Interaction Between Intestinal Serotonin and The Gut Microbiome
He Liu*, Aaron Brown
Biology Department, Morosky College of Health Professions and Sciences, Gannon University, Erie, PA.
*Corresponding Author
He Liu,
Biology Department, Morosky College of Health Professions and Sciences, Gannon University, Erie, PA.
Tel: +18148717871
Fax : +18148715319
E-mail: liu017@gannon.edu
Received: May 11, 2021; Accepted: May 27, 2021; Published: June 04, 2021
Citation: Aaron Brown, He Liu. Interaction Between Intestinal Serotonin and The Gut Microbiome. Int J Anat Appl Physiol. 2021;07(04):192-196. doi: dx.doi.org/10.19070/2572-7451-2100036
Copyright: Aaron Brown�2021. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use,
distribution and reproduction in any medium, provided the original author and source are credited.
Abstract
Greater than 90% of serotonin (5-HT) in the human body is derived from Enterochromaffin (EC) Cells located within the most distal portion of the gastrointestinal tract. Inside of the large intestine can be found a population of trillions of symbiotic microbes. Current evidence shows that these microbes influence host physiology through communication with the nervous system via metabolic byproducts. In the colonic environment, 5-HT is an important signaling molecule for peristalsis, enteric motor and secretory reflexes, and immune responses. 5-HT synthesis within the colon is regulated by host microbiota through stimulation of EC from their short chain fatty acid by products. EC have been shown to release 5-HT into the colonic lumen after stimulation from various non-microbially produced products such as bile acids, allyl isothiocyanate, catecholamines, and tryptamine. Irregular 5-HT signaling has been shown to influence microbial colonization of the colon. Further exploration is necessary to understand the complete mechanism of microbial signaling and colonic 5-HT production.
2.Introduction
3.Serotonin
4.Microbiome Acting On Serotonin
5.Serotonin Acting On The Microbiome
6.Conclusion
7.Acknowledgments
8.References
Keywords
Serotonin; Gut Microbiome; Enterochromaffin Cell.
Introduction
The human gut microbiome consists of a collection of microorganisms,
mainly bacteria, present in the gastrointestinal (GI)
tract. Found with in the human gut microbiome are 1014 resident
microorganisms, of which includes bacteria, viruses, fungi and
protozoa [1]. The distal most portion of the gastrointestinal tract
harbors majority of microorganisms, here their biomass exceeds
1011 cells per gram content [2]. Around 1,000 different bacterial
species are found in the human gut, their relative distribution is
dependent on host age, genetic background, environment, and
lifestyle. More than 70% of these bacterial species come from the
phyla of Firmicutes and Bacteroides [3, 4].
Numerous factors over the course of one�s life contribute to the
development of the gut microbiomes ecosystem. One�s method
of delivery at birth, either vaginally or cesarean, alters the microbial
environment. Vaginal birth has been shown to promote the
proper development of the gut microbiome, due to the child�s
exposure to the mother�s vaginal flora while passing through the
birth canal [5]. Other factors such as infant consumption of breast
milk or formula, social and outdoor exposure, diet and lifestyle, as
well as antibiotic use, can alter the microbial ecosystem [3, 4]. For
example, a short-term change to either a strict animal or plantbased
diet alters the gut microbial content within 24 hours and
can be reversed in 48 hours after cessation of the strict diet [6].
The gut is known to interact with the brain via the enteric nervous
system, the hypothalamus-pituitary-adrenal-gland (HPA) axis, and
the central nervous system. The gut microbiome has the ability
to alter brain development, functionality, and behavior by modifying
immune, endocrine, metabolic, and neural signaling [7]. In
general, through the bidirectional communication network between
the gut and the brain via multiple systems such as the spinal
cord, the peripheral nervous system, and the endocrine system, the gut microbiome regulates many physiological and psychiatric
processes in the body [3, 4, 8, 9]. For example, more than half of
all the patients with irritable bowel syndrome exhibit mood disorders,
modifications of the gut microbiome can cause depressive
behaviors, and antibiotic exposure is known to have psychiatric
side-effects such as anxiety, panic, and major depressive disorder
[9].
