Long-term treatment with morphine increases the D-serine content in the rat brain by regulating the mRNA and protein expressions of serine racemase and D-amino acid oxidase

d-Serine Increases Release of Acetylcholine in Rat Submandibular Glands

d-serine has been observed in submandibular gland tissue in rats, but its functions remain to be clarified. Oral administration of d-serine, but not l-serine, increased its concentrations in the submandibular gland and pilocarpine-induced salivary secretion. In vivo microdialysis was used to collect the d- and l-enantiomers of amino acids from local interstitial fluid in the rat submandibular gland. The proportion of the d-form of serine in interstitial fluid was higher than that in plasma or saliva. Perfusion of the rat submandibular gland with d-serine and l-glutamic acid via the submandibular gland artery resulted in a significant increase in salivary secretion after stimulation of muscarinic receptors with carbachol. In vivo microdialysis applied to the submandibular glands of rats showed that infusion of d-serine along with l-glutamate through the microdialysis probe significantly elevated acetylcholine levels in local interstitial fluids in the submandibular glands of anesthetized rats as compared to that with l-glutamate alone in an N-methyl-d-aspartate receptor glycine site antagonist-sensitive manner. These results indicate that d-serine augments salivary secretion by increasing acetylcholine release in the salivary glands.

D-serine metabolism in the medial prefrontal cortex, but not the hippocampus, is involved in AD/HD-like behaviors in SHRSP/Ezo

Attention-deficit/hyperactivity disorder (AD/HD) is a mild neurodevelopmental disorder with inattention, hyperactivity, and impulsivity as its core symptoms. We previously revealed that an AD/HD animal model, juvenile stroke-prone spontaneously hypertensive rats (SHRSP/Ezo) exhibited functional abnormalities in N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex. D-serine is an endogenous co-ligand that acts on the glycine-binding site of NMDA receptors, which is essential for the physiological activation of NMDA receptors. We herein performed neurochemical and pharmacological behavioral experiments to elucidate dysfunctions in D-serine metabolism (namely, biosynthesis and catabolism) associated to AD/HD. The serine enantiomers ratio (D-serine/D-serine + L-serine, DL ratio) in the medial prefrontal cortex (mPFC) and hippocampus (HIP) was lower in SHRSP/Ezo than in its genetic control. The level of D-amino acid oxidase (DAAO, D-serine degrading enzyme) was higher in the mPFC, and the level of serine racemase (SR, D-serine biosynthetic enzyme), was lower in the HIP in SHRSP/Ezo. Thus, changes in these enzymes may contribute to the lower DL ratio of SHRSP/Ezo. Moreover, a microinjection of a DAAO inhibitor into the mPFC in SHRSP/Ezo increased DL ratio and attenuated AD/HD-like behaviors, such as inattention and hyperactivity, in the Y-maze test. Injection into the HIP also increased the DL ratio, but had no effect on behaviors. These results suggest that AD/HD-like behaviors in SHRSP/Ezo are associated with an abnormal D-serine metabolism underlying NMDA receptor dysfunction in the mPFC. These results will contribute to elucidating the pathogenesis of AD/HD and the development of new treatment strategies for AD/HD.

Free d-Amino Acids in Salivary Gland in Rat

Free d-amino acids, which are enantiomers of l-amino acids, are found in mammals, including humans, and play an important role in a range of physiological functions in the central nervous system and peripheral tissues. Several d-amino acids have been observed in saliva, but their origin and the enzymes involved in their metabolism and catabolism remain to be clarified. In the present study, large amounts of d-aspartic acid and small amounts of d-serine and d-alanine were detected in all three major salivary glands in rat. No other d-enantiomers were detected. Protein expression of d-amino acid oxidase and d-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic d-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the d-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The N-methyl-d-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that d-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.

