Glutamate Levels and Resting Cerebral Blood Flow in Anterior Cingulate Cortex Are Associated at Rest and Immediately Following Infusion of S-Ketamine in Healthy Volunteers

Esketamine in depression: putative biomarkers from clinical research

The discovery of racemic (R, S)-ketamine as a rapid-acting antidepressant and the subsequent FDA approval of its (S)-enantiomer, esketamine, for treatment-resistant depression (TRD) are significant advances in the development of novel neuropsychiatric therapeutics. Esketamine is now recognized as a powerful tool for addressing persistent symptoms of TRD compared to traditional oral antidepressants. However, research on biomarkers associated with antidepressant response to esketamine has remained sparse and, to date, has been largely extrapolated from racemic ketamine studies. Genetic, proteomic, and metabolomic profiles suggest that inflammation and mitochondrial function may play a role in esketamine's antidepressant effects, though these preliminary results require verification. In addition, neuroimaging research has consistently implicated the prefrontal cortex, striatum, and anterior cingulate cortex in esketamine's effects. Esketamine also shows promise in perioperative settings for reducing depression and anxiety, and these effects appear to correlate with increased peripheral biomarkers such as brain-derived neurotrophic factor and serotonin. Further indications are likely to be identified with the continued repurposing of racemic ketamine, providing further opportunity for biomarker study and mechanistic understanding of therapeutic effects. Novel methodologies and well-designed biomarker-focused clinical research trials are needed to more clearly elucidate esketamine's therapeutic actions as well as biologically identify those most likely to benefit from this agent, allowing for the improved personalization of antidepressant treatment.

The Trajectory of Prefrontal GABA Levels in Initially Antipsychotic-Naïve Patients With Psychosis During 2 Years of Treatment and Associations With Striatal Cerebral Blood Flow and Outcome

BACKGROUND

GABAergic (gamma-aminobutyric acidergic) function in the prefrontal cortex seems dysfunctional in patients with first-episode psychosis, but the impact of longer-term treatment and relationship to clinical outcomes and striatal activity are unknown.

METHODS

A longitudinal study of 39 antipsychotic-naïve and benzodiazepine-free patients with psychosis (22.4 ± 5.4 years, 64% women) and 54 matched healthy control participants (HCs) (22.2 ± 4.3 years, 61% women) who were followed up after 6 weeks (28 patients, 51 HCs), 6 months (17 patients, 47 HCs), and 2 years (21 patients, 43 HCs) was completed. GABA levels in the dorsal anterior cingulate cortex and striatal resting cerebral blood flow were assessed on a 3T magnetic resonance scanner at all visits.

RESULTS

GABA levels in the dorsal anterior cingulate cortex were significantly lower in patients at baseline and after 6 weeks but not after 6 months or 2 years. Analyses of groups separately revealed decreased GABA levels after 2 years in HCs but stable levels in patients. Treatment increased striatal resting cerebral blood flow after 6 weeks and 6 months but not after 2 years. GABA levels were negatively associated with striatal resting cerebral blood flow in both groups at all visits. Last, lower baseline GABA levels in patients were related to less functional improvement after 2 years.

CONCLUSIONS

The findings suggest a different trajectory of GABA levels and striatal perfusion in first-episode patients over 2 years of antipsychotic treatment compared with HCs and indicate a downregulatory role of prefrontal GABAergic function on the striatum. Moreover, abnormally low prefrontal GABA level at illness onset may be a marker for a more severe prognosis.

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.

Distinctive effects of NMDA receptor modulators on cerebral microcirculation in a schizophrenia mouse model

Substantial evidence demonstrates that schizophrenia patients have altered cerebral microcirculation. However, little is known regarding how cerebral microcirculatory blood flow (microCBF) changes in schizophrenia. Here, using time-lapse two-photon imaging of individual capillaries, we demonstrated a substantial decrease in cerebral microcirculation in a mouse model of schizophrenia. The involvement of NMDA receptor (NMDAR) functions was investigated to understand further the mechanism of microcirculation reduction in this animal model. Administration of D-serine, a selective full agonist at the glycine site of NMDAR, significantly increased the microCBF in the schizophrenia mouse. Interestingly, administration of GNE-8324, a GluN2A-selective positive allosteric modulator that selectively enhances NMDAR-mediated synaptic responses in inhibitory but not excitatory neurons, had no effect on the microCBF of the schizophrenia mice. Together, these data indicated that NMDAR participated in the regulation of microcirculation in schizophrenia using a mechanism dependent on the tonic NMDAR signaling and the selective modulation of inhibitory neuron activity. Further studies are warranted to establish NMDAR's role in modulating microcirculation in schizophrenia.

