Evidence that the large neutral amino acid L-valine decreases electrically-evoked release of 5-HT in rat hippocampus in vivo

Nutritional importance of tryptophan for improving treatment in depression and diabetes

The importance of nutrients in our diet is becoming increasingly recognized. From the viewpoint of protein synthesis and other physiologic and metabolic functions, all amino acids are important, but some of these amino acids are not synthesized endogenously. This subset, called essential amino acids, comprise dietarily indispensable nutrients. Tryptophan, an essential amino acid, is the sole precursor of neuronal as well as peripheral serotonin (5-hydroxytryptamine). Its systemic or oral administration increases serotonin synthesis because tryptophan hydroxylase, the rate-limiting enzyme of 5-hydroxytryptamine biosynthesis, is physiologically unsaturated with its substrate. Central serotonin is implicated in a number of psychiatric illnesses, including depression, and in responses to stress. Acting peripherally, serotonin affects vasoconstriction, intestinal motility, control of T cell–mediated immunity, and liver and pancreatic functions. Depression and diabetes are 2 highly prevalent diseases that often coexist. There is evidence that occurrence of depression is 2–3 times higher in people with diabetes mellitus. A comorbid condition of diabetes and depression worsens the treatment and increases risk for death. Stress, known for its causal role in depression, can also enhance risk for diabetes. Stress-induced decreases in the circulating levels of tryptophan can impair brain and pancreatic serotonin-dependent functions to precipitate these diseases. The importance of tryptophan supplementation for improving therapeutic intervention in depression and diabetes is the focus of this article. A deficiency of this essential amino acid may enhance risk for depression as well as diabetes, and can also weaken treatment efficacy of medicinal compounds for treating these diseases. Guidelines for optimal levels of circulating tryptophan can help if supplements of this amino acid can improve treatment efficacy.

The Antidepressant-Like Effect of Lactate in an Animal Model of Menopausal Depression

BACKGROUND

This study aimed to investigate the antidepressant-like effect of lactate and elucidate its mechanisms in ovariectomized rats with repeated stress.

METHODS

Two experiments were conducted on female rats in which all groups, except normal, were ovariectomized and underwent immobilization for 14 days. Lactate was administered orally (100, 250, and 500 mg/kg) for 14 consecutive days, and the rats' cutaneous body temperature was measured during the same period. Depression-like behavior in rats was assessed by the tail suspension test (TST) and forced swimming test (FST). Furthermore, enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry were conducted to evaluate the changes that occurred in the neurotransmitter levels and activity.

RESULTS

The lactate 100 and 250 groups had reduced time spent immobile in TST and FST and decreased peripheral body temperature. In ELISA tests, the lactate 250 group expressed elevated levels of serotonin and dopamine in many brain areas. Tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), and protein kinase C (PKC) immunoreactive cells showed increased density and cell counts in lactate administered groups.

CONCLUSION

Results indicated that lactate has an antidepressant effect that is achieved by activation of PKC and upregulation of TH and TPH expression, which eventually leads to enhanced serotonin and dopamine levels in the menopausal rat's brain.

Improving therapeutics in anorexia nervosa with tryptophan

A growing body of evidence suggests that our diet is an important contributing factor in the development, management and prevention of a number of psychiatric illnesses. Tryptophan, an essential amino acid, is the sole precursor of neurotransmitter 5-hydroxytryptamine (5-HT; serotonin). Administration of tryptophan can boost serotonin neurotransmission to produce therapeutically important effects in serotonin deficiency disorders. Anorexia nervosa (AN) an eating disorder associated with high levels of psychiatric comorbidity including psychosis, hyperactivity, depression and anxiety has highest lethality of all psychiatric illnesses. Evidence suggests that excessive dieting and food restriction can decrease brain tryptophan and serotonin in AN patients to precipitate depression, psychosis and hyperactivity. There are currently no FDA approved pharmacological treatments available for AN patients; antidepressants and antipsychotics, largely used to treat associated psychiatric comorbidities are also not very effective. The aim of this non-systematic review article is to evaluate and document a potential importance of tryptophan supplementation in improving therapeutics in AN patients.

Beneficial Effects of a Protein Free, High Carbohydrate Meal on Rat Coping Behavior and Neurotransmitter Levels During Heat Stress

This study examined effects of a single protein-free (PROT-free), high carbohydrate meal (0% protein, 82% carbohydrate) or control meal (20% protein, 62% carbohydrate) on coping behavior and neurotransmitter levels of male Fischer 344 rats. Meals varying in protein/carbohydrate ratio differentially affect transport of neurotransmitter precursors into the brain and alter neurotransmission. The meals were studied in animals exposed to heat stress and normothermic control animals. Coping behavior was assessed using the Porsolt swim test (N = 39). In other rats, dopamine (DA), epinephrine (EPI), norepinephrine (NE) and serotonin in the striatum were assessed using in vivo microdialysis (N = 35). In control-fed rats, heat stress impaired Porsolt performance in comparison to normothermic controls (p <0.05). The PROT-free, high carbohydrate meal protected animals from adverse effects of stress on coping performance as it reduced the effect of heat stress more than 90%. Significant changes in striatal DA and EPI efflux were observed as a consequence of the nutritional manipulation and heat exposure. It is concluded a single PROT-free, high carbohydrate meal improves ability to cope with heat stress and alters neurotransmission. The neurochemical basis for these effects may be a change in DA efflux, although other mechanisms cannot be ruled out.

