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Seasonal difference in brain serotonin
transporter binding predicts symptom severity
in patients with seasonal affective disorder
Brenda Mc Mahon,
1,2
Sofie B. Andersen,
1
Martin K. Madsen,
1
Liv V. Hjordt,
1,2
Ida Hageman,
3
Henrik Dam,
3
Claus Svarer,
1
Sofi da Cunha-Bang,
1,2
William Baare
´,
4
Jacob Madsen,
5
Lis Hasholt,
6
Klaus Holst,
2,7
Vibe G. Frokjaer
1
and Gitte M. Knudsen
1,2
Cross-sectional neuroimaging studies in non-depressed individuals have demonstrated an inverse relationship between daylight
minutes and cerebral serotonin transporter; this relationship is modified by serotonin-transporter-linked polymorphic region short
allele carrier status. We here present data from the first longitudinal investigation of seasonal serotonin transporter fluctuations in
both patients with seasonal affective disorder and in healthy individuals. Eighty
11
C-DASB positron emission tomography scans
were conducted to quantify cerebral serotonin transporter binding; 23 healthy controls with low seasonality scores and 17 patients
diagnosed with seasonal affective disorder were scanned in both summer and winter to investigate differences in cerebral serotonin
transporter binding across groups and across seasons. The two groups had similar cerebral serotonin transporter binding in the
summer but in their symptomatic phase during winter, patients with seasonal affective disorder had higher serotonin transporter
than the healthy control subjects (P= 0.01). Compared to the healthy controls, patients with seasonal affective disorder changed
their serotonin transporter significantly less between summer and winter (P50.001). Further, the change in serotonin transporter
was sex- (P= 0.02) and genotype- (P= 0.04) dependent. In the patients with seasonal affective disorder, the seasonal change in
serotonin transporter binding was positively associated with change in depressive symptom severity, as indexed by Hamilton
Rating Scale for Depression – Seasonal Affective Disorder version scores (P=0.01). Our findings suggest that the development
of depressive symptoms in winter is associated with a failure to downregulate serotonin transporter levels appropriately during
exposure to the environmental stress of winter, especially in individuals with high predisposition to affective disorders.
1 Neurobiology Research Unit, Rigshospitalet and Centre for Integrated Molecular Brain Imaging, Section 6931, Blegdamsvej 9,
2100 Copenhagen, Denmark
2 Faculty of Health and Medical Sciences, University of Copenhagen, Denmark
3 Psychiatric Centre Copenhagen, Blegdamsvej 9, 2100 Copenhagen, Denmark
4 Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Hvidovre Hospital,
Kettega
˚rd alle
´30, 2650 Hvidovre, Denmark
5 PET and Cyclotron Unit, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark
6 Department of Cellular and Molecular Medicine, University of Copenhagen, Blegdamsvej 3B, 2100 Copenhagen, Denmark
7 Department of Biostatistics, University of Copenhagen, Øster Farimagsgade 5, 1014 Copenhagen, Denmark
Correspondence to: Professor Gitte Moos Knudsen,
Neurobiology Research Unit,
Build. 6931, Rigshospitalet, 9
Blegdamsvej, DK-2100 Copenhagen, Denmark
E-mail: gmk@nru.dk
Keywords: seasonal affective disorder; serotonin; serotonin transporter; serotonin transporter linked polymorphic region; PET
doi:10.1093/brain/aww043 BRAIN 2016: 139; 1605–1614 |1605
Received October 08, 2015. Revised December 26, 2015. Accepted January 28, 2016. Advance Access publication March 19, 2016
ßThe Author (2016). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email:
journals.permissions@oup.com
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Abbreviations: 5-HTTLPR = serotonin-transporter-linked polymorphic region; BMI = body mass index; BP
ND
= non-displaceable
binding potential; MDI = Major Depression Inventory; PSQI = Pittsburgh Sleep Quality Index; SIGH-SAD = Hamilton Rating Scale
for Depression – Seasonal Affective Disorder version; SPAQ = Seasonal Pattern Assessment Questionnaire
Introduction
In Scandinavia as well as other countries at Northern lati-
tudes, people are subjected to long and dark winters.
Although well tolerated by most inhabitants, 5% of
the Copenhagen population experience symptoms consist-
ent with seasonal affective disorder and an additional
10% suffer from sub-syndromal seasonal affective dis-
order (Dam et al., 1998), a more moderate condition
where diagnostic criteria for depression are not met.
