Modulation of the NMDA receptor by cyanide: enhancement of receptor-mediated responses.

The effect of cyanide on the N-methyl-D-aspartate (NMDA)-stimulated increase in cytosolic free calcium ([Ca++]i) was studied by microfluorescence in fura-2-loaded cerebellar granule cells. The response to NMDA was enhanced by NaCN over a concentration range of 20 to 100 microM. These concentrations of NaCN in the absence of NMDA had no effect on basal [Ca++]i. In comparison, NaCN did not affect K+-depolarization-induced [Ca++]i elevation. The NaCN potentiation of NMDA-evoked [Ca++]i elevation was blocked by addition of Mg++ and by the NMDA receptor antagonists 2-amino-5-phosphono-valeric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine maleate. Pretreatment of the cells with pregnenolone sulfate or arachidonate, known modulators of the NMDA receptor, enhanced NaCN action. The voltage-sensitive calcium channel blockers nifedepine and diltiazem did not affect the NaCN-induced potentiation. Additionally, the NaCN action was not altered when tetrodotoxin was used to block Na+ channel-mediated glutamate release. In patch-clamp studies, NaCN increased the amplitude and duration of NMDA-stimulated whole-cell currents. NaCN also enhanced the NMDA receptor response in single-channel patch-clamp experiments. In the outside-out patch recording configuration, NaCN increased the NMDA receptor channel opening frequency without affecting single-channel conductance or mean channel open time. These results indicate that cyanide interacts directly with the NMDA receptor channel complex to enhance receptor-mediated responses.