{
  "id": "serotonin",
  "title": "Serotonin",
  "lang": "en",
  "source_lang": "en",
  "summary": "Serotonin is a monoamine signaling molecule that works as a neurotransmitter in the brain and as a hormone-like regulator in the gut, blood, and other tissues.",
  "category": "health",
  "subcategory": "anatomy-physiology",
  "type": "concept",
  "aliases": [
    "5-hydroxytryptamine",
    "5-HT",
    "enteramine (historical name)"
  ],
  "generated_at": "2026-10-08",
  "model": "pro/claude-haiku-5-5",
  "url": "https://aiwiki.page/en/serotonin/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/serotonin/",
      "json": "https://aiwiki.page/zh/serotonin.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "Serotonin, or 5-hydroxytryptamine (5-HT), is a small indoleamine molecule that acts as a chemical messenger in both animals and humans. It is best known as a neurotransmitter that shapes mood, sleep, appetite, and cognition, but most of the body's serotonin is actually made in the gut, where it regulates digestive movement and secretion. Its name reflects its early discovery in blood serum and its ability to constrict blood vessels. Because serotonin reaches so many organs and signals through many receptor types, it sits at the center of research in neuroscience, pharmacology, and medicine.\n\nThe molecule's history runs from the 1860s to the mid-twentieth century. Independent groups in Italy and the United States identified the same substance under different names before it was recognized as one compound. Serotonin is produced from the amino acid tryptophan in two enzymatic steps, and it is broken down by enzymes into inactive products. Its effects depend on the tissue involved: in the brain, serotonin acts through synapses on a family of receptors, while in the gut and blood it acts as a local and circulating signal that influences motility, clotting, and inflammation.\n\nThis entry first covers the chemistry and discovery of serotonin, then its synthesis, distribution, transport, and receptors. It then describes its functions in the central and peripheral nervous systems, its roles in other organisms, its use and misuse in pharmacology, and its emerging links to cancer. The discussion keeps to well-established findings and notes where interpretations remain contested."
    },
    {
      "heading": "Chemical structure and classification",
      "text": "Serotonin is an amino acid-derived indoleamine, a type of biogenic monoamine derived from tryptophan. Its molecular formula is C₁₀H₁₂N₂O, and its systematic name is 3-(2-aminoethyl)-1H-indol-5-ol. The molecular weight is about 176 g/mol, and the pure compound is a white powder with a melting point near 168 °C. Chemically it is built from an indole core, which is a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. An ethylamine side chain sits on the pyrrole portion, and a hydroxyl group sits on the benzene portion at carbon 5. Because the molecule carries both the aromatic indole system and a basic amine, it is classed as a small molecule of the chemical family known as organic indoles, and the wider biogenic amine group also includes catecholamines such as dopamine and noradrenaline.\n\nThe formal name 5-hydroxytryptamine (5-HT) points to the phenolic hydroxyl at carbon 5, which is the feature that separates serotonin from its close relatives. Tryptamine lacks this group, and melatonin carries a methoxy group in that position and an acetylated side chain, which is why the pineal product behaves differently in the body. The hydroxyl and amine groups also take part in hydrogen bonding; in the crystal structure of the free base, neighbouring molecules link through these groups into a three-dimensional network. Receptor recognition depends on this arrangement, and most of the pharmacology described in later sections turns on how drugs fit around this polar end of the molecule.\n\nThe molecule's physical behaviour follows from its two faces. The amine has a pKa of about 9.97 and the phenol about 10.16 in water, so serotonin is largely protonated and positively charged at physiological pH. The compound is water-soluble, with a reported solubility of about 25.5 mg/mL, which allows it to circulate in plasma, although most circulating serotonin is actually taken up into platelets. The indole ring is hydrophobic, so the molecule is amphiphilic rather than purely polar. Its charge is a central reason it crosses membranes poorly.\n\nThat limited membrane passage matters most at the blood–brain barrier. Mammalian serotonin exists in two largely separate pools, central and peripheral, divided by this barrier. In the rat, a classic study found that intravenously given serotonin at doses up to about 0.435 mg/kg did not enter brain tissue, and that peripheral metabolism accounted for its disappearance. A 2008 study reported that brain serotonin can leave the brain through the serotonin transporter, so the barrier is not a one-way wall in every sense. By contrast, the precursors tryptophan and 5-hydroxytryptophan (5-HTP) cross readily, and 5-HTP is converted to serotonin once inside. This is why the brain must synthesise its own serotonin from imported precursors rather than relying on circulating supplies.\n\nSerotonin also shares structural features with other monoamine transmitters, and this shared chemistry helps explain pharmacological overlap. Its primary amine on a two-carbon chain resembles the side chains of dopamine and noradrenaline, while its aromatic ring differs by being an indole rather than a catechol. The same enzyme family degrades these molecules: monoamine oxidase-A (MAO-A) preferentially metabolises serotonin, adrenaline and noradrenaline, while monoamine oxidase-B prefers other substrates. Because the enzymes and transporters that handle these amines are partly shared, drugs designed to act on one monoamine system can influence serotonin as well."
    },
    {
      "heading": "Discovery and naming",
      "text": "Physiologists noticed serotonin's effects long before they knew what the substance was. Reports of a substance in blood serum that narrowed blood vessels go back to the nineteenth century, and Ludwig and Schmidt are often cited for observations made as early as 1868. The compound was not isolated until the twentieth century, and its identification came from two separate lines of research that were not at first known to be the same molecule.\n\nThe first line began in Italy. In the 1930s Vittorio Erspamer, working at the University of Pavia, was studying a substance from the enterochromaffin cells of the gut lining. Colour tests led him to conclude that the substance was not adrenaline but was probably an indole. With M. Vialli, he extracted it from rabbit gastric mucosa and reported it in 1937 under the name enteramine. Its activity was mainly seen as a stimulant of smooth muscle, which is why the gut was the focus of this work.\n\nThe second line developed at the Cleveland Clinic. Irvine Page was investigating the causes of hypertension and suspected a factor carried in the blood, but a particular substance kept appearing in his samples. Working with Maurice Rapport and Arda Green, he isolated the compound as a crystalline vasoconstrictor from blood serum, and the group published the result in 1948. The name was coined by Corcoran, a colleague of Page, by combining the Latin word for serum, serum, with a Greek term for tone, tonic, reflecting both where the substance was found and its first recognised effect.\n\nThe two lines converged over the following years. In 1949 Rapport's group established the chemical structure as 5-hydroxytryptamine (5-HT), and once the structure was known, chemists could synthesise the molecule rather than purify it from blood. In 1952 Erspamer and Asero, writing in Nature, identified enteramine as 5-hydroxytryptamine, which showed that the gut hormone and the serum vasoconstrictor were the same compound. Intestinal serotonin had received less attention than it might have because the brain findings came so soon afterwards.\n\nBetty Twarog brought the story into neuroscience. She had found that serotonin relaxed mussel muscle, a result she had submitted in 1952 but which was not published until 1954, and she then moved to the Cleveland Clinic to work in Page's laboratory. Using a highly sensitive bioassay based on the isolated heart of a hard-shell clam, she and Page reported in 1953 that serotonin was present in the brains of dogs, rats, and rabbits. Page was initially sceptical of testing brain tissue at all. Some accounts date the brain detection to 1952, reflecting when the work was done, but 1953 is the date of the published report that is usually cited.\n\nSources\n\n- Serotonin history and enteramine: https://www.nature.com/articles/1395355\n- Serotonin history overview: https://pmc.ncbi.nlm.nih.gov/articles/PMC10483994\n- Vittorio Erspamer (Wikipedia): https://en.wikipedia.org/wiki/Vittorio_Erspamer\n- Maurice M. Rapport (Wikipedia): https://en.wikipedia.org/wiki/Maurice_M._Rapport\n- Betty Twarog (Wikipedia): https://en.wikipedia.org/wiki/Betty_Twarog"
