The functions and regulation of tryptophan pyrrolase

Mechanisms of Neuronal Damage in Acute Hepatic Porphyrias

Porphyrias are a group of congenital and acquired diseases caused by an enzymatic impairment in the biosynthesis of heme. Depending on the specific enzyme involved, different types of porphyrias (i.e., chronic vs. acute, cutaneous vs. neurovisceral, hepatic vs. erythropoietic) are described, with different clinical presentations. Acute hepatic porphyrias (AHPs) are characterized by life-threatening acute neuro-visceral crises (acute porphyric attacks, APAs), featuring a wide range of neuropathic (central, peripheral, autonomic) manifestations. APAs are usually unleashed by external "porphyrinogenic" triggers, which are thought to cause an increased metabolic demand for heme. During APAs, the heme precursors δ-aminolevulinic acid (ALA) and porphobilinogen (PBG) accumulate in the bloodstream and urine. Even though several hypotheses have been developed to explain the protean clinical picture of APAs, the exact mechanism of neuronal damage in AHPs is still a matter of debate. In recent decades, a role has been proposed for oxidative damage caused by ALA, mitochondrial and synaptic ALA toxicity, dysfunction induced by relative heme deficiency on cytochromes and other hemeproteins (i.e., nitric oxide synthases), pyridoxal phosphate functional deficiency, derangements in the metabolic pathways of tryptophan, and other factors. Since the pathway leading to the biosynthesis of heme is inscribed into a complex network of interactions, which also includes some fundamental processes of basal metabolism, a disruption in any of the steps of this pathway is likely to have multiple pathogenic effects. Here, we aim to provide a comprehensive review of the current evidence regarding the mechanisms of neuronal damage in AHPs.

Tryptophan Metabolism and Related Pathways in Psychoneuroimmunology: The Impact of Nutrition and Lifestyle

In the past, accelerated tryptophan breakdown was considered to be a feature of clinical conditions, such as infection, inflammation, and malignant disease. More recently, however, the focus has changed to include the additional modulation of tryptophan metabolism by changes in nutrition and microbiota composition. The regulation of tryptophan concentration is critical for the maintenance of systemic homeostasis because it integrates essential pathways involved in nutrient sensing, metabolic stress response, and immunity. In addition to tryptophan being important as a precursor for the synthesis of the neurotransmitter serotonin, several catabolites along the kynurenine axis are neuroactive. This emphasizes the importance of the immunometabolic fate of this amino acid for processes relevant to neuropsychiatric symptoms. In humans, besides hepatic catabolism, there is usually a strong relationship between immune activation-associated tryptophan breakdown and increased levels of biomarkers, such as neopterin, which has particular relevance for both acute and chronic diseases. A shift towards neopterin synthesis during oxidative stress may indicate a corresponding decrease in tetrahydrobiopterin, a cofactor of several mono-oxygenases, providing a further link between tryptophan metabolism and serotonergic and catecholaminergic neurotransmission. The psychoneuroimmunological consequences of tryptophan metabolism and the susceptibility of this pathway to modulation by a variety of nutritional and lifestyle-related factors have important implications for the development of both diagnostic and treatment options.

Fatigue in inflammatory rheumatic disorders: pathophysiological mechanisms

Today, inflammatory rheumatic disorders are effectively treated, but many patients still suffer from residual fatigue. This work presents pathophysiological mechanisms of fatigue. First, cytokines can interfere with neurotransmitter release at the preterminal ending. Second, a long-term increase in serum concentrations of proinflammatory cytokines increase the uptake and breakdown of monoamines (serotonin, noradrenaline and dopamine). Third, chronic inflammation can also decrease monoaminergic neurotransmission via oxidative stress (oxidation of tetrahydrobiopterin [BH4]). Fourth, proinflammatory cytokines increase the level of enzyme indoleamine-2, 3-dioxygenase activity and shunt tryptophan away from the serotonin pathway. Fifth, oxidative stress stimulates astrocytes to inhibit excitatory amino acid transporters. Sixth, astrocytes produce kynurenic acid that acts as an antagonist on the α7-nicotinic acetylcholine receptor to inhibit dopamine release. Jointly, these actions result in increased glutamatergic and decreased monoaminergic neurotransmission. The above-described pathophysiological mechanisms negatively affect brain functioning in areas that are involved in fatigue.

Sodium Sulfite Exacerbates Allograft Vasculopathy and Affects Tryptophan Breakdown in Murine Heterotopic Aortic Transplantation

Graft vasculopathy is the main feature of chronic rejection in organ transplantation, with oxidative stress being a major trigger. Inflammation-associated prooxidant processes may be controlled by antioxidants; however, interference with redox-regulated mechanisms is a complex endeavor. An essential feature of the cellular immune response is the acceleration of tryptophan (Trp) breakdown, leading to the formation of several bioactive catabolites. Long-term activation of this immunobiochemical pathway contributes to the establishment of a tolerogenic environment, thereby supporting allograft survival. Herein, the impact of the antioxidant sodium sulfite on the development of graft vasculopathy was assessed in murine aortic transplantation. Allogeneic (BALB/c to C57BL/6) heterotopic murine aortic transplantations were performed. Animals were left untreated or were treated with 10 μl of 0.1 M, of 0.01 M sodium sulfite, or of 0.1 M sodium sulfate, intraperitoneally once/day, until postoperative day (POD) 100. Grafts were assessed by histology, immunohistochemistry, and adhesion molecule gene expression. Serum concentrations of tryptophan and its catabolite kynurenine (Kyn) were measured. On day 100, graft vasculopathy was significantly increased upon treatment with 0.1 M sodium sulfite, compared to allogeneic untreated controls (p = 0.004), which correlated with a significant increase of α-smooth-muscle-actin, Vcam-1, and P-selectin. Serum Kyn concentrations increased in the allogeneic control group over time (p < 0.05, POD ≥ 50), while low-dose sodium sulfite treatment (0.01 M) treatment resulted in a decrease in Kyn levels over time (p < 0.05, POD ≥ 10), compared to the respective baselines (p < 0.05). Longitudinal analysis of serum metabolite concentrations in the different treatment groups further identified an overall effect of sodium sulfite on Kyn concentrations. Antioxidative treatment may result in ambivalent consequences. Our data reveal that an excess of antioxidants like sodium sulfite can aggravate allograft vasculopathy, which further highlights the challenges associated with interventions that interfere with the complex interplay of redox-regulated inflammatory processes.

