Selective Depression of the Spinal Polysynaptic Reflex by the NMDA Receptor Antagonists in an Isolated Spinal Cord in vitro

Abstract

1. The effects of N-methyl-D-aspartate (NMDA) receptor glycine-binding site antagonists 7-chlorokynurenate (7-Clkyn) and (±)-3-amino-1-hydroxy-2-pyrrolidone (HA-966) on spinal reflexes in an isolated spinal cord that was maintained in Mg 2+-free medium in vitro were examined. The actions of 7-Clkyn and HA-966 were compared with those of the channel-site antagonist (i.e., dizocilpine) and NMDA-binding site antagonists—that is, 3-[(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) and DL-2-amino-5-phosphonovalerate (APV). 2. 7-Clkyn and HA-966 produced a selective depression of the polysynaptic reflex (PSR) while negligibly affecting the activity of the monosynaptic reflex (MSR). The PSR was also differentially suppressed by dizocilpine, CPP and APV. The PSR inhibitory activity of the NMDA antagonists was in the following order: dizocilpine>CPP>APV=7-Clkyn>HA-966. 3. The inhibitory effects of 7-Clkyn on PSR were markedly antagonized by the simultaneous application of D-serine, an agonist for the NMDA receptor glycine-binding sites. However, PSR inhibition by dizocilpine and CPP was unaffected. 4. Inhibition of the PSR by 7-Clkyn persisted in the presence of strychnine, which markedly increased the PSR activity by itself. 5. These findings suggest that the NMDA receptor glycine-binding sites play a role in generating the NMDA receptor-mediated PSR in the spinal cord in vitro.