    },
    {
      "heading": "Biosynthesis",
      "text": "The pathway starts with L-tryptophan, an essential amino acid that the body cannot make and must obtain from food. In the human beings, tryptophan (TRP) is an essential amino acid wholly derived from dietary sources. The opening reaction is carried out by tryptophan hydroxylase (TPH), an iron-dependent enzyme that adds a hydroxyl group to the ring of tryptophan, producing 5-hydroxytryptophan (5-HTP). TPH hydroxylates L-Trp into 5-hydroxy-L-Trp (5-HTP) using Fe(II) as co-factor and molecular oxygen and tetrahydrobiopterin (BH_4_) as co-substrates. Because TPH also sets the pace of the whole route, changes in its catalytic activity translate into corresponding changes in the neuronal serotonin content. Increases or decreases in TPH catalytic activity can therefore lead to corresponding changes in the neuronal content of 5-HT.\n\nThe second step is handled by aromatic L-amino acid decarboxylase (AADC), which removes the carboxyl group from 5-HTP and leaves serotonin behind. 5-HTP is subsequently decarboxylated by aromatic amino acid decarboxylase to form serotonin. AADC is not dedicated to this route, since it also takes part in making other monoamines. AADC also catalyzes the formation of other monoamines and thus is not specific for serotonin synthesis. By contrast, TPH is specific and acts as the rate-limiting enzyme for serotonin synthesis. In practice, the commitment to serotonin is therefore made at the hydroxylation step.\n\nTwo isoforms in different tissues\n\nMammals carry two TPH genes. There are two distinct TPH homologies in humans, TPH1 and TPH2, which are separately located on chromosomes 11 and 12, respectively, sharing 71% sequence identity in amino acids. TPH1 was recognized first and is the main source of peripheral serotonin, whereas TPH2 is confined to neurons and is thought to control most central synthesis. Tryptophan hydroxylase-1 (TPH1), the long-recognized isoform, is responsible for the synthesis of most peripheral 5-HT, whereas tryptophan hydroxylase-2 (TPH2) is neuronal specific. TPH1 is expressed in the enterochromaffin cells of the gut and in the pineal gland, where its product can also feed melatonin synthesis. TPH1 plays a role in the peripheral serotonin generation, primarily expressed in the enterochromaffin cells of the gut and in the pineal gland, where it produces 5-HT as a precursor of melatonin synthesis.\n\nKnockout experiments in mice show how the division of labor works in practice. Removing TPH2 sharply lowered brain serotonin, but peripheral levels were not substantially reduced; only when both isoforms were removed did peripheral serotonin also fall markedly. This resulted in dramatically reduced central 5-HT levels in Tph2 knockout (TPH2KO) and Tph1/Tph2 double knockout (DKO) mice; and substantially reduced peripheral 5-HT levels in DKO, but not TPH2KO mice.\n\nCompetition with the kynurenine pathway\n\nOnly a small share of dietary tryptophan ends up as serotonin, because most of it is directed elsewhere. Published figures differ considerably. One review states that ingested tryptophan is largely shunted to the kynurenine pathway (95%) while only minor portions (1%–2%) are sequestered for 5‐HT production. Another accounting for normal physiology estimates roughly approximately 5% (0.05 g) of TRP of food origin is metabolized through the TRP-SER pathway, 85% (0.85 g) through the TRP-KYN pathway, and 10% going to gut bacteria. The kynurenine route begins with TDO, which is mostly active in the liver, or with IDO, an enzyme that is induced by inflammation. In contrast, tryptophan oxidation under influence of the inflammation-inducible enzyme IDO occurs in various cell types, is not limited by a decrease in tryptophan levels and may even lead to tryptophan depletion.\n\nBecause the two routes draw on the same substrate, heavy flux through the kynurenine pathway can reduce the tryptophan available for serotonin. Excess tryptophan catabolism likely depletes the necessary tryptophan needed for the biosynthesis of serotonin. The interplay runs in both directions: in Tph1-mutant mice, plasma and brain levels of tryptophan and its metabolites, including 5-HT, 5-hydroxy indoleacetic acid, indolepropionic acid, indole-3-acetic acid, kynurenine, and xanthurenic acid, are decreased, which suggests that TPH1 activity also shapes tryptophan levels beyond the serotonin route. Even so, the regulation of this balance is poorly understood. However, the two pathways are extremely unequal in their ability to degrade tryptophan, and little is known about the mechanisms maintaining the balance between them."
    },
    {
      "heading": "Distribution across the body",
      "text": "Most of the body's serotonin is made outside the brain. Although relatively rare (<1% of total intestinal epithelia), these unique cells produce >90% of the body's serotonin. These are the enterochromaffin cells of the gut lining. Published figures for their share vary: one review gives up to 95% of total body serotonin, while others settle on about 90%. The gap is small, and the broad picture is consistent across sources. One widely cited account also credits these cells with all plasma 5‐HT, so the circulating pool is largely gut-derived.\n\nMost of that gut-made serotonin does not stay free in the bloodstream. Approximately 95% of the amount of serotonin released in blood by enterochromaffin cells is absorbed and stored in platelets, using the serotonin transporter (SERT). The route runs through the portal circulation: serotonin made in the mucosa reaches the portal vessels and is taken up by platelets, which then carry it into the systemic circulation where it is stored in platelets that enter the systemic circulation. Platelets therefore act as a reservoir that can release serotonin locally when they are activated during clotting.\n\nThe central pool is much smaller. Estimates place only a few percent of the body's total in the central nervous system, with one review putting the figure at about 5% of the total. Brain serotonin is nonetheless functionally important. The serotonin system innervates nearly every region of the brain, even though it only constitutes ∼ 1/200000 of all CNS neurons in humans. In other words, a very small population of neurons sends its axons throughout the brain.\n\nThese central neurons sit mainly in the raphe nuclei, a set of clusters along the midline of the brainstem. The raphe nuclei divide into two groups. The more rostral nuclei located in the midbrain and rostral pons – caudal linear nucleus, dorsal raphe, and median raphe – have projections which extend to forebrain structures. Those forebrain projections are involved in mood regulation, memory, and sleep-wake cycle. The caudal nuclei take a different course: raphe magnus, raphe obscurus, and raphe pallidus – primarily project to spinal cord structures. The dorsal raphe is the largest of these nuclei, yet less than 50% of the neurons within this region are serotonergic, with clusters of 5-HT neurons interdigitated with non-5-HT neurons.\n\nThe pineal gland is a special case, because it uses serotonin as an intermediate step in making a different signal. Pinealocytes take up tryptophan and convert it to serotonin. An enzyme called arylalkylamine N-acetyltransferase (AANAT) then acetylates serotonin to N-acetylserotonin, and a further enzyme methylates that compound to form melatonin, a hormone that tracks the light–dark cycle serotonin, which is acetylated, by arylalkylamine N-acetyltransferase (AANAT) to N-acetylserotonin (NAS) that is converted to melatonin by acetylserotonin O-methyltransferase (ASMT). AANAT is the key regulatory point: activity of the rate-limiting enzyme in melatonin synthesis - serotonin N-acetyltransferase (NAT) - is low during daylight and peaks during the dark phase. Norepinephrine from sympathetic nerve endings drives this rise, which links the serotonin pathway to the body's enzyme-mediated circadian timing. The pineal gland also releases serotonin itself, and one study found that this locally released serotonin feeds back on the gland to sharpen norepinephrine-stimulated melatonin release."
