Metabotropic Glutamate Receptor 5 and Glutamate Involvement in Major Depressive Disorder: A Multimodal Imaging Study

Ketamine and rapid antidepressant action: new treatments and novel synaptic signaling mechanisms

Ketamine is an open channel blocker of ionotropic glutamatergic N-Methyl-D-Aspartate (NMDA) receptors. The discovery of its rapid antidepressant effects in patients with depression and treatment-resistant depression fostered novel effective treatments for mood disorders. This discovery not only provided new insight into the neurobiology of mood disorders but also uncovered fundamental synaptic plasticity mechanisms that underlie its treatment. In this review, we discuss key clinical aspects of ketamine's effect as a rapidly acting antidepressant, synaptic and circuit mechanisms underlying its action, as well as how these novel perspectives in clinical practice and synapse biology form a road map for future studies aimed at more effective treatments for neuropsychiatric disorders.

Preliminary Study of White Matter Abnormalities and Associations With the Metabotropic Glutamate Receptor 5 to Distinguish Bipolar and Major Depressive Disorders

Background

Understanding distinct neurobiological mechanisms underlying bipolar disorder (BD) and major depressive disorder (MDD) is crucial for accurate diagnosis and the discovery of novel and more effective targeted treatments. Previous diffusion-weighted MRI studies have suggested some common frontotemporal corticolimbic system white matter (WM) abnormalities across the disorders. However, critical to the development of more precise diagnosis and treatment is identifying distinguishing abnormalities. Promising candidates include more prominent frontotemporal WM abnormalities observed in BD in the uncinate fasciculus (UF) that have been associated with frontal-amygdala functional dysconnectivity, and with suicide that is especially high in BD. Prior work also showed differentiation in metabotropic glutamate receptor 5 (mGlu5) abnormalities in BD versus MDD, which could be a mechanism affected in the frontotemporal system. However, associations between WM and mGlu5 have not been examined previously as a differentiator of BD. Using a multimodal neuroimaging approach, we examined WM integrity alterations in the disorders and their associations with mGluR5 levels.

Methods

Individuals with BD (N = 21), MDD (N = 10), and HC (N = 25) participated in structural and diffusion-weighted MRI scanning, and imaging with [18F]FPEB PET for quantification of mGlu5 availability. Whole-brain analyses were used to assess corticolimbic WM matter fractional anisotropy (FA) across BD and MDD relative to HC; abnormalities were tested for associations with mGlu5 availability.

Results

FA corticolimbic reductions were observed in both disorders and altered UF WM integrity was observed only in BD. In BD, lower UF FA was associated with lower amygdala mGlu5 availability (p < .05).

Conclusions

These novel preliminary findings suggest important associations between lower UF FA and lower amygdala mGlu5 levels that could represent a disorder-specific neural mechanism in which mGluR5 is associated with the frontotemporal dysconnectivity of the disorder.

Region- and time- specific effects of ketamine on cerebral blood flow: a randomized controlled trial

There is intriguing evidence suggesting that ketamine might have distinct acute and delayed neurofunctional effects, as its acute administration transiently induces schizophrenia-like symptoms, while antidepressant effects slowly emerge and are most pronounced 24 h after administration. Studies attempting to characterize ketamine's mechanism of action by using blood oxygen level dependent (BOLD) imaging have yielded inconsistent results regarding implicated brain regions and direction of effects. This may be due to intrinsic properties of the BOLD contrast, while cerebral blood flow (CBF), as measured with arterial spin labeling, is a single physiological marker more directly related to neural activity. As effects of acute ketamine challenge are sensitive to modulation by pretreatment with lamotrigine, which inhibits glutamate release, a combination of these approaches should be particularly suited to offer novel insights. In total, 75 healthy participants were investigated in a double blind, placebo-controlled, randomized, parallel-group study and underwent two scanning sessions (acute/post 24 h.). Acute ketamine administration was associated with higher perfusion in interior frontal gyrus (IFG) and dorsolateral prefrontal cortex (DLPFC), but no other investigated brain region. Inhibition of glutamate release by pretreatment with lamotrigine abolished ketamine's effect on perfusion. At the delayed time point, pretreatment with lamotrigine was associated with lower perfusion in IFG. These findings underscore the idea that regionally selective patterns of CBF changes reflect proximate effects of modulated glutamate release on neuronal activity. Furthermore, region- specific sustained effects indicate both a swift restoration of disturbed homeostasis in DLPFC as well changes occurring beyond the immediate effects on glutamate signaling in IFG.

Brain-wide changes in excitation-inhibition balance of major depressive disorder: a systematic review of topographic patterns of GABA- and glutamatergic alterations

The excitation-inhibition (E/I) imbalance is an important molecular pathological feature of major depressive disorder (MDD) as altered GABA and glutamate levels have been found in multiple brain regions in patients. Healthy subjects show topographic organization of the E/I balance (EIB) across various brain regions. We here raise the question of whether such EIB topography is altered in MDD. Therefore, we systematically review the gene and protein expressions of inhibitory GABAergic and excitatory glutamatergic signaling-related molecules in postmortem MDD brain studies as proxies for EIB topography. Searches were conducted through PubMed and 45 research articles were finally included. We found: i) brain-wide GABA- and glutamatergic alterations; ii) attenuated GABAergic with enhanced glutamatergic signaling in the cortical-subcortical limbic system; iii) that GABAergic signaling is decreased in regions comprising the default mode network (DMN) while it is increased in lateral prefrontal cortex (LPFC). These together demonstrate abnormal GABA- and glutamatergic signaling-based EIB topographies in MDD. This enhances our pathophysiological understanding of MDD and carries important therapeutic implications for stimulation treatment.

