The role of anthranilic acid in the increase of depressive symptoms and major depressive disorder during treatment for hepatitis C with pegylated interferon-α2a and oral ribavirin

BACKGROUND

Tryptophan metabolism via the kynurenine pathway is considered the link between the immune and endocrine systems. Dysregulation of serotonergic transmission can stem from the direct influence of interferon-α on the activity of serotonergic receptors 5-HT1A and 5-HT2A, and from its indirect effect on tryptophan metabolism. Induction of the kynurenine pathway increases the concentration of neurotoxic kynurenine metabolites, and the activity of kynurenine derivatives is linked to the onset of depression. The aim of our study was to evaluate the relationships between depressive symptoms and kynurenine, tryptophan, anthranilic acid and kynurenic acid concentrations, indolamine 2,3-dioxygenase (IDO) activity and tryptophan availability to the brain.

METHODS

The study followed a prospective longitudinal cohort design. We evaluated 101 patients with chronic hepatitis C who were treated with pegylated interferon-α2a, and 40 controls who were awaiting treatment. We evaluated the relationships between total score on the Montgomery-Åsberg Depression Rating Scale and kynurenine, tryptophan, anthranilic acid and kynurenic acid concentrations, IDO activity and tryptophan availability to the brain. A logistic regression model was adapted for the diagnosis of major depressive disorder at each time point, taking into account changes in parameters of the kynurenine pathway between a given time point and the baseline measurement.

RESULTS

Of the treated patients, 44% fulfilled the criteria for major depressive disorder at least once during the 24 weeks of treatment. Anthranilic acid concentrations were significantly increased compared to baseline for all time points except week 2. Tryptophan availability showed a significant decrease (β = -0.09, p = 0.01) only in week 12 of treatment. Over time, kynurenine, tryptophan and anthranilic acid concentrations, as well as IDO activity and tryptophan availability to the brain, were significantly associated with total score on the Montgomery-Åsberg Depression Rating Scale. A logistic regression model revealed that participants with decreased tryptophan availability to the brain at 12 weeks of treatment and participants with increased anthranilic acid concentrations at week 24 of treatment were at increased risk for diagnosis of major depressive disorder (odds ratios 2.92 and 3.59, respectively).

LIMITATIONS

This study had an open-label design in a population receiving naturalistic treatment.

CONCLUSION

The present study provides the first direct evidence of the role of anthranilic acid in the pathogenesis of inflammation-induced major depressive disorder during treatment for hepatitis C with pegylated interferon-α2a.

Kynurenine pathway metabolism and the neurobiology of treatment-resistant depression: Comparison of multiple ketamine infusions and electroconvulsive therapy

Current first-line antidepressants can take weeks or months to decrease depressive symptoms. Low dose ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, shows potential for a more rapid antidepressant effect, with efficacy also evident in previously treatment-resistant populations. However, a greater understanding of the physiological mechanisms underlying such effects is required. We assessed the potential impact of ketamine infusion on neurobiological drivers of kynurenine pathway metabolism in major depression (HPA axis hyperactivity, inflammation) in patients with treatment-resistant depression compared to gender-matched healthy controls. Furthermore, we assessed these biomarkers before and after electroconvulsive therapy (ECT), which is currently the gold standard for management of treatment-resistant depression. As previously demonstrated, treatment with ketamine and ECT was associated with improved depressive symptoms in patients. At baseline, waking cortisol output was greater in the ECT cohort, kynurenine was greater in the ketamine cohort, and kynurenic acid was lower in patients compared to healthy controls, although inflammatory markers (IL-6, IL-8, IL-10 or IFN-γ) were similar in patients and controls. Furthermore, in patients who responded to ECT, the cortisol awakening response was decreased following treatment. Despite a trend towards reduced kynurenine concentrations in those who responded to ketamine, ketamine was not associated with significant alterations in any of the biomarkers assessed.