Large neutral amino acids: dietary effects on brain neurochemistry and function

The ingestion of large neutral amino acids (LNAA), notably tryptophan, tyrosine and the branched-chain amino acids (BCAA), modifies tryptophan and tyrosine uptake into brain and their conversion to serotonin and catecholamines, respectively. The particular effect reflects the competitive nature of the transporter for LNAA at the blood-brain barrier. For example, raising blood tryptophan or tyrosine levels raises their uptake into brain, while raising blood BCAA levels lowers tryptophan and tyrosine uptake; serotonin and catecholamine synthesis in brain parallel the tryptophan and tyrosine changes. By changing blood LNAA levels, the ingestion of particular proteins causes surprisingly large variations in brain tryptophan uptake and serotonin synthesis, with minimal effects on tyrosine uptake and catecholamine synthesis. Such variations elicit predictable effects on mood, cognition and hormone secretion (prolactin, cortisol). The ingestion of mixtures of LNAA, particularly BCAA, lowers brain tryptophan uptake and serotonin synthesis. Though argued to improve physical performance by reducing serotonin function, such effects are generally considered modest at best. However, BCAA ingestion also lowers tyrosine uptake, and dopamine synthesis in brain. Increasing dopamine function in brain improves performance, suggesting that BCAA may fail to increase performance because dopamine is reduced. Conceivably, BCAA administered with tyrosine could prevent the decline in dopamine, while still eliciting a drop in serotonin. Such an LNAA mixture might thus prove an effective enhancer of physical performance. The thoughtful development and application of dietary proteins and LNAA mixtures may thus produce treatments with predictable and useful functional effects.

Effect of chronic protein ingestion on tyrosine and tryptophan levels and catecholamine and serotonin synthesis in rat brain

OBJECTIVES

Previous studies have shown that brain tyrosine (TYR) levels and catecholamine synthesis rate increase in rats as chronic dietary protein content increases from 2 to 10% (% weight). A single protein, casein, was examined. The present study explores how TYR levels and catecholamine synthesis (and tryptophan (TRP) levels and serotonin synthesis) change when different proteins are ingested chronically over the same range of dietary protein contents.

METHODS

Male rats ingested for 8 days diets contain 2 or 10% protein (zein, gluten, casein, soy protein, or alpha-lactalbumin). On the last day, they were killed 2.5 hours into the dark period, 30 minutes after receiving an injection of m-hydroxybenzylhydrazine, an inhibitor of aromatic l-amino acid decarboxylase. Brain samples were analyzed for amino acids, including 5-hydroxytryptophan (index of serotonin synthesis rate) and dihydroxyphenylalanine (index of catecholamine synthesis rate), by HPLC-electrochemical detection.

RESULTS

TYR levels and catecholamine synthesis rate in brain were unaffected by the particular protein ingested. However, TRP levels and serotonin synthesis rate varied markedly, depending on the protein ingested, with effects being most prominent in the 10% protein groups. The effect of dietary protein on brain TRP correlated very highly with its effect on serotonin synthesis.

DISCUSSION

The results indicate that the protein ingested can chronically modify TRP levels and serotonin synthesis in brain, but not TYR levels or catecholamine synthesis, with effects most distinct at an adequate level of protein intake (10%).

Effects of postnatal treadmill exercise on apoptotic neuronal cell death and cell proliferation of maternal-separated rat pups

Early adverse experiences resulting from maternal separation may lead to neuronal cell death and they can eventually cause memory impairment. In this study, we investigated the effects of postnatal treadmill exercise on the long-term memory capability, spatial learning ability, and depressive state, on the 5-hydroxytryptamine (5-HT, serotonin) synthesis and tryptophan hydroxylase (TPH) expression in the dorsal raphe nuclei, and on the apoptosis and cell proliferation in the hippocampal dentate gyrus of rat pups following maternal separation. The rat pups in the maternal separation groups were separated from their respective mothers on the postnatal day 14. The rat pups in the maternal separation group showed depressive state with deceased memory capability and learning ability compared to the rat pups in the maternal care group. Postnatal treadmill exercise increased memory capability and learning ability and alleviated depressive state of the rat pups in the maternal separation group. The 5-HT synthesis and TPH expression in the dorsal raphe nuclei and cell proliferation in the hippocampal dentate gyrus were significantly decreased in the maternal-separated rat pups, and postnatal treadmill exercise increased 5-HT synthesis, the TPH expression, and the cell proliferation. In contrast, apoptotic neuronal cell death in the hippocampal dentate gyrus was significantly increased in the maternal-separated rat pups, and postnatal treadmill exercise suppressed the maternal separation-induced apoptosis. The present results demonstrated that postnatal treadmill exercise alleviated maternal separation-induced depression with decrease of memory capability and learning ability, by suppressing apoptotic neuronal cell death and by enhancing cell proliferation.

WITHDRAWN: Is acute tryptophan depletion a valid method to assess central serotonergic function?