Effects of Psychotherapy on Glutamatergic Neurotransmission

INTRODUCTION

Psychodynamic psychotherapy is an effective and widely used treatment for major depressive disorder (MDD); however, little is known about neurobiological changes associated with induced symptom improvement.

METHODS

Proton magnetic resonance spectroscopy with a two-dimensional J-resolved sequence served to test the relationship between glutamate (Glu) and glutamine (Gln) levels, measured separately in pregenual anterior cingulate cortex (pgACC) and the anterior midcingulate cortex (aMCC) as a control region, with change in depression symptoms after 6 months of weekly psychodynamic psychotherapy sessions in MDD patients. Depressed (N = 45) and healthy (N = 30) subjects participated in a baseline proton magnetic resonance spectroscopy measurement and a subgroup of MDD subjects (N = 21) then received once-a-week psychodynamic psychotherapy and participated in a second proton magnetic resonance spectroscopy measurement after 6 months. Change in depression symptoms was assessed using the Hamilton Depression Rating Scale (HAMD).

RESULTS

Higher pretreatment pgACC Gln concentrations in MDD patients compared to healthy controls were associated with symptom severity. Patients and controls did not differ regarding Gln levels in aMCC nor regarding Glu levels in both regions. The association of pgACC Gln concentration and severity of depressive symptoms was reversed after 6 months of psychotherapy in MDD subjects. Regarding Gln in aMCC as well as Glu in both regions, there were no significant associations with improvement of depressive symptoms in the course of psychotherapy.

DISCUSSION

Findings indicate specific regional effects of psychodynamic psychotherapy on glutamatergic neurotransmission and thereby highlight the key role of the pgACC in both depression pathophysiology and recovery.

Nicotine Use and Metabotropic Glutamate Receptor 5 in Individuals With Major Depressive and Posttraumatic Stress Disorders

Metabotropic glutamate receptor 5 (mGluR5) dysregulation has been implicated in the pathophysiology of many psychiatric disorders, as well as nicotine use and dependence. We used positron emission tomography with [18F]FPEB to measure mGluR5 availability in vivo in 6 groups: (1) nicotine users (NUs) without other psychiatric comorbidities (n = 23); (2) comparison controls (CCs) without nicotine use or psychiatric comorbidities (n = 38); (3) major depressive disorder subjects with concurrent nicotine use (MDD-NU; n = 19); (4) MDD subjects without concurrent nicotine use (MDD-CC; n = 20); (5) posttraumatic stress disorder subjects with concurrent nicotine use (PTSD-NU; n = 17); and (6) PTSD subjects without concurrent nicotine use (PTSD-CC; n = 16). The goal of the study was to test the hypothesis that mGluR5 availability in key corticolimbic regions of interest (ROIs) is different in NU with versus without comorbid psychiatric disorders (ROI: dorsolateral prefrontal cortex [dlPFC], orbitofrontal cortex [OFC], ventromedial prefrontal cortex [vmPFC], anterior cingulate cortex [ACC], amygdala, hippocampus). We found that NU had 11%-13% lower mGluR5 availability in OFC, vmPFC, dlPFC, and ACC as compared with CC, while PTSD-NU had 9%-11% higher mGluR5 availability in OFC, dlPFC, and ACC compared with PTSD. Furthermore, relationships between mGluR5 availability and psychiatric symptoms varied as a function of psychiatric diagnosis among NUs. NU showed a negative correlation between mGluR5 and smoking cravings and urges (r's = -0.58 to -0.70, p's = 0.011 - 0.047), while PTSD-NU had the reverse relationship (r's = 0.60-0.71, p's = 0.013-0.035 in ACC, vmPFC, and dlPFC). These findings have substantial implications for our understanding of glutamate homeostasis in psychiatric subgroups and for identifying key neural phenotypes among NU. mGluR5 is a potential treatment target for precision medicine in individuals with nicotine use.

Increase in thalamic cerebral blood flow is associated with antidepressant effects of ketamine in major depressive disorder

Ketamine is a promising treatment option for patients with Major Depressive Disorder (MDD) and has become an important research tool to investigate antidepressant mechanisms of action. However, imaging studies attempting to characterise ketamine's mechanism of action using blood oxygen level-dependent signal (BOLD) imaging have yielded inconsistent results- at least partly due to intrinsic properties of the BOLD contrast, which measures a complex signal related to neural activity. To circumvent the limitations associated with the BOLD signal, we used arterial spin labelling (ASL) as an unambiguous marker of neuronal activity-related changes in cerebral blood flow (CBF). We measured CBF in 21 MDD patients at baseline and 24 h after receiving a single intravenous infusion of subanesthetic ketamine and examined relationships with clinical outcomes. Our findings demonstrate that increase in thalamus perfusion 24 h after ketamine administration is associated with greater improvement of depressive symptoms. Furthermore, lower thalamus perfusion at baseline is associated both with larger increases in perfusion 24 h after ketamine administration and with stronger reduction of depressive symptoms. These findings indicate that ASL is not only a useful tool to broaden our understanding of ketamine's mechanism of action but might also have the potential to inform treatment decisions based on CBF-defined regional disruptions.

