A physiological method to selectively decrease brain serotonin release

Dietary tryptophan depletion alters the faecal bacterial community structure of compulsive drinker rats in schedule-induced polydipsia

RATIONALE

Compulsive behaviour, present in different psychiatric disorders such as obsessive-compulsive disorder, schizophrenia and drug abuse, is associated with altered levels of serotonin (5-hydroxytryptamine, 5-HT). The gut microbiota regulates tryptophan (TRP) metabolism and may affect global 5-H synthesis in the enteric and central nervous systems, suggesting a possible involvement of gut microbiota in compulsive spectrum disorders.

OBJECTIVES

The present study investigated whether chronic TRP depletion by diet alters the faecal bacterial community profiles of compulsive versus non-compulsive rats in schedule-induced polydipsia (SIP). Peripheral plasma 5-HT and brain-derived neurotrophic factor (BDNF) levels were evaluated.

METHODS

Wistar rats were selected as High Drinkers (HD) or Low Drinkers (LD) according to their SIP behaviour and were fed for 14 days with either a TRP-free diet (T-) or a TRP-supplemented diet (T+). The faecal bacterial community structure was investigated with 16S rRNA gene-targeted denaturing gradient gel electrophoresis (DGGE) fingerprinting analysis.

RESULTS

Compulsive HD rats showed a lower bacterial diversity than LD rats, irrespectively of the diet. The TRP-depleted HD rats, the only group increasing compulsive licking in SIP, showed a reduction of bacterial evenness and a highly functionally organized community compared with the other groups, indicating that this bacterial community is more fragile to external changes due to the dominance of a low number of species. The chronic TRP depletion by diet effectively reduced peripheral plasma 5-HT levels in both HD and LD rats, while plasma BDNF levels were not altered.

CONCLUSIONS

These results highlight the possible implication of reduced microbial diversity in compulsive behaviour and the involvement of the serotonergic system in modulating the gut brain-axis in compulsive spectrum disorders.

The regulation of liver cytochrome P450 expression and activity by the brain serotonergic system in different experimental models

Introduction: Cytochrome P450 (CYP) metabolizes vital endogenous (steroids, vitamins) and exogenous (drugs, toxins) substrates. Studies of the last decade have revealed that the brain dopaminergic and noradrenergic systems are involved in the regulation of CYP. Recent research indicates that the brain serotonergic system is also engaged in its regulation.Areas covered: This review focuses on the role of the brain serotonergic system in the regulation of liver CYP expression. It shows the effect of lesion and activation of the serotonergic system after peripheral or intracerebral injections of neurotoxins, serotonin precursor, or serotonin (5-HT) receptor agonists. An opposite role of the hypothalamic paraventricular and arcuate nuclei and 5-HT receptors present therein in the regulation of CYP is described. The engagement of those nuclei in the neuroendocrine regulation of CYP by hypothalamic releasing or inhibiting hormones, pituitary hormones, and peripheral gland hormones are shown.Expert opinion: In general, the brain serotonergic system negatively regulates liver cytochrome P450. However, the effects of serotonergic agents on the enzyme expression depend on their mechanism of action, the route of administration (intracerebral/peripheral), as well as on local intracerebral site of injection and 5-HT receptor-subtypes present therein.

Measuring endogenous changes in serotonergic neurotransmission with [11C]Cimbi-36 positron emission tomography in humans

Developing positron emission tomography (PET) radioligands for the detection of endogenous serotonin release will enable the investigation of serotonergic deficits in many neuropsychiatric disorders. The present study investigates how acute challenges that aim to increase or decrease cerebral serotonin levels affect binding of the serotonin 2A receptor (5-HT_2AR) agonist radioligand [¹¹C]Cimbi-36. In a randomized, double-blind, placebo-controlled, three-arm design, 23 healthy volunteers were PET scanned twice with [¹¹C]Cimbi-36: at baseline and following double-blind assignment to one of three interventions (1) infusion of the selective serotonin reuptake inhibitor (SSRI) citalopram preceded by oral dosing of the 5-HT_1AR antagonist pindolol, (n = 8) (2) acute tryptophan depletion (ATD) (n = 7) and (3) placebo (n = 8). Two-sample t-tests revealed no significant group differences in percent change of neocortical [¹¹C]Cimbi-36 binding from baseline to intervention between placebo and citalopram/pindolol (p = 0.4) or between placebo and ATD (p = 0.5). Notably, there was a significantly larger within-group variation in 5-HT_2AR binding after intervention with citalopram/pindolol, as compared with placebo (p = 0.007). These findings suggest that neither ATD nor a combination of citalopram and pindolol elicit acute unidirectional changes in serotonin levels sufficient to be detected with [¹¹C]Cimbi-36 PET in neocortex. We suggest that the large interindividual variation in 5-HT_2AR binding after citalopram/pindolol reflects that after an acute SSRI intervention, individuals respond substantially different in terms of their brain serotonin levels. Our observation has a potential impact for the understanding of patient responses to SSRI.

Abnormal Serotonin Levels During Perinatal Development Lead to Behavioral Deficits in Adulthood

Serotonin (5-HT) is one of the best-studied modulatory neurotransmitters with ubiquitous presynaptic release and postsynaptic reception. 5-HT has been implicated in a wide variety of brain functions, ranging from autonomic regulation, sensory perception, feeding and motor function to emotional regulation and cognition. The role of this neuromodulator in neuropsychiatric diseases is unquestionable with important neuropsychiatric medications, e.g., most antidepressants, targeting this system. Importantly, 5-HT modulates neurodevelopment and changes in its levels during development can have life-long consequences. In this mini-review, we highlight that exposure to both low and high serotonin levels during the perinatal period can lead to behavioral deficits in adulthood. We focus on three exogenous factors that can change 5-HT levels during the critical perinatal period: dietary tryptophan depletion, exposure to serotonin-selective-reuptake-inhibitors (SSRIs) and poor early life care. We discuss the effects of each of these on behavioral deficits in adulthood.

An Update on the Role of Serotonin and its Interplay with Dopamine for Reward

The specific role of serotonin and its interplay with dopamine (DA) in adaptive, reward guided behavior as well as drug dependance, still remains elusive. Recently, novel methods allowed cell type specific anatomical, functional and interventional analyses of serotonergic and dopaminergic circuits, promising significant advancement in understanding their functional roles. Furthermore, it is increasingly recognized that co-release of neurotransmitters is functionally relevant, understanding of which is required in order to interpret results of pharmacological studies and their relationship to neural recordings. Here, we review recent animal studies employing such techniques with the aim to connect their results to effects observed in human pharmacological studies and subjective effects of drugs. It appears that the additive effect of serotonin and DA conveys significant reward related information and is subjectively highly euphorizing. Neither DA nor serotonin alone have such an effect. This coincides with optogenetically targeted recordings in mice, where the dopaminergic system codes reward prediction errors (PE), and the serotonergic system mainly unsigned PE. Overall, this pattern of results indicates that joint activity between both systems carries essential reward information and invites parallel investigation of both neurotransmitter systems.

