Brain serotonin synthesis rates in rhesus monkeys determined by [11C]alpha-methyl-L-tryptophan and positron emission tomography compared to CSF 5-hydroxyindole-3-acetic acid concentrations

A central serotonin regulating gene polymorphism (TPH2) determines vulnerability to acute tryptophan depletion-induced anxiety and ventromedial prefrontal threat reactivity in healthy young men

Serotonin (5-HT) has long been implicated in adaptive emotion regulation as well as the development and treatment of emotional dysregulations in mental disorders. Accumulating evidence suggests a genetic vulnerability may render some individuals at a greater risk for the detrimental effects of transient variations in 5-HT signaling. The present study aimed to investigate whether individual variations in the Tryptophan hydroxylase 2 (TPH2) genetics influence susceptibility for behavioral and neural threat reactivity dysregulations during transiently decreased 5-HT signaling. To this end, interactive effects between TPH2 (rs4570625) genotype and acute tryptophan depletion (ATD) on threat reactivity were examined in a within-subject placebo-controlled pharmacological fMRI trial (n = 51). A priori genotype stratification of extreme groups (GG vs. TT) allowed balanced sampling. While no main effects of ATD on neural reactivity to threat-related stimuli and mood state were observed in the entire sample, accounting for TPH2 genotype revealed an ATD-induced increase in subjective anxious arousal in the GG but not the TT carriers. The effects were mirrored on the neural level, such that ATD specifically reduced ventromedial prefrontal cortex reactivity towards threat-related stimuli in the GG carriers. Furthermore, the ATD-induced increase in subjective anxiety positively associated with the extent of ATD-induced changes in ventromedial prefrontal cortex activity in response to threat-related stimuli in GG carriers. Together the present findings suggest for the first time that individual variations in TPH2 genetics render individuals susceptible to the anxiogenic and neural effects of a transient decrease in 5-HT signaling.

Molecular imaging of autism spectrum disorder

Autism spectrum disorder (ASD) is a condition with onset in early childhood characterized by marked deficits in interpersonal interactions and communication and by a restricted and repetitive range of interests and activities. This review points out key recent findings utilizing molecular imaging including magnetic resonance spectroscopy (MRS) and nuclear neuroimaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). MRS indicates an excitatory/inhibitory imbalance in high-functioning autism. Dysfunction of neurotransmitter and glucose metabolism has been demonstrated by PET and SPECT. Levels of serotonin synthesis in typically developing children are approximately twice those of adults; after the age of 5 years, levels decrease to those of adults. In contrast, levels of serotonin synthesis of children with ASD increase between ages 2 and 15 to 1.5-times adult values. The dopamine transporter is increased in the orbitofrontal cortex of men with ASD. The serotonin transporter is reduced in the brains of children, adolescents, and adults with ASD. Reduced serotonin receptors in the thalamus of adults with ASD are associated with communication difficulties. Glucose metabolism is reduced in the brains of people with ASD. Molecular imaging will provide the preliminary data for promising therapeutic interventions.

Cortico-limbic connectivity in MAOA-L carriers is vulnerable to acute tryptophan depletion

INTRODUCTION

A gene-environment interaction between expression genotypes of the monoamine oxidase A (MAOA) and adverse childhood experience increases the risk of antisocial behavior. However, the neural underpinnings of this interaction remain uninvestigated. A cortico-limbic circuit involving the prefrontal cortex (PFC) and the amygdala is central to the suppression of aggressive impulses and is modulated by serotonin (5-HT). MAOA genotypes may modulate the vulnerability of this circuit and increase the risk for emotion regulation deficits after specific life events. Acute tryptophan depletion (ATD) challenges 5-HT regulation and may identify vulnerable neuronal circuits, contributing to the gene-environment interaction.

METHODS

Functional magnetic resonance imaging measured the resting-state state activity in 64 healthy males in a double-blind, placebo-controlled study. Cortical maps of amygdala correlation identified the impact of ATD and its interaction with low- (MAOA-L) and high-expression variants (MAOA-H) of MAOA on cortico-limbic connectivity.

RESULTS

Across all Regions of Interest (ROIs) exhibiting an ATD effect on cortico-limbic connectivity, MAOA-L carriers were more susceptible to ATD than MAOA-H carriers. In particular, the MAOA-L group exhibited a larger reduction of amygdala connectivity with the right prefrontal cortex and a larger increase of amygdala connectivity with the insula and dorsal PCC.

