Corticosterone Alters AMPAR Mobility and Facilitates Bidirectional Synaptic Plasticity

Background: The stress hormone corticosterone has the ability both to enhance and suppress synaptic plasticity and learning and memory processes. However, until today there is very little known about the molecular mechanism that underlies the bidirectional effects of stress and corticosteroid hormones on synaptic efficacy and learning and memory processes. In this study we investigate the relationship between corticosterone and AMPA receptors which play a critical role in activity-dependent plasticity and hippocampal-dependent learning. Methodology/Principal Findings: Using immunocytochemistry and live cell imaging techniques we show that corticosterone selectively increases surface expression of the AMPAR subunit GluR2 in primary hippocampal cultures via a glucocorticoid receptor and protein synthesis dependent mechanism. In agreement, we report that corticosterone also dramatically increases the fraction of surface expressed GluR2 that undergo lateral diffusion. Furthermore, our data indicate that corticosterone facilitates NMDAR-invoked endocytosis of both synaptic and extra-synaptic GluR2 under conditions that weaken synaptic transmission. Conclusion/Significance: Our results reveal that corticosterone increases mobile GluR2 containing AMPARs. The enhanced lateral diffusion properties can both facilitate the recruitment of AMPARs but under appropriate conditions facilitate the loss of synaptic AMPARs (LTD). These actions may underlie both the facilitating and suppressive effects of corticosteroid hormones on synaptic plasticity and learning and memory and suggest that these hormones accentuate synaptic efficacy

AP-2 levels in primary hippocampal neurons treated with corticosterone.

<p>Hippocampal neurons (DIV 22) were treated with vehicle (lane 1–4) or 100 nM corticosterone (lane 5–8) for 3 h and harvested directly in sample buffer. The immunoblots were probed with two different anti-AP-2 (from Sigma and BD biosciences), anti-α-tubulin and anti-pan-actin antibodies. The positions of molecular weight standards (kDa) are indicated at left. No difference in AP-2 expression levels between control and corticosterone treated neurons is observed.</p

Corticosterone induces a delayed enhancement of the mEPSC amplitude.

<p>A) mEPSC amplitude and (B) frequency after treatment with vehicle or corticosterone. C) Normalized frequency histogram for the distribution of the amplitude of mEPSCs in hippocampal primary neurons after control treatment or treatment with corticosterone. A shift toward larger amplitudes was observed after hormone treatment. D) Cumulative frequency histogram shows a marked shift toward larger amplitude mEPSCs after corticosterone treatment. ^*P<0.05.</p

Corticosterone treatment dramatically modifies the endocytic properties of AMPARs.

<p>A) Rapid endocytosis of synaptic (punctate) and extrasynaptic (diffuse) AMPARs induced by activation of NMDARs in control hippocampal neurons. Scale bar, 20 µm. B) Rapid endocytosis of synaptic (punctate) and extrasynaptic (diffuse) AMPARs induced by activation of NMDARs in corticosterone-treated hippocampal neurons. Scale bar, 20 µm. C) Binned and averaged fluorescence values from punctate (red) and diffuse (blue) regions during and after NMDAR stimulation in control treated cells. Data reflect error bars show±SEM (4 cells for each condition with 14 punctate and 17 diffuse regions). D) Binned and averaged fluorescence values from punctate (red) and diffuse (blue) regions during and after NMDAR stimulation in corticosterone treated cells. Data reflect error bars show±SEM (4 cells for each condition with 14 punctate and 17 diffuse regions).</p

Glucocorticoid receptor activation promotes surface AMPA receptor expression.

<p>A) Representative images of hippocampal neurons at DIV13 treated with vehicle, 30 nM and 100 nM corticosterone for 3 h and stained for surface GluR1 (red) and GluR2 (green). B) Quantification of GluR1 and GluR2 intensity after treatment with vehicle and 0.3–100 nM corticosterone for 3 h. C) Quantification of surface GluR1 and GluR2 intensity after treatment with vehicle for 3 h and 30 nM corticosterone for 1 or 3 h. In addition cells were incubated for 3 h with CORT, washed and incubated in regular medium for 21 h (3 h+21 h). D) Quantification of total GluR1 and GluR2 intensity after treatment with vehicle and 30 nM corticosterone for 3 h. E) Quantification of surface GluR1 and GluR2 intensity of primary hippocampal neurons. Cells were treated with vehicle, 100 µM cycloheximide or 500 nM RU 38486 for 3 h followed 30 min later with vehicle or 30 nM corticosterone applications for 3 h. F) Representative Western blots show expression of GluR1, GluR2, transferrin receptor (TrfR), actin and tubulin in total and surface fraction of biotinylated primary hippocampal cultures treated with vehicle (−) or 100 nM corticosterone (+) for 3 h. G) Quantitative analysis of surface expression of GluR1, GluR2 and transferrin receptor (TrfR) in biotinylated primary hippocampal neurons treated with 100 nM corticosterone for 3 h. H) Quantitative analysis of total expression of GluR1, GluR2, transferrin receptor (TrfR), actin and tubulin in biotinylated primary hippocampal neurons treated with 100 nM corticosterone for 3 h.</p