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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

Alterations in the kynurenine pathway (KP) neuroactive metabolites are implicated in psychiatric illnesses. Investigating the functional outcomes of an altered kynurenine pathway metabolism in vivo in rodents may help elucidate novel therapeutic approaches. The current protocol combines biochemical and behavioral approaches to investigate the impact of an acute kynurenine challenge in rats.

Abstract

The kynurenine pathway (KP) of tryptophan degradation has been implicated in psychiatric disorders. Specifically, the astrocyte-derived metabolite kynurenic acid (KYNA), an antagonist at both N-methyl-d-aspartate (NMDA) and α7 nicotinic acetylcholine (α7nACh) receptors, has been implicated in cognitive processes in health and disease. As KYNA levels are elevated in the brains of patients with schizophrenia, a malfunction at the glutamatergic and cholinergic receptors is believed to be causally related to cognitive dysfunction, a core domain of the psychopathology of the illness. KYNA may play a pathophysiologically significant role in individuals with schizophrenia. It is possible to elevate endogenous KYNA in the rodent brain by treating animals with the direct bioprecursor kynurenine, and preclinical studies in rats have demonstrated that acute elevations in KYNA may impact their learning and memory processes. The current protocol describes this experimental approach in detail and combines a) a biochemical analysis of blood kynurenine levels and brain KYNA formation (using high-performance liquid chromatography), b) behavioral testing to probe the hippocampal-dependent contextual memory (passive avoidance paradigm), and c) an assessment of sleep-wake behavior [telemetric recordings combining electroencephalogram (EEG) and electromyogram (EMG) signals] in rats. Taken together, a relationship between elevated KYNA, sleep, and cognition is studied, and this protocol describes in detail an experimental approach to understanding function outcomes of kynurenine elevation and KYNA formation in vivo in rats. Results obtained through variations of this protocol will test the hypothesis that the KP and KYNA serve pivotal roles in modulating sleep and cognition in health and disease states.

Introduction

The KP is responsible for degrading nearly 95% of the essential amino acid tryptophan1. In the mammalian brain, kynurenine taken into astrocytes is metabolized into the neuroactive small molecule KYNA primarily by the enzyme kynurenine aminotransferase (KAT) II2. KYNA acts as an antagonist at NMDA and α7nACh receptors in the brain2,3,4, and also targets signaling receptors including the aryl hydrocarbon receptor (AHR) and the G-protein coupled receptor 35 (GPR35)5,6....

Protocol

Our experimental protocols were approved by the University of Maryland Institutional Animal Care and Use Committee and followed the National Institutes of Health’s Guide for the Care and Use of Laboratory Animals.

NOTE: Adult male Wistar rats (250–350 g) were used in all experiments. Separate cohorts of animals were used for biochemical analysis, behavioral experiments, and sleep-wake recordings. The animals were housed in a temperature-controlled facility at the Maryland Psychiatric Research Center. They were kept on a 12/12 h light-dark cycle, with lights on at zeitgeber time (ZT) 0 and lights off at ZT 12. The animals receive....

Results

To validate the use of an intraperitoneal kynurenine injection as a method to elevate the brain KYNA, an HPLC analysis of tissue was performed. Standard curves (Figure 1) were constructed using the associated software and allowed for the quantification of the tissue samples. Representative chromatograms for kynurenine and KYNA are presented in Figure 2. Kynurenine was observed at a retention time of 6 min, and KYNA had a retentio.......

Discussion

For a reliable assessment of KYNA in the brain after a peripheral kynurenine administration, it is critical to combine and interpret biochemical and functional experiments. Here, we present a detailed protocol that permits new users to establish effective methods for measuring the plasma kynurenine and brain KYNA of rats. The measurement of kynurenine in the plasma confirmed the accurate injection and the measurement of the metabolite KYNA confirms the de novo synthesis in the brain. There are several advantages.......

Disclosures

The authors have nothing to disclose.

