The antidepressant effect of ketamine is not associated with changes in occipital amino acid neurotransmitter content as measured by [(1)H]-MRS

Psychometric properties of the 23-Item Clinician Administered Dissociative States Scale (CADSS) in a psychological trauma population

OBJECTIVE

Dissociative symptoms are both a pathological consequence of exposure to psychological trauma as well as a side effect of N-methyl-d-aspartate (NMDA) receptor antagonist medications; therefore, accurate and valid assessment of these symptoms is important. The psychometric properties of the 23-item Clinician Administered Dissociative States Scale (CADSS) have been characterized in the ketamine and esketamine literatures. Here, we examine its performance in a sample with and without posttraumatic stress disorder (PTSD) and a history of exposure to psychological trauma.

METHODS

Participants with a history of psychological trauma with (N = 148) and without (N = 100) the diagnosis of PTSD and healthy participants without a psychiatric disorder or history of trauma (N = 28) were assessed with the 23-item CADSS and other psychometric and neuropsychological assessments. Analyses were performed to examine internal consistency, convergent and discriminant validity, factor structure, differential performance in populations reported to be more or less likely to report dissociative symptoms (e.g., patients with and without PTSD), and sensitivity to change resulting from exposure to trauma-related sights and sounds.

RESULTS

The 23-item CADSS was found to have high internal consistency (Cronbach's alpha 0.91) and a single-factor structure. CADSS total scores in trauma-exposed participants with PTSD were higher than those in trauma-exposed participants without PTSD and non-traumatized non-PTSD participants. Finally, veterans with Iraq combat-related PTSD showed a significant increase in CADSS total score after exposure to combat-related slides and sounds.

CONCLUSION

The 23-item CADSS, already validated as a tool to measure dissociation related to administration of NMDA receptor antagonist medication, performs in a reliable and valid manner in the assessment of dissociation in psychologically traumatized participants.

Cortical thickness of the posterior cingulate cortex is associated with the ketamine-induced altered sense of self: An ultra-high field MRI study

Subanesthetic doses of ketamine induce an antidepressant effect within hours in individuals with treatment-resistant depression while it furthermore induces immediate but transient psychotomimetic effects. Among these psychotomimetic effects, an altered sense of self has specifically been associated with the antidepressant response to ketamine as well as psychedelics. However, there is plenty of variation in the extent of the drug-induced altered sense of self experience that might be explained by differences in basal morphological characteristics, such as cortical thickness. Regions that have been previously associated with a psychedelics-induced sense of self and with ketamine's mechanism of action, are the posterior cingulate cortex (PCC) and the pregenual anterior cingulate cortex (pgACC). In this randomized, placebo-controlled, double-blind cross-over magnetic resonance imaging study, thirty-five healthy male participants (mean age ± standard deviation (SD) = 25.1 ± 4.2 years) were scanned at 7 T. We investigated whether the cortical thickness of two DMN regions, the PCC and the pgACC, are associated with disembodiment and experience of unity scores, which were used to index the ketamine-induced altered sense of self. We observed a negative correlation between the PCC cortical thickness and the disembodiment scores (R = -0.54, p < 0.001). In contrast, no significant association was found between the pgACC cortical thickness and the ketamine-induced altered sense of self. In the context of the existing literature, our findings highlight the importance of the PCC as a structure involved in the mechanism of ketamine-induced altered sense of self that seems to be shared with different antidepressant agents with psychotomimetic effects operating on different classes of transmitter systems.

Acute Dissociation and Ketamine's Antidepressant and Anti-Suicidal Ideation Effects in a Midazolam-Controlled Trial

OBJECTIVE

We sought to explore relationships of acute dissociative effects of intravenous ketamine with change in depression and suicidal ideation and with plasma metabolite levels in a randomized, midazolam-controlled trial.

METHODS

Data from a completed trial in suicidal, depressed participants (n = 40) randomly assigned to ketamine was used to examine relationships between ketamine treatment-emergent dissociative and psychotomimetic symptoms with pre/post-infusion changes in suicidal ideation and depression severity. Nonparametric correlational statistics were used. These methods were also used to explore associations between dissociative or psychotomimetic symptoms and blood levels of ketamine and metabolites in a subset of participants (n = 28) who provided blood samples immediately post-infusion.

RESULTS

Neither acute dissociative nor psychotomimetic effects of ketamine were associated with changes in suicidal ideation or depressive symptoms from pre- to post-infusion. Norketamine had a trend-level, moderate inverse correlation with dissociative symptoms on Day 1 post-injection (P  = .064; P =.013 removing 1 outlier). Dehydronorketamine correlated with Clinician-Administered Dissociative States Scale scores at 40 minutes (P = .034), 230 minutes (P = .014), and Day 1 (P = .012).

CONCLUSION

We did not find evidence that ketamine's acute, transient dissociative, or psychotomimetic effects are associated with its antidepressant or anti-suicidal ideation actions. The correlation of higher plasma norketamine with lower dissociative symptoms on Day 1 post-treatment suggests dissociation may be more an effect of the parent drug.

Ketamine: Mechanisms and Relevance to Treatment of Depression

Major depressive disorder (MDD) is a leading cause of suicide in the world. Monoamine-based antidepressant drugs are a primary line of treatment for this mental disorder, although the delayed response and incomplete efficacy in some patients highlight the need for improved therapeutic approaches. Over the past two decades, ketamine has shown rapid onset with sustained (up to several days) antidepressant effects in patients whose MDD has not responded to conventional antidepressant drugs. Recent preclinical studies have started to elucidate the underlying mechanisms of ketamine's antidepressant properties. Herein, we describe and compare recent clinical and preclinical findings to provide a broad perspective of the relevant mechanisms for the antidepressant action of ketamine.

A systematic review of the incidence of medical serious adverse events in sub-anesthetic ketamine treatment of psychiatric disorders

BACKGROUND

Limited published data exists that collates serious adverse outcomes involving ketamine as a psychiatric intervention. This systematic review assesses the reported incidence of medical serious adverse events (MSAEs), including but not limited to cardiovascular events, in patients receiving sub-anesthetic doses of ketamine for psychiatric disorders to guide practitioners during treatment planning, risk-benefit analyses, and the informed consent process.

METHODS

Pubmed database was searched for clinical trials of sub-anesthetic ketamine for psychiatric disorders in non-pregnant adult patients. Of the 2275 articles identified, 93 met inclusion criteria, over half of which were published in 2017 or later. Only studies that reported adverse events were included, and the incidence of MSAEs was calculated.

RESULTS

Of the 3756 participants who received at least one sub-anesthetic dose of ketamine, four participants experienced a MSAE, resulting in an incidence of approximately 0.1 % of individuals. The four MSAEs resolved without reported sequelae. Eighty-three percent of studies reported screening for medical illness and exclusion of high-risk patients. There were no serious cardiac adverse events or deaths observed in any participants; however, most trials' study designs excluded those with high cardiovascular complication risk.

LIMITATIONS

Most studies were small, underpowered for detecting rare MSAEs, at potential high-risk of bias of non-report of MSAEs, and limited mostly to intranasal and intravenous routes.

CONCLUSIONS

Findings suggest that with basic medical screening there is a very low incidence of MSAEs including adverse cardiac or cerebrovascular events in individuals receiving sub-anesthetic ketamine for psychiatric disorders.

Factors Associated with Antidepressant Effects of Ketamine: A Reanalysis of Double-Blind Randomized Placebo-Controlled Trial of Intravenous Ketamine for Treatment-Resistant Depression

INTRODUCTION

Predictors of treatment response to intravenous ketamine remain unclear in patients with treatment-resistant depression (TRD); therefore, this study aimed to clarify these predictors using the US National Institutes of Health database of clinical trials.

METHODS

Data from a placebo-controlled, double-blind, randomized controlled trial were used to assess the efficacy of intravenous ketamine in adult patients with TRD (NCT01920555). For the analysis, data were used from the participants who had received therapeutic doses of intravenous ketamine (i. e., 0.5 and 1.0 mg/kg). Logistic and multivariable regression analyses were conducted to explore the demographic and clinical factors associated with response to treatment or changes in the Hamilton Depression Rating Scale 6 items (HAM-D-6) total score.

RESULTS

This study included 31 patients with TRD (13 women; mean±standard deviation age, 48.4±10.9 years). Logistic regression analysis showed that the age of onset was positively correlated with treatment response after three days of ketamine administration (β=0.08, p=0.037); however, no association was observed between treatment response and age, sex, baseline HAM-D-6 total score, or dissociative score assessed with the Clinician-Administered Dissociative States Scale 40 min after ketamine infusion. Multiple regression analysis showed that no factors were correlated significantly with the percentage change in the HAM-D-6 total score three days after ketamine administration.

DISCUSSION

Later disease onset correlates with a better treatment response three days after ketamine infusion in patients with TRD. Glutamatergic signal transmission may be impaired in patients with an earlier onset of depression, resulting in decreased neuroplasticity, which diminishes ketamine response.

