Chronic variable physical stress during the peripubertal-juvenile period causes differential depressive and anxiogenic effects in the novelty-seeking phenotype: functional implications for hippocampal and amygdalar brain-derived neurotrophic factor and the mossy fibre plasticity

Abstract

Experimentally naive rats show variance in their locomotor reactivity to novelty, some displaying higher (HR) while others displaying lower (LR) reactivity, associated with vulnerability to stress. We employed a chronic variable physical stress regimen incorporating intermittent and random exposures of physical stressors or control handling during the peripubertal-juvenile period to assess interactions between stress and the LRHR phenotype in depressive- and anxiety-like behaviors on the forced swim and social interaction tests, respectively. A decrease in immobility in the forced swim test along with a decrease in social contact in the social interaction test were observed in the juvenile HRs, coupled with increases in brain-derived neurotrophic factor (BDNF) mRNA in the hippocampus and in the basolateral amygdala with chronic variable physical stress. In contrast, an increase in immobility in the forced swim test and a decrease in social contact was observed in the LR counterparts coupled with an increase in the BDNF mRNA in the basolateral amygdala following chronic variable physical stress. Furthermore, chronic physical stress led to increased H3 and H4 acetylation at the P2 and P4 promoters of the hippocampal BDNF gene in the HR rats that is associated with increased suprapyramidal mossy fibre (SP-MF) terminal field volume. In contrast, chronic variable physical stress led to decreased H4 acetylation at the P4 promoter, associated with decreased SP-MF volume in the LR rats. These findings show dissociation in depressive- and anxiety-like behaviors following chronic variable physical stress in the juvenile HR animals that may be mediated by increased levels of BDNF in the hippocampus and in the amygdala, respectively. Moreover, chronic variable physical stress during the peripubertal-juvenile period results in opposite effects in depressive-like behavior in the LRHR rats by way of inducing differential epigenetic regulation of the hippocampal BDNF gene that, in turn, may mediate mossy fibre sprouting.

Figure 1

Percent time spent in social interaction (A) and in immobility in forced swim test (B) in LRHR rats exposed to chronic variable physical stress (PHY) or handled as controls (CONT, n=6 per experimental group). Means are plotted in bar graphs ± SEMs. *: p ≤ 0.05.

Figure 2

Levels of acH3 at the BDNF P2 promoter (A), and acH4 at the BDNF P4 promoter (B) in the LRHR rats exposed to chronic variable physical stress (PHY) or handled as controls (CONT, n=6 per experimental group). Fold changes in Ct values of immunoprecipitated samples obtained from LRHR rats exposed to physical stress or handled controls were calculated with the Ct values obtained from intermediary responders (phenotype control), and quantified using real-time PCR. Data are expressed as means ± SEMs. *: p ≤ 0.05. Sonicated chromatin was run on a 2% agarose gel to confirm proper fragmentation (roughly 400 bp from a representative gel in C).

Figure 3

Panels A, B, C and D constitute x-ray film-exposed images of representative coronal hemisections of the hippocampus that were radioactively labeled with an antisense cRNA probe against the BDNF mRNA in the LRHR rats following chronic variable physical stress or control handling (n=6 per experimental group). Means for optical densities ± SEMs are plotted by bar graphs for signal in the CA3 field (E) and the DG (F) of the hippocampus. *: p ≤ 0.05. Scale bar = 250 μm.

Figure 4

Panels A, B, C and D constitute x-ray film-exposed images of representative sections showing basolateral amygdala that were radioactively labeled with an antisense cRNA probe against the BDNF mRNA in the LRHR rats following chronic variable physical stress or control handling (n=6 per experimental group). Means for optical densities ± SEMs are plotted by a bar graph (E). *: p ≤ 0.05. Scale bar = 250 μm.

Figure 5

An illustration of a coronal hemisection of the dorsal hippocampus depicting the major hippocampal subdivisions and the two compartments of the mossy fibre system (SP-MF and IIP-MF) in which quantitative estimates were performed in the Timm-stained tissue.

Figure 6

Coronal hemisections of the dorsal hippocampus showing Timm-stained mossy fibre terminal fields that are counterstained by cresyl violet representing LR control (A), LR chronic variable physical stress (B), HR control (C) and HR chronic variable physical stress (D) groups (n=6 per experimental group). Arrows are pointing at the SP-MF compartment of the mossy fibre projections. Data pertaining to estimated SP-MF terminal field volume (E) and total mossy fibre terminal field volume (F, SP-MF + IIP-MF) are expressed as means ± SEMs in bar graphs. *: p ≤ 0.05. Scale bar = 250 μm.