Ketamine Modulates the Neural Correlates of Reward Processing in Unmedicated Patients in Remission From Depression

Abstract

Background
Ketamine as an antidepressant improves anhedonia as early as 2h post-infusion. These drug effects are thought to be exerted via actions on reward-related brain areas—yet, these actions remain largely unknown. Our study investigates ketamine’s effects during the anticipation and receipt of an expected reward, after the psychotomimetic effects of ketamine have passed, when early antidepressant effects are reported.

Methods
We examined ketamine’s effects during the anticipation and receipt of expected rewards on pre-defined brain areas, namely the dorsal and ventral striatum, the ventral tegmental area, the amygdala and the insula. We have recruited 37 male and female participants who remitted from depression and were free from symptoms and antidepressant treatments at the time of the scan. Participants were scanned, 2h after drug administration, in a double-blind cross over design (ketamine:0.5mg/kg and placebo) while performing a monetary reward task.

Results
A significant main effect of ketamine, across all ROIs, was observed during the anticipation and feedback phases of win and no-win trials. The drug effects were particularly prominent in the nucleus accumbens and putamen, which showed increased activation upon the receipt of smaller rewards compared to neutral. The levels of (2R,6R)-HNK, 2h post-infusion, significantly correlated with the activation observed in the ventral tegmental area for that contrast.

Conclusions
These findings demonstrate that ketamine can produce detectable changes in reward-related brain areas, 2h after infusion, which occur without symptom changes and support the idea that ketamine might improve reward-related symptoms via modulation of response to feedback.

Full text

Journal Pre-proof
Ketamine Modulates the Neural Correlates of Reward Processing in Unmedicated
Patients in Remission from Depression
Vasileia Kotoula, PhD, Argyris Stringaris, MD, PhD, Nuria Mackes, PhD, Ndabezinhle
Mazibuko, MBBS, Peter.C.T. Hawkins, PhD, Maura Furey, PhD, H Valerie Curran,
Mitul.A. Mehta, PhD
PII: S2451-9022(21)00163-4
DOI: https://doi.org/10.1016/j.bpsc.2021.05.009
Reference: BPSC 804
To appear in: Biological Psychiatry: Cognitive Neuroscience and
Neuroimaging
Received Date: 14 March 2021
Revised Date: 26 April 2021
Accepted Date: 23 May 2021
Please cite this article as: Kotoula V., Stringaris A., Mackes N., Mazibuko N., Hawkins P.C.T, Furey
M., Curran H.V. & Mehta M.A., Ketamine Modulates the Neural Correlates of Reward Processing in
Unmedicated Patients in Remission from Depression, Biological Psychiatry: Cognitive Neuroscience and
Neuroimaging (2021), doi: https://doi.org/10.1016/j.bpsc.2021.05.009.
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© 2021 Published by Elsevier Inc on behalf of Society of Biological Psychiatry.

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Ketamine Modulates the Neural Correlates of Reward
Processing in Unmedicated Patients in Remission from
Depression
Vasileia Kotoula, PhD1, Argyris Stringaris, MD, PhD2, Nuria Mackes, PhD1, Ndabezinhle
Mazibuko, MBBS1, Peter.C.T Hawkins, PhD1, Maura Furey, PhD3, H Valerie Curran4 and
Mitul.A. Mehta, PhD1
1. Department of Neuroimaging, Institute of Psychiatry, Psychology and Neuroscience,
King’s College London
2. Mood Brain and Development Unit, Emotion and Development Branch, National Institute
of Mental Health, National Institutes of Health, Bethesda, Maryland.
3. Janssen Pharmaceuticals of Johnson and Johnson Inc., San Diego, CA, United States.
4. Clinical Psychopharmacology Unit, University College London.
For submission to Biological Psychiatry as a Research Article, November 2020
Running Title: Ketamine modulates reward-processing brain areas
Key Words: Ketamine, (2R,6R)-HNK, VTA, Reward-processing, MID, Feedback,
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Correspondance
Vasileia Kotoula, PhD
Department of Neuroimaging
Institute of Psychiatry, Psychology and Neuroscience
King’s College London
De Crespigny Park, SE5 8AF
London, UK
Phone: +44 20 3228 3058
Fax: +44 20 3228 3058
e-mail: vasileia.kotoula@kcl.ac.uk
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Abstract
Background: Ketamine as an antidepressant improves anhedonia as early as 2h post-
infusion. These drug effects are thought to be exerted via actions on reward-related brain
areas—yet, these actions remain largely unknown. Our study investigates ketamine’s effects
during the anticipation and receipt of an expected reward, after the psychotomimetic effects
of ketamine have passed, when early antidepressant effects are reported.
Methods: We examined ketamine’s effects during the anticipation and receipt of
expected rewards on pre-defined brain areas, namely the dorsal and ventral striatum, the
ventral tegmental area, the amygdala and the insula. We have recruited 37 male and female
participants who remitted from depression and were free from symptoms and antidepressant
treatments at the time of the scan. Participants were scanned, 2h after drug administration, in
a double-blind cross over design (ketamine:0.5mg/kg and placebo) while performing a
monetary reward task.
Results: A significant main effect of ketamine, across all ROIs, was observed during
the anticipation and feedback phases of win and no-win trials. The drug effects were
particularly prominent in the nucleus accumbens and putamen, which showed increased
activation upon the receipt of smaller rewards compared to neutral. The levels of (2R,6R)-
HNK, 2h post-infusion, significantly correlated with the activation observed in the ventral
tegmental area for that contrast.