Ketamine and the neurobiology of depression: Toward next-generation rapid-acting antidepressant treatments

Ketamine has emerged as a transformative and mechanistically novel pharmacotherapy for depression. Its rapid onset of action, efficacy for treatment-resistant symptoms, and protection against relapse distinguish it from prior antidepressants. Its discovery emerged from a reconceptualization of the neurobiology of depression and, in turn, insights from the elaboration of its mechanisms of action inform studies of the pathophysiology of depression and related disorders. It has been 25 y since we first presented our ketamine findings in depression. Thus, it is timely for this review to consider what we have learned from studies of ketamine and to suggest future directions for the optimization of rapid-acting antidepressant treatment.

Ketamine's acute effects on negative brain states are mediated through distinct altered states of consciousness in humans

Ketamine commonly and rapidly induces dissociative and other altered states of consciousness (ASCs) in humans. However, the neural mechanisms that contribute to these experiences remain unknown. We used functional neuroimaging to engage key regions of the brain's affective circuits during acute ketamine-induced ASCs within a randomized, multi-modal, placebo-controlled design examining placebo, 0.05 mg/kg ketamine, and 0.5 mg/kg ketamine in nonclinical adult participants (NCT03475277). Licensed clinicians monitored infusions for safety. Linear mixed effects models, analysis of variance, t-tests, and mediation models were used for statistical analyses. Our design enabled us to test our pre-specified primary and secondary endpoints, which were met: effects of ketamine across dose conditions on (1) emotional task-evoked brain activity, and (2) sub-components of dissociation and other ASCs. With this design, we also could disentangle which ketamine-induced affective brain states are dependent upon specific aspects of ASCs. Differently valenced ketamine-induced ASCs mediated opposing effects on right anterior insula activity. Participants experiencing relatively higher depersonalization induced by 0.5 mg/kg of ketamine showed relief from negative brain states (reduced task-evoked right anterior insula activity, 0.39 SD). In contrast, participants experiencing dissociative amnesia showed an exacerbation of insula activity (0.32 SD). These results in nonclinical participants may shed light on the mechanisms by which specific dissociative states predict response to ketamine in depressed individuals.

Brain-based correlates of antidepressant response to ketamine: a comprehensive systematic review of neuroimaging studies

Ketamine is an effective antidepressant, but there is substantial variability in patient response and the precise mechanism of action is unclear. Neuroimaging can provide predictive and mechanistic insights, but findings are limited by small sample sizes. This systematic review covers neuroimaging studies investigating baseline (pre-treatment) and longitudinal (post-treatment) biomarkers of responses to ketamine. All modalities were included. We performed searches of five electronic databases (from inception to April 26, 2022). 69 studies were included (with 1751 participants). There was substantial methodological heterogeneity and no well replicated biomarker. However, we found convergence across some significant results, particularly in longitudinal biomarkers. Response to ketamine was associated with post-treatment increases in gamma power in frontoparietal regions in electrophysiological studies, post-treatment increases in functional connectivity within the prefrontal cortex, and post-treatment increases in the functional activation of the striatum. Although a well replicated neuroimaging biomarker of ketamine response was not identified, there are biomarkers that warrant further investigation.

Neurometabolite changes in response to antidepressant medication: A systematic review of 1H-MRS findings

Selective serotonin reuptake inhibitors (SSRIs), serotonin and noradrenaline reuptake inhibitors (SNRIs), and (es)ketamine are used to treat major depressive disorder (MDD). These different types of medication may involve common neural pathways related to glutamatergic and GABAergic neurotransmitter systems, both of which have been implicated in MDD pathology. We conducted a systematic review of pharmacological proton Magnetic Resonance Spectroscopy (1H-MRS) studies in healthy volunteers and individuals with MDD to explore the potential impact of these medications on glutamatergic and GABAergic systems. We searched PubMed, Web of Science and Embase and included randomized controlled trials or cohort studies, which assessed the effects of SSRIs, SNRIs, or (es)ketamine on glutamate, glutamine, Glx or GABA using single-voxel 1H-MRS or Magnetic Resonance Spectroscopic Imaging (MRSI). Additionally, studies were included when they used a field strength > 1.5 T, and when a comparison of metabolite levels between antidepressant treatment and placebo or baseline with post-medication metabolite levels was done. We excluded animal studies, duplicate publications, or articles with 1H-MRS data already described in another included article. Twenty-nine studies were included in this review. Fifteen studies investigated the effect of administration or treatment with SSRIs or SNRIs, and fourteen studies investigated the effect of (es)ketamine on glutamatergic and GABAergic metabolite levels. Studies on SSRIs and SNRIs were highly variable, generally underpowered, and yielded no consistent findings across brain regions or specific populations. Although studies on (es)ketamine were also highly variable, some demonstrated an increase in glutamate levels in the anterior cingulate cortex in a time-dependent manner after administration. Our findings highlight the need for standardized study and acquisition protocols. Additionally, measuring metabolites dynamically over time or combining 1H-MRS with whole brain functional imaging techniques could provide valuable insights into the effects of these medications on glutamate and GABAergic neurometabolism.

Association of the delayed changes in glutamate levels and functional connectivity with the immediate network effects of S-ketamine

Ketamine shows rapid antidepressant effects peaking 24 h after administration. The antidepressant effects may occur through changes in glutamatergic metabolite levels and resting-state functional connectivity (rsFC) within the default mode network (DMN). A multistage drug effect of ketamine has been suggested, inducing acute effects on dysfunctional network configuration and delayed effects on homeostatic synaptic plasticity. Whether the DMN-centered delayed antidepressant-related changes are associated with the immediate changes remains unknown. Thirty-five healthy male participants (25.1 ± 4.2 years) underwent 7 T magnetic resonance spectroscopy (MRS) and resting-state functional magnetic resonance imaging (rsfMRI) before, during, and 24 h after a single S-ketamine or placebo infusion. Changes in glutamatergic measures and rsFC in the DMN node pregenual anterior cingulate cortex (pgACC) were examined. A delayed rsFC decrease of the pgACC to inferior parietal lobe (family-wise error corrected p (p_FWEc) = 0.018) and dorsolateral prefrontal cortex (PFC; p_FWEc = 0.002) was detected that was preceded by an immediate rsFC increase of the pgACC to medial PFC (p_FWEc < 0.001) and dorsomedial PFC (p_FWEc = 0.005). Additionally, the immediate rsFC reconfigurations correlated with the delayed pgACC glutamate (Glu) level increase (p = 0.024) after 24 h at trend level (p = 0.067). Baseline measures of rsFC and MRS were furthermore associated with the magnitude of the respective delayed changes (p's < 0.05). In contrast, the delayed changes were not associated with acute psychotomimetic side effects or plasma concentrations of ketamine and its metabolites. This multimodal study suggests an association between immediate S-ketamine-induced network effects and delayed brain changes at a time point relevant in its clinical context.

The Relationship Between Acute Dissociative Effects Induced by Ketamine and Treatment Response in Adolescent Patients with Treatment-Resistant Depression

Objective: Ketamine has proven effective as a rapid-acting antidepressant agent. Several adult studies have investigated the association between ketamine's acute dissociative effects and depression response, but no studies have examined the association in adolescents with treatment-resistant depression (TRD). Methods: We conducted a secondary data analysis of 16 adolescent participants who participated in a randomized, single-dose, midazolam-controlled crossover trial of ketamine in adolescents with depression. We examined the association between the acute dissociative symptoms (measured at 60 minutes following start of infusion using the Clinician-Administered Dissociative States Scale [CADSS], and its three subscales: depersonalization, derealization, amnesia) and response and depression symptom improvement at 1'day (using the Montgomery-Åsberg Depression Rating Scale). Results: Within the ketamine group, there were no significant associations between dissociation symptoms or CADSS subscale scores and magnitude of depression symptom improvement or likelihood of ketamine response. When receiving midazolam, there was no significant association between overall dissociation symptoms and magnitude or likelihood of response of depressive symptoms. Higher levels of symptoms on the 'depersonalization' CADSS subscale when receiving midazolam were associated with less improvement in depression symptoms at 1 day following infusion. Conclusions: In contrast to some adult literature, the current data do not show a relationship between acute dissociative effects and antidepressant response to ketamine in pediatric patients with TRD. Interpretation may be limited by the small sample size, reducing the power to detect small or medium associations. Future research should utilize larger samples to more definitively measure the magnitude of association between acute dissociative symptoms and later antidepressant response to ketamine and to assess the relationship to trial design (e.g., crossover vs. parallel trial, comparison condition utilized and number of infusions) within both adult and pediatric populations. ClinicalTrials.gov identifier: NCT02579928.

