Psychedelic drugs have a somewhat circular definition. Classed as hallucinogenic psychoactive drugs which have the primary effect of triggering non-ordinary states of consciousness - termed 'psychedelic' experiences.
Many of the known psychedelic substances have been found to bind to and activate the brain's serotonin 5-HT2A receptors. ( see Wikipedia )
Although it has been experimentally found that activating the 5-HT2A receptors leads to a marked increase in spontaneous (glutamate-mediated) synaptic activity in the prefrontal cortex, exactly how and why this leads to psychedelia remains unknown.
Further reading : Proceedings of the National Academy of Sciences
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