CONCLUSION

MAOA-L carriers were more susceptable to a central 5-HT challenge in cortico-limbic networks. Such vulnerability of the cortical serotonergic system may contribute to the emergence of antisocial behavior after systemic challenges, observed as gene-environment interaction. Hum Brain Mapp 38:1622-1635, 2017. © 2016 Wiley Periodicals, Inc.

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Tryptophan hydroxylase(2) gene polymorphisms predict brain serotonin synthesis in the orbitofrontal cortex in humans

Brain regional serotonin synthesis can be estimated in vivo using positron emission tomography (PET) and α-[((11))C]methyl-L-tryptophan ((11)C-AMT) trapping (K*) as a proxy. Recently, we reported evidence of lower normalized (11)C-AMT trapping in the orbitofrontal cortex (OBFC) of subjects meeting the criteria for an impulsive and/or aggressive behavioral phenotype. In this study, we examined whether part of the variance in OBFC serotonin synthesis is related to polymorphisms of the gene that encodes for the indoleamine's rate-limiting enzyme in the brain, tryptophan hydroxylase-2 (TPH(2)). In all, 46 healthy controls had PET (11)C-AMT scans and were genotyped for 11 single-nucleotide polymorphisms (SNPs) distributed across the TPH(2) gene and its 5' upstream region. Several TPH(2) SNPs were associated with lower normalized blood-to-brain clearance of (11)C-AMT in the OBFC. Dose-effect relationships were found for two variants (rs6582071 and rs4641527, respectively, located in the 5' upstream region and intron 1) that have previously been associated with suicide. Associations in the OBFC remained statistically significant in a mixed larger sample of patients and controls. These results suggest that in humans, genetic factors might partly account for variations in serotonin synthesis in the OBFC.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

Measuring serotonin synthesis: from conventional methods to PET tracers and their (pre)clinical implications

The serotonergic system of the brain is complex, with an extensive innervation pattern covering all brain regions and endowed with at least 15 different receptors (each with their particular distribution patterns), specific reuptake mechanisms and synthetic processes. Many aspects of the functioning of the serotonergic system are still unclear, partially because of the difficulty of measuring physiological processes in the living brain. In this review we give an overview of the conventional methods of measuring serotonin synthesis and methods using positron emission tomography (PET) tracers, more specifically with respect to serotonergic function in affective disorders. Conventional methods are invasive and do not directly measure synthesis rates. Although they may give insight into turnover rates, a more direct measurement may be preferred. PET is a noninvasive technique which can trace metabolic processes, like serotonin synthesis. Tracers developed for this purpose are α-[¹¹C]methyltryptophan ([¹¹C]AMT) and 5-hydroxy-L-[β-¹¹C]tryptophan ([¹¹C]5-HTP). Both tracers have advantages and disadvantages. [¹¹C]AMT can enter the kynurenine pathway under inflammatory conditions (and thus provide a false signal), but this tracer has been used in many studies leading to novel insights regarding antidepressant action. [¹¹C]5-HTP is difficult to produce, but trapping of this compound may better represent serotonin synthesis. AMT and 5-HTP kinetics are differently affected by tryptophan depletion and changes of mood. This may indicate that both tracers are associated with different enzymatic processes. In conclusion, PET with radiolabelled substrates for the serotonergic pathway is the only direct way to detect changes of serotonin synthesis in the living brain.

Brain serotonin synthesis in adult males characterized by physical aggression during childhood: a 21-year longitudinal study

BACKGROUND

Adults exhibiting severe impulsive and aggressive behaviors have multiple indices of low serotonin (5-HT) neurotransmission. It remains unclear though whether low 5-HT mediates the behavior or instead reflects a pre-existing vulnerability trait.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, positron emission tomography with the tracer alpha-[(11)C]methyl-L-tryptophan ((11)C-AMT) was used to compare 5-HT synthesis capacity in two groups of adult males from a 21-year longitudinal study (mean age +/- SD: 27.1+/-0.7): individuals with a history of childhood-limited high physical aggression (C-LHPA; N = 8) and individuals with normal (low) patterns of physical aggression (LPA; N = 18). The C-LHPA males had significantly lower trapping of (11)C-AMT bilaterally in the orbitofrontal cortex and self-reported more impulsiveness. Despite this, in adulthood there were no group differences in plasma tryptophan levels, genotyping, aggression, emotional intelligence, working memory, computerized measures of impulsivity, psychosocial functioning/adjustment, and personal and family history of mood and substance abuse disorders.

