Translational Research Investigating Psilocybin: Underlying neurobiological mechanisms

This dissertation has taken an interdisciplinary approach to investigate and give insights into psilocybin’s neurobiological underpinnings. Psilocybin is a naturally occurring serotonergic psychedelic and is the main ingredient in hallucinogenic mushrooms. Research has demonstrated that psilocybin profoundly activates the serotonin (5-hydroxytryptamine) 2A (5-HT2A) receptors. Upon activation of these receptors in the brain, psilocybin induces profound perceptual changes such as hallucinations and illusions, altering one’s self-perception, bodily image and mood. There is a growing interest in psilocybin’s therapeutic utility, focusing on psychiatric disorders, such as treatment-resistant depression. Despite psilocybin’s therapeutic potential, the underlying neurobiological mechanisms of psilocybin remain elusive. Therefore, this dissertation aimed to understand and delineate the acute dose-dependent effects of psilocybin on an epigenetic, neurobiological and cognitive level. First, we summarized all literature on the vertical (rodent vs human) and horizontal (healthy vs disease) dose-time-dependent effects of the classic psychedelics psilocybin, lysergic acid diethylamide (LSD), ayahuasca and n,n-dimethyltryptamine (DMT). The review reported in chapter 2 discussed that there is a general paucity of data and that most clinical studies predominantly focus on the effects of psychedelics on mood and anxiety, whereas the pharmacokinetic/pharmacodynamic (PK/PD) relationship and the molecular-induced changes have been largely overlooked. A translational framework that establishes the dose-dependent effects of psychedelics on the brain by investigating the PK/PD relationships in humans and rodents was proposed. The translational framework incorporates cognitive outcomes such as pattern separation and cognitive flexibility, since these paradigms are objective and highly translational. Furthermore, it was discussed that it is important to clinically evaluate the physiological effects of psychedelics on an (epi)genetic and protein level and correlate these alterations to the psychedelic-induced changes in mood, behavior and cognition. We first investigated the dose-dependent effects of psilocybin on behavior and mood in healthy rats. When focusing on locomotion, anxiety, pattern separation, and sensorimotor gating, we found that psilocybin dose-dependently impairs all these outcomes. The experimental studies described in chapter 3 furthermore showed that high doses of psilocybin decrease glutamate and GABA levels in the hippocampus, whereas no differences were seen in the prefrontal cortex of the rats. The hallucinogenic potency of psilocybin was clearly demonstrated in head-twitch response experiments with mice. Taken together, from the behavioral and microdialysis experiments detailed in chapters 3 and 4, we concluded that the adverse behavioral effects of psilocybin at high doses are likely instigated by pre-synaptic inhibitory 5-HT1A receptor stimulation. 244 Therefore, if there are pro-cognitive effects of psilocybin then these would be apparent at lower doses. In chapter 5, we reported the results of further investigations on the role of the 5-HT1A receptors on cognition. We used the biased agonist NLX-101, which selectively targets the post-synaptic 5-HT1A heteroreceptors in the brain. The therapeutic potential of NLX-101 to alleviate cognitive impairment associated with schizophrenia (CIAS) was investigated in sub-chronic ketamine-treated rats. It was shown that NLX-101 decreased schizophrenia-like deficits in spatial pattern separation, cognitive flexibility and sensorimotor gating. We concluded that targeting post-synaptic 5-HT1A heteroreceptors might have beneficial effects on CIAS on multiple domains, and therefore NLX-101 could constitute a promising treatment strategy to relieve cognitive dysfunction in schizophrenia patients. In chapter 6, we introduced the concept of circulating microRNAs (cimiRNAs). CimiRNAs are small non-coding RNA molecules of 19-25 nucleotides that can be found in bodily fluids such as blood, tears, saliva and cerebrospinal fluid. MiRNAs are involved in post-transcriptional regulation of gene expression and therefore modulate a large variety of biological pathways. The literature analysis summarized in chapter 6 provided an overview of the cimiRNAs that are associated with psychiatric and neurodegenerative disorders. The review focused on five disorders, namely Alzheimer’s disease, Parkinson disease, Bipolar disorder, Schizophrenia and Major depression disorder. The similarities and differences in dysregulated cimiRNAs within and between the disorders were examined. Furthermore, we discussed the differences in dysregulated miRNAs when measuring cimiRNAs in different bodily fluids. Overall, we provided a comprehensive assessment of the potential value of cimiRNAs as biomarkers for brain pathologies. We concluded that cimiRNAs have the potential to early diagnose central nervous system disorders and even evaluate the prognosis of therapies, yet technical and methodological approaches need to be improved before cimiRNAs are operational clinical biomarkers. Finally, chapter 7 summarizes the effects of psilocybin we have observed on the blood plasma cimiRNA signature profiles in healthy human volunteers. The cimiRNA signature profile was investigated 6 hours and seven-days post psilocybin treatment to investigate its acute and long-term effects. The cimiRNAs were sequenced utilizing the state-of-the-art technique next-generation sequencing (NGS). We showed that psilocybin dysregulated 42 cimiRNAs acutely and eight cimiRNAs in the long-term. The long-term cimiRNAs were associated to a diverse variety of pathways and processes, including 245 glycolysis, RNA polymerase, fatty acid degradation and axon guidance, whereas the acutely dysregulated cimiRNAs were associated to neuroplasticity and glutamatergic signaling pathways. Next, to provide a deeper understanding of the underlying mechanisms of psilocybin, different cytokines were analyzed in the plasma samples. It was found that psilocybin decreased tumor necrosis factor alpha (TNFα) levels acutely. An interesting correlation between the levels TNFα and miR-1468-5p in the psilocybin treated group was found. Based on literature, there is indeed a link between miR-1468-5p and TNFα, yet the functional role of psilocybin in this relationship still remains elusive. We concluded in chapter 7 that psilocybin specifically alters the cimiRNA profile in healthy volunteers both acutely and in the long-term, yet additional human studies are required to validate these preliminary results.