Input-specific Plasticity of N-methyl-D-aspartate Receptor-mediated Synaptic Responses in Neonatal Rat Motoneurons

Lumbar motoneurons can be activated monosynaptically by two glutamatergic synaptic inputs: the segmental dorsal root (DR) and the descending ventrolateral funiculus (VLF). To determine whether their N-methyl-d-aspartate (NMDA) receptors are independent, we used (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine-hydrogen-maleate (MK-801), known to induce a use-dependent irreversible block of NMDA receptors (NMDARs). In the presence of MK-801 (in bath) and non-NMDA antagonists (in bath, to isolate NMDARs pharmacologically), we first stimulated the DR. After MK-801 blockade of DR synaptic input, the VLF was stimulated. Its response was found to be not significantly different from its control value, suggesting that the DR stimulus activated very few, if any, receptors also activated by VLF stimulation. Similar findings were obtained if the stimulation order was reversed. Both inputs also elicited a polysynaptic NMDAR-mediated response. Evoking the DR polysynaptic response in the presence of MK-801 eliminated the corresponding VLF response; the reverse did not occur. Surprisingly, when MK-801 was washed from the bath, both the DR and the VLF responses could recover, although the recovery of the DR monosynaptic and polysynaptic responses was reliably greater than those associated with the VLF. Recovery was prevented if extrasynaptic receptors were activated by bath-applied NMDA in the presence of MK-801, consistent with the possibility that recovery was due to movement of extrasynaptic receptors into parts of the membrane accessible to transmitter released by DR and VLF stimulation. These novel findings suggest that segmental glutamatergic inputs to motoneurons are more susceptible to plastic changes than those from central nervous system white matter inputs at this developmental stage.