Seasonal affective disorder is characterized by season-
triggered depression and encompasses feelings of hopeless-
ness and blameworthiness, loss of energy, impaired
concentration, hyperphagia and hypersomnia (Rosenthal
et al., 1984). Risk factors for developing seasonal affective
disorder include being female, with females being afflicted
between 2–40 times more often than males (Partonen,
1995), young age (Magnusson and Partonen, 2005) and
being a serotonin-transporter-linked polymorphic region
(5-HTTLPR) short allele carrier (S-carrier) (Rosenthal et
al., 1998). There is additional evidence for seasonal affect-
ive disorder being related to serotonin dysfunction: the
disordercanbeeffectivelytreatedwitheitherbright
light therapy or with a serotonin transporter reuptake in-
hibitor (Thaler et al., 2011), the effects of bright light can
be reversed by lowering cerebral serotonin levels by tryp-
tophan depletion (Lam et al., 1996; Neumeister et al.,
1997a,b, 1998), and dietary (Miller, 2005) or pharmaco-
logical (O’Rourke et al., 1989; Dilsaver and Jaeckle,
1990; Partonen and Lonnqvist, 1996) enhancement of
serotonin transmission alleviates seasonal affective dis-
order symptoms. Further, serotonin transporter function
in platelets is enhanced in seasonal affective disorder
(Willeit et al., 2008). Intriguingly, these risk factors are
also uniquely associated with differences in cerebral sero-
tonin transporter levels: healthy females have higher sero-
tonin transporter density in the midbrain than males
(Erritzoe et al., 2010), cerebral serotonin transporter dens-
ity declines with age (Buchert et al., 2006; Kalbitzer et al.,
2009; Erritzoe et al., 2010), and several studies suggest
that the 5-HTTLPR genotype is related to cerebral sero-
tonin transporter density (Willeit and Praschak-Rieder,
2010).
The season-dependent fluctuation in cerebral serotonin
transporter has been examined in a number of neuroima-
ging studies conducted in healthy volunteers. Early single
photon emission computerized tomography (SPECT) stu-
dies using fewer serotonin transporter-specific radioligands,
were inconclusive (Neumeister et al., 2000; Koskela et al.,
2008; Cheng et al., 2011). However, PET studies of healthy
males and females consistently found higher serotonin
transporter binding in certain brain regions in the winter
than in the summer. One study examined 29 Germans with
11
C-McN5652 PET (Buchert et al., 2006) and two studies
used the selective serotonin transporter radiotracer
11
C-
DASB [
11
C-labelled 3-amino-4-(2-dimethylaminomethyl-
phenylsulfanyl)benzonitrile] and PET in 88 Canadians
(Praschak-Rieder et al., 2008) and in 57 Danes (Kalbitzer
et al., 2010). In the latter study, a significant gene envir-
onment interaction effect was found, with S-carriers dis-
playing larger seasonal serotonin transporter fluctuations
in putamen as compared to long allele (L
A
/L
A
) carriers,
with the peak in serotonin transporter levels around
winter solstice (Kalbitzer et al., 2010). By contrast, two
later PET studies reported no effect of season on cerebral
serotonin transporter binding, one using
11
C-DASB in 63
male UK citizens (Murthy et al., 2010) and another using
11
C-MADAM (
11
C-labelled-N,N-dimethyl-2-(2-amino-4-
methylphenylthio)benzylamine) in 40 male Swedes
(Matheson et al., 2015). In general, these cross-sectional
studies did not take relevant factors, such as S-carriers
status, sex, traveling habits, night shift work, seasonality
and mood, into account.
Surprisingly, in spite of seasonal affective disorder repre-
senting a unique model for investigating the relationship
between serotonin transporter availability in the brain
and season-related mood variations, no studies have so
far examined patients with seasonal affective disorder
both in their asymptomatic and in their symptomatic
phases. A single study investigated 11 patients with sea-
sonal affective disorder in their symptomatic phase and
11 non-depressed healthy volunteers with the non-selective
dopamine transporter and serotonin transporter radioli-
gand
123
I-b-CIT and SPECT and reported lower thalamic-
hypothalamic serotonin transporter binding in patients with
seasonal affective disorder compared to healthy controls
(Willeit et al., 2000).
In the present study we aimed, for the first time in a
longitudinal study design, to characterize how patients
with seasonal affective disorder regulate the serotonin
transporter across seasons, if gender and S-carrier status
modifies this regulation, and to what extent serotonin
transporter changes can predict symptom severity. We
hypothesized that in the winter, patients with seasonal af-
fective disorder have higher cerebral serotonin transporter
levels than healthy controls, whereas the levels are compar-
able in the summer. Moreover, we expected a positive as-
sociation between change in serotonin transporter and in
seasonal affective disorder symptom severity.
