A Single Dose of LSD Does Not Alter Gene Expression of the Serotonin 2A Receptor Gene (HTR2A) or Early Growth Response Genes (EGR1-3) in Healthy Subjects

LSD has regained interest for psychiatric research and clinical applications, yet several pharmacological effects remain incompletely understood. One notable phenomenon is the rapid development of tolerance to LSD's psychological and physiological effects with repeated dosing; early human and animal work implicates adaptations at the serotonin 5-HT2A receptor as a likely mechanism. Preclinical studies also report that acute LSD increases expression of immediate early genes such as EGR1 and EGR2 in rodent cortex via 5-HT2A receptor stimulation, and repeated dosing can reduce 5-HT2A receptor binding in frontal cortex. Liechti and colleagues set out to examine whether a single oral dose of LSD alters transcription of the 5-HT2A receptor gene (HTR2A) and early growth response genes (EGR1-3) in humans. Because direct measurement of brain gene expression in healthy people is not feasible, the investigators used peripheral whole blood mRNA as a cautious biomarker for central transcriptional responses. Their hypothesis was that LSD would acutely alter HTR2A expression and increase EGR1 and EGR2 expression in humans in a manner analogous to rodent findings.

The study used a randomised, double-blind, placebo-controlled, crossover design with two balanced experimental sessions and washout periods of at least 7 days. LSD (100 µg oral capsule) and matched placebo were each administered at 9:00 AM in standardised hospital-room sessions; participants remained supervised for the first 12 h and were monitored overnight. Although 24 healthy volunteers (12 men, 12 women) took part in the trial overall, gene expression measurements were obtained from a subset of 15 participants (7 men, 8 women; mean age 28.5 ± 5.8 years; mean weight 68 ± 8 kg; mean BMI 22.0 ± 2.0 kg/m2). Inclusion and exclusion criteria required physical and mental health, limited lifetime illicit drug exposure, and other standard protections; only two of the 15 gene-expression participants had used a hallucinogen once previously. Peripheral blood was collected into PAXgene tubes before drug administration (baseline) and at 1.5 and 24 h post-dosing. The 1.5 h time point was chosen to coincide with the expected plasma peak of LSD and the 24 h time point because partial tolerance has been documented by then in prior human work. RNA extraction followed standardised procedures with quality control (A260/A280 > 1.9; RQI > 7). Reverse transcription and quantitative real-time PCR (qPCR) were performed for HTR2A and EGR1-3, along with six candidate reference genes (ACTB, GAPDH, ALAS1, RPL13A, PPIA, RRN18S). The two least stable reference genes (GAPDH, PPIA) were excluded and normalisation used ACTB, ALAS1, RPL13A, and RRN18S. PCR runs were performed in triplicate, PCR efficiency estimated with LinRegPCR, and normalisation conducted with qBasePlus software. Plasma LSD concentrations were sampled at multiple time points up to 24 h and measured by LC–MS/MS (lower limit of quantification 0.05 ng/ml); detailed pharmacokinetic parameters were reported elsewhere. For statistical analysis, baseline gene expression values were set to 1 and post-dose values expressed as fold-change from baseline. Paired t-tests compared LSD versus placebo at each time point, and repeated-measures ANOVAs tested effects of time (0, 1.5, 24 h), with sex added as a between-subjects factor to evaluate moderation. Analyses were also standardised to mean age, body weight, and peak plasma concentration of LSD. The significance threshold was p < 0.05 and no correction for multiple comparisons was applied.

Three 24 h samples had insufficient RNA and were excluded from the gene-expression analyses. Overall, neither HTR2A nor EGR1, EGR2, or EGR3 mRNA levels in whole blood changed after administration of 100 µg LSD compared with placebo at 1.5 or 24 h. The repeated-measures ANOVAs showed no significant time effects for gene expression following LSD or placebo. Reported statistics were: HTR2A (LSD: F2,24 = 0.02, P = 1.0; placebo: F2,26 = 1.24, P = 0.3), EGR1 (LSD: F2,24 = 1.14, P = 0.3; placebo: F2,26 = 1.9, P = 0.2), EGR2 (LSD: F2,24 = 1.20, P = 0.3; placebo: F2,26 = 2.67, P = 0.09), and EGR3 (LSD: F2,24 = 1.17, P = 0.2; placebo: F2,24 = 0.08, P = 0.9). Sex did not moderate the effects, and standardising the data for age, body weight, or peak plasma LSD concentration produced similar null findings. The authors also note elsewhere in the trial that participants reported subjective effects that tracked plasma LSD levels and that no acute pharmacological tolerance was evident within 12–24 h after a single dose.

Liechti and colleagues interpret the principal finding as an absence of acute transcriptional changes in HTR2A and EGR1-3 in peripheral blood following a single 100 µg dose of LSD in healthy volunteers. The lack of an effect on HTR2A mRNA aligns with prior animal work that did not find acute changes in HTR2A gene expression in rat brain regions, but the investigators caution that receptor availability can change without altered gene transcription — for example through receptor internalisation or other post-transcriptional mechanisms. Preclinical studies reporting decreased 5-HT2A receptor binding after repeated LSD administration were cited as potentially reflecting such non-transcriptional adaptations. The null results for EGR1 and EGR2 contrast with rodent studies showing rapid, cortex-localised increases in those immediate early genes after LSD. The authors emphasise the key methodological difference that the animal studies measured brain tissue while the present work assessed blood cells; consequently, it remains possible that LSD induces transient central transcriptional responses that are not detectable peripherally. The investigators also highlight that no evidence of acute pharmacological tolerance was observed in the participants within 12–24 h, consistent with subjective effects tracking plasma concentrations and with other single-dose human data. Several limitations acknowledged by the study team temper the conclusions. Only a single, moderate dose was tested and the sample size for gene-expression analyses was relatively small. Sampling was limited to 1.5 and 24 h post-dose, so transient changes occurring between these time points could have been missed; rodent increases in EGR2, for example, have been observed up to about 5 h. Finally, peripheral blood is an imperfect surrogate for brain transcriptional activity, and adaptations relevant to tolerance or downstream signalling may be brain-specific. The authors therefore conclude that repeated-dose and more extensive temporal sampling studies, ideally with direct central measures where possible, are needed to determine whether LSD alters gene expression in humans after chronic administration.

A single acute oral dose of 100 µg LSD did not change the peripheral blood expression of the HTR2A gene or the early growth response genes EGR1-3 in healthy subjects at 1.5 or 24 h after administration. The findings indicate that this single-dose regimen did not produce detectable peripheral transcriptional markers of neuroadaptation, but they do not exclude central or repeated-dose effects that were not assessed in this study.