Desensitization of NMDA receptors depends of their association with plasma membrane sodium-calcium exchangers in lipid nanoclasters

The plasma membrane Na+/Ca2+-exchanger (NCX) has recently been shown to regulate Ca2+-dependent N-methyl-O_SCPLOWDC_SCPLOW-aspartate receptor (NMDAR) desensitization, suggesting tight interaction of NCXs and NMDARs in lipid nanoclaster or \"rafts\". To evaluate possible role of this interaction we studied effects of Li+ on NMDA-elicited whole-cell currents and Ca2+ responses of rat cortical neurons in vitro before and after cholesterol extraction by methyl-{beta}-cyclodextrin (M{beta}CD). Substitution Li+ for Na+ in the external solution caused a concentration-dependent decrease of steady-state NMDAR currents from 440 {+/-} 71 pA to 111 {+/-} 29 pA in 140 mM Na+ and 140 mM Li+, respectively. Li+ inhibition of NMDAR currents disappeared in the absence of Ca2+ in the external solution (Ca2+-free), suggesting that Li+ enhanced Ca2+-dependent NMDAR desensitization. Whereas the cholesterol extraction with M{beta}CD induced NMDAR current decrease to 136 {+/-} 32 pA in 140 mM Na+ and 46 {+/-} 15 pA in 140 mM Li+, the IC50 values for the Li+ inhibition were similar (about 44 mM Li+) before and after this procedure. In Ca2+-free Na+ solution steady-state NMDAR currents after the cholesterol extraction were 47 {+/-} 6 % of control currents. Apparently this amplitude decrease was not Ca2+-dependent. In 1 mM Ca2+ Na+ solution the Ca2+-dependent NMDAR desensitization was greater when cholesterol was extracted. Obviously, this procedure promoted its development. In agreement, Li+ and KB-R7943, an inhibitor of NCX, both considerably reduced NMDAR-mediated Ca2+ responses. The cholesterol extraction itself caused a decrease of NMDAR-mediated Ca2+ responses and, in addition, abolished the effects of Li+ and KB-R7943. Taken together our data suggest that NCXs downregulate the Ca2+-dependent NMDAR desensitization. Most likely, this is determined by co-localization and tight functional interaction of NCX and NMDAR molecules in membrane lipid rafts. Their destruction is accompanied by an enhancement of NMDAR desensitization and a loss of NCX-selective agent effects on NMDARs.