Tryptophan depletion affects compulsive behaviour in rats: strain dependent effects and associated neuromechanisms

RATIONALE

Compulsive behaviour, present in different psychiatric disorders, such as obsessive-compulsive disorder, schizophrenia and drug abuse, is associated with altered levels of monoamines, particularly serotonin (5-hydroxytryptamine) and its receptor system.

OBJECTIVES

The present study investigated whether 5-HT manipulation, through a tryptophan (TRP) depletion by diet in Wistar and Lister Hooded rats, modulates compulsive drinking in schedule-induced polydipsia (SIP) and locomotor activity in the open-field test. The levels of dopamine, noradrenaline, serotonin and its metabolite were evaluated, as well as the 5-HT_2A and 5-HT_1A receptor binding, in different brain regions.

METHODS

Wistar rats were selected as high (HD) or low (LD) drinkers according to their SIP behaviour, while Lister hooded rats did not show SIP acquisition. Both strains were fed for 14 days with either a TRP-free diet (T-) or a TRP-supplemented diet (T+) RESULTS: The TRP depletion diet effectively reduced 5-HT levels in the frontal cortex, amygdala and hippocampus in both strains of rats. The TRP-depleted HD Wistar rats were more sensitive to 5-HT manipulation, exhibiting more licks on SIP than did the non-depleted HD Wistar rats, while the LD Wistar and the Lister Hooded rats did not exhibit differences in SIP. In contrast, the TRP-depleted Lister Hooded rats increased locomotor activity compared to the non-depleted rats, while no differences were found in the Wistar rats. Serotonin 2A receptor binding in the striatum was significantly reduced in the TRP-depleted HD Wistar rats.

CONCLUSIONS

These results suggest that alterations of the serotonergic system could be involved in compulsive behaviour in vulnerable populations.

Simplified dietary acute tryptophan depletion: effects of a novel amino acid mixture on the neurochemistry of C57BL/6J mice

BACKGROUND

Diet and nutrition can impact on the biological processes underpinning neuropsychiatric disorders. Amino acid (AA) mixtures lacking a specific neurotransmitter precursor can change the levels of brain serotonin (5-HT) or dopamine (DA) in the central nervous system. The availability of these substances within the brain is determined by the blood-brain barrier (BBB) that restricts the access of peripheral AA into the brain. AA mixtures lacking tryptophan (TRP) compete with endogenous TRP for uptake into the brain across the BBB, which in turn leads to a decrease in central nervous 5-HT synthesis.

OBJECTIVE

The present study compared the effects of a simplified acute tryptophan depletion (SATD) mixture in mice on blood and brain serotonergic and dopaminergic metabolites to those of a commonly used acute tryptophan depletion mixture (ATD Moja-De) and its TRP-balanced control (BAL).

DESIGN

The SATD formula is composed of only three large neutral AAs: phenylalanine (PHE), leucine (LEU), and isoleucine (ILE). BAL, ATD Moja-De, or SATD formulas were delivered to adult male C57BL/6J mice by gavage. TRP, monoamines, and their metabolites were quantified in blood and brain regions (hippocampus, frontal cortex, amygdala, caudate putamen, and nucleus accumbens).

RESULTS

Both ATD Moja-De and SATD significantly decreased levels of serum and brain TRP, as well as brain 5-HIAA and 5-HT compared with BAL. SATD reduced HVA levels in caudate but did not alter total DA levels or DOPAC. SATD decreased TRP and serotonergic metabolites comparably to ATD Moja-De administration.

CONCLUSION

A simplified and more palatable combination of AAs can manipulate serotonergic function and might be useful to reveal underlying monoamine-related mechanisms contributing to different neuropsychiatric disorders.

Dual serotonergic signals: a key to understanding paradoxical effects?

Neuroscientists have been puzzled by the fact that acute administration of a selective serotonin reuptake inhibitor (SSRI) produces results that are, at times, compatible with either decreases or increases in serotonergic neurotransmission. Furthermore, the underlying cause of the delayed onset of antidepressant effects of SSRI treatment has remained obscure. It has recently been reported that serotonergic raphe neurons co-release glutamate and that serotonergic and glutamatergic components constitute a dual signal with behaviorally distinct effects. We discuss the consequences of these novel findings and propose a framework for understanding the controversial effects of acute SSRI administration. Furthermore, we suggest that the delayed remedial onset of SSRI treatment could be explained by an initial reduction of the glutamatergic component of the dual serotonergic signal.

Chronic tryptophan deprivation attenuates gating deficits induced by 5-HT(1A), but not 5-HT₂ receptor activation

The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) exerts a multifaceted function in the modulation of information processing, through the activation of multiple receptor families. In particular, stimulation of 5-HT(1A) and 5-HT(2A) receptors leads to sensorimotor gating impairments and perceptual perturbations. Previous evidence has shown that chronic deprivation of L-tryptophan (TRP), the precursor of 5-HT, results in marked reductions of 5-HT brain levels, as well as neuroplastic alterations in 5-HT(1A) and 5-HT(2A) expression and/or signaling. Building on these premises, in the present study we tested whether a prolonged TRP deprivation may differentially impact the roles of these receptors in the regulation of the prepulse inhibition (PPI) of the acoustic startle reflex, a dependable index of gating. Male Sprague-Dawley rats were fed for 14 days with either a regimen with negligible TRP content (TR-) or the same diet supplemented of TRP (TR+). At the end of this schedule, rats were treated with the prototypical 5-HT(1A) receptor agonist 8-OH-DPAT (62.5-250 μg/kg, subcutaneous, s.c.) or the 5-HT₂ receptor agonist DOI (0.25-1 mg/kg, s.c.). Notably, the PPI deficits induced by 8-OH-DPAT in TR- rats were significantly milder than those observed in their TR+ counterparts; these effects were fully prevented by the 5-HT(1A) antagonist WAY-100135 (10 mg/kg, intraperitoneal). Conversely, TRP deprivation did not affect the PPI-disrupting properties of DOI. These findings suggest that prolonged 5-HT depletion attenuates the influence of 5-HT(1A), but not 5-HT₂ receptors on sensorimotor gating, confirming the distinct mechanisms of these two targets in PPI regulation.

No effect of acute tryptophan depletion on phosphodiesterase inhibition--related improvements of short-term object memory in male Wistar rats

OBJECTIVE

To further explore the implication of the serotonin (5-HT) system in the improvement of rat short-term object recognition after administration of the type 2 phosphodiesterase inhibitor (PDE-I) BAY 60-7550 and the type 5 PDE-I vardenafil, the effect of PDE2 and PDE5 inhibition upon central amino acid levels, 5-HT, and related parameters were measured after applying acute tryptophan depletion (ATD).

