N-methyl-D-aspartate receptors (NMDARs) are key players in synaptic transmission and plasticity underlying learning and memory. Consequently, synaptic dysfunction of NMDARs has been implicated in the pathophysiology of Alzheimer´s disease (AD).Aims: to unravel possible alterations of GluN1, the obligatory subunit of NMDARs, in different excitatory synapses in the hippocampus.Methods: We used histoblot and SDS-digested freeze-fracture replica labelling (SDS-FRL) techniques.Results: Histoblots showed that GluN1 expression was significantly reduced in the caudate putamen, cortex and in the hippocampus in a laminar-dependent manner, at 12-month-old APP/PS1 mice age compared to age-matched wild type mice but was unaltered at 1 and 6 months. Using quantitative SDS-FRL, we unraveled the molecular organization of GluN1 in various synaptic connections at high spatial resolution in the CA1 and CA3 fields and the DG of the hippocampus in 12-month-old APP/PS1 mice. Density of GluN1 was high at synaptic sites throughout the hippocampus in wild type mice. In APP/PS1 mice, labelling density for GluN1 in excitatory synapses on spines was significantly reduced in DG granule cell-perforant pathway synapses, CA3 pyramidal cell-mossy fiber synapses, excitatory synapses established on CA1 pyramidal cells and on interneurons in the stratum lacunosum-moleculare but not in the stratum radiatum.Conclusions: Our data demonstrate an age-dependent reduction of NMDARs at different excitatory synapses in APP/PS1 mice that may have a significant impact on cognitive function in APP/PS1 mice.Grant PID2021-125875OB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and “ERDF A way of making Europe”, and Junta de Comunidades de Castilla-La Mancha (SBPLY/21/180501/000064)