1606 |BRAIN 2016: 139; 1605–1614 B. Mc Mahon et al.
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Materials and methods
Participants
Healthy volunteers and potential patients with seasonal affect-
ive disorder were recruited through advertisements posted on
the internet and in newspapers. The exclusion criteria were
smoking, past or present neurological or psychiatric (ICD-10)
disorders, use of drugs with known effects on the serotonin
system, use of recreational illegal drugs including cannabis
within the last week or more than 10 times in total (cannabis
was allowed up to 50 times in total), significant medical his-
tory, known retinal pathology, use of photosensitizing medica-
tions, travelling to destinations with a different climate 6
months prior to any of the scans, or night shift work.
Individuals with seasonal affective disorder were required not
to have received bright light therapy or psychotropic drugs as
treatment of their seasonal affective disorder in the past year.
All participants were within a body mass index (BMI) of 19–
28 kg/m
2
. Subjects that met the initial screening criteria were
asked to fill in the Seasonal Pattern Assessment Questionnaire
(SPAQ) (Rosenthal et al., 1984), a self-assessment question-
naire that evaluates seasonal variations in sleep, social activity,
mood, body weight, appetite and energy. The score on each
item is summed to obtain a global seasonality score, which
indexes the degree of seasonality symptoms [range: 0–24,
global seasonality score (GSS) 410 consistent with seasonal
affective disorder] (Kasper et al., 1989). Healthy volunteers
were required to have a maximum GSS of 10, reporting no
problems with seasonality, whereas those with seasonal affect-
ive disorder were required to have a GSS 511 and state that
seasonality was a least a moderate problem. Seasonal affective
disorder candidates were assessed by trained psychiatrists both
in summer and winter. The seasonal affective disorder diagno-
sis was established when subjects met the ICD-10 diagnostic
criteria for major depression and the seasonal affective dis-
order criteria described by Rosenthal et al. (1984). All referred
candidates underwent a Schedules for Clinical Assessment in
Neuropsychiatry (SCAN) interview (Wing et al., 1990) to ex-
clude any other axis I or axis II disorders before final inclu-
sion. The Hamilton Rating Scale for Depression – Seasonal
Affective Disorder version (SIGH-SAD) (Williams et al.,
1988) was used to quantify symptom severity both in
summer and winter.
In total 36 patients were referred for psychiatric assessment.
Of these, 12 were excluded due to co-morbidity or failure to
meet diagnostic seasonal affective disorder criteria. All eligible
subjects were screened with respect to S-carrier/L
A
/L
A
carrier
status prior to inclusion. As previous data suggest a larger
seasonal change in serotonin transporter availability in healthy
S-carriers compared to L
A
/L
A
homozygotes, we chose to in-
clude only S-carriers in the healthy control group. To investi-
gate genotype effects in seasonal affective disorder cohorts we
included an L
A
/L
A
group of six individuals in the seasonal
affective disorder group.
The scan sequence was randomized so that half of the indi-
viduals were scanned for the first time in the summer, the
other half for the first time in winter; defined as a 12-week
interval centred around the winter or summer solstice. All par-
ticipants underwent a medical and neurological examination
before each PET scan and were found to be normal. They
all had normal findings on routine blood tests and their cere-
bral MRI scans were without any pathological findings. To
measure seasonal fluctuations in mood and sleep, participants
filled out online versions of the Major Depression Inventory
(MDI) (range: 0–50, 421 indicates depressed mood) (Bech et
al., 2001; Olsen et al., 2003) and the Pittsburgh Sleep Quality
Index (PSQI) (global scores range: 0–21, 45 indicates sleep
disturbances) (Buysse et al., 1989). Information regarding men-
strual cycle length, timing of current cycle and use of hormo-
nal contraceptives were obtained from female participants on
the day of the PET scan. There were no drop-outs in the
healthy control group, but one subject was excluded due to
technical problems with the PET image. Seven patients with
seasonal affective disorder were lost to follow-up: one individ-
ual failed to go into spontaneous summer remission and six
individuals decided to leave the study before follow-up for
various personal reasons; none of them left the study because
of the treatment restriction.
