Sara Fidalgo

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.

Age-related changes in melatonin release in the murine distal colon

Constipation and fecal impaction are conditions of the bowel whose prevalence increases with age. Limited information is known about how these conditions manifest; however, functional deficits are likely to be due to changes in signaling within the bowel. This study investigated the effects of age on colonic mucosal melatonin (MEL) release and the consequences this had on colonic motility. Electrochemical measurements of MEL overflow demonstrated that both basal and mechanically stimulated MEL release decreased with age. The MEL/serotonin also decreased with increasing age, and the trend was similar to that of MEL overflow, suggestive that age-related changes were primarily due to a reduction in MEL levels. Levels of N-acetylserotonin and the N-acetylserotonin/serotonin ratio were reduced with age, providing an explanation for the reduction in MEL release. Decreases in colonic motility were observed in animals between 3 and 24 months old. Exogenous application of MEL could reverse this deficit in aged colon. In summary, we propose that the age-related decline in MEL release may be due to either decreases or alterations in mechanosensory channels and/or a loss in levels/activity of the N-acetyltransferase enzyme responsible for the synthesis of N-acetylserotonin. Decreases in MEL release may explain the decreases in colonic motility observed in 24 month old animals and could offer a new potential therapeutic treatment for age-related constipation.

Impaired colonic motility and reduction in tachykinin signalling in the aged mouse

Ageing is associated with an increased incidence of constipation in humans. The contribution that the ageing process makes to this condition is unclear. The aim of this study was to determine the effects of age on faecal output and colonic motility in male C57BL/6J mice and to determine the role that altered tachykinin signalling plays in this process. Total faecal output recorded over a 24h period decreased with age due to a reduction in the number of pellets produced and their water content. These changes occurred in the absence of any significant change in food and water intake. There was an increase in the amount of faecal matter stored in the isolated colon with age which caused a proportional increase in colonic length. Analysis of colonic motility using an artificial pellet demonstrated that pellets moved in a stepwise fashion through the colon. There was an age-related increase in pellet transit time due to decreases in the step distance, velocity, and frequency of stepwise movements. These changes were reversed using the neurokinin 2 (NK2) receptor agonist neurokinin A. Addition of the NK2receptor antagonist GR159897 significantly increased transit time in the young animals by decreasing step distance, velocity and frequency, but was without effect in the aged colon. In summary, the ageing C57BL/6J mouse shows an impaired motility phenotype. These effects appear, at least in part, to be due to an attenuation of tachykinin signalling via NK2 receptors.

The TNF-α antagonist etanercept reverses age-related decreases in colonic SERT expression and faecal output in mice

Treatment for chronic constipation in older people is challenging and the condition has a major impact on quality of life. A lack of understanding about the causes of this condition has hampered the development of effective treatments. 5-HT is an important pro-kinetic agent in the colon. We examined whether alterations in colonic 5-HT signalling underlie age–related changes in faecal output in mice and whether these changes were due to an increase in TNF-α. Components of the 5-HT signalling system (5-HT, 5-HIAA, SERT) and TNF-α expression were examined in the distal colon of 3, 12, 18 and 24-month old mice and faecal output and water content monitored under control conditions and following the administration of etanercept (TNF-α inhibitor; 1 mg Kg−1). Faecal output and water content were reduced in aged animals. Age increased mucosal 5-HT availability and TNF-α expression and decreased mucosal SERT expression and 5-HIAA. Etanercept treatment of old mice reversed these changes, suggesting that age-related changes in TNFα expression are an important regulator of mucosal 5-HT signalling and pellet output and water content in old mice. These data point to “anti-TNFα” drugs as potential treatments for age-related chronic constipation.

Chromatographic analysis of age-related changes in mucosal serotonin transmission in the murine distal ileum

BackgroundIn the upper bowel, alterations in motility and absorption of key nutrients have been observed as part of the normal ageing process. Serotonin (5-HT) is a key signalling molecule in the gastrointestinal tract and is known to influence motility, however little is known of how the ageing process alters 5-HT signalling processes in the bowel.ResultsAn isocratic chromatographic method was able to detect all 5-HT precursors and metabolites. Using extracellular and intracellular sampling approaches, we were able to monitor all key parameters associated with the transmission process. There was no alteration in the levels of tryptophan and 5-HTP between 3 and 18 month old animals. There was a significant increase in the ratio of 5-HT:5-HTP and an increase in intracellular 5-HT between 3 and 18 month old animals suggesting an increase in 5-HT synthesis. There was also a significant increase in extracellular 5-HT with age, suggesting increased 5-HT release. There was an age-related decrease in the ratio of intracellular 5-HIAA:extracellular 5-HT, whilst the amount of 5-HIAA did not change with age. In the presence of an increase in extracellular 5-HT, the lack of an age-related change in 5-HIAA is suggestive of a decrease in re-uptake via the serotonin transporter (SERT).ConclusionsWe have used intracellular and extracellular sampling to provide more insight into alterations in the neurotransmission process of 5-HT during normal ageing. We observed elevated 5-HT synthesis and release and a possible decrease in the activity of SERT. Taken together these changes lead to increased 5-HT availability and may alter motility function and could lead to the changes in adsorption observed in the elderly.