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.

Neuroplasticity, the Prefrontal Cortex, and Psychopathology-Related Deviations in Cognitive Control

A basic survival need is the ability to respond to, and persevere in the midst of, experiential challenges. Mechanisms of neuroplasticity permit this responsivity via functional adaptations (flexibility), as well as more substantial structural modifications following chronic stress or injury. This review focuses on prefrontally based flexibility, expressed throughout large-scale neuronal networks through the actions of excitatory and inhibitory neurotransmitters and neuromodulators. With substance use disorders and stress-related internalizing disorders as exemplars, we review human behavioral and neuroimaging data, considering whether executive control, particularly cognitive flexibility, is impaired premorbidly, enduringly compromised with illness progression, or both. We conclude that deviations in control processes are consistently expressed in the context of active illness but operate through different mechanisms and with distinct longitudinal patterns in externalizing versus internalizing conditions.

Prefrontal Glutamate Neurotransmission in PTSD: A Novel Approach to Estimate Synaptic Strength in Vivo in Humans

Background

Trauma and chronic stress are believed to induce and exacerbate psychopathology by disrupting glutamate synaptic strength. However, in vivo in human methods to estimate synaptic strength are limited. In this study, we established a novel putative biomarker of glutamatergic synaptic strength, termed energy-per-cycle (EPC). Then, we used EPC to investigate the role of prefrontal neurotransmission in trauma-related psychopathology.

Methods

Healthy controls (n = 18) and patients with posttraumatic stress (PTSD; n = 16) completed ¹³C-acetate magnetic resonance spectroscopy (MRS) scans to estimate prefrontal EPC, which is the ratio of neuronal energetic needs per glutamate neurotransmission cycle (V_TCA/V_Cycle).

Results

Patients with PTSD were found to have 28% reduction in prefrontal EPC (t = 3.0; df = 32, P = .005). There was no effect of sex on EPC, but age was negatively associated with prefrontal EPC across groups (r = -0.46, n = 34, P = .006). Controlling for age did not affect the study results.

Conclusion

The feasibility and utility of estimating prefrontal EPC using ¹³C-acetate MRS were established. Patients with PTSD were found to have reduced prefrontal glutamatergic synaptic strength. These findings suggest that reduced glutamatergic synaptic strength may contribute to the pathophysiology of PTSD and could be targeted by new treatments.

Differential Role of mGluR5 in Cognitive Processes in Posttraumatic Stress Disorder and Major Depression

Background

A robust literature supports the role of the metabotropic glutamate receptor type 5 (mGluR5) in cognitive functioning. mGluR5 is also implicated in the pathophysiology of posttraumatic stress disorder (PTSD) and major depressive disorder (MDD), which are characterized by cognitive alterations. However, the relationship between mGluR5 and cognition in MDD and PTSD has not yet been directly investigated. To address this gap, we examined the relationship between in vivo mGluR5 availability and cognition in PTSD, MDD, and matched healthy adults (HA).

Methods

Individuals with PTSD (N = 28) and MDD (N = 21), and HA (N = 28) were matched for age, gender, and smoking status. Participants completed 18F-FPEB positron emission tomography (PET) scan, psychiatric and cognitive assessments.

Results

Across models examining the relationship between mGluR5 availability and different domains of cognition across diagnostic groups, only the interaction of diagnosis*attention was significant (F 4,64 = 3.011, P = .024). Higher mGluR5 availability was associated with poorer attention in PTSD in 4 frontolimbic regions of interests (ROI's: OFC (r = -.441, P = .016), vmPFC (r = -.408, P = .028), dlPFC (r = -.421, P = .023), hippocampus (r = -.422, P = .025). By contrast, mGluR5 availability in the MDD group was positively related to Attention (ATTN) in the OFC (r = .590, P = .006), vmPFC (r = .653, P = .002), and dlPFC (r = .620, P = .004). Findings in the hippocampus for MDD followed the same pattern but did not survive correction for multiple comparisons (r = .480, P = .036). ATTN and mGluR5 availability were not significantly related in the HA group. Of note, in MANOVA analyses group*ATTN interaction results in the OFC did not survive multiple comparisons (P = .046). All other findings survived correction for multiple comparisons and remained significant when covarying for potential confounds (eg, depressed mood).

Conclusions

We observed a significant relationship between frontolimbic mGluR5 availability and performance on tests of attention in individuals with MDD and PTSD. This finding aligns with animal work showing dysregulation in mGluR5 in cognitive functioning, and differed as a function of diagnosis. Results suggest interventions targeting mGluR5 may help bolster cognitive difficulties, highlighting the importance of employing different mGluR5 directed treatment strategies in MDD and PTSD.

Differences in mGluR5 Availability Depending on the Level of Social Avoidance in Drug-Naïve Young Patients with Major Depressive Disorder

BACKGROUND

Previous research has shown that metabotropic glutamate receptor-5 (mGluR5) signaling is significantly involved in social avoidance. We investigated the relationship between levels of social avoidance and mGluR5 availability in drug-naïve young patients with major depressive disorder (MDD).

