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Towards a possible aetiology for depressions?
© Liu et al; licensee BioMed Central Ltd. 2007
Received: 27 October 2005
Accepted: 14 September 2007
Published: 14 September 2007
Since a genetic disposition for depression is probable, there ought to be biochemical changes. Increased peptide levels with relevant bioactivities have been found in urine in a previous investigation, which may be such changes.
Urine from patients with severe depression according to ICD 10 have been run on reversed phase High Performance Liquid Chromatography, and off line mass spectrometry was performed on some of these peptides.
We find overlapping patterns of peptide peaks in severe depression, but with considerable individuality. Mass spectrometry shows that some of these peptides are probably of dietary origin, because their sequences are found only in certain dietary proteins. Opioids from casein and gliadin are typical examples.
Our data show that the disposition must be polygenetic because some peptide peaks with the same bioactivity are of different length in different patients, but with the same diagnosis. However, some of the peaks are common Peptide increase in urine is found when break down is deficient, and the data presented agree with reports on peptidase deficiencies in depression. Antidepressant drugs decrease the peptide level after about 3 weeks.
Considerable evidence indicates a genetic disposition for severe depressions [1–4], which of necessity entails chemical changes. The disease takes time to develop, which probably points to unknown substances increasing and, or decreasing until they reach a critical level. We have previously found increased low molecular weight peptides (fragments of proteins) in urine from patients with depression [5, 6] diagnosed according to The Diagnostic Manual of Mental Disorder, 3rd edition (DSMIII). A peptide fraction was found that stimulated the uptake of serotonin (5-HT) into platelets  Compounds with opioid activity were also found. On account of the confusing and varied patterns and levels of compounds found in urinary profiles from subjects with depression, we wanted to study a severely ill group to try to tease out what is typical. Some of the peptides have been purified guided by serotonin uptake stimulation in platelets , opioid receptor binding and/or antibody binding assay . Other peptides have been purified using their mass-spectrometric molecular weight as a guide.
Patients and methods
Severe Depression and the level of peptides.
For pattern analysis the first morning urine and for purification purposes a complete 24 diuresis were collected under supervision (The pattern and levels of compounds were not statistically different comparing morning urine to a 24 h diuresis) and frozen. After thawing, the pH was measured and creatinine determined by the Clinical Chemical Laboratory at Rikshospitalet using standard technique. 0.5 ml urine was pipetted into Costar Spin-x centrifuge filter units (205 Broadway, Cambridge Ma 02139, USA) with cellulose acetate filters of pore radius 0.22 μm and centrifuged at 4000 × g for 30 minutes at 20°C. Filtrate equivalent to 250 nano-moles of creatinine was applied to the column. The column was a C-18 reverse phase column(Vydac C-18 column 0.5 × 25 cm, Hesperia, Ca, USA) detailed elsewhere . Standards obtained from Calbiochem-Novabiochem, AG, Läufelingen, CH-4448, Switzerland and Bachem (Bubendorf, Switzerland) were analyzed after every 11 HPLC runs and spiked urine runs were used when needed.
Was performed on Sephadex G-25 columns to separate high and low molecular weight compounds (Dimensions 1.6 × 90 cm run in 0.5 M acetic acid at 0.4 ml/min, application volume 10 ml). After rota-vapor concentration 10 ml of filtered low MW fractions were run on P2 gels (1.6 × 90 cm) again in 0.5 M acetic acid. Off line alkaline hydrolysis and neutralization of 5% aliquots and ninhydrin colouring in an acetate/cyanide buffer was read at 570 nm . This has the advantage that tryptophan is not destroyed by the hydrolysis and the amino acids have equimolar absorption at this wavelength with this method.
Material obtained from the chromatography peaks was analyzed on the PeSciex API 2000 LC/MS/MS system. The peptides were dissolved in 50% by volume methanol/water/0.01 M formic acid, filtered through Millex GS 0.22 μm filter unit from Millipore, and run in the positive ion mode, and if possible also in the negative ion mode. A blank was run prior to each test sample and because of the high ability for peptides to bind to surfaces, prolonged washing was required. Fragmentation patterns (MS/MS) were compared to that of commercially available standard peptides. The theoretical mass was calculated from tables provided by Micromass UK Ltd, Manchester, UK and compared to the found mass. We chose the average mass different from the mono-isotopic mass. This is the isotopic mix usually found in nature today. To calculate the MW the MW for each amino acid with one Hydrogen removed and an OH group removed is added with addition of one H for the N terminal amino acid and either water (17 daltons) or 16 daltons for C terminal amide and +1 for the positive charge N terminally of NH3 +. The reason for this is that one molecule of water is lost for each peptide bond formed.
