Adenosine A receptor inhibition reduces synaptic and cognitive hippocampal alterations in Fmr1 KO mice.

In fragile X syndrome (FXS) the lack of the fragile X mental retardation protein (FMRP) leads to exacerbated signaling through the metabotropic glutamate receptors 5 (mGlu5Rs). The adenosine A receptors (ARs), modulators of neuronal damage, could play a role in FXS. A synaptic colocalization and a strong permissive interaction between A and mGlu5 receptors in the hippocampus have been previously reported, suggesting that blocking ARs might normalize the mGlu5R-mediated effects of FXS. To study the cross-talk between A and mGlu5 receptors in the absence of FMRP, we performed extracellular electrophysiology experiments in hippocampal slices of Fmr1 KO mouse. The depression of field excitatory postsynaptic potential (fEPSPs) slope induced by the mGlu5R agonist CHPG was completely blocked by the AR antagonist ZM241385 and strongly potentiated by the AR agonist CGS21680, suggesting that the functional synergistic coupling between the two receptors could be increased in FXS. To verify if chronic AR blockade could reverse the FXS phenotypes, we treated the Fmr1 KO mice with istradefylline, an AR antagonist. We found that hippocampal DHPG-induced long-term depression (LTD), which is abnormally increased in FXS mice, was restored to the WT level. Furthermore, istradefylline corrected aberrant dendritic spine density, specific behavioral alterations, and overactive mTOR, TrkB, and STEP signaling in Fmr1 KO mice. Finally, we identified AR mRNA as a target of FMRP. Our results show that the pharmacological blockade of ARs partially restores some of the phenotypes of Fmr1 KO mice, both by reducing mGlu5R functioning and by acting on other AR-related downstream targets.