Reduced level of glutamic acid decarboxylase-67 kDa in the prefrontal cortex in major depression

Abstract

Accumulating evidence suggests dysfunction of the gamma-aminobutyric acid (GABA) system in major depressive disorder (MDD). Neuroimaging studies consistently report reductions of cortical GABA in depressed patients. Our post-mortem analyses demonstrate a reduction in the density and size of GABAergic interneurons in the dorsolateral prefrontal cortex (DLPFC) in MDD. The goal of this study was to test whether the level of glutamic acid decarboxylase (GAD), the GABA synthesizing enzyme, will also be reduced in the same cortical region in MDD. Levels of GAD-65 and GAD-67 proteins were investigated by Western blotting in samples from the DLPFC (BA 9) in 13 medication-free subjects with MDD, and 13 psychiatrically healthy controls. The overall amount of GAD-67 was significantly reduced (-34%) in depressed subjects compared to matched controls. Since recent neuroimaging studies have demonstrated that antidepressants modulate GABA levels, additional experiments were performed to examine the levels of GAD in eight depressed subjects treated with antidepressant medications. Levels of GAD-67 were unchanged in these depressed subjects compared to their respective controls (n=8). The overall amounts of GAD-65 were similar in depressed subjects compared to matched controls, regardless of antidepressant medication. Reduced levels of GAD-67, which is localized to somata of GABA neurons, further support our observation of a decreased density of GABAergic neurons in the PFC in depression. It is likely that a decrease in GAD-67 accounts for the reduction in GABA levels revealed by neuroimaging studies. Moreover, our data support previous neuroimaging observations that antidepressant medication normalizes GABA deficits in depression.

Figure 1

Relationship between the optical density values of Western-blotted protein immunoreactivities and protein amounts for GAD-67, GAD-65, and tubulin. Wells were loaded with three concentrations of cortical tissue consisting of 10, 20, and 40 μg total protein.

Figure 2

Medication-free MDD subjects vs controls. Immunoblots of GAD-67 (A), GAD-65 (C) and tubulin from two representative pairs of control and medication-free MDD subjects used in the analysis; (B) significant decrease in the GAD-67 immunoreactivity (−34%) was observed in depressed subjects (n=13) as compared to controls (n=13); (D) amount of GAD-65 immunoreactivity from control (n=13) and depressed (n=13) subjects. Normalized optical density values for the individual subjects (circles) and mean values (horizontal lines) are presented.

Figure 3

GAD-67 immunoreactivity from individual medication-free MDD subjects expressed as percentages of values from paired control subjects. Each bar is an average of duplicate comparisons.

Figure 4

Medicated MDD subjects vs controls. Immunoblots of GAD-67 (A), GAD-65 (C) and tubulin from two representative pairs of control and medicated MDD subjects used in the analysis; (B) amount of GAD-67 immunoreactivity from control (n=8) and depressed (n=8) subjects; (D) amount of GAD-65 immunoreactivity from control (n=8) and depressed (n=8) subjects. Normalized optical density values for the individual subjects (circles) and mean values (horizontal lines) are presented.