Serotonin-immunoreactive neurons and mast cells in the mouse esophagus suggest involvement of serotonin in both motility control and neuroimmune interactions

Neurochemical classification of serotonin-immunoreactive neurons co-innervating motor endplates in the mouse esophagus

Serotonin immunoreactivity was previously found in myenteric neurons co-innervating motor endplates in the mouse esophagus striated muscle and an involvement in motility control was suggested. However, it is not known if other neuroactive substances are present in these neurons and to what extent they co-localize. First, vasoactive intestinal peptide (VIP) was established as a bona fide marker for putative inhibitory myenteric neurons by evaluating co-localization with neuronal nitric oxide synthase (nNOS) and neuropeptide Y (NPY). Then, co-localization of serotonin and VIP was tested in co-innervating axons on motor endplates, which were visualized with α-bungarotoxin (α-BT) by multilabel immunofluorescence. Myenteric ganglia were also surveyed for co-localization in neuronal perikarya and varicosities. nNOS, NPY, and VIP were completely co-localized in enteric co-innervating nerve terminals on motor endplates. After co-staining with VIP, we found (a) serotonin (5-HT)-positive nerve endings without VIP (44% of 5-HT-positively innervated endplates), (b) 5-HT- and VIP-positive endings without co-localization (35%), and (c) 5-HT- and VIP-positive endings with co-localization (21%). About one-fifth of nerve terminals on motor endplates containing 5-HT originate from putative inhibitory peptidegic nitrergic neurons. However, the majority represents a different population presumably subserving different functions.

Tryptophan Metabolites as Biomarkers for Esophageal Cancer Susceptibility, Metastasis, and Prognosis

BACKGROUND

Perturbation of tryptophan (TRP) metabolism contributes to the immune escape of cancer; however, the explored TRP metabolites are limited, and their efficacy in clarifying the susceptibility and progression of esophageal cancer (EC) remains ambiguous. Our study sought to evaluate the effects of the TRP metabolic profile on the clinical outcomes of EC using a Chinese population cohort; and to develop a risk prediction model targeting TRP metabolism.

METHOD

A total of 456 healthy individuals as control subjects and 393 patients with EC who were followed up for one year as case subjects were enrolled. Quantification of the plasma concentrations of TRP and its metabolites was performed using HPLC-MS/MS. The logistic regression model was applied to evaluate the effects of the clinical characteristics and plasma metabolites of the subjects on susceptibility and tumor metastasis events, whereas Cox regression analysis was performed to assess the overall survival (OS) of the patients.

RESULTS

Levels of creatinine and liver enzymes were substantially correlated with multiple metabolites/metabolite ratios in TRP metabolism, suggesting that hepatic and renal function would exert effects on TRP metabolism. Age- and sex-matched case-control subjects were selected using propensity score matching. Plasma exposure to 5-HT was found to be elevated 3.94-fold in case subjects (N = 166) compared to control subjects (N = 203), achieving an AUC of 0.811 for predicting susceptibility event. Subsequent correlation analysis indicated that a higher plasma exposure to 5-HIAA significantly increased the risk of lymph node metastasis (OR: 2.16, p = 0.0114). Furthermore, it was figured out that OS was significantly shorter for patients with elevated XA/KYN ratio (HR: 1.99, p = 0.0016), in which medium and high levels of XA/KYN versus low level had a significantly lower OS (HR: 0.48, p = 0.0080 and HR: 0.42, p = 0.0031, respectively).

CONCLUSION

This study provides a pivotal basis for targeting endogenous TRP metabolism as a potential therapeutic intervention.

Study on the relationship of depression, anxiety, lifestyle and eating habits with the severity of reflux esophagitis

Background

The etiology of reflux esophagitis (RE) is multi-factorial. This study analyzed the relationship of depression, anxiety, lifestyle and eating habits with RE and its severity and further explored the impact of anxiety and depression on patients’ symptoms and quality of life.

Methods

From September 2016 to February 2018, a total of 689 subjects at Xuanwu Hospital Capital Medical University participated in this survey. They were divided into the RE group (patients diagnosed with RE on gastroscopy, n = 361) and the control group (healthy individuals without heartburn, regurgitation and other gastrointestinal symptoms, n = 328). The survey included general demographic information, lifestyle habits, eating habits, comorbidities, current medications, the gastroesophageal reflux disease (GERD) questionnaire (GerdQ), the Patient Health Questionnaire-9 depression scale and the General Anxiety Disorder-7 anxiety scale.

