Chromatographic analysis of age-related changes in mucosal serotonin transmission in the murine distal ileum
© Parmar et al; licensee Chemistry Central Ltd. 2012
Received: 23 November 2011
Accepted: 27 February 2012
Published: 11 April 2012
In 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.
An 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).
We 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.
KeywordsNeurotransmission Chromatography Ileum Serotonin Ageing Enterochromaffin cell
As part of the normal process of ageing, significant changes are known to arise within most of our organs and tissues, and the gastrointestinal (GI) tract has proved no exception [1, 2]. The incidence of functional GI disorders increases in the upper bowel as we age, including absorptional disorders [2, 3]. These disorders often lead to mal-absorption in the elderly which has been described as the ‘anorexia of ageing’ theory [4, 5]. Although these disorders are well documented, little is known about the physiological changes that underlie these conditions. Age-related alterations have been observed in ion transporters, intestinal transit and the structures of the bowel [6–9], including an age-related reduction in neuronal density or number (mainly cholinergic neurons), particularly within the myenteric plexus [6, 10, 11].
Despite these studies, there appears to be an overall lack of functional data that has examined the effects of ageing on motility. Where studies exist in humans conflicting results have been observed . This may be due to a combination of factors such as dietary alterations and co-morbidities as well as multiple drug therapies in the elderly, making it difficult to assess the impact that age-related changes have on these conditions. Animal models which are not influenced by such intrinsic and extrinsic factors have provided vital information about the impact ageing has on the GI tract in the absence of disease.
It is possible that the age-related changes may in part be due to alterations in signalling within the GI tract. Serotonin (5-HT), an important neurotransmitter and paracrine signalling molecule, is known to have a variety of biological functions. Specifically within the GI tract, it is largely responsible for the maintenance of both nutrition absorption and motility [12, 13]. In addition, it is the intestinal mucosa that contains the largest amount of 5-HT in relation to the rest of the body, with 95% residing within enterochromaffin (EC) cells . EC cells are thought to act as an important interface between the hostile environment of the gut lumen, and the neurons which are unable to penetrate it.
Given the important role the EC cell plays within the GI tract, it is understandable that the measurement of 5-HT, its precursors and metabolites should be fully monitored during normal ageing. Much work has been carried out on how components of the 5-HT signalling mechanism are altered in various disease states, although very few studies have fully elucidated changes in 5-HT signalling from EC cells [16–18]. This is mainly due to the limitations of the methods utilised and the assumptions that are placed on the resultant experimental data to explain observations. For example, studies that have utilised electroanalytical techniques, can only provide insight into levels of 5-HT overflow, which can be used to gain some insight into release and clearance of 5-HT, but cannot provide insight into the regulation of 5-HT synthesis or metabolism [19–21].
Despite the various techniques available to assess components of 5-HT signalling such as electrochemical sensors [22–25], enzyme-linked immunosorbent assay (ELISA) , fluorescence  and immunohistochemistry (IHC) [16, 28], their uses remain limited as only one or two parameters involved in the process of 5-HT neurotransmission can be measured. However, in contrast, high performance liquid chromatography (HPLC) [18, 29, 30] is capable of measuring all parameters noted in Scheme 1. Within the field of gastroenterology, HPLC has been predominantly used to measure levels of analytes in tissue homogenates [18, 28, 30] and hence only intracellular neurochemicals are monitored, meaning the release of 5-HT from EC cells is entirely missed. On the other hand, there are methods such as microdialysis [31, 32] that only measure extracellular levels of 5-HT and so intracellular neurochemicals are not taken into account.
In this study, we investigated murine distal ileum tissue samples from two different age groups (3 and 18 months). We discuss how the two methods of sampling can allow for all processes of 5-HT neurotransmission to be monitored. Finally, in relation to Scheme 1, we highlight the key age-related changes observed within the 5-HT neurotransmission mechanism and discuss their possible connections with the prevalence of dysmotility in the elderly.