CONCLUSIONS/SIGNIFICANCE

These results force a re-examination of the low 5-HT hypothesis as central in the biology of violence. They suggest that low 5-HT does not mediate current behavior and should be considered a vulnerability factor for impulsive-aggressive behavior that may or may not be expressed depending on other biological factors, experience, and environmental support during development.

Chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain

BACKGROUND

Serotonin (5-HT) is a neurotransmitter with important roles in the regulation of neurobehavioral processes, particularly those regulating affect in humans. Drugs that potentiate serotonergic neurotransmission by selectively inhibiting the reuptake of serotonin (SSRIs) are widely used for the treatment of psychiatric disorders. Although the regulation of serotonin synthesis may be an factor in SSRI efficacy, the effect of chronic SSRI administration on 5-HT synthesis is not well understood. Here, we describe effects of chronic administration of the SSRI citalopram (CIT) on 5-HT synthesis and content in the mouse forebrain.

METHODOLOGY/PRINCIPAL FINDINGS

Citalopram was administered continuously to adult male C57BL/6J mice via osmotic minipump for 2 days, 14 days or 28 days. Plasma citalopram levels were found to be within the clinical range. 5-HT synthesis was assessed using the decarboxylase inhibition method. Citalopram administration caused a suppression of 5-HT synthesis at all time points. CIT treatment also caused a reduction in forebrain 5-HIAA content. Following chronic CIT treatment, forebrain 5-HT stores were more sensitive to the depleting effects of acute decarboxylase inhibition.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrate that chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain. Furthermore, our results indicate that chronic 5-HT reuptake inhibition renders 5-HT brain stores more sensitive to alterations in serotonin synthesis. These results suggest that the regulation of 5-HT synthesis warrants consideration in efforts to develop novel antidepressant strategies.

5-Hydroxy-L-[beta-11C]tryptophan versus alpha-[11C]methyl-L-tryptophan for positron emission tomography imaging of serotonin synthesis capacity in the rhesus monkey brain

The purpose of this study was to compare two positron emission tomography (PET) tracers that were developed to follow serotonin (5HT) synthesis by performing sequential PET scanning of the same rhesus monkey (n=4) on the same day. alpha-[11C]Methyl-L-tryptophan ([11C]AMT) and 5-Hydroxy-L-[beta-11C]tryptophan ([11C]HTP) are substrates in the first and second enzymatic steps, respectively, in the biosynthesis of 5HT. Regional net accumulation rate constants were derived from kinetic (two-tissue compartment model with irreversible tracer trapping) and graphic (Patlak) analyses, using the arterial plasma concentrations as input. The kinetic data analysis showed that the rate constant for the transfer of [11C]HTP into the brain (K1) was higher than that for [11C]AMT in the striatum and thalamus but was similar in other brain regions. The rate constant for tracer trapping (k3) was also higher for [11C]HTP than for [11C]AMT in the striatum (0.046+/-0.024 versus 0.019+/-0.006 min(-1)) and thalamus (0.039+/-0.013 versus 0.016+/-0.007 min(-1)). In agreement with previously reported regional HTP accumulation rates, the net accumulation rate constant (K(acc)) for [11C]HTP was also higher in these regions than in other brain regions; this is in contrast to the uniform distribution of [11C]AMT K(acc) values. This suggests that the regional net accumulation rates obtained with these two PET tracers will be of different magnitude, which might be related to the activity of each targeted enzyme.

Acute prefrontal cortex TMS in healthy volunteers: Effects on brain 11C-αMtrp trapping

