cAMP Signaling in Brain is Decreased in Unmedicated
Depressed Patients and Increased by Treatment with a Selective
Serotonin Reuptake Inhibitor
Masahiro Fujita, MD, PhD
1
, Erica M. Richards, MD, PhD
2
, Mark J. Niciu, MD, PhD
2
, Dawn F.
Ionescu, MD
2
, Sami S. Zoghbi, PhD
1
, Jinsoo Hong, PhD
1
, Sanjay Telu, PhD
1
, Christina S.
Hines, MD, PhD
1
, Victor W. Pike, PhD
1
, Carlos A. Zarate Jr, MD
2,*
, and Robert B. Innis, MD,
PhD
1,*
1
Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health,
Bethesda, MD, United States
2
Experimental Therapeutics & Pathophysiology Branch, National Institute of Mental Health,
National Institutes of Health, Bethesda, MD, United States
Abstract
Basic studies exploring the importance of the cyclic adenosine monophosphate (cAMP) cascade in
major depressive disorder (MDD) have noted that the cAMP cascade is downregulated in MDD
and upregulated by antidepressant treatment. We investigated cAMP cascade activity by
using
11
C-(
R
)-rolipram to image phosphodiesterase-4 (PDE4) in unmedicated MDD patients and
after approximately eight weeks of treatment with a selective serotonin reuptake inhibitor
(SSRI).
11
C-(
R
)-rolipram PET scans were performed in 44 unmedicated patients during a major
depressive episode and 35 healthy controls. Twenty-three of the 44 patients had a follow-up
11
C-
(
R
)-rolipram PET scan approximately eight weeks after treatment with an SSRI. Patients were
moderately depressed (Montgomery-Åsberg Depression Rating Scale=30±6) and about half were
treatment-naïve.
11
C-(
R
)-Rolipram binding was measured using arterial sampling to correct for
individual differences in radioligand metabolism. We found in unmedicated MDD patients
widespread, ~20% reductions in
11
C-(
R
)-rolipram binding compared to controls (P=0.001). SSRI
treatment significantly increased rolipram binding (12%, P<0.001) with significantly greater
increases observed in older patients (P<0.001). Rolipram binding did not correlate with severity of
baseline symptoms, and increased rolipram binding during treatment did not correlate with
symptom improvement. In brief, consistent with the results of basic studies, PDE4 was decreased
in unmedicated MDD patients and increased after SSRI treatment. The lack of correlation between
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Correspondence: Masahiro Fujita, MD, PhD, Molecular Imaging Branch, National Institute of Mental Health, Bldg. 10, Rm. B1D43,
10 Center Dr, MSC-1026, Bethesda, MD 20892-1026, USA, Phone: 301-451-8898, Fax: 301-480-3610, fujitam@mail.nih.gov.
*
These two authors equally contributed.
Dr. Ionescu: Dpt. Psychiatry, Massachusetts General Hospital & Harvard Medical School, Boston, MA, USA
Dr. Hines: Dpt. Psychiatry, Univ. of Texas Health Sciences Center, San Antonio; South Texas Veterans Healthcare System, TX, USA
Conflict of Interest
Dr. Zarate is listed as a co-inventor on a patent for the use of ketamine and its metabolites in major depression. Dr. Zarate has assigned
his rights in the patent to the US government but will share a percentage of any royalties that may be received by the government. All
other authors have no conflict of interest to disclose.
HHS Public Access
Author manuscript
Mol Psychiatry
. Author manuscript; available in PMC 2017 April 24.
Published in final edited form as:
Mol Psychiatry
. 2017 May ; 22(5): 754–759. doi:10.1038/mp.2016.171.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
PDE4 binding and depressive symptoms could reflect the heterogeneity of the disease and/or the
heterogeneity of the target, given that PDE4 has four subtypes. These results suggest that PDE4
inhibitors, which increase cAMP cascade activity, may have antidepressant effects.
Keywords
cAMP; second messenger; selective serotonin reuptake inhibitor; positron emission tomography;
major depressive disorder
Introduction
The key role of the cyclic adenosine monophosphate (cAMP) cascade in depression is
thought to involve both pathological changes in unmedicated patients as well as a common
pathway for various antidepressants to produce antidepressant effects. Human postmortem
studies in individuals with depressive disorders have indicated low cAMP signaling
1–3
.
Correspondingly, multiple rodent studies and one human postmortem study showed that
various forms of chronic, but not acute, administration of antidepressants upregulate cAMP
signaling
2, 4
. Based on these findings, the cAMP theory of depression posits low cAMP
signaling in unmedicated patients and, commensurately, upregulation of cAMP signaling as
a mechanism of antidepressant treatment. Here, we sought to examine these two theories in
patients with major depressive disorder (MDD) using the positron emission tomographic
(PET) radioligand
11
C-(
R
)-rolipram, a reversible inhibitor of phosphodiesterase-4 (PDE4).