Morphine-induced inhibition of Ca2+ -dependent d-serine release from astrocytes suppresses excitability of GABAergic neurons in the nucleus accumbens

The nucleus accumbens (NAc) plays a critical role in addictive drug-induced behavioral changes. d-serine is present at high levels in the brain and is involved in the regulation of N-methyl-d-aspartate glutamate (NMDA)-dependent synaptic activity. In this study, we aimed to examine the involvement of d-serine in morphine addiction. Morphine decreased the NMDA receptor-mediated excitatory postsynaptic currents and excitability of GABAergic neurons in the NAc, while exogenous d-serine alleviated the effects of morphine. Morphine reduced extracellular d-serine levels in rat NAc or in primary culture of astrocytes through inhibition of intracellular Ca²⁺ signals and blockade of d-serine release from cell vesicles. Morphine induced robust internalization of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate acid receptor (AMPAR) in primary cultured astrocytes. Moreover, administration of exogenous d-serine to rats inhibited the development of locomotor sensitization to morphine, attenuated the morphine-induced potentiation on conditioned place preference and suppressed the morphine-enhanced expression of p-CREB and ΔFosB in the NAc. Overall, our results showed that morphine inhibited d-serine release from astrocytes through modulation of AMPAR-mediated Ca²⁺ influx, and led to the inhibition of postsynaptic excitability of GABAergic neurons in the NAc. This work may provide a new insight into the underlying mechanisms of morphine addiction.

D-serine in the midbrain periaqueductal gray contributes to morphine tolerance in rats

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.

Whole-genome expression profile in zebrafish embryos after chronic exposure to morphine: identification of new genes associated with neuronal function and mu opioid receptor expression

BACKGROUND

A great number of studies have investigated changes induced by morphine exposure in gene expression using several experimental models. In this study, we examined gene expression changes during chronic exposure to morphine during maturation and differentiation of zebrafish CNS.

RESULTS

Microarray analysis showed 254 genes whose expression was identified as different by at least 1.3 fold change following chronic morphine exposure as compared to controls. Of these, several novel genes (grb2, copb2, otpb, magi1b, grik-l, bnip4 and sox19b) have been detected for the first time in an experimental animal model treated with morphine. We have also identified a subset of genes (dao.1, wls, bnip4 and camk1γb) differentially expressed by chronic morphine exposure whose expression is related to mu opioid receptor gene expression. Altered expression of copb2, bnip4, sox19b, otpb, dao.1, grik-l and wls is indicative of modified neuronal development, CNS patterning processes, differentiation and dopaminergic neurotransmission, serotonergic signaling pathway, and glutamatergic neurotransmission. The deregulation of camk1γb signaling genes suggests an activation of axonogenesis and dendritogenesis.

CONCLUSIONS

Our study identified different functional classes of genes and individual candidates involved in the mechanisms underlying susceptibility to morphine actions related to CNS development. These results open new lines to study the treatment of pain and the molecular mechanisms involved in addiction. We also found a set of zebrafish-specific morphine-induced genes, which may be putative targets in human models for addiction and pain processes.

Gabapentin and (S)-pregabalin decrease intracellular D-serine concentrations in PC-12 cells

The effects of gabapentin (GBP) and (S)-pregabalin (PGB) on the intracellular concentrations of d-serine and the expression of serine racemase (SR) in PC-12 cells were determined. Intracellular d-serine concentrations were determined using an enantioselective capillary electrophoresis assay with laser-induced fluorescence detection. Increasing concentrations of GBP, 0.1-20μM, produced a significant decrease in d-serine concentration relative to control, 22.9±6.7% at 20μM (*p<0.05), with an IC(50) value of 3.40±0.29μM. Increasing concentrations of PGB, 0.1-10μM, produced a significant decrease in d-serine concentration relative to control, 25.3±7.6% at 10μM (*p<0.05), with an IC(50) value of 3.38±0.21μM. The compounds had no effect on the expression of monomeric-SR or dimeric-SR as determined by Western blotting. The results suggest that incubation of PC-12 cells with GBP and PGB reduced the basal activity of SR, which is most likely a result of the decreased Ca(2+) flux produced via interaction of the drugs with the α(2)-δ subunit of voltage-gated calcium channels. d-Serine is a co-agonist of the N-methyl d-aspartate receptor (NMDAR) and reduced d-serine concentrations have been associated with reduced NMDAR activity. Thus, GBP and PGB may act as indirect antagonists of NMDAR, a mechanism that may contribute to the clinical effects of the drugs in neuropathic pain.