High-frequency repetitive transcranial magnetic stimulation (rTMS) of the left dorsolateral prefrontal cortex (LDLPFC) is a technique with purported efficacy as a treatment for major depression. Here, we assessed in vivo, in healthy volunteers, the effect of acute rTMS of the LDLPFC, relative to the stimulation of the left occipital cortex (LOC), on brain regional serotonin synthesis capacity, using the [(11)C]-alpha-methyl-tryptophan ((11)C-alphaMtrp)/PET method. Ten subjects were studied twice, once following rTMS of the LDLPFC and once following rTMS of the LOC in a randomized counterbalanced order. Three blocks of 15 trains of 10 Hz rTMS were delivered 10 min apart. Behavioural and autonomic measures were recorded before and after each rTMS session. Comparisons of TMS-related changes in regional normalized brain uptake and trapping of (11)C-alphaMtrp (K*) values were carried out using SPM99. Statistically significant regional differences were identified on the basis of an extent threshold of 50 voxels, with a peak threshold of p=0.005 uncorrected. Behavioural and autonomic measures were unaffected by rTMS. Relative to LOC stimulation, LDLPFC rTMS was associated with marked changes in normalized K* in limbic areas, with significantly lower values in the left parahippocampal gyrus (BA 28) and the right insula (BA 13), and higher values in the right cingulate gyrus (BA 31) and cuneus (BA 18). These findings indicate that acute rTMS of the LDLPFC in healthy volunteers modulates aspects of tryptophan/5-HT metabolism in limbic areas. Such adaptive changes may contribute to the mechanism of action of prefrontal rTMS in major depression.

alpha-[11C]Methyl-L-tryptophan trapping in the orbital and ventral medial prefrontal cortex of suicide attempters

Low serotonin neurotransmission is thought to increase vulnerability to suicidal behavior. To test this hypothesis, we measured brain regional serotonin synthesis, as indexed by PET and alpha-[(11)C]methyl-L-tryptophan trapping, in 10 patients who had made a high-lethality suicide attempt and 16 healthy controls. Compared to healthy controls, suicide attempters had reduced normalized alpha-[(11)C]methyl-L-tryptophan trapping in orbital and ventromedial prefrontal cortex. alpha-[(11)C]Methyl-L-tryptophan trapping in these regions correlated negatively with suicide intent. Low serotonin synthesis in the prefrontal cortex might lower the threshold for suicidal behavior.

Stability of α-[11C]methyl-l-tryptophan brain trapping in healthy male volunteers

PURPOSE

The purpose of this study was to assess the reproducibility in healthy volunteers of alpha-[11C]methyl-L-tryptophan (alpha[11C]MT) brain trapping imaging with positron emission tomography (PET), using volumes of interest (VOIs) and voxel-based image analysis.

METHODS

Six right-handed healthy male volunteers (34.3+/-10.9 years) with a negative family history for psychiatric disorders were scanned twice in the resting condition, 22+/-17 days apart. An unbiased semiautomatic segmentation of the brain was used to define VOIs. The trapping constant K* (ml g(-1) min(-1)) for alpha[11C]MT was calculated for the whole brain and seven brain regions using the graphical method for irreversible tracers. In addition, parametric maps of K* were obtained from dynamic scans using the same method. Comparison of test and retest K* functional images was performed using SPM99. Student's paired t statistic was applied for comparisons of alpha[11C]MT brain trapping in a priori selected VOIs.

RESULTS

alpha[11C]MT brain trapping in VOIs showed a mean variability 2.6+/-1.8% (0.3-5%) for absolute and 1.5+/-2.1% (1.4-4.1%) for normalized K*. Intraclass correlations between test and retest conditions were 0.61+/-0.34 for absolute K* values and 0.73+/-0.20 for K* values normalized by global mean. SPM99 analysis using a height threshold of p=0.05 (two tailed) and an extent threshold of 100 voxels showed no significant differences between scans.

CONCLUSION

Rest measurements in healthy male volunteers of the trapping constant for alpha[11C]MT, using PET, appeared to be stable during an average interval of 3 weeks.

Significance of abnormalities in developmental trajectory and asymmetry of cortical serotonin synthesis in autism

The role of serotonin in prenatal and postnatal brain development is well documented in the animal literature. In earlier studies using positron emission tomography (PET) with the tracer alpha[(11)C]methyl-l-tryptophan (AMT), we reported global and focal abnormalities of serotonin synthesis in children with autism. In the present study, we measured brain serotonin synthesis in a large group of autistic children (n = 117) with AMT PET and related these neuroimaging data to handedness and language function. Cortical AMT uptake abnormalities were objectively derived from small homotopic cortical regions using a predefined cutoff asymmetry threshold (>2 S.D. of normal asymmetry). Autistic children demonstrated several patterns of abnormal cortical involvement, including right cortical, left cortical, and absence of abnormal asymmetry. Global brain values for serotonin synthesis capacity (unidirectional uptake rate constant, K-complex) values were plotted as a function of age. K-complex values of autistic children with asymmetry or no asymmetry in cortical AMT uptake followed different developmental patterns, compared to that of a control group of non-autistic children. The autism groups, defined by presence or absence and side of cortical asymmetry, differed on a measure of language as well as handedness. Autistic children with left cortical AMT decreases showed a higher prevalence of severe language impairment, whereas those with right cortical decreases showed a higher prevalence of left and mixed handedness. Global as well as focal abnormally asymmetric development in the serotonergic system could lead to miswiring of the neural circuits specifying hemispheric specialization.

