Pharmacological fMRI: Effects of subanesthetic ketamine on resting-state functional connectivity in the default mode network, salience network, dorsal attention network and executive control network

This randomized, double-blind, placebo-controlled cross-over study (n=17) investigates the effects of subanesthetic ketamine (105 mg/70kg) on resting-state functional connectivity in healthy male subjects and found an increase of connectivity between the executive control network (i.e. prefrontal cortex ) and other resting-state networks, such as the anterior cingulum and the frontal gyrus, and decreased connectivity between executive control network and salience network. Increased connectivity is taken to reflect positive psychotic symptoms (e.g. delusions, conceptional disorganization, hallucinatory behavior), whereas the decreased connectivity was taken to reflect negative psychotic symptoms (e.g. difficulties in abstract thinking, withdrawal) and as a sign of decreased visual perception in these subjects.

Abstract

Background: Subanesthetic dosages of the NMDAR antagonist, S-Ketamine, can cause changes in behavior in healthy subjects, which are similar to the state acute psychosis and are relevant in translational schizophrenia research. Functional magnetic resonance imaging (fMRI) can be used for non-hypothesis-driven analysis of brain connectivity. The correlation between clinical behavioral scores and neuroimaging can help to characterize ketamine effects on healthy brains in resting state.

Method: Seventeen healthy, male subjects (mean: 27.42 years, SD: 4.42) were administered an infusion with S-Ketamine (initial bolus 1 mg/kg and continuous infusion of 0.015625 mg/kg/min with dosage reduction −10%/10 min) or saline in a randomized, double-blind, cross-over study. During infusion, resting state connectivity was measured and analyzed with a seed-to-voxel fMRI analysis approach. The seed regions were located in the posterior cingulate cortex, intraparietal sulcus, dorsolateral prefrontal cortex and fronto-insular cortex. Receiver operating characteristics (ROC) were calculated to assess the accuracy of the ketamine-induced functional connectivity changes. Bivariate Pearson correlation was used for correlation testing of functional connectivity changes with changes of clinical scores (PANSS, 5D-ASC).

Results: In the executive network (ECN), ketamine significantly increases the functional connectivity with parts of the anterior cingulum and superior frontal gyrus, but no significant correlations with clinical symptoms were found. Decreased connectivity between the salience network (SN) and the calcarine fissure was found, which is significantly correlated with negative symptoms (PANSS) (R2 > 0.4).

Conclusion: Decreased ketamine-induced functional connectivity in the salience network may qualify as accurate and highly predictive biomarkers for ketamine induced negative symptoms.”

Authors: Felix Mueller, Francesco Musso, Markus London, Peter de Boer, Norman Zacharias & Georg Winterer

Summary

S-Ketamine can cause changes in behavior similar to acute psychosis in healthy subjects. Functional magnetic resonance imaging can be used to analyze brain connectivity.

  1. Introduction

S-Ketamine is an anesthetic and analgesic agent that mimics positive and negative symptoms in healthy subjects similar to acute psychosis or schizophrenia. It is also a promising medication for treatment resistant depression, but it is still unknown whether psychotomimetic side effects can be fully segregated from the therapeutic effect.

fMRI can measure regional and global brain connectivity with high spatial and temporal resolution, and can reveal intrinsic resting state networks. Ketamine research uses mainly two different analysis approaches: global brain connectivity and seed-based analysis.

Global connectivity analysis showed that ketamine increased resting-state functional connectivity across widely distributed networks, and that this increased connectivity correlated with positive schizophrenia symptoms.

Ketamine effects on RS-fMRI functional connectivity in healthy subjects were extensively investigated by recent research using Region of Interest (ROI)-analyses. Bonhomme et al. (2016) studied ketamine resting-state effects in different clinical sedation levels of their subjects without a predefined ketamine plasma levels.

Ketamine increases connectivity between the hippocampus and the DLPFC, cingulate, precuneal, cerebellar and basal ganglia regions, and the medial visual network with the cerebellum and visual cortex. Additionally, ketamine decreases connectivity between the auditory and somatosensory cortex and the insula. Bonhomme et al. (2016) used a target-controlled infusion approach with the Domino model to reach different levels of sedation in their subjects. They found that deeper sedation decreased DMN functional connectivity.

