Low glial numbers in the amygdala in major depressive disorder

Background: Functional imaging studies implicate the prefrontal cortex and amygdala in major depressive disorder and bipolar disorder, and glial decreases have been reported in the prefrontal cortex. Here, glia and neurons were counted in the amygdala and entorhinal cortex in major depressive disorder, bipolar disorder, and control cases. Methods: Tissue blocks from major depressive disorder (7), bipolar disorder (10), and control (12) cases, equally divided between right and left, were cut into 50 mum sections and stained with the Nissl method. One major depressive disorder and all but two bipolar disorder cases had been treated with lithium or valproate. Neurons and glia were counted using stereological methods. Results: Glial density and the glia/neuron ratio were substantially reduced in the amygdala in major depressive disorder cases. The reduction was mainly accounted for by counts in the left hemisphere. No change was found in neurons. Average glia measures were not reduced in bipolar disorder cases; however, bipolar disorder cases not treated with lithium or valproate had significant glial reduction. Similar but smaller changes were found in the entorhinal cortex. Conclusions: Glia are reduced in the amygdala in major depressive disorder, especially on the left side. The results suggest that lithium and valproate may moderate the glial reduction.