Selective decrease in serotonin synthesis rate in rat brainstem raphe nuclei following chronic administration of low doses of amitriptyline: an effect compatible with an anti-migraine effect

The effects of chronic, low-dose amitriptyline on serotonin (5-HT) synthesis rate were measured in rat brain using autoradiography and the trapping of alpha-[14C]-methyl-L-tryptophan (alpha-[14C]-MTrp). Rats received amitriptyline (2 mg/kg per day) or saline via intraperitoneal osmotic minipumps for 21 days. Amitriptyline had no effect on any physiological parameters measured, or on free or total plasma tryptophan levels. However, amitriptyline exerted selective decreases of 15% and 17% (P < 0.001) in serotonin synthesis rates in the dorsal and median raphe nuclei, respectively. There was no reduction in any of the projection areas studied, including the cerebral cortex, hippocampus, thalamus, hypothalamus or striatum. The data suggest that chronic low doses of amitriptyline can lead to sustained 5-HT re-uptake inhibition selectively in the raphe nuclei, an effect compatible with tonic activation of 5-HT(1A) autoreceptors and inhibition of 5-HT synthesis. The failure of chronic amitriptyline treatment to affect 5-HT synthesis rate in the projection areas may ensure an adequate regulation of pain pathways implicated in migraine headache, an effect possibly related to amitriptyline anti-migraine efficacy.

Effects of tryptophan depletion on acute smoking abstinence symptoms and the acute smoking response

Given the putative role of serotonin in the modulation of smoking withdrawal and the central actions of nicotine, this study examined the affective and neuroelectric correlates of smoking abstinence and cigarette smoking following depletion of the serotonin precursor, tryptophan. In a randomized, double-blind two session (tryptophan depletion [TD] vs. nondepletion), placebo-controlled design, spectrally analyzed electroencephalogram (EEG), self-ratings of withdrawal symptoms and mood states were assessed in 18 male cigarette smokers before smoking abstinence, 5 h postsmoking abstinence and again following sham smoking and the smoking of one cigarette. Compared to a nutritionally balanced amino acid (AA) mixture containing tryptophan (i.e., placebo mixture), oral ingestion of a similar mixture devoid of tryptophan resulted in a 70% reduction of plasma tryptophan but failed to alter the appearance or reversal (by acute cigarette smoking) of withdrawal symptoms, negative mood states and increased slow wave EEG in male smokers deprived of cigarettes. These results, although not supporting a role for the serotonergic system in acute smoking and early smoking abstinence symptoms, are discussed in relation to the neuropharmacology of smoking behavior and suggestions for future work.

Reduction in serotonin synthesis following acute and chronic treatments with paroxetine, a selective serotonin reuptake inhibitor, in rat brain: an autoradiographic study with alpha-[14C]methyl-L-tryptophan(2)

Serotonin (5-HT) synthesis rates were calculated on the basis of the assumption that trapping of alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) is directly related to brain 5-HT synthesis. In the first series of experiments, an acute intraperitoneal injection of paroxetine (10 mg/kg) produced a significant reduction in 5-HT synthesis in brain structures containing serotonergic cell bodies (the dorsal, median, and pallidum raphe nuclei), as well as in most projection areas: the ventral tegmental area, median forebrain bundle, hippocampus CA3 region, and nigrostriatal structures (substantia nigra, lateral and medial caudate nuclei). The reductions in the projection areas were greater (between 25 and 53%) than in those areas containing serotonergic cell bodies (between 18 and 23%). In the cerebral cortex, 5-HT synthesis rates were not modified by acute paroxetine treatment. In a second series of experiments, rats were treated with paroxetine (10 mg/kg/day, s.c., delivered by osmotic minipumps) for 14 days. There was a marked decrease (39-69%) in 5-HT synthesis in every structure examined. In conclusion, the present data suggest that the effects of paroxetine on 5-HT synthesis in the cerebral cortex are different from its effects in the cell body area of the brainstem.