Conclusions: These findings demonstrate that ketamine can produce detectable
changes in reward-related brain areas, 2h after infusion, which occur without symptom
changes and support the idea that ketamine might improve reward-related symptoms via
modulation of response to feedback.
This research has been registered at clinicaltrials.gov
URL: https://clinicaltrials.gov/ct2/show/NCT04656886
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ClinicalTrials.gov Identifier: NCT04656886
Background
Major Depressive Disorder (MDD) is characterised by altered reward processing and
a reduced ability to modulate behaviour as a function of rewards (1). Deficits in reward
processing can precede the onset of depression(2), are linked to anhedonia and persist during
remission (3, 4). Ketamine, an NMDA receptor antagonist, produces robust antidepressant
effects that occur as early as 2h after drug infusion, peak at 24h and last up to one week (5).
In relation to reward processing, the drug improves anhedonia, a symptom known to be
resistant to standard anti-depressant treatment (6). To our knowledge however, no study has
examined whether ketamine’s ability to improve anhedonia is the result of direct modulation
of reward processing areas that is not secondary to changes in symptoms. In this study, we
have used a well-validated fMRI task, the monetary incentive delay (MID) task (7) in order to
examine whether the drug engages brain areas involved in reward processing, two hours after
its administration, in a relatively large sample of treatment-free and symptom-free remitted
depressed volunteers.
In the brain, reward processing is mainly subserved by regions that are part of the
mesocorticolimbic pathway (8). Imaging studies that have used the MID task to examine
reward processing in healthy volunteers showed that striatal regions, especially the caudate
and the putamen, but also the insula and frontal brain areas are activated during the
anticipation phase of the MID, when a monetary reward is expected (9). During the feedback
phase of the task when the expected reward is delivered, a similar set of brain regions appear
to be involved (10). In depression, recent meta-analyses showed that the ventral striatum
(VS), the caudate and the putamen present with decreased activation during the anticipation
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and feedback phases of the MID (2, 11-14). This hypoactivation of reward processing areas
observed in depression also persists in remission with studies indicating that compared to
healthy controls remitted depressed volunteers show blunted responses to reward (3) and
decreased activation in prefrontal (4) and striatal (15) regions during loss anticipation and
outcomes. Given the central role of reward processing in depression, compounds that target
these areas are considered promising candidates for alleviating depression, including
anhedonia(16).
Ketamine improves anhedonia as early as two hours after a single infusion, although
the neural basis of these effects is only beginning to be understood. Using [18F] FDG-PET
imaging at two hours post dosing, glucose metabolism in the dorsal anterior cingulate cortex
(dACC) and the putamen correlated with reduced anhedonia in patients with treatment-
resistant bipolar depression (17). In MDD patients, reductions in anhedonia correlated with
increased glucose metabolism in the dACC and hippocampus (18). Anhedonia is not a unitary
construct with separable components including reward anticipation and feedback or delivery
(19) as measured by the MID. Research in non-human primates suggests that ketamine
treatment could ameliorate blunted anticipatory responses to appetitive stimuli by
normalizing brain activation in the sub-genual anterior cingulate cortex (sgACC) (20). One
study in patients with depression investigated ketamine-induced changes in brain activity and
anhedonia using a reward-related fMRI task, demonstrating a reduction in sgACC
hyperactivity to positive feedback in 14 patients tested within five days of a ketamine
infusion (21). The fact that the changes in the metabolism and activation of reward associated
brain areas temporally overlap with symptom changes makes it difficult to determine whether
these changes are due to the primary effects of the drug or are secondary to the effect of
ketamine on depressive symptoms. While these PET and fMRI studies provide insights about
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the neural mechanisms that accompany ketamine’s early antidepressant action, the effects on
brain regions associated with anticipatory and feedback components of reward tasks during
the emergent period of the antidepressant response (2-24hours) are not known.
At a neuronal level, ketamine and its main metabolite, norketamine, indirectly activate
the post-synaptic AMPA receptors and trigger molecular pathways, including BDNF and
mTOR pathways that lead to an increase in synaptic plasticity, which has been linked to the
antidepressant effects of the drug. (for review see (22)). Another metabolite, (2R,6R)-HNK
can bind and activate AMPA receptors directly, and thus trigger the initiation of plasticity-
related molecular processes (23). In animal models of anhedonia, changes in plasticity
markers following ketamine have been linked to increased activations of the reward pathways
that are mainly mediated by dopamine (for review see (24)). While direct actions of (2R,6R)-
HNK are a candidate for such improvements, its action as an antidepressant remains to be
tested in humans and links between this metabolite and anhedonia related changes in brain
activations have yet to be observed.
In this study, we aimed to investigate the effects of ketamine on task performance and
functional brain response to the MID task two hours post-infusion – the time at which early
antidepressant effects are reported – in a cohort of participants who remitted from depression.
We chose to recruit treatment free, remitted depressed participants since they present with
altered brain activations in reward related areas (3, 25, 26) that might resemble those
observed in depression and would allow the examination of ketamine’s effects without the
confounds of antidepressant treatment or concurrent symptom change. We have focused on
specific regions of interest associated with reward processing who are activated during the
MID task, namely the striatum, the VTA, the amygdala and the insula (2, 9-12). We
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hypothesise that ketamine would increase activation in those areas. We also examine cortical
areas associated with reward in a exploratory whole brain analysis. The difference in the
activation between ketamine and placebo in the sgACC was included in an exploratory
analysis, post-hoc analysis. Moreover, we measured the levels of ketamine’s metabolites to
explore whether (2R, 6R)-HNK levels correlate with any ketamine related changes in the
activation of reward processing brain areas.
