562. Repeated psilocybin administration does not induce rewarding effects and increases ventral tegmental dopaminergic activity during withdrawal

Abstract Background Psilocybin has shown therapeutic potential for several neuropsychiatric disorders; however, concerns remain regarding its abuse liability and long-term neurobiological effects following repeated use. While acute psychedelic administration is generally considered to have low addictive potential, the consequences of repeated exposure on reward processing, withdrawal-related behaviors, and mesolimbic dopaminergic (DA) function have not been systematically investigated. Understanding how repeated psilocybin administration affects the ventral tegmental area (VTA) is critical for evaluating its safety profile, given that drugs of abuse are typically associated with reduced DA neuronal firing in the VTA during withdrawal. Aims & Objectives To determine whether a 7-consecutive-day psilocybin (10 mg/kg, i.p.) administration induces rewarding or withdrawal-related effects and to characterize VTA-DA neuronal activity during the withdrawal phase.

Method: A cohort of male Sprague-Dawley rats (n=20) was subjected to the CPP protocol. Animals were habituated to the apparatus on day 1, and baseline compartment preference was assessed on day 2. During these sessions, rats had free access to both compartments for 15 minutes. From days 3 to 9, rats received a vehicle (0.9% saline, i.p.) injection in the morning (8:00 AM) and either vehicle or psilocybin in the afternoon (1:00 PM). During the conditioning sessions, behavioral assessments of psychedelic-associated and compulsive-like behaviors were performed. Immediately after treatment, animals were confined to one CPP compartment for 20 minutes. Rewarding effects were evaluated on the following day (day 10). A separate cohort of rats (n=18) received vehicle or psilocybin for seven consecutive days. Behavioral assessments were conducted on days 1 and 7 of treatment and on days 8 and 9 (24- and 48-hour withdrawal). Following the final assessment, rats were anesthetized, and in vivo extracellular recordings of VTA-DA neurons were performed.

Results: Rats treated with psilocybin exhibited a reduced body weight gain starting on day 2 (p<0.01), which persisted until the end of the treatment period (p<0.0001). Psilocybin did not elicit rewarding effects, as the time spent in the psilocybin-paired compartment was not increased (p>0.05). Across the 7-day treatment period, psychedelic-associated behaviors, such as head-twitch (p<0.001) and wet-dog shake (p<0.01), progressively decreased. Likewise, compulsive- and obsessive-like behaviors, including grooming (p<0.0001), body licking (p<0.001), and scratching (p<0.01), were abolished compared with vehicle-treated animals. Besides these differences observed during the treatment period, no withdrawal-related symptoms were detected 24h or 48h after cessation of psilocybin administration (p>0.05). Additionally, forty-eight hours after the last psilocybin administration, VTA-DA neurons exhibited increased activity in the psilocybin-treated group (p<0.01), with no differences in the number of recorded cells per track (p>0.05). Discussion & Conclusions Our findings corroborate previous evidence showing that psilocybin does not induce rewarding effects, supporting its no potential to promote addiction. Despite repeated exposure, treatment cessation did not trigger withdrawal-related symptoms typically observed during withdrawal from drugs of abuse. Additionally, dopaminergic activity in the ventral tegmental area was increased during psilocybin withdrawal, a pattern that contrasts with the hypodopaminergic state commonly observed during withdrawal from drugs of abuse. Together, these findings suggest that psilocybin engages mesolimbic dopaminergic pathways in a manner distinct from drugs of abuse, supporting its potential therapeutic utility.