Effect of acute administration of L-tryptophan on the release of 5-HT in rat hippocampus in relation to serotoninergic neuronal activity: an in vivo microdialysis study

Direct and indirect mechanisms by which the gut microbiota influence host serotonin systems

Mounting evidence highlights the pivotal role of enteric microbes as a dynamic interface with the host. Indeed, the gut microbiota, located in the lumen of the gastrointestinal (GI) tract, influence many essential physiological processes that are evident in both healthy and pathological states. A key signaling molecule throughout the body is serotonin (5-hydroxytryptamine; 5-HT), which acts in the GI tract to regulate numerous gut functions including intestinal motility and secretion. The gut microbiota can modulate host 5-HT systems both directly and indirectly. Direct actions of gut microbes, evidenced by studies using germ-free animals or antibiotic administration, alter the expression of key 5-HT-related genes to promote 5-HT biosynthesis. Indirectly, the gut microbiota produce numerous microbial metabolites, whose actions can influence host serotonergic systems in a variety of ways. This review summarizes the current knowledge regarding mechanisms by which gut bacteria act to regulate host 5-HT and 5-HT-mediated gut functions, as well as implications for 5-HT in the microbiota-gut-brain axis.

Activation of 5-HT1A and 5-HT7 receptors enhanced a positively reinforced long-term memory

Memory is one of the most important capabilities of our mind since it determines our individuality. Memory formation involves different stages: acquisition, consolidation and retrieval. There are many studies about early stages, however little is known about memory retrieval. Retrieval is the use of learned information and represents a big problem in patients with memory deficits where the main issue is that they can learn but cannot remember. Previous findings have demonstrated that 5-hydroxytryptamine (5-HT) is a neurotransmitter involved in memory process. Hence, here we are exploring the role of 5-HT in memory retrieval by using its metabolic precursor l-tryptophan and several ligands at 5-HT1A and 5-HT7 receptors. Experimental protocol consisted of evaluating conditioned responses (%CR) after one week of interruption following autoshaping sessions for memory formation; a decrease of %CR was interpreted as memory decay. Systemic administration of: (1) l-tryptophan (50 and 100 mg/kg), (2) 5-HT1A receptor agonist 8-OH-DPAT (0.031 and 0.062 mg/kg), (3) the selective antagonist 5-HT1A receptor WAY 100635 (0.3 and 0.6 mg/kg), (4) the 5-HT7 receptor agonist, LP 211, in a dose-dependent manner (1, 2.5, 5.0 and 10.0 mg/kg) enhanced memory retrieval. Further, the 5-HT7 receptor antagonist, SB 269970 (10.0 mg/kg), had no effect. Finally, SB 269970 (10.0 mg/kg) significantly blocked memory retrieval enhancement produced by 10.0 mg/kg LP 211, but not that induced by 2.5 mg/kg LP 211.These results, taken together, suggest that activation of 5-HT1A and 5-HT7 receptors enhanced memory retrieval and these receptors may be therapeutic targets to improve long-term memory retrieval.

Plasma Tryptophan/Large Neutral Amino Acids Ratio in Domestic Dogs Is Affected by a Single Meal with High Carbohydrates Level

Aim of this study was to evaluate the plasma ratio between l-tryptophan (TRP) and five large neutral amino acids (isoleucine + leucine + phenylalanine + tyrosine + valine) (5LNAAs) after a single meal with high carbohydrates level. Five female Labrador Retrievers were involved. Each dog was fed three different meals: M1 (a mix of puffed rice, minced meat and olive oil), M2 (puffed rice and olive oil) and M3 (commercial dry food usually consumed) once in the morning per day for 30 days. Blood was collected right before the first meal (t0) and after 2, 4, 6, 8, 10 and 24 h. Plasma amino acids’ concentrations were measured using an HPLC (High-performance liquid chromatography) method with fluorimetric detection. Plasmatic TRP concentrations showed no significant difference between M1, M2 and M3 samples at any sampling time. M2 led to a decrease in 5LNAAs levels and consequently led to a significant higher TRP/5LNAAs ratios in the 6 h period after the provision of carbohydrates, compared to both M1 and M3. In addition, the mean TRP/5LNAAs ratio was significantly higher in M2 than in M3 at t8 and t10. These results indicate that meal composition affects TRP/5LNAAs ratio and possibly, TRP bioavailability.

Intravenous tryptophan administration attenuates cortisol secretion induced by intracerebroventricular injection of noradrenaline

This study was conducted to investigate the possibility of suppression of stress-induced cortisol (CORT) secretion by tryptophan (TRP) administration and to better understand its regulatory mechanisms by using a noradrenaline (NA) injection into the third ventricle (3V) as a stress model in cattle. A total of 25 Holstein steers with a cannula in the 3V were used. First, the increase in CORT secretion was observed following a NA injection into the 3V in a dose-dependent manner, verifying the appropriateness of this treatment as a stress model of CORT secretion (Experiment 1). The effect of prior-administration of TRP into peripheral blood with a dose that has been demonstrated to increase brain 5-hydroxytryptamine levels on the elevation of plasma CORT induced by NA or corticotropin-releasing hormone (CRH) was then examined (Experiment 2). The prior administration of TRP suppressed NA-induced, but not CRH-induced, CORT elevation. These results suggest that an increase in TRP absorption into peripheral blood could suppress the stress-induced CORT secretion in cattle via the attenuation of the stimulatory effect of NA on the hypothalamic CRH release.

