Glial and glutamatergic markers in depression, alcoholism, and their comorbidity

MR spectroscopic studies of the brain in psychiatric disorders

The measurement of brain metabolites with magnetic resonance spectroscopy (MRS) provides a unique perspective on the brain bases of neuropsychiatric disorders. As a context for interpreting MRS studies of neuropsychiatric disorders, we review the characteristic MRS signals, the metabolic dynamics,and the neurobiological significance of the major brain metabolites that can be measured using clinical MRS systems. These metabolites include N-acetylaspartate(NAA), creatine, choline-containing compounds, myo-inositol, glutamate and glutamine, lactate, and gamma-amino butyric acid (GABA). For the major adult neuropsychiatric disorders (schizophrenia, bipolar disorder, major depression, and the anxiety disorders), we highlight the most consistent MRS findings, with an emphasis on those with potential clinical or translational significance. Reduced NAA in specific brain regions in schizophrenia, bipolar disorder, post-traumatic stress disorder, and obsessive–compulsive disorder corroborate findings of reduced brain volumes in the same regions. Future MRS studies may help determine the extent to which the neuronal dysfunction suggested by these findings is reversible in these disorders. Elevated glutamate and glutamine (Glx) in patients with bipolar disorder and reduced Glx in patients with unipolar major depression support models of increased and decreased glutamatergic function, respectively, in those conditions. Reduced phosphomonoesters and intracellular pH in bipolar disorder and elevated dynamic lactate responses in panic disorder are consistent with metabolic models of pathogenesis in those disorders. Preliminary findings of an increased glutamine/glutamate ratio and decreased GABA in patients with schizophrenia are consistent with a model of NMDA hypofunction in that disorder. As MRS methods continue to improve, future studies may further advance our understanding of the natural history of psychiatric illnesses, improve our ability to test translational models of pathogenesis, clarify therapeutic mechanisms of action,and allow clinical monitoring of the effects of interventions on brain metabolicmarkers

Astrocytic hypertrophy in anterior cingulate white matter of depressed suicides

Increasing evidence suggests that cortical astrocytic function is disrupted in mood disorders and suicide. The fine neuroanatomy of astrocytes, however, remains to be investigated in these psychiatric conditions. In this study, we performed a detailed morphometric analysis of 3D-reconstructed gray and white matter astrocytes in Golgi-impregnated anterior cingulate cortex (ACC) samples from depressed suicides and matched controls. Postmortem ACC samples (BA24) from 10 well-characterized depressed suicides and 10 matched sudden-death controls were obtained from the Quebec Suicide Brain Bank. Golgi-impregnated protoplasmic astrocytes (gray matter, layer VI) and fibrous astrocytes (adjacent white matter) were reconstructed, and their morphometric features were analyzed using the Neurolucida software. For each cell, the soma size as well as the number, length, and branching of processes were determined. The densities of thorny protrusions found along the processes of both astrocytic subtypes were also determined. Protoplasmic astrocytes showed no significant difference between groups for any of the quantified parameters. However, fibrous astrocytes had significantly larger cell bodies, as well as longer, more ramified processes in depressed suicides, with values for these parameters being about twice as high as those measured in controls. These results provide the first evidence of altered cortical astrocytic morphology in mood disorders. The presence of hypertrophic astrocytes in BA24 white matter is consistent with reports suggesting white matter alterations in depression, and provides further support to the neuroinflammatory theory of depression.

The stressed synapse: the impact of stress and glucocorticoids on glutamate transmission

Mounting evidence suggests that acute and chronic stress, especially the stress-induced release of glucocorticoids, induces changes in glutamate neurotransmission in the prefrontal cortex and the hippocampus, thereby influencing some aspects of cognitive processing. In addition, dysfunction of glutamatergic neurotransmission is increasingly considered to be a core feature of stress-related mental illnesses. Recent studies have shed light on the mechanisms by which stress and glucocorticoids affect glutamate transmission, including effects on glutamate release, glutamate receptors and glutamate clearance and metabolism. This new understanding provides insights into normal brain functioning, as well as the pathophysiology and potential new treatments of stress-related neuropsychiatric disorders.

Fluoxetine affects GluK2 editing, glutamate-evoked Ca(2+) influx and extracellular signal-regulated kinase phosphorylation in mouse astrocytes

BACKGROUND

We sought to study the effects of chronic exposure to fluoxetine - a selective serotonin reuptake inhibitor (SSRI) and specific 5-HT(2B) receptor agonist in astrocytes - on the expression of kainate receptors (GluK1-5) in cultured astrocytes and in intact brains in mice and on GluK2 editing by adenosine deaminase acting on RNA (ADAR), as well as the ensuing effects of fluoxetine on glutamate-mediated Ca(2+) influx and extracellular signal-regulated kinase (ERK)(1/2) phosphorylation in astrocytes.

METHODS

We performed reverse transcription-polymerase chain reaction (PCR) to assess mRNA expression. We analyzed RNA editing with amplification refractory mutation system PCR and complementary DNA sequencing. Protein expression and ERK phosphorylation were assessed using Western blots. We studied gene silencing with specific small interfering RNAs (siRNA), and we studied intracellular Ca(2+) using fluorometry.

RESULTS

All GluK subunits were present in the brain in vivo, and GluK2-5 subunits were present in cultured astrocytes. Fluoxetine upregulated GluK2 and ADAR2. Enhanced GluK2 editing by fluoxetine abolished glutamate-mediated increases in intra cellular Ca(2+) and ERK(1/2) phosphorylation. Enhanced editing of GluK2 was prevented by siRNA against the 5-HT(2B) receptor or ADAR2.

LIMITATIONS

Limitations of our study include the use of an in vitro system, but our cultured cells in many respects behave like in vivo astrocytes.

CONCLUSION

Fluoxetine alters astrocytic glutamatergic function.

Towards a glutamate hypothesis of depression: an emerging frontier of neuropsychopharmacology for mood disorders

Half a century after the first formulation of the monoamine hypothesis, compelling evidence implies that long-term changes in an array of brain areas and circuits mediating complex cognitive-emotional behaviors represent the biological underpinnings of mood/anxiety disorders. A large number of clinical studies suggest that pathophysiology is associated with dysfunction of the predominant glutamatergic system, malfunction in the mechanisms regulating clearance and metabolism of glutamate, and cytoarchitectural/morphological maladaptive changes in a number of brain areas mediating cognitive-emotional behaviors. Concurrently, a wealth of data from animal models have shown that different types of environmental stress enhance glutamate release/transmission in limbic/cortical areas and exert powerful structural effects, inducing dendritic remodeling, reduction of synapses and possibly volumetric reductions resembling those observed in depressed patients. Because a vast majority of neurons and synapses in these areas and circuits use glutamate as neurotransmitter, it would be limiting to maintain that glutamate is in some way 'involved' in mood/anxiety disorders; rather it should be recognized that the glutamatergic system is a primary mediator of psychiatric pathology and, potentially, also a final common pathway for the therapeutic action of antidepressant agents. A paradigm shift from a monoamine hypothesis of depression to a neuroplasticity hypothesis focused on glutamate may represent a substantial advancement in the working hypothesis that drives research for new drugs and therapies. Importantly, despite the availability of multiple classes of drugs with monoamine-based mechanisms of action, there remains a large percentage of patients who fail to achieve a sustained remission of depressive symptoms. The unmet need for improved pharmacotherapies for treatment-resistant depression means there is a large space for the development of new compounds with novel mechanisms of action such as glutamate transmission and related pathways. This article is part of a Special Issue entitled 'Anxiety and Depression'.