Results and discussion
Intracellular and extracellular sampling
Utilising both intracellular and extracellular sampling strategies, ratios and levels of important signalling molecules can be monitored which accurately reflect on the process of transmission providing more insight than conventional approaches.
Alterations in serotonin signalling
Overall we have observed an increase in the synthesis (Figure 2D) and release of 5-HT (Figure 3B). Our data also suggest an age-related decrease in the clearance of 5-HT (Figure 4A) in the distal ileum. The net effect of these age-related changes is an increase in the availability of 5-HT in 18 month old ileum. As 5-HT is well known to be associated with motility this increase in luminal 5-HT may have two potential effects: (i) increase in ileal motility due to over activation of cholinergic and nitregic pathways or (ii) desensitize 5-HT3 receptors present on myenteric and submucosal IPANs (shown in Scheme 1), leading to a disconnection to neuronal pathways in the bowel wall. Either is likely to alter the mixing within the lumen and effect adsorption. The latter could result in poor adsorption of nutrients potentially lead to mal-absorption. Such alteration in mucosal signalling mechanism may potentially explain the variety of age-related gastrointestinal conditions observed in the elderly and the onset of anorexia with ageing.
All animal experiments were carried out in compliance with the relevant laws and institution guidelines. C57BL/6 male mice (3 and 18 months old) were euthanized by cervical dislocation. Segments of distal ileum were removed and placed in oxygenated (95% O2 5% CO2) Krebs buffer solution, pH 7.4 (composition in mM: 117 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2 MgCl2, 1.2 NaH2PO4, 25 NaHCO3, and 11 glucose, all obtained from Sigmal Aldrich, UK) prior to sample preparation.
Two separate assays were carried out for both tissue segments in order to obtain concentrations of signalling molecules both within and released from the mucosa.
Extracellular sampling assay
For the measurement of extracellular signalling molecules, 1 cm segments of distal ileum were transferred to a well containing Krebs buffer solution. The tissue samples were then cut along the mesenteric border and pinned out into a modified tissue culture plate forming a whole mount preparation. Fabrication of this modified well plate has been described previously . Once the tissue section was pinned within the individual well, the tissue was washed twice using Krebs buffer to remove any waste or excised mucosa which may have arisen during tissue transfer. After this, all excess Krebs buffer was removed from the well leaving only the pinned out tissue. 1.5 mL of Krebs buffer (ambient temperature) was then added to the well and the tissue was left to stand for 30 min. After this time, a 250 μL aliquot of the buffer solution was taken and added to an Eppendorf tube containing 250 μL of 0.1M ice cold perchloric acid. The mixture was then centrifuged at 13,200 g at 4°C for 10 min. After centrifugation, the supernatant from the sample was filtered using Phenex RC membrane 0.2 μm 4 mm syringe filters (Phenomenex) to remove any particulates. These samples were then stored on ice prior to HPLC analysis. The pinned out tissue was then imaged for surface area measurements.
Intracellular sampling assay
For the measurement of intracellular signalling molecules, 1 cm long segments of distal ileum were transferred to an inverted Petri dish over ice. The tissue sections were cut along the mesenteric border and laid flat with the mucosal layer uppermost. The tissue was rinsed with ice cold Krebs buffer to remove any waste from the tissue surface. Subsequently, the mucosal layer was scraped off containing the cells required for intracellular analysis using a scalpel from an approximately 1 cm2 segment of the tissue samples. The mucosal scraping was transferred to an Eppendorf tube and homogenised in 500 μL of ice cold perchloric acid for 2 minutes. The tissue samples were then centrifuged at 13,200 g at 4°C for 10 min. After centrifugation, the supernatant from the sample was filtered as above and stored on ice prior to HPLC analysis. The remaining tissue segment following mucosal scraping was pinned out to be imaged for surface area measurements.
