Ketamine-induced reduction in mGluR5 availability is associated with an antidepressant response: an [11C]ABP688 and PET imaging study in depression

The receptor hypothesis and the pathogenesis of depression: Genetic bases and biological correlates

Depression has become one of the most prevalent neuropsychiatric disorders characterized by anhedonia, anxiety, pessimism, or even suicidal thoughts. Receptor theory has been pointed out to explain the pathogenesis of depression, while it is still subject to debate. Additionally, gene abnormality accounts for nearly 40-50% of depression risk, which is a significant factor contributing to the onset of depression. Accordingly, studying on receptors and their gene abnormality are critical parts of the research on internal causes of depression. This review summarizes the pathogenesis of depression from six of the most related receptors and their associated genes, including N-methyl-D-aspartate receptor, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, glucocorticoid receptor, 5-hydroxytryptamine receptor, GABA_A receptor α2, and dopamine receptor; and several "non-classic" receptors, such as metabotropic glutamate receptor, opioid receptor, and insulin receptor. These receptors have received considerable critical attention and are highly implicated in the onset of depression. We begin by providing the biological mechanisms of action of these receptors on the pathogenesis of depression. Then we review the historical and social context about these receptors. Finally, we discuss the limitations of the current state of knowledge and outline insights on future research directions, aiming to provide more novel targets and theoretical basis for the early prevention, accurate diagnosis and prompt treatment of depression.

Body mass index and variability in hippocampal volume in youth with major depressive disorder

BACKGROUND

The hippocampus has been implicated in major depressive disorder (MDD), in both adults and youth. However, possible sources of variability for the hippocampus have not been well delineated. Here, we explored the relationship between body mass index (BMI) and hippocampal volume in youth with MDD.

METHODS

Twenty-two controls (9 male, 13 female, 12-24 years), 24 youth with MDD and normal BMI (12 male, 12 female, 14-24 years), and 20 youth with MDD and high BMI (14 male, 6 female, 13-22 years) underwent magnetic resonance (MR) imaging and spectroscopy (1H-MRS). Hippocampal volume was determined through manual tracing of high-resolution anatomical T1 scans, and LCModel quantified neurochemical concentrations. Intracranial volume was used as a covariate in analysis to control for effects of brain volume on hippocampus.

RESULTS

In youth with MDD and normal BMI, right hippocampal volume was reduced (p = 0.006, Bonferroni) and a trend for reduced left hippocampal volume was noted when compared to healthy controls (p = 0.054, Bonferroni). Left hippocampal volumes were negatively associated with BMI in youth with MDD and high BMI group (r = -0.593, p = 0.006). No associations were found between the right hippocampus and BMI and there were no group differences for metabolite concentrations.

LIMITATIONS

Larger sample sizes would enable researchers to explore overweight vs obese groups and effect of sex in MDD-BMI groups.

CONCLUSIONS

BMI may account for some of the variability observed in previous studies of hippocampal volume in MDD, and therefore BMI impacts should be considered in future analyses.

Bias evaluation and reduction in 3D OP-OSEM reconstruction in dynamic equilibrium PET studies with 11C-labeled for binding potential analysis

Iterative image reconstruction is widely used in positron emission tomography. However, it is known to contribute to quantitation bias and is particularly pronounced during dynamic studies with ¹¹C-labeled radiotracers where count rates become low towards the end of the acquisition. As the strength of the quantitation bias depends on the counts in the reconstructed frame, it can differ from frame to frame of the acquisition. This is especially relevant in the case of neuro-receptor studies with simultaneous PET/MR when a bolus-infusion protocol is applied to allow the comparison of pre- and post-task effects. Here, count dependent changes in quantitation bias may interfere with task changes. We evaluated the impact of different framing schemes on quantitation bias and its propagation into binding potential (BP) using a phantom decay study with ¹¹C and 3D OP-OSEM. Further, we propose a framing scheme that keeps the true counts per frame constant over the acquisition time as constant framing schemes and conventional increasing framing schemes are unlikely to achieve stable bias values during the acquisition time range. For a constant framing scheme with 5 minutes frames, the BP bias was 7.13±2.01% (10.8% to 3.8%) compared to 5.63±2.85% (7.8% to 4.0%) for conventional increasing framing schemes. Using the proposed constant true counts framing scheme, a stabilization of the BP bias was achieved at 2.56±3.92% (3.5% to 1.7%). The change in BP bias was further studied by evaluating the linear slope during the acquisition time interval. The lowest slope values were observed in the constant true counts framing scheme. The constant true counts framing scheme was effective for BP bias stabilization at relevant activity and time ranges. The mean BP bias under these conditions was 2.56±3.92%, which represents the lower limit for the detection of changes in BP during equilibrium and is especially important in the case of cognitive tasks where the expected changes are low.

Study of influence of the glutamatergic concentration of [18F]FPEB binding to metabotropic glutamate receptor subtype 5 with N-acetylcysteine challenge in rats and SRM/PET study in human healthy volunteers

Altered glutamate signaling is thought to be involved in a myriad of psychiatric disorders. Positron emission tomography (PET) imaging with [¹⁸F]FPEB allows assessing dynamic changes in metabotropic glutamate receptor 5 (mGluR5) availability underlying neuropathological conditions. The influence of endogenous glutamatergic levels into receptor binding has not been well established yet. The purpose of this study was to explore the [¹⁸F]FPEB binding regarding to physiological fluctuations or acute changes of glutamate synaptic concentrations by a translational approach; a PET/MRS imaging study in 12 healthy human volunteers combined to a PET imaging after an N-acetylcysteine (NAc) pharmacological challenge in rodents. No significant differences were observed with small-animal PET in the test and retest conditions on the one hand and the NAc condition on the other hand for any regions. To test for an interaction of mGuR5 density and glutamatergic concentrations in healthy subjects, we correlated the [¹⁸F]FPEB BP_ND with Glu/Cr, Gln/Cr, Glx/Cr ratios in the anterior cingulate cortex VOI; respectively, no significance correlation has been revealed (Glu/Cr: r = 0.51, p = 0.09; Gln/Cr: r = -0.46, p = 0.13; Glx/Cr: r = -0.035, p = 0.92).These data suggest that the in vivo binding of [¹⁸F]FPEB to an allosteric site of the mGluR5 is not modulated by endogenous glutamate in vivo. Thus, [¹⁸F]FPEB appears unable to measure acute fluctuations in endogenous levels of glutamate.

Advances in novel molecular targets for antidepressants

Depression is the most common psychiatric illness affecting numerous people world-wide. The currently available antidepressant treatment presents low response and remission rates. Thus, new effective antidepressants need to be developed or discovered. Aiming to give an overview of novel possible antidepressant drug targets, we summarized the molecular targets of antidepressants and the underlying neurobiology of depression. We have also addressed the multidimensional perspectives on the progress in the psychopharmacological treatment of depression and on the new potential approaches with effective drug discovery.