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.

Effects of tryptophan depletion and tryptophan loading on the affective response to high-dose CO2 challenge in healthy volunteers

RATIONALE

It has been reported that in panic disorder (PD), tryptophan depletion enhances the vulnerability to experimentally induced panic, while the administration of serotonin precursors blunts the response to challenges.

OBJECTIVES

Using a high-dose carbon dioxide (CO(2)) challenge, we aimed to investigate the effects of acute tryptophan depletion (ATD) and acute tryptophan loading (ATL) on CO(2)-induced panic response in healthy volunteers.

METHODS

Eighteen healthy volunteers participated in a randomized, double-blind placebo-controlled study. Each subject received ATD, ATL, and a balanced condition (BAL) in separate days, and a double-breath 35% CO(2) inhalation 4.5 h after treatment. Tryptophan (Trp) manipulations were obtained adding 0 g (ATD), 1.21 g (BAL), and 5.15 g (ATL) of l-tryptophan to a protein mixture lacking Trp. Assessments consisted of a visual analogue scale for affect (VAAS) and panic symptom list. A separate analysis on a sample of 55 subjects with a separate-group design has also been performed to study the relationship between plasma amino acid levels and subjective response to CO(2).

RESULTS

CO(2)-induced subjective distress and breathlessness were significantly lower after ATD compared to BAL and ATL (p < 0.05). In the separate-group analysis, ΔVAAS scores were positively correlated to the ratio Trp:ΣLNAA after treatment (r = 0.39; p < 0.05).

CONCLUSIONS

The present results are in line with preclinical data indicating a role for the serotonergic system in promoting the aversive respiratory sensations to hypercapnic stimuli (Richerson, Nat Rev Neurosci 5(6):449-461, 2004). The differences observed in our study, compared to previous findings in PD patients, might depend on an altered serotonergic modulatory function in patients compared to healthy subjects.

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.

Free tryptophan/large neutral amino acids ratios in blood plasma do not predict cerebral spinal fluid tryptophan concentrations in interleukin-1-induced anorexia

Peripheral administration of interleukin-1 (IL-1) reduces food intake and affects brain serotonergic activity, suggesting a causal relationship. Furthermore, IL-1 increases the brain concentrations of the serotonin precursor, tryptophan (TRP), by unclear mechanism(s). We aimed at confirming the link between IL-1 administration, raised brain TRP concentrations and the development of anorexia, and at investigating the mechanisms of TRP entry into the brain. Thirty adult, overnight fasted Sprague-Dawley rats were randomly assigned to i.p. injections of 1 mug/kg BW of IL-1 alpha (n=10) or vehicle (n=10), or to pair-feeding with IL-1 animals (n=10). After 2 h, food intake, blood plasma concentrations of total TRP, free TRP, large neutral amino acids (LNAA; competing with TRP for brain entry) were measured. Cerebral spinal fluid (CSF) TRP concentrations were also measured. TRP brain availability was assessed by calculating the plasma ratio free TRP/LNAA. Following IL-1 injection, food intake significantly declined in IL-1 rats, which was paralleled by decreased plasma free TRP and increased plasma LNAA. Despite a decrease in the free TRP/LNAA ratios in plasma, IL-1 significantly increased concentrations of TRP in CSF. These data show that the acute peripheral administration of IL-1 induces anorexia and raises CSF TRP levels. Considering the possible role of the raised CSF TRP in influencing brain serotonin activity, it is postulated that increased serotonergic neurotransmission could be involved in IL-1 induced anorexia.

Tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area

To assess the effects of increased hypothalamic tryptophan (TRP) availability on 5-HT content in preoptic area on thermoregulation and work production during exercise on treadmill, 20.3 microM of L-TRP (n=7) or 0.15M NaCl (n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18 m min(-1) 5% inclination). Exercise time to fatigue (min), and workload (kgm) were analysed. Core temperature was measured by telemetry. At fatigue, brains were quickly removed and preoptic area (POA), hypothalamus (HP), frontal cortex (FC), hippocampi (HC) were rapidly dissected and frozen immediately in dry ice. Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured by HPLC. TRP-exercised rats showed the highest content of 5-HT in the POA and the lowest in the hippocampi compared to the rested and SAL-exercised rats. An inverse relationship between TF and a direct correlation with body temperature changes and POA-5HT levels were observed. A correlation between HC 5-HT content and TF was also found. However, there was no correlation between HC 5-HT content and changes in Tb at fatigue. Finally, our results bring further evidences that increased 5-HT content in POA is involved with an increase in heat production during exercise. In addition, the direct correlation of 5-HT level in hippocampi and TF of TRP-exercised rats suggests that this brain area is also related to motor activity control during exercise. In conclusion, our data indicated that tryptophan-induced central fatigue in exercising rats is related to serotonin content in preoptic area.

