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

Our experimental protocols were approved by the University of Maryland Institutional Animal Care and Use Committee and followed the National Institutes of Health&#8217;s Guide for the Care and Use of Laboratory Animals.
NOTE: Adult male Wistar rats (250&#8211;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 Psychiatr...

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The authors have nothing to disclose.

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...

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 &#945;7nACh 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. In experimental animals, elevations in brain KYNA have been shown to impair their cognitive performance in an array of behavioral assays2,7,8,9,10. An emerging hypothesis suggests that KYNA plays an integral role in modulating cognitive functions by impacting sleep-wake behavior11, thus further supporting the role of astrocyte-derived molecules in modulating the neurobiology of sleep and cognition12.
Clinically, elevations in KYNA have been found in cerebrospinal fluid and post-mortem brain tissue from patients with schizophrenia13,14,15,16, a debilitating psychiatric disorder characterized by cognitive impairments. Patients with schizophrenia are also often plagued by sleep disturbances that may exacerbate the illness17. Understanding the role of KP metabolism and KYNA in modulating a relationship between sleep and cognition, particularly between learning and memory, may lead to the development of novel therapies for treating these poor outcomes in schizophrenia and other psychiatric illnesses.
A reliable and consistent method for the measurement of KP metabolites is important to assure that the research emerging from various institutions can be integrated into the scientific understanding of KP biology. Presently, we describe the methodology to measure kynurenine in rat plasma and KYNA in the rat brain by high-performance liquid chromatography (HPLC). The present protocol, which makes use of a fluorimetric detection in the presence of Zn2+, was first developed by Shibata18 and more recently adapted and optimized to derivatize with 500 mM zinc acetate as the post-column reagent, allowing for the detection of endogenous, nanomolar amounts of KYNA in the brain11.
To stimulate the de novo endogenous KYNA production as described in the present protocol, the direct bioprecursor kynurenine is injected intraperitoneally (i.p.) in rats. In combination with biochemical assessments to determine the degree of KYNA production, the impacts of a kynurenine challenge on the hippocampal-dependent memory (passive avoidance paradigm) and the sleep-wake architecture (EEG and EMG signals) is also investigated11. A combination of these techniques allows for the study of the biochemical and functional impact of a kynurenine challenge in vivo in rats.