Methods
Participants
37 remitted depressed volunteers (21 female, mean age= 28.5 years) took part in a
randomised double-blind, placebo controlled, cross-over study. The MINI International
Neuropsychiatric Interview was used to confirm history of depression and remission at study
entry. Inclusion criteria included a minimum of three months of no antidepressant treatment,
prior to taking part. The exclusion criteria included any history of other psychiatric or
neurological disorder, a previous adverse response to ketamine; any medical conditions that
affect hepatic, renal or gastrointestinal functions; cardiac abnormalities; hypertension; a
significant history of substance abuse or a positive test for drugs of abuse at screening or a
study day; nicotine use (>5 cigarettes per day), alcohol (>28 units/week) and caffeine (> 6
cups per day) or any MRI contraindications. All participants gave written informed consent
for the study, which was approved by the Psychiatry, Nursing and Midwifery Research Ethics
Subcomittee (reference: HR-14/15-0650)
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Study procedures
Participants who met eligibility criteria were randomized to receive either a single
intravenous infusion of ketamine (0.5 mg/kg) or placebo (0.9% saline solution) during the
first session and the alternative treatment in the second session. Ketamine and saline were
administered during a 40min steady state infusion(27)and the sessions were at least 7 days
apart. Participants were scanned 2h after the end of the infusion.
Scales and questionnaires
The Psychotomimetic States Inventory (PSI) was used to assess the psychotomimetic
symptoms that ketamine might produce (28) and completed at the end of each infusion. A
greater PSI score indicates more drug-induced psychotomimetic experiences.
The Snaith-Hamilton Pleasure Scale (SHAPS) was used to assess anhedonia at the
beginning of each scanning session as well as 2h after each infusion (29). Due to multiple
administration during the study, instructions to the SHAPS were modified asking participants
to rate their ability to experience pleasure at the time of the assessment. Higher SHAPS
scores indicate higher levels of anhedonia present.
Image acquisition and Preprocessing
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All scans were acquired using a GE MR750 3-Tesla scanner and a 16-channel head
coil. Functional scans were obtained using T2* sensitive gradient-echo echo-planar imaging
(EPI) (repetition time [TR]= 2000ms, echo time [TE]=30ms, flip angle= 750, field of view
[FoV]=214mm, slice thickness=3mm, number of slices=42). The initial four volumes of each
timeseries were discarded to minimise steady-state effects on the signal amplitude. A total of
414 volumes were analysed for each timeseries acquired. A T1-weighted MPRAGE scan
(FoV= 204mm, TR=7.3ms, TE=3ms, 256x256x156 matrix, slice thickness=1.2mm) was
acquired on each session and was used for the reconstruction of a DARTEL template (30).
All structural and functional data were analysed using SPM-12. Pre-processing steps
included realignment of the scans for each session as well as between sessions, co-
registration to the MPRAGE image and normalization using the DARTEL flow fields. The
normalised images were then smoothed using an 8mm FWHM kernel. During the first level
modelling, the six motion parameters estimated during the realignment were used as
regressors along with frame wise displacement (31). One participant was excluded from the
analysis due to excessive movement - head motion exceeded 3mm and frame-wise
displacement was no more than 1mm.There were no significant differences (Paired t-test,
p>.05) in the framewise displacement between the ketamine and placebo conditions
(Ketamine: mean framewise displacement = 0.091mm, SD0.057, Placebo: mean framewise
displacement = 0.083mm, SD0.045).
The MID task
The version of the task closely followed that described in Knutson et al., 2001 with a
detailed description in supplemental materials. The task consists of 96 trials of different
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reward magnitudes (High win trials, Low win trials, Neutral trials), signalled by the initial
cue. The cue image is followed by a variable delay after which a target appears on the screen
and participants have to respond with a left button press. During the feedback phase, the
outcome of the trial and the total amount won are presented to participants. For the
anticipation phase of the task, three regressors were created corresponding to different reward
magnitudes associated with the task cues: “High win anticipation”, “Low win anticipation”
and “Neutral anticipation”. The feedback phase of the win and no-win trials of the task were
modelled separately and four regressors were created: “High win feedback”, “Low win
feedback”, “High no-win feedback” and “Low no-win feedback”. All the anticipation and
feedback contrasts were examined separately for the ketamine and placebo session and
compared between the two drug conditions. A more detailed description of the task and the
contrasts that were examined for this study is included in the Supplemental Material.
Regions of interest definition
The ROIs we selected comprised the amygdala, the ventral and dorsal striatum, the
VTA and the insula. The bilateral ROI for the ventral striatum (NAc) was defined as
described in Montgomery et al., (2006), based on previous work from Mawlawi et al., (2001).
The amygdala, the dorsal striatum, the VTA and the insula were anatomically defined using
the FSL Harvard-Oxford atlas (34). The bilateral ROI for the sgACC was defined as in
Morris et al., (2019) and included Brodmann area 25. All ROIs were thresholded for grey
matter with the minimal probability index set at 20% and binarized. The mean beta estimates
from the first-level modelling were extracted for each ROI using MarsBar. The ROI values
were extracted for each subject and for each contrast, for the ketamine and placebo sessions
and were analysed in SPSS version 25.
