We found no difference in regionalserotonin transporter binding potential (5-HTT BP) between 20 depressed and20 healthy subjects (analysis of variance, effect of diagnosis on regional5-HTT BP, F1,38 = 0.09-1.4 [P = .77-.24]).Horizontal lines indicate group means.
Correlations between the DysfunctionalAttitudes Scale (DAS) scores and serotonin transporter binding potential (5-HTTBP) in some of the larger regions in depressed subjects. The following highlysignificant correlations were found: prefrontal cortex (P < .001), anterior cingulate (P = .003),bilateral putamen (P < .001), and bilateralthalamus (P = .001).
Comparison of regional serotonintransporter binding potential (5-HTT BP) between 8 depressed subjects withseverely negativistic dysfunctional attitudes (Dysfunctional Attitudes Scalescore >190) and 20 healthy subjects. For regions primarily sampling serotonergicnerve terminals (anteromedial prefrontal cortex, anterior cingulate, and bilateralcaudate, putamen, and thalamus), the 5-HTT BP was significantly greater inthe depressed group (F1,26 = 5.6-12.2 [P = .03-.002]). The midbrain 5-HTT BP was not significantlydifferent (F1,26 = 0.5 [P = .49]).Horizontal lines indicate group means.
Jeffrey H. Meyer, Sylvain Houle, Sandra Sagrati, Anna Carella, Doug F. Hussey, Nathalie Ginovart, Verdell Goulding, James Kennedy, Alan A. Wilson. Brain Serotonin Transporter Binding Potential Measured With Carbon11–Labeled DASB Positron Emission TomographyEffects of Major Depressive Episodes and Severity of DysfunctionalAttitudes. Arch Gen Psychiatry. 2004;61(12):1271–1279. doi:10.1001/archpsyc.61.12.1271
Although brain serotonin transporter (5-HTT) density has been investigated
in subjects with a history of major depressive episodes (MDE), there has never
been an investigation of brain 5-HTT during a current MDE. Brain 5-HTT binding
potential (BP) may have an important role during MDE due to major depressive
disorder, because the 5-HTT regulates extracellular 5-HT. The BP is an index
of receptor density. Carbon 11–labeled 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile
(DASB) positron emission tomography (PET) is the first brain imaging technique
that can measure the 5-HTT BP in cortical and subcortical brain regions in
vivo. The purposes of this study were to investigate 5-HTT BP during MDE and
to determine the relationship between 5-HTT BP and negativistic dysfunctional
attitudes during MDE. Dysfunctional attitudes are negatively biased assumptions
and beliefs regarding oneself, the world, and the future. Our recent publication
of increased serotonin2 BP in MDE with severely negativistic dysfunctional
attitudes suggests that this subgroup of MDE subjects has very low levels
of extracellular serotonin.
Regional 5-HTT BP was measured in 20 nonsmoking medication-free (≥3
months) depressed subjects and 20 age-matched nonsmoking, medication-free,
healthy subjects using [11C]DASB PET. Dysfunctional attitudes were
measured using the Dysfunctional Attitudes Scale.
No difference in regional 5-HTT BP was found between MDE and healthy
subjects; however, the subgroup of MDE subjects with highly negativistic dysfunctional
attitudes had significantly higher 5-HTT BP compared with healthy subjects
in brain regions mainly sampling serotonergic nerve terminals (prefrontal
cortex, anterior cingulate, thalamus, bilateral caudate, and bilateral putamen;
average, 21% greater; F1,26, 5.6-12.2 [P values,
.03-.002]). In the MDE subjects, increased 5-HTT BP was strongly associated
with more negativistic dysfunctional attitudes in brain regions primarily
sampling serotonergic nerve terminals (prefrontal cortex, anterior cingulate,
thalamus, caudate, and putamen; r = 0.64-0.74
[P values, .003 to <.001]).