Serotonin, or 5-HT, is produced in both the brain and the gastrointestinal
tract, with greater than 90% being produced via EC
in the distal gut lumen. 5-HT modulates numerous physiological
processes from mood disorders to gastrointestinal motility
[10]. The two main rate-limiting enzymes for 5-HT synthesis are
tryptophan hydroxylase 1 (Tph1), found in EC, and tryptophan
hydroxylase 2 (Tph2) in various neurons of the brain [10]. The
gut microbiome has been shown to upregulate Tph1 synthesis,
and thus 5-HT production from their SCFA metabolites [11].
Butyric acid, a common microbial waste product of fermentation,
specifically has been found to offer a significant role in 5-HT
production, as an inducer for the Tph1 transcription factor ZBP-
89 [11]. Numerous other stimulatory factors such as bile acids
(Deoxycholic acid and lithocholic acid), allyl isothiocyanate, and
catecholamines have been found to contribute to the release of
5-HT from EC [12, 13].
It is known that serotonin is produced via enterochromaffin cells
within the gut mucosal layer and that this gut-derived serotonin
has numerous physiological roles. In this review, we summarize
the literature regarding the relationship between the gut microbiome
and serotonin. Our goal is to present the current understanding
of how the gut microbiome contributes to the synthesis of
serotonin and the effects serotonin has on the peripheral systems.
Serotonin
Serotonin, 5-hydroxytryptamine or 5-HT, is a neurotransmitter
derived from the essential amino acid tryptophan with diverse
physiological functions. In the brain, it is involved in the modulation
of many behavioral processes such as mood and reward. In
the central nervous system (CNS) 5-HT is predominantly produced
by neurons. However, most serotonin in the body is outside
the CNS, to interact with a variety of seven families of serotonin
receptors [14]. The blood-brain barrier separates serotonin
in the CNS from that in the peripheral systems. More than 90%
of 5-HT in the body is produced in the peripheral systems via
the enterochromaffin cells (EC), which are specialized endocrine
cells in the intestinal epithelia [15, 16].
The rate-limiting enzyme of the 5-HT synthesis in the EC is
Tph1, converting L-tryptophan to L-5-hydroxytryptophan, which
is further converted by L-amino acid decarboxylase (AAAD) into
5-HT [17-19]. In neurons, a different hydroxylase, Tph2, is used
[16, 20]. The amino acid L-tryptophan is one of the nine essential
amino acids that the human body cannot produce and therefore
must be acquired from diet.
5-HT signaling in the gastrointestinal tract and the central nervous
systems occurs through the activation of seven families of
serotonergic receptors: 5-HT1 through 5-HT7. In the gastrointestinal
tract 5-HT3 and 5-HT4 are the most prominent. However,
they operate differently; the 5-HT3 receptor is a ligand-gated
Na+ and K+ cation channel, while the 5-HT4 receptor is a metabotropic
G protein-coupled receptor (GPCR) [21]. 5-HT3 is
highly expressed in afferent vagal neurons, which are known to
innervate the intestinal mucosa and upon activation can directly
communicate with the CNS [22]. 5-HT3 receptors are also known
to regulate colonic motility and peristalsis [23]. While the GPCR
5-HT4 regulates gastrointestinal motility, visceral pain, immune
regulation, and epithelial secretions [24].
Thus, 5-HT is multi-functional as a growth factor, paracrine factor,
and a neurotransmitter, with a vast majority of 5-HT being
derived from EC in the gut. A host of physiological processes
are regulated by 5-HT including depression, sleep patterns, food
appetite, libido, and temperature homeostasis [14, 25-27]. 5-HT
has been shown to modulate gastrointestinal motility, platelet
function, enteric motor and secretory reflexes, immune responses,
bone development, and cardiac functionality. It has also been
shown that the microbiota can impact hippocampal 5-HT levels
showing a connection between the gut and brain serotonergic system
[28]. Different concentrations of 5-HT can alter the microbial
composition of the colonic environment. Increased levels of
gut 5-HT secretion produce a microbial environment that has a
higher probability of leading to severe colitis, showing a connection
between the serotonin-microbiota axis and gut inflammation
[29]. Irregular 5-HT signaling has been observed in gastrointestinal
disorders such as inflammatory bowel disease (IBD) and colorectal
cancer [29]. It is evident that regulated 5-HT production is
essential to numerous physiological processes, and that a combination
of host genetics, physiology, and the gut microbiota play a
significant role in regulating 5-HT production.