METHOD

Wistar rats were orally administered saline or a protein-carbohydrate mixture with or without tryptophan (TRP). TRP-depleted animals additionally received an oral vehicle injection or the PDE inhibitors BAY 60-7550 or vardenafil at a dose known to improve object memory performance.

RESULTS

Although ATD significantly decreased TRP levels in the hippocampus 2 h after administration, 5-HT levels appeared only moderately affected, without any changes observed in the amount of 5-HIAA or 5-HT turnover rate. Moreover, no effects of PDE inhibition upon 5-HT or related parameters were observed.

CONCLUSION

Changes in 5-HT neurotransmitter activity might be excluded as a potential underlying mechanism of the previously reported ability of PDE inhibitors to improve short-term object memory in rats. It is suggested that a decrease in cerebral blood flow potentially underlies ATD-induced object memory deficits, most likely due to decrease in NO synthesis.

Neonatal tryptophan depletion and corticosterone supplementation modify emotional responses in adult male mice

The serotonergic system and the hypothalamic-pituitary-adrenal (HPA) axis are crucially involved in the regulation of emotions. Specifically, spontaneous and/or environmentally mediated modulations of the functionality of these systems early in development may favour the onset of depressive- and anxiety-related phenotypes. While the independent contribution of each of these systems to the emergence of abnormal phenotypes has been detailed in clinical and experimental studies, only rarely has their interaction been systematically investigated. Here, we addressed the effects of reduced serotonin and environmental stress during the early stages of postnatal life on emotional regulations in mice. To this aim, we administered, to outbred CD1 mouse dams, during their first week of lactation, a tryptophan deficient diet (T) and corticosterone via drinking water (C; 80μg/ml). Four groups of dams (animal facility rearing, AFR; T treated, T; C treated, C; T and C treated, TC) and their male offspring were used in the study. Maternal care was scored throughout treatment and adult offspring were tested for: anhedonia (progressive ratio schedule); anxiety-related behaviour (approach-avoidance conflict paradigm); BDNF, dopamine and serotonin concentrations in selected brain areas. T, C and TC treatments reduced active maternal care compared to AFR. Adult TC offspring showed significantly increased anxiety- and anhedonia-related behaviours, reduced striatal and increased hypothalamic BDNF and reduced dopamine and serotonin in the prefrontal cortex and their turnover in the hippocampus. Thus, present findings support the view that neonatal variations in the functionality of the serotonergic system and of HPA axis may jointly contribute to induce emotional disturbances in adulthood.

Serotonin modulates the effects of Pavlovian aversive predictions on response vigor

Updated theoretical accounts of the role of serotonin (5-HT) in motivation propose that 5-HT operates at the intersection of aversion and inhibition, promoting withdrawal in the face of aversive predictions. However, the specific cognitive mechanisms through which 5-HT modulates withdrawal behavior remain poorly understood. Behavioral inhibition in response to punishments reflects at least two concurrent processes: instrumental aversive predictions linking stimuli, responses, and punishments, and Pavlovian aversive predictions linking stimuli and punishments irrespective of response. In the current study, we examined to what extent 5-HT modulates the impact of instrumental vs Pavlovian aversive predictions on behavioral inhibition. We used acute tryptophan depletion to lower central 5-HT levels in healthy volunteers, and observed behavior in a novel task designed to measure the influence of Pavlovian and instrumental aversive predictions on choice (response bias) and response vigor (response latencies). After placebo treatment, participants were biased against responding on the button that led to punishment, and they were slower to respond in a punished context, relative to a non-punished context. Specifically, participants slowed their responses in the presence of stimuli predictive of punishments. Tryptophan depletion removed the bias against responding on the punished button, and abolished slowing in the presence of punished stimuli, irrespective of response. We suggest that this set of results can be explained by a role for 5-HT in Pavlovian aversive predictions. These findings suggest additional specificity for the influence of 5-HT on aversively motivated behavioral inhibition and extend recent models of the role of 5-HT in aversive predictions.

Deficient serotonin neurotransmission and depression-like serotonin biomarker alterations in tryptophan hydroxylase 2 (Tph2) loss-of-function mice

Probably the foremost hypothesis of depression is the 5-hydroxytryptamine (5-HT, serotonin) deficiency hypothesis. Accordingly, anomalies in putative 5-HT biomarkers have repeatedly been reported in depression patients. However, whether such anomalies in fact reflect deficient central 5-HT neurotransmission remains unresolved. We employed a naturalistic model of 5-HT deficiency, the tryptophan hydroxylase 2 (Tph2) R439H knockin mouse, to address this question. We report that Tph2 knockin mice have reduced basal and stimulated levels of extracellular 5-HT (5-HT(Ext)). Interestingly, cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) and fenfluramine-induced plasma prolactin levels are markedly diminished in the Tph2 knockin mice. These data seemingly confirm that low CSF 5-HIAA and fenfluramine-induced plasma prolactin reflects chronic, endogenous central nervous system (CNS) 5-HT deficiency. Moreover, 5-HT(1A) receptor agonist-induced hypothermia is blunted and frontal cortex 5-HT(2A) receptors are increased in the Tph2 knockin mice. These data likewise parallel core findings in depression, but are usually attributed to anomalies in the respective receptors rather than resulting from CNS 5-HT deficiency. Further, 5-HT(2A) receptor function is enhanced in the Tph2 knockin mice. In contrast, 5-HT(1A) receptor levels and G-protein coupling is normal in Tph2 knockin mice, indicating that the blunted hypothermic response relates directly to the low 5-HT(Ext). Thus, we show that not only low CSF 5-HIAA and a blunted fenfluramine-induced prolactin response, but also blunted 5-HT(1A) agonist-induced hypothermia and increased 5-HT(2A) receptor levels are bona fide biomarkers of chronic, endogenous 5-HT deficiency. Potentially, some of these biomarkers could identify patients likely to have 5-HT deficiency. This could have clinical research utility or even guide pharmacotherapy.

Simultaneous alterations of brain and plasma serotonin concentrations and liver cytochrome P450 in rats fed on a tryptophan-free diet

Our previous study suggested involvement of the brain serotonergic system in the regulation of liver cytochrome P450 (CYP). The aim of the present study was to demonstrate simultaneous responsiveness of liver CYP and the peripheral and brain serotonergic systems to a tryptophan deficient diet during three days and one or three weeks of ingestion. The concentrations of serotonin, noradrenaline, dopamine and their metabolites were measured in blood plasma, the hypothalamus and brain stem of male rats. The enzyme activity and protein levels in the liver were determined for isoforms CYP1A, CYP2A, CYP2B, CYP2C6, CYP2C11, CYP2D and CYP3A. A three-day tryptophan-free diet increased serotonin content in the hypothalamus (but not in the brain stem or plasma). After one week, the level of serotonin was not changed in the brain, but was markedly increased in the plasma. A three week tryptophan restriction significantly reduced the concentration of serotonin in the brain and plasma. Changes in CYP2C6 and CYP2C11 (an increase and a decrease, respectively) were maintained throughout the experiment, while those found in other CYP isoforms varied, which usually resulted in a gradual increase in the enzyme activity within three weeks. The observed alterations in liver CYPs suggest involvement of both central and peripheral serotonin in the regulation of liver CYP expression whose mechanism is discussed. In conclusion, a deficit in tryptophan in the diet may be responsible for very serious food-cytochrome P450 and food-drug metabolism interactions. Interactions of this type may also refer to drugs acting via serotonergic system.

