Biochemical aspects of Tourette's Syndrome

Kynurenine (KYN) is the first stable metabolite of the kynurenine pathway, the major route of tryptophan. (TRP) metabolIsm. In the liver, cortisol-inducible tIyptophan-2,3-dioxygenase (TDO) is the first enzyme and rate limiting step. In extrahepatic tissues, it is superceded by indoleamine-2,3-dioxygenase (IDO), an enzyme with a wider substrate specificity. Earlier work in this research group has found substantial elevations in plasma KYN in fasting Tourette's Syndrome (TS) patients with normal TRP and neopterin. The aim of our initial pilot study was to confirm this increase in KYN in fasting human TS patients compared with normal controls, and to see how changes in diet :ay influence certain kynurenine pathway variables. However, we failed to detect a change in plasma KYN, TRP, kynurenic acid (KYNA), neopterin or cortisol between the fasting TS and control groups. Moreover, none of the variables was affected by dietary status, and thus candidates selected for the larger cross-sectional study were permitted to eat and drink freely on the day that blood samples were submitted, but were requested to avoid products containing caffeine, aspirin or nicotine. In the cross-sectional study, TS patients exhibited significantly higher plasma KYN concentrations than controls, although the magnitude of the change was much smaller than originally found. This may be due to differences in detection procedure and the seasonal fluctuation of some biochemical variables, notably cortisol. The generalised increase in neopterin in the TS subject group, suggests a difference in the activity of cytokine-inducible IDO as a likely source for this elevated KYN. Other kynurenine pathway metabolites, specifIcally TRP, 3-hydroxykynurenine (HKY), 3-hydroxyanthranilic acid (HAA) and KYNA were unchanged. In view of recent speculation of the potential therapeutic effects of nicotine in TS, the lower KYN concentrations observed in TS smokers, compared with non-smoking TS patients, was another interesting finding. Tic-like movements, such as head-shakes (HS), which occur in rodents both spontaneously and following diverse drug treatments, closely resemble tic behaviours in humans. The animal tic model was used to examine what effects nicotine may have on shaking behaviours and on selected TRP metabolites. Acute systemic administration of nicotine to mice, produced a dose-dependent reduction in HS frequency (induced by the 5-HT2A/2C agonist DOl), which appeared to be mediated via central nicotinic cholinergic receptors, since mecamylamine pretreatment abolished this effect. Conversely, twice daily subcutaneous injections of nicotine for 7 days, led to an increase in spontaneous and DOI-induced HS. Chronic nicotine also caused a significant elevation m plasma and whole brain KYN concentrations, but plasma TRP, HKY, HAA and KYNA were unaltered. In addition, no change in brain 5-HT or 5-HIAA concentrations or 5-HT turnover, was found. Despite contrasting results from human and animal studIes, a role for nicotine in the mediation of tic-like movements is indicated. The relevance of the kynurenine pathway to TS and the potential role played by nicotine in modifying tic-like behaviours is discussed.

Investigation into the biochemical changes in Tourette Syndrome with a potential for pharmacological manipulation

Kynurenine (KYN) is the first stable metabolite of the kynurenine pathway, which accounts for over 95% of tryptophan metabolism. Two previous studies by this research group reported elevated plasma KYN in Tourette syndrome (TS) patients when compared with age and sex matched controls and another study showed that KYN potentiated 5-HT2A-mediated head-shakes (HS) in rodents. These movements have been suggested to model tics in TS. This raised the questions how KYN acts in eliciting this response and whether it is an action of its own or of a further metabolite along the kynurenine pathway. In the liver, where most of the kynurenine pathway metabolism takes place under physiological conditions, the first and the rate limiting enzyme is tryptophan-dioxygenase (TDO) which can be induced by cortisol. In extrahepatic tissues the same step of the pathway is catalyzed by indoleamine-dioxygenase (IDO), which is induced by cytokines, predominantly interferon-y (INF-y). Plasma neopterin, which shows parallel increase with KYN following immune stimulation, was also found elevated in one of these studies positively correlating with KYN. In the present work animal studies suggested that KYN potentiates and quinolinic acid (QUINA) dose dependently inhibits the 5-HT2A-mediated HS response in mice. The potentiating effect seen with KYN was suggested to be an effect of KYN itself. Radioligand binding and phosphoinositide (PI) hydrolysis studies were done to explore the mechanisms by which kynurenine pathway metabolites could alter a 5-HT2A-receptor mediated response. None of the kynurenine pathway metabolites tested showed direct binding to 5-HT2A-receptors. PI hydrolysis studies with KYN and QUINA showed that KYN did not have any effect while QUINA inhibited 5-HT2A-mediated PI hydrolysis. Plasma cortisol determination in TS patients with elevated plasma KYN did not show elevated plasma cortisol levels, suggesting that the increase of plasma KYN in these TS patients is unlikely to be due to an increased TDO activity induced by increased cortisol. Attention deficit hyperactivity disorder (ADHD) is commonly associated with TS. Salivary cortisol detected in a group of children primarily affected with ADHD showed significantly lower salivary cortisol levels when compared with age and sex matched controls. Plasma tryptophan, KYN, neopterin, INF-y and KYN/tryptophan ratio and night-time urinary 6-sulphatoxymelatonin (aMT6s) excretion measured in a group of TS patients did not show any difference in their levels when compared with age and sex matched controls, but TS patients failed to show the expected positive correlation seen between plasma INF-y, neopterin and KYN and the negative correlation seen between plasma KYN and night-time urinary aMT6s excretion seen in healthy controls. The relevance of the kynurenine pathway, melatonin secretion and cortisol to Tourette Syndrome and associated conditions and the mechanism by which KYN and QUINA alter the 5-HT2A-receptor mediated HS response are discussed.

Biosynthesis and analysis of plant oxylipins

The term oxylipin is applied to the generation of oxygenated products of polyunsaturated fatty acids that can arise either through non-enzymatic or enzymatic processes generating a complex array of products, including alcohols, aldehydes, ketones, acids and hydrocarbon gases. The biosynthetic origin of these products has revealed an array of enzymes involved in their formation and more recently a radical pathway. These include lipoxygenases and α-dioxygenase that insert both oxygen atoms in to the acyl chain to initiate the pathways, to specialised P450 monooxygenases that are responsible for their downstream processing. This latter group include enzymes at the branch points such as allene oxide synthase, leading to jasmonate signalling, hydroperoxide lyase, responsible for generating pathogen/pest defensive volatiles and divinyl ether synthases and peroxygenases involved in the formation of antimicrobial compounds. The complexity of the products generated raises significant challenges for their rapid identification and quantification using metabolic screening methods. Here the current developments in oxylipin metabolism are reviewed together with the emerging technologies required to expand this important field of research that underpins advances in plant-pest/pathogen interactions.