Comparisons of Nociceptive Behavior and NMDA receptor‐Mediated Synaptic Transmission between Mice Lacking D‐Serine Production and Degradation Enzymes

Abstract

D‐serine is the endogenous co‐agonist for the glycine‐binding site of the N‐methyl‐D‐aspartate (NMDA) glutamate receptor. D‐serine is produced by the enzyme serine racemase (SR) and is degraded by the enzyme D‐amino acid oxidase (DAO). Co‐activation of spinal NMDA receptors by D‐serine is known to play a significant role in the etiological mechanism of neuropathic and inflammatory pain. The present study aims to obtain further information on how D‐serine and its related enzymes (SR and DAO) are involved in the modulation of persistent pain.The experiments were performed on both SR knock‐out (SR‐KO) mice and the DAO‐mutant mice that lack the enzymatic activity of DAO. We have previously shown that SR‐KO mice have less than half of the D‐serine content in the spinal cord and that DAO‐mutant mice have a significantly elevated level of D‐serine.The DAO‐mutant and SR‐KO mice used in this study are kindly provided by R. Konno (International University of Health and Welfare, Tochigi, Japan).First, the nociceptive behavioral responses to formalin that is subcutaneously injected into a hind paw were analyzed. Results showed that the responses in the second phase, which are known be associated with NMDA receptor activation, were augmented both in SR‐KO mice and in DAO‐mutant mice.Second, tight‐seal whole‐cell recordings were made from neurons in the superficial dorsal horn (SDH) in the lumbar spinal slices that were prepared from SR‐KO mice or from DAO‐mutant mice. Non‐NMDA and NMDA receptor‐mediated excitatory postsynaptic currents (non‐NMDAEPSCs and NMDA‐EPSCs, respectively) were pharmacologically isolated. Results show that the ratio of the amplitudes of NMDA‐EPSCs to that of non‐NMDA‐EPSCs was significantly smaller in SR‐KO mice and larger in DAO‐mutant mice compared with each corresponding control group and that the time constant of decay time course of NMDA‐EPSCs was significantly longer in SR‐KO mice.Finally, quantitative real‐time RT‐PCR experiments were performed on RNA extracted from the SDH tissue. The results showed that the subunit composition of NMDA receptors altered in SR‐KO mice and DAO‐mutant mice. The most significant observation is that the expression level of the NR2B subunit of NMDA receptors increased in the SR‐KO mice.These observations might indicate that D‐serine and its related enzymes (SR and DAO) influence the activation level of NMDA receptors and their subunit composition and thus affect nociceptive synaptic transmission in the SDH of the spinal cord and influence nociceptive behavior.Support or Funding InformationThis work was supported by JSPS KAKENHI Grant Number JP18K16462 (EK).