A unified model of ketamine's dissociative and psychedelic properties

Ketamine is an N-methyl-d-aspartate antagonist which is increasingly being researched and used as a treatment for depression. In low doses, it can cause a transitory modification in consciousness which was classically labelled as 'dissociation'. However, ketamine is also commonly classified as an atypical psychedelic and it has been recently reported that ego dissolution experiences during ketamine administration are associated with greater antidepressant response. Neuroimaging studies have highlighted several similarities between the effects of ketamine and those of serotonergic psychedelics in the brain; however, no unified account has been proposed for ketamine's multi-level effects - from molecular to network and psychological levels. Here, we propose that the fast, albeit transient, antidepressant effects observed after ketamine infusions are mainly driven by its acute modulation of reward circuits and sub-acute increase in neuroplasticity, while its dissociative and psychedelic properties are driven by dose- and context-dependent disruption of large-scale functional networks. Computationally, as nodes of the salience network (SN) represent high-level priors about the body ('minimal' self) and nodes of the default-mode network (DMN) represent the highest-level priors about narrative self-experience ('biographical' self), we propose that transitory SN desegregation and disintegration accounts for ketamine's 'dissociative' state, while transitory DMN desegregation and disintegration accounts for ketamine's 'psychedelic' state. In psychedelic-assisted psychotherapy, a relaxation of the highest-level beliefs with psychotherapeutic support may allow a revision of pathological self-representation models, for which neuroplasticity plays a permissive role. Our account provides a multi-level rationale for using the psychedelic properties of ketamine to increase its long-term benefits.

Neuroimaging-Derived Biomarkers of the Antidepressant Effects of Ketamine

Major depressive disorder is a highly prevalent psychiatric disorder. Despite an extensive range of treatment options, about a third of patients still struggle to respond to available therapies. In the last 20 years, ketamine has gained considerable attention in the psychiatric field as a promising treatment of depression, particularly in patients who are treatment resistant or at high risk for suicide. At a subanesthetic dose, ketamine produces a rapid and pronounced reduction in depressive symptoms and suicidal ideation, and serial treatment appears to produce a greater and more sustained therapeutic response. However, the mechanism driving ketamine's antidepressant effects is not yet well understood. Biomarker discovery may advance knowledge of ketamine's antidepressant action, which could in turn translate to more personalized and effective treatment strategies. At the brain systems level, neuroimaging can be used to identify functional pathways and networks contributing to ketamine's therapeutic effects by studying how it alters brain structure, function, connectivity, and metabolism. In this review, we summarize and appraise recent work in this area, including 51 articles that use resting-state and task-based functional magnetic resonance imaging, arterial spin labeling, positron emission tomography, structural magnetic resonance imaging, diffusion magnetic resonance imaging, or magnetic resonance spectroscopy to study brain and clinical changes 24 hours or longer after ketamine treatment in populations with unipolar or bipolar depression. Though individual studies have included relatively small samples, used different methodological approaches, and reported disparate regional findings, converging evidence supports that ketamine leads to neuroplasticity in structural and functional brain networks that contribute to or are relevant to its antidepressant effects.

A Participant-Level Integrative Data Analysis of Differential Placebo Response for Suicidal Ideation and Nonsuicidal Depressive Symptoms in Clinical Trials of Intravenous Racemic Ketamine

BACKGROUND

Clinical trials of intravenous (IV) racemic (R,S)-ketamine (hereafter referred to as IV ketamine) have consistently reported rapid and substantial reductions in overall depressive symptoms compared with saline (inactive placebo) or midazolam (active placebo). The evidence for IV ketamine's specific effects on suicidal ideation is less clear, however. This study sought to examine whether differential placebo (saline or midazolam) response to overall depressive symptoms vs suicidal ideation may help explain these divergent findings.

METHODS

Data for this participant-level integrative data analysis were drawn from 151 participants across 10 studies, and linear regression was used to examine the relationship between placebo response for suicidal ideation vs other depressive symptoms indexed from standard rating scales-specifically, depressed mood, anhedonia, anxiety, and guilt-over time.

RESULTS

For participants receiving saline placebo (n = 46), greater placebo response was observed for suicidal ideation compared with other symptoms indexed from standard depression rating scales, except for anxiety. For those receiving midazolam placebo (n = 105), greater placebo response was observed for suicidal ideation compared with depressed mood or anhedonia, and no significant differences were observed when comparing suicidal ideation with anxiety or guilt.

CONCLUSIONS

Taken together, the results provide preliminary evidence of a differential placebo response for suicidal ideation vs other depressive symptoms, while anxiety and suicidal ideation appear to produce similar placebo response profiles. These findings may help explain the more modest findings in clinical IV ketamine trials for suicidal ideation than overall depression.

At-home, sublingual ketamine telehealth is a safe and effective treatment for moderate to severe anxiety and depression: Findings from a large, prospective, open-label effectiveness trial

BACKGROUND

At-home Ketamine-assisted therapy (KAT) with psychosocial support and remote monitoring through telehealth platforms addresses access barriers, including the COVID-19 pandemic. Large-scale evaluation of this approach is needed for questions regarding safety and effectiveness for depression and anxiety.

METHODS

In this prospective study, a large outpatient sample received KAT over four weeks through a telehealth provider. Symptoms were assessed using the Patient Health Questionnaire (PHQ-9) for depression, and the Generalized Anxiety Disorder scale (GAD-7) for anxiety. Demographics, adverse events, and patient-reported dissociation were also analyzed. Symptom trajectories were identified using Growth Mixture Modeling, along with outcome predictors.

RESULTS

A sample of 1247 completed treatment with sufficient data, 62.8 % reported a 50 % or greater improvement on the PHQ-9, d = 1.61, and 62.9 % on the GAD-7, d = 1.56. Remission rates were 32.6 % for PHQ-9 and 31.3 % for GAD-7, with 0.9 % deteriorating on the PHQ-9, and 0.6 % on the GAD-7. Four patients left treatment early due to side effects or clinician disqualification, and two more due to adverse events. Three patient subpopulations emerged, characterized by Improvement (79.3 %), Chronic (11.4 %), and Delayed Improvement (9.3 %) for PHQ-9 and GAD-7. Endorsing side effects at Session 2 was associated with delayed symptom improvement, and Chronic patients were more likely than the other two groups to report dissociation at Session 4.

CONCLUSION

At-home KAT response and remission rates indicated rapid and significant antidepressant and anxiolytic effects. Rates were consistent with laboratory- and clinic-administered ketamine treatment. Patient screening and remote monitoring maintained low levels of adverse events. Future research should assess durability of effects.

Predicting Antidepressant Effects of Ketamine: the Role of the Pregenual Anterior Cingulate Cortex as a Multimodal Neuroimaging Biomarker

BACKGROUND

Growing evidence underscores the utility of ketamine as an effective and rapid-acting treatment option for major depressive disorder (MDD). However, clinical outcomes vary between patients. Predicting successful response may enable personalized treatment decisions and increase clinical efficacy.

METHODS

We here explored the potential of pregenual anterior cingulate cortex (pgACC) activity to predict antidepressant effects of ketamine in relation to ketamine-induced changes in glutamatergic metabolism. Prior to a single i.v. infusion of ketamine, 24 patients with MDD underwent functional magnetic resonance imaging during an emotional picture-viewing task and magnetic resonance spectroscopy. Changes in depressive symptoms were evaluated using the Beck Depression Inventory measured 24 hours pre- and post-intervention. A subsample of 17 patients underwent a follow-up magnetic resonance spectroscopy scan.

RESULTS

Antidepressant efficacy of ketamine was predicted by pgACC activity during emotional stimulation. In addition, pgACC activity was associated with glutamate increase 24 hours after the ketamine infusion, which was in turn related to better clinical outcome.

CONCLUSIONS

Our results add to the growing literature implicating a key role of the pgACC in mediating antidepressant effects and highlighting its potential as a multimodal neuroimaging biomarker of early treatment response to ketamine.

Higher glutamatergic activity in the medial prefrontal cortex in chronic ketamine users

BACKGROUND

The medial prefrontal cortex (mPFC) plays an important role in depression and addiction. Previous studies have shown alterations in glutamatergic activity in the mPFC following the administration of ketamine in patients with depression and healthy controls. However, it remains unclear whether chronic, nonmedical use of ketamine affects metabolites in the mPFC.

METHODS

Using proton magnetic resonance spectroscopy, we measured metabolites (glutamate and glutamine [Glx]; phosphocreatine and creatine [PCr+Cr]; myo-inositol; N-acetyl-aspartate; and glycerophosphocholine and phosphocholine [GPC+PC]) in the mPFC of chronic ketamine users (n = 20) and healthy controls (n = 43). Among ketamine users, 60% consumed ketamine once per day or more, 10% consumed it every 2 days and 30% consumed it every 3 or more days. Using analysis of covariance, we evaluated between-group differences in the ratios of Glx:PCr+Cr, myo-inositol:PCr+Cr, N-acetyl-aspartate:PCr+Cr and GPC+PC:PCr+Cr.

