A single injection of psychedelic or non-hallucinogenic drugs that activate the serotonin 5-HT2A receptor can produce antidepressant- and anxiety-reducing effects in mice with a depression-like condition, lasting up to 15 days. The non-hallucinogenic drug lisuride was effective, suggesting hallucinogenic properties are not required for antidepressant action. In mice lacking the 5-HT2A receptor, the psychedelic DOI was ineffective, but psilocybin still worked, indicating psilocybin's effects can occur through other mechanisms. Blocking other serotonin or dopamine receptors did not stop psilocybin's effects in these mice. These results suggest that 5-HT2A receptor agonists can relieve depression through multiple pathways, independent of whether they cause hallucinations.
Activation of the serotonin 2A (5-HT2A) receptor suppresses fear expression but does not affect retention of fear extinction. Using the psychedelic DOI in mice, the authors found that 5-HT2A receptor activation reduced anxiety-like avoidance behavior and diminished fear expression in multiple tasks, including passive avoidance and auditory fear conditioning. The effect depended on 5-HT2A receptors in the amygdala: local infusion of a 5-HT2A antagonist into the amygdala reversed the effect, while local DOI infusion into the amygdala was sufficient to suppress fear expression. These findings clarify a neural mechanism by which psychedelics may reduce fear, but provide no evidence that they enhance fear extinction memory.