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

The role of dopamine in visual imagery-An experimental pharmacological study

Mental imagery enables people to simulate experiences in their minds without the presence of an external stimulus. The underlying biochemical mechanisms are poorly understood but there is vague evidence that dopamine may play a significant role. A better understanding at the biochemical level could help to unravel the mechanisms of mental imagery and related phenomena such as aphantasia (= lack of voluntary mental imagery), but also opens up possibilities for interventions to enhance or restore mental imagery. To test the hypothesis that acute dopamine depletion leads to a decrease in the strength of mental imagery, N = 22 male participants will be administered an amino acid mixture containing branched-chain amino acids (BCAAs) and tryptophan (TRP) to transiently reduce dopamine synthesis and further N = 22 male participants will receive a placebo. Plasma prolactin (PRL) levels are determined as a peripheral marker of brain dopamine function. The strength of mental imagery will be measured before and after ingestion of the BCAA/TRP mixture using the method of mental imagery priming. Additional exploratory analyses will use genetic data to investigate possible effects of variations on dopaminergic gene loci (e.g., DAT1) on dopamine levels and strength of mental imagery. The results show […].

Phenylketonuria and the brain

Classic phenylketonuria (PKU) is caused by defective activity of phenylalanine hydroxylase (PAH), the enzyme that coverts phenylalanine (Phe) to tyrosine. Toxic accumulation of phenylalanine and its metabolites, left untreated, affects brain development and function depending on the timing of exposure to elevated levels. The specific mechanisms of Phe-induced brain damage are not completely understood, but they correlate to phenylalanine levels and on the stage of brain growth. During fetal life, high levels of phenylalanine such as those seen in maternal PKU can result in microcephaly, neuronal loss and corpus callosum hypoplasia. Elevated phenylalanine levels during the first few years of life can cause acquired microcephaly, severe cognitive impairment and epilepsy, likely due to the impairment of synaptogenesis. During late childhood, elevated phenylalanine can cause alterations in neurological functioning, leading to ADHD, speech delay and mild IQ reduction. In adolescents and adults, executive function and mood are affected, with some of the abnormalities reversed by better control of phenylalanine levels. Altered brain myelination can be present at this stage. In this article, we review the current knowledge about the consequences of high phenylalanine levels in PKU patients and animal models through different stages of brain development and its effect on cognitive, behavioural and neuropsychological function.

Acute depletion of dopamine precursors in the human brain: effects on functional connectivity and alcohol attentional bias

Individuals who abuse alcohol often show exaggerated attentional bias (AB) towards alcohol-related cues, which is thought to reflect reward conditioning processes. Rodent studies indicate that dopaminergic pathways play a key role in conditioned responses to reward- and alcohol-associated cues. However, investigation of the dopaminergic circuitry mediating this process in humans remains limited. We hypothesized that depletion of central dopamine levels in adult alcohol drinkers would attenuate AB and that these effects would be mediated by altered function in frontolimbic circuitry. Thirty-four male participants (22-38 years, including both social and heavy drinkers) underwent a two-session, placebo-controlled, double-blind dopamine precursor depletion procedure. At each visit, participants consumed either a balanced amino acid (control) beverage or an amino acid beverage lacking dopamine precursors (order counterbalanced), underwent resting-state fMRI, and completed behavioral testing on three AB tasks: an alcohol dot-probe task, an alcohol attentional blink task, and a task measuring AB to a reward-conditioned cue. Dopamine depletion significantly diminished AB in each behavioral task, with larger effects among subjects reporting higher levels of binge drinking. The depletion procedure significantly decreased resting-state functional connectivity among ventral tegmental area, striatum, amygdala, and prefrontal regions. Beverage-related AB decreases were mediated by decreases in functional connectivity between the fronto-insular cortex and striatum and, for alcohol AB only, between anterior cingulate cortex and amygdala. The results support a substantial role for dopamine in AB, and suggest specific dopamine-modulated functional connections between frontal, limbic, striatal, and brainstem regions mediate general reward AB versus alcohol AB.

Dopamine and light: effects on facial emotion recognition

Bright light can affect mood states and social behaviours. Here, we tested potential interacting effects of light and dopamine on facial emotion recognition. Participants were 32 women with subsyndromal seasonal affective disorder tested in either a bright (3000 lux) or dim light (10 lux) environment. Each participant completed two test days, one following the ingestion of a phenylalanine/tyrosine-deficient mixture and one with a nutritionally balanced control mixture, both administered double blind in a randomised order. Approximately four hours post-ingestion participants completed a self-report measure of mood followed by a facial emotion recognition task. All testing took place between November and March when seasonal symptoms would be present. Following acute phenylalanine/tyrosine depletion (APTD), compared to the nutritionally balanced control mixture, participants in the dim light condition were more accurate at recognising sad faces, less likely to misclassify them, and faster at responding to them, effects that were independent of changes in mood. Effects of APTD on responses to sad faces in the bright light group were less consistent. There were no APTD effects on responses to other emotions, with one exception: a significant light × mixture interaction was seen for the reaction time to fear, but the pattern of effect was not predicted a priori or seen on other measures. Together, the results suggest that the processing of sad emotional stimuli might be greater when dopamine transmission is low. Bright light exposure, used for the treatment of both seasonal and non-seasonal mood disorders, might produce some of its benefits by preventing this effect.

Acute phenylalanine/tyrosine depletion of phasic dopamine in the rat brain

RATIONALE

Dopamine plays a critical role in striatal and cortical function, and depletion of the dopamine precursors phenylalanine and tyrosine is used in humans to temporarily reduce dopamine and probe the role of dopamine in behavior. This method has been shown to alter addiction-related behaviors and cognitive functioning presumably by reducing dopamine transmission, but it is unclear what specific aspects of dopamine transmission are altered.

OBJECTIVES

We performed this study to confirm that administration of an amino acid mixture omitting phenylalanine and tyrosine (Phe/Tyr[-]) reduces tyrosine tissue content in the prefrontal cortex (PFC) and nucleus accumbens (NAc), and to test the hypothesis that Phe/Tyr[-] administration reduces phasic dopamine release in the NAc.

METHODS

Rats were injected with a Phe/Tyr[-] amino acid mixture, a control amino acid mixture, or saline. High-performance liquid chromatography was used to determine the concentration of tyrosine, dopamine, or norepinephrine in tissue punches from the PFC and ventral striatum. In a separate group of rats, phasic dopamine release was measured with fast-scan cyclic voltammetry in the NAc core after injection with either the Phe/Tyr[-] mixture or the control amino acid solution.

RESULTS

Phe/Tyr[-] reduced tyrosine content in the PFC and NAc, but dopamine and norepinephrine tissue content were not reduced. Moreover, Phe/Tyr[-] decreased the frequency of dopamine transients, but not their amplitude, in freely moving rats.

CONCLUSIONS

These results indicate that depletion of tyrosine via Phe/Tyr[-] decreases phasic dopamine transmission, providing insight into the mechanism by which this method modifies dopamine-dependent behaviors in human imaging studies.

