Postnatal development and properties of N-methyl-d-aspartate (NMDA) receptors were studied with whole-cell and outside-out patch-clamp techniques in interneurons and fluorescence-labelled motoneurons in rat spinal cord slices. Both the absolute amplitude of NMDA-induced currents and currents normalized with respect to the motoneuron capacitance increased significantly at postnatal days 10–13 when compared to the responses evoked at postnatal days 2–3. The mean amplitude of the responses to kainate also increased in motoneurons of postnatal days 10–13. Single-channel currents induced by low concentrations of glutamate, exhibited four distinct amplitude levels corresponding to 19.2 ± 2.4 pS, 38.4 ± 3.5 pS, 56.3 ± 2.4 pS and 69.6 ± 3.7 pS. In contrast, the conductance of single channels, recorded under identical conditions, in rat spinal cord interneurons was less, 15.3 ± 3.2 pS, 29.9 ± 5.4 pS, 46.7 ± 4.8 pS and 62.4 ± 3.9 pS. The high (56/70 pS) conductance single-channel openings in motoneuron patches were sensitive to NMDA receptor inhibitors d-2-amino-5-phosphonovalerate, 7-chlorokynurenic acid and ifenprodil. Whole-cell NMDA-evoked currents were blocked in a voltage-dependent manner by extracellular Mg2+ with an apparent dissociation constant for Mg2+ binding at 0 mV of 1.8 ± 0.5 mm. The conductance and relative distribution of NMDA receptor channel openings induced by 1 μm glutamate in patches isolated from the motoneurons were independent of age from postnatal day 4 to 14.
The results suggest that the properties of NMDA receptor channels in motoneurons differ from those in spinal cord interneurons and cells transfected with NR1/NR2 subunits.