Microbiome Acting On Serotonin
EC act as pressure sensors for the lumen of the colon, where
an increase in pressure can increase luminal 5-HT concentrations
[30]. EC also act as polymodal chemosensors activated by multiple
substances. One study identified three categories of stimulating
substances that promote EC to release 5-HT, including allyl
isothiocyanate (AITC) - a chemical irritant from dietary wasabi
and mustard, fatty acid prokaryotic fermentation products (butyrate,
isobutyrate, and isovalerate), and host-derived catecholamines
(dopamine, epinephrine, and norepinephrine) [12]. Bile acids
are also thought to have a stimulatory effect on EC to release
5-HT. Two bile acids deoxycholic acid (DCA) and lithocholic acid
(LCA) activate a cell-surfaced G-protein coupled receptor, TGR5,
to release the peristaltic transmitter 5-HT in to the lumen [31].
In addition, endospore-forming microbes, such as Clostridium
sporogenes, produce the microbial metabolites cholate, deoxycholate,
p-aminobenzoate (PABA), a-tocopherol, and tyramine,
which have been shown to promote 5-HT synthesis from cultured
rat colonic EC [9].
The effect of microbial stimulation of 5-HT release may or may
not depend on EC. Some microbes may directly participate in
the chemical synthesis of 5-HT. Multiple bacterial strains are
able to produce 5-HT in vitro using tryptophan as the substrate
[32]. Tryptophan decarboxylases are also present in the intestinal
microbiota. For example, two bacterial species, Clostridium
sporogenes and Ruminococcus gnavus express enzymes capable
of decarboxylating tryptophan to tryptamine [33]. One model
suggests that dietary, non-assimilated tryptophan in the colonic lumen is converted by secreted prokaryotic decarboxylases in to
tryptamine, which induces serotonin release from guinea pig EC
[33, 34]. The level of tryptamine in feces was significantly elevated
in mice with normal, non-pathogen microbiota, compared
to germ-free mice [35]. The increased presence of tryptamine in
conventional mice, indicates that the gut microbiome plays a role
in the production of the EC stimulator, tryptamine. Interestingly
it was demonstrated that tryptamine activates the 5-HT4 receptor
and increases anion-dependent fluid secretions in the colon,
which is key for regulating gut motility mechanisms like peristalsis
[24, 36].
Additionally, the gut microbiota stimulates 5-HT production in
EC through its prokaryotic metabolites, mainly short-chain fatty
acids (SCFAs) (Figure 1). SCFAs are produced by the gut microbiota
through the process of fermentation utilizing incomplete carbohydrates
such as starch and fiber. The majority of SCFAs found
in the intestinal lumen are acetate, propionate, and butyrate, none
of which can be produced by human cells [37, 38]. These three
SCFAs are produced in various bacterial species through different
metabolic pathways. Acetate is produced mostly by enteric
bacteria such as Akkermansia muciniphila, Bacteroides spp., Bifidobacterium
spp., Prevotella spp., Ruminococcus spp.[39]. Propionate
is produced in two different pathways; the succinate pathway
by Bacteroidetes spp. and the lactate pathway by Firmicutes
spp. Butyrate is produced through the classical pathway, which
involves the condensation of conventional acetyl-CoA by various
Firmicutes spp [31]. Butyrate has been identified as the preferred
energy source for host colonocytes [39]. The general ratio
of acetate, propionate, and butyrate is approximately 3:1:1, a ratio
heavily influenced by host diet and intestinal microbial species
composition [33, 39], as different phyla in the gut microbiome
are enriched with different cocktails of prokaryotic glycosidases,
lipases, and peptidases [6]. Before propionate and butyrate are
further metabolized by the liver (propionate) and the colon (butyrate),
these SCFA molecules directly activate G-protein coupled
receptors and affect host physiology [39], including promoting
5-HT synthesis in EC [10]. SCFAs have been shown to upregulate
Tph1 levels in EC, and as Tph1 is the rate-limiting enzyme
in the synthesis of serotonin, SCFAs thus likely drive 5-HT production
by EC [29]. In vitro, human EC treated with sodium acetate
and sodium butyrate results in increased Tph1 expression
[10]. Butyrate appears to play a direct role in the production of
Tph1 mRNA. Butyrate is known as a histone deacetylase (HDAC)
inhibitor; this allows butyrate to prevent the removal of acetyl
groups from DNA histone proteins [40]. This action allows DNA
to remain accessible to larger transcription factors. ZBP-89 is one
such transcription factor found in colonic EC that is a butyrateinduced
zinc finger which binds to GC-rich DNA elements [40]. In
one mouse study [40], it was shown that ZBP-89 directly binds
a mouse Tph1 DNA promoter region, thus elevating transcription
of Tph1 mRNA (Figure 1) [11, 39, 40]. When the Tph1 gene is
knocked out (KO) in a mouse model, mice lack >90% of intestinal
and serum 5-HT levels, further emphasizing the importance
of Tph1 in 5-HT synthesis [41]. Acetate and propionate in the
large intestine lumen drive GPR43+ host colonocytes to secrete
Glucagon-like-peptide-1 (GLP-1) and Peptide YY (PYY) [22, 39].