The Effects of Acute Dopamine Precursor Depletion on the Cognitive Control Functions of Performance Monitoring and Conflict Processing: An Event-Related Potential (ERP) Study

Studies using medications and psychiatric populations implicate dopamine in cognitive control and performance monitoring processes. However, side effects associated with medication or studying psychiatric groups may confound the relationship between dopamine and cognitive control. To circumvent such possibilities, we utilized a randomized, double-blind, placebo-controlled, within-subjects design wherein participants were administered a nutritionally-balanced amino acid mixture (BAL) and an amino acid mixture deficient in the dopamine precursors tyrosine (TYR) and phenylalanine (PHE) on two separate occasions. Order of sessions was randomly assigned. Cognitive control and performance monitoring were assessed using response times (RT), error rates, the N450, an event-related potential (ERP) index of conflict monitoring, the conflict slow potential (conflict SP), an ERP index of conflict resolution, and the error-related negativity (ERN) and error positivity (Pe), ERPs associated with performance monitoring. Participants were twelve males who completed a Stroop color-word task while ERPs were collected four hours following acute PHE and TYR depletion (APTD) or balanced (BAL) mixture ingestion in two separate sessions. N450 and conflict SP ERP amplitudes significantly differentiated congruent from incongruent trials, but did not differ as a function of APTD or BAL mixture ingestion. Similarly, ERN and Pe amplitudes showed significant differences between error and correct trials that were not different between APTD and BAL conditions. Findings indicate that acute dopamine precursor depletion does not significantly alter cognitive control and performance monitoring ERPs. Current results do not preclude the role of dopamine in these processes, but suggest that multiple methods for dopamine-related hypothesis testing are needed.

Age moderates the effect of acute dopamine depletion on passive avoidance learning

Despite extensive links between reinforcement-based learning and dopamine (DA), studies to date have not found consistent effects of acute DA reduction on reinforcement learning in both men and women. Here, we tested the effects of reducing DA on reward- and punishment-based learning using the deterministic passive avoidance learning (PAL) task. We tested 16 (5 female) adults (ages 22-40) in a randomized, cross-over design to determine whether reducing global DA by administering an amino acid beverage deficient in the DA precursors, phenylalanine and tyrosine (P/T[-]), would affect PAL task performance. We found that P/T[-] beverage effects on PAL performance were modulated by age. Specifically, we found that P/T depletion significantly improved learning from punishment with increasing participant age. Participants committed 1.49 fewer passive avoidance errors per additional year of age (95% CI, -0.71 - -2.27, r=-0.74, p=0.001). Moreover, P/T depletion improved learning from punishment in adults (ages 26-40) while it impaired learning from punishment in emerging adults (ages 22-25). We observed similar, but non-significant trends in learning from reward. While there was no overall effect of P/T-depletion on reaction time (RT), there was a relationship between the effect of P/T depletion on PAL performance and RT; those who responded more slowly on the P/T[-] beverage also made more errors on the P/T[-] beverage. When P/T-depletion slowed RT after a correct response, there was a worsening of PAL task performance; there was no similar relationship for the RT after an incorrect response and PAL task performance. Moreover, among emerging adults, changes in mood on the P/T[-] beverage negatively correlated with learning from reward on the P/T[-] beverage. Together, we found that both reward- and punishment-based learning are sensitive to central catecholamine levels, and that these effects of acute DA reduction vary with age.

Dopamine precursors depletion impairs impulse control in healthy volunteers

The aim of the present study was to decipher the role of the dopamine system in impulse control. Impulsive actions entail (i) activation of the motor system by an impulse, which is an urge to act and (ii) a failure to suppress that impulse, when inappropriate, in order to prevent an error. These two aspects of action impulsivity can be experimentally disentangled in conflict reaction time tasks such as the Simon task, which measures susceptibility to acting on spontaneous impulses (as well as the proficiency of suppressing these impulses). In 12 healthy volunteers performing a Simon task, dopamine availability was reduced with an amino acid drink deficient in the dopamine precursors, phenylalanine and tyrosine. Classic behavioral measures were augmented with an analysis of the electromyographic activity of the response effectors. Electromyography allows one to detect covert activations undetectable with strictly behavioral measures and further reveals the participants' ability to quickly suppress covert activations before they result in an overt movement. Following dopamine depletion, compared with a placebo condition, participants displayed comparable impulse activation but were less proficient at suppressing the interference from this activation. These results provide evidence that the dopamine system is directly involved in the suppression of maladaptive response impulses.

Tyrosine-free amino acid mixtures reduce physiologically-evoked release of dopamine in a selective and activity-dependent manner

Depletion of the catecholamine precursor tyrosine using tyrosine-free amino acid mixtures is an important tool in neuropsychological studies, and often considered dopamine selective on the basis of neuropharmacological studies. However, little is known of the effects of tyrosine depletion when catecholamine neurons are activated physiologically. Here we investigated the effect of tyrosine-free amino acid mixtures on catecholamine release evoked in vivo using a stimulation paradigm aimed to approximate the phasic firing pattern of these neurons that accompanies cognitive and behavioural change. Dopamine and noradrenaline release was monitored by microdialysis in rat medial prefrontal cortex (mPFC) and striatum (chloral hydrate anaesthesia, perfusion medium containing 1 µM cocaine). Electrical stimulation of the medial forebrain bundle (MFB) caused a short-lasting, frequency-dependent increase in dopamine and noradrenaline. A full tyrosine-free amino acid mixture reduced the release of dopamine in mPFC and striatum, across a range of stimulation frequencies, and the effect was greater as stimulation frequency increased. Similar results were obtained using a smaller tyrosine-free amino acid mixture. In the same experiments showing decreased dopamine, neither tyrosine-free mixture of amino acids significantly altered stimulation-evoked release of noradrenaline. These results show that tyrosine depletion using tyrosine-free amino acid mixtures causes a selective, activity-dependent decrease in dopamine release when dopamine neurons are driven physiologically.

Dietary tyrosine/phenylalanine depletion effects on behavioral and brain signatures of human motivational processing

Dopamine (DA) neurotransmission is critical for motivational processing. We assessed whether disruption of DA synthesis in healthy controls using an amino-acid beverage devoid of catecholamine precursors (tyrosine-phenylalanine depletion (TPD)) would blunt recruitment of the nucleus accumbens (NAcc) by rewards. Sixteen controls ingested each of a tyr/phe-depleting beverage (DEP) or a tyr/phe-balanced (BAL) control beverage in two laboratory visits. Five hours after consumption of each drink, subjects underwent functional magnetic resonance imaging while they viewed anticipatory cues to respond to a target to either win money or avoid losing money. TPD did not exert main effects on mood or on task behavior, but affected brain activation. In right NAcc, TPD blunted activation by anticipation of high rewards. In left NAcc, recruitment anticipating high rewards was modulated by individual differences in mood change across the DEP drink day, where subjects whose mood worsened following TPD (relative to within-day mood change under BAL conditions) also showed lower activation under DEP conditions relative to BAL conditions. Exploratory analysis indicated that TPD qualitatively blunted the voxel-wise spatial extent of suprathreshold activation by reward anticipation. Finally, loss outcomes activated anterior insula under DEP conditions but not under BAL conditions. These data indicate that: (1) dietary depletion of catacholamine precursors will blunt dopaminergic mesolimbic activity, and (2) in controls, synthetic pathways of this neurocircuitry maintain sufficient buffering capacity to resist an effect on motivated behavior. Additional studies are needed to determine if clinical populations would show similar resistance to behavioral effects of TPD.