Platelets were obtained from healthy post-puberty males and were prepared as described . Because the Fura-2 calcium marker leaks out of platelets that were cooled to 4°C during the preparation, the platelets were prepared with glucose and pyruvate to ensure viability at 20°C . For buffers and details see Pedersen et al 1999 . Platelet count was adjusted to 1.5 × 108 platelets per ml with buffer.
Serotonin uptake into platelets
The re-suspended platelets were divided in aliquots of 450 μl stored at 4°C  and when used pre-incubated for 10 min at 37°C. For some unknown reason far more stable uptake data were obtained if stored at 4 degrees . The aliquots were incubated with various concentrations of the peptides in 25 μl buffer and for 4 min, and after 2 min (14C)-5-HT (Amersham Life Science, UK) in 25 μl was added to a final concentration of 1 μM and 41530 cpm and uptake run for 2 min, the time when the uptake is still linear. For details see Pedersen et al 1999 .
This factor and other peptides were purified as described [7, 8]. Briefly: a complete 24 h diuresis was reduced by evaporation under reduced pressure (rota-vapor) to 10 ml and applied to gel filtration on G-25 to separate protein from low MW compounds. Peptides were separated from amino acids and salts as described  and subsequently the low MW fractions were filtered on P2 gels in 0.5 M acetic acid. Composition of purified peptides was found by hydrolysis in 6 M HCl (Merck) with traces of phenol to protect tyrosine and phenylalanine, in closed glass ampoules at 110°C. for 14 hours. HCL was removed over KOH and P2O5 in vacuum. Amino acid analysis was performed on the Alpha plus amino acid analyzer (Pharmacia, Uppsala, Sweden) using the ninhydrin technique as ordained for the apparatus. The average from three analyses is given under each peptide. Peptide separation form amino acids is as described .
Gel filtration characteristics of low MW peaks separated by size.
Serotonin uptake into platelets
Probable structure of some mass spectrometric peaks not shown in fig 5.
Kav of peak
HPLC Co-chromatography with
beta-casomorphineb 1–5 amide
From the composite peak with Kav = 0.61 a compound with MW+1 = 505.4 (fig 5) yielded on amino acids Y = 0.8; P = 1.9; traces of G and E = 1. Found mass+1 = 505.4 (Expected mass +1 = 505.54). This fits gliadin-morphine 1–4 deamidated or Y-P-E-P.
Also in the Kav = 0.66 and 0.61 peak beta-casomorphine 3–6 with amino acid composition F = 0.9; P = 2; G = 1.4. (Glycine always appear higher than expected even in standard synthetic compounds). Found MW +1 = 417.5(Expected = 417.4), which thus agrees with F-P-G-P. (Figure 5, the MW came out as 417.6) or casomorphine 3–6.
MW+1 = 357.2 from the Kav = 0.66 peak likewise fits Lactoferrin 310–313 or SP-P-G or theoretical MW+1 = 357.37 and hydrolysis gave P(2), S(0,9) and G (1.4).
With a MW+1 of MW+1 = 489.3 (Expected = 489.5) probably from kappa-casein has a reasonable structure of L-P-Y-P (casein kappa 56–59). Amino acid composition was quite varying but V = 1, Y = 0.6, P = 2. (Glycine was sometimes found but varied from 0 to 1.)
We also obtained a peak that eluted with substance P (co-chromatography after spiking). However, it clogged (for unknown reason) the capillary electrode in the mass spectrometer, so we were unable to obtain its mass and fragmentation pattern.