Ketamine decreases neuronally released glutamate via retrograde stimulation of presynaptic adenosine A1 receptors

Ketamine produces a rapid antidepressant response in patients with major depressive disorder (MDD), but the underlying mechanisms appear multifaceted. One hypothesis, proposes that by antagonizing NMDA receptors on GABAergic interneurons, ketamine disinhibits afferens to glutamatergic principal neurons and increases extracellular glutamate levels. However, ketamine seems also to reduce rapid glutamate release at some synapses. Therefore, clinical studies in MDD patients have stressed the need to identify mechanisms whereby ketamine decreases presynaptic activity and glutamate release. In the present study, the effect of ketamine and its antidepressant metabolite, (2R,6R)-HNK, on neuronally derived glutamate release was examined in rodents. We used FAST methodology to measure depolarization-evoked extracellular glutamate levels in vivo in freely moving or anesthetized animals, synaptosomes to detect synaptic recycling ex vivo and primary cortical neurons to perform functional imaging and to examine intracellular signaling in vitro. In all these versatile approaches, ketamine and (2R,6R)-HNK reduced glutamate release in a manner which could be blocked by AMPA receptor antagonism. Antagonism of adenosine A1 receptors, which are almost exclusively expressed at nerve terminals, also counteracted ketamine's effect on glutamate release and presynaptic activity. Signal transduction studies in primary neuronal cultures demonstrated that ketamine reduced P-T286-CamKII and P-S9-Synapsin, which correlated with decreased synaptic vesicle recycling. Moreover, systemic administration of A1R antagonist counteracted the antidepressant-like actions of ketamine and (2R,6R)-HNK in the forced swim test. To conclude, by studying neuronally released glutamate, we identified a novel retrograde adenosinergic feedback mechanism that mediate inhibitory actions of ketamine on glutamate release that may contribute to its rapid antidepressant action.

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.

Measuring the effects of ketamine on mGluR5 using [18F]FPEB and PET

The metabotropic glutamate receptor 5 (mGluR5) is a promising treatment target for psychiatric disorders due to its modulatory effects on glutamate transmission. Using [11C]ABP688, we previously showed that the rapidly acting antidepressant ketamine decreases mGluR5 availability. The mGluR5 radioligand [18F]FPEB offers key advantages over [11C]ABP688; however, its suitability for drug challenge studies is unknown. We evaluated whether [18F]FPEB can be used to capture ketamine-induced effects on mGluR5. Seven healthy subjects participated in three [18F]FPEB scans: a baseline, a same-day post-ketamine, and a 24-h post-ketamine scan. The outcome measure was VT/fP, obtained using a two-tissue compartment model and a metabolite-corrected arterial input function. Dissociative symptoms, heart rate and blood pressure increased following ketamine infusion. [18F]FPEB VT/fP decreased by 9% across the cortex after ketamine infusion, with minimal difference between baseline and 24-h scans. Compared to our previous work using [11C]ABP688, the magnitude of the ketamine-induced change in mGluR5 was smaller using [18F]FPEB; however, effect sizes were similar for the same-day post-ketamine vs. baseline scan (Cohen's d = 0.75 for [18F]FPEB and 0.88 for [11C]ABP688). [18F]FPEB is therefore able to capture some of the effects of ketamine on mGluR5, but [11C]ABP688 appears to be more suitable in drug challenge paradigms designed to probe glutamate transmission.

A Review of Molecular Imaging of Glutamate Receptors

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Systemic neuro-dysregulation in depression: Evidence from genome-wide association

Depression is the world's leading cause of disability. Greater understanding of the neurobiological basis of depression is necessary for developing novel treatments with improved efficacy and acceptance. Recently, major advances have been made in the search for genetic variants associated with depression which may help to elucidate etiological mechanisms. The present review has two major objectives. First, we offer a brief review of two major biological systems with strong evidence for involvement in depression pathology: neurotransmitter systems and the stress response. Secondly, we provide a synthesis of the functions of the 269 genes implicated by the most recent genome-wide meta-analysis, supporting the importance of these systems in depression and providing insights into other possible mechanisms involving neurodevelopment, neurogenesis, and neurodegeneration. Our goal is to undertake a broad, preliminary stock-taking of the most recent hypothesis-free findings and examine the weight of the evidence supporting these existing theories and highlighting novel directions. This qualitative review and accompanying gene function table provides a valuable resource and guide for basic and translational researchers, with suggestions for future mechanistic research, leveraging genetics to prioritize studies on the neurobiological processes involved in depression etiology and treatment.

Assessment of Relationship of Ketamine Dose With Magnetic Resonance Spectroscopy of Glx and GABA Responses in Adults With Major Depression: A Randomized Clinical Trial

IMPORTANCE

A single subanesthetic dose of ketamine produces an antidepressant response in patients with major depressive disorder (MDD) within hours, but the mechanism of antidepressant effect is uncertain.

OBJECTIVE

To evaluate whether ketamine dose and brain glutamate and glutamine (Glx) and γ-aminobutyric acid (GABA) level responses to ketamine are related to antidepressant benefit and adverse effects.

DESIGN, SETTING, AND PARTICIPANTS

This randomized, parallel-group, triple-masked clinical trial included 38 physically healthy, psychotropic medication-free adult outpatients who were in a major depressive episode of MDD but not actively suicidal. The trial was conducted at Columbia University Medical Center. Data were collected from February 2012 to May 2015. Data analysis was conducted from January to March 2020.

INTERVENTION

Participants received 1 dose of placebo or ketamine (0.1, 0.2, 0.3, 0.4, or 0.5 mg/kg) intravenously during 40 minutes of a proton magnetic resonance spectroscopy scan that measured ventro-medial prefrontal cortex Glx and GABA levels in 13-minute data frames.

MAIN OUTCOMES AND MEASURES

Clinical improvement was measured using a 22-item version of the Hamilton Depression Rating Scale (HDRS-22) 24 hours after ketamine was administered. Ketamine and metabolite blood levels were measured after the scan.

RESULTS

A total of 38 individuals participated in the study, with a mean (SD) age of 38.6 (11.2) years, 23 (60.5%) women, and 25 (65.8%) White patients. Improvement in HDRS-22 score at 24 hours correlated positively with ketamine dose (t36 = 2.81; P = .008; slope estimate, 19.80 [95% CI, 5.49 to 34.11]) and blood level (t36 = 2.25; P = .03; slope estimate, 0.070 [95% CI, 0.007 to 0.133]). The lower the Glx response, the better the antidepressant response (t33 = -2.400; P = .02; slope estimate, -9.85 [95% CI, -18.2 to -1.50]). Although GABA levels correlated with Glx (t33 = 8.117; P < .001; slope estimate, 0.510 [95% CI, 0.382 to 0.638]), GABA response did not correlate with antidepressant effect. When both ketamine dose and Glx response were included in a mediation analysis model, ketamine dose was no longer associated with antidepressant effect, indicating that Glx response mediated the relationship. Adverse effects were related to blood levels in men only (t5 = 2.606; P = .048; estimated slope, 0.093 [95% CI, 0.001 to 0.186]), but Glx and GABA response were not related to adverse effects.

CONCLUSIONS AND RELEVANCE

In this study, intravenous ketamine dose and blood levels correlated positively with antidepressant response. The Glx response correlated inversely with ketamine dose and with antidepressant effect. Future studies are needed to determine whether the relationship between Glx level and antidepressant effect is due to glutamate or glutamine.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT01558063.

Immune dysregulation in depression: Evidence from genome-wide association

A strong body of evidence supports a role for immune dysregulation across many psychiatric disorders including depression, the leading cause of global disability. Recent progress in the search for genetic variants associated with depression provides the opportunity to strengthen our current understanding of etiological factors contributing to depression and generate novel hypotheses. Here, we provide an overview of the literature demonstrating a role for immune dysregulation in depression, followed by a detailed discussion of the immune-related genes identified by the most recent genome-wide meta-analysis of depression. These genes represent strong evidence-based targets for future basic and translational research which aims to understand the role of the immune system in depression pathology and identify novel points for therapeutic intervention.

mGluR5 mediates ketamine antidepressant response in susceptible rats exposed to prenatal stress

BACKGROUND

New insights have recently been gained into ketamine's potential anti-depressive effects. However, the mechanisms that underlie ketamine's rapid antidepressant activity still remain a mystery.