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Ketamine’s metabolites
Blood samples were collected at the beginning of each study session, immediately
after the drug infusion, and 2h after the end of the infusion. Ketamine, norketamine and the
two isoforms of hydroxynorketamine ((2R, 6R)-HNK; (2S, 6S)-HNK) were measured in
these samples. The values were used as a correlates with the ROI data to explore whether
changes in brain activations induced by ketamine were related to the plasma exposure to
ketamine and its main metabolites.
Statistical analyses
The overall effect of treatment on each task contrast was examined using a mixed-
effects model in SPSS. Each contrast was explored further by comparing the ROI activation
between ketamine and placebo using a paired t-test and within each treatment session by
using a one-sample t-test. Bonferroni correction for multiple comparisons was applied
(p=0.008).
In order to examine whether the ketamine metabolite levels, 2h post infusion, would
predict the ROI activation under ketamine, we performed robust regressions. The placebo
beta values were used as a covariate in this analysis to account for individual differences in
brain activations and FDR correction was applied.
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Results
Subjective Effects of ketamine
The established increase in psychotomimetic effects on ketamine were shown with the
psychotomimetic states inventory (PSI) total score (Ketamine: mean=48.4, SD=±22.9,
Placebo: Mean=15.1, SD=±10.6) and six subscales (see Fig. 1). The immediate effects of
ketamine were as expected, and the low placebo scores also aligned with expectations for this
group of remitted depressed volunteers who did not experience any significant symptoms
including anhedonia. This was also confirmed by the SHAPS, which as expected, indicated
very low levels of anhedonia pre-infusion (pre-placebo mean score=22.7, SD=±5.6, pre-
ketamine mean score =21.8, SD= ± 5.4, Wilcoxon signed test, Z=-0.811 p>.05) that remained
unchanged after ketamine (2h post-ketamine mean score=21.9, SD=±5.3, Wilcoxon signed
rank test, Z=-0.981 p>.05).
The MID task
Task performance
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The total amount of money won during the task did not significantly differ (Paired t-
test, p>.05) between the ketamine and placebo sessions (Ketamine: 45.1, SD=±5.5, Placebo:
43.3, SD=±9.1). For reaction times there was a main effect of reward magnitude with faster
responses for high win trials (F(2,36)=23.2, p<.0001) and no interaction with drug.
Brain activations on placebo
The brain activations during the anticipation and feedback phases of the MID task,
aligned with expectations based on previous studies (see Figure S1).
Ketamine’s effects on the MID task
For the whole brain analyses there were no differences between the ketamine and
placebo sessions.
The a priori defined ROIs were examined for all the contrasts that were created for the
MID task, and here we present the specific contrasts for which ROI activation significantly
changed between the ketamine and placebo sessions. The statistical values for the main
effects are provided in the text, and for the ROIs, in the figures and legends.
Anticipation phase
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A main effect of ketamine increasing activity was identified for the anticipation of all
win trials compared to neutral trials across the predefined ROIs (F(1,36)=9.261, p=.003). No
main drug effect was identified when the anticipation phases of high and low win trials
compared to neutral trials were examined separately or compared to each other. When
individual ROIs were examined separately for each of the anticipation contrasts, ketamine
produced significant changes in the NAc and caudate, when anticipation of high win trials
was contrasted to neutral trials (Fig.2A). This finding, however, did not survive testing for
multiple comparisons.
Feedback phase – win trials
A main effect of ketamine increasing activity was identified for the feedback phase of
low win trials compared to neutral trials across the predefined ROIs (F (1,36)=4.563,
p<.001).
When the feedback phase of win trials was explored further, ketamine, compared to
placebo, increased activations in the NAc and the putamen during the feedback phase of low
win trials compared to neutral trials (Fig. 2B). This effect survived correction for multiple
comparisons.
Feedback phase – no-win trials
A main effect of ketamine, across all predefined ROIs, was observed when the
feedback phase of all the no-win trials was contrasted to the neutral trials (F(1,36)=5.467,
p<.001) and when the feedback phase of high no-win trials was compared to neutral trials
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(F(1,36)=5.859, p=0.016). For individual ROIs none of these effects survived correction for
multiple comparisons (Fig. 2C-E).
Feedback phase- win trials vs no-win trials
A main effect of ketamine, across all predefined ROIs, was identified when all the
win trials were compared to the no-win trials (F(1,36)=5.036, p<.001), but no single ROI
showed a significant change by itself after correction for multiple comparisons (Fig.2F)
Association of ROIs activation with (2R,6R)-HNK levels
A positive correlation was identified, using robust regression, between the VTA
activation, 2h post ketamine and the plasma levels of (2R, 6R)-HNK, 2h post the ketamine
infusion (n=22, pFDR= 0.03). This correlation was identified when the feedback phase of low
win trials was contrasted to that of neutral trials (Fig.3). A positive correlation was also
identified for the activation of the caudate, 2h post ketamine and the plasma levels of (2R,
6R)-HNK when high no win trials were contrasted to neutral trials. This finding did not
survive testing for multiple comparisons.
There were no relationships between ROI values and ketamine, norketamine and (2S,
6S)-HNK plasma levels for any of the task contrasts.