    },
    {
      "heading": "Transport, storage, and inactivation",
      "text": "Once serotonin has been released or has entered the circulation, its signal depends on how it is packaged, taken back up, and broken down. Packaging is handled by vesicular monoamine transporters (VMATs). In neurons, these proteins pull serotonin from the cytoplasm into synaptic vesicles, so that it can be released from nerve terminals when needed. Mammals have two closely related transporters, VMAT1 and VMAT2. Platelets use the same machinery, but the literature disagrees on which isoform dominates. One review identifies VMAT2 as the main isoform responsible for vesicular transport in platelets, while another describes VMAT-1 as the granule transporter.\n\nRemoval from the synapse depends on the serotonin transporter (SERT), which is encoded by the SLC6A4 gene. SERT sits on the presynaptic membrane and pumps serotonin back into the nerve terminal, which ends its action at the receptor. This is also why SERT is such an important pharmacological target. Selective serotonin reuptake inhibitors and related antidepressants block the transporter, so serotonin stays in the extracellular space for longer. The same transporter operates outside the nervous system. Once 5-HT enters the circulation, it is sequestered inside platelets by antidepressant-sensitive 5-HT transporters and then into secretory granules by the vesicular monoamine transporter.\n\nPlatelets are a special case because they cannot make serotonin themselves. Platelets are not able to synthesize serotonin, but take it up from plasma via 5-HTT, store it in dense granules, and release it into the blood during their activation. The granules hold serotonin at very high concentrations. Inside the dense granules, serotonin is sequestered to millimolar concentrations, while free concentrations in the blood are in the low nanomolar range. Uptake is regulated: plasma membrane SERT levels and platelet serotonin uptake initially rise as plasma serotonin increases, but fall below normal if the plasma level keeps rising. Released serotonin then feeds back on platelet activation, since activated platelets release serotonin, thereby amplifying platelet activation and the recruitment of circulating platelets.\n\nInactivation is mostly enzymatic. MAO-mediated deamination is the primary metabolic pathway of serotonin inactivation. The enzyme involved is monoamine oxidase (MAO). MAO-A has about 120-fold higher affinity for serotonin than MAO-B, so MAO-A is the main isozyme involved in serotonin degradation. The first product is an unstable aldehyde, 5-hydroxyindoleacetaldehyde, rather than the acid itself. Aldehyde dehydrogenase then converts this intermediate into 5-hydroxyindoleacetic acid (5-HIAA). A minor alternative route reduces the aldehyde to 5-hydroxytryptophol, which represents less than 1% of serotonin turnover in urine, whereas 5-HIAA is the major metabolite.\n\nBecause 5-HIAA is the main end product, it is used as a marker of serotonin turnover. Clinically, 5-hydroxy indoleacetic acid is used as a proxy for serotonin measurement through a 24-hour urine test, and its primary use is in diagnosing and monitoring carcinoid tumors, a subset of serotonin-secreting neuroendocrine tumors. Its concentration reflects the metabolic activity of serotonin rather than the amount of signaling itself."
    },
    {
      "heading": "Receptors and signaling",
      "text": "Serotonin acts on a large receptor family, and most of its members are G protein–coupled receptors (GPCRs), which transmit signals through intracellular messengers instead of forming an open pore. The classification has changed over time. The Serotonin Club's nomenclature committee placed the main receptor types in the range 5-HT1 to 5-HT4, and it described 5-HT5 to 5-HT7 as recombinant receptors defined by their molecular cloning. Today, seven families (5-HT1 to 5-HT7) are standard in the literature.\n\nWithin the GPCR group, the families couple to different second messengers. The 5-HT1 subfamily (5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F) inhibits adenylyl cyclase, whereas the 5-HT2 subfamily (5-HT2A, 5-HT2B and 5-HT2C) stimulates phospholipase C. Receptors 5-HT4, 5-HT6 and 5-HT7 are generally described as raising cyclic AMP. Because the same molecule binds several receptor types with different affinities, the downstream effect depends on which receptors a given cell expresses. For example, 5-HT1A and 5-HT7 have been shown to play partly opposing roles in fear learning.\n\nCounts of receptor subtypes vary by source. One review of emotional learning cites fourteen 5-HT receptor subtypes, another reports eighteen subtypes across seven families, of which thirteen are GPCRs, and older classifications list fifteen. Part of the difference lies in provisional entries: a few subtypes, such as 5-HT1E and 5-HT5B, are recorded as gene products without an established function in native tissue.\n\nThe 5-HT3 receptor is the exception to the GPCR pattern. It is a ligand-gated ion channel of the Cys-loop family, built from five subunits. When serotonin binds, the channel admits sodium, potassium and calcium ions, which depolarizes the cell within milliseconds. This makes 5-HT3 the only ionotropic serotonin receptor and the basis of fast excitatory transmission in both the central and peripheral nervous systems. Its rapid action is also why 5-HT3 antagonists are used against chemotherapy-induced and postoperative nausea.\n\nSerotonin signals across several spatial scales. In autocrine signaling, a cell releases serotonin and responds to it itself. Mammary epithelial cells, for example, synthesize serotonin, secrete it, and act on their own receptors. In paracrine signaling, the molecule acts on nearby cells. Enterochromaffin cells in the intestine release serotonin that activates 5-HT3 receptors on vagal afferent fibers, and in rat taste buds, serotonin released from taste receptor cells excites afferent fibers through 5-HT3 receptors while inhibiting neighboring cells through 5-HT1A receptors.\n\nIn the brain, serotonin is often described as acting through two modes. In wiring transmission, release at a synapse produces a fast, one-to-one signal confined to a narrow cleft of roughly 20 nm. In volume transmission, serotonin diffuses through the extracellular space and reaches receptors hundreds of nanometers to microns away, allowing one release event to influence several targets. Electron microscopy has found serotonergic varicosities at only about 20 to 50 percent of synaptic connections, which supported the view that volume signaling predominates. Newer work suggests that both modes coexist. Under tonic or low-frequency firing, serotonin transporters rapidly clear the transmitter and keep the signal local. During high-frequency or synchronized firing, or when transporters are inhibited, serotonin can exceed uptake capacity and spread into the surrounding tissue."