Blood 5-hydroxytryptamine, 5-hydroxyindoleacetic acid and melatonin levels in patients with either Huntington's disease or chronic brain injury

Following a study of oxidative tryptophan metabolism to kynurenines, we have now analysed the blood of patients with either Huntington's disease or traumatic brain injury for levels of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and melatonin. There were no differences in the baseline levels of these compounds between patients and healthy controls. Tryptophan depletion did not reduce 5-HT levels in either the controls or in the patients with Huntington's disease, but it increased 5-HT levels in patients with brain injury and lowered 5-HIAA in the control and Huntington's disease groups. An oral tryptophan load did not modify 5-HT levels in the patients but increased 5-HT in control subjects. The tryptophan load restored 5-HIAA to baseline levels in controls and patients with brain injury, but not in those with Huntington's disease, in whom 5-HIAA remained significantly depressed. Melatonin levels increased on tryptophan loading in all subjects, with levels in patients with brain injury increasing significantly more than in controls. Baseline levels of neopterin and lipid peroxidation products were higher in patients than in controls. It is concluded that both groups of patients exhibit abnormalities in tryptophan metabolism, which may be related to increased inflammatory status and oxidative stress. Interactions between the kynurenine, 5-HT and melatonin pathways should be considered when interpreting changes of tryptophan metabolism.

Exercise, serum free tryptophan, and central fatigue

Brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis and release increase during running. This increase in 5HT function may promote central fatigue and contribute to suboptimal physical performance. The rise in brain TRP is reputed to result from exercise-induced elevations in serum nonesterified fatty acid (NEFA) concentrations, which dissociate TRP from albumin in blood and increase the serum free TRP pool. But, as discussed in this article, ample evidence exists that the serum free TRP pool does not control brain TRP uptake. The clearest data are dietary, but pharmacologic data in exercising rats also support this conclusion. Changes in the serum levels of amino acids that compete with TRP for brain uptake appear also not to explain the rise in brain TRP. The mechanism is therefore not presently known. The link between the rise in brain TRP and 5HT synthesis/release is not simple: a rise in brain TRP stimulates 5HT synthesis/release in actively firing neurons. Hence, during exercise, only 5HT neurons that are firing should increase 5HT production/release when brain TRP rises. It is not known which 5HT neurons fire during exercise; the 5HT neurons that respond to exercise-induced increases in brain TRP are therefore not known. Hence, it is not possible to conclude which 5HT neurons contribute to the generation of central fatigue. Because some 5HT neurons control specific functions important to physical performance (e.g., respiration), the current understanding of 5HT neuronal function in central fatigue might benefit from the study of specific 5HT pathways during exercise.

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.

Branched-chain amino acids and brain function

Branched-chain amino acids (BCAAs) influence brain function by modifying large, neutral amino acid (LNAA) transport at the blood-brain barrier. Transport is shared by several LNAAs, notably the BCAAs and the aromatic amino acids (ArAAs), and is competitive. Consequently, when plasma BCAA concentrations rise, which can occur in response to food ingestion or BCAA administration, or with the onset of certain metabolic diseases (e.g., uncontrolled diabetes), brain BCAA concentrations rise, and ArAA concentrations decline. Such effects occur acutely and chronically. Such reductions in brain ArAA concentrations have functional consequences: biochemically, they reduce the synthesis and the release of neurotransmitters derived from ArAAs, notably serotonin (from tryptophan) and catecholamines (from tyrosine and phenylalanine). The functional effects of such neurochemical changes include altered hormonal function, blood pressure, and affective state. Although the BCAAs thus have biochemical and functional effects in the brain, few attempts have been made to characterize time-course or dose-response relations for such effects. And, no studies have attempted to identify levels of BCAA intake that might produce adverse effects on the brain. The only "model" of very high BCAA exposure is a very rare genetic disorder, maple syrup urine disease, a feature of which is substantial brain dysfunction but that probably cannot serve as a useful model for excessive BCAA intake by normal individuals. Given the known biochemical and functional effects of the BCAAs, it should be a straightforward exercise to design studies to assess dose-response relations for biochemical and functional effects and, in this context, to explore for adverse effect thresholds.

Alpha-lactalbumin-enriched diets enhance serotonin release and induce anxiolytic and rewarding effects in the rat

Among food proteins, alpha-lactalbumin (LAC) has the highest ratio of tryptophan (Trp) over its competitor amino acids. Consequently, contrary to casein (CAS), LAC ingestion increases Trp access to the brain leading to enhanced serotonin (5-HT) synthesis. As an index of serotonergic activity, we assessed extracellular 5-HT in response to LAC ingestion, using microdialysis, and performed behavioural tests in rats in order to characterise the suggested improvements of mood observed in humans after ingestion of this protein. Rats were fed with diets enriched either in LAC or CAS as control, acutely (30 min meals) or chronically (3 and 6 days). A 30 min LAC meal significantly increased 5-HT release in the medial hypothalamus. This effect disappeared after 3 and 6 days of diet. The basal premeal 5-HT levels were increasingly enhanced by the LAC diet. Compared to a CAS meal, LAC increased the percentage of time spent on the open arms of the elevated plus maze and the number of visits to the centre of the open field, suggesting an anxiolytic-like effect. A single LAC meal decreased sucrose consumption, while 3 or 6 days diets enhanced it, reflecting an appetitive and/or rewarding action. In conclusion, LAC ingestion induces anxiolytic-like and rewarding effects possibly related to serotonergic activation. Shifting transiently, the commonly consumed CAS-enriched to LAC-enriched diets may induce beneficial effects on mood.