Neural expression of nicotine's antidepressant properties during tryptophan depletion: an EEG study in healthy volunteers at risk for depression

Nicotine amelioration of serotonergically mediated mood dysregulation may contribute to the comorbidity between cigarette smoking and depression, a disorder which is associated with aberrant activation and hemispheric asymmetry in frontal and posterior cortical regions. This randomized, double-blind study in 20 healthy volunteers with a positive family history of depression examined the effects of transdermal nicotine on mood and EEG changes accompanying transient reductions in serotonin induced by acute tryptophan depletion (ATD). Increased self-ratings of depressed mood and elevation in left frontal high alpha power (decreased activation) were evidenced with ATD (vs. balanced mixture) in participants treated with the placebo but not the nicotine treated group. Nicotine alone increased vigor and posterior high alpha bilaterally, and during ATD it prevented the reduction in left frontal high alpha that was evident in the placebo patch group. These findings indicate that in depression prone individuals, nicotine acts to stabilize the mood lowering and associated frontal functional asymmetry elicited by an acute decrease in brain serotonin.

Effects of acute tryptophan depletion on prefrontal-amygdala connectivity while viewing facial signals of aggression

BACKGROUND

Reduced levels of serotonin (5-HT) within prefrontal cortex (PFC)-amygdala circuits have long been implicated in impulsive aggression. However, whether lowering 5-HT alters the dynamic interplay between the PFC and the amygdala has not been directly tested in humans. It is known that manipulating 5-HT via acute tryptophan depletion (ATD) causes variable effects on brain responses to a variety of emotional stimuli, but it remains unclear whether ATD affects functional connectivity in neural networks involved in processing social signals of aggression (e.g., angry faces).

METHODS

Thirty healthy individuals were enrolled in a randomized, double-blind, placebo-controlled ATD study. On each treatment, brain responses to angry, sad, and neutral faces were measured with functional magnetic resonance imaging. Two methods (psycho-physiological-interaction in a general linear model and dynamic causal modeling) were used to assess the impact of ATD on the functional connectivity between PFC and amygdala.

RESULTS

Data from 19 subjects were available for the final analyses. A whole-brain psycho-physiological-interaction in a general linear model showed that ATD significantly modulated the connectivity between the amygdala and two PFC regions (ventral anterior cingulate cortex and ventrolateral PFC) when processing angry vs. neutral and angry vs. sad but not sad vs. neutral faces. Dynamic causal modeling corroborated and extended these findings by showing that 5-HT depletion reduced the influence of processing angry vs. neutral faces on circuits within PFC and on PFC-amygdala pathways.

CONCLUSIONS

We provide strong support for neurobiological accounts positing that 5-HT significantly influences PFC-amygdala circuits implicated in aggression and other affective behaviors.

Effects of maternal L-tryptophan depletion and corticosterone administration on neurobehavioral adjustments in mouse dams and their adolescent and adult daughters

Major depressive disorder (MDD), a pathology characterized by mood and neurovegetative disturbances, depends on a multi-factorial contribution of individual predisposition (e.g., diminished serotonergic transmission) and environmental factors (e.g., neonatal abuse or neglect). Despite its female-biased prevalence, MDD basic research has mainly focused on male rodents. Most of present models of depression are also devalued due to the fact that they typically address only one of the aforementioned pathogenetic factors. In this paper we first describe the basic principles behind mouse model development and evaluation and then articulate that current models of depression are intrinsically devalued due to poor construct and/or external validity. We then report a first attempt to overcome this limitation through the design of a mouse model in which the genetic and the environmental components of early risk factors for depression are mimicked together. Environmental stress is mimicked through the supplementation of corticosterone in the maternal drinking water while biological predisposition is mimicked through maternal access to an L-tryptophan (the serotonin precursor) deficient diet during the first week of lactation. CD1 dams and their offspring exposed to the L-tryptophan deficient diet (T) and to corticosterone (80mg/l; C) were compared to animal facility reared (AFR) subjects. T and C mice served as intermediate reference groups. Adolescent TC offspring, compared to AFR mice, showed decreased time spent floating in the forced-swim test and increased time spent in the open sectors of an elevated 0-maze. Adult TC offspring showed reduced preference for novelty, decreased breakpoints in the progressive ratio operant procedure and major alterations in central BDNF levels and altered HPA regulation. The route of administration and the possibility to control the independent variables predisposing to depressive-like symptoms disclose novel avenues towards the development of animal models with increased external and construct validity. Furthermore, the observation that, compared to adult subjects, adolescent mice display an opposite profile suggests that peri-pubertal developmental processes may interact with neonatal predispositions to calibrate the adult abnormal phenotype.

Mechanism of acute tryptophan depletion: is it only serotonin?

The method of acute tryptophan depletion (ATD), which reduces the availability of the essential amino acid tryptophan (TRP), the dietary serotonin (5-hydroxytryptamine (5-HT)) precursor, has been applied in many experimental studies. ATD application leads to decreased availability of TRP in the brain and its synthesis into 5-HT. It is therefore assumed that a decrease in 5-HT release and subsequent blunted neurotransmission is the underlying mechanism for the behavioural effects of ATD. However, direct evidence that ATD decreases extracellular 5-HT concentrations is lacking. Furthermore, several studies provide support for alternative underlying mechanisms of ATD. This may question the utility of the method as a selective serotonergic challenge tool. As ATD is extensively used for investigating the role of 5-HT in cognitive functions and psychiatric disorders, the potential of alternative mechanisms and possible confounding factors should be taken into account. It is suggested that caution is required when interpreting ATD effects in terms of a selective serotonergic effect.

Mechanism-based pharmacokinetic-pharmacodynamic modeling of the estrogen-like effect of ginsenoside Rb1 on neural 5-HT in ovariectomized mice

We sought to develop a mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) model to characterize the effects of ginsenoside Rb1 (Rb1) and estradiol (E(2)) on neural 5-hydroxytryptamine (5-HT) concentration in ovariectomized mice. PK data of Rb1 and E(2) were obtained in plasma and brain. Brain levels of 5-HT, tryptophan (TRP), 5-hydroxytryptophan (5-HTP), and 5-hydroxyindoleacetic acid (5-HIAA) were determined after a single intravenous injection of Rb1 (20mg/kg) and E(2) (0.2mg/kg) in ovariectomized mice. The activities of tryptophan hydroxylase (TPH), aromatic amino acid decarboxylase (AAAD), and monoamine oxidase (MAO) were also evaluated. Rb1 and E(2) elevated neural 5-HT levels via TPH activation and MAO inhibition, respectively. Effects were well described by the mechanism-based PK-PD model. The net effect of increased 5-HT induced by MAO inhibition is greater than TPH activation. The increased brain levels of 5-HT induced by Rb1 and E(2) were well described by the present PK-PD model, suggesting the use and further development of this mechanism-based model for the effects of ginsenoside on brain 5-HT levels.