Cerebral blood flow in striatum is increased by partial dopamine agonism in initially antipsychotic-naïve patients with psychosis

BACKGROUND

Resting cerebral blood flow (rCBF) in striatum and thalamus is increased in medicated patients with psychosis, but whether this is caused by treatment or illness pathology is unclear. Specifically, effects of partial dopamine agonism, sex, and clinical correlates on rCBF are sparsely investigated. We therefore assessed rCBF in antipsychotic-naïve psychosis patients before and after aripiprazole monotherapy and related findings to sex and symptom improvement.

METHODS

We assessed rCBF with the pseudo-Continuous Arterial Spin Labeling (PCASL) sequence in 49 first-episode patients (22.6 ± 5.2 years, 58% females) and 50 healthy controls (HCs) (22.3 ± 4.4 years, 63% females) at baseline and in 29 patients and 49 HCs after six weeks. RCBF in striatum and thalamus was estimated with a region-of-interest (ROI) approach. Psychopathology was assessed with the positive and negative syndrome scale.

RESULTS

Baseline rCBF in striatum and thalamus was not altered in the combined patient group compared with HCs, but female patients had lower striatal rCBF compared with male patients (p = 0.009). Treatment with a partial dopamine agonist increased rCBF significantly in striatum (p = 0.006) in the whole patient group, but not significantly in thalamus. Baseline rCBF in nucleus accumbens was negatively associated with improvement in positive symptoms (p = 0.046), but baseline perfusion in whole striatum and thalamus was not related to treatment outcome.

CONCLUSIONS

The findings suggest that striatal perfusion is increased by partial dopamine agonism and decreased in female patients prior to first treatment. This underlines the importance of treatment effects and sex differences when investigating the neurobiology of psychosis.

Diversifying autism neuroimaging research: An arterial spin labeling review

Cognition and brain homeostasis depends on cerebral blood flow to secure adequate oxygen and nutrient distribution to the brain tissue. Altered cerebral blood flow has previously been reported in individuals diagnosed with autism spectrum condition in comparison to non-autistics. This phenomenon might suggest cerebral blood flow as a potential biomarker for autism spectrum condition. Major technological advancement enables the non-invasive and quantitative measurement of cerebral blood flow via arterial spin labeling magnetic resonance imaging. However, most neuroimaging studies in autistic individuals exploit the indirect blood oxygen level dependent functional magnetic resonance imaging signal instead. Therefore, this review examines the use of arterial spin labeling to further investigate the neurobiology of the autism spectrum condition. Followed by a comparison of results from molecular imaging and arterial spin labeling studies and a discussion concerning the future direction and potential of arterial spin labeling in this context. We found that arterial spin labeling study results are consistent with those of molecular imaging, especially after considering the effect of age and sex. Arterial spin labeling has numerous application possibilities besides the quantification of cerebral blood flow, including assessment of functional connectivity and arterial transit time. Therefore, we encourage researchers to explore and consider the application of arterial spin labeling for future scientific studies in the quest to better understand the neurobiology of autism spectrum condition.

