The serotonin hallucinogen 5-MeO-DMT alters cortico-thalamic activity in freely moving mice: Regionally-selective involvement of 5-HT1A and 5-HT2A receptors

Riga and colleagues situate their study within growing interest in the neurobiology of serotonergic hallucinogens, both as tools to model psychotic symptoms and as potential therapeutics for mood and anxiety disorders. They note that indoleamine hallucinogens such as 5-MeO-DMT bind with high affinity to several serotonin receptors, notably 5-HT1A and 5-HT2A receptors, and that previous preclinical work has linked the psychotomimetic effects of these drugs to cortical 5-HT2A receptor activation. At the same time, oscillatory activity across cortico-thalamic networks is central to sensory processing and cognition, and is modulated by serotonin; alterations in these rhythms have been reported in psychiatric disorders and after psychotomimetic drug administration. Prior recordings in anaesthetised animals showed that 5-MeO-DMT reduces low-frequency cortical oscillations and alters pyramidal neuron discharge, with indications that 5-HT1A receptors contribute to these effects. The present study set out to characterise how 5-MeO-DMT affects local field potential oscillations and interregional coherence in medial prefrontal cortex (mPFC), primary visual cortex (V1) and mediodorsal thalamus (MD) of freely moving mice. The investigators specifically probed the relative involvement of 5-HT1A and 5-HT2A receptors by comparing wild-type (WT) and 5-HT2A receptor knockout (KO2A) mice and by using the 5-HT1A antagonist WAY-100635. The stated aims were to advance understanding of the neural basis of hallucinations and to identify potential targets for treatment, and to expand knowledge about serotonergic hallucinogen actions in brain regions relevant to mood disorders.

Adult male C57BL/6 mice were used: homozygous 5-HT2A receptor knockouts (KO2A) and WT littermates, aged 9–16 weeks. Animals were maintained on a 12 h light/dark cycle with food and water ad libitum and experimental procedures complied with European regulations. Group sizes and experimental allocations are reported in the extracted text for key comparisons. Pharmacological treatments were given subcutaneously in a fixed volume (10 ml/kg). The principal test compound was 5-MeO-DMT at 5 mg/kg (expressed as free base). The study also used the selective 5-HT1A receptor antagonist WAY-100635, but the extracted text does not clearly report the antagonist dose used. On recording days mice received two injections 30 min apart according to a within-subjects scheme: saline + saline (SAL + SAL) or saline + 5-MeO-DMT (SAL + 5-MeO-DMT); KO2A animals additionally received WAY-100635 in experiments testing prevention of 5-MeO-DMT effects. Each mouse was treated randomly once with each pharmacological treatment and experiments included a wash-out period of at least one week between sessions. Electrophysiology was performed in freely moving mice. A total of 24 animals (14 WT, 10 KO2A) were implanted with electrodes under isoflurane anaesthesia. Stereotaxic placements targeted mPFC (AP +2.2, L −0.3, DV −2.0 mm), V1 (AP −3.6, L −2.5, DV −0.5 mm) and, in a subset (9 WT and 9 KO2A), the mediodorsal thalamus (MD; AP −1.2, L −0.4, DV −3 mm). Signals were acquired at 3.2 kHz, bandpass filtered between 0.1 and 100 Hz, then downsampled tenfold for analysis. Recordings took place in a 40 × 40 cm open field; mice were habituated for 4–5 days before recordings and data were collected for 30 min after each injection. Histological verification of electrode sites was performed post-mortem. Data analysis used wavelet-based power and coherence measures in MATLAB. Frequency bands were defined in the extracted text as delta (0.2–4 Hz), theta (4–10 Hz), beta (10–30 Hz) and gamma (30–80 Hz). Signals were averaged in 5-min epochs; injection periods (the first 5 min after each injection) were excluded. Treatment effects were quantified as areas under the curve (AUCs) over 5–30 min post-injection and expressed as normalised percent changes for comparisons. Statistical tests included paired and independent Student’s t-tests and two- or three-way ANOVAs (factors such as genotype, area and band or treatment and time), followed by Duncan post-hoc tests; significance was set at the 95% confidence level (two-tailed).

