Distinct synaptic and extrasynaptic NMDA receptors identified in dorsal horn neurones of the adult rat spinal cord

1. Using patch-clamp recordings, properties of single-channel and synaptic currents mediated by N-methyl-D-aspartate receptors (NMDARs) were examined in substantia gelatinosa (SG) neurones of adult rat spinal cord slices. 2. In somatic outside-out patches, high- and low-conductance NMDAR channels were present. The low-conductance channels exhibited asymmetrical transitions between the main (44 pS) and subconductance (19 pS) levels, suggesting that they arise from NR2D subunit-containing receptors. The high-conductance channels (main conductance, 57 pS) were blocked by ifenprodil, an NR2B subunit selective blocker. 3. Ifenprodil had no effect on NMDA-EPSCs. The double-exponential decay time course and the apparent K-d for Mg2+ of NMDA-EPSCs suggested the expression of NR2A subunit-containing receptors at the synapse. 4. These results indicate that different NMDAR subtypes are expressed in subsynaptic and extrasynaptic regions of adult SG neurones, which may have differential roles in nociception.