FMR1 and fragile X syndrome: Consistent with the cAMP hypothesis of FXS, we found that PACAP reversed mGluR-LTD in the CA3-CA1 hippocampal synapse in wild-type mice and reduced exaggerated mGluR-LTD in Fmr1 KO, thus correcting a synaptic defect typically observed in FXS mouse models (Costa et al., 2018).