A High-performance Liquid Chromatography Measurement of Kynurenine and Kynurenic Acid: Relating Biochemistry to Cognition and Sleep in Rats

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 brain^2,3,4, 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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Wistar rats	Charles River Laboratories		adult male, 250-350 g
L-kynurenine sulfate	Sai Advantium
ReproSil-Pur C18 column (4 mm x 150 mm)	Dr. Maisch GmbH
EZ Clips	Stoelting Co.	59022
Mounting materials screws	PlasticsOne	00-96 X 1/16
Nonabsorbable Sutures	MedRep Express	699B	CP Medical Monomid Black Nylon Sutures, 4-0, P-3, 18", BOX of 12
Absorbable Sutures	Ethicon	J310H	4-0 Coated Vicryl Violet 1X27'' SH-1
Dental Cement	Stoelting Co.	51458
Drill Bit	Stoelting Co.	514551	0.45 mm
Name	Company	Catalog Number	Comments
Alliance HPLC system
E2695 separation module	Waters	176269503
2475 fluorescence detector	Waters	186247500
post-column reagent manager	Waters	725000556
Lenovo computer	Waters	668000249
Empower software	Waters	176706100
Name	Company	Catalog Number	Comments
Passive avoidance box for rat
Extra tall MDF sound attenuating cubicle	MedAssociates	ENV-018MD	Interior: 22" (W) x 22" (H) x 16" (D)
Center channel modulator shuttle box chamber	MedAssociates	ENV-010MC
Stainless steel grid floor for rat	MedAssociates	ENV-010MB-GF
Auto guillotine door	MedAssociates	ENV-010B-S
Quick disconnect shuttle grid floor harness for rat	MedAssociates	ENV-010MB-QD
Stimulus light, 1" white lens, mounted on modular panel	MedAssociates	ENV-221M
Sonalert module with volume control for rat chamber	MedAssociates	ENV-223AM
SmartCtrl 8 input/16 output package	MedAssociates	DIG-716P2
8 Channel IR control for shuttle boxes	MedAssociates	ENV-253C
Infrared source and dectector array strips	MedAssociates	ENV-256
Tabletop interface cabinet, 120 V 60 Hz	MedAssociates	SG-6080C
Dual range constant current aversive stimulation module	MedAssociates	ENV-410B
Solid state grid floor scrambler module	MedAssociates	ENV-412
Dual A/B shock control module	MedAssociates	ENV-415
2' 3-Pin mini-molex extension	MedAssociates	SG-216A-2
10' Shock output cable, DB-9 M/F	MedAssociates	SG-219G-10
Shuttle shock control cable 15', 6	MedAssociates	SG-219SA
Small tabletop cabinet and power supply, 120 V 60 Hz	MedAssociates	SG-6080D
PCI interface package	MedAssociates	DIG-700P2-R2
Shuttle box avoidance utility package	MedAssociates	SOF-700RA-7
Name	Company	Catalog Number	Comments
Sleep-Wake Monitoring Equipment
Ponehmah software	Data Sciences International (DSI)	PNP-P3P-610
MX2 8 Source Acquisition interface	Data Sciences International (DSI)	PNM-P3P-MX204
Dell computer, Optiplex 7020, Windows 7, 64 bit	Data Sciences International (DSI)	271-0112-013
Dell 19" computer monitor	Data Sciences International (DSI)	271-0113-001
Receivers for plastic cages, 8x	Data Sciences International (DSI)	272-6001-001
Cisco RV130 VPN router	Data Sciences International (DSI)	RV130
Matrix 2.0	Data Sciences International (DSI)	271-0119-001
Network switch	Data Sciences International (DSI)	SG200-08P
Neuroscore software	Data Sciences International (DSI)	271-0171-CFG
Two biopotential channels transmitter, model TL11M2-F40-EET	Data Sciences International (DSI)	270-0134-001