This animal study assesses the effects of tryptamine and phenethylamine psychedelics (psilocin, LSD, mescaline and dimethoxybromoamphetamine (DOB)) using EEG in freely moving rats. The researchers found that all psychedelic’s caused a global decrease in EEG activity. The overall results were almost identical to the effects from human EEG studies, proving that the method has robust translational validity.
“Serotonergic psychedelics are recently gaining a lot of attention as a potential treatment of several neuropsychiatric disorders. Broadband desynchronization of EEG activity and disconnection in humans have been repeatedly shown; however, translational data from animals are completely lacking. Therefore, the main aim of our study was to assess the effects of tryptamine and phenethylamine psychedelics (psilocin 4 mg/kg, LSD 0.2 mg/kg, mescaline 100 mg/kg, and DOB 5 mg/kg) on EEG in freely moving rats. A system consisting of 14 cortical EEG electrodes, co-registration of behavioral activity of animals with subsequent analysis only in segments corresponding to behavioral inactivity (resting-state-like EEG) was used in order to reach a high level of translational validity. Analyses of the mean power, topographic brain-mapping, and functional connectivity revealed that all of the psychedelics irrespective of the structural family induced overall and time-dependent global decrease/desynchronization of EEG activity and disconnection within 1-40 Hz. Major changes in activity were localized on the large areas of the frontal and sensorimotor cortex showing some subtle spatial patterns characterizing each substance. A rebound of occipital theta (4-8 Hz) activity was detected at later stages after treatment with mescaline and LSD. Connectivity analyses showed an overall decrease in global connectivity for both the components of cross-spectral and phase-lagged coherence. Since our results show almost identical effects to those known from human EEG/MEG studies, we conclude that our method has robust translational validity.”
Authors: Čestmír Vejmola, Filip Tylš, Václava Piorecká, Vlastimil Koudelka, Lukáš Kadeřábek, Tomáš Novák & Tomáš Páleníček
Psychedelics are drugs that dramatically affect human perception, cognition and emotions. They are divided into two main chemical classes, tryptamine derivatives and phenethylamine derivatives, and produce similar effects via their agonistic action on 5-HT2A receptors.
In order to examine the effects of psychedelics in animal models, scientists have to deal with behavioral experiments, such as evaluation of locomotion, exploratory behavior, sensorimotor processing, stereotyped behaviors, cognitive tasks, etc. EEG can be used as a whole-brain neuroimaging tool in awake and freely moving animals.
Human EEG studies with serotonergic psychedelics consistently report a broadband spectral power decrease (delta to gamma) and a decrease in functional connectivity and integrity of networks, while MEG studies show a decrease in oscillations within the gamma range.
Early animal EEG studies with psychedelics showed that the signal was reduced in amplitude and desynchronized. Recent studies have focused on specific areas of the brain and have shown the same trend.
We recorded the EEG from 14 cortical surface electrodes implanted on freely moving rats and used a model of resting-state-like EEG to analyze the EEG changes induced by psilocin, LSD, mescaline, and DOB.
Experiments were performed on adult male Wistar rats in a controlled temperature and humidity room with a regular twelve-hour light/dark cycle. Ethical approval was given by the National Committee for the Care and Use of Laboratory Animals, CZ.
The following substances were used: psilocin, LSD, mescaline hydrochloride and 2,5-dimethoxy-4-bromoamphetamine. The doses were established with respect to experimental drugs potencies, as tested using drug-discrimination tests.
The rats were anesthetized by inhalation of Isoflurane and mounted in a stereotactic frame with atraumatic ear-bars. The skull was cleared off and electrodes were placed on the frontal association cortex, primary motor cortex, medial parietal association cortex, lateral parietal association cortex, and temporal association cortex. We drilled holes into the skulls of rats and implanted gold-plated electrodes in the regions of interest. The electrodes were fixed to the skulls with Dentalon dental cement and the rats recovered from the anesthesia well.
The experiments were conducted during the daytime, 7 days after surgery, with the rats being connected to the EEG system in their home cages. The animals were scored for active behavior and inactivity, and handled for a few seconds if a suspicion of sleep was observed.
EEG signal preprocessing and analysis
Psychedelic effects on QEEG in rats are congruent among active/inactive behavior, but spectral power has been shown to be altered by rat behavior. We used only segments corresponding to behavioral inactivity for further processing. The EEG data were bandpass filtered with a linear FIR filter, segmented according to behavioral activity and inactivity scoring, and then subjected to editing using the Neuroguide software. The selected signal was tested for reliability and then subjected to Fast Fourier Transform.
Absolute power spectra and coherence analyses were performed directly by the Neuroguide software. Coherence was calculated as an absolute value of coherency (cross-spectral coherence) and the imaginary part of coherency (phase-lagged coherence) was calculated using 30 intra-hemispheric electrode pairs and six inter-hemispheric electrode pairs.
Statistics and visualization
Due to high interindividual variability, the computed data were normalized by the Box-Cox Ratio transformation and tested for normality. ANOVA was used to compare the effect of specific treatments.
The significance level was set to P 0.05 for all statistical analyses. Topographic maps depicting the distribution of significant spectral power change were created using the method of 3D spline mapping and visualizations were performed using the Python software. We plotted a probability distribution of all relative connectivity changes from the baseline for each subject, electrode pair, and frequency band, and then plotted pie charts depicting behavioral activity.
The behavioral activity was assessed as the ratio of active behavior to inactivity in each 10-min epoch. The active substances had no effect on the behavior.