    },
    {
      "heading": "Central nervous system functions",
      "text": "Within the brain, serotonin-producing neurons are concentrated in the raphe nuclei, a set of cell clusters along the midline of the brainstem. Only a small share of the neurons in the mammalian brain can synthesize serotonin, well under 0.1 percent. Their axons nonetheless branch widely through the central nervous system. Most of the cell bodies sit in the rostral group, and its corticolimbic fibers reach the amygdala, hippocampus, thalamus and basal ganglia, regions that shape emotion, stress responses and cognition. Corticolimbic projections from the rostral group to the amygdala, hippocampus, thalamus, and basal ganglia modulate emotion, mood, response to stress, and cognition.\n\nSerotonergic projections are associated with several behavioral domains. Mood regulation is the best-known, but the same pathways also contribute to regulation of appetite, sleep cycles, movement, and sexual function. The link to learning and memory has a long history. In the 1970s, Eric Kandel showed in the sea slug Aplysia that serotonin raised cyclic AMP levels in sensory neurons, a change that contributed to memory formation. In general, global lowering of 5-HT levels did not affect spatial learning except when using MDMA as depleting agent. Narrower manipulations give different results: depleting serotonin from one region, the supramammillary nucleus, impaired water-maze performance and disrupted hippocampal theta activity in rats. However, when depleting 5-HT from the supra-mammillary nucleus (SUM), inefficient MWM performance was evident. Hippocampal high-frequency theta was abolished during MWM training.\n\nAt the circuit level, serotonin contributes to the balance between excitation and inhibition in cortical and limbic networks. Its effect is not uniform, because the outcome depends on the receptor subtype, the target cell type, and the brain region involved.\n\nHuman work often relies on acute tryptophan depletion, a method that lowers central serotonin synthesis. For central serotonin production to occur, tryptophan first needs to gain access to the central nervous system (CNS) via the blood-brain barrier. In practice, researchers give volunteers an amino acid drink without tryptophan. Studies have shown that acute tryptophan depletion reduced tryptophan levels in the brain by up to 70%. In one trial of adults over 60 with recovered depression, depletion of plasma free tryptophan was 71% at 4 and 7 hours after the active drink. That study found no evidence of mood change at any time in either group, although the ATD/RD group showed a significant decrease compared with placebo on a cognitive screening test. Other studies report different results. Depletion can produce low mood in people with a history of depression, and the mood lowering effect of TD depends upon the genotype for a functional polymorphism in the promoter region of the 5-HT transporter (5-HTT), according to preliminary evidence. Overall, the effects appear to vary with population, sex and genetics.\n\nSerotonin's role in anxiety and emotional processing is also under study, and rodent work shows that timing matters. In mice, acute and chronic fluoxetine treatment reduced associative fear learning without affecting memory recall and had opposite effects on anxiety-like behaviour. Acute fluoxetine decreased serotonergic activity in the DR, while chronic treatment led to serotonergic activity that was indistinguishable from that of control levels. These findings point to distinct subpopulations of dorsal raphe neurons as candidates for regulating fear and anxiety, but the underlying mechanisms are not yet fully resolved."
    },
    {
      "heading": "Peripheral functions",
      "text": "Although serotonin is best known for its effects in the brain, most of the body's supply is made outside the central nervous system. Enterochromaffin (EC) cells within the GI mucosa are specialized enteroendocrine cells that synthesize and secrete between 90% and 95% of total body serotonin. In the gut, the released molecule works as a local messenger that shapes intestinal motility and fluid secretion. Peristaltic, secretory, vasodilatory, vagal, and nociceptive reflexes are initiated when EC cells release 5-HT. The vagal pathway is also the one that links gut serotonin to the sensation and reflex of nausea and vomiting, which is why serotonin receptors became drug targets for these symptoms.\n\nSerotonin reaches the bloodstream in a tightly controlled way. In the blood, serotonin is taken up and stored by platelets and is released during blood coagulation. Only a small fraction stays dissolved in plasma: approximately only 2% of the blood serotonin is free in the fluids and can act directly as a hormone. Platelets cannot make the molecule themselves, since platelets lack TpH and thus cannot produce serotonin on their own. Their stored supply therefore depends on uptake from the gut. The effect on vessel walls depends on which receptor is engaged. Serotonin can narrow or widen vessels, and it contributes to platelet aggregation, reinforcing the clot that forms at an injury. Serotonin released from platelets at a wound therefore links the clotting response to the later stages of tissue repair, although the details of this role are still being worked out.\n\nSerotonin also takes part in inflammation. Its effect on the intestinal lining depends on the receptor: serotonin can act both as a pro-inflammatory and anti-inflammatory signalling molecule in the intestinal mucosa via activation of serotonin receptors (5-HT7 or 5-HT4 receptors, respectively). Through receptors on immune cells, serotonin can alter how these cells respond to injury and infection, which places it in the broader immune system network. Its role in tissue repair is similarly receptor- and context-dependent, and research in this area has moved beyond the gut to include bone and metabolic tissues.\n\nThe enteric nervous system is a separate peripheral network that runs along the length of the gastrointestinal tract and operates largely independently of the brain. Its neurons include serotonergic cells in the myenteric plexus. Within the bowel, serotonin is synthesized by the enterochromaffin (EC) subtype of enteroendocrine cell and by serotonergic neurons of the myenteric plexus. Within the bowel, the two sources operate in different ways. EC cells release their stores in response to mechanical and chemical stimuli, while enteric neurons use serotonin as a local signal for neighbouring nerve cells and muscle. The two systems share a common enzymatic first step, but the neuronal and mucosal cells rely on different tryptophan hydroxylase isoforms, which allows each population to be regulated separately.\n\nThe gut's serotonin output is also shaped by the environment within the intestine. Bacteria within the lumen of the bowel influence serotonin synthesis and release by enterochromaffin cells. This link between microbes and hormone-like signalling is one reason gut serotonin is studied as part of the digestive system's interaction with the wider body. Serotonin from the gut is also linked to metabolism: studies in mice suggest that gut-derived serotonin affects how the liver and fat tissue respond to fasting. Clinically, a number of serotonin signalling pharmacotherapeutic targets for treatment of nausea, diarrhoea or constipation have already been developed, showing how peripheral receptor biology translates into treatment."