METHODS

We used a rat prenatal stress (PS) model of depression to explore the functional role of mGluR5 in ketamine's rapidly induced antidepressant activity. Effects of the antidepressants imipramine, escitalopram, ketamine, and fluoxetine were compared. AAV-mGluR5 and AAV-shRNA-mGluR5 were constructed to overexpress and knockdown hippocampal mGluR5 respectively.

RESULTS

This study shows that mGluR5, which is associated with depression-like behaviors, is increased in susceptible rats exposed to prenatal stress, and that ketamine could significantly alleviate these stress-induced effects. RU-38486 down-regulated expression of mGluR5 and up-regulated NR1. MPEP and CHPG also altered expression of both mGluR5 and NR1. Notably, hippocampal overexpression of mGluR5 in wild type rats changed NR1 and PSD-95 expression and induced depression-like behavior that could be blocked by ketamine activity. Further, knockdown of hippocampal mGluR5 in PS-S rats restored normal levels of mGluR5, NR1, and PSD-95, and alleviated depression-like behavior.

LIMITATIONS

The entire rat hippocampus was used for this study, but the role of mGluR5 may vary by sub-region.

CONCLUSION

These results suggest that hippocampal mGluR5 may play a key role in mediating the rapid antidepressant effects of ketamine in a prenatal stress model of depression. This provides a novel therapeutic target in clinical treatment of depression.

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.

Presynaptic dopamine deficit in minimally conscious state patients following traumatic brain injury

Dopaminergic stimulation has been proposed as a treatment strategy for post-traumatic brain injured patients in minimally conscious state based on a clinical trial using amantadine, a weak dopamine transporter blocker. However, a specific contribution of dopaminergic neuromodulation in minimally conscious state is undemonstrated. In a phase 0 clinical trial, we evaluated 13 normal volunteers and seven post-traumatic minimally conscious state patients using 11C-raclopride PET to estimate dopamine 2-like receptors occupancy in the striatum and central thalamus before and after dopamine transporter blockade with dextroamphetamine. If a presynaptic deficit was observed, a third and a fourth 11C-raclopride PET were acquired to evaluate changes in dopamine release induced by l-DOPA and l-DOPA+dextroamphetamine. Permutation analysis showed a significant reduction of dopamine release in patients, demonstrating a presynaptic deficit in the striatum and central thalamus that could not be reversed by blocking the dopamine transporter. However, administration of the dopamine precursor l-DOPA reversed the presynaptic deficit by restoring the biosynthesis of dopamine from both ventral tegmentum and substantia nigra. The advantages of alternative pharmacodynamic approaches in post-traumatic minimally conscious state patients should be tested in clinical trials, as patients currently refractory to amantadine might benefit from them.

Characterization of [11C]PXT012253 as a PET Radioligand for mGlu4 Allosteric Modulators in Nonhuman Primates

PURPOSE

Modulation of presynaptic metabotropic glutamate receptor 4 (mGlu₄) by an allosteric ligand has been proposed as a promising therapeutic target in Parkinson's disease and levodopa-induced dyskinesia. A positron emission tomography (PET) ligand for an allosteric site of mGlu₄ may provide evidence that a clinical drug candidate reaches and binds the target. A carbon-11-labeled PET radioligand binding an allosteric site of mGlu₄, [¹¹C]PXT012253, has been recently developed. Here, we describe the detailed characterization of this novel radiolabeled mGlu₄ ligand in nonhuman primates.

PROCEDURES

[¹¹C]PXT012253 binding in the brain of cynomolgus monkeys, under the baseline and blocking conditions with the structurally different mGlu₄ allosteric ligand PXT002331, currently in clinical trials for Parkinson's disease, was quantified with compartment and graphical modeling approaches using a radiometabolite-corrected plasma input function. Whole-body biodistribution of [¹¹C]PXT012253 was then assessed using PET/x-ray computed tomography to estimate the human effective doses of [¹¹C]PXT012253 for further clinical studies.

RESULTS

[¹¹C]PXT012253 displayed binding in mGlu₄-expressing regions in the brain of cynomolgus monkeys. Brain regional time-activity curves of [¹¹C]PXT012253 were well described in the two-tissue compartment model (2TC). Total distribution volume was stably estimated using Logan plot and multilinear analysis (MA1) although 2TC showed unstable values in some cases. Competition with PXT002331 showed high specific binding in the total distribution volume. Whole-body PET showed high accumulation of [¹¹C]PXT012253 in the liver, kidney, heart, and brain in the initial phase. The radioligand was excreted through both the gastrointestinal and the urinary tracts. Effective dose of [¹¹C]PXT012253 was estimated to be 0.0042 mSv/MBq.

CONCLUSIONS

[¹¹C]PXT012253 was shown to be a promising PET radioligand for mGlu₄ allosteric modulators in the monkey brain. MA1 would be the choice of quantitative method. Further development of [¹¹C]PXT012253 in human subjects is warranted.

Metabotropic glutamate receptor 5 in bulimia nervosa

Bulimia nervosa (BN) shares central features with substance-related and addictive disorders. The metabotropic glutamate receptor subtype 5 (mGlu5) plays an important role in addiction. Based on similarities between binge eating and substance-related and addictive disorders, we investigated mGlu5 in vivo in 15 female subjects with BN and 15 matched controls. We measured mGlu5 distribution volume ratio (DVR) with positron emission tomography (PET) using [11 C]ABP688. In BN mGlu5 DVR was higher in the anterior cingulate cortex (ACC), subgenual prefrontal cortex, and straight gyrus (p < 0.05). In BN, higher mGlu5 DVR in various brain regions, including ACC, pallidum, putamen, and caudate, positively correlated with "maturity fears" as assessed using the Eating Disorder Inventory-2 (p < 0.05). In BN and controls, smokers had globally decreased mGlu5 DVR. We present the first evidence for increased mGlu5 DVR in BN. Our findings suggest that pharmacological agents inhibiting mGlu5 might have a therapeutic potential in BN.

Neuroimaging Biomarkers for Predicting Treatment Response and Recurrence of Major Depressive Disorder

The acute treatment duration for major depressive disorder (MDD) is 8 weeks or more. Treatment of patients with MDD without predictors of treatment response and future recurrence presents challenges and clinical problems to patients and physicians. Recently, many neuroimaging studies have been published on biomarkers for treatment response and recurrence of MDD using various methods such as brain volumetric magnetic resonance imaging (MRI), functional MRI (resting-state and affective tasks), diffusion tensor imaging, magnetic resonance spectroscopy, near-infrared spectroscopy, and molecular imaging (i.e., positron emission tomography and single photon emission computed tomography). The results have been inconsistent, and we hypothesize that this could be due to small sample size; different study design, including eligibility criteria; and differences in the imaging and analysis techniques. In the future, we suggest a more sophisticated research design, larger sample size, and a more comprehensive integration including genetics to establish biomarkers for the prediction of treatment response and recurrence of MDD.

