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

Abstract

Background: Alteration of glutamatergic neurotransmission in the prefrontal cortex (PFC) may contribute to the pathophysiology of alcoholism and major depressive disorder (MDD). Among glial cells, astrocytes are mostly responsible for recycling synaptic glutamate by uptake through excitatory amino acid transporters 1 and 2 (EAAT1 and EAAT2), and conversion to glutamine with glutamine synthetase (GS). Low density of astrocytes in the PFC of "uncomplicated' alcoholics and MDD subjects may parallel altered glutamate transporters and GS in the PFC.

Methods: Immunohistochemistry and Western blotting for glutamate transporters, GS and glial fibrillary acidic protein (GFAP) were applied to postmortem tissue of the left orbitofrontal cortex from 13 subjects with MDD, 13 with alcoholism, 10 with comorbid alcoholism plus MDD (MDA), and 13 non-psychiatric controls. Area fraction of immunoreactivity was measured in sections, and protein levels in Western blots.

Results: EAAT2 immunoreactivity was significantly lower in MDD and MDA subjects than in controls. EAAT1 levels were lower in MDA and MDD subjects as compared to controls, while GS levels in MDA were significantly lower than in alcoholics and controls, and lower in MDD subjects than in alcoholics. Area fraction of GFAP was lower in MDD, but not in MDA subjects as compared to controls or alcoholics.

Limitations: High variability of protein levels in some groups and effects of antidepressant treatment, although appearing to be limited, cannot be fully evaluated.

Conclusions: There are differential changes in the expression of glial glutamatergic markers in depression and alcoholism, suggesting a depletion of certain aspects of glutamatergic processing in depression.

Fig. 1

The top four panels are representative micrographs of EAAT2 immunoreactivity in the ORB of normal controls (A), MDD subjects without alcoholism (MDD) (B), MDD subjects with comorbid alcoholism (MDA) (C), and alcoholics (D). E) is a graph summarizing the results of measurements and the statistical comparisons of the area fraction of EAAT2 immunoreactivity in the same groups of subjects. F) High magnification picture of EAAT2 immunoreactive astrocytes in layer III of a control subject. Note the localization of immunoreactivity to the fine branches of astrocyte processes. MDD: Subjects with diagnosis of Major depressive disorder but not alcoholism; MDA: Subjects with comorbid diagnoses of major depressive disorder and alcoholism: ALC: Subjects with a diagnosis of alcoholism, and without diagnosis of comorbid major psychiatric disorder. Bar in C applies to A, B, C and D panels.

Fig. 2

Representative Western blots (A and C), and graphs (B and D) summarizing the quantification of the relative levels of EAAT2 and EAAT1 from Western blots for the four diagnostic groups in this study. Abbreviations as in Fig. 1.

Fig. 3

Representative Western blots (A), and graph (B) summarizing the quantification of the relative levels of glutamine synthetase (GS) from Western blots for the four diagnostic groups in this study. Abbreviations as in Fig. 1.

Fig. 4

Scatter plots illustrating the negative correlation between levels of GS and levels of EAAT2 in control (A) and alcohol-dependent subjects (B).

Fig. 5

The top four panels are representative micrographs of GFAP immunoreactivity in the ORB of normal controls (A), MDD subjects without alcoholism (B), MDA subjects (C), and alcoholics (D). Panel E summarizes the results of measurements and the statistical comparisons of the area fraction of EAAT2 immunoreactivity in the same groups of subjects. Abbreviations as in Fig. 3. Panel F shows a representative Western blot for GFAP protein in the ORB and summarizes in a graph the results for GFAP levels in the four groups. Abbreviations as in Fig. 1.

Fig. 6

Scatter plots illustrating the correlation between onset of disorder (A and B), duration (C) or age (D and E) and the levels of EAAT1 or GFAP.