Gene expression patterns in the hippocampus and amygdala of endogenous depression and chronic stress models

Abstract

The etiology of depression is still poorly understood, but two major causative hypotheses have been put forth: the monoamine deficiency and the stress hypotheses of depression. We evaluate these hypotheses using animal models of endogenous depression and chronic stress. The endogenously depressed rat and its control strain were developed by bidirectional selective breeding from the Wistar-Kyoto (WKY) rat, an accepted model of major depressive disorder (MDD). The WKY More Immobile (WMI) substrain shows high immobility/despair-like behavior in the forced swim test (FST), while the control substrain, WKY Less Immobile (WLI), shows no depressive behavior in the FST. Chronic stress responses were investigated by using Brown Norway, Fischer 344, Lewis and WKY, genetically and behaviorally distinct strains of rats. Animals were either not stressed (NS) or exposed to chronic restraint stress (CRS). Genome-wide microarray analyses identified differentially expressed genes in hippocampi and amygdalae of the endogenous depression and the chronic stress models. No significant difference was observed in the expression of monoaminergic transmission-related genes in either model. Furthermore, very few genes showed overlapping changes in the WMI vs WLI and CRS vs NS comparisons, strongly suggesting divergence between endogenous depressive behavior- and chronic stress-related molecular mechanisms. Taken together, these results posit that although chronic stress may induce depressive behavior, its molecular underpinnings differ from those of endogenous depression in animals and possibly in humans, suggesting the need for different treatments. The identification of novel endogenous depression-related and chronic stress response genes suggests that unexplored molecular mechanisms could be targeted for the development of novel therapeutic agents.

Figure 1

The endogenous depression model, the Wistar–Kyoto More Immobile (WMI) strain, shows depressive behavior not linked to fear/anxiety (a–d). Chronic restraint stress (CRS) increases adrenocortical function consistently in all four strains (e and f). (a) In the forced swim test (FST), immobility scores of the WMI and WKY Less Immobile (WLI) animals differ significantly across generations. (b) Time spent in the inner circle of the open-field test, (c) total distance traveled and (d) movement traces of representative WMI and WLI animals. (e) Plasma corticosterone levels were consistently elevated after CRS in all four strains and (f) adrenal weights were consistently greater in the CRS group of all strains. ^*P<0.01, ^**P<0.001.

Figure 2

An across-experiment, gene-to-gene mapping was created for 10 112 gene symbols. Scatter plots are shown for observed effects obtained from the chronic restraint stress-no stress (CRS-NS) microarray experiment (vertical axis) and the Wistar–Kyoto More Immobile-WKY Less Immobile (WMI-WLI) microarray experiment (horizontal axis) by applying the linear models of Equation (1) and (2). The statistics are differences in condition effects, α_2g−α_1g, for the amygdala (a) and hippocampus (b), and differences in brain region-related effects, γ_2g−γ_1g (c).

Figure 3

(a) Validation of genes differentially expressed in Wistar–Kyoto More Immobile (WMI) and WKY Less Immobile (WLI) hippocampi or amygdala (n=6 per strain) by real-time reverse transcription-polymerase chain reaction (RT-PCR). The correlation between fold change in the Affymetrix microarray experiment and real-time RT-PCR determination of relative quantification ratios are shown (Pearson's correlation, r=0.720, P=0.002). (b) Validation of genes differentially expressed in chronic restraint stress (CRS) vs no stress (NS) (n=12 per treatment) hippocampi or amygdala by real-time RT-PCR. The correlation between fold change in the Illumina array experiment and real-time RT-PCR determination of relative quantification ratios are shown (Pearson's correlation, r=0.725, P=0.008). (c) Lack of correlation between absolute fold change and significant P-value (–log P) from the microarray analyses for genes with expression changes validated by real-time RT-PCR.