Role of P2X7 receptor-mediated IL-18/IL-18R signaling in morphine tolerance: multiple glial-neuronal dialogues in the rat spinal cord

The glial function in morphine tolerance has been explored, but its mechanisms remain unclear. Our previous study has showed that microglia-expressed P2X7 receptors (P2X7R) contribute to the induction of tolerance to morphine analgesia in rats. This study further explored the potential downstream mechanisms of P2X7R underlying morphine tolerance. The results revealed that the blockade of P2X7 receptor by P2X7R antagonist or targeting small interfering RNA (siRNA) reduced tolerance to morphine analgesia in the pain behavioral test and spinal extracellular recordings in vivo and whole-cell recording of the spinal cord slice in vitro. Chronic morphine treatment induced an increase in the expression of interleukin (IL)-18 by microglia, IL-18 receptor (IL-18R) by astrocytes, and protein kinase Cγ (PKCγ) by neurons in the spinal dorsal horn, respectively, which was blocked by a P2X7R antagonist or targeting siRNA. Chronic morphine treatment also induced an increased release of D-serine from the spinal astrocytes. Further, both D-amino acid oxygenase (DAAO), a degrading enzyme of D-serine, and bisindolylmaleimide α (BIM), a PKC inhibitor, attenuated morphine tolerance. The present study demonstrated a spinal mechanism underlying morphine tolerance, in which chronic morphine triggered multiple dialogues between glial and neuronal cells in the spinal cord via a cascade involving a P2X7R-IL-18-D-serine-N-methyl-D-aspartate receptor (NMDAR)-PKCγ-mediated signaling pathway.

PERSPECTIVE

The present study shows that glia-neuron interaction via a cascade (P2X7R-IL-18-D-serine-NMDAR-PKCγ) in the spinal cord plays an important role in morphine tolerance. This article may represent potential new therapeutic targets for preventing morphine analgesic tolerance in clinical management of chronic pain.

Interactions of the potent D-amino acid oxidase inhibitor CBIO with morphine in pain and tolerance to analgesia

A series of experiments using technologies of gene mutation and silencing as well as chemical biology have demonstrated that spinal D-amino acid oxidase (DAAO) contributes to the development of central sensitization-mediated chronic pain and might be a potential molecular target for the treatment of chronic pain. DAAO inhibitors are now under clinical investigations for the management of chronic neuropathic pain. This study examined the interactions between morphine and the DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) in pain and analgesia tolerance mainly in the formalin test. Given subcutaneously CBIO acutely interacted with morphine in analgesia in an additive manner both in the acute nociception settings (the formalin acute phase nociception, hot-plate test and tail immersion test) and in formalin-induced tonic pain. Bi-daily exposure of CBIO given subcutaneously for 7 days did not produce self-tolerance to analgesia or cross-tolerance to morphine whereas 7-day subcutaneous morphine induced self-tolerance to analgesia but not cross-tolerance to CBIO. More importantly, subcutaneous co-administrations or even single dose of CBIO completely prevented or reversed morphine tolerance to analgesia (exhibited by a single dose or a dose-response curve of morphine) in both formalin-induced acute phase nociception and tonic phase pain. These results, for the first time, identified DAAO as an efficacious molecule mediating morphine tolerance, in addition to clarifying the complex interactions between morphine and DAAO inhibitors probed by CBIO, and provided a pharmacological basis for DAAO inhibitors in combination with morphine to clinically manage pain.

Capillary electrophoresis-laser-induced fluorescence (CE-LIF) assay for measurement of intracellular D-serine and serine racemase activity

An enantioselective capillary electrophoresis-laser-induced fluorescence (CE-LIF) method for the analysis of D-serine (D-Ser) in cellular matrices has been developed. The assay involves derivatization with FITC followed by CE-LIF using 0.5 mM hydroxyl propyl-β-cyclodextrin in borate buffer [80 mM, pH 9.3]. The method was able to resolve D-Ser and L-Ser with an enantioselectivity (α) of 1.03 and a resolution (R(s)) of 1.37. Linearity was established from 0.25 to 100.00 μM. The assay was also able to enantioselectively resolve 6 additional amino acid racemates. The method was applied to the determination of intracellular D-Ser concentrations in PC-12, C6, 1312N1, and HepG2 cell lines. This method was used to determine the concentration-dependent increases in D-Ser and associated EC₅₀ values produced by L-Ser and the concentration-dependent decreases in d-Ser and associated IC₅₀ values produced by glycine, a competitive inhibitor of serine racemase (SR). Western blot analysis determined that the PC-12 and C6 cell lines contained monomeric and dimeric forms of SR while the 1321N1 and HepG2 cells contained only the monomeric form. Although the SR dimer has been identified as the active form of the enzyme, all four of the tested cell lines expressed enzymatically active SR.