The final sample included 23 healthy S-carriers with low
seasonality scores (13 females, GSS: 4.8 2.1, age: 26 6
years) and 17 patients with seasonal affective disorder (nine
females, 11 S-carriers, GSS: 14.1 2.2, age: 27 9 years), all
values given as mean standard deviation (SD). The groups
were comparable with respect to age {unpaired t-test of mean
age [(age
winter
+ age
summer
) / 2], P= 0.55}, sex (Fishers exact
test, P40.99) and BMI (unpaired t-test summer: P= 0.15
and winter: P= 0.32). Detailed sample characteristics are
included in Table 1.
The study was approved by The Copenhagen Region Ethics
Committee (H-1-2010-085 with amendments and KF-01-
2006-20 with amendment 21971/220225, H-1-2010-91 and
H-2-2010-108) and performed in accordance with the
Declaration of Helsinki II. All subjects gave informed written
consent prior to participation
Genotyping, plasma amino acids and
hormone data
Analysis of the serotonin transporter length polymorphism carrier
status was performed on DNA purified from saliva, as described
in the Supplementary material. Immediately before all PET scans,
blood was drawn for determination of plasma tryptophan as well
as the tryptophan load relative to its amino acid carrier competi-
tors (Knudsen et al., 1990). In a subsample of females, oestradiol
and progesterone levels were measured in serum collected on the
day of the PET scan and analysed as detailed previously (Frokjaer
et al., 2015).
MRI data acquisition
Participants were scanned on a 3 T Siemens Magnetom Trio
(n= 31) or Verio MR scanner (n= 9). High-resolution 3D T
1
-
weighted magnetization prepared rapid gradient echo was used
for tissue classification and T
2
-weighted turbo spin echostructural
images were used for brain-masking. Images were acquired as
previously described (Madsen et al., 2011).
PET imaging
All PET scans were conducted using a Siemens ECAT High-
Resolution Research Tomography scanner operating in 3D list-
Seasonal serotonin transporter changes in patients with SAD BRAIN 2016: 139; 1605–1614 |1607
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mode. Following a 6-min transmission scan, dynamic PET
scans were acquired over 90 min after injection of
11
C-DASB
[593 13 (range 536–612) MBq] over 20 s into the antecubi-
tal vein. PET acquisition and quantification were performed as
previous described (Frokjaer et al., 2009, 2015) and
11
C-DASB
radiosynthesis as described elsewhere (Lehel et al., 2009).
The quantification of
11
C-DASB was done using the
Multilinear Reference Tissue Model with a fixed k
2
’
(MRTM2) (Ichise et al., 2003), to generate the BP
ND
(non-
displaceable binding potential) using cerebellum as a reference
region. We chose whole brain serotonin transporter binding as
our primary outcome measure because (i)
11
C-DASB binding
potentials are highly correlated across brain regions suggesting
that serotonin transporter is regulated globally putatively
through raphe nuclei serotonergic firing (Erritzoe et al.,
2010); and (ii) seasonal serotonin transporter changes have
been described in various brain regions (Willeit et al., 2000,
2008; Reimold et al., 2007; Praschak-Rieder et al., 2008;
Kalbitzer et al., 2010). A volume-weighted average of whole
brain
11
C-DASB binding potential (global BP
ND
) was calcu-
lated based on 17 volume-weighted grey matter segmented
brain regions (amygdala, anterior cingulate gyrus, caudate,
entorhinal cortex, hippocampus, insula cortex, medial inferior
frontal gyrus, medial inferior temporal gyrus, occipital cortex,
orbitofrontal cortex, parietal cortex, posterior cingulated
gyrus, putamen, sensorimotor cortex, superior frontal gyrus,
superior temporal gyrus, and thalamus):
Global BPND ¼XBPNDx volumex
ðÞ
=Xvolumexð1Þ
Statistical analysis
Based on previous test-retest studies
11
C-DASB BP
ND
has a
variability of 3.7% and a reliability of 0.89 (Kim et al.,
2006), thus eight subjects are needed to detect a 20% defer-
ence in BP
ND
.
Group and seasonal differences in oestradiol and progester-
one levels (females only), psychometric scores, BMI, plasma
tryptophan load, k
2
’, non-displaceable binding (as proxy:
AUC
cerebellum
) and injected DASB mass/kg were tested with
paired or unpaired Students t-tests, as appropriate, two-tailed
P-values were adopted throughout all analyses. The correlation
between the psychometric scores (PSQI global score versus
MDI and SIGH-SAD versus MDI) was tested by linear correl-
ation regression. Multicollinerity between continuous variables
in multiple regression analysis was tested by calculation of the
variance inflation factor (VIF) (1 / 1 – R
2
)withaR
2
threshold of
0.75. A significance level of P= 0.05 was adopted throughout all
analyses. All results are expressed as means SD.
