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Serotonergic abnormalities are hypothesized to contribute to obsessive-compulsive disorder (OCD). This study used positron emission tomography (PET) with the radioligand [11C]MDL 100907 to examine whether the distribution of one of the serotonin receptors, the 5-HT2A receptor, is altered in OCD.
Nineteen OCD subjects, free of psychiatric medications and depression, and 19 matched healthy controls underwent PET scans following injection of [11C]MDL 100907. Total distribution volumes (VT) were derived by kinetic analysis using the arterial input function. Two measures of 5-HT2A availability were computed (BPND and BPP). Groups were compared using a region of interest (ROI) analysis and voxelwise analysis of spatially normalized parametric maps. ROIs included cortical regions (orbitofrontal, dorsolateral prefrontal, medial prefrontal, anterior cingulate, temporal, parietal, occipital, and insular cortex) and limbic regions (entorhinal cortex, parahippocampal gyrus, and medial temporal lobe).
No significant group differences were observed in [11C]MDL 100907 BPND or BPP in the ROIs or in the voxelwise analysis of BPND maps. There was a significant correlation in the orbitofrontal cortex between [11C] MDL 100907 binding and age of onset, with earlier age of onset associated with higher binding.
In contrast to prior reports, people with OCD (free of psychiatric medications and depression) are not characterized as a group by major changes in 5-HT2A availability in cortical or limbic brain regions. Further research is warranted to examine potential differences in 5-HT2A availability between early and late onset OCD and to assess 5-HT2A function in relation to other neurotransmitter systems implicated in OCD.
Obsessive-compulsive disorder (OCD) is characterized by intrusive, distressing thoughts, images, or impulses (obsessions) and repetitive mental or behavioral acts (compulsions). Brain imaging studies suggest that OCD is associated with hyperactivity in a brain circuit involving the orbitofrontal cortex (OFC), striatum, and thalamus (1), and this hyperactivity may reflect glutamatergic dysfunction in OFC-striatal pathways (2-5). Serotonergic dysfunction has also been implicated based on the efficacy of serotonin reuptake inhibitors (SRIs) for OCD symptoms; some studies also found abnormalities in peripheral measures of serotonin function and in behavioral or physiological responses following challenges with serotonergic probes in OCD patients (6, 7). However, it remains unclear whether there are serotonergic abnormalities in OFC-striatal circuits in OCD.
In vivo imaging with positron emission tomography (PET) enables a direct examination of the serotonin system in the living human brain, using radioligands to visualize multiple 5-HT receptor types and reuptake transporters (8). Two PET studies examined the distribution of the serotonin transporter in the striatum, but neither found abnormalities in OCD (9, 10). On the other hand, 5-HT2A receptors in the OFC have been implicated in OCD because of their location on glutamatergic excitatory pyramidal neurons that project to the striatum and on GABAergic inhibitory interneurons that modulate these pyramidal cells (11, 12). Thus, reduced functioning or availability of 5-HT2A receptors in the OFC could lead to abnormalities in glutamatergic neurotransmission, potentially explaining the OFC-striatal hyperactivity found in some OCD imaging studies. Others have suggested that SRIs work in OCD by activating OFC 5-HT2A receptors, thereby reducing glutamatergic hyperactivity and decreasing OCD symptoms (2, 5, 13). If so, then baseline differences in 5-HT2A availability might explain individual differences in SRI response.
Two PET studies support a link between 5-HT2A receptors and OCD. Perani and colleagues (14) found small, widespread reductions in cortical 5-HT2A availability in OCD subjects compared to healthy controls using the selective 5-HT2A radioligand [11C]MDL 100907 (15). However, the sample size was small (n=9 OCD, n=6 controls), and the groups were not matched for age. In contrast, Adams and colleagues (16) did not find significant differences in cortical 5-HT2A receptor binding between OCD patients (n=15) and healthy controls (n=15). They did find increased 5-HT2A binding in the caudate nucleus in OCD. However, the Adams study used [18F] altanserin, a radioligand that is less selective for 5-HT2A receptors than [11C]MDL 100907 and whose modeling is complicated by the fact that it has radiolabeled metabolites that enter the brain (17). Moreover, the density of 5-HT2A receptors is very low in the caudate (18), such that measuring 5-HT2A binding with any currently available PET radioligand is not reliable due to low signal to noise ratio.
