Endocannabinoid System Involvement in Exercise-Induced Enhancement of Fear Extinction in Mice

Abstract

The endocannabinoid (eCB) system has emerged as a promising target for enhancing fear extinction learning, which has therapeutic implications for the treatment of stress and anxiety disorders that rely primarily on exposure-based therapies. Although previous investigations have used pharmacological approaches to enhance fear extinction, there is a strong rationale to investigate the efficacy of non-pharmacological approaches (e.g., exercise) shown to activate the eCB system. PURPOSE: To examine the effects of exercise on the extinction of conditioned fear, anxiety-like behaviors, and eCB adaptations in cortico-limbic regions. METHODS: ICR/CD1 male mice (N = 26) completed a series of behavioral tests prior to and following a fear-conditioning (day 1; FC) and fear-extinction (days 2-5; FEXT) protocol. Following FC, mice were randomly assigned to caging containing either an unlocked (EX) or locked (CON) running wheel, with unlimited access until 24 hours following the last FEXT session. Mice were sacrificed 48 hours after the last behavioral test in order to examine central eCB tissue content. Data were analyzed using a series of one-way and mixed model ANOVAs, Pearson correlations, and Cohen’s d effect sizes. RESULTS: EX mice exhibited a significant reduction in anxiety-like behaviors from pre to post compared to the CON mice (p < 0.05). Although both groups experienced less freezing over time, EX mice exhibited significantly less freezing on days 2 (d = 0.39), 3 (d = 0.50), 4 (d = 1.16), and 5 (d = 0.89) compared to CON mice (p < 0.05). Additionally, moderate to strong negative correlations were found between wheel running revolutions and freezing time on days 2 through 5, indicating that mice who ran more in between extinction sessions tended to freeze less during subsequent sessions. Lastly, EX mice exhibited significantly greater amygdalar eCB content and significantly less hippocampal eCB content compared to CON mice (p < 0.05). CONCLUSION: These preliminary results suggest that voluntary exercise enhances fear- extinction and reduces anxiety-like behaviors in mice, possibly due to eCB system adaptations in brain regions involved in regulating fear and anxiety responses. Supported by the UW Virginia Horne Henry Fund and the Advancing a Healthier Wisconsin Endowment at the Medical College of Wisconsin.