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Exploratory analysis
Ketamine, 2h post its administration did not produce any significant changes in the
activation of the sgACC in any of the task contrasts that were examined. The results of this
analysis are presented in the Supplementary Material (Figure S2).
Discussion
Ketamine, approximately 2h after its administration, modulated brain activity during
the MID task, in areas that are important for reward processing. To our knowledge our study
is the first to demonstrate that ketamine can produce detectable changes in the activation of
brain areas that are important for reward processing and anhedonia 2h after infusion, without
concurrent changes in depressive symptoms and the confounding effects of antidepressant
treatment.
Previous studies have shown that ketamine, 24h after its administration normalises
some of the connectivity changes observed in depression (35, 36) as well as reducing
hyperactivation in the sgACC during a reward processing task (37). All these effects, at the
time-point when they were observed, were accompanied by improvements in depressive
symptoms and thus could either be attributed to the primary effects of the drug on neural
processes that are affected in depression or could be the secondary effect of symptom
changes that ketamine produces. In our cohort of remitted depressed volunteers, depressive
symptoms and anhedonia were not present and did not change with ketamine suggesting that
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the drug can directly modulate reward-related neural processes (17, 18, 21) producing
differential effects depending on the task contrast.
Ketamine increased the activation of the NAc, the putamen, the insula and the caudate
when the feedback phase of win and no-win trials was compared to that of neutral trials
(Figure 1B-1D). Recent meta-analyses have shown that striatal regions present with
decreased activations during the anticipation and feedback phase of the MID task in patients
with a mixture of mood disorders (2, 12). Moreover, striatal hypofunction persists during
remission (15) and altered brain activations in those areas could also contribute to the blunted
responses to positive feedback that characterises remitted depressed individuals (38).
Remitted depressed and depressed individuals also demonstrate heightened neural responses
to negative feedback (39) which has been related to anhedonia.
The fact that ketamine, during the feedback phase of the MID task, approximately 2h
post-administration, altered the activation within the mesolimbic reward pathway provides a
plausible mechanism by which ketamine could modulate abnormal responses to positive and
negative feedback. Additionally, ketamine’s effects are more prominent for the feedback
phase of no win trials which could indicate that the drug increases the salience of these trials
in our remitted depressed cohort. This effect could increase motivation especially in relation
to no win trials, and be beneficial for anhedonia. Several of the brain areas where ketamine-
induced alterations were observed in our study are also target areas for antidepressant
treatments with different pharmacology (40) and changes in their activation and connectivity
predicts treatment response (41, 42). Taken together these findings indicate that the effects
observed in our study, 2h post ketamine, could be relevant to symptoms’ improvement in
depression. However, in order to fully understand the consequence of these changes in the
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modulation of specific symptoms such as anhedonia and guilt (39), studies in actively
depressed patients will be needed.
In our study, we found preliminary evidence to link the changes in brain activity with
the levels of an active metabolite of ketamine, (2R, 6R)-HNK. The increases in brain activity
in the VTA during the feedback phase of low win trials positively correlated with the levels
of (2R, 6R)-HNK. Increased VTA activity during the feedback phase of a task that does not
involve new learning is rather unexpected. It is possible, that ketamine might increase
sensitivity to negative feedback. As a result, the negative outcomes of the no win trials would
be perceived as unexpected and trigger new learning which would be associated with
increased activation of the VTA(43, 44). The increased plasticity accompanying ketamine’s
antidepressant action might also be contributing to that effect.
It has been suggested that direct activation of AMPA receptors by (2R, 6R)-HNK
triggers the plasticity-related pathways, mediating ketamine’s antidepressant action (23).
Brain areas of the mesolimbic pathway receive dense glutamatergic input and glutamate
receptors of this pathway are crucial for synaptic plasticity (45). While there is no direct
evidence of increased plasticity after ketamine in patients, PET studies support this
conclusion through increased glucose metabolism, which correlates with improvements in
depression symptoms and anhedonia in the VS, dACC and putamen 2h post-infusion (17, 18).
Taken together with studies of Lally et al (17, 18), our findings demonstrate the potential
value of concurrent measurement of brain metabolism, functional modulation of brain
activity, symptom changes and metabolites levels in building a model of the effects of
ketamine in improving specific symptoms.
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This study has a number of limitations. First, the absence of a healthy volunteer group
does not allow the direct characterisation of impairments in reward processing in our remitted
group and thus establish whether the effect of ketamine is towards a normalization of these
changes. Additionally, to our knowledge no other study exists looking at the effects of
ketamine in reward processing, 2h post administration, at healthy volunteers that might assist
with the interpretability of our findings.
Second, most of the ketamine associated changes have been identified during the
feedback phase of the MID task highlighting the role of positive and negative outcomes for
reward processing and anhedonia. The strength of the MID task design is in the reward
anticipation phase with fewer trials contributing to the feedback contrasts, thus future studies
using a reward task designed to focus on outcomes will help in replicating the feedback
effects, as well the potential relationships with anticipation effects. While it remains possible
that the effects during feedback are a consequence of the drug effects during anticipation, this
is unlikely as both increases and decreases in activity were observed during feedback on
ketamine versus placebo. These differential effects also do not fit with an interpretation of the
drug effect being understood as a change in neurovascular coupling. Additionally, in our
results we observe that ketamine has differential effects during the feedback phase of win and
no win trials, potentially indicating that the drug might produce more profound effects during
no win condition or even punishment. Our version of the MID task does not have a loss
condition and thus does not allow us to determine the specificity of our effects during reward
trials or explore any potential effects that ketamine might have when monetary rewards are
lost instead of not gained by participants. Future studies that are better powered to look at
feedback, including loss and no win as well win trials are needed on order to address that
question.
