Kynurenine to tryptophan ratio as a biomarker of acute stress in fish
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Date
2022-01-17
Authors
Wish, Jade
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Abstract
The aim of this study was to determine the kynurenine (KYN) to tryptophan (TRP) ratio in fish liver and brain tissue to assess its usefulness as a biomarker of acute stress exposure. The activity of the IDO enzyme in the kynurenine pathway is known to be activated during periods of stress, infection, inflammation, and immunomodulation. The enzyme activity was measured indirectly using the ratio of TRP and its primary metabolite KYN. Laboratory held rainbow trout (Oncorhynchus mykiss) were subjected to an acute physical stressor and kynurenine, TRP, serotonin (5-HT), and cortisol were measured in liver and brain tissue extracts sampled at 4- and 48-hrs post-stress exposure. The analytical method used to extract and detect our analytes was based on lyophilization; liquid-solid extraction followed by isotope dilution high-performance liquid chromatography positive ion electrospray tandem mass spectrometry. The [KYN]/[TRP] ratio (KTR) was greater in fish liver and brain in the 48-hrs post-stress exposure group (n=8) relative to controls (n=8, p<0.05); a similar increase was not observed in fish in the 4-hr post-stress exposure group. The levels of TRP in the liver and brain samples did not decrease substantially and the increase in the KTR can be attributed to the increase in KYN levels. Hepatic and brain cortisol levels were also elevated in fish from both stress-induced groups relative to their respective controls implying that cortisol responded more quickly to the stressful stimulus than KYN, TRP, and 5-HT. My results suggest that (i) cortisol responds more quickly to an induced stressor than KYN and TRP and (ii) KTR is a promising acute stress diagnostic biomarker in fish (iii) KTR increases is due to KYN increasing and TRP slightly decreasing. The broad scope of TRP metabolism in organisms suggests that the KTR may also be a useful biomarker for stress in different species. Efforts are ongoing to assess whether the KTR can be used as a biomarker for chronic stress in fish exposed to aquatic contaminants and other environmental stressors and if similar assessments can be made on tissues collected via non-lethal approaches.
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Keywords
Kynurenine to tryptophan ratio, Tryptophan metabolism, acute stress biomarker, Indoleamine 2,3-dioxygenase, cortisol induction, Rainbow trout, Liver and brain tissues, HPLC-MS/MS