Disturbance of serotonergic neurotransmission in patients with postmyocardial infarction and depression

The objective of this study was to assess the hypothesis that patients who develop depression after myocardial infarction (MI) have a lower level of brain serotonergic neurotransmission through measurement of plasma free fraction of L-tryptophan and intensity-dependence auditory-evoked potentials (IDAEPs). A cross-sectional study was carried out in 74 adults after MI. Thirty-four patients suffered from depression and 40 patients did not demonstrate depressive symptoms. We measured the free fraction, bound and total plasma L-tryptophan, and neutral amino acids as well as recording IDAEPs. Patients who developed depression after MI showed a significantly lower level in the free fraction of L-tryptophan and in the ratios of free fraction of L-tryptophan/total L-tryptophan and free fraction of L-tryptophan/neutral amino acids. It is noteworthy that the slope of the amplitude/stimulus intensity functions (ASF slope) of the N1/P2 component was significantly higher post-MI in depressed patients. Higher ASF slope of the N1/P2 component associated with a low free fraction of L-tryptophan in plasma reflect a low brain serotonergic neurotransmission. These findings suggest an important deterioration of brain serotonergic activity as a pathophysiological mechanism in post-MI patients for the development of clinical depression. Therefore, we propose these biochemical and electrophysiological procedures as noninvasive clinical indicators of brain serotonergic activity in these patients.

Alteration in plasma corticosterone levels following long term oral administration of lead produces depression like symptoms in rats

Lead toxicity is known to induce a broad range of physiological, biochemical and behavioral dysfunctions that may result in adverse effects on several organs, including the central nervous system. Long-term exposure to low levels of lead (Pb(2+)) has been shown to produce behavioral deficits in rodents and humans by affecting hypothalamic-pituitary-adrenal (HPA) axis. These deficits are thought to be associated with altered brain monoamine neurotransmission and due to changes in glucocorticoids levels. This study was designed to investigate the effects of Pb(2+)exposure on growth rate, locomotor activity, anxiety, depression, plasma corticosterone and brain serotonin (5-HT) levels in rats. Rats were exposed to lead in drinking water (500 ppm; lead acetate) for 5 weeks. The assessment of depression was done using the forced swimming test (FST). Estimation of brain 5-HT was determined by high-performance liquid chromatography with electrochemical detection. Plasma corticosterone was determined by spectrofluorimetric method. The present study showed that long term exposure to Pb(2+) significantly decreased the food intake followed by the decrease in growth rate in Pb(2+)exposed rats as compared to control group. No significant changes in open field activity were observed following Pb(2+)exposure while significant increase in anxiogenic effect was observed. Increased plasma corticosterone and decreased 5-HT levels were exhibited by Pb(2+)exposed rats as compared to controls. A significant increase in depressive like symptoms was exhibited by Pb(2+)exposed rats as compared to control rats. The results are discussed in the context of Pb(2+) inducing a stress-like response in rats leading to changes in plasma corticosterone and brain 5-HT levels via altering tryptophan pyrrolase activity.

The new '5-HT' hypothesis of depression: cell-mediated immune activation induces indoleamine 2,3-dioxygenase, which leads to lower plasma tryptophan and an increased synthesis of detrimental tryptophan catabolites (TRYCATs), both of which contribute to the onset of depression

This paper reviews the body of evidence that not only tryptophan and consequent 5-HT depletion, but also induction of indoleamine 2,3-dioxygenase (IDO) and the detrimental effects of tryptophan catabolites (TRYCATs) play a role in the pathophysiology of depression. IDO is induced by interferon (IFN)γ, interleukin-6 and tumor necrosis factor-α, lipopolysaccharides and oxidative stress, factors that play a role in the pathophysiology of depression. TRYCATs, like kynurenine and quinolinic acid, are depressogenic and anxiogenic; activate oxidative pathways; cause mitochondrial dysfunctions; and have neuroexcitatory and neurotoxic effects that may lead to neurodegeneration. The TRYCAT pathway is also activated following induction of tryptophan 2,3-dioxygenase (TDO) by glucocorticoids, which are elevated in depression. There is evidence that activation of IDO reduces plasma tryptophan and increases TRYCAT synthesis in depressive states and that TDO activation may play a role as well. The development of depressive symptoms during IFNα-based immunotherapy is strongly associated with IDO activation, increased production of detrimental TRYCATs and lowered levels of tryptophan. Women show greater IDO activation and TRYCAT production following immune challenge than men. In the early puerperium, IDO activation and TRYCAT production are associated with the development of affective symptoms. Clinical depression is accompanied by lowered levels of neuroprotective TRYCATs or increased levels or neurotoxic TRYCATs, and lowered plasma tryptophan, which is associated with indices of immune activation and glucocorticoid hypersecretion. Lowered tryptophan and increased TRYCATs induce T cell unresponsiveness and therefore may exert a negative feedback on the primary inflammatory response in depression. It is concluded that activation of the TRYCAT pathway by IDO and TDO may be associated with the development of depressive symptoms through tryptophan depletion and the detrimental effects of TRYCATs. Therefore, the TRYCAT pathway should be a new drug target in depression. Direct inhibitors of IDO are less likely to be useful drugs than agents, such as kynurenine hydroxylase inhibitors; drugs which block the primary immune response; compounds that increase the protective effects of kynurenic acid; and specific antioxidants that target IDO activation, the immune and oxidative pathways, and 5-HT as well.