Targeting Homer1a for Rapid Antidepressant Effects

In this issue of Neuron, Holz et al. (2019) show that the synaptic protein Homer1a switches mGluR5 signaling to increase AMPA receptor activity for the rapid antidepressant actions of sleep deprivation.

mGluR5 receptor availability is associated with lower levels of negative symptoms and better cognition in male patients with chronic schizophrenia

Consistent findings postulate disturbed glutamatergic function (more specifically a hypofunction of the ionotropic NMDA receptors) as an important pathophysiologic mechanism in schizophrenia. However, the role of the metabotropic glutamatergic receptors type 5 (mGluR5) in this disease remains unclear. In this study, we investigated their significance (using [¹¹C]ABP688) for psychopathology and cognition in male patients with chronic schizophrenia and healthy controls. In the patient group, lower mGluR5 binding potential (BP_ND) values in the left temporal cortex and caudate were associated with higher general symptom levels (negative and depressive symptoms), lower levels of global functioning and worse cognitive performance. At the same time, in both groups, mGluR5 BP_ND were significantly lower in smokers (F_[27,1] = 15.500; p = .001), but without significant differences between the groups. Our findings provide support for the concept that the impaired function of mGluR5 underlies the symptoms of schizophrenia. They further supply a new perspective on the complex relationship between tobacco addiction and schizophrenia by identifying glutamatergic neurotransmission—in particularly mGluR5—as a possible connection to a shared vulnerability.

Default mode network connectivity change corresponds to ketamine's delayed glutamatergic effects

Ketamine exerts rapid antidepressant effects peaking 24 h after a single infusion, which have been suggested to be reflected by both reduced functional connectivity (FC) within default mode network (DMN) and altered glutamatergic levels in the perigenual anterior cingulate cortex (pgACC) at 24 h. Understanding the interrelation and time point specificity of ketamine-induced changes of brain circuitry and metabolism is thus key to future therapeutic developments. We investigated the correlation of late glutamatergic changes with FC changes seeded from the posterior cingulate cortex (PCC) and tested the prediction of the latter by acute fractional amplitude of low-frequency fluctuations (fALFF). In a double-blind, randomized, placebo-controlled study of 61 healthy subjects, we compared effects of subanesthetic ketamine infusion (0.5 mg/kg over 40 min) on resting-state fMRI and MR-Spectroscopy at 7 T 1 h and 24 h post-infusion. FC decrease between PCC and dorsomedial prefrontal cortex (dmPFC) was found at 24 h post-infusion (but not 1 h) and this FC decrease correlated with glutamatergic changes at 24 h in pgACC. Acute increase in fALFF was found in ventral PCC at 1 h which was not observed at 24 h and inversely correlated with the reduced dPCC FC towards the dmPFC at 24 h. The correlation of metabolic and functional markers of delayed ketamine effects and their temporal specificity suggest a potential mechanistic relationship between glutamatergic modulation and reconfiguration of brain regions belonging to the DMN.

In vivo metabotropic glutamate receptor 5 availability-associated functional connectivity alterations in drug-naïve young adults with major depression

There has been increasing interest in glutamatergic neurotransmission as a putative underlying mechanism of depressive disorders. We performed [¹¹C]ABP688 positron emission tomography (PET) and resting-state functional magnetic resonance imaging (rs-fMRI) in drug-naïve young adult patients with major depression to examine alterations in metabotropic glutamate receptor-5 (mGluR5) availability, and to investigate their functional significance relating to neural systems-level changes in major depression. Sixteen psychotropic drug-naïve patients with major depression without comorbidity (median age: 22.8 years) and fifteen matched healthy controls underwent [¹¹C]ABP688 PET imaging and 3-T MRI. For mGluR5 availability, we quantified [¹¹C]ABP688 binding potential (BP_ND) using the simplified reference tissue model. Seed-based functional connectivity analysis was performed using rs-fMRI data with regions derived from quantitative [¹¹C]ABP688 PET analysis as seeds. In region-of-interest (ROI)-based and voxel-based analyses, the [¹¹C]ABP688 BP_ND was significantly lower in patients than in controls in the prefrontal cortex ROI and in voxel clusters within the prefrontal, temporal, and parietal cortices, and supramarginal gyrus. The [¹¹C]ABP688 BP_ND seed-based functional connectivity analysis showed significantly less negative connectivity from the inferior parietal cortex seed to the fusiform gyrus and inferior occipital cortex in patients than in controls. The correlation patterns between [¹¹C]ABP688 BP_ND and functional connectivity strength (β) for the superior prefrontal cortex seed were opposite in the depression and control groups. In conclusion, using a novel approach combining [¹¹C]ABP688 PET and rs-fMRI analyses, our study provides a first evidence of lower mGluR5 availability and related functional connectivity alterations in drug-naïve young adults with major depression without comorbidity.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Radiosynthesis of the diastereomerically pure (E)-[11 C]ABP688

We report an efficient protocol for the radiosynthesis of diastereomerically pure (E)-[¹¹ C]ABP688, a positron emission tomography (PET) tracer for metabotropic glutamate type 5 (mGlu5) receptor imaging. The protocol reliably provides sterile and pyrogen-free formulation of (E)-[¹¹ C]ABP688 suitable for preclinical and clinical PET imaging with >99% diastereomeric excess (d.e.), >99% overall radiochemical purity (RCP), 14.9 ± 4.3% decay-corrected radiochemical yield (RCY), and 148.86 ± 79.8 GBq/μmol molar activity in 40 minutes from the end of bombardment.

Metabotropic Glutamate Receptor Subtype 5 in Alcohol-Induced Negative Affect

Allosteric modulators of metabotropic glutamate 5 receptors (mGlu5 receptors) have been identified as a promising treatment to independently alleviate both negative affective states and ethanol-seeking and intake. However, these conditions are often comorbid and might precipitate one another. Acute and protracted ethanol withdrawal can lead to negative affective states. In turn, these states are primary drivers of alcohol relapse, particularly among women. The current review synthesizes preclinical studies that have observed the role of mGlu5 receptor modulation in negative affective states following ethanol exposure. The primary behavioral assays discussed are ethanol-seeking and intake, development and extinction of ethanol-associated cues and contexts, behavioral despair, and anxiety-like activity. The work done to-date supports mGlu5 receptor modulation as a promising target for mediating negative affective states to reduce ethanol intake or prevent relapse. Limitations in interpreting these data include the lack of models that use alcohol-dependent animals, limited use of adolescent and female subjects, and a lack of comprehensive evaluations of negative affective-like behavior.

In vivo evidence for dysregulation of mGluR5 as a biomarker of suicidal ideation

Recent evidence implicates dysregulation of metabotropic glutamatergic receptor 5 (mGluR5) in pathophysiology of PTSD and suicidality. Using positron emission tomography and [18F]FPEB, we quantified mGluR5 availability in vivo in individuals with PTSD (n = 29) and MDD (n = 29) as a function of suicidal ideation (SI) to compare with that of healthy comparison controls (HC; n = 29). Volume of distribution was computed using a venous input function in the five key frontal and limbic brain regions. We observed significantly higher mGluR5 availability in PTSD compared with HC individuals in all regions of interest (P's = 0.001-0.01) and compared with MDD individuals in three regions (P's = 0.007). mGluR5 availability was not significantly different between MDD and HC individuals (P = 0.17). Importantly, we observed an up-regulation in mGluR5 availability in the PTSD-SI group (P's = 0.001-0.007) compared with PTSD individuals without SI. Findings point to the potential role for mGluR5 as a target for intervention and, potentially, suicide risk management in PTSD.

