Psilocybin has a narrow therapeutic window as an antidepressant treatment

Psilocybin, a naturally occurring psychedelic compound in magic mushrooms, shows promise as a novel intervention with a single administration inducing rapid and long-lasting antidepressant effects. However, there are limited studies on the optimal dosing required for the beneficial effects of psilocybin given its side effects. To address this gap, we investigated in Wistar rats whether a single psilocybin administration (0.1, 0.32, 1.0, and 3.2 mg/kg) had antidepressant-like effects in the forced swim test (FST), a pro-social effect in the social interaction test (SIT), and the ability to alter pleasure using the sucrose preference test (SPT). We also examined the dose-response relationships of psilocybin on the head-twitch response (HTR), locomotor activity, body temperature, and weight gain. Furthermore, we explored whether the brain-derived neurotrophic factor (BDNF) levels in the hippocampus and prefrontal cortex (PFC) paralleled the behavioral changes observed after psilocybin. In the FST, psilocybin induced dose-dependent inverted-U-shaped responses with only the intermediate dose of 0.32 mg/kg producing short and long-term antidepressant-like effects. A similar pattern was observed for the SIT, the SPT, and the HTR. In contrast, the high doses of psilocybin (1.0 and 3.2 mg/kg), while deprived of anti-depressant-like activity, significantly reduced body temperature, locomotor activity, and body weight gain. BDNF levels in the hippocampus and PFC increased dose-dependently after psilocybin, but linearly suggesting a dissociation between high BDNF levels and the observed antidepressant-like behaviors. Our results indicated that there is a narrow window for the therapeutic potential of psilocybin, with 0.32 mg/kg effectively producing antidepressant-like effects without the accompanying adverse effects observed only at higher doses.