Seasonal changes in global serotonin transporter BP
ND
were
analysed in multiple regression models of various complexities
to investigate:
(i) If global BP
ND
differs between groups in either summer or
winter, using absolute global BP
ND
as outcome variable and par-
ameters known to affect SERT binding BMI (Erritzoe et al.,
2010), age (Frokjaer et al., 2009; Erritzoe et al., 2010), genotype
(Willeit and Praschak-Rieder, 2010) and sex (Kalbitzer et al.,
2009) as covariates: Global BP
ND season
group BMI
season
age
season
genotype sex.
(ii) If change in serotonin transporter across seasons (i.e.
BP
ND
=BP
ND
winter – BP
ND
summer) differs between patients
with seasonal affective disorder and healthy controls, using
BP
ND
as an outcome variable and group as variable of interest.
As seasonal affective disorder is more common in young individ-
uals (Magnusson, 2000), females (Magnusson, 2000), and pos-
sibly in S-carriers (Rosenthal et al., 1998) we included age, sex,
genotype, and group sex interaction (but not BMI, as BMI
changes is part of the seasonal affective disorder symptomatology)
as covariates: Global BP
ND
group sex sex by group
mean age genotype. In a post hoc analysis, we also examined
three additional brain regions of relevance for depression: the
raphe nuclei, hippocampus and anterior cingulate cortex.
(iii) If the relative BP
ND
(rel BP
ND
=BP
ND
/winterBP
ND
)ad-
justed for sex and genotype predicts seasonal symptom devolve-
ment in seasonal affective disorder, defining the outcome variable
as the relative difference in SIGH-SAD score [rel SIGH-
SAD = (winter score – summer score) / winter score]: rel BP
ND
rel SIGH-SAD sex genotype.
Statistical data analyses were carried out in GraphPad Prism
version 6, GraphPad Instat version 3 and R version 3.1.
Results
Sample characteristics
Objective ratings evaluated by the psychiatrists and subject-
ive mood ratings reported by the participants were highly
correlated. SIGH-SAD scores and MDI scores correlated
positively for both summer: n= 17, estimate = 0.89 SIGH-
SAD scores per MDI score, r
2
= 0.23, P= 0.05, and winter:
n= 17, estimate = 0.52 SIGH-SAD scores per MDI score,
r
2
= 0.34, P= 0.01. As expected, individuals with seasonal
affective disorder had significantly higher MDI, PSQI and
SIGH-SAD scores in the winter compared to the summer
(Table 1), but similar MDI and PSQI scores as the healthy
controls in the summer (P= 0.20 and 0.41, respectively).
The group difference in winter for PSQI and MDI scores
was large (P50.0001 for both scores). Across all partici-
pants, winter and summer MDI and PSQI global score
were highly correlated, n= 40, summer: estimate = 1.2
PSQI global score per MDI score, r
2
= 0.37, P50.0001
and winter: estimate = 3.0 PSQI global score per MDI
score, r
2
= 0.55, P50.0001.
We did not observe seasonal differences in BMI, plasma
tryptophan, k
2
’ or non-displaceable binding in any of the
two groups (Table 1). In a subset of the female participants,
we showed that serum oestradiol and progesterone were
similar across seasons, suggesting no significant difference in
timing of menstrual cycle in summer and winter (Table 1).
Coincidently, the healthy control group received a lower
injected DASB mass/kg bodyweight in the summer (Table
1); the maximal dose given was 0.05 mg/kg whereas the
maximal dose given in the winter was 0.13 mg/kg.
However, when tested as a covariate in the statistical
models, injected DASB mass/kg did not change the outcome
1608 |BRAIN 2016: 139; 1605–1614 B. Mc Mahon et al.
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of group differences and therefore this variable was not
included in any of the final models.
Serotonin transporter binding
Seasonal effects across groups
In the summer, subjects with seasonal affective disorder
and healthy controls had comparable global BP
ND
levels
[n= 40, estimate = 0.02 BP
ND
, 95% confidence interval
(CI) = 0.073 to 0.033, R
2
= 0.20, df = 34, P= 0.45].
In the winter, patients with seasonal affective disorder
had higher global BP
ND
compared to the healthy controls
(n= 40, estimate = 0.06 BP
ND
, 95% CI = 0.013 to 0.101,
R
2
= 0.27, df = 34, P= 0.01) (Fig. 1).