METHODS

Twenty non-smoking patients and eighteen matched non-smoking healthy controls underwent [¹¹C]ABP688 positron emission tomography (PET) and magnetic resonance imaging scans. The binding potential (BP_ND) of [¹¹C]ABP688 was obtained using the simplified reference tissue model. Patients' level of social avoidance was assessed using the Social Avoidance and Distress Scale (SADS). For [¹¹C]ABP688 BP_ND, the region-of-interest (ROI)-based between-group comparisons and correlations with SADS scores were investigated. The frontal cortices were chosen as a priori ROIs based on previous PET investigations in MDD, and on literature underscoring the importance of the frontal cortex in social avoidance.

RESULTS

Independent samples t-tests revealed no significant differences in [¹¹C]ABP688 BP_ND in the frontal cortices between the MDD patient group as a whole and healthy controls. One-way analysis of variance with post-hoc tests revealed significantly lower BP_ND in the bilateral superior frontal cortex (SFC) and left middle frontal cortex (MFC) in MDD patients with low levels of social avoidance (L-SADS) than in healthy controls. The L-SADS patients also had significantly lower BP_ND in the medial part of the right SFC than both MDD patients with high levels of social avoidance (H-SADS) and healthy controls. The L-SADS patients also showed significantly lower BP_ND in the orbital parts of the SFC, MFC, and inferior frontal cortex than H-SADS patients. No significant group differences were found between H-SADS patients and healthy controls. The ROI-based correlation analysis revealed significant positive correlations between social avoidance levels and frontal [¹¹C]ABP688 BP_ND in the entire patients.

CONCLUSION

Our exploratory study shows significant differences in frontal mGluR5 availability depending on the level of social avoidance in drug-naïve non-smoking MDD patients, suggesting that social avoidance should be considered as one of the clinical factors involved in mGluR5 signaling changes in depression.

Intrinsic Connectivity Networks of Glutamate-Mediated Antidepressant Response: A Neuroimaging Review

Conventional monoamine-based pharmacotherapy, considered the first-line treatment for major depressive disorder (MDD), has several challenges, including high rates of non-response. To address these challenges, preclinical and clinical studies have sought to characterize antidepressant response through monoamine-independent mechanisms. One striking example is glutamate, the brain's foremost excitatory neurotransmitter: since the 1990s, studies have consistently reported altered levels of glutamate in MDD, as well as antidepressant effects following molecular targeting of glutamatergic receptors. Therapeutically, this has led to advances in the discovery, testing, and clinical application of a wide array of glutamatergic agents, particularly ketamine. Notably, ketamine has been demonstrated to rapidly improve mood symptoms, unlike monoamine-based interventions, and the neurobiological basis behind this rapid antidepressant response is under active investigation. Advances in brain imaging techniques, including functional magnetic resonance imaging, magnetic resonance spectroscopy, and positron emission tomography, enable the identification of the brain network-based characteristics distinguishing rapid glutamatergic modulation from the effect of slow-acting conventional monoamine-based pharmacology. Here, we review brain imaging studies that examine brain connectivity features associated with rapid antidepressant response in MDD patients treated with glutamatergic pharmacotherapies in contrast with patients treated with slow-acting monoamine-based treatments. Trends in recent brain imaging literature suggest that the activity of brain regions is organized into coherent functionally distinct networks, termed intrinsic connectivity networks (ICNs). We provide an overview of major ICNs implicated in depression and explore how treatment response following glutamatergic modulation alters functional connectivity of limbic, cognitive, and executive nodes within ICNs, with well-characterized anti-anhedonic effects and the enhancement of "top-down" executive control. Alterations within and between the core ICNs could potentially exert downstream effects on the nodes within other brain networks of relevance to MDD that are structurally and functionally interconnected through glutamatergic synapses. Understanding similarities and differences in brain ICNs features underlying treatment response will positively impact the trajectory and outcomes for adults suffering from MDD and will facilitate the development of biomarkers to enable glutamate-based precision therapeutics.

The Effect of the Low Glutamate Diet on the Reduction of Psychiatric Symptoms in Veterans With Gulf War Illness: A Pilot Randomized-Controlled Trial

The objective of this pilot study was to examine the effects of the low glutamate diet on anxiety, post-traumatic stress disorder (PTSD), and depression in veterans with Gulf War Illness (GWI). The low glutamate diet removes dietary excitotoxins and increases consumption of micronutrients which are protective against glutamatergic excitotoxicity. This study was registered at ClinicalTrials.gov (NCT#03342482). Forty veterans with GWI completed psychiatric questionnaires at baseline and after 1-month following the low glutamate diet. Participants were then randomized into a double-blind, placebo-controlled crossover challenge with monosodium glutamate (MSG; a dietary excitotoxin) vs. placebo over three consecutive days per week, with assessments on day three. Data were analyzed across the full sample and with participants categorized by baseline symptom severity. Pre-post-dietary intervention change scores were analyzed with Wilcoxon signed-rank tests and paired sample t-tests across the full sample, and changes across symptom severity categories were analyzed using ANOVA. Crossover challenge results were analyzed with linear mixed modeling accounting for challenge material (MSG v. placebo), sequence (MSG/placebo v. placebo/MSG), period (challenge week 1 v. week 2), pre-diet baseline symptom severity category (minimal/mild, moderate, or severe), and the challenge material^*symptom severity category interaction. A random effect of ID (sequence) was also included. All three measures showed significant improvement after 1 month on the diet, with significant differences between baseline severity categories. Individuals with severe psychological symptoms at baseline showed the most improvement after 1 month on the diet, while those with minimal/mild symptoms showed little to no change. Modeling results from the challenge period demonstrated a significant worsening of anxiety from MSG in only the most severe group, with no significant effects of MSG challenge on depression nor PTSD symptoms. These results suggest that the low glutamate diet may be an effective treatment for depression, anxiety, and PTSD, but that either (a) glutamate is only a direct cause of symptoms in anxiety, or (b) underlying nutrient intake may prevent negative psychiatric effects from glutamate exposure. Future, larger scale clinical trials are needed to confirm these findings and to further explore the potential influence of increased micronutrient intake on the improvements observed across anxiety, PTSD, and depression.