Zonisamide regulates basal ganglia transmission via astroglial kynurenine pathway

To clarify the anti-parkinsonian mechanisms of action of zonisamide (ZNS), we determined the effects of ZNS on tripartite synaptic transmission associated with kynurenine (KYN) pathway (KP) in cultured astrocytes, and transmission in both direct and indirect pathways of basal ganglia using microdialysis. Interactions between cytokines [interferon-γ (IFNγ) and tumor-necrosis factor-α (TNFα)] and ZNS on astroglial releases of KP metabolites, KYN, kynurenic-acid (KYNA), xanthurenic-acid (XTRA), cinnabarinic-acid (CNBA) and quinolinic-acid (QUNA), were determined by extreme liquid-chromatography with mass-spectrometry. Interaction among metabotropic glutamate-receptor (mGluR), KP metabolites and ZNS on striato-nigral, striato-pallidal GABAergic and subthalamo-nigral glutamatergic transmission was examined by microdialysis with extreme liquid-chromatography fluorescence resonance-energy transfer detection. Acute and chronic ZNS administration increased astroglial release of KYN, KYNA, XTRA and CNBA, but not QUNA. Chronic IFNγ administration increased the release of KYN, KYNA, CNBA and QUNA, but had minimal inhibitory effect on XTRA release. Chronic TNFα administration increased CNBA and QUNA, but not KYN, KYNA or XTRA. ZNS inhibited IFNγ-induced elevation of KYN, KYNA and QUNA, but enhanced IFNγ-induced that of CNBA. TNFα-induced rises in CNBA and QUNA were inhibited by ZNS. ZNS inhibited striato-nigral GABAergic, striato-pallidal GABAergic and subthalamo-nigral glutamatergic transmission via activation of groups II and III mGluRs. ZNS enhanced astroglial release of endogenous agonists of group II mGluR, XTRA and group III mGluR, CNBA. Activated endogenous mGluR agonists inhibited transmission in direct and indirect pathways of basal ganglia. These mechanisms contribute to effectiveness and well tolerability of ZNS as an adjunct treatment for Parkinson's disease during l-DOPA monotherapy. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

The kynurenine pathway of tryptophan degradation as a drug target

In mammalian cells, the essential amino acid tryptophan is degraded primarily by the kynurenine pathway, a cascade of enzymatic steps containing several biologically active compounds. Metabolites of this pathway, collectively termed 'kynurenines', have been shown to be involved in many diverse physiological and pathological processes. In particular, fluctuations in the levels of kynurenines have discrete effects on the nervous and immune systems. A considerable number of pharmacological tools have recently become available to probe the kynurenine pathway experimentally. Some of these 'kynurenergic' agents can be envisioned to be of therapeutic value, especially in the treatment of diseases that are associated with impaired kynurenine pathway metabolism.

Interferon-alpha-induced changes in tryptophan metabolism. relationship to depression and paroxetine treatment

BACKGROUND

Tryptophan (TRP) degradation into kynurenine (KYN) by the enzyme, indoleamine-2,3-dioxygenase, during immune activation may contribute to development of depressive symptoms during interferon (IFN)-alpha therapy.

METHODS

Twenty-six patients with malignant melanoma were randomly assigned in double-blind fashion to receive either placebo or paroxetine, beginning 2 weeks before IFN-alpha treatment and continuing for the first 12 weeks of IFN-alpha therapy. At treatment initiation and at 2, 4, and 12 weeks of IFN-alpha treatment, measurements of TRP, KYN, and neopterin (a marker of immune activation), were obtained, along with structured assessments of depression, anxiety, and neurotoxicity.

RESULTS

Regardless of antidepressant treatment status, all patients exhibited significant increases in KYN, neopterin, and the KYN/TRP ratio during IFN-alpha therapy. Among antidepressant-free patients, patients who developed major depression exhibited significantly greater increases in KYN and neopterin concentrations and more prolonged decreases in TRP concentrations than did nondepressed, antidepressant-free patients. Moreover, in antidepressant-free patients, decreases in TRP correlated with depressive, anxious, and cognitive symptoms, but not neurovegetative or somatic symptoms. No correlations were found between clinical and biological variables in antidepressant-treated patients.

CONCLUSIONS

The results suggest that reduced TRP availability plays a role in IFN-alpha-induced depressive symptoms, and paroxetine, although not altering the KYN or neopterin response to IFN-alpha, attenuates the behavioral consequences of IFN-alpha-mediated TRP depletion.