Group effects across seasons (BP
ND
)
An example of a seasonal affective disorder patient’s
11
C-DASB PET image in summer and winter is shown in
Fig. 2. We found a significant group effect when comparing
seasonal change, BP
ND
(=BP
ND
winter BP
ND
summer),
adjusted for genotype, sex, age and sex group interaction
(n= 40, estimate = 0.10 BP
ND
,P50.001) (Fig. 3A). We
found a significant effect of genotype (S-carriers 4L
A
L
A
,
P= 0.04) and of sex (females 4males: P= 0.02), whereas
we did not see any effect of age (P= 0.21). The group dif-
ference in BP
ND
was driven by the female participants,
with a significant sex group interaction effect (P= 0.03);
in the winter, females with seasonal affective disorder upre-
gulate whereas healthy females downregulate the serotonin
transporter (sex-contrasts: seasonal affective disorder fe-
males versus healthy control females, estimate = 0.10
BP
ND
,P50.001, adjusted for all pair-wise comparisons
by the Tukey post hoc test procedure) (Supplementary
Table 1).
In a post hoc analysis, we examined if the differences in
global BP
ND
could be replicated in brain regions of rele-
vance for depression and this generated results similar to
the global BP
ND
: raphe nuclei (P= 0.004), hippocampus
(P= 0.03), anterior cingulate (P= 0.0001).
Relation between change in relative symptom
severity and seasonal binding potential
In the seasonal affective disorder group, the relative change
in binding potential was positively correlated to relative
change in depressive symptom severity, as indexed by
SIGH-SAD scores: relative seasonal serotonin transporter
change predicted relative seasonal SIGH-SAD change
Table 1 Sample characteristics and radioligand variables
Summer Winter Paired t-test P-value
Healthy controls, n=23
Clinical data
MDI score 5.4 3.6 5.0 3.5 0.49
PSQI GS 3.7 2.1 3.6 1.8 0.79
BMI (kg/m
2
) 23.1 2.1 22.9 2.1 0.42
Biochemistry
Tryptophan load (n= 14) 0.13 0.02 0.13 0.02 0.86
Oestradiol (nmol/l) (n= 10) 0.13 0.07 0.24 0.14 0.06
Progesterone (nmol/l) (n= 11) 1.56 1.00 4.1 7.93 0.32
Radioligand variables
Non-displaceable binding (Bq/ml)
a
18634 2650 18489 3351 0.77
k
2
’ (per min) 0.07 0.01 0.07 0.001 0.57
Injected mass (mg/kg) 0.02 0.01 0.04 0.03 0.001
Seasonal affective disorder patients, n=17
Clinical data
MDI score 6.4 4.2 21.4 7.9 50.001
PSQI GS 4.5 1.8 6.5 2.3 0.02
SIGH-SAD score 2.1 2.3 23.1 8.8 50.001
BMI (kg/m
2
) 22.3 2.5 22.1 2.5 0.29
Biochemistry
Tryptophan load 0.14 0.03 0.13 0.02 0.07
Oestradiol (nmol/l) (n= 8) 0.19 0.20 0.18 0.18 0.81
Progesterone (nmol/l) (n= 7) 6.6 13.7 0.84 0.40 0.32
Radioligand
Non-displaceable binding (Bq/ml)
a
18516 3747 17600 3684 0.07
k
2
’ (per min) 0.07 0.01 0.07 0.01 0.48
Injected mass (mg/kg) 0.02 0.03 0.03 0.06 0.68
Data are shown as mean SD.
a
As evaluated by AUC
cerebellum.
Seasonal serotonin transporter changes in patients with SAD BRAIN 2016: 139; 1605–1614 |1609
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(n= 17, estimate = 0.83 SIGH-SAD score per BP
ND
, 95%
CI = 0.28 to 1.38, R
2
= 0.47, df = 13, P= 0.01) (adjusted
for genotype and sex) (Fig. 3B). In the correlation analysis,
genotype constituted a significant covariate (P
S-car-
riers
= 0.04), whereas sex did not (P= 0.94).