    },
    {
      "heading": "Evolution and other organisms",
      "text": "Reviewers describe serotonin as a phylogenetically old compound, and they locate it in animals, plants, and some bacteria. Serotonin is a phylogenetically ancient compound found in animals, plants, and some bacteria. In eukaryotes, its core chemistry depends on two enzymes that act on aromatic amino acids, aromatic amino acid hydroxylase (AAAH) and aromatic amino acid decarboxylase (AAAD). Accounts of the early chemistry describe primitive hydroxylases that accepted several aromatic amino acids and yielded antioxidant products, serotonin among them.\n\nOne hypothesis links the origin of the pathway to atmospheric oxygen. Because the hydroxylation step consumes molecular oxygen, one review argues that the pathway probably first operated in single-celled organisms that could photosynthesize, since the early atmosphere held little oxygen. The persistence of the pathway is often explained by endosymbiosis. In this model, cyanobacteria and α-proteobacteria were taken up by primitive eukaryotic cells and later became chloroplasts and mitochondria, which carried their biochemistry forward. Melatonin, a serotonin derivative, has also been detected in these same bacterial groups, where it seems to have worked as an antioxidant and free-radical scavenger.\n\nThe pathway differs in plants. There, tryptophan is decarboxylated to tryptamine by tryptophan decarboxylase (TDC), followed by serotonin biosynthesis catalyzed by tryptamine 5-hydroxylase (T5H), so the order of the two steps is reversed compared with vertebrates. Plant indoleamines are thought to contribute across the plant life cycle, from reproduction to morphogenesis and environmental sensing. In animals, the signaling system is also old: serotonin and its receptors have been reported in planarians, the nematode C. elegans, and the fruit fly Drosophila, and the receptors share significant orthology across these species. Receptor evolution is also dated in broad terms. One estimate suggests that the diversification of 5-HT receptors unfolded over roughly 750 million years, and another places the divergence of the major 5-HT G protein–coupled receptor families before the split between vertebrates and invertebrates, about 600 million years ago. These figures are estimates and vary between analyses.\n\nThe nematode C. elegans has been a useful model for dissecting these pathways. The canonical route runs through the enzyme tryptophan hydroxylase (TPH-1) in neurons. A 2022 study found that a second route, using phenylalanine hydroxylase (PAH-1), generates substantial serotonin in non-neuronal tissues. Most of the serotonin in these worms is incorporated into N-acetylserotonin-derived glucosides, which remain in the body and are further modified by the carboxylesterase CEST-4. The expression pattern of CEST-4 suggests that serotonin or its derivatives move between tissues, and bacterial indole production also interacts with this metabolism. The authors propose that these metabolites may have signaling roles of their own, which means that some earlier serotonin-dependent phenotypes may not have been caused by serotonin alone."
    },
    {
      "heading": "Clinical pharmacology",
      "text": "Most clinically important drugs that act on serotonin work at one of three points: the transporter that clears the transmitter from the synapse, the receptors it binds to, or the enzymes that break it down. The best-known group is the selective serotonin reuptake inhibitors (SSRIs). They block SERT on presynaptic terminals, so serotonin stays in the extracellular space longer and its concentration rises. SSRIs are prescribed mainly for depression and are also used for anxiety-disorders. Their transporter blockade is almost immediate, yet clinical benefit usually takes several weeks to appear. That lag is one reason researchers have questioned whether a simple rise in serotonin explains the therapeutic effect.\n\nReceptor-selective drugs show more directly how the receptor family can be split into separate therapeutic targets. The triptans, such as sumatriptan, are agonists at 5-HT1B and 5-HT1D receptors and remain a first-line option for moderate to severe migraine attacks. Their mode of action was once attributed mainly to narrowing of cranial blood vessels through 5-HT1B receptors. Later work emphasized other effects, including inhibition of trigeminal nerve activity and suppression of the release of calcitonin gene-related peptide, and most authors now regard the mechanism as a combination. Lasmiditan, a selective 5-HT1F agonist approved in the United States in 2019, was developed to relieve migraine without the vasoconstrictive action of the triptans. Antiemetics such as ondansetron block 5-HT3 receptors, which are ligand-gated ion channels in the gut and the brainstem, and they are used to prevent nausea during chemotherapy. Many second-generation antipsychotics combine antagonism at 5-HT2A receptors with blockade of dopamine D2 receptors, a combination thought to shape their side-effect profile. These drugs are used in schizophrenia and in some forms of bipolar-disorder.\n\nThe serotonin hypothesis of depression, which holds that low serotonin causes depressive symptoms, remains contested. A systematic umbrella review by Joanna Moncrieff and colleagues, published in Molecular Psychiatry, examined 17 systematic reviews, meta-analyses, and large data-set studies covering serotonin metabolites, receptor binding, transporter levels, tryptophan depletion, and SERT gene studies. The authors concluded that there was no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity. The review also found some evidence that long-term antidepressant use lowers serotonin concentrations, and it noted that prior drug exposure was not reliably excluded in several imaging studies. Critics responded that the review had methodological weaknesses and selective reporting, and they argued that the serotonin system remains implicated in depression. The disagreement therefore concerns how the existing evidence should be weighed, not only what the data show.\n\nTryptophan depletion is the most direct experimental probe. In a 1996 study of 15 women with recurrent depression who were in recovery and off medication, a tryptophan-free amino acid mixture reduced plasma tryptophan by about 75% and produced clinically significant depressive symptoms in ten of them. Subsequent studies found that depletion triggered relapse-like symptoms in many patients who were in remission while taking SSRIs. A prospective study reported that the mood response to depletion predicted later depressive episodes with a sensitivity of 78% and a specificity of 80%. Results in other groups have been less consistent. In a study of 14 fully remitted former patients who were off medication, depletion had no significant effect on mood, and a meta-analysis of healthy volunteers found no effect in most of them. Some researchers interpret these findings as showing that depletion reverses processes involved in antidepressant action, rather than revealing a simple deficit in serotonin.\n\nResearch on tryptophan supplementation, which raises the precursor rather than lowering it, has produced results that vary with the condition studied and the design of the trial. Because precursor levels, serotonin synthesis, and drug exposure are tangled together in clinical populations, the depletion and supplementation literature is best read as a set of partial, method-dependent findings rather than as proof of a single causal pathway. Randomized trials that compare these approaches directly remain the strongest source of evidence for judging their clinical relevance."
    },
    {
      "heading": "Cancer and emerging research",
      "text": "Mitogenic activity\n\nSerotonin's growth-promoting effects have been reported in several tissues, including vascular smooth muscle, neurons, and hepatocytes. Since its discovery, serotonin has been linked to cellular proliferation in several types of tissues. The gut epithelium is the best-studied case. Enterochromaffin cells there are the main source of the body's serotonin, producing 90% of the body's serotonin, yet the proliferative signal appears to come mainly from neuronal serotonin rather than from the mucosal pool. In one mouse study, epithelial growth tracked the TPH2-dependent neuronal supply, and removing the TPH1-dependent enterochromaffin source did not alter mucosal proliferation. Work on mice lacking the serotonin transporter points to a relay. Blocking either 5-HT2A receptors or muscarinic receptors prevented the extra growth, which suggests that serotonin acts on submucosal cholinergic neurons, and their output then reaches the epithelium (ketanserin and scopolamine each prevented the increase in mucosal growth and proliferation).\n\nEarly pharmacological work pointed the same way. In 1982, Tutton and Barkla reported that two serotonin uptake inhibitors, citalopram and fluoxetine, suppressed cell division in dimethylhydrazine-induced colonic tumours in rats, and retarded the growth of 2 out of 3 lines of human colonic tumours grown as xenografts.\n\nTumor-derived serotonin\n\nSome tumors appear to make their own serotonin. Triple-negative breast cancer lines were found to express higher levels of tryptophan hydroxylase 1 (TPH1) than hormone-responsive breast cancer cell lines. In a 2016 study, knock-down of TPH1 expression using siRNA significantly reduced the proliferation of MDA-MB-231 cells, and serotonin stimulated invasion and proliferation through the 5-HT7 receptor. The authors proposed an autocrine loop in which the tumor cells synthesize serotonin that feeds back on their own receptors. Similar mitogenic effects have been described in liver cancer, where serotonin induces a pro-proliferative effect in hepatocellular carcinoma (HCC) cells, and a proposed mediator is the transcription factor FOXO3a. Most of this evidence comes from cell culture, which a 2024 review notes was primarily studied in in vitro systems.\n\nConflicting evidence and drug repurposing\n\nThe direction of serotonin's effect on tumors is not consistent across studies. A 2024 review describes the evidence as contradictory findings, particularly in studies using knockout mice for specific receptors or peripheral serotonin deficiency, and it attributes part of the disagreement to the dose-dependent mitogenic effects of serotonin, for example, higher doses of 5-HT stimulate cell growth, while lower concentrations induce vasoconstriction. Other work reports that exogenous serotonin accelerated tumor growth and enhanced malignancy in mice.\n\nThese mixed results affect efforts to repurpose serotonergic drugs. Epidemiological studies have suggested that SSRIs have the potential to reduce the risk of many cancers, including kidney, breast, colorectal, and liver cancers, although retrospective studies are not uniform in this respect. Most supporting data remain preclinical. For example, in one review of sertraline, current evidence is predominantly preclinical. In vitro, four serotonergic drugs (tropisetron, imipramine, ketanserin, and cyproheptadine) were reported to induce apoptosis and inhibit cell migration with varying efficacies in gastric cancer cells.\n\nImmunomodulation\n\nA growing line of research connects serotonin to the immune system in tumors. A review of the peripheral serotonergic system emphasizes its immunomodulatory activities in human cancers. In mouse models, inhibiting peripheral serotonin synthesis has had measurable effects: treatment with a TPH1 inhibitor (telotristat ethyl [TE]) in this study reduced tumour progression and simultaneously enhanced the effectiveness of anti‐PD1 therapy in mice. Whether these findings extend to patients, and how serotonin's immune effects interact with its direct effects on tumor cells, remain open questions."
