Characterization of the single-channel properties of NMDA receptors in laminae I and II of the dorsal horn of neonatal rat spinal cord.

The single-channel properties of native NMDA receptors in laminae I and II of the dorsal horn of the neonatal rat spinal cord were studied using outside-out patch-clamp techniques. These receptors were found to have several features that distinguish them from native NMDA receptors elsewhere in the CNS. Single-channel currents activated by NMDA (100 nm) and glycine (10 microm) exhibited five distinct amplitude components with slope-conductance values of 19.9 +/- 0.8, 32.9 +/- 0.6, 42.2 +/- 1.1, 53.0 +/- 1.0 and 68.7 +/- 1.5 pS. Direct transitions were observed between all conductance levels but transitions between 69-pS openings and 20-, 33- and 42-pS openings were rare. There was no significant difference in the frequency of direct transitions from 42- to 20-pS compared to 20- to 42-pS transitions. The Kb (0 mV) for Mg2+ was 89 microm. The Mg2+ unblocking rate constant was similar to other reported values. However, the Mg2+ blocking rate constant was larger than other reported values, suggesting an unusually high sensitivity to Mg2+. The NR2B subunit-selective antagonist, ifenprodil, had no significant effect on overall channel activity but significantly decreased the mean open time of 53-pS openings. These results suggest neonatal laminae I and II NMDA receptors are not simply composed of NR1 and NR2B subunits or NR1 and NR2D subunits. It is possible that these properties are due to an as yet uninvestigated combination of two NR2 subunits with the NR1 subunit or a combination of NR3A, NR2 and NR1 subunits.