Kinetic properties of NMDA receptors in embryonic Xenopus spinal neurons

1. The kinetic properties of embryonic Xenopus spinal cord neuron N- methyl-D-aspartate (NMDA)-activated receptors (NMDAR) were examined at the single-channel level. These receptors have a main conductance state of 50 pS and make occasional sojourns to a subconductance level of ~40 pS. The open channel lifetime is 1.3 ms at -80 mV and in 1 mM Ca²⁺-, Mg²⁺-free solution. Extracellular Mg²⁺ blocks the channel at a rate of 2.9 x 10⁸ M^{- 1} s^-1 and with a K(d) of ~20 μM at -80 mV. In patches with only one channel active, the closed interval duration distribution requires at least four exponentials to be fit. The time constant of one closed interval component decreases with increasing NMDA concentration. Kinetic modeling indicates that these NMDA receptors open at a rate of 230 s^-1 and close at a rate of 167 s^-1, that in the absence of desensitization the maximum probability of being open is ~0.5, and that this probability is half- maximal at ~150 μM NMDA.