Psilocybin therapy increases cognitive and neural flexibility in patients with major depressive disorder

Psilocybin has shown promise for the treatment of mood disorders, which are often accompanied by cognitive dysfunction including cognitive rigidity. Recent studies have proposed neuropsychoplastogenic effects as mechanisms underlying the enduring therapeutic effects of psilocybin. In an open-label study of 24 patients with major depressive disorder, we tested the enduring effects of psilocybin therapy on cognitive flexibility (perseverative errors on a set-shifting task), neural flexibility (dynamics of functional connectivity or dFC via functional magnetic resonance imaging), and neurometabolite concentrations (via magnetic resonance spectroscopy) in brain regions supporting cognitive flexibility and implicated in acute psilocybin effects (e.g., the anterior cingulate cortex, or ACC). Psilocybin therapy increased cognitive flexibility for at least 4 weeks post-treatment, though these improvements were not correlated with the previously reported antidepressant effects. One week after psilocybin therapy, glutamate and N-acetylaspartate concentrations were decreased in the ACC, and dFC was increased between the ACC and the posterior cingulate cortex (PCC). Surprisingly, greater increases in dFC between the ACC and PCC were associated with less improvement in cognitive flexibility after psilocybin therapy. Connectome-based predictive modeling demonstrated that baseline dFC emanating from the ACC predicted improvements in cognitive flexibility. In these models, greater baseline dFC was associated with better baseline cognitive flexibility but less improvement in cognitive flexibility. These findings suggest a nuanced relationship between cognitive and neural flexibility. Whereas some enduring increases in neural dynamics may allow for shifting out of a maladaptively rigid state, larger persisting increases in neural dynamics may be of less benefit to psilocybin therapy.

Dynamic alterations in the central glutamatergic status following food and glucose intake: in vivo multimodal assessments in humans and animal models

Fluctuations of neuronal activities in the brain may underlie relatively slow components of neurofunctional alterations, which can be modulated by food intake and related systemic metabolic statuses. Glutamatergic neurotransmission plays a major role in the regulation of excitatory tones in the central nervous system, although just how dietary elements contribute to the tuning of this system remains elusive. Here, we provide the first demonstration by bimodal positron emission tomography (PET) and magnetic resonance spectroscopy (MRS) that metabotropic glutamate receptor subtype 5 (mGluR5) ligand binding and glutamate levels in human brains are dynamically altered in a manner dependent on food intake and consequent changes in plasma glucose levels. The brain-wide modulations of central mGluR5 ligand binding and glutamate levels and profound neuronal activations following systemic glucose administration were further proven by PET, MRS, and intravital two-photon microscopy, respectively, in living rodents. The present findings consistently support the notion that food-associated glucose intake is mechanistically linked to glutamatergic tones in the brain, which are translationally accessible in vivo by bimodal PET and MRS measurements in both clinical and non-clinical settings.

Effect of age on brain metabotropic glutamate receptor subtype 5 measured with [18F]FPEB PET

OBJECTIVE

Metabotropic glutamate receptor subtype 5 (mGluR5) is integral to the brain glutamatergic system and cognitive function. This study investigated whether aging is associated with decreased brain mGluR5 availability.

METHODS

Cognitively normal participants (n = 45), aged 18 to 84 years, underwent [18F]FPEB positron emission tomography scans to quantify brain mGluR5. Distribution volume (VT) was computed using a venous or arterial input function and equilibrium modeling from 90 to 120 min. In the primary analysis, the association between age and VT in the hippocampus and association cortex was evaluated using a linear mixed model. Exploratory analyses assessed the association between age and VT in multiple brain regions. The contribution of gray matter tissue alterations and partial volume effects to associations with age was also examined.

RESULTS

In the primary analysis, older age was associated with lower [18F]FPEB binding to mGluR5 (P = 0.026), whereas this association was not significant after gray matter masking or partial volume correction to account for age-related tissue loss. Post hoc analyses revealed an age-related decline in mGluR5 availability in the hippocampus of 4.5% per decade (P = 0.007) and a non-significant trend in the association cortex (P = 0.085). An exploratory analysis of multiple brain regions revealed broader inverse associations of age with mGluR5 availability, but not after partial volume correction.

CONCLUSION

Reductions in mGluR5 availability with age appear to be largely mediated by tissue loss. Quantification of [18F]FPEB binding to mGluR5 may expand our understanding of age-related molecular changes and the relationship with brain tissue loss.

