5-Hydroxytryptophan Alleviates Inflammation-Associated Mood Disturbance in Two Mouse Models of Colitis

Abstract

Anxiety and depression states are common in patients with inflammatory bowel disease (IBD) and correlate with disease activity suggesting a biologic link to inflammation. Experimentally lowered serum levels of tryptophan, the amino acid precursor of serotonin, can induce mood disturbance. We previously reported that in patients with active Crohn's disease serum tryptophan levels can be severely depressed while kynurenine levels are elevated. This elevated kynurenine/tryptophan ratio reflects increased expression of the immunomodulatory enzyme indoleamine 2,3 dioxygenase (IDO) present in active IBD and colitis models. In the current study we aimed to determine if enteral delivery of 5-hydroxytryptophan (5-HTP), the tryptophan-based serotonin precursor, improved mood disturbance or affected colitis severity in two mouse models. Dextran sodium sulfate (DSS) colitis was induced in Bl6 mice with 3% DSS given over 3 5-day courses, with 10-day recovery periods. 2,4,6 trinitrobenzene sulfonic acid (TNBS) colitis was induced in BALB/c mice using 4 escalating doses of TNBS (0.35 mg to 1.2 mg). In each experiment there were 3 groups of 7-10 mice: 1) control group (PBS gavage), 2) colitis control group (PBS gavage), and 3) colitis group with 5-HTP (0.5 mg gavage before testing). Behavior was filmed and scored at three time points: pre-colitis, after acute colitis, and after chronic colitis. Anxiety-like behavior was tested using the light-dark preference test (LDT). Depressive behavior was tested using the tail suspension test (TST). Locomotor activity (LMA) was tested at each time-point to rule out potential physical impairment prior to behavioral testing. Colitis activity was followed clinically (diarrhea, hematochezia, and weight loss) and assessed by histology and myeloperoxidase (MPO) activity. No differences in clinical or histology parameters were identified between colitis groups. Similarly, MPO activity did not differ with or without 5-HTP supplementation in both DSS and TNBS colitis. Locomotor abilities were similar in all 3 groups. No differences in mouse behavior were present at baseline or after acute colitis. Mice with chronic colitis from DSS spent significantly less time in light than controls, indicating anxiety-like behavior. This anxiety behavior was normalized by 5-HTP supplementation. Depressive behavior as measured by TST immobility time was longer in chronic DSS colitis; however, a therapeutic effect of 5-HTP was not observed. In TNBS colitis, mice had fewer entries to the light compared to controls in the LDT. This anxiety-like behavior was normalized by 5-HTP supplementation. There was no effect of TNBS colitis or 5-HTP on immobility as measured by the TST. Table 1 summarizes these findings. Anxiety- and depressive-like behavior is heightened in 2 models of chronic colitis. While anxiety behavior patterns vary by mouse strain, they are normalized by administration of the tryptophan-derived serotonin precursor 5-HTP. Importantly, colitis is not worsened by enteral supplementation with physiologic doses of 5-HTP (i.e. proportional to human dosing). With translational validation, 5-HTP may be considered an intervention for colitis-associated anxiety in IBD patients.