Study of the brain serotonergic system with labeled alpha-methyl-L-tryptophan

alpha-Methyl-L-tryptophan (alpha-MTrp) is an artificial amino acid and an analog of tryptophan (Trp), the precursor of the neurotransmitter serotonin (5-HT). In this article we have summarized available data, which suggest that the measurement of the unidirectional uptake of alpha-MTrp and its conversion to 5-HT synthesis rates is a valid approach for the determination of brain 5-HT synthesis rates. The main feature on which the model is based is the trapping of labeled alpha-MTrp in brain tissue. An overview of opposing opinions, which suggest that there is a need for a metabolic conversion of tracer, is also presented and discussed critically. As with all biological modeling there is likely to be room for improvements of the proposed biological model. In addition, there are a limited number of clearly defined circumstances in which the method is confounded by the metabolism of labeled alpha-MTrp via the kynurenine pathway. Nonetheless, a significant body of evidence suggests that labeled alpha-MTrp is a useful tracer to study brain 5-HT synthesis in most circumstances. Calculation of 5-HT synthesis rates depends on the plasma-free tryptophan concentration, which, according to the balance of arguments in the literature, is a more appropriate parameter than the total-plasma tryptophan. The method, as proposed by us, can be used in conjunction with autoradiographic measurements in laboratory animals, and with positron emission tomography in large animals and humans. We review studies in animals looking at the normal control of 5-HT synthesis and the way in which it is altered by drugs, as well as initial studies investigating healthy humans and patients with neuropsychiatric disorders.

Brain net unidirectional uptake of alpha-[14c]methyl-L-tryptophan (alpha-MTrp) and its correlation with regional serotonin synthesis, tryptophan incorporation into proteins, and permeability surface area products of tryptophan and alpha-MTrp

The uptake and trapping constants for labeled tryptophan (Trp) via the serotonin (5-hydroxytryptamine; 5-HT) metabolic pathway and for the incorporation of Trp into proteins, and alpha-[14C]methyl-L-tryptophan (alpha-MTrp) were measured. Measurements were done in rats treated with either saline or probenecid (200 mg/kg). In addition, the blood-brain barrier (BBB) permeability surface area products for Trp (PS(T)) and alpha-MTrp (PSalpha) were measured in normal rats. The results suggest that, in both groups of rats, there is a highly significant correlation (p < 0.05; Pearson Product Moment Correlation (PPMC) between the brain uptake and trapping constants for alpha-MTrp and those of Trp via the 5-HT metabolic pathway, but there is no significant correlation (p > 0.05; PPMC) between either of these constants and the PS products of either compound. There is also no significant correlation (p > 0.05; PPMC) between the constant for the Trp incorporation into proteins with any of the other parameters. For all parameters, except Trp incorporation into proteins (alpha-MTrp is not incorporated into proteins), there was a highly significant correlation (p < 0.001) between the quantities measured for Trp and alpha-MTrp. The data presented here strongly suggests that the brain uptake and trapping of alpha-MTrp relates to brain 5-HT synthesis, and does not relate to the BBB transport or protein incorporation of Trp. On the basis of these results, as well as those previously reported, we concluded that trapping (unidirectional uptake) of alpha-MTrp can be converted to the 5-HT synthesis rates in the brain. From this also follows that labeled alpha-MTrp is a good tracer for in vivo evaluation of the brain 5-HT synthesis.

Pet studies of serotonin synthesis in the human brain

The method for measuring serotonin synthesis in human brain uses [11C]alpha-methyl-L-tryptophan as a tracer and positron emission tomography. The alpha-methyl-L-tryptophan is converted to alpha-methylserotonin, which is not a substrate for monoamine oxidase and therefore accumulates in the brain. In a pilot study published recently, rates of serotonin synthesis were found to be higher in men than in women. This was due to the lower plasma free tryptophan in the women under the experimental conditions used, and does not necessarily reflect the situation in all circumstances. Acute tryptophan depletion lowered brain serotonin synthesis by 90% or more. Patients with borderline personality disorder, who exhibit emotional lability and impulsivity, may have lower brain serotonin synthesis rates than healthy controls.