Effects of restraint stress on the regulation of hippocampal glutamate receptor and inflammation genes in female C57BL/6 and BALB/c mice

The two strains of inbred mice, BALB/c and C57BL/6, are widely used in pre-clinical psychiatry research due to their differences in stress susceptibility. Gene profiling studies in these strains have implicated the inflammation pathway as the main contributor to these differences. We focused our attention on female mice and tested their response to 5- or 10-day exposure to restraint stress. We examined the stress induced changes in the regulation of 11 inflammatory cytokine genes and 12 glutamate receptor genes in the hippocampus of female BALB/c and C57BL/6 mice using quantitative PCR. Elevated proinflammatory cytokine genes include Tumor Necrosis Factor alpha (TNFa), nuclear factor kappa-light-chain-enhancer of activated B cells (NFKB), Interleukin 1 alpha (IL1a), Interleukin 1 receptor (IL1R), Interleukin 10 receptor alpha subunit (IL10Ra), Interleukin 10 receptor beta subunit (IL10Rb), and tumor necrosis factor (TNF) super family members. Our results show that BALB/c and C57BL/6 mice differ in the genes induced in response to stress exposure and the level of gene regulation change. Our results show that the gene regulation in female BALB/c and C57BL/6 mice differs between strains in the genes regulated and the magnitude of the changes.

Targeting of Metabotropic Glutamate Receptors for the Development of Novel Antidepressants

Since discovering that ketamine has robust antidepressant effects, the glutamatergic system has been proposed as an attractive target for the development of novel antidepressants. Among the glutamatergic system, metabotropic glutamate (mGlu) receptors are of interest because mGlu receptors play modulatory roles in glutamatergic transmission, consequently, agents acting on mGlu receptors might not exert the adverse effects associated with ketamine. mGlu receptors have eight subtypes that are classified into three groups, and the roles of each mGlu receptor subtype in depression are being investigated. To date, the potential use of mGlu5 receptor antagonists and mGlu2/3 receptor antagonists as antidepressants has been actively investigated, and the mechanisms underlying these antidepressant effects are being delineated. Although the outcomes of clinical trials using an mGlu5 receptor negative allosteric modulator and an mGlu2/3 receptor negative allosteric modulator have not been encouraging, these trials have been inconclusive, and additional trials using other compounds with more appropriate profiles are needed. In contrast, the roles of group III mGlu receptors have not yet been fully elucidated because of a lack of suitable pharmacological tools. Nonetheless, investigations of the use of mGlu4 and mGlu7 receptors as drug targets for the development of antidepressants have been ongoing, and some interesting evidence has been obtained.

Molecular Imaging of mGluR5 Availability with [11C]ABP68 in Glutaminase Heterozygous Mice

Many PET tracers enable determination of fluctuations in neurotransmitter release, yet glutamate specifically can not be visualized in a noninvasive manner. Several studies point to the possibility of visualizing fluctuations in glutamate release by changes in affinity of the mGluR5 radioligand [11C]ABP688. These studies use pharmacological challenges to alter glutamate levels, and so probe release, but have not measured chronic alterations in receptor occupancy due to altered neurotransmission relevant to chronic neuropsychiatric disorders or their treatment. In this regard, the GLS1 heterozygous mouse has known reductions in activity of the glutamate-synthetic enzyme glutaminase, brain glutamate levels and release. We imaged this model to elucidate glutamatergic systems. Dynamic [11C]ABP688 microPET scans were performed for mGluR5. Western blot was used as an ex vivo validation. No significant differences were found in BPND between WT and GLS1 Hets. SPM showed voxel-wise increased in BPND in GLS1 Hets compared to WT consistent with lower synaptic glutamate. This was not due to alterations in mGluR5 levels, as western blot results showed lower mGluR5 levels in GLS1 Hets. We conclude that because of the chronic glutaminase deficiency and subsequent decrease in glutamate, the mGluR5 protein levels are lowered. Due to these decreased endogenous glutamate levels, however, there is increased [11C]ABP688 binding to the allosteric site in selected regions. We speculate that lower endogenous glutamate leads to less conformational change to the receptors, and thus higher availability of the binding site. The lower mGluR5 levels, however, lessen [11C]ABP688 binding in GLS1 Hets, in part masking the increase in binding due to diminished endogenous glutamate levels as confirmed with voxel-wise analysis.

Acute Ketamine Infusion in Rat Does Not Affect In Vivo [11C]ABP688 Binding to Metabotropic Glutamate Receptor Subtype 5

Detecting changes in metabotropic glutamate receptor 5 (mGluR5) availability through molecular imaging with the positron emission tomography (PET) tracer [¹¹C]ABP688 is valuable for studying dysfunctional glutamate transmission associated with neuropsychiatric disorders. Using an infusion protocol in rats, we visualized the acute effect of subanesthetic doses of ketamine on mGluR5 in rat brain. Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist known to increase glutamate release. Imaging was performed with a high-affinity PET ligand [¹¹C]ABP688, a negative allosteric modulator of mGluR5. Binding did not change significantly from baseline to ketamine in any region, thereby confirming previous literature with other NMDA receptor antagonists in rodents. Hence, in rats, we could not reproduce the findings in a human setup showing significant decreases in the [¹¹C]ABP688 binding after a ketamine bolus followed by ketamine infusion. Species differences may have contributed to the different findings in the present study of rats. In conclusion, we could not confirm in rats that endogenous glutamate increases by ketamine infusion are reflected in [¹¹C]ABP688 binding decreases as was previously shown for humans.

Sex differences in [11C]ABP688 binding: a positron emission tomography study of mGlu5 receptors

Purpose

The purpose of this study was to assess, in a large sample of healthy young adults, sex differences in the binding potential of [¹¹C]ABP688, a positron emission tomography (PET) tracer selective for the metabotropic glutamate type 5 (mGlu5) receptor.

Methods

High resolution [¹¹C]ABP688 PET scans were acquired in 74 healthy volunteers (25 male, 49 female, mean age 20 ± 3.0). Mean binding potential (BP_ND = f_ND * (B_avail / K_D)) values were calculated in the prefrontal cortex, striatum, and limbic regions using the simplified reference tissue model with cerebellar grey matter as the reference region.

Results

[¹¹C]ABP688 BP_ND was significantly higher in men compared to women in the prefrontal cortex (p < 0.01), striatum (p < 0.001), and hippocampus (p < 0.05). Whole-brain BP_ND was 17% higher in men. BP_ND was not related to menstrual phase in women.

Conclusions

Binding availability of mGlu5 receptors as measured by PET [¹¹C]ABP688 is higher in healthy men than women. This likely represents a source of variability in [¹¹C]ABP688 studies and could have relevance for sex differences in cognitive-behavioral functions and neuropsychiatric disorders.

Effect of (Z)-isomer content on [11C]ABP688 binding potential in humans

PURPOSE

To determine how the low-affinity (Z)-isomer of the radiotracer [¹¹C]ABP688 affects binding potential values in vivo in humans.

METHODS

High-resolution [¹¹C]ABP688 PET scans were acquired on 74 healthy volunteers (25 male, 49 female, mean age 20 ± 3.0). The relative contents of (E)- and (Z)-isomers were determined prior to injection using analytical high-performance liquid chromatography [r_t(E) = 10 min, r_t(Z) = 8.5 min]. Mean binding potential [BP_ND = f_ND * (B_avail/K_D)] values were calculated in the striatum, limbic regions, and prefrontal cortex using the simplified reference tissue model with cerebellar grey matter as reference.

RESULTS

Mean ± SD (E)-isomer content in [¹¹C]ABP688 production was 92 ± 3.8% (range 78-97%). Percent (E)-isomer was positively correlated with BP_ND in the striatum (ρ = 0.28, p = 0.015) and limbic regions (ρ = 0.25, p = 0.036). In multiple regression analysis, sex (β = 0.39, p = 0.001) and (E)-isomer content (β = 0.23, p = 0.040) were significant predictors of BP_ND.