We have found substantial increase in peptide excretion in severe depression. To our surprise the level was found to be decreased in treated patients. Increase in peptides especially in urine is usually due to peptidase deficiency or inhibition [13, 14]. The presented data indicate that severely depressed patients show hyper-peptiduria as would be expected if peptidase defects were present. Peptidase deficiencies have been found in depression and melancholic states [15–19]. Because the level and pattern of peptides differ, and differing chain lengths of peptides with the same activities are found, this points to a heterogeneity of genetics. Different families may have a different sets of dysfunctional enzymes probably peptidases or peptidase binding proteins. This may explain the difficulty of pinpointing a specific gene as the genetic disposition in depression. We suggest that we have depression causing genes which may cause formation of different but overlapping peptidases or peptidase regulating proteins, and that the mediators of depression may be peptides regulating the uptake and release of different transmitters. Since antidepressant medication reduced the level of peptides (table 1) this could be due to peptidase induction as has been shown for neuroleptic medication. It remains to be seen if peptide level correlates with the degree of depression. The presence of opioids may explain the psychotic features of our patients because opioids have been shown to cause increased dopamine in the synaptic cleft by inhibiting reuptake  An exogenous supply of peptides may also explain the often seen and peculiar fluctuating course of the disorder with morning worsening and afternoon a relative high or agitation. Increased reuptake of serotonin into platelets may have relevance to the reported changes in serotonin transport into platelets in depression [21, 22]. Both the tri-cyclic antidepressants and selective serotonin re-uptake inhibitors have the opposite effect on serotonin uptake. We have previously found the tri-peptide but amidated, in autistic patients' urine and this tri-peptide stimulated the serotonin transporter mediated uptake of serotonin in hamster ovarial cells transfected with the human serotonin transporter gene . That tri-peptide doubled the serotonin content of platelets when injected into pups subcutaneously . 5 HT 2a receptors are increased in brain tissue and platelets in depression  which would agree with decreased levels in the synaptic cleft due to stimulated uptake. Also the decreased 5HIAA (5-hydroxy indole-acetic acid) in suicidal patients fit an increased uptake . The tri-peptide sequence has only been found in reelin, which is a matrix proteinase . That food derived peptides are taken up has been demonstrated [28, 29] and is increased by peptidase defects . Depression has been found in cases of coeliac disease [31, 32] indicating that such a mechanism is not unreasonable. Considerable differences in rates in different cultures may also thus be explainable. Other groups have found increases in some peptides in depression. Thus TRH (pE-H-P-NH2) has been found increased in CSF [33, 34]; beta-endorphin  and an unspecified opioid fraction 1 measured by receptor binding [36, 37]. Substance P was also found increased in CSF , and delta sleep factor increased in plasma [39, 40]. Also plasma arginine vasopressin increase in depression was inversely related to daytime motor activity . Furthermore post partum psychosis may be mediated by different human caso-morphines  and shows depressive traits. Since peptides in general are excellent peptidase inhibitors  and peptides also have strong tendencies to form complexes [44, 45] and bind to other molecules and membranes [46, 47], a rather complex situation with varied results can be envisioned. Bell shaped dose responses are common to many peptides and add to this complicated picture . The nature of some of these peptides is shared with schizophrenia and may constitute the common features of these disorders . Possibly relevant to a gut-brain axis in depression is epidemiological data showing very high frequency of depression in irritable bowel syndrome [49–51]. Future work is needed to elucidate if there is a correlation of peptide increase and degree of depression and to look for any correlations between individual peptide peaks and specific symptoms of this disorder.
In severe cases of hospitalized patients with depression, we find peptide increase in urine. After treatment the level is decreased. Some of these peptides affect serotonin uptake and others show exorphine like characteristics. Our study is based on a small number of patients due to problems of getting untreated ones. We do not know how general or specific these findings are to the depression spectrum. However, our data are compatible with reported decreased peptidase levels found by others in depression. If correct the profound heterogeneity casts doubt on most double blind treatment trials in depression.
We wish to thank Major Ecbo's Foundation and Haldis and Josef Andresens foundation for support.