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Impact of nutrition on canine behaviour: current status and possible mechanisms

Each year, millions of dogs worldwide are abandoned by their owners, relinquished to animal shelters, and euthanised because of behaviour problems. Nutrition is rarely considered as one of the possible contributing factors of problem behaviour. This contribution presents an overview of current knowledge on the influence of nutrition on canine behaviour and explores the underlying mechanisms by which diet may affect behaviour in animals. Behaviour is regulated by neurotransmitters and hormones, and changes in the availability of their precursors may influence behaviour. Tryptophan, the precursor of serotonin, may affect the incidence of aggression, self-mutilation and stress resistance. The latter may also be influenced by dietary tyrosine, a precursor to catecholamines. As diet composition, nutrient availability and nutrient interactions affect the availability of these precursors in the brain, behaviour or stress resistance may be affected. PUFA, especially DHA, have an important role as structural constituents in brain development, and dietary supply of n-3 and n-6 PUFA could modify aspects of the dopaminergic and serotonergic system and, consequently, cognitive performance and behaviour. Finally, persistent feeding motivation between meals can increase stereotyped behaviour and aggression and decrease resting time. This feeding motivation may be altered by dietary fibre content and source. At present, few studies have been conducted to evaluate the role of nutrition in canine (problem) behaviour through the above mentioned mechanisms. Studies that explore this relationship may help to improve the welfare of dogs and their owners.

Heterogeneity of serum tryptophan concentration and availability to the brain in patients with the chronic fatigue syndrome

We assessed the serotonin status of patients with the chronic fatigue syndrome (CFS). Tryptophan (Trp) availability to the brain, expressed as the ratio of concentration of serum Trp to the sum of those of its five competitors (CAA), and other parameters of Trp disposition were compared in 23 patients with the CFS and 42 healthy controls. The serum [free Trp]/[CAA] ratio was 43% higher in CFS patients, due to a 48% higher [free Trp]. [Total Trp] was also significantly higher (by 19%) in CFS patients, and, although the [total Trp]/[CAA] ratio did not differ significantly between the control and patient groups, the difference became significant when the results were co-varied with age and gender. [CAA] was not significantly different between groups, but was significantly lower in females, compared to males, of the CFS patient group. We have established normal ranges for Trp disposition parameters and propose criteria for defining the serotonin-biosynthetic status in humans. We have provisionally identified two subgroups of CFS patients, one with normal serotonin and the other with a high serotonin status. The relevance of our findings to, and their implications for, the pharmacological and other therapies of the chronic fatigue syndrome are discussed.

Serotonin-related tryptophan in children with insulin-dependent diabetes

In the course of the present research in school children with insulin-dependent diabetes mellitus, we observed that the free fraction of L-tryptophan, the free fraction of L-tryptophan/total L-tryptophan, and the free fraction of L-tryptophan/neutral amino acids ratios, are significantly reduced. The decrease of free fraction of L-tryptophan in plasma with a concomitant decrease of the free fraction of L-tryptophan/neutral amino acids ratio suggest a decrease in the transport of the precursor amino acid to the brain and in the serotonin synthesis rate, similar to that observed in diabetic animals. This finding may be of relevance in the pathophysiology and in the clinical picture, which could be related to an alteration of serotonin metabolism and neurotransmission in the brain and may be possibly related to neuropsychiatric disorders in diabetic school children. Thus we propose that the free fraction of L-tryptophan and the free fraction of L-tryptophan/neutral amino acids ratios may be clinically useful as indicators of brain serotonergic activity in these patients. In our laboratory, we are currently obtaining additional data on the functional role of the brain serotonergic system in humans to further support the relevance of our results.

No early effects of electroconvulsive therapy on tryptophan metabolism and disposition in endogenous depression

The possibility that a single electroconvulsive therapy (ECT) could increase tryptophan (Trp) availability to the brain for 5-hydroxytryptamine (5-HT, serotonin) synthesis was examined in 10 depressed patients before and during the 1st hour following an ECT and in 4 control (minor ear, nose, and throat surgical) subjects receiving similar premedication. Trp availability to the brain, expressed as the serum Trp: competing amino acid ratio, and related aspects of Trp disposition were not significantly altered by ECT any differently than from preoperative stress and premedication. We suggest that Trp availability to the brain and, hence, cerebral 5-HT synthesis are not altered in depressed patients early after a single ECT.