5-HT(1A) receptor imaging in the human brain: effect of tryptophan depletion and infusion on [(18)F]MPPF binding

The 5-HT(1A) receptor has been implicated in a variety of physiological processes, psychiatric disorders, and neurodegenerative disorders. [(18)F]MPPF is a useful radioligand for quantitative imaging of 5-HT(1A) receptors in human subjects. Previous studies have shown that the binding of some radioligands is sensitive to changes in neurotransmitter concentration, whereas in other cases, binding is not affected. In the present study we investigated if [(18)F]MPPF binding to the 5-HT(1A) receptor is sensitive to changes in 5-HT. Changes in 5-HT levels were achieved by influencing its synthesis through tryptophan depletion, including a tryptophan-free amino acid drink 4.5 h prior to the PET scan and tryptophan infusion (10 mg/ml, 50 mg/kg, 30 min, starting 60 min prior to the PET scan). Binding of [(18)F]MPPF in the brain of six healthy, male volunteers was compared in these two conditions. Mean binding potentials in the medial temporal cortex, cortical regions, and raphe nucleus did not significantly differ between the two conditions. The results of the study show that, under the experimental conditions used, [(18)F]MPPF binding was not affected. It is hypothesized that the increases in 5-HT levels needed to produce a measurable effect on [(18)F]MPPF binding would be significantly greater than that possible with tryptophan manipulation. Therefore, in pathological conditions, where such large increases in 5-HT levels are not expected, [(18)F]MPPF seems a useful ligand to measure 5-HT(1A) receptor distribution without the interference of endogenous 5-HT.

The role of serotonin in human mood and social interaction. Insight from altered tryptophan levels

Alterations in brain tryptophan levels cause changes in brain serotonin synthesis, and this has been used to study the implication of altered serotonin levels in humans. In the acute tryptophan depletion (ATD) technique, subjects ingest a mixture of amino acids devoid of tryptophan. This results in a transient decline in tissue tryptophan and in brain serotonin. ATD can result in lower mood and increase in irritability or aggressive responding. The magnitude of the effect varies greatly depending on the susceptibility of the subject to lowered mood or aggressivity. Unlike ATD, tryptophan can be given chronically. Tryptophan is an antidepressant in mild to moderate depression and a small body of data suggests that it can also decrease aggression. Preliminary data indicate that tryptophan also increases dominant behavior during social interactions. Overall, studies manipulating tryptophan levels support the idea that low serotonin can predispose subjects to mood and impulse control disorders. Higher levels of serotonin may help to promote more constructive social interactions by decreasing aggression and increasing dominance.

Effects of dietary sucrose on hippocampal serotonin release: a microdialysis study in the freely-moving rat

The effects of dietary supplementation with either sucrose or starch (50 g/kg regular food for 2 weeks) on central 5-hydroxytryptamine (5HT; serotonin) release were investigated in freely-moving rats. It has been suggested that the amount of transmitter that serotoninergic neurons release might be altered by food intake. We monitored the effects of sucrose and starch on concentrations of extracellular 5HT, its metabolite 5-hydroxyindoleacetic acid (5HIAA), gamma-aminobutyric acid (GABA) and dopamine in the hippocampus, using in vivo microdialysis. The major finding was that baseline levels of extracellular hippocampal 5HT in rats with ad libitum access to food supplemented with sucrose were significantly higher compared with the starch control group. We then verified that sucrose supplementation affected the potency of S(+)fenfluramine to increase hippocampal 5HT levels. In both groups of rats, acute intraperitoneal injection (1 mg/kg) of this anorectic drug induced a response curve of the extracellular hippocampal 5HT levels, with a shape that corresponded with earlier data for different brain areas often using up to 10-fold higher doses of S(+)fenfluramine. Nevertheless, we showed that throughout the experiment the absolute values of the sucrose response curve remained higher than in the starch group. On the other hand, S(+)fenfluramine exerted longer lasting effects in the starch group, as compared with the sucrose group. Significant decreases in levels of extracellular hippocampal 5HIAA levels following S(+)fenfluramine administration were simultaneously observed. A practical implication of the present findings is that dietary sucrose may bias the results of studies investigating brain serotoninergic mechanisms and the effects of (anorectic) drugs interacting with 5HT systems in the hippocampus.

Can nutrient supplements modify brain function?