Beneficial estrogen-like effects of ginsenoside Rb1, an active component of Panax ginseng, on neural 5-HT disposition and behavioral tasks in ovariectomized mice

Decreased 5-hydroxytryptamine (5-HT) concentration in the brain has been linked to central nervous system dysfunctions, especially in menopausal women. Ginsenoside Rb1, a potential phytoestrogen, has been shown to improve central nervous system dysfunctions, comparable to the estrogen treatment. To investigate the estrogen-like effects of ginsenoside Rb1 on neural 5-HT disposition and behavioral tasks, we quantified the concentrations of 5-HT and other related endogenous substances in the frontal cortex and striatum of ovariectomized mice. The activities of tryptophan hydroxylase (TPH), aromatic amino acid decarboxylase (AAAD) and monoamine oxidase (MAO) were also measured to evaluate the synthesis and metabolism of neural 5-HT. Our work shows that both ginsenoside Rb1 and estradiol increased the neural 5-HT concentration. Ginsenoside Rb1 and estradiol administration resulted in elevated TPH and depressed MAO activities, indicating that modulating the synthesis and metabolism of neural 5-HT successfully elevated 5-HT concentration. Ginsenoside Rb1 and estradiol also improved object recognition and decreased immobility time in the forced swimming test. However, a pretreatment with clomiphene (an estrogen receptor antagonist) blocked the beneficial effects of ginsenoside Rb1 and estradiol, suggesting that the estrogen-like effects of ginsenoside Rb1 were estrogen receptor-dependent.

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

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Acute tryptophan depletion potentiates 3,4-methylenedioxymethamphetamine-induced cerebrovascular hyperperfusion in adult male Wistar rats

The serotonergic (5-hydroxytryptamine; 5-HT) dysfunction found in depression may affect not only brain function (mood) but also cerebrovascular control. Similar, but possibly occult, disturbances may also be induced by 3,4-methylenedioxymethamphetamine-induced neurotoxicity (MDMA, or "ecstasy"). Acute tryptophan depletion (ATD) is widely used to identify vulnerability to depression, and we hypothesized that repeated MDMA administration would increase the sensitivity of rats to this acute serotonergic challenge. In this study, male Wistar rats were injected with MDMA (20 mg kg(-1), twice daily for 4 days) and challenged 3 weeks later with ATD, induced by intragastric administration of a nutritional mixture with tryptophan (TRP) removed. Cerebral metabolism (CMRG) and blood flow (CBF) were measured in parallel groups of animals following ATD by using quantitative [(14)C]2-deoxyglucose and [(14)C]iodoantipyrine autoradiographic techniques, respectively. A significant reduction in paroxetine binding to 5-HT transporter sites in MDMA-treated rats indicated 5HT terminal depletion, whereas the plasma TRP/sum large neutral amino acids ratio was reduced by 40% following ATD. Under all experimental conditions, the normal close correlation between CBF and metabolic demand was maintained. However, a global analysis of all brain regions revealed a significant decrease in the overall ratio of CBF to CMRG after ATD in control animals, whereas a higher ratio was observed after ATD in the MDMA-treated group. This increase in blood flow relative to cerebral metabolism suggests an ATD-induced loss of cerebrovascular tone in MDMA-treated animals that could have pathophysiological consequences and might conceivably contribute to the behavioral dysfunction of depression.

Stress-mediated decreases in brain-derived neurotrophic factor as potential confounding factor for acute tryptophan depletion-induced neurochemical effects

Acute tryptophan depletion (ATD) is extensively used to investigate the implication of serotonin (5-hydroxytryptamine; 5-HT) in the onset and treatment of depression and cognitive disorders. Brain-derived neurotrophic factor (BDNF) is strongly linked to the 5-HT system and plays an essential role in mood and memory processes. The present study investigated the effects of ATD upon BDNF in serum, hippocampus and prefrontal cortex in the rat to further explore the underlying mechanism of ATD. ATD significantly decreased peripheral tryptophan (TRP) levels and moderately interrupted 5-HT metabolism 4h after administration of the nutritional mixture. Although no direct effects of ATD upon serum or brain BDNF concentrations were found, a stress-mediated, decrease in BDNF was observed in the prefrontal cortex. Moreover, brain TRP levels correlated positively with BDNF in both the prefrontal cortex and hippocampus. Thus, BDNF-mediated mechanisms due to ATD and/or its application stress might underlie ATD-induced neurochemical and behavioural alterations.

Reconciling the role of serotonin in behavioral inhibition and aversion: acute tryptophan depletion abolishes punishment-induced inhibition in humans

The neuromodulator serotonin has been implicated in a large number of affective and executive functions, but its precise contribution to motivation remains unclear. One influential hypothesis has implicated serotonin in aversive processing; another has proposed a more general role for serotonin in behavioral inhibition. Because behavioral inhibition is a prepotent reaction to aversive outcomes, it has been a challenge to reconcile these two accounts. Here, we show that serotonin is critical for punishment-induced inhibition but not overall motor response inhibition or reporting aversive outcomes. We used acute tryptophan depletion to temporarily lower brain serotonin in healthy human volunteers as they completed a novel task designed to obtain separate measures of motor response inhibition, punishment-induced inhibition, and sensitivity to aversive outcomes. After a placebo treatment, participants were slower to respond under punishment conditions compared with reward conditions. Tryptophan depletion abolished this punishment-induced inhibition without affecting overall motor response inhibition or the ability to adjust response bias in line with punishment contingencies. The magnitude of reduction in punishment-induced inhibition depended on the degree to which tryptophan depletion reduced plasma tryptophan levels. These findings extend and clarify previous research on the role of serotonin in aversive processing and behavioral inhibition and fit with current theorizing on the involvement of serotonin in predicting aversive outcomes.

