title: D-serine in the midbrain periaqueductal gray contributes to morphine tolerance in rats.
abstract: BACKGROUND
  The N-methyl-D-aspartate subtype of glutamate receptor plays a critical role in morphine tolerance. D-serine, a co-agonist of N-methyl-D-aspartate receptor, participates in many physiological and pathophysiological processes via regulating N-methyl-D-aspartate receptor activation. The purinergic P2X7 receptor activation can induce the D-serine release in the central nervous system. This study aimed to investigate the role of the ventrolateral midbrain periaqueductal gray D-serine in the mechanism of morphine tolerance in rats. The development of morphine tolerance was induced in normal adult male Sprague-Dawley rats through subcutaneous injection of morphine (10 mg/kg). The analgesic effect of morphine (5 mg/kg, i.p.) was assessed by measuring mechanical withdrawal thresholds in rats with an electronic von Frey anesthesiometer. The D-serine concentration and serine racemase expression levels in the ventrolateral midbrain periaqueductal gray were evaluated through enzyme-linked immunosorbent assay and Western blot analysis, respectively. The effects of intra-ventrolateral midbrain periaqueductal gray injections of the D-serine degrading enzyme D-amino acid oxidase and antisense oligodeoxynucleotide targeting the P2X7 receptor on chronic morphine-treated rats were also explored.
  
  
  RESULTS
  We found that repeated morphine administrations decreased the antinociceptive potency of morphine evidenced by the percent changes in mechanical pain threshold in rats. By contrast, the D-serine contents and the expression levels of the serine racemase protein were upregulated in the ventrolateral midbrain periaqueductal gray in morphine-tolerant rats. The development of morphine tolerance was markedly alleviated by intra-ventrolateral midbrain periaqueductal gray injections of D-amino acid oxidase or antisense oligodeoxynucleotide targeting the P2X7 receptor.
  
  
  CONCLUSIONS
  Our data indicate that the development of antinociceptive tolerance to morphine is partially mediated by ventrolateral midbrain periaqueductal gray D-serine content, and the activation of the ventrolateral midbrain periaqueductal gray P2X7 receptor is an essential prelude to D-serine release. These results suggest that a cascade involving P2X7 receptor-D-serine-N-methyl-D-aspartate receptor mediated signaling pathway in the supraspinal mechanism of morphine tolerance.
authors.affiliation: Department of Pain Medicine, Affiliated Hospital of Zunyi Medical University, Zunyi, Guizhou, China Guizhou Key Laboratory of Anesthesia and Organ Protection, Zunyi Medical University, Zunyi, Guizhou, China.
authors.forename: Song
authors.initials: S
authors.lastname: Cao
authors.affiliation: Research Center for Medicine and Biology, Zunyi Medical University, Zunyi, Guizhou, China xiaozhi1971@163.com.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Xiao
authors.affiliation: Graduate School, Zunyi Medical University, Zunyi, Guizhou, China.
authors.forename: Mengjie
authors.initials: M
authors.lastname: Sun
authors.affiliation: Graduate School, Zunyi Medical University, Zunyi, Guizhou, China.
authors.forename: Youyan
authors.initials: Y
authors.lastname: Li
mesh_terms: D000698:Analgesia; D000818:Animals; D003605:D-Amino-Acid Oxidase / Q000378:metabolism; D004361:Drug Tolerance; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009838:Oligodeoxyribonucleotides / Q000494:pharmacology; D017288:Pain Threshold / Q000187:drug effects; D010487:Periaqueductal Gray / Q000378:metabolism*; D019998:Racemases and Epimerases / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D058486:Receptors, Purinergic P2X7 / Q000378:metabolism; D012694:Serine / Q000378:metabolism*; D015854:Up-Regulation / Q000187:drug effects; D000099300:Serine Racemase
chemical_list: D003605:D-Amino-Acid Oxidase; D009020:Morphine; D009838:Oligodeoxyribonucleotides; D019998:Racemases and Epimerases; D058486:Receptors, Purinergic P2X7; D012694:Serine; D000099300:Serine Racemase
keywords: D-serine; Morphine tolerance; P2X7 receptor; midbrain periaqueductal gray