CONCLUSIONS

Even modest levels of (Z)-[¹¹C]ABP688 can reduce estimates of tracer binding in vivo. Future studies should use production methods that enrich levels of (E)-[¹¹C]ABP688, report tracer isomer ratios, and account for this factor in their analyses.

Glutamatergic neurometabolite levels in major depressive disorder: a systematic review and meta-analysis of proton magnetic resonance spectroscopy studies

Alterations in glutamatergic neurotransmission are implicated in the pathophysiology of depression, and the glutamatergic system represents a treatment target for depression. To summarize the nature of glutamatergic alterations in patients with depression, we conducted a meta-analysis of proton magnetic resonance (¹H-MRS) spectroscopy studies examining levels of glutamate. We used the search terms: depress* AND (MRS OR "magnetic resonance spectroscopy"). The search was performed with MEDLINE, Embase, and PsycINFO. The inclusion criteria were ¹H-MRS studies comparing levels of glutamate + glutamine (Glx), glutamate, or glutamine between patients with depression and healthy controls. Standardized mean differences (SMD) were calculated to assess group differences in the levels of glutamatergic neurometabolites. Forty-nine studies met the eligibility criteria, which included 1180 patients and 1066 healthy controls. There were significant decreases in Glx within the medial frontal cortex (SMD = -0.38; 95% CI, -0.69 to -0.07) in patients with depression compared with controls. Subanalyses revealed that there was a significant decrease in Glx in the medial frontal cortex in medicated patients with depression (SMD = -0.50; 95% CI, -0.80 to -0.20), but not in unmedicated patients (SMD = -0.27; 95% CI, -0.76 to 0.21) compared with controls. Overall, decreased levels of glutamatergic metabolites in the medial frontal cortex are linked with the pathophysiology of depression. These findings are in line with the hypothesis that depression may be associated with abnormal glutamatergic neurotransmission.

Exploratory genome-wide association analysis of response to ketamine and a polygenic analysis of response to scopolamine in depression

Growing evidence suggests that the glutamatergic modulator ketamine has rapid antidepressant effects in treatment-resistant depressed subjects. The anticholinergic agent scopolamine has also shown promise as a rapid-acting antidepressant. This study applied genome-wide markers to investigate the role of genetic variants in predicting acute antidepressant response to both agents. The ketamine-treated sample included 157 unrelated European subjects with major depressive disorder (MDD) or bipolar disorder (BD). The scopolamine-treated sample comprised 37 unrelated European subjects diagnosed with either MDD or BD who had a current Major Depressive Episode (MDE), and had failed at least two adequate treatment trials for depression. Change in Montgomery-Asberg Depression Rating Scale (MADRS) or the 17-item Hamilton Depression Rating Scale (HAM-D) scale scores at day 1 (24 h post-treatment) was considered the primary outcome. Here, we conduct pilot genome-wide association study (GWAS) analyses to identify potential markers of ketamine response and dissociative side effects. Polygenic risk score analysis of SNPs ranked by the strength of their association with ketamine response was then calculated in order to assess whether common genetic markers from the ketamine study could predict response to scopolamine. Findings require replication in larger samples in light of low power of analyses of these small samples. Neverthless, these data provide a promising illustration of our future potential to identify genetic variants underlying rapid treatment response in mood disorders and may ultimately guide individual patient treatment selection in the future.

Attenuation of Antidepressant Effects of Ketamine by Opioid Receptor Antagonism

OBJECTIVE

In addition to N-methyl-d-aspartate receptor antagonism, ketamine produces opioid system activation. The objective of this study was to determine whether opioid receptor antagonism prior to administration of intravenous ketamine attenuates its acute antidepressant or dissociative effects.

METHOD

In a proposed double-blind crossover study of 30 adults with treatment-resistant depression, the authors performed a planned interim analysis after studying 14 participants, 12 of whom completed both conditions in randomized order: placebo or 50 mg of naltrexone preceding intravenous infusion of 0.5 mg/kg of ketamine. Response was defined as a reduction ≥50% in score on the 17-item Hamilton Depression Rating Scale (HAM-D) score on postinfusion day 1.

RESULTS

In the interim analysis, seven of 12 adults with treatment-resistant depression met the response criterion during the ketamine plus placebo condition. Reductions in 6-item and 17-item HAM-D scores among participants in the ketamine plus naltrexone condition were significantly lower than those of participants in the ketamine plus placebo condition on postinfusion days 1 and 3. Secondary analysis of all participants who completed the placebo and naltrexone conditions, regardless of the robustness of response to ketamine, showed similar results. There were no differences in ketamine-induced dissociation between conditions. Because naltrexone dramatically blocked the antidepressant but not the dissociative effects of ketamine, the trial was halted at the interim analysis.

CONCLUSIONS

The findings suggest that ketamine's acute antidepressant effect requires opioid system activation. The dissociative effects of ketamine are not mediated by the opioid system, and they do not appear sufficient without the opioid effect to produce the acute antidepressant effects of ketamine in adults with treatment-resistant depression.

Metabotropic Glutamate Receptor 5 in the Medial Prefrontal Cortex as a Molecular Determinant of Pain and Ensuing Depression

Pain and depression affect one another, and this bidirectional interaction implies the existence of common or interacting neural pathways. Among the neural circuits relevant to negative affection, the medial prefrontal cortex (mPFC) is known to be involved in both pain and depression. Persistent stress from physical pain and mental distress can evoke maladaptive changes in mPFC circuits to induce depression. Conversely, the unpleasant mood condition alters mPFC circuits to distort the appraisal of aversion and make individuals vulnerable to pain. In this article, recent findings regarding mPFC in chronic pain and/or depression are reviewed, with particular focus on the metabotropic glutamate receptor 5 (mGluR5). Although the involvement of mGluR5 within the mPFC in both pain and depressive disorders has been extensively studied, there are controversies regarding changes in the activity of the mPFC during chronic pain and depression, and the functional roles of mGluR5 on altered mPFC activity. We discuss alterations in the availability of mGluR5 in the mPFC in these disorders, its role in behavioral manifestations, and its possible influence on cellular subpopulations that mediate dysfunction in the mPFC. We also propose molecular mechanisms that may cause expressional changes in mGluR5 within the mPFC circuitry.

Imaging glutamate redistribution after acute N-acetylcysteine administration: A simultaneous PET/MR study

Glutamate is the most abundant excitatory neurotransmitter in the human brain, but in vivo imaging of acute fluctuations in glutamatergic levels has not been well established. The purpose of this study was to examine acute changes in glutamate after stimulation with N-acetylcysteine (NAC) using a simultaneous positron emission tomography/magnetic resonance spectroscopy (PET/MRS) approach. Ten healthy adult males were examined in two scanning sessions, and 5g NAC was administered 1 h prior to one of the scan sessions. Simultaneous PET/MR data were acquired using an integrated 3T PET/MR scanner. Glutamate (Glu), glutamine (Gln), and glutamate + glutamine (Glx) levels were assessed from MRS data collected from the basal ganglia with PRESS and from the left prefrontal cortex with PRESS and MEGAPRESS, and mGluR5 binding (BP_ND) was assessed from PET data collected with [¹⁸F]PSS232. NAC administration was associated with a significant reduction in Glx and Gln in the basal ganglia spectra, and in Glx in the frontal MEGAPRESS spectra (p < 0.05); no differences in [¹⁸F]PSS232 BP_ND were observed with NAC, although a correlation between pre-/post-treatment Glx and baseline BPnd was found. The MRS-visible Glx signal is sensitive to acute fluctuations in glutamate. The change in Glx was mostly driven by a change in Gln, lending weight to the notion that Gln can provide a proxy marker for neurotransmitter/synaptic glutamate. [¹⁸F]PSS232 binding is not sensitive to acute glutamate shifts independently, but was associated with the extent of glutamate liberation upon NAC stimulation.

