Contribution of the Serotonin 5-HT2A Receptor to the Therapeutic Effect of Psilocin on Social Behavior Deficits in Mice Repeatedly Exposed to Social Defeat Stress

ABSTRACT Psychedelics such as psilocybin and lysergic acid diethylamide (LSD) exert hallucinogenic effects through stimulation of serotonin 5-HT2A receptors (5-HT2ARs) in the cerebral cortex. In recent years, numerous reports have demonstrated that psychedelics are effective in treating various psychiatric disorders such as major depressive disorder (MDD), treatment-resistant depression (TRD), and anxiety-related disorders. We have previously reported that administration of psilocin, the active metabolite of psilocybin, produces antidepressant-like effects in mice. Furthermore, we found that this effect is mediated by 5-HT2AR activation. Since depression and other psychiatric disorders often lead to impairments in social behavior (e.g., social avoidance), the present study examined the effects of psilocin on social avoidance behavior in mice subjected to chronic social defeat stress (CSDS), a widely used model that closely models human psychosocial stress. Mice exposed to CSDS exhibited social avoidance behavior, whereas psilocin administration before the onset of CSDS had little effect on this behavior. In contrast, psilocin administration after the completion of CSDS ameliorated social avoidance in CSDS-exposed mice. This effect was blocked by pretreatment with a 5-HT2AR antagonist, indicating that psilocin exerts its therapeutic effects through 5-HT2AR activation. Taken together, psilocin exerts therapeutic effects on social avoidance behavior after stress through activation of 5-HT2AR, but not preventive effects when administered before stress, suggesting that psilocin may promote stress resilience rather than resistance.