To determine whether the distribution of OFC 5-HT2A receptors is abnormal in OCD, we examined 5-HT2A availability in 19 adults with OCD and 19 matched healthy controls using PET and the selective 5-HT2A radioligand [11C]MDL 100907. Because this is the largest OCD sample ever studied, we also examined 5-HT2A availability in other cortical and limbic regions where reliable quantification is possible (19). Based on current models of OCD, we hypothesized that OCD subjects would have decreased 5-HT2A availability in the OFC compared to healthy controls. We also explored whether 5-HT2A availability was associated with OCD severity, OCD symptom dimensions, or age of OCD onset.
The institutional review board of the New York State Psychiatric Institute/Columbia University approved this study. Written informed consent was obtained from each subject after study procedures were explained. Subjects were recruited by advertisements and word-of-mouth.
Participants were between the ages of 18 and 55, had no significant medical problems, were not pregnant or nursing, had no current or past neurological disorder (other than Tic Disorder), and were free of psychoactive medications. Women who were using hormonal contraceptives or were postmenopausal were excluded. OCD subjects met OCD criteria from the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) for at least one year and had no other current Axis I psychiatric disorder (except Specific Phobia in one subject). None were receiving OCD treatment (medication or cognitive-behavioral therapy [CBT]) at the time of study participation. Healthy controls had no current or past DSM-IV Axis I disorder; none had a family history of OCD as assessed by the Family History Screen (20). Groups were matched for age, sex, ethno-racial categories, and current nicotine use.
Diagnoses were made by clinical interview and confirmed with the Structured Clinical Interview for DSM-IV (21). Medical health was confirmed by physical exam, blood tests, urinalysis, urine drug screen and electrocardiogram. On the day of the PET scan, OCD and depressive severity were assessed by a trained rater using the Yale-Brown Obsessive Compulsive Scale (Y-BOCS, scale range: 0 to 40; (22, 23)) and the Hamilton Depression Rating Scale (HAM-D, 17-item, scale range: 0 to 50; (24)). The Y-BOCS checklist was used to generate scores for each OCD patient along five different symptom dimensions (contamination and cleaning, taboo thoughts, doubt and checking, symmetry and ordering, and hoarding), using the approach recommended by Pinto and colleagues (25-27).
PET was performed with the ECAT EXACT HR+ (Siemens/CTI, Knoxville, TN) operated in the 3D mode. Prior to the PET scan, an arterial catheter was inserted into the radial artery of each subject after completion of the Allen test and infiltration of the skin with 1% lidocaine. A venous catheter was inserted into a forearm vein on the opposite side. Head movement was minimized with a polyurethane head immobilization system (Soule Medical). Following a 10 min transmission scan, [11C]MDL100907 was injected intravenously over 30 seconds. Emission data were collected for 120 minutes, as 21 frames of increasing duration. Previous analysis of [11C]MDL 100907 regional uptake established that 120 minutes of data were required for stable estimates of 5-HT2A availability in all cortical regions (19). Projection data were corrected for attenuation, scatter and random coincidences and reconstructed by filtered backprojection to a 128×128×63 array (Shepp filter, 0.5 cycles/ray, voxel dimensions 1.7×1.7×2.4 mm3).
Following [11C]MDL 100907 injection, arterial samples were collected with an automated sampling system every 10 seconds for the first two minutes, every 20 seconds for the 3rd and 4th minutes, and manually thereafter at longer intervals (total = 31 samples). Samples were processed as described elsewhere (29). Six additional samples (collected at 2, 16, 30, 50, 70 and 90 min) were processed to measure the fraction of plasma activity representing unmetabolized parent compound (30). The metabolite-corrected input function and clearance of the parent compound (CL, L·hr−1) were calculated following published methodology (31).
T1-weighted anatomical MR images for region drawing and coregistration were acquired on a GE 1.5T Signa Twin Speed Scanner. Segmentation was performed following previously published methods (30, 32). MRI scans were reviewed by a neuroradiologist and confirmed to be free of gross structural abnormalities.