Discussion
This is the first study to compare seasonal fluctuations in
cerebral serotonin transporter binding in people with and
without seasonal affective disorder. First, we found that in
the winter, but not in the summer, individuals with sea-
sonal affective disorder have higher cerebral serotonin
transporter binding than people without seasonality symp-
toms. Second, seasonal affective disorder individuals regu-
late their cerebral serotonin transporter binding differently
than individuals without seasonality symptoms: there is a
significant group effect when comparing BP
ND
in seasonal
affective disorder individuals versus in healthy individuals;
having seasonality symptoms, S-carrier status and being
female makes you less likely to reduce your cerebral sero-
tonin transporter binding in the winter. Third, among the
seasonal affective disorder individuals, a relative larger
change in serotonin transporter binding from winter to
summer is associated with relatively more depressive symp-
toms. Overall, our findings suggest that seasonal affective
disorder-prone individuals are unable to appropriately
adjust their serotonin transporter binding levels to accom-
modate the environmental stressor of winter, thereby elicit-
ing the symptoms of seasonal affective disorder. We can of
course not rule out that the changes in serotonin trans-
porter are appropriate adjustments to a the depressive con-
dition, but this does not seem to be a sensible
interpretation, given that higher serotonin transporter
density generally is associated with lower serotonin levels
(Jennings et al., 2006) and given that blocking of the sero-
tonin transporter often is used to treat seasonal affective
disorder (Pjrek et al., 2009).
As mentioned above, the only small study in patients
with seasonal affective disorder that was carried out in
the winter only, reported that as measured with a non-
selective SPECT radioligand, patients had lower thalamic
serotonin transporter binding compared to non-depressed
individuals (Willeit et al., 2000). In a subsequent post
hoc analysis, we investigated this and found significantly
higher thalamic binding potential in patients with seasonal
affective disorder in the winter compared to healthy
controls.
By DSM-IV definition, seasonal affective disorder is con-
sidered a sub-specifier of major depressive disorder and
therefore, it makes sense to relate our findings to the out-
come from studies in patients with major depressive dis-
order. In a recent review of cross-sectional molecular
neuroimaging studies of major depressive disorder patients
and healthy controls, it was established that in various
brain regions, serotonin transporter binding was higher in
patients with major depressive disorder (Savitz and Drevets,
2013). As an explanation, the authors suggest that a chron-
ically higher expression of serotonin transporter leads to
lower serotonin levels and decreased serotonergic neuro-
transmission. Another two PET studies investigated symp-
tom severity versus serotonin transporter binding: Meyer et
al. (2004) reported a positive association between serotonin
transporter binding in various brain regions and scores of
the Dysfunctional Attitudes Scale in depressed subjects, but
not in healthy controls, while Cannon and co-workers
(2007) found, in patients with bipolar disorder, that sero-
tonin transporter binding correlated positively with anxiety
ratings in insular cortex and dorsal cingulate cortex. In the
Figure 1 Seasonal effects across groups. No difference in serotonin transporter BP
ND
was found across groups in summer (P= 0.45),
whereas patients with seasonal affective disorder had higher serotonin transporter compared to healthy controls in the winter (P= 0.01). Binding
potential values are adjusted for differences in age, BMI, sex and 5-HTTLPR genotype. SAD = seasonal affective disorder.
1610 |BRAIN 2016: 139; 1605–1614 B. Mc Mahon et al.
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latter study, no correlations were found between serotonin
transporter binding and ratings derived from the Hamilton,
the Montgomery–A
˚sberg Depression Rating Scale or the
Inventory of Depressive Symptomatology. Notably, these
cross-sectional studies do not involve comparisons to sero-
tonin transporter binding in the patients’ symptom-free
phase and accordingly, the findings cannot be directly com-
pared to our longitudinal design study.
In accordance with observations from our population-
based study (Kalbitzer et al., 2010), we found a gene en-
vironment interaction effect in this sample of patients with
seasonal affective disorder; the S-carriers had a significantly
larger seasonal serotonin transporter binding change
compared to the L
A
/L
A
-carriers. A limitation of the study
is that it was designed to include only S-carriers in the
control group, which means that we cannot conclude any-
thing about L
A
L
A
carriers in this group. In continuation of
this, the control group was carefully selected to include
only subjects without seasonality symptoms, which means
that the control group cannot be taken as representative for
the population as a whole.