Ventromedial prefrontal cortex/anterior cingulate cortex Glx, glutamate, and GABA levels in medication-free major depressive disorder

Glutamate (Glu) and gamma-aminobutyric acid (GABA) are implicated in the pathophysiology of major depressive disorder (MDD). GABA levels or GABAergic interneuron numbers are generally low in MDD, potentially disinhibiting Glu release. It is unclear whether Glu release or turnover is increased in depression. Conversely, a meta-analysis of prefrontal proton magnetic resonance spectroscopy (¹H MRS) studies in MDD finds low Glx (combination of glutamate and glutamine) in medicated MDD. We hypothesize that elevated Glx or Glu may be a marker of more severe, untreated MDD. We examined ventromedial prefrontal cortex/anterior cingulate cortex (vmPFC/ACC) Glx and glutamate levels using ¹H MRS in 34 medication-free, symptomatic, chronically ill MDD patients and 32 healthy volunteers, and GABA levels in a subsample. Elevated Glx and Glu were observed in MDD compared with healthy volunteers, with the highest levels seen in males with MDD. vmPFC/ACC GABA was low in MDD. Higher Glx levels correlated with more severe depression and lower GABA. MDD severity and diagnosis were both linked to higher Glx in vmPFC/ACC. Low GABA in a subset of these patients is consistent with our hypothesized model of low GABA leading to glutamate disinhibition in MDD. This finding and model are consistent with our previously reported findings that the NMDAR-antagonist antidepressant effect is proportional to the reduction of vmPFC/ACC Glx or Glu levels.

Met carriers of the BDNF Val66Met polymorphism show reduced Glx/NAA in the pregenual ACC in two independent cohorts

The Met allele of the Val66Met SNP of the BDNF gene (rs6265) is associated with impaired activity-dependent release of brain-derived neurotrophic factor (BDNF), resulting in reduced synaptic plasticity, impaired glutamatergic neurotransmission, and morphological changes. While previous work has demonstrated Val66Met effects on magnetic resonance spectroscopy (MRS) markers of either glutamatergic metabolism (Glx) or neuronal integrity (NAA), no study has investigated Val66Met effects on these related processes simultaneously. As these metabolites share a metabolic pathway, the Glx/NAA ratio may be a more sensitive marker of changes associated with the Val66Met SNP. This ratio is increased in psychiatric disorders linked to decreased functioning in the anterior cingulate cortex (ACC). In this study, we investigated the correlation of the Val66Met polymorphism of the BDNF gene with Glx/NAA in the pregenual anterior cingulate cortex (pgACC) using MRS at 3 Tesla (T) (n = 30, all males) and 7 T (n = 98, 40 females). In both cohorts, Met carriers had lower Glx/NAA compared to Val homozygotes. Follow-up analyses using absolute quantification revealed that the Met carriers do not show decreased pgACC glutamate or glutamine levels, but instead show increased NAA compared to the Val homozygotes. This finding may in part explain conflicting evidence for Val66Met as a risk factor for developing psychiatric illnesses.

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.

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.

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.

Investigating the effects of ebselen, a potential new lithium mimetic, on glutamate transmission

Ebselen, a potential new lithium-like drug, treatment reduced brain glutamate, as measured by PET imaging using the mGluR5 radioligand [¹⁸ F]FPEB in rats.

PET imaging of mGluR5 in Alzheimer's disease

BACKGROUND

Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and may constitute an important therapeutic target in Alzheimer's disease (AD) by mediating the synaptotoxic action of amyloid-β oligomers. We utilized the positron emission tomography (PET) radioligand [18F]FPEB to investigate mGluR5 binding in early AD.

METHODS

Sixteen individuals with amnestic mild cognitive impairment (MCI) due to AD or mild AD dementia who were positive for brain amyloid were compared to 15 cognitively normal (CN) participants who were negative for brain amyloid. Diagnostic groups were well balanced for age, sex, and education. Dynamic PET scans were acquired for 60 min, starting at 60 min after the initial administration of up to 185 MBq of [18F]FPEB using a bolus-plus-constant-infusion method (Kbol = 190 min). Equilibrium modeling with a cerebellum reference region was used to estimate [18F]FPEB binding (BPND) to mGluR5. Analyses were performed with and without corrections for gray matter atrophy and partial volume effects.

RESULTS

Linear mixed model analysis demonstrated a significant effect of group (p = 0.011) and the group × region interaction (p = 0.0049) on BPND. Post hoc comparisons revealed a significant reduction (43%) in mGluR5 binding in the hippocampus of AD (BPND = 0.76 ± 0.41) compared to CN (BPND = 1.34 ± 0.58, p = 0.003, unpaired t test) participants, and a nonsignificant trend for a reduction in a composite association cortical region in AD (BPND = 1.57 ± 0.25) compared to CN (BPND = 1.86 ± 0.63, p = 0.093) participants. Exploratory analyses suggested additional mGluR5 reductions in the entorhinal cortex and parahippocampal gyrus in the AD group. In the overall sample, hippocampal mGluR5 binding was associated with episodic memory scores and global function.

CONCLUSIONS

[18F]FPEB-PET revealed reductions in hippocampal mGluR5 binding in early AD. Quantification of mGluR5 binding in AD may expand our understanding of AD pathogenesis and accelerate the development of novel biomarkers and treatments.

