N-Methyl-n-aspartate receptors (NMDA receptors) are known to be permeable to Na+ and Ca2+ ions. In this study, we tested whether polyamines (putrescine, spermidine, spermine), organic cations found in cells, can permeate NMDA receptors expressed in Xenopus laevis oocytes and HEK293 cells. It was found that polyamines, especially spermidine, can permeate NMDA channels expressed from GluN1/GluN2A or GluN1/GluN2B activated by glycine and glutamate. Furthermore, spermidine and Ca2+ influx through NMDA receptors was observed in the presence of Mg2+, although Na+ influx was strongly inhibited by Mg2+. The Km values for spermidine influx through GluN1/GluN2A and GluN1/GluN2B were 2.2 mM and 2.7 mM, respectively in the presence of isotonic extracellular ion solutions. Spermidine uptake by NMDA receptors was dependent on the presence of glycine and glutamate, and inhibited by Ca2+ and by memantine, an NMDA receptor channel blocker. The Km values for Ca2+ influx through GluN1/GluN2A and GluN1/GluN2B were 4.6 mM and 33 mM, respectively, under the same ionic conditions. The results indicate that spermidine and Ca2+, but not Na+, can permeate NMDA receptors in the presence of Mg2+. Spermidine, if released locally from presynaptic terminals (where its concentration is high in synaptosomes and synaptic vesicles) could permeate NMDA receptors and play a role in synaptic plasticity mediated by NMDA receptors together with Ca2+. (C) 2015 Elsevier Inc. All rights reserved.