The HPLC apparatus consisted of a Jasco HPLC pump (Model: PU-980) and Rheodyne manual injector equipped with a 20 μl loop. A Kinetic® ODS 2.6 μm 100 mm x 2.1 mm i.d. analytical column with a KrudKatcher™ Ultra in-line filter (Phenomenex®, Macclesfield, UK) was employed. The HPLC system was used in a completely isocratic mode for the detection of the signalling molecules. The HPLC system was run at a flow rate of 100 μL min-1 and an injected sample volume of 20 μL was used. CHI1001A potentiostat (CH Instruments, Austin, TX, USA) was used to control the detector voltage and record the current. A 3 mm glassy carbon electrode (flow cell, BAS) served as the working electrode and was used with a Ag|AgCl reference electrode and a stainless steel block as the auxiliary electrode. Amperometric recordings were carried out, where the working electrode was set at a potential of +850 mV vs Ag|AgCl reference electrode. Control and data collection/processing were handled through the CHI1001A software.
The stock buffer for the mobile phase was comprised of the following: 0.1 M sodium acetate, 0.1 M citric acid and 27 μM disodium ethylene-diamine-tetra-acetate (EDTA) dissolved in 1 L of deionised water. This was then buffered to pH 3.0. The mobile phase was prepared with the stock buffer mixed with methanol in the ratio of 8:2 (v/v) and degassed after mixing.
Standards and accuracy
Standard solutions were prepared from 500 μM stock standards of each analyte and were made up in 0.1 M perchloric acid (Σ). Each of the standard solutions were prepared on the day of analysis and stored at 4°C prior to injection. A calibration plot was obtained by running individually, a range of concentrations of 5-HTP, 5-HT, tryptophan and 5-HIAA (all obtained from Sigma Aldrich). Concentrations investigated were from the range of 0.1 -20 μM for 5-HT and 0.01-10 μM for 5-HTP, tryptophan and 5-HIAA. The peak areas obtained from chromatographic analysis of all injected samples were converted to concentrations using the calibration responses of the neurochemical standards.
Data analysis and interpretation
As the concentration of the neurochemicals was analysed in both intracellular and extracellular conditions, a normalisation method was required that could be utilised in both approaches. We have utilised normalisation by tissue area using pixel counting algorithms as this can be utilised for both sampling approaches. The tissue area calculation by pixel counting algorithm is widely used in a variety of scientific fields [42, 43]. The process of data normalisation utilised within this study has been described elsewhere . Briefly, following chromatographic analysis for intracellular and extracellular levels of the neurochemicals, the pinned out tissue sections were photographed using a digital camera (7 megapixel) and the resultant image was analysed to calculate the surface area in pixels using IMAGE J. From the intracellular samples the area of the removed mucosal samples were used, whilst for the extracellular samples the total area of the tissue was analysed. Once all intracellular and extracellular images were converted to surface area in cm2, the concentration data was normalised to the surface area, to allow for comparison between both sampling assays. All data are shown as mean ± standard error of the mean (S.E.M) and n refers to the number of single tissue samples per animal included within the data set. Concentrations of individual neurochemicals were compared between 3 and 18 month old animals using the student t-test, whilst ratios of neurochemicals were analysed using the Mann Whitney test.
We have utilised two sampling approaches which provide more suitable markers to study the full process of 5-HT signalling using chromatography. We have shown that age-related changes in 5-HT signalling occur between 3 and 18 month old animals. Increased synthesis and release coupled with reduced clearance were observed which ultimately leads to increased 5-HT availability within the ileum. Such increases in 5-HT may lead to altered motility which may underlie age-related GI disorders in the upper bowel.
The authors would like to thank BBSRC ‘Ageing bladder and bowel’ grant (BB/G015988/1) for funding.
