Gastroenterology

Gastroenterology

Volume 157, Issue 2, August 2019, Pages 507-521.e4
Gastroenterology

Original Research
Full Report: Basic and Translational—Alimentary Tract
Effects of Serotonin and Slow-Release 5-Hydroxytryptophan on Gastrointestinal Motility in a Mouse Model of Depression

https://doi.org/10.1053/j.gastro.2019.04.022Get rights and content

Background & Aims

Mood disorders and constipation are often comorbid, yet their shared etiologies have rarely been explored. The neurotransmitter serotonin (5-HT) regulates central nervous system and enteric nervous system (ENS) development and long-term functions, including gastrointestinal (GI) motility and mood. Therefore, defects in neuron production of 5-HT might result in brain and intestinal dysfunction. Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in 5-HT biosynthesis. A variant of TPH2 that encodes the R441H substitution (TPH2-R441H) was identified in individuals with severe depression. We studied mice with an analogous mutation (TPH2-R439H), which results in a 60%–80% decrease in levels of 5-HT in the central nervous system and behaviors associated with depression in humans. Feeding chow that contains 5-HTP slow release (5-HTP SR) to TPH2-R439H mice restores levels of 5-HT in the central nervous system and reduces depressive-like behaviors.

Methods

We compared the effects of feeding chow, with or without 5-HTP SR, to mice with the TPH2-R439H mutation and without this mutation (control mice). Myenteric and submucosal plexuses were isolated from all 4 groups of mice, and immunocytochemistry was used to quantify total enteric neurons, serotonergic neurons, and 5-HT–dependent subsets of neurons. We performed calcium imaging experiments to evaluate responses of enteric neurons to tryptamine-evoked release of endogenous 5-HT. In live mice, we measured total GI transit, gastric emptying, small intestinal transit, and propulsive colorectal motility. To measure colonic migrating motor complexes (CMMCs), we isolated colons and constructed spatiotemporal maps along the proximodistal length to quantify the frequency, velocity, and length of CMMCs. We measured villus height, crypt perimeter, and relative densities of enterochromaffin and enteroendocrine cells in small intestinal tissue.

Results

Levels of 5-HT were significantly lower in enteric neurons from TPH2-R439H mice than from control mice. TPH2-R439H mice had abnormalities in ENS development and ENS-mediated GI functions, including reduced motility and intestinal epithelial growth. Total GI transit and propulsive colorectal motility were slower in TPH2-R439H mice than controls, and CMMCs were slower and less frequent. Villus height and crypt perimeter were significantly decreased in colon tissues from TPH2-R439H mice compared with controls. Administration of 5-HTP SR to adult TPH2-R439H mice restored 5-HT to enteric neurons and reversed these abnormalities. Adult TPH2-R439H mice given oral 5-HTP SR had normalized numbers of enteric neurons, total GI transit, and colonic motility. Intestinal tissue from these mice had normal measures of CMMCs and enteric epithelial growth

Conclusions

In studies of TPH2-R439H mice, we found evidence for reduced release of 5-HT from enteric neurons that results in defects in ENS development and GI motility. Our findings indicate that neuron production of 5-HT links constipation with mood dysfunction. Administration of 5-HTP SR to mice restored 5-HT to the ENS and normalized GI motility and growth of the enteric epithelium. 5-HTP SR might be used to treat patients with intestinal dysfunction associated with low levels of 5-HT.

Section snippets

Animals

Male and female TPH2-R439H mice (c57BL6/J–129S6/SvEvTac background) were bred at Duke University (Durham, NC) and shipped to Columbia University Medical Center (New York, NY) or bred at CUMC.34 Experiments were conducted with homozygous wild-type (WT) and TPH2-R439H littermates. 5-HTP immediate release (IR) was administered intraperitoneally in a dose-response fashion, from 1 to 100 mg/kg. The 5-HTP SR (≅1 g/kg/day, 6.7 mg 5-HTP/g chow; created by mixing the 5-HTP in finely crushed chow with a

The TPH2-R439H Polymorphism Results in Decreased Levels of Enteric Neuronal Serotonin and Impedes Enteric Nervous System Development

TPH2 is required for 5-HT production in enteric neurons.13 Immunocytochemistry and calcium (Ca2+) channel imaging were thus used to assess the 5-HT content of the ENS and enteric neurons, respectively, in TPH2-R439H mice and WT littermates. Significantly fewer 5-HT–immunoreactive nerve cell bodies were detected in TPH2-R439H mice than in WT controls (Figure 1 and Supplementary Table 1). This deficit was manifest both in total numbers of serotonergic neurons and as the proportion of total

Discussion

GI problems and mood disorders are often comorbid. Few underlying links, however, are known. Mice with a polymorphism that alters neuronal 5-HT production, TPH2-R439H, were previously found to manifest depression- and anxiety-like behaviors, similar to individuals with an analogous human single-nucleotide polymorphism, TPH2-R441H.21 CNS 5-HT production is known to be deficient in TPH2-R439H mice, and TPH2 is responsible for 5-HT biosynthesis in both the ENS and the CNS. We therefore tested the

Acknowledgements

Author contributions: Kara G. Margolis, Narek Israelyan, and Andrew Del Colle designed and conducted the studies, analyzed data, and wrote the manuscript. Andrew Del Colle, Yeji Park, Albert Xing, Ruth Ann Luna, Dane D. Jensen, Moneek Madra, Virginia Saurman, Ray Rahim, Rocco Latorre, Kimberly Law, William Carson, and Nigel W. Bunnett conducted experiments and acquired data. Mark G. Caron and Jacob P.R. Jacobsen created and provided mice, conducted experiments, and analyzed data. All coauthors

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    Conflicts of interest These authors disclose the following: Jacob P.R. Jacobsen and Marc G. Caron are inventors on US patents pertaining to the adjunct 5-HTP SR method of treatment and hold equity in Evecxia Inc, a company founded to develop a 5-HTP slow-release drug for the management of serotonin-related disorders. The remaining authors disclose no conflicts.

    Funding This work was supported by National Institutes of Health grants DK093786 (Kara G. Margolis), NS15547 (Kara G. Margolis), MH79201 (Mark G. Caron), T35AG044303 (Narek Israelyan), NS102722 (Nigel W. Bunnett), DE026806 (Nigel W. Bunnett), and DK118971 (Nigel W. Bunnett); US Department of Defense grants PR160365 (Kara G. Margolis) and PR170507 (Nigel W. Bunnett); American Gastroenterological Association Student Research Fellowship Award (Narek Israelyan); the Lundbeck Foundation (Jacob P.R. Jacobsen); and gifts from the Phyllis and Ivan Seidenberg Family Fund for Children’s Digestive Health (Kara G. Margolis) and The Lennon Family (Mark G. Caron).

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