Lay abstract

Brain function and health depend on cerebral blood flow to secure the necessary delivery of oxygen and nutrients to the brain tissue. However, cerebral blood flow appears to be altered in autistic compared to non-autistic individuals, potentially suggesting this difference to be a cause and potential identification point of autism. Recent technological development enables precise and non-invasive measurement of cerebral blood flow via the magnetic resonance imaging method referred to as arterial spin labeling. However, most neuroimaging studies still prefer using the physiologically indirect measure derived from functional magnetic resonance imaging. Therefore, this review examines the use of arterial spin labeling to further investigate the neurobiology of autism. Furthermore, the review includes a comparison of results from molecular imaging and arterial spin labeling followed by a discussion concerning the future direction and potential of arterial spin labeling. We found that arterial spin labeling study results are consistent with those of molecular imaging, especially after considering the effect of age and sex. In addition, arterial spin labeling has numerous application possibilities besides the quantification of cerebral blood flow. Therefore, we encourage researchers to explore and consider the application of arterial spin labeling for future scientific studies in the quest to better understand the neurobiology of autism.

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.

Impaired verbal memory function is related to anterior cingulate glutamate levels in schizophrenia: findings from the STRATA study

Impaired cognition is associated with lower quality of life and poor outcomes in schizophrenia. Brain glutamate may contribute to both clinical outcomes and cognition, but these relationships are not well-understood. We studied a multicentre cohort of 85 participants with non-affective psychosis using proton magnetic resonance spectroscopy. Glutamate neurometabolites were measured in the anterior cingulate cortex (ACC). Cognition was assessed using the Brief Assessment for Cognition in Schizophrenia (BACS). Patients were categorised as antipsychotic responders or non-responders based on treatment history and current symptom severity. Inverted U-shaped associations between glutamate or Glx (glutamate + glutamine) with BACS subscale and total scores were examined with regression analyses. We then tested for an interaction effect of the antipsychotic response group on the relationship between glutamate and cognition. ACC glutamate and Glx had a positive linear association with verbal memory after adjusting for age, sex and chlorpromazine equivalent dose (glutamate, β = 3.73, 95% CI = 1.26-6.20, P = 0.004; Glx, β = 3.38, 95% CI = 0.84-5.91, P = 0.01). This association did not differ between good and poor antipsychotic response groups. ACC glutamate was also positively associated with total BACS score (β = 3.12, 95% CI = 0.01-6.23, P = 0.046), but this was not significant after controlling for antipsychotic dose. Lower glutamatergic metabolites in the ACC were associated with worse verbal memory, and this relationship was independent of antipsychotic response. Further research on relationships between glutamate and cognition in antipsychotic responsive and non-responsive illness could aid the stratification of patient groups for targeted treatment interventions.

Glutamate levels across deep brain structures in patients with a psychotic disorder and its relation to cognitive functioning

BACKGROUND

Patients with psychotic disorders often show prominent cognitive impairment. Glutamate seems to play a prominent role, but its role in deep gray matter (DGM) regions is unclear.

AIMS

To evaluate glutamate levels within deep gray matter structures in patients with a psychotic disorder in relation to cognitive functioning, using advanced spectroscopic acquisition, reconstruction, and post-processing techniques.

METHODS

A 7-Tesla magnetic resonance imaging scanner combined with a lipid suppression coil and subject-specific water suppression pulses was used to acquire high-resolution magnetic resonance spectroscopic imaging data. Tissue fraction correction and registration to a standard brain were performed for group comparison in specifically delineated DGM regions. The brief assessment of cognition in schizophrenia was used to evaluate cognitive status.

RESULTS

Average glutamate levels across DGM structures (i.e. caudate, pallidum, putamen, and thalamus) in mostly medicated patients with a psychotic disorder (n = 16, age = 33, 4 females) were lower compared to healthy controls (n = 23, age = 24, 7 females; p = 0.005, d = 1.06). Stratified analyses showed lower glutamate levels in the caudate (p = 0.046, d = 0.76) and putamen p = 0.013, d = 0.94). These findings were largely explained by age differences between groups. DGM glutamate levels were positively correlated with psychomotor speed (r(30) = 0.49, p = 0.028), but not with other cognitive domains.

CONCLUSIONS

We find reduced glutamate levels across DGM structures including the caudate and putamen in patients with a psychotic disorder that are linked to psychomotor speed. Despite limitations concerning age differences, these results underscore the potential role of detailed in vivo glutamate assessments to understand cognitive deficits in psychotic disorders.

