Nitric oxide-independent inhibition by sodium nitroprusside of the native N-methyl-D-aspartate recognition domain in a manner different from that by potassium ferrocyanide.

Binding of 3H-5-methyl-10,11 -dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) was significantly inhibited by the addition of sodium nitroprusside (SNP), a nitric oxide (NO) donor, at a concentration range of 0.1 microM to 0.1 mM in rat brain synaptic membranes. On the contrary, two other NO donors, S-nitroso-N-acetylpenicillamine and S-nitroso-L-glutathione, did not inhibit binding even at 0.1 mM. Similarly potent inhibition of [3H]MK-801 binding was caused by the addition of potassium ferrocyanide, while potassium ferricyanide induced slight inhibition of binding at 0.1 mM. Both SNP and potassium ferrocyanide markedly inhibited binding of [3H]glutamic (Glu) and [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acids, without significantly affecting that of [3H]glycine and [3H]5,7-dichlorokynurenic acid. Further addition of Glu significantly exacerbated the inhibition by both SNP and potassium ferrocyanide at concentrations of 1-10 microM. Potent inhibition was also induced for [3H]MK-801 binding by the treatment of synaptic membranes with either SNP or potassium ferrocyanide, followed by efficient washing which also inhibited [3H]MK-801 binding due to removal of endogenous agonists. By contrast, dithiothreitol clearly differentiated between inhibitory properties of SNP and potassium ferrocyanide on [3H]MK-801 binding in terms of reversibility of the inhibition following pretreatment and subsequent washing. These results suggest that SNP may interfere with opening processes of the native NMDA channel through molecular mechanisms different from those underlying the inhibition by potassium ferrocyanide at the NMDA recognition domain in a manner independent of the generation of NO radicals.