PDE4 is the primary enzyme in brain to metabolize cAMP to the inactive monophosphate,
thereby terminating cAMP signaling. Because of a feedback mechanism, rolipram binding to
PDE4 provides a measure of the activity of this enzyme. Essentially, increased cAMP
stimulates protein kinase A (PKA), which phosphorylates PDE4
5
. This, in turn, increases
both enzymatic activity of PDE4 and rolipram binding affinity
6
. Previous work from our
laboratory confirmed biochemical studies of this phosphorylation effect using in vivo PET
imaging in rats; as predicted, local injection into brain of a PKA activator increased in vivo
binding of
11
C-(
R
)-rolipram to PDE4, and injection of a PKA inhibitor had the opposite
effect
7
.
Because anoxia leads to dephosphorylation of PDE4 within minutes, postmortem
measurements of PDE4 enzymatic activity and rolipram binding require rapid removal of
brain, which is possible in animals but not in humans. For this reason, we used in vivo
binding of
11
C-(
R
)-rolipram to assess cAMP signaling in humans. A previous study from
our laboratory found that, consistent with the cAMP theory of depression,
11
C-(
R
)-rolipram
was decreased by about 20% in all areas of the brain in unmedicated MDD patients currently
experiencing a major depressive episode
8
. Building on this work, the main purpose of the
present study was to test the cAMP theory of the mechanism of antidepressant response.
More specifically, we hypothesized that two months of treatment with an SSRI would
increase
11
C-(
R
)-rolipram binding in the brain of MDD patients experiencing a major
depressive episode and that this increased binding would correlate with symptom
improvement. We also compared rolipram binding between healthy controls and
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unmedicated MDD patients in a larger sample than in our previous study
8
to confirm
downregulation of cAMP signaling in unmedicated patients.
Materials and Methods
Participants
This study was approved by the Institutional Review Board of the National Institute of
Mental Health and the Radiation Safety Committee of the National Institutes of Health.
Informed consent was obtained from all subjects. For the baseline PET scan, participants
included all those in our prior study
8
plus 10 additional control subjects and 16 additional
MDD patients. This brought the total number of participants to 35 controls and 44 MDD
patients, which is more than twice the number enrolled in most PET molecular imaging
studies in brain (Table 1). Because all of these participants were recruited continuously in
the same manner, we report results for all subjects together. The patients met DSM-IV
criteria for MDD
9
and were currently experiencing a major depressive episode without
psychotic features (n=44). All patients were unmedicated at the time of the first
11
C-(
R
)-
rolipram PET scan; about half were treatment-naïve and the other half had been free from
psychotropic medications for an average of 28 months. Patients were required to have a
score of ≥20 on the Montgomery-Åsberg Depression Rating Scale (MADRS)
10
at the time
of the first PET scan. Patients were moderately depressed at that time as assessed by the
MADRS (30±6).
Twenty-three of the 44 unmedicated MDD patients had a second
11
C-(
R
)-rolipram PET scan
7.9±1.7 weeks after starting an SSRI (citalopram for 19 patients, escitalopram for one, and
sertraline for three; all patients received only monotherapy). As long as SSRI treatment was
clinically appropriate, all patients were asked if they were willing to have SSRI treatment
and then have another PET scan. SSRIs were given under this research protocol. No patient
was removed from the study after starting an SSRI because of symptom worsening or
because no therapeutic effect was observed. Clinical characteristics between those who had
only a baseline
11
C-(
R
)-rolipram scan and those who had two PET scans were mostly
similar except for gender balance; only one female patient agreed to have two PET scans.
Healthy controls had no history of a major psychiatric or neurological disorder and no first-
degree relative with a mood or psychotic disorder. Controls and patients were well matched
in terms of sex, age, and cigarette smoking (P>0.80). To study the reproducibility of the PET
measurement, 13 of 35 healthy controls had a second PET scan without medication at an
interval of 8.0±2.1 weeks.
All patients and controls were between 18 and 55 years of age and in good physical health.
See Supplementary Information for more details.
Data acquisition
Evaluation of Symptom Severity—Severity of depressive and anxiety symptoms was
assessed for both control subjects and MDD patients using the MADRS, the 17-item
Hamilton Rating Scale for Depression (HDRS-17)
11
, and the Hamilton Rating Scale for
Anxiety (HAM-A)
12
.
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Brain Imaging—PET, magnetic resonance imaging (MRI), and data processing were
conducted as previously described
8
. After intravenous administration of
11
C-(
R
)-rolipram,
PET images were acquired for 90 minutes. To calculate
11
C-(
R
)-rolipram binding in the
brain—which is not influenced by cerebral blood flow or peripheral clearance—
unmetabolized
11
C-(
R
)-rolipram levels in arterial plasma were measured for 90 minutes.
Because only free
11
C-(
R
)-rolipram enters the brain, plasma free fraction (
f
P
) of
11
C-(
R
)-
rolipram was measured using arterial plasma in each scan. High-resolution anatomical MRI
scans were performed for every subject except two MDD patients who had only clinical
MRI scans. High-resolution MRI scans were used to analyze PET data after transforming
into the single standard space (Montreal Neurological Institute space). Data from the two
patients without high-resolution MRI scans were analyzed in a similar way by using a
template image of
11
C-(
R
)-rolipram as described previously
8
.
