Fragile X Syndrome (FXS), the most common inherited form of intellectual disability, is caused by loss of the fragile X mental retardation protein (FMRP). FMRP is a negative regulator of local mRNA translation downstream of group 1 metabotropic glutamate receptor (Gp1 mGluR) activation. In the absence of FMRP there is excessive mGluR-dependent protein synthesis, resulting in exaggerated mGluR-dependent long-term synaptic depression (LTD) in area CA1 of the hippocampus. Understanding disease pathophysiology is critical for development of therapies for FXS and the question arises of whether it is more appropriate to target excessive LTD or excessive mGluR-dependent protein synthesis. Priming of long-term potentiation (LTP) is a qualitatively different functional consequence of Gp1 mGluR-stimulated protein synthesis at the same population of CA1 synapses where LTD can be induced. Therefore we determined if LTP priming, like LTD, is also disrupted in the Fmr1 knockout (KO) mouse. We found that mGluR-dependent priming of LTP is of comparable magnitude in wild-type (WT) and Fmr1 KO mice. However, whereas LTP priming requires acute stimulation of protein synthesis in WT mice, it is no longer protein synthesis dependent in the Fmr1 KO. These experiments show that the dysregulation of mGluR-mediated protein synthesis seen in Fmr1 KO mice has multiple consequences on synaptic plasticity, even within the same population of synapses. Furthermore, it suggests that there is a bifurcation in the Gp1 mGluR signaling pathway, with one arm triggering synaptic modifications such as LTP priming and LTD and the other stimulating protein synthesis that is permissive for these modifications.
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