Here, we conducted a double-blind, randomized, placebo-controlled cross-over subanesthetic ketamine-drug challenge experiment in healthy male subjects. We assessed the accuracy of a functional biomarker to predict drug effects and correlated drug effects on functional connectivity with clinical symptom changes before and after each scan session.

  1. Methods and materials

This Phase-I study was conducted in compliance with the Ethical principles of the WMA Declaration of Helsinki, the EU Clinical Trial Directive 2001/20/EC and the German Medicines Act.

2.1. Participants

Twenty-four healthy male volunteers were recruited via newspapers and the internet. A medical, neurological and psychiatric examination was performed prior to study inclusion, and male subjects were recruited exclusively to avoid additional screenings, e.g. pregnancy tests, prior to the study.

17 subjects were included in the study, of which 2 dropped out because of adverse events. 5 subjects were excluded because of prematurely terminated RS-fMRI scans, and 8 subjects were excluded because of excessive head motion during RS-fMRI echo planar imaging sessions.

2.2. Study design

This study used a randomized, double-blind, placebo-controlled cross-over design with S-ketamine and saline intravenously given right before the start and during the MR scan. The subjects received a psychopathological evaluation by using PANSS and 5D-ASC.

A test was conducted with the following subitems: oceanic boundlessness, dread of ego dissolution, visionary restructuralization, auditory alterations, and vigilance reduction. This evaluation was repeated directly after the MR scan with 0.1 mg/kg S-ketamine administered as a bolus, and 0.015625 mg/kg/min thereafter with a dosage reduction of 10% every 10 min. Ketamine plasma levels were not measured during this experiment, but the infusion regime was chosen to reach relatively stable plasma levels. The infusion rate was 0.02 mg/kg/min, and the plasma levels were between 0.25 g/ml and lower than 0.48 g/ml during the whole experiment.

2.3. fMRI data acquisition

The MRI scans were performed using a 3 T MR Scanner (Trio, Siemens, Erlangen, Germany) and consisted of a structural MR scan during minute 0 – 7, a task-related MR sequence using a visual oddball task from minute 8 to 34, and a resting-state EPI session of 20 min.

2.4. fMRI data processing

The scan sessions were shortened to the same length to avoid different resting-state periods as an additional source of confounds, and to avoid selecting subjects with more positive symptoms over less affected subjects.

The head motion of the included volumes between ketamine and placebo were compared using the six realignment parameters from the fMRI preprocessing described in Van Dijk et al. (2012).

Functional EPI data were preprocessed using Statistical Parametric Mapping 12 (SPM 12), and then segmented into grey matter, white matter and cerebrospinal fluid.

2.5. FMRI analysis and ROC curves

Preprocessed data were analyzed by using the CONN connectivity toolbox V15. Motion related artifacts were reduced with session specific realignment parameters, despiking, nuisance regression of motion related components, linear detrending and band-pass filtering.

To analyze seed-to-voxel data, spherical ROIs were defined in the PCC, intraparietal sulcus, DLPFC, ECN and fronto-insular cortex. The ROI volume size was chosen accordingly to the values used in the publication by Woodward et al., 2011. To analyze functional networks, mean BOLD-signal time courses were extracted, correlated with every other voxel in the brain using bivariate Pearson correlation, and then transformed to z-scores using Fisher r-to-z transformation. Two-sided between condition contrasts were performed using paired-t-tests. This fMRI data analysis is performed on one data set, but for four different seed regions separately. The cluster extent threshold is lowered after FDR-correction to p-value 0.0125 for further analysis, and ROC curves are calculated for the delta values.

2.6. Correlation testing with symptom scores

The PANSS and 5D ASC subitems were tested before and after every scan session. The differences between ketamine and placebo conditions were tested using paired t-tests, and correlations between fMRI signal change and clinical score were tested using Pearson two-tailed correlations.

