Role of D-serine in superficial dorsal horn neuron

D-serine (DS) is an endogenous ligand that especially binds to glycine binding site on N-methyl D-aspartate (NMDA) receptor, a key excitatory neuro transmitter receptor in the central nervous system. When glia cell is activated in spinal cord after peripheral nerve injury, gliotransmitter such as inflammatory cytokines (IL-beta, TNF alpha-, IL-6 etc), ATP, and glutamate are released to neuronal synaptic membrane, suggesting that they modulate synaptic transmission in spinal pain transmission. In this study, we investigated the effect of DS that it is known for gliotransmitter releasing from astrocyte in spinal cord using electrophysiology and immunohistochemistry methods, and we identified the difference of its role between normal and peripheral nerve injury model. When exogenous NMDA was applied to spinal slices at a holding potential of-50 mV, the amplitude of NMDA induced inward current was increased in the presence of DS. Moreover, DS induced transient outward currents were observed at a holding potential of 0 mV, and this current was inhibited by glycine receptor antagonist, strychnine (2 mu M), suggesting that the DS-induced outward current is mediated by glycine receptor. In addition, we examined these effects of DS after 10 days operation of spare nerve injury (SNI). Compared with naive rats, the activation of NMDA receptor by DS was augmented, but inhibitory effect mediated by glycine receptor was decreased in SNI model rats. Finally, we investigated the activation of pERK in spinal cord after SNI. As the result, pERK immuno-positive cell were upregulated in injury side, but not in intact side. In the present study, we have demonstrated that DS in spinal dorsal horn neuron has two distinct actions, one is an excitatory action mediated by NMDA receptor, the other is an inhibitory action mediated by glycine receptor. Interestingly, the excitatory action by DS after peripheral nerve injury was more sensitive than physiological condition, whereas its inhibitory action after peripheral nerve injury was less than naive one. These findings may play an important role for the development of neuropathic pain.