Irrespective of group, our data show that seasonal sero-
tonin transporter fluctuations are particularly prominent in
the female participants, in accordance with their higher fre-
quency of affective disorders, compared to males
(Magnusson and Partonen, 2005). Differences in sex
Figure 3 Group effects across seasons and correlation to seasonal affective disorder symptoms. (A) Cerebral serotonin trans-
porter change across seasons between patients with seasonal affective disorder and healthy controls adjusted for sex, age, genotype and
sex group interaction effects. The BP
ND
(BP
ND
winter BP
ND
summer) was significantly different between groups (P50.0001). (B) Relative
change in symptom severity [SIGH-SAD scores winter-summer difference relative to winter (rel SIGH-SAD)] was significantly associated with
relative difference in global cerebral serotonin transporter binding (rel BP
ND
=BP
ND
/BP
ND
winter) (n= 17, estimate = 0.83, R
2
= 0.47,
P= 0.01).
Figure 2 The
11
C-DASB PET image of a patient with seasonal affective disorder in summer and in winter. Cerebral serotonin
transporter binding in a 22-year-old female S-carrier scanned symptom-free in the summer (left) and during winter when she had severe
depressive symptoms and a SIGH-SAD score of 27 (right). The quantified
11
C-DASB PET image is overlaid on a T
1
-weighted structural magnetic
resonance image. The patient had the highest cerebral serotonin transporter in the winter.
Seasonal serotonin transporter changes in patients with SAD BRAIN 2016: 139; 1605–1614 |1611
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hormone profiles are likely to shape differences in suscep-
tibility to affective disorders (Patton et al., 2014) and oes-
tradiol fluctuations are known to augment the risk of
depression (Munk-Olsen et al., 2006; Freeman et al.,
2014; Frokjaer et al., 2015). Further, oestradiol levels
have, in a large Norwegian study, been reported to show
small, but significant seasonal fluctuation with a peak in
June and a nadir in October (Bjornerem et al., 2006).
Thus, seasonal fluctuations in oestradiol levels may add
to the vulnerability to depression. Finally, in a randomized
clinical trial where 60 healthy females underwent interven-
tion with either placebo or a gonadotrophin-releasing hor-
mone agonist, it was found that the combination of large
oestradiol decreases and higher serotonin transporter bind-
ing at follow-up relative to baseline interacted in predicting
depressive responses to gonadotrophin-releasing hormone
agonist manipulation (Frokjaer et al., 2015). This increase
in serotonin transporter binding may represent a mechan-
ism by which sex hormone manipulation triggers depressive
symptoms.
Notably, for all participants low mood and poor sleep
quality were highly correlated both in summer and winter.
We suggest that this may be due to a common regulation of
mood and sleep mediated by serotonin (Murillo-Rodriguez
et al., 2012) or through interaction between mood and
sleep. Sleep disturbances often coincide with a diagnosis
of depression (Vandeputte and de Weerd, 2003;
BaHammam et al., 2015) and disruptions of circadian
rhythms are common in depression, in particular seasonal
affective disorder, and vice versa, depressive mood causes
rumination and increased latency to sleep.
In conclusion, we find evidence that the development of
depressive symptoms in winter is due to a failure to down-
regulate serotonin transporter appropriately during expos-
ure to the environmental stress of winter, especially in
individuals with high risk profiles for affective disorders.
We suggest that the increased serotonin transporter
causes low levels of endogenous serotonin and thus facili-
tates symptoms of seasonal affective disorder. However, to
confidently establish whether changes in serotonin trans-
porter binding represent a primary event or a secondary
compensatory regulation prompted by changes in serotonin
levels, it is necessary to conduct a biannual assessment of
serotonin levels in a seasonal affective disorder cohort, e.g.
by quantification of serotonin 4 receptors (Haahr et al.,
2014). Our data suggest that intervention with selective
serotonin re-uptake inhibitors may be particularly effective
in female or S-carrier patients with seasonal affective dis-
order and that stratification according to sex and genotype
may be warranted in a reanalysis of previously conducted
trials in seasonal affective disorder.
Acknowledgements
We wish to thank all the participants for kindly joining the
research project. We thank the John and Birthe Meyer
Foundation for the donation of the cyclotron and PET
scanner. The excellent technical assistance of Lone
Ibsgaard Freyr, Agnete Dyssegaard, Bente Dall, Sussi
Larsen, Gerda Thomsen and Svitlana Olsen is gratefully
acknowledged. Part of the data was presented on a poster
at the 27
th
European College of Neuropsychopharmacology
Congress in October 2014.
Funding
This project was funded by the Lundbeck Foundation, the
Danish Council for Independent Research (D.F.F.), a scho-
lar from Dr Ejlif Trier-Hansen and wife Ane Trier-Hansen
and a Brain Mind and Medicines scholar. G.M.K. serves as
consultant for H. Lundbeck A/S.
Supplementary material
Supplementary material is available at Brain online.
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