The suitability of [11C]-alpha-methyl-L-tryptophan as a tracer for serotonin synthesis: studies with dual administration of [11C] and [14C] labeled tracer

The tracer [11C]-alpha-methyl-L-tryptophan (alphaMTP) has been used to measure brain serotonin synthesis rates with positron emission tomography (PET). To address questions about the accuracy of the kinetic model, [14C]alphaMTP was used to directly measure conversion to [14C]-alpha-methyl-serotonin (alphaM5HT) in monkeys that had been previously studied with PET and [11C]alphaMTP. Four male, fasted, isoflurane-anesthetized rhesus monkeys were studied with [11C]alphaMTP and PET. Immediately after the initial 3-hour scan, a second dose of [11C]alphaMTP was coinjected with 1 mCi of [14C]alphaMTP, and additional PET data were collected. Approximately 90 minutes after the second alphaMTP administration, the animals were killed with an overdose of phenobarbital, and brain samples from 21 regions were taken and analyzed by HPLC. Minimal conversion of alphaMTP to alphaM5HT occurred; HPLC analysis of 14C radioactivity showed that greater than 96% of the total counts were in fractions corresponding to the alphaMTP peak. Brain concentrations of serotonin, tryptophan, 5-hydroxyindole-3-acetic acid, and alphaMTP also were determined fluorometrically using external quantification. Patlak plots generated from PET images acquired over 3 hours showed no time period of linear increase, and final slopes were not significantly different from zero, consistent with the finding of minimal conversion to [14C]alphaM5HT. These data indicate that in the 3-hour period after injection, [11C]alphaMTP is acting predominantly as a tracer of tryptophan uptake, not serotonin synthesis.

Alpha[C-11]methyl-L-tryptophan PET maps brain serotonin synthesis and kynurenine pathway metabolism

Alpha[C-11]methyl-L-tryptophan (AMT) is an analog of tryptophan used with positron emission tomography for the measurement of serotonin synthesis in humans. Several attempts have been made to estimate the serotonin synthesis rate from plasma and brain kinetic data of AMT using the same model as that applied for the measurement of the glucose metabolic rate with 2-deoxyglucose. However, although AMT is similar to 2-deoxyglucose with regard to an irreversible pool of tracer uptake, there are important differences between the two tracers and how the model can be applied. These differences include transport at the blood-brain barrier and the presence of a large unmetabolized pool of AMT, precluding the method from providing the absolute serotonin synthesis rate. Despite this limitation, the unidirectional uptake rate constant (K-complex) values have been found to be stable within an individual, and the rank order of regional brain values for K-complex are consistent with the rank order for serotonin content in human brain. Furthermore, changes in K-complex with age, gender, and disease states are consistent with previously reported biochemical measurements of serotonin in brain tissue. The authors suggest, therefore, that the K-complex is an index of serotonin synthesis which they have termed the "serotonin synthesis capacity." The authors argue that AMT is a useful tracer for study of serotonergic mechanisms, and under certain pathologic states, of metabolism by means of the kynurenine pathway.

Biochemical and autoradiographic measurements of brain serotonin synthesis rate in the freely moving rat: a reexamination of the alpha-methyl-L-tryptophan method

Biochemical approaches were used in freely moving rats to determine, under steady-state conditions, the brain/arterial plasma partition coefficients of L-tryptophan and alpha-[3H]methyl-L-tryptophan, from which the lumped constant for the alpha-methyl-L-tryptophan method of estimating the rate of brain serotonin synthesis is calculated. The lumped constants were significantly different in the various structures examined: 0.149 +/- 0.003 in the raphe dorsalis, 0.103 +/- 0.002 in the raphe centralis, 0.087 +/- 0.003 in the reticular formation, and 0.62 +/- 0.08 in the pineal gland. From these data we proposed a two-compartment model to calculate the rate of serotonin synthesis by quantitative autoradiography using a three-time point experiment. Rates of synthesis for the raphe dorsalis and the reticular formation (620 +/- 57 and 80 +/- 35 pmol/g of tissue/min, respectively) were similar to those measured simultaneously by biochemical means, but rates were 50% higher for the raphe centralis (568 +/- 90 vs. 381 +/- 31 pmol/g of tissue/min). The lack of dynamic equilibrium of the tracer between plasma and tissue pools may explain the discrepancy between the two methods. Our findings did not confirm previous data, indicating that the application of the autoradiographic method to measure the rate of brain serotonin synthesis using alpha-methyl-L-tryptophan as tracer has limitations.