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In summary, this study demonstrates that ketamine, 2h post administration, could
produce detectable changes in brain areas that are part of the mesolimbic pathway involved in
reward processing. These changes were not secondary to symptom changes in our cohort of
remitted depressed volunteers. During the feedback phase of low win and high no-win trials,
changes in brain activity correlate with the levels of (2R, 6R)-HNK. These findings support a
model whereby ketamine improves reward processing deficits via enhanced anticipation of
reward and modulation of responses to negative feedback, and also highlight the importance
of the drug metabolite levels in understanding ketamine’s antidepressant and anti-anhedonic
actions. Future studies examining the role of ketamine’s metabolites during reward
processing task in depression would contribute to our understanding of ketamine’s
antidepressant action.
Acknowledgements
This work was supported with founding from Johnson and Johnson via a research grant
awarded to University College London and King’s College London (Grant Code: 23034).
This paper represents independent research part-supported by a scholarship supported from
the National Institute for Health Research (NIHR) Biomedical Research Centre at South
London and Maudsley NHS Foundation Trust and King’s College London. The views
expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the
Department of Health and Social Care. The authors of this paper would like to thank the
Centre for Neuroimaging Sciences’ research stuff for all their hard work and support
throughout this research project.
Disclosures
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Dr Curran’s research is supported by the UK Medical Research Council and NIHR; she has
consulted for Janssen on esketamine. Dr Mehta has received funding from J&J, Lundbeck
and Takeda and has acted as a consultant for Lundbeck and Takeda. Dr Furey is an employee
of Janssen Research and Development. All other authors report no biomedical financial
interests or potential conflicts of interest.
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References
1. Whitton AE, Treadway MT, Pizzagalli DA (2015): Reward processing dysfunction in
major depression, bipolar disorder and schizophrenia. Curr Opin Psychiatry. 28:7-12.
2. Keren H, O'Callaghan G, Vidal-Ribas P, Buzzell GA, Brotman MA, Leibenluft E, et
al. (2018): Reward Processing in Depression: A Conceptual and Meta-Analytic Review
Across fMRI and EEG Studies. Am J Psychiatry. 175:1111-1120.
3. Pechtel P, Dutra SJ, Goetz EL, Pizzagalli DA (2013): Blunted reward responsiveness
in remitted depression. J Psychiatr Res. 47:1864-1869.
4. Schiller CE, Minkel J, Smoski MJ, Dichter GS (2013): Remitted major depression is
characterized by reduced prefrontal cortex reactivity to reward loss. J Affect Disord. 151:756-
762.
5. Murrough JW, Soleimani L, DeWilde KE, Collins KA, Lapidus KA, Iacoviello BM,
et al. (2015): Ketamine for rapid reduction of suicidal ideation: a randomized controlled trial.
Psychol Med. 45:3571-3580.
6. Argyropoulos SV, Nutt DJ (2013): Anhedonia revisited: is there a role for dopamine-
targeting drugs for depression? J Psychopharmacol. 27:869-877.
7. Knutson B, Westdorp A, Kaiser E, Hommer D (2000): FMRI visualization of brain
activity during a monetary incentive delay task. Neuroimage. 12:20-27.
8. Arias-Carrion O, Stamelou M, Murillo-Rodriguez E, Menendez-Gonzalez M, Poppel
E (2010): Dopaminergic reward system: a short integrative review. Int Arch Med. 3:24.
9. Wilson RP, Colizzi M, Bossong MG, Allen P, Kempton M, Mtac, et al. (2018): The
Neural Substrate of Reward Anticipation in Health: A Meta-Analysis of fMRI Findings in the
Monetary Incentive Delay Task. Neuropsychol Rev. 28:496-506.
Journal Pre-proof

23
10. Oldham S, Murawski C, Fornito A, Youssef G, Yucel M, Lorenzetti V (2018): The
anticipation and outcome phases of reward and loss processing: A neuroimaging meta-
analysis of the monetary incentive delay task. Hum Brain Mapp. 39:3398-3418.
11. Ng TH, Alloy LB, Smith DV (2019): Meta-analysis of reward processing in major
depressive disorder reveals distinct abnormalities within the reward circuit. Transl
Psychiatry. 9:293.
12. Zhang B, Lin P, Shi H, Ongur D, Auerbach RP, Wang X, et al. (2016): Mapping
anhedonia-specific dysfunction in a transdiagnostic approach: an ALE meta-analysis. Brain
Imaging Behav. 10:920-939.
13. Nielson DM, Keren H, O'Callaghan G, Jackson SM, Douka I, Vidal-Ribas P, et al.
(2020): Great Expectations: A Critical Review of and Suggestions for the Study of Reward
Processing as a Cause and Predictor of Depression. Biol Psychiatry.
14. Stringaris A, Vidal-Ribas Belil P, Artiges E, Lemaitre H, Gollier-Briant F, Wolke S,
et al. (2015): The Brain's Response to Reward Anticipation and Depression in Adolescence:
Dimensionality, Specificity, and Longitudinal Predictions in a Community-Based Sample.
Am J Psychiatry. 172:1215-1223.