Dopamine precursor depletion impairs structure and efficiency of resting state brain functional networks

Spatial patterns of functional connectivity derived from resting brain activity may be used to elucidate the topological properties of brain networks. Such networks are amenable to study using graph theory, which shows that they possess small world properties and can be used to differentiate healthy subjects and patient populations. Of particular interest is the possibility that some of these differences are related to alterations in the dopamine system. To investigate the role of dopamine in the topological organization of brain networks at rest, we tested the effects of reducing dopamine synthesis in 13 healthy subjects undergoing functional magnetic resonance imaging. All subjects were scanned twice, in a resting state, following ingestion of one of two amino acid drinks in a randomized, double-blind manner. One drink was a nutritionally balanced amino acid mixture, and the other was tyrosine and phenylalanine deficient. Functional connectivity between 90 cortical and subcortical regions was estimated for each individual subject under each dopaminergic condition. The lowered dopamine state caused the following network changes: reduced global and local efficiency of the whole brain network, reduced regional efficiency in limbic areas, reduced modularity of brain networks, and greater connection between the normally anti-correlated task-positive and default-mode networks. We conclude that dopamine plays a role in maintaining the efficient small-world properties and high modularity of functional brain networks, and in segregating the task-positive and default-mode networks. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Dopamine and light: dissecting effects on mood and motivational states in women with subsyndromal seasonal affective disorder

BACKGROUND

Despite evidence that bright light can improve mood, the neurobiology remains poorly understood. Some evidence implicates the catecholamines. In the present study, we measured the effects of transiently decreasing dopamine (DA) synthesis on mood and motivational states in healthy women with mild seasonal mood changes who were tested in either bright or dim light.

METHODS

On 2 test days, participants slept overnight in a light-controlled room. On the morning of each session, half of the participants awoke to gradual increases of bright light, up to 3000 lux, and half to dim light (10 lux). For all participants, DA was reduced on 1 of the test days using the acute phenylalanine/tyrosine depletion (APTD) method; on the other day, they ingested a nutritionally balanced control mixture (BAL). Beginning 4 hours postingestion, participants completed subjective mood questionnaires, psychological tests and a progressive ratio breakpoint task during which they worked for successive units of $5.

RESULTS

Thirty-two women participated in our study. The APTD lowered mood, agreeableness, energy and the willingness to work for monetary reward. The effects on energy and motivation were independent of light, while the effects on mood and agreeableness were seen in the dim condition only, being prevented by bright light.

LIMITATIONS

Acute phenylalanine/tyrosine depletion might affect systems other than DA. The sample size was small.

CONCLUSION

These results suggest that increased DA function may be responsible for some of the beneficial effects of light, while adding to the evidence that the neurobiology of mood and motivational states can be dissociated.

Effects of acute dopamine precusor depletion on immediate reward selection bias and working memory depend on catechol-O-methyltransferase genotype

Little agreement exists as to acute dopamine (DA) manipulation effects on intertemporal choice in humans. We previously found that catechol-O-methyltransferase (COMT) Val158Met genotype predicts individual differences in immediate reward selection bias among adults. Moreover, we and others have shown that the relationship between COMT genotype and immediate reward bias is inverted in adolescents. No previous pharmacology studies testing DA manipulation effects on intertemporal choice have accounted for COMT genotype, and many have included participants in the adolescent age range (18-21 years) as adults. Moreover, many studies have included female participants without strict cycle phase control, although recent evidence demonstrates that cyclic estradiol elevations interact with COMT genotype to affect DA-dependent cognition. These factors may have interacted with DA manipulations in past studies, potentially occluding detection of effects. Therefore, we predicted that, among healthy male adults (ages 22-40 years), frontal DA tone, as indexed by COMT genotype, would interact with acute changes in DA signaling to affect intertemporal choice. In a double-blind, placebo-controlled design, we decreased central DA via administration of an amino acid beverage deficient in the DA precursors, phenylalanine and tyrosine, and tested effects on immediate reward bias in a delay-discounting (DD) task and working memory (WM) in an n-back task. We found no main effect of beverage on DD or WM performance but did find significant beverage*genotype effects. These results suggest that the effect of DA manipulations on DD depends on individual differences in frontal DA tone, which may have impeded some past efforts to characterize DA's role in immediate reward bias in humans.

Effects of acute dopamine precusor depletion on immediate reward selection bias and working memory depend on catechol-O-methyltransferase genotype

Little agreement exists as to acute dopamine (DA) manipulation effects on intertemporal choice in humans. We previously found that catechol-O-methyltransferase (COMT) Val158Met genotype predicts individual differences in immediate reward selection bias among adults. Moreover, we and others have shown that the relationship between COMT genotype and immediate reward bias is inverted in adolescents. No previous pharmacology studies testing DA manipulation effects on intertemporal choice have accounted for COMT genotype, and many have included participants in the adolescent age range (18-21 years) as adults. Moreover, many studies have included female participants without strict cycle phase control, although recent evidence demonstrates that cyclic estradiol elevations interact with COMT genotype to affect DA-dependent cognition. These factors may have interacted with DA manipulations in past studies, potentially occluding detection of effects. Therefore, we predicted that, among healthy male adults (ages 22-40 years), frontal DA tone, as indexed by COMT genotype, would interact with acute changes in DA signaling to affect intertemporal choice. In a double-blind, placebo-controlled design, we decreased central DA via administration of an amino acid beverage deficient in the DA precursors, phenylalanine and tyrosine, and tested effects on immediate reward bias in a delay-discounting (DD) task and working memory (WM) in an n-back task. We found no main effect of beverage on DD or WM performance but did find significant beverage*genotype effects. These results suggest that the effect of DA manipulations on DD depends on individual differences in frontal DA tone, which may have impeded some past efforts to characterize DA's role in immediate reward bias in humans.

From anticipation to action, the role of dopamine in perceptual decision making: an fMRI-tyrosine depletion study

During simple sensorimotor decision making, neurons in the parietal cortex extract evidence from sensory information provided by visual areas until a decision is reached. Contextual information can bias parietal activity during the task and change the decision-making parameters. One type of contextual information is the availability of reward for correct decisions. We tested the hypothesis that the frontal lobes and basal ganglia use contextual information to bias decision making to maximize reward. Human volunteers underwent functional MRI while making decisions about the motion of dots on a computer monitor. On rewarded trials, subjects responded more slowly by increasing the threshold to decision. Rewarded trials were associated with activation in the ventral striatum and prefrontal cortex in the period preceding coherent dot motion, and the degree of activation predicted the increased decision threshold. Decreasing dopamine transmission, using a tyrosine-depleting amino acid mixture, abolished the reward-related corticostriatal activation and eliminated the correlation between striatal activity and decision threshold. These observations provide direct evidence that some reward-related functional MRI signals in the striatum are the result of dopamine neuron activity and demonstrate that mesolimbic dopamine transmission can influence perceptual and decision-making neural processes engaged to maximize reward harvest.

Reliance on habits at the expense of goal-directed control following dopamine precursor depletion

RATIONALE

Dopamine is well known to play an important role in learning and motivation. Recent animal studies have implicated dopamine in the reinforcement of stimulus-response habits, as well as in flexible, goal-directed action. However, the role of dopamine in human action control is still not well understood.