Effects of acute physical exercise on central serotonergic systems

This paper reviews data concerning the effects of acute physical exercise (treadmill running) in trained rats. Works from the 1980's have established that acute running increases brain serotonin (5-hydroxytryptamine: 5-HT) synthesis in two ways. Lipolysis-elicited release of free fatty acids in the blood compartment displaces the binding of the essential amino acid tryptophan to albumin, thereby increasing the concentration of the so-called "free tryptophan" portion, and because exercise increases the ratio of circulating free tryptophan to the sum of the concentrations of the amino acids that compete with tryptophan for uptake at the blood-brain barrier level, tryptophan enters markedly in the brain compartment. However, this marked increase in central tryptophan levels increases only to a low extent brain 5-HT synthesis, as assessed by the analysis of 5-hydroxyindoleacetic acid levels, thereby suggesting that exercise promotes feedback regulatory mechanisms. Indirect indices of 5-HT functions open the possibility that acute exercise-induced increases in 5-HT biosynthesis are associated with (or lead to) increases in 5-HT release. Lastly, the hypothesis that training and/or acute exercise triggers changes in 5-HT receptors has been examined in several studies; actually, both positive and negative results have been reached. Taken together, all these data support the need for future studies on the functional effects of exercise on 5-HT, including those related to the hypothesis that the positive mood effects of exercise rely (partly or totally) on central serotonergic systems.

Changes in blood and plasma serotonergic measurements in rats--effect of nicotine and/or exposure to different stresses

Effects of two kinds of stress on plasma and whole blood serotonergic measurements were studied in nicotine administered rats. Plasma tryptophan levels increased after footshock stress. Stress given to nicotine administered rats resulted in decrease in plasma tryptophan levels. Although there was no change in 5-HT levels in whole blood, footshock increased plasma levels of 5-HT, and restraint stress decreased its levels. Plasma and whole blood 5-HIAA levels increased in rats given stress and nicotine. 5-HIAA/5-HT ratio (the turnover rate of 5-HT) in plasma and blood increased only in rats given stress and nicotine. These results indicate that nicotine administration may decrease stress induced increase in plasma 5-HT levels by increasing its degradation.

Effects of nicotine and electric footshock on peripheral serotonergic measures and on platelet aggregation in whole blood of rats

The effects of nicotine and electric foot shock as well as of their combination on blood serotonergic measures and on whole blood aggregation have been analyzed. In rats subjected to electric footshock a rise (p < 0.05) in plasma but not in whole blood serotonin was observed, whereas this parameter was not influenced in nicotine-treated rats when compared to the control group. The combination of nicotine with electric footshock only slightly increased plasma serotonin and showed no effect on whole blood serotonin, but 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of serotonin (5-HT), as well as the 5-HIAA/5-HT ratio were markedly increased (p < 0.01) suggesting an enhanced turnover of 5-HT under these conditions. The collagen-induced aggregation in whole blood was not influenced in nicotine-, in footshock- nor in combined-treated rats when compared to the controls. Our data indicate that stress as well as the combination of stress with nicotine may affect the serotonergic system which is in contrast to the exposure to nicotine alone.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

The spinal actions of nonsteroidal anti-inflammatory drugs and the dissociation between their anti-inflammatory and analgesic effects

The traditional classification of nonsteroidal anti-inflammatory drugs (NSAIDs) as exclusively 'peripherally acting' agents is no longer valid. For many of these agents there is a growing body of evidence in favour of an additional central mechanism for their anti-inflammatory and analgesic effects. This view is further supported by the recent discovery that a substantial component of the hyperalgesia and allodynia that characterise postinjury hypersensitivity occurs in the CNS, notably the spinal dorsal horn. An important corollary is that inhibition of central nociceptive processing may represent an important analgesic mode of action for those NSAIDs that are effective in the management of pain after tissue injury. Historically, attempts to group this heterogeneous class of compounds into a single entity are largely derived from the observation that the majority of clinically useful NSAIDs are weak organic acids (pKa 3 to 5), bind extensively to plasma albumin (= 99%), and inhibit (to varying degrees) prostaglandin synthesis. However, the significance of these various unifying features is becoming increasingly obscure. While inhibition of prostaglandin synthesis apparently remains an important analgesic mode of action for NSAIDs both in the periphery and the CNS, other mechanisms should be considered. Some NSAIDs, in addition to their effects on prostaglandin synthesis, also affect the synthesis and activity of other neuroactive substances believed to have key roles in processing nociceptive input within the dorsal horn. It has been argued that these other actions, in conjunction with inhibition of prostaglandin synthesis, may synergistically augment the effects of NSAIDs on spinal nociceptive processing. Despite much effort, it remains a formidable task to assess the significance of these differential mechanisms upon clinical pain states. In the meantime, however, it may be possible, on the basis of in vivo studies, to evaluate the impact of putative spinal analgesic mechanisms that are unrelated to inhibition of prostaglandin synthesis. This approach has recently been extended to include the identification of pharmacokinetic and clinical correlates of these derived in vivo parameters, and in this way attempt to demonstrate clinical relevance.

Relationships between lower plasma L-tryptophan levels and immune-inflammatory variables in depression

Despite much research, the pathophysiology underlying lower L-tryptophan (L-TRP) availability in major depression has remained elusive. The present study investigates whether lower L-TRP availability in major depression is related to immune activation which may occur in that illness and is known to modulate L-TRP metabolism. Toward this end, the authors have measured the following in depressed patients and normal control subjects: plasma levels of L-TRP, and the competing amino acids (CAA) valine, leucine, isoleucine, tyrosine, and phenylalanine, together with indices of immune function such as haptoglobin (Hp) and transferrin (Tf) plasma levels, dipeptidyl peptidase IV (DPP IV) serum activity, and mitogen-induced culture supernatant interleukin-6 (Il-6) production. Both plasma levels of L-TRP and the L-TRP/CAA ratio were significantly lower in major depressed subjects as compared with healthy control subjects. There were significant correlations between plasma L-TRP levels, on the one hand, and Tf plasma levels, DPP IV activity (both positive), Il-6 production, and Hp plasma levels (both negative), on the other. Up to 63.7% of the variance in L-TRP plasma concentrations could be explained by DPP IV, Hp, Il-6 values, and gender. Up to 50% of the variance in the L-TRP/CAA ratio could be explained by Hp values (negative correlation) and gender. It is hypothesized that lower plasma L-TRP availability in major depression may be related to the immune response in that illness.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Brain tryptophan uptake and sodium-potassium ATPase activity in long-term streptozotocin diabetic rats

