A single dose of psilocybin increases synaptic density and decreases 5-HT2A receptor density in the pig brain

Nakul Ravi Raval, Annette Johansen, Lene Lundgaard Donovan, Nidia Fernandez Ros,
Brice Ozenne, Hanne Demant Hansen, Gitte Moos Knudsen

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 November 2020

doi : 10.20944/preprints202011.0742.v1

 

Abstract :

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 4.4% 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 PFC for [3H]Cimbi-36 only.

Seven days post-intervention, there was still significantly higher SV2A density in hippocampus (+9.24%) and PFC (+6.1%) whereas there were no longer any differences in 5-HT2AR density. Our findings suggest that psilocybin’s antidepressive actions are linked to increased persistent synaptogenesis and possibly also to an acute decrease in 5-HT2AR density.

Keywords : Psilocybin, psychedelics, neuroplasticity, SV2A, 5-HT2A, depression, autoradiography, functional-selectivity

 

1. Introduction

Serotonergic psychedelic drugs have for centuries been extensively used in religious practices and also recreationally [1]. The neurobiological and behavioral effects in mammals are mediated through stimulation of the brain serotonin 2A receptor (5-HT2AR) as reviewed by Vollenweider et al. [2,3]. Upon ingestion of psilocybin, a tryptamine psychedelic [1], it quickly dephosphorylates to the active compound psilocin which has a high affinity to 5-HT2AR, but also to other 5-HT receptors such as 5-HT1AR and 5-HT2CR [1,4,5].

Psychedelic stimulation of 5-HT2AR, a G-protein-coupled receptor (GPCR), has recently shown potential as an anxiolytic and antidepressant therapy. Some clinical studies suggest that a single dose of psilocybin rapidly and effectively relieves symptoms in depression and anxiety, with effects that persist long after the psychedelic experience [6–9]. Research in rodents suggests that psilocybin, lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-iodoamphetamine (DOI), N,Ndimethyltryptamine (DMT) and alkaloids like harmine, tetrahydroharmine, and harmaline (present in ayahuasca) induce structural neuroplasticity and alter the expression of important proteins like VGLUT1, BDNF, and MAP2 [10–13]. The mechanism behind these synaptic changes is hypothesized to be exerted via the 5-HT2AR pathway [10].

Changes in synaptic density in brain regions associated with emotional processing, i.e., hippocampus and prefrontal cortex (PFC), may play a vital role in the pathophysiology of mood disorders, e.g. major depressive disorder. Both post-mortem human brain [14,15] and in vivo[16] studies in depressed individuals have shown a loss of synapses through the down-regulation of synaptic proteins and genes. Hence, upregulation of presynaptic proteins and an increase in synaptic density may be associated with the potential antidepressive effects of psychedelics.

Synaptic vesicle protein 2A (SV2A) is an integral 12-transmembrane domain glycoprotein expressed in synaptic vesicles throughout the brain [17], and SV2A density is thought to reflect presynaptic density [18]. The levetiracetam derivative UCB-J, which binds selectively to SV2A, has in its radiolabeled form been shown to correspond to synaptic density as measured with the wellcharacterized presynaptic protein synaptophysin [19–21].

Classical receptor binding assay studies have demonstrated that 5-HT2AR (and other GPCRs) exist in two affinity states, a high- and a low- affinity state [22–24]. The affinity states of the receptors are considered to represent different functional states of the receptor, high-affinity being functionally active (activation of Gαi1-protein pathway) in contrast to the low-affinity state (activation of canonical Gαq/11-protein pathway) [25]. Whereas 5-HT2AR antagonists bind to the total pool of 5-HT2AR, 5-HT2AR agonists bind to the high-affinity state GPCRs [26]. Stimulation of 5-HT2AR leads to rapid receptor internalization [27]. This endosomal internalization may lead to lysosomal degradation and down-regulation of 5-HT2AR, as extensively reviewed by Gray J.A. and Roth B.L. [28].

In the present study, we hypothesize that a psychedelic dose of psilocybin increases presynaptic density as reflected in SV2A protein levels in the pig brain. We also test the hypothesis that the availability of 5-HT2AR is decreased after agonist stimulation with psilocybin.

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