Results

The mean age and sex ratio of the two groups were similar. Multivariate logistic regression analysis identified the following factors as related to the onset of RE (p < 0.05): low education level; drinking strong tea; preferences for sweets, noodles and acidic foods; sleeping on a low pillow; overeating; a short interval between dinner and sleep; anxiety; depression; constipation; history of hypertension; and use of oral calcium channel blockers. Ordinal logistic regression analysis revealed a positive correlation between sleeping on a low pillow and RE severity (p = 0.025). Depression had a positive correlation with the severity of symptoms (r_s = 0.375, p < 0.001) and patients’ quality of life (r_s = 0.306, p < 0.001), whereas anxiety showed no such association.

Conclusions

Many lifestyle factors and eating habits were correlated with the onset of RE. Notably, sleeping on a low pillow was positively correlated with RE severity, and depression was positively related to the severity of symptoms and patients’ quality of life.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12876-021-01717-5.

The role of high-resolution manometry in the assessment of upper gastrointestinal involvement in systemic sclerosis: a systematic review

Systemic sclerosis (SSc) affects the upper gastrointestinal (GI) system in 90% of patients. High-resolution manometry (HRM) assesses esophageal dysmotility, but its role in diagnosis and follow-up remains unclear. The objectives of this systematic review were to investigate the role of HRM in the assessment of SSc-associated upper GI involvement and to evaluate the correlation between HRM abnormalities and clinical characteristics and the effects of therapeutic interventions on HRM findings. Fifteen articles were included. Most (11/15) studies were of very good or good quality. Most studies assessed correlations between esophageal symptoms and esophageal dysmotility. Two studies assessed the effectiveness of buspirone and reported HRM findings. Studies assessing upper GI symptoms using validated questionnaires, such as the University of California Scleroderma Clinical Trial Consortium Gastrointestinal Tract 2.0 or Gastrointestinal Symptoms Severity Index score, found an association between absent contractility on HRM and upper GI symptoms, but even asymptomatic patients often have esophageal body dysmotility on HRM. Esophageal dysmotility positively correlates with the presence of interstitial lung disease on high-resolution computed tomography and reduced diffusion capacity (< 0.8 of predicted value). Trials investigating the effect of buspirone demonstrate both increased lower esophageal sphincter resting pressure and reduced upper GI symptoms. Most studies report on limited patient numbers and retrospective data. Potential bias was minimized using quality appraisal. HRM findings correlate to upper GI symptoms when assessed by validated questionnaires and can detect response to therapy in buspirone trials. Esophageal body dysmotility on HRM positively correlates with the presence of interstitial lung disease. KEY POINTS: • Esophageal body dysmotility on HRM correlates with presence of ILD. • HRM findings seem to correspond to clinical symptom alleviation in interventional trials, but data are still limited. • At present HRM, a procedure with a high negative burden to the patient, offers little to no role in the therapeutic strategy.

Traditional Tibetan medicine Anzhijinhua San attenuates ovalbumin-induced diarrhea by regulating the serotonin signaling system in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Tibetan medicine has been practiced for 3800 years. Anzhijinhua San (AZJHS), which is a traditional Tibetan medicine, has been effective in the treatment of indigestion, anorexia and cold diarrhea. However, the effects of AZJHS on allergic diarrhea have not been reported.

AIM OF THE STUDY

The aim of the present study was to elucidate the effect of AZJHS on experimental ovalbumin-induced diarrhea and elucidate its possible mechanism.

MATERIALS AND METHODS

Female BALB/c mice were sensitized by intraperitoneal injection with 50 μg ovalbumin (OVA) and 1 mg alum in saline twice during a 2-week period. From day 28, mice were orally challenged with OVA (50 mg) every other day for a total of ten times. AZJHS (46.8 and 468.0 mg/kg) was orally administered every other day from day 0-46. Food allergy symptoms were evaluated. OVA- specific IgE, 5-HT and its metabolites in serum were determined. Immunohistochemical and histopathology were performed in gastrointestinal tract tissues. 5-HT-related gene expression was assayed in the colon.

