¹Mcgill University, Psychiatry, Montreal, Canada, ²University of Padova, Department of Pharmaceutical and Pharmacological Sciences, Padova, Italy

Background: Major depression (MD) is a prevalent and debilitating disorder, characterized by anhedonia, persistent low mood, and learned helplessness, affecting up to 21% of adults over a lifetime. Chronic stress is a major environmental contributor to MD, and preclinical models of chronic mild stress (CMS) allow investigation of its behavioral and neurobiological consequences. Behavioral assays, including the sucrose preference test (SPT) and forced swim test (FST), are used to measure anhedonia and learned helplessness, respectively. Despite advances, few studies have explored rapid-acting compounds capable of reversing chronic stress-induced depressive-like behaviors.

Aims & Objectives: This study aimed to evaluate the behavioral impact of a 30-day CMS paradigm in male Sprague-Dawley rats and to determine the potential acute antidepressant effects of 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) on stress-induced anhedonia and learned helplessness.

Method: Male Sprague-Dawley rats were exposed to three to four daily mild stressors plus one overnight stressor for 30 days, including FST, stroboscopic lighting, noise, tail pinch, cold room, cage tilt, food/water deprivation, wet bedding, and exposure to foreign objects. Sucrose preference and FST immobility were assessed weekly to track behavioral changes. On day 25, rats underwent stereotaxic implantation of epidural cortical electrodes for electrocorticography (ECoG) and neck muscle electrodes for electromyography (EMG) to monitor stress-induced alterations in sleep-wake patterns. Following CMS exposure, animals received a single high dose of 5-MeO-DMT (10 mg/kg, i.p.) or vehicle (95% NaCl, 5% DMSO, i.p.). ECoG and EMG recordings were conducted to evaluate sleep-wake activity, including REM and non-REM sleep. Data were analyzed using repeated measures ANOVA and post hoc tests, with significance set at p < 0.05.

Results: CMS exposure significantly decreased sucrose preference (%) in all stressed animals at week 1 (72.99 ± 6.43 %) and week 4 (67.76 ± 7.13 %), as compared to control at week 1 (92.50 ± 2.44 %, p < 0.05) and week 4 (98.10 ± 0.71 %, p < 0.05). Average weekly body weight (g) gain was also stunted in stressed animals (9.81 ± 1.21 g), as compared to control animals (18.56 ± 1.647 g, p < 0.05). FST immobility time (s) selectively increased in stressed animals from week 1 (128.42 ± 12.62 s) to week 4 (189.21 ± 14.03 s, p < 0.05). Acute administration of 5-MeO-DMT reversed CMS-induced anhedonia, restoring sucrose preference to baseline levels (92.58 ± 2.663 %), while stressed animals treated with vehicle remained anhedonic (76.72 ± 7.31 %, p < 0.05). ECoG analyses suggested that CMS reduced REM sleep, an effect that was normalized following 5-MeO-DMT treatment.

Discussion & Conclusions: Chronic unpredictable stress reliably induced anhedonia and altered body weight gain in male Sprague-Dawley rats. Differential responses between SPT and FST indicate that anhedonia and learned helplessness are mediated by distinct mechanisms. Importantly, acute 5-MeO-DMT selectively reversed anhedonia-like behavior and normalized sleep disturbances, highlighting its potential as a rapid-acting antidepressant targeting reward-related deficits induced by chronic stress. These findings provide preclinical evidence supporting the therapeutic potential of 5-MeO-DMT for stress-induced depressive phenotypes and underscore its ability to selectively ameliorate specific depressive-like symptoms.