Acute tryptophan depletion in C57BL/6 mice does not induce central serotonin reduction or affective behavioural changes

Acute tryptophan depletion is extensively used to investigate the implication of serotonin in the onset of depressive disorders. In rats, it lowers peripheral tryptophan and decreases central serotonin concentrations. We aimed to establish the rat model of acute tryptophan depletion in the mouse for potential application as serotonin challenge tool in genetic mouse models of depression. Pharmacokinetic and behavioural effects of a tryptophan-free diet were examined in Swiss and C57BL/6 mice. Peripheral amino acids were measured and central tryptophan and serotonin concentrations were compared with anxiety and depression-like behaviour in the elevated zero-maze, forced swimming test or tail suspension test. While acute tryptophan depletion resulted in a 74% reduction of the plasma ratio tryptophan to the sum of other large neutral amino acids in Swiss mice 1h after administration (2x10 ml/kg, 30 min interval), there was only a 40% reduction in C57BL/6 mice. The latter did not show anxiety in the elevated zero-maze or increased immobility in the forced swimming test or tail suspension test. A higher dose (2x20 ml/kg) with a longer interval (60 min) reduced the ratio with 68% in C57BL/6 mice, lowered hippocampal serotonin turnover and had no functional effect when tested in the elevated zero-maze and forced swimming test. These findings have important implications for the use of acute tryptophan depletion in general and in particular for its application in mice. Although in healthy mice no clear central serotonin or functional effects were observed, further research is indicated using mice with pre-existing serotonin dysfunction, as they might be more vulnerable to acute tryptophan depletion.

Nutritional tryptophan restriction impairs plasticity of retinotectal axons during the critical period

The use-dependent specification of neural circuits occurs during post-natal development with a conspicuous influence of environmental factors, such as malnutrition that interferes with the major steps of brain maturation. Serotonin (5-HT), derived exclusively from the essential aminoacid tryptophan, is involved in mechanisms of development and use-dependent plasticity of the central nervous system. We studied the effects of the nutritional restriction of tryptophan in the plasticity of uncrossed retinotectal axons following a retinal lesion to the contralateral retina during the critical period in pigmented rats. Litters were fed through their mothers with a low tryptophan content diet, based on corn and gelatin, a complemented diet with standard tryptophan requirements for rodents or standard laboratory diet. The results suggest a marked reduction in the plasticity of intact axons into denervated territories in the tryptophan restricted group in comparison to control groups. Tryptophan complementation between PND10-21 completely restored retinotectal plasticity. However, the re-introduction of tryptophan after the end of the critical period (between PND28-P41) did not restore the sprouting ability of uncrossed axons suggesting a time-dependent effect to the reversion of plasticity deficits. Tryptophan-restricted animals showed a reduced activity of matrix metalloproteinase-9 and altered expressions of phosphorylated forms of ERK1/2 and AKT. Our results demonstrate the influence of this essential aminoacid as a modulator of neural plasticity during the critical period through the reduction of serotonin content which alters plasticity-related signaling pathways and matrix degradation.

Effect of acute tryptophan depletion on noradrenaline and dopamine in the rat brain

In human subjects, the acute tryptophan (TRP) depletion (ATD) paradigm has been shown to have effects on mood and cognition. It is assumed that these effects are mediated through the serotonin system. In this study, we have examined the effects of ATD on the central concentrations of the monoamine transmitters, noradrenaline (NA) and dopamine (DA) as well as on serotonin (5-HT). Effects on NA and DA could also affect mood and cognition. Following oral administration of TRP-containing (TRP+) and TRP-free (TRP-) amino acid mixtures, neurotransmitter concentrations and free plasma TRP concentrations were determined by High Performance Liquid Chromatography (HPLC) with electrochemical detection. Free plasma TRP was significantly and substantially reduced (79%) in rats given a TRP- amino acid mixture when compared with those given a TRP+ mixture. ATD also significantly decreased 5-HT and 5-hydroxyindolacetic acid in the frontal cortex, remaining cortex and hippocampus, but did not significantly reduce these in the striatum. Furthermore, ATD did not significantly alter the concentration of NA and DA in any brain region examined. This study demonstrates that the administration of a TRP- amino acid mixture in rats can reduce free plasma TRP to levels comparable to those reported in human studies. These results indicate that behavioural and cognitive changes produced by ATD in preclinical or clinical studies are likely to be due to specific effects on the serotonergic system.

Phosphodiesterase 2 and 5 inhibition attenuates the object memory deficit induced by acute tryptophan depletion

The underlying mechanism of short-term memory improvement after inhibition of specific phosphodiesterases (PDEs) is still poorly understood. The present study aimed to reveal the ability of PDE5 and PDE2 inhibitors, that increase cyclic guanosine monophosphate (cGMP) and both cyclic adenosine monophosphate (cAMP) and cGMP, respectively, to reverse an object recognition deficit induced by acute tryptophan depletion. Acute tryptophan depletion is a pharmacological challenge tool to lower central serotonin (5-hydroxytryptamine; 5-HT) levels by depleting the availability of its dietary precursor tryptophan. Short-term object memory was tested in male Wistar rats by exposing them to the object recognition task. First, the effects of acute tryptophan depletion upon object recognition 2 h after administration of the nutritional mixture were established. Subsequently, acute tryptophan depletion was combined with the PDE5 inhibitor vardenafil (1, 3 and 10 mg/kg) or with the PDE2 inhibitor BAY 60-7550 (0.3, 1 and 3 mg/kg), 30 min prior to testing. Acute tryptophan depletion significantly lowered plasma tryptophan levels and impaired object recognition performance. Vardenafil (3 and 10 mg/kg) and BAY 60-7550 (3 mg/kg) were able to attenuate the acute tryptophan depletion induced object recognition impairment. Thus, both PDE5 and PDE2 inhibition improved short-term object recognition performance after an acute tryptophan depletion induced deficit. The underlying mechanisms, however, remain poorly understood and further studies are needed to determine whether the present findings can be explained by a direct effect of enhanced cAMP and cGMP levels upon 5-HT activity, or even other neurotransmitter systems, and possibly an interaction with synthesis of nitric oxide or effects upon cerebral blood flow function.

Low plasma tryptophan in carcinoid patients is associated with increased urinary cortisol excretion

BACKGROUND

Previously we observed in patients suffering from a metastatic carcinoid tumor that irritability, aggression and lack of impulse control are associated with low levels of plasma tryptophan and presumably with low brain serotonin function. In rats we showed that a diet of low tryptophan resulted in higher stress responses and higher corticosterone production. Here we tested in carcinoid patients whether tryptophan depletion due to tumor 5-HT overproduction is associated with high cortisol production.

METHODS

Urinary excretion of cortisol, serotonin, 5-hydroxyindole acetic acid (the main metabolite of serotonin a marker of tumor activity), plasma levels of tryptophan and platelet content of serotonin (index of peripheral serotonin synthesis) were determined in metastatic midgut carcinoid patients. Patients (N = 25) were divided into two groups based on their plasma tryptophan levels (< or = 25 micromol/l, n = 12 and > or = 49 micromol/l, n = 13).

RESULTS

Carcinoid patients with low plasma tryptophan levels had significantly higher urinary excretion of free cortisol (p < 0.01), independent of tumor activity. The inter-individual differences in the low tryptophan group, however, were substantial.

CONCLUSIONS

In a subgroup of the patients suffering from metastatic carcinoid disease the cerebral access of plasma tryptophan is impaired, thus rendering cerebral serotonin neurotransmission suboptimal and leading to hypercortisolism. The present study provides further support to the idea that low serotonergic function is a risk for developing stress-associated psychopathology.