EC express the GLP-1and PYY Y1 receptors [42], and upon appropriate
ligand binding EC cells release 5-HT into the colonic
lumen [22] (Figure 1). Butyrate functions in a direct manner by
upregulating Tph1 mRNA, while acetate and propionate stimulate
release of GLP-1 and PYY, which then signal EC to release
5-HT. Thus, for EC cells, butyrate likely drives 5-HT production
while acetate and propionate regulate 5-HT release.
Figure 1. The release of 5-HT and upregulation of TpH1 mRNA via short chain fatty acid signaling from the gut microbiome.
In germ-free mice, serum concentrations of 5-HT tend to be lower than colonized mice. Mice raised with a human gut microbiome exhibited more than twice the Tph1 transcription and a 20% increasein Tph1 compared to germ-free mice [11]. When the microbiome was introduced to germ-free mice, they exhibited elevated Tph1 expression and restoration of serum 5-HT concentration [10, 43]. These observations show that the gut microbiome does influence Tph1 expression and in turn, 5-HT production.
Serotonin Acting On The Microbiome
The influence that the gut microbiome has on host physiology
through its metabolic byproducts, neural influence, and immune
modulation is becoming more apparent in the scientific community.
However, less is known about the change in the ecosystem of
the gut microbiome in response to host physiological irregularities.
Many patients with gastrointestinal disorders such as inflammatory
bowel disease (IBD) tend to exhibit dysbiosis of the gut
microbiome and have irregular 5-HT signaling [29]. It is unclear as to whether irregular 5-HT signaling leads to microbial dysbiosis
or vice versa. However, one study showed [29] that varied 5-HT
signaling regulated gut bacteria growth in a species-specific manner
that created an environment more susceptible to colitis. When
5-HT was introduced to the gut lumen at increasing concentrations,
bacterial growth was inhibited, and obligate anaerobes were
the most affected. The researchers in one study [29] compared
the gut microbial composition of Tph1 knockout (KO) mice to
conventional wild-type (WT) mice. The Tph1 KO mice showed
a significant difference in the composition of their microbial
ecosystem compared to the WT mice, further emphasizing that
host genetic background strongly determines a host�s unique
microbiota signature. Further, in Tph1 KO mice, fecal sample
analysis revealed significantly decreased levels of the SCFAs acetate,
butyrate, and propionate [29]. In contrast, the number of
endospore-forming bacteria in the gut is increased as levels of
luminal 5-HT are increased. It is known that about 50% of 5-HT
produced in the gut is the product of endospore-forming bacteria
from families such as Clostridiaceae and Turicibacteraceae [44].
These results suggest that not only does the gut microbiome affect
5-HT synthesis, but also that 5-HT has the ability to alter the
microbial composition. 5-HT signaling, and endospore-forming
bacteria colonization may form a positive feedback mechanism,
that interfaces with host status of genetics, diet, and environmental
influence on development of gut bacterial species.
Conclusion
Approximately 10-100 trillion communalistic microbial organisms
colonize the human body in various sites including the oral cavity,
skin, female vagina, and most prominently the distal portion
of the gastrointestinal tract [45]. The bacterial organisms found
in the colon play a pivotal role in human health, and specifically
5-HT production. One of the main ways in which the gut bacteria
can influence 5-HT production is through the synthesis of
SCFA metabolites from fiber and starch fermentation. Butyrate
specifically has been identified as one SCFA that promotes the
upregulation of the 5-HT rate-limiting enzyme, Tph1. Interestingly
it was also identified that increased 5-HT concentrations in
the gut lumen promote the growth of spore-forming bacteria,
and these same spore-forming bacteria produce metabolites that
promote further synthesis of 5-HT. Some gut bacterial organisms
exhibit symbiotic relationships with their human hosts. One example
is the production of SCFA�s, bacterial organisms can produce
SCFA�s as byproducts of fermentation. However, humans
alone are not capable of producing SCFA�s. Bacterial organisms
use the gastric environment to grow and ferment the dietary
starch and fiber to produce energy. While humans use the SCFA�s
for various purposes from energy sources to signal transduction.