Image analysis was performed blind to diagnosis using MEDx software (Medical-Numerics, Inc., Germantown MD) following procedures described elsewhere (33). PET data were realigned to a reference frame with the realignment tool in the SPM2 software package (34). Realigned PET data were coregistered to individual subjects’ MRI image using a maximization of mutual information algorithm (35) as implemented in SPM2.
Regions of interest (ROIs) and region of reference (cerebellum; CER) boundaries were drawn on the MRI according to criteria derived from brain atlases and published reports (see 32 for original references). A segmentation-based method was used for neocortical regions, and a direct identification method was used for subcortical regions (29). Regional volumes were computed (gray matter voxels only for neocortical regions) using the statistics tool in MEDx. For bilateral regions, right and left regions were summed. The 11 ROIs were: orbitofrontal cortex (OFC, see Figure S1 in the Supplement which illustrates the derivation of this primary ROI); dorsolateral prefrontal cortex (DLPFC); medial prefrontal cortex (MPFC); anterior cingulate cortex (ACC); temporal cortex (TEM); parietal cortex (PAR); occipital cortex (OCC); insular cortex (INS); entorhinal cortex (ENT); parahippocampal gyrus (PHG); and medial temporal lobe (MTL, a spatially weighted average of 5 limbic structures: uncus, amygdala, hippocampus, ENT, and PHG). All had sufficient 5-HT2A receptor density (mean BPND of ≥ 0.5, as defined below) to provide a reliable specific binding signal for analysis (36).
[11C] MDL100907 distribution volumes (VT, mL·cm−3), the ratio of total regional activity to arterial plasma activity at equilibrium (37), were estimated in each ROI using two-tissue compartment analysis (2TC) with metabolite-corrected arterial plasma input function. Brain activity was corrected for blood activity contribution assuming a constant 5% blood volume fraction (38). Data were fitted to the 2TC model by nonlinear least-squares regression, using in-house developed software implemented in MATLAB (The math Works, Inc, Natick MA).
Derivation of 5-HT2A receptor parameters was based on the assumption that human cerebellum is virtually devoid of 3H- and 11C-labeled MDL100907 specific binding (39) so that cerebellum VT (VT CER) was representative of equilibrium nondisplaceable binding (VND). Two measures of [11C]MDL100907 equilibrium specific binding (binding potential) were calculated (40): 1) BPP, the ratio at equilibrium of specifically bound radiotracer to unmetabolized tracer in arterial plasma, derived as the difference between VT in the ROI and VT CER,; and 2) BPND, the ratio at equilibrium of specifically bound radiotracer to nondisplaceable [free plus nonspecifically bound] radiotracer in tissue, derived as the ratio of BPP to VT CER. Pharmacological interpretations of these outcome measures are
where Bavail is the regional density of 5-HT2A receptors available to bind radioligand in vivo (nmol·L−1), KD (nmol·L−1) is the in vivo affinity of the tracer for 5-HT2A receptors, fp (unitless) is the fraction of [11C] MDL100907 in arterial plasma not bound to plasma proteins, and fND (unitless) is the free fraction of free plus non-specifically bound [11C] MDL100907 in brain tissue (40). BPND was chosen a priori as the primary outcome measure.
Volumes of the 11 ROIs and the cerebellum were examined for group differences using two sample t-tests. Ominbus tests of group differences were performed with linear mixed-model analysis (LMM) for BPND and BPP, with regions as repeated measure and diagnostic group as between subject factor. MTL was not included in the LMM analysis as it is a composite of other regions. Exploratory analyses also compared OCD subjects and controls on each parameter within each R0I using two sample t-tests. Within the OCD group, the relationship between 5-HT2A availability and OCD severity (measured by the Y-BOCS), each of the five OCD symptom dimensions (i.e., contamination and cleaning, taboo thoughts, doubt and checking, symmetry and ordering, and hoarding), and age of OCD onset was examined using the Pearson's product-moment correlation, correcting for age (because 5-HT2A availability decreases with age (19)). All statistical tests were 2-tailed with level of significance α= 0.05.