RESULTS

Chronic ketamine users showed significantly higher Glx:PCr+Cr ratios than healthy controls (median 1.05 v. 0.95, p = 0.008). We found no significant differences in myoinositol:PCr+Cr, N-acetyl-aspartate:PCr+Cr or GPC+PC:PCr+Cr ratios between the 2 groups. We found a positive relationship between N-acetyl-aspartate:PCr+Cr and Glx:PCr+Cr ratios in the healthy control group (R = 0.345, p = 0.023), but the ketamine use group failed to show such an association (ρ = 0.197, p = 0.40).

LIMITATIONS

The cross-sectional design of this study did not permit causal inferences related to higher Glx:PCr+Cr ratios and chronic ketamine use.

CONCLUSION

This study provides the first evidence that chronic ketamine users have higher glutamatergic activity in the mPFC than healthy controls; this finding may provide new insights relevant to the treatment of depression with ketamine.

Is (R)-ketamine a Potential Therapeutic Agent for Treatment-Resistant Depression with Less Detrimental Side Effects? A Review of Molecular Mechanisms Underlying Ketamine and its Enantiomers

Approximately one-third of individuals with major depressive disorder are resistant to conventional antidepressants (i.e., monoamine-based therapies), and, even among respondents, a proper therapeutic effect may require weeks of treatment. Ketamine, a racemic mixture of the two enantiomers, (R)-ketamine and (S)-ketamine, is an N-methyl-d-aspartate receptor (NMDAR) antagonist and has been shown to have rapid-acting antidepressant properties in patients with treatment-resistant depression (TRD). Although (R)-ketamine has a lower affinity for NMDAR, it presents greater potency and longer-lasting antidepressant properties, with no major side effects, than racemic ketamine or (S)-ketamine in preclinical findings. Thereby, ketamine and its enantiomers have not only an antagonistic effect on NMDAR but also a strong synaptogenic-modulatory effect, which is impaired in TRD pathophysiology. In this review, we summarize the current evidence regarding the modulation of neurotransmission, neuroplasticity, and neural network activity as putative mechanisms of these rapid-acting antidepressants, highlighting differences on intracellular signaling pathways of synaptic proteins such as mammalian target of rapamycin (mTOR), extracellular signal-regulated kinase (ERK) and brain-derived neurotrophic factor (BDNF). In addition, we discuss probable mechanisms involved in the side effects of ketamine and its enantiomers.

Relationship Between Dissociation and Antidepressant Effects of Esketamine Nasal Spray in Patients With Treatment-Resistant Depression

BACKGROUND

In this post-hoc analysis, data from 2 positive, pivotal, phase 3 trials of esketamine nasal spray (ESK) in treatment-resistant depression (TRD)-short-term study (TRANSFORM-2) and maintenance study (SUSTAIN-1)-were analyzed to evaluate the relationship between dissociation and antidepressant effects of ESK.

METHODS

Analysis by responder status, correlation analysis, and mediation analysis were performed to assess the relationships between peak Clinician Administered Dissociative States Scale (CADSS) scores after first (day 1) and last (day 25) ESK dose and change in Montgomery-Åsberg Depression Rating Scale (MADRS) total scores at the first (day 2) and last assessments (day 28) in TRANSFORM-2 and peak CADSS after first maintenance ESK dose and time to relapse in SUSTAIN-1 (only for mediation analysis).

RESULTS

In TRANSFORM-2, the percentage of responders (>50% reduction in MADRS) at day 2 and day 28 did not significantly differ between patients who did vs did not manifest significant dissociation (peak CADSS scores >4 or ≤4, respectively) following the first ESK dose. Spearman correlation coefficients between dissociation and depression improvement were nonsignificant and close to zero. CADSS scores did not significantly mediate the reduction in MADRS at day 2 or 28 in TRANSFORM-2 or the time to depression relapse in SUSTAIN-1. The mean difference in MADRS between ESK and active-control arms persisted beyond day 2 without significant change across time, although the mean peak CADSS scores significantly decreased across consecutive doses and fewer patients experienced significant dissociation after the last ESK dose compared with the first.

CONCLUSION

Within the dose range tested, the dissociative and antidepressant effects of ESK were not significantly correlated.

TRIAL REGISTRATION

NCT02417064 (TRANSFORM-1); NCT02418585(TRANSFORM-2); NCT02493868 (SUSTAIN-1).

Acute effects of ketamine on the pregenual anterior cingulate: linking spontaneous activation, functional connectivity, and glutamate metabolism

Ketamine exerts its rapid antidepressant effects via modulation of the glutamatergic system. While numerous imaging studies have investigated the effects of ketamine on a functional macroscopic brain level, it remains unclear how altered glutamate metabolism and changes in brain function are linked. To shed light on this topic we here conducted a multimodal imaging study in healthy volunteers (N = 23) using resting state fMRI and proton (¹H) magnetic resonance spectroscopy (MRS) to investigate linkage between metabolic and functional brain changes induced by ketamine. Subjects were investigated before and during an intravenous ketamine infusion. The MRS voxel was placed in the pregenual anterior cingulate cortex (pgACC), as this region has been repeatedly shown to be involved in ketamine's effects. Our results showed functional connectivity changes from the pgACC to the right frontal pole and anterior mid cingulate cortex (aMCC). Absolute glutamate and glutamine concentrations in the pgACC did not differ significantly from baseline. However, we found that stronger pgACC activation during ketamine was linked to lower glutamine concentration in this region. Furthermore, reduced functional connectivity between pgACC and aMCC was related to increased pgACC activation and reduced glutamine. Our results thereby demonstrate how multimodal investigations in a single brain region could help to advance our understanding of the association between metabolic and functional changes.

Use of ketamine and esketamine for depression: an overview of systematic reviews with meta-analyses

PURPOSE

To summarize the evidence of efficacy and safety of the use of ketamine and esketamine for depression.

METHODS

A literature search was performed in Medline, the Cochrane Library, LILACS, and CRD until November 2020. We included systematic reviews with meta-analyses of randomized controlled trials on the use of ketamine and esketamine in adult patients with depression. Two authors independently performed the study selection and data extraction. The AMSTAR-2 tool was used to appraise the quality of included reviews.

RESULTS

A total of 118 records were identified, and 11 studies fully met the eligibility criteria. Compared to control, ketamine improved the clinical response at 40 min to 1 week and clinical remission at 80 min to 72 h, and esketamine improved both outcomes at 2 h to 4 weeks. Ketamine and esketamine also had a beneficial effect on the depression scales score and suicidality. For adverse events, oral ketamine did not show significant change compared to control, while intranasal esketamine showed difference for any events, such as dissociation, dizziness, hypoesthesia, and vertigo. Most reviews were classified as "critically low quality," and none of them declared the source of funding of the primary studies and assessed the potential impact of risk of bias in primary studies.

CONCLUSION

Ketamine and esketamine showed a significant antidepressant action within a few hours or days after administration; however, the long-term efficacy and safety are lacking. In addition, the methodological quality of the reviews was usually critically low, which may indicate the need for higher quality evidence in relation to the theme.

Ketamine and other glutamate receptor modulators for depression in adults with unipolar major depressive disorder

BACKGROUND

Many studies have recently been conducted to assess the antidepressant efficacy of glutamate modification in mood disorders. This is an update of a review first published in 2015 focusing on the use of glutamate receptor modulators in unipolar depression.

OBJECTIVES

To assess the effects - and review the acceptability and tolerability - of ketamine and other glutamate receptor modulators in alleviating the acute symptoms of depression in people with unipolar major depressive disorder.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), Ovid MEDLINE, Embase and PsycINFO all years to July 2020.  We did not apply any restrictions to date, language or publication status.

SELECTION CRITERIA

Double- or single-blinded randomised controlled trials (RCTs) comparing ketamine, memantine, esketamine or other glutamate receptor modulators with placebo (pill or saline infusion), other active psychotropic drugs, or electroconvulsive therapy (ECT) in adults with unipolar major depression.

DATA COLLECTION AND ANALYSIS

Three review authors independently identified studies, assessed trial quality and extracted data. The primary outcomes were response rate (50% reduction on a standardised rating scale) and adverse events. We decided a priori to measure the efficacy outcomes at different time points and run sensitivity/subgroup analyses. Risk of bias was assessed using the Cochrane tool, and certainty of the evidence was assessed using GRADE.