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Vardenafil increases cell proliferation in the dentate gyrus through enhancement of serotonin expression in the rat dorsal raphe

This study was conducted to evaluate the effects of vardenafil (Levitra), a phosphodiesterase-5 (PDE-5) inhibitor, on cell proliferation in the hippocampal dentate gyrus and on 5-hyroxytryptamine (5-HT, serotonin) synthesis and tryptophan hydroxylase (TPH) expression in the rat dorsal raphe nucleus. Male Sprague-Dawley rats were divided into 6 groups (n=5 in each group): a control group, a 0.5 mg/kg-1 day vardenafil-treated group, a 1 mg/kg-1 day vardenafil-treated group, a 2 mg/kg-1 day vardenafil-treated group, a 1 mg/kg-3 day vardenafil-treated group, and a 1 mg/kg-7 day vardenafil-treated group. 5-bromo-2'-deoxyuridine (BrdU) immunohistochemistry was then performed to evaluate cell proliferation in the dentate gyrus. In addition, 5-HT and TPH immunohistochemistry was conducted to evaluate serotonin expression in the dorsal raphe. The results revealed that treatment with vardenafil increased cell proliferation in the dentate gyrus and enhanced 5-HT synthesis and TPH expression in the dorsal raphe in a dose- and duration-dependent manner. The findings demonstrate that the increasing effect of vardenafil on cell proliferation is closely associated with the enhancing effect of vardenafil on serotonin expression under normal conditions.

L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications

An essential component of the human diet, L-tryptophan is critical in a number of metabolic functions and has been widely used in numerous research and clinical trials. This review provides a brief overview of the role of L-tryptophan in protein synthesis and a number of other metabolic functions. With emphasis on L-tryptophan's role in synthesis of brain serotonin, details are provided on the research uses of L-tryptophan, particularly L-tryptophan depletion, and on clinical trials that have been conducted using L-tryptophan supplementation. The ability to change the rates of serotonin synthesis in the brain by manipulating concentrations of serum tryptophan is the foundation of much research. As the sole precursor of serotonin, experimental research has shown that L-tryptophan's role in brain serotonin synthesis is an important factor involved in mood, behavior, and cognition. Furthermore, clinical trials have provided some initial evidence of L-tryptophan's efficacy for treatment of psychiatric disorders, particularly when used in combination with other therapeutic agents.

Additive subthreshold dose effects of cannabinoid CB(1) receptor antagonist and selective serotonin reuptake inhibitor in antidepressant behavioral tests

The main clinically used antidepressant drugs are selective monoamine reuptake inhibitors, including selective serotonin reuptake inhibitors (citalopram, sertraline), selective dopamine reuptake inhibitor (nomifensine) and selective noradrenaline reuptake inhibitor (reboxetine), but they have various side effects. Because cannabinoid CB(1) receptor antagonists (SR141716A, AM251) enhance monoamine release, they might be beneficial in the therapy of affective disorders. We hypothesized that the use of monoamine reuptake inhibitors in combination with cannabinoid CB(1) receptor antagonists would allow a lower dose of monoamine reuptake inhibitors to be used in the therapy of depression, thereby reducing or eliminating the side effects. To test this hypothesis, we examined the combination of SR141716A or AM251 with citalopram, sertraline, nomifensine or reboxetine at subthreshold doses to see whether these combinations would show an additive effect in the forced swimming test and the tail suspension test with mice. Subthreshold doses of cannabinoid CB(1) receptor antagonist and selective serotonin reuptake inhibitors, which separately had no effect on the immobility of mice in the tests, showed a clear effect when the drugs were administered at 40 and 30 min, respectively, before the tests, without any change of motor activity. Therefore, the use of subthreshold doses of these agents in combination might be useful to enhance mainly serotonergic neurotransmission, and to reduce or eliminate the side effects of citalopram and sertraline.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA-) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 microM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA- lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA- had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 microM) was administered in perfusate, LNAA- still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.

Effect of Acanthopanax senticosus on 5-hydroxytryptamine synthesis and tryptophan hydroxylase expression in the dorsal raphe of exercised rats

Acanthopanax senticosus Harms (AS) is classified into the family of Araliaceae. The plant has been used as an analeptic aid, which improves weakened physical status and strength. Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter and tryptophan hydroxylase (TPH) catalyzes the rate-f the raphe nuclei. These are associated with "central fatigue hypotheses" in the brain. In the present study, the effects of Acanthopanax senticosus on the time to exhaustion by treadmill exercise and on 5-HT synthesis and TPH expression in the dorsal raphe were investigated by immunohistochemistry. In the present results, Acanthopanax senticosus increased the time to exhaustion by treadmill running and it suppressed the exercise-induced increase of 5-HT synthesis and TPH expression. Acanthopanax senticosus was effective as caffeine for increasing the exhaustion time in treadmill running and for reducing the exercise-induced increase of 5-HT synthesis and TPH expression in the dorsal raphe. The present study shows that Acanthopanax senticosus reduces fatigue during exercise by the inhibition of exercise-induced 5-HT synthesis and TPH expression in the dorsal raphe.

A long-term ecstasy-related change in visual perception

RATIONALE

The present study provides the first evidence of the long-term consequences of ecstasy use on visual processes thought to reflect serotonergic functions in the occipital lobe. Methylenedioxymethamphetamine ("ecstasy") is known to cause lasting changes to the serotonin system in animals, and convergent evidence suggests that similar changes occur in human ecstasy users. Other research suggests that serotonin may be involved in lateral inhibition between orientation sensitive neurons in the occipital lobe, and that disruption to the serotonin system causes an increase in the magnitude of the tilt aftereffect illusion that is known depend on those neurons.

OBJECTIVES

The aim of the present study was to determine if ecstasy users have detectable changes in occipital lobe behavioural functioning, as revealed by the tilt aftereffect illusion.