Serum, liver and brain tryptophan concentrations and brain Na+K(+)-ATPase activity were studied in streptozotocin diabetic rats after an acute tryptophan load. Results show that tryptophan administration in the experimental diabetic group produces a generalized fall in tryptophan uptake in all the brain regions studied, though it does not increase serum and hepatic tryptophan concentrations. These parameters are normalized in insulin-treated diabetic rats. With regard to Na+K(+)-ATPase, diabetic animals showed a diminished and unchanged activity; whereas, the other two experimental groups showed a gradual decrease and a negative correlation with brain tryptophan uptake.

Disturbances in dexamethasone suppression test and lower availability of L-tryptophan and tyrosine in early puerperium and in women under contraceptive therapy

This study investigates the function of the hypothalamic-pituitary-adrenal (HPA)-axis and the availability of L-tryptophan and tyrosine to the brain in postpartum women and in women taking long-term oral contraceptives. To this end, we have measured the following parameters in 50 women (i.e. 9 normal controls, 10 women taking oral contraceptives, and 31 postpartum females): plasma cortisol, L-tryptophan, tyrosine and the amino acids (CAA) known to compete with them for transport through the blood-brain barrier. We have determined the effects of 1 mg of dexamethasone on the above-mentioned biological markers in postpartum females. Plasma cortisol and tyrosine were significantly higher and lower, respectively, in puerperium and in women under contraceptive therapy as opposed to normal controls. L-Tryptophan was significantly lower in postpartum females, whilst the L-tryptophan/CAA ratio did not differ across the three study groups. Postpartum females revealed a significant negative relationship between the availability of L-tryptophan to the brain and postpartum mood, as measured by Zung's Depression and Anxiety Scales and State Anxiety Inventory. Dexamethasone had a significant suppressive effect on L-tryptophan/CAA and tyrosine/CAA ratios, with cortisol nonsuppression appearing in 82% of the postpartum females.

Characterization of the L-tryptophan transport system in the liver of growing rats

In order to characterize the system of L-tryptophan (TRP) transport into liver during the growing period of 10 to 42 days, the changes of tryptophan 2,3-dioxygenase (TDO) activity, levels of serum, liver, brain, and muscle TRP, and the rate and mode of TRP uptake into isolated hepatocytes were examined in male Wistar rats. Liver TDO activity increased rapidly at 16 days of age. A marked and rapid decrease in free serum TRP level occurred before weanling, while a small decrease in total serum TRP level was found after weanling. The change of liver TRP level was similar to that of free serum TRP level and correlated well. There was a significant inverse correlation between liver TDO activity and either free serum TRP level or liver TRP level. A rapid change in TRP level did not occur in brain and muscle during the growing period. The concentrations of brain and muscle TRP correlated better with those of total serum TRP than with those of free serum TRP. The rate of TRP uptake into hepatocytes isolated from rats aged 10 days was lower than that from rats aged 21 and 42 days. The former hepatocytes were lacking in a high-affinity saturable transport component for TRP uptake which was present in the latter ones. The present results indicate that a great change in the system of TRP transport into liver occurs in growing rats, and that in suckling rats a high level of free serum TRP contributes to the efficient transport of the amino acid into the liver.

Decreased plasma tryptophan concentration in major depression: relationship to melancholia and weight loss

Plasma total tryptophan (TRP) concentration was significantly lower in 31 patients with major depression compared to a healthy control group. The ratio of plasma TRP concentration to that of other branch chain amino acids (the TRP:BCAA ratio) was also decreased. Further analysis revealed that the decrease in plasma TRP and TRP:BCAA ratio was most apparent in patients with major depression and melancholia. Overall, women but not men had significantly decreased plasma tryptophan concentrations, perhaps because of a contributory effect of weight loss; this latter effect, however, could not be distinguished clearly from a diagnosis of melancholia. Our data suggest that in some depressed patients, reductions in plasma tryptophan availability may contribute to abnormalities in brain 5-hydroxytryptamine function.

In vivo evidence that insulin does not inhibit hepatic tryptophan pyrrolase activity in rats

Previous reports have indicated that insulin administration triggers an early increase in plasma tryptophan (TRP) levels in fasted rats. Then, the present study was undertaken to investigate the putative role of liver tryptophan pyrrolase (TPO) in this short-term effect of insulin. In 24 hr fasted rats, doses of insulin that triggered an increase in plasma TRP levels (i.e., 2-3 I.U./kg, 1 hr) did not alter either holoenzyme or total enzyme activity. In another series of experiments, the administration of insulin (2 I.U./kg) to 24 hr fasted rats promoted biphasic time effects on plasma TRP levels and liver TPO activity. Thus, insulin initially triggered a rise in plasma TRP (without any change in liver TPO activity) and then increased liver TPO activity whilst plasma TRP returned toward control levels. In addition, hypercorticosteronemia was evidenced throughout the first phasis. Lastly, the influence of insulin administration (2 I.U./kg) on fasting-induced TPO induction was analysed. Whereas fasting increased liver TPO activity in a time-dependent manner, insulin administration (2 I.U./kg, 30 min) did not modify either plasma TRP or liver TPO activity. The data reported herein bring evidence that the effect of insulin administration on circulating TRP is not mediated by an inhibition of hepatic TPO.