<table>
	<tbody>
		<tr>
			<td>Wistar rats</td>
			<td>Charles River Laboratories</td>
			<td></td>
			<td>adult male, 250-350 g</td>
		</tr>
		<tr>
			<td>L-kynurenine sulfate</td>
			<td>Sai Advantium</td>
			<td></td>
			<td></td>
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			<td>ReproSil-Pur C18 column (4 x 150 mm)</td>
			<td>Dr. Maisch GmbH</td>
			<td></td>
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			<td>Stoelting Co.</td>
			<td>59022</td>
			<td></td>
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			<td>Mounting materials screws</td>
			<td>PlasticsOne</td>
			<td>00-96 X 1/16</td>
			<td></td>
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		<tr>
			<td>Nonabsorbable Sutures</td>
			<td>MedRep Express</td>
			<td>699B</td>
			<td>CP Medical Monomid Black Nylon Sutures, 4-0, P-3, 18&#34;, BOX of 12</td>
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			<td>Absorbable Sutures</td>
			<td>Ethicon</td>
			<td>J310H</td>
			<td>4-0 Coated Vicryl Violet 1X27&#39;&#39; SH-1</td>
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			<td>Dental Cement</td>
			<td>Stoelting Co.</td>
			<td>51458</td>
			<td></td>
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			<td>Drill Bit</td>
			<td>Stoelting Co.</td>
			<td>514551</td>
			<td>0.45 mm</td>
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			<td>Name</td>
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			<td>Catalog Number</td>
			<td>Comments</td>
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			<td>Alliance HPLC system</td>
			<td></td>
			<td></td>
			<td></td>
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			<td>E2695 separation module</td>
			<td>Waters</td>
			<td>176269503</td>
			<td></td>
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			<td>2475 fluorescence detector</td>
			<td>Waters</td>
			<td>186247500</td>
			<td></td>
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			<td>post-column reagent manager</td>
			<td>Waters</td>
			<td>725000556</td>
			<td></td>
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			<td>Lenovo computer</td>
			<td>Waters</td>
			<td>668000249</td>
			<td></td>
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			<td>Empower software</td>
			<td>Waters</td>
			<td>176706100</td>
			<td></td>
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		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Passive avoidance box for rat</td>
			<td></td>
			<td></td>
			<td></td>
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		<tr>
			<td>Extra tall MDF sound attenuating cubicle</td>
			<td>MedAssociates</td>
			<td>ENV-018MD</td>
			<td>Interior: 22&#34;W x 22&#34;H x 16&#34;D</td>
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		<tr>
			<td>Center channel modulator shuttle box chamber</td>
			<td>MedAssociates</td>
			<td>ENV-010MC</td>
			<td></td>
		</tr>
		<tr>
			<td>Stainless steel grid floor for rat</td>
			<td>MedAssociates</td>
			<td>ENV-010MB-GF</td>
			<td></td>
		</tr>
		<tr>
			<td>Auto guillotine door</td>
			<td>MedAssociates</td>
			<td>ENV-010B-S</td>
			<td></td>
		</tr>
		<tr>
			<td>Quick disconnect shuttle grid floor harness for rat</td>
			<td>MedAssociates</td>
			<td>ENV-010MB-QD</td>
			<td></td>
		</tr>
		<tr>
			<td>Stimulus light, 1&#34; white lens, mounted on modular panel</td>
			<td>MedAssociates</td>
			<td>ENV-221M</td>
			<td></td>
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			<td>Sonalert module with volume control for rat chamber</td>
			<td>MedAssociates</td>
			<td>ENV-223AM</td>
			<td></td>
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			<td>SmartCtrl 8 input/16 output package</td>
			<td>MedAssociates</td>
			<td>DIG-716P2</td>
			<td></td>
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		<tr>
			<td>8 Channel IR control for shuttle boxes</td>
			<td>MedAssociates</td>
			<td>ENV-253C</td>
			<td></td>
		</tr>
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			<td>Infrared source and dectector array strips</td>
			<td>MedAssociates</td>
			<td>ENV-256</td>
			<td></td>
		</tr>
		<tr>
			<td>Tabletop interface cabinet, 120 V 60 Hz</td>
			<td>MedAssociates</td>
			<td>SG-6080C</td>
			<td></td>
		</tr>
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			<td>Dual range constant current aversive stimulation module</td>
			<td>MedAssociates</td>
			<td>ENV-410B</td>
			<td></td>
		</tr>
		<tr>
			<td>Solid state grid floor scrambler module</td>
			<td>MedAssociates</td>
			<td>ENV-412</td>
			<td></td>
		</tr>
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			<td>Dual A/B shock control module</td>
			<td>MedAssociates</td>
			<td>ENV-415</td>
			<td></td>
		</tr>
		<tr>
			<td>2&#39; 3-Pin mini-molex extension</td>
			<td>MedAssociates</td>
			<td>SG-216A-2</td>
			<td></td>
		</tr>
		<tr>
			<td>10&#39; Shock output cable, DB-9 M/F</td>
			<td>MedAssociates</td>
			<td>SG-219G-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Shuttle shock control cable 15&#39;, 6</td>
			<td>MedAssociates</td>
			<td>SG-219SA</td>
			<td></td>
		</tr>
		<tr>
			<td>Small tabletop cabinet and power supply, 120 V 60 Hz</td>
			<td>MedAssociates</td>
			<td>SG-6080D</td>
			<td></td>
		</tr>
		<tr>
			<td>PCI interface package</td>
			<td>MedAssociates</td>
			<td>DIG-700P2-R2</td>
			<td></td>
		</tr>
		<tr>
			<td>Shuttle box avoidance utility package</td>
			<td>MedAssociates</td>
			<td>SOF-700RA-7</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Sleep-Wake Monitoring Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ponehmah software</td>
			<td>Data Sciences International (DSI)</td>
			<td>PNP-P3P-610</td>
			<td></td>
		</tr>
		<tr>
			<td>MX2 8 Source Acquisition interface</td>
			<td>Data Sciences International (DSI)</td>
			<td>PNM-P3P-MX204</td>
			<td></td>
		</tr>
		<tr>
			<td>Dell computer, Optiplex 7020, Windows 7, 64 bit</td>
			<td>Data Sciences International (DSI)</td>
			<td>271-0112-013</td>
			<td></td>
		</tr>
		<tr>
			<td>Dell 19&#34; computer monitor</td>
			<td>Data Sciences International (DSI)</td>
			<td>271-0113-001</td>
			<td></td>
		</tr>
		<tr>
			<td>Receivers for plastic cages, 8x</td>
			<td>Data Sciences International (DSI)</td>
			<td>272-6001-001</td>
			<td></td>
		</tr>
		<tr>
			<td>Cisco RV130 VPN router</td>
			<td>Data Sciences International (DSI)</td>
			<td>RV130</td>
			<td></td>
		</tr>
		<tr>
			<td>Matrix 2.0</td>
			<td>Data Sciences International (DSI)</td>
			<td>271-0119-001</td>
			<td></td>
		</tr>
		<tr>
			<td>Network switch</td>
			<td>Data Sciences International (DSI)</td>
			<td>SG200-08P</td>
			<td></td>
		</tr>
		<tr>
			<td>Neuroscore software</td>
			<td>Data Sciences International (DSI)</td>
			<td>271-0171-CFG</td>
			<td></td>
		</tr>
		<tr>
			<td>Two biopotential channels transmitter, model TL11M2-F40-EET</td>
			<td>Data Sciences International (DSI)</td>
			<td>270-0134-001</td>
			<td></td>
		</tr>
	</tbody>
</table>

a high-performance liquid chromatography measurement of kynurenine and kynurenic acid: relating biochemistry to cognition and sleep in rats

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 &#945;7 nicotinic acetylcholine (&#945;7nACh) 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.

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...

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

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.

