Contributions of NMDA receptors to network recruitment and rhythm generation in spinal cord cultures

N-methyl-d-aspartic acid (NMDA) receptors are implicated in fictive locomotion; however, their precise role there is not clear. In cultures of dissociated cells from foetal rat spinal cord, synchronous bursting (but not fictive locomotion) can be induced by disinhibition, which is produced by blocking glycinergic and gamma-aminobutyric acid (GABA)A-dependent synaptic conductances. In this study, we investigate the role of NMDA-R in rhythm generation during disinhibition with multielectrode arrays and patch-clamp. We previously determined that bursting activity is generated by repetitive recruitment of a network through recurrent excitation. Blocking NMDA-R with d(-)-2-amino-5-phosphonopentanoic acid (APV) decreased the burst duration, suggesting a role of such receptors in the maintenance of high network activity during the bursts. In addition, APV reduced burst rate in about a third of the experiments, suggesting a contribution of NMDA-R in network recruitment. When (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrate (AMPA)/kainate receptors were blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in the presence of disinhibition, the burst rate was reduced and burst onset was slowed in two-thirds of the experiments. In the remaining experiments, bursting ceased completely with CNQX. Neither APV nor CNQX changed the spatial patterns of activity in the network, suggesting a co-operation of both receptors in rhythm generation. While NMDA alone was not able to create a rhythm, it accelerated bursting in the presence of disinhibition, made it more regular and slowed down network recruitment. These effects were most likely due to the depolarization of the interneurons in the network. We conclude that NMDA-R contribute to rhythm generation in spinal cultures by supporting recurrent excitation and network recruitment and by depolarizing the network.