A Single Dose of Psilocybin Increases Synaptic Density and Decreases 5-HT 2A Receptor Density in the Pig Brain

Int J Mol Sci. 2021 Jan 15;22(2):835. doi: 10.3390/ijms22020835.


A single dose of psilocybin, a psychedelic and serotonin 2A receptor (5-HT2AR) agonist, may be associated with antidepressant effects. The mechanism behind its antidepressive action is unknown but could be linked to increased synaptogenesis and down-regulation of cerebral 5-HT2AR. Here, we investigate if a single psychedelic dose of psilocybin changes synaptic vesicle protein 2A (SV2A) and 5-HT2AR density in the pig brain. Twenty-four awake pigs received either 0.08 mg/kg psilocybin or saline intravenously. Twelve pigs (n = 6/intervention) were euthanized one day post-injection, while the remaining twelve pigs were euthanized seven days post-injection (n = 6/intervention). We performed autoradiography on hippocampus and prefrontal cortex (PFC) sections with [3H]UCB-J (SV2A), [3H]MDL100907 (5-HT2AR antagonist) and [3H]Cimbi-36 (5-HT2AR agonist). One day post psilocybin injection, we observed 4.42% higher hippocampal SV2A density and lowered hippocampal and PFC 5-HT2AR density (-15.21% to -50.19%). These differences were statistically significant in the hippocampus for all radioligands and in the PFC for [3H]Cimbi-36 only. Seven days post-intervention, there was still significantly higher SV2A density in the hippocampus (+9.24%) and the PFC (+6.10%), whereas there were no longer any differences in 5-HT2AR density. Our findings suggest that psilocybin causes increased persistent synaptogenesis and an acute decrease in 5-HT2AR density, which may play a role in psilocybin's antidepressive effects.

Keywords: 5-HT2A; SV2A; autoradiography; depression; functional-selectivity; neuroplasticity; psilocybin; psychedelics.

MeSH terms

  • Animals
  • Antidepressive Agents / administration & dosage
  • Autoradiography
  • Brain / drug effects*
  • Female
  • Hallucinogens / administration & dosage
  • Hippocampus / drug effects
  • Ligands
  • Prefrontal Cortex / drug effects
  • Psilocybin / administration & dosage*
  • Receptor, Serotonin, 5-HT2A / metabolism*
  • Swine
  • Synapses / drug effects*


  • Antidepressive Agents
  • Hallucinogens
  • Ligands
  • Receptor, Serotonin, 5-HT2A
  • Psilocybin

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