Saline injections produced modest changes in spectral power that the authors subtracted from drug effects. In WT mice saline produced a small but significant decrease in gamma power in all examined areas and slight increases in delta, theta and beta powers in V1 and MD; effects ranged approximately between 93 ± 1% and 115 ± 4% of pre-saline values. Saline did not affect interregional coherence in WT mice. KO2A mice showed similar small saline-induced changes in power (93 ± 3% to 121 ± 4%) and minor increases in some coherences (approximately 101.8 ± 0.4% to 105.0 ± 1.2%). In WT mice, administration of 5-MeO-DMT (SAL + 5-MeO-DMT) increased theta and gamma power in mPFC and increased delta power in V1; in MD there was a marginal decrease in beta power. Inter-regional coherence changes in WT animals included increased mPFC–V1 coherence in the beta band, increased mPFC–MD coherence in theta and beta bands, and increased V1–MD coherence in the beta band. Sample sizes reported for power/coherence analyses were typically n = 10 WT and n = 9 KO2A for mPFC/V1, while MD recordings had smaller WT samples (n = 5) and KO2A n = 9. KO2A mice showed larger and more widespread spectral changes after 5-MeO-DMT. In mPFC, 5-MeO-DMT increased the power of all examined bands (delta, theta, beta, gamma). In V1, the drug increased delta, theta and beta power, with the most pronounced effect being an elevation of V1 delta power to 258 ± 44% of pre-drug values. In MD, a significant increase in delta power was observed, while higher-frequency MD power was largely unaffected. Coherence increases in KO2A mice were extensive: mPFC–V1 coherence rose in delta, theta and beta bands; mPFC–MD coherence increased in delta, theta and beta bands (with a notable beta band increase to 122 ± 2% of pre-drug values); and V1–MD coherence increased marginally in delta and significantly in beta. A three-way ANOVA (genotype × area × band) highlighted significant interactions: the most affected areas were mPFC and V1, with a genotype-dependent elevation of V1 delta power in KO2A mice compared with WT. Post-hoc comparisons confirmed differences between genotypes in V1 and identified the V1 delta increase in KO2A as distinct from effects in other areas and bands. Regarding coherences, mPFC–MD was the most affected pair and showed the strongest beta-band increase in KO2A mice. WAY-100635 alone (first administration comparison) increased beta power in the MD of KO2A mice but did not alter coherence measures in any genotype. Critically, pretreatment of KO2A mice with WAY-100635 prevented essentially all 5-MeO-DMT-induced increases in band power across areas and abolished 5-MeO-DMT effects on interregional coherence, with the exception of delta V1–MD coherence which was not fully prevented. Statistical details for prevention analyses were reported in supplementary tables referenced in the extracted text.

The authors interpret their findings as supporting a role for simultaneous disruption of V1 activity and cortico-thalamic (mPFC–MD) circuits in 5-MeO-DMT’s hallucinatory effects. They emphasise that 5-MeO-DMT produced larger oscillatory changes in cortical areas than in the MD thalamus and that effects were generally more pronounced in KO2A mice, implicating 5-HT1A receptor activation as a principal mediator of the observed electrophysiological changes. Riga and colleagues suggest that, because 5-HT2A receptors are absent in KO2A mice, the larger effects seen in that genotype indicate that concurrent 5-HT2A activation in WT animals may attenuate or counterbalance 5-HT1A-mediated actions; this is consistent with opposite actions of the two receptors on pyramidal cell function and with prior data showing differential receptor contributions. The discussion highlights region- and band-specific patterns: increases in mPFC theta and gamma power and enhanced mPFC–MD coherence could relate to antidepressant-like mechanisms previously linked to increased frontal theta and enhanced cortico-thalamic connectivity. In contrast, the strong increase in V1 delta power in KO2A mice is linked to visual cortical hyperactivity and to mechanisms underlying visual hallucinations; the authors note parallels with human EEG/fMRI findings following psychedelics. They also consider the divergent effects of 5-MeO-DMT in awake versus anaesthetised preparations, proposing that differences in cortical excitation–inhibition balance and in whether 5-HT1A receptors act preferentially on GABAergic interneurons or pyramidal cells may account for opposite delta-band directions under different states. The role of coherence is discussed as an index of inter-area synchrony; 5-MeO-DMT increased beta-band coherence broadly, with the strongest changes in mPFC–MD, which the authors link to dense reciprocal connectivity. Prevention of most effects by WAY-100635 in KO2A mice is taken as further evidence for 5-HT1A receptor involvement, although one coherence (V1–MD delta) was less sensitive to antagonist pretreatment. Finally, the authors acknowledge the main limitation: the difficulty of extrapolating rodent electrophysiological findings directly to human brain function. They nevertheless note similarities between the present mouse results and reported human effects of 5-MeO-DMT and related serotonergic agents, and suggest that 5-HT1A antagonists might merit exploration for treating visual hallucinations and that some electrophysiological outcomes could relate to antidepressant properties.

Riga and colleagues conclude that 5-MeO-DMT simultaneously alters population activity in PFC–MD circuits and in primary visual cortex, changes that they link to the visual hallucinations and introspective states induced by the drug. The largest power effect was observed in V1 delta band, whereas the most affected synchrony was between mPFC and MD. The more pronounced effects in 5-HT2A knockout mice and their prevention by the 5-HT1A antagonist WAY-100635 support a preferential action of 5-MeO-DMT on 5-HT1A receptors. The authors suggest that 5-HT1A antagonists could have potential utility for treating visual and perhaps other forms of hallucinations, and they propose that increases in PFC theta power and mPFC–MD coherence may relate to putative antidepressant actions of the drug. Overall, the study is presented as contributing to understanding mechanisms of psychedelic agents and the brain circuits they affect.