Fig. 3. The effect of voluntary chronic aerobic exercise on serum tryptophan metabolites and enzyme activities. Tryptophan metabolites (tryptophan [TRP], kynurenine [KYN], kynurenic acid [KYNA], 3-OH-kynurenine [3-HK]) and their ratios (KYN/TRP ratio, 3-HK/KYN ratio, KYNA/KYN ratio and KYNA/3-HK ratio) in C57BL/6 mice of 3-month-old, 6-month-old and 8-month-old (n=5 per timepoint) subjected to voluntary chronic aerobic exercise by inserting a running wheel in the cage for previous 1 month and without running wheel in control group (n=9 in 3-month-old and 6-monthold, and n=10 in 8-month-old). The black horizontal line in each box represents the median, with the boxes representing the interquartile range. ^*p<0.05, ^**p<0.01 (Mann-Whitney U test).