Psilocybin induces dose-dependent changes in functional network organization in rat cortex

Abstract

Psilocybin produces an altered state of consciousness in humans and is associated with complex spatiotemporal changes in brain networks. Given the emphasis on rodent models for mechanistic studies, there is a need for characterization of the effect of psilocybin on brain-wide network dynamics. Previous rodent studies of psychedelics, using electroencephalogram, have primarily been done with sparse electrode arrays that offered limited spatial resolution precluding network level analysis, and have been restricted to lower gamma frequencies. Therefore, in the study, we used electroencephalographic recordings from 27 sites (electrodes) across rat cortex (n=6 male, 6 female) to characterize the effect of psilocybin (0.1 mg/kg, 1 mg/kg, and 10 mg/kg delivered over an hour) on network organization as inferred through changes in node degree (index of network density) and connection strength (weighted phase-lag index). The removal of aperiodic component from the electroencephalogram localized the primary oscillatory changes to theta (4-10 Hz), medium gamma (70-110 Hz), and high gamma (110-150 Hz) bands, which were used for the network analysis. Additionally, we determined the concurrent changes in theta-gamma phase-amplitude coupling. We report that psilocybin, in a dose-dependent manner, 1) disrupted theta-gamma coupling [p<0.05], 2) increased frontal high gamma connectivity [p<0.05] and posterior theta connectivity [p≤0.049], and 3) increased frontal high gamma [p<0.05] and posterior theta [p≤0.046] network density. The medium gamma frontoparietal connectivity showed a nonlinear relationship with psilocybin dose. Our results suggest that high-frequency network organization, decoupled from local theta-phase, may be an important signature of psilocybin-induced non-ordinary state of consciousness.

Keywords: High gamma oscillations; Phase-Amplitude Coupling; Psilocybin; Psychedelic; Rat; weighted Phase-Lag Index.

Figure 1.. Schematic showing the experimental design and timeline.

Each rat (n=12, 6 male, 6 female) received 0.9% saline and three doses of psilocybin (0.1 mg/kg, 1 mg/kg, and 10 mg/kg) as a continuous infusion over the course of an hour. Each infusion session was separated by 5–7 days and was conducted in a counter-balanced manner. On the right-side of the experiment timeline is shown an image of the rat cranium indicating the EEG, ground, and reference electrode location.

Figure 2.. Psilocybin altered global peak oscillatory frequencies and amplitudes in a dose-dependent manner.

(A-C) Global spectrograms averaged across rats (n=12, 6 male, 6 female) depicting the difference between each dose (0.1 mg/kg, 1 mg/kg, and 10 mg/kg) and saline. Psilocybin/saline infusion started at time=0 and stopped at time=60 minutes. Warm colors indicate higher spectral power while cool colors indicate lower spectral power relative to saline. (D) Representative global power spectrum averaged over minutes −10 to 0 (left) and 10–20 (right) of 10 mg/kg psilocybin dose. The FOOOF algorithm was used to model and remove the 1/f component from the original power spectrum to quantify band-specific peak frequencies and amplitudes. Displayed here are the original power spectrum (blue), the aperiodic component (red), and the oscillatory component (gold). For visualization the original and aperiodic spectra are log-transformed (left y-axis) and the oscillatory spectra are on a linear scale (right y-axis). (E-J) Changes in peak frequency and amplitude in the theta, medium gamma, and high gamma bands. The 1 mg/kg of psilocybin slowed theta peak frequency and increased medium gamma peak frequency. The 10 mg/kg psilocybin dose caused a significant decrease in theta peak frequency. The 10 mg/kg dose initially increased medium gamma peak frequency, but towards the end of infusion the peak frequency slowed relative to saline. The 1 mg/kg and 10 mg/kg doses of psilocybin decreased theta amplitude and increased medium and high gamma amplitudes relative to saline. The data are provided as mean ± standard error of the mean. *p<0.05, FDR-corrected post-hoc comparisons.

Figure 3.. Effect of psilocybin on rat movement

3D gyroscope activity averaged in 10-minute bins plotted as mean ± standard error of the mean. The 1 mg/kg dose briefly increased rat head movements, whereas the 10 mg/kg dose resulted in a quiescent state with minimal movement, thereby dissociating the increased gamma power from movement. *p<0.05, FDR-corrected post-hoc comparisons between saline and psilocybin doses.

Figure 4.. Intravenous psilocybin dose-dependently disrupted theta-gamma coupling.

Following psilocybin infusion, both the 1 mg/kg and 10 mg/kg doses resulted in decoupling of theta phase from medium (A) and high (B) gamma amplitudes in a dose-dependent fashion. PAC was not altered by the 0.1 mg/kg dose. Each grid represents a 10-minute average of PAC values across all rats (n=12). Each square indicates an electrode in the layout described in Figure 1. White and black asterisks: p<0.05, FDR-corrected post-hoc comparisons.

Figure 5.. Intravenous psilocybin induced broad reorganization of theta and gamma cortical connectivity patterns.

wPLI connectivity differences between psilocybin and saline averaged across rats (n=12) and over 10-minute bins. Red lines indicate increased wPLI relative to saline, blue lines indicate decreased wPLI. Dots indicate electrode location corresponding to the electrode map shown in Figure 1. Dot size indicates node degree magnitude. Yellow dots show significantly increased node degree relative to saline. Only connections that were significantly different (p<0.05) from saline following FDR-correction are displayed. Beginning halfway through infusion, the 1 mg/kg psilocybin dose caused sparse increases in frontoparietal theta connectivity (A), increased medium gamma frontal connectivity, but decreased posterior connectivity (B), and caused broad frontoparietal increases in high gamma connectivity (C). The 10 mg/kg psilocybin dose had a similar effect on cortical connectivity, but was amplified in a dose-dependent fashion, in particular causing large increases in theta posterior connectivity (A), increasing, then decreasing medium gamma frontoparietal connectivity (B), and increasing frontoparietal high gamma connectivity (C).