Effect of mGluR2 positive allosteric modulation on frontostriatal working memory activation in schizophrenia

Negative symptoms and cognitive deficits contribute strongly to disability in schizophrenia, and are resistant to existing medications. Recent drug development has targeted enhanced NMDA function by increasing mGluR2/3 signaling. However, the clinical utility of such agents remains uncertain, and markers of brain circuit function are critical for clarifying mechanisms and understanding individual differences in efficacy. We conducted a double-blind, placebo-controlled, randomized cross-over (14 day washout) pilot study evaluating adjunctive use of the mGluR2 positive allosteric modulator AZD8529 (80 mg daily for 3 days), in chronic stable patients with schizophrenia (n = 26 analyzed). We focused on 3 T fMRI response in frontostriatal regions during an n-back working memory task, testing the hypothesis that AZD8529 produces fMRI changes that correlate with improvement in negative symptoms and cognition. We found that AZD8529 did not produce significant group-average effects on symptoms or cognitive accuracy. However, AZD8529 did increase n-back fMRI activation in striatum (p < 0.0001) and anterior cingulate/paracingulate (p = 0.002). Greater drug-versus-placebo effects on caudate activation significantly correlated with greater reductions in PANSS negative symptom scores (r = -0.42, p = 0.031), and exploratory correlations suggested broader effects across multiple symptom domains and regions of interest. These findings demonstrate that fMRI responses to mGluR2 positive modulation relate to individual differences in symptom reduction, and could be pursued for future biomarker development. Negative clinical results at the group level should not lead to premature termination of investigation of this mechanism, which may benefit an important subset of individuals with schizophrenia. Imaging biomarkers may reveal therapeutic mechanisms, and help guide treatment toward specific populations.

Ready for a "breakthrough" with ketamine? A look at recent pharmacological insights!

PURPOSE OF REVIEW

To update pharmacological insights on ketamine integrating information from different disciplines for developing steps to "breakthrough" approaches in clinical challenges.

RECENT FINDINGS

Pharmacokinetic/pharmacodynamic (PK/PD) models have incorporated recirculation, ketamine metabolites, drug-drug interaction, and covariates such as age. Ketamine-induced relief from treatment-resistant depression has been explained by "disinhibition" of gamma-aminobutyric acid-ergic interneurons and synaptogenic mechanisms requiring neurotrophic signals. Neuroimaging/electroencephalographic investigations have shown an increase in gamma spectral power in healthy volunteers and patients with depression, but also opposite changes in functional network connectivity after subanesthetic ketamine. Volunteer data may not be transferable to clinical conditions. Altered states of consciousness induced by subanesthetic ketamine have been described by disruption of resisting-state functional networks and frontoparietal connectivity with preservation of multisensory and sensor-motor networks. This has been interpreted as a "disconnected consciousness".

SUMMARY

More precise PK/PD models may improve the ketamine use regimen. The findings from research on depression are an important discovery because ketamine's impact on neuronal plasticity and synaptogenesis in human brain disease has directly been documented. Psychic adverse effects with subanesthetic ketamine are related to a "disconnected consciousness". Overall, progress has been made, but the "breakthrough" still has to come.

Association between cannabinoid 1 receptor availability and glutamate levels in healthy controls and drug-free patients with first episode psychosis: a multi-modal PET and 1H-MRS study

Cannabinoid 1 receptor and glutamatergic dysfunction have both been implicated in the pathophysiology of schizophrenia. However, it remains unclear if cannabinoid 1 receptor alterations shown in drug-naïve/free patients with first episode psychosis may be linked to glutamatergic alterations in the illness. We aimed to investigate glutamate levels and cannabinoid 1 receptor levels in the same region in patients with first episode psychosis. Forty volunteers (20 healthy volunteers, 20 drug-naïve/free patients with first episode psychosis diagnosed with schizophrenia/schizoaffective disorder) were included in the study. Glutamate levels were measured using proton magnetic resonance spectroscopy. CB1R availability was indexed using the distribution volume (VT (ml/cm3)) of [11C]MePPEP using arterial blood sampling. There were no significant associations between ACC CB1R levels and ACC glutamate levels in controls (R = - 0.24, p = 0.32) or patients (R = - 0.10, p = 0.25). However, ACC glutamate levels were negatively associated with CB1R availability in the striatum (R = - 0.50, p = 0.02) and hippocampus (R = - 0.50, p = 0.042) in controls, but these associations were not observed in patients (p > 0.05). Our findings extend our previous work in an overlapping sample to show, for the first time as far as we're aware, that cannabinoid 1 receptor alterations in the anterior cingulate cortex are shown in the absence of glutamatergic dysfunction in the same region, and indicate potential interactions between glutamatergic signalling in the anterior cingulate cortex and the endocannabinoid system in the striatum and hippocampus.

