Peripheral Etanercept Administration Normalizes Behavior, Hippocampal Neurogenesis, and Hippocampal Reelin and GABAA Receptor Expression in a Preclinical Model of Depression

Abstract

Depression is a serious psychiatric disorder frequently comorbid with autoimmune disorders. Previous work in our lab has demonstrated that repeated corticosterone (CORT) injections in rats reliably increase depressive-like behavior, impair hippocampal-dependent memory, reduce the number and complexity of adult-generated neurons in the dentate gyrus, decrease hippocampal reelin expression, and alter markers of GABAergic function. We hypothesized that peripheral injections of the TNF-α inhibitor etanercept could exert antidepressant effects through a restoration of many of these neurobiological changes. To test this hypothesis, we examined the effect of repeated CORT injections and concurrent injections of etanercept on measures of object-location and object-in-place memory, forced-swim test behavior, hippocampal neurogenesis, and reelin and GABA β2/3 immunohistochemistry. CORT increased immobility behavior in the forced swim test and impaired both object-location and object-in-place memory, and these effects were reversed by etanercept. CORT also decreased both the number and complexity of adult-generated neurons, but etanercept restored these measures back to control levels. Finally, CORT decreased the number of reelin and GABA β2/3-ir cells within the subgranular zone of the dentate gyrus, and etanercept restored these to control levels. These novel results demonstrate that peripheral etanercept has antidepressant effects that are accompanied by a restoration of cognitive function, hippocampal neurogenesis, and GABAergic plasticity, and suggest that a normalization of reelin expression in the dentate gyrus could be a key component underlying these novel antidepressant effects.

Keywords: antidepressant; depression; etanercept; hippocampus; neurogenesis; reelin.

FIGURE 1

Schematic representation of the experimental design used for the study. All rats received the same amount of handling, injections, and behavioral testing. The injection top up periods in between each behavioral test were implemented to ensure that the effects of CORT did not wane over time.

FIGURE 2

Effect of CORT and etanercept on body weight. The CORT rats weighed significantly less than the vehicle and etanercept rats on days 14 (p < 0.05) and days 21 (p < 0.05). In addition, the CORT + etanercept rats weighed significantly less than the vehicle or etanercept rats on days 14 (p < 0.05) and 21 (p < 0.05). All data are represented as means ± standard error of the mean.

FIGURE 3

Effect of CORT and etanercept on FST behavior. (A) Shows the effect of treatment on time spent immobile. The CORT rats spent significantly more time immobile than did the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). (B) Shows the effect of treatment on time spent struggling. The CORT rats spent significantly less time struggling compared to the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). (C) Shows the effect of treatment on time spent swimming. The CORT rats spent significantly less time swimming than did the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). All data are represented as means ± standard error of the mean.

FIGURE 4

Effect of CORT and etanercept on cognition. (A) Shows a schematic of the object location memory test. (B) Shows the discrimination ratio in this test for the rats in each group. The CORT rats had a significantly lower discrimination ratio than did the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). (C) Shows a schematic of the object-in-place memory test. (D) Shows the discrimination ratio in this test displayed by the rats in each group. The CORT rats had a significantly lower discrimination ratio than did the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). All data are represented as means ± standard error of the mean.

FIGURE 5

Effect of CORT and etanercept on DCX-ir and dendritic complexity. CORT treatment significantly impaired hippocampal neurogenesis, and etanercept treatment restored this. (A) Shows representitive photomicrographs of DCX expression in the dentate gyrus (scale bar = 200 μm). A higher magnification image is pictured in the insets (Scale bar = 50 μm). (B) Shows the effect of treatment on the number of DCx-ir cells in the subgranular zone. CORT-treated rats had significantly fewer DCX-ir cells than did the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). (C) Shows representative photomicrographs of the six categories of dendritic complexity. (D) Shows the quantified categorization of dendritic complexity after analysis of a subset of DCX-ir cells. In general, the CORT rats had a higher percentage of cells in category 1 and category 2 and lower percentage of cells in categories 5 and 6, demonstrating that DCX-ir neurons are less well developed after CORT treatment. All data are represented as means ± standard error of the mean.

FIGURE 6

Effect of CORT and etanercept on the number of reelin-ir neurons. (A) Shows representative photomicographs of reelin-ir neurons in the dentate gyrus (scale bar = 200 μm). We focused our attention on reelin-ir cells in the proliferative subgranular zone. (B) Shows quantified values of reelin-ir neurons. The CORT rats had significantly fewer reelin-ir cells compared to the vehicle, CORT + etanercept, and etanercept rats (p < 0.05). All data are represented as means ± standard error of the mean.

FIGURE 7

Effect of CORT and etanercept on the number of GABA_A β2/3-ir cells. (A) Shows representative photomicrographs of GABA_A β2/3 expression along the subgranular zone (scale bar = 200 μm). Arrows point to cell bodies showing GABA_A β2/3-ir. Note that in the vehicle rats, GABA_A β2/3 immunostaining appears to be concentrated on the plasma membrane of the cell bodies, whereas after CORT treatment the labeling is more diffuse and can be observed along the cytoplasm indicating receptor internalization. Etanercept partially rescues this effect. (B) Shows the quantified number of GABA_A β2/3-ir cells in each group. The CORT rats had significantly fewer GABA_A β2/3-ir cells compared to the vehicle and CORT + etanercept rats (p < 0.05). All data are represented as means ± standard error of the mean.