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.

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.

Association between prefrontal glutamine levels and neuroticism determined using proton magnetic resonance spectroscopy

There is growing evidence for GABA and glutamate-glutamine dysfunction in the pathogenesis of mood and anxiety disorders. It is important to study this pathology in the early phases of the illness in order to develop new approaches to secondary prevention. New magnetic resonance spectroscopy (MRS) measures allow determining glutamine, the principal metabolite of synaptic glutamate that is directly related to glutamate levels in the synaptic cleft, as well as glutamate and GABA. In contrast to previous investigations, this study used community-based recruitment methods and a combined categorical and dimensional approach to psychopathology. In the study protocol, neuroticism was defined as the primary outcome. Neuroticism shares a large proportion of its genetic variance with mood and anxiety disorders. We examined young adult participants recruited from the general population in a cross-sectional study using 3-T 1H-MRS with one voxel in the left dorsolateral prefrontal cortex (DLPFC). The total sample of N = 110 (61 females) included 18 individuals suffering from MDD and 19 individuals suffering from DSM-IV anxiety disorders. We found that glutamine and glutamine-to-glutamate ratio were correlated with neuroticism in the whole sample (r = 0.263, p = 0.005, and n = 110; respectively, r = 0.252, p = 0.008, and n = 110), even when controlling for depression and anxiety disorder diagnoses (for glutamine: beta = 0.220, p = 0.047, and n = 110). Glutamate and GABA were not significantly correlated with neuroticism (r = 0.087, p = 0.365, and n = 110; r = -0.044, p = 0.645, and n = 110). Lack of self-confidence and emotional instability were the clinical correlates of glutamate-glutamine dysfunction. In conclusion, this study suggests that prefrontal glutamine is increased in early phases of mood and anxiety disorders. Further understanding of glutamate-glutamine dysfunction in stress-related disorders may lead to new therapeutic strategies to prevent and treat these disorders.

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.

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.

Administration of Sub-anesthetic Dose of Ketamine and Electroconvulsive Treatment on Alternate Week Days in Patients with Treatment Resistant Depression: A Double Blind Placebo Controlled Trial

INTRODUCTION AND BACKGROUND

Patients with depression who fail to respond to at least two antidepressants in their current episode are considered to have Treatment Resistant Depression (TRD). ECT is an effective treatment of TRD but cognitive side effects limit its use. Ketamine elicits a rapid antidepressant response in sub-anesthetic repeated doses. ECT and ketamine may be modulating the glutamate system, therefore when administered in an interleaved fashion, they could have a synergistic effect.

METHODS

15 TRD patients were recruited and 12 were included in the analysis. Patients were randomly assigned to an ECT + iv. ketamine or ECT + iv. placebo (midazolam). At baseline and before each infusion, depression severity scales were administered. At baseline, halfway through and at the end of the study, cognitive tests were administered.

RESULTS

There was no difference between the ketamine and placebo arms, per change in 17-item Hamilton Depression Scores (HAM-D), Young Mania Rating Scores or cognitive tests. Per HAMD scores, 3 ECT +ketamine subjects (42%) showed early remission (HAMD < 8) and maintained euthymia for 3 additional visits. None of the ECT +midazolam subjects (0%) achieved early remission. This difference showed a trend level significance (Chi square P-Value = 0.0910).

CONCLUSION

The results of the study were limited due to the small sample size. However, a trend level difference in rates of early remission was seen, suggesting that ketamine + ECT may lead to a faster symptom relief. A larger sample size is needed for statistical confirmation.

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.

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.

Brain glutamate in medication-free depressed patients: a proton MRS study at 7 Tesla

BACKGROUND

The possible role of glutamate in the pathophysiology and treatment of depression is of intense current interest. Proton magnetic resonance spectroscopy (MRS) enables the detection of glutamate in the living human brain and meta-analyses of previous MRS studies in depressed patients have suggested that glutamate levels are decreased in anterior brain regions. Nevertheless, at conventional magnetic field strengths [1.5-3 Tesla (T)], it is difficult to separate glutamate from its metabolite and precursor, glutamine, with the two often being measured together as Glx. In contrast, MRS at 7 T allows clear spectral resolution of glutamate and glutamine.

METHOD

We studied 55 un-medicated depressed patients and 50 healthy controls who underwent MRS scanning at 7 T with voxels placed in anterior cingulate cortex, occipital cortex and putamen (PUT). Neurometabolites were calculated using the unsuppressed water signal as a reference.

RESULTS

Compared with controls, depressed patients showed no significant difference in glutamate in any of the three voxels studied; however, glutamine concentrations in the patients were elevated by about 12% in the PUT (p < 0.001).

CONCLUSIONS

The increase in glutamine in PUT is of interest in view of the postulated role of the basal ganglia in the neuropsychology of depression and is consistent with elevated activity in the descending cortical glutamatergic innervation to the PUT. The basal ganglia have rarely been the subject of MRS investigations in depressed patients and further MRS studies of these structures in depression are warranted.