Changes of renin-angiotensin system-related aminopeptidases in early stage Alzheimer's disease

&NA; Activities of aminopeptidases A, B, and N (ApA, ApB & ApN) and insulin‐regulated aminopeptidase (IRAP) have been seen to be decreased amongst patients with Alzheimers disease (AD). All of these enzymes are involved with the brain renin‐angiotensin system which is believed to be involved with learning and memory. This study aimed to explore the time course and the mechanisms underlying these changes. Serum samples were collected from 45 AD patients at the start of the study, and again 13 months later (n = 37). The control group was 22 healthy, older, adults. Enzyme activity was determined at two substrate concentrations to allow Michaelis‐Menten analysis of the enzyme activity. The results indicated that there was decreased activity of ApA, ApB and ApN amongst AD patients but no difference in serum IRAP activity. There were no associations between enzyme activity and age, gender nor scores on psychomotor tests. Consideration of the data for the two time points for AD patients showed that the changes in ApB occurred at an early stage of the disease and persisted, whilst those of ApA and ApN only became apparent at later stages of the disease. Although differences in Michaelis‐Menten parameters were not statistically significant, consideration of the values suggested that the decrease in ApB activity may be a result of changes in enzyme protein conformation, whilst that of ApN may be a consequence of decreased enzyme expression. Importantly, the different time courses of the effects and the differential changes in enzyme affinity and expression indicated that the observed changes with progression of AD were not a ‘class effect’ for serum aminopeptidases but were idiosyncratic for the individual enzymes. HighlightsDecreased serum aminopeptidases A, B and N activity in Alzheimers disease.Persistent decrease in activity of aminopeptidase B from early stage of disease.Decreased activity of aminopeptidase A and N only in later stage of the disease.

Changes in murine anorectum signaling across the life course

Increasing age is associated with an increase in the incidence of chronic constipation and fecal impaction. The contribution of the natural aging process to these conditions is not fully understood. This study examined the effects of increasing age on the function of the murine anorectum.

Prenatal alcohol exposure reduces 5-HT concentration in mouse intestinal muscle and mucosa

The influence of prenatal alcohol exposure on the serotoninergic system in the brain has been well studied, however its influence on the serotoninergic system in the gastrointestinal system remains unknown. The objective of the study was to use a mouse model of prenatal alcohol exposure to investigate the effects on serotonin and its metabolites and precursors in colonic tissue. This study used treatment of mouse breeding harems with 5% ethanol with saccharin via drinking water throughout pregnancy and compared the results with a saccharin control group. Tryptophan, serotonin (5-HT) and 5- hydroxyindoleacetic acid (5-HIAA) concentrations were measured in the longitudinal muscle myenteric plexus (LMMP) and mucosa of intestinal tissue by high-performance liquid chromatography (HPLC). Decreased 5-HT concentrations in mucosa and LMMP (females only) were observed in prenatally exposed mice compared to controls. Increases in mucosal and LMMP tryptophan concentration were only observed in prenatally exposed female mice. In conclusion, prenatal alcohol exposure causes a decrease in conversion of tryptophan to 5-HT in both muscle and mucosa although the effect is more pronounced in females. The observed sex difference may be related to changes associated with the estrous cycle.

Low-dose chronic prenatal alcohol exposure abolishes the pro-cognitive effects of angiotensin IV

&NA; Prenatal ethanol exposure (PAE) in humans results in a spectrum of disorders including deficits in learning and memory. Animal models to date have typically used high ethanol doses but have not identified the biochemical changes underlying the cognitive deficit. This study used treatment of mouse breeding harems with 5% ethanol via drinking water throughout pregnancy and lactation and explored the behavioural consequences in the progeny at 3–6 months of age using the open field test, novel object recognition test and elevated plus maze to measure anxiety and memory consolidation. The effects of angiotensin IV on behaviour of the progeny were also determined. The results indicated that PAE increased anxiety‐like behaviour as determined in the open field test in male but not female progeny. In control animals, angiotensin IV enhanced memory consolidation in males, but this effect was abolished by PAE. The abolition of the pro‐cognitive effect of angiotensin IV was not a consequence of increased anxiety, and there was some evidence of a long‐lasting anxiolytic effect of angiotensin IV in the male PAE progeny. These results suggest that PAE may act via alteration of the actions of the brain renin‐angiotensin system to impair memory consolidation, but these effects may be partially sex‐dependent. HighlightsPrenatal exposure to 5% ethanol increased anxiety in male but not female progeny.Prenatal exposure to 5% ethanol abolished the precognitive effects of angiotensin IV in male progeny.Angiotensin IV induced anxiolysis in males exposed to 5% ethanol in utero.

Acute paraquat exposure impairs colonic motility by selectively attenuating nitrergic signalling in the mouse.

Paraquat, a common herbicide, is responsible for large numbers of deaths worldwide through both deliberate and accidental ingestion. Previous studies have eluded that the bioavailability of paraquat increases substantially with increasing dose and that these changes may in part be due to the effects that these high concentrations have on the gastrointestinal tract (GI tract). To date, the actions of acute, high concentrations (20mM for 60 min) of paraquat on the GI tract, particularly the colon which is a major site of paraquat absorption, are unknown. This study examined the effects of acute paraquat administration on colonic motility in the C57BL/6 mouse. Acute paraquat exposure decreased colonic motility and the amplitude of colonic migrating motor complexes (CMMCs), which are major motor patterns involved in faecal pellet propulsion. In isolated segments of distal colon, paraquat increased resting tension and markedly attenuated electrical field stimulation-evoked relaxations. Pharmacological dissection of paraquats mechanism of action on both the CMMCs and field stimulated tissue using the nitric oxide synthase inhibitor NG-nitro-L-arginine and direct measurement of NO release from the myenteric plexus, demonstrated that paraquat selectively attenuates nitrergic signalling pathways. These changes did not appear to be due to alterations in colonic oxidative stress, inflammation or complex 1 activity, but were most likely caused by paraquats ability to act as a redox couple. In summary, these data demonstrate that acute paraquat exposure attenuates colonic transit. These changes may facilitate the absorption of paraquat into the circulation and so facilitate its toxicity.