The role of 5-HT2A receptor signalling in psychedelic drug action

Thesis

Recent interest in psychedelic drugs as a treatment for psychiatric disorders has provoked investigation into their mechanisms of action at the 5-HT2A receptor. A central question is which signalling pathways mediate the hallucinogenic and therapeutic effects of these drugs and whether they are common. Notably, there are recent reports of 5-HT2A receptor agonists with therapeutic potential in preclinical models that lack hallucinogenic effects. Biased signalling at the 5-HT2A receptor is one possible explanation for this. This thesis aimed to explore the signalling properties of psychedelic and non-psychedelic drugs and examine the roles of specific 5-HT2A receptor-linked pathways in hallucinogenic and neuroplastic effects of psychedelic drugs, using preclinical models. 

The signalling properties of psychedelic and non-psychedelic 5-HT2A receptor agonists were compared using assays of Gq and β-arrestin2 signalling in human neuroblastoma SH-SY5Y cells and rat C6 glioma cells. Non-psychedelic drugs did not demonstrate consistent differences in ligand bias compared to psychedelic drugs but consistently displayed lower signalling efficacy across all assays.  

The role of β-arrestin2 in hallucinogenic and neuroplastic effects was then investigated using β-arrestin2 KO mice. Compared to WT controls, β-arrestin2 KO mice displayed no differences in plasticity-related gene expression evoked by psilocin or DOI and β-arrestin KO mice demonstrated a minor reduction in psilocin-induced head-twitches while the responses to DOI or LSD were unchanged.  

Next, in vitro experiments discovered evidence of a previously unreported 5-HT2A receptor signalling pathway associated with NAADP-mediated lysosomal Ca2+ release via TPC2. This signalling pathway was revealed on the basis of pharmacological manipulation of the 5-HT2A receptor agonist-stimulated Ca2+ response in C6 and SH-SY5Y cells, and findings that 5-HT2A receptor agonists increased cellular levels of NAADP. Further, TPC2 KO caused a reduction in psilocin-evoked head-twitches and plasticity-related gene expression, compared to WT controls.  

Finally, experiments examined the roles of β-arrestin2, Gq and NAADP signalling in psychedelic drug-induced dendritogenesis in primary cortical neurons. Dendritogenesis was increased by psychedelic drug exposure, and this response was reduced by βarrestin2 KO. Experiments found little evidence of a role for Gq or NAADP signalling in psilocin- or DOI-evoked dendritogenesis. 

The findings of this thesis identify low 5-HT2A receptor signalling efficacy as a mechanism that differentiates psychedelic from non-psychedelic drugs. Further, a novel 5-HT2A receptor signalling pathway involving NAADP was identified. Finally, new insights into the signalling pathways underpinning the hallucinogenic and neuroplastic effects of psychedelic drugs are provided which will aid optimisation of psychedelic drugs for therapeutic use.

Authors: Ippolito, AL

• DOI: 10.5287/ora-gogmeoqn7
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: neuropharmacology; psychedelic drugs; biased signalling
• Subjects: Neuropsychopharmacology