Over the past 40 y, several lines of investigation have shown that the chemistry and function of both the developing and the mature brain are influenced by diet. Examples are the effect of folate deficiency on neural tube development during early gestation, the influence of essential fatty acid deficiency during gestation and postnatal life on the development of visual function in infants, and the effects of tryptophan or tyrosine intake (alone or as a constituent of dietary protein) on the production of the brain neurotransmitters derived from them (serotonin and the catecholamines, respectively). Sometimes the functional effects are clear and the underlying biochemical mechanisms are not (as with folate and essential fatty acids); in other cases (such as the amino acids tyrosine and tryptophan), the biochemical effects are well understood, whereas the effect on brain function is not. Despite the incomplete knowledge base on the effects of such nutrients, investigators, physicians, and regulatory bodies have promoted the use of these nutrients in the treatment of disease. Typically, these nutrients have been given in doses above those believed to be required for normal health; after they have been given in pure form, unanticipated adverse effects have occasionally occurred. If this pharmacologic practice is to continue, it is important from a public safety standpoint that each nutrient be examined for potential toxicities so that appropriate purity standards can be developed and the risks weighed against the benefits when considering their use.

Enhancement in extracellular serotonin levels by 5-hydroxytryptophan loading after administration of WAY 100635 and fluoxetine

It has been demonstrated that synthesis of serotonin (5-HT) is dependent on the availability of precursor, as well as the activity of 5-HT neurons. In the present series of experiments, we examined the effects of precursor (5-HTP) loading on extracellular hypothalamic 5-HT after administration of fluoxetine alone or in combination with WAY 100635, a selective 5-HT1A antagonist. In the first experiment, fluoxetine alone (10 mg/kg i.p.) caused 5-HT levels to significantly increase to 150% of basal levels. Subsequent administration of 5-HTP at 10, 20, and 40 mg/kg i.p. caused 5-HT levels to further increase to a maximum value of 254%, 405%, and 618%, respectively. In the second experiment, either vehicle or WAY 100635 (1 mg/kg/hour s.c.) was infused, then fluoxetine (10 mg/kg i.p.) and 5-HTP (10 mg/kg i.p.) were administered. By itself, WAY 100635 led to a slight but significant increase in hypothalamic 5-HT levels one hour after the start of administration (130% of basal levels). In the WAY 100635-treated group, fluoxetine caused an increase to 240% of basal levels after one hour, which rose to 290% of basal levels after two hours. Subsequent administration of 5-HTP further increased 5-HT levels to 580% of basal levels after one hour. In the vehicle-treated group, fluoxetine caused an increase of 160% of basal levels which was stable over two hours, and subsequent administration of 5-HTP led to a slight increase in 5-HT levels of 220% after one hour. These results suggest that combining blockade of 5-HT1A autoreceptors with 5-HT uptake inhibition results in a synergistic increase in synthesis and release of 5-HT when precursor is administered.

Cerebrospinal fluid concentrations of tryptophan and 5-hydroxyindoleacetic acid in Macaca mulatta: diurnal variations and response to chronic changes in dietary protein intake

In rats, dietary protein is known to influence brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis. However, few studies have examined this relationship in primates (including humans). We therefore studied the effect in monkeys of changes in chronic protein intake on plasma and cerebrospinal fluid (CSF) concentrations of TRP and 5-hydroxyindoleacetic acid (5HIAA), the principal 5HT metabolite. Juvenile male monkeys (Macacca mulatta) consumed for sequential 4-week periods diets differing in protein content (approximately 23%-->approximately 16%--> approximately 10%-->approximately 6% protein [%-energy/day]). Each day, food was presented as a morning meal of fruit, and an afternoon meal consisting of a pelleted, commercial diet and fruit. During week 4 on each diet, blood and CSF were sampled diurnally via indwelling catheters. Plasma and CSF TRP varied diurnally and with dietary protein content. On all diets, CSF TRP declined modestly in the morning, and increased in the afternoon; the magnitude of the increments varied directly with dietary protein content. Diurnal variations were absent for CSF 5HIAA; however, CSF 5HIAA varied directly with chronic dietary protein content. We conclude that dietary protein content can chronically influence CSF TRP concentrations in monkeys. The variation in CSF 5HIAA suggests chronic protein intake may influence serotonin synthesis and turnover, perhaps via changes in TRP concentrations.

Comparison of the effects of alpha-methyl-p-tyrosine and a tyrosine-free amino acid load on extracellular noradrenaline in the rat hippocampus in vivo

Peripheral administration of an amino acid load lacking tyrosine and its precursor, phenylalanine, causes a lowering of central tyrosine levels. The aim of the present study was to examine the effects of tyrosine depletion on extracellular noradrenaline using microdialysis. Extracellular noradrenaline was measured in hippocampus of the anaesthetized rat under both baseline conditions (with reuptake inhibitor, desipramine, in the perfusion medium) and following administration of the alpha2-adrenoreceptor antagonist, idazoxan. The tyrosine free amino acid load did not alter either baseline noradrenaline or the twofold rise in noradrenaline evoked by idazoxan compared with saline controls. In contrast, the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, caused a marked reduction in baseline extracellular noradrenaline and abolished the rise induced by idazoxan. In conclusion, the present data indicate that under the conditions used, a tyrosine-free amino acid mixture may not be an effective means to interfere with central noradrenaline function. This contrasts with recent findings demonstrating that the tyrosine-depletion approach can be used to decrease presynaptic dopamine function.