OBJECTIVES

We present the first investigation of the effect of reducing dopamine function in healthy volunteers on the balance between habitual and goal-directed action control.

METHODS

The dietary intervention of acute dietary phenylalanine and tyrosine depletion (APTD) was adopted to study the effects of reduced global dopamine function on action control. Participants were randomly assigned to either the APTD or placebo group (ns = 14) to allow for a between-subjects comparison of performance on a novel three-stage experimental paradigm. In the initial learning phase, participants learned to respond to different stimuli in order to gain rewarding outcomes. Subsequently, an outcome-devaluation test and a slips-of-action test were conducted to assess whether participants were able to flexibly adjust their behaviour to changes in the desirability of the outcomes.

RESULTS

APTD did not prevent stimulus-response learning, nor did we find evidence for impaired response-outcome learning in the subsequent outcome-devaluation test. However, when goal-directed and habitual systems competed for control in the slips-of-action test, APTD tipped the balance towards habitual control. These findings were restricted to female volunteers.

CONCLUSIONS

We provide direct evidence that the balance between goal-directed and habitual control in humans is dopamine dependent. The results are discussed in light of gender differences in dopamine function and psychopathologies.

Acute dopamine and/or serotonin depletion does not modulate mismatch negativity (MMN) in healthy human participants

RATIONALE

Schizophrenia is commonly associated with impairments in pre-attentive change detection, as represented by reduced mismatch negativity (MMN). While the neurochemical basis of MMN has been linked to N-methyl-D: -aspartic acid (NMDA) receptor function, the roles of the dopaminergic and/or the serotonergic systems are not fully explored in humans.

OBJECTIVES

The aim of the present study was to investigate the effects of acutely depleting dopamine (DA) and serotonin (5-hydroxytryptamine, 5-HT) alone or simultaneously by depleting their amino acid precursors on MMN in healthy participants.

METHODS

Sixteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject's duration MMN was assessed under four acute treatment conditions separated by a 5-day washout period: balanced amino acid control (no depletion), tyrosine/phenylalanine depletion (to reduce DA neurotransmission), tryptophan depletion (to reduce 5-HT neurotransmission) and tryptophan/tyrosine/phenylalanine depletion (to reduce DA and 5-HT neurotransmission simultaneously).

RESULTS

Acute depletion of either DA and 5-HT alone or simultaneously had no effect on MMN.

CONCLUSIONS

These findings suggest that modulation of the dopaminergic and serotonergic systems acutely does not lead to changes in MMN.

Dopamine precursor depletion improves punishment prediction during reversal learning in healthy females but not males

INTRODUCTION

The neurotransmitter dopamine has frequently been implicated in reward processing but is also, increasingly, implicated in punishment processing. We have previously shown that both patients with Parkinson's disease and healthy individuals with low dopamine (DA) synthesis are better at reversal learning based on punishment than reward. Here, we extend these prior findings by examining the effects of artificially reducing DA synthesis in healthy individuals performing this previously employed task.

METHODS

In a double-blind, placebo-controlled crossover design, we applied the acute tyrosine and phenylalanine depletion (ATPD) procedure to reduce global DA synthesis in 15 female and 14 male subjects. Each subject performed the reward- and punishment-based reversal-learning paradigm.

RESULTS

There was a significant three-way interaction between ATPD, the valence of the outcome signalling reversal and the gender of the participants. Examination of punishment and reward-based reversals separately revealed that this was driven by a significant improvement in punishment processing in female but not male subjects following DA depletion.

CONCLUSIONS

Reducing DA synthesis in healthy individuals shifted sensitivity of performance from reward to punishment processing. Gender differences in DA synthesis might underlie the selectivity of this effect to female subjects. Such gender biases may go some way towards explaining the gender biases in certain psychiatric disorders such as depression and Parkinson's disease.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

The role of dopamine in alcohol self-administration in humans: individual differences

OBJECTIVE

To clarify dopamine's role in alcohol self-administration in a heterogeneous sample of drinkers using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Sixteen men with variable drinking histories were characterized on their ethanol-induced cardiac response, a marker previously proposed to index dopamine system reactivity and vulnerability to alcohol abuse. During separate sessions participants were administered (i) a nutritionally balanced (BAL) amino acid (AA) mixture, (ii) a mixture lacking the dopamine precursors, phenylalanine and tyrosine, and (iii) APTD followed by the dopamine precursor, L-DOPA. Five hours after AA administration, participants could earn units of alcohol using a progressive ratio breakpoint task.

RESULTS

Alcohol self-administration was reduced in the APTD and APTD+L-DOPA conditions relative to the BAL condition. In both cases the changes were predicted by ethanol-induced cardiac change.

CONCLUSIONS

The motivation to drink is likely regulated by more than one neurobiological mechanism. Individual differences in cardiac responsivity to ethanol might provide a peripheral marker of responsiveness to pharmacological manipulations of dopamine.

Differential effects of acute serotonin and dopamine depletion on prepulse inhibition and p50 suppression measures of sensorimotor and sensory gating in humans

Schizophrenia is associated with impairments of sensorimotor and sensory gating as measured by prepulse inhibition (PPI) of the acoustic startle response and P50 suppression of the auditory event-related potential respectively. While serotonin and dopamine play an important role in the pathophysiology and treatment of schizophrenia, their role in modulating PPI and P50 suppression in humans is yet to be fully clarified. To further explore the role of serotonin and dopamine in PPI and P50 suppression, we examined the effects of acute tryptophan depletion (to decrease serotonin) and acute tyrosine/phenylalanine depletion (to decrease dopamine) on PPI and P50 suppression in healthy human participants. In addition, we also examined for the first time, the effects of simultaneous serotonin and dopamine depletion (ie combined monoamine depletion) on PPI and P50 suppression. The study was a double-blind, placebo-controlled cross-over design in which 16 healthy male participants completed the PPI and P50 paradigms under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Selective depletion of dopamine had no significant effect on either PPI or P50 suppression, whereas selective serotonin depletion significantly disrupted PPI, but not P50 suppression. Finally, the simultaneous depletion of both serotonin and dopamine resulted in significant reduction of both PPI and P50 suppression. We suggest these results can be explained by theories relating to optimal levels of monoaminergic neurotransmission and synergistic interactions between serotonergic and dopaminergic systems for normal 'gating' function. These findings suggest that a dysfunction in both serotonin and dopamine neurotransmission may, in part, be responsible for the gating deficits observed in schizophrenia, and their normalization following administration of atypical antipsychotic drugs.

Effects of selective and combined serotonin and dopamine depletion on the loudness dependence of the auditory evoked potential (LDAEP) in humans

BACKGROUND

The loudness dependence of the auditory evoked potential (LDAEP) has been suggested as a possible in vivo measure of central serotonin function. However, more recent studies suggest that the LDAEP may be modulated by multiple neuromodulatory systems in addition to the serotonergic system. Accordingly we further examined the effects of selective serotonin, dopamine and simultaneous serotonin and dopamine depletion on the LDAEP in healthy subjects.