MATERIALS AND METHODS

Thirty ecstasy users and 34 non-drug using controls were compared on the magnitude of the tilt aftereffect illusion following adaptation to stimuli oriented at 15 and 40 degrees from vertical.

RESULTS

Ecstasy users who had not used amphetamines for 115 days or more had a larger average tilt aftereffect than non-drug using controls after adaptation to 40 degrees stimuli but not after adaptation to 15 degrees stimuli. Additionally, there was no difference between non-drug using controls and ecstasy users who had used amphetamines within the last 61 days at either adaptation angle.

CONCLUSIONS

The results were consistent with the proposal that ecstasy-related damage to the serotonin system causes behavioural changes on tests of visual perception processes that are thought to reflect serotonergic functions in the occipital lobe.

Effects of rapid tryptophan depletion on salivary cortisol in older people recovered from depression, and the healthy elderly

Reduced serotonin (5-HT) function and abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis are thought to play a role in the aetiology of major depression. We sought to examine this issue in the elderly by assessing the effects of lowering brain 5-HT on salivary and plasma cortisol in elderly patients who had recovered from at least one episode of major depression and in a healthy, age matched comparison group. A double-blind, cross-over design involving administration of two nutritionally balanced amino acid mixtures (with or without tryptophan) was used. Salivary cortisol was measured at intervals before and after the drink. There was no effect of acute tryptophan depletion (ATD) on salivary cortisol (ATD by time; F=0.97, df=7,210, p=0.454) but a significant interaction between group and time (F=3.91, df=7,210, p=0.010). Healthy subjects showed a marked increase in cortisol levels 2-3 hours into the procedure regardless of drink composition while recovered depressed subjects did not. In elderly patients who had recovered from depression there was no evidence of greater vulnerability of hypothalamic 5-HT pathways to 5-HT depletion. However, they demonstrated reduced reactivity of the HPA axis compared to healthy subjects.

Acute tryptophan depletion. Part I: rationale and methodology

OBJECTIVE

Acute tryptophan depletion (ATD) is an experimental technique that has been widely used over the last decade to investigate the role of serotonin (5-HT) in a variety of disorders. This review, the first of two articles, describes the rationale behind this technique and provides detail on how it is applied in research settings.

METHOD

The authors outline the development of this technique with reference to the seminal literature and more recent findings from neuroimaging and neuroendocrine studies. This is supplemented by the authors' clinical experience of over 5 years of continuous experimental work with this paradigm in over 50 subjects.

RESULTS

Acute tryptophan depletion is a method that significantly reduces central 5-HT in human subjects. Non-serotonergic explanations of the effects of ATD have not been confirmed, supporting the specificity of this method.

CONCLUSIONS

The ATD technique is a valid method of manipulating central 5-HT levels. The second article in this series will review the application of ATD in depression, anxiety and other psychiatric conditions.

A low-protein diet alters rat behavior and neurotransmission in normothermic and hyperthermic environments

Dietary protein contains amino acids used in the brain for synthesis of neurotransmitters. Although information on pre- and post-natal exposure to low-protein diets in rodents is available, little is known about effects of such diets on adult animals. Therefore, the behavioral and neurochemical consequences of exposure to a brief (11 days), low (4%)-protein diet in animals exposed to normothermic and hyperthermic test conditions were examined. In separate groups of animals, the Porsolt Swim test and elevated plus maze were administered. These tasks are sensitive to nutritional and/or environmental manipulations. In other groups of rats exposed to the same dietary and environmental conditions, dopamine, norepinephrine, epinephrine, and serotonin in the striatum were assessed using microdialysis. In the Porsolt swim test, which assesses coping behavior, performance was impaired under normothermic and hyperthermic conditions in animals on the low-protein diet. Performance on the plus maze, a measure of exploration and anxiety, was altered in the hyperthermic condition by low protein, with the diet increasing exploration. Microdialysis detected increased norepinephrine in the striatum of hyperthermic animals on the low-protein diet. This study demonstrates that changes in stress-related behaviors of adult animals occur following brief exposure to low-protein diets.

Effects of reduced endogenous 5-HT on the in vivo binding of the serotonin transporter radioligand11C-DASB in healthy humans

Although abnormal serotonin (5-HT) function is implicated in a range of mental disorders, there is currently no method to directly assess 5-HT synaptic levels in the living human brain. The in vivo binding of some dopamine (DA) radioligands such as (11)C-raclopride is affected by fluctuations in endogenous DA, thus providing an indirect measure of DA presynaptic activity. Attempts to identify a serotonergic radiotracer with similar properties have proved unsuccessful. Here, we investigated in humans the effects of reduced synaptic 5-HT on the in vivo binding of the 5-HT transporter (SERT) radioligand (11)C-DASB, using Positron Emission Tomography (PET) and the rapid tryptophan depletion (RTD) technique. Eight (8) subjects (5M, 3F) were scanned with (11)C-DASB under control and reduced endogenous 5-HT conditions, in a within-subject, double-blind, counterbalanced, crossover design. Regional distribution volumes (V(T)) were calculated using kinetic modeling and metabolite-corrected arterial input function. (11)C-DASB specific binding was estimated as binding potential (BP) and specific to nonspecific equilibrium partition coefficient (V(")(3)), using the cerebellum as reference region. RTD caused small but significant mean reductions in (11)C-DASB V(T) (-6.1%) and BP (-4.5%) across brain regions, probably explained by a concomitant reduction in (11)C-DASB plasma free fraction (f(1)) of similar magnitude. No significant change in (11)C-DASB V(")(3) was observed between control and reduced 5-HT conditions. Nor was there a significant relationship between the magnitude of tryptophan depletion and change in BP and V(")(3) across individual subjects. These results suggest that (11)C-DASB in vivo binding is not affected by reductions in endogenous 5-HT.