RESULTS

Severe symptoms of allergic diarrhea were observed in the model group (diarrhea, anaphylactic response, and rectal temperature). AZJHS (46.8 and 468.0 mg/kg) significantly reduced mouse diarrhea and significantly prevented the increases in OVA-specific IgE levels (P < 0.05), which challenge with OVA. AZJHS (46.8 and 468.0 mg/kg) significantly prevented the increases in 5-HT-positive cells. The nuclei of EC cells in the AZJHS (46.8 and 468.0 mg/kg) group increased in size and the secretory granules were fewer in number compared with those in the model group. AZJHS (46.8 and 468.0 mg/kg) significantly increased the relative fold changes of 5-HTP and 5-HT compared with the model group. The mRNA expression of the serotonin transporter (Sert) and serotonin receptor 3A (Htr3a) was significantly decreased after the 10th challenge with OVA, and AZJHS (46.8 and 468.0 mg/kg) significantly increased these levels.

CONCLUSIONS

We demonstrated that the administration of AZJHS attenuated OVA-induced diarrhea by regulating the serotonin pathway. These results indicated that AZJHS may be a potential candidate as an anti-allergic diarrhea agent.

Serotonin-induced contractile responses of esophageal smooth muscle in the house musk shrew (Suncus murinus)

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) is a regulatory factor in motility of the gastrointestinal tract including the esophagus. Although we proposed that vagal cholinergic and mast cell-derived non-cholinergic components including serotonin coordinately shorten the esophagus, the precise mechanism of serotonin-induced contractions in the suncus esophagus is still unclear. Therefore, the aims of this study were to determine characteristics of contractile responses induced by serotonin and to identify 5-HT receptor subtypes responsible for regulating motility in the suncus esophagus.

METHODS

An isolated segment of the suncus esophagus was placed in an organ bath, and longitudinal or circular mechanical responses were recorded using a force transducer.

KEY RESULTS

Serotonin evoked contractile responses of the suncus esophagus in the longitudinal direction but not in the circular direction. Tetrodotoxin did not affect the serotonin-induced contractions. Pretreatment with a non-selective 5-HT receptor antagonist or double application of 5-HT₁ and 5-HT₂ receptor antagonists blocked the serotonin-induced contractions. 5-HT₁ and 5-HT₂ receptor agonists, but not a 5-HT₃ receptor agonist, evoked contractile responses in the suncus esophagus.

CONCLUSION & INFERENCES

The findings suggest that serotonin induces contractile responses of the longitudinal smooth muscle in the muscularis mucosae of the suncus esophagus that are mediated via 5-HT₁ and 5-HT₂ receptors on muscle cells. The serotonin-induced contractions might contribute to esophageal peristalsis and emetic response.

Enteric co-innervation of striated muscle in the esophagus: still enigmatic?

The existence of a distinct ganglionated myenteric plexus between the two layers of the striated tunica muscularis of the mammalian esophagus has represented an enigma for quite a while. Although an enteric co-innervation of vagally innervated motor endplates in the esophagus has been suggested repeatedly, it was not possible until recently to demonstrate this dual innervation. Twenty-two years ago, we were able to demonstrate that motor endplates in the rat esophagus receive dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus. Meanwhile, a considerable amount of data has been gathered on enteric co-innervation and its occurrence in the esophagus of a variety of species including humans, its neurochemistry, spatial relationships on motor endplates, ontogeny and possible functional roles. These data underline the significance of this newly discovered innervation component, although its function in vivo is still largely unknown. The aim of this review, which is an update of our previous paper (Wörl and Neuhuber in Histochem Cell Biol 123(2):117-130. doi: 10.1007/s00418-005-0764-7 , 2005a), is to summarize the current knowledge about enteric co-innervation of esophageal striated muscle and to provide some hints as to its functional significance.

The 5-HT1A receptor agonist buspirone improves esophageal motor function and symptoms in systemic sclerosis: a 4-week, open-label trial

BACKGROUND

Acute administration of the oral 5-HT1A receptor agonist buspirone, which is commonly used as an anxiolytic drug, may improve compromised lower esophageal sphincter function. In an open-label trial we assessed the effects of buspirone on esophageal motor function and symptoms in patients with esophageal involvement associated with systemic sclerosis (SSc).