Serotonin transporters play an important role during depression. The
magnitude of regional 5-HTT BP can provide a vulnerability to low levels of
extracellular serotonin and symptoms of extremely negativistic dysfunctional
Serotonin (5-HT) may be abnormally regulated during major depressiveepisodes (MDE) due to major depressive disorder. This assertion is based largelyon abnormalities of serotonin turnover during MDE. For example, the levelof the cerebrospinal fluid metabolite of serotonin, 5-hydroxyindoleaceticacid, is often low during MDE, especially when suicidal ideation is present.1,2 Prolactin release is increased afteradministration of fenfluramine, a 5-HT–releasing drug, and it has beenobserved that the prolactin release after fenfluramine administration is attenuatedduring MDE.3,4 A role for serotoninto modulate mood has been proposed, because mood lowering after tryptophandepletion is often observed in subjects with either a family history of depressiveepisodes or a history of depressive episodes.5,6
The primary mechanism by which extracellular levels of serotonin maybe low during an MDE due to major depressive disorder is unknown. It has beenproposed that increased serotonin transporter (5-HTT) density could lead toincreased serotonin clearance from extracellular regions in the brain. Theanswer to this fundamentally important question is not known because thereare no investigations of 5-HTT density in subjects who are in the midst ofa depressive episode due to major depressive disorder. Instead, a number ofpostmortem studies have investigated subjects with a history of an MDE. Thesestudies usually do not distinguish between unipolar disorder and bipolar disorder,and the samples of unipolar subjects are often small. Most of these postmorteminvestigations report decreased 5-HTT density7- 9 orno difference in 5-HTT density10- 12 betweendepressed and healthy subjects. Although the regions investigated varied acrossthese studies, all sampled the prefrontal cortex.
It is difficult for postmortem investigations to determine the relationshipbetween 5-HTT density abnormalities and the presence of a current depressiveepisode, because the clinical data are gathered retrospectively. In theory,with receptor-ligand imaging in vivo, it is quite feasible to collect clinicaldata and determine the binding potential (BP), an index of receptor density,during a current depressive episode.
In the past, the barrier to investigating the 5-HTT BP with receptor-ligandimaging was the absence of a method that was both valid for multiple brainregions and reliable. Quantification of the 5-HTT BP in most brain regions,including the prefrontal cortex, was not possible with either of the 2 previousimaging methods, including carbon 11 [11C]–labeled(+)McN5652positron emission tomography (PET) and iodine I 123–labeled 2β-carbomethoxy-3β-(4-123I-iodophenyl)-tropane (CIT) single-photon emission computed tomography(SPECT). To our knowledge, the test-retest reliability for regional 5-HTTBP found with [123I]β-CIT SPECT or [11C](+)-McN5652PET has not been published. Relative to specific binding, [11C](+)McN5652had very high nonspecific uptake, and 5-HTT BP values were only detectablein the thalamus13 and possibly the basal gangliaand midbrain (although measuring 5-HTT BP in the latter 2 regions may requireconcurrent arterial sampling of the radiotracer).14 TheSPECT radiotracer, [123I]β-CIT, has been used to measure 5-HTTsites in the midbrain region in depressed subjects.15,16 However,[123I]β-CIT has similar affinity for the dopamine transporter(DAT) and the 5-HTT17,18; hencethe midbrain [123I]β-CIT is a combined measure of DAT BP inthe substantia nigra and 5-HTT BP in the raphe nuclei.
