Psilocybin induces rapid and persistent growth of dendritic spines in frontal cortex in vivo

Abstract

Psilocybin is a serotonergic psychedelic with untapped therapeutic potential. There are hints that the use of psychedelics can produce neural adaptations, although the extent and timescale of the impact in a mammalian brain are unknown. In this study, we used chronic two-photon microscopy to image longitudinally the apical dendritic spines of layer 5 pyramidal neurons in the mouse medial frontal cortex. We found that a single dose of psilocybin led to ∼10% increases in spine size and density, driven by an elevated spine formation rate. The structural remodeling occurred quickly within 24 h and was persistent 1 month later. Psilocybin also ameliorated stress-related behavioral deficit and elevated excitatory neurotransmission. Overall, the results demonstrate that psilocybin-evoked synaptic rewiring in the cortex is fast and enduring, potentially providing a structural trace for long-term integration of experiences and lasting beneficial actions.

Keywords: antidepressant; dendrites; hallucinogen; medial prefrontal cortex; neural plasticity; psilocybin; pyramidal neuron; serotonergic psychedelic; structural remodeling; synapse.

Figure 1.. Psilocybin increases the density and size of dendritic spines in the mouse medial frontal cortex.

(A) Head-twitch responses as a function of dose, tested on 82 C57BL/6J mice. (B) Time course of head-twitch responses after administrating psilocybin (1 mg/kg, i.p.), averaged from 2 males and 2 female C57BL/6J mice. Line, moving average. (C) Timeline for the learned helplessness assay. (D) The proportion of escape failure for all animals in Test 1 and Test 2, i.e., before and after psilocybin (1 mg/kg, i.p.), ketamine (10 mg/kg, i.p.), and saline administration. (E) The change in escape failure, from Test 1 to Test 2, for susceptible animals for psilocybin, ketamine, and saline treatments. (F) Imaging setup. (G) Fixed coronal section from Thy1^GFP mice. (H) Timeline for the longitudinal imaging study. (I) Example field of view. (J) Effects of psilocybin or saline treatment on spine density, plotted as fold-change from baseline value on Day -3. Mean ± SEM. (K, L) Similar to (J), plotted separately for females and males. (M–O) Similar to (J–L) for spine head width. Sample sizes and details of the statistical analyses are provided in Table S1. See also Figure S1.

Figure 2.. Psilocybin elevates the formation rate of dendritic spines.

(A) Example field of view. Purple arrowhead, stable spine. Green arrowhead, new spine. (B) Effects of psilocybin or saline treatment on the formation rates of dendritic spines for female and male mice, plotted as difference from baseline value on Day -1. Mean ± SEM. (C) Similar to (B) for elimination rates. (D) Fraction of spines newly formed on Day 1 that remained stable on Day 7 and Day 34 for female and male mice. Filled circles, individual dendritic segments. Sample sizes and details of the statistical analyses are provided in Table S1. See also Figure S2.

Figure 3.. Mechanistic details revealed by ketanserin pretreatment and electrophysiological characterizations.

(A) Head-twitch responses after administrating psilocybin without saline pre-treatment (10 min prior; n = 3 mice), psilocybin (1 mg/kg, i.p.) with ketanserin pre-treatment (1 mg/kg, 10 min prior; n = 3), and saline with ketanserin pre-treatment (n = 4). (B) Timeline for the experiment. (C) Effects of psilocybin or saline treatment on spine density in animals pretreated with ketanserin, plotted as fold-change from baseline value on Day -3. Mean ± SEM. (D) Similar to (C) for spine head width. (E) Effects of psilocybin or saline treatment on the formation rates of dendritic spines for female and male mice pretreated with ketanserin, plotted as difference from baseline value on Day -1. (F) Representative traces of mEPSCs recorded from putative layer 5 pyramidal neurons of Cg1/M2 in brain slices. (G) Grouped and cumulative distribution plots of mEPSC frequency for animals that received psilocybin or saline 24 h before recording. Each open circle denotes a cell (n = 25 cells from 4 females for saline; 24 cells from 4 females for psilocybin; 19 cells from 5 males for saline; 23 cells from 4 males for psilocybin). (H) Similar to (G) for mEPSC amplitude. Sample sizes and details of the statistical analyses are provided in Table S1. See also Figure S3.

Figure 4.. Region-specific effects of psilocybin.

(A) Stitched confocal image of a coronal brain section from a Thy1^GFP mouse. (B) Magnified images showing apical and basal dendritic segments. (C) Images of apical dendrites in Cg1/M2. (D) Effects of psilocybin and saline on spine density for apical dendrites in Cg1/M2. Open circles, individual dendritic segments. Gray line, mean ± SEM. (E) Similar to (D) for spine protrusion length. (F) Similar to (D) for spine head width. (G – J) Similar to (C – F) for PrL/IL. (K – N) Similar to (C – F) for M1. (O – R) Similar to (C – F) for basal dendrites in Cg1/M2. Sample sizes and details of the ANOVA models are provided in Table S1. See also Figure S4.