Effect of Ketamine on Human Neurochemistry in Posterior Cingulate Cortex: A Pilot Magnetic Resonance Spectroscopy Study at 3 Tesla

Ketamine is a powerful glutamatergic long-lasting antidepressant, efficient in intractable major depression. Whereas ketamine's immediate psychomimetic side-effects were linked to glutamate changes, proton MRS (1H-MRS) showed an association between the ratio of glutamate and glutamine and delayed antidepressant effect emerging ∼2 h after ketamine administration. While most 1H-MRS studies focused on anterior cingulate, recent functional MRI connectivity studies revealed an association between ketamine's antidepressant effect and disturbed connectivity patterns to the posterior cingulate cortex (PCC), and related PCC dysfunction to rumination and memory impairment involved in depressive pathophysiology. The current study utilized the state-of-the-art single-voxel 3T sLASER 1H-MRS methodology optimized for reproducible measurements. Ketamine's effects on neurochemicals were assessed before and ∼3 h after intravenous ketamine challenge in PCC. Concentrations of 11 neurochemicals, including glutamate (CRLB ∼ 4%) and glutamine (CRLB ∼ 13%), were reliably quantified with the LCModel in 12 healthy young men with between-session coefficients of variation (SD/mean) <8%. Also, ratios of glutamate/glutamine and glutamate/aspartate were assessed as markers of synaptic function and activated glucose metabolism, respectively. Pairwise comparison of metabolite profiles at baseline and 193 ± 4 min after ketamine challenge yielded no differences. Minimal detectable concentration differences estimated with post hoc power analysis (power = 80%, alpha = 0.05) were below 0.5 μmol/g, namely 0.39 μmol/g (∼4%) for glutamate, 0.28 μmol/g (∼10%) for Gln, ∼14% for glutamate/glutamine and ∼8% for glutamate/aspartate. Despite the high sensitivity to detect between-session differences in glutamate and glutamine concentrations, our study did not detect delayed glutamatergic responses to subanesthetic ketamine doses in PCC.

Associations Between Cognitive Function and Levels of Glutamatergic Metabolites and Gamma-Aminobutyric Acid in Antipsychotic-Naïve Patients With Schizophrenia or Psychosis

BACKGROUND

Abnormal glutamate and GABA (gamma-aminobutyric acid) levels have been found in the early phase of schizophrenia and may underlie cognitive deficits. However, the association between cognitive function and levels of glutamatergic metabolites and GABA has not been investigated in a large group of antipsychotic-naïve patients.

METHODS

In total, 56 antipsychotic-naïve patients with schizophrenia or psychotic disorder and 51 healthy control subjects underwent magnetic resonance spectroscopy to measure glutamate, glutamate+glutamine (Glx), and GABA levels in dorsal anterior cingulate cortex (ACC) and glutamate and Glx levels in left thalamus. The cognitive domains of attention, working memory, and IQ were assessed.

RESULTS

The whole group of antipsychotic-naïve patients had lower levels of GABA in dorsal ACC (p = .03), and the subgroup of patients with a schizophrenia diagnosis had higher glutamate levels in thalamus (p = .01), but Glx levels in dorsal ACC and thalamus did not differ between groups. Glx levels in dorsal ACC were positively associated with working memory (logarithmically transformed: b = -.016 [higher score indicates worse performance], p = .005) and attention (b = .056, p = .035) in both patients and healthy control subjects, although the association with attention did not survive adjustment for multiple comparisons.