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

The mechanisms of action of the rapid antidepressant effects of ketamine, an N-methyl-D-aspartate glutamate receptor antagonist, have not been fully elucidated. This study examined the effects of ketamine on ligand binding to a metabotropic glutamatergic receptor (mGluR5) in individuals with major depressive disorder (MDD) and healthy controls. Thirteen healthy and 13 MDD nonsmokers participated in two [11C]ABP688 positron emission tomography (PET) scans on the same day-before and during intravenous ketamine administration-and a third scan 1 day later. At baseline, significantly lower [11C]ABP688 binding was detected in the MDD as compared with the control group. We observed a significant ketamine-induced reduction in mGluR5 availability (that is, [11C]ABP688 binding) in both MDD and control subjects (average of 14±9% and 19±22%, respectively; P<0.01 for both), which persisted 24 h later. There were no differences in ketamine-induced changes between MDD and control groups at either time point (P=0.8). A significant reduction in depressive symptoms was observed following ketamine administration in the MDD group (P<0.001), which was associated with the change in binding (P<0.04) immediately after ketamine. We hypothesize that glutamate released after ketamine administration moderates mGluR5 availability; this change appears to be related to antidepressant efficacy. The sustained decrease in binding may reflect prolonged mGluR5 internalization in response to the glutamate surge.

Clinical Findings Documenting Cellular and Molecular Abnormalities of Glia in Depressive Disorders

Depressive disorders are complex, multifactorial mental disorders with unknown neurobiology. Numerous theories aim to explain the pathophysiology. According to the “gliocentric theory”, glial abnormalities are responsible for the development of the disease. The aim of this review article is to summarize the rapidly growing number of cellular and molecular evidences indicating disturbed glial functioning in depressive disorders. We focus here exclusively on the clinical studies and present the in vivo neuroimaging findings together with the postmortem molecular and histopathological data. Postmortem studies demonstrate glial cell loss while the in vivo imaging data reveal disturbed glial functioning and altered white matter microstructure. Molecular studies report on altered gene expression of glial specific genes. In sum, the clinical findings provide ample evidences on glial pathology and demonstrate that all major glial cell types are affected. However, we still lack convincing theories explaining how the glial abnormalities develop and how exactly contribute to the emotional and cognitive disturbances. Abnormal astrocytic functioning may lead to disturbed metabolism affecting ion homeostasis and glutamate clearance, which in turn, affect synaptic communication. Abnormal oligodendrocyte functioning may disrupt the connectivity of neuronal networks, while microglial activation indicates neuroinflammatory processes. These cellular changes may relate to each other or they may indicate different endophenotypes. A theory has been put forward that the stress-induced inflammation—mediated by microglial activation—triggers a cascade of events leading to damaged astrocytes and oligodendroglia and consequently to their dysfunctions. The clinical data support the “gliocentric” theory, but future research should clarify whether these glial changes are truly the cause or simply the consequences of this devastating disorder.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Upregulation of prefrontal metabotropic glutamate receptor 5 mediates neuropathic pain and negative mood symptoms after spinal nerve injury in rats

Patients with chronic pain easily accompany the negative mood symptoms such as depression and anxiety, and these disturbances in turn affect the aversive perception of pain. However, the underlying mechanisms are largely unknown. We hypothesized that the alteration of metabotropic glutamate receptor 5 (mGluR5) in the brain region underlies such a comorbidity of aversive states. We scanned the brain of chronic neuropathic pain model rats using positron emission tomography (PET) technique with an mGluR5-selective radiotracer [11C] ABP688 and found various brain regions with higher or lower level of mGluR5 compared to control rats. Among the brain areas, a prominent upregulation of mGluR5 was shown in the prelimbic region (PrL) of the medial prefrontal cortex (mPFC) of chronic neuropathic pain animals. A pharmacological blockade of upregulated mGluR5 in the PrL ameliorated the negative symptoms including tactile hypersensitivity and depressive-like behavior, which relieved the subjects from the unpleasant state of chronic neuropathic pain condition. Conversely, lentiviral overexpression of the mGluR5 in the PrL of naïve rats successfully induced comorbid pain and negative moods. Our data provide deeper insight into the shared mechanism of pain perception and negative emotions, identifying a therapeutic target for the treatment of chronic pain and mood disorders.

Altered metabotropic glutamate receptor 5 markers in PTSD: In vivo and postmortem evidence

Posttraumatic stress disorder (PTSD) is a prevalent and highly disabling disorder, but there is currently no targeted pharmacological treatment for it. Dysfunction of the glutamate system has been implicated in trauma and stress psychopathology, resulting in a growing interest in modulation of the glutamate system for the treatment of PTSD. Specifically, the metabotropic glutamate receptor 5 (mGluR5) represents a promising treatment target. We used [¹⁸F]FPEB, a radioligand that binds to the mGluR5, and positron emission tomography (PET) to quantify in vivo mGluR5 availability in human PTSD vs. healthy control (HCs) subjects. In an independent sample of human postmortem tissue, we investigated expression of proteins that have a functional relationship with mGluR5 and glucocorticoids in PTSD. We observed significantly higher cortical mGluR5 availability in PTSD in vivo and positive correlations between mGluR5 availability and avoidance symptoms. In the postmortem sample, we observed up-regulation of SHANK1, a protein that anchors mGluR5 to the cell surface, as well as decreased expression of FKBP5, implicating aberrant glucocorticoid functioning in PTSD. Results of this study provide insight into molecular mechanisms underlying PTSD and suggest that mGluR5 may be a promising target for mechanism-based treatments aimed at mitigating this disorder.