- Wade PR: Aging and Neural Control of the GI Tract: I. Age-related changes in the enteric nervous system 10.1152/ajpgi.00091.2002. Am J Physiol Gastrointest Liver Physiol. 2002, 283: G489-G495.View ArticleGoogle Scholar
- Thomson ABR: Small intestinal disorders in the elderly. Best Pract Res Clin Gastroenterol. 2009, 23: 861-874. 10.1016/j.bpg.2009.10.009.View ArticleGoogle Scholar
- Firth M, Prather CM: Gastrointestinal motility problems in the elderly patient. Gastroenterology. 2002, 122: 1688-1700. 10.1053/gast.2002.33566.View ArticleGoogle Scholar
- Blundell JE: Understanding anorexia in the elderly: formulating biopsychological research strategies. Neurobiol Aging. 1988, 9: 18-20.View ArticleGoogle Scholar
- Morley JE, Thomas DR: Anorexia and aging: pathophysiology. Nutrition. 1999, 15: 499-503. 10.1016/S0899-9007(99)00057-X.View ArticleGoogle Scholar
- Johnson RJR, Schemann M, Santer RM, Cowen T: The effects of age on the overall population and on subpopulations of myenteric neurons in the rat small intestine. J Anat. 1998, 192: 479-488. 10.1046/j.1469-7580.1998.19240479.x.View ArticleGoogle Scholar
- Freeman HJ, Quamme GA: Age-related changes in sodium-dependent glucose transport in rat small intestine. Am J Physiol Gastrointest Liver Physiol. 1986, 251: G208-G217.Google Scholar
- O'Mahony D, O'Leary P, Quigley EMM: Aging and Intestinal Motility. Drugs Aging. 2002, 19: 515-527. 10.2165/00002512-200219070-00005.View ArticleGoogle Scholar
- Hasler WL: Small Intestinal Motility. Physiology of the Gastrointestinal Tract. 2006, Academic, Burlington, 935-964. Fourth EditionView ArticleGoogle Scholar
- Phillips RJ, Kieffer EJ, Powley TL: Aging of the myenteric plexus: neuronal loss is specific to cholinergic neurons. Auton Neurosci. 2003, 106: 69-83. 10.1016/S1566-0702(03)00072-9.View ArticleGoogle Scholar
- Saffrey MJ: Ageing of the enteric nervous system. Mech Ageing Dev Ageing Gut. 2004, 125: 899-906.View ArticleGoogle Scholar
- Bertrand PP, Bertrand RL: Serotonin release and uptake in the gastrointestinal tract. Auton Neurosci. 2010, 153: 47-57. 10.1016/j.autneu.2009.08.002.View ArticleGoogle Scholar
- Gershon M: Review article: roles played by 5-hydroxytryptamine in the physiology of the bowel. Aliment Pharmacol Ther. 1999, 13: 15-30. 10.1046/j.1365-2036.1999.00437.x.View ArticleGoogle Scholar
- Raybould HE, Cooke HJ, Christofi FL: Sensory mechanisms: transmitters, modulators and reflexes doi:10.1111/j.1743-3150.2004.00477.x. Neurogastroenterol Motil. 2004, 16: 60-63. 10.1111/j.1743-3150.2004.00477.x.View ArticleGoogle Scholar
- Bertrand PP, Kunze WAA, Furness JB, Bornstein JC: The terminals of myenteric intrinsic primary afferent neurons of the guinea-pig ileum are excited by 5-hydroxytryptamine acting at 5-hydroxytryptamine-3 receptors. Neuroscience. 2000, 101: 459-469. 10.1016/S0306-4522(00)00363-8.View ArticleGoogle Scholar
- Coates MD, Mahoney CR, Linden DR, Sampson JE, Chen J, Blaszyk H, Crowell MD, Sharkey KA, Gershon MD, Mawe GM, Moses PL: Molecular defects in mucosal serotonin content and decreased serotonin reuptake transporter in ulcerative colitis and irritable bowel syndrome. Gastroenterology. 2004, 126: 1657-1664. 10.1053/j.gastro.2004.03.013.View ArticleGoogle Scholar
- Costedio M, Hyman N, Mawe G: Serotonin and Its Role in Colonic Function and in Gastrointestinal Disorders. Dis Colon Rectum. 2007, 50: 376-388. 10.1007/s10350-006-0763-3.View ArticleGoogle Scholar