Effects of dexamethasone on the availability of L-tryptophan and on the insulin and FFA concentrations in unipolar depressed patients

Several authors have shown that the availability of L-tryptophan (L-TRP) in the serum is lower in patients with major depression than in controls. It has recently been reported that the administration of a dose of dexamethasone sufficient to cause cortisol suppression also caused significant decrements in the availability of L-TRP. In order to elucidate the putative pathophysiological mechanisms underlying this decreased L-TRP disposition, the authors examined 37 depressed women categorized according to the DSM-III. L-TRP, the sum of five competing amino acids (CAA), the L-TRP/CAA ratio, and insulin and free fatty acid (FFA) levels were determined both before and after oral administration of 1 mg dexamethasone. The availability of L-TRP was significantly lower in depressed women with melancholia compared with simple major and minor depressives. The baseline disposal of L-TRP was related neither to insulin nor to FFA concentrations Dexamethasone administration significantly reduced the L-TRP and L-TRP/CAA values and increased FFA and insulin levels. Postdexamethasone L-TRP and FFA levels were significantly and positively correlated.

The relationships between the cortisol responses to dexamethasone and to L-5-HTP, and the availability of L-tryptophan in depressed females

In order to investigate the relationships between the hypothalamic-pituitary-adrenal (HPA)-axis activity, the central serotonergic neurotransmission, and the peripheral metabolism of l-tryptophan (L-TRP), the authors measured the following: the postdexamethasone cortisol values, the cortisol responses to 125 mg 5-hydroxy-L-tryptophan (L-5-HTP) orally, and the total L-TRP/competing amino acids (CAA) ratio in 64 depressed females. Severely depressed females showed significantly lower values for L-TRP/CAA, significantly higher postdexamethasone cortisol values, and cortisol responses to L-5-HTP as compared with minor depressives. Dexamethasone nonsuppressors showed significantly lower L-TRP/CAA values as compared with suppressors. The cortisol responses to dexamethasone were significantly and negatively correlated with the availability of L-TRP. The cortisol responses to L-5-HTP were not related to either the availability of L-TRP or to the postdexamethasone cortisol values.

The effects of glucocorticoids on the availability of L-tryptophan and tyrosine in the plasma of depressed patients

There is evidence that a functional deficit of serotonin/noradrenaline and/or of their precursors L-tryptophan (L-TRP)/tyrosine and disorders in the hypothalamic-pituitary-adrenal (HPA) axis are linked to the pathophysiology of severe depressions. Several reports suggest a reciprocal relationship between these neurotransmitters and HPA axis activity. In order to investigate the effect of glucocorticoid excess on the availability of L-TRP and tyrosine to the brain, we measured urinary cortisol (UC) excretion in 24-h urine, and the availability of both amino acids before and after treatment with 1 mg dexamethasone in 26 depressed patients. We found no relationship between UC excretion and the availability of either amino acid. Dexamethasone significantly suppressed the availability of L-TRP (P less than 10(-5] and of tyrosine (P = 0.005). Major depressed patients with melancholia exhibited a significantly lower availability of L-TRP than minor depressives (P = 0.007).

The decreased availability of L-tryptophan in depressed females: clinical and biological correlates

1. The plasma levels of L-tryptophan (L-TRP) and the sum of five competing amino acids (CAA) namely tyrosine, phenylalanine, valine, leucine, isoleucine, were determined in 79 depressed females categorized according to the DSM-III. 2. In these patients the authors measured several parameters known to affect the availability of the above amino acids, i.e. triidothyronine (FT3) and thyroxine (FT4), vanilylmandelic acid (VMA), noradrenaline and adrenaline in 24 hr urine, the sex hormonal and nutritional state. 3. The 1 mg dexamethasone suppression test was performed and the pre and postdexamethasone cortisol and adrenocorticotropic hormone (ACTH) levels were determined at 8 a.m. 4. L-TRP and the ratio L-TRP/CAA were significantly lower in severely depressed females (296.X3, 296.X4) as compared with minor (300.40, 309.00) and simple major depressives (296.X2). The ratio L-TRP/CAA performed well as a clinical tool separating melancholic from minor depression. 5. FT3, FT4, VMA and noradrenaline were significantly increased in the severely depressed females, but these data did not correlate with the availability of L-TRP. Neither baseline cortisol nor the sex hormonal, nor the nutritional state related to the L-TRP data. The ratio L-TRP/CAA was significantly and negatively correlated with the postdexamethasone cortisol and ACTH values.

Suppressant effects of dexamethasone on the availability of plasma L-tryptophan and tyrosine in healthy controls and in depressed patients

Formation in the brain of serotonin from L-tryptophan (L-TRP) and noradrenaline from tyrosine are pathways related to the pathophysiology of major depression and to the regulation of the hypothalamic-pituitary-adrenal (HPA) axis. In the past, decrements in L-TRP availability and disorders in the HPA axis have repeatedly been observed in major depressed patients; both factors were shown to be inversely correlated. In order to investigate the relationships between glucocorticosteroid activity and the availability of L-TRP and tyrosine, the authors measured L-TRP, tyrosine, valine, leucine, isoleucine and phenylalanine in baseline conditions and after treatment with 1 mg dexamethasone in 16 healthy controls and in 50 depressed patients. The ratios between L-TRP and tyrosine and the sums of the amino acids known to compete with them during transport across the blood-brain barrier were computed as an index of (respectively) the serotonin and noradrenaline synthesis in the brain. We found significantly decreased plasma L-TRP and tyrosine levels after treatment with dexamethasone compared with basal levels. Accordingly, the plasma ratios between L-TRP and tyrosine and the sum of the competing amino acids were significantly reduced by dexamethasone administration. It was hypothesized that through these actions of dexamethasone on peripheral amino acids, the central noradrenaline and serotonin control over the HPA-axis could be altered.