MAIN RESULTS

Thirty-one new studies were identified for inclusion in this updated review. Overall, we included 64 studies (5299 participants) on ketamine (31 trials), esketamine (9), memantine (5), lanicemine (4), D-cycloserine (2), Org26576 (2), riluzole (2), atomoxetine (1), basimglurant (1), citicoline (1), CP-101,606 (1), decoglurant (1), MK-0657 (1), N-acetylcysteine (1), rapastinel (1), and sarcosine (1). Forty-eight studies were placebo-controlled, and 48 were two-arm studies. The majority of trials defined an inclusion criterion for the severity of depressive symptoms at baseline: 29 at least moderate depression; 17 severe depression; and five mild-to-moderate depression. Nineteen studies recruited only patients with treatment-resistant depression, defined as inadequate response to at least two antidepressants. The majority of studies investigating ketamine administered as a single dose, whilst all of the included esketamine studies used a multiple dose regimen (most frequently twice a week for four weeks). Most studies looking at ketamine used intravenous administration, whilst the majority of esketamine trials used intranasal routes. The evidence suggests that ketamine may result in an increase in response and remission compared with placebo at 24 hours odds ratio (OR) 3.94, 95% confidence interval (CI) 1.54 to 10.10; n = 185, studies = 7, very low-certainty evidence). Ketamine may reduce depression rating scale scores over placebo at 24 hours, but the evidence is very uncertain (standardised mean difference (SMD) -0.87, 95% CI -1.26 to -0.48; n = 231, studies = 8, very low-certainty evidence). There was no difference in the number of participants assigned to ketamine or placebo who dropped out for any reason (OR 1.25, 95% CI 0.19 to 8.28; n = 201, studies = 6, very low-certainty evidence). When compared with midazolam, the evidence showed that ketamine increases remission rates at 24 hours (OR 2.21, 95% CI 0.67 to 7.32; n = 122,studies = 2, low-certainty evidence). The evidence is very uncertain about the response efficacy of ketamine at 24 hours in comparison with midazolam, and its ability to reduce depression rating scale scores at the same time point (OR 2.48, 95% CI 1.00 to 6.18; n = 296, studies = 4,very low-certainty evidence). There was no difference in the number of participants who dropped out of studies for any reason between ketamine and placebo (OR 0.33, 95% CI 0.05 to 2.09; n = 72, studies = 1, low-certainty evidence). Esketamine treatment likely results in a large increase in participants achieving remission at 24 hours compared with placebo (OR 2.74, 95% CI 1.71 to 4.40; n = 894, studies = 5, moderate-certainty evidence). Esketamine probably results in decreases in depression rating scale scores at 24 hours compared with placebo (SMD -0.31, 95% CI -0.45 to -0.17; n = 824, studies = 4, moderate-certainty evidence). Our findings show that esketamine increased response rates, although this evidence is uncertain (OR 2.11, 95% CI 1.20 to 3.68; n = 1071, studies = 5, low-certainty evidence). There was no evidence that participants assigned to esketamine treatment dropped out of trials more frequently than those assigned to placebo for any reason (OR 1.58, 95% CI 0.92 to 2.73; n = 773, studies = 4,moderate-certainty evidence). We found very little evidence for the remaining glutamate receptor modulators. We rated the risk of bias as low or unclear for most domains, though lack of detail regarding masking of treatment in the studies reduced our certainty in the effect for all outcomes.

AUTHORS' CONCLUSIONS

Our findings show that ketamine and esketamine may be more efficacious than placebo at 24 hours. How these findings translate into clinical practice, however, is not entirely clear. The evidence for use of the remaining glutamate receptor modulators is limited as very few trials were included in the meta-analyses for each comparison and the majority of comparisons included only one study. Long term non-inferiority RCTs comparing repeated ketamine and esketamine, and rigorous real-world monitoring are needed to establish comprehensive data on safety and efficacy.

A qualitative and quantitative account of patient's experiences of ketamine and its antidepressant properties

BACKGROUND

Ketamine is central to one of the most rapidly growing areas of neuroscientific research into novel treatments for depression. Limited research has indicated that the psychedelic properties of ketamine may play a role in its antidepressant effects.

AIM

The aim of the current study was to explore the psychedelic experiences and sustained impact of ketamine in major depressive disorder.

METHODS

In the current study, ketamine (0.44 mg/kg) was administered to 32 volunteers with major depressive disorder in a crossover design with the active-placebo remifentanil, in a magnetic resonance imaging (MRI) environment. The 11-dimension altered states of consciousness questionnaire and individual qualitative interviews were used to capture the acute psychedelic experience. The Montgomery-Asberg Depression Rating Scale and further interviewing explored lasting effects. The second qualitative interview took place ⩾3 weeks post-ketamine.

RESULTS

Greater antidepressant response (reduction in Montgomery-Asberg Depression Rating Scale at 24 h) correlated with the 11-dimension altered states of consciousness dimensions: spirituality, experience of unity, and insight. The first qualitative interview revealed that all participants experienced perceptual changes. Additional themes emerged including loss of control and emotional and mood changes. The final interview showed evidence of a psychedelic afterglow, and changes to perspective on life, people, and problems, as well as changes to how participants felt about their depression and treatments.

CONCLUSIONS

The current study provides preliminary evidence for a role of the psychedelic experience and afterglow in ketamine's antidepressant properties. Reflexive thematic analysis provided a wealth of information on participants' experience of the study and demonstrated the psychedelic properties of ketamine are not fully captured by commonly used questionnaires.

Dissociative symptoms with intravenous ketamine in treatment-resistant depression exploratory observational study

There is evidence for ketamine use in treatment-resistant depression (TRD). Several safety and tolerability concerns arise regarding adverse drug reactions and specific subpopulations. This paper aims to investigate the relationship between dissociative and psychometric measures in course of intravenous ketamine treatment in TRD inpatients with major depressive disorder and bipolar disorder.This study result represents safety data in a population of 49 inpatients with major depressive disorder and bipolar disorder subjects receiving eight 0.5 mg/kg of ketamine intravenous infusions, with a duration of 40 min each, as an add-on treatment to standard-of-care pharmacotherapy, registered in the naturalistic observational protocol of the tertiary reference unit for mood disorders (NCT04226963). The safety psychometrics assessed dissociation and psychomimetic symptomatology with the Clinician-Administered Dissociative States Scale (CADSS) the Brief Psychiatric Rating Scale (BPRS).The significant differences in CADSS scores between measurements in course of the treatment were observed (P = .003). No significant differences between BPRS measurements were made after infusions. In each case, both BPRS and CADSS values dropped to the "absent" level within 1 hour from the infusion. Neither CADSS nor BPRS scores were associated with the treatment outcome.The study demonstrates a good safety profile of intravenous ketamine as an add-on intervention to current psychotropic medication in TRD. The abatement of dissociation was observed in time with no sequelae nor harm. The study provides no support for the association between dissociation and treatment outcome.This study may be underpowered due to the small sample size. The protocol was defined as a study on acute depressive symptomatology without blinding.

Ketamine-50 years in use: from anesthesia to rapid antidepressant effects and neurobiological mechanisms

Over the past 50 years, ketamine has solidified its position in both human and veterinary medicine as an important anesthetic with many uses. More recently, ketamine has been studied and used for several new indications, ranging from chronic pain to drug addiction and post-traumatic stress disorder. The discovery of the rapid-acting antidepressant effects of ketamine has resulted in a surge of interest towards understanding the precise mechanisms driving its effects. Indeed, ketamine may have had the largest impact for advancements in the research and treatment of psychiatric disorders in the past few decades. While intense research efforts have been aimed towards uncovering the molecular targets underlying ketamine's effects in treating depression, the underlying neurobiological mechanisms remain elusive. These efforts are made more difficult by ketamine's complex dose-dependent effects on molecular mechanisms, multiple pharmacologically active metabolites, and a mechanism of action associated with the facilitation of synaptic plasticity. This review aims to provide a brief overview of the different uses of ketamine, with an emphasis on examining ketamine's rapid antidepressant effects spanning molecular, cellular, and network levels. Another focus of the review is to offer a perspective on studies related to the different doses of ketamine used in antidepressant research. Finally, the review discusses some of the latest hypotheses concerning ketamine's action.

Analysis of Antidepressant Activity of Huang-Lian Jie-Du Decoction Through Network Pharmacology and Metabolomics

Depressive disorder is a common mental disorder characterized by depressed mood and loss of interest or pleasure. As the Herbal medicines are mainly used as complementary and alternative therapy for depression. This study aimed at exploring antidepressant activity of Huang-lian Jie-du Decoction (HLJDD), and evaluating active components and potential depression-associated targets. HLJDD was administered on chronic unpredictable mild stress-induced (CUMS) depressive mice. Behavior evaluation was performed through force swimming test (FST), novelty-suppressed feeding test (NSF), and open field test (OFT). Active components of HLJDD, potential targets, and metabolic pathways involved in depression were explored through systemic biology-based network pharmacology assay, molecular docking and metabonomics. FST assay showed that CUMS mice administered with HLJDD had significantly shorter immobility time compared with control mice. Further, HLJDD alleviated feeding latency of CUMS mice in NSFand increased moving distance and duration in OFT. In the following network pharmacology assay, thirty-eight active compounds in HLJDD were identified based on drug-like characteristics, and pharmacokinetics and pharmacodynamics profiles. Moreover, forty-eight molecular targets and ten biochemical pathways were uncovered through molecular docking and metabonomics. GRIN2B, DRD, PRKCA, HTR, MAOA, SLC6A4, GRIN2A, and CACNA1A are implicated in inhibition of depressive symptoms through modulating tryptophan metabolism, serotonergic and dopaminergic synaptic activities, cAMP signaling pathway, and calcium signaling pathway. Further network pharmacology-based analysis showed a correlation between HLJDD and tryptophan metabolism. A total of thirty-seven active compounds, seventy-six targets, and sixteen biochemical pathways were involved in tryptophan metabolism. These findings show that HLJDD acts on potential targets such as SLC6A4, HTR, INS, MAO, CAT, and FoxO, PI3K/Akt, calcium, HIF-1, and mTOR signaling pathways, and modulates serotoninergic and dopaminergic synaptic functions. In addition, metabonomics showed that tryptophan metabolism is the primary target for HLJDD in CUMS mice. The findings of the study show that HLJDD exhibited antidepressant effects. SLC6A4 and MAOA in tryptophan metabolism were modulated by berberine, baicalein, tetrahydroberberine, candicine and may be the main antidepressant targets for HLJDD.

