5-HT2A receptor activation is dispensable for psilocybin-induced fear extinction and neuroplasticity in mice

Psilocybin facilitates fear extinction in rodents, but its clinical utility is hampered by 5-hydroxytryptamine (5-HT) 2A-receptor-mediated hallucinogenic effects. This study aimed to explore whether 5-HT2A receptor is obligatory for psilocybin's fear extinction and neuroplasticity effect.

Male C57BL/6J mice underwent auditory-cue fear conditioning. Ketanserin (5-HT2A/2C antagonist) or MDL100907 (selective 5-HT2A antagonist) was administered 30 min before a single psilocybin (2.5 mg/kg, intraperitoneal) or vehicle injection. Mice were fear-conditioned 2 days before psilocybin administration, extinction training commenced 30 min after the psilocybin injection, and extinction testing was performed 24 h later. The mice were euthanized 1.5 h after behavioral testing in the MDL100907 antagonism experiment, and hippocampal and medial prefrontal cortex (mPFC) tissues were collected. Western blotting and enzyme-linked immunosorbent assay (ELISA) were used to detect brain-derived neurotrophic factor (BDNF) protein expression. Immunofluorescence staining was used to assess the number of doublecortin-positive cells in the hippocampal dentate gyrus (DG).

In extinction testing, mice that received psilocybin combined with either ketanserin or MDL100907 still exhibited a significantly lower freezing response than those treated with antagonist alone. MDL100907 pretreatment failed to block the psilocybin-evoked upregulation of BDNF in both hippocampus and mPFC. Likewise, the increase in doublecortin (DCX)-positive newborn neurons in the DG induced by psilocybin remained intact after MDL100907 blockade.

The effects of psilocybin in facilitating fear extinction and increasing neuroplasticity in fear-conditioned mice are not dependent on 5-HT2A receptor activation, which supports that its therapeutic and hallucinogenic effects can be dissociated.