- Linden DR, Chen J-X, Gershon MD, Sharkey KA, Mawe GM: Serotonin availability is increased in mucosa of guinea pigs with TNBS-induced colitis 10.1152/ajpgi.00488.2002. Am J Physiol Gastrointest Liver Physiol. 2003, 285: G207-G216.View ArticleGoogle Scholar
- Patel BA: Continuous amperometric detection of co-released serotonin and melatonin from the mucosa in the ileum. Analyst. 2008, 133: 516-524. 10.1039/b717034c.View ArticleGoogle Scholar
- Patel BA, Bian X, Quaiserova-Mocko V, Galligan JJ, Swain GM: In vitro continuous amperometric monitoring of 5-hydroxytryptamine release from enterochromaffin cells of the guinea pig ileum. Analyst. 2007, 132: 41-47. 10.1039/b611920d.View ArticleGoogle Scholar
- Bertrand PP: Real-time detection of serotonin release from enterochromaffin cells of the guinea-pig ileum. Neurogastroenterol Motil. 2004, 16: 511-514. 10.1111/j.1365-2982.2004.00572.x.View ArticleGoogle Scholar
- Bertrand RL, Senadheera S, Markus I, Liu L, Howitt L, Chen H, Murphy TV, Sandow SL, Bertrand PP: A Western Diet Increases Serotonin Availability in Rat Small Intestine. Endocrinology. 2011, 152: 36-47. 10.1210/en.2010-0377.View ArticleGoogle Scholar
- Patel BA: Electroanalytical approaches to study signaling mechanisms in the gastrointestinal tract. Neurogastroenterol Motil. 2011, 23: 595-605. 10.1111/j.1365-2982.2011.01708.x.View ArticleGoogle Scholar
- Patel BA, Arundell M, Parker KH, Yeoman MS, O'Hare D: Microelectrode investigation of neuroneal ageing from a single identified neurone. Phys Chem Chem Phys. 2010, 12: 10065-10072.View ArticleGoogle Scholar
- Zhao H, Bian X, Galligan JJ, Swain GM: Electrochemical measurements of serotonin (5-HT) release from the guinea pig mucosa using continuous amperometry with a boron-doped diamond microelectrode. Diam Relat Mater. 2010, 19: 182-185. 10.1016/j.diamond.2009.10.004.View ArticleGoogle Scholar
- Bertrand PP, Barajas-Espinosa A, Neshat S, Bertrand RL, Lomax AE: Analysis of real-time serotonin (5-HT) availability during experimental colitis in mouse. Am J Physiol Gastrointest Liver Physiol. 2010, 298: G446-G455. 10.1152/ajpgi.00318.2009.View ArticleGoogle Scholar
- Qi S-d, Tian S-l, Xu H-x, Sung J, Bian Z-x: Quantification of luminally released serotonin in rat proximal colon by capillary electrophoresis with laser-induced fluorescence detection. Anal Bioanal Chem. 2009, 393: 2059-2066. 10.1007/s00216-009-2655-6.View ArticleGoogle Scholar
- Bian X, Patel B, Dai X, Galligan JJ, Swain G: High Mucosal Serotonin Availability in Neonatal Guinea Pig Ileum Is Associated With Low Serotonin Transporter Expression. Gastroenterology. 2007, 132: 2438-2447. 10.1053/j.gastro.2007.03.103.View ArticleGoogle Scholar
- Lwin A, Patel BA: High performance liquid chromatography method for the detection of released purinergic and biogenic amine signaling molecules from in vitro ileum tissue. J Sep Sci. 2010, 33: 1538-1545. 10.1002/jssc.200900853.View ArticleGoogle Scholar
- Chau RMW, Patel BA: Determination of serotonin, melatonin and metabolites in gastrointestinal tissue using high-performance liquid chromatography with electrochemical detection. Biomed Chromatogr. 2009, 23: 175-181. 10.1002/bmc.1100.View ArticleGoogle Scholar
