Functional Dysregulation in Stress-Induced Modulation of Synaptic Plasticity in a Mouse Model of Fragile X Syndrome

Date

2015-04-30

Authors

Ghilan, Mohamed

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Abstract

The fragile X mental retardation protein (FMRP) is an important regulator of protein translation, and a lack of FMRP expression leads to a cognitive disorder known as fragile X syndrome (FXS). Clinical symptoms characterizing FXS include learning impairments and heightened anxiety in response to stressful situations. The Fmr1-/y mouse has previously been shown to have deficits in context discrimination and novel object recognition tasks, which primarily rely on the dentate gyrus (DG) region of the hippocampal formation, but not in the Morris water maze (MWM) or the elevated plus-maze tasks, which primarily depend on the Cornu Ammonis (CA1) region. Furthermore, previous research has demonstrated N-methyl-D-aspartate receptor (NMDAR)-associated synaptic plasticity impairments in the DG but not in the CA1. However, the impact of acute stress on synaptic plasticity in the Fmr1-/y hippocampus has not been examined. The current study sought to extend previous behavioural investigations in the Fmr1-/y mouse, as well as examine the impact of stress on activation of the hypothalamic-pituitary-adrenal (HPA)-axis and on hippocampal synaptic plasticity. To further characterize hippocampus-dependent behaviour in this mouse model, the DG-dependent metric change spatial processing and CA1-dependent temporal order discrimination tasks were evaluated. The results reported here support previous findings and demonstrate that Fmr1-/y mice have performance deficits in the DG-dependent task but not in the CA1-dependent task, suggesting that previously reported subregional differences in NMDAR-associated synaptic plasticity deficits in the hippocampus of the Fmr1-/y mouse model may also manifest as selective behavioural deficits in hippocampus-dependent tasks. In addition, following acute stress, mice lacking FMRP showed a faster elevation of the glucocorticoid corticosterone and a more immediate impairment in long-term potentiation (LTP) in the DG. Stress-induced LTP impairments were rescued by administering the glucocorticoid receptor (GR) antagonist RU38486. Administration of RU38486 also enhanced LTP in Fmr1-/y mice in the absence of acute stress to wild-type levels, and this enhancement was blocked by application of the NMDAR antagonist 2-amino-5-phosphonopentanoic acid. These results suggest that a loss of FMRP results in enhanced GR signalling that may adversely affect NMDAR-dependent synaptic plasticity in the DG. Finally, synaptic plasticity alterations reported in this work were found to be specific to the DG and were unidirectional, i.e., restricted to LTP, as NMDAR- and metabotropic glutamate receptor (mGluR)-LTD were both unaffected by acute stress in the DG or the CA1 regions. This study offers new insights into synaptic plasticity impairments in the Fmr1-/y mouse model, and suggests stress and GRs as important contributors to learning and memory deficits in FXS.

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Keywords

Hippocampus, Synaptic Plasticity, Fragile X Syndrome, Stress, Anxiety, Glucocorticoids, LTP, LTD, Corticosterone, Dentate Gyrus, Learning and Memory, Electrophysiology

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