Modulation of NMDA-mediated intrinsic membrane properties of ascending commissural interneurons in neonatal rat spinal cord

In this paper, the modulation of ascending commissural interneurons by N-methyl-D-aspartate was investigated in neonatal rats by using retrograde labeling and whole-cell patch clamp. Data shows these interneurons can be divided into three types (single spike, phasic, and tonic) based on their firing patterns. A hyperpolarization-activated nonselective cation current and persistent inward current are expressed in these interneurons. The parameters studied (n = 48) include: resting membrane potential (-59.2 ± 0.8 mV), input resistance (964.4 ± 49.3 MΩ), voltage threshold (-39.5 ± 0.6 mV), rheobase (13.5 ± 0.7 pA), action potential height (55.6 ± 2.2 mV), action potential half-width (2.8 ± 0.1 ms), afterhyperpolarization magnitude (16.1 ± 1.2 mV) and half-decay (217.9 ± 10.7ms). 10 µM N-methyl-D-aspartate increases excitability of ascending commissural interneurons by depolarizing the membrane potential, hyperpolarizing voltage threshold, reducing rheobase, and shifting the frequency-current relationship to the left. N-methyl-D-aspartate enhances persistent inward currents but reduces hyperpolarization-activated nonselective cation currents. This research uncovers unique ionic and intrinsic properties of ascending commissural interneurons which can be modulated by major excitatory neurotransmitters such as N-methyl-D-aspartate to potentially facilitate left-right alternation during locomotion.