- Ishida J, Yoshitake T, Fujino K, Kawano K, Kehr J, Yamaguchi M: Serotonin monitoring in microdialysate from rat brain by microbore-liquid chromatography with fluorescence detection. Anal Chim Acta. 1998, 365: 227-232. 10.1016/S0003-2670(97)00616-8.View ArticleGoogle Scholar
- Westerink BHC: Analysis of biogenic amines in microdialysates of the brain. J Chromatogr B: Biomed Sci Appl. 2000, 747: 21-32. 10.1016/S0378-4347(00)00338-8.View ArticleGoogle Scholar
- Amatore C, Arbault S, Bonifas I, Bouret Y, Erard M, Guille M: Dynamics of Full Fuson During Vesicular Exocytotic Events: Release of Adrenaline by Chromaffin Cells. ChemPhysChem. 2003, 4: 147-154. 10.1002/cphc.200390024.View ArticleGoogle Scholar
- Omiatek DM, Dong Y, Heien ML, Ewing AG: Only a Fraction of Quantal Content is Released During Exocytosis as Revealed by Electrochemical Cytometry of Secretory Vesicles. ACS Chem Neurosci. 2010, 1: 234-245. 10.1021/cn900040e.View ArticleGoogle Scholar
- Braskie MN, Wilcox CE, Landau SM, O'Neil JP, Baker SL, Madison CM, Kluth JT, Jagust WJ: Relationship of striatal dopamine synthesis capacity to age and cognition. J Neurosci. 2008, 28: 14320-14328. 10.1523/JNEUROSCI.3729-08.2008.View ArticleGoogle Scholar
- Dejesus OT, Endres CJ, Shelton SE, Nickles RJ, Holden JE: Noninvasive assessment of aromatic L-amino acid decarboxylase activity in aging rhesus monkey brain in vivo. Synapse. 2001, 39: 58-63. 10.1002/1098-2396(20010101)39:1<58::AID-SYN8>3.0.CO;2-B.View ArticleGoogle Scholar
- Lloyd KG, Hornykiewicz O: Occurrence and distribution of aromatic L-amino acid (L-DOPA) decarboxylase in the human brain. J Neurochem. 1972, 19: 1549-1559. 10.1111/j.1471-4159.1972.tb05099.x.View ArticleGoogle Scholar
- Yeoman M: Synapse specific changes in serotonin signalling contribute to age-related changes in the feeding behaviour of the pond snail, Lymnaea. In Age. 2007, 29: 111-112.Google Scholar
- Miguez JM, Aldegrunde A, Paz-Valinas L, Recio J, Sanchez-Barcelo E: Selective changes in the contents of noradrenaline, dopamine and serotonin in rat brain areas during aging. J Neural Transm. 1999, 106: 1089-1098. 10.1007/s007020050225.View ArticleGoogle Scholar
- Yoshimoto K, Yoshida T, Sorimachi Y, Hirano A, Takeuchi Y, Ueda S, Yasuhara M: Effects of age and ethanol on dopamine and serotonin release in the rat nucleus accumbens. Physiol Behav. 1998, 64: 347-351. 10.1016/S0031-9384(98)00067-5.View ArticleGoogle Scholar
- van Dyck CH, Malison RT, Seibyl JP, Laruelle M, Klumpp H, Zoghbi SS, Baldwin RM, Innis RB: Age-related decline in central serotonin transporter availability with [123I][beta]-CIT SPECT. Neurobiol Aging. 2000, 21: 497-501. 10.1016/S0197-4580(00)00152-4.View ArticleGoogle Scholar
- Chubb C, Inagaki Y, Sheu P, Cummings B, Wasserman A, Head E, Cotman C: BioVision: An application for the automated image analysis of histological sections. Neurobiol Aging. 2006, 27: 1462-1476. 10.1016/j.neurobiolaging.2005.08.023.View ArticleGoogle Scholar
- McAtee P, Hallett I, Johnston J, Schaffer R: A rapid method of fruit cell isolation for cell size and shape measurements. Plant Meth. 2009, 5: 5-10.1186/1746-4811-5-5.View ArticleGoogle Scholar
- Parmar L, Morgan LD, Patel BA: Intracellular and extracellular sampling to monitor the neurotransmission process using a chromatographic method. Anal Meth. 2011, 3: 2770-2776. 10.1039/c1ay05520h.View ArticleGoogle Scholar
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