Ichimiya et al19 used [11C](+)McN5652PET to investigate the thalamus 5-HTT BP in 7 subjects with major depressivedisorder. The data from these 7 subjects were pooled with those of 6 subjectswho had bipolar disorder. That study did not address whether 5-HTT BP wasabnormal during a current MDE. Only 5 subjects with a current MDE and majordepressive disorder were enrolled in the study.19
Carbon 11–labeled 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile(DASB) positron emission tomography (PET) is a new advance in brain imagingthat measures 5-HTT BP in multiple brain regions, including the prefrontalcortex.20- 24 [11C]DASB is highly selective, showing nanomolar affinity for the 5-HTTand negligible affinity for other receptors.20,21 Themain advantage of [11C]DASB is that it has a much higher ratioof specific binding to nonspecific binding in vivo. As a result, with [11C]DASB PET, 5-HTT BP values are quantifiable in prefrontal cortexand reasonably high in the basal ganglia and thalamus, even without arterialsampling.22,23 Furthermore, the5-HTT BP found with [11C]DASB PET is reliable for most brain regionsunder test-retest conditions.24
This study had 2 main purposes. The first was to investigate the prefrontaland subcortical (thalamus, basal ganglia, and midbrain) 5-HTT BP in subjectswho are currently experiencing an MDE secondary to major depressive disorder.The first purpose focused on whether brain 5-HTT abnormalities occur duringa current MDE, an issue not resolved in earlier postmortem studies8- 12 orthe PET investigation of thalamic 5-HTT BP by Ichimiya et al.19
The second main purpose was to determine the relationship between 5-HTTBP and dysfunctional attitudes in MDE. Dysfunctional attitudes are negativelybiased assumptions and beliefs regarding oneself, the world, and the future.A modest degree of negativism is found in healthy subjects, whereas a highproportion of subjects with MDE have severely negativistic thinking. Our recentinvestigations suggest that a subgroup of subjects with MDE, rather than allsubjects with MDE, have very low levels of extracellular serotonin. A previousstudy from our group found that increasing 5-HT agonism with a single doseof d-fenfluramine in healthy subjects lowered dysfunctional attitudes tremendouslytoward optimism (highly significant effect, F1,25 = 17[P < .001]).25 Thisfinding suggested a role for serotonin in humans as a modulator of dysfunctionalattitudes. Given that serotonin can function as a modulator of dysfunctionalattitudes, it seemed quite possible that low levels of serotonin could contributeto the increased severity of dysfunctional attitudes that often occur duringMDE. In a second investigation in MDE subjects, we found that negativisticdysfunctional attitudes were associated with increased cortex serotonin2 BP.25 Furthermore, MDE subjects withseverely negativistic dysfunctional attitudes had increased serotonin2 BP compared with healthy subjects. Both findings were highly significant,especially in the prefrontal cortex (r = 0.56[P = .009] and F1,19 = 11[P = .003], respectively).25 Inanimal models, serotonin2 receptor density may increase after certain5-HT–depleting paradigms (that reduce serotonin synthesis or storagefor ≥2 weeks26,27) and decreaseafter certain 5-HT–increasing paradigms (monoamine oxidase inhibitionfor ≥2 weeks28,29). The BPis proportional to receptor density. A plausible explanation for the relationshipbetween serotonin2 BP and dysfunctional attitudes is that subjectswho have a form of MDE with severely negativistic dysfunctional attitudesalso have had an extended period of low levels of extracellular serotoninin the cerebral cortex.
We hypothesized that MDE subjects with greater 5-HTT BP would have moreseverely negativistic dysfunctional attitudes. The underlying model for thishypothesis is that MDE subjects with greater 5-HTT BP will remove more extracellularserotonin, have lower levels of extracellular serotonin, and experience moresevere symptoms of pessimism. The prefrontal cortex region is the primarylocation for this hypothesis, because it is typically sampled in investigationsreporting increased serotonin2 density in drug-free depressed suicidevictims.12,30 Furthermore, thisregion had the strongest correlation between the Dysfunctional Attitudes Scale(DAS) and serotonin2 BP in a previous study by our group.25 We also examined the relationship between dysfunctionalattitudes and other sampled brain regions, including the midbrain. However,the 5-HTT in the midbrain region is proximal to serotonergic cell bodies31 and would not be expected to directly modulate serotoninlevels in distant brain regions.
Twenty subjects with an MDE and major depressive disorder (mean age,35 years [SD, 11 years]; 9 men and 11 women) and another 20 age-matchedhealthy subjects (mean age, 35 years [SD, 11 years]; 10 men and 10 women)were recruited. Ages ranged from 19 to 52 years. Healthy subjects were agematched within 3 years to depressed patients. All MDE and healthy subjectswere physically healthy, had no history of alcohol or substance abuse or neurotoxinuse, were free of psychotropic drug use for longer than 3 months plus 5 half-livesof any medication, and were nonsmoking.
