PTSD patients and 16 healthy controls. PTSD diagnosis was confirmed using the Posttraumatic Diagnostic Scale (PDS), and symptom severity was assessed with the Impact of Event Scale-Revised (IES-R). Blood samples were collected for transcriptome analysis using Agilent SurePrint G3 microarrays and DNA methylation analysis using Infinium MethylationEPIC BeadChips. Statistical analyses included ANOVA, t-tests, and correlation analyses. Integrative analysis compared mouse hippocampal transcriptomes after fear memory retrieval and human blood transcriptomes in relation to reexperiencing symptoms.

Pharmacological and optogenetic experiments in mice showed that increasing cAMP levels enhanced fear memory retrieval and maintenance, while decreasing cAMP levels impaired these processes. Integrative analysis of mouse and human transcriptomes revealed significantly reduced mRNA expression of PDE4B (a cAMP-degrading enzyme) in PTSD patients with more severe reexperiencing symptoms and in mouse hippocampi after fear memory retrieval. Lower PDE4B mRNA levels were correlated with decreased DNA methylation of a locus within PDE4B in PTSD patients. Specifically, PDE4B/Pde4b expression levels were negatively correlated with PTSD reexperiencing symptoms, downregulated after fear memory retrieval in mice, and lower in patients than in controls. In mice, decreased *Pde4b* mRNA levels were observed in both the hippocampus and peripheral blood after fear memory retrieval. Co-expression analysis identified RAP1A as strongly co-expressed with PDE4B, and both were downregulated in relation to reexperiencing symptoms and fear memory retrieval. Mediation analysis showed that PDE4B DNA methylation status, particularly at cg14227435, affected PTSD reexperiencing symptoms by altering PDE4B mRNA expression levels.

The findings strongly support the hypothesis that cAMP signaling pathway activation plays a crucial role in PTSD reexperiencing symptoms. The downregulation of PDE4B, leading to increased cAMP levels, appears to enhance traumatic memory consolidation and reconsolidation, thereby contributing to the persistent reexperiencing of traumatic memories. The concordance between human and mouse transcriptome changes supports the validity of the study's integrative approach. The involvement of RAP1A further suggests a complex interplay between cAMP and MAPK signaling pathways. The impact of PDE4B DNA methylation on gene expression highlights epigenetic mechanisms influencing PTSD.

This study demonstrates a strong link between cAMP signaling, PDE4B expression, and PTSD reexperiencing symptoms. The downregulation of PDE4B, leading to increased cAMP, enhances fear memory, contributing to reexperiencing. Future research should focus on larger, sex-balanced human studies to validate these findings and investigate the causal relationship between PDE4B dysregulation and PTSD. Further research into the interaction between cAMP and other signaling pathways, such as MAPK, is warranted. Targeting the cAMP signaling pathway might offer new therapeutic avenues for PTSD.

The study's limitations include a relatively small sample size in the human study, the inclusion of only female participants, and a lack of investigation into the effects of cAMP manipulation on memories beyond fear memories. The generalizability of findings from peripheral blood to the brain requires further investigation.