3.1. Head motion

A group-level paired t-test showed no significant difference between ketamine and placebo in mean head motion and mean rotation.

3.2. Ketamine effects on resting state functional networks

The DLPFC showed increased correlation with several regions in the ketamine condition, including the anterior cingulum, superior frontal gyrus, medial temporal lobe, angular gyrus and superior temporal lobe. The fronto-insular cortex showed decreased correlation with several regions, including the calcarine fissure. Ketamine increased fMRI correlations in the anterior cingulum, superior frontal gyrus, calcarine fissure and SN, which were highly specific and sensitive to differentiate between ketamine and placebo condition.

3.3. Correlation testing with symptom scores

Ketamine increased clinical scores in PANSS and 5D-ASC, and there were three significant correlations between clinical scores and functional connectivity: negative symptoms with ECN-L. calcarine fissure, positive symptoms with SN-R. calcarine fissure, and VIR with DMN-L. superior parietal lobule.

  1. Discussion

In the present study, subanesthetic ketamine effects on four resting-state fMRI networks were investigated in healthy subjects. The ECN and SN showed increased functional connectivity with resting network hubs, and metabolic activity in the prefrontal cortex was increased.

Ketamine-induced RS-fMRI signal changes were associated with decreased connectivity of the ECN and SN seed with the calcarine fissure in the occipital lobe, which represents a part of the primary visual cortex. This finding could be related to impaired NMDAR-related neural processing.

Ketamine-induced decreases in functional connectivity were observed in the somatosensory network, but not in the default mode network (DMN). This suggests that the direction of ketamine effects might be dependent on the particular network of investigation.

The negative symptoms were positively correlated with the SN and calcarine fissure connectivity. Subjects with minimal difference between pre- and postsession PANSS score showed increased negative connectivity, which could indicate increased inhibition of the visual cortex by ketamine. This study suggests that negative symptoms are caused by neuronal inhibition in the executive control and sensory areas, whereas positive symptoms are likely caused by excitation/ loss of inhibition in different brain areas.

We didn’t find any anticorrelations in contrast to Bonhomme et al., and we hypothesize that the PCC seed doesn’t represent the whole DMN and that deeper sedation might have caused the anticorrelations to be diminished.

Recent studies have used whole-brain connectivity and region-specific analysis approaches to investigate ketamine effects. Whole-brain connectivity showed increased connectivity both in cortical and subcortical regions, and a correlation between increased connectivity in thalamus and striatum and reduced negative symptoms of the PANSS was observed. Ketamine-induced increases of connectivity were consistent with the reported global effects, but we did not find significant correlations with the PANSS positive symptoms scale. This could be due to differences in the study designs, or because our hypothesis-based approach was too conservative.

Ketamine is used to mimic clinical symptoms of acute schizophrenia in healthy subjects. However, the results of a study on schizophrenia patients are difficult to reconcile with the results of a study on ketamine-induced changes in RS-fMRI.

4.1. Limitations

The selection of male subjects, the shortening of the session to a similar length, the lack of measurement of ketamine plasma concentration levels, the use of seed-to-voxel approach and the use of predefined ROIs limited the results of this study to cortical areas exclusively.

  1. Conclusion

We found significant ketamine effects on two RS-fMRI functional networks, and the clinical symptom scores were also increased in ketamine condition. However, decreased functional connectivity between SN and calcarine fissure was strongly correlated with negative symptoms.

Contributors

Felix Mueller, Georg Winterer, Norman Zacharias, Francesco Musso, Marcus London and Peter de Boer undertook the fMRI data processing, statistical analysis and writing of this manuscript.

Funding and disclosure

Georg Winterer has received sponsorship to attend scientific meetings and speaker honoraria from Janssen Pharmaceutica and Johnson & Johnson Pharmaceutical Research & Development. Peter de Boer and Marcus London are employees at Janssen Pharmaceutica/Janssen-Cilag.

Study details

Compounds studied
Ketamine

Topics studied
Neuroscience

Study characteristics
Placebo-Controlled Double-Blind Within-Subject Randomized Bio/Neuro

Participants
17

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