Calculation of
11
C-(R)-Rolipram Binding in Brain—
11
C-(
R
)-Rolipram binding levels
were measured by compartmental modeling as total distribution volume (
V
T
/
f
P
)
13
in 10
large preselected regions covering most brain areas: frontal, parietal, lateral temporal,
occipital, medial temporal, and anterior cingulate cortices; caudate; putamen; thalamus; and
cerebellum. Right- and left-side data were combined for each region. Two additional
analyses were performed. First, to investigate possible changes in rolipram binding in small
regions,
V
T
/
f
P
was calculated in each volume element (i.e., voxel) of the images by Logan
plot
14
, and parametric images were created where each voxel value was
V
T
/
f
P
. These
parametric images were analyzed using Statistical Parametric Mapping (SPM) version 2008
(SPM8; Wellcome Trust Centre for Neuroimaging, London, United Kingdom). Second, to
eliminate the influence of individual differences in gray matter volume, partial volume
correction
15
was applied to the parametric images using MRI images segmented to gray and
white matter. Rolipram binding levels in the 10 regions were subsequently measured.
Statistical Analysis—Group comparisons and correlation/regression analyses were
performed for both regional and voxel data. Correlation/regression analyses were performed
by controlling for possible confounding factors such as age and gender. See Supplementary
Information for details. Results are shown as mean ± SD.
Results
Baseline scans without medications: The larger sample size in the current study
confirmed the previous finding of decreased
11
C-(R)-rolipram binding in unmedicated MDD
patients
The current study expanded our previous work to include a larger sample size of 35 healthy
controls and 44 unmedicated patients
8
. Our results confirmed our previous finding of
widespread significant decreases in
11
C-(
R
)-rolipram binding. The decrease was highly
significant, with an average change of −18% (Fig. 1, P=0.001, F=11.45, df=1,77 for group
difference in the 10 regions analyzed using repeated measures, two-way analysis of variance
with regions as the within subjects factor, Cohen’s d=0.77). The magnitude of the decrease
was similar across the 10 large preselected regions across brain (range= −17% to −21%).
The analysis using each voxel data and SPM confirmed widespread and similar magnitudes
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of decrease across brain areas found by the analysis based on large regions. SPM analysis
with no global normalization detected highly significant decreases in most brain areas with
family-wise-error (FWE)-corrected P<0.001. After applying global normalization, i.e.,
adjusting the average between controls and unmedicated patients, no small region in
unmedicated patients showed a significantly greater or smaller decrease than other brain
regions.
Other findings in this larger sample of controls vs. unmedicated patients were similar to
those reported previously. The decrease in rolipram binding was not caused by the
potentially smaller volume of brain regions in patients reported in the literature
17
because
PET data corrected for volume of gray matter (i.e., partial volume correction) showed highly
significant decreases in rolipram binding in unmedicated patients (−15%, P=0.005, F=8.31,
df=1,75, Cohen’s d=0.67; two patients who did not have high resolution structural MRI were
not included). Patients who smoked cigarettes (n=10) had significantly lower rolipram
binding (−23%) than non-smoking patients (n=34) (P=0.027, F=5.25, df=1,42, Cohen’s
d=0.71), although healthy controls showed no difference (3%) between smokers and non-
smokers. After removing eight smokers from the control group and 10 smokers from the
unmedicated patients group, 34 non-smoker unmedicated patients showed 14% lower
rolipram binding than 27 non-smoker controls (P=0.025, F=5.309, df=1,59, Cohen’s
d=0.60). Neither history of prior antidepressant treatment, comorbid anxiety disorders, nor
gender affected rolipram binding in unmedicated patients (P>0.21).
Age and gender were possible confounding factors that affected baseline rolipram binding
The current larger sample allowed us to investigate possible confounding factors that might
have affected rolipram binding. The previous smaller sample
8
did now allow us to perform
such investigations. In particular, age and gender affected rolipram binding, and these
confounding factors might have made it difficult to detect a possible relationship between
severity of symptoms and rolipram binding. Specifically, our analyses showed that, when
controlling for age, no correlation between baseline rolipram binding and severity of
symptoms was observed. In addition, interactions between rolipram binding, symptom
severity, and age were found in male, but not female, patients.
Significant correlations between baseline rolipram binding and MADRS scores were
observed in 32 male patients in nine of 10 large regions (P=0.008–0.029). However, in all
regions for male patients, rolipram binding was significantly negatively correlated with age
(P<0.039 in all regions). Furthermore, older male patients tended to show more severe
symptoms as measured by the MADRS (P=0.061). When controlling for age, the partial
correlation between baseline rolipram binding and MADRS score was significant only in the
frontal cortex (P=0.039 without correction for multiple comparisons) but not in other areas.
An SPM regression analysis that controlled for age in male patients detected no significant
relationship between severity of symptoms and rolipram binding. Taken together, these
analyses suggest that age was a key factor in the correlation between rolipram binding and
MADRS score, but that there was inadequate evidence of a relationship between rolipram
binding and MADRS score. Notably, female patients showed no correlation between
baseline rolipram binding and severity of symptoms or age (P>0.14). In addition, healthy
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