    }
  ],
  "links": [
    "neurotransmitter",
    "neuroscience",
    "pharmacology",
    "medicine",
    "amino-acid",
    "molecule",
    "chemistry",
    "organic-chemistry",
    "hormone",
    "enzyme",
    "platelet",
    "central-nervous-system",
    "neuron",
    "ion-channel",
    "synapse",
    "digestive-system",
    "immune-system",
    "microorganism",
    "metabolism",
    "anxiety-disorders",
    "migraine",
    "schizophrenia",
    "bipolar-disorder",
    "evidence-based-medicine"
  ],
  "sources": [
    {
      "title": "Synthesis and Properties of Serotonin_Chemicalbook",
      "url": "https://www.chemicalbook.com/article/synthesis-and-properties-of-serotonin.htm"
    },
    {
      "title": "Frontiersin",
      "url": "https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2020.00219/xml"
    },
    {
      "title": "Serotonin (C10H12N2O) properties",
      "url": "https://www.webqc.org/compound.php?compound=Serotonin"
    },
    {
      "title": "Crystal structure of serotonin - PMC - NIH",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC8983975/"
    },
    {
      "title": "Indole",
      "url": "https://en.wikipedia.org/wiki/Indole"
    },
    {
      "title": "5-Hydroxytryptophan: A precursor of serotonin influences regional blood-brain barrier breakdown, cerebral blood flow, brain edema formation, and neuropathology",
      "url": "https://www.researchgate.net/publication/334556708_5-Hydroxytryptophan_A_precursor_of_serotonin_influences_regional_blood-brain_barrier_breakdown_cerebral_blood_flow_brain_edema_formation_and_neuropathology"
    },
    {
      "title": "Brain and blood indole metabolites after peripheral administration of14C-5-HT in rat",
      "url": "https://link.springer.com/article/10.1007/BF00966020"
    },
    {
      "title": "Augmented brain 5‐HT crosses the blood–brain barrier through the 5‐HT transporter in rat - Nakatani - 2008 - European Journal of Neuroscience - Wiley Online Library",
      "url": "https://onlinelibrary.wiley.com/doi/10.1111/j.1460-9568.2008.06201.x"
    },
    {
      "title": "5 Hydroxytryptophan - an overview",
      "url": "https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/5-hydroxytryptophan"
    },
    {
      "title": "Monoamine - an overview",
      "url": "https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/monoamine"
    },
    {
      "title": "Indole derivatives",
      "url": "https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/5130327"
    },
    {
      "title": "The Discovery of Serotonin and its Role in Neuroscience",
      "url": "https://www.nature.com/articles/1395355"
    },
    {
      "title": "Expanding the concept of serotoninomics: perspectives for serotonin studies in the 20’s of the 21st century - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC10483994"
    },
    {
      "title": "Serotonin",
      "url": "https://www.ebsco.com/research-starters/anatomy-and-physiology/serotonin"
    },
    {
      "title": "Serotonergic Mechanisms Regulating the GI Tract: Experimental Evidence and Therapeutic Relevance - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC5526216"
    },
    {
      "title": "Vittorio Erspamer",
      "url": "https://en.wikipedia.org/wiki/Vittorio_Erspamer"
    },
    {
      "title": "Maurice M. Rapport",
      "url": "https://en.wikipedia.org/wiki/Maurice_M._Rapport"
    },
    {
      "title": "Betty Twarog",
      "url": "https://en.wikipedia.org/wiki/Betty_Twarog"
    },
    {
      "title": "Techniques for in vivo serotonin detection in the brain: State of the art - Zhao - 2023 - Journal of Neurochemistry - Wiley Online Library",
      "url": "https://onlinelibrary.wiley.com/doi/10.1111/jnc.15865"
    },
    {
      "title": "Serotonin, and the Past and Future of LSD",
      "url": "https://maps.org/news/bulletin/serotonin-and-the-past-and-future-of-lsd/"
    },
    {
      "title": "Kynurenine Pathway of Tryptophan Metabolism in Neuropsychiatric Disorders: Pathophysiologic and Therapeutic Considerations",
      "url": "https://www.cpn.or.kr/journal/view.html?doi=10.9758%2Fcpn.2020.18.4.507"
    },
    {
      "title": "Structural characterization of human tryptophan hydroxylase 2 reveals L-Phe as the superior regulatory domain ligand relevant for serotonin biosynthesis",
      "url": "https://www.biorxiv.org/content/10.1101/2022.09.20.508773.full.pdf"
    },
    {
      "title": "TPH2 - an overview",
      "url": "https://www.sciencedirect.com/topics/neuroscience/tph2"
    },
    {
      "title": "Cryo-EM Structure and Activator Screening of Human Tryptophan Hydroxylase 2",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9420949/"
    },
    {
      "title": "Genetic Disruption of Both Tryptophan Hydroxylase Genes Dramatically Reduces Serotonin and Affects Behavior in Models Sensitive to Antidepressants",
      "url": "https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0003301"
    },
    {
      "title": "Tryptophan Hydroxylase - an overview",
      "url": "https://www.sciencedirect.com/topics/neuroscience/tryptophan-hydroxylase"
    },
    {
      "title": "Tryptophan‐derived serotonin‐kynurenine balance in immune activation and intestinal inflammation - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC9292703/"
    },
    {
      "title": "Serotonin, Kynurenine, and Indole Pathways of Tryptophan Metabolism in Humans in Health and Disease - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC12899273/"
    },
    {
      "title": "The kynurenine pathway activities in a sub-Saharan HIV/AIDS population",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4545362/"
    },
    {
      "title": "Modulation of NAD+ metabolic pathways for treatment of disease",
      "url": "https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10722529"
    },
    {
      "title": "Tryptophan Hydroxylase 1 Regulates Tryptophan and Its Metabolites",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12071428/"
    },
    {
      "title": "Regulating the balance between the kynurenine and serotonin pathways of tryptophan metabolism - Li - 2017 - The FEBS Journal - Wiley Online Library",
      "url": "https://febs.onlinelibrary.wiley.com/doi/10.1111/febs.14026"
    },
    {
      "title": "Enterochromaffin Cells Are Gut Chemosensors that Couple to Sensory Neural Pathways: Cell",
      "url": "https://www.cell.com/cell/fulltext/S0092-8674(17)30595-0"
    },
    {
      "title": "Enterochromaffin Cells–Gut Microbiota Crosstalk: Underpinning the Symptoms, Pathogenesis, and Pharmacotherapy in Disorders of Gut-Brain Interaction",
      "url": "https://www.jnmjournal.org/journal/view.html?uid=1767&vmd=Full"
    },
    {
      "title": "Regional differences in nutrient‐induced secretion of gut serotonin - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC5371566/"
    },
    {
      "title": "Serotonin Uptake - an overview",
      "url": "https://www.sciencedirect.com/topics/immunology-and-microbiology/serotonin-uptake"
    },
    {
      "title": "Serotonin as a Mitogen in the Gastrointestinal Tract: Revisiting a Familiar Molecule in a New Role - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S2352345X21000977"
    },
    {
      "title": "Serotonin Neurons in the Dorsal and Medial Raphe Nuclei: from Single-Cell Transcriptomes to Whole-Brain Projections",