15. Hammar A, Neto E, Clemo L, Hjetland GJ, Hugdahl K, Elliott R (2016): Striatal
hypoactivation and cognitive slowing in patients with partially remitted and remitted major
depression. Psych J. 5:191-205.
16. Krystal AD, Pizzagalli DA, Smoski M, Mathew SJ, Nurnberger J, Jr., Lisanby SH, et
al. (2020): A randomized proof-of-mechanism trial applying the 'fast-fail' approach to
evaluating kappa-opioid antagonism as a treatment for anhedonia. Nat Med. 26:760-768.
17. Lally N, Nugent AC, Luckenbaugh DA, Ameli R, Roiser JP, Zarate CA (2014): Anti-
anhedonic effect of ketamine and its neural correlates in treatment-resistant bipolar
depression. Transl Psychiatry. 4:e469.
Journal Pre-proof

24
18. Lally N, Nugent AC, Luckenbaugh DA, Niciu MJ, Roiser JP, Zarate CA, Jr. (2015):
Neural correlates of change in major depressive disorder anhedonia following open-label
ketamine. J Psychopharmacol. 29:596-607.
19. Husain M, Roiser JP (2018): Neuroscience of apathy and anhedonia: a transdiagnostic
approach. Nat Rev Neurosci. 19:470-484.
20. Alexander L, Gaskin PLR, Sawiak SJ, Fryer TD, Hong YT, Cockcroft GJ, et al.
(2019): Fractionating Blunted Reward Processing Characteristic of Anhedonia by Over-
Activating Primate Subgenual Anterior Cingulate Cortex. Neuron. 101:307-320 e306.
21. Morris LS, Costi S, Tan A, Stern ER, Charney DS, Murrough JW (2020): Ketamine
normalizes subgenual cingulate cortex hyper-activity in depression.
Neuropsychopharmacology.
22. Yang C, Yang J, Luo A, Hashimoto K (2019): Molecular and cellular mechanisms
underlying the antidepressant effects of ketamine enantiomers and its metabolites. Transl
Psychiatry. 9:280.
23. Zanos P, Moaddel R, Morris PJ, Georgiou P, Fischell J, Elmer GI, et al. (2016):
NMDAR inhibition-independent antidepressant actions of ketamine metabolites. Nature.
533:481-486.
24. Rincon-Cortes M, Grace AA (2020): Antidepressant effects of ketamine on
depression-related phenotypes and dopamine dysfunction in rodent models of stress. Behav
Brain Res. 379:112367.
25. Whitton AE, Kakani P, Foti D, Van't Veer A, Haile A, Crowley DJ, et al. (2016):
Blunted neural responses to reward in remitted major depression: A high-density event-
related potential study. Biol Psychiatry Cogn Neurosci Neuroimaging. 1:87-95.
Journal Pre-proof

25
26. Geugies H, Mocking RJT, Figueroa CA, Groot PFC, Marsman JC, Servaas MN, et al.
(2019): Impaired reward-related learning signals in remitted unmedicated patients with
recurrent depression. Brain. 142:2510-2522.
27. Murrough JW, Perez AM, Pillemer S, Stern J, Parides MK, aan het Rot M, et al.
(2013): Rapid and longer-term antidepressant effects of repeated ketamine infusions in
treatment-resistant major depression. Biol Psychiatry. 74:250-256.
28. Mason OJ, Morgan CJ, Stefanovic A, Curran HV (2008): The psychotomimetic states
inventory (PSI): measuring psychotic-type experiences from ketamine and cannabis.
Schizophr Res. 103:138-142.
29. Snaith RP, Hamilton M, Morley S, Humayan A, Hargreaves D, Trigwell P (1995): A
scale for the assessment of hedonic tone the Snaith-Hamilton Pleasure Scale. Br J Psychiatry.
167:99-103.
30. Ashburner J (2007): A fast diffeomorphic image registration algorithm. Neuroimage.
38:95-113.
31. Siegel JS, Power JD, Dubis JW, Vogel AC, Church JA, Schlaggar BL, et al. (2014):
Statistical improvements in functional magnetic resonance imaging analyses produced by
censoring high-motion data points. Hum Brain Mapp. 35:1981-1996.
32. Montgomery AJ, Mehta MA, Grasby PM (2006): Is psychological stress in man
associated with increased striatal dopamine levels?: A [11C]raclopride PET study. Synapse.
60:124-131.
33. Mawlawi O, Martinez D, Slifstein M, Broft A, Chatterjee R, Hwang DR, et al. (2001):
Imaging human mesolimbic dopamine transmission with positron emission tomography: I.
Accuracy and precision of D(2) receptor parameter measurements in ventral striatum. J
Cereb Blood Flow Metab. 21:1034-1057.
Journal Pre-proof

26
34. Desikan RS, Segonne F, Fischl B, Quinn BT, Dickerson BC, Blacker D, et al. (2006):
An automated labeling system for subdividing the human cerebral cortex on MRI scans into
gyral based regions of interest. Neuroimage. 31:968-980.
35. Abdallah CG, Averill LA, Collins KA, Geha P, Schwartz J, Averill C, et al. (2017):
Ketamine Treatment and Global Brain Connectivity in Major Depression.
Neuropsychopharmacology. 42:1210-1219.
36. Evans JW, Szczepanik J, Brutsche N, Park LT, Nugent AC, Zarate CA, Jr. (2018):
Default Mode Connectivity in Major Depressive Disorder Measured Up to 10 Days After
Ketamine Administration. Biol Psychiatry. 84:582-590.