Healthy subjects underwent screening to rule out Axis I disorders (currentor in remission),32 current suicidal ideation,and history of self-harm behavior, anger dyscontrol, or impulsive behavior.For each subject, written consent was obtained after the procedures had beenfully explained.
The 20 subjects with MDE were obtained from a larger sample of 37 medication-freesubjects with MDE who had enrolled in a study of antidepressant occupancy.24,33 All were followed up for a minimumof 2 months by a psychiatrist (J.H.M.). The other 17 subjects were excludedon the basis of the criteria of nonsmoking and comorbid Axis I anxiety disorders.Diagnosis of MDE secondary to major depressive disorder was based on the StructuredClinical Interview for DSM-IV for Axis I disorders(SCID), (S.S.)34 and a consultation by a psychiatrist(J.H.M). For subjects with MDE, the minimum severity of depression for enrollmentwas based on a cutoff score of 16 on the 17-item Hamilton Depression RatingScale. The mean Hamilton Depression Rating Scale score was 20 (SD, 4). Theserecruiting methods are similar to what has been described previously.25,35 Exclusion criteria included MDE withpsychotic symptoms, bipolar disorder (type I or II), comorbid Axis I diagnoses,and history of self-harm or suicidality outside of episodes of depression,anger dyscontrol, impulsive behavior, and neuroleptic use. The exclusion ofself-harm, anger dyscontrol, and impulsivity (taken from the Structured ClinicalInterview for DSM-IV Axis II Disorders)36 ruledout severe borderline personality disorder behaviors. People who have severeborderline personality disorder behaviors have abnormal severity of dysfunctionalattitudes.37 Fourteen of the 20 depressed patientshad never received a trial of antidepressant treatment. No subject with MDEhad received antidepressant treatment within the past 3 months. The recruitmentof medication-free subjects has become a recognized standard in receptor-ligandimaging studies of MDE.25,35,38- 40
All patients received common blood tests to rule out medical causesof disturbed mood (measurement of thyroid function, electrolyte levels, andcomplete blood cell count). Three of the MDE subjects who had received anantidepressant trial in the past also had a history of substance use (marijuana)that did not meet criteria for the SCID diagnosis of drug abuse. These 3 patientshad not used marijuana within the previous year. Each underwent urine drugscreening, the results of which were negative. All healthy subjects underwenturine drug screening.
All patients gave written consent after the procedure had been fullyexplained. The study and recruitment procedures were approved by the researchethics board for human subjects at the Centre for Addiction and Mental Health,Toronto, Ontario.
Additional information recorded included version A of the DAS; visualanalog scales for mood, anxiety, and energy; and the Scale for Suicide Ideation.41 Version A of the DAS is a 40-item self-report thatrequires subjects to indicate to what degree they agree with an individualstatement. Each endorsement is converted into a score between 1 and 7. Thetotal score for endorsement of pessimistic statements and disagreement withoptimistic statements becomes the final measurement from the DAS. The DASdetects negativistic thinking during episodes of MDE,42,43 hasvery good test-retest reliability across 6 to 8 weeks (r = 0.83-0.84),44,45 andhas a high degree of internal consistency (Cronbach α = 0.85-0.87).45,46
The synthesis and imaging of [11C]DASB has been describedpreviously.20- 23 Beforethe PET scan, an intravenous bolus of 10 mCi (370 MBq) of [11C]DASBwas injected. The [11C]DASB was of high radiochemical purity (>95%)and high specific activity (950 ± 270 mCi/μmol [35 ± 10GBq/μmol] at the time of injection). The PET images were obtained usinga GEMS 2048-15B camera (Scanditronix Medical, General Electric, Uppsala, Sweden;intrinsic resolution, full width at half maximum, 5.5 mm). The images werecorrected for attenuation using a germanium Ge 68 transmission scan and reconstructedby filtered back projection (Hanning filter).