- Allan MG: Twin studies of affective illness. Arch Gen Psychiatry. 1976, 33: 1476-1489.View ArticleGoogle Scholar
- Bertelsen A, Harvald B, Hauge M: A Danish twin study of manic-depressive disorders. Br J Psychiatry. 1977, 130: 330-351.View ArticlePubMedGoogle Scholar
- Cavoret RJ, O'Gorman TW, Heywood EC, Troughton E: Genetic and environmental factors in major depression. J Affect Disord. 1985, 9: 155-164. 10.1016/0165-0327(85)90095-3.View ArticleGoogle Scholar
- Wender PH, Kety SS, Rosenthal D, Schulsinger F, Ortmann J, Lunde I: Psychiatric Disorders in the Biological and adoptive families of adopted individuals with affective disorders. Arch Gen Psychiatry. 1986, 43: 923-929.View ArticlePubMedGoogle Scholar
- Sælid G, Haug JO, Heiberg T, Reichelt KL: Peptide containing fractions in depression. Biol Psychiatry. 1985, 20: 245-256. 10.1016/0006-3223(85)90054-X.View ArticlePubMedGoogle Scholar
- Reichelt KL, Edminson PD, Toft KG: Urinary peptides in schizophrenia and depression. Stress Med. 1984, 1: 169-181. 10.1002/smi.2460010307.View ArticleGoogle Scholar
- Pedersen OS, Liu Y, Reichelt KL: Serotonin uptake stimulating peptide found in plasma of normal individuals and in some autistic urines. J Pept Res. 1999, 53: 641-646. 10.1034/j.1399-3011.1999.00058.x.View ArticlePubMedGoogle Scholar
- Reichelt WH, Reichelt KL: The possible role of peptides derived from food proteins in diseases of the nervous system. Epilepsy and other Neurological Disorders in Coeliac Disease. Edited by: Gobbi G. 1997, London: John Libbey & Comp Ltd, 225-235.Google Scholar
- Reichelt WH, Ek J, Stensrud MB, Reichelt KL: Peptide excretion in celiac disease. J Pediatr Gastroenterol Nutr. 1998, 26: 305-309. 10.1097/00005176-199803000-00012.View ArticlePubMedGoogle Scholar
- Lingjaerde O: Uptake of serotonin in blood platelets: dependence on sodium and chloride, and inhibition by choline. FEBS Lett. 1969, 3: 103-106. 10.1016/0014-5793(69)80108-0.View ArticlePubMedGoogle Scholar
- Böhlen P, Castillo F, Ling R, Guillemin R: An efficient procedure for the separation of peptides from amino acids and salts. Int J Pept Protein Res. 1980, 16: 306-310.View ArticlePubMedGoogle Scholar
- Calabrese EJ, Baldwin LA: Hormesis: U-shaped dose responses and their centrality in toxicology. Trends Pharmacol Sci. 2001, 22: 285-291. 10.1016/S0165-6147(00)01719-3.View ArticlePubMedGoogle Scholar
- Abassi Z, Golomb E, Keiser HR: Neutral endopeptidase inhibition increases urinary excretion and plasma level of Endothelin. Metabolism. 1992, 41: 683-685. 10.1016/0026-0495(92)90303-R.View ArticlePubMedGoogle Scholar
- Watanabe Y, Kojima-Kumatsu T, Iwaki-Egewa S, Fujimoto Y: Increased excretion of proline-containing peptides in dipeptidyl-peptidase IV deficient rats. Res Commun Chem Pathol Pharmacol. 1993, 81: 323-350.PubMedGoogle Scholar
- Maes M, Demeester I, Vanhoof G, Scharpe S, Bosmans E, Vandervorst C, Verkerk R, Minner B, Suy E, Raus J: Decreased serum dipeptidyl-peptidase IV activity in major depression. Biol Psychiatry. 1991, 30: 577-586. 10.1016/0006-3223(91)90027-J.View ArticlePubMedGoogle Scholar
- Maes M, Scharpe S, Meltzer HY, Suy E, Cosyns P, Calabrese J: Lower angiotensin 1 converting enzyme activity in melancholic subjects: a pilot study. Biol Psychiatry. 1992, 32: 621-624. 10.1016/0006-3223(92)90075-B.View ArticlePubMedGoogle Scholar
- Maes M, Goosens F, Scharpe S, Meltzer HY, D'Hondt P, Cosyns P: Lower serum prolyl-endopeptidase enzyme activity in major depression: further evidence that peptidases play a role in patho-physiology of depression. Biol Psychiatry. 1994, 35: 545-552. 10.1016/0006-3223(94)90101-5.View ArticlePubMedGoogle Scholar