METHODS

Thirty consecutive patients with SSc and symptomatic esophageal involvement, despite treatment with proton pump inhibitors, underwent high resolution manometry and chest computed tomography for assessment of motor function and esophageal dilatation, respectively. Regurgitation, heartburn, dysphagia, and chest pain severity was subjectively scored by visual analog scales. Manometric parameters (primary endpoint) and symptom severity (secondary endpoint) were re-examined after 4-week daily administration of 20 mg buspirone. Other medications remained unchanged.

RESULTS

Eight patients did not complete the trial because of buspirone-associated dizziness (n = 2), or nausea (n = 2), or reluctancy to undergo final manometry. In the remaining 22 patients lower esophageal sphincter (LES) resting pressure increased from 7.7 ± 3.9 to 12.2 ± 4.6 mmHg (p = 0.00002) after buspirone administration; other manometric parameters did not change. Statistical analysis revealed negative correlation between individual increases in resting LES pressure and supra-aortic esophageal diameter (r = -0.589, p = 0.017), suggesting a more beneficial effect in patients with less severely affected esophageal function. Heartburn and regurgitation scores decreased at 4 weeks compared to baseline (p = 0.001, and p = 0.022, respectively).

CONCLUSION

Our findings warrant more conclusive evaluation with a double-blind controlled study; however, buspirone could potentially be given under observation for objective improvement in all patients with SSc who report reflux symptoms despite undergoing standard treatment.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02363478 Registered: 21-02-2014.

Involvement of catecholaminergic neurons in motor innervation of striated muscle in the mouse esophagus

Enteric co-innervation is a peculiar innervation pattern of striated esophageal musculature. Both anatomical and functional data on enteric co-innervation related to various transmitters have been collected in different species, although its function remains enigmatic. However, it is unclear whether catecholaminergic components are involved in such a co-innervation. Thus, we examined to identify catecholaminergic neuronal elements and clarify their relationship to other innervation components in the esophagus, using immunohistochemistry with antibodies against tyrosine hydroxylase (TH), vesicular acetylcholine transporter (VAChT), choline acetyltransferase (ChAT) and protein gene product 9.5 (PGP 9.5), α-bungarotoxin (α-BT) and PCR with primers for amplification of cDNA encoding TH and dopamine-β-hydroxylase (DBH). TH-positive nerve fibers were abundant throughout the myenteric plexus and localized on about 14% of α-BT-labelled motor endplates differing from VAChT-positive vagal nerve terminals. TH-positive perikarya represented a subpopulation of only about 2.8% of all PGP 9.5-positive myenteric neurons. Analysis of mRNA showed both TH and DBH transcripts in the mouse esophagus. As ChAT-positive neurons in the compact formation of the nucleus ambiguus were negative for TH, the TH-positive nerve varicosities on motor endplates are presumably of enteric origin, although a sympathetic origin cannot be excluded. In the medulla oblongata, the cholinergic ambiguus neurons were densely supplied with TH-positive varicosities. Thus, catecholamines may modulate vagal motor innervation of esophageal-striated muscles not only at the peripheral level via enteric co-innervation but also at the central level via projections to the nucleus ambiguus. As Parkinson's disease, with a loss of central dopaminergic neurons, also affects the enteric nervous system and dysphagia is prevalent in patients with this disease, investigation of intrinsic catecholamines in the esophagus may be worthwhile to understand such a symptom.

Beneficial effect of the 5-HT1A receptor agonist buspirone on esophageal dysfunction associated with systemic sclerosis: A pilot study

BACKGROUND

Esophageal involvement in systemic sclerosis (SSc) carries significant morbidity and is empirically managed with domperidone, albeit with questionable efficacy. The oral 5-HT1A receptor agonist buspirone may enhance esophageal peristalsis and lower esophageal sphincter (LES) function in healthy volunteers.

AIM

We aimed to test the hypothesis that buspirone may exert a beneficial acute effect on esophageal motor dysfunction in symptomatic patients with SSc.

METHODS

Twenty consecutive patients with SSc reporting esophageal symptoms underwent high-resolution manometry before and 30 minutes after administration of buspirone (10 mg). Ten other patients received domperidone (10 mg) and served as control group. Changes in LES resting and residual pressure, amplitude, duration, and velocity of distal esophageal body contractions were examined.

