Hippocampal granule neuron number and dentate gyrus volume in antidepressant-treated and untreated major depression

Abstract

Smaller hippocampal volume is reported in major depressive disorder (MDD). We hypothesize that it may be related to fewer granule neurons (GN) in the dentate gyrus (DG), a defect possibly reversible with antidepressants. We studied age-, sex-, and postmortem interval-matched groups: no major psychopathology (controls); unmedicated-MDD; and MDD treated with serotonin reuptake inhibitors (MDD*SSRI) or tricyclics (MDD*TCA). Frozen right hippocampi were fixed, sectioned (50 μm), immunostained with neuronal nuclear marker (NeuN), and counterstained with hematoxylin. GN and glial number, and DG and granule cell layer (GCL) volumes were stereologically estimated. Fewer GNs in the anterior DG were present in unmedicated-MDDs compared with controls (p=0.013). Younger age of MDD onset correlated with fewer GNs (p=0.021). Unmedicated-MDDs had fewer mid-DG GNs than MDD*SSRIs (p=0.028) and controls (p=0.032). Anterior GCL glial number did not differ between groups. Anterior/mid GCL volume was smaller in unmedicated-MDDs vs controls (p=0.008) and larger in MDD*SSRIs vs unmedicated-MDDs (p<0.001), MDD*TCAs (p<0.001), and controls (p<0.001). Anterior GCL volume and GN number (r=0.594, p=0.001), and mid DG volume and GN number (r=0.398, p=0.044) were correlated. Anterior DG capillary density correlated with GN number (p=0.027), and with GCL (p=0.024) and DG (r=0.400, p=0.047) volumes. Posterior DG volume and GN number did not differ between groups. Fewer GNs in unmedicated-MDD without fewer neuronal progenitor cells, as previously reported, suggests a cell maturation or survival defect, perhaps related to MDD duration. This may contribute to a smaller hippocampus and is potentially reversed by SSRIs. Postmortem studies are correlative and animal studies are needed to test implied causal relationships.

Figure 1

Granule neurons are identified by immunocytochemistry and cell number and region volumes were estimated by stereology. (a) Neuronal nuclear marker NeuN labels granule neurons (GNs) in the dentate gyrus (DG) and neurons in the hilus and cornu ammonis (CA) regions of the human hippocampus. The region of interest identified for GN counting by stereology is the granule cell layer (GCL; indicated by arrow); volume measures were taken for the GCL as well as the full DG, including also the subgranular zone (SGZ, red) and molecular layer (ML, blue). DG and GCL area were outlined on coronal human hippocampal sections at regular interval along the anterior-posterior axis to estimate DG volume. The anterior, mid and posterior DG was identified as follows: the anterior DG courses along the optic tract for ∼15 mm from the center of the posterior margin of the anterior commissure to the end of the optic tract, including medially the uncal hippocampus and laterally the anterior body. The mid DG spans ∼9 mm to the end of the lateral geniculate. The posterior DG starts when the lateral geniculate disappears, extending ∼16 mm posteriorly and is contiguous to the lateral and inferior pulvinar. We did not analyze the supracallosal continuation of the hippocampus posterior body, which constitutes the induseum griseum. (b) Magnification of the GCL shown in (a); GCL, SGZ, and ML layers are indicated. Nuclei of glial cells are seen in blue, stained with hematoxylin. The optical disector with fractionator defines the sampling frames across the entire region of interest. GNs are visualized at 600 × magnification for cell counting. Cells touching the green line are included in the counting while cells touching the red line are not.

Figure 2

Granule neuron number and DG and GCL volume along the rostrocaudal axis of the human hippocampus. (a) Untreated subjects with major depressive disorder (MDD) have fewer granule neurons than controls and SRRI-treated MDDs. MDDs show fewer granule neurons (GNs) in the anterior and mid DG compared with controls without psychopathology or treatment. In the mid DG, MDDs treated with selective serotonin reuptake inhibitors (MDD*SSRI) have more GNs than untreated MDDs (MDD). In MDDs treated with tricyclic antidepressants (MDD*TCA), GN number does not differ from the other groups in any DG subregion. (b) Dentate gyrus (DG) and granule cell layer (GCL) volume are larger with antidepressant treatment in major depressive disorder. ANOVA showed anterior DG volume differs between groups, with no post hoc significance change. Mid DG and GCL volumes are larger in MDDs treated with selective serotonin reuptake inhibitors (MDD*SSRI) compared with untreated depressed subjects (MDD) and controls with no psychopathology or treatment. Repeated-measures MANOVA, where anterior and mid position are repeated measures for GCL and DG volumes, showed DG volume is larger in MDD*SSRI vs MDD (p<0.001), MDDs treated with tricyclic antidepressants (MDD*TCA, p=0.020) and controls (p=0.007); GCL volume is smaller in MDD vs controls (p=0.008) and larger in MDD*SSRI vs MDD (p<0.001), MDD*TCA (p<0.001), and controls (p<0.001).

Figure 3

Granule neuron number and dentate gyrus volume correlations with clinical characteristics and capillary characteristics. Subjects that did not have information regarding age of onset of first episode of major depression or lifetime number of episodes of major depression were excluded from the analysis. (a) In untreated MDDs, younger age of MDD onset correlated with fewer granule neurons (GNs) in anterior DG. Age of MDD onset showed no correlation with GN number in mid (r=−0.294) or posterior DG (r=0.149), not shown. (b) More lifetime depressive episodes correlated with smaller mid GCL volume and there was no correlation with anterior (r=−0.099) and posterior (r=−0.159) GCL volume or with any DG subregion volume (not shown). Note: GN=granule neuron; GCL=granule cell layer. (c) GN number in anterior DG correlates with number of capillaries per mm³ in the whole sample. Correlations within groups are not significant. (d) GN number in anterior DG correlates with the number of bifurcations per capillary in the whole sample. Correlations within groups are not significant, including in subjects with major depression treated with tricyclic antidepressants (not shown). Note: Controls=subjects with no Axis I or II psychiatric diagnosis; MDDs=untreated subjects with major depression; MDD*SSRI=subjects with major depression treated with selective serotonin reuptake inhibitors; MDD*TCA=subjects with major depression treated with tricyclic antidepressants.