The relationship between human host and bacterial organism may
have once been viewed as adversarial, but very may well be one
of companionship.
Thus, the field of the microbiota-gut-brain (MGB) axis is supported
by numerous studies that identify the importance of the
gut microbiome and its metabolites in the production of 5-HT,
specifically its EC upregulation of the rate-limiting enzyme Tph1.
We also presented the research on the influence that 5-HT can
have on the composition of the gut microbial environment. Further
investigation is necessary to understand the overarching impact
the gut microbiome plays on human health, as well as how
lifestyle, diet, genetics, and host physiology may impact the composition
of the human gut microbiome. Further investigation in
to the gut microbiome and its symbiotic relationship with human
health is necessary for the advancement of our scientific community
and various healthcare sectors. Questions include how can an
individual�s gut microbiota change with age, diet, exercise, and or
injury. Do some microbial signatures make us more or less prone
to mental disorders or physical disease?
Acknowledgments
The authors thank Dr. Gary Vanderlaan at Gannon University for reviewing the manuscript and providing valuable suggestions before submission.
References
- Gill SR, Pop M, Deboy RT, Eckburg PB, Turnbaugh PJ, Samuel BS, et al. Metagenomic analysis of the human distal gut microbiome. Science. 2006 Jun 2;312(5778):1355-9. Pubmed PMID: 16741115.
- Walter J, Ley R. The human gut microbiome: ecology and recent evolutionary changes. Annu Rev Microbiol. 2011;65:411-29. Pubmed PMID: 21682646.
- Singh RK, Chang HW, Yan D, Lee KM, Ucmak D, Wong K, et al. Influence of diet on the gut microbiome and implications for human health. J Transl Med. 2017 Apr 8;15(1):73. Pubmed PMID: 28388917.
- Peirce JM, Alvi�a K. The role of inflammation and the gut microbiome in depression and anxiety. J Neurosci Res. 2019 Oct;97(10):1223-1241. Pubmed PMID: 31144383.
- Dominguez-Bello MG, De Jesus-Laboy KM, Shen N, Cox LM, Amir A, Gonzalez A, et al. Partial restoration of the microbiota of cesarean-born infants via vaginal microbial transfer. Nat Med. 2016 Mar;22(3):250-3. Pubmed PMID: 26828196.
- David LA, Maurice CF, Carmody RN, Gootenberg DB, Button JE, Wolfe BE, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014 Jan 23;505(7484):559-63. Pubmed PMID: 24336217.
- Szoke H, Kov�cs Z, B�kkon I, Vagedes J, Szab� AE, Hegyi G, et al. Gut dysbiosis and serotonin: intestinal 5-HT as a ubiquitous membrane permeability regulator in host tissues, organs, and the brain. Rev Neurosci. 2020 May 26;31(4):415-425. Pubmed PMID: 32007948.
- O'Mahony SM, Clarke G, Borre YE, Dinan TG, Cryan JF. Serotonin, tryptophan metabolism and the brain-gut-microbiome axis. Behav Brain Res. 2015 Jan 15;277:32-48. Pubmed PMID: 25078296.
- Rogers GB, Keating DJ, Young RL, Wong ML, Licinio J, Wesselingh S. From gut dysbiosis to altered brain function and mental illness: mechanisms and pathways. Mol Psychiatry. 2016 Jun;21(6):738-48. Pubmed PMID: 27090305.
- Yano JM, Yu K, Donaldson GP, Shastri GG, Ann P, Ma L, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015 Apr 9;161(2):264-76. Pubmed PMID: 25860609.
- Reigstad CS, Salmonson CE, Rainey JF 3rd, Szurszewski JH, Linden DR, Sonnenburg JL, et al. Gut microbes promote colonic serotonin production through an effect of short-chain fatty acids on enterochromaffin cells. FASEB J. 2015 Apr;29(4):1395-403. Pubmed PMID: 25550456.
- Bellono NW, Bayrer JR, Leitch DB, Castro J, Zhang C, O'Donnell TA, et al. Enterochromaffin Cells Are Gut Chemosensors that Couple to Sensory Neural Pathways. Cell. 2017 Jun 29;170(1):185-198.e16. Pubmed PMID: 28648659.