METHODS

The study employed a placebo-controlled, double-blind, cross over design. Fourteen subjects were tested under four acute treatment conditions: placebo (balanced amino acid drink), tryptophan (serotonin) depletion (ATD), tyrosine/phenylalanine (dopamine) depletion (ATPD) and combined tryptophan/tyrosine/phenylalanine (serotonin and dopamine) depletion (CMD). Testing was conducted 5.5 h post-depletion and changes in the amplitude of the N1/P2 at varying intensities (60, 70, 80, 90, 100 dB) were examined at C(Z).

RESULTS

Greater than 80% plasma precursor depletion was achieved across all conditions. Despite significant depletion of monoamine precursors, ATD, (p = 0.318), ATPD (p = 0.061) and CMD (p = 0.104) had no effects on the LDAEP (60-100 dB).

CONCLUSION

Acute serotonin and dopamine depletion did not modulate the LDAEP. This finding adds support to growing evidence that the LDAEP is insensitive to acute changes in serotonin and dopamine neurotransmission.

Effects of acute tyrosine/phenylalanine depletion on the selective processing of smoking-related cues and the relative value of cigarettes in smokers

RATIONALE

Acute tyrosine/phenylalanine depletion (ATPD) is a validated neurobiological challenge that results in reduced dopaminergic neurotransmission, allowing examination of the effects of a hypodopaminergic state on craving-related processes.

OBJECTIVES

We studied 16 nonabstaining smokers (>10 cigarettes/day; 9 males; age 20-33 years) to whom was administered a tyrosine/phenylalanine-free mixture (TYR/PHE-free) and a balanced amino acid mixture (BAL) in a double-blind, counterbalanced, crossover design.

METHODS

Subjective cigarette craving, attentional bias to smoking-related word cues, relative value of cigarettes, negative mood, and expired carbon monoxide (CO) levels were measured at various timepoints through 300 min. Participants smoked at hourly intervals to prevent acute nicotine withdrawal during testing.

RESULTS

The TYR/PHE-free mixture, as compared to the BAL mixture, was associated with a greater increase in CO levels from baseline (p = 0.01). Adjusting for the potential confounding influence of between-condition differences in CO levels across time, TYR/PHE-free mixture was associated with increased demand for cigarettes (p = 0.01) and decreased attentional bias toward smoking-related words (p = 0.003). There were no significant differences between conditions in either subjective craving or depressed or anxious mood (p values > 0.05).

CONCLUSION

Among nonabstaining daily smokers, acute dopaminergic depletion via ATPD may influence smoking behavior and indices of smoking-related motivation, such as attentional bias to smoking cues and relative cigarette value, which are not readily captured by subjective craving.

Tyrosine depletion alters cortical and limbic blood flow but does not modulate spatial working memory performance or task-related blood flow in humans

Dopamine appears critical in regulating spatial working memory (SWM) within the PFC of non-human primates; however findings in humans are less clear. Recent studies of the effects of global depletion of dopamine via acute tyrosine/phenylalanine depletion (TPD) on SWM task performance have yielded inconsistent results, which may be partly related to task differences. These previous studies do not address whether TPD can directly impair PFC functioning. The current study investigated the effects of TPD on (1) regional cerebral blood flow (rCBF) during a SWM n-back task using H(2) (15)O Positron Emission Tomography (PET), and (2) behavioural performance on three different SWM tasks. Ten healthy males were scanned twice: once following a placebo (balanced) amino acid mixture and once following an equivalent mixture deficient in tyrosine/phenylalanine (TPD condition). Participants completed two additional delayed-response tasks to examine whether differences in response demands influenced TPD effects on performance. TPD resulted in widespread increases in rCBF, with maximum increases in the region of the parahippocampal gyrus bilaterally, left inferior frontal gyrus, and the putamen. TPD related rCBF reductions were observed in the medial frontal gyrus bilaterally, right inferior temporal gyrus and the pons. Despite widespread changes in blood flow following TPD, no specific effects on SWM neural networks or task performance were observed. The use of three different SWM tasks suggests that task differences are unlikely to account for the lack of effects observed. These findings question the capacity of TPD to consistently modulate dopamine function and SWM neural networks in humans.

Region-specific effects of a tyrosine-free amino acid mixture on amphetamine-induced changes in BOLD fMRI signal in the rat brain

BACKGROUND

Acute depletion of tyrosine using a tyrosine-free amino acid mixture offers a novel dietary approach to inhibit activated dopamine pathways in the brain. This study investigated the potential of in vivo functional magnetic resonance imaging (fMRI) methods as a noninvasive means to detect effects of tyrosine depletion on dopamine function.

METHODS

Changes in blood-oxgenation level dependent (BOLD) contrast induced by administration of the dopamine-releasing agent, amphetamine (3 mg/kg i.v.), were measured in halothane-anaesthetised rats.

RESULTS

Amphetamine evoked changes in BOLD signal intensity with the greatest effects observed in the nucleus accumbens (-7.7%), prefrontal cortex (-13.6%), and motor cortex (+12.5%). Pretreatment with a tyrosine-free amino acid mixture attenuated the response to amphetamine in some regions (nucleus accumbens and prefrontal cortex), but not others (motor cortex). Amphetamine itself had no effect in thalamus and hippocampus but, surprisingly, increased the BOLD signal after the amino acid mixture.

CONCLUSION

These experiments demonstrate that amphetamine evokes region-specific changes in the BOLD signal in rats, and that this effect is attenuated in some but not all regions by tyrosine depletion. The data support the application of fMRI techniques for studying the effects of tyrosine depletion on dopamine function in animals and also humans.

Effects of acute tyrosine depletion on subjective craving and selective processing of smoking-related cues in abstinent cigarette smokers

We investigated the impact of the administration of a tyrosine-depleting amino acid mixture compared to a balanced mixture on measures of mood, craving and selective processing of smoking-related cues in healthy cigarette smokers instructed to abstain from smoking for 12 h prior to, and during, the experiment. A modified stroop task was used to index selective processing of smoking-related cues. We observed evidence for an increase in subjective craving among males, and an attenuation of the selective processing of smoking-related cues compared to control cues among females, in the tyrosine-depleting condition compared to the balanced condition. No effects of mixture were observed on measures of subjective mood. These results tentatively support for the role of dopaminergic neurotransmission in mediating the response of cigarette smokers to smoking-related cues. In addition, these results also provide further evidence for sex differences in the factors that maintain cigarette smoking, in particular with respect to conditioned reinforcement of smoking behaviour, and suggest that the relationship between subjective craving and selective processing of smoking-related cues may differ in males and females.

Acute serotonin and dopamine depletion improves attentional control: findings from the stroop task

Schizophrenia is associated with impairments of attentional control on classic experimental paradigms such as the Stroop task. However, at a basic level the neurochemical mechanisms that may be responsible for such impairments are poorly understood. In this study, we sought to investigate the influence of brain monoamine function on Stroop task performance in healthy participants using the established methods of acute dietary serotonin, dopamine, and combined monoamine depletion. The study was a double-blind placebo controlled design in which 12 healthy male participants completed the Stroop task under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Decreased Stroop interference indicating improved attentional control was observed after both tryptophan depletion and tyrosine/phenylalanine depletion, while there was no significant change in interference after combined monoamine depletion. Findings suggest that reduced tonic dopamine or serotonin activity within specific neural circuits (such as the striatum, anterior cingulate, or prefrontal cortex) may play a critical role in attentional control, possibly by improving gating of information via reducing noise in monoaminergic systems. These findings enhance our understanding of the neurochemical basis of attentional control and the possible cause of attentional control deficits in schizophrenia.