To obtain a measure of the 5-HTT BP with region-of-interest data, wechose the noninvasive method of Logan et al47 (hereafterreferred to as the Logan method) implemented within PMOD software (PMOD TechnologiesLtd, Zurich, Switzerland).48
We used the noninvasive Logan method because it provides valid measurementsof 5-HTT BP, and the between-subject variance in 5-HTT BP is low for mostbrain regions. The coefficient of variation (standard deviation/mean) usingthe Logan method47 varied from 25% in the prefrontalcortex and midbrain to 15% in the putamen. Furthermore, the 5-HTT BP foundwith this method is very reproducible. In our data sets,24 theabsolute difference in the test-retest measurement of the 5-HTT BP with applicationof the Logan method47 averages below 15% ofthe 5-HTT BP itself in all regions except the dorsolateral prefrontal cortexand midbrain, for which it is higher at about 15% to 20% of the mean 5-HTTBP. The less reliable test-retest measurement of 5-HTT BP in the dorsolateralprefrontal cortex region and midbrain reflects a lesser sensitivity of [11C]DASB to detect the 5-HTT BP in these regions with this method. Themeasurement of 5-HTT BP with the noninvasive Logan method47 isan underestimate of the same 5-HTT BP found with invasive arterial samplingand the single-tissue compartment model. This is quite acceptable becausethe between-subject variance of 5-HTT BP is also lower with the noninvasiveLogan method,47 and the regional 5-HTT BP foundwith the 2 methods is very strongly correlated. In general, we find that the5-HTT BP measured using reference tissue–based methods is highly correlatedwith the ratio of the kinetically determined distribution volumes betweenregions with specific binding and the reference region.23 Aspart of our assessment of the noninvasive Logan method,47 wedetermined distribution volumes in the thalamus, striatum, frontal cortex,and cerebellum in a set of healthy subjects using an arterial input functionand a single-tissue compartment model. Ratios of distribution volumes in regionswith specific binding to the distribution volume of the cerebellum were determinedfor each brain region, and this ratio was highly correlated with the bindingpotential found with the noninvasive Logan method (r = 0.97).
For each region, except the midbrain, analyses were also carried outusing the modified simplified reference tissue method,49 whichis also suitable for [11C]DASB PET.50 Thelesser reversibility of radiotracer uptake in the midbrain precluded routineuse of the modified simplified reference tissue model in this region.
A key assumption in the noninvasive models is that there is a referenceregion that does not contain specifically bound radioligand. For [11C]DASB, the cerebellum is a suitable reference region because studiesreport either undetectable51 or extremely low5-HTT density.52,53 More recently,it was found that the 5-HTT density in the cerebellum is less than 3% of thestriatal value using the Western blot method (Stephen J. Kish, PhD, YoshiakiFurukawa, MD, Li-Jan Chang, MSc, Junchao Tong, PhD, N.G., A.A.W., S.H., andJ.H.M., unpublished data, May 2004).