- Bunnett NW. Neuro-humoral signalling by bile acids and the TGR5 receptor in the gastrointestinal tract. J Physiol. 2014 Jul 15;592(14):2943-50. Pubmed PMID: 24614746.
- Berger M, Gray JA, Roth BL. The expanded biology of serotonin. Annu Rev Med. 2009;60:355-66. Pubmed PMID: 19630576.
- Gershon MD. 5-Hydroxytryptamine (serotonin) in the gastrointestinal tract. Curr Opin Endocrinol Diabetes Obes. 2013 Feb;20(1):14-21. Pubmed PMID: 23222853.
- Gershon MD, Tack J. The serotonin signaling system: from basic understanding to drug development for functional GI disorders. Gastroenterology. 2007 Jan;132(1):397-414. Pubmed PMID: 17241888.
- Wikoff WR, Anfora AT, Liu J, Schultz PG, Lesley SA, Peters EC, et al. Metabolomics analysis reveals large effects of gut microflora on mammalian blood metabolites. Proc Natl Acad Sci U S A. 2009 Mar 10;106(10):3698- 703. Pubmed PMID: 19234110.
- H�glund E, �verli �, Winberg S. Tryptophan Metabolic Pathways and Brain Serotonergic Activity: A Comparative Review. Front Endocrinol (Lausanne). 2019 Apr 8;10:158. Pubmed PMID: 31024440.
- Kitahama K, Ikemoto K, Jouvet A, Araneda S, Nagatsu I, Raynaud B, et al. Aromatic L-amino acid decarboxylase-immunoreactive structures in human midbrain, pons, and medulla. J Chem Neuroanat. 2009 Oct;38(2):130-40. Pubmed PMID: 19589383.
- Heredia DJ, Gershon MD, Koh SD, Corrigan RD, Okamoto T, Smith TK. Important role of mucosal serotonin in colonic propulsion and peristaltic reflexes: in vitro analyses in mice lacking tryptophan hydroxylase 1. J Physiol. 2013 Dec 1;591(23):5939-57. Pubmed PMID: 24127620.
- Nichols DE, Nichols CD. Serotonin receptors. Chemical reviews. 2008 May 14;108(5):1614-41.
- Lund ML, Egerod KL, Engelstoft MS, Dmytriyeva O, Theodorsson E, Patel BA, et al. Enterochromaffin 5-HT cells - A major target for GLP-1 and gut microbial metabolites. Mol Metab. 2018 May;11:70-83. Pubmed PMID: 29576437.
- Thompson AJ, Lummis SC. 5-HT3 receptors. Curr Pharm Des. 2006;12(28):3615-30. Pubmed PMID: 17073663.
- Bhattarai Y, Williams BB, Battaglioli EJ, Whitaker WR, Till L, Grover M, et al. Gut Microbiota-Produced Tryptamine Activates an Epithelial G-Protein- Coupled Receptor to Increase Colonic Secretion. Cell Host Microbe. 2018 Jun 13;23(6):775-785.e5. Pubmed PMID: 29902441.
- Folk GE Jr, Long JP. Serotonin as a neurotransmitter: a review. Comp Biochem Physiol C Comp Pharmacol Toxicol. 1988;91(1):251-7. Pubmed PMID: 2905227.
- Vanhoutte PM. Serotonin: beyond the brain. ACS Chem Neurosci. 2013 Jan 16;4(1):26-7. Pubmed PMID: 23336041.
- El-Merahbi R, L�ffler M, Mayer A, Sumara G. The roles of peripheral serotonin in metabolic homeostasis. FEBS Lett. 2015 Jul 8;589(15):1728-34. Pubmed PMID: 26070423.
- Clarke G, Grenham S, Scully P, Fitzgerald P, Moloney RD, Shanahan F, et al. The microbiome-gut-brain axis during early life regulates the hippocampal serotonergic system in a sex-dependent manner. Mol Psychiatry. 2013 Jun;18(6):666-73. Pubmed PMID: 22688187.
- Kwon YH, Wang H, Denou E, Ghia JE, Rossi L, Fontes ME, et al. Modulation of Gut Microbiota Composition by Serotonin Signaling Influences Intestinal Immune Response and Susceptibility to Colitis. Cell Mol Gastroenterol Hepatol. 2019;7(4):709-728. Pubmed PMID: 30716420.