Effects of tryptophan deficiency on prepulse inhibition of the acoustic startle in rats

RATIONALE

Serotonin (5-HT) plays a key role in the pathophysiology of psychotic disorders, presumably through a modulation of dopamine (DA) transmission. Reduction of 5-HT signaling has been suggested to enhance dopaminergic responses in animal models of psychosis. An intriguing naturalistic strategy to reduce 5-HT brain content is afforded by the dietary restriction to its precursor, l-tryptophan (TRP).

OBJECTIVE

We investigated the impact of a TRP-deficient diet in rats on the prepulse inhibition of the startle (PPI), a measure of sensorimotor gating which is typically impaired by psychotomimetic substances.

MATERIALS AND METHODS

After either short-term (6 h) or long-term (14 days) TRP deprivation, rats were tested for startle reflex and PPI. Moreover, we assessed the impact of both TRP deprivation regimens on PPI reduction induced by the psychotomimetic substance d-amphetamine (AMPH).

RESULTS

Both TRP-deficient regimens failed to significantly affect PPI responses. However, chronic, but not short-term, TRP-deficient diet induced a significant sensitization to the effects of AMPH (1.25-2.5 mg/kg, subcutaneous). The enhanced predisposition to PPI disruption elicited by prolonged TRP deprivation was completely reversed 24 h after reinstatement of TRP in the diet, as well as pretreatment with antipsychotic drugs haloperidol (0.1 mg/kg, intraperitoneal) and clozapine (5 mg/kg, intraperitoneal), which exert their therapeutic action mostly through blockade of DA D(2) receptors.

CONCLUSIONS

The present results confirm and extend previous findings on the impact of serotonergic signaling in the modulation of DA transmission in schizophrenia and point to chronic TRP deprivation as a potential model of environmental manipulation that may produce a sensitization to psychotic-like symptoms induced by dopaminergic activation.

Low tryptophan diet increases stress-sensitivity, but does not affect habituation in rats

Cerebral dysfunction of 5-HT (serotonin) has been associated with stress response and with affective disorders. Stress alone is insufficient to induce depression, since only a minor proportion of subjects that have experienced stressful life events develop depressive episodes. We investigated whether long-term brain 5-HT depletion induced in rats by a diet with low content of its precursor tryptophan affects stress-responsiveness in rats. Stress-sensitivity was measured through various physiological parameters and by measuring the rats' response to acoustic stimuli. One group of rats was subjected to daily acoustic stimulus sessions for 5 days. Other groups received both immobilization stress and acoustic stimulus sessions daily for either 9 days (chronic experiment) or 1 day (acute experiment). A low tryptophan diet led to decreases in plasma tryptophan levels, low ratio of tryptophan/large neutral amino acid, whole blood 5-HT, and neuronal 5-HT content in the Dorsal and Median Raphe Nuclei, as well as altered c-fos expression in the brain. Without concomitant immobilization, the diet alone did not affect reactivity and habituation to acoustic stimuli, although plasma corticosterone levels, but not the adrenal weights, were increased on day 5. Low tryptophan and chronic immobilization stress together with the acoustic testing procedure increased adrenal weight, plasma corticosterone levels and reactivity to the acoustic stimuli, but not the rate of habituation to acoustic stimuli. These results show that cerebral dysfunction of serotonin achieved through a low tryptophan diet, increases the sensitivity of rats to external and stressful stimuli, but does not impair the capacity to adapt to these stimuli. Accordingly, brain-serotonin modulates reactivity to stress, but not stress coping.

Lack of evidence for reduced prefrontal cortical serotonin and dopamine efflux after acute tryptophan depletion

RATIONALE

Acute tryptophan depletion (ATD) is a widely used method to study the role of serotonin (5-HT) in affect and cognition. ATD results in a strong but transient decrease in plasma tryptophan and central 5-HT synthesis and availability. Although its use is widespread, the evidence that the numerous functional effects of ATD are caused by actual changes in 5-HT neuronal release is not very strong. Thus far, decreases in 5-HT efflux (thought to reflect synaptic release) were only reported after chronic tryptophan depletion or when ATD was combined with blockade of 5-HT reuptake.

OBJECTIVE

With the current experiment, we aimed to study the validity of the method of ATD by measuring the extent to which it reduces the efflux of 5-HT (and dopamine) in the prefrontal cortex in the absence of reuptake blockage.

MATERIALS AND METHODS

We simultaneously measured in freely moving animals plasma tryptophan via a catheter in the jugular vein and 5-HT and DA efflux in the medial prefrontal cortex through microdialysis after ATD treatment.

RESULTS

ATD reduced plasma tryptophan to less than 30% of control, without affecting 5-HT or DA efflux in the prefrontal cortex, indicating that even strong reductions of plasma tryptophan do not necessarily result in decreases in central 5-HT efflux.

CONCLUSION

The present experiment showed that reductions in plasma tryptophan, similar to values associated with behavioural effects, do not necessarily reduce 5-HT efflux and suggest that the cognitive and behavioural effects of ATD may not be (exclusively) due to alterations in 5-HT release.

Acute and chronic tryptophan depletion differentially regulate central 5-HT1A and 5-HT 2A receptor binding in the rat

RATIONALE

Tryptophan depletion is used to reduce central serotonergic function and to investigate its role in psychiatric illness. Despite widespread clinical use, its effects on serotonin (5-HT) receptors have not been well characterized.

OBJECTIVE

The aim of this study was to examine the effect of acute (ATD) and chronic tryptophan depletion (CTD) on free-plasma tryptophan (TRP), central TRP and 5-HT and brain 5-HT(1A) and 5-HT(2A) receptor binding in the rat.

METHODS

TRP and 5-HT were measured by high-performance liquid chromatography and receptor levels determined by homogenate radioligand binding and in-vitro receptor autoradiography.

RESULTS

Free-plasma TRP, central TRP and central 5-HT levels were significantly and similarly reduced by ATD and 1- and 3-week CTD compared to controls. ATD significantly reduced 5-HT(1A) binding in the dorsal raphe (14%) but did not significantly alter postsynaptic 5-HT(1A) binding (frontal cortex, remaining cortex and hippocampus) or 5-HT(2A) binding (cortex and striatum). One-week CTD did not significantly alter cortical 5-HT(2A) binding or postsynaptic 5-HT(1A) binding. Furthermore, 3-week CTD did not significantly alter 5-HT(1A) binding but significantly increased cortical 5-HT(2A) binding without affecting striatal or hippocampal levels. In the CTD 1 and 3-week groups, rat body weight was significantly decreased as compared to controls. However, weight loss was not a confounding factor for decreased cortical 5-HT(2A)-receptor binding.

CONCLUSION

ATD-induced reduction in somatodendritic 5-HT(1A) autoreceptor binding may represent an intrinsic 'homeostatic response' reducing serotonergic feedback in dorsal raphe projection areas. In contrast, the increase in 5-HT(2A) receptor after CTD may be a compensatory response to a long-term reduction in 5-HT.

