Post-traumatic stress disorder (PTSD) is a common psychological condition characterized by delayed and persistent mental disturbances that stem from exposure to significant traumatic events in one's lifetime. These events may include, but are not limited to, life-threatening accidents, witnessing traumatic occurrences, being threatened with a weapon, experiencing physical attacks, sexual assault, or combat situations during war (Kessler, 2000). While PTSD affects roughly 6 % of the general population, it is seen in 25 to 35 % of individuals who have undergone severe traumatic experiences (Ressler et al., 2022). Symptoms of PTSD include intrusive memories tied to the traumatic event, distress when confronted with trauma-related cues and avoidance of those cues, negative changes in cognition and mood, and heightened arousal and reactivity. The currently recognized effective treatment for PTSD is exposure-based, trauma-focused cognitive behavioral therapy, which aids in regulating the brain's fear response. Regarding pharmacotherapy, the options currently available are limited to a few antidepressants that focus on reducing symptoms rather than addressing the underlying pathophysiology of PTSD (Ressler et al., 2022). Consequently, there is an urgent need for the development of medications that repair the pathological changes, which could pave the way for personalized treatment plans.

The pathophysiology of PTSD has been extensively explored in the literature. Prior studies have firmly established a significant correlation between hippocampus and PTSD (Lin et al., 2024; Shin et al., 2006). The hippocampus plays a crucial role in the formation and retrieval of contextual fear memories (Malin and McGaugh, 2006; Marek et al., 2018). Structural changes in the hippocampus, such as a reduction in volume, have been linked to emotional and cognitive impairments in individuals diagnosed with PTSD (Weis et al., 2021). Beyond the hippocampus, the amygdala and cerebral cortex are also implicated in PTSD, as numerous studies have demonstrated a correlation between the severity of PTSD symptoms and the activity alterations of these brain regions (Shin et al., 2006; Malin and McGaugh, 2006; Jin and Maren, 2015). At the molecular level, chronic low-grade neuroinflammation has been observed in individuals with PTSD (Speer et al., 2018). Research indicates that patients with PTSD exhibit increased circulating levels of pro-inflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), IL-6, and interferon-γ (IFN-γ) (Maloley et al., 2019). Furthermore, experimental studies conducted on animal models of PTSD, specifically those subjected to single prolonged stress (SPS), have shown elevated inflammation in the brain (Wang et al., 2018). These inflammatory responses are associated with heightened anxiety and difficulties in extinguishing fear memories (Dong et al., 2020). Notably, reducing inflammation in the brain has been shown to significantly alleviate anxiety and facilitate the extinction of fear memories (Sadeghi et al., 2024).

AMPK, an evolutionarily conserved serine-threonine kinase, functions as an energy sensor within cells. It becomes activated in response to decreases in the ATP/AMP ratio (Nani et al., 2021). In addition to maintaining cellular energy homeostasis, AMPK also regulates the activity of the inflammatory mediator nuclear factor kappa-B (NF-κB) and its downstream targets, such as IL-1β, IL-6, TNF-α, iNOS, and cyclooxygenase-2 (COX-2) (Nani et al., 2021). When AMPK is inhibited, this inhibition triggers the production of inflammatory factors via the NF-κB pathway (Saito et al., 2019). Therefore, treatment strategies aimed at activating AMPK may effectively reduce central inflammation, thereby alleviating the symptoms of PTSD.

Irisin, a myokine induced by exercise, is released through the proteolytic cleavage of fibronectin type III domain-containing protein 5 (FNDC5) (Lourenco et al., 2019). It was initially identified in peripheral tissues. However, an increasing body of evidence has demonstrated its expression in the central nervous system, especially in the hippocampus and prefrontal cortex (Lourenco et al., 2019). Within the brain, it is likely that irisin is secreted locally by neurons and glial cells to regulate neural functions (Choi et al., 2024). Previous studies have demonstrated that irisin can exert neuroprotective functions by reducing neuroinflammation through the activation of AMPK. For instance, Wang et al. have shown that irisin improves depressive-like behaviors in rats via the AMPK pathway (Wang and Pan, 2016). Irisin does not directly activate AMPK. Recent studies suggest that it may act through αV/β5 receptors: 1) Binding to αV/β5 may activate plasma membrane calcium channels to induce endoplasmic reticulum Ca2+ release, thereby stimulating AMPK activation (Li et al., 2024); 2) αV/β5 activation enhances energy expenditure (e.g., glucose uptake/fatty acid oxidation), elevating the Adenosine Monophosphate/Triphosphate (AMP/ATP) ratio to promote AMPK activation (Wang et al., 2024). Consequently, it is plausible that irisin may have the potential to ameliorate PTSD symptoms through its anti-neuroinflammatory effects mediated by the AMPK pathway.

In this study, we hypothesized that irisin might alleviate anxiety and fear memory extinction impairment associated with PTSD by modulating AMPK activity. The single prolonged stress (SPS) animal model, which is effective in replicating the pathological conditions seen in PTSD patients, was utilized for our investigation. First, we investigated the correlation between irisin levels in the brain regions including hippocampus, cerebral cortex, and amygdala and the behavioral deficits induced by SPS. This initial step was crucial in establishing a baseline understanding of how changes in irisin levels in the brain might be related to the emergence of PTSD-like behaviors. Subsequently, we delved into the potential beneficial effects of exogenous irisin on SPS-induced behavior deficits. Alongside this, we explored the underlying mechanisms, with a particular emphasis on the AMPK-mediated inflammatory pathway.