- Tsukamoto K, Ariga H, Mantyh C, Pappas TN, Yanagi H, Yamamura T, et al. Luminally released serotonin stimulates colonic motility and accelerates colonic transit in rats. Am J Physiol Regul Integr Comp Physiol. 2007 Jul;293(1):R64-9. Pubmed PMID: 17442783.
- Ge X, Pan J, Liu Y, Wang H, Zhou W, Wang X. Intestinal Crosstalk between Microbiota and Serotonin and its Impact on Gut Motility. Curr Pharm Biotechnol. 2018;19(3):190-195. Pubmed PMID: 29804531.
- Bosi A, Banfi D, Bistoletti M, Giaroni C, Baj A. Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease. Int J Tryptophan Res. 2020 Jun 11;13:1178646920928984. Pubmed PMID: 32577079.
- Williams BB, Van Benschoten AH, Cimermancic P, Donia MS, Zimmermann M, Taketani M, et al. Discovery and characterization of gut microbiota decarboxylases that can produce the neurotransmitter tryptamine. Cell Host Microbe. 2014 Oct 8;16(4):495-503. Pubmed PMID: 25263219.
- Takaki M, Mawe GM, Barasch JM, Gershon MD, Gershon MD. Physiological responses of guinea-pig myenteric neurons secondary to the release of endogenous serotonin by tryptamine. Neuroscience. 1985 Sep;16(1):223- 40. Pubmed PMID: 2940472.
- Marcobal A, Kashyap PC, Nelson TA, Aronov PA, Donia MS, Spormann A, et al. A metabolomic view of how the human gut microbiota impacts the host metabolome using humanized and gnotobiotic mice. ISME J. 2013 Oct;7(10):1933-43. Pubmed PMID: 23739052.
- Jenkins TA, Nguyen JC, Polglaze KE, Bertrand PP. Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut- Brain Axis. Nutrients. 2016 Jan 20;8(1):56. Pubmed PMID: 26805875.
- Yang NJ, Chiu IM. Bacterial Signaling to the Nervous System through Toxins and Metabolites. J Mol Biol. 2017 Mar 10;429(5):587-605. Pubmed PMID: 28065740.
- Topping DL, Clifton PM. Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides. Physiol Rev. 2001 Jul;81(3):1031-64. Pubmed PMID: 11427691.
- Koh A, De Vadder F, Kovatcheva-Datchary P, B�ckhed F. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell. 2016 Jun 2;165(6):1332-1345. Pubmed PMID: 27259147.
- Essien BE, Grasberger H, Romain RD, Law DJ, Veniaminova NA, Saqui- Salces M, et al. ZBP-89 regulates expression of tryptophan hydroxylase I and mucosal defense against Salmonella typhimurium in mice. Gastroenterology. 2013 Jun;144(7):1466-77, 1477.e1-9. Pubmed PMID: 23395646.
- Savelieva KV, Zhao S, Pogorelov VM, Rajan I, Yang Q, Cullinan E, et al. Genetic disruption of both tryptophan hydroxylase genes dramatically reduces serotonin and affects behavior in models sensitive to antidepressants. PLoS One. 2008;3(10):e3301. Pubmed PMID: 18923670.
- Bai L, Merchant JL. Transcription factor ZBP-89 cooperates with histone acetyltransferase p300 during butyrate activation of p21waf1 transcription in human cells. J Biol Chem. 2000 Sep 29;275(39):30725-33. Pubmed PMID: 10899165.
- Hata T, Asano Y, Yoshihara K, Kimura-Todani T, Miyata N, Zhang XT, et al. Regulation of gut luminal serotonin by commensal microbiota in mice. PLoS One. 2017 Jul 6;12(7):e0180745. Pubmed PMID: 28683093.
- Fung TC, Vuong HE, Luna CDG, Pronovost GN, Aleksandrova AA, Riley NG, et al. Intestinal serotonin and fluoxetine exposure modulate bacterial colonization in the gut. Nat Microbiol. 2019 Dec;4(12):2064-2073. Pubmed PMID: 31477894.
- Sochocka M, Donskow-Lysoniewska K, Diniz BS, Kurpas D, Brzozowska E, Leszek J. The Gut Microbiome Alterations and Inflammation-Driven Pathogenesis of Alzheimer's Disease-a Critical Review. Mol Neurobiol. 2019 Mar;56(3):1841-1851. Pubmed PMID: 29936690.