The potential of 1H-MRS in CNS drug development

RATIONALE

Proton magnetic resonance spectroscopy (¹H-MRS) is a cross-species neuroimaging technique that can measure concentrations of several brain metabolites, including glutamate and GABA. This non-invasive method has promise in developing centrally acting drugs, as it can be performed repeatedly within-subjects and be used to translate findings from the preclinical to clinical laboratory using the same imaging biomarker.

OBJECTIVES

This review focuses on the utility of single-voxel ¹H-MRS in developing novel glutamatergic or GABAergic drugs for the treatment of psychiatric disorders and includes research performed in rodent models, healthy volunteers and patient cohorts.

RESULTS

Overall, these studies indicate that ¹H-MRS is able to detect the predicted pharmacological effects of glutamatergic or GABAergic drugs on voxel glutamate or GABA concentrations, although there is a shortage of studies examining dose-related effects. Clinical studies have applied ¹H-MRS to better understand drug therapeutic mechanisms, including the glutamatergic effects of ketamine in depression and of acamprosate in alcohol dependence. There is an emerging interest in identifying patient subgroups with 'high' or 'low' brain regional ¹H-MRS glutamate levels for more targeted drug development, which may require ancillary biomarkers to improve the accuracy of subgroup discrimination.

CONCLUSIONS

Considerations for future research include the sensitivity of single-voxel ¹H-MRS in detecting drug effects, inter-site measurement reliability and the interpretation of drug-induced changes in ¹H-MRS metabolites relative to the known pharmacological molecular mechanisms. On-going technological development, in single-voxel ¹H-MRS and in related complementary techniques, will further support applications within CNS drug discovery.

Central nervous system-related safety and tolerability of add-on ketamine to antidepressant medication in treatment-resistant depression: focus on the unique safety profile of bipolar depression

BACKGROUND

There is evidence supporting the use of ketamine in treatment-resistant depression (TRD). However, there are some safety and tolerability concerns associated with ketamine. This study aimed to investigate ketamine's safety and tolerability to the central nervous system and to assess the relationship between dissociative symptomology and psychometric outcomes during and after intravenous ketamine treatment concurrent with treatment by varying psychotropic medications in treatment-refractory inpatients with major depressive disorder (MDD) and bipolar disorder (BP).

METHODS

A total of 49 patients with MDD and BP were included in this study. The subjects were administered ketamine and were assessed for changes using an observational protocol.

RESULTS

No antidepressants were associated with psychomimetic symptomatology except for citalopram (p = 0.019). Patients treated with citalopram showed a higher intensity of psychomimetic symptomatology. The use of classic mood-stabilizers was significantly associated with an increase in psychomimetic symptomatology according to the Brief Psychiatric Rating Scale (BPRS; lamotrigine p = 0.009, valproate p = 0.048, lithium p = 0.012). No sequelae were observed.

CONCLUSIONS

Despite the limitations that this study may be underpowered due to the small sample size, the sample consisted of a heterogeneous TRD population in a single site, and there no blinding of who underwent only acute ketamine administration, our observations indicate ketamine use requires close safety and tolerability monitoring with regards to psychomimetic and dissociative symptoms in TRD-BP and careful management for MDD patients.ClinicalTrials.gov identifier: NCT04226963.

Glutamate and GABA Homeostasis and Neurometabolism in Major Depressive Disorder

Major depressive disorder (MDD) is a leading cause of distress, disability, and suicides. As per the latest WHO report, MDD affects more than 260 million people worldwide. Despite decades of research, the underlying etiology of depression is not fully understood. Glutamate and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitters, respectively, in the matured central nervous system. Imbalance in the levels of these neurotransmitters has been implicated in different neurological and psychiatric disorders including MDD. ¹H nuclear magnetic resonance (NMR) spectroscopy is a powerful non-invasive method to study neurometabolites homeostasis in vivo. Additionally, ¹³C-NMR spectroscopy together with an intravenous administration of non-radioactive ¹³C-labeled glucose or acetate provides a measure of neural functions. In this review, we provide an overview of NMR-based measurements of glutamate and GABA homeostasis, neurometabolic activity, and neurotransmitter cycling in MDD. Finally, we highlight the impact of recent advancements in treatment strategies against a depressive disorder that target glutamate and GABA pathways in the brain.

Neurobiological biomarkers of response to ketamine

As a field, psychiatry is undergoing an exciting paradigm shift toward early identification and intervention that will likely minimize both the burden associated with severe mental illnesses as well as their duration. In this context, the rapid-acting antidepressant ketamine has revolutionized our understanding of antidepressant response and greatly expanded the pharmacologic armamentarium for treatment-resistant depression. Efforts to characterize biomarkers of ketamine response support a growing emphasis on early identification, which would allow clinicians to identify biologically enriched subgroups with treatment-resistant depression who are more likely to benefit from ketamine therapy. This chapter presents a broad overview of a range of translational biomarkers, including those drawn from imaging and electrophysiological studies, sleep and circadian rhythms, and HPA axis/endocrine function as well as metabolic, immune, (epi)genetic, and neurotrophic biomarkers related to ketamine response. Ketamine's unique, rapid-acting properties may serve as a model to explore a whole new class of novel rapid-acting treatments with the potential to revolutionize drug development and discovery. However, it should be noted that although several of the biomarkers reviewed here provide promising insights into ketamine's mechanism of action, most studies have focused on acute rather than longer-term antidepressant effects and, at present, none of the biomarkers are ready for clinical use.

Early life stress and glutamate neurotransmission in major depressive disorder

Early life stress (ELS) and glutamate neurotransmission have been implicated in the pathophysiology of major depressive disorder (MDD). In non-human primates, ELS was positively correlated with cortical Glx (i.e., glutamate + glutamine). However, the relationship between ELS and cortical glutamate in adult patients with MDD is not fully known. Using ¹H Magnetic Resonance Spectroscopy (MRS), we conducted exploratory analyses measuring occipital cortical glutamate and glutamine levels in 36 medication-free patients with MDD. In a subsample (n=11), we measured dynamic glutamate/glutamine cycling (V_cycle) using advanced ¹³C MRS methods. ELS history was assessed using Early-life Trauma Inventory (ETI). Exploratory analyses suggest a relationship between ETI and glutamine as reflected by a significant positive correlation between ETI scores and occipital glutamine (r_s=0.39, p=0.017) but not glutamate. Post-hoc analyses showed that the association with glutamine was driven by the ETI emotional abuse (ETI-EA) subscale (r_s=0.39, p=0.02). V_cycle correlation with ETI was at trend level (r_s=0.55, p=0.087) and significantly correlated with ETI-EA (r_s=0.67, p=0.03). In this small sample of patients with MDD, those with childhood emotional abuse appear to have increased occipital glutamate neurotransmission as reflected by increased glutamate/glutamine cycling and glutamine level. Future studies would be needed to confirm this pilot evidence and to examine whether ELS effects on glutamate neurotransmission underlie the relationship between ELS and psychopathology.

The relationship between subjective effects induced by a single dose of ketamine and treatment response in patients with major depressive disorder: A systematic review

OBJECTIVE

The relationship between ketamine's hallucinogenic- and dissociative-type effects and antidepressant mechanism of action is poorly understood. This paper reviewed the correlation between subjective effects defined by various psychometric scales and observed clinical outcomes in the treatment of patients with Major Depressive Disorder (MDD).

METHODS

Based on PRISMA guidelines, we reviewed the dissociative and psychotomimetic mental state induced with ketamine during MDD treatment. Our selected studies correlated depression rating with validated scales collected at regular intervals throughout the study period such as the Clinician-Administered Dissociative States Scale (CADSS), Brief Psychiatric Rating Scale (BPRS), and the 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC). We excluded studies with bipolar depression or with repeated dosing and no single-dose phase. We included 8 of 556 screened reports.

RESULTS

Two of five CADSS studies found significant negative correlations between increases in CADSS scores and depression scores. One of six BPRS studies demonstrated correlations between BPRS scores and depression scores. The 5D-ASC's one study found no correlation with the MADRS.