Normal levels of tryptophan hydroxylase immunoreactivity in the dorsal raphe of depressed suicide victims

A variety of evidence suggests that serotonin neurotransmission is altered in the brain of suicide victims and depressed patients. While numerous post-mortem studies have investigated serotonin transporters and receptors, few studies have examined the biosynthetic integrity of the rate-limiting enzyme, tryptophan hydroxylase (TPH), in post-mortem specimens of depressed suicide subjects. Therefore, the aim of the present study was to test the hypothesis that the levels of TPH immunoreactivity (IR) are altered in specific subnuclei of the dorsal raphe (DR) in depressed suicide victims. Suicide victims with a confirmed diagnosis of major depression were matched with non-psychiatric controls based on age, gender and post-mortem interval. Frozen tissue sections containing the DR were selected from two anatomical levels and processed for TPH radioimmunocytochemistry. The optical density corresponding to the regional levels of TPH-IR was quantified in specific subnuclei of the DR from the film autoradiographic images. No significant differences in the levels of TPH-IR were found in any DR subnuclei between depressed suicide victims and control subjects. The lack of change in TPH-IR levels does not necessarily imply that serotonin synthesis or neurotransmission is not altered in the brain of depressed subjects. Many factors influence and regulate serotonin synthesis, and it is conceivable that alterations exist at other levels of regulation of serotonin biosynthesis in depression. Our findings indicate that TPH biosynthesis, at least at the protein level, is not significantly altered in the DR of depressed suicide victims.

Effects of Paeonia radix on 5-hydroxytryptamine synthesis and tryptophan hydroxylase expression in the dorsal raphe of exercised rats

Paeonia radix is the root of Paeonia japonica MIYABE, a perennial plant classified in the family Paeoniaceae. In the present study, the effects of Paeonia radix on performance in treadmill exercise, and 5-hydroxytryptamine (5-HT) synthesis and tryptophan hydroxylase (TPH) expression in the dorsal raphe were investigated. Time to exhaustion in treadmill exercise was increased and exercise-induced increases in 5-HT synthesis and TPH expression in the dorsal raphe were shown to be suppressed by Paeonia radix treatment; 5-HT synthesis and TPH expression were inhibited by Paeonia radix treatment under resting conditions as well. In sum, treatment with Paeonia radix, inhibiting 5-HT synthesis and TPH expression, may bring about reduced fatigue, both during exercise and the resting state.

Effects of acupuncture on the 5-hydroxytryptamine synthesis and tryptophan hydroxylase expression in the dorsal raphe of exercised rats

In the present study, the effects of acupuncture on the endurance treadmill exercise, serotonin (5-hydroxytryptamine, 5-HT) synthesis, and tryptophan hydroxylase (TPH) expression in the dorsal raphe nuclei were investigated. Acupunctural stimulation were bilaterally inserted into the locus of Zusanli in each hind leg for 20 min. Onset of exhaustion time on the treadmill running was increased significantly by acupunctural stimulation and exercise-induced increments of the 5-HT synthesis and TPH expression in the dorsal raphe were suppressed by acupunctural stimulation. Based on the results, it can be suggested that the suppressive effects of acupunctural stimulation on the 5-HT synthesis and TPH expression in the dorsal raphe during exercise is one of ergogenic mechanisms of acupuncture.

Up-regulation of tryptophan hydroxylase expression and serotonin synthesis by sertraline

The neurotransmitter serotonin is involved in a variety of brain functions, and abnormal changes in serotonin neurotransmission are associated with an array of psychiatric disorders, including depression. Sertraline is a selective serotonin reuptake inhibitor (SSRI) and an effective antidepressant. Sertraline increases the serotonin concentration in the synaptic cleft by a short-term action; however, clinical improvement is observed only after several weeks, suggesting that the therapeutic effect may be caused by long-term alterations in serotonin transmission. We determined the effects of sertraline on serotonin synthesis in vivo and in vitro. Long-term treatment of rats with sertraline up-regulated mRNA and protein levels of the serotonin-synthesizing enzyme tryptophan hydroxylase (TPH), as determined by in situ hybridization and immunocytochemistry, respectively. In vitro studies using RBL-2H3 cells also showed an increase in mRNA and protein levels of TPH by sertraline, as determined by Northern blot and immunoblot analyses, respectively. This was accompanied by increases in the levels of TPH enzymatic activity and total serotonin. These data demonstrate that in addition to the known short-term action as an uptake blocker, sertraline also exerts a long-term effect on the serotonin neurotransmission by enhancing serotonin synthesis. A similar effect was observed with another SSRI, fluoxetine, but not with the non-SSRI chlorpromazine. The up-regulation of TPH gene expression by sertraline was attenuated by the protein kinase A (PKA) inhibitor N-[2-(p-bromocinnamylamine)-ethyl]-5-isoquinolinesulfonamine, suggesting that a mechanism involving the PKA signaling pathway might at least in part mediate the long-term therapeutic action.

