Modulation Of Glycine Site Of NMDA Receptor Contributes To The Integration Of Nociceptive Processing In The Spinal Cord And Anterior Cingulate Cortex

The N-methyl-D-aspartate (NMDA) receptor has a number of agonist sites andregulation sites. The occupation of both glutamate and glycine binding sites isnecessary for the activation of NMDA receptor. Glycine is termed as "co-agonist" ofglutamate. Whether the glycine site of NMDA receptor is saturated or not is an issueunder debate for many years. Levels of the glycine in extracellular and cerebrospinalfluid are sufficient to saturate the high affinity NMDA-associated glycine site.Furthermore, the addition of exogenous glycine to brain slice preparations generallyhas no effect on responses to NMDA receptor agonists. However, several lines ofevidence have demonstrated that addition of glycine site agonists produce enhancementof NMDA receptor–mediated responses in vivo and in vitro. The glycine transporterslocated on glial cells can lower extracellular glycine in the vicinity of synaptic NMDAreceptors to subsaturating concentrations.NMDA receptor involves in nociceptive processing, including sensation andemotion of pain. Spinal NMDA receptor plays a crucial role in nociceptivetransmission and contributes to central sensitization after tissue injury andinflammation. The anterior cingulate cortex (ACC) is an important part of the limbicsystem. Accumulated evidence indicates that ACC contributes to pain-related negativeemotion. High level of D-serine has been found in the mammalian brain and localizedto astrocytes. As D-serine acts as a potent and selective agonist for thestrychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serineis a potential endogenous ligand for the glycine site of NMDA receptor.The present study was designed to determine the effect of D-serine on NMDAreceptor function in the spinal cord and ACC, and the contributions of glycine site tonociceptive processing and pain emotion.1) Using extracelluar electrophysiological recording, we examined the effect ofD-serine on responses of spinal wide dynamic range (WDR) neurons in normaland carrageenan-inflammed ratsIn carrageenan-injected rats, the threshold of C response was lower than thatof normal rats. As compared with normal rats, the responses of WDR neurons toreceptive stimuli were stronger in carrageenan-injected rats. Administration ofD-serine in the spinal cord markedly enhanced the C response of WDR neurons innormal but not in carrageenan-injected rats. These data suggest that the glycinesite of spinal NMDA receptor is not saturated in normal conditions, but saturatedfollowing carrageenan-induced inflammation.2) Using whole-cell recordings in the ACC slices, the effects of D-serine andd-amino acid oxidase, the D-serine metabolizing enzyme on NMDA current wereinvestigated in preparations from normal and monoarthritic pain modelsD-serine enhanced the NMDA-induced current in the ACC neurons both innormal and monoarthritic pain model rats, with more prominent in the latter.Superfusion of DAAO for 10 min attenuated NMDA current. These resultssuggest that the glycine site is not saturated in normal and monoarthritic rats.D-serine is an endogenous ligand for the glycine site of ACC NMDA receptor.The subunit composition of NMDA receptor might change after CFA-inducedarthritis.3) Combination of the formalin induced conditioning place avoidance (F-CPA)model and the ACC microinjection, the effects of glycine site antagonist,7-chlorokynurenate, and DAAO on pain emotion were exploredMicroinjection of 7-CK (4 nmol) into the ACC attenuated F-CPA withoutaffecting formalin-induced acute pain behavior. These results suggest that glycinesite of NMDA receptor contribute to pain emotion. DAAO (50 μg) microinjectedinto the ACC blocked F-CPA but not formalin-induced acute pain behavior either.In the absence of paw injection of formalin, neither 7-CK nor DAAO inducedpreference or avoidance. These results support the role of ACC in pain affect andindicate for the first time that the endogenous D-serine is necessary for theinduction of pain emotion.