The BP found with these noninvasive methods represents the ratio ofspecifically bound radiotracer to radiotracer in free and nonspecific compartmentsat equilibrium. This BP is equal to (1/V2) × Bmax/Kd54,55 where V2 represents free and nonspecific binding at equilibrium; Bmax, receptor density; and Kd, the dissociation constant. For [11C]DASB PET, V2 is sufficiently similar among subjects that it may beconsidered a constant.23
Regions chosen were bilateral anteromedial prefrontal cortex (Brodmannareas included were part of 8, 9, and 10), left and right dorsolateral prefrontalcortex (part of Brodmann areas 9 and 46), anterior cingulate, left and rightcaudate, left and right putamen, left and right thalamus, and midbrain. Prefrontalregions were included because indexes of increased serotonin2 receptordensity have been reported in prefrontal cortex in suicide victims12,30,56- 59 andmedication-free depressed subjects who are suicidal12,30 orhave severely negativistic thinking.25 Theanterior cingulate was chosen because metabolic abnormalities have been reportedin this general region in subgroups of depressed subjects.60,61 Otherregions (thalamus, basal ganglia, and midbrain) were chosen because the 5-HTTdensity is high in these regions.51- 53
To assist with the region-of-interest measurement, each subject underwenta magnetic resonance imaging scan (GE Signa 1.5-T scanner; GE Medical Systems,Milwaukee, Wis) (spin-echo sequence proton density–weighted image; x,y, and z voxel dimensions, 0.78, 0.78, and 3 mm, respectively). Regions ofinterest were found using a semiautomated method62,63 verifiedby visual assessment with reference to a coregistered magnetic resonance imagingscan. The only exception was the midbrain region, which was drawn within thesuperior 2 planes to the pons with reference to the coregistered magneticresonance imaging scan. These methods have been described in more detail previously.62
There was no effect of age on regional 5-HTT BP in the healthy samplefor any region (prefrontal cortex, anterior cingulate, bilateral caudate,bilateral putamen, bilateral thalamus, and midbrain) (1-way analysis of variance[ANOVA], F1,18 = 0.01-3.0 [P = .92-.10]).There was no effect of sex on 5-HTT BP in these regions within the healthysample (1-way ANOVA, F1,18 = 0.001-1.0 [P = .97-.32]). The results for this analysis in peripheralregions using the modified simplified reference tissue model49,50 weresimilar (effect of age, 1-way ANOVA, F1,18 = 0.03-1.8[P = .73-.20]; effect of sex, 1-way ANOVA,F1,18 = 0.008-0.6 [P = .93-.45]).
In the entire sample, MDE had no effect on regional 5-HTT BP for anyindividual region (1-way ANOVA, F1,38 = 0.09-1.4 [P = .77-.24]); in fact, the mean regional 5-HTTBP within each group was very similar (Figure1). The results for this analysis in peripheral regions using themodified simplified reference tissue model49,50 weresimilar (effect of depression, 1-way ANOVA, F1,38 = 0.1-1.3[P = .75-.26]).
In MDE subjects, in all regions primarily sampling 5-HT nerve terminals(ie, everywhere except the midbrain), there were strong correlations betweenincreasing DAS scores and increasing 5-HTT BP (Figure 2). In healthy subjects, the correlations between DAS andregional 5-HTT BP were nonsignificant. The correlation coefficients betweenthe DAS and regional 5-HTT BP in healthy subjects were compared with the correlationcoefficients between the DAS and regional 5-HTT BP found in MDE subjects.We performed the comparison by transforming the correlation coefficients into z scores with the Fisher transformation so that they thencould be compared using the function of the gaussian distribution.64 For all regions mainly sampling 5-HT nerve terminals,the correlation between regional 5-HTT BP and DAS was significantly differentbetween MDE and healthy subjects (Table).For all of these analyses, extremely similar results were obtained using regional5-HTT BP found with the modified simplified reference tissue method.49,50
Severely negativistic dysfunctional attitudes in the MDE subjects weredefined as a DAS score of greater than 190. The cutoff score of 190 was chosenbecause this score is 3 SDs above the mean DAS score of our healthy subjects.Eight subjects with MDE had a DAS score greater than 190, and their regional5-HTT BP was compared with that of the 20 healthy subjects. For all peripheralregions, the 5-HTT BP was higher in the MDE subjects with highly abnormalDAS scores (Figure 3). For these analyses,similar results were obtained using regional 5-HTT BP found with the modifiedsimplified reference tissue method.49,50
To our knowledge, this was the first investigation of regional 5-HTTBP with a selective radioligand in drug-free depressed subjects, and therewere 3 main findings. First, there was no difference in regional 5-HTT BPin the entire sample of depressed subjects compared with the healthy subjects.Second, depressed subjects with greater regional 5-HTT BP had higher levelsof dysfunctional attitudes. Third, a subgroup of depressed subjects with extremelyhigh levels of dysfunctional attitudes had increased regional 5-HTT BP comparedwith healthy subjects.
