An Assessment of Cognitive Function in Irritable Bowel Syndrome

This study is based on the hypothesis that the disruption of this brain-gut axis in irritable bowel syndrome (IBS) is a consequence of increased degradation of tryptophan along the kynurenine pathway. The investigators aim to fully characterize this putative metabolic abnormality and determine its impact on both gastrointestinal (GI) and central nervous system (CNS) signaling by examining the relationship between individual pathway metabolites, GI symptoms and cognitive processing in IBS patients

An increased degradation of tryptophan along the kynurenine pathway has been reported in both depression and IBS (Clarke et al 2009a; Fitzgerald et al 2008; Myint et al 2007). Although such studies have suggested an alteration in the production of quinolinic acid as a consequence of this disruption, actual levels of this NMDA receptor agonist remain to be measured and are essential to the full characterization of pathway disruption. The relevance of this strategy is confirmed by studies that have implicated peripheral quinolinic acid measures as surrogate marker of disease activity in juvenile idiopathic inflammatory myopathies (Rider et al 2002). Moreover it has been demonstrated that increasing plasma levels of this neurotoxic metabolite can influence CNS processes (Yan et al 2005) and that an increased peripheral production of kynurenine can increase central quinolinic acid concentrations (Raison et al 2009a). A correlation between alterations in the kynurenine pathway, GI disturbances and cognitive outcomes remains to be fully defined. In this study, the investigators propose to obtain a complete profile of plasma kynurenine pathway metabolites in IBS patients with and without comorbid depression. Temporal instability in symptom profile is a hallmark of IBS and represents a considerable obstacle to biomarker discovery (Clarke et al 2009b). The preliminary data generated here will potentially be used as the basis for future grant applications that will propose a longitudinal study of these putative biomarker candidates.

BACKGROUND/SIGNIFICANCE

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder accounting for up to 20% of cases presenting at gastroenterologist clinics in the western world with an unexplained female predominance (Ersryd et al 2007). Although the typical symptoms are commonly experienced in the general population, the abdominal pain and disturbances in defecatory function experienced by IBS sufferers are on a scale that positions the disorder as one of the leading causes of work absenteeism and presenteeism in western societies. Clearly the burden it places on the individual sufferer and society as a whole is substantial and is further compounded by its draining of healthcare resources (Clarke et al 2009b).

Despite the widespread prevalence of the disorder is it still poorly understood and is largely characterized by the lack of a reliable validated biological marker. This is reflected in the reliance on symptom based diagnostic criteria in conjunction with the exclusion of other gastrointestinal disorders (Drossman 2006). Further complications arise in the form of the psychiatric comorbidity so frequently observed among the IBS population (Spiller 2004). It is perhaps unsurprising then that treatment options are inadequate and advances in our understanding of the disorder are urgently required to address the unmet medical needs of its sufferers.

Tryptophan is an essential amino acid and its role as a precursor to many biologically active agents ensures its importance in the consideration of a variety of disease states including schizophrenia (Barry et al 2009; Schwarcz and Pellicciari 2002), depression (Myint et al 2007) and inflammatory bowel disease (Forrest et al 2003). Although much of the attention to date has focused on the production of serotonin from tryptophan, the often-overlooked kynurenine metabolic pathway (Appendix figure 1) consumes over 95% of the available peripheral tryptophan in mammals (Clarke et al 2009a). The first step in this cascade involves the conversion of tryptophan to kynurenine with reports suggesting that the majority of CNS concentrations of the metabolite are drawn from the periphery (Myint et al 2007). This is an important consideration and validates the use of plasma tryptophan metabolites in biomarker studies. This first rate limiting step in the metabolic cascade is catalysed by either the ubiquitous indoleamine-2,3-dioxygenase (IDO) or the largely hepatic based tryptophan 2,3-dioxygenase (TDO). Crucially in the context of stress related disorders like IBS and depression, the activity of both enzymes can be induced by agents which are stress responsive-IDO by inflammatory mediators and TDO by corticosteroids (Ruddick et al 2006).

Once formed, kynurenine essentially participates in two important but alternate metabolic scenarios-one leading to the formation of the neuroprotective kynurenic acid and the other leading to the production of the neurotoxic quinolinic acid (Ruddick et al 2006). The neuroprotective effects of kynurenic acid are attributed to its competitive blockade of the NMDA receptor at the glycine co-agonist site. Conversely quinolinic acid has excititoxic properties due to its potent activation of the NR2A and NR2B NMDA receptor subunits (Schwarcz and Pellicciari 2002). Clearly any imbalance in the production of these bioactive ligands could lead to profound disturbances in CNS glutamatergic signaling and thus affect glutamate-driven behaviours such as learning and memory.

