Positron emission tomography and fluorodeoxyglucose studies of metabolic hyperfrontality and psychopathology in the psilocybin model of psychosis

Earlier research implicated the serotonergic system in psychedelic and endogenous psychotic states because of structural similarities between psilocybin, LSD and serotonin and because these drugs appear to exert effects via 5-HT2 receptor activation. Positron emission tomography (PET) with [F-18]-fluorodeoxyglucose (FDG) allows direct investigation of regional cerebral glucose metabolism (CMRglu) and thereby offers a way to link receptor pharmacology, regional brain activity, and phenomenology. Previous PET studies in schizophrenia have reported heterogeneous findings (hypofrontality in many chronic patients, but hyperfrontality in some acute cases), prompting hypotheses that acute psychotic symptom formation may be associated with increased frontal activity and altered cortico‑striato‑thalamic (CST) loops. Vollenweider and colleagues set out to use the psilocybin model of psychosis in healthy volunteers to test whether excessive serotonergic activation produces a hyperfrontal metabolic pattern and whether such metabolic changes correlate with psychotomimetic symptoms, in particular ego disturbance and hallucinations. The study therefore combined psychometric assessment with FDG‑PET imaging to compare baseline and psilocybin states within subjects and to relate regional metabolic changes to measures of altered consciousness and schizophrenia‑like symptoms.

Fifteen healthy volunteers (8 males, 7 females; mean age 33.3 ± 4.8 years) were recruited and screened to exclude personal or first‑degree family histories of major psychiatric disorders, illicit drug abuse, and abnormal medical or MRI findings. Personality scales from the Freiburg Personality Inventory were also used as exclusion criteria. Written consent was obtained. The overall experimental design was an open, within‑subject study in two phases: a preliminary drug tolerance/exposure phase and a PET imaging phase. The extracted text indicates that psilocybin FDG‑PET scans were performed in 10 volunteers as part of the Phase II PET studies; however, the sample allocation across arms and the reasons for the reduction from 15 to 10 for the psilocybin PET scans are not fully clarified in the extracted text. Each subject served as their own control and underwent a baseline and two drug FDG‑PET scans at monthly intervals. Subjects were told they would receive a baseline scan and two drug scans (psilocybin, ketamine or d‑amphetamine) but were not blinded to receiving an active drug; the specific drug on each drug day was not disclosed to the subject. Groups were balanced so that in the overall sample five subjects received psilocybin plus ketamine, five ketamine plus amphetamine, and five psilocybin plus amphetamine; this report focuses on the psilocybin condition only. Psilocybin (5 mg capsules) was dosed orally at 15 mg for subjects ≤50 kg and 20 mg for subjects >51 kg, administered 90 minutes before FDG injection so the FDG uptake would coincide with peak drug effects. Psychopathology and altered‑states assessments were applied before and immediately after each PET scan using the AMDP inventory (manic‑depression and schizophrenia syndrome scores and subscales), the Ego Pathology Inventory (EPI; global and five subscales), the SCL‑90‑R symptom checklist, and the Altered States of Consciousness questionnaire (APZ; global score and three subscales OSE, AIA, VUS). Plasma psilocybin levels were sampled during PET via an arterial line and measured by HPLC. FDG‑PET data were acquired using a CTI/Siemens tomograph with dynamic scanning during the first 48 minutes after tracer infusion and a static scan thereafter. FDG (mean dose ~177.6 MBq) was infused over 3 minutes, and multiple arterial blood samples (23) were collected for input functions and plasma measures. Parametric images of rCMRglu were generated using an autoradiographic method, transformed to stereotaxic (Talairach) space, and a standardised set of regions of interest (ROIs) placed bilaterally across frontal, temporal, parietal, occipital cortices and subcortical structures (including caudate, putamen, thalamus, cerebellum). Regional glucose utilisation was expressed as absolute CMRglu (µmol/100 g/min) and as relative CMRglu (ratio to whole‑brain mean). Metabolic ratios (for example frontal to occipital) were calculated to assess hyper‑ versus hypofrontal patterns, and changes from baseline to psilocybin were expressed as percent differences. Statistical analyses were performed with SAS and STATISTICA; the extracted text states that each subject served as their own control and that correlations between metabolic changes and psychometric scores were examined, but full details of all statistical tests are not completely reported in the provided extraction.