To examine if there were small focal regions of group differences that the ROI analyses did not detect, an additional voxelwise analysis was performed. One tissue compartment analysis (1TC) was applied to each voxel, and a VT map was generated for each subject. While the 2TC approach was considered more parsimonious for ROI analysis, the 1TC model was used for voxelwise analysis due to its greater stability in the setting of lower signal to noise ratio, and the similarity of the outcome measures between the two methods when applied to ROI data. BPND maps were generated using VT CER as an estimate of VND as before. Each subject's data were nonlinearly normalized into a common template space (MNI T1-weighted single subject MRI), by first transforming their MRI images into the template space using SPM2 software, and applying the transformation parameters to the BPND images. Data were smoothed with a 12mm isotropic Gaussian filter. Analysis was restricted to voxels with BPND values exceeding 20% of the overall image mean. Comparisons of group means were performed using the False Discovery Rate correction for multiple comparisons, with corrected significance level α = 0.05.
One-hundred and thirty-nine OCD subjects were evaluated. Seventy-five were ineligible: most (n=54) because of a comorbid medical or psychiatric disorder, some because they were on psychiatric medication (n=18), and the rest (n=3) for miscellaneous reasons. Of the 64 eligible OCD subjects, 40 declined participation, primarily due to concerns about the arterial line and/or radiation exposure. Twenty-four OCD subjects agreed to participate, but five did not complete the PET scan (arterial line could not be placed, n=3; declined after signing consent, n=2).
The demographic and clinical characteristics of the 19 OCD subjects and 19 matched healthy controls who completed PET and MRI scans are shown in Table 1. There was no group difference in mean age (t =−0.06, df = 36, p = 0.95). Gender and ethnicity distributions were matched. There were no significant group differences in days since last menstrual period in females (p > 0.56). One subject in each group currently used nicotine.
OCD subjects had clinically meaningful OCD symptoms, with a mean Y-BOCS score of 26 and a range from 20 (moderate OCD) to 36 (extreme OCD). All five OCD symptom dimensions were represented (i.e., contamination and cleaning, taboo thoughts, doubt and checking, symmetry and ordering, and hoarding), and each patient had symptoms in more than one dimension with two exceptions: one had only hoarding symptoms and one had only contamination and cleaning symptoms. All but seven OCD subjects were SRI-naïve. These seven had been off SRI medication for an average of 44 weeks (SD= 52; range: 10-156 weeks). Only two had been exposed to other psychotropic medications (buproprion, n=1; olanzapine, n=1). All but three were CBT-naïve.
PET scan parameters are presented in Table 2. There were no significant group differences in the injected dose, specific activity at injection time, injected mass, peripheral clearance of [11C] MDL100907, or fraction of [11C] MDL100907 in arterial plasma not bound to plasma proteins. There was no significant difference in the volume of distribution in the cerebellum (VT CER), assumed to be representative of equilibrium nondisplaceable binding (VND).
Volumes of ROIs are presented in Table 3. Region-wise comparisons showed no statistically significant group differences in ROI volumes, with two exceptions: OCD subjects had significantly smaller insula and parietal cortex than controls; neither remained significant after False Discovery Rate correction (41) for multiple comparisons (n = 11, excluding MTL).
The two measures of 5-HT2A availability, BPND (the ratio at equilibrium of specifically bound radiotracer to nondisplaceable tracer) and BPP (the ratio at equilibrium of specifically bound radiotracer to unmetabolized tracer in arterial plasma) are presented in Table 4 for each ROI. In LMM analyses, there was no significant effect of group for either outcome measure (BPND: effect of diagnosis: F(1,36) = 0.493, p = 0.487, effect of region: F(9,36) = 120, p < 0.001, region by diagnosis interaction F(9,36) = 0.956, p = 0.49; BPP: effect of diagnosis: F(1,36) = 0.273, p = 0.60, effect of region: F(9,36) = 87, p < 0.001, region by diagnosis interaction F(9,36) = 1.01, p = 0.44). Likewise, region-wise comparisons revealed no significant differences between groups in any region. A scatter plot for OFC BPND values in OCD subjects and healthy controls is shown in Figure 1.
