Chronic Corticosterone Exposure Persistently Elevates the Expression of Memory-Related Genes in the Lateral Amygdala and Enhances the Consolidation of a Pavlovian Fear Memory

<div><p>Chronic exposure to stress has been widely implicated in the development of anxiety disorders, yet relatively little is known about the long-term effects of chronic stress on amygdala-dependent memory formation. Here, we examined the effects of a history of chronic exposure to the stress-associated adrenal steroid corticosterone (CORT) on the consolidation of a fear memory and the expression of memory-related immediate early genes (IEGs) in the lateral nucleus of the amygdala (LA). Rats received chronic exposure to CORT (50 μg/ml) in their drinking water for 2 weeks and were then titrated off the CORT for an additional 6 days followed by a 2 week ‘wash-out’ period consisting of access to plain water. Rats were then either sacrificed to examine the expression of memory-related IEG expression in the LA or given auditory Pavlovian fear conditioning. We show that chronic exposure to CORT leads to a persistent elevation in the expression of the IEGs Arc/Arg3.1 and Egr-1 in the LA. Further, we show that rats with a history of chronic CORT exposure exhibit enhanced consolidation of a fear memory; short-term memory (STM) is not affected, while long-term memory (LTM) is significantly enhanced. Treatment with the selective serotonin reuptake inhibitor (SSRI) fluoxetine following the chronic CORT exposure period was observed to effectively reverse both the persistent CORT-related increases in memory-related IEG expression in the LA and the CORT-related enhancement in fear memory consolidation. Our findings suggest that chronic exposure to CORT can regulate memory-related IEG expression and fear memory consolidation processes in the LA in a long-lasting manner and that treatment with fluoxetine can reverse these effects.</p></div

Chronic exposure to CORT persistently enhances the expression of memory-related IEGs and synaptically-localized proteins in the LA.

<p>(<b>A</b>) Schematic of the behavioral protocol. Rats received either Water or CORT in their drinking water (50 μg/ml) for 2 weeks. Half the rats were sacrificed at the end of CORT exposure period. The other half was titrated off the CORT (25 μg/ml, 12.5 μg/ml) and given a 2 week period of exposure to water alone (‘wash-out’) prior to being sacrificed. (<b>B</b>) Mean (±SEM) immunoreactivity for GluR1, synaptophysin and BDNF (Water: n = 9; CORT: n = 9) from area CA3 in rats sacrificed on the last day of CORT exposure. (<b>C</b>) Mean (±SEM) immunoreactivity for GluR1 (Water: n = 9; CORT: n = 9), synaptophysin (Water: n = 9; CORT: n = 9), Arc/Arg3.1 (Water: n = 9; CORT: n = 9), Egr-1 (Water: n = 9; CORT: n = 9), BDNF (Water: n = 9; CORT: n = 9), acetyl-H3 (Water: n = 7; CORT: n = 9), phospho-ERK 1 and phospho-ERK 2 (Water: n = 8; CORT: n = 9) from the LA in rats sacrificed on the last day of CORT exposure. (<b>D</b>) Representative Western blots for each protein in (C). (<b>E</b>) Mean (±SEM) immunoreactivity for GluR1, synaptophysin, and BDNF (Water: n = 9; CORT: n = 9) from area CA3 in rats sacrificed on the last day of the wash-out period. (<b>F</b>) Mean (±SEM) immunoreactivity for GluR1 (Water: n = 9; CORT: n = 8), synaptophysin (Water: n = 9; CORT: n = 9), Arc/Arg3.1 (Water: n = 9; CORT: n = 8), Egr-1 (Water: n = 9; CORT: n = 8), BDNF (Water: n = 9; CORT: n = 9), acetyl-H3 (Water: n = 9; CORT: n = 8), phospho-ERK 1 and phospho-ERK 2 (Water: n = 9; CORT: n = 8) from the LA in rats sacrificed on the last day of the wash-out period. (<b>G</b>) Representative Western blots for each protein in (F). For each experiment, protein levels have been normalized to GAPDH levels for each sample and expressed as a percentage of the Water group. (*) <i>p</i> < 0.05 relative to Water group.</p

Fluoxetine treatment following chronic CORT exposure reverses the CORT-induced enhancement of fear memory consolidation.

<p>(<b>A</b>) Schematic of the behavioral protocol. Rats received either Water (n = 7) or CORT (n = 7; 50 μg/ml) for 2 weeks followed by CORT titration (25 μg/ml, 12.5 μg/ml). Rats then received 3 weeks of either Water (n = 6) or fluoxetine (FLX; n = 7; 333 μg/ml). At the end of the fluoxetine exposure period, rats were tested in the elevated plus maze (EPM). Rats were then fear conditioned with 2 tone-shock pairings and tested for STM (2 hr later) and LTM (24 hr later). (<b>B</b>) Mean (±SEM) time spent exploring the open and closed arms of the EPM in each group. (<b>C</b>) Mean (±SEM) post-shock freezing scores in each group following each conditioning trial. (<b>D</b>) Mean (±SEM) STM scores across each trial in each group. (<b>E</b>) Mean (±SEM) LTM scores across each trial in each group.</p