Acute psilocybin and ketanserin effects on cerebral blood flow: 5-HT2AR neuromodulation in healthy humans

Psilocybin, via its active metabolite psilocin, is a serotonergic psychedelic that shows rapid and sustained therapeutic effects in disorders such as major depression and anxiety. Prior work indicates that stimulation of the serotonin 2A receptor (5-HT2AR) underlies the acute subjective effects of classical psychedelics, and that 5-HT2ARs are expressed both on cortical neurons and on vascular smooth muscle where they can mediate vasoconstriction. Functional neuroimaging studies using BOLD fMRI and EEG have reported large changes in functional connectivity during acute psychedelic experiences, while a small number of arterial spin labelling (ASL) studies have reported reductions in cerebral blood flow (CBF) after psilocybin. However, inconsistencies remain across those reports with respect to regional patterns and possible vascular vs neuronal mechanisms, and no in vivo human studies have directly contrasted a 5-HT2AR agonist and antagonist to dissociate receptor-specific vascular and neurophysiological effects.

Larsen and colleagues conducted a randomised, single-blind, within-subject crossover study in 28 healthy volunteers (10 females; mean age 33 ± 8 years) comparing oral psilocybin and oral ketanserin. Recruitment ran from September 2018 to March 2021; 44 people were screened and 16 were excluded. Each participant completed two intervention sessions separated by at least three weeks (mean interval 31 days). Seventeen participants received psilocybin first and 11 received ketanserin first. Psilocybin dosing was weight-adjusted in 3 mg capsules (mean dose 0.26 ± 0.04 mg/kg; absolute mean 19.8 ± 3.7 mg); ketanserin was given as a single 20 mg tablet. These doses were chosen to target comparable peak 5-HT2AR occupancy (~70%) approximately 1–3 hours after administration. Ethics approvals and informed consent procedures were reported. Participants completed pre-drug MRI scans (structural, pseudo-continuous ASL (pcASL), and, for a subset, time-of-flight angiography) and post-drug pcASL at predefined timepoints. On psilocybin days pcASL scans were acquired at ~40, 80, 130 and 300 minutes post-dose; blood samples for plasma psilocin level (PPL) and subjective drug intensity (SDI, single-item 0–10) were taken after each scan. On ketanserin days pcASL scans were acquired pre-drug and at ~80 minutes post-drug; SDI was also recorded. PPL was measured from antecubital venous plasma by ultra-performance liquid chromatography–tandem mass spectrometry and reported in mg/L reflecting free (unconjugated) psilocin. Imaging was performed on two 3 T Siemens Magnetom Prisma scanners (MR1: 15 participants, 2D single-PLD pcASL; MR2: 13 participants, 3D multi-PLD pcASL). For the 13 participants scanned on MR2 a TOF MR angiography sequence was acquired to estimate internal carotid artery (ICA) diameter. PcASL images were quality checked and preprocessed using standard pipelines (motion correction, distortion correction, coregistration to T1, quantification to absolute CBF using BASIL in FSL with a single-compartment kinetic model, normalisation to MNI space and smoothing). Global grey-matter weighted CBF and 13 bilateral regional CBF values were computed using grouped AAL3 atlas ROIs (prefrontal, temporal, parietal, occipital, ACC, PCC, thalamus, amygdala, putamen, caudate, hippocampus, insula, OFC). The cerebellum was excluded due to inconsistent coverage. ICA diameter was quantified with an in-house semi-automatic tool applied to TOF images: user-guided selection of the ICA followed by 2D region growing and 3D reconstruction, with diameter calculated as the median across slices. Statistical analysis used linear mixed-effects models (LME) with participant as a random effect and age, sex and scanner as fixed effects. Separate LMEs were fitted for each outcome (global CBF, 13 regional CBF values, ICA diameter) and each exposure (PPL or SDI). For the 13 regional tests p-values were adjusted using Dunnett’s method to control the family-wise error rate (pFWER); global CBF and ICA diameter were treated as independent tests. Ketanserin effects were analysed using pre-drug vs +80-min comparisons; drug-by-time interaction models compared psilocybin and ketanserin effects. A statistical threshold of p (or pFWER) < 0.05 was used. An exploratory receptor-occupancy analysis applied an Emax model using previously estimated EC50 and maximal occupancy parameters. The authors report numbers of included scans: 124 pcASL scans from psilocybin days (70 MR1, 54 MR2) and 49 scans from ketanserin days (30 MR1, 19 MR2). Missing scans and excluded data due to motion or image quality were described; analyses for ketanserin were restricted to pre and +80-min scans to align occupancy windows.