Rats subjected to extended L-tryptophan restriction during early postnatal stage exhibit anxious-depressive features and structural changes

Serotonin transmission dysfunction has been suggested to play an important role in depression and anxiety. This study reports the results of a series of experiments in which rats were subjected to extended maize-based tortilla diets during early postnatal stages. This diet contains only approximately 20% of the L-tryptophan in normal diets of laboratory rodents. Compared with controls, experimental rats displayed a significant increase of immobility counts in the forced swimming test and exhibited anxiety-like behavior in the elevated plus maze test after 1 month of diet treatment. Low levels of serotonin contents were found in prefrontal cortex, striatum, hippocampus, and brainstem using high-performance liquid chromatography. Immunocytochemical reactions against 5-Bromo-2'-deoxyuridine revealed a significant decrease in the proliferation rate for the subgranular zone of dentate gyrus. c-Fos expression after the forced swimming test was found reduced in prefrontal cortex, dentate gyrus, CA1, and hilus of hippocampus and amygdala. Moreover, dendrite arbor atrophy and decreased spine density were evident in Golgi-Cox-impregnated CA1 pyramidal neurons. Abnormal dendrite swelling in dentate gyrus granule cells was also observed. These findings indicate an involvement of hyposerotoninergia in emotional disturbance produced by L-tryptophan restriction during critical developmental stages and suggest that neuroplasticity changes might underlie these changes.

Augmented cocaine-induced accumbal dopamine efflux, motor activity and place preference in rats fed with a tryptophan-deficient diet

In the present study we demonstrate that consumption of a tryptophan-deficient diet for a period of 14 days decreased the striatal serotonin and 5-hydroxyindolacetic acid tissue content in rats, whereas the level of dopamine remained unchanged. Under this condition of diminished serotonergic tone, a challenge dose of cocaine (10mg/kg, i.p.) significantly increased motor activity and dopamine extracellular content in the nucleus accumbens compared to rats fed with a balanced diet. We moreover found that pretreatment with cocaine (7 and 10mg/kg, i.p.) produced a significant increase in preference for a cocaine-associated environment in the tryptophan-deficient group compared to control rats. Our experiments show that a low tone of serotonergic system, augments the behavioural reinforcing effect of cocaine and that this effect may be due to a increased cocaine-induced accumbal dopamine release. These data indicate that a tryptophan-deficient diet alters the behavioural and neurochemical effect of psychostimulants, such as cocaine, and suggest an important role of serotonin in modulation of these effects.

Tryptophan-deficient diet increases the neurochemical and behavioral response to amphetamine

The present study examined the effects of a tryptophan-deficient diet on behavioral and neurochemical response to amphetamine. A tryptophan-deficient diet (14 days) decreased striatal serotonin and 5-hydroxyindolacetic acid content in rats. Under the latter conditions, amphetamine increased dopamine efflux in striatum and nucleus accumbens and produced a greater increase in motor activity when compared to controls. These results indicate how response to psychostimulants might be altered in the presence of a tryptophan-deficient diet.

The effect of tryptophan depletion on the action of haloperidol in MK-801-treated rats

We investigated the effect of tryptophan depletion (tryptophan-free mixture) on locomotor activity in an animal model of schizophrenia, induced by acute administration of 5R,10S-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801), and the influence of the tryptophan-free mixture on the action of the typical antipsychotic haloperidol. Male rats were pre-treated with haloperidol 60 min after receiving the tryptophan-free mixture (or water). We measured total distance travelled in an open field during a 90-min period. Administration of the tryptophan-free mixture resulted in decreased levels of tryptophan, serotonin and its metabolite 5-hydroxyindolacetic acid in the frontal cortex. Serotonin depletion increased the total distance travelled by MK-801-treated rats, modified the inhibitory effect of haloperidol and normalized the locomotor activity pattern in the model of schizophrenia-like behaviour. The effect of the tryptophan-free mixture combined with the classical antipsychotic haloperidol in MK-801-treated rats indicates the possibly important role of the serotonergic system in the action of antipsychotics.

Fluoxetine-induced changes in body weight and 5-HT1Areceptor-mediated hormone secretion in rats on a tryptophan-deficient diet

Tryptophan depleting protocols are commonly used to study the role of serotonin in mood disorders. The present study examined the impact of a tryptophan-deficient diet and fluoxetine on the serotonergic regulation of neuroendocrine function and body weight. We hypothesized that the regulation of postsynaptic 5-HT1Areceptors is dependent on the levels of 5-HT in the synapse. Rats on a control or a tryptophan-deficient diet received daily injections of saline or fluoxetine (5 or 10 mg·kg-1·day-1ip) from day 7 to day 21. The tryptophan-deficient diet produced a 41% reduction in the level of 5-HT but no change in the density of [3H]paroxetine-labeled 5-HT transporters. Treatment with fluoxetine inhibited the gain in weight in rats maintained on the control diet. The tryptophan-deficient diet produced a significant loss in body weight that was not significantly altered by treatment with fluoxetine. Treatment with fluoxetine produced a dose-dependent desensitization of hormone responses to injection of the 5-HT1Areceptor agonist (±)8-hydroxy-2-(di- n-propylamino)tetralin ((±)8-OH-DPAT). The tryptophan-deficient diet produced an increase in the basal levels of corticosterone but did not alter the basal levels of ACTH or oxytocin. Also, this diet inhibited the magnitude of 8-OH-DPAT-induced increase in plasma levels of ACTH and oxytocin but did not impair the ability of fluoxetine to desensitize the 5-HT1Areceptor-mediated increase in plasma hormones. These data suggest that a reserve of 5-HT enables fluoxetine to desensitize postsynaptic 5-HT1Areceptors in the hypothalamus. In conclusion, the profound physiological changes induced by tryptophan depletion may complicate the interpretation of studies using this experimental approach.

Tryptophan as a link between psychopathology and somatic states

OBJECTIVE

Several somatic illnesses are associated with psychiatric comorbidity. Evidence is provided that availability of the essential amino acid tryptophan, which is the precursor of serotonin, may cause this phenomenon.

METHODS

We performed a database search to find relevant articles published between 1966 and 2002. For our search strategy, we combined several diseases from the categories hormonal, gastrointestinal, and inflammatory with the search terms "tryptophan" and "serotonin."

RESULTS

The catabolism of tryptophan is stimulated under the influence of stress, hormones and inflammation by the induction of the enzymes tryptophan pyrrolase (in the liver) and IDO (ubiquitous). Because of the reduction in blood levels of tryptophan under these circumstances the formation of cerebral serotonin is decreased.

CONCLUSIONS

It is argued that the coupling of peripheral tryptophan levels and cerebral serotonin levels has physiological significance. The clinical implications and therapeutic consequences of changes in tryptophan and consequently serotonin metabolism are discussed.