CONCLUSIONS

The findings suggest a positive association between glutamatergic metabolites and cognitive function that do not differ between patients and healthy control subjects. Moreover, our data indicate that decreased GABAergic levels in dorsal ACC are involved in schizophrenia and psychotic disorder, whereas increased glutamate levels in thalamus seem to be implicated in schizophrenia pathophysiology. The findings imply that first-episode patients with cognitive deficits may gain from glutamate-modulating compounds.

The potential of 1H-MRS in CNS drug development

RATIONALE

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

OBJECTIVES

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

RESULTS

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

CONCLUSIONS

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

Treatment response after 6 and 26 weeks is related to baseline glutamate and GABA levels in antipsychotic-naïve patients with psychosis

BACKGROUND

Poor response to dopaminergic antipsychotics constitutes a major challenge in the treatment of psychotic disorders and markers for non-response during first-episode are warranted. Previous studies have found increased levels of glutamate and γ-aminobutyric acid (GABA) in non-responding first-episode patients compared to responders, but it is unknown if non-responders can be identified using reference levels from healthy controls (HCs).

METHODS

Thirty-nine antipsychotic-naïve patients with first-episode psychosis and 36 matched HCs underwent repeated assessments with the Positive and Negative Syndrome Scale and 3T magnetic resonance spectroscopy. Glutamate scaled to total creatine (/Cr) was measured in the anterior cingulate cortex (ACC) and left thalamus, and levels of GABA/Cr were measured in ACC. After 6 weeks, we re-examined 32 patients on aripiprazole monotherapy and 35 HCs, and after 26 weeks we re-examined 30 patients on naturalistic antipsychotic treatment and 32 HCs. The Andreasen criteria defined non-response.

RESULTS

Before treatment, thalamic glutamate/Cr was higher in the whole group of patients but levels normalized after treatment. ACC levels of glutamate/Cr and GABA/Cr were lower at all assessments and unaffected by treatment. When compared with HCs, non-responders at week 6 (19 patients) and week 26 (16 patients) had higher baseline glutamate/Cr in the thalamus. Moreover, non-responders at 26 weeks had lower baseline GABA/Cr in ACC. Baseline levels in responders and HCs did not differ.

CONCLUSION

Glutamatergic and GABAergic abnormalities in antipsychotic-naïve patients appear driven by non-responders to antipsychotic treatment. If replicated, normative reference levels for glutamate and GABA may aid estimation of clinical prognosis in first-episode psychosis patients.

Glutamate in schizophrenia: Neurodevelopmental perspectives and drug development

Research into the neurobiological processes that may lead to the onset of schizophrenia places growing emphasis on the glutamatergic system and brain development. Preclinical studies have shown that neurodevelopmental, genetic, and environmental factors contribute to glutamatergic dysfunction and schizophrenia-related phenotypes. Clinical research has suggested that altered brain glutamate levels may be present before the onset of psychosis and relate to outcome in those at clinical high risk. After psychosis onset, glutamate dysfunction may also relate to the degree of antipsychotic response and clinical outcome. These findings support ongoing efforts to develop pharmacological interventions that target the glutamate system and could suggest that glutamatergic compounds may be more effective in specific patient subgroups or illness stages. In this review, we consider the updated glutamate hypothesis of schizophrenia, from a neurodevelopmental perspective, by reviewing recent preclinical and clinical evidence, and discuss the potential implications for novel therapeutics.

The neurophysiology of ketamine: an integrative review

The drug ketamine has been extensively studied due to its use in anaesthesia, as a model of psychosis and, most recently, its antidepressant properties. Understanding the physiology of ketamine is complex due to its rich pharmacology with multiple potential sites at clinically relevant doses. In this review of the neurophysiology of ketamine, we focus on the acute effects of ketamine in the resting brain. We ascend through spatial scales starting with a complete review of the pharmacology of ketamine and then cover its effects on in vitro and in vivo electrophysiology. We then summarise and critically evaluate studies using EEG/MEG and neuroimaging measures (MRI and PET), integrating across scales where possible. While a complicated and, at times, confusing picture of ketamine’s effects are revealed, we stress that much of this might be caused by use of different species, doses, and analytical methodologies and suggest strategies that future work could use to answer these problems.

