Ongoing episode of major depressive disorder is not associated with elevated plasma levels of kynurenine pathway markers
Introduction
The etiology of major depressive disorder (MDD) is regarded as multi-causal (Belmaker and Agam, 2008), and the pathophysiology still remains elusive. A connection between depression and alterations in the immune system has been found in numerous studies (Smith, 1991, Maes et al., 1993, Sluzewska et al., 1996, Dantzer and Kelley, 2007, Kim et al., 2007, Leonard and Maes, 2012). The concept that inflammatory factors could cause depression was supported by findings that recombinant human cytokines led to depression in 30–45% of patients in clinical trials of various cancers and viral hepatitis (Miyaoka et al., 1999, Musselman et al., 2001). The causality seems clear in depression induced by cytokine therapy, and experimental studies have revealed detailed information about the interplay between the immune system and the central nervous system (CNS) (Dantzer et al., 2011). In a recent study we showed that plasma concentrations of a range of cytokines, including pro- and anti-inflammatory cytokines, and T-helper (Th)1 and Th2 cytokines, were elevated during an MDD depressive episode, but normalized after recovery (Dahl et al., 2014).
The development of depressive symptoms upon interferon therapy is suggested to be caused by activation of the enzyme indoleamine 2,3-dioxygenase (IDO) (Myint and Kim, 2014), as this enzyme is known to be stimulated by proinflammatory cytokines (Capuron et al., 2003, Wichers et al., 2005). IDO converts the essential amino acid tryptophan (TRP) into kynurenine (KYN), which is then further metabolized along the kynurenine pathway. TRP is also the precursor for serotonin (5-HT), a neurotransmitter regarded as essential for regulation of mood (Hamon and Blier, 2013). Therefore, in states of inflammation, TRP is being catabolized via the kynurenine pathway rather than being used for serotonin production. The degradation of TRP can also be carried out by the enzyme tryptophan 2,3-dioxygenase (TDO), which is stimulated by cortisol. Together IDO and TDO regulate the first step of TRP degradation. KYN is further catabolized to kynurenic acid (KA), or alternatively via a separate branch of the pathway to 3-hydroxy kynurenine (3-OH KYN) and other metabolites, including quinolinic acid (QA), and eventually on to the final product of the pathway, nicotinamide adenine dinucleotide (NAD).
It has been suggested that increasing kynurenine pathway activity may lead to depression in different ways. First, it may decrease the availability of TRP for serotonin synthesis (Capuron et al., 2003), and reduced activity in the serotonin system may subsequently lead to depressive symptoms (Neumeister et al., 2004). Furthermore, the TRP catabolites (TRYCATs) formed along the KYN pathway are able to cause neurotoxicity and regulate glutamate neurotransmission. The formation of 3-OH KYN leads to the production of reactive oxygen species and initiates neuronal apoptosis (Okuda et al., 1998, Stone et al., 2001). The metabolites QA and KA both bind to the N-methyl-d-aspartate (NMDA) receptor; QA is an agonist (Stone, 1993, de Carvalho et al., 1996), while KA is an antagonist (Stone, 1993, Hilmas et al., 2001). Interestingly, it has been shown that ketamine – acting as an NMDA receptor antagonist and thus having the potential to interfere with the KYN pathway activity – is an effective therapy against treatment-resistant depression (Zarate et al., 2006) and suicidality (Price et al., 2009). A recent animal model study confirmed that peripheral inflammation induces KYN pathway activity in the brain, and that the resulting depressive-like effects can be specifically counteracted by ketamine (Walker et al., 2013).
The KYN/TRP ratio reflects the activity of the initial enzymes of the pathway (i.e., IDO and TDO), and is increased by inflammatory cytokines. Measures of other metabolites produced by the kynurenine pathway and their ratios reflect the activity of other subsequent enzymes, as well as the balance in activity between the two main branches of the pathway (Ormstad et al., 2013). TRP has to compete with certain amino acids, the so-called competing amino acids (CAAs), for the transport over the blood–brain barrier (BBB). Thus, the index [(tryptophan titer × 100)/CAAs titer], the so-called TRP index, can be used as a measure of TRP availability in the brain (Wichers et al., 2005). Moreover, since TRP availability is a limiting factor in the synthesis of 5-HT, the TRP index is also an indicator of 5-HT production in the brain; this could be relevant to the development of depressive symptoms.
