Xin Fang

Bio: Xin Fang is an academic researcher from Coventry Health Care. The author has contributed to research in topics: Neuropathic pain & Noxious stimulus. The author has an hindex of 2, co-authored 2 publications receiving 25 citations.

Pharmacodynamic Effects of a d-Amino Acid Oxidase Inhibitor Indicate a Spinal Site of Action in Rat Models of Neuropathic Pain

Abstract: Inhibition of d-amino acid oxidase (DAAO) activity is a potential target for the treatment of chronic pain. Here we characterized the effects of systemic administration of the DAAO inhibitor 4H-furo[3,2-b]pyrrole-5-carboxylic acid (SUN) in rat models of neuropathic and inflammatory pain. Oral administration of SUN dose dependently attenuated tactile allodynia induced by ligation of the L5 spinal nerve (SNL) and similarly reversed thermal hyperalgesia produced by chronic constriction injury. In addition, SUN was efficacious against complete Freund's adjuvant-induced thermal hyperalgesia. In these models, maximal reversal of pain-related behaviors corresponded with maximum rates of increase in brain and plasma d-serine concentrations, indicative of full inhibition of DAAO activity. To investigate the possible site(s) of action, we recorded spontaneous nerve activity and mechanically evoked responses of central spinal cord dorsal horn neurons and compared these with spontaneous activity of peripheral dorsal root filaments in anesthetized SNL model animals. Oral SUN reduced spontaneous activity in both central and peripheral recordings at doses and pretreatment times that corresponded to reduced mechanical allodynia in behavioral experiments. After intravenous administration of SUN, the onset of action for this central effect was rapid (maximal effects within 30 minutes), but was abolished by severing afferent inputs to the dorsal horn. Overall, these results indicate that inhibition of DAAO in peripheral afferent spinal circuits reduced spontaneous neuronal activity to attenuate pain-related behaviors in rat models of neuropathic and inflammatory pain.

In vivo electrophysiological recording techniques for the study of neuropathic pain in rodent models.

Abstract: Neuropathic pain develops following nerve injury, and is a chronic pain syndrome that can persist long after repair of a wound or removal of the neurological insult. This condition remains poorly treated, not least because of a lack of mechanism-based therapeutics. Clinically, neuropathic pain is characterized by three major symptoms: thermal or mechanical allodynia (pain sensation in response to previously non-noxious stimuli); hyperalgesia (enhanced pain sensation to noxious stimulation); and spontaneous, ongoing pain. These clinical symptoms can be modeled in rodent neuropathic pain models using behavioral and electrophysiological readouts. This unit describes techniques designed to record pathophysiological electrical activity associated with neuropathic pain at the level of the periphery, in single fibers of primary sensory neurons, and from wide dynamic range (WDR) neurons of the dorsal horn of the spinal cord. These techniques can be employed in both naive animals and in animal models of neuropathy to investigate fundamental mechanisms contributing to the neuropathic pain state and the site, mode, and mechanism of action of putative analgesics. Curr. Protoc. Pharmacol. 66:11.15.1-11.15.26. © 2014 by John Wiley & Sons, Inc. Keywords: electrophysiology; peripheral nerve; ectopic discharge; spinal cord; dorsal horn neurons; neuropathic pain

Human D-Amino Acid Oxidase: Structure, Function, and Regulation

Abstract: D-Amino acid oxidase (DAAO) is an FAD-containing flavoenzyme that catalyzes with absolute stereoselectivity the oxidative deamination of all natural D-amino acids, the only exception being the acidic ones. This flavoenzyme plays different roles during evolution and in different tissues in humans. Its three-dimensional structure is well conserved during evolution: minute changes are responsible for the functional differences between enzymes from microorganism sources and those from humans. In recent years several investigations focused on human DAAO, mainly because of its role in degrading the neuromodulator D-serine in the central nervous system. D-Serine is the main coagonist of N-methyl D-aspartate receptors, i.e., excitatory amino acid receptors critically involved in main brain functions and pathologic conditions. Human DAAO possesses a weak interaction with the FAD cofactor; thus, in vivo it should be largely present in the inactive, apoprotein form. Binding of active-site ligands and the substrate stabilizes flavin binding, thus pushing the acquisition of catalytic competence. Interestingly, the kinetic efficiency of the enzyme on D-serine is very low. Human DAAO interacts with various proteins, in this way modulating its activity, targeting, and cell stability. The known properties of human DAAO suggest that its activity must be finely tuned to fulfill a main physiological function such as the control of D-serine levels in the brain. At present, studies are focusing on the epigenetic modulation of human DAAO expression and the role of post-translational modifications on its main biochemical properties at the cellular level.

Effects of quercetin on chronic constriction nerve injury in an experimental rat model.

Abstract: Flavonoids are popular substances in the literature, with proven effects on cardiovascular, neoplastic and neurodegenerative diseases. Antioxidant effect is the most pronounced and studied one. Among thousands of flavonoids, quercetin (QUE) is a prototype with significant antioxidant effects. This study aims to demonstrate the effects of QUE in an experimental rat model of chronic constriction injury (CCI). A two-level study was designed with 42 adult Wistar rats that were randomly assigned to different groups. In the first part, animals in sham, control, quercetin, morphine and gabapentine groups received chronic constriction injury to their sciatic nerves and received a single dose of QUE, morphine and gabapentine. In the second part, different dose regimens of QUE were administered to different groups of animals. Pre-injury and post-injury assessments for mechanical hypersensitivity, thermal sensitivity, locomotor activity and anxiety were recorded and statistical comparisons were performed between different groups. Comparison of QUE with morphine and gabapentine has revealed significant effects of this agent in the current chronic constriction injury model. QUE was significantly superior to Gabapentine and morphine in terms of alleviating mechanical and thermal hypersensitivity. Additionally, pre-injury administration of QUE for 4 days demonstrated long-term effectiveness on mechanical hypersensitivity. This preliminary report the on effects of QUE in a chronic constriction injury model proved significant effects of the agent, which should be supplemented with different studies using different dose regimens.

Novel human D-amino acid oxidase inhibitors stabilize an active-site lid-open conformation.

Abstract: The NMDAR (N-methyl-D-aspartate receptor) is a central regulator of synaptic plasticity and learning and memory. hDAAO (human D-amino acid oxidase) indirectly reduces NMDAR activity by degrading the NMDAR co-agonist D-serine. Since NMDAR hypofunction is thought to be a foundational defect in schizophrenia, hDAAO inhibitors have potential as treatments for schizophrenia and other nervous system disorders. Here, we sought to identify novel chemicals that inhibit hDAAO activity. We used computational tools to design a focused, purchasable library of compounds. After screening this library for hDAAO inhibition, we identified the structurally novel compound, ‘compound 2’ [3-(7-hydroxy-2-oxo-4-phenyl-2H-chromen-6-yl)propanoic acid], which displayed low nM hDAAO inhibitory potency (Ki=7 nM). Although the library was expected to enrich for compounds that were competitive for both D-serine and FAD, compound 2 actually was FAD uncompetitive, much like canonical hDAAO inhibitors such as benzoic acid. Compound 2 and an analog were independently co-crystalized with hDAAO. These compounds stabilized a novel conformation of hDAAO in which the active-site lid was in an open position. These results confirm previous hypotheses regarding active-site lid flexibility of mammalian D-amino acid oxidases and could assist in the design of the next generation of hDAAO inhibitors.