The neurobiology of depression, ketamine and rapid-acting antidepressants: Is it glutamate inhibition or activation?

The discovery of the antidepressant effects of ketamine has opened a breakthrough opportunity to develop a truly novel class of safe, effective, and rapid-acting antidepressants (RAADs). In addition, the rapid and robust biological and behavioral effects of ketamine offered a unique opportunity to utilize the drug as a tool to thoroughly investigate the neurobiology of stress and depression in animals, and to develop sensitive and reproducible biomarkers in humans. The ketamine literature over the past two decades has considerably enriched our understanding of the mechanisms underlying chronic stress, depression, and RAADs. However, considering the complexity of the pharmacokinetics and in vivo pharmacodynamics of ketamine, several questions remain unanswered and, at times, even answered questions continue to be considered controversial or at least not fully understood. The current perspective paper summarizes our understanding of the neurobiology of depression, and the mechanisms of action of ketamine and other RAADs. The review focuses on the role of glutamate neurotransmission - reviewing the history of the "glutamate inhibition" and "glutamate activation" hypotheses, proposing a synaptic connectivity model of chronic stress pathology, and describing the mechanism of action of ketamine. It will also summarize the clinical efficacy findings of putative RAADs, present relevant human biomarker findings, and discuss current challenges and future directions.

Intraoperative ketamine for prevention of depressive symptoms after major surgery in older adults: an international, multicentre, double-blind, randomised clinical trial

BACKGROUND

Ketamine is a general anaesthetic with anti-depressant effects at subanaesthetic doses. We hypothesised that intraoperative administration of ketamine would prevent or mitigate postoperative depressive symptoms in surgical patients.

METHODS

We conducted an international, randomised clinical trial testing the effects of intraoperative administration of ketamine [0.5 mg kg^-1 (Lo-K) or 1.0 mg kg^-1 (Hi-K)] vs control [saline placebo (P)] in patients ≥60 yr old undergoing major surgery with general anaesthesia. We administered the Patient Health Questionnaire-8 before the operation, on postoperative day (POD) 3 (primary outcome), and on POD30 to assess depressive symptoms, a secondary outcome of the original trial.

RESULTS

There was no significant difference on POD3 in the proportion of patients with symptoms suggestive of depression between the placebo [23/156 (14.7%)] and combined ketamine (Lo-K plus Hi-K) [61/349 (17.5%)] groups [difference = -2.7%; 95% confidence interval (CI), 5.0% to -9.4%; P=0.446]. Of the total cohort, 9.6% (64/670; 95% CI, 7.6-12.0%) had symptoms suggestive of depression before operation, which increased to 16.6% (84/505; 95% CI, 13.6-20.1%) on POD3, and decreased to 11.9% (47/395; 95% CI, 9.1-15.5%) on POD30. Of the patients with depressive symptoms on POD3 and POD30, 51% and 49%, respectively, had no prior history of depression or depressive symptoms.

CONCLUSIONS

Major surgery is associated with new-onset symptoms suggestive of depression in patients ≥60 yr old. Intraoperative administration of subanaesthetic ketamine does not appear to prevent or improve depressive symptoms.

CLINICAL TRIALS REGISTRATION

NCT01690988.

The effects of ketamine on prefrontal glutamate neurotransmission in healthy and depressed subjects

The ability of ketamine administration to activate prefrontal glutamate neurotransmission is thought to be a key mechanism contributing to its transient psychotomimetic effects and its delayed and sustained antidepressant effects. Rodent studies employing carbon-13 magnetic resonance spectroscopy (13C MRS) methods have shown ketamine and other N-methyl-D-aspartate (NMDA) receptor antagonists to transiently increase measures reflecting glutamate-glutamine cycling and glutamate neurotransmission in the frontal cortex. However, there are not yet direct measures of glutamate neurotransmission in vivo in humans to support these hypotheses. The current first-level pilot study employed a novel prefrontal 13C MRS approach similar to that used in the rodent studies for direct measurement of ketamine effects on glutamate-glutamine cycling. Twenty-one participants (14 healthy and 7 depressed) completed two 13C MRS scans during infusion of normal saline or subanesthetic doses of ketamine. Compared to placebo, ketamine increased prefrontal glutamate-glutamine cycling, as indicated by a 13% increase in 13C glutamine enrichment (t = 2.4, p = 0.02). We found no evidence of ketamine effects on oxidative energy production, as reflected by 13C glutamate enrichment. During ketamine infusion, the ratio of 13C glutamate/glutamine enrichments, a putative measure of neurotransmission strength, was correlated with the Clinician-Administered Dissociative States Scale (r = -0.54, p = 0.048). These findings provide the most direct evidence in humans to date that ketamine increases glutamate release in the prefrontal cortex, a mechanism previously linked to schizophrenia pathophysiology and implicated in the induction of rapid antidepressant effects.

Ketamine and Ceftriaxone-Induced Alterations in Glutamate Levels Do Not Impact the Specific Binding of Metabotropic Glutamate Receptor Subtype 5 Radioligand [18F]PSS232 in the Rat Brain

Several studies showed that [¹¹C]ABP688 binding is altered following drug-induced perturbation of glutamate levels in brains of humans, non-human primates and rats. We evaluated whether the fluorinated derivative [¹⁸F]PSS232 can be used to assess metabotropic glutamate receptor 5 (mGluR5) availability in rats after pharmacological challenge with ketamine, known to increase glutamate, or ceftriaxone, known to decrease glutamate. In vitro autoradiography was performed on rat brain slices with [¹⁸F]PSS232 to prove direct competition of the drugs for mGluR5. One group of rats were challenged with a bolus injection of either vehicle, racemic ketamine, S-ketamine or ceftriaxone followed by positron emission tomography PET imaging with [¹⁸F]PSS232. The other group received an infusion of the drugs during the PET scan. Distribution volume ratios (DVRs) were calculated using a reference tissue model. In vitro autoradiography showed no direct competition of the drugs with [¹⁸F]PSS232 for the allosteric binding site of mGluR5. DVRs of [¹⁸F]PSS232 binding in vivo did not change in any brain region neither after bolus injection nor after infusion. We conclude that [¹⁸F]PSS232 has utility for measuring mGluR5 density or occupancy of the allosteric site in vivo, but it cannot be used to measure in vivo fluctuations of glutamate levels in the rat brain.

Test-retest variability of [11 C]ABP688 estimates of metabotropic glutamate receptor subtype 5 availability in humans

[¹¹ C]ABP688 is a positron emission tomography (PET) radioligand that binds selectively to metabotropic glutamate type 5 receptors (mGluR5). The use of this tracer has identified receptor binding changes in clinical populations, and has been informative in drug occupancy studies. However, previous studies have found significant increases in [¹¹ C]ABP688 binding in the later scan of same-day comparisons, and estimates of test-retest reliability under consistent scanning conditions are not available. The objective of this study was to assess the variability of [¹¹ C]ABP688 binding in healthy people in scans performed at the same time of day. Two [¹¹ C]ABP688 scans were acquired in eight healthy volunteers (6 women, 2 men) using a high-resolution research tomograph (HRRT). Scans were acquired 3 weeks apart with start times between 10:00am and 1:30pm. Mean mGluR5 binding potential (BP_ND ) values were calculated across cortical, striatal and limbic brain regions. Participants reported on subjective mood state after each scan and blood samples were drawn for cortisol analysis. No significant change in BP_ND between scans was observed. Variability in BP_ND values of 11-21% was observed across regions, with the greatest change in the hippocampus and amygdala. Reliability was low to moderate. BP_ND was not statistically related to scan start time, subjective anxiety, serum cortisol levels, or menstrual phase in women. Overall, [¹¹ C]ABP688 BP_ND estimates show moderate variability in healthy people. Reliability is fair in cortical and striatal regions, and lower in limbic regions. Future research using this ligand should account for this in study design and analysis.