It is well established that the kynurenine pathway markers in the blood are increased in cytokine-therapy-induced depression (Capuron et al., 2003, Wichers et al., 2005). Similarly, increased kynurenine pathway activation has been observed in suicidal patients, both in peripheral blood (Sublette et al., 2011) and in the cerebrospinal fluid (CSF) (Erhardt et al., 2013, Bay-Richter et al., 2014). Sublette et al. (2011) found an increased KYN/TRP ratio, which is indicative of an activated pathway, in the peripheral blood of suicide attempters, while Erhardt et al. (2013) and Bay-Richter et al. (2014) reported increased levels of the NMDA-receptor agonist QA in the CSF of suicide attempters.
There are some inconsistent previous reports regarding the kynurenine pathway activity in the blood of patients suffering from MDD. Gabbay et al. (2010) found that the activity of this pathway was elevated in patients with MDD and melancholia, but not elevated in MDD without melancholia. Myint et al. (2007) found that MDD patients had a higher KYN/TRP ratio than normal controls. However, after six weeks, as the depression rating scores were significantly reduced relative to baseline, the kynurenine pathway activity showed a further significant elevation. In a recent study, Myint et al. (2013) reiterated these findings. Hughes et al. (2012) found in MDD that the KYN/TRP ratio was elevated independent of kynurenine pathway activation, based on the findings that the plasma TRP concentration was reduced but the mRNA levels of IDO1 and IDO2 mRNA were not elevated. There are also inconsistent reports from cohort studies. Elovainio et al. (2012) found that IDO activity may be a risk factor for future depression, while Quak et al. (2014) found no indication that the relationship between inflammation and depressive symptoms is mediated by TRP degradation, as evidenced by the KYN/TRP ratio.
These inconsistencies could be due to low sample size, different recruitment procedures and thus patient characteristics, lack of longitudinal design, or patients not being drug-naive at the start of treatment. Thus, we here studied kynurenine pathway metabolites in a representative MDD outpatient sample (which excluded patients with bipolar features) who were drug-free at start of treatment and were known to have increased levels of cytokines (Dahl et al., 2014) The study had a 12-week follow-up period.
Our aim was to determine whether the kynurenine pathway activity in peripheral blood is elevated in patients with an ongoing MDD depressive episode, as assessed by the KYN/TRP ratio. Further we investigated whether the ratios of key neuroactive metabolites (KYN/KA and QA/KA) were altered and whether TRP was depleted (as shown by a reduced TRP index). Lastly, we studied the relationship of kynurenine pathway markers to the cytokine changes previously found in the same cohort. A follow-up design was used to increase inference about the potential role of the kynurenine pathway in patients with MDD depressive episode.
Section snippets
Study design
At baseline, clinical data were collected and blood was drawn for the analysis of TRP, TRP catabolites, and CAAs from 50 MDD patients who were free of psychotropic and immune-modulating medications and from 34 healthy controls. The patients then received 12 weeks of “treatment as usual”, after which clinical data collection and blood sampling for analysis were repeated. Of the 50 patients who were enrolled, 43 completed the follow-up part of the study; the 7 dropouts were due to death (n = 1) and
No increase in KYN pathway activation in MDD patients
The plasma levels of TRP, KYN, KA, QA, CAAs and the ratios KYN/TRYP, KYN/KA, QA/KA and TRP/CAAs were not significantly different in depressed patients compared to healthy controls at baseline. These data are presented in Table 2.
Melancholic depression and recurrent depression
Thirty-eight of the patients (76%) had melancholic depression at baseline. The baseline levels of TRP, KYN, KA, QA and CAAs, as well as the ratios of KYN/TRYP, KYN/KA, QA/KA, and TRP index, did not differ significantly between patients with melancholic and
Discussion
Elevated kynurenine pathway activity may play a causative role in the development of depression (Leonard, 2010, Chopra et al., 2011, Dantzer et al., 2011, Maes et al., 2011). This pathway has thus far been studied mainly in animal models and in relation to cytokine-therapy-induced depression (Dantzer et al., 2011; Leonard and Maes, 2012). It is important to understand whether this biological pathway is generally activated or dysregulated in patients with depressive symptoms, or whether it
Role of the funding source
The study is fully funded by Ringerike Psychiatric Center and Vestre Viken Hospital Trust, Norway.
Conflict of interest statement
The authors declare no conflict of interest.
Acknowledgements
The authors would like to thank Vestre Viken Hospital Trust and Ringerike Psychiatric Center for funding the study. We would also like to thank the South-Eastern Norway Regional Health Authority's (HSØ) Norwegian Research Network on Mood Disorders (NORMOOD) for initiating the study, and Tove Hæreid Otterstad for her coordination of the NORMOOD project. Further, we are indebted to Anne Skjerstein for taking care of the blood samples and the logistics. Finally, we express our gratitude to the
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