Caffeine inhibits exercise-induced increase in tryptophan hydroxylase expression in dorsal and median raphe of Sprague-Dawley rats

Effect of caffeine on the expression of tryptophan hydroxylase (TPH), rate limiting enzyme of serotonin synthesis, in dorsal and median raphe was investigated via immunohistochemistry. In exercise groups, Sprague-Dawley rats were put on treadmill running for 30 min per day for 6 consecutive days. On the seventh day, animals of control-with-caffeine group were injected subcutaneously with 4 mg/kg caffeine, while control-without-caffeine group were injected with 0.9% NaCl, sacrificed 2 h later. Exercise-with-caffeine group and exercise-without-caffeine group were injected with caffeine and NaCl, respectively; all-out time was determined 1 h after injection, and then sacrificed. Caffeine increased all-out time in exercised rats, and inhibited the exercise-induced elevation in TPH expression. The suppressive effect of caffeine on TPH expression in exercised rats can be suggested as one possible ergogenic mechanism of caffeine.

Tryptophan hydroxylase regulation. Drug-induced modifications that alter serotonin neuronal function

Tryptophan hydroxylase is the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin. A variety of drugs are known to diminish the function of this enzyme, and possibly cause damage to serotonin neurons. These include the substituted amphetamines methamphetamine and 3,4-methylenedioxy-methamphetamine, as well as L-DOPA, the most common therapy for Parkinsons Disease. In view of the important role for dopamine in the effects of these drugs on tryptophan hydroxylase and on serotonin neurons, we tested whether dopamine could alter the activity of this important enzyme. We found that dopamine-derived quinones, but not dopamine, inactivate tryptophan hydroxylase and convert the protein to a redox-cycling quinoprotein. This posttranslational modification of tryptophan hydroxylase could play a role in the drug-induced reduction in serotonin synthesis.

L-DOPA-quinone inactivates tryptophan hydroxylase and converts the enzyme to a redox-cycling quinoprotein

Tryptophan hydroxylase, the initial and rate limiting enzyme in the biosynthesis of serotonin (5-HT), is inactivated by the quinone of L-DOPA. L-DOPA itself has no effect on enzyme activity. The inactivation of tryptophan hydroxylase could be prevented by glutathione (GSH), dithiothreitol, cysteine, and ascorbic acid but not by scavengers of hydrogen peroxide (catalase), hydroxyl radical (DMSO), or superoxide (superoxide dismutase). All cysteinyl residues within tryptophan hydroxylase are modified after treatment with L-DOPA-quinone as revealed by loss of DTNB-reactivity and formation of cysteinyl-DOPA residues. L-DOPA-quinone also converts tryptophan hydroxylase to a redox-cycling quinoprotein. These results suggest a possible mechanism of 5-HT neuronal damage in Parkinson's Disease by a redox-cycling quinoprotein.

Peroxynitrite inactivates tryptophan hydroxylase via sulfhydryl oxidation. Coincident nitration of enzyme tyrosyl residues has minimal impact on catalytic activity

Tryptophan hydroxylase, the initial and rate-limiting enzyme in serotonin biosynthesis, is inactivated by peroxynitrite in a concentration-dependent manner. This effect is prevented by molecules that react directly with peroxynitrite such as dithiothreitol, cysteine, glutathione, methionine, tryptophan, and uric acid but not by scavengers of superoxide (superoxide dismutase), hydroxyl radical (Me(2)SO, mannitol), and hydrogen peroxide (catalase). Assuming simple competition kinetics between peroxynitrite scavengers and the enzyme, a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25 degrees C and pH 7.4 was estimated. The peroxynitrite-induced loss of enzyme activity was accompanied by a concentration-dependent oxidation of protein sulfhydryl groups. Peroxynitrite-modified tryptophan hydroxylase was resistant to reduction by arsenite, borohydride, and dithiothreitol, suggesting that sulfhydryls were oxidized beyond sulfenic acid. Peroxynitrite also caused the nitration of tyrosyl residues in tryptophan hydroxylase, with a maximal modification of 3.8 tyrosines/monomer. Sodium bicarbonate protected tryptophan hydroxylase from peroxynitrite-induced inactivation and lessened the extent of sulfhydryl oxidation while causing a 2-fold increase in tyrosine nitration. Tetranitromethane, which oxidizes sulfhydryls at pH 6 or 8, but which nitrates tyrosyl residues at pH 8 only, inhibited tryptophan hydroxylase equally at either pH. Acetylation of tyrosyl residues with N-acetylimidazole did not alter tryptophan hydroxylase activity. These data suggest that peroxynitrite inactivates tryptophan hydroxylase via sulfhydryl oxidation. Modification of tyrosyl residues by peroxynitrite plays a relatively minor role in the inhibition of tryptophan hydroxylase catalytic activity.

