Molecular and functional properties of synaptically activated NMDA receptors in neonatal motoneurons in rat spinal cord slices

The functional properties of N-methyl-d-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSC) were studied in fluorescence-labell ed motoneurons in thin spinal cord slices. The deactivation of NMDA recepto r EPSCs in motoneurons voltage-clamped at +40 mV was independent of intensi ty or location of stimulation and of postnatal age [tau(fast) = 28.5 +/- 4. 6 ms (63.6 +/- 8.8%) and tau(slow) = 165.6 +/- 49.6 ms]. In the presence of 1 mm Mg2+ the amplitude of NMDA receptor EPSCs was voltage-dependent. Bolt zmann analysis of the relationship between peak NMDA receptor EPSC amplitud e and membrane potential suggested an apparent K-d of Mg2+ (at 0 mV) of 0.8 7 mm. Nonstationary variance analysis of NMDA receptor EPSCs gave an estima ted single-channel conductance of 59 +/- 14 pS. Direct measurement of the N MDA receptor channel openings in outside-out patches isolated from motoneur ons indicated the presence of single-channel conductance levels of 21.8 +/- 2.8 pS, 37.1 +/- 3.2 pS, 49.6 +/- 5.1 pS and 69.6 +/- 4.2 pS. Single-cell RT-PCR analysis of mRNA revealed that NR1, NR2A-D and NR3A transcripts were expressed in motoneurons. These results suggest that specific assembly of NMDA receptor subunits in motoneurons determines the functional and pharmac ological properties of the receptors in these cells.