Activation of N-methyl-D-aspartate receptors (NMDARs) requires the binding of a co-agonist, either D-serine or glycine, in addition to glutamate. Changes in occupancy of the co-agonist binding site are proposed to modulate neural networks including those controlling swimming in frog tadpoles. Here, we characterize regulation of the NMDAR co-agonist binding site in mammalian spinal locomotor networks. Blockade of NMDARs by D(−)-2-amino-5-phosphonopentanoic acid (D-APV) or 5,7-dichlorokynurenic acid reduced the frequency and amplitude of pharmacologically induced locomotor-related activity recorded from the ventral roots of spinal-cord preparations from neonatal mice. Furthermore, D-APV abolished synchronous activity induced by blockade of inhibitory transmission. These results demonstrate an important role for NMDARs in murine locomotor networks. Bath-applied D-serine enhanced the frequency of locomotor-related but not disinhibited bursting, indicating that co-agonist binding sites are saturated during the latter but not the former mode of activity. Depletion of endogenous D-serine by D-amino acid oxidase or the serine-racemase inhibitor erythro-beta-hydroxy-L-aspartic acid (HOAsp) increased the frequency of locomotor-related activity, whereas application of L-serine to enhance endogenous D-serine synthesis reduced burst frequency, suggesting a requirement for D-serine at a subset of synapses onto inhibitory interneurons. Consistent with this, HOAsp was ineffective during disinhibited activity. Bath-applied glycine (1-100 µM) failed to alter locomotor-related activity, whereas ALX 5407, a selective inhibitor of glycine transporter-1 (GlyT1), enhanced burst frequency, supporting a role for GlyT1 in NMDAR regulation. Together these findings indicate activity-dependent and synapse-specific regulation of the co-agonist binding site within spinal locomotor networks, illustrating the importance of NMDAR regulation in shaping motor output.
- motor control
- central pattern generator
- spinal cord
- Copyright © 2016, Journal of Neurophysiology