The kynurenine pathway (KP) of tryptophan degradation has been implicated in psychiatric disorders. Specifically, the astrocyte-derived metabolite ...

This method combines biochemical and behavioral approaches to help answer key questions in the neuroscience field about the impact of an acute kynurenine challenge in rats. The main advantage of the technique is that it provides a reliable and consistent method for the measurement of kynurenine pathway metabolites in rat brain and plasma. Demonstrating the procedure will be Carly Fabian, a student research assistant from my laboratory.At the appropriate experimental end point after intraperitoneal kynurenine administration, thaw frozen harvested plasma on wet ice and dilute the thawed samples with ultrapure water. Transfer 100 microliters of diluted sample into individual containers of 25 microliters of 6%perchloric acid and vortex in centrifuge the samples. Then, transfer 100 microliters of supernatants from each tube into individual 250 microliter microcentrifuge tubes for kynurenine and kynurenic acid determination.For kynurenic acid measurement from brain samples, thaw the frozen harvested brain samples on dry ice and individually weigh each sample on a precise analytical balance. When all of the samples have been weighed, move the tubes onto wet ice and dilute each tissue sample at a one to 10 weight by volume ratio with ultrapure water, followed by homogenization with a sonicator. Aliquot 100 microliters of each sample slurry to a new tube then add 25 microliters of 25%perchloric acid for vortexing and centrifugation.Then, transfer 100 microliters of supernatant into individual 250 microliter microcentrifuge tubes for kynurenic acid determination. To prepare the high performance liquid chromatography, or HPLC, machine for kynurenine analysis, first place the individual HPLC intake tubes into the mobile phase, ultrapure water and 100%acetonitrile containers. Then, program the instrument to run 5%acetonitrile and 95%ultrapure water at 0.5 milliliters per minute and allow the HPLC machine to run for 20 to 30 minutes.When a stable pressure and baseline have been achieved, change the solution composition to a 5%acetonitrile and 95%sodium acetate mobile phase and equilibrate the instrument for another 30 minutes. While the instrument is equilibrating, turn on the lamp in the fluorescence detector. Prior to the derivitization, pump the zinc acetate solution separately through a peristaltic pump that combines with the mobile phase post column and turn on the post column pump to a flow rate of 0.1 milliliters per minute for about 20 minutes, until it reaches a stable pressure of about 500 psi.When the instrument is ready for the analysis, set the system software to control the sample sequence parameters and to allow the autoinjection of multiple samples. In the Run Sample'screen, click File'and drag down to Create New Sample Set'selecting Empty'in the new window. Individually list all of the standards and samples in the order they should be run, with the standards assayed at the beginning and end of the sequence.In the function column, designate the standards and samples and set the program to inject water between every five samples. Set the run time for each sample to 15 minutes and inject 20 microliters of the samples from the standards, followed by 20 microliters of the plasma samples and 30 microliters of the brain homogenate samples as experimentally appropriate. Then, set the excitation and emission wavelengths according to the compound being measured and run the sequence.To quantify the data, open the Browse Project'screen and under the Sample Sets'tab, double click on the sample set to be quantified. From the list of injections, highlight all of the standards and right click to select Process'in the new window. Then, use the drop-down menu to select Calibrate and Quantitate'Highlight all of the standards again, right click and select Review'When the chromatograms open, integrate and calibrate each standard to generate the standard curve.Then, return to the Sample Sets'tab and double click on the sample set to quantitate and review as demonstrated, integrating and quantitating each sample to output the concentration of each sample based on the previously created standard curve. HPLC analysis of tissue samples harvested from kynurenine administered animals reveal a kynurenine retention time of about six minutes and a kynurenic acid retention time of about 11 minutes. The quantification of plasma kynurenine and hippocampal kynurenic acid reveals that acute kynurenine injection increases brain kynurenic acid levels with a peak achieved at two hours post-injection that returns to baseline at four hours post-injection.Although no group differences are observed during passive avoidance paradigm training trials, control animals display a significant increase in latency to enter the dark side during testing trials, indicating contextual learning, while animals injected with kynurenine 24 hours previously do not display the same increase in latency, demonstrating a deficit in learning. Further, assessment of the impact of an acute kynurenine elevation during the light phase on sleep-wake architecture indicates a reduction in the total rapid eye movement sleep duration during the first two hours after kynurenine injection. These data are mirrored by an increase in wake duration during these time periods and a slight reduction in non-rapid eye movement sleep, demonstrating that an acute kynurenine elevation also causes disturbances in sleep-wake dynamics.Following this procedure, other methods like behavioral analysis can be performed to answer additional questions about the role of elevated kynurenic acid formation in modulating cognition and sleep.

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JoVE Journal

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10.4K ビュー.  University of Maryland School of Medicine. This method combines biochemical and behavioral approaches to help answer key questions in the neuroscience field about the impact of an acute kynurenine challenge in rats. The main advantage of the technique is that it provides a reliable and consistent method for the measurement of kynurenine pathway metabolites in rat brain and plasma. Demonstrating the procedure will be Carly Fabian, a student research assistant from my laboratory.At the appropriate experimental end point after int...