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.

Inhibition of restraint-induced anorexia by injected tryptophan

Tryptophan injected at doses of 50mg/kg did not alter 24 h cumulative food intake and growth rate in rats. A single episode of 2 h restraint stress decreased food intake and growth rate of saline and tryptophan injected rats. The decreases of both food intake and growth rate were smaller in tryptophan injected (food intake 23.9% p<0.05; growth rate 2.9% p<0.05) than saline injected (food intake 78.5% p<0.01; growth rate 6.1% p<0.01) rats suggesting that tryptophan administration inhibits restraint-induced anorexia. Following an acute challenge with 2h restraint increases of 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxyindoleacetic acid (5-HIAA) but not tryptophan were greater in tryptophan injected than saline injected rats. The findings imply that tryptophan-induced increases of brain 5-HT and 5-HIAA have little effect on functional serotoninergic activity under basal conditions but a facilitatory effect on functional response occurs in conditions of increased serotoninergic neuronal activity such as during stress.

Headache induced by serotonergic agonists--a key to the interpretation of migraine pathogenesis?

Serotonergic agonists such as m-chlorophenylpiperazine (m-CPP) and fenfluramine may induce migraine attacks. This has led to opposing theories concerning the role of 5-hydroxytryptamine (5HT) in triggering migraine attacks; is there hyperfunction or hypofunction of the central serotonergic system. Our review of the literature strongly suggests that m-CPP and fenfluramine provoke migraine attacks by stimulating, directly or indirectly, the 5HT2C/5HT2B receptors, although there is no total agreement with this interpretation. Central 5HT hypersensitivity in migraine patients, probably due to 5HT neuronal depletion, is proposed on the basis of review of electrophysiological tests and neuroendocrine challenge paradigms.

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.

Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists

At clinically relevant doses, selective serotonin (5-HT) reuptake inhibitors (SSRIs) and MAO inhibitors (MAOIs) increase the extracellular concentration of 5-HT in the midbrain raphé nuclei, thereby activating inhibitory somatodendritic 5-HT1A autoreceptors. Consequently, the firing activity of 5-HT neurons is reduced and the enhancement of extracellular 5-HT concentration in forebrain is dampened. Overriding this feedback by using antagonists of 5-HT1A autoreceptors permits SSRIs to produce a marked increase of extracellular 5-HT in the forebrain. Hence, combined treatment with an SSRI and a 5-HT1A antagonist increases the extracellular concentration of 5-HT more so than the former drug alone. The treatment of patients with major depression using an SSRI and pindolol, a 5-HT1A/ beta-adrenoceptor antagonist, markedly reduced the latency of the antidepressant response in previously untreated patients and induced a rapid improvement in treatment-resistant patients.

Acute effects of L-tryptophan on brain extracellular 5-HT and 5-HIAA levels in chronic experimental portal-systemic encephalopathy

Portal-systemic encephalopathy (PSE) is associated with increased brain turnover of serotonin (5-hydroxytryptamine; 5-HT). Despite this metabolic increase, neuronal release of 5-HT is unaltered in neocortex of portacaval shunted (PCS) rats. In the present study, frontal neocortical extracellular 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) levels were determined in PCS rats and sham-operated controls prior to, as well as, after acute challenge with L-tryptophan (L-TRP; a bolus dose of 280 mg/kg i.p. followed by 5 consecutive hourly doses of 50 mg/kg). Neither basal 5-HT nor 5-HIAA extracellular levels were significantly altered in PCS rats compared to controls. L-TRP administration resulted in unaltered extracellular 5-HT but elevated 5-HIAA levels in PCS and sham rats. These findings do not suggest that changes in brain neuronal 5-HT release play any major functional role in the pathogenesis of chronic PSE. The present data also emphasize the importance of distinguishing between brain 5-HT metabolism and brain 5-HT release.

Differential potentiation of L-tryptophan-induced head-twitch response in mice by cocaine and sertraline

Using selective monoamine uptake blockers and appropriate selective monoamine receptor antagonists, we have previously shown that cocaine enhances the frequency of 5-HT2A receptor-mediated 5-hydroxytryptophan (5-HTP)-induced head-twitch response (HTR) in mice via inhibition of serotonin uptake. Concomitantly, cocaine prevented the maximal producible HTR frequency via simultaneous indirect stimulation of the inhibitory presynaptic 5-HT1A and postsynaptic alpha 2 receptors. In the present study, we have investigated the effects of cocaine and the selective 5-HT (sertraline), norepinephrine (nisoxetine) and dopamine (GBR 12935) uptake inhibitors on the L-tryptophan-induced HTR in the presence of a nonselective monoamine oxidase inhibitor, tranylcypromine. We utilized two experimental protocols where cocaine or sertraline were administered either after (protocol 1) or prior to (protocol 2) L-tryptophan injection. Cocaine potentiated the ability of L-tryptophan to induce HTR to a greater extent in protocol 1, whereas sertraline induced a greater effect in protocol 2. However, in our earlier study cocaine (and also sertraline) up to 10 mg/kg produced a similar degree of potentiation in both experimental protocols on the 5-HTP-induced HTR. Furthermore, as in the latter study on the 5-HTP-induced HTR, in the present investigation nisoxetine potently attenuated whereas GBR 12935 did not modulate the induced HTR. The results show that the respective serotonergic and noradrenergic effects of cocaine also operate on the L-tryptophan-induced HTR. The differential effects of cocaine and sertraline in experimental protocols 1 and 2 on the L-tryptophan- versus 5-HTP-induced HTRs suggest that cocaine has additional effects on the conversion of L-tryptophan to 5-HT.