Recent progress in the study of IBS has largely been brought about by a theoretical view of the condition as a disorder of the brain-gut axis (Ohman and Simren 2007). Within this construct, stressful insults are now thought to be integral to the destabilization of normal brain-gut axis signaling. Higher brain centres are involved both in the processing of afferent GI signals and the communication of changes in an individuals emotional state to the gastrointestinal tract (Mayer 2000a; Mayer 2000b). The emotional motor system, consisting of the limbic system and paralimbic structures such as the medial prefrontal cortex, amygdala and hypothalamus, is one such centre. The importance of these structures in the maintenance of normal cognitive function implies that brain-gut axis perturbations could manifest as cognitive impairment across a number of domains.

Other lines of evidence also support an altered neuropsychological state in IBS. An emerging body of research points to the presence of a low grade inflammation in this disorder (Barbara and Stanghellini 2009; Clarke et al 2009b). It has recently been reported that baseline interleukin-6 levels, the cytokine that is most consistently reported to be elevated in IBS (Clarke et al 2009b), is predictive of cognitive symptoms of depression at follow up in a longitudinal study (Gimeno et al 2009). It has also been demonstrated that patients with chronic fatigue syndrome, a disorder that has also been linked with increased inflammatory mediators (Raison et al 2009b), have associated cognitive deficits (Thomas and Smith 2009). Previous studies from our laboratory have highlighted the relationship between the inflammatory profile in IBS and an increased degradation of tryptophan along the kynurenine pathway (Clarke et al 2009a; Fitzgerald et al 2008). Moreover, cancer patients treated by cytokine immunotherapy experience related and persistent cognitive alterations (Capuron et al 2001) and the cognitive alterations accompanying infection are also present in apyretic sick subjects (Capuron et al 1999). The increased tryptophan degradation observed following this immunotherapy is considered to be a key mediator of the resulting neuropsychological alterations (Capuron et al 2002; Raison et al 2009a). Cumulatively, these studies suggest that IBS is a disorder possessing a molecular signature (increased, inflammation and disturbed tryptophan metabolism) indicative of cognitive deficits.

However, despite the uncovering of this molecular signature in IBS, the widespread acceptance of the brain-gut axis model and the high psychiatric comorbidity, a comprehensive cognitive profiling of the IBS patient population has not been performed. The cognitive assessments that have been reported support a cognitive phenotype in the disorder but to date have focused on singular aspects of affective memory performance (Kilkens et al 2005; Kilkens et al 2004) or verbal IQ deficits (Dancey et al 2009). Moreover, there has been, to our knowledge no study correlating cognitive function with mechanistically-oriented biomarkers.

Before the benefits of such a correlation can be accrued however, a more complete assessment of cognitive dysfunction in IBS is urgently required. The Cambridge Neuropsychological Test Automated Battery (CANTAB) is a computerized cognitive assessment program that covers within its remit a broad section of cognitive domains (Fray and Robbins 1996). The battery has been validated in the assessment of cognitive dysfunction across a wide range of disease states including psychiatric disorders such as anxiety (Kaplan et al 2006) and both unipolar and bipolar mood disorders (Elliott et al 1996; Sweeney et al 2000). Insights gained from neuroimaging studies in IBS which highlight the higher brain centres functioning abnormally in the syndrome (Mayer et al., 2008) can be used to inform the initial choice of tests to be applied to IBS subjects. Investigations in other disorders that are reported to have an inflammatory component such as chronic fatigue disorder where assessments of free recall, reaction time and vigilance in conjunction with the Stroop colour-word interference task to measure distraction from irrelevant stimuli have yielded interesting results (Thomas and Smith 2009). The specific executive function assessments which can be applied include the Stocking of Cambridge Task (SOC) which assesses a subject's ability to rearrange a set of balls in a specified minimum number of moves and minimum time and the intradimensional/extradimensional (ID/ED) attentional set shifting task (adapted from the Wisconcin Card Sorting Test (WCST)) used to assess a subjects ability to maintain attention. CANTAB visual memory tasks such as the spatial recognition memory task (SRM) and the paired associates learning task (PAL) can be used to assess deficits in memory (Michopoulos et al 2008). Taken together, these tests are sensitive to dysfunction across a number of different brain regions (Sweeney et al 2000). Recently the cognitive assessments available in the CANTAB battery have been used in conjunction with measures of clinical insight to assess the association between poor insight and cognitive impairment in schizophrenia (Donohoe et al 2009). This may also be relevant to IBS where an increased awareness of and concern about bodily sensations and GI functioning is well established (Posserud et al 2009). It will also be important to rule out alternative explanations for any alterations that are observed included task irrelevant thoughts related to the disease. Few attempts have been made to link such findings to GI symptoms and putative biological marker candidates.