Acknowledgements

The present study was funded in part by the National Institutes of Health (R01 NS102209) and a donation from the Clare E. Forbes Trust.

....

Materials

NameCompanyCatalog NumberComments
Wistar ratsCharles River Laboratoriesadult male, 250-350 g
L-kynurenine sulfateSai Advantium
ReproSil-Pur C18 column (4 x 150 mm)Dr. Maisch GmbH
EZ ClipsStoelting Co.59022
Mounting materials screwsPlasticsOne00-96 X 1/16
Nonabsorbable SuturesMedRep Express699BCP Medical Monomid Black Nylon Sutures, 4-0, P-3, 18", BOX of 12
Absorbable SuturesEthiconJ310H4-0 Coated Vicryl Violet 1X27'' SH-1
Dental CementStoelting Co.51458
Drill BitStoelting Co.5145510.45 mm
NameCompanyCatalog NumberComments
Alliance HPLC system
E2695 separation moduleWaters176269503
2475 fluorescence detectorWaters186247500
post-column reagent managerWaters725000556
Lenovo computerWaters668000249
Empower softwareWaters176706100
NameCompanyCatalog NumberComments
Passive avoidance box for rat
Extra tall MDF sound attenuating cubicleMedAssociatesENV-018MDInterior: 22"W x 22"H x 16"D
Center channel modulator shuttle box chamberMedAssociatesENV-010MC
Stainless steel grid floor for ratMedAssociatesENV-010MB-GF
Auto guillotine doorMedAssociatesENV-010B-S
Quick disconnect shuttle grid floor harness for ratMedAssociatesENV-010MB-QD
Stimulus light, 1" white lens, mounted on modular panelMedAssociatesENV-221M
Sonalert module with volume control for rat chamberMedAssociatesENV-223AM
SmartCtrl 8 input/16 output packageMedAssociatesDIG-716P2
8 Channel IR control for shuttle boxesMedAssociatesENV-253C
Infrared source and dectector array stripsMedAssociatesENV-256
Tabletop interface cabinet, 120 V 60 HzMedAssociatesSG-6080C
Dual range constant current aversive stimulation moduleMedAssociatesENV-410B
Solid state grid floor scrambler moduleMedAssociatesENV-412
Dual A/B shock control moduleMedAssociatesENV-415
2' 3-Pin mini-molex extensionMedAssociatesSG-216A-2
10' Shock output cable, DB-9 M/FMedAssociatesSG-219G-10
Shuttle shock control cable 15', 6MedAssociatesSG-219SA
Small tabletop cabinet and power supply, 120 V 60 HzMedAssociatesSG-6080D
PCI interface packageMedAssociatesDIG-700P2-R2
Shuttle box avoidance utility packageMedAssociatesSOF-700RA-7
NameCompanyCatalog NumberComments
Sleep-Wake Monitoring Equipment
Ponehmah softwareData Sciences International (DSI)PNP-P3P-610
MX2 8 Source Acquisition interfaceData Sciences International (DSI)PNM-P3P-MX204
Dell computer, Optiplex 7020, Windows 7, 64 bitData Sciences International (DSI)271-0112-013
Dell 19" computer monitorData Sciences International (DSI)271-0113-001
Receivers for plastic cages, 8xData Sciences International (DSI)272-6001-001
Cisco RV130 VPN routerData Sciences International (DSI)RV130
Matrix 2.0Data Sciences International (DSI)271-0119-001
Network switchData Sciences International (DSI)SG200-08P
Neuroscore softwareData Sciences International (DSI)271-0171-CFG
Two biopotential channels transmitter, model TL11M2-F40-EETData Sciences International (DSI)270-0134-001

References

  1. Leklem, J. E. Quantitative aspects of tryptophan metabolism in humans and other species: a review. The American Journal of Clinical Nutrition. 24 (6), 659-672 (1971).
  2. Pocivavsek, A., Notarangelo, F. M., Wu, H. Q., Bruno, J. P., Schwarcz, R., Pletnikov, M. V., Waddington, J. L.

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