      "url": "https://www.biorxiv.org/content/10.1101/674697.full.pdf"
    },
    {
      "title": "Neuroanatomy, Nucleus Raphe - StatPearls - NCBI Bookshelf",
      "url": "https://www.ncbi.nlm.nih.gov/sites/books/NBK544359/"
    },
    {
      "title": "Collateralized dorsal raphe nucleus projections: a mechanism for the integration of diverse functions during stress - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC3156417/"
    },
    {
      "title": "A brief review about melatonin, a pineal hormone - PMC - NIH",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC10118741/"
    },
    {
      "title": "The Pineal Gland and Melatonin",
      "url": "https://vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/pineal.html"
    },
    {
      "title": "Exploring the serotonin‐probiotics‐gut health axis: A review of current evidence and potential mechanisms - Akram - 2024 - Food Science & Nutrition - Wiley Online Library",
      "url": "https://onlinelibrary.wiley.com/doi/full/10.1002/fsn3.3826"
    },
    {
      "title": "Serotonin modulates melatonin synthesis as an autocrine neurotransmitter in the pineal gland",
      "url": "https://www.pnas.org/doi/10.1073/pnas.2113852118"
    },
    {
      "title": "The Vesicular Monoamine Content Regulates VMAT2 Activity through Gαq in Mouse Platelets - Journal of Biological Chemistry",
      "url": "https://www.jbc.org/article/S0021-9258(19)58270-9/fulltext"
    },
    {
      "title": "Immuno-Thrombotic Effects of Platelet Serotonin",
      "url": "https://www.intechopen.com/chapters/55699"
    },
    {
      "title": "JCI - Interactions between integrin αIIbβ3 and the serotonin transporter regulate serotonin transport and platelet aggregation in mice and humans",
      "url": "https://www.jci.org/articles/view/33374"
    },
    {
      "title": "Frontiersin",
      "url": "https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2019.00032/xml"
    },
    {
      "title": "Molecular mechanisms of SERT in platelets: regulation of plasma serotonin levels - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/20729489/"
    },
    {
      "title": "5-Hydroxyindoleacetaldehyde",
      "url": "https://en.wikipedia.org/wiki/5-Hydroxyindoleacetaldehyde"
    },
    {
      "title": "5 Hydroxyindole - an overview",
      "url": "https://www.sciencedirect.com/topics/medicine-and-dentistry/5-hydroxyindole"
    },
    {
      "title": "Biochemistry, 5 Hydroxyindoleacetic Acid - StatPearls - NCBI Bookshelf",
      "url": "https://www.ncbi.nlm.nih.gov/sites/books/NBK551684/"
    },
    {
      "title": "Serotonin metabolism by monoamine oxidase in rat primary astrocyte cultures - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/1700071/"
    },
    {
      "title": "[5-hydroxytryptamine (serotonin) receptors--nomenclature and classification of types and subtypes] - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/7586016/"
    },
    {
      "title": "Serotonin receptors: subtypes, functional responses and therapeutic relevance - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/7667401/"
    },
    {
      "title": "The role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and its interaction in emotional learning and memory",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4528280/"
    },
    {
      "title": "Modulation of the Serotonergic Receptosome in the Treatment of Anxiety and Depression: A Narrative Review of the Experimental Evidence",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7918669/"
    },
    {
      "title": "The discovery, structure, and function of 5-HTR1E serotonin receptor",
      "url": "https://biosignaling.biomedcentral.com/articles/10.1186/s12964-023-01195-0"
    },
    {
      "title": "The 5-HT3 Receptor Channel: Function, Activation and Regulation",
      "url": "https://link.springer.com/chapter/10.1007/978-3-642-57083-4_21"
    },
    {
      "title": "The serotonin-gated 5-HT3 receptor: a tale of functions and structures of a prototypical pentameric ligand-gated ion channel",
      "url": "https://www.nature.com/articles/s41401-026-01763-3"
    },
    {
      "title": "5-HT3 Receptor - an overview",
      "url": "https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/5-ht3-receptor"
    },
    {
      "title": "Serotonin 3 Receptor - an overview",
      "url": "https://www.sciencedirect.com/topics/neuroscience/serotonin-3-receptor"
    },
    {
      "title": "Predicted conformations of 5-HT3 receptor ion channels are modified by subunit D",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12172986/"
    },
    {
      "title": "Serotonin as a volume transmission signal in the “simple nervous system” of mollusks: From axonal guidance to behavioral orchestration",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9679287/"
    },
    {
      "title": "Intestinal serotonin acts as a paracrine substance to mediate vagal signal transmission evoked by luminal factors in the rat - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC2278417/"
    },
    {
      "title": "Intraluminal volume homeostasis: A common sertonergic mechanism among diverse epithelia - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/22046455/"
    },
    {
      "title": "A paracrine signaling role for serotonin in rat taste buds: expression and localization of serotonin receptor subtypes",
      "url": "https://journals.physiology.org/doi/full/10.1152/ajpregu.00572.2003"
    },
    {
      "title": "Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10704479/"
    },
    {
      "title": "Serotonin signaling at cilia synapses - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S095943882500025X"
    },
    {
      "title": "Coding principles and mechanisms of serotonergic transmission modes",
      "url": "https://www.nature.com/articles/s41380-025-02930-4"
    },
    {
      "title": "Serotonergic Signaling Rewired: A Lipid Raft-Controlled Model of Synaptic Transmission Grounded in the Fundamental Parameters of Biological Systems",
      "url": "https://www.mdpi.com/2075-1729/16/1/118"
    },
    {
      "title": "The role of serotonin in declarative memory: A systematic review of animal and human research - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S0149763422002184"
    },
    {
      "title": "Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4728667/"
    },
    {
      "title": "(PDF) Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis",
      "url": "https://www.researchgate.net/publication/291376617_Influence_of_Tryptophan_and_Serotonin_on_Mood_and_Cognition_with_a_Possible_Role_of_the_Gut-Brain_Axis"
    },
    {
      "title": "Effects of Acute Tryptophan Depletion on Mood and Cognitive Functioning in Older Recovered Depressed Subjects - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/abs/pii/S106474811261728X"
    },
    {
      "title": "Serotonin Function During Depression",
      "url": "https://clinicaltrials.gov/study/NCT00033787"
    },
    {
      "title": "The Role of Dorsal Raphe Nucleus Serotonergic Systems in Emotional Learning and Memory in Male BALB/c Mice - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S0306452223004530"
    },
    {