Liver tryptophan pyrrolase. A major determinant of the lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL mice

The lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL, compared with -non-preferring CBA, mice is caused by a decrease in circulating tryptophan availability to the brain secondarily to a higher liver tryptophan pyrrolase activity associated with a higher circulating corticosterone concentration. Activity or expression of liver tryptophan pyrrolase and/or their induction by glucocorticoids may be important biological determinants of predisposition to alcohol consumption.

Physical exercise and brain monoamines: a review

A review of the literature on the relationship of exercise to mental health strongly suggests that the two are closely linked. Thus, physical activity has been reported to reduce depression symptoms and anxiety, and to improve coping with stress. Logically, many investigators have then tried to determine the physiological mechanisms that are responsible for this mood-elevating effect of exercise. Among the current hypotheses, those regarding the endorphin and the monoamine systems have received a great deal of attention. On that basis, the respective effects of physical exercise on brain dopamine, noradrenaline and serotonin transmission are reviewed herein. In addition, suggestions are advanced for future research in this field.

Effect of latent iron deficiency on 5-hydroxytryptamine metabolism in rat brain

Eight weeks of latent iron deficiency in weaned rats maintained on an experimental low iron content diet (18-20 mg/kg) did not significantly alter the packed cell volume and hemoglobin concentration; however, the hepatic and brain nonheme iron contents decreased by 66% and 21% (p less than 0.001), respectively. The tryptophan concentration decreased by 31% and 34% in liver and brain, respectively, in rats on experimental diet (p less than 0.01). The brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid contents were reduced by 21% and 23% (p less than 0.01 and p less than 0.02), respectively. However, in the brain, weight, protein, DNA, and the activities of monoamine oxidase, aldehyde dehydrogenase, and liver tryptophan oxygenase were found to remain unaltered. When rehabilitated with a diet containing 390 mg/kg iron, rats previously maintained on the experimental diet for 2 weeks showed partial recovery in tryptophan levels both in liver and brain. However, brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels remained unaltered. The hepatic iron content improved without any change in brain iron content. The latent iron deficiency produced significant alterations in the metabolism of 5-hydroxytryptamine and brain iron content that could not be recovered 2 weeks after the iron rehabilitation.

Determination of serum kynurenine and hepatic tryptophan dioxygenase activity by high-performance liquid chromatography

The status of the oxidative metabolism of L-tryptophan is usually evaluated by the determination of tryptophan metabolites in serum or urine and/or the activities of various oxidative enzymes in tissues. I have developed assays for serum kynurenine and hepatic tryptophan dioxygenase (TDO) activity based on the determination of kynurenine (KYN) by isocratic, reverse phase HPLC with spectrophotometric detection at 365 nm. Sample pretreatment prior to HPLC requires little more than perchloric acid precipitation of serum or a TDO incubation mixture. The analytical recovery for the serum assay was 101 +/- 2%, while the run-to-run coefficient of variation at normal KYN levels was approximately 8%. Serum KYN levels in 40 apparently healthy fasting humans were normally distributed and ranged from 0.27 to 0.69 microgram/ml (mean +/- SD: 0.47 +/- 0.1). Serum KYN in predialysis specimens from a group of 20 patients with chronic renal failure demonstrated a highly significant increase (mean +/- SD: 0.83 +/- 0.35 microgram/ml; P less than 0.001) as compared to the reference population. It is possible that such an increase might contribute to the pathophysiology of the uremic state. The analytical recovery of KYN from TDO incubation mixtures was approximately 90%. There was no evidence for the onward metabolism of KYN during the assay of whole liver homogenates. The mean (+/- SD) TDO activity of rat liver homogenates preincubated with ascorbate and hematin was 2.3 +/- 0.8 mumol/h/g wet wt (30 degrees C). The sensitivity, specificity, and convenience of these two methods suggest that they are suitable for routine use in the investigation of the biology and pathology of oxidative tryptophan metabolism.

Increase in tryptophan oxygenase activity in alcoholic patients

This study was conducted in order to assess whether chronic excessive alcohol consumption affects the activity of the liver enzyme tryptophan oxygenase which is rate limiting along the most important pathway of tryptophan catabolism. Five alcoholics were studied twice, once shortly after admission to an inpatient unit and the second time 1 month later. On each study day patients were given a tryptophan load of 50 mg/kg. Kynurenine in the urines (which reflects tryptophan oxygenase activity) was measured for a period of 6 hr following the load and showed a significantly enhanced activity of the enzyme shortly after cessation of drinking. This increased activity could explain the lowered tryptophan levels we have previously reported in alcoholics. The increase in enzyme activity may have been mediated by a rise in glucocorticoid hormones. In all instances, plasma cortisol measured hourly for 6 hr after the start of the experiment, was higher shortly after cessation of drinking than 1 month later.