Mood is indirectly related to serotonin, norepinephrine and dopamine levels in humans: a meta-analysis of monoamine depletion studies

Dysfunction in the monoamine systems of serotonin (5-HT), norepinephrine (NE) and dopamine (DA) may causally be related to major depressive disorder (MDD). Monoamine depletion studies investigate the direct effects of monoamines on mood. Acute tryptophan depletion (ATD) or para-chlorophenylalanine (PCPA) deplete 5-HT, acute phenylalanine/tyrosine depletion (APTD) or alpha-methyl-para-tyrosine (AMPT) deplete NE/DA. Available depletion studies found conflicting results in heterogeneous populations: healthy controls, patients with previous MDD in remission and patients suffering from MDD. The decrease in mood after 5-HT and NE/DA depletion in humans is reviewed and quantified. Systematic search of MEDLINE and EMBASE (1966-October 2006) and cross-references was carried out. Randomized studies applying ATD, PCPA, APTD or AMPT vs control depletion were included. Pooling of results by meta-analyses was stratified for studied population and design of the study (within or between subjects). Seventy-three ATD, 2 PCPA, 10 APTD and 8 AMPT studies were identified of which 45 ATD and 8 APTD studies could be meta-analyzed. 5-HT or NE/DA depletion did not decrease mood in healthy controls. 5-HT or NE/DA depletion slightly lowered mood in healthy controls with a family history of MDD. In drug-free patients with MDD in remission, a moderate mood decrease was found for ATD, without an effect of APTD. ATD induced relapse in patients with MDD in remission who used serotonergic antidepressants. In conclusion, monoamine depletion studies demonstrate decreased mood in subjects with a family history of MDD and in drug-free patients with MDD in remission, but do not decrease mood in healthy humans. Although depletion studies usefully investigate the etiological link of 5-HT and NE with MDD, they fail to demonstrate a causal relation. They presumably clarify a vulnerability trait to become depressed. Directions for further investigation of this vulnerability trait are proposed.

Affective bias and response modulation following tyrosine depletion in healthy adults

Acute phenylalanine/tyrosine depletion (ATPD) has been used to transiently lower central nervous system dopamine activity in animals and humans. Findings suggest that ATPD may impact dopamine transmission in limbic and striatal regions. Impact on cognitive functions has varied across studies, although several recent reports suggest that affective processing in the context of a go/no-go response control task may be impaired during ATPD. In this study, response control under affective vs nonaffective conditions was examined in healthy individuals who underwent either ATPD or a balanced condition in a between-subjects design. Effects of ATPD were validated through its effects on serum prolactin secretion. ATPD resulted in significantly increased prolactin levels relative to the balanced mixture. Although there were no differences in self-reported mood between the groups, individuals in the ATPD condition demonstrated diminished sensitivity to positively valenced words and seemingly enhanced sensitivity to negatively valenced words in an affective go/no-go task. They also showed difficulties in modulating ongoing behavior in a nonaffective go/no-go task when responses had to be intermittently inhibited then immediately restarted. Basic motor functions were not impacted. Findings are discussed in relation to dopamine's role in switching signals within neural networks that are important for response modulation and affective control.

Brain dopamine and kinematics of graphomotor functions

Three experiments were performed in an attempt to achieve a better understanding of the effect of dopamine on handwriting. In the first experiment, kinematic aspects of handwriting movements were compared between healthy participants and patients with Parkinson's disease (PD) on their usual dopaminergic treatment and following withdrawal of dopaminergic medication. In the second experiment, the writing performance of healthy participants with a hyperechogenicity of the substantia nigra as detected by transcranial sonography (TCS) was compared with the performance of healthy participants with low echogenicity of the substantia nigra. The third experiment examined the effect of central dopamine reduction on kinematic aspects of handwriting movements in healthy adults using acute phenylalanine and tyrosine depletion (APTD). A digitising tablet was used for the assessment of handwriting movements. Participants were asked to perform a simple writing task. Movement time, distance, velocity, acceleration and measures of fluency of handwriting movements were measured. The kinematic analysis of handwriting movements revealed that alterations of central dopaminergic neurotransmission adversely affect movement execution during handwriting. In comparison to the automatic processing of handwriting movements displayed by control participants, participants with an altered dopaminergic neurotransmission shifted from an automatic to a controlled processing of movement execution. Central dopamine appears to be of particular importance with regard to the automatic execution of well-learned movements.

Lack of effect of acute dopamine precursor depletion in nicotine-dependent smokers

RATIONALE

Nicotine increases dopamine (DA) release but its role in nicotine dependence remains unclear.

OBJECTIVE

To assess the role of DA in nicotine craving and self-administration using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Fifteen nicotine-dependent men ingested, a minimum of 3days apart, a nutritionally balanced amino acid (AA) mixture (BAL), a mixture deficient in the catecholamine precursors, phenylalanine and tyrosine, and APTD followed by the immediate DA precursor, L-DOPA. Beginning 3h after ingestion of the AA mixture, subjects smoked 4 cigarettes. Craving, mood, and other aspects of subjective state were assessed with self-report scales. Smoking puff topography was measured with a computerized flowmeter.

RESULTS

APTD did not change smoking puff topography, cigarette craving, or subjective effects of smoking.

CONCLUSIONS

The findings suggest that in nicotine-dependent smokers craving for cigarettes, subjective effects of nicotine, and the self-administration of freely available cigarettes are largely unrelated to acute changes in DA neurotransmission.

In rats chronically treated with clozapine, tyrosine depletion attenuates the clozapine-induced in vivo increase in prefrontal cortex dopamine and norepinephrine levels

We previously reported that depletion of brain tyrosine attenuated the acute clozapine (CLZ)-induced increase in medial prefrontal cortex (MPFC) dopamine (DA) levels. This effect was now examined after chronic CLZ treatment. Male rats received CLZ (10 mg kg(-1) day(-1)) in drinking water for 21 days. On day 18, a cannula was stereotaxically implanted over the MPFC. A microdialysis probe was inserted on day 20. On day 21 after a stable baseline was reached, rats received an acute injection of vehicle (VEH) or a tyrosine- and phenylalanine-free mixture of neutral amino acid [NAA(-)] (total 1 g kg(-1), i.p., two injections, 1 h apart) followed by CLZ (10 mg kg(-1), i.p.) or VEH. Basal tyrosine or norepinephrine (NE) levels were not different between the groups, but basal DA was higher in the group treated chronically with CLZ (p<0.05). Acute CLZ (10 mg kg(-1), i.p.) increased MPFC DA and NE levels to 370% and 510% of baseline, respectively, and similarly in rats chronically pretreated with CLZ or VEH. NAA(-) did not affect basal MPFC DA or NE levels but significantly attenuated acute CLZ-induced DA (220% of baseline) and NE (330% of baseline) levels (p<0.01) in rats pretreated chronically with CLZ or with VEH. These data demonstrate that even after chronic CLZ administration, the acute CLZ-induced increases in MPFC DA and NE levels depend on the availability of brain tyrosine. Judicious manipulation of brain tyrosine levels may provide a useful probe as well as a mechanism for enhancing psychotropic drug actions.