37. Morris LS, Costi S, Tan A, Stern ER, Charney DS, Murrough JW (2020): Ketamine
normalizes subgenual cingulate cortex hyper-activity in depression.
Neuropsychopharmacology. 45:975-981.
38. McCabe C, Mishor Z, Cowen PJ, Harmer CJ (2010): Diminished neural processing of
aversive and rewarding stimuli during selective serotonin reuptake inhibitor treatment. Biol
Psychiatry. 67:439-445.
39. Martin-Soelch C (2009): Is depression associated with dysfunction of the central
reward system? Biochem Soc Trans. 37:313-317.
40. Stoy M, Schlagenhauf F, Sterzer P, Bermpohl F, Hagele C, Suchotzki K, et al. (2012):
Hyporeactivity of ventral striatum towards incentive stimuli in unmedicated depressed
patients normalizes after treatment with escitalopram. J Psychopharmacol. 26:677-688.
41. Avissar M, Powell F, Ilieva I, Respino M, Gunning FM, Liston C, et al. (2017):
Functional connectivity of the left DLPFC to striatum predicts treatment response of
depression to TMS. Brain Stimul. 10:919-925.
Journal Pre-proof

27
42. Salomons TV, Dunlop K, Kennedy SH, Flint A, Geraci J, Giacobbe P, et al. (2014):
Resting-state cortico-thalamic-striatal connectivity predicts response to dorsomedial
prefrontal rTMS in major depressive disorder. Neuropsychopharmacology. 39:488-498.
43. Ranaldi R (2014): Dopamine and reward seeking: the role of ventral tegmental area.
Rev Neurosci. 25:621-630.
44. Saunders BT, Richard JM (2011): Shedding light on the role of ventral tegmental area
dopamine in reward. J Neurosci. 31:18195-18197.
45. Eisenhardt M, Leixner S, Lujan R, Spanagel R, Bilbao A (2015): Glutamate
Receptors within the Mesolimbic Dopamine System Mediate Alcohol Relapse Behavior. J
Neurosci. 35:15523-15538.
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Figure Legends
Fig 1. Ketamine administration produced robust psychotomimetic effects as measured by the
PSI, right after the infusion. Significant increases were observed in the total score as well as
the six PSI subscales for the ketamine session compared to the placebo session (paired t-test,
p<.05).
Fig. 2. The activation of our pre-defined ROIs was examined for the anticipation (A) and
feedback phase of the high and low win and no win trials (B-F). The beta values extracted for
each contrast were compared between the ketamine and placebo sessions. All significant
comparisons (paired t-test, p<.05) are indicated with an asterisk. When the feedback phase of
the low win trials was contrasted to the feedback phase of neutral trials the ventral
striatum/nucleus accumbens and the dorsal striatum/putamen presented with significant
increases 2h post ketamine compared to placebo (B) and this result survived Bonferroni
correction for multiple comparisons (pCORR = 0.008), indicated with a red asterisk. The ROIs
that were significantly activated (pFDR_CORR<.05) for the same contrast in the placebo session
alone are indicated with a cross. The task activations under placebo are presented in more
detail in the Supplementary Material
Fig. 3. A.The levels of (2R,6R)-HNK, as measured 2h post- infusion, significantly correlated
(rs= 0.33, p= 0.03) with the activation (beta values) of the Ventral Tegmental Area during the
ketamine session and when the feedback phase of low win trials was contrasted to that of
neutral trials. This finding remained significant (pFDR CORR = 0.033) when a robust regression
was performed using the placebo beta values as a covariate to account for individual
differences in brain activation during that contrast. B. The blood concentrations for ketamine
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and its main metabolites were measured at the end of the 40min infusion and 2h post
infusion.
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Anh ed oni a Co g nit i ve
Diso rg an i sati o n
De lusio nal Thin king Mani a Paranoia Perceptual
Dist o rti o ns
Total Score
Ketamine
Placebo
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Anhedonia Cognitive
Disorganization
Delusional
Thinking Mania Paranoia Perceptual
Distortions Total Score
Ketamine 8.33(±4.6)16.47(±8.1)5.63(±6.0)5.57(±3.9) 4.01(±4.7)8.37(±5.6)48.37(±22.9)
Placebo 5.43(±2.5)4.10(±3.4)1.47(±2.9)2.77(±2.5)0.66(±1.4)0.67(±1.5)15.1(±10.5)
Figure 1.
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The Psychotomimetic Effects of Ketamine
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0.0
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Feedback of win and no win trials
All win trials vs All no win trials
Ket ami ne Pl ac e bo
Figure 2.
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ROIs Activation on Ketamine for Differ ent MID Contr asts
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Plasma Concentration Levels (ng/ml)
Plasma concentrations
Ketamine and Metabolite
End of In fus io n
Pos t i nf usio n ( 2h)
(2R,6R)-HNK (2S,6S)-HNK
Norketamine
Ketamine
0
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-1. 5 -1 -0.5 00.5 11.5 22.5 3
(2R,6R)-HNK
Ventral Tegmental Area (ketamine betas)
Feedback of win trials
Low win trials vs Neutral trials
rs=0.331
p FDR CORR = 0.03
Correlation of VTA activati on with (2R,6R)-KNH levels
❉
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❉
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Figure 3.
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