Effects of acute tryptophan depletion on plasma and cerebrospinal fluid tryptophan and 5-hydroxyindoleacetic acid in normal volunteers

Brain serotonin synthesis and metabolism (turnover), as indicated by CSF concentrations of 5-hydroxyindoleacetic acid (5-HIAA), may depend on plasma concentrations of the essential amino acid L-tryptophan (TRP). We investigated the biochemical effects of acute plasma TRP depletion (ATD) in normal volunteers undergoing a 36-h CSF collection via lumbar drain. Six subjects who were in good health were put on a low-TRP diet (160 mg/day) 24 h before lumbar puncture; this diet was continued for the first 22 h of the CSF collection. At hour 22, subjects ingested a TRP-deficient 15-amino acid drink shown previously to deplete plasma TRP. Total plasma TRP, free plasma TRP, and CSF TRP subsequently decreased 86.3, 86.5, and 92.3%, respectively. CSF 5-HIAA decreased by 32.8%. Plasma total and free TRP concentrations were both decreased at approximately 2 h following ingestion of the TRP-free amino acid drink and were lowest approximately 6 h after ATD; CSF TRP and 5-HIAA were decreased at 2.5 h and approximately 4 h after ATD, respectively. CSF TRP was lowest 8.0 h later. CSF 5-HIAA continued to decrease 14 h after the TRP-deficient amino acid drink was given.

Rapid depletion of plasma tryptophan: a review of studies and experimental methodology

Evidence that the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays a role in the pathophysiology of mood disorders has been accumulating over the past three decades. Recent studies on this neurotransmitter have extended across the spectrum of psychiatric disorder, suggesting a role for 5-HT in psychosis, aggression, eating disorders and addiction. However, much of the evidence has come from post-mortem examination of the brain or measures of peripheral rather than central 5-HT function. The technique of tryptophan depletion allows investigation of brain 5-HT function in living subjects by examining the behavioural responses to this pharmacological challenge. This review considers the current status of tryptophan depletion as an experimental technique and discusses the implications of findings both in affective disorders and in a range of other psychiatric syndromes. MEDLINE and PSYCHLIT searches were completed for the years 1966 to November 1996 using the key words 'serotonin', '5-hydroxytryptamine', 'tryptophan' and 'depletion'. In addition relevant journals were hand-searched for the period from 1980 to December 1996. Forty-four double-blind studies in humans and three clinical case reports were identified; these cover a range of psychiatric disorders including mood disorders and psychoses, anxiety and eating disorders and specific behaviours such as appetite, aggression and craving. The studies reviewed utilized a variety of differing methodologies reducing the extent to which results can be generalized. A series of studies in depressed patients (before and after treatment with antidepressants) and their first-degree relatives have shown the importance of an intact 5-HT system in the action of antidepressants and offer new insights into the biology of affective disorder. The mood change induced by tryptophan depletion may predict those patients likely to respond to 5-HT-specific drugs. Rapid tryptophan depletion has also been reported to exacerbate both panic and aggression in vulnerable individuals. Effects in other disorders are conflicting and further research is needed to clarify these findings.

Differential effects of restraint stress on hippocampal 5-HT metabolism and extracellular levels of 5-HT in streptozotocin-diabetic rats

Streptozotocin (STZ)-elicited diabetes reduces central serotonin (5-hydroxytryptamine, 5-HT) synthesis/metabolism, but whether this reduction leads to decreased release of 5-HT has only scarcely been investigated. We have thus analysed the impact of STZ diabetes on hippocampal extracellular 5-HT levels both under basal conditions and during restraint stress, a procedure known to stimulate hippocampal 5-HT synthesis/metabolism and release. The pretreatment with STZ (3 weeks beforehand) and the 1 h restraint session respectively decreased and increased hippocampal 5-HT metabolism, as assessed by tissue analysis of 5-HT and 5-hydroxyindoleacetic acid. On the other hand, hippocampal microdialysis revealed no difference in basal levels of extracellular 5-HT levels in (conscious) vehicle- and STZ-pretreated rats, but a differential effect of restraint. Thus, extracellular 5-HT levels increased throughout restraint (maximal increase: 194%) in vehicle-, but not in STZ-pretreated rats. In the latter rat group, plasma corticosterone levels were, however, increased, thus indicating a significant aversiveness to stress. Lastly, because anxiety-related behaviours may be affected by hippocampal serotonergic systems, resting and restrained vehicle- and STZ-pretreated rats were compared (immediately after stress) in an elevated plus-maze of anxiety. Pretreatment with STZ reduced the percent number of open arm entries and the number of closed arm entries, indicating increased anxiety and reduced locomotor activity, respectively. Restraint tended to increase anxiety-related behaviours in all rats, but this trend never reached significance. Our results confirm that gross analyses of 5-HT metabolism do not yield information on 5-HT release, and suggest that the prevalence of diabetes among patients suffering affective disorders could be related to the lack of hippocampal serotonergic response to aversive stimuli.

Molecular mechanism of the inactivation of tryptophan hydroxylase by nitric oxide: attack on critical sulfhydryls that spare the enzyme iron center

Tryptophan hydroxylase (TPH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin (5-HT), is irreversibly inactivated by nitric oxide (NO). We have expressed brain TPH as a recombinant glutathione-S-transferase fusion protein and delineated the catalytic domain of the enzyme as the region spanning amino acids 99-444. Highly purified TPH catalytic core, like the native enzyme from brain, is inactivated by NO in a concentration-dependent manner. Removal of iron from TPH produces an apoenzyme with low activity that can be reconverted to its highly active holo-form by the addition of ferrous iron. Apo-TPH exposed to NO cannot be reactivated by iron. Treatment of holo-TPH (iron-loaded) with the disulfide 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB) causes an inactivation of TPH that is readily reversed by dithiothreitol (DTT). DTNB-treated TPH [sulfhydryl (SH)-protected] exposed to NO is returned to full activity by thiol reduction with DTT. The inactivation of native TPH by NO cannot be reversed by either iron or DTT. These data indicate that NO inactivates TPH by selective action on critical SH groups (i.e., cysteine residues) while sparing catalytic iron sites within the enzyme. The results are interpreted with reference to the substituted amphetamines, which are neurotoxic to 5-HT neurons, that inactivate TPH in vivo and are now known to produce NO and other reactive oxygen species in vivo.