Neither OCD severity nor any of the five symptom dimensions (current or lifetime) were significantly correlated with [11C] MDL100907 BPND or BPP in any ROI when adjusting for age. In contrast, there was a significant association only in the OFC between age of OCD onset and both BPND and BPP when adjusting for age (BPND: ρ = −0.68, p=0.002; BPP: ρ =−.57, p=0.015). The association with BPND survived False Discovery Rate correction (41) for multiple comparisons (n=10 ROIs, excluding MTL), whereas the association with BPP did not. The relationship between OFC BPND and age of OCD onset is illustrated in Figure 2.
Mean BPND maps are shown in Figure 3. 5-HT2A availability was high in cortical and limbic regions and low in the striatum, in agreement with the ROI analysis and the known distribution of 5-HT2A in the human brain (18). In the SPM analysis, no voxels or clusters of voxels were significantly different between the groups following False Discovery Rate correction.
This study compared 5-HT2A availability in the brains of OCD and healthy subjects. Contrary to our hypothesis, we failed to detect group differences in 5-HT2A availability in the OFC, in any of the other ROIs, or in the voxelwise analysis. Within the OCD group, regional 5-HT2A availability was not significantly correlated with OCD severity or symptom dimensions. However, there was an association between earlier age of OCD onset and greater 5-HT2A availability in the OFC when adjusting for age.
The lack of group differences in 5-HT2A availability in cortical and limbic ROIs conflicts with Perani and colleagues (14). They reported small, widespread cortical decreases in 5-HT2A availability in OCD using [11C] MDL100907. However, their sample consisted of only 9 patients and 6 controls, and the groups were not matched for age. 5-HT2A availability is known to decrease with age (19). On the other hand, the lack of group differences in cortical and limbic regions is consistent with the findings of Adams and colleagues (16) who used [18F] altanserin, a less selective 5-HT2A radiotracer than [11C] MDL100907, to study 15 OCD and 15 healthy control subjects. Adams and colleagues also reported increased 5-HT2A binding in the caudate nucleus in OCD. In a larger sample and using [11C] MDL100907, we found no caudate group differences (BPND: OCD = − 0.02 ± 0.11, healthy controls = 0.02 ± 0.07; p = 0.24, effect size = 0.39). However, given the low density of caudate 5-HT2A receptors in the human brain (18), it is difficult to quantify specific binding reliably with [11C] MDL100907 (caudate BPND ≤ 0.02 observed here) or [18F] altanserin (caudate BPND ≤ 0.13; (16)). Thus, any findings in this region should be treated with caution.
Despite the absence of group differences in 5-HT2A availability, we observed a strong and significant association between earlier age of OCD onset and higher 5-HT2A availability in the OFC. The voxelwise analysis showed a similar relationship (although it did not reach threshold for statistical significance) and suggested that this relationship was homogeneous across the OFC ROI. This finding should be viewed with caution given that only five subjects had early onset OCD (i.e., ≤ 10 years of age). Nevertheless, it does warrant a comment. A post-hoc review of the data suggested that those with early onset OCD had a trend toward higher 5-HT2A availability than those with later onset OCD; healthy controls were in-between. This raises the intriguing possibility that the pathophysiology of OCD (including the role of the 5-HT2A receptor) in early versus late-onset OCD might differ, as suggested by others (42-45). However, other factors cannot be excluded. For example, although OCD severity did not differ between those with early versus later OCD onset (mean Y-BOCS [SD]: 25.6 [1.5] versus 26.5 [4.4]), those with early onset were more likely to be SRI naïve (1 of 5 versus 6 of 13, with one subject missing age of OCD onset). Further research on potential biological differences between early and late onset OCD is warranted.