An important function of the 5-HTT is to remove extracellular serotonin.This concept is primarily based on the findings that selective serotonin reuptakeinhibitors inhibit the reuptake of 5-HT65 andthat knockout mice without the 5-HTT have increased levels of extracellularserotonin.66 The findings of greater 5-HTTBP in MDE subjects with high levels of dysfunctional attitudes is consistentwith this role. Greater 5-HTT BP could result in greater clearance of serotonin.On the basis of our previous findings,25 theMDE subjects with more negativistic thinking should have lower levels of extracellularserotonin. (Previous findings were that increased serotonin release afterd-fenfluramine administration decreased dysfunctional attitudes toward optimism.25 Higher serotonin2 BP was found in MDEwith severe dysfunctional attitudes.25 Otherinvestigators have reported greater serotonin2 density after 5-HT–depletingparadigms of ≥2 weeks’ duration26,27 andreduced serotonin2 density after certain serotonin-increasing paradigmsof ≥2 weeks’ duration28,29).The association between negativistic thinking and higher 5-HTT BP is interpretedto reflect a vulnerability to greater extracellular clearance of serotoninwhen the 5-HTT BP is high. Greater extracellular clearance of serotonin thenresults in more severe symptoms. The earlier investigations25 suggestthat depressed patients with greater dysfunctional attitudes have a greaterloss of extracellular serotonin, and the present study proposes that greater5-HTT BP is a mechanism that could contribute to greater extracellular serotoninloss.
This interpretation suggests that the 5-HTT and DAT have similar rolesof increasing vulnerability to symptoms during MDE. In a previous report ofa separate sample of subjects, we found that MDE subjects with the lowestDAT BP were not motor retarded, whereas MDE subjects with higher DAT BP wereimpaired on neuropsychological measurement of motor speed.35 Thecorrelation between DAT BP and motor retardation was very strong (r = 0.86 [P = .006]),and the subjects were also carefully recruited (drug free, nonsmoking, andno comorbidity).35 The interpretation of thisprevious study was that greater DAT BP provides vulnerability to greater clearanceof dopamine, resulting in lower levels of extracellular dopamine and moresevere motor impairment, whereas lower DAT BP reduces clearance of extracellulardopamine, resulting in near-normal extracellular dopamine levels and negligiblemotor impairment.
We are characterizing the sample of MDE subjects with more severe dysfunctionalattitudes as having more severe serotonergic disturbances with greater serotonin2 BP in the previous sample,25 and withgreater 5-HTT BP in the current sample. This may have implications for suiciderisk due to MDE. The DAS is highly correlated with the Beck Hopelessness Scaleduring MDE,67,68 and greater scoreson the Beck Hopelessness Scale are associated with higher likelihood of suicide.69,70 Therefore, it is possible that duringMDE, in some people, elevated 5-HTT BP may lead to lower serotonin levels,less stimulation via major serotonergic signal transduction pathways, andelevated serotonin2 BP. It may be that these processes mediategreater negativistic thinking and eventually result in an elevated risk ofsuicide.
The correlations between DAS and 5-HTT BP were very strong in the brainregions that primarily sampled serotonergic nerve terminals. This is not surprising,because the 5-HTT BP tends to associate across these regions within subjects.For example, subjects with higher 5-HTT BP in the frontal cortex tend to havehigher 5-HTT BP in the striatum.
In a large earlier postmortem study ( 53 patients vs 107 subjectswith no history of a depressive episode), it was found that the 5-HTT densitywas lower in the patient sample.7- 9 Thereare several major differences in sampling between the earlier study and thepresent one. In our study, all subjects were in the midst of a current MDEdue to major depressive disorder. The previous study sampled patients whohad a history of unipolar or bipolar depressive episodes and were not necessarilyin the midst of a depressive episode at the time of measurement. Another keydifference was that the subjects in the present study were about a decadeyounger than the subjects in the postmortem study. If there are any long-termeffects of mood disorder illnesses on 5-HTT density, the results of the studieswould be expected to differ. Future longitudinal studies of 5-HTT BP in untreatedremitted depressed subjects may help resolve these questions.