Effects of tryptophan and/or acute running on extracellular 5-HT and 5-HIAA levels in the hippocampus of food-deprived rats

The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability (and in turn 5-HT synthesis) alter 5-HT release in the hippocampus of food-deprived rats. To this end, we compared the respective effects of acute exercise, administration of tryptophan, and the combination of both treatments, upon extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. All rats were trained to run on a treadmill before implantation of the microdialysis probe and 24 h of food deprivation. Acute exercise (12 m/min for 1 h) increased in a time-dependent manner extracellular 5-HT levels (maximal increase: 47%), these levels returning to their baseline levels within the first hour of the recovery period. Besides, exercise-induced increases in extracellular 5-HIAA levels did not reach significance. Acute administration of a tryptophan dose (50 mg/kg i.p.) that increased extracellular 5-HIAA (but not 5-HT) levels in fed rats, increased within 60 min extracellular 5-HT levels (maximal increase: 55%) in food-deprived rats. Whereas 5-HT levels returned toward their baseline levels within the 160 min that followed tryptophan administration, extracellular 5-HIAA levels rose throughout the experiment (maximal increase: 75%). Lastly, treatment with tryptophan (60 min beforehand) before acute exercise led to marked increases in extracellular 5-HT and 5-HIAA levels (maximal increases: 100% and 83%, respectively) throughout the 240 min that followed tryptophan administration. This study indicates that exercise stimulates 5-HT release in the hippocampus of fasted rats, and that a pretreatment with tryptophan (at a dose increasing extracellular 5-HT levels) amplifies exercise-induced 5-HT release.

Effect of valine on 5-HT-mediated prolactin release in healthy volunteers, and on mood in remitted depressed patients

BACKGROUND

Animal experimental studies suggest that the amino acid valine may decrease brain serotonin (5-HT) function by inhibiting the transport of the 5-HT precursor, L-tryptophan, across the blood barrier. The aim of the present study was to assess whether valine could decrease brain 5-HT function in healthy subjects and provoke symptomatic relapse in recently remitted depressed patients taking antidepressant drug treatment.

METHOD

We studied the effect of valine (30 g) on the prolactin (PRL) response to the 5-HT releasing agent, D-fenfluramine, in healthy male subjects and on the mood of 12 remitted depressed patients taking either selective serotonin re-uptake inhibitors (n = 10) or lithium and amitriptyline (n = 2).

RESULTS

Valine significantly lowered the PRL response to D-fenfluramine in healthy subjects. In the remitted depressives, valine caused a mild but detectable lowering of mood on a number of measures but only one patient experienced a significant relapse in mood.

CONCLUSIONS

Valine administration may decrease brain 5-HT neurotransmission in humans. This effect could explain the mild increase in depressive symptoms in patients taking 5-HT-potentiating drugs.

Dieting decreases plasma tryptophan and increases the prolactin response to d-fenfluramine in women but not men

We studied the effect of 3 weeks of moderate calorie restriction on 5-HT-mediated prolactin (PRL) release in healthy volunteers using the 5-HT-releasing agent d-fenfluramine. In women, dieting significantly lowered plasma total and free tryptophan (TRP) and increased the PRL response to d-fenfluramine. None of these measures were altered in men who dieted. These findings add to the data indicating that dieting alters brain 5-HT function in women, perhaps as a consequence of reducing the availability of plasma TRP.

Effects of chlorisondamine and restraint on cortical [3H]ketanserin binding, 5-HT2A receptor-mediated head shakes, and behaviours in models of anxiety

A recent study has indicated that ganglionic transmission mediates acute restraint-elicited increases in brain tryptophan (5-HT precursor) levels, 5-HT synthesis and (possibly) release. Because restraint-induced release of 5-HT has been shown to be associated with a paradoxical increase in cortical 5-HT2A receptor binding, we have examined the influence of 5-HT synthesis/release upon cortical 5-HT2A receptor binding and 5-HT2A receptor-mediated head shakes in 3-hr restrained rats pretreated with the ganglionic blocker chlorisondamine. In keeping with past reports regarding the effects of restraint and ganglionic blockade upon anxiety, we have also measured the behavioural effects of restraint and/or chlorisondamine in two animal models of anxiety, the elevated plus-maze and the social interaction test. Chlorisondamine pretreatment (2.5 mg/kg, 20 min beforehand) prevented restraint-elicited defaecation and body weight decreases. Although stress amplified the head shake response to the injection of the 5-HT2A/5-HT2C receptor agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI, 1 or 2 mg/kg 2 hr after the end of restraint), cortical [3H]ketanserin binding remained unaltered. Chlorisondamine treatment was inactive, except for the amplification of the head shake response to DOI (2 mg/kg) in restrained rats. When exposed to the social interaction test, neither restraint nor chlorisondamine affected social interaction, locomotion, or rearings. In the elevated plus-maze, the percent number of open arms entered and the total number of arms entered were decreased by acute restraint, whilst chlorisondamine pretreatment was inactive.