Glial reduction in amygdala in major depressive disorder is due to oligodendrocytes

Abstract

Background A previous study reported reductions in glial density and glia/neuron ratio in the amygdala of individuals with major depressive disorder (MDD), without a change in neuronal density. It is not known, however, whether this glial loss is due to astrocytes, oligodendrocytes, or microglia. Methods Tissue samples, equally from the right and left hemispheres, were obtained from subjects diagnosed with MDD ( n = 8), bipolar disorder (BD) ( n = 9), or no psychiatric disorders ( n = 10). Sections were stained immunohistochemically for S-100β (for astrocytes) and human leukocyte antigen (for microglia), and with the Nissl method. In Nissl-stained sections, oligodendrocytes have more compact, darker-stained nuclei, whereas astrocytes and microglia have larger, lighter-stained nuclei, with more granular chromatin. Neurons are larger, with a nucleolus and stained cytoplasm. The density of glia was determined with stereologic methods. Results The density of total glia and oligodendrocytes in the amygdala was significantly lower in MDD than in control subjects, but not significantly lower in BD compared with control subjects. The decreases were largely accounted for by differences in the left hemisphere. There was no significant decrease in astrocyte or microglia density in MDD or BD subjects. Conclusions The glial cell reduction previously found in the amygdala in MDD is primarily due to oligodendrocytes.