Effect of acute tyrosine depletion in using a branched chain amino-acid mixture on dopamine neurotransmission in the rat brain

Central dopamine function is reduced by decreasing the availability of the catecholamine precursor, tyrosine, using a tyrosine-free amino acid mixture containing multiple large neutral as well as branched chain amino-acids, which compete with tyrosine for uptake into the brain. Current mixtures are cumbersome to make and administer, and unpalatable to patients and volunteers. Here, we investigate whether individual or limited amino-acid combinations could reduce brain tyrosine levels and hence dopamine function. Measurements of regional brain tyrosine levels, catecholamine and indoleamine synthesis (L-DOPA and 5-HTP accumulation, respectively) were used to identify an effective paradigm to test in neurochemical, behavioral and fos immunocytochemical models. Administration of leucine or isoleucine, or a mixture of leucine, isoleucine, and valine reduced tyrosine and 5-HTP, but not L-DOPA accumulation. A mixture of leucine, valine, and isoleucine supplemented with tryptophan reduced brain tyrosine and L-DOPA, but not 5-HTP. In microdialysis experiments this amino-acid mixture reduced basal and amphetamine-evoked striatal dopamine release, as well as amphetamine-induced hyperactivity. This mixture also reduced amphetamine-induced fos expression in striatal areas. In conclusion, the present study identified a small combination of amino acids that reduces brain tyrosine and dopamine function in a manner similar to mixtures of multiple amino acids. This minimal mixture may have use as a dopamine reducing paradigm in patient and volunteer studies.

Combined D1/D2 receptor stimulation under conditions of dopamine depletion impairs spatial working memory performance in humans

RATIONALE

The mesocortical dopamine system is regarded as an important modulator of working memory. While it has been established that stimulation of the D1/D2 receptor in primates can improve spatial working memory performance, findings in humans are less consistent. Recent studies in humans suggest that global depletion of dopamine via tyrosine/phenylalanine depletion may impair spatial working memory performance, although these results are also inconsistent, and it has been suggested that task differences may partly underlie the inconsistent findings.

OBJECTIVES

This study had two aims: (1) to investigate the effects of acute tyrosine depletion (TPD) on a number of working memory tasks and (2) to examine whether stimulation of D1/D2 receptors under conditions of TPD can attenuate or "reverse" TPD-induced working memory impairments.

METHODS

Eighteen healthy male participants performed a spatial working memory delayed-recognition task, non-spatial working memory task and spatial n-back task on three separate occasions, after TPD, TPD and pergolide (D1/D2 agonist), and placebo.

RESULTS

TPD did not impair working memory performance on any of the tasks administered. However, stimulation of D1/D2 receptors under TPD conditions caused a subtle impairment in spatial working memory performance.

CONCLUSIONS

The finding that D1/D2 stimulation under TPD conditions impairs working memory highlights the complexity of functional effects of augmenting dopaminergic transmission within a dopamine-depleted state. The lack of TPD-related effects on a range of working memory tasks questions the reliability of TPD as a modulator of dopamine function and working memory performance in humans.

The effects of acute tyrosine and phenylalanine depletion on spatial working memory and planning in healthy volunteers are predicted by changes in striatal dopamine levels

RATIONALE

Dopamine (DA) is considered important in the modulation of tasks of spatial working memory. However, the findings from studies in humans to date are mixed. While this may be due to the characteristics of the tasks used, it is also possible that these findings are explained by variable central effects of the manipulations used.

OBJECTIVE

To test the effects of acute tyrosine and phenylalanine depletion (TPD, which reduces synthesis and release of brain DA) on cognitive function and relate changes in performance accuracy to the central effects of TPD measured with [11C]raclopride positron emission tomography (PET).

METHODS

Fourteen participants were given tests of spatial working memory, planning, verbal memory span and trial-and-error learning after acute TPD, seven of whom also received PET scans to measure changes in striatal DA levels.

RESULTS

Although TPD produced a clear reduction in tyrosine and phenylalanine availability to the brain, no impairments on any of the cognitive tests were observed. However, changes in spatial working memory and planning accuracy after TPD showed a highly significant relationship with the changes in striatal DA levels.

CONCLUSIONS

Our findings suggest that the effects of TPD on spatial working memory and planning may be unreliable due to the variability of the changes in brain DA levels achieved with this manipulation.

Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

RATIONALE

The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.

OBJECTIVES

The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, alpha2-adrenoceptor-mediated regulation) were characterized in the LC.

METHODS

Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.

RESULTS

NA in the LC decreased via local infusion of Ca2+-free medium (-42+/-5%) or the sodium channel blocker tetrodotoxine (TTX) (-47+/-8%) but increased (333+/-40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor alpha-methyl-p-tyrosine (250 mg kg(-1), i.p.) and the vesicle depletory drug reserpine (5 mg kg(-1), i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the alpha2-adrenoceptor agonist clonidine (0.1-100 microM) decreased NA (E(max)=-79+/-5%) in the LC, whereas the opposite effect (E(max)=268+/-53%) was observed with the alpha2A-adrenoceptor antagonist BRL44408 (0.1-100 microM). This suggests a tonic modulation of NA release through local alpha2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1-100 microM) administered into the LC increased NA in the LC (E(max)=223+/-40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.

CONCLUSION

Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.

The subjective and cognitive effects of acute phenylalanine and tyrosine depletion in patients recovered from depression

Although there is evidence for the involvement of dopamine (DA) in unipolar depression, no published study has yet used the technique of acute phenylalanine and tyrosine depletion (APTD), a dietary intervention that selectively lowers DA synthesis, in order to investigate the role of DA in mood disturbance. Tyrosine and phenylalanine depleted and placebo amino acid drinks were administered to 20 patients recovered from depression in a double-blind, placebo-controlled, crossover design. Measures included subjective effects, Hamilton Depression Rating Scale scores, and a comprehensive battery of well-validated computerized cognitive tests. APTD induced a substantial reduction in the ratio of plasma tyrosine and phenylalanine to large neutral amino acids. However, relapse of depressive symptoms was not seen. Although performance on most cognitive tests was unaffected, there was a selective effect on decision-making, with APTD causing participants to bet significantly less. In conclusion, These results suggest a specific role for the involvement of DA in reward/punishment processing in humans. While APTD did not induce relapse in any participant, it did cause patients recovered from depression to show lowered sensitivity to reward in a gambling game. It is hypothesized that tests involving reward/punishment processing are preferentially affected by DA depletion, and that a more complete account of depression is likely to result from considering the roles played by serotonin, noradrenaline, and DA in mediating the various cognitive and clinical symptoms, including anhedonia.