CONCLUSIONS

Ketamine's dissociative and psychotomimetic effects were correlated with depression changes in 37.5% of studies, but most studies did not examine this relationship and future studies should consider this association since it appears important for MDMA and psilocybin therapies.

Effect of Ketamine on Limbic GABA and Glutamate: A Human In Vivo Multivoxel Magnetic Resonance Spectroscopy Study

INTRODUCTION

Converging evidence suggests that ketamine elicits antidepressant effects via enhanced neuroplasticity precipitated by a surge of glutamate and modulation of GABA. Magnetic resonance spectroscopic imaging (MRSI) illustrates changes to cerebral glutamate and GABA immediately following ketamine administration during dissociation. However, few studies assess subacute changes in the first hours following application, when ketamine's antidepressant effects emerge. Moreover, ketamine metabolites implicated in its antidepressant effects develop during this timeframe. Thus, this study aimed to investigate subacute changes in cerebral Glx (glutamate + glutamine), GABA and their ratio in seven brain regions central to depressive pathophysiology and treatment.

METHODS

Twenty-five healthy subjects underwent two multivoxel MRS scans using a spiral encoded, MEGA-edited LASER-localized 3D-MRSI sequence, at baseline and 2 h following intravenous administration of racemic ketamine (0.8 mg/kg bodyweight over 50 min). Ketamine, norketamine and dehydronorketamine plasma levels were determined at routine intervals during and after infusion. Automated region-of-interest (ROI)-based quantification of mean metabolite concentration was used to assess changes in GABA+/total creatine (tCr), Glx/tCr, and GABA+/Glx ratios in the thalamus, hippocampus, insula, putamen, rostral anterior cingulate cortex (ACC), caudal ACC, and posterior cingulate cortex. Effects of ketamine on neurotransmitter levels and association with ketamine- and metabolite plasma levels were tested with repeated measures analyses of variance (rmANOVA) and correlation analyses, respectively.

RESULTS

For GABA+/tCr rmANOVA revealed a measurement by region interaction effect (p_uncorr < 0.001) and post hoc pairwise comparisons showed a reduction in hippocampal GABA+/tCr after ketamine (p_corr = 0.02). For Glx/tCr and GABA+/Glx neither main effects of measurement nor measurement by region interactions were observed (all p_uncorr > 0.05). Furthermore, no statistically significant associations between changes in any of the neurotransmitter ratios and plasma levels of ketamine, norketamine, or dehydronorketamine were observed (p_corr > 0.05).

CONCLUSION

This study provides evidence for decreased hippocampal GABA+/tCr ratio 2 h following ketamine administration. As MRS methodology measures total levels of intra- and extracellular GABA, results might indicate drug induced alterations in GABA turnover. Our study in healthy humans suggests that changes in GABA levels, particularly in the hippocampus, should be further assessed for their relevance to ketamine´s antidepressant effects.

Increasing doses of ketamine curtail antidepressant responses and suppress associated synaptic signaling pathways

Clinical findings show that a single subanesthetic dose of ketamine elicits rapid antidepressant effects. Accumulating data suggests that ketamine blocks the N-methyl-D-aspartate receptor and results in specific effects on intracellular signaling including increased brain-derived neurotrophic factor (BDNF) protein expression, which augments synaptic responses required for rapid antidepressant effects. To further investigate this potential mechanism for ketamine's antidepressant action, we examined the effect of increasing ketamine doses on intracellular signaling, synaptic plasticity, and rapid antidepressant effects. Given that ketamine is often used at 2.5-10 mg/kg to examine antidepressant effects and 20-50 mg/kg to model schizophrenia, we compared effects at 5, 20 and 50 mg/kg. We found that intraperitoneal (i.p.) injection of low dose (5 mg/kg) ketamine produces rapid antidepressant effects, which were not observed at 20 or 50 mg/kg. At 5 mg/kg ketamine significantly increased the level of BDNF, a protein necessary for the rapid antidepressant effects, while 20 and 50 mg/kg ketamine did not alter BDNF levels in the hippocampus. Low concentration ketamine also evoked the highest synaptic potentiation in the hippocampal CA1, while higher concentrations significantly decreased the synaptic effects. Our results suggest low dose ketamine produces antidepressant effects and has independent behavioral and synaptic effects compared to higher doses of ketamine that are used to model schizophrenia. These findings strengthen our knowledge on specific signaling associated with ketamine's rapid antidepressant effects.

Dorsolateral prefrontal γ-aminobutyric acid in patients with treatment-resistant depression after transcranial magnetic stimulation measured with magnetic resonance spectroscopy

BACKGROUND

The therapeutic mechanism of repetitive transcranial magnetic stimulation (rTMS) for treatment-resistant depression (TRD) may involve modulation of γ-aminobutyric acid (GABA) levels. We used proton magnetic resonance spectroscopy (MRS) to assess changes in GABA levels at the site of rTMS in the left dorsolateral prefrontal cortex (DLPFC).

METHODS

In 26 adults with TRD, we used Mescher–Garwood point-resolved spectroscopy (MEGA-PRESS) spectral-editing MRS to measure GABA in the left DLPFC before and after standard clinical treatment with rTMS. All participants but 1 were medicated, including 12 patients on GABA agonist agents.

RESULTS

Mean GABA in the DLPFC increased 10.0% (p = 0.017) post-rTMS in the overall sample. As well, GABA increased significantly in rTMS responders (n = 12; 23.6%, p = 0.015) but not in nonresponders (n = 14; 4.1%, p = not significant). Changes in GABA were not significantly affected by GABAergic agonists, but clinical response was less frequent (p = 0.005) and weaker (p = 0.035) in the 12 participants who were receiving GABA agonists concomitant with rTMS treatment.

LIMITATIONS

This study had an open-label design in a population receiving naturalistic treatment.

CONCLUSION

Treatment using rTMS was associated with increases in GABA levels at the stimulation site in the left DLPFC, and the degree of GABA change was related to clinical improvement. Participants receiving concomitant treatment with a GABA agonist were less likely to respond to rTMS. These findings were consistent with earlier studies showing the effects of rTMS on GABA levels and support a GABAergic model of depression.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Ketamine and depression: a narrative review

Depression is the third leading cause of disability in the world. Depressive symptoms may be reduced within several weeks after the start of conventional antidepressants, but treatment resistance concerns one-third of patients who fail to achieve recovery. Over the last 20 years, ketamine, an antagonist of the N-methyl-D-aspartate receptor, has been described to have antidepressant properties. A literature review was conducted through an exhaustive electronic search. It was restricted to Cochrane reviews, meta-analyses, and randomized controlled trials (RCTs) of ketamine for major depressive disorder and/or bipolar disorder. This review included two Cochrane reviews, 14 meta-analyses and 15 trials. Ketamine was studied versus placebo, versus other comparators and as an anesthetic adjuvant before electroconvulsive therapy. In 14 publications, ketamine provided a rapid antidepressant effect with a maximum efficacy reached at 24 hrs. Its effect lasted for 1–2 weeks after infusion, but a longer-term effect is little reported. Ketamine does not seem to improve depressive symptoms at the end of electroconvulsive sessions. Safety and tolerability profiles with ketamine at low single dose are generally good in depressed patients. However, there is a lack of data concerning ketamine with repeated administration at higher doses. The clinical use of ketamine is increasing. Intranasal (S)-ketamine has recently been approved for depression by the Food and Drug Administration. It could be a promising treatment in depressed patients with suicidal ideation. Collectively, the level of proof of efficacy remains low and more RCTs are needed to explore efficacy and safety issues of ketamine in depression.

Ketamine has distinct electrophysiological and behavioral effects in depressed and healthy subjects

Ketamine's mechanism of action was assessed using gamma power from magnetoencephalography (MEG) as a proxy measure for homeostatic balance in 35 unmedicated subjects with major depressive disorder (MDD) and 25 healthy controls enrolled in a double-blind, placebo-controlled, randomized cross-over trial of 0.5 mg/kg ketamine. MDD subjects showed significant improvements in depressive symptoms, and healthy control subjects exhibited modest but significant increases in depressive symptoms for up to 1 day after ketamine administration. Both groups showed increased resting gamma power following ketamine. In MDD subjects, gamma power was not associated with the magnitude of the antidepressant effect. However, baseline gamma power was found to moderate the relationship between post-ketamine gamma power and antidepressant response; specifically, higher post-ketamine gamma power was associated with better response in MDD subjects with lower baseline gamma, with an inverted relationship in MDD subjects with higher baseline gamma. This relationship was observed in multiple regions involved in networks hypothesized to be involved in the pathophysiology of MDD. This finding suggests biological subtypes based on the direction of homeostatic dysregulation and has important implications for inferring ketamine's mechanism of action from studies of healthy controls alone.