Psilocybin produced a transient psychotomimetic syndrome in the volunteers, emerging 20–30 minutes after oral administration, peaking about 60 minutes later and persisting 1–2 hours before resolving by 5–6 hours. The phenomenology unfolded in three phases: an initial vegetative/disturbance phase, a peak period with derealization, depersonalization, altered sense of time/space (OSE), sensory alterations and hallucinations (VUS), and loosening of ego boundaries (AIA, EPI), and a terminal recovery phase. During the peak with eyes closed, 80% of subjects reported elementary visual disturbances and 70% reported complex visual hallucinations; 40% reported vivid recollections tied to emotional imagery. Ego pathology increased on the EPI global score, with moderate impairments in Ego identity and Ego demarcation and larger changes in Ego consistency and Ego activity; Ego vitality showed little change. FDG‑PET showed a global increase in cerebral glucose metabolism with a mean whole‑brain CMRglu increase of 19.9 ± 4.8% (p < .01). The largest regional absolute increases were bilateral frontomedial cortex (left/right: 23.5%/25.6%), frontolateral cortex (25.4%/22.7%), anterior cingulate, temporomedial cortex (22.9%/27.1%), and thalamus (21.9%/25.0%). Smaller increases occurred in somatosensory (9.7%/19.1%), motor (14.1%/16.0%), occipitomedial cortex (13.7%/15.1%), and putamen (21.0%/15.5%). Pairwise comparisons indicated that increases in frontal and temporomedial cortex were significantly greater than in occipital cortex and the right putamen (p < .05), and anterior cingulate increases exceeded those in left somatosensory cortex, right putamen and occipital cortex (p < .01). Several regional increases correlated with dose per body weight (mg/kg PO): frontal, temporolateral and motor cortex increases in the left hemisphere and frontal and posterior cingulate increases in the right hemisphere (p values ranged from < .02 to < .05). Metabolic ratio analysis showed a significant rise in the frontomedial:occipitomedial ratio in both hemispheres (~9.7% left/9.5% right), indicative of a hyperfrontal pattern. The frontolateral:temporolateral ratio changed slightly but significantly. Some frontocortical to putamen ratios increased in the right hemisphere. At baseline there was a left>right asymmetry in frontolateral and sensorimotor cortex; psilocybin enhanced the frontolateral asymmetry and abolished the sensorimotor asymmetry. Correlations between psychometrics and metabolic changes linked the metabolic pattern to symptoms. The AMDP schizophrenia syndrome score correlated positively with increases in left temporolateral cortex (p < .03) and putamen (p < .04). The AMDP Hallucinatory‑disintegration subscale and APZ VUS and OSE scores (hallucinations, derealization/depersonalization) showed negative correlations with the frontomedial:temporolateral ratio (Hall p < .008; VUS p < .04; OSE p < .003), suggesting greater hallucinatory/ego‑dissolution symptoms where fronto‑temporal balance shifted. Global EPI (EPIglo) correlated positively with increases extending from left frontolateral to right frontomedial cortex, while APZ AIA (anxious ego dissolution) correlated with anterior cingulate increases. Ego‑identity and Ego‑demarcation impairments correlated with right frontomedial increases and with frontomedial:anterior cingulate or frontomedial:temporomedial ratios in relevant hemispheres. The schizophrenia syndrome and EPI global score also correlated bilaterally with increases in putamen and left caudate. These pattern of correlations were reported to parallel some metabolic findings seen in acute psychotic episodes in schizophrenic patients.

Vollenweider and colleagues interpret the findings as evidence that psilocybin, a mixed 5‑HT2/5‑HT1 receptor agonist, induces a metabolic hyperfrontality that parallels aspects of acute psychotic symptom formation. The regional pattern—marked metabolic stimulation of frontomedial and frontolateral cortex, anterior cingulate, temporomedial cortex, thalamus and to a lesser extent basal ganglia—fits with the idea that excessive serotonergic activation can disrupt CST feedback loops and the proposed thalamic gating/filter function, producing sensory overload of frontal regions and related derealization, depersonalization and ego‑disintegration phenomena. The authors note that the regional metabolic increases broadly correspond to brain regions rich in 5‑HT2 receptors but caution that the changes cannot be attributed solely to 5‑HT2 activation; regulatory mechanisms and other neurotransmitter systems are likely involved. Comparison with a ketamine model—where psychotomimetic doses also increase global CMRglu and produce frontal hypermetabolism despite NMDA receptor antagonism—led the investigators to suggest a common downstream effect in cortico‑striatal pathways. Because 5‑HT2 and NMDA receptors are located on cortical GABAergic neurons, disruption of GABAergic regulation in cortico‑striatal circuits is proposed as a possible shared substrate. The discussion also raises the potential contribution of dopaminergic activation, directly or indirectly, to the observed hyperfrontality. Relating imaging to phenomenology, the study reports positive correlations between hyperfrontality (and anterior cingulate activity) and measures of ego pathology, derealization and anxious ego dissolution, and links left temporal and basal ganglia changes with hallucinations and the AMDP schizophrenia syndrome score. The authors place these correlations alongside PET and SPECT findings from acutely psychotic, often drug‑naive, schizophrenic patients in which hyperfrontal patterns have been observed, arguing that the psilocybin model reproduces several metabolic features of acute psychosis and differs from the hypofrontality typically reported in chronic schizophrenia. The investigators conclude that the psilocybin model is a useful heuristic tool to explore the role of 5‑HT2 receptor activation in psychosis, while acknowledging that additional work is required to disentangle the roles of other neurotransmitters such as dopamine and to clarify the precise mechanisms linking receptor activation to the observed metabolic and clinical effects. The extracted text does not provide a formal limitations section; however, methodological features that could affect interpretation are apparent from the report and include the open (non‑blinded) design and the modest number of psilocybin PET scans reported. The investigators explicitly state that metabolic changes are unlikely to reflect only 5‑HT2 activation and call for further studies to investigate indirect effects, including dopaminergic contributions.