Whenever multiple regions are investigated in a brain imaging study,one should consider how multiple comparisons influence the interpretationof results. In this study, the 2 major findings (correlation between 5-HTTBP and dysfunctional attitudes during depression and higher 5-HTT BP in depressedsubjects with severe dysfunctional attitudes) were present in all 6 regionsof serotonin nerve terminal areas. The probability of having significant findingsin all 6 brain regions sampling serotonergic nerve terminals by independentchance alone is exceedingly small. The probability of 6 significant, independentfindings is 0.056 or 1.6 × 10−8. Therefore, it is unlikely that the findings of this studyrepresent independent chance events.
The results of this study show strong significant correlations betweenthe 5-HTT BP and dysfunctional attitudes. However, there are limitations.Not all brain regions were analyzed; rather, we chose brain regions in whichthe 5-HTT BP could be reliably and validly measured. The 5-HTT BP is proportionalto both 5-HTT density and affinity. Even so, the combined measurement of densityand affinity obtained with the BP does not alter our interpretations, becauseboth 5-HTT density and affinity should contribute to serotonin clearance.We also acknowledge that 5-HTT density and affinity are not the only indicesof 5-HTT function. For example, it is possible for desensitization processesto occur for some receptors without changing either the density or the affinityof receptors. We cannot measure serotonin directly in the human brain, andin our interpretations we made inferences about extracellular serotonin basedon the strong relationships between manipulations of serotonin and dysfunctionalattitudes, as well as measures of serotonin2 receptors and dysfunctionalattitudes in our previous investigations.25 Wealso recognize that there are alternative interpretations for the correlationsbetween dysfunctional attitudes and regional 5-HTT BP. For example, it couldbe that in MDE with high dysfunctional attitudes, there is a greater densityof neurons expressing 5-HTT, a greater density of dendrites expressing 5-HTT,a wider range of neurons expressing 5-HTT, an unusual regulation of 5-HTTin response to some other process, etc. The importance of the explanationwe presented is that it most straightforwardly accounts for both existingknowledge regarding 5-HTT function65,66 andthe relationships between dysfunctional attitudes and serotonin measures inearlier investigations.25
This study was the first investigation of the 5-HTT BP in drug-free,nonsmoking, currently depressed subjects with a reasonably large sample sizeusing a specific radioligand. We found that MDE subjects with more negativisticthinking had greater 5-HTT BP in MDE and that a subgroup of MDE subjects withseverely negativistic thinking had a 21% higher 5-HTT BP compared with healthysubjects. There was no difference in 5-HTT BP between the entire sample ofMDE and the healthy subjects. The findings are important because they suggesta role for 5-HTT in the pathophysiology of serotonin-related symptoms. Duringan MDE, elevated 5-HTT BP may result in excessive extracellular clearanceof serotonin, providing vulnerability to serotonin-related symptoms. Thisinterpretation is consistent with previous investigations that associate serotoninagonist effects (ie, d-fenfluramine effect and serotonin2 BP) withdysfunctional attitudes.25 Our data argue thatthe 5-HTT BP is strongly associated with a symptom of excessive dysfunctionalattitudes within an MDE rather than the MDE itself.
Correspondence: Jeffrey H. Meyer, MD, PhD,FRCPC, 11th Floor, Centre for Addiction and Mental Health, Clarke Division,250 College St, Toronto, Ontario, CanadaM5T 1R8 (email@example.com).
Submitted for Publication: March 20, 2003;final revision received December 10, 2003; accepted June 23, 2004.
Funding/Support: This study was supported bythe National Alliance for Research in Schizophrenia and Depression (GreatNeck, NY) and Eli-Lilly Canada (Toronto, Ontario). Dr Meyer is supported bythe Canadian Institutes of Health Research New Investigator program (Ottawa,Ontario).
Acknowledgment: We thank research assistantsAlex Kecojevic and Corey Jones and technicians Kevin Cheung, Armando Garcia,Li Jin, MSc, and Ruiping Guo.