NMDA receptors in preBotzinger complex neurons can drive respiratory rhythm independent of AMPA receptors.

The role of AMPA receptors (AMPARs) in generation and propagation of respiratory rhythm is well documented both in vivo and in vitro, whereas the functional significance of NMDA receptors (NMDARs) in preBötzinger complex (preBötC) neurons has not been explored. Here we examined the interactions between AMPARs and NMDARs during spontaneous respiratory rhythm generation in slices from neonatal rats in vitro. We tested the hypothesis that activation of NMDARs can drive respiratory rhythm in the absence of other excitatory drives. Blockade of NMDARs with dizocilpine hydrogen maleate (MK-801, 20 microM) had a negligible effect on respiratory rhythm and pattern under standard conditions in vitro, whereas blockade of AMPARs with NBQX (0.5 microM) completely abolished respiratory activity. Removal of extracellular Mg2+ to relieve the voltage-dependent block of NMDARs maintained respiratory rhythm without a significant effect on period, even in the presence of high NBQX concentrations (<or= 100 microM). Removal of Mg2+ increased inspiratory-modulated inward current peak (I(I)) and charge (Q(I)) in preBötC neurons voltage-clamped at -60 mV by 245% and 309%, respectively, with respect to basal values. We conclude that the normal AMPAR-mediated postsynaptic current underlying respiratory drive can be replaced by NMDAR-mediated postsynaptic current when the voltage-dependent Mg2+ block is removed. Under this condition, respiratory-related frequency is unaffected by changes in I(I), suggesting that the two can be independently regulated.