RESULTS

Esophageal hypomotility and hypotensive LES was found in 63% and 67% of patients, respectively. Demographic and clinical characteristics, including baseline manometric parameters, were comparable between groups. Resting pressure of LES increased after buspirone from 9.42 ± 2.6 to 11.53 ± 3.4 mmHg (p = 0.0002 by paired t-test), but not after domperidone; a trend for increase of amplitude of contractions was also observed after buspirone (p = 0.09). Comparison of the individual changes revealed that buspirone was superior to domperidone in enhancing LES pressure ( + 2.11 ± 2.0 versus -0.45 ± 2.3 mmHg, p = 0.006). No significant effects of either drug were noted on other examined parameters of esophageal function.

CONCLUSION

The beneficial acute effect of buspirone on impaired LES function associated with SSc suggests a role of 5-HT1A receptor-mediated interactions in these patients. Prospective studies to examine whether buspirone is of long-term therapeutic value for SSc-associated esophageal disease are warranted.

Histomorphology and immunohistochemistry of the lower esophageal sphincter of the least shrew (Cryptotis parva)

The biochemical and histopathological changes in the lower esophageal sphincter (LES) in the pathogenesis of gastroesophageal reflux disease have gained interest. The least shrew is able to vomit in response to emetogens and provides a good model to study the histology of this phenomenon relative to the published reports in the commonly used but vomit-incompetent laboratory species. The LES is located at the junction of the esophagus and stomach. It typically closes at rest and opens in response to swallowing. Our findings demonstrate that the least shrew does not have a well-defined LES, lacks esophageal glands and has a mucosal valve-like projection from the terminal end of the esophagus before joining the gastric epithelium at the lesser curvature. In addition, the least shrew has thoracic and abdominal components prior to joining the gastric epithelium. The mucosal lining of the esophagus is folded, becoming clearly convoluted and forming a bucket-like structure at the level of the esophageocardiac junction (ECJ). No significant differences are to be found between the structure and thickness of the wall before and after the ECJ. Thus, the ECJ forming the LES is relatively less complex than those of other mammals including man. The distribution of enterochromaffin (EC) cells is confined to the lamina propria of the junction and is not associated with the cardiac glands, suggesting its functional involvement with the smooth muscle in and around the ECJ. In conclusion, the least shrew's anatomical sphincter appears ill-defined and is replaced by a less sturdy valve-like mucosal flap.

Gastro oesophageal reflux disease (GORD)-related symptoms and its association with mood and anxiety disorders and psychological symptomology: a population-based study in women

BACKGROUND

Psychopathology seems to play a role in reflux pathogenesis and vice versa, yet few population-based studies have systematically investigated the association between gastro-oesophageal reflux disease (GORD) and psychopathology. We thus aimed to investigate the relationship between GORD-related symptoms and psychological symptomatology, as well as clinically diagnosed mood and anxiety disorders in a randomly selected, population-based sample of adult women.

METHODS

This study examined data collected from 1084 women aged 20-93 yr participating in the Geelong Osteoporosis Study. Mood and anxiety disorders were identified using the Structured Clinical Interview for DSM-IV-TR Research Version, Non-patient edition (SCID-I/NP), and psychological symptomatology was assessed using the General Health Questionnaire (GHQ-12). GORD-related symptoms were self-reported and confirmed by medication use where possible and lifestyle factors were documented.

RESULTS

Current psychological symptomatology and mood disorder were associated with increased odds of concurrent GORD-related symptoms (adjusted OR 2.1, 95% CI 1.3-3.5, and OR 3.0, 95% CI 1.7-5.6, respectively). Current anxiety disorder also tended to be associated with increased odds of current GORD-related symptoms (p = 0.1). Lifetime mood disorder was associated with a 1.6-fold increased odds of lifetime GORD-related symptoms (adjusted OR 1.6, 95% CI 1.1-2.4) and lifetime anxiety disorder was associated with a 4-fold increased odds of lifetime GORD-related symptoms in obese but not non-obese participants (obese, age-adjusted OR 4.0, 95% CI 1.8-9.0).

CONCLUSIONS

These results indicate that psychological symptomatology, mood and anxiety disorders are positively associated with GORD-related symptoms. Acknowledging this common comorbidity may facilitate recognition and treatment, and opens new questions as to the pathways and mechanisms of the association.

Modulation of esophageal afferent pathways by 5-HT3 receptor inhibition

BACKGROUND

The study aims were to investigate whether neural pathways involving 5-HT3 receptors mediate: (i) distension-induced upper esophageal sphincter (UES) relaxation reflex, (ii) esophageal sensitivity to acid and electrical stimuli, and (iii) viserosomatic sensitization following acid exposure.