Inhibition of rat liver tryptophan pyrrolase activity and elevation of brain tryptophan concentration by administration of DL-alpha-amino-beta-pyridinepropanoic acid (pyridylalanine) analogs

Single doses of DL-alpha-amino-beta-(2-pyridine)propanoic acid (2-PA, 100 mg/kg) significantly decreased the holoenzyme and apoenzyme activities of rat liver tryptophan pyrrolase (TP) and increased brain tryptophan, serotonin (5-HT) and 5-hydroxyindole-3-ylacetic acid concentrations. 2-PA had no inhibitory effect on either of the enzyme activities in vitro, but its expected metabolites were effective. Single doses of DL-alpha-amino-beta-(3-pyridine)propanoic acid (3-PA, 100 mg/kg) decreased only the holoenzyme activity and elevated brain tryptophan and its metabolites levels in rats. 3-PA and its metabolite, 3-pyridylpyruvate, inhibited only the holoenzyme activity in vitro. DL-alpha-Amino-beta-(4-pyridine)propanoic acid (4-PA) caused significant changes in liver TP (holo- and apoenzyme forms) activity and brain tryptophan concentration only after repeated administration (100 mg/kg/day). 4-PA was a weak inhibitor of the holoenzyme, but its metabolites apparently inhibited the holo- and apoenzyme activities in vitro. These findings suggest that PA analogs (and/or their metabolites) increased brain tryptophan (and hence 5-HT synthesis) by directly inhibiting liver TP activity.

Peripheral and central short-term effects of fusaric acid, a DBH inhibitor, on tryptophan and serotonin metabolism in the rat

Fusaric acid (FA) administration to the rats promoted one hour later a large decrease in plasma total tryptophan (TRP), without affecting either plasma free TRP or lipolysis, as measured by plasma non esterified fatty acid concentration. The previous change was associated with hypoinsulinemia, hyperglycemia and increased plasma corticosterone level. Regression analysis revealed a significant correlation between brain TRP and the percentage of plasma TRP which was free (i.e. unbound to albumin), both increased by FA injection. The increase in brain TRP promoted an increased brain serotonin synthesis, as measured by the enhanced brain and CSF 5-HIAA levels. Valine pretreatment, which blocks TRP entry into the brain, completely prevented FA-induced brain TRP and brain 5-HIAA increases. These results suggest that the increased brain serotonergic turnover following FA treatment was due to a peripheral action of the drug upon TRP disposition. The latter effect may be caused (i) by in vivo peripheral alterations in catecholaminergic metabolism and (ii) by FA chemical structure since in vitro experiments revealed that FA was able to displace TRP binding to albumin, thus increasing the plasma free TRP pool.

Quantification of the importance of individual steps in the control of aromatic amino acid metabolism

The quantitative importance of the individual steps of aromatic amino acid metabolism in rat liver was determined by calculation of the respective Control Coefficients (Strengths). The Control Coefficient of tryptophan 2,3-dioxygenase for tryptophan degradation was determined in a variety of physiological conditions and with a range of activities of tryptophan 2,3-dioxygenase. The Control Coefficient varied from 0.75 with basal enzyme activity to 0.25 after maximal induction of the enzyme by dexamethasone. The remainder of the control for tryptophan degradation was associated with the transport of the amino acid across the plasma membrane, with only very small contributions from kynureninase and kynurenine hydroxylase. The Control Coefficients of tyrosine aminotransferase for tyrosine degradation were approx. 0.70 and 0.20 with basal and dexamethasone-induced tyrosine aminotransferase activities respectively; the Control Coefficients of the transport of the amino acid into the cell were 0.22 and 0.58 respectively. Phenylalanine hydroxylase was found to have a Control Coefficient for the degradation of phenylalanine of approx. 0.50 under conditions of basal enzyme activity; after maximal activation by glucagon, the Control Coefficient decreased to 0.12. The transport of phenylalanine was responsible for the remaining control in the pathway. These results have important implications, directly for the regulation of aromatic amino acid metabolism in the liver, and indirectly for the regulation of neuroamine synthesis in the brain.

Dominant social status facilitates the behavioral effects of serotonergic agonists

The effects of dominance rank on the behavioral responses to drugs that enhance central serotonergic function were examined in 45 adult male vervet monkeys living in 15 stable social groups. Each group contained 3 adult males, 3 adult females, and their immature offspring. Dominance rank was assessed by measuring success in intermale agonistic encounters. In every group one male was clearly the dominant, or alpha male, and the other two males were subordinate. Males from 5 groups received 3 doses of the serotonin reuptake inhibitor fluoxetine (0.5, 1.0 and 2.0 mg/kg/day); those from a second set of 5 groups received 3 doses of the receptor agonist quipazine (0.25, 0.50 and 1.0 mg/kg/day); those from a third set of 5 groups received the serotonin precursor tryptophan (10, 20 and 40 mg/kg/day). The 3 drug treatments produced strikingly similar behavioral effects. Each produced dose-dependent increases in approaching, grooming, resting and eating and decreases in locomoting, avoiding, being vigilant and being solitary. Dominant males were significantly more responsive behaviorally to all 3 drugs than were subordinate males: the increase or decrease in each behavioral measure was larger in dominant than in subordinate males. In combination with previous studies, these data suggest that dominant and subordinate males differ in the drug sensitivity of their serotonergic systems.

Ethanol increases rat liver tryptophan oxygenase: evidence for corticosterone mediation

Acute administration of ethanol (4.0 g/kg) intragastrically or intraperitoneally induced rat liver tryptophan oxygenase (TO) activity 3-4 fold 4-5 hr later. Ethanol administration increased the concentration of plasma free tryptophan and free fatty acids. Pretreatment with a beta-receptor blocker, propranolol, modified the latter responses without affecting the TO induction due to ethanol. The rise of the level of plasma free tryptophan due to ethanol was too small to influence TO activity. Liver tryptophan concentration and TO half-life was unchanged after ethanol administration. Ethanol administration increased the concentration of plasma corticosterone sufficiently to increase TO activity. Pretreatment with a glucocorticoid antagonist blocked this TO response to ethanol. The increased TO activities found after ethanol or corticosterone treatment were influenced in the same manner and to the same extent by cycloheximide. Taken together it is concluded that ethanol induces TO through a rise of glucocorticoid hormones and not by a tryptophan-linked mechanism.