Diet-induced changes in serum cholesterol concentrations do not alter tryptophan hydroxylation rate or serotonin concentrations in gerbil brain

The relationship between serum cholesterol concentrations and serotonin synthesis rate in brain was examined in Mongolian gerbils chronically fed diets containing 20% fat (safflower oil, beef tallow or butterfat) with or without added cholesterol (0.5%, dry weight). After 22 days on these diets, circulating cholesterol concentrations ranged between approximately 1.5 and approximately 20 mumol/ml. Despite this enormous range, in vivo tryptophan hydroxylation rate, and serotonin and 5-hydroxyindoleacetic acid concentrations in cerebral cortex, hypothalamus and brainstem did not differ significantly among the diet groups. Tryptophan concentrations in serum and brain were also unaffected. These results do not support the hypothesis that the link between depression, suicide and violent deaths and below-normal or reduced serum cholesterol concentrations in humans involves an alteration in serotonin synthesis and/or release by brain neurons.

Behavioral effects of dietary neurotransmitter precursors: basic and clinical aspects

The levels and possibly function of several neurotransmitters can be influenced by the supply of their dietary precursors. The neurotransmitters include serotonin, dopamine, noradrenaline, histamine, acetylcholine and glycine, which are formed from tryptophan, tyrosine, histidine, choline and threonine. Tryptophan has been tested more than the other precursors in clinical trials and is currently available in some countries for the treatment of depression. Other uses for tryptophan and the therapeutic potential of other neurotransmitter precursors have not been tested adequately. Given the relative lack of toxicity of dietary components, further clinical trials with neurotransmitter precursors should be carried out.

Exercise and brain neurotransmission

Physical exercise influences the central dopaminergic, noradrenergic and serotonergic systems. A number of studies have examined brain noradrenaline (norepinephrine), serotonin (5-hydroxytryptamine; 5-HT) and dopamine with exercise. Although there are great discrepancies in experimental protocols, the results indicate that there is evidence in favour of changes in synthesis and metabolism of monoamines during exercise. There is a possibility that the interactions between brain neurotransmitters and their specific receptors could play a role in the onset of fatigue during prolonged exercise. The data on the effects of branched chain amino acid (BCAA) supplementation and 'central fatigue' seem to be conflicting, although recent studies suggest that BCAA supplementation has no influence on endurance performance. There are numerous levels at which central neurotransmitters can affect motor behaviour; from sensory perception, and sensory-motor integration, to motor effector mechanisms. However, the crucial point is whether or not the changes in neurotransmitter levels trigger or reflect changes in monoamine release. Until recently most studies were done on homogenised tissue, which gives no indication of the dynamic release of neurotransmitters in the extracellular space of living organisms. Recently, new techniques such as microdialysis are voltammetry were introduced to measure in vivo release of neurotransmitters. Microdialysis can collect virtually any substance from the brain of a freely moving animal with a limited amount of tissue trauma. This method allows measurement of local neurotransmitter release during on-going behavioural changes such as exercise. The results of the first studies using these methods indicate that the release of most neurotransmitters is influenced by exercise. Although the few studies that have been published to date show some discrepancies, we feel that these recently developed and more sophisticated in vivo methods will improve our insight into the relationship between the monoamine and other transmitters during exercise. Continued quantitative and qualitative research needs to be conducted so that a further understanding of the effects of exercise on brain neurotransmission can be gained.