METHODS

In Study I, in a double-blind crossover trial (n = 9) esophageal sensory and pain thresholds to electrical stimulation were measured in the esophagus, midsternum, and the foot, before subjects were randomized to receive either Ondansetron (8 mg i.v.) or NaCl (0.9% w/v). HCl (0.15 mol L(-1)) was then infused into distal esophagus and electrical thresholds were reassessed. Following electrical sensory threshold testing, subjects received a second esophageal infusion of HCl to evaluate esophageal sensitivity to acid. In Study II (N = 10), frequencies of distension-induced UES relaxation responses were scored before and after treatment with Ondansetron and NaCl in a double-blind crossover trial.

KEY RESULTS

In Study I, ondansetron had no effect on esophageal sensitivity to HCl or acid-induced sensitization. However, blockade of 5-HT3 receptors did reduce midsternum somatic pain thresholds. Sixty minutes after esophageal acid exposure, pain thresholds were significantly lower in the ondansetron arm (mean Δ-1.36 ± 0.4 mA) when compared with NaCl (mean Δ-0.14 ± 0.58 mA) (P < 0.05). In Study II, 5-HT3 receptor blockade had no significant effect on UES relaxation reflex.

CONCLUSIONS & INFERENCES

This study does not support the hypothesis that in health, 5-HT3 receptors play a significant role in esophago-UES distention-induced relaxation reflex and esophageal sensitivity to acid or electrical stimulation. It does provide new evidence for involvement of 5-HT3 receptors in viscerosomatic sensitization.

5-HT2A receptor antagonists inhibit hepatic stellate cell activation and facilitate apoptosis

BACKGROUND

5-hydroxytryptamine (5-HT) receptors are upregulated in activated hepatic stellate cells (HSCs), and are therefore thought to play an important role in their activation.

AIM

The aim of this study was to determine whether 5-HT2A receptor antagonists affect the activation or apoptosis of HSCs in vitro and/or in vivo.

METHODS

For the in vitro experiments, the viability, apoptosis and wound healing ability of LX-2 cells were examined after treatment with various 5-HT2A receptor antagonists. Levels of HSC activation markers (procollagen type I, α-SMA, TGF-β and Smad 2/3) were measured. For in vivo experiments, rats were divided into three groups: (i) a control group, (ii) a disease group, in which cirrhosis was induced by thioacetamide (iii) a treatment group, in which cirrhosis was induced and a 5-HT2A receptor antagonist (sarpogrelate, 30 mg/kg) was administered.

RESULTS

5-HT2A , but not 5-HT2B receptor mRNA increased with time upon HSC activation. 5-HT2A receptor antagonists (ketanserin and sarpogrelate) inhibited viability and wound healing in LX-2 cells and induced apoptosis. Expression of α-SMA and procollagen type I was also inhibited. In the in vivo study, lobular inflammation was reduced in the sarpogrelate-treated group, but there was only slight and statistically insignificant attenuation of periportal fibrosis. Expression of α-SMA, TGF-β and Smad 2/3 was also reduced in the treatment group.

CONCLUSIONS

5-HT2A receptor antagonists can reduce inflammation and the activation of HSCs in this cirrhotic model.

Serotonin: from top to bottom

Serotonin is a monoamine neurotransmitter, which is phylogenetically conserved in a wide range of species from nematodes to humans. In mammals, age-related changes in serotonin systems are known risk factors of age-related diseases, such as diabetes, faecal incontinence and cardiovascular diseases. A decline in serotonin function with aging would be consistent with observations of age-related changes in behaviours, such as sleep, sexual behaviour and mood all of which are linked to serotonergic function. Despite this little is known about serotonin in relation to aging. This review aims to give a comprehensive analysis of the distribution, function and interactions of serotonin in the brain; gastrointestinal tract; skeletal; vascular and immune systems. It also aims to demonstrate how the function of serotonin is linked to aging and disease pathology in these systems. The regulation of serotonin via microRNAs is also discussed, as are possible applications of serotonergic drugs in aging research and age-related diseases. Furthermore, this review demonstrates that serotonin is potentially involved in whole organism aging through its links with multiple organs, the immune system and microRNA regulation. Methods to investigate these links are discussed.