Anesthetics: from modes of action to unconsciousness and neurotoxicity

Modern anesthetic compounds and advanced monitoring tools have revolutionized the field of medicine, allowing for complex surgical procedures to occur safely and effectively. Faster induction times and quicker recovery periods of current anesthetic agents have also helped reduce health care costs significantly. Moreover, extensive research has allowed for a better understanding of anesthetic modes of action, thus facilitating the development of more effective and safer compounds. Notwithstanding the realization that anesthetics are a prerequisite to all surgical procedures, evidence is emerging to support the notion that exposure of the developing brain to certain anesthetics may impact future brain development and function. Whereas the data in support of this postulate from human studies is equivocal, the vast majority of animal research strongly suggests that anesthetics are indeed cytotoxic at multiple brain structure and function levels. In this review, we first highlight various modes of anesthetic action and then debate the evidence of harm from both basic science and clinical studies perspectives. We present evidence from animal and human studies vis-à-vis the possible detrimental effects of anesthetic agents on both the young developing and the elderly aging brain while discussing potential ways to mitigate these effects. We hope that this review will, on the one hand, invoke debate vis-à-vis the evidence of anesthetic harm in young children and the elderly, and on the other hand, incentivize the search for better and less toxic anesthetic compounds.

Ketamine-Associated Brain Changes: A Review of the Neuroimaging Literature

Major depressive disorder (MDD) is one of the most prevalent conditions in psychiatry. Patients who do not respond to traditional monoaminergic antidepressant treatments have an especially difficult-to-treat type of MDD termed treatment-resistant depression. Subanesthetic doses of ketamine-a glutamatergic modulator-have shown great promise for rapidly treating patients with the most severe forms of depression. As such, ketamine represents a promising probe for understanding the pathophysiology of depression and treatment response. Through neuroimaging, ketamine's mechanism may be elucidated in humans. Here, we review 47 articles of ketamine's effects as revealed by neuroimaging studies. Some important brain areas emerge, especially the subgenual anterior cingulate cortex. Furthermore, ketamine may decrease the ability to self-monitor, may increase emotional blunting, and may increase activity in reward processing. Further studies are needed, however, to elucidate ketamine's mechanism of antidepressant action.

Impact of midazolam vs. saline on effect size estimates in controlled trials of ketamine as a rapid-acting antidepressant

The goal of this study was to infer the effectiveness of midazolam as a comparator in preserving the blind in ketamine studies for mood disorders through patient-level analyses of efficacy trial outcomes. In this integrative data analysis (k = 9, N = 367 patients with mood disorders), clinical outcomes were compared across four groups: ketamine (midazolam-controlled), ketamine (saline-controlled), midazolam, and saline. Ketamine doses ranged from 0.5 to 0.54 mg/kg and midazolam doses ranged from 0.02 to 0.045 mg/kg. The baseline-to-Day 1 effect size was d = 0.7 (95% CI: 0.4-0.9) for ketamine (midazolam) versus midazolam and d = 1.8 (95% CI: 1.4-2.2) for ketamine (saline) versus saline. The effect of ketamine relative to control was larger in saline-controlled studies than in midazolam-controlled studies (t(276) = 2.32, p = 0.02). This was driven by a comparatively larger effect under midazolam than saline (t(111) = 5.40, p < 0.0001), whereas there was no difference between ketamine (midazolam) versus ketamine (saline) (t(177) = 0.65, p = 0.51). Model-estimated rates of response (with 95% CI) yielded similar results: ketamine (midazolam), 45% (34-56%); ketamine (saline), 46% (34-58%); midazolam, 18% (6-30%); saline, 1% (0-11%). The response rate for ketamine was higher than the control condition for both saline (t(353) = 7.41, p < 0.0001) and midazolam (t(353) = 4.59, p < 0.0001). Studies that used midazolam as a comparator yielded smaller effects of ketamine than those which used saline, which was accounted for by greater improvement following midazolam compared to saline.

Anxiety during ketamine infusions is associated with negative treatment responses in major depressive disorder

About 20 to 30 percent of patients with Major Depressive Disorder (MDD) do not respond to standard treatment and are considered treatment-resistant. The N-methyl-d-aspartate (NMDA) glutamate receptor antagonist ketamine has demonstrated rapid antidepressant effects in treatment-resistant MDD, but it is unknown whether its acute psychological effects are related to the later antidepressant effect. Therefore, we investigated the association between antidepressant responses to ketamine and the quality of ketamine-induced psychological experiences in MDD. A total of 31 patients (M = 49.5 ± 11.2 years, 16 women) were treated with three ketamine infusions per week (0.5 mg/ kg over 40 min) administered for two consecutive weeks. Depression severity was assessed using the Montgomery-Åsberg Depression Rating Scale (MADRS) at baseline, after four and 24 h and at end of treatment. The 5-Dimensional Altered States of Consciousness Rating Scale (5D-ASC) was applied four hours after the first infusion to assess the subjective quality of acute psychological effects. Patients with a ≥ 50% MADRS reduction from baseline to end of treatment were considered as responders. After six infusions, 17 of 31 patients (55%) showed a response to ketamine treatment, while 14 patients (45%) had no response. Anxiety-related experiences induced by ketamine were significantly higher in non-responders. Percentage MADRS reduction after four hours and individual levels of ketamine-induced anxiety were predictive of a response at end of treatment. The study demonstrated the considerable impact of ketamine-induced anxiety on the antidepressant efficacy of ketamine. It underpins the importance of considering patients' subjective experiences and underlines the possibility of a phenotypic response predictor.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Depression as a Neuroendocrine Disorder: Emerging Neuropsychopharmacological Approaches beyond Monoamines

Depression is currently recognized as a crucial problem in everyday clinical practice, in light of ever-increasing rates of prevalence, as well as disability, morbidity, and mortality related to this disorder. Currently available antidepressant drugs are notoriously problematic, with suboptimal remission rates and troubling side-effect profiles. Their mechanisms of action focus on the monoamine hypothesis for depression, which centers on the disruption of serotonergic, noradrenergic, and dopaminergic neurotransmission in the brain. Nevertheless, views on the pathophysiology of depression have evolved notably, and the comprehension of depression as a complex neuroendocrine disorder with important systemic implications has sparked interest in a myriad of novel neuropsychopharmacological approaches. Innovative pharmacological targets beyond monoamines include glutamatergic and GABAergic neurotransmission, brain-derived neurotrophic factor, various endocrine axes, as well as several neurosteroids, neuropeptides, opioids, endocannabinoids and endovanilloids. This review summarizes current knowledge on these pharmacological targets and their potential utility in the clinical management of depression.

Acute low-dose ketamine produces a rapid and robust increase in plasma BDNF without altering brain BDNF concentrations

Peripheral BDNF changes after ketamine administration have been proposed as a biomarker for brain BDNF changes. However, published data are conflicting and come from studies in paired animal groups. This study determined the time course of plasma BDNF concentrations following the administration of a single 10 mg/kg dose of ketamine by different routes of administration in rats. Brain BDNF concentrations in prefrontal cortex, hippocampus and cortex were measured in the same animals. Ketamine administration resulted in a rapid and robust increase in plasma BDNF concentrations that were sustained for 240 min. In contrast, there were no changes in brain BDNF concentrations in prefrontal cortex, hippocampus or cortex and there were no correlations between peripheral and central BDNF concentrations. These data suggest that peripheral BDNF is unlikely to be a useful biomarker of acute central BDNF changes following ketamine.

A comprehensive regional neurochemical theory in depression: a protocol for the systematic review and meta-analysis of 1H-MRS studies in major depressive disorder

BACKGROUND

Magnetic resonance spectroscopy (MRS) is a non-invasive analytical technique that investigates the presence and concentrations of brain metabolites. In the context of major depressive disorder (MDD), MRS has revealed regional biochemical changes in GABA, glutamate, and choline across different brain compartments. Technical and methodological advances in MRS data acquisition, in particular proton-based 1H-MRS, have resulted in a significant increase in the incidence of reports utilizing the technique for psychiatric disorder research and diagnosis. The most recent comprehensive meta-analysis reviewing MRS in MDD stems from 2006. Using contemporary systemic reviews and meta-analysis, the aim is to first test a neurochemical circuit-based theory of depression and then to determine if clinical scores relate to metabolite concentrations before and during treatment.

METHODS

Region-specific metabolite changes in MDD will be assessed by systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). Inclusion criteria will include participant age (18 to 65), English language studies, known regions of interest, and detailed documentation of 1H-MRS procedures. Reported brain regions will be standardized according neuroanatomical expertise allowing increased power of the meta-analysis. Regions of interest will initially include the hippocampus, thalamus, prefrontal cortex, anterior and posterior cingulate gyri, parietal lobe, and basal ganglia. Exclusion criteria will include comorbid psychiatric illness and drug use. Two independent reviewers will undertake all data extraction, while a third reviewer will check for reviewer discrepancies. Statistical analysis will be performed using STATA supplemented by Metan software and SPSS.

DISCUSSION

This data will shed new light on the biochemical basis of depression in different brain regions, thereby highlighting the potential of MRS in identifying biomarkers and generating models of MDD and treatment response.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42018091494.