Brain-specific Phgdh deletion reveals a pivotal role for L-serine biosynthesis in controlling the level of D-serine, an N-methyl-D-aspartate receptor co-agonist, in adult brain

In mammalian brain, D-serine is synthesized from L-serine by serine racemase, and it functions as an obligatory co-agonist at the glycine modulatory site of N-methyl-D-aspartate (NMDA)-selective glutamate receptors. Although diminution in D-serine level has been implicated in NMDA receptor hypofunction, which is thought to occur in schizophrenia, the source of the precursor L-serine and its role in D-serine metabolism in adult brain have yet to be determined. We investigated whether L-serine synthesized in brain via the phosphorylated pathway is essential for D-serine synthesis by generating mice with a conditional deletion of D-3-phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95). This enzyme catalyzes the first step in L-serine synthesis via the phosphorylated pathway. HPLC analysis of serine enantiomers demonstrated that both L- and D-serine levels were markedly decreased in the cerebral cortex and hippocampus of conditional knock-out mice, whereas the serine deficiency did not alter protein expression levels of serine racemase and NMDA receptor subunits in these regions. The present study provides definitive proof that L-serine-synthesized endogenously via the phosphorylated pathway is a key rate-limiting factor for maintaining steady-state levels of D-serine in adult brain. Furthermore, NMDA-evoked transcription of Arc, an immediate early gene, was diminished in the hippocampus of conditional knock-out mice. Thus, this study demonstrates that in mature neuronal circuits L-serine availability determines the rate of D-serine synthesis in the forebrain and controls NMDA receptor function at least in the hippocampus.

Metabolism of the neuromodulator D-serine

Over the past years, accumulating evidence has indicated that D-serine is the endogenous ligand for the glycine-modulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. D-Serine is synthesized in glial cells and neurons by the pyridoxal-5' phosphate-dependent enzyme serine racemase, and it is released upon activation of glutamate receptors. The cellular concentration of this novel messenger is regulated by both serine racemase isomerization and elimination reactions, as well as by its selective degradation catalyzed by the flavin adenine dinucleotide-containing flavoenzyme D-amino acid oxidase. Here, we present an overview of the current knowledge of the metabolism of D-serine in human brain at the molecular and cellular levels, with a specific emphasis on the brain localization and regulatory pathways of D-serine, serine racemase, and D-amino acid oxidase. Furthermore, we discuss how D-serine is involved with specific pathological conditions related to N-methyl-D-aspartate receptors over- or down-regulation.

Expression of NR3B but not NR2D subunit of NMDA receptor in human blood lymphocytes can serve as a suitable peripheral marker for opioid addiction studies

Glutamate is critically involved in opioid addiction. It has been suggested that neurotransmitter receptors expression in peripheral blood lymphocytes may reflect brain status. In the present study, using Real-time PCR, the mRNA expression of NR2D and NR3B subunits of NMDA glutamate receptor has been investigated in peripheral blood lymphocytes of four groups each comprising of 25 male individuals: opioid addicts, methadone maintained patients, long-term abstinent former opioid addicts, and non-addicted control subjects. We found that NR2D subunit mRNA expression was not changed in all three test groups in comparison to control subjects. However, the NR3B mRNA expression was significantly up-regulated by the factors 9.11 (P<0.001), 10.07 (P<0.001) and 4.08 (P<0.05) in abstinent, addicted and methadone maintained subjects, respectively relative to control group. As a conclusion, our data indicate that the transcriptional level of the NR2D subunit of NMDA receptor is not regulated by chronic opioid addiction. However, it seems that the over-expression of NR3B subunit of NMDA receptor is a long lasting result of opioid abuse. In addition, considerable decrease in the up-regulated state of the NR3B subunit by methadone may represent another benefit of methadone therapy for opioid addicts and may serve as a suitable marker to evaluate the successfulness of therapy.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).