- Maes M, DeMeester I, Verkerk R, DeMedts P, Wauters A, Vanhoof G, Vandoolaeghe E, Neels H, Scharpe S: Lower serum dipeptidyl peptidase IV activity in treatment resistant major depression: relationships with immune-inflammatory markers. Psychoneuroendocrinology. 1997, 22: 65-78. 10.1016/S0306-4530(96)00040-6.View ArticlePubMedGoogle Scholar
- Elgun S, Keskinege A, Kumbassar H: Dipeptidyl peptidase IV and adenosine deaminase activity. Decrease in depression. Psychoneuroendocrinology. 1999, 24: 823-832. 10.1016/S0306-4530(99)00039-6.View ArticlePubMedGoogle Scholar
- Hole K, Bergslien AA, Jørgensen H, Berge O-G, Reichelt KL, Trygstad OE: A peptide containing fraction from schizophrenia which stimulates opiate receptors and inhibits dopamine uptake. Neuroscience. 1979, 4: 1139-1147. 10.1016/0306-4522(79)90062-9.View ArticleGoogle Scholar
- Paul SM, Rehavi M, Skolnic P, Ballenger JC, Goodwin FK: Depressed patients have decreased binding of tritiated imipramine to platelet "tansporter". Arch Gen Psychiatry. 1981, 38: 1315-1317.View ArticlePubMedGoogle Scholar
- Oxenkrug GF: The content and uptake of 5-HT by blood platelets in depressive patients. J Neural Transm. 1979, 45: 285-289. 10.1007/BF01247145.View ArticlePubMedGoogle Scholar
- Keller F: 2nd annual report to EU Commission project BMH4-CT 96-0730. 1998, 1-10.Google Scholar
- Persico AM, Baldi A, Reichelt KL, Gonzales A, Keller F: Serotonin uptake-stimulating peptides extracted for urines of autistic patients: potential significance for the pathogenesis of autistic disorders. Amer Neurosci. Meet Los Angeles. 1998, Abs no 2Google Scholar
- Pandey GN: Altered serotonin function in suicide. Ann N Y Acad Sci. 1997, 836: 182-200. 10.1111/j.1749-6632.1997.tb52360.x.View ArticlePubMedGoogle Scholar
- Åsberg ML, Träskman L, Thoren P: 5-HIAA in the cerebrospinal fluid: A biochemical suicide predictor. Arch Gen Psychiatry. 1976, 38: 1193-1197.View ArticleGoogle Scholar
- Quattrocchi CC, Wannenes F, Persico AM, Ciafre SA, d'Arcangelo G, Farace MG, Keller F: Reelin is a Serine Protease of the Extracellular Matrix. J Biol Chem. 2002, 277: 303-309. 10.1074/jbc.M106996200.View ArticlePubMedGoogle Scholar
- Chabance B, Marteau P, Rambaud JC, Migliore-Samour D, Boynard M, Perrotin P, Guillet R, Jolles P, Fiat AM: Casein peptide release and passage to the blood in humans during digestion of milk and yogurt. Biochimie. 1998, 80: 155-165. 10.1016/S0300-9084(98)80022-9.View ArticlePubMedGoogle Scholar
- Gardner MLG: Absorption of intact proteins and peptides. Physiology of the Gastrointestinal Tract. Edited by: Johnson LR. 1994, New York: Raven Press, 1795-1782. 3Google Scholar
- Mahe S, Tome D, Dumontier AM, Desjeux JF: Absorption of intact morphiceptin by diisopropylfluorophosphate-treated rabbit ileum. Peptides. 1989, 10: 45-52. 10.1016/0196-9781(89)90074-0.View ArticlePubMedGoogle Scholar
- Hallert C, Åstrøm J, Sedvall G: Psychic disturbances in adult celiac disease III. Reduced central momoamine metabolism and signs of depression. Scand J Gastroenterol. 1982, 17: 25-28.View ArticlePubMedGoogle Scholar
- Corvaglia L, Catamo R, Pepe G, Lazzari R, Corvaglia E: Depression in adult untreated celiac subjects: diagnosed by the pediatrician. Am J Gastroenterol. 1999, 94: 839-843. 10.1111/j.1572-0241.1999.00956.x.View ArticlePubMedGoogle Scholar
- Kirkegaard C, Faber J, Hummer L, Rogowski P: Increased levels of TRH in cerebrospinal fluid from patients with endogenous depression. Psychoneuroendocrinology. 1979, 4: 227-235. 10.1016/0306-4530(79)90006-4.View ArticlePubMedGoogle Scholar
- Banki CM, Bissette G, Arato M, Nemeroff CB: Elevation of Immunoreactive CSF TRH in depressed patients. Am J Psychiatry. 1988, 145: 1526-1531.View ArticlePubMedGoogle Scholar
- Darko DF, Risch SC, Gillin JC, Golshan S: Association of beta-endorphin with specific clinical symptoms of depression. Am J Psychiatry. 1992, 149: 1162-1167.View ArticlePubMedGoogle Scholar
