Elevated plasma kynurenine in Tourette syndrome

Immune-mediated animal models of Tourette syndrome

An autoimmune diathesis has been proposed in Tourette syndrome (TS) and related neuropsychiatric disorders such as obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, autism and anorexia nervosa. Environmental triggers including infection and xenobiotics are hypothesized to lead to the production of brain-directed autoantibodies in a subset of genetically susceptible individuals. Although much work has focused on Group A Streptococcus (GAS), the role of this common childhood infection remains controversial. Animal model studies based on immune and autoantibody findings in TS have demonstrated immunoglobulin (Ig) deposits and stereotypic movements and related behavioral disturbances reminiscent of TS following exposure to GAS, other activators of host anti-microbial responses, soluble immune mediators and anti-GAS or anti-neuronal antibodies. Demonstration of the ability to recreate these abnormalities through passive transfer of serum IgG from GAS-immunized mice into naïve mice and abrogation of this activity through depletion of IgG has provided compelling evidence in support of the autoimmune hypothesis. Immunologically-based animal models of TS are a potent tool for dissecting the pathogenesis of this serious neuropsychiatric syndrome.

Plasma kynurenine and related measures in tic disorder patients

OBJECTIVE

Increased plasma kynurenine has been reported in tic disorder patients, and this observation has been suggested to be indicative of immune dysregulation. In the present study, we examined plasma levels of kynurenine and related molecules in a group of tic disorder patients.

METHODS

Plasma concentrations of tryptophan, kynurenine, cortisol, and neopterin were determined in Dutch tic disorder patients (N = 59), and healthy volunteers (N = 32). Group means were compared and age-controlled intra-individual correlations between tic severity and plasma levels of these molecules were examined.

RESULTS

No significant differences were found between patient and control groups in plasma levels of tryptophan, kynurenine, and cortisol concentrations, nor in the kynurenine/tryptophan ratio. However, neopterin was significantly (p = 0.035) higher in patients (mean = 5.13 nmol/l) than in controls (mean = 3.30 nmol/l). Plasma levels of these molecules did not correlate with tic severity, with the exception of tryptophan (r = -0.289, p = 0.049). In patients, plasma neopterin correlated with kynurenine (r = 0.438, p = 0.002); in healthy subjects, tryptophan correlated with kynurenine (r = 0.670, p < 0.001).

CONCLUSION

While the observed elevation in plasma neopterin is consistent with immune activation in a subset of tic disorder patients, metabolism of tryptophan through the kynurenine pathway appears to be unaltered in tic disorder patients.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Obsessive-compulsive disorder and tic syndromes

The phenomenology of OCD and TS seem to match perfectly with the existing conceptualization of the functional relationship between frontal cortical and subcortical circuits. Failed editing of thoughts and impulses, perseverative patterns, and inhibitory deficits are the most convenient descriptors of the symptoms, and some operationalized measures can capture evidence for such deficits in TS and OCD patients. Beyond these expectations borne from conceptual models and some broad patterns of distributed metabolic disturbances in neuroimaging studies, a specific causal pathology within CSPT circuitry needs to be identified in these disorders. This is not a criticism of the existing studies of TS and OCD; to the contrary, the scarcity of pathologic material, the limits of resolution of existing technologies, and the heterogeneity of the phenotypes make the accomplishments of these studies more impressive. As clinicians strive to integrate clinical and scientific findings into coherent models for the pathophysiology of OCD and TS, it is useful to identify practical and effective strategies for therapeutic interventions.

Endogenous neurotoxins from tryptophan

In most tissues, including brain, a major proportion of the tryptophan which is not used for protein synthesis is metabolised along the kynurenine pathway. Long regarded as the route by which many mammals generate adequate amounts of the essential co-factor nicotinamide adenine dinucleotide, two components of the pathway are now known to have marked effects on neurones. Quinolinic acid is an agonist at the N-methyl-D-aspartate sensitive subtype of glutamate receptors in the brain, while kynurenic acid is an antagonist and, thus, a potential neuroprotectant. A third kynurenine, 3-hydroxykynurenine, is involved in the generation of free radicals which can also damage neurones. Quinolinic acid is increasingly implicated in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease, while kynurenic acid has become a standard for the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke.

Tic disorders

The picture that emerges is one of complex behavioral phenotypes that alter over the course of CNS development. While many cases are mild and may not come to medical attention, others are chronic and disabling. The aim of this article is to provide an overview of recent progress in understanding phenomenology, epidemiology, genetics, neurobiology, and treatment of tic disorders.

Tics and fits. The current status of Gilles de la Tourette syndrome and its relationship with epilepsy

The Gilles de la Tourette syndrome (GTS) is a neurobehavioural disorder characterized by multiple motor and vocal tics. Accessory symptoms include coprophenomena, echophenomena and obsessive-compulsive behaviours. There is a significant genetic contribution to their aetiology. Simple tics may be mistaken for myoclonic epilepsy and complex tics for complex partial seizures. Like epilepsy, GTS is a brain disorder that leads to general disturbances in behaviour not just the cardinal symptoms (i.e. tics or seizures).

Neurochemical and some related psychopharmacological aspects of Tourette's syndrome: an update

Neurochemical investigations of Tourette's syndrome (TS) suggest that the symptoms of this disorder may be the result of an imbalance among several neurotransmitter and/or neuromodulator systems. Neurochemicals which have been studied included: catecholamines; acetylcholine; tryptophan and its metabolites; the amino acids γ-aminobutyric acid (GABA), glutamate, phenylalanine and p-tyrosine; trace amines; opioid peptides; cyclic AMP and androgenic hormones. A suitable animal model of TS would do much to advance our understanding of this disorder, and there are some interesting recent developments in this regard.