Prenatal exposure to methyl mercury in rats: focus on changes in kynurenine pathway

Previous studies showed learning and memory deficits following prenatal exposure to methyl mercury (MMC) in rats. Considering the described dysfunction in several neurotransmission systems after MMC exposure, one can surmise that changes in the kynurenine pathway could also be involved in an altered brain functional development. Thus we focused our attention on the potential alteration in the production of tryptophan metabolites by prenatal MMC exposure. For this purpose, brains were removed, at postnatal days 21 and 60, from rats treated, at gestational day 8, with saline or a single dose of MMC (8 mg/kg). The levels of tryptophan, glutamic, aspartic, kynurenic, anthranilic, and quinolinic acids were determined in hippocampal tissues of both groups of rats. No change was detected in the concentration of aspartic, glutamic, and kynurenic acids in 21- and 60-day-old exposed rats in comparison with age-matched controls. On the contrary, at 21 days of age but not at 60 days, we found a very significant reduction of anthranilic acid and, in parallel, an increase of quinolinic acid levels in MMC-exposed rats in comparison with control animals. Finally in the same brain area, tryptophan levels were significantly increased both at 21 and 60 days of postnatal life. These results suggest that an imbalance in the kynurenine pathway could be involved in the toxic effects induced by MMC on brain development.

Effects of nicotine on head-shakes and tryptophan metabolites

RATIONALE

Nicotine appears to ameliorate the tics of Tourette syndrome. There is evidence that plasma concentrations of the tryptophan metabolite kynurenine may be elevated in this condition. Rodent head-shakes have been proposed as a putative model of Tourette syndrome and are potentiated by kynurenine.

OBJECTIVES

To determine the effects of acute and chronic nicotine on mouse head-shakes, and to study whether nicotine influences brain and plasma levels of kynurenine and certain of its further metabolites in this species.

METHODS

Behavioural and biochemical studies.

RESULTS

Acute (-10 min) administration of (-)-nicotine, or the nicotinic agonist (+)-epibatidine, dose dependently attenuated head-shakes induced by the 5-HT2A/2C receptor agonist +/-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). This attenuation was inhibited by the nicotinic receptor antagonist mecamylamine. Acute nicotine did not affect either spontaneous head-shakes or plasma and brain kynurenine. Fifteen hours after the last of twice daily injections of nicotine (1.6 mg/kg for 7 days), the frequency of spontaneous and DOI-induced head-shakes was significantly potentiated and there was a significant elevation of both plasma and brain kynurenine, although no differences were detected in plasma concentrations of tryptophan, kynurenic acid, 3-hydroxykynurenine or 3-hydroxyanthranilic acid. Brain levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid were also unaffected. In contrast, all these measures were unchanged 15 h after a single nicotine dose (1.6 mg/kg).

CONCLUSIONS

The acute studies indicate that head-shakes induced by DOI are indeed inhibited by nicotinic receptor agonists and suggest that this is not a consequence of an increase in kynurenine. While a role for kynurenine or its metabolites in increasing the head-shake rate after chronic nicotine cannot be excluded, alternative explanations included alterations in the expression or functional status of nicotinic receptor components and further work will be required to characterise this effect.

Tryptophan metabolism in pregnant sheep: increased fetal kynurenine production in response to maternal tryptophan loading

OBJECTIVE

The effects of a tryptophan load on the plasma concentration of kynurenine, the precursor for the production in the brain of the neuroactive products kynurenic acid and quinolinic acid, were determined in pregnant sheep at midgestation and late gestation and in nonpregnant sheep.

STUDY DESIGN

Pregnant ewes were given an intravenous infusion of 100 mg/kg L-tryptophan during 2 hours at 95 to 98 days' gestation (n = 4) or 135 to 138 days' gestation (n = 10). Nonpregnant ewes (n = 6) were studied in late estrus. Arterial blood samples taken from 2 hours before to 48 hours after the start of the infusion were used for analysis of plasma tryptophan, kynurenine, and cortisol concentrations.

RESULTS

Tryptophan loading at both gestational ages resulted in significantly greater increases in kynurenine concentrations in fetal plasma (at 95-98 days' gestation, from 5.7 +/- 1.2 micromol/L [baseline] to 247.9 +/- 86.7 micromol/L (peak); at 135-138 days' gestation, from 9.0 +/- 2.3 micromol/L [baseline] to 289.0 +/- 194.0 micromol/L [peak]) than in maternal plasma [at 95-98 days' gestation, from 4.6 +/- 0.8 micromol/L [baseline] to 118.0 +/- 79.7 micromol/L [peak]; at 135-138 days' gestation, from 4.8 +/- 2.9 micromol/L [baseline] to 98.3 +/- 67.8 micromol/L [peak]). It took longer for kynurenine concentrations to return to basal values in the fetus (24-30 hours) than in the ewe (8-12 hours). The kynurenine responses in pregnant and nonpregnant ewes were not different from each other.

CONCLUSION

The production of kynurenine from tryptophan is significantly greater in the fetal lamb than in the pregnant or nonpregnant adult ewe.