Metabotropic Glutamatergic Receptor 5 and Stress Disorders: Knowledge Gained From Receptor Imaging Studies

The metabotropic glutamatergic receptor subtype 5 (mGluR5) may represent a promising therapeutic target for stress-related psychiatric disorders. Here, we describe mGluR5 findings in stress disorders, particularly major depressive disorder (MDD), highlighting insights from positron emission tomography studies. Positron emission tomography studies report either no differences or lower mGluR5 in MDD, potentially reflecting MDD heterogeneity. Unlike the rapidly acting glutamatergic agent ketamine, mGluR5-specific modulation has not yet shown antidepressant efficacy in MDD and bipolar disorder. Although we recently showed that ketamine may work, in part, through significant mGluR5 modulation, the specific role of mGluR5 downregulation in ketamine's antidepressant response is unclear. In contrast to MDD, there has been much less investigation of mGluR5 in bipolar disorder, yet initial studies indicate that mGluR5-specific treatments may aid in both depressed and manic mood states. The direction of modulation needed may be state dependent, however, limiting clinical feasibility. There has been relatively little study of posttraumatic stress disorder or obsessive-compulsive disorder to date, although there is evidence for the upregulation of mGluR5 in these disorders. However, while antagonism of mGluR5 may reduce fear conditioning, it may also reduce fear extinction. Therefore, studies are needed to determine the role mGluR5 modulation might play in the treatment of these conditions. Further challenges in modulating this prevalent neurotransmitter system include potential induction of significant side effects. As such, more research is needed to identify level and type (positive/negative allosteric modulation or full antagonism) of mGluR5 modulation required to translate existing knowledge into improved therapies.

A brain network model for depression: From symptom understanding to disease intervention

Understanding the neural substrates of depression is crucial for diagnosis and treatment. Here, we review recent studies of functional and effective connectivity in depression, in terms of functional integration in the brain. Findings from these studies, including our own, point to the involvement of at least four networks in patients with depression. Elevated connectivity of a ventral limbic affective network appears to be associated with excessive negative mood (dysphoria) in the patients; decreased connectivity of a frontal-striatal reward network has been suggested to account for loss of interest, motivation, and pleasure (anhedonia); enhanced default mode network connectivity seems to be associated with depressive rumination; and diminished connectivity of a dorsal cognitive control network is thought to underlie cognitive deficits especially ineffective top-down control of negative thoughts and emotions in depressed patients. Moreover, the restoration of connectivity of these networks-and corresponding symptom improvement-following antidepressant treatment (including medication, psychotherapy, and brain stimulation techniques) serves as evidence for the crucial role of these networks in the pathophysiology of depression.

Metabotropic Glutamate Receptor 5 as a Target for the Treatment of Depression and Smoking: Robust Preclinical Data but Inconclusive Clinical Efficacy

The ability of novel pharmacological compounds to improve outcomes in preclinical models is often not translated into clinical efficacy. Psychiatric disorders do not have biological boundaries, and identifying mechanisms to improve the translational bottleneck between preclinical and clinical research domains is an important and challenging task. Glutamate transmission is disrupted in several neuropsychiatric disorders. Metabotropic glutamate (mGlu) receptors represent a diverse class of receptors that contribute to excitatory neurotransmission. Given the wide, yet region-specific manner of expression, developing pharmacological compounds to modulate mGlu receptor activity provides an opportunity to subtly and selectively modulate excitatory neurotransmission. This review focuses on the potential involvement of mGlu5 receptor disruption in major depressive disorder and substance and/or alcohol use disorders. We provide an overview of the justification of targeting mGlu5 receptors in the treatment of these disorders, summarize the preclinical evidence for negatively modulating mGlu5 receptors as a therapeutic target for major depressive disorders and nicotine dependence, and highlight the outcomes of recent clinical trials. While the evidence of mGlu5 receptor negative allosteric modulation has been promising in preclinical investigations, these beneficial effects have not translated into clinical efficacy. In this review, we identify key challenges that may contribute to poor clinical translation and provide suggested approaches moving forward to potentially improve the translation from preclinical to clinical domains. Such approaches may increase the success of clinical trials and may reduce the translational bottleneck that exists in drug discovery for psychiatric disorders.

Antagonism at the NR2B subunit of NMDA receptors induces increased connectivity of the prefrontal and subcortical regions regulating reward behavior

RATIONALE

Evidence indicates that ketamine's rapid antidepressant efficacy likely results from its antagonism of NR2B-subunit-containing NMDA receptors (NMDAR). Since ketamine equally blocks NR2A- and NR2B-containing NMDAR, and has affinity to other receptors, NR2B-selective drugs might have improved therapeutic efficiency and side effect profile.

OBJECTIVES

We aimed to compare the effects of (S)-ketamine and two different types of NR2B-selective antagonists on functional brain networks in rats, in order to find common circuits, where their effects intersect, and that might explain their antidepressant action.

METHODS

The experimental design comprised four parallel groups of rats (N = 37), each receiving (S)-Ketamine, CP-101,606, Ro 25-6981 or saline. After compound injection, we acquired resting-state functional magnetic resonance imaging time series. We used graph theoretical approach to calculate brain network properties.

RESULTS

Ketamine and CP-101,606 diminished the global clustering coefficient and small-worldness index. At the nodal level, all compounds induced increased connectivity of the regions mediating reward and cognitive aspects of emotional processing, such as ventromedial prefrontal cortex, septal nuclei, and nucleus accumbens. The dorsal hippocampus and regions involved in sensory processing and aversion, such as superior and inferior colliculi, exhibited an opposite effect.

CONCLUSIONS

The effects common to ketamine and NR2B-selective compounds were localized to the same brain regions as those reported in depression, but in the opposite direction. The upregulation of the reward circuitry might partially underlie the antidepressant and anti-anhedonic effects of the antagonists and could potentially serve as a translational imaging phenotype for testing putative antidepressants, especially those targeting the NR2B receptor subtype.

Glutamatergic Dysfunction and Glutamatergic Compounds for Major Psychiatric Disorders: Evidence From Clinical Neuroimaging Studies

Excessive glutamate release has been linked to stress and many neurodegenerative diseases. Evidence indicates abnormalities of glutamatergic neurotransmission or glutamatergic dysfunction as playing an important role in the development of many major psychiatric disorders (e.g., schizophrenia, bipolar disorder, and major depressive disorder). Recently, ketamine, an N-methyl-d-aspartate antagonist, has been demonstrated to have promisingly rapid antidepressant efficacy for treatment-resistant depression. Many compounds that target the glutamate system have also become available that possess potential in the treatment of major psychiatric disorders. In this review, we update evidence from recent human studies that directly or indirectly measured glutamatergic neurotransmission and function in major psychiatric disorders using modalities such as magnetic resonance spectroscopy, positron emission tomography/single-photon emission computed tomography, and paired-pulse transcranial magnetic stimulation. The newer generation of antidepressants that target the glutamatergic system developed in human clinical studies is also reviewed.