Twenty-eight participants contributed pcASL data, though some scans were missed or excluded for quality reasons as reported. Across the psilocybin condition observed ranges were: PPL 0–27.6 mg/L, SDI 0–10, and CBF 22.9–77.0 ml/100g/min. On ketanserin days CBF ranged 34.5–84.2 ml/100g/min. All participants reported psychoactive effects after psilocybin; ketanserin produced negligible subjective effects (SDI mean 0.23, median 0, range 0–3 at +80 min). Global CBF decreased following psilocybin and this decrease was significantly associated with both plasma psilocin and subjective intensity. Specifically, LME results showed a negative association between global CBF and PPL (beta = −0.38 ml/100g/min per mg/L psilocin; 95% CI in the extracted text = [−0.56, −0.20]; p < 0.0001). A negative association was also reported between global CBF and SDI (beta = −0.75 ml/100g/min per unit SDI; p < 0.0001); the extracted 95% confidence interval for this estimate appears inconsistently ordered in the text, but the authors report an estimated maximal global CBF decrease of 11.6% at peak PPL or SDI. Regionally, almost all cortical ROIs showed numerically negative associations between PPL and CBF; statistically significant negative associations (pFWER < 0.02) were observed in the parietal, occipital, temporal and prefrontal cortices, and in the ACC and PCC. Associations between SDI and regional CBF were significant in parietal, occipital, prefrontal and temporal cortices (pFWER < 0.0009) and in the ACC, PCC and putamen (pFWER < 0.008). An exploratory analysis using estimated receptor occupancy produced similar associations. Ketanserin intake did not produce statistically significant changes in global CBF (p = 0.35) nor in regional CBF (all pFWER > 0.19), and it did not change ICA diameter (p = 0.99). Direct drug-by-time comparisons at ~80 minutes showed a significant interaction in the parietal cortex: psilocybin produced a larger CBF decrease than ketanserin (beta = −7.86 ml/100g/min; 95% CI [−12.99, −2.74]; pFWER = 0.0005). No other regions displayed significant drug-by-time interactions after correction. ICA diameter decreased with psilocybin and this change was associated with both PPL and SDI. The extracted LMEs reported a beta of −0.02 mm per mg/L psilocin (95% CI [−0.03, −0.015], p < 0.0001) and −0.03 mm per unit SDI (95% CI [−0.038, −0.014], p < 0.0001). These reductions correspond to an estimated 10.5% decrease in ICA diameter at peak PPL and a 6.6% decrease at peak SDI. A drug-by-time interaction for ICA diameter was significant (beta = −0.39 mm; 95% CI [−0.649, −0.133] mm; pFWER = 0.007), indicating a greater decrease after psilocybin than after ketanserin.

Larsen and colleagues interpret their results as showing that acute psilocybin administration produces widespread reductions in cerebral blood flow that are significantly associated with both plasma psilocin concentrations and momentary subjective drug intensity. They report up to an 11.6% reduction in global CBF at peak drug effect and provide the first in vivo human evidence that psilocybin constricts the internal carotid artery, with an estimated 10.5% reduction in ICA diameter at peak PPL. By contrast, oral ketanserin at a 20 mg dose—expected to produce high 5-HT2AR occupancy—did not produce substantial changes in CBF or ICA diameter, which the authors interpret as evidence that tonic endogenous 5-HT2AR signalling plays a limited role in resting CBF regulation. Positioning these findings relative to previous research, the authors note general consistency with earlier ASL studies that reported reduced CBF after psilocybin, while acknowledging regional discrepancies (for example, limited thalamic effects in their data compared with some prior reports). Methodological heterogeneity between studies (route of administration, ASL sequence type, sample size) is invoked as a likely contributor to such differences. The stronger statistical associations observed with SDI than with plasma psilocin are discussed in light of possible non-linear relations between plasma concentration and receptor occupancy and the bimodal distribution of SDI scores; the authors caution that linear models relating PPL to neural changes may not fully capture such relations. The discussion highlights a potential vascular contribution to the observed CBF decreases. Psilocin’s action at vascular 5-HT2ARs and prior preclinical reports of carotid vasoconstriction provide a plausible mechanism whereby reduced ICA calibre and altered blood flow velocity could influence pcASL quantification and lead to underestimated CBF. The authors also consider alternative explanations for apparent discrepancies between psilocybin effects on CBF and previously reported changes in cerebral glucose metabolism (CMRglc), suggesting possible decoupling of flow and metabolism or additional receptor contributions (for example 5-HT1A agonism) that may differentially affect haemodynamics and metabolic demand. Several limitations are acknowledged: the single-blind design (participant blinding is challenging in psychedelic studies), imaging across two scanners with different pcASL sequences (2D single-PLD vs 3D multi-PLD), incomplete data for some scans and participants, potential hysteresis effects (insufficient data to model ascent vs descent phases), increased head motion during psychedelic states, and absence of end-tidal CO2 measures which can influence CBF estimation. The authors also note some grey-matter weighted CBF estimates lay outside typical ranges for pcASL but retained these data after quality considerations. Finally, they caution that findings from healthy volunteers may not generalise directly to clinical populations, but argue the results inform physiological mechanisms that should be considered in future work. The authors recommend that future studies assessing psychedelic effects on haemodynamics should include direct measures of vascular calibre and blood flow velocity (for example phase-contrast mapping and angiography), consider multimodal imaging (simultaneous [18F]-FDG PET and pcASL), and, where possible, use more selective pharmacological probes to disentangle neuronal from vascular contributions to observed signals. They conclude that their data support an asymmetric 5-HT2AR modulation of CBF and highlight the importance of accounting for vascular effects when interpreting neuroimaging signals during psychedelic drug action.