The effects of an inhibitor of tryptophan 2,3-dioxygenase and a combined inhibitor of tryptophan 2,3-dioxygenase and 5-HT reuptake in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3-pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO), and a novel inhibitor 709W92 ((E)-6-fluoro-3-[2-(4-pyridyl)vinyl]-1H-indole), of both TDO and 5-hydroxytryptamine (5-HT) reuptake, were examined on tryptophan catabolism, cerebrospinal fluid (CSF) concentrations of tryptophan and 5-HT and serotonergic-mediated physiology and behaviour in the rat. The catabolism of L-[ring-2-14C]tryptophan in vivo was completely inhibited by prior administration of 709W92. 709W92, but not 680C91, potentiated head-twitch produced by 5-hydroxytryptophan, prevented head-twitch and whole brain 5-HT depletion produced by p-chloroamphetamine and rapidly decreased dorsal raphe firing. Both 709W92 and 680C91 elevated CSF tryptophan by up to 260% of basal concentration. A maximally effective dose of 680C91 elevated a global measure of brain extracellular 5-HT (CSF 5-HT) to concentrations similar to those seen maximally after exogenous tryptophan administration (approx 170% of basal). Maximally effective doses of 709W92 increased CSF 5-HT to concentrations comparable to those seen after tryptophan and 5-HT reuptake inhibitor coadministration (approx 900% of basal) and to concentrations greater than those achieved maximally with serotonergically active antidepressant monotherapy (approx 500% of basal). 709W92 did not elevate CSF 5-HT to concentrations associated with the serotonin syndrome (approx 3000% of basal). The combined TDO inhibitor/5-HT reuptake inhibitor, 709W92, showed anxiolytic activity in the rat-pup vocalization model of anxiety. These results show that 709W92 (a novel inhibitor of both TDO and 5-HT reuptake), can produce an elevation of CSF 5-HT similar to that achieved with a serotonin reuptake inhibitor/tryptophan combination therapy but with a more sustained timecourse; such compounds may therefore have superior antidepressant efficacy in the clinic.

Serotonin and alcohol intake, abuse, and dependence: findings of animal studies

Despite a relatively large body of literature on the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake, the functional significance of serotonergic neurotransmission and its relationship to alcohol intake, abuse, and dependence remains to be fully elucidated. In part two of this review, the experimental (animal) data is summarized along two lines: the effects of serotonergic manipulations on the intake of alcohol, and the effects of acute and chronic alcohol intake, as well as the withdrawal of chronic alcohol, on the serotonergic system. It is concluded that serotonin mediates ethanol intake as a part of its larger role in behavior modulation, such that increases in serotonergic functioning decrease ethanol intake, and decreased serotonergic functioning increases ethanol intake. Ethanol produces transient increases in serotonergic functioning that activate the mesolimbic dopaminergic reward system. The results are discussed in light of recent theories describing the regulatory role of serotonin in general behavior.

Effect of acute tryptophan depletion on mood and appetite in healthy female volunteers

Twelve healthy female subjects received the following three drinks in a double-blind, semi-balanced, cross over design: (a) 50 g of amino acids without L-tryptophan (LTP); (b) 50 g of amino acid with LTP (balanced); (c) plain water. Compared to both the balanced amino acid mixture and plain water, the LTP drink significantly lowered plasma total and free tryptophan at 4.5 h. However, compared to the two control conditions, there was no effect of the LTP drink on subjective ratings of mood or hunger. Similarly, the LTP drink did not alter significantly either total calorie intake or the macronutrient content of a test meal 5 h after drink ingestion.

Anxiolytic effects of the 5-HT1A receptor agonist ipsapirone in the rat: neurobiological correlates

Ipsapirone, a 5-HT1A receptor agonist, is a psychoactive compound with anxiolytic and antidepressive properties. Concerning the mechanism of action of ipsapirone, most studies point towards an interaction with the central serotonergic system. For characterization of the anxiolytic properties of ipsapirone, the ultrasonic vocalization test, a rat model based on conditioned anxiety, was used. In this test ipsapirone time and dose dependently inhibited the shock induced ultrasonic vocalization of rats after systemic application of ipsapirone, indicating its potent anxiolytic properties. To gain further insight into the mechanism underlying the anxiolytic activity of ipsapirone, the effects of this compound on the central serotonergic system of rats were characterized. Extracellular single unit recordings, measuring the firing rate of serotonergic dorsal raphe neurons in anaesthetized rats, showed that ipsapirone dose and time dependently suppressed neuronal firing. In the hippocampus, one of the projection areas of the serotonergic dorsal raphe neurons, systemic application of ipsapirone resulted in a dose- and time-dependent reduction in serotonergic neurotransmission, measured as a reduced serotonin output in microdialysis experiments with freely moving rats. The time- and dose-effect curves reflecting the anxiolytic, electrophysiological, and biochemical effects correlated well with each other for different doses of ipsapirone. It is concluded that ipsapirone exhibits its anxiolytic effects, at least partially, by stimulating presynaptic somatodendritic 5-HT1A receptors in the brain stem raphe nuclei (i.e., the dorsal raphe), resulting in attenuation of presynaptic cell firing and subsequent inhibition of serotonergic neurotransmission in the limbic system (i.e., the hippocampus).

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

L-Valine competes with tryptophan for transport into the brain and has previously been shown to decrease brain 5-HT synthesis. In the present study, the effect of L-valine on electrically evoked hippocampal 5-HT release was determined in the anaesthetized rat using microdialysis. In control animals two electrical stimulations of the dorsal raphe nucleus 120 min apart (S1 and S2, respectively) released similar amounts of 5-HT. In contrast, in animals which received L-valine (200 mg/kg) between stimulations, S2 released a significantly smaller amount of 5-HT than did S1, although basal 5-HT release was unchanged. The data demonstrate that L-valine decreases the electrically-evoked release of 5-HT in hippocampus in vivo.