EEG absolute mean power spectra
Repeated-measures ANOVA revealed a significant effect of treatment in all frequency bands, specifically for delta, theta, alpha, beta, high beta, and gamma frequency bands.
All psychedelics decreased power within the whole evaluated frequency range of 1 – 40 Hz, but DOB and LSD effects remained after 80 – 90 min.
Topographic maps of EEG power spectra
Figure 2C shows that all of the drugs induced overall decreases in EEG activity, with psilocin and LSD causing the most significant decreases, whereas mescaline and DOB induced the most significant decreases.
theta and alpha power in the temporal and visual cortices for LSD and mescaline appeared in the last epoch.
The global connectivity patterns of active substances are demonstrated by the probability distribution graphs of all coherence changes.
All treatments induced a decrease in synchronization between electrode pairs over a cortical surface, but phenethylamine derivatives were more potent in inducing connectivity changes compared to tryptamine derivatives.
All psychedelics decreased the absolute spectral power across the whole of the tested frequency range, and all of the substances also considerably decreased brain connectivity. Psilocin had the most pronounced effects during the first epoch and by the last epoch, the effect almost vanished.
All psychedelics induced a decrease in power across the tested frequencies, but LSD and mescaline induced a biphasic effect with increases in theta and alpha band above the visual cortex. These increases may be linked to the stimulatory effects of these drugs.
DOB had minimal impact on mean activity within the theta and alpha bands and in contrast showed a tiny peak at 8 Hz. Compared to the other substances, DOB-induced decreases in activity were most prominent in the high beta and gamma bands.
Early animal studies investigating changes in EEG signal under psychedelic intoxication were methodologically disparate and mostly evaluated by visual inspection. However, more recent animal studies recording local field potentials (LFPs) after treatment with various psychedelics revealed similar effects. In contrast to several human studies, our data show a robust decrease within the high beta and gamma ranges, and a similar effect as has been shown in humans using EEG or MEG.
Using methods of source localization in rats, we are able to see a similar pattern of decrease in connectivity in animals following treatment with phenethylamine psychedelics. This suggests that the observed general decrease in connectivity is the second epiphenomena of psychedelic effects in animals.
This study is the first to show a direct comparison of several psychedelics with such consistent results and translational validity.
We demonstrated that all psychedelics were associated with decreased activity and global connectivity, and that there were only modest differences between them.
There are many references to psychedelic drugs, including the altered states of consciousness rating scale (OAV), the tryptamine world, phenethylamine world, and the psychedelic world of piHKAL. Human brain activity under LSD is decomposed into connectome-harmonic components, which reveal dynamical repertoire re-organization, and a theory of conscious states informed by neuroimaging research with psychedelic drugs. In 2003, Body S, Kheramin S, Ho MY, Miranda F, Bradshaw CM, Szabadi E, et al. studied the effects of DOI (2,5-dimethoxy-4-iodoamphetamine) and ketanserin on rat behavior. A double-blind, placebo-controlled dose-effect study of psilocybin in healthy humans revealed a positive association between psilocybin-induced spiritual experiences and insightfulness and synchronization of neuronal oscillations.
LSD, psilocybin, and psilocin have been shown to have various effects on the brain, including gamma coherence, brain activity, and subjective reports, and the effects of ayahuasca on the brain have also been studied. The hallucinogen DOI and the natural hallucinogen 5-MeO-DMT reduce low-frequency oscillations in the rat prefrontal cortex, and are reversed by antipsychotic drugs. Wood J, Kim Y, Moghaddam B, Goda SA, Piasecka J, Olszewski M, Kasicki S, Hunt MJ. The psychedelic drug 4-bromo-2,5-dimethoxyphenethylamine (2C-B) induces behavioral, neurochemical and pharmaco-EEG profiles in rats, which differ between active and inactive locomotor states. The mGlu2/3 receptor agonist LY379268 induces EEG power spectra and coherence in the ketamine model of psychosis. Quantitative EEG and the Frye and Daubert standards of admissibility. Phase lag index is a method for assessing functional connectivity from multi channel EEG and MEG with diminished bias from common sources.
The authors discuss the use of coherence, phase differences, phase shift, and phase lock in EEG/ERP analyses, as well as the use of non-linear dynamics in neuroscience to explain how neural rhythms are preserved in larger brains. Pharmaco-EEG studies in animals have shown that dopamine D2 receptor-mediated effects are different from the first temporal phase of action, and that the delayed temporal dopaminergic effects of LSD are mediated by a mechanism different from the first temporal phase of action. Apomorphine induces changes in cortical EEG activity in rats, which are mediated by D-1 and D-2 dopamine receptors. A twelve-electrode cortical EEG system was evaluated in freely moving rats to study the effects of hallucinogenic and stimulatory amphetamine derivatives. The results showed that activation of serotonin 2A receptors underlies the psilocybin-induced effects on oscillations, N170 visual-evoked potentials, and visual hallucinations.
The authors of this work are V, FT, VP, VK, LK, TN, TP, and they are all responsible for the accuracy of the work.
Dr. TP and FT declare that they are involved in clinical trials with psilocybin and MDMA outside the submitted work, and the remaining authors declare no potential conflict of interest.
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Authors associated with this publication with profiles on BlossomTomáš Páleníček
Tomas Palinek is a researcher and psychiatrist in the Czech Republic where he studies a variety of psychedelics at the NIHM.