A New Perspective on Using Glycols in Glutamate Biosensor Design: From Stabilizing Agents to a New Containment Net

Glutamate is a major excitatory neurotransmitter in the brain. It is involved in many normal physiological brain activities, but also neurological disorders and excitotoxicity. Hence, glutamate measurement is important both in clinical and pre-clinical studies. Pre-clinical studies often use amperometric biosensors due to their low invasiveness and the relatively small size of the devices. These devices also provide fast, real-time measurements because of their high sensitivity. In the present study, diethylene glycol (DEG), neopentyl glycol (NPG), triethylene glycol (TEG), and glycerol (GLY) were used to increase the long-term stability of glutamate biosensors. The evaluation was made by measuring variations of the main enzymatic (VMAX and KM) and analytical (Linear Region Slope (LRS)) parameters. Of the glycols tested, TEG was the most promising stabilizer, showing about twice as high VMAX maintained over a greater duration than with other stabilizers tested. It is also yielded the most stable linear region slope (LRS) values over the study duration. Moreover, we highlighted the ability of glycols to interact with enzyme molecules to form a containment network, able to maintain all the layered components of the biosensor adhering to the transducer.

Ketamine induced changes in regional cerebral blood flow, interregional connectivity patterns, and glutamate metabolism

Several imaging studies have attempted to characterize the contribution of glutamatergic dysfunction to functional dysconnectivity of large-scale brain networks using ketamine models. However, findings from BOLD imaging studies are conflicting, in part because the signal stems from a complex interaction between blood flow, blood volume, and oxygen consumption. We used arterial spin labelling imaging to measure regional cerebral blood flow (rCBF) in a group of healthy volunteers during a saline and during a ketamine infusion. We examined changes in rCBF and interregional connectivity patterns, as well as their associations with clinical symptom severity and Glx (glutamate + glutamine) assessed with magnetic resonance spectroscopy. We report a regionally selective pattern of rCBF changes following ketamine administration and complex changes in interregional connectivity patterns. We also found that the increase in rCBF in the bilateral putamen and left hippocampus was positively correlated with ketamine induced clinical symptom severity while anterior cingulate rCBF during the ketamine challenge was negatively correlated with change in hippocampal Glx. Our study adds to the efforts to empirically confirm putative links between an NMDA receptor blockage and dysconnectivity of large-scale brain networks, specifically the salience, executive control and default mode networks, suggesting that a glutamatergic imbalance may contribute to dysconnectivity. Development of glutamatergic compounds that alleviate disease burden, possibly through normalizing glutamate excess related increased rCBF, is direly needed.

Glutamate levels in the anterior cingulate cortex in un-medicated first episode psychosis: a proton magnetic resonance spectroscopy study

Converging lines of evidence suggest that glutamatergic dysfunction may contribute to the pathophysiology of first episode psychosis. We investigated whether first episode psychosis patients free from all pharmacological treatments and illicit substances show cortical glutamatergic alterations. One-hundred and eleven volunteers including 65 healthy volunteers and 46 first episode psychosis patients free from all pharmacological treatments (28 drug naïve) underwent a proton magnetic resonance spectroscopy scan measuring glutamate levels in the bilateral anterior cingulate cortex. Symptom severity was measured using the Positive and Negative Syndrome Scale (PANSS) and cognition was measured using the Wechsler Adult Intelligence Scale (WAIS) digit symbol test. There were no differences in glutamate levels between patients and controls. These findings remained unchanged when adjusting for the effects of age, sex and ethnicity or when restricting the analyses to patients who were both medication naïve to all pharmacological treatments and illicit substances. Whilst these findings do not preclude glutamatergic alterations in psychosis, methodological advances are needed for us to investigate whether patients show alterations in other aspects of glutamate function, such as pre-synaptic glutamate or release.