Catechol-o-methyltransferase inhibition improves set-shifting performance and elevates stimulated dopamine release in the rat prefrontal cortex

The Val158Met polymorphism of the human catechol-O-methyltransferase (COMT) gene affects activity of the enzyme and influences performance and efficiency of the prefrontal cortex (PFC); however, although catecholaminergic neurotransmission is implicated, the underlying mechanisms remain elusive because studies of the role of COMT in PFC function are sparse. This study investigated the effect of tolcapone, a brain-penetrant COMT inhibitor, on a rat model of attentional set shifting, which is dependent on catecholamines and the medial PFC (mPFC). Additionally, we investigated the effect of tolcapone on extracellular catecholamines in the mPFC using microdialysis in awake rats. Tolcapone significantly and specifically improved extradimensional (ED) set shifting. Tolcapone did not affect basal extracellular catecholamines, but significantly potentiated the increase in extracellular dopamine (DA) elicited by either local administration of the depolarizing agent potassium chloride or systemic administration of the antipsychotic agent clozapine. Although extracellular norepinephrine (NE) was also elevated by local depolarization and clozapine, the increase was not enhanced by tolcapone. We conclude that COMT activity specifically affects ED set shifting and is a significant modulator of mPFC DA but not NE under conditions of increased catecholaminergic transmission. These data suggest that the links between COMT activity and PFC function can be modeled in rats and may be specifically mediated by DA. The interaction between clozapine and tolcapone may have implications for the treatment of schizophrenia.

Selective effects of acute serotonin and catecholamine depletion on memory in healthy women

There is converging evidence that brain serotonin and dopamine may selectively modulate learning and memory in humans. However, this has not been directly demonstrated. In the current study, we used the method of amino acid precursor depletion to explore the effects of low serotonin and catecholamine function on memory in healthy female volunteers. Participants completed three experimental sessions: (i) tryptophan depletion (TD to lower 5-HT); (ii) tyrosine and phenylalanine depletion (TPD to lower catecholamines); and (iii) a balanced control condition (Bal). All testing was conducted in a double-blind, placebo-controlled, crossover design. Cognitive and mood assessments were performed at baseline and 5 h after ingesting the amino acid mixture. Consistent with previous studies, TD impaired declarative memory consolidation on a structured word-learning task, while TPD, acting to lower brain dopamine availability, impaired spatial working memory. No secondary deficits were observed on measures of attention, short-term memory or subjective mood state. These findings suggest that low brain serotonin versus dopamine selectively impairs memory performance in humans. This may shed light on the role of these neurotransmitters in disorders that are characterized by significant memory impairment.

The effects of tyrosine depletion in normal healthy volunteers: implications for unipolar depression

RATIONALE

In recent years, there has been a growing interest in the role of dopamine (DA) both in the pathogenesis of unipolar depression and in motivated behaviour. The innovative technique of acute tyrosine depletion presents an opportunity to characterise further its function in these domains.

OBJECTIVE

The present study examined the physiological, subjective and cognitive effects of acute tyrosine depletion in healthy volunteers.

METHODS

A double-blind, placebo-controlled, parallel group design was employed. Half of the participants ingested a balanced amino-acid mixture (BAL) and the other half received an identical mixture except that tyrosine and phenylalanine were absent (TYR-free). Plasma amino acid concentrations and subjective ratings were monitored at both baseline (T(0)) and 5 h following consumption (T(5)) of the mixtures. A comprehensive neuropsychological test battery was also administered at T(5).

RESULTS

Relative to the BAL group, the reduction in TYR availability to the brain was more marked in the TYR-free group. Employment of psychological rating scales revealed that, compared with the BAL group, the TYR-free group became less content and more apathetic. For the affective go/no-go task, whilst the BAL group exhibited a happy latency bias, the TYR-free group demonstrated a sad latency bias. Furthermore, in the decision-making task, the rate at which the TYR-free group increased their bets in response to more likely outcomes was lower than that of the BAL group. Taken together, these neuropsychological findings strikingly paralleled those reported in previous investigations of unipolar depression. The experimental groups could not be differentiated on any of the other neuropsychological measures, including more classical assessments of fronto-executive function.

CONCLUSION

These findings are consistent with the hypothesis that dopaminergic factors are particularly involved in disrupted affect/reward-based processing characteristic of clinical depression.

Monoamine depletion in psychiatric and healthy populations: review

A number of techniques temporarily lower the functioning of monoamines: acute tryptophan depletion (ATD), alpha-methyl-para-tyrosine (AMPT) and acute phenylalanine/tyrosine depletion (APTD). This paper reviews the results of monoamine depletion studies in humans for the period 1966 until December 2002. The evidence suggests that all three interventions are specific, in terms of their short-term effects on one or two neurotransmitter systems, rather than on brain protein metabolism in general. The AMPT procedure is somewhat less specific, affecting both the dopamine and norepinephrine systems. The behavioral effects of ATD and AMPT are remarkably similar. Neither procedure has an immediate effect on the symptoms of depressed patients; however, both induce transient depressive symptoms in some remitted depressed patients. The magnitude of the effects, response rate and quality of response are also comparable. APTD has not been studied in recovered major depressive patients. Despite the similarities, the effects are distinctive in that ATD affects a subgroup of recently remitted patients treated with serotonergic medications, whereas AMPT affects recently remitted patients treated with noradrenergic medications. The evidence also suggests that ATD and APTD affect different cognitive functions, in particular different memory systems. Few studies investigated cognitive effects of the procedures in patients. Patients who are in remission for longer may also be vulnerable to ATD and AMPT, but the relationship with prior treatment is much weaker. For these patients, individual vulnerability markers are the more important determinants of depressive response, making these techniques potentially useful models of vulnerability to depression.

Effects of serotonin and catecholamine depletion on interleukin-6 activation and mood in human volunteers

There is increasing evidence that depression and related neurotic illnesses are associated with alterations in immune function that may contribute to their pathogenesis. For example, clinical and experimental studies have shown that abnormal HPA-axis activation and monoamine neurotransmission may be related to an increased release of proinflammatory cytokines from stimulated lymphocytes in the periphery and brain. In the present investigation, the effects of tryptophan depletion (TD) on unstimulated plasma interleukin-6 (IL-6) concentrations were investigated in order to determine whether acute changes in serotonin (5-HT) neurotransmission would induce a proinflammatory response in healthy individuals. The effects of TD were compared with the analogous procedure of tyrosine depletion (TPD), which reduces catecholamine metabolism in humans. Thirteen female participants completed three experimental sessions: TD, TPD and a balanced-control condition (B). Mood-ratings and blood sampling were performed at baseline and 5 h after the administration of the mixtures. Analyses revealed that TD and TPD markedly reduced tryptophan and tyrosine/phenylalanine levels, respectively. No changes in plasma IL-6 production or ratings of lowered mood were observed, however, subjects did report feeling more fatigued after TD. These findings indicate that a transient disruption in global monoamine function does not stimulate a proinflammatory response of IL-6 in normal volunteers.

Tyrosine depletion attenuates the behavioural stimulant effects of amphetamine and cocaine in rats

Neurochemical studies show that a tyrosine-free amino acid mixture depletes brain tyrosine and decreases dopamine synthesis and release. Here, we tested whether such a mixture would reduce the behavioural effects of amphetamine and other psychostimulants. A tyrosine-free amino acid mixture decreased the behavioural activation induced by both D-amphetamine (2 mg/kg s.c.) and cocaine (2 mg/kg s.c.). In contrast, the activation induced by the dopamine agonist, apomorphine (0.75 and 5 mg/kg s.c.), or the 5-hydroxytryptamine releasing agent, p-chloroamphetamine (2 mg/kg s.c.) was not altered. These findings provide behavioural evidence that tyrosine-free amino acid mixtures reduce presynaptic dopamine function in the brain.