Chronic Stress Triggers Expression of Immediate Early Genes and Differentially Affects the Expression of AMPA and NMDA Subunits in Dorsal and Ventral Hippocampus of Rats.

Anibal Pacheco,Esteban Aliaga,Macarena Tejos-Bravo,Mauricio Muñoz,Nicolas A Parra-Fiedler,Alexandra García-Pérez,Paulina S Rojas,Jenny L Fiedler,Felipe I Aguayo,Gonzalo García-Rojo,Felipe A Olave,Claudio S Parra

doi:10.3389/fnmol.2017.00244

Abstract

Previous studies in rats have demonstrated that chronic restraint stress triggers anhedonia, depressive-like behaviors, anxiety and a reduction in dendritic spine density in hippocampal neurons. In this study, we compared the effect of repeated stress on the expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in dorsal and ventral hippocampus (VH). Adult male Sprague-Dawley rats were randomly divided into control and stressed groups, and were daily restrained in their motion (2.5 h/day) during 14 days. We found that chronic stress promotes an increase in c-Fos mRNA levels in both hippocampal areas, although it was observed a reduction in the immunoreactivity at pyramidal cell layer. Furthermore, Arc mRNAs levels were increased in both dorsal and VH, accompanied by an increase in Arc immunoreactivity in dendritic hippocampal layers. Furthermore, stress triggered a reduction in PSD-95 and NR1 protein levels in whole extract of dorsal and VH. Moreover, a reduction in NR2A/NR2B ratio was observed only in dorsal pole. In synaptosomal fractions, we detected a rise in NR1 in dorsal hippocampus (DH). By indirect immunofluorescence we found that NR1 subunits rise, especially in neuropil areas of dorsal, but not VH. In relation to AMPA receptor (AMPAR) subunits, chronic stress did not trigger any change, either in dorsal or ventral hippocampal areas. These data suggest that DH is more sensitive than VH to chronic stress exposure, mainly altering the expression of NMDA receptor (NMDAR) subunits, and probably favors changes in the configuration of this receptor that may influence the function of this area.

Highlights

The hippocampus is a heterogeneous structure along its dorso-ventral axis, displaying a differentiated synaptic network with other brain areas, which determines its influence in different functions (Fanselow and Dong, 2010; Strange et al, 2014)
Considering that chronic restraint stress induces cognitive alterations, anhedonia, depression-like and anxiety-like behaviors, we explored whether chronic stress modifies the expression of c-Fos and Arc immediately early genes (IEG) and both NMDA and amino-3-hydroxy5-methyl-4-isoxazole propionic acid receptor (AMPAR) subunits differently in dorsal hippocampus (DH) and ventral hippocampus (VH), along with PSD-95, which is a neuronal PDZ protein that associates with receptors and cytoskeletal elements at the synapse
These results demonstrate the activation of the HPA axis, suggesting that animals did not adapt to daily exposure to the homotypic stressor

Summary

Introduction

The hippocampus is a heterogeneous structure along its dorso-ventral axis, displaying a differentiated synaptic network with other brain areas, which determines its influence in different functions (Fanselow and Dong, 2010; Strange et al, 2014). CA1 and subiculum send efferent back to the deep layers of the entorhinal cortex While this classic tri-synaptic network participates in memory and learning processes, current evidence reveals an alternative circuitry in which DG neurons project to CA2, which in turn connects to CA1 pyramidal cells (Kohara et al, 2014). This circuitry establishes a direct connection between DH and VH, supporting a pathway to modulate brain areas linked to ventral CA1; for instance, prefrontal cortex and basolateral amygdala (Kohara et al, 2014). Behavioral studies have revealed that CA2 circuits are crucial for social (Sotomayor-Zárate et al, 2010), temporal (Mankin et al, 2015) and probably emotional aspects of memory

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