      "title": "Acute tryptophan depletion evokes negative mood in healthy females who have previously experienced concurrent negative mood and tryptophan depletion - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC2705725/"
    },
    {
      "title": "Acute dietary tryptophan depletion impairs maintenance of \"affective set\" and delayed visual recognition in healthy volunteers - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/11351939"
    },
    {
      "title": "Diverse Metabolic Roles of Peripheral Serotonin",
      "url": "https://academic.oup.com/endo/article/158/5/1049/3058742"
    },
    {
      "title": "The roles of peripheral serotonin in metabolic homeostasis - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S001457931500455X"
    },
    {
      "title": "The Serotonin Signaling System: From Basic Understanding To Drug Development for Functional GI Disorders - Gastroenterology",
      "url": "https://www.gastrojournal.org/article/S0016-5085(06)02436-X/fulltext"
    },
    {
      "title": "Exploring the serotonin‐probiotics‐gut health axis: A review of current evidence and potential mechanisms - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC10867509/"
    },
    {
      "title": "Non-conventional features of peripheral serotonin signalling — the gut and beyond",
      "url": "https://www.nature.com/articles/nrgastro.2017.51"
    },
    {
      "title": "Enzyme Promiscuity in Serotonin Biosynthesis, From Bacteria to Plants and Humans",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9048412/"
    },
    {
      "title": "Melatonin Synthesis and Function: Evolutionary History in Animals and Plants - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC6481276/"
    },
    {
      "title": "Frontiers",
      "url": "https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.873555/full"
    },
    {
      "title": "Evolution of Serotonin: Sunlight to Suicide - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S1569733910700692"
    },
    {
      "title": "CHAPTER 1.1 - Evolution of Serotonin: Sunlight to Suicide",
      "url": "https://www.researchgate.net/publication/223058309_CHAPTER_11_-_Evolution_of_Serotonin_Sunlight_to_Suicide"
    },
    {
      "title": "Host–microbe serotonin metabolism - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/pii/S1043276024001954"
    },
    {
      "title": "Invertebrate serotonin receptors: a molecular perspective on classification and pharmacology",
      "url": "https://journals.biologists.com/jeb/article/221/19/jeb184838/33818/Invertebrate-serotonin-receptors-a-molecular"
    },
    {
      "title": "Parallel pathways for serotonin biosynthesis and metabolism in C. elegans",
      "url": "https://www.nature.com/articles/s41589-022-01148-7"
    },
    {
      "title": "The serotonin theory of depression: a systematic umbrella review of the evidence",
      "url": "https://www.nature.com/articles/s41380-022-01661-0"
    },
    {
      "title": "A leaky umbrella has little value: evidence clearly indicates the serotonin system is implicated in depression",
      "url": "https://www.nature.com/articles/s41380-023-02095-y"
    },
    {
      "title": "Serotonin 5-HT1B/1D Receptor Agonists in Migraine",
      "url": "https://link.springer.com/article/10.2165/00023210-199810040-00005"
    },
    {
      "title": "Full article: Are 5-HT1 receptor agonists effective anti-migraine drugs?",
      "url": "https://www.tandfonline.com/doi/full/10.1080/14656566.2021.1910235"
    },
    {
      "title": "Lasmiditan mechanism of action – review of a selective 5-HT1F agonist",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7288483/"
    },
    {
      "title": "Relapse of depression after rapid depletion of tryptophan - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/abs/pii/S0140673696070444"
    },
    {
      "title": "Tryptophan depletion and risk of depression relapse: a prospective study of tryptophan depletion as a potential predictor of depressive episodes - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/abs/pii/S0006322300008933"
    },
    {
      "title": "The effect of tryptophan depletion on mood in medication-free, former patients with major affective disorder - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/9094147/?dopt=Abstract"
    },
    {
      "title": "Tryptophan depletion and depressive vulnerability - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/abs/pii/S0006322399000955"
    },
    {
      "title": "Serotonin as a Mitogen in the Gastrointestinal Tract: Revisiting a Familiar Molecule in a New Role - Cellular and Molecular Gastroenterology and Hepatology",
      "url": "https://www.cmghjournal.org/article/S2352-345X(21)00097-7/fulltext"
    },
    {
      "title": "Neuronal serotonin regulates growth of the intestinal mucosa in mice - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/22609381/"
    },
    {
      "title": "Influence of inhibitors of serotonin uptake on intestinal epithelium and colorectal carcinomas",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2011088/"
    },
    {
      "title": "Tryptophan hydroxylase 1 and 5-HT7 receptor preferentially expressed in triple-negative breast cancer promote cancer progression through autocrine serotonin signaling",
      "url": "https://link.springer.com/article/10.1186/s12943-016-0559-6"
    },
    {
      "title": "Serotonin promotes the proliferation of serum-deprived hepatocellular carcinoma cells via upregulation of FOXO3a",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3601970/"
    },
    {
      "title": "Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities - Chen - 2024 - Clinical and Translational Medicine - Wiley Online Library",
      "url": "https://onlinelibrary.wiley.com/doi/10.1002/ctm2.1750"
    },
    {
      "title": "Serotonin promotes aggressive features in breast cancer cells by modulating proliferation and migration, hormone receptors and HER2 expression - ScienceDirect",
      "url": "https://www.sciencedirect.com/science/article/abs/pii/S0303720725002436"
    },
    {
      "title": "Citalopram exhibits immune-dependent anti-tumor effects by modulating C5aR1^+^ TAMs",
      "url": "https://www.biorxiv.org/content/10.1101/2024.10.15.618536.full.pdf"
    },
    {
      "title": "Repurposing the Antidepressant Sertraline: A Systematic Scoping Review of Its Anticancer Mechanisms",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12392137/"
    },
    {
      "title": "Repositioning serotonergic antagonists as therapeutic agents in gastric cancer: induction of apoptosis, inhibition of cell migration, and cell cycle arrest in AGS cells - PubMed",
      "url": "https://pubmed.ncbi.nlm.nih.gov/41477555/"
    },
    {
      "title": "Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11213692/"
    },
    {
      "title": "Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities - PMC",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC11213692/"
    },
    {
      "title": "Repurposing the Antidepressant Sertraline: A Systematic Scoping Review of Its Anticancer Mechanisms - Blum - 2025 - Pharmacology Research & Perspectives - Wiley Online Library",
      "url": "https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1002/prp2.70168"
    }
  ],
  "license": "https://creativecommons.org/licenses/by/4.0/"
}