Our study has several strengths. First, it is the largest OCD study to measure 5-HT2A receptor availability using PET and to conduct ROI and voxelwise analyses. The observed difference in BPND and BPP measures between the two groups was very small in all ROIs, with relatively narrow 95% confidence intervals that were centered typically around zero and included zero, providing confidence in our null results. The voxelwise analysis was also negative. With regards to the OFC, our hypothesized region of interest, our sample size provided 80% power to detect an effect size of 0.93 and a group difference in BPND of at least 23% if it existed. However, the empirical effect size for BPND that we observed for the OFC was 0.05. More than 6000 subjects in each group would be needed for this difference to reach significance given the within-group variability we observed.
Second, OCD subjects were well-matched to healthy controls on age, sex, and ethnicity. Moreover, none of the OCD subjects had current major depression, and only four had ever had a major depressive episode. This is important because some PET studies have found 5-HT2A abnormalities in people with depression (46, 47). Additionally, the OCD sample was relatively uncontaminated by SRI treatment: 12 were SRI naïve, and the other seven had been free of SRIs for an average of 44 weeks. These factors as well as our larger sample likely explain why we had different findings than Perani and colleagues (14).
Finally, we used [11C] MDL100907, the first truly selective radioligand for the 5-HT2A receptor, which has excellent signal to noise properties and good test-retest reliability (19). In addition, we measured the arterial input function enabling the quantitative derivation of distribution volumes and confirmation of the lack of group differences in cerebellar distribution volume, a critical issue when calculating BPND and BPP.
Like all PET studies, this study also had limitations. First, [11C] MDL100907 provides reliable data only in cortical and limbic brain regions where 5-HT2A receptor density is relatively high (19). Thus, our study cannot address whether there are differences in 5-HT2A distribution in the thalamus, striatum, or midbrain. Moreover, this PET method can detect differences in 5-HT2A binding (reflecting the affinity or density of 5-HT2A receptors) but cannot assess 5-HT2A function. Second, our results cannot exclude the possibility of 5-HT2A alterations in certain homogeneous subtypes of OCD. As noted above, only five of our 19 OCD subjects had early onset OCD (i.e., ≤ 10 years of age), and only three reported OCD in a first-degree relative; this may be an inevitable trade-off of recruiting OCD adults who were free of medication and depression. Finally, although the largest OCD sample to study 5-HT2A availability with PET, our sample size was still relatively small to detect associations between clinical features and 5-HT2A findings. Given this, it was surprising to observe the strong association between earlier age of OCD onset and higher OFC 5-HT2A availability.
In conclusion, we did not find differences in 5-HT2A availability in cortical and limbic regions in OCD subjects who were free of psychiatric medication and comorbid major depression. However, earlier onset of OCD was associated with greater 5-HT2A receptor availability in the OFC. Further research on potential biological differences between early and late-onset OCD is warranted. Future studies should also assess 5-HT2A function in the OFC-striatal brain circuit in relation to the glutamatergic system that has been implicated in OCD.
This study was supported by the National Institute of Mental Health (R01 MH073915 to HBS) and by the New York State Office of Mental Health. We thank staff of the Anxiety Disorders Clinic and the Division of Translational Imaging for help with study administration (Donna Vermes), patient recruitment and coverage in the PET suite (Raphael Campeas, Carolyn Rodriguez, Lawrence Kegeles, Nina Urban, Ragy Girgis), database management (Andrew Schmidt and Page Van Meter), and expert research assistance (Jessica McCarthy and Rena Staub). We thank Drs. Marc Laruelle and Michael Liebowitz for advising on the original study design and Dr. Jay Gingrich for discussions about glutamatergic and serotonergic brain systems.
This study was supported by the National Institute of Mental Health (R01 MH073915 to HBS) and by the New York State Office of Mental Health. No other sources of biomedical funding supported this study. In the last two years, Dr. Simpson has received medication at no-cost from Janssen Pharmaceuticals for another NIMH-funded study and consulting fees from Pfizer Inc for advice regarding the medication Lyrica, and research funds from Neuropharm Ltd and from Transcept Pharmaceuticals. Dr. Slifstein has received consulting fees from GlaxoSmithKline and Amgen Inc. and research funding from Pierre-Fabre Inc. Dr. Abi-Dargham has received consulting fees from Lundbeck, Boehringer Ingelheim, research funding from GSK, and lecture fees from BMS-Otsuka, Merck and Sunovion.
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