- Agren H, Terenius L: Depression and CSF endorphin fraction 1: seasonal variation and higher levels in unipolar than bipolar patients. Psychiatry Res. 1983, 10: 303-311. 10.1016/0165-1781(83)90077-X.View ArticlePubMedGoogle Scholar
- Terenius L, Wahlstrom A, Agren T: Naloxone (narcan)treatment in depression: Clinical observations and effects on CSF endorphins and monoamine metabolites. Psychopharmacology (Berl). 1997, 54: 31-33. 10.1007/BF00426537.View ArticleGoogle Scholar
- Rimon R, Le Greves P, Nyberg F, Heikkila L, Salmela L, Terenius L: Elevation of substance P-like peptides in the CSF of psychiatric patients. Biol Psychiatry. 1984, 19: 509-516.PubMedGoogle Scholar
- Westrin A, Ekman R, Träskman-Bendz L: High delta sleep-inducing peptide-like immuno-reactivity in plasma in suicidal patients with major depressive disorder. Biol Psychiatry. 1998, 43: 734-739. 10.1016/S0006-3223(97)00254-0.View ArticlePubMedGoogle Scholar
- Westrin Å, Engström G, Ekman R, Träskman-Bendz L: Correlations between plasma-neuropeptides and temperament dimensions differ between suicidal patients and healthy controls. J Affect Disord. 1998, 49: 45-54. 10.1016/S0165-0327(97)00197-3.View ArticlePubMedGoogle Scholar
- Van Londen L, Kerkhof GA, Van den Berg F, Goekoop JG, Zwinderman KH, Frankhuijzen-Siervogel AC: Plasma arginine vasopressin and motor activity in major depression. Biol Psychiatry. 1998, 43: 196-204. 10.1016/S0006-3223(97)80433-7.View ArticlePubMedGoogle Scholar
- Lindstrøm LH, Nyberg F, Terenius , Bauer K, Besev G, Gunne LM, Lyrenaas S, Wildeck-Lund G, Lundberg B: CSF and plasma beta-casomorphin like opioid peptides in post-partum psychosis. Am J Psychiatry. 1984, 141: 1059-1066.View ArticlePubMedGoogle Scholar
- LaBella FL, Geiger JD, Glavin G: Administration of peptides inhibits the degradation of endogenous peptides. The dilemma of distinguishing direct from indirect effects. Peptides. 1985, 6: 645-660. 10.1016/0196-9781(85)90168-8.View ArticlePubMedGoogle Scholar
- Burhol K, Jensen TG, Florholmen TG, Jorde H, Vonen B, Olsen R: Protein-binding and aggregation of somatostatin in human plasma. Ital J Gastroenterol. 1966, 18: 1-6.Google Scholar
- Kastin AJ, Casillanos PF, Fischman PJ, Profitt JK, Graf MV: Evidence for peptide aggregates. Pharmacol Biochem Behav. 1984, 21: 969-974.View ArticlePubMedGoogle Scholar
- Rocetti G, Venturella F, Roda IG: Enkephalin binding system in human plasma III Comparative protection of different peptides. Neurochem Res. 1988, 13: 221-224. 10.1007/BF00971536.View ArticleGoogle Scholar
- Meneszo NY, Khatchaturian C: Peptides bound to albumin. Life Sci. 1986, 39:Google Scholar
- Boteva K, Lieberman J: Reconsidering the classification of schizophrenia and manic-depressive illness-a critical analysis and new conceptual model. World J Biol Psychiatry. 2003, 4: 81-92.View ArticlePubMedGoogle Scholar
- Addolorato G, Capristo E, Stefanini GG, Gasbarrini G: Inflammatory bowel disease: a study of the association between anxiety and depression, physical morbidity, and nutritional status. Scand J Gastroenterol. 1997, 32: 1013-1021.View ArticlePubMedGoogle Scholar
- Haug TT, Mykletun A, Dahl AA: Are anxiety and depression related to gastrointestinal symptoms in the general population?. Scand J Gastroenterol. 2005, 37: 294-298. 10.1080/003655202317284192.View ArticleGoogle Scholar
- Alander T, Svärdsudd K, Johansson SE, Agreus L: Psychological illness is commonly associated with functional gastrointestinal disorders and is important to consider during patient consultation: a population-based study. BMC Med. 2005, 3: 8-20. 10.1186/1741-7015-3-8.PubMed CentralView ArticlePubMedGoogle Scholar
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